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COLORANTS FOR NON-TEXTILE APPLICAT10NS Edited by H. S. FREEMAN

Textile Engineering, Chemistry and Science North Carolina State University Ra1eig h, NC 27695 -8302 USA A. T. PETERS

Chemistry and Forensic Science University of Bradford Bradford BD7 IDP UK

E LSEVIE R

2000

ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands

0 2000 Elsevier Science B.V. All rights reserved This work is protected under copyright by Elsevier Science, and the following terms and conditions apply to its use: Photocopying Single photocopies of single chapters may be made for personal use as allowed by national copyright laws. Permission of the Publisher and payment of a fee is required for all other photocopying, including multiple or systematic copying. copying for advertising or promotional purposes. resale, and all forms of document delivery. Special rates are available for educational institutions that wish to make photocopies for non-profit educational classroom use Permissions may be sought directly from Elsevier Science Rights & Permisxma Department. PO Box 800, Oxford OX5 IDX. UK: phone. (+44) 1865 843830. fax: (+44) 1865 853333, e-mail: permi~sions~elsevier.co.uk. You may also contact Rights & Permissions directly through Elsebier's home page (http.//www.elsevier.nl). selecting first 'Customer Support'. then 'General Information', then 'Permissinns Query Form' In the USA, users may clear permissions and make payment, through the Copyright Clearance Center. Inc.. 222 Rosewood Drive, Danvers, MA 01923, USA: phone: (978) 7508400, fax. (978) 7504744. and in the U K through the Copyright Licensing Agency Rapid Clearance Service (CLARCS), YO Tottenham Court Road.London WIPOLP, UK: phone. (+44) 171 631 5 5 5 5 : fax: (+44) 171 631 5500. Other countries may have a local reprographic rights agency for payments Derivative Works Tables of contents may he reproduced for internal circulation. but permishion of El*evier Science i s required for externai resale or dimibution of such material Permission of the Publisher is required for all other derivative work,, including compilation* and tran\lalions. Electronic Storage or Usage Permission of the Publisher is required to store or use electronically an) material contained in this wurl. including any chapter or part of a chapter Except as outlined aboke, no part of t h i h work may he reproduced. *tored in a retrieral ry\tem or tianhmitted In any form or by any means. electronic, mechanical, photocopying, recording or otherwire. uithout prior written p e r m l \ h n of the Publisher. Address permissions requests to. El*evier Science Rights & Permiraion, Department, at the mail. fax and e-mail addre$ae\ noted above Notice No re\ponsibility is aaumed by the Publi\her tor an) Injury andlor damage to per\on\ nr propert) 8 , d matter of products liability. negligence or otherui*e. or from any use or operation of any method\. prnduct,. in\tructmns or ideas contained in the material herein. Because of rapid advances in the medical sciences. in particular. independent verification of diagnoses and drug dosage5 \hould be made.

First edition 2000 Library of Congress Cataloging in Publication Data A catalog record from the Library of Congress has been applied for.

ISBN: 0-444-82888-5 @The paper used in this publication meets the requirements of ANSIiNISO 239.48-1992 (Permanence of Paper). Printed in The Netherlands.

V

Preface While the previous volumes in our color chemistry series have dealt mainly with colorants for textile application, we have decided that our final volume would concentrate on dyes aimed at non-textile markets. It seemed t o us that much of the new basic research is taking place in this area and that this is the principal way forward in color chemistry. With this in mind, the present thirteen-chapter volume covers the chemistry of natural and synthetic dyes that are designed to enhance the quality of our life apart from textiles. The first group of chapters covers the design, synthesis, properties and application technology pertaining to dyes for digital printing and photography. We believe that the reader will be pleased with the breadth and depth of information presented in each case. Of particular interest is the discussion of strategies for the design of dyes in these categories, with emphasis on enhancing technical properties. In view of certain new developments, the ink-jet chapter includes results from studies pertaining to dyes for textiles. The three chapters comprising section I1 of this volume cover the broad subject of dyes for food, drug and cosmetic applications and then provide an in-depth look at dyes for biomedical applications and molecular recognition. The chapter on dyes for molecular recognition places emphasis on applications in the biological sciences, including sensory materials and artificial receptors. While the former two topics have been covered elsewhere in the past, we believe that the present chapters are unequalled in scope. Section I11 provides an in-depth review of the design of laser dyes and dye-based functional materials. In the first of the two chapters, the major principles of laser operation are summarized. This is followed by a discussion on spectroscopic properties, such as activation and deactivation of absorbed light by laser dyes. Approaches to the development of new laser dyes are presented. The second chapter pertains to the synthesis of dicyanopyrazine-based multifunctional dyes. The visible and fluorescence spectra of these dyes in solution and the solid state are correlated with their three-dimensional molecular structures. Molecular stacking behavior and solid state properties of these “multifunctional” dye materials are presented. The final group of chapters pertains to natural dyes and dyes for natural substrates. In recent years, the impression among certain consumers that “natural” is betterlsafer has generated much interest in the use of natural dyes rather than synthetics. This has led to a few short discussion papers in which the environmental advantages t o using natural dyes have been questioned. The

vi

initial chapter in this group provides both a historical look a t natural dyes and a comprehensive compilation of natural dye structures and their sources. Though natural dyes are of interest as colorants for textiles, selected ones are used primarily in food and cosmetics. Chapter ten provides an update on the author’s previous reviews pertaining to structure-color-relationships among precursors employed in the coloration of hair. Chemical constitutions characterizing hair dye structures are presented, along with a summary of available precursors and their environmental properties. Similarly, the chapter on leather dyes covers constitutions and nomenclature, in addition to providing interesting perspectives on the origin and use of leather, the dyeing of leather, and key environmental issues. This volume is concluded with another look at colors in nature. In this case, rather than revisiting colors in plant life, an interesting chapter dealing with color in the absence of colorants is presented. Chapter twelve covers basic concepts of color science, which is a subject not presented heretofore in our volumes, and illustrates how 3-D assemblies leading to a plethora of colors are handled in nature. It is our hope that this atypical “color chemistry’’ chapter will invoke ideas that lead t o the design of useful colorants. The chapters presented in this volume demonstrate that color chemistry still has much to offer individuals with inquiring minds who are searching for a career path. The work presented herein demonstrates the creativity of today’s color chemists and the wide variety of interesting non-textile areas from which a career can be launched. The editors are grateful to the authors who appreciated the need for a book of this type and took the time to share their expertise.

H.S. Freeman A.T. Peters

vii

Contributors Roy R. Bradbury

Hexagon Tower Floor 8, Zeneca Specialties, PO Box 42, Hexagon House Blackey, Manchester M9 8ZS, UK

Kathryn K. Carr

Hexagon Tower Floor 8, Zeneca Specialties, PO Box 42, Hexagon House Blackey, Manchester M9 8ZS, UK

John F. Corbett

Bristol-Meyers Squibb, Stamford, CT 06922

Harold S. Freeman

North Carolina State University, Box 8301, Raleigh, NC 27695. USA

Jog0 de Moura

Departmento de Quimica, Universidada da Madeira Praga do Municipio, 9000 Funchal, Portugal

Michael Filosa

Polaroid Research Laboratories, Polaroid Corporation, Cambridge, MA 02139

Zbigniew J. Hinz


Masahiko Inouye

Department of Applied Materials Science, Osaka Prefecture University, Sakai, Osaka 599-853 1, Japan

Masaru Matsuoka

Kyoto Women’s University, Imakumano, Higashiyama, Kyoto 605, Japan

Theodore Pavlopoulos

U S . Naval Command, Control and Ocean Surveillance Center Research, Development, Test and Evaluation Division Code 361, San Diego, CA 92152

viii

Arnold T. Peters

Chemistry & Chemical Technology, University of Bradford, Bradford, West Yorkshire BD7 lDP, UK

Alois G. Piintener

Pulverweg 13, CH-43 10 Rheinfelder, Switzerland

Ulrich Schlesinger

Im Unterworth 39, D-79589 Binzen, Germany

Joseph F. Senackerib

Tri-K Industries, Montvale, NJ 07645

Dr. Mohan Srinivasarao

N.C. State University, Box 8301, Raleigh, NC 27695

David Waller


ix

CONTENTS

...................................................... Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Preface

v vii

.

I PRINTING AND IMAGING TECHNOLOGIES Chapter 1

Dyes for ink jet printing Kathryn Carr 1 2 3

4

5 6

3.1 3.2 3.3 4.1 4.2 4.3 4.4 5.1 5.2 6.1 6.2

7 8

Chapter 2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ink jet technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photorealistic ink jet printing . . . . . . . . . . . . . . . . . . . . . . . . Photorealistic ink jet printers . . . . . . . . . . . . . . . . . . . . . . . . Dyes for photorealistic ink jet printing . . . . . . . . . . . . . . . . . . Specialmedia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aqueous ink jet dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Blackdyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magentadyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yellowdyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cyandyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non-aqueous ink jet dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . Solvent inks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hotmeltinks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ink jet printing as a manufacturing process . . . . . . . . . . . . . . . . Colour filters for liquid crystal displays . . . . . . . . . . . . . . . . . Textile ink jet printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 2 2 2 3 4 5 9 16 22 25 25 26 27 27 30 32 33

Thermal transfer printing Roy Bradbury 1 2

3

2.1 2.2 2.3 2.4

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal wax transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Colourant donor sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wax transfer colourants . . . . . . . . . . . . . . . . . . . . . . . . . . . Receiver sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dye diffusion thermal transfer. D2T2 . . . . . . . . . . . . . . . . . . .

35 36 36 36 38 39 40

X

3.1 Dye sheet (dye donor sheet) . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Receiver sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Dye diffusion thermal transfer dyes . . . . . . . . . . . . . . . . . . . . 4 Lightfastness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 3

Dyes used in photography David Waller. Zbigniew J . Hinz. and Michael Filosa

1 2

3 4

5 6 7

8

I1

.

41 42 43 51 59 60

2.1 2.2 2.3 2.4 2.5 2.6 2.7 4.1 4.2 4.3 4.4

7.1 7.2 7.3 7.4 7.5 7.6

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conventional photographic dyes . . . . . . . . . . . . . . . . . . . . . . Developers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yellow couplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magenta couplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cyan couplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Two-equivalent couplers . . . . . . . . . . . . . . . . . . . . . . . . . . . Novel couplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Novel chromogenic development . . . . . . . . . . . . . . . . . . . . . Silver dye bleach process . . . . . . . . . . . . . . . . . . . . . . . . . . . Dye diffusion processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polaroid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kodak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fuji . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Agfa-Gevaert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Opacification dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bleachable filter dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensitizing dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The structure of sensitizing dyes . . . . . . . . . . . . . . . . . . . . . . The structure of merocyanines . . . . . . . . . . . . . . . . . . . . . . . Some additional important dye classes . . . . . . . . . . . . . . . . . . Requirements for sensitization . . . . . . . . . . . . . . . . . . . . . . . Three color spectral sensitization . . . . . . . . . . . . . . . . . . . . . Infrared sensitization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

61 61 64 67 70 74 81 83 86 87 90 90 96 100 104 105 107 109 112 115 117 119 120 124 125

FD&C AND MEDICAL DYES

Chapter 4

Color additives for foods. drugs. and cosmetics Joseph F. Senackerib

1

The historical development of certified color additives . . . . . . . . 1.1 Certified colors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

131 131

xi

2 3 4 5

6 7

Chapter 5

1.2 1.3 1.4 1.5 2.1

5.1 5.2 5.3

Federal food. drug and cosmetic act of 1938 . . . . . . . . . . . . . . Color additive amendments of 1960 (Public Law 86-618) . . . . . Provisions of the color additive amendments of 1960 . . . . . . . . Provisionally listed colors . . . . . . . . . . . . . . . . . . . . . . . . . . A survey of certified color additives . . . . . . . . . . . . . . . . . . . . Classification of FDA approved color additives . . . . . . . . . . . . Properties of color additives . . . . . . . . . . . . . . . . . . . . . . . . . Evaluation and analysis of color additives . . . . . . . . . . . . . . . . World-wide regulations and permitted color additives . . . . . . . . European commission (EC) . . . . . . . . . . . . . . . . . . . . . . . . . Japanese ministry of health and welfare (MHW) . . . . . . . . . . . US food and drug administration (FDA) . . . . . . . . . . . . . . . . . The future of color additives . . . . . . . . . . . . . . . . . . . . . . . . . General References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

133 133 134 139 141 141 163 167 171 171 172 172 186 187

Biomedical application of dyes Jog0 C.V. Pais de Moura Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dyes in bioanalysis and medical diagnostics . . . . . . . . . . . . . . . 2.1 DNA sequencing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Cancer detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Virus detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Cell detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Bacteria detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Analysis of blood gases. pH and fluid electrolytes . . . . . . . . . . 2.7 Membrane potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 Human chorionic gonadotropin (pregnancy testing) . . . . . . . . . 2.9 Cholesterol assay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10 Glucose assay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11 Bilirubin assay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.12 Diagnosis of periodontal diseases . . . . . . . . . . . . . . . . . . . . . 2.13 Investigation of protein chemistry and enzyme activity with diazonium salts . . . . . . . . . . . . . . . . . . . . . . . . 2.14 Miscellaneous analytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Dyes as therapeutic agents . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Anti-cancer drugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Photodynamic therapy and other photobiological applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Antiviral agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Antifungal and antibacterial agents . . . . . . . . . . . . . . . . . . . . 3.5 Other biomedical applications . . . . . . . . . . . . . . . . . . . . . . . 4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 2

189 190 190 193 195 198 201 203 205 206 206 206 207 208 209 211 212 212 215 224 225 226 221 228

xii

6

Chapter 6

References

.....................................

228

Functional dyes directed for molecular recognition: chromogenic and fluorescent receptors Masahiko Inouye 1 2 3 4 5 6 7 8

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Historical perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Metal cations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Ammonium cations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Inorganic anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Organic anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Neutral molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Saccharides and their derivatives . . . . . . . . . . . . . . . . . . . . . 5.2 Other neutral molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

238 239 240 241 250 256 256 258 260 260 263 267 268 268

.

111 FUNCTIONAL MATERIALS

Chapter 7

Laser dyes: structure and spectroscopic properties Theodore G . Pavlopoulos 1 2

3

4

2.1 2.2 2.3 3.1 3.2 3.3 3.4 3.5 3.6 4.1 4.2 4.3

5

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principles of lasers and dye lasers . . . . . . . . . . . . . . . . . . . . . Molecules as electronic dipoles . . . . . . . . . . . . . . . . . . . . . . Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dyelasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fluorescence and molecular structure . . . . . . . . . . . . . . . . . . . Activation and deactivation of organic molecules by light . . . . . Aromatic hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitrogen-heteroaromatics . . . . . . . . . . . . . . . . . . . . . . . . . . Other aromatic hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . Organic dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of molecular structure on fluorescence . . . . . . . . . . . . . Triplet-triplet absorption of organic compounds . . . . . . . . . . . . Triplet-triplet absorption in laser dyes . . . . . . . . . . . . . . . . . . Vibronic spin-orbit interactions in heterocyclics . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spectroscopic parameters that affect laser action . . . . . . . . . . . .

275 277 277 279 283 285 285 287 289 290 291 291 292 293 293 295 297

xiii

5.1 The gain equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 The rate equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Excitation sources for dye lasers ...................... 6.1 Pulsed lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Flashlamp excitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Long-pulse flashlamp and cw laser excitation . . . . . . . . . . . . . 6.4 Other potential excitation sources . . . . . . . . . . . . . . . . . . . . . 7 Spectroscopic test equipment . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Minimum equipment required . . . . . . . . . . . . . . . . . . . . . . . 7.2 Quantum fluorescence yields . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Triplet extinction coefficients . . . . . . . . . . . . . . . . . . . . . . . 7.4 Triplet photo-selection spectroscopy . . . . . . . . . . . . . . . . . . . 7.5 Measuring triplet state life times . . . . . . . . . . . . . . . . . . . . . . 7.6 Measuring spectroscopic parameters with Thiel’s jet stream method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Laserdyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 New laser dyes from molecular engineering . . . . . . . . . . . . . . 8.3 New laser dyes from spectroscopic studies . . . . . . . . . . . . . . . 8.4 Oligophenylenes. oxazoles, and benzoxazole . . . . . . . . . . . . . 8.5 Coumarin laser dyes .............................. 8.6 Rhodamine laser dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7 Other commercial laser dyes . . . . . . . . . . . . . . . . . . . . . . . . 8.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Quasi-aromatic laser dyes ........................... 9.1 syn-Bimane laser dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Di- a-pyridylamino-BF2 complex . . . . . . . . . . . . . . . . . . . . . 9.3 Pyrromethene-BF2 laser dyes . . . . . . . . . . . . . . . . . . . . . . . . 10 Final Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1 High-efficiency laser dyes . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 summary ..................................... 11 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Chapter 8

297 298 299 300 300 302 302 303 303 305 306 310 311 312 313 313 314 315 317 321 322 324 326 326 326 329 330 333 333 334 334 334

Multifunctional dye materials from new dicyanopyrazine chromophores Masaru Matsuoka 1 2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Syntheses of dicyanopyrazine related dyes . . . . . . . . . . . . . . . . 2.1 Syntheses of pyrazine dyes derived from DAMN 1 . . . . . . . . . . 2.2 Syntheses of pyrazine dyes derived from DCDCP 17 . . . . . . . . 2.3 Syntheses of pyrazine dyes derived from DADCP 29 . . . . . . . .

339 340 341 343 346

xiv

3

Molecular structure. chromophoric system and their functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Structure in the molecular level and their conformational changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Spectral properties and molecular stacking of pyrazine dyes in solution and the solid state . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Molecular stacking and solid state fluorescence of pyrazine dyes 3.4 Self-assembling and solid state functionality of pyrazine dyes . . 4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

348 348 354 360 374 379 380

.

IV NATURAL COLOWSUBSTRATES

Chapter 9

Natural dyes Alois G. Puntener and Ulrich Schlesinger Introduction and historical overview . . . . . . . . . . . . . . . . . . . . Isotins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alizarine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aliphatic polyenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flavonoids and neoflavanoids . . . . . . . . . . . . . . . . . . . . . . . Porphyrin derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tanning agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pigments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Purple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Indigo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Alizarine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Kermesic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Carminic acid (American cochineal) . . . . . . . . . . . . . . . . . . . 2.6 Saffron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 Bixin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 Flavonoids and neoflavanoids . . . . . . . . . . . . . . . . . . . . . . . 2.9 Chlorophyll . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10 Haemoglobin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 1 Tanning agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.12 Naphthoquinone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Dye Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Woolandsilk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Cotton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Leather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

382 383 387 392 398 404 405 407 414 415 415 418 423 425 427 428 430 430 436 437 437 438 439 439 439 441 443

xv

4

5

6 7 8

Chapter 10

Paper and wood dyeing . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cosmetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Painters’ and artists’ colours . . . . . . . . . . . . . . . . . . . . . . . . Ink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dyes for foodstuffs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fastness and shades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Safety. toxicology and ecology ....................... Use of natural dyes today . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Recommended literature and software program . . . . . . . . . . . . 8.2 Literature cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 3.6 3.7 3.8 3.9

Synthetic dyes for human hair John F. Corbett 1 2

3

4

Chapter 11

446 446 447 449 449 451 452 452 453 453 453 453

2.1 2.2 2.3 2.4 3.1 3.2 3.3

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidation hair dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Historical development . . . . . . . . . . . . . . . . . . . . . . . . . . . . The chemistry of oxidation dyeing . . . . . . . . . . . . . . . . . . . . Commercial dye palettes . . . . . . . . . . . . . . . . . . . . . . . . . . . Air oxidation dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Non-oxidative hair dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . Historical development . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of nitro-dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other non-oxidative hair colorants . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

456 457 457 459 463 466 467 467 471 475 476

Leather dyes Alois G . Puntener 1 2

3

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tanning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dyeing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The isoelectric point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Finishing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Origin and use of leather . . . . . . . . . . . . . . . . . . . . . . . . . . . Demand for leather dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of leather dyes in drum application . . . . . . . . . . . Classification of leather dyes in finishing . . . . . . . . . . . . . . . . Classification of fur dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . Dyeingleather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

478 478 478 479 480 480 481 482 482 483 484 484

Erratum for 'Colorants for Non-Textile Applications' (Eds: H.S. Freeman & A.T. Peters) (ISBN 0-444-82888-5) pages xvi and xvii

.

2.9 3 3.1 3.2 3.3 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 5 5.1 5.2 5.3 5.4 5.5 5.6 6 6.1 6.2 7 7.1 7.2 7.3 7.4 7.5 7.6 7.7 8 8.1 8.2 8.3 8.4 9 9.1 9.2 9.3 9.4 9.5

Classification of fur dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . Dyeing leather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Requirements in leather dyeing . . . . . . . . . . . . . . . . . . . . . . . Dyeing theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Trends over the last 100 years in leather dyeing . . . . . . . . . . . . The first synthetic cationic leather dyes . . . . . . . . . . . . . . . . . . The first anionic dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of the azo dyes . . . . . . . . . . . . . . . . . . . . . . . . Aciddyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Special azo dyes for leather . . . . . . . . . . . . . . . . . . . . . . . . . . Direct dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal complexes and mordant dyes . . . . . . . . . . . . . . . . . . . . Special metal complexes for leather . . . . . . . . . . . . . . . . . . . . New development trends in leather dyeing . . . . . . . . . . . . . . . . Computer recipe prediction . . . . . . . . . . . . . . . . . . . . . . . . . . Trichromatic dye systems . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulphur dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drum pigmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactive dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Liquid dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nomenclature and structure of leather dyes . . . . . . . . . . . . . . . Availability of leather dyes . . . . . . . . . . . . . . . . . . . . . . . . . . Structural formulas of leather dyes . . . . . . . . . . . . . . . . . . . . . Practical aspects of dyeing and dyeing auxiliaries . . . . . . . . . . . Carriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Change of pH in the dyebath . . . . . . . . . . . . . . . . . . . . . . . . . Influence of anionic auxiliaries . . . . . . . . . . . . . . . . . . . . . . . Influence of electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . Auxiliaries which react with the dyes . . . . . . . . . . . . . . . . . . . Amphoteric auxiliaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Top dyeings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fastness properties and how to improve and test them . . . . . . . . Wet fastness properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . Water repellents (waterproofing) ...................... Light fastness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . International standard methods for determining the colour fastness of leather . . . . . . . . . . . . . . . . . . . . . . . . . . . Trends in health and environmental regulation . . . . . . . . . . . . . Life cycle and risk assessments . . . . . . . . . . . . . . . . . . . . . . . Risk assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Regulations and laws . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pathways of leather dyes into the environment . . . . . . . . . . . . . Disposal of dyed leather goods .......................

484 484 484 484 485 490 490 491 491 492 492 494 495 498 499 500 502 503 504 505 506 508 508 508 534 534 534 535 535 537 537 538 538 538 539 540 543 544 544 545 546 551 553

10 11

Chapter 12

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Structural colors: nano-optics in the biological world Mohan Srinivasarao Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CIEcolorspace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General methods of color production ..................... Colors due to interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . Colors due to diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Colors due to scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Color generation on wings . . . . . . . . . . . . . . . . . . . . . . . . . . . Butterfly wings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Eyeshine of butterflies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Birdfeathers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Color specification and color vision ..................... 7.1 Polarization colors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Color properties of cholesteric liquid crystals . . . . . . . . . . . . . . . 8.1 Color by selective reflection . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Application of CLCs to optical limiting . . . . . . . . . . . . . . . . . . . 10 Color due to vibrational transitions . . . . . . . . . . . . . . . . . . . . . . 10.1 Is water blue? If so. why? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Color Palettes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 2 3 3.1 3.2 3.3 4 4.1

Index

554 554

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558 559 563 564 569 571 574 574 584 588 588 590 594 594 597 597 597 600 601 601 604 607

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Colorants for Nan-Textile Applications H.S. Freeman and A.T. Peters (Editors) 2000 Elsevier Science B.V.

1

1 Dyes for Ink Jet Printing K. CARR

Zeneca Specialties, PO Box 42, Hexagon House, Blackley, Manchester M9 8ZS, United Kingdom

1. INTRODUCTION Ink jet printing has made considerable progress during the last ten years and finds application in a variety of areas. It is used in the work place, in photographic imaging, in producing large colour posters and in other industrial applications such as marking, coding and addressing of packaging materials. The ink jet process itself possesses a number of advantages over more traditional printing methods and, as a consequence, the technology is moving into market sectors such as textile printing and the fabrication of colour filters for liquid crystal displays. Several factors play a key role in the successful operation of ink jet systems, including hardware design, computerized colour management, paper and media in general, colorants and ink formulations. During the early days of ink jet printing, much of the patent activity was related to the use or modification of dyes, which were originally designed for other application areas such as textile coloration. From the mid 1980’s onwards, however, research effort has been focused on improving the physical properties of ink jet dyes in their own right. A significant proportion of all the new developments is aimed at novel colorants for office systems. Since the bulk of office printers use water-based inks, these developments predominantly feature aqueous dyes of one form or another. Some of the more recent aspects of ink jet dye evolution, which have appeared in the patent literature, will be discussed in the sections that follow.

2. INK JET TECHNOLOGY Ink jet technology falls into two main categories, namely continuous and drop-on-demand

[1,2]. Continuous printing relies on the selective deflection of ink droplets to form the required

image. To facilitate this process, the droplets are charged electrostatically. Continuous ink jet printers tend to find greater application in industrial systems where they are used for printing a range of materials from carpets to packaging and labels. Resolution tends to be lower compared to drop-on-demand printing. Inks in this sector are mainly solvent based, and are used for marking non-porous substrates such as glass and metal or where fast drytimes are required. Black is the dominant colour. Materials used in inks for continuous ink jet printing are not required to meet the same high purity levels as those used in drop-on-demand printers.

Drop-on-demand printing, as the name suggests, involves the expulsion of ink droplets only when they are required to form part of an image. There are two subdivisions within this category, depending on the method used to fire the droplets. Piezo printing relies on the controlled deformation of a piezoelectric crystal, which generates a pressure wave within the ink chamber and culminates in the firing of a droplet. Piezo printers can operate using aqueousbased, solvent-based or hot melt inks. Thermal or bubble jet technology relies on the generation of a bubble of vapour in the ink chamber by rapidly heating a tiny resistor situated just behind the nozzles. The controlled expansion of this bubble causes a droplet of ink to be expelled. Thermal printers depend on a minimum amount of water being present in the ink for bubble generation and for this reason tend to use aqueous inks. In addition to black, the three primary subtractive colours yellow, magenta and cyan are also important. All of these materials must meet very stringent purity requirements before they can be used, especially in thermal applications where a phenomenon known as kogation can occur. This is basically a charring process that leads to deposits on the resistors and can have a marked detrimental effect on firing ability. Drop-on-demand printing is the dominant technology for office-type applications.

3. PHOTOREALISTIC INK JET PRINTING 3.1. Photorealistic ink jet printers Photorealistic ink jet printing is a relatively recent trend that has been inspired by the public’s desire to edit, print and reproduce photographic images. A number of associated technical advances have made this trend possible, including the availability of affordable printers capable of producing photographic image quality and the development of the digital camera. Other items such as inexpensive scanners have become available and have also played a role in prompting interest in this area. Printers may be dedicated to producing photographic quality images or they may be generalpurpose machines that have photo-capability. There have been huge improvements in resolution levels, to facilitate the production of photorealistic images and some dedicated printers can achieve 1440 x 720 and even 1200 x 1200 dots per inch [3]. Higher resolution is achieved at the cost of print speed and it can take, on average, ten minutes to produce a full colour A4 image. Drop volumes in general are considerably smaller and are typically around 14 picolitres for a 1440 dpi machine [4]. The number of colour levels needed to create the illusion of continuous tone has increased and some printers feature six inks rather than the usual four in order to achieve this. The additional two inks are Zighr versions of the cyan and magenta. 3.2. Dyes for photorealistic ink jet printing One of the major uses of photorealistic ink jet printing is in taking pictures of people and it is very important to be able to reproduce skin tones reliably. For this reason, a reddish yellow in the ink set is critical. This, together with a bluish magenta and a greener cyan, constitutes the photographic trichromat. This is distinct from the trichromat of process colours used in printing magazines and newspapers. Higher chroma dyes are desirable in order to produce vibrant images, and robustness requirements are strict since many photographs are placed in areas where they may be exposed to high light intensities and varying

3

humidity. The nature of the media is an important consideration when selecting dyes for this area.

3.3. Special media The successful creation of ink jet printed images of photorealistic quality and robustness relies to a considerable extent on the use of appropriate substrate material. Plain paper has given way to special media that give the look and feel of a silver halide photograph. Careful choice of the materials used in the construction of these media is critical in order to ensure that the necessary level of water resistance, light fastness and print quality are obtained. A typical advanced ink jet medium comprises a multilayer structure that serves to control the behaviour of the ink droplets after deposition (Figure 1) [5].

Figure 1. Schematic diagram of an advanced ink jet medium. Ink receiving coating: Substrate: Backcoat:

This may be a single or multiple ink absorptive material containing dye-fixing components. Coated or treated paper, plastic film, metallised foil, fabric etc. Maintains the dimensional stability of the media.

One of the largest technical challenges for ink jet printing is increasing print speed. This is a function of the capabilities of the printer itself and the time required for the imaged media to dry. The latter is important in order to avoid the transfer of ink to any other surface with which it is placed in contact. The majority of inks are aqueous based, with small amounts of high boiling organic solvents such as glycols, so the media must have high water absorptivity to achieve a fast drytime. Drying mechanisms include absorptive drying and evaporative drying, the former being the dominant mode. Absorptive drying can be broken down into two components, namely capillary drying and molecular diffusion. In the case of porous media, the absorptive drying process involves capillary movement of the ink from the surface into the bulk. With coated media, absorptive drying involves molecular diffusion of the ink vehicle into the bulk medium through ink-media interaction. As well as ink surface tension, absorptivity depends on factors such as the hydrophiliclhydrophobic balance within the receiver layer and makes the correct choice of

4

polymeric binder very important. Receiver polymers include polyvinyl alcohol, gelatin, polyvinyl pyrrolidone, acrylate and acrylamide polymers, cellulose, and polyurethane derivatives. In general, the higher the coating weight of the receiver layer, the lower the drytime 161. Water-fastness and general control of dye migration through the medium is achieved by the addition of fixing materials or mordants. Most aqueous dyes are made up of hydrophobic 'cores' to which ionic groups such as sulfonic or carboxylic acids are appended. Mordants are selected which are able to complex these dyes through chemical and physical means. Since most dyes are anionic in nature, mordants tend to be cationic and are typically polymers containing quaternary ammonium, phosphonium and sulfonium functionality. Other coating components include amorphous silica or silica gels and their derivatives, which are included for their ink absorptivity. Oxides or hydrates of aluminum, calcium and magnesium are also employed, to provide unique properties such as gloss, vividness of colour or to contribute to dye fixation. Optical brighteners, surfactants, cross-linkers and plasticisers may also be used in combination with UV and light stabilizers. The surface energy and pH of the medium are important factors affecting ink and dye behaviour in the medium and this must be taken into account when considering the overall composition of the receiver layer. The nature and formulation of the ink being used as well as the end use of the printed output play a major role in defining the type medium that will be required. This has led to considerable patent activity over the last few years in this area [7]. 4. AQUEOUS INK JET DYES

Colorants comprising the first generation of ink jet dyes were selected for their vividness (high chroma), good aqueous solubility, and the stable inks derived therefrom which gave a reliable printing performance. Ink jet printers have become considerably more sophisticated over the years, and the demand for dyes and inks capable of producing very high print quality has grown [I]. The properties of an effective aqueous dye include the following: Good tinctorial strength High water solubility (5-20%) High water-fastness on the substrate employed Rapid fixation after deposition Good durability, especially light-fastness High chroma colours Good thermal stability (for thermal ink jet systems) Low toxicity Ink formulation research has complemented dye development and has been aimed towards achieving the following print properties: Good optical density No feathering Minimal black-to-colour bleed Uniform and controlled drop spreading

5

Good water-fastness Rapiddrytime Smear resistance Media compatibility In addition to the above print qualities, the ink formulation must also be compatible with the materials from which the ink cartridge is constructed. It must promote dye solubility, prevent clogging or crusting of the nozzles, and ensure a long shelf life. 4.1. Black Dyes In the early days of ink jet printing, inks were formulated using dyes from other application areas after they had been rigorously purified. At this stage, an understanding of the physical properties required by chromophores in order to be effective ink jet materials was still in its infancy. Initially, highly soluble molecules were used primarily because they gave stable, reliable ink formulations. CI Food Black 2 (l),a simple disazo structure with four sulphonic acid groups, was used extensively and gave good neutral black shades [1,2].

1

Black is an important colour in any printing application because of the need to be able to print good quality text. One of the major problems associated with highly soluble dyes such as 1 is poor water-fastness, and handling a print with slightly moist fingers could result in serious smudging of the image or text, especially with plain paper. The desire to improve waterfastness has been the driving force behind much of the research into new dyes for ink jet printing. Initially, modified versions of CI Food Black 2 emerged, but they showed little improvement in water-fastness. It was only in the late 1980’s that a real step-change in performance became available, due to the introduction of dyes that had differential solubility. The ideal water-fast dye is required to have high water solubility for ease of use in the ink but low water solubility after deposition on the paper. Most aqueous inks are formulated slightly alkaline (pH 7.5-10) and most plain papers are slightly acidic (pH 4.5-6.5). The differential solubility approach involves incorporating into the chromophore functional groups with a pKa value between 6.5 and 8. In the alkaline ink, the groups are ionized and confer good solubility, whereas on the paper considerably fewer groups are ionized, resulting in a dramatic reduction in solubility. The functional group that has been used most successfully is the carboxylic acid group. In practice, this has meant the synthesis of new dyes in which a large proportion of the original sulphonic acid groups are replaced by carboxylic acid groups [8,9];

6

compare, for example, formula 2, one of the original chromophore types designed to have differential solubility.

&N\\

N

n=Oorl x = 1or2 2

There has been a considerable amount of research into black dyes for ink jet printing since these early developments. Chromophores are being sought which, in addition to high waterfastness, have additional attributes such as higher print optical density, improved print quality, reduced black-to-colour bleed, enhanced smear fastness and better reliability. Some of the major contributions will be discussed in this section, in which the chromophores are divided according to the number of conjugated azo linkages present in the molecule. The greatest level of attention appears to be focused on disazo dyes, as judged by the amount of patent activity in this area.

4.1.1. Disazo Dyes Since the introduction of chromophores such as 2, further developments have resulted in derivatives with a greater proportion of carboxylic acid groups to achieve still higher waterfastness. In order to facilitate this, it has been necessary to design modified intermediates for use in dye synthesis. Examples such as 3 and 4 feature the intermediate N-carboxyphenyl Gamma acid (2-amino-8-hydroxy-6-sulphonaphthalene) which is conveniently prepared via a Bucherer reaction involving 2,8-dihydroxy-6-sulphonaphthalene and 4-aminobenzoic acid [ 10,l I].

+

3; Ar =

4;Ar =

*

4.1.2. Trisazo Dyes The synthesis of larger chromophores of the trisazo variety frequently relies on the tetrazotization and coupling of larger aromatic diamines as a means of extending the conjugation within the molecule, to produce good, neutral black shades. In the past, benzidine was used extensively but its use is now restricted because of its toxicity. One example of a benzidine alternative is used in the synthesis of CI Direct Black 168 (5).

R

HO,S

SO,H

NH2

5 R=H

More recently, analogues of CI Direct Black 168 have been disclosed and which are reported to have improved solubility and stability in ink formulations [12-141. Dye 6, for example, with a ,,A of 630nm, is reported to give deep black prints with excellent lightfastness and water-fastness. Polyamine triazinyl substituents have been used in other trisazo dyes to promote rapid fixation and good abrasion resistance [15]. Carboxylated trisazo chromophores such as 7 have been reported and are claimed to have good wet-fastness and optical density [ 161; as with other carboxy analogues, the ammonium salt is preferred.

8

7 Complex polyfunctional molecules such as 7 are prepared via a series of steps in which careful control of pH is essential to minimise the generation of impurities. The sequence begins with the diazotization and coupling of 5-aminoisophthalic acid to l-amino-8-hydroxy-3,6disulphonaphthalene (H-acid) under slightly acidic conditions to ensure coupling occurs ortho to the amino group. The next step involves diazotization and coupling of 5-acetylamino-2aminobenzoic acid under alkaline conditions (pH 8), this time ortho to the hydroxyl group. Alkaline hydrolysis releases the amino group required for the formation of the final azo linkage via coupling to 3-aminophenol under alkaline conditions. Control of regiochemistry of this final bond formation is difficult and a mixture of isomers is usually obtained. However, this can be desirable since it often results in better solubility and a more neutral shade.

4.1.3. Tetrakisazo dyes There are relatively few tetrakisazo chromophores reported in the patent literature. These dyes appear to be somewhat simpler, with most of the structural modifications needed to achieve improvements in performance being introduced on the ends of the molecules. As before, carboxylated species are used to achieve water-fastness [17,18]. In the case of compound 8, for example, it was found beneficial to incorporate dicarboxyalkylamino groups to achieve good ink stability and reliability. This dye, with a,A at 632nm, is reported to have a high tinctorial strength and gives high definition images with good fastness properties, especially wet-fastness on plain, acid papers. N,C O N ,H , NH40Zc>

H O a N - c 0f C ~ ONZH NH 44

q/N,&N*N D""" NH,O,S

SO,NH,

8

9

4.1.4. Metallised dyes Metallised disazo dyes such as 9 have been proposed for use in ink jet printing but they are not always neutral in shade and have to be mixed with small amounts of a second component such as a yellow [ 191. The use of a mixture is claimed to offer improved solution stability and reliability of printing. Fastness properties, especially light-fastness, are reported to be good. In other dyes, the two components have been covalently linked together via the triazine moiety.

L = Colourless ligand, e.g. water 9

4.2. Magenta dyes The identification of a magenta chromophore for use in ink jet printing which possesses all the desirable attributes in a single molecule has yet to be realized. Many properties such as high chroma and good light-fastness appear to be mutually exclusive. The number and variety of magenta dye patents reflects the extent of the effort in this area. In this section, the dyes have been divided according to chromophore and the discussion will focus on the more recent developments in each individual area. 4.2.1. Azo Dyes The vast majority of patents claiming novel magenta dyes for use in ink jet printing describe azo dyes of one type or another. Before describing the different types of dye structure, it is worth focusing on one important feature of this type of chromophore. Tautomerism is a common phenomenon of dyes that feature phenolic coupling components. This is an equilibrium process which lies in favour of the hydrazo rather than the true azo form (Figure 2) [1,21. a

S0,Na N

Na0,S

+

\

N

’

~

a E , N , fS0,Na i

A

.c

S0,Na

Na0,S

\

’

Figure 2. Tautomerism in arylazonaphthol-based dyes.

S0,Na

10

In comparison, dyes featuring arylamino coupling components exist almost exclusively in the azo form (Figure 3).

Figure 3. Structure of arylazoamine-based dyes. Tautomerism can have a profound effect on the properties of dyes, especially on their robustness. In particular, the deterioration of many dyes on exposure to light often occurs as a direct result of the attack of singlet oxygen at a vulnerable site in the chromophore. In the case of hydrazo dyes, the point at which the 'azo' linkage is attached to the coupling component is most susceptible to attack. The resulting Ene-type process results in scission of the azo bond. This mechanism is not prevalent in dyes that exist in the true azo form. It can be seen, therefore, that a careful and subtle choice must be made between chromophores within a very similar group depending on the particular end application being considered. The magenta dyes described within this section are divided according to the type of coupling component upon which they are based. These are in general well-known, commercially available materials and are chosen for a particular effect such as hue, chroma, and light-fastness.

4.2.1.1. H-Acid based dyes l-Amino-8-hydroxy-3,6-disulphonaphthalene (H-acid) has been used in the preparation of aqueous magenta dyes for many years. These dyes tend to be bright, bluish-red, colorants with good water solubility. Light-fastness is generally moderate. The added benefit of using a polyfunctional component such as H-acid is that further elaboration is convenient and dyes can be 'fine tuned' reasonably easily to give optimum performance in a particular environment. Among the simpler structural types of dyes in this group are those based on N-acyl, benzoyl or -tosyl derivatives of H-acid. Compound 10 for example absorbs at 517 and 530nm and is claimed to afford good wet-fastness and light-fastness on recording media such as plain paper [20]. Its synthesis is straightforward and involves the diazotization and coupling of 4aminotoluene-3-sulphonicacid with N-benzoyl H-acid at pH 8.

11

10 Somewhat more elaborate and perhaps rather more common magenta dyes for ink jet applications are those featuring triazinyl derivatives of H-acid, e.g. compound 11 [21]. The triazinyl moiety is introduced easily using the commercially available intermediate cyanuric chloride (12).

11

12

Careful control of temperature, stoichiometry and pH allows three different substituents to be appended to the symmetrical triazine ring, if desired, in three sequential SNA reactions. Control of this stepwise process is facilitated primarily by the reduction in reactivity of the triazine system after each successive substitution reaction. As the highly electronegative chlorine atoms are replaced, the ring becomes less electron deficient and therefore less reactive in the SNA process. In practical terms, this implies performing the first substitution at O"C, the second around 30°C and the third between 50 and 70°C depending on the nature of the nucleophile being employed. In ink jet dyes, triazine substituents include aromatic and aliphatic amines, thiols and alcohols, as well as the N H 2 and OH groups. Halogen substituents can give rise to hydrolytic instability problems in the ink and are not generally used. Compound 11 is assembled by first preparing the dichlorotriazinyl derivative of H-acid by reaction with a slurry of cyanuric chloride in acetone and water at pH 6. After two hours, a solution of diazotized 2-amino-4,5-dimethylbenzene sulphonic acid is added and the pH of the mixture adjusted to 7 to effect coupling adjacent to the hydroxyl group. Rather than isolate the highly reactive dichlorotriazinyl dye, the mixture is diluted with water and used directly in the next stage of the synthesis. In the final two steps, both of which involve triazine substitution reactions, the less reactive nucleophile is usually added first. Thus, a solution of 4-

12

aminosalicylic acid is added and the mixture is stirred overnight at room temperature and a pH of 6-7. The resulting monochlorotriazinyl dye can now be safely isolated, typically by precipitation, which is brought about by the addition of 20% aq. NaCl and subsequent filtration. The final step in the preparation of compound 11 involves heating an aqueous solution of the monochloro compound and ethanolamine for three hours (Scheme 1). The product is isolated as described above and desalinated, usually by dialysis. Compound 11 is reported to give bright magenta ink jet prints on plain paper which have good light- and wet-fastness. The desire to improve water-fastness, especially on plain paper, has generated a number of technical approaches designed to achieve this goal. The use of carboxylic acid groups, where possible instead of sulphonic acid groups, as discussed in the section on black dyes, is one means of obtaining an improvement in performance. However, the nature of the intermediates commonly used in magenta dye synthesis means the molecules will still have a degree of sulfonation and the differential solubility effect is reduced somewhat.

4.2.1.2. Gamma acid based dyes A second aminonaphthol to find application as a coupling component in the synthesis of magenta dyes is 2-amino-8-hydroxy-6-sulphonaphthalene(Gamma acid). Magenta dyes based on Gamma acid tend to be more hypsochromic and somewhat lower in chroma than those based on H-acid. Owing to the fact that Gamma acid dyes exist almost exclusively in the true azo form, light-fastness is usually superior to that of the H-acid dyes. The structurally simpler members of this family tend to be acylated compounds such as CI Acid Red 37 (13) but the more recent examples in the patent literature are triazinyl derivatives [22,23].

Na0,S

13 A typical synthesis of magenta dyes based on Gamma acid varies somewhat from that employed in the case of the H-acid analogues. In this case, nitroanilines tend to be employed and the second, latent, amino group is generated by selective reduction once the diazotization and coupling stage is complete. In order to ensure that the formation of the azo linkage occurs adjacent to the amino group, the coupling process is carried out under acidic conditions (pH 45). Reaction with cyanuric chloride and subsequent condensation with other nucleophiles occurs at the end of the synthetic sequence and is effected under similar conditions to those described for H-acid dyes. Scheme 2 provides a representative synthetic sequence in detail.

13 CI N A N

-

A )&

Cyanuric chloride Water/Acetone

Na0,S

S0,Na

0-5 Degrees PH 6

NaO,S

CI

S0,Na

0-5 Degrees

Me

N A N Me Me

2.

ANAC1

H,N-OH 70 Degrees

HO CO,H

Me Me

11

Scheme 1. Synthesis of magenta dye 11.

PH 7 Me

14

@ & ; -

Sodium Water sulphide

NO,

\ /

pH 4-5

HO,S

$ Na0,S

I

NH2

Cyanuric chloride 0-5 Degrees

Na0,S N

\ /

HO,S

HO,S

\ /

\ /

PH 7 50 Degrees

b

H

pH 7-7.5 30 Degrees 4

\ /

OH

N

14

S

Na0,S

pH 7-8 S0,Na

HO,S CI

/

Scheme 2. Synthesis of a Gamma acid-based magenta dye. The dye (14) formed in Scheme 2 absorbs at 518nm. It is claimed to have high water and ink solubility and to give prints with good water- and light-fastness [23].

4.2.1.3. Metallized azo dyes Probably one of the best ways to achieve an improvement in light-fastness is to protect the vulnerable azo linkage by metal complexation. However, dyes of this type invariably suffer from a reduction in chroma and, to some extent, poorer solubility. This is believed to be attributable to the formation of sheet like aggregates in some cases [24]. Metals that have been used in this area include copper [25] and mixtures of chromium and cobalt [26]. The copper complex 15 is an example of this type of dye.

15

S0,Li

15 (L = Colourless ligand, e.g. water) 4.2.2. Non-Azo dyes There are far fewer magenta dyes belonging to the non-azo category, especially for use in aqueous inks. Of the dyes that are known, virtually all are xanthenes of one type or another. 4.2.2.1. Xanthenes Xanthene dyes with their narrow absorption curves and tendency to fluoresce can be used to produce very bright ink jet prints, and one particular compound to have found extensive application in this area is CI Acid Red 52 (16). Unfortunately, the light-fastness of this chromophoric system is particularly poor.

I

S0,Na

16 The bulk of synthetic developments to date have focused on improving the water-fastness of this system. To this end, xanthenes were modified by incorporation of arylamino substituents instead of the diethylamino groups found in CI Acid Red 52. Furthermore, the arylamino moieties were themselves substituted with carboxylic acid functionality, and the dyes isolated as the ammonium salts, in order to induce differential solubility behaviour and improve waterfastness [27]. Compound 17 is an example of a modified xanthene that is reported to give bright magenta prints on plain paper with good water-fastness and improved light-fastness.

16

NH,O,C

6’” 17

Synthesis of the xanthene dyes begins with 3,6-dichlorosulphonefluorescein(18), which is treated with the appropriate amine in the presence of zinc chloride in sulpholane for three hours at elevated temperatures. After cooling, the reaction mixture is quenched into iced water and the precipitated product collected. The dye is converted to the ammonium salt and dialyzed as normal.

18 4.3. Yellow Dyes Yellow dyes for use in ink jet printing appear to be almost exclusively azo dyes in various forms. The nature of the chemistry used to access many of these dyes is very similar to that used in the preparation of the magenta dyes. For the purpose of discussion, it is convenient to divide the different structures into two main groups according to whether they are based on carbocyclic or heterocyclic intermediates. 4.3.1. Carbocyclic Azo Dyes 4.3.1.1. Dyes based on phenyl azo components One of the earliest dyes to be used in ink jet printing falls into this category. CI Direct Yellow 132 (19) comprises two identical, very simple azo systems joined together via a ureido linkage and gives attractive mid-yellow prints with good light-fastness [1,2]. The small size of the chromophoric portion of this dye makes it undesirable for use in its own right since it would produce prints with very poor water-fastness. Hence, two units are linked together, usually by treatment with phosgene.

17

NHCONH SO,H

HO,S

19 The development of materials with higher water-fastness led to the emergence of carboxylated chromophores such as 20 [28], employed as the ammonium salt in order to maximise the differential solubility effect.

HO,C

/o-

I. NaNOdHCl

HO,C

2 s a n i l i d HO,C e

CO,H

@ 0. The merocyanines are nonionic dyes, with a structure that resonates between charge neutral and the dipolar extremes (Figure 11). They are more soluble in nonpolar solvents than the ionic cyanines, and can exhibit very strong solvatochromic effects [138]. In addition, just like cyanines, their solution absorption shifts to longer wavelengths as length of the conjugated carbon chain is increased (Table 12). Table 12 Structure and solution absorbance of some merocyanine dyes

benzothiazole - 3-0x0dihydrothianapthene

benzothiazole - rhodanine

n

hmax

0

432 528 605 635

1

2 3

n

n

hmax

0 1 2 3

indolenine. thio. bartituric acid

447 558 640

0 1 2 3

hmax

495 595 697

Merocyanines dyes (120) often contain groups that can be alkylated to give alkylmercapto compounds (121). These, in turn, can condense with compounds containing reactive methyl (122) or methylene (123) groups t o give complex dyes (124, 125). Some of these complex dyes were found to behave as powerful sensitizers and supersensitizers [139].

117

120

I

H5C2

124

125

s

7.3. Some additional important dye classes Two additional classes of compounds related t o cyanines and worth

mentioning are the azacyanine and the styryl dyes. The chief difference between the true cyanines and azacyanines is in the conjugated carbon chain joining the nuclei. One or more carbons of the chain are replaced by a nitrogen. The position of the nitrogen atoms in the chain determine whether the particular azacyanine behaves as a sensitizing or as a desensitizing dye. A dye such as 8-azacyanine (126)is a sensitizer, while dyes such as 9-azacyanine (127)or 8,9-diazacyanine (128)are potent desensitizers.

QJap / \

“I

\

126

127

128

Styryl dyes have two nitrogens connected to each other through a chain of conjugated carbon double bonds, but unlike cyanines, one of the nitrogens is not part of a heterocyclic nucleus (Figure 12).

Figure 12. The general structure of a styryl dye where n = 0 or 1. Few of the early styryl dyes were found to be useful sensitizers until the synthesis of Pinaflavol (129) in 1922, which proved to be the most powerful sensitizer then known for green light [140].

my“\“ ’ \

,Q

129

119

7.4. Requirements for sensitization One of the most important requirements a photographic sensitizing dye must fulfill is to be strongly adsorbed to the silver halide surface. If the dye is not in intimate contact with the silver halide grain, there can be no energy or electron transfer and hence no spectral sensitizing action. All photographic films and papers are multi-component systems. They include silver halides, gelatin, dispersing agents, hardening agents, stabilizing agents, chemical sensitizers, imaging dyes, couplers, and other addenda. Many of these components compete for space on the crystal surface and can in the worst case displace the sensitizing dye [1411. In the 1970’s, polarographic potentials obtained for various dyes were correlated with photographic performance. For the particular pAg emulsion used, it was discovered that if the reduction potential of a given dye was -0.81 eV or lower, the sensitizer performed well. If the value was higher the dye either sensitized poorly or not at all. This “crossover potential” between good and bad sensitizing dyes, however, was a function of the type of emulsion used and the conditions of exposure. If the pAg was lowered or the chemical sensitization altered, dyes, with reduction potentials as high as -0.54 eV, were found to spectrally sensitize the emulsion. If the exposures were done in a vacuum, dyes with reduction potentials as high as -0.25 eV worked. Many of the best dyes have reduction potentials on the order of -1.00 eV. Varying the oxidation potential of a dye also had an effect, unrelated to sensitizing ability. Dyes with oxidation potentials of 0.85 eV or less often fogged emulsions. Some dyes, behaved as chemical sensitizers rather than spectral sensitizers. Others formed undesirable complexes with silver ions, or reacted additively with developers to accelerate the development rate, with consequent loss of discrimination between exposed and unexposed silver. All of the electrochemical studies were performed on monomeric dyes in solution. So far, no one has successfully measured the redox potentials of sensitizing dyes or their Jaggregates while adsorbed to a silver halide surfaces [1421. The stereochemistry of the dye is another important feature that influences sensitizing properties. In order to sensitize, the chromophoric system of the dye, which in a cyanine includes the nitrogen containing nuclei, must lie in the same plane. It was found that excessive crowding could alter the planarity of the dye resulting in a loss of sensitizing ability (Figure 13).

120

Slightly crowded 2-naphthoquinaldine Is still planar and a good sensitizer.

In the highly crowded phenanthradin dye, planarity is compromised and the dye is not a sensitizer.

Figure 13. A n example of how steric crowding can force a dye out of planarity in sensitizing dyes. The best sensitizing dyes are compact as well as planar. Dyes with methyl or ethyl meso-substituents in the carbocyanine chain are more compact than their unsubstituted analogues. Compact molecules have fewer degrees of freedom available and are more likely to encounter other dye molecules with the same conformation. This makes it easier for these dyes to form into well-defined aggregates [1431. All sensitizing dyes, whether good or bad, have another common feature. To a greater or lesser extent, all reduce the intrinsic sensitivity of silver. Often, the magnitude of this effect can be controlled by carefully choosing the type of silver halide crystal (cubic, octahedral, or tabular), the chemical sensitization, and the amount of sensitizing dye used. Cubic grains are desensitized less than octahedral grains. Tabular grains with their large surface area can accept much larger concentrations of dye, but not without exhibiting similar desensitizing effects. The optimal concentration for individual dyes may vary, but a good carbocyanine dye can be used in concentrations as high as 200 milligrams per mole silver. Good dicarbocyanine sensitizers, which desensitize more strongly than the corresponding carbocyanines, must be used in much lower concentrations of approximately 60 milligrams per mole silver [144].

7.5. Three color spectral sensitization General purpose black and white films require spectral sensitizers that provide a single emulsion layer with overall panchromatic sensitivity. Color films, in contrast, require three separate, wavelength specific emulsions. Each emulsion must be sensitive over a specific wavelength range and have a distinct region of maximum sensitivity. The blue region extends from the ultraviolet t o 500 nm with a peak sensitization around 450 nm. The green region is defined as extending from 450 nm to approximately 600 nm, with peak sensitivity around 545 nm. The

121

red region requires sensitivity from about 550 nm to 690 nm with a peak sensitivity a t about 635 nm [145]. Photographic emulsions using octahedral or cubic silver halides have adequate spectral sensitivity below 425 nm. Blue sensitizing dyes (Figure 14), however, must be added t o fill the remainder of the blue region, as well as to shift the absorption maximum to the desired 450 nm. This is achieved by using one or more blue sensitizing dyes. In contrast, tabular grains, because of their thinness, and consequently much lower intrinsic sensitivity, require additional sensitizing dyes for sensitivity on the short wavelength side as well [1461.

\SO: 133

132

CI

I H3C

.. \

(CH2)2COOe

134

Figure 14. Examples of currently used blue sensitizing dyes.

135

122

Silver halides have little or no sensitivity to green light and the sensitizing dyes chosen must provide all the sensitivity required. Most of the dyes used to sensitize in the green region are carbocyanines (Figure 15). They are often used as combinations of long green and short green sensitizers [1471.

'03d

136

138

q/ \

139

p-0 w

/ \ o

C2H5

/ \

/ \

140

Figure 15. Examples of currently used green sensitizing dyes.

123

143

144

CI

\

145

146

147

124

Most red sensitizing dyes are also carbocyanines. Combinations of several dyes are often required t o properly represent the red spectral region. Examples of red sensitizing dyes used today are 143-148[148].

7.6. Infrared sensitization Silver halide can also be made sensitive to infrared light. For photographic purposes, the infrared region is defined as starting at 700 nm and extending far beyond. It is possible to find a few carbocyanines (149), heavily substituted with electron-donating substituents that provide efficient sensitization in the near infrared region (700 nm to 750 nm). Most often, however, the dyes used for longer infrared sensitization are dicarbocyanine (150) or tricarbocyanines (151) [1491. When compared with dyes used to sensitize emulsions in the visible regions, infrared sensitizers (Figure 16), with a few exceptions, showed lower sensitivitieslspeeds, act as potent desensitizers, and do not J-aggregate well. The dyes were also more sensitive to the effects of emulsion additives, atmospheric oxygen, and elevated temperatures. Despite the difficulties, photographic emulsions have been successfully sensitized out to 1100 nm (154).

149

NIR carbocyanine with an absorption maximum at 720 nm and an extent to 750 nm.

150

Dicarbocyaninedye sensitizes at an absorption maximum of 780 nm and an extent to 830 nm.

Figure 16. Examples of efficient infrared sensitizing dyes that form strong Jaggregates. The concept of chain-stiffened infrared sensitizing dyes (see 151-154) was introduced to improve the performance and stability, especially at longer wavelengths [1501.

125

152

q - - 153

/

/

/

/

/

154

8. REFERENCES 1.

2. 3. 4. 5. 6. 7. 8.

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126

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Colorants for Non-Textile Applications H.S.Freeman and A.T. Peters (Editors) 2000 Elsevier Science B.V. All rights reserved.

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4 Color Additives for Foods, Drugs, and Cosmetics JOSEPH F. SENACKERIB

Tri-K Industries, Montvale, NJ 07645 1. THE HISTORICAL DEVELOPMENT OF CERTIFIED COLOR ADDITIVES The use of synthetic organic color additives in food products in the United States was first regulated by an Act of Congress on August 2 , 1886. This Act authorized the addition of coloring matter to butter. A second Act followed on June 6, 1896, when Congress recognized coloring matter as a legitimate constituent of cheese. In the Appropriations Act of May 1900 for the Department of Agriculture, the use of coloring matter was recognized by Congress as a problem that might affect the health of the nation. Under the allocation of funds for the general expense of the Bureau of Chemistry was included an item, “To enable the Secretary of Agriculture to investigate the character of proposed food preservatives and coloring matters to determine their relation to digestion and health and to establish the principles which should guide their usage”. Under this authority, the secretary of Agriculture issued several food inspection decisions relating to the coloring of foods. The passage of the Federal Food and Drugs Act of June 1906 brought the question of the use of coloring matters in foods under Government supervision. Before that time, harmless colors were available and generally used, but the publicity given to the indiscriminate use of questionable products resulted in Government legislation. There are cases on record where toxic colorants, such as chrome yellow (lead chromate) and red lead (Pb304)were used in foods and many cases of poisoning were diagnosed as an overindulgence in sweets.

1.1. Certified colors The Food Inspection Decision No. 76 of July 13, 1907 created certified colors and so attempts were made to put an end to the indiscriminate use of impure or unsafe coloring matter in foods. A list of seven color additives was accepted as harmless, these compounds being chosen based upon a study of the color additives then in use for the coloring of foods. This investigation included a very detailed and exhaustive search of the literature concerning the chemistry and toxicity of these coal-tar colors, a study of the law of various countries and states regarding their use and also included many chemical examinations in the Bureau of Chemistry laboratories. The color additives permitted were as follows: Red Shades Amaranth (Formerly FD&C Red No. 2) Ponceau 3R (Formerly FD&C Red No. 1 and Ext. D&C Red No. 15)

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Erythrosine (Now FD&C Red No. 3) Orange Shade Orange I (Formerly FD&C Orange No. 1 and Ext. D&C Orange No. 3) Yellow Shade Naphthol Yellow S (Formerly FD&C Yellow No. 1 now External D&C Yellow No. 7) Green Shade Light Green SF Yellowish (Formerly FD&C Green No. 2 ) Blue Shade Indigo Disulfo Acid (Now FD&C Blue No. 2) Provision was made for the addition of new color additives to these seven. The Bureau of Chemistry’s rules of selection prevailed because the color additives subsequently added were done so after appropriate pharmacological and toxicological tests had proven them harmless. These color additives and their year of admission are listed below: Tartrazine (now FD&C Yellow No. 5) 1916 Yellow AB (formerly FD&C Yellow No. 3) 1918 Yellow OB (formerly FD&C Yellow No. 4) 1918 Sudan 1 * 1918 Butter Yellow * 1918 1922 Guinea Green B (formerly FD&C Green No. 1) 1927 Fast Green FCF (now FD&C Green No. 3) 1929 Ponceau SX (now FD&C Red No. 4) 1929 Sunset Yellow (now FD&C Yellow No. 6) 1929 Brilliant Blue FCF (now FD&C Blue No. 1) *Sudan 1 and Butter Yellow were removed from the list of permitted color additives in 1919. Under the coal-tar color regulations of the 1906 Act a system of voluntary cert$cation was set up. Manufacturers submitted representative samples of batches of color additives to the FDA for analysis and approval. Batches that met the specification for identity and purity were assigned lot numbers by the FDA. The period from 1907 to 1914 saw the “certified color” business firmly established in the United States but still dependent upon foreign countries for its supply of basic raw materials and intermediates such as 2-naphthol, phthalic anhydride, and dimethylaniline, so a very large proportion of coal-tar colors was still being imported. With the outbreak of World War I in August 1914, there came an abrupt halt to the supply of raw materials and finished dyes, and the industry was forced into the production of coal-tar intermediates and colors of sufficient purity to meet the standards set up by the Department of Agriculture. As time went on, the supply and purity of the necessary intermediates increased and by 1937 nearly 500,000 lbs. of certified food color was produced in the US.

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1.2. Federal Food, Drug and Cosmetic Act of 1938 As time passed, inadequacies in the 1906 law and the regulations became apparent. Changes were deemed essential to increase protection of the public health and welfare. These changes were not only in food color additive applications, but in colorants also used by the drug and cosmetic industries. On June 25, 1938, Congress passed the Federal Food, Drug & Cosmetic Act of 1938. This law became effective on January 1, 1940. This Act contained the requirement that only certified coal-tar colors could be used in foods, drugs and cosmetics. The use of any uncertified coal-tar color was considered an adulteration and, as such, was a misdemeanor, punishable under the law. Under this Act three different groups of color additives were listed: a. FD&C Color additives that were certifiable for use in coloring foods, drugs and cosmetics. b. D&C Color additives that were certifiable for use in ingested and externally applied drugs and cosmetics but not in foods. They constituted a group of dyes and pigments considered suitable for use in drugs and cosmetics for internal use when coming in contact with mucous membranes or when ingested only occasionally. c. External D&C Color additives which constituted a group that was not certifiable for use in products intended for ingestion, but which were considered suitable in products that were externally applied. These color additives were not permitted for use in products that came in contact with mucous membranes.

1.3. Color additive amendments of 1960 (Public Law 86-618) This law amended the Food, Drug and Cosmetic Act of 1938. Under this new law the Secretary of Health, Education and Welfare (HEW) is required to list separately color additives for use in foods, drugs and cosmetics, to the extent that these listed color additives are suitable and safe when used in accordance with published regulations. Under the 1938 law coal-tar colors could not be used in foods, drugs and cosmetics unless they were listed by the U.S. Food and Drug Administration (FDA) as “harmless and suitable for use”. The law also called for certification of batches of these listed color additives with or without harmless diluents. A Supreme Court decision defined the “harmless” principle as meaning harmless regardless of the quantity of the “coal-tar color“ which was being used. Under this ruling the FDA had to decertify a color additive if any quantity or concentration of the color caused harm, even though a lesser quantity or amount related to its actual use level was perfectly safe. Because of this “harmless per se” principle the FDA delisted seven of the FD&C color additives, and started action to delist a great number of the D&C color additives, many of which were essential to the production of drug and cosmetic products. The need for enactment of a new law to permit the continued use of color additives was clearly stated by the Secretary of HEW in a letter to the Vice President.

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1.4. Provisions of the color additive amendments of 1960 In 1960, Congress amended the Federal Food, Drug and Cosmetic Act to require premarket safety clearance for colors. The new law also established a way to achieve provisional and permanent listing of color additives. Some important changes made by the 1960 Act were: a. Uniform criteria of admissibility: The law did away with the differences in legal requirements and treatment between the so-called coal-tar colors and other color additives. b. Safety of use principle: The law substituted the “harmless per se” interpretation formerly used with the requirement that color additives under the condition of use specified in the regulations are safe. c. Certification and exemptions from certification: The new law provided for listing and certification of batches of color additives as required under the old law, but also permitted the Secretary to grant exemptions from the requirement of certification where certification was not necessary to protect the health of the public. d. Effective date and transitional provisions: The Amendment became effective upon enactment, and provided for provisional listings, pending completion of the scientific investigations needed as a basis for making determinations as to listing of such color additives under the new permanent provisions of the bill. The Color Additive Amendments of 1960 also provided that the Secretary, in determining whether a color additive shall be listed for use in foods, drugs and cosmetics, consider the scientific data which establish safety under condition of use. In determining whether a proposed use is safe, he also shall consider the probable consumption, the cumulative effect, safety factors, and the availability of any needed practicable methods of analysis for determining the identity and quantity of the pure dye, intermediates and impurities contained in the color additives, as well as the amount of additive in, or on, any such food, drug or cosmetic, and any substance formed in or on such food, drug or cosmetic because of the use of the color additive. One exception exists within the law; that exception is in Section 601 of the Federal Food, Drug and Cosmetic Act of 1938 which allows the use of synthetic organic colorants (known generically as Coal-Tar Dyes in 1938) in hair dye products when such products contained adequate directions for use and prominent cautionary statements. The burden of supplying the necessary scientific data for permanent listing fell on the producers and users of the color additives. Because of the great expense involved in doing the pharmacological and chemical experiments necessary for inclusion in a petition for permanent listing, work was initially begun on only those colors which were of the greatest economic importance to the food, drug and cosmetic industries, and many of the previously certifiable colors were delisted by default. The Color Additive Amendments of 1960 provided for a 2 1/2-year grace period for color additives that were commercially available before it was enacted. These colors were classified as “provisionally listed,” so that the testing could be completed to meet the new proof of safety requirements. It further provided that the Secretary of Health, Education and Welfare had the power to postpone the original closing date for such period or periods as he finds necessary to carry out the purposes of the law.

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The Cosmetic Toiletry and Fragrance Association (CTFA) then known as The Toilet Goods Association, the Certified Color Manufacturers’ Association (CCMA) formerly the Certified Color Industry Committee and the Pharmaceutical Manufacturers’ Association (PMA), representing the manufacturers and users of color additives, immediately started the work of providing the required data. To comply with the color additive pre-market safety clearance requirements, manufacturers of finished goods and manufacturers of color additives contributed money (based on sales) to support the necessary testing program. The responsibility for coordinating the testing of specific colors was divided among the industry associations. Each had an assigned responsibility to coordinate the research that was being conducted in independent laboratories. Color additive testing for external applied colors required only acute oral lethality determinations in two species, viz. 90-day rabbit dermal toxicity studies and life-time skin painting studies in mice. Testing for colors subject to ingestion required the studies described above, plus range-finding and chronic (lifetime) toxicity studies in two species. At the onset of the toxicity testing program, in addition to the chronic rat studies required of all the colorants, seven year dog feeding studies were initiated on 4 of the FD&C color additives to see if this longer term chronic feeding study would give more information on chronic toxicity than the 2to 2 1/2 year rat feeding study then still in use. In late 1970, a scientific event occurred which was to postpone FDA’s final consideration of industry’s petitions while an entire new round of toxicological testing was conducted. In December 1970, a paper was published in Russia alleging that the color Amaranth was responsible for embryotoxic effects. Because of the similarities between FD&C Red No. 2 and Amaranth, the Russian study was a cause for concern among industry and FDA scientists, even though the chemical composition of the compound tested by the Russians was unclear, and there were serious deficiencies in the Russian test methodology. Early in February 1971, FDA mandated a second series of tests in order to determine whether color additives had potential for adverse effects on animal reproduction. Thus, multigeneration reproduction (rats) and teratology (rats and rabbits) studies were required on all FD&C and D&C color additives subject to ingestion. FDA set a deadline of December 31, 1971 for the initiation of tests. In all, FDA’s new testing requirements applied to 25 different FD&C and D&C colors. As FDA recognized at the time, neither teratology nor reproduction tests had been included in the earlier found of studies for several reasons. Adequate animal models for predicting teratologic and reproductive effects on humans had not yet been developed in 1960. It took the thalidomide disaster in 1961 to alert the scientific community to the importance of animal models for these effects and the evolution of scientifically acceptable test methods took several years longer. Until the Russian study, moreover, there was no cause for suspecting that the certified colors might have any adverse effect on reproduction and thus there was no reason to require testing to determine whether such effects might occur. As a result of FDA’s decision, industry was once again faced with the challenge of developing test protocols, obtaining sufficient funding to finance the tests, and selecting test laboratories. Complicating these tasks was the need to obtain the cooperation of all four affected industries.

136

While this was going on, FDA decided to delist FD&C Violet No. 1 because it was to be carcinogenic based on foreign studies conducted on an uncertified batch of the color. FDA delisted this color in April 1973. Because of the Russian study indicating that FD&C Red No. 2 might be embryotoxic and carcinogenic, FDA initiated its own feeding studies and industry conducted some metabolic studies. Disparities in test methods and results led to endless debate, climaxed by the formation of a special advisory committee to resolve the scientific differences. Simultaneously, FDA had requested extensive usage data from industry to assist in devising revised tolerances for the ingestion of FD&C Red No. 2. However, in February 1976, FDA revoked the provisional listing for FD&C Red No. 2 because of “public concern”, this provoked extensive litigation. FDA’s action in delisting the color was upheld in the courts, and the denial of the color additive petition was upheld in the administrative proceeding. Feeding studies on FD&C Red No. 4, sponsored by PMA, had produced a number of bladder lesions in the test animals. PMA concluded that the lesions were related solely to catheterization of the test animals during testing, not to the test color. Nonetheless, FDA delisted the color in September 1976. This had a major impact on the producers of maraschino cherries, for whom the continued availability of FD&C Red No. 4 was critical. After an administrative hearing, the FDA Commissioner upheld the denial of the food additive petition. FD&C Red No. 4 was subsequently petitioned and approved for an external use only color but still retains the designation FD&C Red No. 4. Throughout the course of the color additive litigation, industry had assumed that the 21 uncertified colors contained on the 1964 Harvey list would be permanently listed by FDA on its own initiative. In 1972, after FDA had published revised regulations embodying the Second Circuit Court’s decision, the CTFA wrote to FDA requesting that the Agency fulfill FDA Deputy Commissioner Harvey’s pledge to list the 21 colors not requiring certification on its own initiative. FDA responded that it would review the available chemical and toxicological data on the colors and determine whether the course urged by CTFA could be followed. In October 1974, FDA published a Federal Register notice announcing that petitions for the 21 colors would be required. A survey of users of color additives was immediately undertaken and it was determined that 13 of the colors were not, and had never been, used for coloring purposes, although they had been frequently included in consumer products for other reasons. After bringing this information to FDA’s attention, FDA delisted the 13 colors in April 1975. Information was obtained by the CTFA on the chemistry and use data for six of the remaining colors in anticipation of filing petitions. These petitions were submitted in December 1975, and in May 1976 FDA advised that the petitions would be considered acceptable if the same eye-area tests were conducted for the six colors that had been required for all other uncertified colors. On February 4, 1977, FDA published a final order which postponed the closing date for the use of many provisionally listed color additives, provided certain investigations were undertaken by industry, and that data be submitted to the agency on a prescribed schedule. The thrust of the February order was to require additional long-term testing in two animal species thus requiring industry to repeat all the chronic toxicity studies on the ingested colors that had been performed in the 1960’s. The reason given by FDA for these additional requirements was

137

the changing scientific standards for the evaluation of color additives. The new tests were required for 32 FD&C and D&C colors subject to incidental ingestion. For 8 of the 32 colors it was decided the cost of testing would be too expensive in relation to the colors' commercial value. After industry informed the FDA of this decision, the colors were delisted. For a ninth color, caramel, FDA decided not to require the additional studies. The February 1977 regulation extending the provisional listings was challenged in a lawsuit brought by the Health Research Group (HRG) and its parent organization, Public Citizen. HRG alleged that FDA had authority to postpone closing dates only to allow completion of ongoing tests, not to require new tests. They also alleged that, even if within FDA's statutory authority, the decision to postpone the closing dates to January 1981 was arbitrary and capricious. The HRG arguments were rejected by the District Court in a decision handed down in September 1977. The District Court first held that FDA has broad discretion to extend a color's provisional listing pending the development of the scientific data necessary for an informed decision on the color's permanent listing. The court then reviewed the administrative record and concluded that it provided a rational basis for FDA's postponement of the closing dates for the 32 colors. The court stressed that the new testing assures that the Commissioner will have the most complete information possible with which to take final action on these additives, and that the continued provisional listings maintain the commercial availability of color additives not shown to be harmful in any way. HRG entered an appeal from the district court's decision. They withdrew their appeal, however, after FDA entered stipulation establishing procedures for notifying HRG of any additional extensions of the provisional listings. Pursuant to FDA's February 1977 regulation, industry promptly moved forward with a program for testing the FD&C and D&C colors. The studies conducted on these color additives also supported permanent listing for other color additives. Five of the colors being tested were closely related chemically to an additional five colors, so that the data on the tested color would also be applicable to its related color. Although the studies were commenced promptly, a number of developments occurred during the course of testing that made it impossible to meet the FDA time-table. Petitions were submitted to the FDA requesting postponements of the closing dates for the 23 color additives. After evaluating the petitions, FDA proposed to postpone the closing dates for each of the 23 colors. FDA outlined a staggered set of new closing dates that would extend the provisional listings from between one to three years. FDA was unable to publish its final rule before the January 1981 closing date, because President Reagan signed an Executive Order in January 1981 directing all agencies to postpone issuing all projected regulations for sixty days. At the earliest opportunity, FDA obtained permission from the Office of Management and Budget to proceed with a final rule, and that rule was published in the Federal Register in March 1981. In the preamble to the rule, FDA reaffirmed its prior determination that "its review of progress reports on each color under test demonstrates that there continues to be no public health or safety concerns with any of the 23 color additives." The rule adopted the staggered closing dates contained in FDA's proposal.

138

HRG filed suit in the District Court to contest these extensions in March 1981. The parties subsequently moved for summary judgment and the District Court upheld FDA’s extension of the provisional listings in November 1981. From 1982 through 1984, FDA granted permanent listing to 13 of the 23 color additives covered by the March 1981 extension. Four of the remaining 10 color would eventually be granted permanent listing. However, for 6 others, D&C Red No. 9 (and related D&C Red No. 8), D&C Red No. 19 (and related D&C Red No. 37), FD&C Red No 3 (for cosmetic use) and D&C Orange No. 17 FDA decided to conduct a detailed scientific review because the testing showed carcinogenicity in some animals when fed at extremely high levels. After a complete review, the decision to delist these 6 colors was made based on a restrictive interpretation of the Delaney Clause, which assumes zero-risk as the only acceptable standard. Industry argued that there is no risk to humans and that the tumors produced in the test animals were caused by a secondary (indirect) mechanism. As evidence, industry presented the data for the quantitative risk assessment on humans which in the worst case, for ingested use, ranged from one in one million to one in 120 million. In the worst case, for external use, the range was from one in 3.2 million to one in 830 trillion. Industry also raised the issues of selective penetration and inconstant mixtures and the FDA extend the provisional listing of the color additives to allow full debate and careful resolution of the policy issues relating to the interpretation of the Delaney Clause and the scientific issue relating to risk assessment. The final decision was made to delist the 6 colors based on the interpretation that the Delaney Clause only permitted a zerorisk standard. Before 1960 there were 118 certified colors on the approved list, 19 food colors, 70 drug and cosmetic colors and 29 external drug and cosmetic colors. Today, there are 43 certified colors on the approved list, 9 FD&C colors, 30 D&C colors, 2 External D&C colors and 2 colors specifically for medical devices. Many of the colors are approved for use in more then one category. As examples, FD&C Blue No. 1 can be used in foods, drugs and cosmetics and D&C Yellow No. 10 can be used in drugs, cosmetics and medical devices. Listed in Table 1 are all of the color additives that the FDA requires be certified. The figures were supplied by the FDA and are reported in thousands of pounds. Before 1960 the designations FD&C, D&C and External D&C related to the allowable use for the product, that is, FD&C indicated permitted use for foods, drugs and cosmetics, D&C indicated permitted use for ingested drugs and cosmetics, and External D&C indicated permitted use in external drugs and cosmetics. Today, these designations do not indicate with certainty the permitted use of the product. An External D&C is still only permitted for topical use; however, the FD&C designation does not always indicate the product is permitted in foods, drugs and cosmetics. FD&C Red No. 4 is not permitted for food use and is only permitted for external use in drugs and cosmetics. FD&C Red No. 3 is permitted for food and ingested drug use but is not permitted for any use in cosmetics. These changes occurred because of the mandated testing required by the FDA which found that products like FD&C Red No.4 were not suitable for use in food applications. It is the FDA’s intention to eventually change the designations of those color additives that are now designated incorrectly.

139

1.5. Provisionally listed colors There are still a number of colors that have not yet received permanent listing. These provisionally listed colors are many of the FD&C and D&C lake colors. In total there are 29 lake colors that still have provisional listings. Six of these are FD&C lakes and 23 are D&C lakes. These lake colors still have some minor questions or concerns that remain to be resolved. Table 1 Thousands of pounds of colorants certified annually by the FDA Colorants* FD&C Primary FD&C Blue No. 1 FD&C Blue No. 2 FD&C Green No. 3 FD&C Red No. 3 FD&C Red No. 40 FD&C Yellow No. 5 FD&C Yellow No. 6 Citrus Red No. 2 Subtotal FD&C Lakes FD&C Blue No. 1 FD&C Blue No. 2 FD&C Red No. 40 FD&C Yellow No. 5 FD&C Yellow No. 6 Subtotal F D d C Recertifications D&C Primary D&C Green No. 5 D&C Green No. 6 D&C Green No. 8 D&C Orange No. 4 D&C Orange No. 5 D&C Red No. 6 D&C Red No. 7 D&C Red No. 17 D&C Red No. 21 D&C Red No. 22 D&C Red No. 27 D&C Red No. 28 D&C Red No. 30

1993

1994

1995

363.0 109.2 12.3 227.9 2,904.4 1,960.8 1,856.5 2.0

455.6 245.3 11.8 181.8 3,257.7 1,761.4 2,192.3 2.4 8,008.3

432.5 194.2 10.4 203.2 3,172.0 2,200.6 2,102.6 2.5

144.0 85.4 468.1 914.5 642.1

167.2 110.7 568.2 972.2 625.0

7,436.2

1996

1997

446.4 116.5 12.0 206.1 3,447.7 2,025.7 2,122.2 0.9

8,318.0

8,377.5

414.0 172.4 9.8 205.8 3,167.2 2,086.3 1,934.9 1.9

159.1 144.2 597.5 1,139.7 720.3

189.1 107.9 678.2 975.6 810.6

210.2 158.3 664.6 1,223.8 787.1

7,992.3

2,254.1

2,443.3

2,760.8

2,761.4

61.5

3,044.0

54.3

3.0 1.1 40.7 4.6 5.1 11.3 0.0 1.o 4.2 2.1 2.5 8.9 4.5

9.8 1.6 44.8 9.8 6.2 17.6 0.0 4.7 4.2 1.8 3.1 9.2 25.4

5.1 1.2 20.8 14.9 3.8 26.6 0.0 1.9 7.8 5.7 3.1 10.0 6.8

0.1 1.4 19.6 4.9 3.5 16.2 1.7 5.5 7.4 1.7 6.2 12.1 20.7

4.6 0.7 25.3 25.4 4.5 33.9 7.6 0.0 5.7 4.6 1.6 14.6 34.0

136.7

63.5

57.8

140

D&C Red No. 33 D&C Red No. 36 D&C Violet No. 2 D&C Yellow No. 8 D&C Yellow No. 10 D&C Yellow No. 11 Phthalocyaninato Cu Subtotal D&C Lakes D&C Blue No. 1 D&C Orange No. 5 D&C Red No. 6 D&C Red No. 7 Table 1 (continued) Colorants D&C Red No. 21 D&C Red No. 22 D&C Red No. 27 D&C Red No. 28 D&C Red No. 30 D&C Red No. 33 D&C Red No. 34 D&C Red No. 36 D&C Yellow No. 5 D&C Yellow No. 6 D&C Yellow No. 10 Subtotal D & C Recertifications Externalprimary Colors FD&C Red No. 4 Ext. D&C Violet No. 2 Ext. D&C Yellow No. 7 Subtotal Ext. Recertifications FD&C Mixtures Totals

24.0 11.9 2.3 2.3 48.4 0.0 0.0

12.6 8.5 1.o 4.0 87.5 2.8 0.0

25.0 13.2 2.5 6.0 94.2 4.7 >o. 1

13.5 6.7 2.1 5.0 77.8 1.6 0.6

24.6 16.3 1.6 3.6 79.9 3.9 >o.1

177.9

254.6

253.3

208.3

292.4

1.3 7.4 207.3 293.1

0.4 7.0 239.9 230.2

0.0 6.8 149.0 237.9

1.3 4.8 154.8 358.8

0.0 3.3 144.5 360.2

1993 25.5 1.2 114.4 13.1 57.0 1.6 7.5 0.0 12.4 3.0 92.1

1994 9.3 1.3 128.6 12.8 55.4 9.7 7.0 0.0 13.2 14.3 93.7

1995 46.5 6.3 100.9 15.7 31.4 3.5 0.0 0.0 2.4 2.7 109.9

1996 20.6 5.5 99.4 17.1 50.4 14.9 12.3 0.8 5.0 0.0 82.9

1997 16.0 4.2 80.5 17.6 46.3 22.1 16.9 0.0 5.7 18.9 83.4

836.9

822.8

713.0

828.6

819.6

8.8 6.4 0.0

24.0 9.1 0.5 33.6

7.0 5.4 0.0

12.4

26.3 11.3 0.1

2.5 1.2

30.1 11.9 0.0 42.0 4.9 1.3

3.0 0.0

5.9 1.o

8.7 1.o

10,889.4

11675.4

12159.2

12280.9

12274.5

28.7

15.2

34.7

* The corresponding CI names are provided in Table 2.

23.2

24.3

21.0

37.7

141

2. A SURVEY OF CERTIFIED COLOR ADDITIVES Certified colorants may be classified into groups according to chemical structures. AZO COLORANTS. This group includes the greatest number of colorants on the certified list. This group can be divided into four types: a. Unsulfonated compounds that are insoluble in water but soluble in aromatic solvents and oils; a typical example is D&C Red 17. b. Insoluble compounds that contain sulfonic acid groups in the ortho position to the azo bond. These color additives are converted by a permissible precipitant into an insoluble metal salt; examples include D&C Red 7 and Red D&C Red 34. c. Azo compounds that contain one or more sulfonic acid or carboxylic groups to produce water solubility. The hydrophilic group is generally in the meta-or para-position to the azo group; examples are FD&C Red 40 and D&C Orange 4. d. Unsulfonated compounds that are precipitated directly on coupling and contain no groups capable of salt formation; an example of this group is D&C Red 36. TRIPHENYLMETHANE COLORANTS. This is a group of sulfonated dyes (derived from the corresponding basic dyes) containing two or more sulfonic acid groups. Examples of this group are FD&C Blue 1 and FD&C Green 3. XANTHENE (FLUORAN) COLORANTS. This group of colors generally appear very bright and sometimes fluorescent; examples of this group are D&C Yellow 7 and D&C Red 27. QUlNOLINE COLORANTS. There are only two quinoline dyes permitted by the FDA. They are D&C Yellow 11 and D&C Yellow 10. D&C Yellow 10 is a mixture of a mono and a disulfonated derivative of D&C Yellow 11. ANTHRAQUINONE COLORANTS. The certified anthraquinone color additives fall into three sub-groups: a. Sulfonated acid types which are water soluble; an example is D&C Green 5 b. Unsulfonated types which are oil soluble; an example is D&C Green 6 c. Hydroxyanthraquinone types; examples are D&C Violet 2 which is oil soluble, and its sulfonated derivative, External D&C Violet 2 which is water soluble. INDIGOID COLORANTS. There are three certified colors in this group. D&C Blue 6, D&C Red 30 and FD&C Blue 2. D&C Blue 6 and D&C Red 30 are pigments. FD&C Blue 2 is a water soluble colorant made by sulfonation of D&C Blue 6. PYRENE COLORANTS. D&C Green 8 is the only pyrene colorant on the certified list. NITRO COLORANTS. There is only one nitro dye permitted, viz. External D&C Yellow 7. [PHTHALOCYAhVNATO (2-)] COPPER. This medical device colorant is approved only for coloring sutures intended for general and ophthalmic surgery.

2.1. Classification of FDA approved color additives All approved color additives are classified by the FDA into categories. These categories are established based on the permitted use. The categories established are foods, drugs, cosmetics, and medical devices. Many of the colorants are listed in more than one category, but the restrictions on the products may be different from category to category.

142

Table 2 shows the chemical structures of all of the currently approved FDA color additives that require certification. All of these colors are presently being manufactured, with the exception of D&C Brown No. 1 for which the FDA specifications are too restrictive for practical manufacture. These color additives can be certified as the dye or the lake pigment produced from the dye. The FDA distinguishes the difference by identifying the dye as the straight color, for example, FD&C Blue No. 1 or D&C Red No. 33 and the lake as the salt, that is, FD&C Blue No. 1 aluminum lake or D&C Red No. 33 barium lake. The FD&C lakes are prepared by extending, on a substratum of alumina, a salt prepared from one of the previously certified FD&C dyes by combining the dye with the basic radical aluminum or calcium. The D&C lake pigments are also produced by combining with a basic radical. However, it is not required that the dye be certified before producing the lake pigment, and many more basic radicals and substrates are permitted. The substrates permitted are alumina, blanc fixe (barium sulfate), gloss white, clay, titanium dioxide, zinc oxide, talc, rosin, aluminum benzoate, or any combination of two or more of these substrates. Table 2 Chemical structures of colors that require certification.

S03Na

S03Na

H o FD&C Blue No. 2 (CI Food Blue 1)

FD&C Blue No. 1 (CI Food Blue 2 )

S03Na

OH

FD&C Green No. 3 (CIFood Green 31

FD&C Red No. 3 (CI Food Red 141

143

6

Na03S

\

N'/ NHQ

H3C

S03Na

FD&C Red No. 40 (GI Food Red 17)

FD&C Red No. 4 (CI Food Red 1)

HO N a O 3 S O N ' / \" 8/

HO N T ! N a O 3S ON"

c

~

~

N

FD&C Yellow No. 5 (CI Food Yellow 4)

~

S03Na

FD&C Yellow No. 6 (CI Food Yellow 3)

Citrus Red No. 2 (GI Solvent Red 80)

Orange B ( GI Acid Orange 137)

D&C Blue No. 4 (GI Acid Blue 9)

D&C Blue No. 6 (GI Vat Blue 1)

144

D&C Blue No. 9 (GI Vat Blue 9)

D&C Brown No. 1(GI Acid Orange 24)

NaO$

Na03S.

D&C Green No. 5 (GI Acid Green 25)

D&C Green No. 6 (GI Solvent Green 3) HO

H

O

G

a

S03Na Na03S

\

D&C Green No. 8 (GI Solvent Green 7) Br

N a O 3 S o f l \N /3

D&C Orange No. 4 (CI Acid Orange 7)

Br

D&C Orange No. 5 (GI Solvent Red 72)

D&C Orange No. 10 (GI Solvent Red 73)

145

D&C Orange No. 11(CI Acid Red 95)

D&C Red No. 6 (CI Pigment Red 57) HO

D&C Red No. 7 (CI Pigment Red 57:l) D&C Red No. 17 (CI Solvent Red 23)

Br

Br Br

Br

0

D&C Red No. 21 (CI Solvent Red 43) Br

D&C Red No. 22 (CI Acid Red 87)

Br

CI

D&C Red No. 27 (CI Solvent Red 48)


146

D&C Red No. 30 (CI Vat Red 1)

D&C Red No. 31 (CI Pigment Red 64:U

S03Na


D&C Red No. 34 (CI Pigment Red 63:l)

D&C Red No. 36 (CI Pigment Red 4)

0

bH

D&C Violet No. 2 (CI Solvent Violet 13)

D&C Red No. 39 (CI Pigment Red 100)

0

OH

Ext. D&C Violet No. 2 (CI Acid Violet 43)

147

HO

0

D&C Yellow No. 7 (CI Solvent Yellow 94)

Ext. D&C Yellow No. 7 (CI Acid Yellow 1)

Na03S

0

S03Na

D&C Yellow No. 8 (CI Acid Yellow 7 3 ) D&C Yellow No. 10 (CI Acid Yellow 3)

% \

0

D&C Yellow NO. 11(CI Solvent Yellow 33)

[Phthalocyaninato(2-)]Copper (CI. Pigment Blue 15)

The basic radicals permitted are sodium, potassium, aluminum, barium, calcium, strontium, or zirconium. Table 3 includes all of the certified color additives with the official FDA name, the most common trade names, the chemical classifications as designated by the

148

FDA, the Color Index (CI) number, the shade of the color additive, the typical process used to manufacture the color and the use restrictions as defined by the Code of Federal Regulations, 21 CFR, section 74.101 through section 82.2707a. Table 3 Classification of Certified Color Additives FDA Name Trade Name Tmical Manufacturinp Process Chemical Class (CI Number CAS Registry No.) Shade Use and Restrictions Foods: Drugs: Cosmetics: Medical Devices: FD&C Blue No. 1 Brilliant Blue FCF Condensation of benzaldehyde-2sulfonic acid with a-(N-ethylanilino)toluenesulfonic acid (benzylethylanilinesulfonic acid) Triphenylmethane CI No. 42090 2650-18-2 Greenish Blue Foods: General use. Drugs: General use. Cosmetics: General use. Medical Devices: Not listed. FD&C Blue No. 2 Indigotine, Indigotin IA Sulfonation of indigo Indigoid CI NO.73015 860-22-0 Deep Blue Foods: General use. Drugs: Permitted for ingested drugs. May not be used in topical applications. May be used in surgical sutures made of nylon 66 (adipic acid and hexamethylene diamine) but may not exceed 1.0% weight. Cosmetics: Not listed. Medical Devices: Not listed. FD&C Green No. 3 Fast Green FCF Condensation of 4-hydroxybenzaldehyde-2-sulfonic acid with a-N-ethylanilino) toluenesulfonic acid Triphenylmethane CI No. 42053 2353-45-9 Bluish Green Foods: General use. Drugs: Permitted

149

for general use except not permitted in eye area. Cosmetics: Permitted for general use except not permitted for use in eye area. Medical Devices: Not listed. FD&C Yellow No. 5 Tartrazine Condensation of psulfonic acid with oxalacetic ester, coupling of the product with diazotized sulfanilic acid, then hydrolysis of the ester with NaOH; or condensation of phenylhydrazine-4sulfonic acid with dihydroxytartaric acid Phenylhydrazine-PyrazoloneCI No. 19140 1934-21-0 Greenish Yellow Foods: General use. Drugs: General use. Cosmetics: General use. Medical Devices: Not listed. FD&C Yellow No. 6 Sunset Yellow FCF Coupling of diazotized sulfanilic acid with 2-naphthol-6-sulfonic Monoazo CI NO.15985 2783-94-0 Reddish Yellow Foods: General use. Drugs: Permitted for general use except not permitted in eye area. Cosmetics: General use except not permitted in eye area. Medical Devices: Not listed. FD&C Red No. 3 Erythrosine Iodination of fluorescein (D&C Yellow No. 7) Xanthene CI NO.45430 16423-68-0 Bluish Pink Foods: General use. Drugs: Ingested use only. Cosmetics: Not permitted. Medical Devices: Not listed. FD&C Red No. 4 Ponceau SX Coupling of diazotized l-amino-2,4dimethylbenzene-5-sulfonicacid with 1-naphthol-4-sulfonicacid Monoazo CI NO.14700 4548-53-2 Yellowish Red Foods: Not permitted. Drugs: External use only. Not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: Not listed.

150

FD&C Red No. 40 Allura Red AC Coupling of diazotized 5-amino 4-methoxy2-toluenesulfonic acid with 6-hydroxy-2-naphthalene sulfonic acid -Monoazo CI NO. 16035 25956-17-6 Yellowish Red Foods: General use. Drugs: General use. Cosmetics: General use. Medical Devices: Not listed. Orange B Acid Orange 137 Condensation of phenylhydrazine-4-sulfonic acid with oxalacetic ester, coupling of the product with diazotized naphthionic acid Pyrazolone CI No. 19235 15139-76-1 53060-70-1 Reddish Yellow Foods: May only be used to color casings or surfaces of frankfurters and sausages. May not exceed 150 parts per million by weight of the finished food. Drugs: Not listed. Cosmetics: Not listed. Medical Devices: Not listed. Citrus Red No. 2 Solvent Red 80 Coupling of diazotized 2,5dimethoxyaniline with 2- naphthol Monoazo CI NO. 12156 6358-53-8 Scarlet Foods: May only be used for coloring orange skins not intended for use in processing or if used for processing must be designated as Packinghouse elimination. May not exceed 2 parts per million by weight of the whole fruit. Drugs: Not listed. Cosmetics: Not listed. Medical Devices: Not listed. D&C Blue No. 4 Alphazurine FG Diammonium salt of FD&C Blue No. 1 Triphenylmethane CI No. 42090 6371-85-3 Bright Greenish Blue Foods: Not listed. Drugs: External use only. Not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: Not listed. D&C Blue No. 6 Indigo Convert N-phenylglycine into pseudoindoxyl by fusion with sodium amide in the presence of a mixture of potassium and sodium hydroxides and sodium cyanide then oxidize Indigoid Deep Blue CI NO.73000 482-89-3

151

Foods: Not listed. Drugs: Not listed. Cosmetics: Not listed. Medical Devices: Permitted for coloring surgical sutures as follows: May not exceed 0.2% by weight for polyethylene terephthalate sutures intended for general surgical use; may not exceed 0.25% by weight for plain or chromic collagen absorbable sutures for general surgical use; may not exceed 0.5% by weight for plain or chromic collagen absorbable sutures for ophthalmic surgical use; may not exceed 0.5%by weight for polypropylene sutures intended for general surgical use; and may not exceed 0.5% by weight for coloring polydioxanone synthetic absorbable suturesintended for general and ophthalmic surgical use. D&C Blue No. 9 Carbanthrene Blue Chlorination of indanthrene Anthraquinone CI No. 69825 130-20-1 Dull Greenish Blue Foods: Not listed. Drugs: May only be used for coloring cotton and silk surgical sutures intended for general and ophthalmic use. Color may not exceed 2.5%by weight. Cosmetics: Not listed. Medical Devices: Not listed D&C Green No. 5 Alizarin Cynine Green F Condensation of leucoquinizarin with 4-toluidine and sulfonation Anthraquinone Dull Bluish Green CI No. 61570 4403-90-1 Foods: Not listed. Drugs: May only be used for coloring surgical sutures made of nylon 6 (poly-[ecaprolactaml) or nylon 66 (adipic acid and hexamethylenediamine) for general surgery. Color may not exceed 0.6%by weight. Cosmetics: General use. Medical Devices: Not listed. Condensation of leucoquinizarin with D&C Green No. 6 Quinizarin Green SS 4-toluidine Anthraquinone Dull Bluish Green CI No. 61565 128-80-3 Foods: Not listed. Drugs: External use only. Not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: May not exceed 0.03% by weight when used in contact lenses. May be used t o color surgical sutures as follows: may not exceed 0.75% by weight when used for polyethylene terephthalate sutures intended for general and ophthalmic use; may not exceed 0.1% by weight when used in polyglycolic acid sutures with a diameter greater than U.S.P. size 8-0 which are intended general

152

and ophthalmic use; may not exceed 0.5% by weight for polyglycolic acid with diameters not greater than U.S.P. 8-0 which are intended for general and ophthalmic surgical use; may not exceed 0.21% by weight for poly(glyco1ic acid-co trimethylene carbonate) sutures intended for general use; and not to exceed 0.10% by weight of haptic material coloring polymethylmethacrlate support haptics of intraocular lenses. Sulfonation of pyrene to D&C Green No. 8 Pyranine Concentrated Pyrenetetrasulfonic acid, salting out with NaC1, hydrolysis in NaOH solution, addition of formic acid, and salting out with NaCl Pyrene CI NO. 59040 6358-69-6 Yellowish Green Foods: Not listed. Drugs: External use only. May not exceed 0.01%by weight. May not be used in eye area. Cosmetics: External use only. Not permitted in eye area. May not exceed 0.01% by weight. Medical Devices: Not listed. D&C Brown No. 1 Resorcin Brown Coupling diazotized sulfanilic acid with resorcinol and coupling the product with diazotized 2,4-xylidine in dilute alkali solution Disazo CI NO.20170 1320-07-6 Light Orange Brown Foods: Not listed. Drugs: Not listed. Cosmetics: External use only. Not permitted in eye area. Medical Devices: Not listed. Coupling diazotized sulfanilic acid with 2D&C Orange No. 4 Orange I1 naphthol Monoazo CI NO.15510 633-96-5 Bright Orange Foods: Not listed. Drugs: External use only except not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: Not listed. Dibromofluorescein Bromination of fluorescein (D&C D&C Orange No. 5 Yellow No. 7) Fluoran CI NO.45370:l 596-03-2 Reddish Orange Foods: Not listed. Drugs: External use only except not permitted in eye area. May not exceed 5 milligrams per daily dose. May be used in coloring mouthwashes and dentifrices.

153

Cosmetics: May be used to color lip products in amounts not t o exceed 5.0% by weight. May be used in general for mouthwashes and dentifrices. General use for external products except that it is not permitted in eye area. Medical Devices: Not listed. D&C Orange No. 10 Diiodofluorescein Iodination of fluorescein (D&C Yellow No. 7) Fluoran CI NO.45425~1 38577-97-8 Reddish Orange Foods: Not listed. Drugs: External use only. Not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: Not listed. D&C Orange No. 11 Erythrosine Yellowish Conversion of D&C Orange No. 10 to the Na salt Xanthene CI NO.45425 33239-19-9 Red Foods: Not listed. Drugs: External use only. Not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: Not listed. Coupling of diazotized 6-amino-mD&C Red No. 6 Lithol Rubin B toluenesulfonic acid with 3- hydroxy-2-naphthoic acid Monoazo CI NO.15850 5858-81-1 Medium Red Foods: Not listed. Drugs: General use except not permitted in eye area. The combination of D&C Red No. 6 and D&C Red No. 7 may not exceed 5 milligrams per daily dose. Cosmetics: General use except not permitted in eye area. Medical Devices: Not listed. D&C Red No. 7 Lithol Rubin B Calcium Heating of D&C Red No. 6 with CaClz Monoazo CI NO.15850 5281-04-9 Bluish Red Foods: Not listed. Drugs: General use except not permitted in eye area. The combination of D&C Red No. 7 and D&C Red No. 6 may not exceed 5 milligrams per daily dose. Cosmetics: General use except not permitted in eye area. Medical Devices: Not listed. Coupling of diazotized aminoazobenzene with Toney Red D&C Red No. 17 2- naphthol Disazo CI NO.26100 85-86-9

154

Dull Red Foods: Not listed. Drugs: External use only. Not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: May be used generally for coloring contact lenses. TetrabromofluoresceinBrominationof fluorescein D&C Red No. 21 (D&C Yellow No. 7) CI No. 45380A 15086-94-9 Fluoran Bluish Pink Foods: Not listed. Drugs: General use except not permitted in eye area. Cosmetics: General use except not permitted in eye area. Medical Devices: Not listed. Conversion of D&C Red No. 21 to the Na salt Eosin YS D&C Red No. 22 Xanthene CI NO.45380 17372-87-1 Yellowish Pink Foods: Not listed. Drugs: General use except not permitted in eye area. Cosmetics: General use except not permitted in eye area. Medical Devices: Not listed. D&C Red No. 27 Tetrachlorotetrabromo- Condensation of resorcinol fluorescein with tetrachlorophthalic anhydride and bromination Fluoran CI NO.45410A 13473-26-2 Bluish Pink Foods: Not listed. Drugs: General use except not permitted in eye area. Cosmetics: General use except not permitted in eye area. Medical Devices: Not listed. Conversion of D&C Red No. 27 to the Na salt Phloxine B D&C Red No. 28 Xanthene CI NO.45410 18472-87-2 Bluish Pink Foods: Not listed. Drugs: General use except not permitted in eye area. Cosmetics: General use except not permitted in eye area. Medical Devices: Not listed. Helindone Pink CN Oxidation of 6-chloro-4D&C Red No. 30 methylthioindoxyl; or chlorination of 4,4'-dimethylthioindigo Indigoid CI NO.73360 2379-74-0 Bluish Pink Foods: Not listed. Drugs: General use except not permitted in eye area.

155

Cosmetics: General use except not permitted in eye area. Medical Devices: Not listed. Brilliant Lake Red R Coupling of diazotized aniline with 3D&C Red No. 31 hydroxy-2-naphthoicacid and conversion to the Ca salt Monoazo Bluish Pink CI NO.15800 6371-76-2 Foods: Not listed. Drugs: External use only. Not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: Not listed. D&C Red No. 33 Acid Fuchsin D Coupling of diazotized aniline with 8amino-l-naphthol3,6- disulfonic acid in alkali solution Monoazo CI NO.17200 3567-66-6 Dull Bluish Red Foods: Not listed. Drugs: Dosage unrestricted for external use, mouthwashes and dentifrices. Not permitted in eye area. For ingested drugs, may not exceed 0.75 milligrams per daily dose. Cosmetics: Amount permitted is unrestricted for external use, mouthwashes, breath fresheners and dentifrices. May be used to color lip products in amounts not to exceed 3.0% by weight. Not permitted in eye area. Medical Devices: Not listed. D&C Red No. 34 Deep Maroon Coupling of diazotized 2-naphthylamine-lsulfonic acid with 3- hydroxy-2-naphthoicacid and conversion to the Ca salt Monoazo CI NO.15880 6417-83-0 Maroon Foods: Not listed. Drugs: External use only. Not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: Not listed. D&C Red No. 36 Flaming Red Coupling of diazotized 2-chloro-4nitroaniline with 2-naphthol Monoazo CI NO.12085 2814-77-9 Blazing Red Foods: Not listed. Drugs: Dosage unrestricted for external use. Not permitted in eye area. Permitted for ingested drugs except not permitted for mouth washes and dentifrices. Dosage restriction for drugs taken continuously for less than 1 year is 1.7 milligrams per daily dose. Dosage restriction for drugs taken continuously for longer than 1 year is 1.0 milligrams per daily dose.

156

Cosmetics: Amount permitted is unrestricted for external use. May be used to color lip products in amounts not to exceed 3.0% by weight. Not permitted in eye area. Medical Devices: Not listed. D&C Red No. 39 Alba Red Coupling of diazotized anthranilic acid with N,N-bis-(2-hydroxyethy1)aniline Monoazo CI NO. 13058 6371-55-7 Dark Bluish Red Foods: Not listed. Drugs: Permitted for coloring quaternary ammonium type germicidal solutions only. May not exceed 0.1% by weight of germicidal solution. Cosmetics: Not listed. Medical Devices: Not listed. D&C Violet No. 2 Alizurol Purple SS Condensation of quinizarin with p toluidine; or condensation of l-hydroxy-4-halogenanthra quinone with p-toluidine CI No. 60725 81-48-1 Anthraquinone Dark Bluish Violet Foods: Not listed. Drugs: External use only. Not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: May be used generally to color contact lenses. May be used to color surgical sutures as follows: may not exceed 0.2% by weight for glycolic-lactic acid polyester synthetic absorbable sutures for general and ophthalmic surgical use; may not exceed 0.3% by weight for polydioxanone synthetic absorbable sutures for general and ophthalmic surgical use; may not exceed 0.25% by weight for X-caprolactonelglycolide copolymer synthetic absorbable sutures for general surgical use; may not exceed 0.1% by weight for poly(X-caprolactone) absorbable sutures for general surgical use; and may be used to color polymethylmethacrylate intraocular lenshaptics at levels not to exceed 0.2 % by weight of the haptic material. Ext. D&C Violet No. 2 Alzurol Purple Sulfonation of D&C Violet No.2 Anthraquinone CI NO.60730 4430-18-6 Bluish Violet Foods: Not listed. Drugs: Not listed. Cosmetics: External use only. Not permitted in eye area. Medical Devices: Not listed. D&C Yellow No. 7 Fluorescein Condensation of resorcinol with phthalic anhydride in the presence of ZnC1, or H,SO Fluoran CI NO.45350 2321-07-5 Greenish Yellow Foods: Not listed.

157

Drugs: External use only. Not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: Not listed. Nitration of the di or trisulfonic acid Ext. D&C Yellow No. 7Naphthol Yellow S of l-naphthol or the nitroso compound of the 2,7-disulfonic acid Nitro CI NO.10315 846-70-8 Greenish Yellow Foods: Not listed. Drugs: External use only. Not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: Not listed. Conversion of D&C Yellow No 7 to the Na salt D&C Yellow No. 8 Uranine Xanthene CI NO.45350 518-47-8 Greenish Yellow Foods: Not listed. Drugs: External use only. Not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: Not listed. D&C Yellow No. 10 Quinoline Yellow WS Sulfonation of D&C Yellow No. 11 Quinoline CI NO.47005 8004-92-0 Greenish Yellow Foods: Not listed. Drugs: General use except not permitted in eye area. Cosmetics: General use except not permitted in eye area. Medical Devices: May be used generally to color contact lenses. D&C Yellow No. 11 Quinoline Yellow SS Condensation of quinaldine with phthalic anhydride in the presence of ZnC1, Quinoline CI NO.47000 8003-22-3 Greenish Yellow Foods: Not listed. Drugs: External use only. Not permitted in eye area. Cosmetics: External use only. Not permitted in eye area. Medical Devices: Not listed. Phthalocyaninat0(2-)1 Copper Phthalocyanine Heat phthalonitrile with cuprous chloride or heat phthalic anhydride, phthalimide or phthalamide with a copper salt and urea, cyanoguanidine or 4-toluenesulfonamide and oxide cupric chloride in the presence of ammonium molybdate or arsenic Phthalocyanine CI NO.74160 147-14-8 Off Blue Food: Not listed. Drugs: Not listed.

158

Cosmetics: Not listed. Medical Devices: General use in contact lenses. May be used t o color surgical sutures intended for general and ophthalmic surgery and which are made from polypropylene, polybutester or polybutylene terephthalate monofilament. May not exceed 0.5% by weight. May be used to color polymethylmethacrylate monofilament, which is intended to be used as supporting haptics for intraocular lenses. May not exceed 0.5% by weight. The commissioner of the Food and Drug Administration has the authority under law to exempt from certification any color additive if the commissioner has available facts which demonstrate that the color additive should be listed, and that certification procedure is not necessary in order to protect the public health. The FDA approved color additives that do not require certification are listed in Table 4. This list contains the official FDA name, the compound's identity as described by the FDA, and the use restrictions for each of the colorants. A color additive that is exempt from certification is still required to meet the specifications established by the FDA. It is the responsibility of the manufacturer of the color to assure that the material offered for sale meets the FDA specifications. Table 4 Color Additives Exempt from Certification by the FDA Color Additives Identity Uses & Restrictions Foods & Cosmetics: Alumina A1203 *xHaO Permitted as Lake substrate. Drugs: No restrictions. Medical Devices: Not listed. Foods, Drugs & Cosmetics: Chromium-cobaltChromium oxide & aluminum oxide Cobalt carbonate & Not listed. Medical Devices: Permitted aluminum oxide for use in coloring contact Ferric ammonium citrate

The product produced by the interaction of ferric hydroxide in the presence of ammonia

Foods, Cosmetics & Medical Devices: Not listed. Drugs: May only be used in combination with pyrogallol for coloring plain and chromic catgut sutures intended for

159

general and ophthalmic surgical use. Foods, Drugs & Cosmetics: General use. Medical Devices: Not listed. Foods & Medical Devices: Not listed. Drugs: External use only. Cosmetics: General use. Foods, Cosmetics & Medical Devices: Not listed. Drugs: General use. Food: Not to exceed 30mg per pound of solid food or pint of liquid. Not to exceed 4 grams per ton of chicken feed. Drugs: May be generally used to color ingested drugs only. Cosmetics & Medical Devices: Not listed. Foods & Drugs: General use. Cosmetics & Medical Devices: Not listed.

Annatto and Annatto extract

Extract prepared from Annatto Seeds

Bismuth oxychloride

BiOCl

Calcium carbonate

CaC04

Canthaxanthin

0-carotene-4, 4’-dione

Caramel

Prepared by controlled heat treatment of food grade Dextrose, Invert sugar, Lactose, Malt syrup, Molasses, Starch hydrolysate and fractions thereof, and Sucrose Prepared synthetically Foods, Drugs & Cosmetics: General use. or obtained from natural sources Medical Devices: Not listed. An extract of carminic Foods, Drugs & Cosmetics: acid obtained from the General use. Medical Devices: Not listed. Cochineal bug A green to black powder Foods & Medical Devices: obtained from chlorophyll Not listed. by replacing the methyl & Drugs & Cosmetics: Permitted for coloring phytyl ester groups with dentifrices. Not to exceed alkali and replacing magnesium with copper 0.1% by weight.

&Carotene Cochineal extract; Carmine Potassium sodium copper chloropyhllin

160

Dihydroxyacetone

1,3-dihydroxy-2- propanone

Synthetic iron oxide

Fez03 or Fe304

Ferric ammonium ferrocyanide

Oxidation under acidic conditions with sodium dichromate of ferrous sulfate and sodium ferrocyanide in the presence of ammonium sulfate Fer [Fe(CN)6]3.XHz0

Ferric ferrocyanide Chromium hydroxide green

Cr203.XH20

Chromium oxide green

C1-203 Drugs & Cosmetics:

Guanine

Crystals obtained from fish scales consisting principally of guanine and hypoxanthine 1,2,3-Trihydroxybenzene

Pyrogallol

Foods & Medical Devices: Not listed. Drugs & Cosmetics: External use only. Foods: May be used to color sausage casings. Not to exceed 0.1% by weight. May be used to color dog or cat food. May not exceed 0.25% by weight. Drugs: General use. Ingested usage may not exceed 5 milligrams per day calculated as elemental iron. Cosmetics: General use. Medical Devices: May be used to color contact lenses. Foods & Medical Devices: Not listed. Drugs & Cosmetics: External use only. Foods & Medical Devices: Not listed. Drugs & Cosmetics: External use only. Foods: Not listed. Drugs & Cosmetics: External use only. Medical Devices: Not listed. Foods: Not listed. External use only. Medical Devices: Permitted for use in coloring contact lenses. Foods & Medical Devices: Not listed. Drugs: External use only. Cosmetics: General use. Foods, Cosmetics &

161

Pyrophyllite

Ala03*4SiOa*HzO

Logwood extract

Reddish brown to black solid extracted from the heartwood of the leguminous tree. The material is principally hematein

Mica

KzA14(AlzSisOzo)(OH)4 and HaKA13(Si04)3

Talc

Finely powdered, native, hydrous magnesium silicate containing small amounts of aluminum silicates TiOa

Titanium dioxide

Aluminum powder Finely powdered aluminum prepared

Medical Devices: Not listed. Drugs: May be used in combination with ferric ammonium citrate for coloring plain and chromic catgut sutures intended for ophthalmic surgical use. Foods & Medical Devices: Not listed. Drugs & Cosmetics: External use only. Not permitted in eye area. Foods, Cosmetics & Medical Devices: Not listed. Drugs: May be used to color nylon 66, nylon 6 or silk non-absorbable sutures for general and ophthalmic surgery. May not exceed 1.0% by weight of the sutures. Foods & Medical Devices: Not listed. Drugs: May be used in dentifrices and externally applied products. Cosmetics: General use. Foods & Medical Devices: Not listed. Drugs: General use. Cosmetics: Not listed. Foods: General use. May not exceed 1.0% by weight. Drugs & Cosmetics: General use. Medical Devices: May be used to color contact lenses. Foods & Medical Devices: Not listed.

162

from virgin aluminum Bronze powder

Finely powdered virgin electrolytic copper and zinc

Copper powder

Finely powdered virgin electrolytic copper

Zinc oxide

ZnO

Bismuth citrate

Disodium -copper Guaiazulene

BiCsHaO7

EDTA Disodium {(N,N’-1,2ethanediylbis [N-(carboxymethyl) glycinatol ) (4-) N,N,O,O’,ON,ONl cuprate (2-) 1,4-dimethyl-7isopropyl-azulene

Henna

Dried leaf and petiole of Lawsonia alba Lam

Lead acetate

Pb(OOCCH3)2*3H20

Drugs & Cosmetics: External use only. Foods & medical Devices: Not listed. Drugs: External use only. Cosmetics: General use. Foods & Medical Devices: Not listed. Drugs: External use only. Cosmetics: General use. Foods & Medical Devices: Not listed. Drugs: External use only. Cosmetics: General use. Foods, Drugs & Medical Devices: Not listed. Cosmetics: May be used for coloring hair and scalp products. May not exceed 0.5% weightl volume. May only be used on scalp. Foods, Drugs & Medical Devices: Not listed. Cosmetics: Shampoos only. Foods, Drugs & Medical Devices: Not listed. Cosmetics: External use only. Not in eye area. Foods, Drugs & Medical Devices: Not listed. Cosmetics: May be used only to color hair on the scalp. Foods, Drugs & Medical Devices: Not listed. Cosmetics: May be used to color hair on scalp only. May not exceed 0.6%

163

Silver

Ultramarines

Manganese Violet

Crystalline powder of high purity silver prepared by the reaction of silver nitrate with ferrous sulfate in the presence of nitric, phosphoric and sulfuric acids Na7A6Si6024S3 or any complex having the typical formula Na(ASi0)S

Mn(III)NH4P207

weightlvolume calculated as Pb. Foods, Drugs & Medical Devices: Not listed. Cosmetics: May be used to color finger nail polish. May not exceed 1.0%in final product.

Foods: May be used for coloring salt intended for animal feed. May not exceed 0.5%by weight Drugs & Medical Devices: Not listed. Cosmetics: External use only. Foods, Drugs & Medical Devices: Not listed. Cosmetics: General use.

3. PROPERTIES OF COLOR ADDITIVES When selecting a color t o use there are a number of things to be considered; should a dye or a pigment be used for this application; is the color permitted for the intended use; is the color safe at the intended use level; is the color stable under the conditions of use; is the formula compatible with the color; etc? The difficulties that can occur are almost unlimited and the problems may not be limited to the formulation but can occur with the material or liner that is used in the packaging. Some of the most common problems that occur come from an incompatible pH or the presence of a strong reducing or oxidizing agent. Additional problems, which may not be visible at the time the product is manufactured but can occur during storage, may develop from incompatible ions, excessive heat, microorganisms, or ultraviolet light. Table 5 shows the solubility characteristics of all the certified straight color. Table 6 shows the fastness properties of the certified straight colors.

164

Table 5 Solubilities of certified colors

I

FDA Name

Steric Oleic Acid H 2 0 Acid

FD&C Blue No. 1 FD&C Blue No. 2 FD&C Green No. 3 FD&C Yellow No. 5 FD&C Yellow No. 6 FD&C Red No. 3 FD&C Red No. 4 FD&C Red No. 40 Orange B Citrus Red No. 2 D&C Violet No. 2 D&C Blue No. 4 D&C Blue No.6 D&C Blue No. 9 D&C Green No. 5 D&C Green No. 6 D&C Green No. 8 D&C Yellow No. 7 D&C Yellow No. 8 D&C Yellow No. 10 D&C Yellow No. 11 D&C Orange No. 4 D&C Orange No. 5 D&C Orange No. 10 D&C Orange No. 11 D&C Brown No. 1 D&C Red No. 6 D&C Red No. 7 D&C Red No. 17 D&C Red No. 21 D&C Red No. 22 D&C Red No. 27

C I I IE I IE IE I IE S S C D D IE M IA D IE I S IE D IE IE IE I D S D IE D -

- - S S

Ethyl deOH Ether

--

C S I ss S I S S IE ss S S I S S IE S ss IE S I S S IE ss I S ss ss I S S C S IU I D IU D IA S IE S M I ss SF IA SSF IBF D SF SF IE SF S M I I S S S IE S IB D S IE S S IE S S S S IE S ss I I IA D I 3s-M S IBF D ss SF SF IE D IB ss -

I I I I I IA I I I

Mineral Minerr Xycer- WaX

:to€ Oil - S

01

cetont

Petrol Jelly

- - -

C S C ss I S I I M I S S ss IE IE S ss I I S IE S IE ss IE S IE I ss I S IE S ss IE ss ss S ss S ss ss S Ia sw I S C S C D I I D D D I D I ID I IEW ss IE S M ss ss M IA I IA SSF SSF I ss* ss D SSF D M IE Ia SF IE IA ss I I S S S S ss S I M IE S IE M M ss D D IE IE IA S S S IE IE IA S IE ss ss IE S I I IA I S D D D I IA ss ss S ss S M* ss D Da D SF IE SF IA IE IA D D ss DA -

IA I I I I

ss I I IA

ss ss IA I I

ss ss IA

ss

KIa

ss S IA S

ss ss ss IA IA

ss S

ss ss

C I I IE I IE IE I IE S S C D D IE M IA D IE I S IE D IE IE IE I D S D IE D -

7 165

Table 5 continued IA

D&C Red No. 30 D&C Red No. 31 D&C Red No. 33 D&C Red No. 34 D&C Red No. 36 D&C Red No. 39 Ext. D&C Vlt No. 2 Ext. D&C Yel. No. 7

IA

ss

D D

ss

I

I I I I S I I

- -IE S IE D D D

S I

ss ss

I I D D I I I

I IA S+

ss

ss

ss S

I D

M S S

i i D D IA i i

ss M

Table 6 Fastness Properties of Certified Colors 5%

FDA Name FD&C Blue No. 1 FD&C Blue No. 2 FD&C Green No. 3 FD&C Yellow No. 5 FD&C Yellow No. 6 FD&C Red No. 3 FD&C Red No. 4 FD&C Red No. 40 Orange B Citrus Red No. 2 D&C Violet No. 2 D&C Blue No. 4 D&C Blue No.6 D&C Blue No. 9 D&C Green No. 5 D&C Green No. 6 D&C Green No. 8 D&C Yellow No. 7 D&C Yellow No. 8 D&C Yellow No. 10 D&C Yellow No. 11 D&C Orange No. 4 D&C Orange No. 5 D&C Orange No. 10 D&C Orange No. 11

Light 3 1 3

5

5 6

5 5 5

3 3 6 3

2PYA 6

5

5 5

3

61

4 3 6

7 5 4 2 2 3 3 2

5

2 3

3

10% HC1

4g 5 5 5 5 1PY

I

5 5 5 21k 51 5 51 51 5 51 I I

3P

3P

51 5 71 71 5 5L

I

5 I 5 4AI 2PYA 2nvA

5 51 5 41 IPY

1 nv

10%

NaOH 4

4

2b 5 5 6 5 5 5 51 51

5% FeS04 4r 4

Alum

3Y

4 4 4

d

4 P PZ

4 d 41 41 4 I I 4 I 4d I

4 J

P P

4

4 41

4r Iw 2m Sr 6

JP

41 4 I I 4 I 4d I P 4 I JP

6

n

P

4 6LU 6IU 5 61 5 6s 6

ZP Z

I I P

I P

ox. Red. Agents Agents 2 2 2 3 3 3 3 3 3 3 2 2 6 6 3 3 3 3 3 2 2 3 3 3

1

1 1 1 1 1 1 1 1 1 1 U U 2 2 3 3 3 5 5 3 3 1

R

1

4

166

Table 6 continued D&C Brown No. 1 D&C Red No. 6 D&C Red No. 7 DCC Red No. 17 D&C Red No. 21 D&C Red No. 22 D&C Red No. 27 D&C Red No. 28 D&C Red No. 30 D&C Red No. 31 D&C Red No. 33 D&C Red No. 34 D&C Red No. 36 D&C Red No. 39 Ext. D&C Violet No. 2

Ext. D&C Yellow No. 7

3 5 6 6 2 2 2 3 6 5 5 4 6 2 5

4

5 5 51 61 31 2PY 31 2P 71 5 6 51 61 SY 5

5 4 41 41 31 IPY 3 4P I 4 32 4 4d sx 5

6sly 4d 51 41 5Sr 5 3Sr 6 61U 5 5 41 4d 6Sx 5

5

5

5

I P 4 I 4d 4AId 42

P P 41 41 41 2Y I P I P 4 I 4 I 4

3 3 3 3 4 4 4 4 5 3 3 3 3 3 3

4 U 1 1 1 1 3 2

Zd

4

3

3

P P 41d 41d Id 3d I 2

1 1 1 1 4 4 4

Abbreviations for Tables 5 and 6:

A

May bleed or stain, very sparingly solublg

B

Insoluble in water, soluble in aqueous alkaline solution

C

Practically insoluble, but is useful in neutral or slightly acidic emulsions

D

Practically insoluble, but may be dispersed by grinding and homogenizing

E Practically insoluble in the fatty

F

Soluble or dispersible in oils and waxes, when 10-25%of a fatty acid is present

I

Insoluble

J

Tends to thicken or gel the solution

L

Turns orange in hue

M Moderately soluble (less than 1%) Ox = oxidizing

acid, oil, or wax, but is useful in coloring slightly some waxes

Red = reducing

Solution is usually fluorescent

S

Dissolves (soluble 1%or more)

167

SS Sparingly soluble (less than 0.25%)

t

U

V

Turns violet in hue

W

Becomes tinctorially weaker

alkaline aqueous emulsions

X

Turns yellow in hue.

b

Turns much bluer in hue

Y

Turns yellower in hue.

C

At 25°C

Z

Hazy or cloudy.

d

Hue becomes duller and darker

*

Practically colorless

g

Turns much greener in hue.

+

Not suitable for acid solutions

k

Turns brownish in hue

1

Very poor fastness.

2

Poor fastness.

3

Fair fastness.

In alkaline reducing vats a soluble leuco compound forms

W Not fast to prolonged storage in

m Turns scarlet in hue

Dye destroyed, or solution turns practically colorless

n

Slowly or on standing for some time

4

Moderate fastness.

P

Dye precipitates as heavy metal salt or color acid

5

Good fastness.

r

Turns redder in hue.

6

Very good fastness.

sl

Slightly

7

Excellent fastness.

4. EVALUATION AND ANALYSIS OF COLOR ADDITIVES

A comprehensive review of the methods of analysis used for identification of food, drug, cosmetic, and medical device colors is presented in the Encyclopedia of Industrial Chemical Analysis, Vol. 10, pp. 447-547, John Wiley & Sons, 1970 and in the Official Methods of Analysis of the Association of Official Analytical Chemists, published by the A.O.A.C. Common to all colorants is the evaluation of the color for shade and strength. Dyes are generally evaluated on a visible spectrophotometer at the specific wavelength optimum for that color. Dyes are generally used at very low levels,

168

such as, 0.1% or 0.05%, and the strength is usually not a major factor except if two or more dyes are mixed to produce a desired shade. Pigments, lake pigments and toner colors are evaluated for shade by several different methods depending on the intended application. They will be evaluated side by side against a standard so that the two colors touch to create a line between making the determination of differences less difficult. Some of the common methods used to evaluate shade are as follows. The pure powder color is pressed between two pieces of paper and visually compared. A dispersion of 10% or less color is prepared in an oil such as castor oil or mineral oil by milling with a three roller mill, Hoover muller, etc. A film of uniform thickness is then prepared on a card that is half white and half black and the film is then evaluated visually. A film with the color dispersed in nitrocellulose lacquer is also prepared in the same way and evaluated by visual analysis. Pigment color strength is typically evaluated by using the methods described above and blending the powder color with talc, titanium dioxide, or another color and visually evaluating. Also used are computers with programs designed to evaluate data produced using a visible spectrophotometer. Using this approach the preparation and presentation of the samples is critical. Both wet chemistry and instrumentation methods are used t o evaluate the FDA approved colors. Typical analysis involve titration, extraction, gravimetric analysis, visible and infrared spectrophotometry, high performance liquid chromatography and reverse-phase HPLC, thin layer and ion chromatography, xray fluorescence spectrophotometry, elution column chromatography, and atomic absorption spectroscopy. Additionally, the FDA has bacteria specification of 100 colonies per gram with no gram negatives permitted. Most of the chemical evaluation are for intermediates specific relating to the chemical class of the color. Listed below are some of the more common analytical test used to assure that the specific color meets the FDA requirements. The determination of total color in certifiable color additives and color additive lake pigments. Typical evaluation by visible spectrophotometry or titanium trichloride titration. The determination of total color in certifiable halogenated fluorescein colors. Typical evaluation by gravimetric analysis. The determination of volatile matter in certifiable colors. Typical evaluation by infrared heat moisture analysis. The determination of insoluble matter in certifiable color additives and color additive lake pigments. Typical evaluation by gravimetric analysis. The determination of anthraquinone intermediates in D&C Green No. 5 and Ext. D&C Violet No. 2. Typical evaluation by isooctane extraction. The determination of unsulfonated subsidiary colors and ether-soluble matter in D&C Red No. 6 and D&C Red No. 7 and their lake pigments. Typical evaluation by ether extraction.

169

The determination of sulfonated organic impurities in FD&C Yellow No. 5 . Typical evaluation by reversed-phase high-performance liquid chromatography. The determination of sulfonated organic impurities in FD&C Yellow No. 6. Typical evaluation by reversed-phase high-performance liquid chromatography. The determination of sulfonated organic impurities in FD&C Red No. 40. Typical evaluation by reversed-phase high-performance liquid chromatography. The determination of components of D&C Red No. 27 and D&C Red No. 28. Typical evaluation by thin-layer chromatography and video densitometry. The determination of intermediates, side reaction products, and subsidiary colors in D&C Red No. 27 and D&C Red No. 28. Typical evaluation by reversedphase high-performance liquid chromatography. The determination of acetone soluble matter in D&C Red No. 30. Typical evaluation by acetone extraction. The determination of sodium chloride, sodium sulfate, and sodium iodide in certifiable colors. Typical evaluation by ion chromatography. The determination of soluble barium in certifiable lake pigments. Typical evaluation by gravimetric analysis. The determination of sodium iodide, phthalic acid, and related impurities in FD&C Red No. 3. Typical evaluation by gravity elution column chromatography. The determination of intermediates and reaction by-products in FD&C Blue No. 1, FD&C Green No. 3 and D&C Blue No. 4. Typical evaluation by gravity elution column chromatography. The determination of intermediates in FD&C Red No. 4, D&C Green No. 8, D&C Orange No. 4,D&C Red No. 33, D&C Yellow No. 10, and Ext. D&C Violet No. 2 and their lake pigments. Typical evaluation by gravity elution column chromatography. The determination of intermediates and related impurities in Citrus Red No. 2, D&C Blue No. 6, D&C Red No. 6, D&C Red No. 7, D&C Red No. 17, D&C Red No. 34, D&C Violet No. 2 and D&C Yellow No. 11 and their lake pigments. Typical evaluation by gravity elution column chromatography. The determination of intermediates and related impurities in D&C Orange No. 5 , D&C Red No. 21, D&C Red No. 22, D&C Red No. 27, D&C Red No. 28 and their lake pigments. Typical evaluation by gravity elution column chromatography. The determination of sulfonated organic impurities in FD&C Blue No. 2. Typical evaluation by reversed-phase high-performance liquid chromatography. The determination of subsidiary colors in FD&C Red No. 3. Typical evaluation by reversed-phase high-performance liquid chromatography. The determination of leuco base in FD&C Blue No. 1. Typical evaluation by reversed-phase high-performance liquid chromatography.

170

The determination of intermediates and reaction by-products in D&C Yellow No. 7 and D&C Yellow No. 8. Typical evaluation by reversed-phase highperformance liquid chromatography. The determination of intermediates and related impurities in D&C Red No. 36 and it lake pigment. Typical evaluation by reversed-phase high-performance liquid chromatography. The determination of intermediates and unsulfonated subsidiary colors in D&C Red No. 6, D&C Red No. 7 and their lake pigments. Typical evaluation by reversed-phase high-performance liquid chromatography. The determination of intermediates and subsidiary colors in D&C Red No. 33. Typical evaluation by reversed-phase high-performance liquid chromatography. The determination of aromatic amines in FD&C Yellow No. 5 . Typical evaluation by diazotization and coupling followed by reversed-phase highperformance liquid chromatography. The determination of aromatic amines in FD&C Yellow No. 6. Typical evaluation by diazotization and coupling followed by reversed-phase highperformance liquid chromatography. The determination of unsulfonated aromatic amines in D&C Red No. 33. Typical evaluation by diazotization and coupling followed by reversed-phase high-performance liquid chromatography. The determination of unsulfonated subsidiary color and diethyl ether-soluble matter in D&C Yellow No. 10. Typical evaluation by reversed-phase highperformance liquid chromatography. The determination of p-toluidine in D&C Red No. 6 and D&C Red No. 7. Typical evaluation by reversed-phase high-performance liquid chromatography. The determination of subsidiary colors in FD&C Blue No. 1, FD&C Green No. 3, and D&C Blue No. 4.Typical evaluation by thin layer chromatography. The determination of mono and disulfonated component colors in D&C Yellow No. 10 and its lake pigments. Typical evaluation by thin layer chromatography. The determination of components and subsidiary colors in D&C Orange No. 5, D&C Red No. 21, D&C Red No. 22 and their lake pigments. Typical evaluation by thin layer chromatography. The determination of lower halogenated subsidiary colors in D&C Red No. 27, D&C Red No. 28 and their lake pigments. Typical evaluation by thin layer chromatography. The determination of subsidiary colors in D&C Red No. 36. Typical evaluation by thin layer chromatography. The determination of subsidiary colors in D&C Green No. 8. Typical evaluation by thin layer chromatography. The determination of subsidiary colors in D&C Green No. 5 and Ext. D&C Violet No. 2. Typical evaluation by thin layer chromatography.

171

The determination of lead and arsenic in certifiable colors additives and color additive lake pigments. X-ray fluorescence spectrometry. The determination of chromium in FD&C Blue No. 1, FD&C Green No. 3 and D&C Blue No. 4 and manganese in FD&C Blue No. 1. X-ray fluorescence spectrometry. The determination of mercury in certifiable color additives and color additive lake pigments. Typical evaluation by automated microwave digestion and dedicated mercury analyzer. The quantitative determination of lead (Pb), arsenic (As), antimony (Sb), mercury (Hg), and total heavy metals. Typical evaluation by atomic absorption spectroscopy.

5. WORLDWIDE REGULATIONS AND PERMITTED COLOR ADDITrVES Many countries regulate color additives; however, there are three regulatory bodies that appear to have established the regulations which the other countries use as the basis for their laws. These regulatory bodies are the US Food and Drug Administration (FDA), the European Commission (EC), and the Japanese Ministry of Health and Welfare (MHW). Table 7 lists the color additives permitted for use by the three major regulatory bodies, that is, the FDA, EC and the MHW. Table 8 lists the color additives permitted for use by the FDA and the EC. Table 9 lists the color additives permitted for use by the FDA and the MHW. Table 10 lists the color additives permitted for use by the EC and the MHW. Table 11lists the color additives permitted for use only by the FDA. Table 12 lists the color additives permitted for use only by the EC. Table 13 lists the color additives permitted for use only by the MHW. 5.1 European Commission (EC) The EC does not specifically name the approved colorants but uses the color index number to identify each color. All lake pigments produced from an EC approved dye are permitted for use by the EC member countries. There are specific regulations and requirements for the salts used to produce the lakes. The specification established for free barium has made it very difficult sell D&C Red No. 6 barium lake in the EC regulated countries. Barium, strontium and zirconium compounds in general are prohibited by EC regulations, however, the insoluble barium, strontium and zirconium lake pigments are permitted provided they can pass the test for insolubility. The EC regulations on color additives currently exempt hair dyes but do place certain requirements and restrictions on colorants intended for hair coloring use.

172

5.2 Japanese Ministry of Health and Welfare UVIHW) The MHW has not established a list of permitted inorganic and natural colorants similar to the FDA list of “exempt form certification” colorants. The MHW regulates the use of “inorganic and natural” colors on a case by case during the pre-market approval process. The MHW regulates oxidative hair dyes as quasi-drugs. There are sixty-two (62) active and one thousand and five (1,005)inactive ingredients permitted for use in hair dyes. The specifications and use limitations of these products are published in the first volume of the Japanese Standards of Quasi-Drug Ingredients. The manufacturing and importation of hair coloring products is closely regulated by the MHW. The lake pigments and extended salts of approved dyes are not permitted by the MHW unless expressly authorized by the regulations

5.3 US Food and Drug Administra-tion (FDA) The use of FDA approved color additives was discussed earlier in this chapter. Colorants intended only for use in hair dyes are specifically exempt from listing and certification. All hair dye products are required to have adequate directions for use and warning statements as described in the regulations. These must including directions for a preliminary patch test before use and a warning against use on eyebrows and eyelashes. Listed color additives may also be used in hair coloring products and three such exempt colorants are specifically approved for use as hair colorants, they are, Bismuth Citrate, Henna, and Lead Acetate. The FDA lists three other times when a color additive intended for human use is exempt from certification. These are, if batch certification is shown to be unnecessary for the protection of the public health, if it is intended for investigational use and bears a label to that effect, and if it is used or intended to be used solely for a purpose other than coloring. If an agent that imparts color to a formulation is intended solely for a purpose other than coloring, such use must have proper scientific support. Any colorant may be used in the manufacture of soap as long as the product meets the FDA definition of soap and is properly labeled. The FDA defines soap as “an alkali salt of fatty acids” A product intended for cleansing the human body that makes a claim, such as, ((deodorant soap”, “softens skin”, “moisturizes”, “beautified’, etc. is considered by the FDA to be a cosmetic or a drug product and must use only colors approved for that use.

173

Table 7 Color Additives Approved for Use in the European Community, Japan and the United States United States Euro. Comm. Japanese Color Index Color Index Number Name Name Name Name

FD&C Blue No. 1

42090

Food Blue 2

42090

Blue No.1

FD&C Yellow No. 5

19140

Yellow No.4

19140

Acid Yellow 23; Food Yellow 4

FD&C Yellow No. 6

15985

Yellow No.5

15985

Food Yellow 3

FD&C Green No. 3

42053

Green No.3

42053

Food Green 3

FD&C Red No. 4

14700

Red No.504

14700

Food Red 1

D&C Blue No. 4

42090

Blue No.205

42090

Acid Blue 9

D&C Green No. 5

61570

Green No.201

61570

Acid Green 25

D&C Green No. 6

61565

Green No.202

61565

Solvent Green 3

D&C Green No. 8

59040

Green No.204

59040

Solvent Green 7

D&C Orange No.4

15510

Orange No.205

15510

Acid Orange 7

D&C Orange No. 5

45370

Orange No.201

45370:l

Solvent Red 72

D&C Orange No. 10

45425

Orange No.206

45425:l

Solvent Red 73

D&C Orange No. 11 45425

Orange No.207

45425

Acid Red 95

D&C Red No. 6

15850

Red No.201

15850

Pigment Red 57

D&C Red No. 7

15850

Red No.202

15850:l

Pigment Red 57:l

D&C Red No. 17

26100

Red No.225

26100

Solvent Red 23

D&C Red No. 21

45380

Red No.223

45380:2

Solvent Red 43

174

D&C Red No. 22

45380

Red No.230

45380

Acid Red 87

D&C Red No. 27

45410

Red No.218

45410:l

Solvent Red 48

D&C Red No. 28

45410

Red No. 104

45410

Acid Red 92

D&C Red No. 30

73360

Red No.226

73360

Vat Red 1

D&C Red No. 31

15800

Red No.219

15800:l

Pigment Red 64:1

D&C Red No. 33

17200

Red No.227

17200

Acid Red 33; Food Red 12

D&C Red No. 34

15880

Red No.220

15880:l

Pigment Red 63: 1

D&C Red No. 36

12085

Red No.228

12085

Pigment Red 4

D&C Yellow No. 7

45350

Yellow No.201

45350:l

Acid Yellow 73

D&C Yellow No. 8

45350

Yellow No.202

45350

Acid Yellow 73

D&C Yellow No. 10

47005

Yellow No.203

47005


D&C Yellow No. 11

47000

Yellow No.204

47000

Solvent Yellow 33

D&C Violet No. 2

60725

Violet No.201

60725

Solvent Violet 13

D&C Brown No. 1

20170

Brown No.201

20170

Acid Orange 24

Ext. D&C Yellow No. 10316 No. 7

Yellow No.403

10316

Acid Yellow 1

Ext. D&C Violet No. 2

60730

Violet No.401

60730

Acid Violet 43

p-Carotene

40800

P-Carotene

40800

Food Orange 5

175

Carmine

75470

Natural Red 4

Chlorophyllin-Copper 75810

Sodium Copper

75810

Natural Green 3 Complex Chlorophyllin

Aluminum powder

77000

A1 Powder

77000

Pigment Metal 1

Aluminum hydroxide 77002

Aluminum hydroxide

77002

Pigment White 24

Kaolin

77004

Kaolin

77004

Pigment White 19

Ultramarines

77007

U1tramarine

77007

Pigment Blue 29

p-Carotene

75130

l3 -Carotene

75130

Natural Yellow 26; Natural Brown 5

Annatto

75120

Annatto

75120

Natural Orange 4

Guanine

75170

Guanine

75170

Natural White 1

Barium sulfate

77120

Barium sulfate

77120

Pigment White 21; Pigment White 22

Bismuth oxychloride 77163

Bismuth oxychloride

77163

Pigment White 14; Pigment White 31

Calcium Carbonate

77220

Calcium carbonate

77220

Pigment White 18

Chromium oxide green

77288

Chromium oxide 77288

Pigment Green 17

77289

Pigment Green 18

Carmine

75470

Chromium hydroxide 77289 green

Hydrated chromium oxide

176

Calcium sulfate

77231

Calcium sulfate

77231

Pigment White 25

Iron oxides

77491

Red oxide of iron

77491

Pigment Red 101; Pigment Red 102; Pigment Brown 6; Pigment Brown 7

Iron oxides

77492

Yellow oxide of iron

77492

Pigment Yellow 42; Pigment Yellow 43; Pigment Brown 6; Pigment Brown 7

Iron oxides

77499

Black oxide of iron

77499

Ferric ammonium Ferrocyanide; Ferric Ferrocyanide

77510

Iron Blue

77510

Pigment Black 11;Pigment Brown 6; Pigment Brown 7 Pigment Blue 27

Magnesium carbonate 77713

Magnesium carbonate

77713

Pigment White 18

Manganese Violet

77742

Manganese violet

77742

Pigment Violet 16

Titanium dioxide

77891

Titanium dioxide 77891

Pigment White 6

Zinc oxide

77947

Zinc oxide

Pigment White 4

Aluminum stearate

Aluminum Aluminum stearate stearate

77947

Aluminum Not listed stearate

177

Calcium stearate

Calcium stearate

Calcium stearate

Calcium stearate

Not listed

Caramel

Caramel

Caramel

Caramel

Not listed

Magnesium stearate

Magnesium Magnesium stearate stearate

Magnesium stearate

Not listed

Zinc Stearate

Zinc stearate Zinc stearate

Zinc stearate Not listed

Table 8 Color Additives Approved for use in the United States and the European Community Color Index Number 77400

Color Index Name Pigment Metal 2

United States Name Bronze Powder

Euro. Commun. Name 77400

Japanese Name No history

Copper Powder

77400

No history

77400

FD&C Red No. 40 16035

Not Permitted

16035

Pigment Metal 2 Food Red 17

Silver

No history

77820

None

77820

Table 9 Color Additives Approved for use in the United States and Japan United States Name Mica

Euro. Commun. Name Not Permitted

Japanese Name Mica

Color Index Number 77019

Ultramarines

Not Permitted

Ultramarine

77013

Magnesium oxide

Not Permitted

Magnesium oxide

77711

Color Index Name Pigment White 20; Pigment White 26 Pigment Green

24

None

178

Talc

Not Permitted

Talc

77718

Pigment White 26

Dihydroxyacetone

Not Listed

Dihydroxyacetone

DHA

Not listed

Guaiazulene

Not Permitted

Guaiazulene

Guaiazulene

Not listed

Pyrophyllite

Not Permitted

Pyrophyllite

Pyrophyllite

Not listed

Table 10 Color Additives Approved for use in the European Community and Japan United States Name

Euro. Commun. Name

Japanese Name

Color Index Number

Color Index Name

Not Permitted

10020

Green No. 401 10020

Acid Green 1

Not Permitted

11680

Yellow No.401 11680

Pigment Yellow 1

Not Permitted

11725

Orange No.401 11725

Pigment Orange 1

Not Permitted

12120

Red No.221

12120

Pigment Red 3

Not Permitted

15620

Red No.506

15620

Acid Red 88

Not Permitted

15630

Red No.205

15630

Pigment Red 49

Not Permitted

15630

Red No.207

15630:l

Pigment Red 49: 1

Not Permitted

15630

Red No.206

15630:2

Pigment Red 49:2

Not Permitted

15630

Red No.208

15630:3

Pigment Red 49:3

Not Permitted

15865

Red No.405

15865

Pigment Red 48

Not Permitted

16185

Red No.2

16185


179

Not Permitted

16255

Red No.102

Not Permitted

18820

Yellow No.407 18820

Acid Yellow 11

Not Permitted

20470

Black No.401

20470

Acid Black 1; Basic Black 4; Solvent Brown 12

Not Permitted

45100

Red No.106

45100

Acid Red 52

Not Permitted

45190

Red No.401

45190

Acid Violet 9; Solvent Violet 10

Not Permitted

45350

Yellow No. 202 45350

Acid Yellow 73

Not Permitted

45380

Red No.230

45380

Acid Red 87

Not Permitted

45430

Red No.3

45430


Not Permitted

69825

Blue No.204

69825

Vat Blue 6; Pigment Blue 64

Not Permitted

73000

Blue No.201

73000

Vat Blue 1; Pigment Blue 66

Not Permitted

73015

Blue No.2

73015

Acid Blue 74; Food Blue 1; Pigment Blue 63

Not Permitted

74160

Blue No.404

74160

Pigment Blue 15

Not Permitted

75100

Saffron Extract 75100

16255


Natural Yellow 6; Natural Yellow 19; Natural Red 1

180

Not Permitted

77266

Carbon Black 77266

Pigment Black 6; Pigment Black 7

Not Permitted

77346

Cobalt Aluminum Oxide

77346

Pigment Blue 28; Pigment Green 14

Not Permitted

77480

Gold Leaf

77480

Pigment Metal 3

Not Permitted

Not listed

Capsanthin; Capsicum Capsorubin Tincture

Capsanthin; Capsorubin

Not Permitted

Lactoflavin

Riboflavin

Not listed

Lactoflavin

Table 11 Color Additives Approved for use in the United States Only United States

Name Henna

Name Not Permitted

Euro. Commun.

Japanese

Bismuth citrate disodium

Not Permitted

Not Permitted Bismuth citrate Not listed

Not Permitted

No history

Disodium EDTA-Copper EDTA-Copper

Not listed

Lead Acetate

Not Permitted

Not Permitted Lead acetate

Not listed

Name No history


Color Index Name Natural Orange 6

Table 12 Color Additives Approved for use in the European Community Only United States Name Not Permitted

Euro. Commun. Japanese Name Name 10006 Not Permitted


Color Index Name Pigment Green 8

181

Not Permitted

11710

Not Permitted

11710

Pigment Yellow 3

Not Permitted

11920

Not Permitted

11920

Solvent Orange 1; Food Orange 3

Not Permitted

12010

Not Permitted

12010

Solvent Red 3

Not Permitted

12150

Not Permitted

12150

Solvent Red 1

Not Permitted

12370

Not Permitted

12370

Pigment Red 112

Not permitted

12420

Not Permitted

12420

Pigment Red 7

Not Permitted

12480

Not Permitted

12480

Pigment Brown 1

Not Permitted

12490

Not Permitted

12490

Pigment Red 5

Not Permitted

12700

Not Permitted

12700

Disperse Yellow 16; Solvent Yellow 1

Not Permitted

13015

Not Permitted

13015


Not Permitted

14270

Not Permitted

14270

Acid Orange 6; Food Yellow 8

Not Permitted

14720

Not Permitted

14720

Acid Red 14; Food Red 3; Mordant Blue 7

Not Permitted

14815

Not Permitted

14815

Food Red 2

Not Permitted

15525

Not Permitted

15525

Pigment Red 68

Not Permitted

15580

Not Permitted

15580

Pigment Red 5 1

Not Permitted

15980

Not Permitted

15980

Food Orange 2

Not Permitted

16230

Not Permitted

16230

Acid Orange 10; Food Orange 4

Not Permitted

16290

Not Permitted

16290


Not Permitted

18050

Not Permitted

18050


182

Not Permitted

18130

Not Permitted

18130

Acid Red 155

Not Permitted

18690

Not Permitted

18690

Acid Yellow 121; Solvent Yellow 21

Not Permitted

18736

Not Permitted

18736

Acid Red 180

Not Permitted

18965

Not Permitted

18965


Not Permitted

20040

Not Permitted

20040

Pigment Yellow 16

Not Permitted

21100

Not Permitted

21100

Pigment Yellow 13

Not Permitted

21108

Not Permitted

21108

Pigment Yellow 83

Not Permitted

21230

Not Permitted

21230

Solvent Yellow 29

Not Permitted

24790

Not Permitted

24790

Acid Red 163

Not Permitted

27290

Not Permitted

27290

Acid Red 73

Not Permitted

27755

Not Permitted

27755

Food Black 2

Not Permitted

28440

Not Permitted

28440

Food Black 1

Not Permitted

40215

Not Permitted

40215

Direct Orange 34; Direct Orange 39

Not Permitted

40820

Not Permitted

40820

Food Orange 6

Not Permitted

40825

Not Permitted

40825

Food Orange 7

Not Permitted

40850

Not Permitted

40850

Food Orange 8

Not Permitted

42045

Not Permitted

42045

Acid Blue 1

Not Permitted

42051

Not Permitted

42051

Acid Blue 3

Not Permitted

42080

Not Permitted

42080

Acid Blue 7

Not Permitted

42100

Not Permitted

42100

Acid Green 9

Not Permitted

42170

Not Permitted

42170

Acid Green 22

183

Not Permitted

42510

Not Permitted

42510

Basic Violet 14

Not Permitted

42520

Not Permitted

42520

Basic Violet 2

Not Permitted

42735

Not Permitted

42735

Acid Blue 104

Not Permitted

44045

Not Permitted

44045

Basic Blue 26; Pigment Blue 2; Solvent Blue 4

Not Permitted

44090

Not Permitted

44090

Acid Green 50; Food Green 4

Not Permitted

45220

Not Permitted

45220

Acid Red 50

Not Permitted

45396

Not Permitted

45396

Solvent Orange 16

Not Permitted

45405

Not Permitted

45405

Acid Red 98

Not Permitted

50325

Not Permitted

50325

Acid Violet 50

Not Permitted

50420

Not Permitted

50420

Acid Black 2

Not Permitted

51319

Not Permitted

51319

Pigment Violet 23

Not Permitted

58000

Not Permitted

58000

Mordant Red 11

Not Permitted

60724

Not Permitted

60724

Disperse Violet 27

Not Permitted

61585

Not Permitted

61585

Acid Blue 80

Not Permitted

62045

Not Permitted

62045

Acid Blue 62

Not Permitted

69800

Not Permitted

69800

Vat Blue 4

Not Permitted

71105

Not Permitted

71105

Vat Orange 7; Pigment Orange 43

Not Permitted

73385

Not Permitted

73385

Vat Violet 2; Pigment Violet 36

Not Permitted

73900

Not Permitted

73900

Pigment Violet 19

Not Permitted

73915

Not Permitted

73915

Pigment Red 122

Not Permitted

74100

Not Permitted

74100

Pigment Blue 16

184

Not Permitted

74180

Not Permitted

74180

Direct Blue 86

Not Permitted

74260

Not Permitted

74260

Pigment Green 7

Not Permitted

75125

No history

5125

Natural Yellow 27

Not Permitted

75135

No history

75135

Natural Yellow 27

Not Permitted

75300

No history

75300

Natural Yellow 3

Not Permitted

77015

No history

77015

Pigment Red 101; Pigment Red 102

Not Permitted

77267

No history

77267

Pigment Black 9

Not Permitted

77268:l

No history

77268: 1

Food Black 3

Not Permitted

77745

No history

77745

None

Not Permitted

Acid Red 195

No history

Acid Red 195

Not listed

Not Permitted

Anthocyanins

No history

Anthocyanins Not listed

Not Permitted

Beetroot Red

No history

Beetroot Red

Not listed

Not Permitted

Bromocresol Green

No history

Bromocresol Green

Not listed

Not Permitted

Bromothymol Blue

No history

Bromothymol Blue

Not listed

Table 13 Color Additives Approved for use in Japan Only United States Name

Euro. Commun. Japanese Name Name

Not Permitted

Not Permitted

Yellow No.404

11380

Solvent Yellow 5

Not Permitted

Not Permitted

Yellow No.405

11390

Solvent Yellow 6

Not Permitted

Not Permitted

Orange No.203

12075

Pigment Orange 5

Not Permitted

Not Permitted

Orange No.403

12100

Solvent Orange 2

Color Index Color Index Number Name

185

Not Permitted

Not Permitted

Red No.505

12140

Solvent Orange 7

Not Permitted

Not Permitted

Red No.404

12315

Pigment Red 22

Not Permitted

Not Permitted

Yellow No.406

13065

Acid Yellow 36

Not Permitted

Not Permitted

Orange No.402

14600

Acid Orange 20

Not Permitted

Not Permitted

Red No.203

15585

Pigment Red 53

Not Permitted

Not Permitted

Red No.204

155851

Pigment Red 53: 1

Not Permitted

Not Permitted

Red No.503

16150


Not Permitted

Not Permitted

Red No.502

16155

Food Red 6

Not Permitted

Not Permitted

Yellow No.402

18950

Acid Yellow 40

Not Permitted

Not Permitted

Yellow No.205

21090

Pigment Yellow 12

Not Permitted

Not Permitted

Orange No.204

21110

Pigment Orange 13

Not Permitted

Not Permitted

Red No.501

26105

Solvent Red 24

Not Permitted

Not Permitted

Blue No.202

42052

Acid Blue 5

Not Permitted

Not Permitted

Blue No.203

42052:l

Acid Blue 5

Not Permitted

Not Permitted

Green No.402

42085

Acid Green 3; Food Green 1

Not Permitted

Not Permitted

Green No.205

42095

Acid Green 5 ; Food Green 2

Not Permitted

Not Permitted

Red No.213

45170

Basic Violet 10

Not Permitted

Not Permitted

Red No.214

45170

Basic Violet 10

Not Permitted

Not Permitted

Red No.215

45170

Basic Violet 10

Not Permitted

Not Permitted

Red No.231

45410

Acid Red 92

Not Permitted

Not Permitted

Red No.232

45440

Acid Red 94

Not Permitted

Not Permitted

Red No.105

45440

Acid Red 94

186

Not Permitted

Not Permitted

Blue No.403

61520

Solvent Blue 63

The following is a list of countries with color additive regulations that are based upon one of the three major regulatory bodies. I n many cases the specific country identifies the regulatory group which was the basis for i t regulations, however, regulations for any specific colorant should be checked. FDA Argentina Canada Chile China Columbia Ecuador Guatemala Mexico Panama Peru The Philippines USA Venezuela

EC Belgium Brazil Bulgaria Denmark Finland France Germany Greece Hungary Ireland Italy Luxembourg Malta

The Netherlands New Zealand Norway Poland Portugal Saudi Arabia South Africa Spain Sweden Switzerland Turkey UK

MHW Japan Korea (South) Taiwan

There are also countries with regulations established based on parts taken from the different regulatory groups. These countries are listed below. Hong Kong Indonesia Israel Jamaica Malaysia Australia Thailand Trinidad & Tobago

FDA, EC, & MHW FDA & EC FDA & EC FDA & EC FDA & EC FDA & EC FDA & EC FDA & EC

6. THE FUTURE OF COLOR ADDITIVES

In the United States there are a number of colors t h a t could obtain listing. The primary group would be all those colors that were not tested and automatically delisted. However, these are not the only colors t h a t could be approved; any color

187

that passes t h e testing can be listed. The practical situation is that it is unlikely that a new color additive will be added to the list. The time and cost of testing for approval of a new color additive is not justify based on t h e return that can be obtained from t h e new color. I n t h e EC a new color could be approved if it is submitted before t h e regulations a r e finalized. After the regulations are final it is unlikely that t h e EC will be very receptive to permitting a new color since they a r e now going through t h e process of listing colors and establishing t h e final list. I n J a p a n there is a positive list of approved colors; however, t h e MHW does accept petitions for the addition of a new color. A Japanese company must submit t h e petition, and t h e proper testing must be completed. If such a new color is listed the company that submitted t h e petition usually receives a one or two year exclusive on t h e use of the color. There is an effort underway to harmonize between regulatory bodies the list of permitted colors. The purpose is that formulation developed in one country would be acceptable in other countries. The concept is good b u t it is not likely to happen quickly, if at all, because each regulatory body feels they have p u t considerable time and effort in establishing their color additive regulations a n d they want their regulations to be t h e basis for t h e harmonized regulations. It is likely that no significant changes will occur in color additive regulations for a very long time.

7. GENERAL REFERENCES W.C. Bambridge, Ind. Eng. Chem., 18 (1926) 1329-1331. H.O. Calvery, Am. J. Pharm., 114 (1942) 324-329. Code of Federal Regulations, Food and Drug Administration, Title 21, Parts 1 to 99, Sections 73.1 through 82.2707a, Revised April 1, 1996. Encyclopedia of Chemical Analysis, Vol.10, John Wiley & Sons, Inc., New York, 447-547, 1970. W. Honvitz, (ed.) Official Methods of Analysis of the Association of Official Analytical Chemists, 12th edition, AOAC, Washington, DC, 1975. J.M. Rempe and L.G. Santucci (eds.) International Color Handbook, Second edition, Cosmetic, Toiletry & Fragrance Association, Washington, DC, 1997. D.M. Marmion, Handbook of US. Colorants, Third Edition, John Wiley & Sons, Inc., New York, 1991.

188

J. Noonan, Color Additives in Foods, Handbook of Food Additives, The Chemical Rubber Company, Cleveland, OH, 1968, 25-49. Reports on Certification of Color Additives, Food and Drug Administration, Fiscal years 1993, 1994, 1995, 1996, 1997. K. Venkataraman, The Analytical Chemistry of Synthetic Dyes, John Wiley & Sons, Inc., New York, 1977.

Zuckerman, S., Senackerib, J. Colorants for Foods, Drugs, and Cosmetics. Kirk-Othmer: Encyclopedia of Chemical Technology, Volume 6, Third edition, pp. 561-596, John Wiley & Sons, inc., New York, 1979.

Colorants for Non-Textile Applications H.S. Freeman and A.T. Peters (Editors) @ 2000 Elsevier Science B.V. All rights reserved.

189

5. Biomedical Application of Dyes JOAO C.V. PAIS DE MOURA Departamento de Quimica, Universidade da Madeira, 9000, Funchal, Portugal IBQF, Universidade do Minho, 4710 Braga, Portugal

1. INTRODUCTION The use of dyes in the biomedical area has seen a remarkable growth in research interest and technical importance in recent years, and is probably the most rapidly expanding area of dye chemistry at the present time. Dyes are used, for example, in many diagnostic applications and qualitative and quantitative determinations can often be easily and reliably performed by rapid and economic methods. Such applications range from simple organic reactions for spectroscopic detection and measurement of body fluid analytes t o high definition imaging technology for tumor detection. Dyes can also be used clinically for the treatment of disease. Photodynamic therapy is one example which is currently enjoying much success in the treatment of certain kinds of cancer, such as malignancies of the skin, head and neck, lung and oesoephagus, and is an expanding area, both in terms of the types of tumor that can be treated and its general clinical acceptance. Other therapeutic applications are associated with the antiviral and bactericidal properties of dyes and for example several compounds have been evaluated for the treatment of human immunodeficiency virus (HIV) and other viruses. This paper reviews briefly current and potential biomedical applications of dyes that fall within the general areas of bioanalysis, medical diagnostics and the therapeutic treatment of disease. There are of course numerous other applications of dyes that might be considered to fall within the realms of biomedicine, and these would include for example the important areas of histology, fluorescent biolabelling and fluorescent bioprobes. However space limitations preclude detailed discussion of these here and the interested reader should consult the catalogues of companies specializing in these fields in order to keep up to date with what are very rapidly developing areas.

190

2. DYES IN BIOANALYSIS AND MEDICAL DIAGNOSTICS 2.1. DNA sequencing The human genome comprises three billion nucleotides forming the twenty-two pairs of chromosomes plus two autosomes, each with continuous DNA pieces of 50-500 million nucleotides. The organization and sequence of DNA forming the human genome contains unique information about the source that provides the DNA, and determination of the exact structure of this extremely complex assembly is one of the greatest challenges facing scientists today. Dyes, particularly fluorescent dyes, play a major role in current sequencing procedures and have been largely responsible for the rapid strides that have been made in this area in the last few years. Native DNA consists of two linear polymers, or strands of nucleotides. Each strand is a chain of nucleosides linked by phosphodiester bonds. The two strands are held together in an antiparallel orientation by hydrogen bonds between complementary bases of the nucleotides of the two strands: deoxyadenosine pairs with thymidine and deoxyguanosine pairs with deoxycytidine. Analysis of polynucleotides with currently available techniques provides a spectrum of information ranging from the understanding of the function and control of genes to the non-research applications such as genetic identification, genetic counseling, forensic analysis and medical diagnosis. Presently there are two basic approaches to DNA sequence determination: the dideoxy chain termination method [ll and the chemical degradation method 121. Both methods require the generation of one or more sets of labeled DNA fragments each having a common origin and each terminating with a known base. In most automated DNA sequencing machines, fragments having different terminating bases are labeled with different fluorescent dyes. The labeled fragments are combined and loaded onto the same gel column for electrophoretic separation. Base sequence is determined by analyzing the fluorescent signals emitted by fragments as they pass a stationary detector during the separation process. One of the major difficulties in DNA sequencing techniques is obtaining a set of dyes t o label different fragments. First, it is difficult to find three or more dyes that do not have significantly overlapping emission bands, since the typical emission band halfwidth for organic fluorescent dyes is about 40-80 nanometers. Second, even when dyes with non-overlapping emission bands are found, the set may still be unsuitable for DNA sequencing if the respective fluorescence efficiencies are too low. Third, when several fluorescent dyes are used concurrently, excitation becomes difficult because the absorption bands of the dyes are often widely separated. The most efficient excitation occurs when each dye is illuminated a t the wavelength corresponding to its absorption band maximum. Because of these constraints only a few sets of fluorescent dyes can be used in automated DNA sequencing [3-81.

191

Examples of dyes useful in DNA chain-termination sequencing procedures include Rhodamine 6G (C.I. Basic Red 1) (1) [9], Texas Red (2) [lo] and other rhodamine derivatives [ll-151.

S02CI 1

2

Middendorf and Patonay [16] used near-infrared and infrared cyanine dyes, e.g. (3),for detection and sequencing of nucleic acids. Other examples of cyanine dyes have been described [17-191. COOR

A

R = (CH2)"OH

n = 2,3,6

3

Fluorescein dyes represent another class of dyes used for DNA sequencing. They include Fluorescein (C.I. Acid Yellow 73) [201, dichlorofluoresceins [21-231, and fluorescein-isothiocyanate (FITC) [24]. Theisen et al. synthesized a series of fluorescein phosphoramidites, e.g. (4) [25], and confirmed the high stability of the dye-oligonucleotide linkage during the conditions of DNA synthesis and cleavageldeprotection. To provide enhanced sequencing of DNA, Leonard used fluorescein as a chemical marker covalently bound to a double-stranded oligonucleotide molecule D61. Other examples of dyes for DNA sequencing include rhodol [271, 4methylcoumarin [28], phthalocyanine [291 and thiazoline 1301 chromophores. Nitro Blue Tetrazolium (5) [311 and Methyl Violet (C.I. Basic Violet 1) [321 have been suggested for the same purpose.

192

4

5

Beck [33] described a method of detecting and sequencing nucleic acids by chemiluminescence using a substituted 1,2-dioxetane (6) as chemiluminescent enzyme substrate and Fuller [341 suggested the use of Bromophenol Blue (7)and Xylene Cyanol FF for stopping enzymatic reactions prior to DNA sequencing analysis.

6

7

Hashimoto [35], using Hoechst 33258 (8),an electrochemically active dye, and a gold electrode, was able to achieve sequence-specific gene detection by voltammetry.

193

H 8

Ethidium bromide (9)) a dye used for nucleic acid detection which intercalates in double-stranded DNA (dsDNA) [36-371 has been used for discrimination between wild-type and sickle-cell P-globin alleles [381 and to detect toxic substances in air and ground water [391. Keller 1401 used ethidium bromide for sizing DNA fragments. The method comprises fragmenting enzymatically a piece of DNA at preselected sites, staining stoichiometrically with the dye and examining the fluorescence intensity by laser excitation. The intensity of the fluorescence emissions from each fragment is directly proportional to the fragment length. Other examples of fluorescent dyes having binding affinity to DNA include Texas Red [41-421 and TOT0 [43-441. The interactions of nucleic acids with Acridine Orange (C.I. Basic Orange 14) (10) has been studied [45-481 and Herkstroeter [49] has proposed its use for DNA sequencing analysis.

v

9

10

2.2. Cancer detection No one can be unaware that cancer is one of the leading causes of death in most developed countries and most efforts to cure cancers, once they are present with advanced symptoms, have been disappointing. Consequently increasing attention has been paid t o the identification of high-risk patients and earlier diagnosis of cancerous and pre-cancerous conditions. It is now recognized that cancer is a multi-step process in which the development of malignancy may take many years or decades. Davies et al. have described an in vitro method for screening and diagnosing patients for early stage cancer, or for the identification of premalignant changes which may eventually lead to cancer development 1501. In this method, a cell

194

preparation is made from a tissue sample and a calcium probe, e.g., a fluorescent dye, is added to the sample. Measurement of intracellular Ca2+ was made using the fluorescent probe FURA-2AM (ll),and it was concluded from the data that the calcium content may be useful diagnostically for distinguishing normal, tumor and precancerous cells. Lazo et al. [511 suggested the use of fluoromycin (FLM), a covalently linked product of the bleomycin derivative talisomycin Slob and FITC, for measuring cellular uptake of bleomycin compounds, a group of peptides with antitumor activity widely used in the treatment of various cancers such as squamous cell carcinoma, testicular carcinoma and Hodgkin’s disease. FITC was also used t o label an immune modulator for the diagnosis of malignant tumors [52].

11

Connors and Monosov [53] described a method for determining viability and proliferative capacity of tissue in vitro and for evaluation of antineoplastic drugs using fluorescent dyes such as 5-carboxyfluorescein diacetate (12) and Calcein Blue (13)and Goldenberg used 2-(4’-hydroxyazobenzene)-benzoic acid (HABA) conjugated with avidin for the determination of extent of biotinylation of proteins, and thus useful for the detection of cancerous lesions [54]. N(CH&OOH)2 0

&OH

13

12

Wang et al. described a process for the detection of chromosome structural abnormalities by in situ hybridization using nucleic acid probes [551. The method comprises obtaining a fixed tissue sample from a patient, digesting the fixed

195

sample with an effective amount of proteinase, performing in situ hybridization with probes labeled with Texas Red or Lucifer Yellow, comparing with a normal control and detecting chromosome structural abnormality. Fluorescein and indocyanine have been proposed for labeling tumor-specific monoclonal antibodies such as antimelanoma or anticolon carcinoma antibodies [56]. The dye-labeled antibodies accumulate within a corresponding tumor, making it detectable by fluorescence angiography at a very early stage of development. 3,3’-Dihexyloxacarbocyanine(14) has been used for differential binding of fluorescent membrane-potential-sensitive material to lymphocytes for malignancy determination [571 and DiO-C14(3), a cyanine dye, was used to label cells for the determination of sites of primary or metastatic tumors [MI.

14

The use of the disazo dye 15 as a reagent for stomach cancer diagnosis has been described. In this method, stomach cancer patients were orally given a liquid containing the dye and were examined by endoscopy. According to the authors, the detection rate was 80% us. 28% for the regular endoscopic method [591.

Na03S

Na03S 15

Case and Bekowies developed a method for detecting toxic chemical exposure effects and metabolic activation of carcinogenic chemical agents 1601. The process involves the conversion of a broad spectrum of carcinogenic substances into metabolic intermediates, which can be detected by a colorimetric indicator such as malachite green.

2.3. Virus detection Dorval et al. prepared anti-IgM-IgG and anti-IgA-IgG labeled with indigo for diagnosis of human immunodeficiency virus (H1V)-positive serum [611 and Schwartz suggested a flow cytometer method using Texas Red or Phycoerythrin

196

for lymphocyte determination for the diagnosis of immunological status in AIDS patients [621. The perylene dye 16 was used to prepare antibody-labeled liposomes for AIDS diagnosis [63] and Cubbage et al. developed a fluorimetric assay to detect HIV RNA in HIV-infected H9 cells with a fluorescein-labeled probe for HIV RNA using Evans Blue (C.I. Direct Blue 53) to reduce non specific emission of light andor reduce background fluorescence [641.

16

Recently, several azo dyes (17)were examined for their efficacy against HIV [65]. (2.1.Direct Orange 26, C.I. Direct Red 23, C.I. Direct Red 24 and C.I. Direct Red 26 possess the ability to inhibit the binding of the HIV coat protein gp120 to its target, CD4 glycoprotein present on peripheral blood T4 lymphocytes. According to the authors, these azo dyes are useful in the protection against and treatment of viral infections, and in detecting and quantitating CD4-positive lymphocytic cells.

CI Dir. Red 23

phenyl

4-methylcarboxamidophenyl

CI Dir. Org. 26

phenyl

phenyl

CI Dir. Red 24

2-methyl-4-benzenesulfonic acid 2-methoxyphenyl

CI Dir. Red 26

2-methoxyphenyl

4-naphthalenesulfonic

acid

Davalian et al. described a process for diagnosis of hepatitis B based on photoactive properties of 94benzal-SH-xanthene) and 2-hydroxyethyl-9,lOdibromoanthracene 1661. The method uses an oligonucleotide probe that recognizes a sequence of hepatitis B RNA and is covalently attached to the anthracene compound by means of a carbamate-linking group. A second oligonucleotide probe for hepatitis B RNA is covalently linked t o xanthene compound by means of an amide-linking group.

197

The presence of hepatitis B RNA in the sample causes the dyes to come into close proximity by virtue of the binding of the respective oligonucleotides with the RNA. Upon irradiation of the medium the 9,lO-dibromoanthracene is excited and converts ground state oxygen t o singlet oxygen. The singlet oxygen reacts with the xanthene t o give a xanthone that is fluorescent. The fluorescence is measured photometrically at 390 nm and the amount of light produced by the photoactive indicator is related to the presence of hepatitis B RNA. The same method [66] was applied for the determination of a particular blood group antigen on the surface of a red blood cell, namely, an A group antigen using dye (18)as photoactive indicator precursor and the hydrophobic dye (19) as photosensitizer.

Saitoh and Ishiguro developed a method for the quantification of HCV RNA in the serum samples of patients suffering from chronic hepatitis C using a quinoline dye [67] and Munebayashi suggested the use of TOP0 for the preparation of dye-labeled antigen for detecting hepatitis B virus [68-701.The use of chromogens such as tetramethylbenzidine to detect hepatitis virus [71] and cytomegalovirus [72] by ELISA immunoassay has also been described. Other examples of dyes for virus detection include a carboxymethyl-fluorescein derivative for diagnosis of herpes simplex virus [731 and 4-chloro-l-naphthol-Nacetylneuraminic acid and Fast Red ITR (C.I. Azoic Diazo Component 42) for the detection of influenza virus [74] in clinical samples. Hirschfeld used 3-amino-9-ethyl carbazole as peroxidase substrate to label antibodies for the diagnosis of rubella [75] and Hug1 et al. prepared oligonucleotide probes with dye (20) for the detection of pseudorabies virus [761.

20

Teoule, using Hoechst 33258, was able to detect the nucleic acid of human papilloma virus types 16 and 18 [771.

198

2.4. Cell detection The methods used for cell detection in biological fluids such as blood and urine, are based on the staining properties of dyes or on the enzymatic and immunological properties of cells. Johnson and Schaeper described a method for the detection of leukocyte cells in urine based on leukocyte esterase activity [78]. The process is based on the hydrolysis of the 3-hydroxy-5-phenyl-pyrrole-N-tosylL-alanine ester (Scheme 1, I) by an enzyme t o give the pyrrole (Scheme 1, 11) which then reacts with a diazonium salt, e.g., l-diazo-2-naphthol-4-sulfonic acid (Scheme 1,111)to produce the purple azo dye (Scheme 1, IV).

Scheme I

It has been reported that 2-[(5-carboxy-2-pyridyl)azol-l-naphthol-4-sulfonic acid can be used for spectrophotometric detection of cells in urine using a detection wavelength of 610 nm [791. Reticulocytes are precursors of mature red blood cells and contain ribonucleic acid (RNA). Its detection and enumeration in a blood sample is an indicator of erythropoietic activity, and has diagnostic and prognostic value in acute hemorrhage and hemolytic anemia, and is a measure of response to iron, vitamin B12 and folic acid therapy.

199

Dyes used for stain and to detect reticulocytes include New Methylene Blue (NMB) (C.I. Basic Blue 241, Brilliant Cresyl Blue (BCB), Acridine Orange (C.I. Basic Orange 14) and pyronin Y. However, NMB is non-fluorescent and true precipitated RNA is often difficult to differentiate from precipitated stain. On the other hand, Acridine Orange-stained cells are difficult to separate from the autofluorescence red cell peak, and the reticulocyte count is usually lower than the obtained with NMB. Also, pyronyn has a very low quantum efficiency, leading to very low fluorescent signals. To overcome these problems, the use of Thiazole Orange (21) and Thioflavin T (C.I. Basic Yellow 1) has been suggested for the detection and enumeration of reticulocytes in a blood sample by flow cytometry [80-811. According to the authors [801, the use of Thioflavin T stained reticulocytes in an automatic flow cytometer is particularly advantageous in that there are low fluorescent backgrounds and fluorescent gates may be easily selected by use of an unstained control. Moreover, there is no precipitation of intracellular reticulocyte RNA, whereby the cells need not be fixed. In addition, there is a linear relationship between the fluorescent signal for an individual reticulocyte, which provides information as to reticulocyte age.

21

When unbound to RNA the dye provides little or no fluorescence and can be excited at 488 nm but when it stains the RNA in reticulocytes, the optical properties change dramatically. In particular, the absorption curve shifts t o a longer wavelength (510 nm) and the dye now exhibits strong fluorescence a t about 530 nm. Other examples of fluorescent nucleic acid stains are based on benzothiazole, quinoline and pyridine systems [82]. The dye l-methyl-2-amino-4-pdimethylaminostyrylpyridinium iodide was used to differentiate erythrocytes and reticulocytes by the intensely stained yellowish reticulum of the reticulocytes and the plain green-yellow stained membrane of erythrocytes. Alcian Yellow (22) has been used to stain erythrocytes and platelets for use in solid-phase immunoassays [83].According t o the authors, the immobilized cells maintain the complete antigenic activity and can be stored a t room temperature for a t least six months without any loss of accuracy or sensitivity of immunoassays performed with them.

200

Horan et al. used liposomes containing N-(3-sulfopropyl)-4-(pdidecylaminostyryl) pyridinium as reagent to label and enumerate splenocytes B41. Lymphocytes, like erythrocytes, are blood cells and they include T-cell (T lymphocyte) and B cell (B lymphocyte). In various immunological diseases, measurement of the abnormality in the quantity of T cells (playing an important role in cell-mediated immunity) and B cells (producing humoral antibody) is said to be useful for diagnosis or for grasping the pathology of the disease. A method for the quantification of T cells from a blood sample using microcapsules carrying carboxyfluorescein attached to anti-T-cell antibodies has been claimed [85]. The microcapsules combined with the cells were examined by fluorescence microscopy in a n unbroken state and counted as the number of the corresponding T cells to be analyzed. According to the authors, other watersoluble dyes such as Phycoerythrin, Tetramethylrhodamine and Eosin (2’,4’,5’,6’tetrabromofluorescein) may be used. Other examples of dyes for identification and enumeration of T cells include FITC [861 and propidium iodide [87]. Ledis et al. prepared a reagent system for four-population differential determination of leukocytes by flow cytometry employing cyanine dyes containing sulfonic acid chains such as DiI c3so3-(3) [88] and thiazine dyes such as Sulforhodamine B (23)[89].

23

The authors were able to differentiate the four major leukocyte types found in blood: lymphocytes, monocytes, neutrophils and eosinophils, thus providing a method of leukocyte differential analysis.

20 1

The use of Malachite Green (C.I. Basic Green 4) (24) and ABTS (25) for fecal occult blood test method has been described 1901.

2.5 Bacteria detection Mansour et al. developed a method for the diagnosis of bacteriuria and pyuria that includes the simultaneous detection and quantitation of microrganisms, leukocytes and squamous epithelial cells in a urine specimen [91]. The three cell types are stained with a fluorescent dye such as, ethidium bromide, Thioflavin T ((2.1. Basic Yellow 1; 26), 4',6-diamidino-2-phenylindole (DAPI) or 3,3'dipentyloxacarbocyanine iodide, and the urine specimen is analyzed directly, preferably by a single-flow microfluorimetry technique. The isomaleimide dye (27) has been suggested for the detection and measurement of bacteria in biological materials [921 and DAPI and Propidium Iodide were used together for direct counting of bacteria after electrochemical disinfection treatment 1931.

cie

CH3

26

27

Using this double staining method, dead and viable cells were visualized as red and blue fluorescent cells respectively, using W excitation and epifluorescence microscopy. Other examples of staining dyes include the use of Ruthenium Red for the detection of Streptococcus thermophilus 1941 and Texas Red [951 and Fluorescein [961 t o detect Salmonella. FITC [95], Methylene Blue derivative (28) [97] and pyrylium dye (29) [98] have been claimed for the detection of Escherichia coli in urine. Dye (29) was also effective for the detection and quantification of Pseudomonas aeruginosa [981.

202

28

29

Chun et al., using Fast Garnet GBC salt ((2.1. Azoic Diazo Component 4), developed a method t o detect the presence of Neisseria gonorrhoea based on plactamase enzyme activity [991 and Lawrence et al. suggested a method of detecting candidiasis by assaying for the presence of enzymically active Candida albicans aspartic protease in a biological sample using ABTS as indicator dye [loo]. ABTS was also used to prepare labeled virus, without the inactivation of viral binding sites, and used to distinguish spectrophotometrically Salmonella typhimurium from Klebsiella pneumoniae a t 600 nm [loll. The use of 7-hydroxy-4-methylcoumarin as fluorogenic substrate to distinguish between Mycobacterium gordoneae and Mycobacterium terrae, two closely related species which are difficult to separate biochemically, has been described [102]. Acridine Orange (C.I. Basic Orange 14) has been described as a membrane penetrating stain that will stain the nucleic acids of parasites in blood [1031. However, other blood cells are also permeable to Acridine Orange and thus it will stain to some degree nucleated white blood cells. Thus, where the clinician is not skilled in the identification of the stages of an infection, false positives may occur using a stain of this type. The synthesis and the application of PUR-1 (301,a cationic cyanine dye, which preferentially stains the nucleic acid of blood borne parasites with little or no staining of nucleated red and white blood cells and platelets, has been described [1041. The dye PUR-1 is composed of two parts: the purine portion, which is mainly involved in the complexation with the DNA and the benzothiazole part, which provides the binding to the phosphate of DNA. Two main types of stains are used for the detection of malaria parasites in blood, namely the Romanowsky-type (Field’s) [lo51 and the fluorescent type (Purl and analogues). The former type finds application in morphological studies and is composed of two compounds, Methylene Blue (basic stain) and Eosin (acid stain). The mixture stains the cytoplasm of the malaria parasite pale purplish blue and the chromatin deep red. The fluorescent type stains (PUR-1) are used for rapid detection and quantification purposes. Recently, it has been suggested that PUR-1 analogues can be used for the diagnosis of malaria [1061. Dye (31)had advantages over PUR-1, namely that the leukocytes could be distinguished from malaria parasites and there was no

203

obvious interference by non-parasitic material such as precipitated stain at the dilution employed. Also, distinctive features of asexual and sexual stages of Plasmodium species other than Plasmodium falciparum were discernible.

The fluorogenic and chromogenic substrates used in bacterial diagnostics have recently been reviewed [1071. 2.6. Analysis of blood gases, pH and fluid electrolytes Physiological measurement of blood gases and hydrogen ion concentration (pH) is important for a variety of medical reasons. Myocardial contractility after cardiac surgery is strongly influenced by the acid-base balance of the patient’s blood. Patients with low cardiac output or with severe pulmonary disease show strong signs of acid-base imbalance accompanied by changes in the peripheral circulation or in ventilation-perfusion relationships. Further, the monitoring of blood oxygen or carbon dioxide levels during certain medical procedures, such as cardiopulmonary and cardiovascular by-pass heart surgery has numerous advantages. Determination of blood gases generally involves the use of an optical probe, which includes a body having a tip and one or more sensors. The colorimetric sensor material comprises a water soluble indicator bound to solid support material that changes light absorption characteristics in response t o modifications in the fluid properties, and may be used for pH, C02 andor 0 2 determination. Colorimetric dyes include Ru( l,lO-phenanthroline)3C12 [lo81 for oxygen determination, FITC [log], Phenol Red (32) [108, 1101 and xanthene dye (33) 11111 for pH determination and 1,3-dihydroxypyrene-6,8-disulfonate[1121 for C02 detection.

\

SO3H

32

33

Reagents for calcium determination in body fluids include the use of Rhodamine-123 [1131 and Cresolphthalein (34) [114]. Dye 35, a metal-sensitive

204

triphenylmethane dye, exhibits an excellent ability to complex with divalent cations, like magnesium and calcium ions, and as a result, it undergoes a detectable color transition in the presence of such ions [115].

The use of fluorescent dyes such as Indo-1 (36)and Fura-2 [1161 and o,o’dihydroxyarylazo chelatometric dyes such as Eriochrome Black T (C.I.Mordant Black 11)and calmagite [117-1181for measuring the magnesium concentration in biological fluids (blood, urine) have been described. Delton et al. [119] suggested the use of formazan indicator dye (37)for colorimetric quantification of lithium in blood and Soya et al. [120] described an enzymic-spectrophotometricmethod for potassium determination in body fluids.

36

37

In this method (Scheme 2), adenosine triphosphate (ATP) generated in the pyruvate kinase (PK)-catalyzed formation of pyruvate from phosphoenolpyruvic acid (PEP) and adenosine diphosphate (ADP) is used for phosphorylation of glycerol by glycerol kinase (GK). This is followed by oxidation of glycerol 3phosphate with glycerol 3-phosphate oxidase (G3POD) and the H202 formed can then be measured quantitatively with a leuco dye such as 10-N-methylcarbamoyl-3,7-dimethylamino-10H-phenothiazine, or with a reduced chromogen consisting of a combination of 4-aminoantipyrine (4-AA)and phenol.

PEP

+ ADP

Glycerol + ATP Glycerol-3-phosphate

+0

2 H 2 0 2 + 4-A4 + phenol

PK K+

205

grIuvate + ATP

GK

7Glycerol-3-phosphate

+ ADP

G3POD

7Dihydroxyacetonephosphate

POD

7 Quinone

dye ( h,,=400

+H

,02

nm)

Scheme 2 Methods for measuring intracellular pH, calcium and sodium ions with fluorescent dyes have been reviewed [1211. 2.7. Membrane potential The membrane potential is an important property of many cells and organelles. Changes in membrane potential control or accompany numerous biological processes, such as information transfer in neuronal networks, muscle contraction, and energy transduction during photosynthesis or metabolism. Hoffman and Laris and Sims et al. [122-1231 developed a fluorescence method t o determine the membrane potential of cells using a cyanine dye, 3,3’dipropylthiadicarbocyanine [diS-C3-(5)](38-a).This dye belongs t o the general class of lipophilic cations that move electrophoretically across natural or artificial lipid membranes in response to the prevailing membrane potential. Accumulation of the dye in the compartment bound by the membrane results in characteristic changes in the absorption spectrum of the dye as well as in its fluorescence emission spectrum, both of which can readily be detected at appropriate wavelengths in the visible region. Hoffman’s method has been widely applied t o determine the membrane potential of animal cells [124-1281 and other cyanine dyes such as 3,3’-dipropyloxadicarbocyanine, [129l, diO-C6-(3) (38b) [1301, dipentyloxacarbocyanine [diOCg-(3)1 11311 and 3,3’-diethylthiadicarbocyanine iodide [diS-C2-(5)1 [1321 have proved t o be useful for membrane potential determination.

38

The most important properties of optical probes that require improvement are: voltage sensitivity; absence of photodynamic damage; bleaching; pharmacological

206

side effects induced by the binding. Grinvald et al. [1331 have reported the use of styryl dyes t o improve the quality of fluorescent voltage-sensitive probes.

2.8. Human chorionic gonadotropin (pregnancy detection) In recent years, several immunoassays to detect human chorionic gonadotropin (hCG) as an early indicator of pregnancy have been patented. In general, these pregnancy tests use anti-human chorionic gonadotropin monoclonal antibodies immobilized with a label zone that forms part of a flow path for a liquid sample or liquid reagent in a test device. Dye-labeled conjugates are mixed with the sample and applied to the test strip containing anti-hCG antibody. When hCG is present in the sample, a clearly discernible color appears in the capture zone. Applicable dyes include Remazol Brilliant Blue R (39) [134], Foron Brilliant Blue [1351, sodium fluorescein [1361, MTT tetrazolium salt [1371, Oil Red 0 150 [138], FITC 11391, rhodaminehydrazide [140], C.I. Food Blue 2 and C.I. Food Yellow 4 [141], Palanil Light Red [142], cyanine dye (40) [143] and 4,5-bis(4-methoxyphenyl)-2-(3,4-dimethoxy-4hydroxy)imidazole [144-1451.

2.9. Cholesterol assay Cholesterol in biological fluids can be detected and quantified by colorimetric enzymic analysis using Meldola Blue (C.I. Basic Blue 6) [1461 or 4aminoantipyrine (4-AA) associated with N,N-dialkylaniline derivatives [147-1491. Bates et al. [1501, using 4-AA and phenol were able to determine triglyceride and cholesterol a t 340 and 500 nm, respectively, and Wardas et al. [1511 suggested the use of Bromophenol Blue and Aniline Blue (C.I. Acid Blue 20) as visualizing agents for the detection of cholesterol by thin layer chromatography (TLC). The use of 4-aminoaniline diazonium salt for enzymatic determination of cholesterol in serum has been described 11521. 2.10 Glucose assay Control of glucose in blood is important particularly in the case of patients with diabetes. Self-monitoring of blood glucose by diabetic patients at home is increasingly common and provides a means for frequent measurement of blood glucose. These measurements are important in a number of diabetic circumstances such as pregnancy, unstable diabetic conditions, propensities

201

towards severe ketosis or hypoglycemia and use of portable insulin infusion devices or multiple daily injections. The most common methods for self-monitoring currently in use incorporate a test strip which is placed in contact with a “whole blood” sample. Glucose in blood is reacted with enzymes, such as glucose oxidase and horseradish peroxidase, producing hydrogen peroxide. Hydrogen peroxide oxidizes a chromophore, which yields a color change on the test strip in proportion to the glucose concentration present in the blood. The chromophore system widely employed in a diagnostic test for glucose of the kind described above is the combination of 3-methyl-2-benzothiazolinone hydrazone (MBTH) (scheme 3, I-a) together with N,N-dimethylaniline. These compounds can undergo an oxidation reaction to form a blue colored chromophore (Scheme 3,II). This system suffers from several drawbacks. MBTH is unstable under the action of heat and alkalinity, even in the hydrochloride form. Furthermore, the reaction is most efficient under highly acidic conditions, e.g., pH 2. However, in these conditions, the enzymes employed in the test have little or no activity. In order t o overcome this stability problem and to increase the reactivity of reagents, the use of modified forms of MBTH, such as compound (scheme 3, I-b) [153] has been suggested.

(I) a) R=H

S03Na

b ) R = 0 2 s x Y

Scheme 3

Other dye systems include the use of C.I. Food Red 17, C.I. Food Red 14 and C.I. Food Blue 2 [154], Bromothymol Blue [155], Remazol Yellow GNL (C.I. Reactive Yellow 23) [156], Kayaset Black 151H [157] and leuco-imidazole derivatives [1581.

2.11. Bilirubin assay Bilirubin is a reddish yellow bile pigment, whose presence in blood serum a t too high level indicates jaundice, and its measurement is used as a liver function test. Standard assays used for the quantitative determination employ aqueous solutions of diazonium salts in dilute hydrochloric acid. Bilirubin present in the serum reacts with the diazonium salt and the resulting azobilirubin fragments are determined by measuring the reflectance density a t ca. 600 nm.

208

For example, when determined by coupling it with diazonium salts 41 [159] and 42 [160] the absorbance may be measured a t 535 and 560 nm respectively.

41

42

Other examples of diazonium salts for determination of bilirubin include 4[-

N,N-bis(carboxymethyl)sulfamyll benzenediazonium hexafluorophosphate [1611, 3,5-chlorophenol diazonium tetrafluoroborate [162], sulfanilamide diazonium salts [ 1631, sulfanilic acid [164], 4-(N-dodecylsulfamyl) benzene diazonium tetrafluoroborate [1651 and 2,4-dichlorobenzenediazonium tetrafluoroborate [1661. Urobilinogen, a reduced colorless product of bilirubin, can also be measured in urine as a liver function test using 2,4-dichlorobenzenediazoniumfluoroborate [1671 and 2,2'-dimethylbibenzyl-4,4'-bisdiazoniumtetrafluoroborate [1681. 2.12. Diagnosis of periodontal diseases

There have been various reports on the use of dyes for the diagnosis of periodontal diseases. Some methods are based on the staining properties of dyes such as Alcian Blue [169] and xanthene derivatives [170] and involves the comparison of the staining intensities of samples from periodontal disease to that of an internal control site (sample collected from the healthiest periodontal site in the patient). Other methods described are based on the detection of high levels of aminotransferases in biological fluids. Aminotransferases such as L-alanine aminotransferase (ALT) is an intracellular enzyme widely distributed among mammalian tissues. Following acute tissue injury in the course of disease, trauma or toxicity, damaged cells release ALT into the circulation, interstitial fluid, inflammatory exudate and other body fluids. High levels of aminotransferases can be detected using indicator dyes such as Malachite Green (C.I. Basic Green 4) and Rhodamine B (C.I. Basic Violet 10) [171-1721 and triarylmethane (43) and phenothiazine (44) dyes [173].

43

44

209

Fine has described a method using 4-nitroaniline diazonium salt for the evaluation of gingivitis or advanced periodontitis 11741. Dyes have also been employed as coloring agents for dental restoration, and claimed colorants include Methylene Blue (C.I. Basic Blue 9) , Methylene Green ((2.1.Basic Green 5) and Nile Blue A ((2.1. Basic Blue 12) 117.51, Rhodamine 6GND (C.I. Basic Red 1)and Phthalocyanine Blue (C.1. Pigment Blue 15) [1761 Nuclear Fast Red (45) [177] and Phenol Blue (46) [1781.

45

46

Recently, the use of fluorescein type dyes for direct pH measurement on tooth surfaces has been described 11791. 2.13. Investigation of protein chemistry and enzyme activity with diazonium salts Diazotization of primary arylamines and the coupling reactions of the resultant diazonium ions are extremely important processes in dye chemistry. Applications of these reactions have been widely extended into the biomedical analytical area, and several examples have already been discussed in earlier sections. Diazo coupling is particularly useful in methods for investigating proteins and for determining enzyme activity [1801. Although the procedures do not employ dyes as such, the end result is an azo dye chromophore. Diazo coupling has long been employed in protein chemistry, and in 1915 Pauly first used diazotized sulfanilic acid (Pauly reagent) to couple with tyrosine and histidine residues t o form colored products (47, 48). The resulting azo compounds are colored and several spectrophotometric methods have since been developed for various applications, such as protein labeling, detection of drugs of abuse, diagnosing disease, immunological assays and cancer treatment.

HO3S 47

48

210

Recently, Kozaki et al. [1811 improved the Pauly method for quantitative analysis of L-histidine. The old methods showed low stability of absorbance in reaction solutions directly following the addition of reagents. The authors succeeded in stabilizing the absorbance of the reaction solution for a period of twenty four hours a t 17 "C and verified this as a clinical method for quantification of L-histidine. New methods and improvements in existing methods for the determination of enzyme activity have been developed. For example, Schmidt [1821 described a method for quantification of alkaline phosphatase activity in cells (osteoblasts) using Naphthol AS-BI phosphate (C.I. Azoic Coupling Component 4) (Scheme 4, I), and Fast Red Violet LB. Alkaline phosphatase produces a naphthol (Scheme 4, 11) from the phosphate which reacts with the diazonium salt to produce an azo compound (scheme 4, 111) which becomes associated with the cell membrane. The colored product is extracted with DMSO and the absorbance a t 500 nm can be determined. The enzyme activity is related to the number of cells. A

U

II

mo-y-oH nu

alk. DhosDh.

1

Other examples include the use the 2-methoxy-4-(N-morpholino)benzenediazonium tetrachlorozincate [183] for colorimetric determination of esterases and proteinases in body fluids (to detect leukocytes in urine) and 4amino-2,5-diethoxybenzanilidediazonium chloride (Fast Blue BB) [1841 for detection of aspartate transaminase (AST) in oral fluids as an indicator of periodontal disease. Recently, Ehret et al. [1851 using 4-dimethylamino benzenodiazonium and 6coumarin diazonium salts obtained structural information for acetylcholinesterase including identification of active (Phe330 and Leu332) and peripheral sites (Trp279 and Tyr70).

211

Covalent bonding of proteins to diazonium salts is useful for enzyme immobilization. Matsuki et al. [ 1861 prepared a biologically active substanceimmobilized carrier comprising phosphazene polymer having side chains such as diazotized aromatic amino groups. The polymeric carrier thus prepared was treated with bovine serum albumin (BSA) resulting in immobilization of 94% of BSA after one hour. The phosphazene polymer can be used in affinity chromatography preventing contamination by impure proteins. According to the authors, good separation efficiency is obtained and it has high durability in contrast to carbohydrate carriers that are easily contaminated. The activation of side chains of the carrier by diazotization to bind to DNA covalently has been described to detect genetic defects of patients with Duchenne’s muscular dystrophy [1871. Diazonium salts are used for immobilization of ligands for antibody screening. They include rn-phenylenediamine [188], Fast Scarlet ((3.1. Azoic Diazo Component 12) and Fast Blue B (C.I.Azoic Diazo Component 109) [1891. 2.14. Miscellaneous analytes Diacryl Red MS-N [190] and Propidium Iodide [191] have been used for the detection of antibodies in the serum of rheumatic patients and for the diagnosis of ankylosing spondylitis. Israel et al. [192] used a mixture of C.I. Food Blue 2, C.I. Food Yellow 4 and C.I. Food Red 17 for fast colorimetric analysis of ethanol in body fluids and Rojas et al. 11931 developed a method for the determination of antihistaminic substances such as chlorpheniramine, phenylpropanolamine, phenyleprhine and clemizol using Bromothymol Blue and Methyl Orange (C.I. Acid Orange 52). The use 4-nitrobenzenediazonium ion for the assay of anti-inflammatory drugs such as fentiazac and tiaprofenic acid has been described [194]. This method utilizes Griess reaction where the acidic drugs react with sodium nitrite releasing an equivalent amount of nitrous acid. The latter reacts with 4-nitroaniline and the coupling reagent 1-naphthylamine to form an orange pigment (49) exhibiting hmax at 462 nm.

49

Johnson described a method for identifying drugs of abuse such as marijuana in biological fluids [195], The marijuana’s major urinary metabolite is ll-nor-9carboxy-A-9-tetrahydrocannabinol(50) and can be detected using Fast Blue BB Salt ((2.1. Azoic Diazo Component 20) (51), Fast Blue B Salt (C.I. Azoic Diazo

212

Component 48) and Fast Corinth V Salt (C.I. Azoic Diazo Component 39). The presence of an intense pink-red color was considered a positive test.

50

51

3. DYES AS THERAPEUTIC AGENTS

3.1. Anti-cancer drugs Cancer represents nowadays one of the largest scourges of the modern world, particularly of the developed countries. In such countries, about 20% of the population can be expected to contract cancer and fail to get effective treatment. Every year, millions of persons die of cancer, but progress in cancer treatment is now accelerating rapidly, and average life expectancies of patients diagnosed with almost any types of cancer are now showing significant improvements. Apart from more sensitive early detection techniques, a major contribution to this improvement has been the development of effective anti-cancer drugs. Generally, antitumor agents can be grouped into antibiotic and immunological types. Since both types of antitumor agents do not distinguish between tumor and normal cells, they are strongly toxic to normal cells and produce undesirable side effects, such as alopecia (loss of hair), emesis (nausea), nephrotoxicity, cardiotoxicity, etc. More recently, the use of "Biological Response Modifiers (BRM)" has been suggested. These are proteinaceous substances that regulate the response in living body and are useful in the treatment of malignant tumors in vivo. They include interferon, tumor necrosis factor, lymphotoxin and interleukin. They are less toxic because they are inherently produced in vivo but they have the disadvantage that their large-scale preparation in high-purity and pyrogen-free form is very difficult. Most of the proteinaceous BRMs exhibit no antitumor activity when administered orally. Current research is particularly intense in the area of novel antitumor drug development, and a number of these materials are dyes or dye-related structures. Cyanine dyes have been suggested as being useful for cancer treatment "6-1981. Compound 52 showed high tumor-inhibiting activity in mice injected with P-388 tumor cells [199] and trimethincyanines, having benzothiazole [2001 and naphthothiazole [2011 terminal nuclei had the same effect on mice inoculated with LOX melanoma cells and mice bearing Ehrlich ascites tumors.

213

CHpCH3

HsCH2C 52

Minami et al. screened this type of compound for antitumor activity against P388 leukemia and B16 melanoma and concluded that the replacement of the conjugated chain system between the two nuclei with a saturated aliphatic chain produced a marked decrease in antitumor activity [202]. Rhodacyanine dyes have been described as antitumor agents with a high degree of selectivity against cancer cells, especially carcinoma and melanoma cells [203-2041. These dyes consist of two heterocyclic rings such as thiazoles at both termini of the conjugate system and 4-oxothiazolidine (Rhodanine) at the center of the chain. Compound 53 is an example of a rhodacyanine dye that exhibits antitumor activity [205].

53

Other classes of dyes suggested for cancer therapy are phenothiazines [2061 and phenoxazines [207]. Although many phenothiazines do not show significant antitumor and antineoplastic activity, some of the derivatives help indirectly in decreasing cytotoxic effects caused by radiation and other chemical carcinogens. Additionally, some phenothiazine derivatives provide protection against cancers caused by metabolic activation of carcinogens such as dimethylbenzanthracene. The selective accumulation of phenothiazine derivatives in certain tissues such as brain and melanoma tumors may provide an effective treatment of such tumors. Takayanagi used a mixture of dye 54 and Methylene Blue (C.I. Basic Blue 9) to obtain a water soluble complex with cancer inhibiting activity [2081.

54

214

Connors and Guo described a method for evaluating the effectiveness of drugs in inhibiting the growth of tumor cells by using 3-(4,5-dimethylthiazol-2-y1)-2,5diphenyltetrazolium bromide (MTT) and Propidium Iodide [209]. De Long et al. [2101 used Sudan I, Sudan I11 and Coumarin to provide a model system for the study of anticarcinogen drugs. Finlay et al. [211] used phenylbisbenzimidazole derivatives towards Lewis long cells, human tumor lines HT29 (colon) and FME (melanoma) to potentiate the citotoxicity of anticancer drugs such as etoposide and CI-921. The inhibition of DNA topoisomerases, particularly topoisomerase I1 (topo 11) is now considered to be an important component in the mechanism of action of a large number of the most clinically active anticancer drugs presently available including doxorubicin, mitoxantrone, VP16, camptothecin, topotecam, M-AMSA, VM26 and the ellipticines. These drugs are believed to inhibit top0 I1 by stabilizing a proteiddruglnucleic acid ternary complex termed the cleavable complex. However, these drugs also exhibit other mechanisms of action, such as free radical generation and formation of DNA covalent adducts which contribute to their overall toxicity and poor therapeutic index. Additionally, the failure of these agents to produce long term cures in the major malignancies is probably exacerbated by the presence of the de novo resistance and the development of acquired drug resistance. The use of substituted 1,Canthraquinones 1212-2131 and 1,4bis(aminoalkylamino)-5,8-dihydroxyanthraquinones[214] as anticancer agents has been suggested. Mincher showed that the anthraquinone dye 55 inhibited K562 human tumor cell cultures [215]. Bordin et al. [216] studied the biological properties of benzopsoralen derivatives. Compounds of the type of 56 were found to have strong antiproliferative effects towards Ehrlich cells by two different mechanisms, namely under W - A irradiation and in the dark.

R '0

0 55

0

56

It has been claimed in the literature that cancer cell growth can be inhibited by the use of the dyes Suramine and related polysulfonated azo dyes [2171, Naphthol Blue Black (C.I. Acid Black 1) [218], the triphenylmethane dyes Malachite Green (C.I. Basic Green 4) and Brilliant Green (C.I. Basic Green 1) [219] and the acridine dyes Acriflavine, Proflavine and Acridine Orange (C.I. Basic Orange 14) [2201.

215

One way to increase antitumoral activity is to combine two structures with proven antitumoral activity, namely, an intercalating and an alkylating structure. Compound 57 is such an example and has proved to be an effective cytostatic drug in human carcinomas [2211. The same authors reported the high antitumoral activity of a flavone derivative 58, which inhibit tumor proliferation in vivo. According to the authors, it acts as a growth factor inhibitor and its mechanism of action is connected with inhibition of tyrosine and serine phosphokinases, two enzymes involved in the proliferation of tumor cells.

H3cw N;7 OH

57

0

CH3

58

The antitumor activity of diazonium compounds has also been reported. Okuara prepared new diazo substituted amino acid compounds that showed antitumor activity [222]. One example was O-(2-diazo-3-hydroxy-l-oxopropyl)-Lserine, which inhibited mammary adenocarcinoma MCF-7 cell growth. 3.2. Photodynamic therapy and other photobiological applications The clinical use of light to treat disease has been known for a long time. At the turn of the century Acridine Orange and Eosin were used for the photodestruction of harmful bacteria [223] and tuberculosis of the skin was found t o respond to ultraviolet radiation 12241. At the same time, Eosin and Methylene Blue were examined for use in the phototreatment of cancer. In 1948 the potential of porphyrins and metalloporphyrins in cancer phototherapy was first suggested [2251. In the past two decades a great deal of attention has been focused on improved modalities of cancer treatment, and the use of light to achieve this has proved particularly attractive. The idea that cancer cells can be killed by a non-toxic drug that just needs to be activated by exposure to light has long attracted the interest of scientists, and many advances have been made in recent years. Photodynamic therapy (PDT) is but one specific area of photochemotherapy (the latter term includes all types of photochemical therapeutic treatment not necessarily involving dyes or molecular oxygen), and is now a well established procedure for the treatment of cancer and other diseases. PDT can be regarded as

216

a targeted form of chemotherapy in which the drug itself is harmless, but may be activated by exposure to light. In this technique, cancer cells can be destroyed selectively. In conventional chemotherapy, drugs are used without selectivity and they also attack healthy cells resulting in many unpleasant side effects. On the other hand, PDT can be seen as an extension of radiotherapy where cytotoxicity may be induced at much lower energies of electromagnetic radiation than in conventional radiotherapy. PDT has been defined as the combination of three components, light, photosensitizing drug and molecular oxygen, in order to produce biological damage of therapeutic value. If one of these components is not present, there is no biological effect. The general principles of PDT are straightforward. The photosensitizing drug is normally delivered intravenously (although in some cases oral or topical application is more appropriate), and two or three days later there is some degree of preferential localization in tumor tissue (the exact degree is a matter of considerable debate). The tumor area is then irradiated with intense laser light and this leads to the generation of cytotoxic species and t o the rapid destruction of the host tissue. The technique of PDT is developing rapidly, and there is a need for improved photosensitizing dyes to enable a wider range of tumor types to be treated. Research in this area is intense and it is mainly directed towards porphyrins, porphyrin-type molecules and phthalocyanines. The most widely used sensitizes in PDT is the hematoporphyrin derivative (HPD) and this is a complex mixture of porphyrins prepared from hematoporphyrin (HP) (59) by reaction with acetic acidsulfuric acid followed by treatment with sodium acetate 12261. HP is closely related to the naturally occurring protoporphyrin IX (PP) (60),whose iron complex is haem, the prosthetic group of haemoglobin and mioglobin.

59

60

Commercially available HPD is known as Photofrina and has been approved for PDT treatment of cancer in Canada (since 1993)) United States, Holland, Japan and France. The field of application includes the treatment of superficial

217

bladder and lung cancers, inoperable superficial esophageal and gastric cancers and palliative treatment of advanced lung and esophageal cancer in patients unsuitable for Nd:YAG therapy [227]. Most recently it has been approved for the treatment of certain early stage lung cancers in France, Germany and the USA. The main drawbacks to HPD and PhotofrinB are long-lived skin photosensitivity (three months) and its rather low intrinsic absorbance at the effective absorption wavelength maximum (630 nm). This wavelength is also not optimum for tissue penetration, and somewhat longer wavelengths would be preferable. Second generation photosensitizes are porphyrin derivatives such as tetra(m-hydroxypheny1)chlorin (mTHPC) (61)[228] a photosensitizer that shows an intense absorption at 652 nm and exhibits a high quantum yield for singlet oxygen production, and is an extremely active compound in vivo . Chlorin e6 [2292321, mono-L-aspartyl chlorin e6 (NPe6) [233-2341, mesochlorin e6 (62) [235], chlorin p6 [2361 and lysyl chlorin p6 [237] all show greater PDT anti-tumor activity than HPD.

0

HO

61

62

Others sensitizers for PDT include Lutetium Texaphyrin (63), a near-infrared water-soluble photosensitizer for the treatment of cancers of large tumor size [2381. The PDT efficacy was attributed to the selective uptakehetention of the texaphyrin sensitizes in addition to the depth of light penetration achievable at 732 nm laser irradiation.

218

H3CH2C H3CH2C

63

R = (CH2)30H

Griffths et al. [2391 described the synthesis of poly-substituted zinc phthalocyanines (64) which offer cancer PDT potential. All the dyes showed PDT activity in uiuo although the dicarboxylic acid diamide (64f) was the least effective, in agreement with its poorer singlet oxygen sensitizing properties. The same authors, using fibrosarcoma LSBDl in BDIX rats as model, showed that ZnPC-TSA (64a) can be a more effective sensitizer than polyhaematoporphyrin [2401.

Other hydrophilic zinc phthalocyanines that have been shown to be effective for the PDT of cancer include sulfonated phthalocyanines having gallium [2411 and ruthenium [2421 as central metal atom. Cationic dyes such as rhodamines [243-2441, cyanines [245-2461, triarylmethanes 1247-2481 and thiazines [249-2501 have received attention because of their selective uptake by mitochondria of cancer cells [2511. The reason for this selectivity appears to be related to the negatively charged cell (plasma) and mitochondria1 membrane [226].

219

5-Ethylamino-9-diethylaminobenzo[a]phenothiazinium chloride (EtNBS) (65a) possesses several characteristics that render it a promising candidate for PDT of cancer. EtNBS absorbs intense light at 652 nm, accumulates rapidly intracellularly 12521, and tumor destruction occurs with minimal effects to the vasculature [2531. Furthermore, the drug is eliminated twenty four hours after administration and no damage occurs to the surrounding normal skin following PDT of subcutaneous tumors.

66

65

a)X=S; R=H; R,=Et b)X=O; R=H; R,=H c)X=O; R = I ; R,=H Nile Blue (65b),a benzo[a]phenoxazine dye, is a good tumor marker and has been studied extensively in oncology [207]. Comparative to MB, it appears to be an ineffective photosensitiser because of its low singlet oxygen generating efficiency (@A=0.005).Cincotta [254] studied the photodynamic properties of a series of Nile Blue derivatives in several carcinoma cell lines and observed that ) increased the the presence of heavy atoms in the molecule ( 6 5 ~ greatly phototoxicity of the photosensitiser. The phenoxazine dye 66 is a promising candidate for use in PDT since it is expected to penetrate and accumulate efficiently in tumor cells [2551. N,N'-Bis(2-ethyl-l,3-dioxolane)kryptocyanine (EDKC) (67)is another example of a cationic dye of potential interest for PDT although it suffers from instability in solution [256-2571.

67

Another approach involving structural variations of the porphyrin molecule has been described [258]. Preliminary studies on the photodynamic properties of dye (68) indicate that while photophysical characteristics in the diverse

220

porphyrin-phenothiazine hybrid molecules remain intact, the light induced nuclease activity was markedly influenced by the nature of the linker moiety.

R

d

68 (n = 3-5)

The potential application of PDT t o other therapeutic areas has been the subject of intense research and several photoactive dyes have been evaluated for clinical application. Methylene Blue (C.I. Basic Blue 9) (69a),a phenothiazine dye, is one of the most well-characterized photosensitizers of this type and its virucidal activity was recognized a long time ago [259]. It has been extensively studied for PDT inactivation of viruses in blood [260-2641. Methylene Blue is preferentially retained in cells infected with virus compared with non-infected cells and absorbs light in the range of wavelengths that penetrate deeply into tissues. Additionally, Methylene Blue is a weak mutagen and can be used for inactivation of blood viruses such as hepatitis B virus [2651, human immunodeficiency virus type 1 [265-2661 and herpes simplex virus type 1 [2671. The use of Methylene Blue for photochemical virus inactivation of plasma products has been routinely used in Germany since 1992 [268-2711. The dyes Toluidine 0, Azure A and Thionin [272], benzoporphyrine derivatives [2731 and tetrasulfonated aluminum phthalocyanine [274] have been suggested for similar purposes. The photodynamic inactivation of escherichia coli by Methylene Blue [275] and paramecium by MB [276-2771 and Rhodamine 6G [2781 has been reported. Wagner et al. [279] studied the factors affecting virus photoinactivation by a series of phenothiazine dyes. They found that 1,9-dimethylmethylene blue (69b) had the highest affinity for nucleic acid and the greatest singlet oxygen yield, and this could explain the photosensitizer’s ability to inactivate viruses without adversely affecting anucleate red cells. Methylene Violet ((3.1. Basic Violet 8) (70) [280-2811 and aluminum phthalocyanine tetrasulfonate [282] have been suggested for vesicular stomatitis virus (VSV) inactivation.

22 1

70

69 a) R = H; b) R=CH3

&

Other examples of the use of dyes to induce photodynamic inactivation of viruses include benzoporphyrin derivative monoacid ring A (BPDMA) for feline leukemic virus [283], chlorin e6 for influenza A virus [2841, zinc phthalocyanine [285] and calphostin C (71) [286] for herpes simplex virus (HSV) and silicon phthalocyanine HOSiPcOSi(CH3)2(CH2)3N(CH3)2 [287] and hypocrellin A (72) 12881 for human immunodeficiency virus type 1.

CH3O

CH3

@

CH3O CH3

Ph

OH H3

/

CH3

/

I

\

OH

I

OCO7.(p -OH-Ph) OCH3

0

I

\

OH

71

I

COCH3 OCH3

0

72

Other photoactive dye for inactivating pathogens in body fluids is benzoporphyrin derivative (BPD) [289], Merocyanine 540 [290] and merocyanine dye (73)[291-2921.

73

Recently, Trauner and Hasan reported the use of 8-methoxypsoralen, BPDMA and Photofrin for the photodynamic treatment of rheumatoid and inflammatory arthritis [293]. Horowitz et al. [294] suggested the use of 4’-aminomethyl-4,5’,8trimethylpsoralen combined with light for sterilization of blood transfusion products.

222

Psoralens are naturally occurring compounds that have been used therapeutically for millennia in Asia and Africa. The action of psoralens and light has been used to treat vitiligo and psoriasis and more recently various forms of lymphoma. Psoralen binds to nucleic acid double helices by intercalation between base pairs: adenine, guanine, cytosine and thymine (DNA) or uracil (RNA). Upon absorption of a W A photon the psoralen excited state will react with a thymine or uracil double bond and covalently attach itself to a nucleic acid helix (Scheme 5 ) . The reaction is specific for a thymine (DNA) or uracil (RNA) base and will proceed only if the psoralen is intercalated in a site containing thymine or uracil. The initial photoadduct can absorb a second UVA photon and react with a second thymine or uracil on the opposing strand of the double helix to crosslink the two strands of the double helix.

DNA Strand

Psoralen

DNA Strand

monoadduct OCH3

diadduct

Scheme 5

I

223

Lethal damage to a cell or virus occurs when a psoralen intercalated into a nucleic acid duplex in sites containing two thymines (or uracils) on opposing strands sequentially absorb two W A photons. According to the authors, brominated psoralens are more efficient sensitizers because only one photon of light is required to activate the brominated sensitizer, whereas two photons are required t o activate the non-brominated compound. Moreover, the brominated psoralens may be activated by X-rays as well as W light. As shown in Scheme 6 absorption of a W A photon by a bromopsoralen in the presence of guanine leads t o electron transfer and the formation of free radicals and ultimately to nucleic acid cleavage and viral or cell death.

a1 H I

0

+DNA

aDNA

J

H

hv (UVA)

0

0

Scheme 6

Methods of inactivation of viral and bacterial blood contaminants with photoactive dyes have been described. Platz et al. suggested the use of halogenated psoralens, e.g. (74) combined with W light for decontamination of blood [2951. Br

I

74

Other photoactive dyes for inactivating pathogens in a body fluid include xanthene dyes (e.g. 75) [2961 and 1,2-dioxetanes (e.g. 76) as new antimalarial agents [2971.

224

TH3

OH Br

Br

bH 75

CH(CH3)2 76

Levy suggested a method for eliminating bacteria from water without resort to chemicals [2981. The method is carried out by mixing with water a dye, e.g. Methylene Blue (C.I. Basic Blue 9) or Tryptan Blue (C.I. Direct Blue 141, which stains bacteria contained in the water and irradiating the solution with appropriate radiation sufficient to degrade a significant proportion of the dye and also kill the bacteria stained by the dye. 3.3. Antiviral agents The discovery of new therapeutic agents for the treatment of acquired immune deficiency syndrome (AIDS) is a matter of high priority. There are only a few drugs for clinical use and in recent years a lot of compounds of potential interest in AIDS have been tested. Several dyes were screened for their ability to inhibit human immunodeficiency virus (HIV) [299-3001. Rose Bengal (C.I. Acid Red 94), Eosin Y (C.I. Acid Red 87) and Methylene Blue were used to inactivate replication of HIV by interfering with transcription and translation of viral nucleic acid [3011. Other examples include C.I. Reactive Blue 15 (77)[302] and C.I. Direct Red 79 [303].

77

225

Clanton et al. studied over fifty different commercially available sulfonic acidcontaining dyes for their ability to prevent HIV-1 induced cell killing and in inhibiting HIV-1 replication [304], They observed that compounds having remarkably similar structure, but with differing patterns of sulfonic acid group substitution, had a wide range of potency in inhibiting HIV-1. Chicago Sky Blue (CSB) (78) was highly effective in the inhibition of HIV-1 and was a potent inhibitor of the reverse transcriptase of both HIV-1 and HIV-8. The authors suggested that CSB appears to disrupt the interaction between viral proteins and cell membranes, both in the fusion step early in the infection cycle, and in the development of syncytia in the late stages of virus infection. Recently, Hattori et al. examined the antiviral activity of Cibacron Blue 3GA ((2.1. Reactive Blue 2) and C.I. Reactive Red 120 and concluded that triazine dyes bind to the functional regions of envelope glycoproteins of HIV-1 that play an important role for HIV infection 13051.

Xanthene dyes, such as Rhodamine 123, Rhodamine B (C.I. Basic Violet lo), Rhodamine 6G (C.I. Basic Red 1)and Sulforhodamine B enhanced the antiviral activity of poly r(A-U) towards vesicular stomatitis virus 13061. The antiviral activity of RNA-dye combinations was reviewed by Jamison et al. [3071. 3.4. Antifungal and antibacterial agents Jolly et al. synthesized several carboxyphenylazopyrazoles for textile application and tested them for their antifungal activity [3081. Dyes 79a (R = 2C0,H; R' = 4-OCH3)and b(R = 4CO& R' = 4421) were active.

79

226

A number of dyes have been found to have useful bactericidal properties. For example, the use of antiprotozoal tryptan dyes for veterinary preparations [3091, Crystal Violet (C.I. Basic Violet 3) for treatment of infections by toxoplasma gondii [3101 and escherichia coli [3111 has been described. Other dyes have been suggested for their bactericidal effect against microorganisms such as staphylococcus aureus and epidermitis, escherichia coli, bacillus subtilis and pseudomonas aeruginosa. Such dyes include cyanine [3123161, quinoline [317], thioquinoxalinone [318], triphenylmethane [319-3201 and anthraquinone [321] derivatives. Ogawa et al. [322] reported the growth inhibition of bacillus subtilis by basic dyes and Ben-Chetrit et al. [323] suggested the use of Methyl Blue ((2.1. Acid Blue 22) and Fast Green FCF (C.I. Food Green 3) (80)as a basis for the development of a new approach to drug therapy in systemic lupus erythematosus.

CH~CHI

\

S03Na

80

3.5. Other biomedical applications Dyes have found miscellaneous applications in many other biomedical areas. For example, indocyanine green mixed with human fibrinogen has been employed in tissue laser-welding compositions [324]; blue dextran [325] and (2.1. Disperse Blue 1 (81) [3261 have been used to increase the albumin-binding ability of polymeric prosthetic devices [325]; Chicago Sky Blue, Congo Red (C.I. Direct Red 28) and Evans Blue ((2.1. Direct Blue 53) has been suggested for the prevention and treatment of arterial thrombosis [327]; Brilliant Black BN (C.I. Food Black 1) and Yellow 2G ((2.1. Food Yellow 5 ) have been proposed as agents to support therapy for dissolving urinary calculus by inhibiting oxalic acid absorption in the intestine [3281. The application of dyes such as C.I. Disperse Red 11 incorporated in prosthetic material used to replace blocked or damaged arteries, has been studied [3291.

227

81

Fluorescein has been suggested for coating or impregnating intraocular lenses, either before or after insertion of the lenses into the eye, in order to protect susceptible ocular tissues from light induced damage [330], Redox dyes have been referred for treatment of shock states such as septic and cardiogenic shock. Septic shock is characterized by circulatory insufficiency due to diffuse cell and tissue injury and the pooling of blood in the microcirculation. Cardiogenic shock is the most important fatal complication of acute myocardial infarctus and occurs as a consequence of the decrease in arterial pressure and consequent reduction in coronary blood flow. Both septic and cardiogenic shock have been associated with an increase in the production of nitric oxide (NO). The mechanism by which NO causes vasodilation and hypotension is through activation of guanylate cyclase which results in increased levels of cyclic GMP, that causes relaxation of smooth muscle. Stamler and Loscalzo suggested the use of guanylate cyclase inhibitors such as Methylene Blue (C.I. Basic Blue 9), Toluidine Blue ((2.1. Basic Blue 17) and Neutral Red ((3.1. Basic Red 5) for the treatment of states of shock [3311. Redox organic dyes have also been suggested for the treatment of periodontal disease. The administration of Methylene Blue (C.I. Basic Blue 9) or Patent Blue V ((2.1. Acid Blue 1) t o gingival sulcus or periodontal pockets on patients with periodontal disease produced a highly significant beneficial effect [332]. 4. CONCLUSIONS

It is evident that dyes, or dye-like molecules, currently play a major role in many areas of analytical biochemistry, medical diagnostics and even the treatment and prevention of disease, and this role continues t o increase at a rapid pace. In some applications the color of the dye is essential for the application, and this is particularly true for analytical and diagnostic applications, and is also true in photodynamic therapy. In many other applications the color is irrelevant and it is the relatively large planar structures of dye molecules, with their complex balance of hydrophobicfhydrophilic properties, that enables them to interact subtlely with organisms, cells, and cell constituents, often with high specificity, and then t o exert the desired effect. As new dyes are developed with highly refined structures

228

for specialized bio-applications, the relationship between these molecules and the traditional textile dyes will become increasingly less apparent. Nevertheless, the wealth of knowledge buried in the literature of traditional dye chemistry will continue to prove indispensable in the development of such new materials, and the biochemist or clinician with any involvement with such materials should retain an awareness of classical dye chemistry.

5. ACKNOWLEDGMENTS The author would like to thank Drs. A. Oliveira-Campos and J. Griffths for helpful discussions during the preparation of this chapter.

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Colorants for Non-Textile Applications H.S. Freeman and A.T. Peters (Editors) 2000 Elsevier Science B.V. All rights reserved.

6 Functional Dyes for Molecular Recognition: Chromogenic and Fluorescent Receptors MASAHIKO INOUYE Department of Applied Materials Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan 1. INTRODUCTION

Supramolecular chemistry has been a rapidly growing field during the last decade, the burst of the activity being influenced by the award of a Nobel Prize in 1987 to three of its pioneers, C. J. Pedersen, D. J. Cram, and J.-M. Lehn. Supramolecular chemistry has been defined as chemistry beyond the molecule, the designed chemistry of the intermolecular bond [l-31. The chemistry is a highly interdisciplinary field that not only serves as a basic concept in the understanding of biological events, but one which also leads t o the development of novel molecular materials [41. For example, artificial models of biologically important molecules and molecular systems have revealed the importance of strict complementarity in size, shape, and functional groups at the molecular level for selective intermolecular interaction, i.e., molecular recognition 14,571. Recent investigations in this particular field have tended to shift from static phenomena to dynamic ones, i.e., from simple synthetic hosts to artificial receptors that, upon recognition of a specific substrate, undergo several conjugated functions [5-71. Dyes have long been recognized only as coloring materials. In the past two decades, however, dyes have gained appraisal as information storage, signaling, and transduction molecules. This renaissance of dye chemistry coined a new phrase, viz., "functional dyes." Application of functional dyes is now found in various fields such as pressure- and heat-sensitive dyes for carbonless paper, organic photoconductors in electrophotography, infrared dyes for laser optical recording systems, etc [8,9]. Functional dyes are also attracting much attention from the viewpoint of biology and biochemistry because of their potential for detecting biologically essential molecules [lo]. Thus, the construction of artificial receptors, in which the molecular recognition process is synchronized with the

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signal transduction of the dye moiety of molecule, is of critical importance not only for creating the next field of supramolecular chemistry, but also for producing practical products such as molecular sensors. Functional dyes are suitable for mimicking such signal transduction processes and by combining molecular recognition with dye chemistry, chromogenic and fluorescent receptors were created. This chapter deals with the design and synthesis of chromogenic and fluorescent receptors. The extent of relevant data, however, is too vast to be surveyed within a small chapter, especially for metal cation-recognition. Thus, the section below pertaining to receptors for metal-cations is restricted only to more recent prominent examples. Fortunately, several excellent reviews and books have already appeared for this topic [ll-251. Furthermore, functional dyemodified macromolecules and proteins as well as their utilization at membranes and monolayers are omitted [26-291. Data reported up t o 1994 relating to this chapter are discussed only briefly, since a text published at 1996 provides a review of the prior art [41. 2. HISTORICAL PERSPECTIVE

Artificial molecular recognition chemistry started with the discovery of dibenzo18-crown-6 (1) by C. J. Pedersen in 1967 [30-331. In the strict sense of the term, the first chromogenic receptors are also crown ethers. Indeed, Pedersen did observe W-changes of the benzene units of crown ethers upon recognition of alkali-metal cations [311 (Scheme 1). This factor usually applies to all cases of naturally occurring and artificial receptors, i.e., guest recognition more or less induces electronical perturbation of the receptors. Thus, this chapter is concerned with only the receptors that are rationally designed to possess informationsignaling abilities by means of visible (color) and fluorescence changes upon recognition. The first example of chromogenic receptors with high color selectivity was dyeattached crown ethers, chromoionophores 2, developed by M. Takagi 1341 and a little later 3 by F. Vogtle [351 some twenty years ago. Since then, various chromoionophores have been designed and synthesized aiming at specific detection of particular metal cations. Chromoionophores take advantage of the strong charge-dipole interaction between the bound metal cations and the lone electron pairs of the chromophore part for color changes. This is a main reason why most chromogenic receptors are designed t o recognize metal cations 111-251. On the

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other hand, chromogenic receptors for anions and neutral molecules have recently become accessible as a result of progress of molecular recognition chemistry and of synthetic efforts. In this chapter, chromogenic receptors will be treated in their guest order of cations, anions, and neutral molecules.

Scheme 1.Interaction of K‘ with dibenzo-18-crown-6 (1).

02NPNo2

2

NO2

3

3. CATIONS As mentioned in the introduction, a number of chromogenic and fluorescent

24 1

receptors for metal cations have been reported 111-251. This section introduces several more recent examples. Saccharide derivatives containing ammonium groups will be discussed in Section 5.1.

3.1. Metal cations 3.1.1. Transition- and heavy-metal Cations Transition- and heavy metals such as Hg, Cu, Co, Pb, are significant pollutants due their widespread use, but they can also be essential trace elements in biological systems. Therefore, the development of receptors for heavy transitionmetal cations may be a worthwhile subject in its own right. Recently, several such receptors were designed to recognize some transition- and heavy metal cations [211. Hirose reported that the aromatic Kemp’s acid imide 4 had an especially high transport ability for Hg2+ 1361. They constructed a molecular sensor 5 for Hg2+by substituting a chromogenic azobenzene moiety in place of the alkyl group of 4 1371. They found that after the addition of an acetonitrile solution of Hg(C104)2, the yellow solution of 5 changed color to red (Figure l),and that 5 showed a high response to Hg2+ at concentrations as low as 4.0 X 10-6 M (K, = 8.0 X 104 M-1). The selectivity of 5 for Hg2+ was high compared with that of other transition- and heavy-metal cations (Co2+,Niz+, Cu2+, Zn2+, Cd2+, Pbz+, and Ag+) on the basis of the detection limits, while addition of 1 equiv. of HC1 t o a solution of 5 resulted in the same spectral changes as that for Hg2+. This observation implied that the proton transfer from the carboxylic groups to the azo groups of 5 upon addition of the metal cations was responsible for the color changes. One of the most significant practical applications of chromogenic receptors has been in cell biology. For intracellular applications, NIR chromogenic receptors with water-solubility are desirable because shorter wavelengths are strongly absorbed by the nucleobases of DNA and RNA and by the aromatic amino acid residues of proteins [381. Zwitterionic squarylium dyes are known to absorb intensely beyond 700 nm, so dye-based chromogenic receptors are promising candidates for the above purpose. The water-soluble squarylium dye 6, capable of quantitatively and selectively detecting trace amounts of transition- and heavy metal cations, has been described [391.

242

Et-

Q 4

2!-

5

6

0.8

3 0.6 0

p"

a 0.4 0.2 0.0

350

400

450

X/nm

500

550

600

Figure 1.W - V i s spectra of 5 ( ~ . O X I OM) - ~before and after incremental addition of Hg(C10,), in CH,CN containing 0.01M LiCIO, a s electrolyte. Reprinted from J. Chem. SOC.,Chem. Commun, 1997,297.

243

Complexation of 6 with different metal cations produces a new absorption band, which in many cases is characteristic of the particular metal cation, so trace amounts of Cu2+, Hg2+, Pb2+, Mn2+, La3+, Eu3+, Tb3+, Gd3+, and Lu3+ can be detected selectively. Thus, addition of Cu2+ to a solution of 6 in a mixed solvent of water-acetonitrile (4:l) at pH 6.7 results in a decrease in the intensities of the absorption maxima at 790 and 640 nm, and a sharp new band around 670 nm appears. The complex with Hg2+ gives the same maximum at 670 nm, while the other complexes show their own characteristic maxima. Furthermore, the absorption spectrum of 6*Cu2+is unaffected by the addition of excess alkali- and alkaline-earth metal cations, which are present in large quantities in cytoplasm. For detection of Cu2+, Czarnik published a novel approach, viz., fluorescent “chemodosimeters” [40,41]. Chemodosimeters are similar to those of chemosensors or chromogenic receptors, except that their response reflects a cumulative exposure t o analyte and is therefore not reversible. The nonfluorescent rhodamine B hydrazide 7 of the chemodosimeter is selectively hydrolyzed by the addition of Cu2+to give the fluorescent 8 (Scheme 2).

7

Et2N

+

Et2N

+

NEtp

8 Scheme 2. Use of a chemodosimeter (e.g. rhodamine B) in the detection of Cu2+

244

Recognition of Cu2+ and signaling are stoichiometric and irreversible. The resulting dosimetric analysis of Cu2+was utilized to measure Cu2+ concentrations with linear response to 2 pM Cu2+ in water (Figure 2). No other metal cations produced any fluorescence under appropriate conditions. Although real-time response of this method remains desirable, the chemodosimeter will produce a new insight for practical chemosensors.

22.00

,

Figure 2. Dosimetric analysis of Cu’’.

Spiropyran derivatives 9 are an important class of photo- and thermochromic compounds which can be converted to the corresponding zwitterionic merocyanine isomers 9’ (Scheme 3) [42-441. The isomerization is unique in terms of accompanied large changes of structural and electric characteristics of the molecules. When the factor that affects this isomerization is a chemical species, especially a specific substrate, new functions of the spiropyrans appear, i.e., structural change and signaling based on molecular recognition. Thus, Inouye [27,45-501, Kimura [51-531, and other research groups [54] synthesized various spiropyrans possessing molecular recognition sites 10 - 13, and examined their recognition-signaling abilities.

245

Scheme 3. Molecular recognition involving a zwiterionic merocyanine compound.

Winkler and co-workers extended this spiropyran-based technology to the highly fluorescent photoreversible metal cation sensors with parts per billion sensitivity [55]. They designed and synthesized the 8-hydroxyquinolol-derived spiropyran 14, fluorescence emission of which at 610 nm increased by a factor of 14 upon addition of ZnClz. This fluorescence increase was explained not only in

246

terms of an increase in the concentration of the merocyanine-Zn2+ complex 14’*Zn2+but also t o the coordination of Zn2+ by the fluorophore (Scheme 4). The sensitivity of 14 for Zn2+ in 1:l ethanovwater was determined to be 3.3 ppb. Although the selectivity of 14 for metal cations is not excellent, the high sensitivity for metal cations could lead to a practical useful sensing device. Other chromogenic methods for detecting Zn2+ are based on metal-induced peptide folding [56] and fluorescence probe-modified cyclens [57].

Scheme 4.Interaction between a Zn ion and spiropyran 14.

3.1.2. Chromogenic receptors as logic gates Molecular-based electronic and optoelectronic devices have attracted increasing attention from the viewpoint of ultimate miniaturization of integrated circuit [58601. The function produced by the molecular devices results from the sum of the performance of component functional molecules. These remarkable features of the devices have inspired investigations into furnishing several functional molecules such as molecular sensors, switches, and signal transducers [15,18,21,61-661. In all respects of these functional molecules in response to external physical and/or chemical stimuli, molecular recognition processes play a leading role. The chromogenic and fluorescent receptors so far mentioned are thought to be a device component molecule that has one-inputlone-output functionality. In these systems, chemical binding (the “input”)results in a change in electronic absorption or fluorecence emission (the “output”)from the receptor. In 1993, de Silva reported a fascinating photoionic “AND” gate molecule 15: the fluorescence signal of 15 depends on the presence of two chemical species (proton and sodium cation) [67]. In other words, in this system, two chemical “inputs”

247

result in an “output“ in the form of fluorescence increase (Scheme 5). Although 15 is interpreted as the first rationally designed molecular logic gate poor emission and small “on-off‘signal intensity ratios of 15 remained to be improved.

Scheme 5 . Conversion of compound 15 to a fluorescent cation. Later, an essentially complete digital action was achieved by the same research group, in which the “off” states are virtually nonfluorescent under the conditions of operation [681. They explained the high performance of 16 being at least partly due to the novel arrangement of photohon-responsive modules: the logic molecule 16 has two PET (photoinduced electron transfer) donors, aminomethyl and benzo15-crown-5 moieties, so it is only when both the donors bind H+ and Na+, respectively, that 16 has no PET t o the fluorophore, i.e., fluorescence of the anthracene moiety is observed. Indeed, no fluorescence of 16 (10-6 M) is observable in CH30H unless both H+ and Na+ are present in sufficiently high concentration. A large fluorescence enhancement by a factor of >37-76 resulted in the presence of H+ (10-3 M) and Na+ (10-2 M). This system takes advantage of the design basis that consists of fluorescence-PET competition and careful arrangement of the photohon-active components. Whenever multiple chemical inputs independently produce one output, the molecules can be regarded as an “OR” logic gate. Thus, to be an effective OR counterpart, the receptor moiety must have poor recognition selectivity. With this in mind, various “ O R logic gate type molecules were developed by de Silva [691, Bharadwaj [70,711,and Stoddart 1721.

248

Et,N

16

One of two chemical inputs may be changed by a photo-input t o produce “photo

AND or OR ion to photo” molecular devices. Inouye has reported the crown etherlinked spiropyrans 10 and 11, isomerization of which to the open-chain colored merocyanines 10’ and 11’ was induced by recognition of alkali-metal cations as well as W-irradiation [27,45-501. Thus, according to the perspective of molecular devices, 10 and 11 can be considered to perform “ O R gate type dual-mode signal transduction by synchronizing molecular recognition processes with their photochromism. Also synthesized was an “AND” gate type counterpart, crown spiropyran 17 that could only isomerized to the merocyanine structure 17’(output) for the combination of ionic and photonic inputs (Scheme 6 ) [731.

Scheme 6.Conversion ofmerocyanine 17 to its active form.

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The design of the new crown spirobenzopyran 17 was based on the fact that the crown-bound cations could interact with the ether oxygens of the oxyethylene sidearm and not the phenolate oxygen of the opened merocyanine form 17’. Thus, the absence of strong electrostatic interaction between crown-bound cations and the phenolate oxygen of 17’ would prevent ready thermal isomerization of 17 to 17’*M+,in contrast with 10 and 11, and that the photo-isomerized cationaccommodated merocyanine form 17’*M+would be stabilized by the additional interaction of the lariat oxyethylene sidearm for the cations. The absorption spectra of 17 were scarcely affected upon addition of any of alkali-metal iodides in CHBCN,in contrast to the results for 10 and 11. The 1H NMR spectra of 17 with LiI in CDsCN, however, suggested that the lithium cations were bound t o the macrocycle of 17. Subsequently, irradiation (360 nm) of the alkali-metal iodidescontained CH3CN solutions of 17 gave rise to changes in their spectra, and new absorption bands appeared. The colored solution was stable and did not decolorize thermally in dark conditions a t room temperature even after 30 days. The “AND” gate type receptor developed by Inouye is expected to contribute t o molecularbased device technologies, although significant improvements including selectivity, response, and fatigue resistance are still needed. 3.1.3. Chromogenic receptor-basedpolymers For considering the application of chromogenic receptors to sensory materials, conjugated polymer-based supramolecules are attractive. Polymers possessing molecular recognition sites can take advantage of the high sensitivity of conjugation to external structural perturbations upon recognition of metal cations. In this context, various sensory polymers have been designed and synthesized [74801. Recently, Swager reported interesting examples using this approach. Polythiophene-modified calix [41 arene-based chromogenic receptors were synthesized and which revealed reversible ion-specific absorption changes [till. The choice of the calix[4larene was based on its high affinity for Na+. The ionochromic response of the polymer 18 with addition of Na+ showed an increased effective conjugation length of the polythiophene backbone. Thus, the addition of Na+ in THF solution containing 18 resulted in red shifts (up to 80 nm) of the absorption spectrum, while no or negligible changes were observed for Li+ and K+. The red shifts were explained by the incorporation of Na+ into the binding sites, thus forcing a rotation about the bithiophene axis and resulting in increased orbital 7c-overlap. Also synthesized was a fluorescent sensory poly(pheny1ene thiophene) containing a calix [41 arene-based ionophore [821. On a similar line,

250

Wasielewski and co-workers reported a transition metal-sensitive polymer 19,in which a pseudoconjugated, bipyridine-containing polymer undergoes conformational changes upon recognition of metal cations and which results in a near fully conjugated entity [831. The polymers are highly ionochromic to a wide variety of transition metal cations and main group metal cations. Although, of course, versatility for many cations means low selectivity for a particular cation, the work demonstrated a new approach to sensitive and reversible metal cation responsive polymers.

3.2.Ammonium cations Ammonium cations are by far the most studied organic cations in molecular recognition chemistry [84]. The size of the unsubstituted ammonium cation (NH4+)is near to that of K+, so that binding of ammonium cations by crown ethers and related hosts is similar in many aspects to the binding of the K+ cation. Furthermore, alkylammonium cations are seen in many biological entities, and most of them are chiral. Thus, enantioselective molecular recognition for chiral ammonium cations has been comprehensively investigated [84,85-871. 3.2.1. Chiral chromogenic receptors for chiral ammonium cations [181 Vogtle reported the first chiral chromophoric crown ether 20 in 1984 1881. The discrimination of an enantiomer with 20, however, could not be ascertained in the

25 1

UV-Vis spectra. The first example of successful “color discrimination” for chiral ammonium cations was achieved by Kaneda 189-911. They synthesized a chiral (SSSS)-crown azophenol 21 and observed an enantioselective blue shift (ca. 10 nm) of 21 compared to that of a (RRRR)-counterpart for (5’)-1(naphthy1)ethylamine. Novel chiral discrimination systems were developed by Kubo [921. A proton transfer and conformational change in a chiral calixarene-based receptor 22 induced by recognition of chiral amines caused spectral shifts in two chromophores attached to the binding cavity of the calixarene. They chose (S)-1,l’-binaphthyl groups as a chiral barrier and two indophenol groups as chromophores. The chiral chromogenic receptor 22 was dissolved in ethanol to yield a red solution (Lax = 515.5 nm). Addition of (R)-phenylglycinol to the solution gave rise to a color change from red to blue-violet as a result of the red shift (23 nm) in the band originally at 515.5 nm and the appearance of a new absorption band a t 652.5 nm, while the corresponding enantiomer, (S)-phenylglycinol produced no change under identical conditions. This observation that (R)-phenylglycinol induced color change of 22 was ascribed to ionization of the indophenol chromophore on the one hand (652.5 nm) and a binaphthyl induced hydrophobic effect on the other (538.5 nm) (Figure 3). The chiral chromogenic receptor not only represents a colorimetric sensor for chiral recognition, but also suggests a new general strategy for visual distinction between enantiomers. 3.2.2. Other type of ammonium cations

In addition to amino acids and their derivatives, a variety of ammonium compounds exist in biosystems. The nervous system involves many different kinds of specific interaction with different neurotransmitters that are often amine and ammonium compounds. Because of this complexity, there is great interest in the development of chromogenic receptors for specific neurotransmitters from the viewpoint of histochemistry [93,94]. Inouye has described artificial-signaling acetylcholine receptors, in which the presence of acetylcholine (not other neurotransmitters) induces a large fluorescence enhancement in protic media [45,951. The system consists of a resorcinollacetaldehyde tetrameric receptor 23 and a fluorescent pyrene-modified N-alkylpyridinium compound 24. They found that 24 was bound t o the ionized receptor 234-, and that orange fluorescence of 24 was strongly quenched upon complexation (PET mechanism) in alkaline media (0.01 M KOWMeOH). The fluorescence regeneration was observed by the addition of acetylcholine to the nonfluorescent solution of 234-024(fluorescence enhancement factor: I/I,,>20),

252

UMe

20

M -N b

o

Figure 3. A possible structure for 22-(R)-phenylgIycinol.

21

253

while the fluorescence spectra were not affected by the addition of any of the other low-molecular weight neurotransmitters (Scheme 7; Figure 4). This spectral behavior was attributed to the increasing proportion of the free-pyridinium dye 24 with respect to that of the complexed one by addition of acetylcholine: the association constants of the pyridinium dye 24 and acetylcholine t o the receptor 23 were similar.

234'.24 Nonfluorescence

234-.Acetylcholi ne

24

+

Fluorescence

Scheme 7. Acetylcholine-generatedfluorescence. The detection of acetylcholine was carried out in alkaline media, so the pyridinium dye 24 and acetylcholine are gradually decomposed to the dihydropyridine adduct and choline, respectively, implying that the system cannot be recommended for the time-dependent monitoring of acetylcholine. In order to overcome this problem Shinkai et al. used calix[6]arene-p-sulfonate 25 as a receptor motif [96]. The calixarene has very low pK, values and can include cationic guests with the aid of cation-.rr:interactions. This fact suggests that 25

254

may be useful in the neutral pH region for the detection of acetylcholine where no decomposition of the system and acetylcholine take place. Indeed, fluorescence regeneration of the system 24/25 was observed only by the addition of acetylcholine (I/L, c 3). Although Shinkai’s system required much higher concentration of both the receptor moiety and acetylcholine than those of Inouye’s system to obtain sufficient response, the system is useful for the time-dependent monitoring of acetylcholine in neutral aqueous solution.

Figure 4. Effect of low-molecular weight neurotransmitters on fluorescence spectra.

Another solution for the detection in neutral protic media was developed by Inouye and co-workers 145, 951. They synthesized a fluorescence probe-modified artificial acetylcholine receptor 26 (intramolecular system). The acetylcholine receptor 26 showed weak fluorescence in neutral EtOH, indicating that intramolecular quenching of the fluorescence of the pyrene moiety by the receptor site occurred efficiently. The addition of acetylcholine to the solution produced a strong fluorescence emission (ca. twice) (Scheme 8). The biological significance of

25 5

these receptors lies in the fact that acetylcholine is one of the most abundant neurotransmitters in nerve cells, and that no reliable methods for the chemical transformation of acetylcholine to its fluorescent derivatives in the presence of other neurotransmitters are currently available [97-991.

26 Weak fluorescence

26aAcetylcholine Strong fluorescence

Scheme 8. Acetylcholine-enhanced fluorescence.

A fluorescent PET receptor for another important neurotransmitter, GABA (yaminobutyric acid) was developed by de Silva [loo]. The fluorescent receptor 27 consists of monoaza-18-crown-6 as a recognition site for the ammonium terminal of GABA, while a guanidinium unit serves as a binding site for the carboxylate end. In this fluorescent receptor, an azacrown moiety engages in PET with an anthracene fluorophore: the fluorescence of 27 is quenched by intramolecular PET from the amino groups of the azacrown ring unless t h e cationic substrate is recognized. The receptor 27 showed useful fluorescence enhancement (IlI,, = 2.2) upon addition of GABA in water-methanol, while control receptor without the guanidinium unit from 27 yielded no measurable fluorescence response t o GABA. This finding suggests that 27 consists of the minimal set of components for GABA sensing, i.e., simultaneous binding of cations and anions is essential for the recognition of zwitterionic substrates (Scheme 9) [loll.

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Scheme 9. Simultaneous binding of anions and cations. 4. ANIONS

Although the vast majority of artificial receptors have been designed and synthesized for binding cations or neutral molecules including zwitterions, interest in receptors that recognize anionic substrates is increasing [102-104]. This is possibly because of the fact that molecular recognition of anionic species is seen in a number of biological processes. Thus, several chromogenic receptors for anionic substrates have recently appeared. In this section, chromogenic receptors that possess only anion-binding sites are discussed, and those for ditopic substrates such as zwitterions (section 3.2.) and anionic saccharide derivatives (section 5.1.) are discussed in other sections. 4.1. Inorganic anions The long-lived luminescence (ms) of lanthanide complexes has been utilized in various bioassays because of easy distinction from the short-lived background fluorescence (ps) present in most biological systems 1105-1081. Another advantage of the lanthanide complexes is their high coordination capabilities: the lanthanide complex itself can bind extra anionic species. Along this line, Parker et al, developed macrocyclic europium-phenanthridinium conjugates as luminescent chemosensors for pH and halide ions [log]. They synthesized phenanthridineattached monoamide triphosphinate derivatives of 1,4,7,10-tetraazacyclo-

257

dodecane-Eu3f complex 28. Excitation of 28a at 380 nm (pH 7.4)gave rise to no europium emission, and acidification (pH1.5) with CF3COOH turned on the red europium luminescence. Similarly, N-alkylation of the phenanthridine of 28a yielded the Eu-luminescent 28b. The Eu luminescence was quenched by addition of halide ions. The observed luminescence quenching by C1- was independent of pH (in the range 15-91, and was unaffected by the presence of phosphate, citrate, lactate, and bicarbonate, indicating that the europium complexes can serve as a attractive new sensory systems for pH and for halide ions. Transition metal-bridged macrocycles represent a new class of supramolecules that possess tremendous promise in molecular recognition chemistry [1101. The macrocycles are usually tetranuclear metal complexes, and therefore arranged in square geometries. Hupp’s variant contains visible-light-addressable, luminescence metal-ligand components within a square assembly 11111 that is attractive in the context of molecular sensing application. The light-emitting square shaped complex 29 showed a significant red shift in emission compared to that of monomeric fuc-Re(C0)3C1(4,4’-bpy)z, while decreasing by ca.25-fold the emission intensity attributed to quenching by the Pd(I1) fragments. The emission of 29 is enhanced upon addition of tetraethylammonium perchlorate, implying that the C104- affects the luminescence efficiency indirectly, e.g., by altering the quenching energetics. Although the association constant between 29 and C104was determined to be 900 M-1, the selectivity of 29 for other anionic species remains to be investigated.

28a R = H 28b R = M e

258

4.2. Organic anions Anion recognition is often attained by incorporating Lewis acidic metal cations into the receptor center. Thus, Fabbrizzi appended an anthracenyl group to peripheral nitrogen atoms of tris(2-aminoethy1)amine (tren) which bind Zn2+ to give a fluorescent anion receptor 30 [1121. In 30,the four-coordinate metal center has a vacant site for coordination of an anion. Addition of 4 -N fl(dimethy1amino)benzoate t o a methanol solution of 30 caused a dramatic decrease of the fluorescence intensity, while acetate gave no effect (Scheme 10). The observed fluorescence quenching is caused by PET formation from the Zn2+-bound dimethyaminobenzoate to the photo-excited anthracene unit. The fluorescence of the anthracene moiety was not altered, or showed an intensity decrease of less than 5% in the presence of NOa-, NCS-, and C1-. This work demonstrated that metal-ligand interactions can be conveniently used for anion recognition, and that the appended fluorophore can be used to signal the occurrence of the recognition process. Anslyn developed the practical chemosensor system for citrate, which consisted of the citrate receptor 31 and 5-carboxyfluorescein as an absorption/ fluorescence probe [1131. The system is specific for citrate, so that the system did not respond to other analyte in the absence of citrate. This was demonstrated by analyzing “Coca Cola” and “Diet Coke”, soft drinks containing either no citrate or very low concentrations of citrate, respectively (Scheme 11). PET

30Carboxylate

Scheme 10. Complex formed between compound 30 and a carboxylate ion.

259

H

H

31

31*Citrate

Scheme 11.Molecular recognition system for citrate.

Phosphate and its partial esters are an another class of biologically important 32 as a fluorescent anions. Araki reported 6,6’-bis(hexylamino)-2,2’-bipyridine receptor for phosphoric acid diesters [114]. Increase in diphenyl phosphate concentration in cyclohexane solution of 32 produced absorption changes with isosbestic points. In the presence of excess diphenylphosphate, the spectrum of 32 was identical to that of the HC1 salt of 32, indicating that the spectral change was due t o protonation of 32 by the substrate. Indeed, the presence of an acidic proton in substrates was found to be essential for inducing the absorption changes of 32. Similarly, the fluorescence emission spectrum of 32 altered drastically upon recognition of diphenyl phosphate, viz., from the blue fluorescence of 32 to the green fluorescence of the complex in acetonitrile. The luminescence metalloreceptor 33 for phosphodiesters was designed and synthesized by Watanabe [1151. This luminescence receptor has a neutral bis(acylaminoimidazo1ine) binding site and can recognize anionic and also neutral phosphodiesters with luminescent signal transduction.

260

32-Phosphate 5. NEUTRAL MOLECULES

Uncharged substrates are no longer recognized by strong electrostatic interactions. Hydrogen bonding, van der Waals, hydrophobic, and n: -stacking interactions are used solely and sometimes jointly [116]. In aprotic and nonpolar solvents, hydrogen bonding is the major driving force for recognition of neutral molecules, while in the protic, especially in water, hydrophobic and sr-stacking interactions are predominant. Among neutral molecules, chromogenic and fluoropholic receptors for saccharides have been extensively investigated by Shinkai and other research groups, so this is a reason why the section for saccharide and their derivatives is separated.

5.1. Saccharides and their derivatives Among the many artificial receptors, however, only a few have been shown to be effective for the recognition of the third major class of natural building blocks, saccharides and their derivatives [116,117]. This is possibly because of the threedimensional complexity of saccharide structures and of the great difficulty in distinguishing between the families of closely related stereoisomers, as well as of the extremely weak intermolecular hydrogen bonding abilities of saccharide hydroxyl residues [116,118], Thus, Shinkai used phenylboronic acids as an artificial receptor for saccharides, in which the boronic acids form cyclic esters in water through the formation of strong covalent bonds with saccharides, particularly with those containing cis-diol groups (Scheme 12) [1191. Most of the investigations pertaining to this section were initially carried out by Shinkai and

26 1

co-workers, and an excellent review of this topic up to 1995 was published by them [120]. Thus, we discuss here work appearing after 1995 for the recognition of saccharide derivatives such as nucleosides.

Scheme 12. Molecular recognition of saccharide derivatives.

Oxidation of primary hydroxyl groups of aldoses (monosaccharide) to carboxyl groups produced uronic acid, which are well-known to be important in the biosynthetic process of L-ascorbic acid. Shinkai designed and synthesized fluorescent receptors 34 for uronic acids, which includes both a boronic acid moiety for diol binding and a metal chelate one for carboxylate binding [1211. In the presence of saccharide, the fluorescence intensity of 34 is increased at pH 6-11. This increment is explained by reduced PET from the amino groups to the fluorescent phenanthroline by intensified B-N interaction upon complexation of saccharide with the boronic acid group. In the presence of Zn2+, selectivity for uronic acids (D-glucuronic acid and D-galacturonic acid) was obtained, indicating that uronic acids are bound to 34*Zn2+by a cooperative action of boronic acid-diol complexation and Zn2+-carboxylatecoordination (Scheme 13A). In a similar line, the cationic saccharide derivative glucosamine was targeted by James et al. utilizing a boronic acid-anthracene-azacrown ether conjugate 35 (Scheme 13B) [122]. Both the nitrogen atoms of azacrown and aminomethylphenylboronic acid are expected to participate in PET with the anthracene fluorophore. Thus, this system can be considered t o behave like an AND logic gate (see section 3.1.2.). Indeed, fluorescence increase was observed by the addition of glucosamine hydrochloride, but not of glucose. In the former, the nitrogen of the azacrown is used for the recognition of ammonium groups and the nitrogen of the aminomethylphenylboronic acid for the diol groups of glucosamine hydrochloride.

262

34*Uronic Acid

35*Glucosamine

Scheme 13. Urinic acid and glucosamine interactions. Smith and Taylor extended this approach to a nucleotide, a ribonucleoside 5’triphosphate [123]. They synthesized the fluorescent phenylboronic acid-attached poly(ally1amine) 36, in which the polyammonium chain is expected to bind triphosphate groups of nucleotides. Indeed, titration of 36 with uridide, 5’-UMP2-, and 5’-UDP3- gave negligible changes in fluorescence emission, while 5’-UTP4yielded a small but significant fluorescence increase. Furthermore, the no response of 36 to 2’-deoxy-5’-UTP4- indicated the importance of the cis-diol chelation with the boronic acid (Scheme 14). Chromogenic receptors for nucleosides were also developed by Shinkai [124]. Azobenzene was tethered to phenylboronic acid t o give 37. The azobenzene moiety serves not only as a chromophore but also as a x -stacking unit with nucleobase part of nucleosides, that results in a spectral change of the receptor 37. Although the chromogenic receptors showed large association constants as well as satisfactory difference in the complexation abilities among nucleosides, the spectral change induced upon recognition of nucleosides was small.

263

36*5'-UTP4-

37

Scheme 14.Ionic interactions involving compound 36.

5.2. Other neutral molecules Cyclodextrins (CDs) have been a representative host molecule for evaluating chemical answers concerning intermolecular interactions in water [ 1251. CDs bind organic molecules by hydrophobic andor van der Waals interactions in water, so complementarity in size and shape between CD and the substrate is critical. Among many investigations for artificial receptors utilizing CDs as a recognition moiety, Ueno and co-workers have developed prominent examples of CD-modified chromogenic and fluorescent receptors. Related works before 1995 were wellsurveyed by themselves [126,1271, so only recent examples are presented here. Among many fluorophores, the dansyl (5-dimethylamino-1-naphthylsulfonyl) group is a promising one that emits fluorescence in response to its microenvironment. A variety of dansyl-modified CDs have been designed and synthesized. Ueno et al reported N-dansylleucine-attachedCDs 38 [1281 and Ndansyl-attached CD dimer 39 [1291 as a fluorescence receptor for steroids, terpenols, etc. They observed that the fluorescence intensities of the CD derivatives always decreased upon the substrate recognition, and that the degree of the changes depended on the substrates. The guest-binding abilities of the monensin (natural ionophore for Na+)-dansyl-CD triad system 40 was also studied and found t o be enhanced by the presence of Na+ which may interact with monensin [130]. In this system, the fluorescence intensity decreased on addition of organic substrate such as terpenols. In all cases, the decrease of fluorescence is

264

due to the exclusion of the dansyl moiety from the hydrophobic CD cavity to bulk water associated with the guest binding by CD (Scheme 15). Feiters also synthesized novel dansyl-appended CDs and investigated the self-inclusion and sensor properties [1311.

NH

I

39

Another approach was shown by Osa, who designed bis(naphthy1)CDs 41 that showed the intramolecular eximer emission upon recognition of steroids [132]. Ueno designed a further triad system which consisted of a fluorophore ( N f l dimethylaminobenzoyl groups), CD, and biotin (an avidin-binding site [ 1331) units 42 [134]. The system showed interesting behavior: the fluorescence intensities of the NJV-dimethylaminobenzoyl groups decreased and increased on the addition of steroids in the absence and presence of avidin (protein), respectively. The fluorescence increase may be related with the locational change of the fluorophore moiety from the CD cavity to the more hydrophobic pocket of avidin. Chromophore-attached CDs have also synthesized by Ueno and co-workers [135].

265

Scheme 15. Guest binding associated with CD derivatives.

41

42

Nocera's approach is based on an absorption-energy transfer-emission (AETE) process using lanthanide emission (see section 4.1.) [105-1081. They designed a Tb3f-diethylenetriaminepentaacetate-appended CD 43 as a powerful BTEX (benzene, toluene, ethylbenzene, xylene) and PAH (polycyclic aromatic hydrocarbon) chemosensing receptor [136]. 43, which shows very weak emission in water, is markedly enhanced by the addition of aromatic hydrocarbons. The increase in Tb-emission intensity with added aromatic hydrocarbon is accompanied by the appearance of bands in the fluorescence-excitation spectra

266

that are identical to those of typical absorption of the aromatic substrates, indicating the AETE process (Scheme 16).

Scheme 16. Aromatic hydrocarbon-induced emission enhancement. Finally, it is worthwhile to mention that the creatinine receptor developed by Bell et a1 [137]. Although creatinine is an end product of nitrogen metabolism, current creatinine assays have several limitations. The design of the creatinine receptor 44 is based on the fact that the binding of creatinine by complementary hydrogen bonds will cause the transfer of a proton from the OH group of the receptor to its end nitrogen atom, and that the resulting complex would showed a visible change in color (Scheme 17). Indeed, a solution of 44 in CHzC12 turns from yellow to brownish-orange upon addition of creatinine (Figure 5). The coloration is selective for creatinine; no or negligible color changes were observed in the presence of any analytes existing a t normal blood serum level. The structural and energetic features of these specific interactions have also been investigated using ab initio structure methods [138]. The creatinine receptor illustrates the high possibility of designed chromogenic and fluorescent receptors for the practical quantification of biologically important organic analytes.

267

44

44Creatinine

Scheme 17. Ionic interactions involving the creatine receptor.

6. CONCLUSION AND OUTLOOK

The design and synthesis of chromogenic and fluorescent receptors started from the viewpoint of simple curiosity of supramolecular chemists: what is recognized ? what kind of response will be obtained? Because of their potential for nondestructive detection and high sensitivity, however, they have become of great interest in the development of molecular sensors that change their absorption andor fluorescence properties in response to the presence of biologically important analytes. Although the works described in this chapter were not always purposely directed toward analytical application, some of the receptors and systems seem t o have sufficient potential as practical molecular sensors. Further investigations for chromogenic and fluorescent receptors using the principles of supramolecular chemistry may result in conventional analytical chemistry to be modified to a molecular recognition-based one.

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7. ACKNOWLEDGEMENTS

Thanks are expressed to my co-workers, Jun-ya Chiba, Shiro Ito, Masayoshi Takase, and Haruna Teraoka for the literature search and for assistance in the preparation of this chapter. 8. REFERENCES

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Colorants for Non-Textile Applications H.S.Freeman and A.T. Peters (Editors) 2000 Elsevier Science B.V.

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7 Laser dyes: Structure and Spectroscopic Properties THEODORE G. PAVLOPOULOS U S . Space and Naval Warfare Systems Center, Code D361, San Diego, CA 921525001 1. INTRODUCTION

Sorokin and Lankard [l] and Schafer, et al. [2] reported laser action from organic compounds in solution about 30 years ago. Dye lasers, using laser dyes as the active media, have become one of the most widely used lasers. A key feature of dye lasers is their continuous tunability over a wide spectral region, covering most of the fluorescence spectral region of each laser dye. Dye lasers are liquid lasers. This is an important attribute because it allows the removal of excessive heat from the active liquid media by simply circulating the dye solution. For this reason, dye lasers can be operated at rather high-pulse repetition rates. Because rather high pulse energies can also be obtained from dye lasers, they can be operated at a relatively high-average power. In addition, mode-locked dye lasers can produce ultra short pulses of sub-pic0 second duration. Since very narrow linewidths can also be obtained, dye lasers have become an indispensable tool in spectroscopic research. Studies pertaining to laser dyes have led t o reviews by several authors, e.g., Drexhage 131, Steppe1 [41, and Maeda [51. There are many scientific, industrial, and medical applications that require high-average power, high-pulse energy, and low-cost visible laser light. Unfortunately, the generation of visible coherent radiation possessing these properties is one of the few areas in laser physics that has stubbornly resisted advancement. The present review has been written t o demonstrate that large dye lasers using improved (high-efficiency) laser dyes as the active medium and pumped by large flashlamps can lead to further advances in this field. This review contains guidelines on how t o identify or develop new and improved laser dyes. Under flashlamp excitation, laser dyes presently available show low efficiencies (ratio between optical output to electrical input) in the 0.3 to 0.6 percent range. Only Rhodamine 6G and a few other dyes show efficiencies of about 1 percent [61. According t o Sorokin, et al. [71 and Schmidt and Schafer [81, so-called triplet state losses (TSL) critically affect laser action efficiency. It was found that under

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flashlamp excitation, laser pulse intensity diminished sooner than flashlamp pulse intensity. Triplet-state dye molecules (M,) of concentration (N,) generated during excitation absorb laser light, resulting in TSL. In addition, most photochemical reactions, because of the generally long lifetimes (7,) of triplet-state molecules, take place in the triplet state. Efficient laser dyes exhibit strong fluorescence, i.e., possess a high quantum fluorescence yield (Q,), Q, = 1, and have only a small triplet-triplet (T-T) absorption (small triplet extinction coefficients E&,)) present in the fluorescence (laser action) spectral region (FSR) A,. The quantum fluorescence yield (QJ is defined as the ratio of the number of fluorescence photons emitted (N,), to the number of photons absorbed (NA),Absorption of light by the triplet state molecules is called T-T absorption and, similar to conventional near UV/visible absorption spectroscopy is expressed in L(liter)/mol(mole)cm. For a better understanding of the laser action properties of laser dyes, the major principles of laser operation are briefly reviewed and a short overview of lasers is provided. This is followed by a discussion on spectroscopic properties, such as activation and deactivation of absorbed light by organic compounds, including laser dyes, relationships between fluorescence and molecular structure, and T-T absorption spectra. Experimental data reviewed show that considerable T-T absorption is still present over the FSR in presently available laser dyes. According to Drexhage’s loop rule [31, chromophores appear t o have lower TSL where the oscillation of n-electrons is affected by heteroatom(s) substitutions of carbon-atom(s). It has been suggested by Pavlopoulos [91 that this effect result from the presence of strong vibronic spin-orbit interactions present in heterocyclic systems. Heteroatom substitution introduces triplet state n,n* transitions, which efficiently ‘steal’ intensity from triplet n,n* transitions. Since reduced TSLs result in higher laser action efficiencies, the presence of vibronic spin-orbit interaction in heterocyclic systems is of key importance in laser dye technology. It is concluded that families of many efficient laser dyes may be found among five-and six-membered heterocyclics. Due to strong vibronic spin-orbit interactions present in these compounds, they should possess reduced T-T absorption over their FSR. If the compound also exhibits strong fluorescence, efficient laser action may be observed. Besides the laser dye employed as the active medium, the type of pump sources employed critically determines laser action threshold and efficiency. Therefore, this chapter includes a discussion on pump (excitation) sources of dye lasers. Large flashlamps appear to be the most economical pump sources for dye lasers for generating inexpensive visible laser light. The basic equipment required to record spectroscopic laser dye parameters Q,, E&), and triplet state life times (TJ is presented. Only a brief discussion is given, since detailed information can be obtained from the papers cited.

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In the section that follows, molecular engineering and spectroscopic studies are presented. These are approaches to new laser dyes development. This is followed by a section on spectroscopic and laser action data for some commercially available laser dyes. Three families of so-called quasi-aromatic laser dyes, the synbimanes, 2,2’-di-a-pyridylamino-BF2 (DPA-BF,) complex, and the pyrrometheneBF, complexes are also presented. These compounds possess strong vibronic spinorbit interactions. 2. PRINCIPLES OF LASERS AND DYE LASERS

2.1. Molecules as electronic dipoles All lasers emit stimulated (coherent) radiation, which is highly monochromatic. Many of the spectroscopic, as well as the laser action properties of molecules, can be explained classically by simply viewing molecules as small electronic dipoles (oscillators). Using this description, polarization characteristics of electronic transitions of organic molecules, together with stimulated emission and some of their laser action properties are easily understood. By treating a molecule as an electronic dipole, it is evident that a dipole can only interact (absorb and emit) with electromagnetic radiation (light) that is polarized along its axis. Each electronic transition, whether near Whisible and T-T absorption, fluorescence, or phosphorescence, exhibits its own characteristic polarization. In a rotationally symmetric molecule, electronic dipoles will be aligned (positioned) parallel to either the long or short molecular axis. Consequently, all electronic transitions will be polarized in only these two directions. Because electronic transitions are broadened by vibrational and rotationalholvent interactions, the resulting broad near UV/visible and T-T absorption, fluorescence and phosphorescence bands will, therefore, exhibit their own characteristic polarization. On the other hand, due t o the random orientation of molecules in gases and in liquid solutions, emitted and absorbed radiation of molecules is depolarized. However, crystals may show preferred orientations for absorbing and emitting light. When the length of a dipole in a molecule is increased by substitution of an Hatom, the interaction may cause an increase in the length of the electronic dipole. A longer electronic dipole will absorb and emit light at longer wavelengths. These red shifts are referred to as the bathochromic effect. For weak absorption bands, these red shifts may also be accompanied by an intensification of the absorption band, referred t o as the hyperchromic effect. If the red-shifted near Whisible absorption band is of lowest energy, the fluorescence spectrum will also be shifted to longer wavelengths. The spectral location of T-T absorption bands may also be influenced by substitutions. Consequently, substitution may critically affect the laser action properties of a molecule. These effects will be discussed in more detail in Section 8.3.

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Viewing a molecule as a small, damped electronic dipole (oscillator), stimulated emission can also be explained classically. A dipole may interact with a wave train; a wave train is an electromagnetic oscillation, which has a beginning and an end. The following three interactions with an electromagnetic wave train may occur. The loosely bound electron may absorb the electromagnetic radiation, being elevated (excited) from energy level El to E,. An excited dipole, occupying energy level E,, may loose energy by emitting electromagnetic radiation, returning to energy level E,. This process is referred to as spontaneous emission. Because excited state life times (2) of atoms and molecules are in the nsec range, all emitted electromagnetic wave trains of light have finite lengths (S); we have S = c 7,where c is the speed of light. Consequently, all spontaneous emissions emitted from conventional light sources consist of electromagnetic wave trains, having S in the length of meters. In rare cases, an oscillating (excited) dipole may interact in phase with an incident electromagnetic wave train, increasing the length (monochromaticity, also called coherence length) S of the wave train. This type of deactivation mechanism is called stimulated emission. All three mechanisms are depicted in Figure 1.

“t EpI -7E2

El

El Absorption

Spontaneous Emission

Stimulated Emission

Figure 1. The three fundamental quantum mechanical processes: absorption, spontaneous, and stimulated emission, between two energy levels, E, and E,, with E, > El. Einstein, in his statistical derivation of Max Planck’s black body radiation equation, besides spontaneous emission and absorption, introduced stimulated emission as an equally important second deactivation mechanism of atomic and molecular systems. The word laser stands for light amplification by stimulated emission of radiation. In a laser, the short wave train generated by a spontaneous emission from an atom or molecule interacts in phase with another electron in the excited state E,,

219

lengthening the wave train. This process is repeated, the wave train being reflected back again into the active region by one of the laser mirrors. This causes a considerable lengthening of the wave train, with the wave train becoming more and more monochromatic (coherent) and gaining in intensity. 2.2. Lasers

Lasers are widely used in spectroscopy, isotope separation, material working (e.g., welding), material inspection, recording, holography, display, communications, remote sensing, marketing, surveying, optical alignments, in medical, and many other areas. A laser consists of the following three main parts: the gain or the active medium, an energy (or pump) source, and the optical cavity or the resonator.

AM

HR

w

M

oc

ES

Figure 2. The basic building blocks of a laser are: the active medium (AM), an excitation, or pump source (ES), and the resonant cavity, consisting of two mirrors, and the output coupler (OC). the high reflector (HR), The energy source generates an overpopulation of excited atoms or molecules in the gain (laser) medium. A representation of a laser system is shown in Figure 2. The gain medium could be a gas (gas lasers), a liquid, mostly a dye in solution (dye lasers), or a solid (solid state lasers). The pump source could be another laser, or a flashlamp. In gas lasers, an electric discharge may also provide the required excitation energy. The resonator consists of two laser mirrors which are aligned parallel to each other. The active medium is located between these mirrors. One mirror has 100 percent reflectivity for the laser light and, therefore, is called the high reflector (HR). The other mirror has some transmission for the laser light and is called the output coupler ( 0 0 . (a) Gas lasers. The CO, laser is especially noteworthy among lasers because of its high efficiency, which is in the 5 to 30 percent range. This laser operates in the pulsed and cw (continuous wave) mode with maximum output at 10.6 p [lo]. In principle, a gas laser has no upscaling limitations. Overheating of the active medium can be avoided by circulating the gas.

280

Widely used gas lasers that generate cw near Whisible laser light are the argon ion and krypton ion lasers. Unfortunately, these lasers also exhibit poor efficiencies. Standard argon and krypton models operate at 5 and 4 Watt respectively, which is the power of all laser lines. These lasers are driven by about 12 kWatt of electric power. Table 1 shows the most common lines observed from these two cw lasers. Table 1. Wavelengths in (nm) of laser lines, together with their power outputs P (in W) in their fundamental TEM,, mode, of commercially available one meter model cw argon and krypton ion lasers. Argon Ion Laser Power, TEM,, Multi-line (nm)

457.9-514.5 351.1-363.8

P(W)

5.000 0.100

Single-line (nm)

514.5 501.7 496.5 488.0 476.5 472.7 465.8 457.9 454.4

Krypton Ion Laser Power, TEM,, Multi-line (nm)

P(W)

647.1-676.4 406.7-415.4 337.4-356.4

0.750 0.100 0.150

Single-line (nm)

2.000 0.400 0.700 1.500 0.750 0.300 0.200 0.350 0.120

799.3 752.5 676.4 647.1 568.2 530.9 520.8 482.5 476.2 413.1

0.030 0.100 0.150 0.600 0.200 0.200 0.090 0.045 0.060 0.060

One of the most widely used cw lasers is the heliudneon laser, with laser action at 632.9 nm. Most commercial models operate with efficiencies in the 0.01 percent range and output power in the 0.5 to 0 7 mWatt range. Low efficiencies are also observed from cw cadmium/ helium lasers, with laser lines at 441.5 and 325 nm. Important pulsed lasers are nitrogen lasers, which exhibit laser action at 337 nm, with energies of several mJ/pulse or less, and excimer lasers, which use noble gashalogen mixtures as the active media. They exhibit laser action in the near-

28 1

U V ;ArF (193 nm), KrCl(222 nm), KrF (249 nm), XeCl(308 nm), and XeF (350 nm). These lasers have energies in the 0.1 to 1J/pulse range and are commercially

available. Efficiency for the pulsed copper vapor lasers, lasing at 511 and 578 nm, reaches about 1percent in some models. It is unlikely that we will be able to find new laser media among gases and vapors with new and unusual laser action properties, such as showing efficient laser action in the visible spectrum. (b) Solid state lasers. Only a few solid-state lasers exhibit efficient laser action in the visible portion of the spectrum. The oldest laser, the flashlamp pumped ruby laser, lasing at 694.3 nm [lo] for some special applications, is still commercially available. The active medium is synthetic ruby (A120J, doped with 0.01 t o 0.5 percent chromium. Especially noteworthy is the Nd:YAG laser, which is optically pumped; the host is a crystal of ytterbium aluminum garnet (YAG) with concentrations of Nd ions of about one percent [ l l l . YAG is a thermally robust host material. Other solid host materials, including glass, are also used for Nd. Nd:YAG shows laser action at 1.064 y under flashlamp excitation, with efficiencies from a fraction of 1 to about 2 percent. Qswitching and frequency doubling with BBO @-barium borate) crystals produces coherent radiation at 532 nm. Remarkably, this green line is one of the most widely used visible laser lines, and is often used t o pump laser dyes. Solid state lasers have the following limitations: Solid state media are often difficult to fabricate with a high degree of optical homogeneity, especially in large pieces. Large rods of the crystalline solid-state media can be expensive to produce. Because most of the excitation light is converted into heat, the biggest problem for solid state lasers results from the difficulty of removing excess heat from the medium. Transparent solid media are usually poor thermal conductors. Overheating often results in permanent damage t o the solid. For these reasons, solid state laser devices have upscaling limitations. They exhibit low pulse energies and pulse repetition rates, which result in low-average power outputs.

(c) Semiconductor diode lasers and laser diode arrays. There have been tremendous and very successful efforts devoted t o developing semiconductor junction diode lasers [lo] over the last 20 years. These tiny lasers convert electricity, at 10 to 50 percent efficiency, into near IR laser light. Many different structures are used, and they have the electrical characteristics of semi-conductor diodes. Laser light is emitted from a rectangular area that is generally 3 t o 5 ym wide and about 0.1 t o 2 pm thick. The beam produced diverges strongly in the direction perpendicular to the emitting plane of the emitting layer. Available power levels available range from a few mW t o about 100 mW, with peak power in the MW range. A junction diode laser is depicted in Figure 3.

282 E M i n l N G LAYER

LARGER DIVERGENCE OF LASER BEAM IN PLANE PERPENDICULAR TO ACTIVE LAYER FAR-FIELD LASER BEAM

Figure 3. Single, narrow-stripe semiconductor diode laser Diode lasers for the red portion of the spectral region are also available, although their efficiencies are smaller. New compound semiconductors emitting in the yellow and blue region are still in active development. Diode lasers have found very wide applications in fiber optical communications, optical recording, laser printing, and many other areas. Semiconductor diode laser arrays (DLAs) consist of many small diode lasers, simultaneously produced in a row on one strip (bar) [12,131; strips containing up to 200 single diodes are available. These strips have been stacked on top of each other, producing very compact two-dimensional (2-D) surface-emitting excitation sources. Outputs up to k W for the near IR have been obtained. Larger DLAs, using many bars, are expensive. A major milestone in the development of DLAs has involved pumping Nd:YAG laser efficiently. This has been achieved with conversion efficiencies in the 10 t o 30 percent range. However, eficient cooling of these devices has been difficult. To obtain visible laser light, Q-switching and frequency doubling have produced green laser light at 532 nm. A loss of up to 50 percent is associated with frequency doubling. Using many bars of DLAs, many hundred of watts of power output at 532 nm have been obtained from these devices. This success led to optimistic predictions that DLAs eventually could replace gas, dye, and solid-state lasers not only in the near IR, but also in the visible spectral region. However, these predictions have not been fulfilled. It had been anticipated that all the single diodes would couple coherently and in phase. It appears that this coupling has only partially been achieved. These new laser devices are very compact and user-friendly in the laboratory. Depending on size, the larger laser devices are very expensive. However, as discussed, larger devices experience over-heating because of the poor heat conducting properties of the optically pumped solid Nd:YAG media, severely limiting upscaling.

283

2.3. Dye lasers A typical flashlamp-pumped dye laser is depicted in Figure 4. Excitation is provided by a high-intensity flash, which is generated from the flashlamp by discharging a capacitor bank located in the high-voltage ( H V ) DC power supply. The elliptic mirror configuration efficiently focuses the flashlamp light on the dye cell.

CELL

I

POWER SUPPLY

Figure 4. Outline of a flashlamp pumped dye laser. Continuous tunable laser radiation is obtained by turning the diffraction grating. Other sources employed to pump dye lasers are described in section 6. Circulating the dye solution removes spent dye and provides efficient cooling of the dye solution. The dye solution (the active medium) is positioned within the two resonators, the HR and the OC. In dye lasers, for wavelength selection, the HR is replaced by a tuning element, which can be either a diffraction grating, or a prism. By simply turning the grating, the tuning curve (L), is obtained, covering part of the fluorescence spectral region (FL) of the laser dye. The tuning curve is depicted in Figure 5 . Using several laser dyes possessing overlapping tuning curves, one can span the near ultraviolet (W),visible, and near infrared (IR) regions continuously. This is depicted in Figure 6, which presents a collection of tuning curves (L) of commercially available laser dyes, pumped with a coaxial flashlamp pumped dye laser from Phase-R. From this tuning curve it is apparent that improved laser dyes are needed for the near W h l u e , green, and the redhear IR portions of the spectrum. Recently, new laser dyes (Exalite) have been developed for the near W h i o l e t portion of the spectrum.

284

WAVELENGTH

h

-

Figure 5 . The tuning curve (L), the near Whisible absorption (S-S),fluorescence spectrum (FL) of the laser dye, and the absorption spectrum of the triplet state dye molecules (T-T).

I C

RHOOAMINE

400

500 600 WAVELENGTH h (nm) -+

700

800

Figure 6 . Tuning curves (L) produced from a coaxial flashlamp pumped dye laser, using a collection of commercially available laser dyes. Significantly, using presently available laser dyes, the upscaling of flashlamppumped dye lasers is restricted by the size of the flashlamp. Only small and sturdily constructed flashlamps can be used for pumping, producing short light pulses with short pulse risetime (19). Reducing T-T absorption over the FSR in dyes will allow the use of large flashlamps with a high power rating for pumping,

285

producing long laser pulses. Larger flashlamps, together with higher efficiency laser dyes, should yield high-average power outputs. 3. FLUORESCENCE AND MOLECULAR STRUCTURE 3.1. Activation and deactivation of organic molecules by light For an organic compound to display laser action, it must first exhibit fluorescence. However, most organic compounds will not exhibit fluorescence, due to the presence of radiationless processes that transform absorbed light into heat. These radiationless deactivation processes significantly affect laser action in organic compounds. An abbreviated Jablonski [14,151 energy diagram characteristic of organic molecules is presented in Figure 7. This figure illustrates some of the main electron energy levels in the singlet and triplet manifolds that are important explaining radiative (luminescence) and non-radiative processes in organic compounds, including laser dyes. Vibrational sub-levels broaden all electronic transitions in organic molecules. Between these sub-levels there are the closely spaced rotational and moleculedsolute interaction sub-levels. Electronic absorption and luminescence (fluorescence and phosphorescence) bands produced by organic molecules are, therefore, broad and continuous, sometimes exhibiting vibrational structure. An organic molecule in its ground state has its two outer orbital electrons with their spins aligned anti-parallel to each other. This state is referred to as the ground singlet-state, So A molecule in the singlet ground state is designated by M, and the concentration of these molecules is designated by No. (a) Light of wavelength (v,) can be absorbed by a molecule, exciting one electron ..., S,.This process is referred to into any of the higher singlet levels (bands), S,, S,, as the conventional near W/visible singlet-singlet ( S - S ) absorption A, Upon excitation into one of these sub-levels, the molecule will relax very quickly (in 10" to sec) to the lowest vibrational sub-level. From the singlet levels S,, S,, ...,Sn, the molecule will relax very quickly, radiationless into the singlet state S,. A molecule in the singlet-state S, is designated by M, and the concentration of such molecules by N,. A molecule in the singlet-state S, is designated by M,, etc.

M,

+ hv, + MI, (or M,, M,, etc.).

(1)

All relaxations within electronic sub-levels, as well as relaxations from higher singlet and triplet levels to S, and T, have been omitted in Figure 7. This prevents obscuring some of the features important to our discussion.

286 SINGLET MANIFOLD

TRIPLET MANIFOLD

s3k t Tm

Figure 7. Jablonski's energy diagram. This diagram describes some of the activation and deactivation pathways and energy levels in organic compounds in solution at room temperature, which are relevant for our discussion. Some important features of the diagram have been omitted to preserve simplicity. (b) From the first excited singlet-state level, S,, a molecule may: (i) return to the ground singlet level, So, by emitting fluorescence vF; (ii) return to the ground singlet-state S, radiationless with rate constant (kIc); (iii) undergo photo decomposition, or (iv) its outer electron may switch its spin-orientation and align with its spin parallel to the electron in the lower orbital, producing the tripletstate. This process is also refereed to as spin-orbit interaction (coupling), a firstorder perturbation [161. The rate of this process is determined by the intersystem crossing rate constant (kIsc).Spin-orbit coupling has a very negative effect on laser action efficiency.

M, + M,

+ heat,

MI + products.

(kIc).

(3) (4)

287

(c) The lowest energy triplet-state is designated by TI. A molecule in its lowest and the concentration of these triplet state TI is designated by M, (or ql) molecules by N,. Molecules in higher triplet state levels T,, T,, etc. are designated by &*,M ,: etc. From the triplet state level, TI, a molecule (MJ may: (i) decay t o the ground singlet state, So, by emitting phosphorescence v,; (ii) by a radiationless process, with rate constant &, decay to the ground singlet state, So, or (iii) undergo photodecomposition.

M,?+ M,

+ hv,.

(6)

In solutions at room temperature, generally no phosphorescence is observed and the decay process is covered by the rate constant, k, = lh,, only. Because of the forbidden nature of this transition, their triplet-state lifetime (7,) is rather long. The molecule M, in its triplet state TI, has different spectroscopic and chemical properties from the original molecule in its ground singlet state So. Significantly, from the triplet state level, T,, molecule M, can also absorb light v, becoming excited into higher triplet state levels, T,, T3,..., T,. This process is referred to as T-T absorption 4.

M,

+ hv, + M,:

(or q3, M,4, etc.).

(9)

Because of the fundamental importance of T-T absorption t o laser dyes, this subject is discussed separately in section 4. 3.2. Aromatic hydrocarbons In the gas phase, the ability of a molecule to exhibit fluorescence is strictly determined by its chemical structure. In the case of molecules in solution, fluorescence also depends on the solvent employed. There is a large amount of experimental data available in the literature on the fluorescence and absorption of organic compounds. Fluorescence of organic compounds, as well as the effect of substitution on absorption and fluorescence spectra has been discussed in many review articles and books, including those by Forster [171, Hercules 1181, Parker 1191, Becker [201, Berlman [211, and Wehry [221. In this section, a few experimental observations are presented t o show some general trends on the relationship between the structure and fluorescence of

288

organic compounds. However, despite the richness of experimental data in this area, there always seems to be some exceptions to the trends. Aliphatic and saturated cyclic hydrocarbons do not exhibit fluorescence. Their electrons are either tightly bound or involved in o-bonding. These compounds absorb only in the far UV;and absorption of high-energy radiation often results in o-bond dissociation. Fluorescence is mainly observed from larger compounds that contain multiple conjugated double bonds, resulting in nonlocalized, less strongly bound n: electrons.

Benzene

Tetracene

Naphthalene

Anthracene

Phenanthrene

@ \

Chrysene

Pyrene

Figure 8. Chemical structures of some aromatic hydrocarbons Conjugated compounds start to absorb light near 220 nm, and longer wavelengths promote electrons into the n* anti-bonding orbital, i.e., n: + n* (n:,n:*) transitions. Disruption of bonding due to absorbed light is less extensive. More specifically, fluorescence is almost exclusively observed with molecules containing aromatic and heteroaromatic systems. In solution, practically all unsubstituted aromatic hydrocarbons such as benzene, naphthalene, phenanthrene, anthracene, pyrene, etc., exhibit fluorescence, with their lowest electronic states being the n,n* type. Benzene emits fluorescence in the n e a r - W region, and with an increase in the number of benzene rings present, emission moves t o longer wavelengths. The fluorescence intensities in these compounds vary from weak to strong (Table 2).

289

Table 2. Spectral locations of fluorescence intensity maxima h, in nm, together with the quantum fluorescence yields Q, of some aromatic hydrocarbon [211. Aromatic hydrocarbon Benzene" Naphthalene" Anthracene" Tetraceneb Phenanthrene" Chrysene" Pyrene"

h,(nm)

270 322 380 480 349 361 372

QF

0.07 0.23 0.32

----

0.13 0.14 0.32

Solvents used: "cyclohexane,bbenzene. The diphenylpolyenes, H,C,-(CH=CH),-C,H,, because of the conjugation between the phenyl groups on both ends, show fluorescence. This fluorescence shifts with n = 1 from the violet t o the red with n = 7 because of the increase of conjugation [23]. Substitution of H-atoms by alkyl groups generally does not affect the absorption spectra of aromatic hydrocarbons, or their fluorescence intensity. Electrondonating groups, such as -OH, -CH,O, -NH,, -NHR, -NR,, (where R = alkyl groups), often preserve or enhance fluorescence intensities and also cause a red shift. Halogen atoms of increasing atomic weights, as well as atoms of high atomic weights, show a sharp decrease in, or quenching of, fluorescence intensity due t o an increase in spin-orbit interactions. In parallel, one observes an increase in phosphorescence intensities due to the so-called heavy atom effect [16]. Also, due t o spin-orbit coupling, introduction of the nitro group will in most cases quench fluorescence [16,221. Sulfonic acid group provides water solubility; because this substituent does not participate in the resonance of aromatic compounds, it does not quench fluorescence.

3.3. Nitrogen-heteroaromatics Heteroaromatic molecules are five- and six-membered ring systems that contain one or more atoms other than carbon as a ring member. Substituting a C-atom with an N-atom in aromatic compounds often introduces nonbonding lone pair electrons n. Consequently, an electron may be excited from a n,n* orbital to an

290

n,n* orbital. These transitions in the singlet manifold possess weak intensity, and generally do not have a significant effect on the oscillator strength (0 of the n ,n* S-S absorption spectra of a compound. However, the presence of n,n* singlet states in heteroaromatic compounds is responsible for the difference between their luminescence (fluorescence and phosphorescence) behavior and that of the corresponding non-heteroaromatic hydrocarbon. In heteroaromatic molecules, the n,n* singlet state is very often of lowest energy. This leads to an increase in intersystem crossing rate constant (krsc).In this case only weak, and often, no fluorescence is observed and generally, one observes strong phosphorescence. If the n,n*state is of lower energy than that of the n,n* state, the molecule will exhibit fluorescence. Energy-wise, these two states will often be only slightly separated. Solvent interactions or substituents present may change the order and luminescence properties of these molecules, depending on which state is lower. There is a limited understanding of the luminescence properties of many heterocyclic molecules. 3.4. Other aromatic heterocyclics Aromatic carbonyl compounds such as ketones, aldehydes and carboxylic acids possess n,n* states as the lowest state and, often, show no fluorescence. Again, by introducing appropriate substituents, fluorescence may be observed if the lowest energy state is of the n,n*type. Solvent effects may also become important. Oxygen-containing heterocycles, including those containing carbonyl groups, generally do not exhibit fluorescence, except when they are fused to at least one aromatic ring; an example is presented in Figure 9. Aromatic molecules in which C-atoms have been substituted with atoms of higher atomic masses than oxygen are of no interest as laser dyes. Generally, this type of substitution results in increased spin-orbit coupling. Due to the heavy atom effect [121, a reduction or quenching of fluorescence and increasing phosphorescence yields are observed.

Benzene

a-Pyrone

Coumarin

Figure 9. Coumarin, an oxygen-containing heterocycle.

29 1

Significantly, some six- and five-membered heterocycles which do not exhibit any fluorescence, when combined chemically together into the same molecule, may possess lowest energy singlet 7c,7c* states and, therefore, exhibit fluorescence. These so-called quasi-aromatics systems are described in section 9.

3.5. Organic dyes Among the various classes of organic dyes known [24], the main spectroscopic characteristic of dyes is the presence of a strong, low energy S-S absorption band in the visible portion of the spectrum, which generates the impression of color. Among organic dyes that exhibit strong fluorescence, some also exhibit laser action. Consequently, the spectroscopy of dyes is of considerable importance to our subject. Soon after the first organic dyes were synthesized, theories were developed to relate dye structure to spectroscopic properties. These include the work of Witt, who proposed in 1876 that dyes are produced by combining a chromophore with an auxochromic group [251. This, and related early theories, have been replaced by the resonance theory, which has been further refined by the perturbational molecular orbital theory [26281. However, the classical interpretation of organic dyes by Witt, from the practical point of view, is still valuable for laser dye development. Auxochromic groups are colorless. In addition to the classical auxochromic groups, any atom or group that contains lone-pair-electrons that are in conjugation with a 7c electron system, can be classified as an auxochromic group. Chromophores originally referred to unsaturated groups like -CH=CH-, >C=CC=N-, -C=N, >C=O, -N=N-, a benzene ring, or combinations of these groups. Conjugation results in non-localized 7c electronic systems. For our purpose, aromatic hydrocarbons, nitrogen aromatics, and other heterocyclic (five and sixmembered) compounds are considered as chromophores. The chromophores may be themselves intrinsically colored already and some of them may also show strong fluorescence and, therefore, may also exhibit laser action. In summary, colored organic substances (dyes) are produced by the substitution of H-atom(s) in a chromophore by auxochromic group(s). 3.6. Effects of molecular structure on fluorescence Planar molecules possessing a high degree of symmetry exhibit near UV/visible absorption (S-S) bands that often exhibit well-resolved vibrational bands. Because in planar molecules the movement of 7c electrons is not restricted, strong fluorescence, i.e., a high QF value (Q, = 1) is often observed. This is contrary to other cases where the molecule is not planar or in which steric crowding is present. Due to steric crowding, resonance in molecules may be reduced or even lost. In thc latter case, each conjugated system may independently exhibit an absorption spectrum. It is well known that many organic compounds, including

292

dyes, may only show intense fluorescence when their molecular structure is rigid, planar, and devoid of steric crowding [21,29-331.

q-p

change

'CH;

Biphenyl

Fluorene

acyclic ether

cyclic ethers

change

'

z

0 N

~ to

& n

R

0

single butterfly

or

/

double butterfly

Figure 10. Methods for rendering atoms and groups planar t o the molecular plane An example is biphenyl, with Q, = 0.18, whereas the >CH, bridged fluorene which has a Q, = 0.80 in cyclohexane. Therefore, an often-used method for improving the Q, value of a compound is to restrict a molecule to planarity [341. This applies to planarity of the aromatic rings, as well as to substituents attached to aromatic rings; this is illustrated in Figure 10. 4. TRIPLET-TRIPLET ABSORPTION OF ORGANIC COMPOUNDS

In 1941, Lewis and coworkers observed a new transient absorption spectrum when fluorescein dissolved in boric acid was subjected to intense excitation [351. This spectrum disappeared when the excitation was terminated. The new absorption spectrum was correctly identified to present the spin-forbidden T-T absorption of organic compounds. Intense continuous lamps were used for excitation. However, these lamps generate rather low concentration of triplet state molecules (NJ. The entire field advanced dramatically when pulsed flashlamps were used for excitation (pumping). With flash photolysis, not only in the solid state, but also in gases and liquids, high-concentrations of triplet state molecules (NJ can be generated. [36,371.

293

4.1. Triplet-tripletabsorption in laser dyes Carmichael and Hug, in their review article on T-T absorption, list about 1140 compounds [381. For a considerable percentage of the listings, the &,(A) values are presented. Reviewing these experimental data on T-T absorption of organic compounds, including laser dyes, one finds some surprising trends. Experimentally, one observes the presence of at least one strong T-T absorption band to the long wavelength side of the lowest energy S-S absorption band in most organic compounds. Often, one or more T-T absorption bands of weaker intensity may also be present in this spectral region. The intensity of the strong T-T absorption bands e,(max) generally ranges from 1 to 10x104 Umol cm. This is comparable with the &,(max)values of the lowest energy S-S absorption bands at maximum intensity of organic dyes. In Figure 11, three representative cases (A, B, and C) depicting the spectral location of the one strong T-T absorption band in and near the FSR observed in most organic compounds are presented. For simplicity, we have omitted all the weaker T-T absorption bands. Triplet state losses (TSL) are determined mainly by the presence of the strongest T-T absorption band. Because case C has the lowest T-T absorption of the FSR for the three cases, one can expect to observe laser action from this case only if a high Q, value is also present. Indeed, from T-T absorption measurements on laser dyes, case C is the predominant case for most of presently available laser dyes [39-411. In laser dyes, one generally has: = 10-15 . &,(max)/&,(A,) 8,

(10)

is the S-S absorption (extinction) coefficient.

Dyes are characterized by their strong to very strong lowest energy S-S absorption bands in the visible portion of the spectrum, generating the impression of color. However, having T-T absorption over the FSR that is only about 10 to 15 times weaker than the strong to very strong lowest energy S-S absorption band, results in low laser action efficiencies. Consequently, laser dyes with lower T-T absorption over their FSR should exhibit higher efficiencies. 4.2. Vibronic spin-orbit interactions in heterocyclics Significantly, vibronic spin-orbit interactions present in heterocyclic molecules seem to reduce T-T absorption, and consequently present an important approach in obtaining improved laser dyes. Inspecting the molecular structures of laser dyes, Drexhage made the following observation: “In a dye where the electrons of the chromophore can make a loop when oscillating between the end groups, the triplet yield will be higher than in a related compound where this loop is blocked.” [3]. Blockage of the free movement

294

of electrons by heteroatomb) for C-atoms in the center ring of anthracene to obtain the rhodamine, oxazine, carbazine, and other related dyes seems to result in lower triplet yield and consequently, in higher laser-action efficiencies.

f FL

E

E t

FL

>

E

E t

FL

f

WAVE LENGTH h

-E

Figure 11. This graph depicts the different cases (A, B, C) of possible spectral locations of a strong triplet-triplet (T-T) absorption band relative to the near Whisible absorption (S-S) and fluorescence spectrum (FL), often found in organic compounds. Pavlopoulos [91 suggested that the substitution of carbon atoms by heteroatoms result in a considerable reduction of intensities of T-T transitions. This effect results from an efficient vibronic coupling mechanism between n,n* and n,n* triplet states in heteroatom molecules. To study this proposition, the singlet, &,(A), and triplet extinction coefficient, &,(A), of the lowest energy S-S and T-T transition of anthracene, and its heterocyclic derivatives acridine, phenazine, phenoxazine, and 2,2’-dipyridylamino-BF2(DPA-BF, complex) were measured. The &,(A) values were converted to E,(v) values, and the singlet and triplet oscillator strengths (f, and f,, respectively) were obtained by integration [9].After heteroatom(s1 substitution, the f, values stayed about constant for acridine, phenazine, and phenoxazine, and increased somewhat for the DPA-BF, complex. However, compared to anthracene, the f, values decreased considerably for acridine,

295

phenazine, phenoxazine, and for the DPA-BF, complex. This effect becomes even more pronounced when one follows the trend of the fdfTvalues. The ratio fJfT increases by a factor of about 6 from anthracene going to the DPA-BF, complex. Six different pathways for intersystem crossing and vibronic coupling are possible in N-containing heterocycles [201. Pathway E is the one that applies to laser dyes. This pathway is presented in Figure 12. Here, the lowest energy singlet state from which fluorescence originates is n,n*. There is vibronic coupling (mixing) between the n,n* and n,n* triplet states, with the n,n* absorption stealing intensity from the n,n* absorption. This, in turn, reduces the intensity (oscillation strength f,) of the n,n* T-T transition, and, consequently increases the n,n* absorption intensities present in different regions of the spectrum. This observation is of key importance to laser-dye technology, as it suggests the type of molecular structures that should exhibit low T-T absorption over the FSR.

Figure 12. Spin-orbit and vibrational coupling in nitrogen containing heterocycles (pathway E) Three representative cases (D, E, F) of spectral location of one T-T absorption band in and near the FSR in heterocyclic systems, where strong vibronic spin-orbit interactions, together with high-fluorescence intensities, are presented, are shown in Figure 13. Obviously, case F is the most desirable one, with only small T-T absorption present over the FSR. 4.3. Summary

Although benzene rings carry the spectroscopic property of fluorescence, they also seem to introduce excessive T-T absorption, as pictured in A, B, and C of Figure 11. Most of the laser dyes presented in Chapter 8 contain at least one "unsubstituted" benzene ring, with "unsubstituted" meaning that all six C-atoms are still present. The coumarin dyes have one and the rhodamine dyes have two

296

“unsubstituted” benzene rings present. This is also true for practically all the over 500 laser dyes listed by Maeda [5].

WAVE LENGTH h

-

Figure 13. Different cases (D, E, F) of possible spectral locations of a weak triplettriplet (T-T) absorption band relative to the near Whisible absorption (S-s) and fluorescence spectrum (FL), in organic compounds where strong vibrational spinorbit interactions seem to be present. Spin-orbit coupling has a very negative effect on laser action efficiency. Working against this detrimental effect to laser action efficiency: (i) the decay of triplet state molecules (MJ to the ground singlet state (So) leads to a reduction of their concentration (NT).This effect depends on the length of the triplet state lifetime ( ~ ~ (ii) 1 ; adding triplet state quenchers t o the dye solution may reduce T,, and therefore, further reduce N,; (iii) vibronic spin-orbit interactions in heterocyclic molecules reduce the intensities of T-T absorption bands, and therefore reduce the &,(A,) values. Consequently, efficient laser dyes should contain only five- and six-membered heterocyclics but no “unsubstituted” benzene ring(s).

297

5. SPECTROSCOPIC PARAMETERS THAT AFFECT LASER ACTION Among the very large number of organic compounds known, laser action is observed only from a very small fraction. Even from the small number known laser dyes, some of these dyes may exhibit laser action in a dye laser pumped by one excitation source, but not when pumped by another source. Therefore, one would like to relate the spectroscopic parameters of the laser dye t o spectroscopic parameters of the pump source. The main spectroscopic parameters of laser dyes are k,,, (or QF), &,(A& and 2,. Unfortunately, there is a very serious lack of data on all these parameters for most laser dyes. This makes it difficult and, in most cases, impossible to understand quantitatively most laser action properties, especially laser action threshold and efficiency. With the few data available, we will try, at least qualitatively, to connect spectroscopic parameters with dye laser performance. 5.1. The gain equation The tuning curve depicted in Figure 5 , besides the near Whisible (S-S) and T-T absorption and the fluorescence spectrum FL, shows the spectral region of laser action (L). Only where the gain in the laser medium is larger than the losses in the medium and in the cavity, will laser action occur. The derivation of the gain equation can be found in many publications and books, including articles by Peterson [42] and Thiel [43].In this section, the author will try to provide a qualitative understanding of this equation in order to provide an appreciation for the losses encountered in dye lasers. On exciting an organic compound with very intense light and neglecting photo decomposition, a good approximation is given by equation 11.

No = No’ + N, + N,,

(11)

with No*the depleted No concentration. Losses depend on four absorptiodloss mechanisms: (i) absorption of laser light from the onset of near Whisible (S-S) absorption, &,(A) No’(t),which overlaps the FSR. Laser action will start at diminishing S-S absorption; (ii) under very intense excitation (e.g., from pulsed lasers), losses may also result from excited singlet state absorption (S, + SJ, &,(A) N,(t); (iii) triplet state losses TSL = &,(A) N,(t), and (iv) cavity losses. From all four-loss mechanisms, TSLs are by far the most serious ones. In this article, our attention is focused on reducing &,(A,) values in laser dyes, to obtain improved performance. TSL, i.e., the amount of laser light absorbed by triplet state molecules (M,) is a product that depends on two parameters: (i) the number (concentration) N,(t) of triplet state molecules M, generated during excitation and

298

(ii) the value of their triplet state extinction coefficient (laser action spectral region L). TSL = N,

&,(A,).

&,(A,) over the FSR

(12)

5.2. The rate equation

To estimate TSL, one would like to estimate the concentration of triplet state molecules (N,) during excitation. Therefore one needs to derive the rate equation that describes the build-up and decay of N, molecules as a function of time (t) during excitation. The change of N,, with time (t), i.e., (dNJdt) will be proportional to: (i) the number (concentration) of molecules (N,) populating S, and (ii) the intersystem At the same time, the triplet state molecules & in T, crossing rate constant (kISc). will deactivate radiationless to the ground singlet state So with decay time (7,). Therefore, dNJdt will also depend on the number (concentration) of molecules (N,) in the triplet-state TI and will be inversely proportional to the triplet state lifetime (7,) of these molecules [441: dNJdt = N,(t) k,,, - N,(t)/T, .

(13)

To solve this differential equation for N,(t), it is necessary to know N,(t). N, will follow, i.e. be proportional to the shape of the excitation pulse, P(t). Here, we will consider only two important border cases. (a) Laser or flashlamp excitation with short pulses. Pulse rise time 6 > z, holds; in this

case one has flashlamp pulse rise times in the range of several tens of psec to several msec. For cw excitation, one has 6 = 03. With both excitation sources, the number (concentration) of triplet state molecules reaches equilibrium (N, = constant) and has N, = constant. This implies dNddt = 0; hence we obtain equations 15 and 16. NT

= Nl

kISC ‘T

(15)

9

TSL N, &,(A,)

= N, k,,,

&,(A,)

7,

. (16)

For efficient laser action to occur, kIsc,&,(A,), and z, must be kept small. This article suggests that organic compounds can be obtained where small &,(A,) values are present. Schafer has proposed approaches to improve the efficiency of laser dyes by reducing the triplet state lifetimes (7,) of laser dyes 1461. For Rhodamine and most other laser dyes and with dissolved oxygen from air present, z, will range from about 1 to 10 psec [47]. Oxygen is generally, but not always an efficient triplet state quencher. This subject will be further discussed in Section 7.5. The intersystem crossing constant (k,,,) is related to the Q, value of a compound by equation 17:

QF

k,

= kIC

(17)

+ klSC + kF

With lk, = z, the singlet state lifetime (generally in the 2 to 10 nsec range), and neglecting radiationless transitions, kIc ( = 0) and Q, = 1, for laser dyes, equation 18 is obtained.

k,,,

(1- Q&F

.

(18)

This equation is of key importance for understanding the operation of laser dyes. It explains why a high Q, value of an organic compound is of critical importance for its ability to exhibit laser action. A compound exhibiting a high Q, (Q, = 1) value implies the presence of a small k,,, rate. Because there is always some spinorbit coupling present, k,,, is always # 0 and, therefore, &,is always < 1.

6. EXCITATION SOURCES FOR DYE LASERS Dye lasers are optically pumped. To obtain laser action, one attempts to generate high populations (concentrations) of N, molecules in the first excited

300

singlet-state, S,. Excited singlet state molecules have short fluorescence lifetimes (7,))in the range of 2 to 10 nsec. To overcome losses, one must employ very intense excitation sources. Pulsed lasers and pulsed flashlamps, together with cw lasers, conveniently provide the required high excitation intensities. As pump sources for dye lasers, four distinct excitation sources are used: (i) pulsed lasers emitting fast rise-time pulses 6 (6 in the 5 t o 20 nsec range); (ii) pulsed flashlamps with fast pulse rise time (6 in the 50-nsec to few psec range); (iii) pulsed flashlamps with slow-pulse rise time (6 from several tenths of psec to several msec) and (iv) cw lasers. We will discuss (iii) and (iv) together, because with these pump sources the triplet state concentrations (N,) reach equilibrium values. As pump sources with future potential, we also discuss conventional light sources and diode laser arrays (DLAs). 6.1. Pulsed lasers Because of the short pulse duration of these lasers, generally in the nsec range, equation 17 holds well, and laser action is routinely observed from many dyes with: (i) high Q, values, even if their &,(A,) values are not too small, or (ii) not too high Q, values, but small &,(A,) values, and (iii) all cases between (i) and (ii). However, for dyes with about equal Q, values, the dye with lower &,(A,) values will lase more efficiently. Nitrogen lasers (337 nm) are among the most frequently used excitation source for dye lasers. The smaller models are inexpensive. However, excimer, Q-switched and frequency-doubled, tripled, and quadrupled Nd:YAG, and copper-vapor lasers are also used for excitation. Experimentally, to obtain maximum absorption of the pump light, one tries to match the pump laser light wavelength with the wavelength of maximum S-S absorption of the dye. Strong absorption of the exciting pump light leads to higher outputs of laser light. Upscaling of dye lasers pumped by pulsed lasers is only restricted by the size of the pump laser. Large pulsed lasers are expensive and also have upscaling limitations.

6.2. Flashlamp excitation Flashlamps with pulse lengths (p) of 50 nsec to several psec are widely used to pump dye lasers. Flashlamp pumped dye lasers are rather simple devices. The flashlamps, together with their driving power supplies, are relatively inexpensive. The flashlamp emission spectrum consists of two parts: a black body radiation curve, emitted by the flashlamp plasma, and imposed on it is the pressure broadened line spectrum of the fill gas.

301

Two types of flashlamps are mainly used for dye laser excitation, i.e., coaxial and linear flashlamps. Coaxial flashlamps consist of two cylindrical glass tubes. The dye solution flows through the inner tube. The outer tube, which is terminated by two ring electrodes, forms the actual flashlamp. This provides a very close coupling between the flashlamp and the dye solution, which also cools the flashlamp, eliminating the pump cavity. Coaxial flashlamp-pumped dye lasers have small repetition rates, generally below 1 Hz. Considerable time is needed until the thermal gradients produced by the hot walls of the flashlamp are dissipated. Linear flashlamps do not have this problem. Often, they can be operated up to about 50 Hz, producing higher average power outputs. Because of this attribute, our attention is focused on employing linear flashlamps for pumping large dye lasers. When pumping dye lasers with flashlamps of short pulse rise time, 8 (50 t o 100 nsec), and the accumulation of dye molecules in their triplet state can be neglected and equation 14 should hold well. However, when pumping with flashlamps having a relatively long pulse rise time (100 nsec to several psec), the value of Z, becomes important and equation 14 no longer applies. Dye lasers pumped with flashlamps have distinct advantages over dye lasers pumped with pulsed lasers. The wide wavelength range emitted by flashlamps (from about 200 nm to the near-IR) allows one t o pump multiple laser dyes in the same dye laser system. In this way, the entire spectral range over which dyes exhibit laser action can be covered, namely between about 340 to 1,300 nm. However, dye lasers pumped with flashlamps have a number of limitations. To obtain the required short light pulses (pulse lengths from 50 nsec to several psec), the flashlamps must be driven with high driving voltages, generally in the 20 kV to 25 kV range. These high voltages generate high color temperatures, in the 20,000 OK to 25,000 OK range. The maximum spectral emission intensity of the flashlamp plasma is in the 150 nm range. All radiation of wavelengths smaller than 200 nm is efficiently absorbed by the flashlamp quartz envelope. Strong radiation between 200 and 300 nm contributes t o the photochemical process, reducing laser-dye lifetimes. For dye lasers operating in the yellow to red portion of the spectrum, flashlamp radiation between 200 and 400 nm is often poorly absorbed. Further, the high plasma temperature also generates high gas pressures in the flash lamp. To reduce flashlamp failure (explosion), these devices require sturdy construction by using thick wall tubing, thus necessitating the use of small-size flashlamps having a top energy output limited to about 1000 Joules per pulse. At a 10-Hz repetition rate and using Rhodamine 6G as laser dye, output energies are up to about 10 Joules per pulse, resulting in average power outputs in the 100 W range.

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6.3. Long-pulse flashlamp and cw laser excitation Flashlamps emitting slow rise-time pulses (several tens of psec to several msec) have several important advantages as pump sources for dye lasers. They require driving voltages only in the 6 kV to 10 kV range and the required high-voltage power supplies are of simple construction. These flashlamps operate at lower color temperatures, generating lower gas pressures. Therefore, larger flashlamps with thinner walls can be used, emitting light pulses in the 10 and higher kJoule range. Because of lower color temperatures, the emission intensity maximum of the plasma are now located in the near-UV/blue/green spectral region. This is the spectral region where most laser dyes absorb radiation most efficiently. The reduced radiation intensity in the short-UV region should result in better photostability of laser dyes. Rhodamine 6G is among the few laser dyes that can be pumped by large flashlamps, having slow pulse rise times [48]. Using a 5x103 Umol cm solution of cyclooctatetraene as a triplet state quencher, laser action was observed from a dye laser pumped with a flashlamp emitting 600-psec long-pulses from a 5x105 Umol cm solution of Rhodamine 6G in ethanol. Efficiency was below 0.1 percent. Here, low efficiencies are to be expected because cyclooctatetraene starts absorbing light just below 400 nm [491. The use of improved laser dyes avoids the use of cyclooctatetraene and, consequently, higher efficiency should be obtained. Most cw dye lasers are pumped either by cw argon ion or krypton ion lasers. Depending on the absorption spectrum of the laser dye employed, multi-lines are often used for excitation. Single laser lines and multi-lines available to pump dye lasers are listed in Table 1. 6.4. Other potential excitation sources Besides the main four pump sources discussed above, there are two more potential pump sources for dye lasers. (a). Incoherent light sources. Using a high-pressure arc lamp, cw laser action from Rhodamine 6G was observed by Thiel et al. [47]. As expected, efficiency is very low. Possibly, using improved laser dyes as the active media, this approach could considerably reduce the overall cost of generating cw visible laser light. It would be the most inexpensive method for producing visible cw laser light. This is important, especially with the high prices for cw argon ion or krypton ion laser units.

(b) Diode laser arrays as pump sources for dye lasers. Instead of pumping solidstate lasers with surface-emitting (2-D) DLAs, there seem to be more advantages when using them to pump dye lasers. Such an arrangement should have an almost unlimited upscaling potential to very-high-average power outputs. Contrary to pumping solid-state media, overheating of the liquid media can be avoided by circulating the dye solution. Unfortunately, presently available near-IR laser dyes

303

have low efficiencies and in this respect, there seems to be a need for improved laser dyes. 7. SPECTROSCOPIC TEST EQUIPMENT It has been shown that organic laser dyes must possess some very specific spectroscopic properties to exhibit laser action. Therefore, to conduct a more thorough evaluation on laser dye development, it would necessary t o measure all three important spectroscopic parameters, i.e., kIsc,&,(A,), and 7,. To measure each parameter, this generally requires separate experimental setups. For a limited laser-dye test effort, this section offers suggestions on the minimal test equipment required. This is followed by a discussion of equipment for a more comprehensive laser dye improvement effort. Obtaining experimental data on all three spectroscopic parameters poses considerable experimental difficulties. Therefore, first we will propose the minimal equipment needed and present examples of experimental methods used to measure Q,, E ~ and , z, values. Whether all these spectroscopic parameters can be obtained or not, one would like to perform actual laser action experiments on dyes, preferably using a flashlamp-pumped dye laser. Employing a laser-dye jet stream, Thiel [43,521 has developed a method that allows the measurement of all these parameters with only one experimental setup. Because this method requires a considerable investment in equipment and personnel to operate the equipment, this method will be briefly sketched at the end of this section. 7.1. Minimum equipment required (a) Testing for fluorescence. For a compound to exhibit laser action, it must exhibit strong fluorescence. The determination as to whether the dissolved compound emits no, weak, moderate, or strong fluorescence is simply obtained by using one of the hand-held, commercially available mercury W lamps. Because fluorescence intensity is often solvent-dependent, different solvents are used, starting with ethanol, methanol, 1,4-dioxane, etc. In these solvents, most laser dyes exhibit strong fluorescence, together with high a degree of photostability. To obtain a rough estimate on fluorescence intensity of a new laser dye, visual comparison is used. The sample’s intensity of fluorescence is compared with the fluorescence intensity of a laser dye emitting fluorescence in the same spectral region. All laser dyes have Q, values in the 0.80 to 0.98 range. With a collection of about 8 t o 10 of these laser dyes, one can span the visible spectral region. If the potential laser dye exhibits strong fluorescence, its near-Whisible absorption spectrum is recorded. In most cases, dyes will exhibit a strong, lowest energy S-S absorption band. The presence of a very weak absorption a t the longer

304

wavelengths than the strong S-S absorption band may indicate the presence of an impurity. (b) Testing for laser action. This should be the next step, preferably using a flashlamp pumped dye laser. The tests is carried out in the static mode, i.e., the dye solution circulation system has been disconnected. This has several advantages: (i) it allows the use of a low volume of dye solution, requiring only small amounts of dye; (ii) the cleaning of the dye laser system is much simplified, and (iii) dye photostability is easily determined. If laser action is observed from a flashlamp-pumped dye laser, an efficient laser dye has been identified. Only dyes with low &,(A,) values will show laser action from flashlamp pumped dye lasers. Changing solvents and dye concentration will maximize laser output power. To judge the performance of a new laser dye, comparison is made between t h e power output with the power output of the most efficient laser dye presently available that exhibits laser action in the same spectral region. For dye testing, the author used a small dye laser pumped by a 5-cm-long flashlamp, which delivers pulses up to 15 Joules/pulse of about 200-nsec rise time, which are about 600 nsec long. An elliptic configuration was used to focus the flashlamp light on the dye cell. The dye cell was 5 cm long and had 3 mm inner bore diameter. Only small amounts of dye solution are required for testing. The dye laser sits directly on top of the power supply housing to reduce inductance. By loosening the screws on the flashlamp that connects it to the high voltage terminal, the dye laser can be removed for cleaning and refilling. The power supply delivers up to 15 kV. The dye laser can be used with three pairs of mirrors, which cover the near-UVlviolet, blue/red, and rednear-IR portions of the spectrum. (c) Fluorescence spectra of organic compounds. It is desirable to be able to measure the fluorescence spectra of organic compounds. Spectrophotometers are commercially available. A typical experimental arrangement is shown in Figure 14. The excitation lamp EXL and the monochromator form an angle of go", with the sample (a cuvette) S positioned at the corner. This arrangement reduces unabsorbed excitation light to enter the monochromator. A stable excitation lamp, i.e., a small high-pressure mercury lamp together with a filter F, is employed for excitation. Filter F, has transmission for a specific mercury line. A small high-pressure xenon lamp can also be employed for excitation. A monochromator provides excitation light of appropriate wavelengths.

305 FI n

s

EXL

RECORDER

t

1 “YN 1 AMPLIFIER

-a

,

s

L

e

C

H

MONOCHROMATOR

Figure 14. Experimental arrangement for measuring fluorescence spectra of organic compounds. The lens, L,, collects the fluorescence light, which is focused on the monochromator slit (SL). Positioned in front of the monochromator slit is a filter (FJ, which transmits fluorescence, but absorbs stray excitation light. A photomultiplier (PM) is used as detector. A chopper (CH), in front of the monochromator slit (SL), together with phase sensitive detection allows time averaging over noise. 7.2. Quantum fluorescence yields Experimental difficulties are encountered obtaining accurate absolute Q, values. Measurement errors are generally in the k2 percent range. In all our equations, k,,, is the spectroscopic parameter that is of primary interest. According t o equation 18, the presence of a high Q, (= 1) value of a compound implies the presence of a small k,,, value. However, equation 18 is only of limited use for obtaining accurate values. For example, a Q, value of 0.98 f0.02 yields a meaningless k,,, value. Nevertheless, it would be helpful to have some rough values for Q,. Measuring a Q, of 80 percent suggests that either by rendering the molecule more rigid, or choosing a closely related derivative of the compound, higher Q, values may be exhibited. For a compound exhibiting a Q, value of 95 percent, it may be difficult to further improve its Q, value.

306

To obtain absolute Q, values, it is necessary to measure the number of light quanta emitted as fluorescence light (N,), over the number of quanta (N,) absorbed by the compound. There are several experimental methods available to measure absolute quantum yields. Among them are calorimetric determination [53], photon counting, magnesium oxide as standard, integrating spheres, and thermal blooming [54,551. Methods to measure Q, values, both absolute and relative, have been discussed in a review by Demas and Crosby [56], which also outlines the advantages and disadvantages of the different experimental methods. Because of the experimental difficulties associated with attempting to obtain absolute Q, values, the measurement of relative Q, values seems more appropriate. Several experimental methods are available to measure relative Q, values. These methods involve replacing the standard scatterer with a substitute reference of known Q, value. Comparing the fluorescence intensity of a test compound to a known Q, value (Lee,a quantum yield standard) is experimentally less difficult. Measurement accuracy is generally in the S p e r c e n t range. Data for members of quantum yield standards have been published 1561. 7.3. Triplet extinction coefficients Because T-T absorption over the FSR critically affects laser action efficiency, it is necessary to have equipment available to obtain conveniently and accurately small &,(A,) values of laser dyes. Measuring small &,(A) values presents two experimental challenges. Firstly, it is necessary to generate a sufficiently large concentration of N, molecules, and secondly, to measure that concentration. For laser dyes, due to their high Q, values, only small N, concentrations, as well as small &,(A,) values, are present. Historically, the most widely used method to obtain &&A) values has been the depletion method. Basically, the experimental setup is outlined in Figure 4 with the laser output mirror and the grating removed. A beam of continuous white light passes through the cell. It is focused on the slit of a monochromator, and the intensity of the monitoring light beam is measured with a photomultiplier/oscilloscope combination coupled to a camera. A high concentration of N, molecules is generated in the cell, from discharging the flashlamp. That causes absorption of the continuous light beam. In good approximation, Equation 19 holds.

No = Nd + N,.

(19)

No’ the depleted No concentration is measured at the intensity absorption maximum, A., Negligible T-T absorption at h, is assumed. Because No is known and N,’ has been measured, N, can be obtained by measuring triplet optical densities, OD,(A); see Equation 20:

307

OD,(A) = N, &,(A) d,

(20)

and one obtains &,(A) values, where d is the thickness of the sample. However as depicted in Figure 11, for the majority of organic compounds, one rarely has negligibility of T-T absorption present in the spectral region of strong S-S absorption. Consequently, &,(A) values obtained by the depletion method are generally not very accurate. Among the seven different methods listed by Carmichael and Hug [38] to obtain &,(A) values, the so-called intensity variation method proposed by McClure [57], together with cw laser excitation seem to meet the requirements of simplicity and accuracy [58-61]. If the equipment to measure fluorescence spectra has been assembled as shown in Figure 14, some key parts of equipment needed to measure &,(A) values are available. Other parts of the equipment needed are a liquid-nitrogen quartz Dewar and medium size (one meter) cw argon ion or krypton ion laser. Their laser lines available for excitation are listed in Table 1. The near-UV/visible (325 and 441.5 nm) laser lines from a cw cadmium/ helium can also be used for excitation. The cw laser light is focused with the aid of lens L, on the surface of sample S, containing the glassy solution, generating a high concentration of triplet state molecules (N,), The cw laser should be operated in its TEM,, mode. The intensity profile of laser lines in their TEM,, can be expressed by a Gaussian curve [621, Equation 21: I,(x,y,z) =:I exp ( - r2/a:), (21) With aAthe radius at which the intensity has decreased to l/e of its original value. The experimental arrangement is shown in Figure 15. The sample (S) containing the dye solution is submerged in liquid nitrogen contained in the liquid-nitrogen Dewar flask (LND). The dye is dissolved in a glassy solvent, forming a solid glass. In the solid state, triplet-state life times (7,) are generally much higher compared to those concentrations present in liquid solvents, resulting in increased concentration of triplet state molecules (N,). In our experiments, the sample is the quartz cell (S),d = 2 mm thick and about 10 mm in diameter. The filter (F,) has maximum transmission for the cw laser line used for excitation. This reduces radiation emitted from the plasma tube in the laser from entering the monochromator, possibly saturating the photomultiplier. To vary cw laser excitation intensities I,,, the cw laser pump current is varied. However, excessively reducing the cw laser pump current may lead in a change of the mode-structure of the cw excitation laser light, causing measurement errors.

308

Therefore, the filter (FJ, a neutral density filter, is used t o reduce the intensity (I_)of the cw laser light.

LND

Figure 15. Experimental arrangement to measure triplet-triplet (T-T) absorption spectra of organic compounds. For excitation, a cw laser is employed. With the aid of the lens (L4),the small excitation area is focused into a small hole (H) on the dovetail plate, which is covering the monochromator slit (SL). Light from a monitoring light source (MLS), tungsten or a high-pressure small xenon lamp is focused on a pinhole (PH). After being reflected by the mirror (M), the light passes through the small excitation area, undergoing strong attenuation due to the high concentration N, of triplet state molecules. Through the small hole in the dovetail plate in front of the monochromator slit (SL), some unabsorbed cw laser light will pass; to reduce its intensity, a beam stop (BS) is pasted on the lens To record the intensities I, and I, only, a chopper (CH) is used, together with phase-sensitive detection equipment (a lock-in amplifier) and a strip-chart recorder. This allows one to discriminate unabsorbed cw laser excitation light, fluorescence and phosphorescence emitted from the dissolved dye. Only light intensities I, and I, originating from MLS, are recorded. A power meter (PM) measures cw laser light intensities Iex. Triplet optical densities (OD,) can then be recorded, and these are given by Equation 22:

OD&)

log IL/Io= &,(A) N, d.

(22)

309

in which I, is the intensity of the monitoring light when the cw laser is off (no triplet state molecules are generated), and I, is the intensity with the cw laser on, generating triplet state molecules of concentration N,. Focusing cw laser light on the sample (S), even at low intensities, generates heat. Schlieren and some time small bubbles in the liquid nitrogen are generated, resulting in undesired noise. Placing a magnetic stirring bar into the liquid nitrogen below the sample and stirring it with a magnetic stirrer will generate a large amount of bubbles. The rod that holds the cell has two small holes above the sample. Bubbling gaseous helium through the rod into the liquid nitrogen will quickly eliminate all bubbles, generating a liquid nitrogen environment of high optical quality.

One obtains &,(A) values with the aid of equation 23, which has been derived by McClure [57]. He derived his equation by considering all the activation and deactivation rates of organic molecules in the singlet and triplet manifolds. Ai contains all the rate constants and only relative excitation cw laser power (Lx) measurements need to be recorded. Experimentally, t o obtain E, values, one simply varies the cw laser excitation intensity Iex, while recording OD, values. A plot of l/OD, versus 1/Iex yields a straight line. Extrapolation to the intersection with the ordinate (i.e., l / I e x + 0, or I ~ x+ -) yields l/OD,". At infinite excitation intensity (Iex), all singlet state molecules of original concentration (No)would have been converted into triplet state molecules of concentration, N, (N, = No).The &,(A) values are calculated from the relation:

OD,(A)- = &,(A) N,(77 OK) d.

(24)

Because the measurements were performed at liquid nitrogen temperature (77 OK), there is a volume contraction. The concentration of the dissolved compound N,(77 OK) is higher than N,(RT), the concentration at room temperature (RT). To obtain No(77 OK), it is necessary t o know the contraction factor (p) of the glassy solvent employed: No(77 OK) = p N,(RT).

(25)

The contraction factor is obtained using the following procedure: A 10 cm-long quartz test tube, with an inside diameter of about 10 mm and an outside diameter of about 12 mm is used. The tube has a mark, made with a file, about 1 cm below

310

its top. Weighing the tube empty and filling it up with water to the prescribed level yields the volume (V,). Filling the tube with the glassy solvent up to the mark and weighing yields W,. The tube with the solvent is submerged in liquid nitrogen until frozen. The tube is quickly removed from the liquid nitrogen and filled with water up to the mark and weighed, yielding W,. W, - W, = V, is the reduction of volume due t o contraction and p is obtained from Equation 26: p = (V, - V,) I

v,.

(26) The l/OD,(A)" value is obtained from extrapolation, N,(RT) and the sample thickness d are known, &,(A) can be obtained. In carrying out these experiments, one has to guard against saturation, which will lead t o false &,(A) values. Although a compound might be completely dissolved a t room temperatures, a t liquid nitrogen temperatures the solution may become saturated. However, the saturation concentration is unknown. The procedure to use for guarding against saturation has been described [631. 7.4. Triplet photo-selection spectroscopy To assist laser dye synthesis it is very helpful to have available polarization data on electronic transitions (S-S and T-T) in the chromophore. Although this section may appear somewhat complicated, pertinent polarization experimental data on S-S and T-T transitions are routinely obtained by simply inserting an analyzer (polarizer) A into the light path of the monitoring light beam. This is depicted in Figure 15. Without any additional effort, important polarization data on S-S relative to T-T absorption bands are easily obtained. This method is called photo-selection spectroscopy [64,651. The analyzer is used in two fixed positions. By rotating it go", it becomes parallel or perpendicular t o the polarization of the cw laser beam, and measurement of the two triplet optical densities OD,' and OD,' can be made. OD," and OD,' are the triplet optical densities with the analyzer position parallel or perpendicular to the polarization of the exciting cw laser beam. The degree of polarization (PI is defined by Equation 27:

OD;' P =

OD;'

+

OD,'

-

OD,'

2 cos2w - 1

-

cos2yJ + 3

(27)

in which y is the angle between the absorbing electronic singlet and triplet dipoles (transition moments).

311

On exciting into an S-S absorption band of a symmetric molecule consisting of only one transition, and studying a T-T (or fluorescence, or phosphorescence) band consisting of one transition, one should observe polarization values (P);P = + 0.5, or - 0.33 if the transition is aligned parallel (w= 0') or perpendicular (w= 90') relative to the absorbing S-S transition. However, one generally observes P values, which approach either +0.5 or -0.33 values. P = 0 is observed, if: (i) two overlapping and about equally strong, but differently polarized, T-T transitions are present, or (ii) the S-S absorption band consists of two differently polarized, but about equally strong, S-S transitions. 7.5. Measuring triplet state life times The ability to measure T~ values of laser dyes in solution is desirable. There is a serious deficiency of data on this important spectroscopic parameter. Oxygen from air usually, but not always, acts as an efficient triplet state quencher. The presence of oxygen is sometimes, but not always desirable, because oxygen may be involved in photochemical reactions. Collisions between excited state molecules and solvent molecules are in many cases sufficient t o result in small T~ values in the psec range. Fletcher et al. performed stability tests on 25 oxazole dyes using flashlamp excitation, with or without oxygen present [661. In these experiments, the best lasing conditions were observed in some cases in the absence of oxygen. Under air, good laser-dye lifetimes were obtained for solvents such as ethanol and water mixtures. Basically, to perform triplet state lifetime measurements on organic compounds, including laser dyes, the following method is used. A monitoring light beam, which also could be a cw dye laser, after passing through the cell containing the sample, is focused on a photo-detector connected to a fast oscilloscopelcamera combination. A short pulse from an intense excitation source (pulsed laser or flashlamp), emitting radiation that is strongly absorbed by the compound t o be studied, generates large concentrations of N, molecules. Passing through the small excitation region, the monitoring light beam will experience attenuation from the triplet state molecules generated. The decay curve of the excited (triplet) state molecules is monitored and a logarithmic plot of the monitoring light or cw laser beams recovery time then yields 2, values. Originally, as excitation sources, short light pulses from flashlamps having a pulse-duration of several psec were used. Over the last 20 years, flashlamps have been replaced with pulsed lasers, generally emitting short pulses in the range of 5 to 10 nsec. Depending on the spectral location of the absorption band, either the lines from a nitrogen (337 nm), a frequency tripled (353 nm), or doubled (532 nm) and Q-switched Nd:YAG, and halogedrare gas (excimer) laser are used for excitation; wavelengths of excimer lasers are listed in Section 2.2.

312

More specific experimental information on measurements has been described by Demas 1671.

excited

state

lifetime

7.6. Measuring spectroscopic parameters with Thiel’sjet stream method Thiel developed a new method that allows &,(A), klsc,and 2, values with only one experimental setup to be obtained [43,521.In addition, these values on the triplet state lifetimes (TJ of laser dyes are obtained in the liquid phase, and different solvents can be used. Therefore, this method also allows one t o study the effect of triplet state quenchers (e.g., oxygen) on the intersystem crossing rate (kIsc). The working of this method is best explained by viewing the experimental layout, depicted in Figure 16. A dye-solution jet stream is generated with the aid of a cw dye-laser nozzle. A cw laser beam (E) from an argon ion or krypton ion laser is used for excitation and is focused on the jet stream. This beam is chopped; it is generating excited state molecules of concentration N, and N,, which are swept downstream in the direction (x) with speed (v,). The distribution of these excited molecules occurs inside the JET (see *).

MONOCHROMATOR

PHOTODIODE

77 AMPLIFIER

RECORDER

Figure 16. Equipment used for measuring all three spectroscopic parameters, kISC, E ~ and , 2, with one experimental setup.

313

Below the jet stream, a t a distance of (xJ, a probing laser beam (MI, from a cw dye laser is focused on the jet stream, probing the excited state molecules generated by the laser (E). The cw laser beam (E) used for excitation will generate a concentration distribution of N, following the intensity profile of Equation 21, and N, will be proportional to .:I In this equation, because the dye jet steam is moving in the direction of x, one must insert v, t for x in the expression, r2 = x2 + y2.This provides the distribution for N,(t,x,y) and N,(t,x,y). The differential equations for N, are provided in ref. 47. For solving N,, equation 14 is used. The resulting rather complex differential equations are solved and also provided in ref. 43. Phase-sensitive detection is used and, therefore, very small concentrations of N, and N, can be easily detected. Because the distance (x,) between the cw laser M and E can be varied, the decay of excited state molecules is easily followed, allowing one to obtain z, and zT Using the solutions of the differential equations, together with the absorption data on the compounds studied, one can extract klSC,and &,(A), allowing calculations of the concentrations N, and N,. Together with the obtained value of z, the laser action efficiency and threshold of laser dyes can be calculated.

8 LASERDYES 8.1.Overview In this section the author first discusses approaches to identify and develop new laser dyes from molecular engineering and spectroscopic studies. An overview on selected laser dyes, together with their molecular structures and some of their laser and spectroscopic properties is also provided. In the overview on laser dyes, dyes found either by trial and error or by molecular engineering are presented. After laser action was observed from chloro-aluminum-phthalocyanine (Figure 17) by Sorokin and Lankard [l]and from 3,3’-diethylthiatricarbocyanine(Figure 18) by Schmidt and Schafer [2], practically all organic compounds exhibiting fluorescence, which were either obtainable from chemical supply houses or found on laboratory shelves, were tested for laser action. Either flashlamps or short pulse lasers were used for excitation. Among the tens of thousands of organic compounds that exhibit strong fluorescence, only about 500 exhibit laser action [3-51. Many of the first compounds found to exhibit laser action were organic dyes. Although other groups of organic compounds that were not dyes were identified, all compounds that emit coherent radiation are now called laser dyes. Presently, about 120 laser dyes are commercially available; about half of them were discovered by trial and error and the rest are closely related derivatives of them.

314

Figure 17.

Chloro-aluminum-phthalocyanine

Figure 18.

3,3’-Diethylthiatricarbocyanineiodide

8.2. New laser dyes from molecular engineering Drexhage appears to be the first researcher who attempted to obtain new laser dyes by molecular engineering [32],The approach involved synthesizing a compound that was very close structurally to a known laser dye. The goal is to retain all the desired laser action properties from the prototype laser dye, while hopefully adding additional new and desirable characteristics. These target properties are: (9 laser action in a different spectral region; (ii) higher laser action efficiency; (iii) better photochemical stability, andor (iv) better solubility or solubility in a more desirable solvent that is either less toxic, less flammable, or water-based. Water solubility of laser dyes, and good photostability, are very desirable properties in laser dyes. Water is readily available, does not present a fire hazard, and has very desirable thermo-optical properties. In contrast to most organic solvents, its refractive index changes little with temperature, especially around the freezing point. In summary, aqueous dye solutions maintain good laser beam qualities. Unfortunately, water often quenches fluorescence and often increases photodecomposition. Basically, there are several different approaches generating new laser dyes by molecular engineering: (i) following the classical definition of organic dyes, the

315

same chromophore, is used, but changes in the auxochromic group(s) are made; (ii) this method involves replacing one or more C-atom(s) in the chromophore by a heteroatomb). For example, coumarin is replaced with azacoumarin (Figure 19); (iii) enhancing molecular rigidity of the molecule may improve its Q, value, resulting in higher laser action efficiency. Methods are described in Section 3.6, Figure 10, and (iv) the photostability of dyes (e.g., coumarin) may be improved by switching from a -CH, group to a the -CF, group [681.

Figure 19. Coumarin (left) and Aza-coumarin (right).

8.3. New laser dyes from spectroscopic studies We have pointed out earlier that a rather large percentage of laser dyes could be viewed as dyes in the classical sense, consisting of a chromophore in which one or two aromatic H-atoms have been substituted by auxochromic groups. However, there exists a very large group of chromophores, in which auxochromic group substitutions have not been effected. This raises an important question: is it possible from spectroscopic studies on chromophores t o predict that, after hypothetical auxochromic group substitution, the new compound will, or will not, have low T-T absorption over its FSR? Spectroscopic studies require the measurement of spectral locations, intensities, and polarization of low energy S-S and T-T transitions (absorption bands) in the chromophores. There are only a few spectral locations of S-S relative to T-T absorption bands, together with their polarization (so-called constellations), where, after substitution with interacting groups, the resulting compounds will exhibit low T-T absorption over its FSR 139,691. Experimentally, one simply measures the spectral location(s) of T-T absorption spectra (OD, values) and their polarization (degree of polarization P) relative t o S-S transitions in a chromophore, employing triplet photo-selection spectroscopy using the equipment depicted in Figure 15. In Section 2.1 on the polarization of electronic transitions, the effect of substitution(s), which increase the length of the electronic oscillator, was discussed. This is accomplished by groups that either increase conjugation or are electron donating. All these observations are summarized as follows:

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In a molecule (e.g., a chromophore), electronic transitions that give rise to SS and T-T absorption, fluorescence and phosphorescence bands, have their electronic dipoles positioned in different directions within a molecule. In a symmetric molecule, all these transitions are generally either positioned (polarized) along the short or long molecular axis. Substitution(s) of H-atoms in a chromophore in the direction (or near the direction) of S-S and T-T transitions by groups which either increase conjugation or are electron donating, will lengthen the S-S and T-T dipoles; a red shift of these transitions will be observed. For example, long- or near long-axis substitution(s1 will red shift all S-S and T-T transition polarized along the long molecular axis. All short-axis S-S and T-T transitions will generally not be red-shifted much. The same applies to short-axis substitution(s), which will not significantly affect longaxis polarized transitions. Therefore, knowing the polarization and spectral location of low energy S-S and T-T transitions (bands) within a chromophore, the amount of T-T absorption present over the FSR, can be estimated to a certain degree. (a) Constellation A(1). In the chromophore, the lowest energy S-S and T-T absorption bands are positioned a t a considerable distance from each other, as depicted in Figure 11 (case C) or Figure 13 (case F). Both the S-S and T-T absorption bands (transitions) are positioned parallel to each other and a positive degree of polarization (P) is observed. Substitution in the direction of the transition moments will move both the S-S and T-T absorption bands, as well as the fluorescence band, to longer wavelength. Here, the amount of T-T absorption present in the FSR of the chromophore will be about the same as in the substituted compound. (b) Constellation A(I1). In the chromophore, the lowest S-S and T-T absorption bands are positioned as depicted in Figure 11(case B) or Figure 13 (case E).The SS and T-T transitions are positioned perpendicular to each other and a negative degree of polarization (P) is observed. With substitution in the direction of the T-T transition, the T-T absorption band moves further to the red portion of the spectrum, producing case C as depicted in Figure 11, or case F as depicted in Figure 13.

(c) Constellation A(II1). In the chromophore, the lowest energy S-S and T-T absorption bands are positioned as depicted in Figure 13 (case D) with the T-T absorption band rather sharp and narrow. The S-S and the T-T absorption bands are polarized differently (positioned perpendicular to each other) and a negative degree of polarization (P), is observed. Substitution is in the direction of the S-S transition will red-shifting it over the unaffected T-T absorption band. The fluorescence then falls into a spectral region with no T-T absorption present. Although we have not discussed this possibility, there seems to be no theoretical

317

reasons t o exclude such a spectral arrangement of electronic transitions where no T-T absorption is present at the long wavelength side of the lowest energy S-S absorption band. From various evaluations of substitutions in the 7-position of coumarin, approximate red shifts in the singlet and triplet manifolds are listed in Table 1 of ref. 70. 8.4. Oligophenylenes, oxazoles, and benzoxazole Oligophenylenes such as para-terphenyl (Figure 20) and para-quaterphenyl (Figure 21)) and their derivatives, are widely used laser dyes for the near W to violet portion of the spectrum. All of these compounds can be characterized case C in Figure 11.

Figure 20. para-Terphenyl.

Figure 21. para-Quaterphenyl. The S-S, T-T absorption, and fluorescence FL spectra of para-terphenyl are depicted in Figure 22 [21,39,71]. A strong T-T absorption band is present in the red portion of the spectrum. Considerable T-T absorption is overlapping the FSR. Because these compounds exhibit high Q, values [22], laser action is obtained easily with n e a r - W pulsed lasers. However, flashlamps with short-pulse rise times in the 50-nsec range have t o be used for excitation to obtain laser action at about 340 nm, for para-terphenyl and at about 374 nm for para-quaterphenyl [46,721. The use of these compounds is somewhat limited due to their poor solubility in most solvents. Performing spectroscopic studies on para-terphenyl and para-quaterphenyl, Pavlopoulos and Hammond observed a positive degree of polarization, suggesting a constellation type A(1) is present 1391. Auxochromomic group substitutions (-NH, and -N(C,H,),) were effected on both molecules in the para-position and the resulting derivatives exhibited laser action under nitrogen laser excitation, but with poor photostability.

318

tt

p

1

io.1

I 0

lM)

300

400 WAVELENGTH Unm)

500

600

Figure 22. Near Whisible absorption (S-S), fluorescence (FL), triplet-triplet absorption (T-T), and polarization spectrum (P)of para-terphenyl.

To improve the solubility of the oligophenylenes, several methyl- and methoxysubstituted para-quaterphenyl on the internal ortho position were studied by Giisten et al. [73,74]. A n example of these compounds is shown in Figure 23.

Figure 23.2’,3”-Dimethoxy-para-quaterphenyl. According to Kauffman et al. [75],partial bridging of the rings a t the orthopositions of the aromatic rings in para-quaterphenyl as depicted in Figure 10 resulted in higher efficiencies under flashlamp pumping. Improvements were also obtained when the methoxy groups in the para-position, as depicted in Figure 10, were made cyclic [34].Some of these laser dyes are commercially available. A typical example of such a dye is depicted in Figure 24.

8

319

Pr Pr

Figure 24. 2,7-Bis(6-chromaryl)-9,9’-dipropylfluorene. Many oxazoles and benzoxazoles exhibit strong fluorescence [211. Most of these compounds lase rather effkiently under short-pulsed laser excitation. Often, however, it is difficult to obtain laser action from them using short rise time flashlamp excitation [5]. A typical representative of these compounds is paru-bis(5phenyloxazolyl) benzene (POPOP), depicted in Figure 25. POPOP exhibits laser action a t about 420 nm. Again, this compound is a typical case C, depicted in Figure 11.

Figure 25. paru-Bis(5-phenyloxazolyl) benzene (POPOP). From the positive degree of polarization of the T-T absorption band of POPOP shown in Figure 26, it can be concluded that a constellation A(I) is present. Their two lowest energy electronic transitions, S-S and T-T, are positioned along the long-axis molecular axis (in the p,p’-position). Therefore, auxochromic group substitution along the long-axis should shift the S-S, the fluorescence FL, and the T-T absorption to longer wavelength and this is, indeed, the case [391. The molecular structure of dibutoxy-POPOP is depicted in Figure 27. Its S-S and T-T absorption and polarization spectra (P) together with the fluorescence spectrum (FL) are depicted in Figure 28. After long-axis substitution, the two long-axis polarized S-S and T-T transitions remain aligned parallel to each other and, as expected, again exhibit a positive P value.

320

+

+O.l

I

I

FL

300

350

400

550

500

450

WAVELENGTH

650

600

h(nrn)-

Figure 26. Near UV/visible absorption (S-S),fluorescence (FL), (T-T) absorption, and polarization spectrum (P) of POPOP.

Figure 27. Dibutoxy POPOP.

108

90 ?2

54

36 18

300

350

400

450

WAVELENGTH

-

500

550

600

650

h(nrn)

Figure 28. Near W/visible absorption (S-S),fluorescence (FL), (T-T) absorption, and polarization spectrum (P) of butoxy-POPOP.

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8.5 Coumarin laser dyes

Fusing a benzo group to the nonfluorescing a-pyrone molecule results in coumarin. Substitution of the 7-position by auxochromic groups such as -OH, OCH,, -NH,, -NHCH,, -N(CH,),, and other electron-donating substituents yields coumarin laser dyes. They form an important group of laser dyes that cover the spectral region from 440 t o about 545 nm. The first coumarin laser dye found 7diethylamino-4-methylcoumarin(Coumarin 11, which exhibits laser action at about 460 nm under flashlamp excitation [761. Its structure is shown in Figure 29.

(Coumarin 1). Figure 29. 7-Diethylamino-4-methylcoumarin The amino analogue, 7-amino-4-methylcoumarin(Coumarin 120) shows laser action at 440 nm [77];its structure is shown in Figure 30.

Figure 30. 7-Amino-4 methylcoumarin (Coumarin 120).

The conventional near W/visible absorption (S-S), fluorescence (FL), and (T-T) absorption, together with the polarization spectrum (P) of Coumarin 120, are presented in Figure 31. Clearly, case C depicted in Figure 11 is present, with considerable T-T absorption still overlapping the FSR [39-41,691. This explains the rather low laser action efficiencies of coumarin dyes under flashlamp excitation, generally ranging from 0.3 t o 0.6 percent.

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c

t t

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I I

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&

'L

> 8

u1

4

0 250

3M)

350

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WAVELENGTH hnmj

5w

550

6w

Figure 3 1. Near UV/visible absorption ( S - S ) , fluorescence (FL), (T-T) absorption, and polarization spectrum (P) of Coumarin 120.

cH3m0 CY3

CH3

CF3

Figure 32. AC3F structure. In some coumarin dyes the basic chromophore has been replaced with its heterocyclic analogue, such as aza-coumarin, quinolone, or aza-quinolone. A typical example is AC3F [79], presented in Figure 32. To keep the amino group coplanar, it has been replaced by the single butterfly system depicted in Figure 10. To improve the photostability of this compound, the methyl group in the 4-position has been replaced by -CF, Using molecular engineering, data on about 100 new coumarin derivatives have been published over the last 25 years [3-5,321. Coumarin dyes have the tendency for low photostability. Their degradation products often absorb strongly in the laser action spectral region [78], 8.6. Rhodamine laser dyes In 1967, Sorokin and Lankard [80], using the method of trial and error, reported laser action from Rhodamine 6G under flashlamp excitation. This laser

323

dye exhibits efficient laser action in the 590 nm region and lases with about one percent efficiency in most flashlamp pumped-dye lasers 161. Rhodamine 6G, its chemical structure depicted in Figure 33, still serves as the yardstick by which the efficiency of other laser dyes are measured.

Figure 33. Rhodamine 6G. Because Rhodamine 6G also exhibits good photostability, this laser dye is one of the most often used and studied. Used in cw dye lasers, it is also one of the most efficient laser dyes. It has a very strong and broad lowest energy absorption band, with an extinction coefficient of ~ ~ ( 5 4 =0 )1.1 x lo5 L/mol cm. Interestingly, its ~ ~ ( 5 9 0=) 9.7 x lo3 L/mol cm [41,81] value cannot be termed small, with E, (540)/~,(590)-- 11.It would be very desirable to have available laser dyes in which this ratio is 20 or higher, so that efficient laser action would result. For Rhodamine 6G dissolved in air-saturated ethylene glycol, Thiel [43,47] obtained a value of 2, = 5 psec, for oxygen-saturated, 1.2 psec, and for argonsaturated solution, 20 psec, respectively. Using an ethanollglycerin mixture, he obtained 7, = 0.6 psec, while ethylene glycol gave z, values for Rhodamine 19, Rhodamine B, and Rhodamine 101 in the range of 5 to 6.5 psec. The structure of Rhodamine B is shown in Figure 34.

OcooH Figure 34. Rhodamine B. Fluorescein 1801 and Rhodamine B [81 are two other widely used xanthene laser dyes. Rhodamine dyes seem t o possess the case-C behavior depicted in Figure 11. There is a strong T-T absorption band in Fluorescein at about 1140 nm [821 and in Rhodamine 6G at about 1120 nm 1831.

324

Figure 35. Rhodamine 700. Over the last 25 years, a considerable number of rhodamine dyes and related compounds have been synthesized, most of them showing efficient laser action in the 560 to 800 nm region 13-51. An example of an efficient xanthene laser dye obtained by molecular engineering, is Rhodamine 700 [84,85], the structure of which is depicted in Figure 35. Using the “double butterfly” structure, the amino groups have been rendered rigid and planar (Figure 10). Replacing the phenyl group substituent of Rhodamine 6G by the -CF, group should enhance its photostability, and this efficient dye exhibits laser action between 700 to 800 nm . 8.7. Other commercial laser dyes The dyes in this section allow us to draw some general conclusions about the relationships between molecular structure and efficient laser action. a-NPO (Figure 36) exhibits laser action at about 400 nm [861. This dye also possesses type-C behavior depicted in Figure 11 [371. DCM (Figure 37) shows laser action at about 655 nm [891. Brilliant Sulfaflavin (Figure 38) exhibits laser action at about 540 nm [87]. This compound belongs to the small number of laser dyes in which oxygen from air does not efficiently reduce its triplet state lifetime. However, cyclooctatetraene does act as an efficient triplet state quencher for this dye [881. Oxazine 4 (Figure 39) lases a t about 682 nm 1321, Carbazine 720 (Figure 40) lases at about 720 nm [31, and DTDC (Figure 41) shows laser action at about 760 nm DO].

Figure 36. a-NPO.

325

Y

NC

CN

Figure 37.4-Dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4 H-pyran (DCM).

Figure 38. Brilliant Sulfaflavine.

Figure 39.3,7-Ethylamino-2,8-dimethylphenoxazine-5-ium perchlorate (Oxazine

4).

CH3

0

Figure 40. Carbazine 720.

326

Figure 41. 3-Diethylthiadicarbocyanineiodide (DTDC).

8.8. Summary Most of the laser dyes presented in this section and most of those listed by Maeda [5] contain a t least one five-, or six-membered heterocyclic. This is an indication that vibronic spin-orbit interactions are partially effective in most of the presently available laser dyes. These interactions reduce to some extent T-T absorption intensities over their FSR and therefore are increasing overall lasing efficiencies. However, all these compounds, and most of those listed by Maeda 151, also contain a t least one "unsubstituted" benzene ring. 9. Quus~-AROMATIC LASER DYES

It was suggested in Section 4.3 that efficient laser dyes could be found among compounds that contained no "unsubstituted" benzene rings, but only five- and sixmembered heterocyclics. In this section, we present three families of such quasiaromatic compounds. Because of the presence of strong vibronic spin-orbit interactions in these heterocyclic systems, T-T absorption should be considerably reduced over the FSR. 9.1. syn-Bimane laser dyes Kosower and co-workers reported striking fluorescence in the violet to green region of the spectrum from a new family of heterocyclic compounds that were termed syn-bimanes; the related anti-bimanes exhibited no fluorescence [91-941. Structurally, these syn-bimane are very different from all laser dyes discussed so far (Figure 42).

R2

R2

a Figure 42. syn-Bimane (a) and anti-bimane (b)

0

R2

b

327

Also, the spectroscopic properties of the syn-bimanes are very different from other aromatic compounds and their heterocyclic analogues. Fluorescence is observed from syn-bimanes despite the fact that these molecules do not contain any benzene rings or 6-membered heterocycles. The presence of the two carbonyl groups should not favor fluorescence. Efficient laser action was observed from an aqueous solution of syn(methy1,methyl) bimane under flashlamp pumping [95]; the structure of this molecule is shown in Figure 43. In water and trifluoroethanol, several synbimanes lase as efficiently as some coumarin laser dyes 196,971.

Figure 43. syn-(Methy1,methyl)bimane. Some syn-bimanes are water-soluble and even show exceptional photostability in this solvent under flashlamp excitation. The spectral location of the lowest energy S-S absorption band of the syn-bimanes is very solvent-dependent. As a consequence, the spectral location of the fluorescence maximum is also significantly affected. For example, in 1,4-dioxane, the fluorescence intensity maximum of syn-(methy1,methyl) bimane is located in the violet portion of the spectrum, in ethanol it shifts to the blue, and in water to the blue/green region. Another typical example of a syn-bimane laser dye is syn-(dimethoxyphosphylmethyl, methyl) bimane [96], the structure of which is depicted in Figure 44. The S-S, T-T absorption and fluorescence spectrum FL of this compound are shown in Figure 45

Figure 44. Syn-(Dimethoxyphosphinylmethy1,methyl) bimane.

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6.0

A

8.0

B

7.0

1

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(x 10.3)

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n 250

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Figure 45. Near Whisible absorption (S-S), fluorescence (FL), and (T-T) bimane in A water absorption spectra of syn-(dimethoxyphosphinylmethy1,methyl) and B in 1,4-dioxane as solvents. In spectrum A, with water as solvent, laser action takes place a t 514 nm. In spectrum B, with 1,4-dioxane as solvent, laser action is observed at 438 nm. A rather narrow but only medium intensity T-T absorption band is located near 480 nm [95]. The spectral location of this T-T absorption band seems to be unaffected by change in solvent [981. The T-T absorption band exhibits mixed polarization 195,991, and therefore, none of the constellations is present in the syn-bimanes. From the three spectral locations of T-T absorption bands depicted in Figure 13, case D is operative. Case D is the most undesirable spectral location of a T-T absorption band in the FSR for molecules exhibiting strong vibronic spin-orbit interactions. Nevertheless, laser action is observed a t 438 nm using 1,4-dioxane as

329

solvent, with laser action intensity about the same as that observed for Coumarin 440 [97]. No laser action is observed in ethanol due to overlapping of the fluorescence spectral region with T-T absorption. However, with water as solvent, laser action is observed at 514 nm. The laser action output was about 20 percent higher than that observed from Coumarin 30. Significantly, from about 525 nm t o about 650 nm, little or no T-T absorption was observed. Attempts were made t o shift the fluorescence band of the synbimanes into this region by introducing various substituents into the R, and R, position to enhance the solvent effect. However, these structural modifications were unsuccessful. If they had been successful, one would have produced a constellation A(III), but not by using a red shift of the lowest energy S-S absorption band by molecular substitution, but rather by solvatochromism. Substituting phenyl groups at the long- or short-molecular axis, or utilizing two benzene rings directly fused to the syn-bimane molecule, resulted in a considerable increase in the intensity of the T-T absorption band located in the bluelgreen spectral region. This observation is consistent with the idea that efficient laser dyes should not contain any "unsubstituted" benzene rings [991. 9.2 Di- a-pyridylamino-BF,complex Schafer et al. [loo] reported laser action under nitrogen laser excitation from dia-pyridylamino-BF, (DPA-BF,) complex. However, no laser action was observed under flashlamp pumping, probably because its Q, value is only = 0.40. This compound is a derivative of anthracene and its structure is depicted in Figure 46. It contains heterocyclic moieties, but no "unsubstituted" benzene ring.

Figure 46. Di-a-pyridylamino (DPA)-BF,complex. The S-S, T-T, FL, and polarization spectra arising from this complex are shown in Figure 47. Some of the spectroscopic features of the DPA-BF, still resemble those of anthracene. For instance, its lowest energy S-S absorption band is still located in the near-UV. On the other hand, its oscillator strength (fJ has increased almost fourfold [91. Noteworthy is the rather low T-T absorption over the FSR, making this dye case F type (Figure 13). For the DPA-BF, complex, the ratio &,(max)/&,(h,)= 16, which is larger than the corresponding value for Rhodamine 6G. Because one observes a positive degree of

330

polarization (P), the lowest energy S-S and T-T dipoles are positioned parallel to each. Therefore, a constellation case A(1) is present. Because the lowest energy S-S and T-T transitions in anthracene [9]are positioned along its long molecular axis, they are most likely also positioned along the long molecular axis of the DPABF, complex. Therefore, this molecule might be transformed into an efficient laser dye by long-axis substitution of H-atoms by auxochromic groups. As stated, this type of substitution often increases the Q, values of chromophores, which in turn will support efficient laser action [17-221.

300

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700

WAVELENGTH I\(nrn)-

Figure 47. Near Whisible (S-S), fluorescence (FL), (T-T) absorption, and polarization spectrum (P) of 2,2’-dipyridylamino-BF2(DPA-BF,) complex 9.3 Pyrromethene-BF, laser dyes The pyrromethene-BF, (P-BF,) [loll complexes fulfill the definition of quasiaromatic compounds [102]. Many of the complexes also exhibit high &, values and their general structure is presented in Figure 48.

6 F

Figure 48. Pyrromethene-BF, complex.

3

33 1

Many of the P-BF, complexes show efficient laser action under flashlamp excitation. Some of these P-BF, complexes are commercially available. The 1,3,5,7,8-pentamethyl-P-BF2 complex in ethanol exhibited laser action a t 546 nm and lased about three times more efficiently than Coumarin 545 and about 10 percent less efficient than Rhodamine 6G.These new laser dyes fill the gap in the green portion of the spectrum of the tuning curve in Figure 6.

Figure 49. 1,3,5,7,8-Pentamethy1-2,6-diethyl-P-BF2 complex.

1,3,5,7,8-Pentamethyl-2,6-diethyl-P-BF2, the structure of which is shown in Figure 49, lases a t 567 nm and has an output about three times higher than that of Rhodamine 560. Other P-BF, complexes show similar efficiencies [103-1071. (Figure 50). Another typical P-BF, complex is 2,3,5,6-bis-tetrarnethylene-P-BF2 The (S-S) absorption, fluorescence (FL), and (T-T) absorption spectra of this P-BF, complex are depicted in Figure 51. This compound ranks among the more efficient of P-BF, dyes [107]. From Figure 51 it is apparent that an exceptionally low (&,(A,) = 2.4~10 L/mol ~ cm) T-T absorption is present over the FSR. Because of this low TT absorption, together with a high Q, value, efficient laser action is observed. Indeed, these dyes represent the desired case F of Figure 13, with ~,(max)/E,(A,)= 30. The T-T absorption band of P-BF, complexes consist of two differently polarized transitions [981 and consequently, none of the constellations is present.

complex. Figure 50. 2,3,5,6-Bis-tetramethy1ene-P-BF2

332 100

80

t

ES,&T

(x 103)

40

20

459

5M)

550 WAVELENGTH

600

659

700

750

h(nm)-

Figure 51. Near Whisible (S-S), fluorescence (FL), and triplet-triplet (T-T) complex. spectra of 2,3,5,6-bis-tetramethylene-1,7,8-trimethylp~romethene-BF~ O’Neil 11081, pumping synchronously with 2 W at 532 nm from a Coherent Antaras Nd:YAG laser and using l,3,5,7,8-pentamethy1-P-BF2 complex dissolved in propylene glycol- monoethyl ether, produced 950 mW a t 565 nm. This is about twice the efficiency of Rhodamine 6G. Under the same experimental conditions, Rhodamine 6G dissolved in ethylene glycol produced only 510 mW. The Q, value of 1,3,5,7,8-pentamethy1-P-BF2 complex is 0.995, indicating the presence a small intersystem crossing rate constant (kIsc).Significantly, O’Neil observed for this dye 7, = 453 f29 psec, a surprisingly high T~ value. Such a high value is generally observed in pure (degassed) solutions of organic compounds, without any triplet state quencher, such as oxygen from air, being present. Using all laser lines (from 457.9 to 514.5 nm) of a 5 W cw argon ion laser for acid-P-BF, excitation, the sodium salt of 1,3,5,7,8-pentamethyl-2,6-disulfonic complex (Exciton-556) dissolved in ethylene glycol demonstrated a significant advance in the 530 to 625 nm spectral region over commercial dyes currently available [log]. The best of these laser dyes have peak efficiencies in the order of 35 percent. Coumarin 6 and Rhodamine 110 typically exhibit less than 25 percent disulefficiency in this region. In this region, disodium-1,3,5,7,8-pentamethyl-2,6 fonate-P-BF, complex has a cw efficiency of 45 percent at 553 nm. Rhodamine 6G tested in the same laser produced 1.5 W for 32 percent efficiency. The half-life of P556 was over three times longer than Coumarin 6 or Rhodamine 110. Up to 85 percent conversion efficiency has been obtained from plastic rods doped [1101. For excitation, a with the 1,3,5,7,8-pentamethy1-2,6-diethy1-P-BF2-comp1ex diode-pumped, Q-switched and frequency-doubled Nd:YAG was used. Under the

333

same conditions, Sulforhodamine produced only 37 percent conversion efficiency. The plastic was an acrylic copolymer containing five parts methyl methacrylate and one part hydroxypropyl acrylate. This new solid-state lasing material can be fabricated very cheaply in very large sizes. There are advantages to using plastics as host materials for laser dyes. The handling and storage of solvents is obviated and the used laser material is easily disposable. Using the same excitation source, but employing solutions of some P-BF, complexes in ethanol, similar high-conversion efficiencies were reported by Allik et al. [lll]. 10. FINAL OBSERVATIONS 10.1. High-efficiency laser dyes We have one main question left unanswered. What is the highest efficiency we can expect from laser dyes? To answer this question, we would have to know the lowest values of the spectroscopic parameters (k,,,, &,(A,), and 7,) an organic compound can ever attain. Unfortunately, this information is lacking. Therefore, only some rough estimates can be provided. A related question is: Can the pyrromethene-BF, complexes be designated as high-efficiency laser dyes? This question should actually be divided into two parts. Are the P-BF, complexes highefficiency laser dyes under short-pulse laser excitation or flashlamp excitation, with long pulse rise time? Under short-laser pulse excitation, some P-BF, complexes exhibit about 85percent conversion efficiencies. Therefore, under these conditions, one can classify these P-BF, complexes as high-efficiency laser dyes. On the other hand, the answer is no for flashlamp pumping. The P-BF, complexes do not lase much more efficiently than Rhodamine 6G, although the P-BF, complexes have considerably less T-T absorption over their FSR compared to Rhodamine 6G. However, their exceptionally high T, value of 453 k 29 psec is probably responsible for their rather low efficiency under flashlamp excitation. It appears that oxygen from the air may be totally inefficient as a triplet state quencher for the P-BF, complexes triplet state molecules. Derivatives of the P-BF, complexes, however, may possess T, values in the psec range and should significantly outperform Rhodamine 6G under flashlamp excitation. In fact, a factor of two or more over Rhodamine 6G might be observed. Using large flashlamp emitting long rise-time-pulses, pulse energies in the several 100-Joules range should be obtainable.

Nevertheless, this may not be the most promising approach to producing inexpensive visible laser light. A better approach might be to pump high-efficiency laser dyes with incoherent continuous light sources, as demonstrated by Thiel et al. [471. Success will depend greatly on the spectroscopic parameters of these new laser dyes. At this point, however, it is difficult t o suppress wishful thinking, and

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one should synthesize these new dyes first, rather than merely making predictions.

10.2. Summary After laser action was observed from chloro-aluminum-phthalocyanineand 3,3’diethylthiatricarbo-cyanine iodide about 30 years ago, thousands of compounds that exhibit fluorescence have been evaluated for laser action, mostly under flashlamp and nitrogen laser pumping. To continue fruitful development in this area, interest should be specifically focused on quasi-aromatic compounds. These are compounds comprising five-and six-membered heterocyclic systems. If a high Q, value is present, the compound should be tested for laser action under flashlamp excitation. If the Q, value is not too high, low, or zero, one should then conduct spectroscopic studies to determine if the compound exhibits low T-T absorption over the FSR. If this is the case, a closely related derivative may exhibit stronger fluorescence. Also, obtaining the T-T polarization spectrum may also be helpful. Depending on the value of the degree of polarization recorded, substitution with interacting groups may result in efficient laser dyes. 11 ACKNOWLEDGMENTS The author wishes to thank Mr. W. A. Friday, U.S. Army MICOM, who provided partial funding for his research, Dr. G. Massey, San Diego State University and Dr. J. M. Kauffman, University of the Sciences in Philadelphia, for helpful discussions, Dr. R Moore, my Division Head, for supporting this work, and Dr. E. Thiel, University-GH Siegen (Germany), for making his “Habilitionsschrift” available.

12. REFERENCES 1 P.P. Sorokin and R. Lankard, IBM J. Res. and Dev., 11 (1966) 162. 2 F.P. Schafer, W. Schmidt, and J. Volze, Appl. Phys. Lett., 9 (1966) 306. 3 K.H. Drexhage, in F.P. Schafer, (ed.), Dye Lasers, p. 144, Springer-Verlag, New York, 1990. 4 R. Steppel, in M. Weber, (ed.), Overview in CRC Handbook of Laser Science and Technology Vol. 1, Lasers and Masers, Organic Dye Lasers, p. 298, CRC Press, Boca Raton, Fla., 1982. 5 M. Maeda, Laser Dyes, Academic Press, New York, 1984. 6 J. Duarte, in F.J. Duarte and L.W. Hillman (eds.), Dye Laser Principles, p. 249, Academic Press, New York, 1990. 7 P.P. Sorokin, J.R. Lankard, E.C. Hammond, and V.L. Moruzzi, IBM J. Res. and Dev., 11 (1967) 130. 8 W. Schmidt and F.P. Schafer, Z. Naturforsch., 22A (1967) 1563.

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8 Multifunctional Dye Materials from new Dicyanopyrazine Chromophores MASARU MATSUOKA Kyoto Women’s University, Material Science Laboratory Kitahiyoshi, Imakumano, Higashiyama, Kyoto 605-8501,Japan 1. INTRODUCTION

Pyrazine chemistry based on hydrogen cyanide has been studied extensively by du Pont [ll,Dow Chemicals [21and Sagami Research groups from 1970,who were mainly interested in their biological activities [31. During that period, few derivatives were studied as dye chromophores, but the syntheses of pyrazinophthalocyanines was reported in 1990 [4]. Pyrazine has two nitrogen atoms at the 1,kpositions to replace carbon atoms of the benzene ring, and it is anticipated to have many functionalities and reactivities in comparison to the benzene analogues. The strong electron withdrawing ability of the pyrazine nucleus make possible various nucleophilic substitution reactions in the nucleus. It is also known to act as an electron acceptor and an oxidant. Pyrazine related dyes showed strong fluorescence and are currently of interest in application fields such as emitters for electroluminescence devices, copy preventing inks, solar energy collecting materials, energy-transfer materials, and fluorescence materials for various applications. We have developed new pyrazine dye chemistry and correlations of their multifunctionalities with their molecular structure and in the solid state. The molecular stacking and self-assembling behavior of the pyrazine chromophores is affected by intermolecular x-x interactions and, together with hydrophobic interactions of the alkyl substituents, are especially interesting to correlate with their functionalities. Diaminomaleonitrile (DAMN) as a tetramer of hydrogen cyanide has been known for a long time, but 2,3-dichloro-5,6-dicyanopyrazine (DCDCP) and 2,5-diamino-3,6-dicyanopyrazine(DADCP) are now become available as new industrial raw materials from Nippon Soda. DAMN can be prepared directly from hydrogen cyanide by oligomerization [51 and much related chemistry has been reported 111. Molecular design of new dye chromophores can be performed by using the molecular orbital (MO) method. Three dimensional molecular structure and the

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optimized conformation of a dye chromophore can be simulated by the molecular mechanics (MM) method. Simulation of molecular stacking in aggregates or crystals has now become available by means of the molecular dynamics (MD) method. These computational methodologies are very useful in the design of functional dye materials. Material design of dye aggregates such as single crystals and evaporated thin films is very difficult because the evaluation of intermolecular interactions of dye chromophores in aggregates is not yet clearly understood. On the other hand, many functionalities of dye materials such as nonlinear optical susceptibility, electroluminescence, photoconductivity, and photovoltaic efficiency exclusively depend on the intermolecular n-?r interactions of the chromophores. Many methods are known t o assemble dye molecules to produce functional materials, but it has not yet been established how to correlate their functionalities with the chemical structures of the dye molecule, since it has not been possible to quantitatively evaluate the intermolecular n-n interactions of molecules in the solid state. These are very interesting subjects for us to develop new multifunctional dye materials based on the pyrazine chromophores. We have designed and synthesized many types of pyrazine dyes by using three raw materials, viz., DAMN, DCDCP and DADCP. Their absorption and fluorescence properties both in solution and the solid state were evaluated and correlated with their optimized structures for molecular stacking, crystal morphologies and phase-transfer characters, depending on the substituents. The Stokes shift (SS) and the differences in absorption (Ah) and fluorescence (AF) maximum from solution to the solid state was used to evaluate the intermolecular n-z interactions in the solid state. We have made extensive studies of pyrazine chemistry concerning DAMN [9,1214, 17, 19-21],DCDCP [7,8, 10, 11, 15, 16, 221, DADCP [6, 181, and their derivatives [23-261. In this chapter, the syntheses of dicyanopyrazine related multifunctional dyes are described, and their visible and fluorescence spectra in solution and the solid state are correlated with their three dimensional molecular structures. Some functionalities are correlated with their molecular stacking behavior with respect to their chromophoric systems. 2. SYNTHESES OF DICYANOPYRAZINE RELATED DYES

DAMN 1 and DCDCP 17 are available as chemical reagents, but 17 can be prepared in laboratory from 1 and oxalyl chloride followed by chlorination with thionyl chloride. DADCP 29 can be synthesized by the oxidative coupling of 2,3diamino-3-(phenylthio)-acrylonitrile (obtained from the oligomerization of hydrogen cyanide) in the presence of diphenyldisulfide [6]. The syntheses of 17 and 29 are summarized in Scheme 1.

341

1

BHCN

PhSSPh

17

I

NC

NH2

PhS

NH2

PI Citric acid Sodium citrate

t

29

Scheme 1.Synthesis of DCDCP (17) and DADCP (29). 2.1. Syntheses of pyrazine dyes derived from DAMN (1) The syntheses of pyrazine dyes derived from 1 are summarized in Scheme 2. The amino groups of 1 have moderate basicity and react with the carbonyl group t o form the azomethine bond. The reaction of 1 with 1,2-dicarbonyl compounds gave the 5,6-dicyanopyrazine derivatives 2 and 4, respectively. The 2-methyl group in 2 (X=H, Me, OH, OAc) is reactive with the carbonyl group because of strong electron withdrawing effect of the dicyanopyrazine moiety. Chemical shifts of the 2-methyl group in 2 were observed at around 2.3-2.8 ppm, while that of toluene was observed at 2.09 ppm. Compounds 2 reacted with arylaldehydes to give the styryl type fluorescent dyes 5 [9]. 2,4-Pentanedione was oxidized by selenium dioxide to give 2,3,4-pentanetrione that reacted with 1 to give 3. The acetyl group in 3 was very reactive with the carbonyl compounds and was reacted with arylaldehydes t o give the chalcone derivatives 6 [19]. The reaction of 1 with 1,4-dibromo-2,3-butanedionegave 4 (Y=bromomethyl) together with 2-bromomethyl-2-(bromoacetyl)-4,5-dicyanoimidazole in low yield, which was quantitatively oxidized by DDQ to the corresponding 2H-imidazole D71. The reaction of 4 (Y=bromomethyl) with triphenyl phosphine followed by strong base gave the corresponding Wittig reagent, which reacted with benzaldehyde to give 2,3-bis(styryl)-5,6-dicyanopyrazine 7a together with 2-styryI-3-triphenylphosphoranylidenemethyl-5,6-dicyanopyrazine 8a. Many other arylaldehydes such as cinnamaldehyde, 2-~yridinecarboxaldehyde, 2-furaldehyde, 3carbazolylaldehyde were also reacted with the Wittig reagent from 4 to give the corresponding 2,3-bis(arylethenyl)-5,6-dicyanopyrazines. The reactions were carried out in the presence of sodium ethylate in ethanol, and the corresponding 8 were obtained as by-product. Compounds 8 were stable and can be stored [121.

342

-

4HCN

1

I - - xx NH2

NC

N

CH2Br

NC NHz I DAMN

NC

N

Y

NC

4

-

5

2

1

-

-

NC

N

CH2Br

NC

N

CH2Br

XI

PhqP

+ a ; X=CH

4

8

b ; X=N

-

NC

CN

hv

7

+ NC'

xx N

CH2Br

NC

N 4

CH2Br

NC

N

CH2Br

NC

N 4

Ph

NC

PPh3

-

NC

NC

10

'CN

x:

N?'

N

Pyrazinonaphthalocyanine PPh3

13

12

16

Scheme 2. Synthesis of DAMN-based compounds.

2

343

On the other hand, the reaction of the Wittig reagent of 4 (Y=Ph) with aryldialdehydes such as 1,2-, 1,3-, and 1,4-phthaldialdehydes or 2,6-pyridinedialdehyde gave the analogous bis-dicyanopyrazine fluorescence dyes 9a-9d, respectively 1131. Photoreaction of 7a in benzene under W irradiation gave the photodimer 10a having the cyclobutane ring with the head-to-tail dimerized structure. The evaporated thin film of 7a gave the same photodimer 10a under W irradiation. Single crystals of 7a also gave small amounts of 10a together with insoluble photopolymers in organic solvents. The structure of 10a was characterized by 1H NMR spectra, mass and elemental analysis. Similar photoreaction of 7b in benzene gave the dimer 10b together with the other dimer l l b having two cyclobutane rings with a head-to-head structure. From these results, compounds 7a and 7b were self-assembled in the solid state, but their stacking structure is different 1241. Details of the photodimerization and oligomerization of 7 with respect to their molecular stacking in the solid state are discussed in Section 3.4. The reaction of the Wittig reagent from 4 (Y=bromomethyl) with the 1,2dicarbonyl compound gave the dicyanoquinoxaline 12 which was converted to the corresponding pyrazinonaphthalocyanines 13 [17]. This ring-closure reaction to give 12 proceeded in the presence of sodium hydride in DMF at 120°C. The same reaction of the Wittig reagent with the 1,2-dicarbonylcompound in the presence of sodium ethylate in ethanol under reflux conditions gave the mono-condensation product, 2-acylethenyl-3-triphenylphosphoranylidenemethyl-5,6-di-cyanopyrazine, as reaction intermediate (Scheme 2). The synthesis of pyrazinonaphthalo-cyanine 13 (R=Ph) proceeded in 80% yield in the presence of cuprous chloride in quinoline at 150°C. The reaction of 1 with the 1,3-dicarbonyl compound gave the 7-membered azepine 14, which further reacted with arylaldehydes such as julolidinaldehyde to give the heterocycles 15 1191. The ring-closure reaction between 1 and the 1,3dicarbonyl compound gave the enamine 14 as a predominant tautomer, which reacted with arylaldehydes to give 15. The structures of 14 and 15 were confirmed by 1H NMR spectra. The non planar conjugation systems of 14 and 15 are discussed in Section 3.1 in connection with their absorption spectra. The bromomethyl group in 4 (Y=Ph) has enough electrophilic activity to react with bipyridyls, and the viologen derivatives 16 having a dicyanopyrazine moiety could be synthesized. Their electrochromism accelerated by the pyrazine substituents will be published elsewhere 1231. 2.2. Syntheses of pyrazine dyes derived from DCDCP (17) The syntheses of pyrazine dyes derived from 17 are summarized in Scheme 3. The reaction of DCDCP 17 with various nucleophiles such as amines and thiols gave mono- and di-substituted 5,6-dicyanopyrazines. Both of the chlorine atoms in

344

17 are very reactive to nucleophiles. Reaction of 17 with triethylamine gave diethylaminoethenylpyrazines 18 (R=Et) in 25% yield. In the reaction, 17 oxidized triethylamine to diethylaminoethylene, which reacted with 17 to give 18, and 17 has oxidation properties similar to chloranil [7]. The reaction of 17 with Nethylpiperidine gave the corresponding 2-(cyclohexylamino)ethenyl-3-chloro-5,6dicyanopyrazine 19 and 2-(N-ethylazacyclohexenyl)-3-chloro-5,6-dicyanopyrazine 20 in low yields, respectively. Similar reactions of 17 with N-alkylcyclicamines also gave the corresponding 19 and 20, respectively [81. On the other hand, 17 reacted readily with various enamines to give the corresponding aminoethenylpyrazines 20 in high yields. N-methylindole and Nmethyl-2,6-dimethylpyrrolereacted with 17 to give the product in 60-80% yields, but N-phenylpyrrole did not react a t all. In these reactions, the second chlorine did not react due to the steric character of the first substituent [8]. Similar reaction of 17 with the Fisher’s type bases, a kind of enamine, gave 21 in good yields [161. The second chlorine did not react with the enamine to give the bissubstituted product, but 21 further reacted with other less hindered nucleophiles such as an alkylamine, alcohol or thiol. The reaction of 21 with alkylenedithiol or alkylenediamine then gave the corresponding bis(aminoviny1)pyrazine 116, 221. Dyes 21 with the long N-alkyl group were also synthesized by the same method [221. Alternatively, reaction of 17 with the thiocarbonyl compounds such as thioacetamide or thiourea gave the tetraazatetracyanothianthrene 22a in quantitative yield [7]. It has been reported that similar reactions of thiocarbonyl compounds with 2,3-dihalogenonaphthoquinone,2,3-dichloroquinoxaline and 3,4dichloro-N-phenylmaleimidegave the corresponding polycyclic-1,4-dithiines[271. Tetracyanodihydropyrazino[2,3-b:2’,3’-e]pyrazines 22b were successfully prepared by the intermolecular ring-closure reaction of 2-alkylamino-3-chloro-5,6-dicyanopyrazine in the presence of triethylamine. The yields were affected by the substituent R and 22b (R=H and Me) were obtained in 78% and 43% yields, respectively. N-Phenyl or N-(4-alkylphenyl) derivatives of 22b were synthesized in 60-95% yields [lo]. The reaction of 2,3-bis(methylamino)-5,6-dicyanopyrazine with 17 hardly proceeded because of the steric hindrances of the methyl groups, and 22b (R=Me)was obtained only in 3% yield by this method 171. Both chlorine atoms in 17 are very reactive to nucleophiles and many types of ring-closure reactions have been proposed to give a wide variety of pyrazinoheterocycles [171 (Scheme 4). Pyrrolo[2,3-blpyrazines 23 can be synthesized from 17 and the enamines obtained from the carbonyl compound and alkylamine. The enamine, previously synthesized from 2-butanone and nbutylamine, reacted with 17 to give 2,3-dicyano-5-(N-butyl)-6,7-dimethylpyrrolo[2,3-b]pyrazine 23 (R=n-butyl, R1,R2=Me)in 19% yield. Similar reaction of 17 with cyclohexanone and alkylamine gave the annelated derivative 23

345

(RI,R2=1,4-butylene) in 80% yield. The N-aryl derivatives of 23 could be synthesized from 17 with arylamines followed by cyclization with 1,3-dicarbonyl compounds in the presence of sodium hydride. The reaction of 17 with 4-noctylaniline followed by benzoylacetophenone gave 2,3-dicyano-5-[N-(4octylphenyl)]-6-phenyl-7-benzoylpyrrolo[2,3-b]pyrazinein 26% yield. The corresponding furo[2,3-b]pyrazines 24 (X=CN or COPh) were synthesized from 17 with benzoyl-acetonitrile or benzoylacetophenone in the presence of sodium amide in 40-70% yields, respectively, a s shown in Scheme 4 [17].

..

18

DCDCP

17

-

22

a ; X=S

b ; X=NR

NC

17

17

17

N NC

C N

NC 24

~

N

N

2s

~N B c NC -N

u~

~N

28

Scheme 3. Synthesis of pyrazine dyes from DCDCP (17).

~ Porphyrazine " u ~

28

346

1

R

23

PhCOCH2X

N

a

y

R

23

1) 4-alkylaniline

2) PhCOCH2COPh

NCxNnCoPh

DBN / EtOH

NC

N

N

Ph

a : X=CN

Porphyrazines

24 b : X = COPh C:

X=H

23

R

Scheme 4. Synthesis of compound 23 from DCDCP. Tetrahydropyrazino [2,3-b]indoles 25 were synthesized by the reaction of 17 with the enamine obtained previously from 4-t-butylcyclohexanone and alkylamine, followed by cyclization in 8045% yields. Aromatization of 25 to pyrazino[2,3blindoles 26 was carried out in carbon tetrachloride in the presence of Nbromosuccinimide and benzoylperoxide in 60-75% yields. The solubility of 26 in organic solvents was greatly improved by the introduction of a t-butyl group or of a long chain alkyl group as the N-substituent [ E l . Reaction of 23 or 26 with DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) or DBN (1,5-diazabicyclo[4.3.0lnon-5-ene) in ethanol gave the corresponding porphyrazines 27 (20-30% yields) [17] or 28 (4080% yields) [15] as the annelated pyrazinophthalocyanines, respectively, as shown in Scheme 3. 2.3. Syntheses of pyrazine dyes derived from DADCP (29) Syntheses of pyrazine dyes derived from 29 are summarized in Scheme 5. 2,5Diamino-3,6-dicyanopyrazine(DADCP) 29 can be synthesized by the oxidative

in benzene in the presence of coupling of 2,3-diamino-3-(phenylthio)acrylonitrile citric acid and sodium citrate in 75% yield (Scheme 1). DADCP 29, a new fluorescent chromophore, can be converted to the tetraalkylamino derivatives 30 (R=Me, 76%; R=n-butyl, 5%;R=benzyl, 18%yields) by treating of 29 with excess of alkyliodide in the presence of sodium hydroxide in dimethyl-acetamide. Alkylation of 29 proceeded stepwise, and their absorption and fluorescence maximum shifted to longer wavelength depending on the number of alkyl groups.

341

Reactions of 29 with alkylenediiodide gave the cyclic amino derivatives, pyrrolidinyl 31a in 57% and piperidino 31b in 38% yields, respectively. Reaction of 29 with 1,2-bis(bromomethyl)benzene, 1,8-bis(bromomethyl)naphthalene and 9,10-bis(bromomethyl)-phenanthrenealso proceeded in 63-67%yields [61.

34

30

a ; n=4 31 b ; n=5

35

29 DADCP

36

Ny,;Ntd;CH-NMez HZN

32 Ny,NKN=SMe2 HZN

37

N

I

t

CN

39

Scheme 5. Summary of pyrazine dyes derived from DADCP (29). Hydrolysis of the cyano groups in 29 proceeded stepwise and direct hydrolysis of the cyano group to the carboxylic acid was unsuccessful. Hydrolysis of 29 in conc. sulfuric acid gave the carboxyamide in quantitative yield, and then it could be quantitatively converted to the carboxylic acid in 5% aqueous sodium hydroxide. The acid could be modified to the alkoxycarbonyl derivatives 32 by reacting with an alkyliodide in the presence of DBU in DMF. Further alkylation of 32 gave bis(N-alkylamino) derivatives 33 [6, 181. Acylation of 29 proceeded stepwise to give mixtures of products, such as the N-acyl-, the NJV-bis-acyl-, the N,”-bis-acyl

348

derivatives 34. Their absorption and fluorescence maxima shifted t o shorter wavelength depending on the number of acyl groups. Reaction of 29 with alkyl isocyanate gave the bis-alkylureido 35,and further ring-closure reaction of 35 gave the heterocycles 36. Syntheses and fluorescence properties of the polyazaheterocycles from 29 [25] and the self-assembling behaviors of the related dyes have been studied [26]. The amino groups of 29 are reactive in excess of dimethylformamide and dimethylsulfoxide as reaction solvents and give the corresponding mono- (37)and bis-azomethine (38)which have strong fluorescence, and the mono- (39) and bis-azasulfone derivative (40) having very weak fluorescence, respectively. 3. MOLECULAR STRUCTURE, CHROMOPHORIC SYSTEM AND THEIR FUNCTIONALITY

Color-structure relationships of dye chromophores have traditionally been the most important factors for colorants, but many interests are now focused on their functionalities in aggregates and the solid state. The absorption spectra, fluorescence properties and other functionalities are changed largely by their molecular stacking in the solid state. In this section, the optimized structure of the chromophore was calculated by using the MOPAC method and their structures were correlated with their spectral properties and other functionalities. Conformational changes of dye chromophores were studied from the experimental results. The molecular stacking behavior of the chromophores was correlated with the three dimensional molecular structure, and then to their functionalities in the solid state. Solid state absorption and fluorescence spectra were also used to evaluate the molecular stacking behavior in the solid state. Some X-ray crystal analysis data were correlated with the calculated results obtained using the PPP MO and the MOPAC methods.

3.1. Structures at the molecular level and their conformational changes The dicyanopyrazine moiety has a planar structure and has a strong electron withdrawing ability. Consequently, the related dyes have a strong intramolecular charge-transfer chromophoric system, which is well reproduced by the PPP MO method in planar chromophoric systems. Conformational analysis and the optimized structure of the related dyes was evaluated by the MOPAC method. Dye 18 (R,R=1,4-butylene) showed temperature dependent 1H NMR spectra, in which two sets of the methylene protons in the pyrrolidine ring showed different chemical shifts to each other at room temperature, but which were overlapped completely a t 130°C. Two conformers of 18 can be pictured by rotation of the single bond between the pyrazine ring and the ethenyl group. From the calculated

349

heats of formation for the optimized structures, 18a is more stable than 18b by 2.6 (PM3) and 4.4 kcal/mol (AM1). Rotation of the single bond between the amino nitrogen and the ethenyl carbon in 18a was evaluated by the MOPAC version 6 using the AM1 method [281. The relationship between the calculated heat of formation and the rotation angle in 18a is shown in Figure 1 [lll. The most stable conformation of 18a calculated by the AM1 method has an almost planar structure, but that from the PM3 is bent at the pyrrolidine nitrogen atom. Thus, AM1 suggested an sp2 nitrogen atom while an sp3 nitrogen was suggested by PM3. The results indicate that 18a is the more stable conformation at rotational angles of around 0" and 180". The Arrhenius plot of 18a was obtained from the results of 500 Mz 1H NMR spectra. The heat of activation was calculated from the slope, and determined to be 8.0 kcal/mol, a value which was well reproduced by the MOPAC calculations of 8.7 by the AM1 and 6.8 kcal/mol by the PM3 method, respectively. From these results, it is seen that the MOPAC approaches are very valuable t o in the evaluation of the optimized structure and rotational conformers. It was found that dye 18 has a planar structure with intramolecular charge-transfer character, but has a rotational conformer at higher temperature [ill. 1381

g E

4

x

132-

.# s

c

I

130-

N 'f

NC

N

CI

18b

18a

128-

1264 -200

NC

-100

0

I00

200

Angle (degrea)

AM1

PM3

Figure 1. The relationship between the rotation angle at the C-N bond and the calculated heat of formation of 18a, and the AM1 and PM3 optimized structures of 18a.

350

Dye 19, having similar structure to 18, also existed in two rotational conformers at room temperature, which was confirmed by their 500 MHz 1H NMR spectra. Dye 20 existed in two rotational conformers, as illustrated in Figure 2. The dyes showed two sets of 'H NMR signals that were consistent with the two conformers. Dye 19 has trans-configuration with respect to the ethylene protons, which showed a coupling constant of J= 11.6 ppm. Their ratio in solution was determined from their integral values and found to be 6:lfor 19 and 4:lfor 20 181.

1SA

a

Et

H NC \N

I

198

N

CI

20A

b 208

0.4

03

Figure 2. Two rotational conformers for 19 and 20, and the 500 MHz NMR spectra of 20.

351

The olefinic proton of 20A was observed at 8.42 ppm (a) as a singlet and that of 20B was at 8.30 ppm (a') as a singlet. The ring methylene protons were observed at 2.55 ppm (b) and 2.58 ppm (b') as triplet, respectively [81. The conformer A was

therefore proposed t o be the predominant one, in accordance with the calculated results for 18 [ l l l . The geometry optimization of dye chromophores can be carried out effectively by the MOPAC method. For example, the chalcone derivatives 6 show large solvatochromism depending on the polarity of the solvent used. The chromophoric system of 6 is interrupted at the carbonyl group by the steric requirements of the substituents. The dihedral angle between the two n-conjugation system of 6 (R=Me) was calculated t o be around 67" by the MOPAC method [191, and consequently, no conjugation through the whole molecule was apparent. These results are good in accordance with the experimental data. The completely planar optimized structure for dye 7a was also obtained by the same method [121 and was also confirmed by X-ray crystal analysis [24]. On the other hand, non-planar structures for dyes 9 were also obtained in which the phenyl substituents were twisted from the planar n-conjugation system at around 80-90' t o avoid steric hindrances [131. Optimization of the chromophoric systems of 21 by using the MOPAC method was useful in correlating their chemical structures with their absorption spectra. The heat of formation and the dihedral angle between each n-conjugation system were also useful to ascertain their three dimensional molecular structure. The calculated optimized structures were correlated with their chromophoric systems, substituent effects, molar absorptivity and structural changes affected by large substituents [14,161. Some of the optimized structures obtained from the MOPAC PM3 method are shown in Figure 3. Structural optimization of the 7-membered azepine 14 was conducted using MOPAC methods and revealed the non-planar structure of 14. We also find that 15 has a non-planar structure. The dihedral angle between the azepine and the ethylene moieties is calculated at around 56"(Figure 4). Compound 14 exhibited an equilibrium between the neutral and the deprotonated ion pair species in dimethylsulfoxide, which was confirmed by the 1HNMR spectra. The amino hydrogen of 14 was found to be acidic, and the deprotonated anion will therefore be more stable than the neutral species in polar solvents. Compound 14 had no fluorescence, but 15 had a strong red fluorescence at 610 nm, and is thus a new fluorescent chromophore having a non-planar conjugated system [19]. The fluorescence properties of 15,with respect to its nonplanar conjugated structure, are of interest for further studies. On the other hand, dye 15 showed reversible absorption changes under photoirradiation.

352

O

H

Ncx:mNMe2 NC

6 (R=Me) 67" twisted

fl H

N K C N

/ H

/

/ N N e C N CN

"H".

9

(trans)

A

@ NC

CN

NC NC 2-Y N-

-

\ /

(cis)

21a

21a

*

A 39' twisted

*

A

40' twisted

Figure 3. Optimized structures of 6 (R=Me),9 (1,3-isomer)and 21a using MOPAC methods.

353

Irradiation of 15 at 540 nm in benzene produced a hypsochromic shift of Lax from 546 nm to 527 nm accompanied by an increase of absorbance from 33,400 to 43,500 (E); the absorbance at 370 nm decreased. The reverse process was observed when W light at 370 nm was used for the irradiation (Figure 4). These observations are proposed to arise from the photo-induced conformational changes of the twisted azepine moiety before and after irradiation.

enamine

14

deprotonated

B

A

t

10 min

I

f

c

I

MO

I 400

I

I Hx)

1

I €00

Figure 4. Tautomerism of 14 and the optimized structures of julolidine dye 15, and the photochromism of 15 in benzene.

354

3.2. Spectral properties and molecular stacking of pyrazine dyes in solution and the solid state The dicyanopyrazine moiety of 17 and 29 has strong electron withdrawing ability and related dyes have strong intramolecular charge-transfer chromophoric systems. Introduction of donors into the nucleus produces bathochromic shifts of Laxdepending on the number of substituents and the strength of their donor properties. Dicyanopyrazine related dyes generally have strong fluorescence both in solution and in the solid state. The basic chromophoric system of 29 was compared with that of 2,5-diamino-3,6-dicyanobenzene 41 by using the PPP MO method. The synthesis of 41 needs many steps, starting from the intermolecular condensation of ethyl 4-chloroacetoacetate, and the overall yield of 41 was 10% [61. Chemical modifications of 29 in order to obtain more dye chromophores were relatively easy in comparison with those from 41. Comparison of the MO calculation results, 7c-electron density changes accompanying the first excitation, and the observed absorption and fluorescence properties are summarized in Figure

5. NC

N

NCaNHz

NH2

XN&,

H2N

H2N

X,,,= 458 nrn (log E 3.41) bid. 442 nm ( f = 0.506 ) ,F 559nm (DMF) S.S.

101 nm 2.804 eV €HOMO -8.6957 E!JJMO -3.0269 AEl

0.080

CN

41

29

424 nrn (log E 3.63) 419 ntn ( f = 0.323 ) F516IW1 (DMF) S.S. 92 nrn AE, 2.962 eV €HOMO -8.4818 ELUMO -2.5760

-,b

&id,

0.102

Figure 5. Comparison of the observed spectral properties and the calculated results by the PPP MO method for 29 and 41.

355

Dye 29 produced a bathochromic shift of 34 nm compared with 41,due t o the are well in electron withdrawing ability of the pyrazine ring. The calculated Lax accordance with the observed values, but the oscillator strength of 29 was calculated to be much larger than that of 41. However, reverse experimental results were obtained. The bathochromic shift of 29 from 41 can be explained by the difference in the energy levels of the frontier orbitals, and the relatively much lower ELUMO value of 29 indicates the much stronger electron withdrawing ability of 29 compared with that of 41. The n-electron density changes also indicate the stronger electron withdrawing ability of the ring-nitrogens, consequently resulting in the much stronger intramolecular charge-transfer chromophoric system of 29. From the calculated results, the following substituent effects were deduced: Nalkylation produces a bathochromic shift; acylation of the amino groups produces a hypsochromic shift, and hydrolysis of the cyano groups produces a hypsochromic shift. All these results were well in accord with the experimental results summarized in Table 1 161. Table 1 Substituent effects on the absorption (Lax) and fluorescence spectra (Fmax) of dyes derived from 29 and their spectral changes from solution to solid state Compd. No.

29 30 30 31a 31b 32

R

h a x (log E)

Frnax (soln.) Fmax (solid)

SS a

(nm)

(m)

AF

458 (3.41)" Me 495 (3.64)" ~ - B u 522 (3.68)'

538 594 603 a

-h 643 651

80 99 81

49 48

0.4

526 (3.74)" 492 (3.59)' 458 (3.88)"

601 ' 607 ' 544 =

706 629 644

75 115 86

105 22 100

0.3

63 92

0.8 0.6

Et

0.3

357 (3.84) "

432"

37 38

452 (3.84)' 456 (3.91)'

527 ' 525

590 617

39 40 41

513 (3.74)'

613

-

100

526 (3.61)"

651

699

125

48

424 (3.63)

516 '

524

92

8

34

Quantum

(m) Yield (@)

Me

a

@

75 75 69

.E

a, Stokes shift. b, Measured in 1,Zdimethoxyethane; c, in chloroform; d, in dichloromethane; e, in dimethylformamide; f, in water; g, in acetonitrile. h, Not detectable.

0.1

356

The typical intramolecular charge-transfer chromophoric system of pyrazine dyes is exemplified by dye 5 in which the alkylaniline moiety acts as a donor and the dicyanopyrazine moiety acts as an acceptor. The x-electron density changes accompanying the first excitation of the styryl dye 5 are shown in Figure 6. Decreases of electron density at the aniline moiety (donor) and increases of it at the dicyanopyrazine moiety (acceptor), especially a t the 5-cyano group and the ring nitrogen atoms, are observed [91. They show small solvatochromism (at most 12 nm) depending on the solvent polarity, because the dipole moment difference between the excited and the ground states is relatively small. However, the 3hydroxyl derivative of 5 (X=OH, R=Me) showed large spectral changes induced by hydroxypyrazine-pyrazinonetautomerism.

-0,066

164

0.027 -0.002 0.009 0,045

NGC N q -0.168 -0.Oli’ I 0.129

5

Ooo79+

NEC

)1-0.036

N

0.121 0.136 0.177 Figure 6. The .x-electron density changes accompanying the first excitation of dye 5.

The color in solution changed drastically from yellow in polar solvents (the hydroxypyrazine form 5a, 451 nm in DMSO) to scarlet in nonpolar solvents (the pyrazinone form 5b, 498 nm in chloroform). These observations were well reproduced from the results of PPP MO calculations [91. Similar hydroxypyrazinepyrazinone tautomerism was also observed in the cases of the julolidinyl analogue of 5 , (the hydroxypyrazine 5c and pyrazinone 5d) and the hydroxyl derivative of aminovinyldicyanopyrazine 21b (Figure 7). The hydroxypyrazine absorbed at shorter wavelength and the pyrazinone absorbed at longer wavelength, which was

357

also confirmed by PPP calculations. The tautomerism and solvent effects of 21b are shown in Figure 7. The tautomeric mixtures of 5c and 5d in solution were confirmed by their 1H NMR spectra in comparison with the 3-H derivative 5e [14]. The chemical shifts (ppm) of each proton for 5e were observed at 8.560 (s, 3-H), 6.782 and 7.844 (olefin, d, J=15Hz), 7.096 (s, benzene), 2.764 and 3.298 (t, methylene) and 1.980 (quintet, methylene). These results are well in accordance with the structure of 5e. Those for the tautomeric mixtures of 5c and 5d were observed a t 6.634 (broad, 3-OH), 6.775 and 7.640 (broad, olefin), 6.824 (s, benzene), 2.653 and 2.674, 3.144 and 3.257 (m, methylene) and 1.913 (m, methylene). The broad peaks of the olefin protons indicated mixtures of isomers and therefore both 5c and 5d existed in solution. In the case of 21b, the pyrazinone was predominant in a nonpolar solvent such as chloroform, and the hydroxypyrazine was predominant in polar solvents such as DMSO or alcohol (Figure 7). 472 nm

5a 5~

5b 5d

(R=Me,R'=H) (RR' = (Cb)s)

Me

' U

8

4

P

M $ -e :

a N c K __t

21b Hydroxy pyrazine form Calcd. hmax 465 nrn

NC

H

li

Pyrazinone form 494 nm

Figure 7. Hydroxypyrazine-pyrazinone tautomerism of 5 (X=OH) and 21b,and the solvent effects of 21b.

358

The intramolecular charge-transfer chromophoric system of 22 was evaluated by the PPP MO method and the strong electron withdrawing effects of the dicyanopyrazine moiety was confirmed. The alkyl derivatives of 22b (R=Me) absorbed at 398 nm in ethanol but dye 22b (R=H) absorbed at 440 nm in spite of the lower electron donating property of the imino group (R=H) compared to that of the alkylamino group. Addition of acetic acid t o the ethanol solution of 22b (R=H) produced a hypsochromic shift t o 400 nm, and addition of sodium ethoxide produced a bathochromic shift to 580 nm. From these results, it was concluded that 22b (R=H) exists as the monoanion in ethanol and the bathochromic shift of 40 nm from 22b (R=H) was explained by the acidic character of the imino group. Tautomerism of the 1,kdihydro isomer to the 9,lO-dihydro isomer of 22b was also confirmed. The reaction of 2,3-dich1oro-6,7-dicyano-1,4,5,8-tetraazanaphtha1ene and 1 in DMAC gave the 1,4-dihydro isomer, which spontaneously isomerized to the 9,lO-dihydro isomer. The symmetrical structure of 22b (R=H) is much more stable than the 1,4-isomer. Oxidation of 22b to 2,3,7,8-tetracyano-1,4,5,6,9,10hexaazaanthracene, as a strong electron acceptor, was unsuccessful (Figure 8). The reduction potential of 22b was observed at around -1.30 volt and reversible, and oxidation of 22b hardly proceeded [lo].

DMAC rOOmt0mp.

1xkX,XCN CN

NC NC

N

17

N

1 ,.l-isomer

9 R

22b

*y

1

N NC

y

x N

y

y

N

N

CN

9,lO-isomer

22b (R = H)

Figure 8. Alternative synthesis of 22b (R=H) and tautomerism from the 1,4isomer to the 9,lO-isomer 22b (R=H). The monostyryl dye 42, having similar structure to 8 , showed two absorption maxima at around 400 and 520 nm. The corresponding dye 5 (X=Me) showed a

359

single peak a t 500 nm and the two maxima of 42 were due t o the phosphorane group at 3-position. As a result, 42 showed a large halochromism of 150-190 nm depending on the acid-base equilibrium a t the 3-phosphorane group. Three resonance hybrid structures 42a-c can be considered, in which 42c mainly donates electron to the pyrazine moiety and produces a hypsochromic shift of L a x . Addition of acetic acid to the solution of 42 produced a large bathochromic shift due to protonation at the carbanion in 42b, in which the reverse electron withdrawing effect (inductive effect) of the 3-substituent produced a much more bathochromic shift of Lax to 570nm in comparison with that of 5 (X=Me) a t 500 nm. Resonance hybrid structures and the halochromism of 42 are shown in Figure 9 1121.

1

FNR2

NC N N CH=PPh3

428

42b

z

P

c

42c

Wavelength (nm) Figure 9. Resonance hybrid structures of 42 and the effect of acetic acid on the absorption spectra of 42 in chloroform, AcOH / 42 (moVmo1); 1(O/l), 2 (lO/l), 3 (lOO/l), 4 (1000/1).

360

3.3. Molecular stacking and solid state fluorescence of pyrazine dyes Dicyanopyrazine dyes generally have strong fluorescence in solution and the solid state. It is of interest to study the fluorescence properties of dye chromophores in connection with their energy level in the first excited state. Some characteristics of the ground state of a chromophore can be evaluated from absorption spectra and from the PPP MO method. On the other hand, those in the excited state can be deduced from their fluorescence spectra. DADCP 29, in particular, has a small .n-conjugation system, but has strong fluorescence with a high fluorescence quantum yield of 0.8, and is therefore a good candidate to study the relationship between fluorescence properties and molecular structure in the excited state [6,18]. The differences in &ax and Fmax of chromophores from the solid state to the solution are denoted as A?, and AF, respectively. The Stokes shift (SS) denotes the difference between F,,, and &ax, and is a very useful parameter in discussions of the fluorescence properties of chromophores. The SS value indicates the emission energy level with respect to the first excited state of the chromophore, and a large SS value indicates large structural changes or energy loss during the emission process. These values can be effectively used to evaluate the molecular aggregation behavior or intermolecular interactions of dye chromophores in the solid state, as well as in molecular level. We have reported the molecular stacking and intermolecular x-.n interactions of some naphthoquinones and tetrathiabenzoquinones in which large Ah values correspond to large nonlinear optical susceptibility [29-331. In the following section, some functionalities including spectral properties of the pyrazine chromophores are discussed, in connection with the parameters A?,, AF and SS. Solid state absorption spectra were measured in the vapor deposited thin film and the solid state fluorescence were measured in the same film or in the powder state. Three types of dyes 30, 32 and 33 derived from 29 were used to evaluate spectral properties in solution and the solid state [6]. The spectral properties of 2,5-bis(N,N-dialkylamino)-3,5-dicyanopyrazine(30 and 31) in solution and the solid state are summarized in Table 2. Substituent effects on their spectral properties can be evaluated by using the parameters A?, and A F . At the molecular level, the &ax, Fma, and SS values did not change so much, because intermolecular interactions between the dye molecules are not operative in solution. In the solid state, A?, and AF values changed from positive to negative and were influenced largely by the substituents. Large differences in A 1 were observed among dyes 30a (94 nm), 30b (37 nm) and 30c (66 nm). These differences indicated that the length of the alkyl substituent on the amino groups significantly affected their molecular stacking, due t o steric hindrance between the molecules and the hydrophobic interactions of the alkyl groups.

361

Table 2 Substituent effects in 30 on their &ax and F,,

in solution and the solid state.

30 Dye No.

30a 30b 30c 30d 30e

h a x (m) (soh.) (solid) 496 590 Me2 550 EtZ 513 584 (n-Prl2 518 498 (CH2Ph)Z 488 487 (CH2C&14Me-p)2 496

R,R

31a 31b

526

-(CH&-

492

461 596 417 566

AA (nm)

94 37 66 10 -9 - 65 70 -75 74

Fmax (m) (soh.) (solid) 601 690 602 647 607 664 588 608 590 574

AF

(nm) 89 45 57 20 -16

SS (nm) 105 89 89 100

94

601

706

105

75

607

629

22

115

These substituent effects can be evaluated from the optimized molecular structure obtained by the MOPAC method (Figure 10). Dye 29 has a completely planar structure, but substitution of the alkyl moieties on the amino groups caused some steric hindrance for their molecular stacking. The benzylamino derivative 30d produced a small hypsochromic shift (-8 nm) of Lax in solution compared to that of 30a,and showed a smaller A 1 value of 10 nm. It is proposed that in dye 30d there is significant steric hindrance between the two benzyl groups and none of the intermolecular 7c-7c interactions affecting their solid state spectra are operative; steric factors of the N,N-bisbenzylamino groups in 30d are exemplified in Figure 10 1211. Two of the benzyl groups are oriented perpendicular, and completely overlapped the dicyanopyrazine chromophoric system, thus preventing intermolecular 7c-7c interactions. On the other hand, dyes 30a-30cwere concluded to interact with each other, because of the lower steric hindrance of the alkyl substituents. Dye 30e showed a negative Ah value of -9 nm, which was concluded t o arise from steric factors originating at the additional 4-methyl group, thus preventing intermolecular n-7c interactions. A single peak was observed for compounds 31a (526 nm) and 31b (492 nm) in chloroform, but these dyes showed two absorption peaks in the solid state (Figure 11). Splitting absorption spectrum of 31 was proposed to arise from the separation

362

of the degenerated excited state, caused by the reduction of molecular symmetry in the solid state.

NC

N

NH2

XNXCN

H2N

29

Me2N

30a

NP

A

Figure 10.

B

Optimized structures of dyes 29,30and 31 obtained by the MOPAC PM3 method (A: front view, B: side view) and steric requirements of alkyl groups.

363

The shifts of the two peaks in the solid state from that in solution were almost the same, and this was proposed to arise from separation of the degenerate excited state. These observations have been reported in the solid state absorption spectrum of titanylphthalocyanine 1341. Here, the intermolecular interactions reduced the molecular symmetry of the phthalocyanine chromophore in the solid state and the degenerate excitation energy level in the molecular level (in solution) was split to higher and lower energy levels, thus producing two visible absorption maxima [211. In the cases of 31a and 31b,their molecular symmetry is proposed to be Ci, and the symmetry will be reduced by intermolecular interactions of the cyclic amino moieties. On the other hand, AF values also decreased with increase in the steric hindrance parameters of the alkyl groups, as observed for the Ah values. Fluorescence is emitted from the lowest excited state, and the F,, value of dyes 31a and 31b were observed from the lower energy level at 706 nm for 31a and 629 nm for 31b in the solid state. It is of interest that dyes 30 and 31 have a small nconjugation system, and absorb at around 500 nm in solution, but emit strong red fluorescence in the solid state. These large color differences, accompanying strong fluorescence, can be utilized in fluorescence color filters for display, copypreventing inks, etc.

m

E 2

9

31b

300

400

500 600 Wavelength (nrn)

700

800

Figure 11.Absorption spectra of 31a and 31b recorded on a n evaporated thin film.

364

The substituent effects of R in dyes 32 are summarized in Table 3. Dyes 32 have a planar structure in the n-conjugation system, but the alkyl group R in 32 has an sp3 carbon atom, and shows the "zigzag" conformation ''up or down" from the n-plain. It is proposed that substituent R does not have much effect on their spectra with respect to electronic effects, but may affect the intermolecular n-n interactions. Dyes 32 are valuable candidates to evaluate the effects of molecular stacking on spectral properties in the solid state. No substituent effects of R were observed in the values of Lax, Fmax and SS values in the molecular level, but quite large differences were observed in the Ah values in the solid state. The Ah values decreased with increase of the chain length and the steric character of the substituent R. There are many conformations in respect t o the orientation of the ethyl groups. The optimized structure, having the lowest heat of formation, was obtained for 32a by the MOPAC PM3 method, as shown in Figure 12 [18]. Table 3 Substituent effects of 32 on Amax and Fmax in solution and in the solid state

xx

R02C ,N H2N

NH2

N C02R

32 Dye

No.

R

32a Et n-Bu 32b 3 2 ~ i-Bu 32d n-CloHzl CH2Ph 32e

(soln.) (solid)

AA (nm)

458 456 453 456

62 91 33 26

hmax (nm)

460

520 547 486 482 506

46

Fmax (nm)

(soln.) (solid) 546 546 545 546 548

621 599 602 600 615

AF (m)

75 53 57 54 67

SS

(nm) 88 90

92

90 88

365

Et02CxNxNH EtHN

N

Et

C02Et

33a

Figure 12.

Optimized structures for 32a,33a and 33c obtained by the MOPAC PM3 method.

On the other hand, the X-ray crystal analysis of 32b was successfully conducted and some of the results are shown in Figure 13 [26]. Dye 32b showed a planar nconjugation system, and a zigzag orientation of the butyl group in crystals. The intermolecular hydrophobic interactions, the bent structures of the butyl group, and the intermolecular n-n interactions of the chromophore produced strong molecular stacking in the crystals. The intermolecular overlap between the nearest upper and lower molecules are shown in Figure 13(A), and the interlayer molecular overlaps with the distance of 2.7-2.9 A are shown in Figure 13(B). Interatomic charge-transfer interactions between the amino nitrogen and the

366

carbonyl carbon were also observed in Figure 13(A) [26]. The very short interlayer distance also confirmed strong intermolecular 5c-5c interactions. The large Ah value of 91 nm for 32b in comparison with that of 33 nm for 32c can be explained in terms of the differences in the steric parameters between the more bulky iso-butyl group and less bulky n-butyl group. In the case of 32,no critical band splitting in the solid state absorption spectra was observed, and the substituent R did not affect so much change in the molecular symmetry in the solid state as was observed in the case of 31.

B Figure 13.

X-Ray crystal analysis of dye 32b. Molecular overlap from the frontview of the 7t-plane (A) and the side-view of the 5c -plane (B).

367

Additional substitution of R1 to dyes 32 gave the corresponding dyes 33. Substituent effects of dyes 33 on their Lax and F,, values in solution and the solid state are summarized in Table 4. Table 4 Substituent effects of 33 on Lax and F,, R02C

R'HN

in solution and in the solid state N

NHR'

N

C02R

EX 33

Dye R No. R' 33a Et Et 33b Bu Bu 33c Et CH2Ph 33d i-Pr CHZPh 33e CH2Ph mZph 33f CHzC&CN-p CH2C84CN-p

hax(nm) A), (soln.) (solid) (nm) 514 506 - 8 514 492 -22 504 502 510 530 20 502

Fmax(nm) AF SS (soln.) (solid) (nm) (nm> 614 663 49 100 624 654 30 110 604 646 42 100 598 615 17 96 607 601 - 6 97 596 584 -12 94

It is therefore proposed that intra- and intermolecular steric hindrance factors arising from of R and R1 may affect the absorption spectra in the solid state. Dye 32a has no steric effects, but substitution of additional ethyl groups in dye 33a produced a hypsochromic shift of 8 nm from solution t o the solid state. A similar result was observed in the case of dye 33b,in which a hypsochromic shift of 22 nm was observed. The more bulky butyl group in 33b produced a larger hypsochromic shift in the solid state absorption in comparison with that of dye 33a. Introduction of the benzyl group (33c-33f)as R1 produced a hypsochromic shift in the absorption spectra in solution, and distortion of the chromophoric system in the solid state was therefore concluded from observations of the optimized structure of dye 33c (Figure 12). Intermolecular n-71 interactions of the chromophore are completely prevented by the steric requirements of the benzyl group, in the same manner as in dye 30d. The AF values of dyes 33 also decreased depending on the extent of steric hindrance of the substituents, and negative AF values were observed in the cases of dyes 33e and 33f, in which serious steric hindrance

368

between both of the benzyl groups was clearly evident. New pyrazine fluorescence dyes derived from 29 are thus valuable to obtain longer wavelength fluorescence in the solid state, a property that can be applied in fluorescent functional materials [61. Dye 9 absorbed at around 340-393 nm and emitted a t 440-480 nm in chloroform (Table 5 ) . The SS values changed depending on the orientation of the substituents. Dye 9a (1,2-isomer) and 9c (1,kisomer) showed large SS values of 114, and 127 nm, but 9b (1,3-isomer) showed a value of 83 nm. The pyridine analogue 9d showed the smallest of 47 nm. Solid state fluorescence maximum shifted to longer wavelength in the range of 5-40 nm in comparison with those in solution, which were also influenced by the position of the substituents. These rather small AF values were mainly influenced by the steric hindrance arising from the phenyl groups, which prevented intermolecular ?c-x interactions of the chromophore. Steric hindrances in 9 are explained by the differences in their optimized structures obtained by the MOPAC PM3 method (Figure 3) 1131. Table 5 Substituent effects of 9 on ha, and F,,

“1

NC N C,N- ; N ‘

in solution and in the solid state

Ph 9

Dye No.

X

9a 9b 9c 9d

CH (1,2-bis)

CH(1,3-bis) CH(1,4-bis) N (2,6-bis)

hmax

(m) 366 376 340 393

Fmax (nm) 480

459 467 440

Ph

N’ CN

SS

(m) 114 83 127 47

Fmax(so1id)

AF

(W

(nm)

516 464 507 469

36 5 40 29

On the other hand, large AF values were observed in the case of dyes 12. These dyes absorbed a t around 350-420 nm and emitted a t 410-520 nm in solution, but their solid state fluorescence was affected largely by their molecular stacking, and the AF values changed from 125 to -42 nm (Table 6 ) . It is of interest to note that

369

12 has the same chromophoric system, and has a completely planar structure in the dicyanoquinoxaline moiety, but their molecular stacking was affected largely by the substituents R. The SS values were also affected by the substituents, values from 60 to 120 nm being observed. We intend to explain these results by their structure in the solid state and their molecular stacking behavior in connection with their fluorescence quantum yield in solution in due course. The naphthalocyanine analogue 13 derived from 12 absorbed at around 820 nm and is thus a possible candidate for near infrared absorbing dyes. The solubility of 13 (R=Ph) in organic solvents was very poor, but introduction of long chain alkyl groups (R) improves their solubility [17].

Table 6 Substituent effects in 12 on ha, and F,,

in solution and in the solid state.

l2 Dye No. l2a 12b 12c l2d Ue 12f 1%

R Ph 4-MeOC6H4 4-BrC6H4 Ph, Me Ph, H Me, Me Me, H

hmax

(m) 383 420 393 365 382 358 347

Fmax (nm) 502 518 470 483 460 436 407

SS

Fmax(s0lid)

(m) 119 98 77 118 78 78 60

(m) 470 567 496 441 560 561 497

AF (m) - 32 49 26 - 42

100 125 90

370

Type 23 dyes containing long chain alkyl groups showed a wide melting point range, and also showed phase-transfer character which also influenced the aggregation behavior of the pyrazinophthalocyanine analogues 27 derived from 23 [171. All compounds 23 and 24 have blue to green fluorescence at around 400-530 nm, influenced by the chromophoric system and substituents therein. The SS values of 23 were relatively large (110-140 nm) but those of 24 were small (40-60) nm [171. These differences in both chromophoric systems are of especial interest with respect to their fluorescence properties. Many pyrazinophthalocyanines, viz., 13,27 and 28 were synthesized from their precursors 12,23and 26,respectively. They are phthalocyanine analogues having annelated pyrazine chromophores. Large differences in their fluorescence properties were observed in both chromophores; phthalocyanines are known to have fluorescence [351 but their applications as fluorescence chromophores have not been reported. On the other hand, the pyrazinophthalocyanines 27 have a strong red fluorescence both in solution and the solid state (Table 7). For example, dye 27 absorbed a t 713 nm in chloroform and emitted a t 719 nm, and the SS value was very small, 6 nm [171. Table 7 Visible and fluorescence spectra in chloroform of dyes 27 containing multiple long alkyl groups.

Compd. rnp('C)

27

..

hmax(nrn) Fmax(nrn) SS

27a

82-90

713

719

6

27b

153-164

713

719

6

27C

280-285

712

719

7

27d

>300

712

719

7

37 1

Their spectral properties in solution were not changed by the number and the chain length of the alkyl groups in 27. Molecular aggregations induced by hydrophobic interactions of the long chain alkyl substituents were observed in 27. They showed wide range melting point in spite of their large molecular weight, over 3,000. A series of pyrazinophthalocyanines having long chain alkyl groups [20] exhibit high solubility in nonpolar solvents, crystal morphology and liquid crystal properties, molecular aggregations depending on temperature and solvent polarity, near infrared absorption and red fluorescence. It is of interest t o note that 27a showed critical temperature dependence of the spectral changes over the range 25°C t o 50°C (Figure 14A). The monomeric form was predominant at higher temperature, and aggregates were predominant at room temperature in cyclohexane. The spectra were also affected by solvent polarity, and addition of small amounts of chloroform to the cyclohexane solution of 27a significantly changed the spectra (Figure 14B) [17]. These spectral changes were induced from their molecular aggregation, affected by the hydrophobic interactions of the long chain alkyl groups.

I

I

600

Figure 14.

I

I

700

Wavelength (nm)

I

600

700 Wavelength (nm)

0

Temperature dependence (A) and effect of solvent polarity (B) on the absorption spectra of 27a containing long chain alkyl groups.

372

Other examples of the molecular stacking influence on the absorption spectra and fluorescence spectra were studied in the case of dyes 43, 44 and 45. These have the same chromophoric system as aminovinyldicyanopyrazine 21,but have a long chain N-alkyl group [221. Their structures and spectral properties are summarized in Table 8. Table 8 Effects of N-alkyl chain length of 43-45 on solid state.

43 Dye No.

43a 43b 43c 43d 44a 44b 45a 45b 45c

0

8 10 15

0 8 3 8 15

44 (nm)

n

(CHC13) (solid)

272-273 92-106 95-101 262-270 298-300 150-164 250-251 164-169 148-152

Laxand ,F

472 471 471 473 536 531 560 552 552

585 487 518 561 515 582 619 474 461

in solution and in the

45 Ah (nm) 113 16 47 88 -21 51 59 -78 -85

AF Fmax(nm) (CHC13) (solid) (nm) 540 550 556 558

625 597 610 627

85 47 54 69

SS (MI)

68 79 85 85

They showed a wide range of melting point of up to 14 "C with increase of the chain length of alkyl group, due to the phase-transfer property induced by the intermolecular hydrophobic interactions of the alkyl groups. They also showed similar visible and fluorescence maxima and SS values in solution, and no

313

intermolecular interaction of the dye chromophores were observed in molecular level. However, their solid state absorption and fluorescence spectra changed significantly depending on the chain length of alkyl group. Dye 43a showed exceptionally large Ah (115 nm) and AF (85 nm) values and was concluded to have special molecular stacking in the solid state, a phenomenon that has also been observed in the J-aggregate of some cyanine dyes. A bent structure, for the parent chromophore 21a (43a,n=O), was calculated using MOPAC methods (Figure 3). The AA. values of 43 increased with the length of the alkyl group, which indicated an increase in the intermolecular hydrophobic interactions of the alkyl group. These dyes have a relatively large SS value, arising from the dicyanopyrazine moiety. Dyes 44 and 45 did not show any detectable fluorescence. The large AF values of 43 in the solid state were effective in obtaining longer wavelength fluorescence (orange t o red) from yellow to red colored dyes. The Ah and AF values increased as the number of the methylene units (n) from 8 to 15. On the other hand, dyes 44a and 45b,45c showed large negative Ah values, which indicate the head-to-tail molecular stacking of the paired chromophore.

Me

\

43a

400

Figure 15.

500

600

Wavelength (urn)

7 0

Differences in the absorption spectra of 43a in chloroform ( .... ) and a vapor deposited thin film ( - ).

374

Such hypsochromic shifts in ha,are generally observed in the case of cancellation of dipole moment in the ground state, as is well known in the case of H-aggregates in some cyanine dyes. It has also been established that the parent chromophores of dyes 44 and 45 have a planar structure in comparison with the bent structure (40") of 43 [16]. The intermolecular n-n interactions and electrostatic interactions of 44 and 45 then cancel the large dipole moment predominantly, thus affecting molecular stacking [221. The large difference in Ah values of 137 nm between 45a (n=3)and 45b (n=8) is very critical and is influenced by the length of alkyl group. The differences in the absorption spectra of 43a from solution to the solid state are shown in Figure 15 [161. Time dependence absorption spectra of the vapor deposited thin film of 43b are shown in Figure 16, in which spectral changes from the amorphous state to the assembled structure are observed, accompanied by a bathochromic shift of Lax [221. Dye 43b showed a wide melting point range from 92°C to 106°C and the DSC showed two endothermic peaks at 103°C and 113"C, respectively, which indicates the phase-transfer character of 43b. The TGA of 43b showed that weight loss started at 248"C, an exothermic peak of decomposition at 363°C with weight loss of 40%, and another weight loss of 18% was observed at 603°C.

3.4. Self-assembling and solid state functionality of pyrazine dyes The solid state chemistry of dye molecules is of current interest with respect to correlating their functionality with the molecular stacking behavior in aggregates. Functional dye materials useful as organic nonlinear optical materials, organic photoconductors, and emitters for electroluminescence devices are now being studied with respect to the intermolecular n-n interactions of dye molecules in the solid state. Topochemical photoreaction of organic crystals gives high regioselective and stereoselective products, because the mobility of each molecule is restricted [361. A potentially useful photoreaction is [2n+2n] cycloaddition, in which topochemical dimerization and polymerization proceed [37, 381. The photoreaction of 7a has been studied in solution, in evaporated thin film, and in single crystals. The topochemical intermolecular dimerization reaction in the solid state was confirmed from the reaction products of the head-to-tail type cyclobutane derivative. The same monocyclobutane derivative and/or the biscyclobutane derivative were also isolated from the photolysis of 7b in solution. X-ray crystal analysis of 7a revealed a layered molecular stacking, and the results were correlated with the structure of the light-induced reaction products [24]. The reaction and products are summarized in Scheme 6 . A benzene solution of 7a was irradiated by W light at 365 nm for 20 h, and the photodimer 10 was isolated in 50% yield. The head-to-tail dimerized structure for 10 was confirmed by 1H NMR

375

and mass spectrometry, in tandem with -ray crystal analysis of 7a. In related studies, photocycloaddition of alkyl 4- [2-(4-pyridinyl)ethenyllcinnamate in the solid state has been previously reported to give the head-to-tail dimer [39]. Similar coupling patterns of the protons on the cyclobutane ring are well reproduced in the case of 10. On the other hand, similar photoirradiation of 7b in benzene gave quite different products, giving mixtures of 10b and the corresponding cis-isomer in 67%yield, together with l l b in 33%yield in an overall conversion of 60% of 7b. The three products were isolated by column chromatography [241.

Me

43b

t

I

I

I

I

I

I

Figure 16. Time dependence absorption spectra of the vapor deposited thin film of 43b, 1; (0.1 h), 2; (3 h), 3; (24 h).and the spectra in chloroform ( 1. ....a

The cis-isomer of 10b and its isomerization was confirmed by 1H NMR. Compound l l b has a head-to-head oriented two-layer structure that is connected by two cyclobutane rings. It was produced in solution by dimerization of 7b. The head-to-head type dimer having a single cyclobutane ring was not obtained. The equilibrium between the trans- and the cis-isomer of 10b was attained under photoirradiation at 290 nm or 365 nm, and an isosbestic point was observed at 300

316

nm. On the other hand, photolysis of 7a or 7b in the vapor deposited thin film gave the same dimers of 10. Dye 7a gave two kinds of crystals, one being red colored needle-like crystals (from ethyl acetate) and the other yellow colored crystals (from benzene). These crystals showed different color and different fluorescence at 515 nm (yellow crystal) and 579 nm (red crystal), respectively. They showed different photochemical reactivity; as the yellow crystal polymerized upon irradiation but the red crystals did not react. Similar results were obtained using vapor deposited thin films of 7a. In this case, the yellow film gave 10a and/or oligomers, but the red film was inert. After photobleaching of the yellow film, the products were extracted by chloroform and chromatographed to isolate 10a. From the extract, an oligomer having 18-19 monomer units of was detected by GPC. Irradiation of yellow single crystals of 7a gave trace amounts of 10a, along with several solvent insoluble products. NC

E

NC N

C

H

CN

365 nm benzene

',x-

NC'

'CN

10 (trans) a: X=CH

7

a : X=CH b: X=N

b: X=N

NC

+

CN

+ NC

CN

10 (cis) a : X=CH b: X=N

Scheme 6 . Photolysis of type 7 dyes in solution.

11 a : X=CH b: X=N

377

On the other hand, photoirradiation of the vapor deposited thin film of 7b gave lob,together with small amounts of insoluble polymers, but did not give llb and the cis-isomer of lob.

The reaction was controlled by interlayer distances and molecular orientation in the solid state, which was determined by the X-ray crystal analysis. X-ray crystal analysis of 7a (yellow crystal) revealed a planar layered structure with an interlayer distance of 3.5A. Each molecule in the same layer is oriented in the same direction, and the next nearest lines are oriented in the reverse direction. Alternative orientations of each line were observed in the same plane (Figure 17). The intermolecular distance of the ethylene units necessary to give the photodimer was calculated as 5.7A, and consequently 10a was obtained from the result of the lattice slipping in the crystals under photoirradiation. Such molecular or lattice slipping to give the photodimer has been previously reported [36,40]. Preparation of the red colored single crystals was unsuccessful and the relationship between the crystal structures and non-reactivity of it could not be ascertained. Dye 7a absorbed at 377 nm and emitted at 487 nm in chloroform, and showed a large SS value of 110 nm, which indicates large stabilization of the emission state with respect to the first excitation state. Dye 7b absorbed at 359 nm and emitted at 470 nm, and the SS value was 111 nm [12].The hypsochromic shift of 7b from 7a was attributed to the decrease of the donor property going from a benzene moiety to one of pyridine. Photodimerization of 7 to 10 produced a hypsochromic shift of ha,and F,, because of the interruption of the x-conjugation. The x-conjugation in 11 was completely interrupted between the donor and acceptor moieties and 11 had no color. Dye 10 showed strong bluish green fluorescence, but 7 showed a weak green fluorescence [241. The difference in fluorescence between 7a and 10a is of interest because of the opportunity to study fluorescence behavior from the aspect of three-dimensional molecular structures. It is generally known that the formation of the exciplex and/or intermolecular interactions of a fluorescent chromophore in higher concentration decreases or quenches fluorescence. These solid state photodimerization and oligomerization processes can be used to produce the nonlinear optical (NLO) devices, in which the NLO susceptibility can be changed by photo-masking techniques. Some pyrazine dyes have strong fluorescence even in the solid state and their applications as an emitter for electroluminescence (EL) device are of interest. The EL device was fabricated by conventional vacuum deposition. The chargetransporting material (a-NPD), and then dye 46, were deposited from a tungstenboat onto an IT0 coated glass substrate under 10-5-10-6 Torr. A mixture of magnesium and silver (1O:l)was then deposited from the tungsten-basket onto the dye deposited layer. The thickness of the charge-transporting layer, emitting layer and cathode were 50,67 and 200 nm, respectively, as shown in Figure 18 (A) D31.

378

A

B

Figure 17.

X-ray crystal structure of 7a; interlayer structure (A) and the overlapped pair (B).

The relationship between the applied voltage of the current density and the EL intensity of the device is shown in Figure 18 (B); maximum EL intensity of 0.55 cd m 2 was achieved at dc 14 volt and was observed only under dark conditions. Dye 46 exhibited a low reduction potential of -0.66 V, thus indicating a stronger electron withdrawing molecule compared with the well known emitter tris(8-

319

quinolinato)aluminum, which showed a reduction potential of -1.79 V. Consequently, the low EL intensity of 46 may arise from the unbalance of the energy gap between the emitting layer and the charge-transporting layer [41].

h

v

P

1

n

t

a-NPD,50 nm IT0

I

Figure 18. Structure of the EL device (A), and current density-voltage and luminescence-voltage characteristics (B) for the EL device (I T 0 / aNPD / 46 / MgAg).

4. CONCLUSIONS

New pyrazine dyes for multifunctional dye materials have been developed in a variety of chromophoric systems. Most of them have strong blue to red fluorescence, and are very valuable as fluorescent coloring matters and materials for optoelectronics devices. Pyrazine chromophores are small in molecular size and substituents effects on their intermolecular n-n interactions can be evaluated effectively by X-ray crystal analyses and MOPAC approaches. Relationships between the three dimensional molecular structures and intermolecular n-n interactions will be established in the future, to develop new functional dye

380

materials. The solid state absorption and fluorescence properties of new pyrazine fluorescence dyes derived from 1, 17 and 29 were evaluated from differences of their spectra in solution and the solid state. The parameters AA, AF and SS are valuable parameters for studies of their molecular stacking behavior in the solid state. X-ray crystal analysis and the MOPAC method are also very relevant in evaluating intermolecular interactions in their molecular stacking. The hydrophobic interactions of alkyl groups and the interatomic charge-transfer interactions in the layered crystal structure are also very important in giving many n-electron related functionalities. The effective molecular stacking in the solid state is necessary in order to obtain longer wavelength fluorescence, a property that can be applied in a variety of fluorescent functional materials.

5. REFERENCES 1. 2. 3. 4.

5. 6. 7.

8.

9. 10. 11. 12. 13. 14. 15. 16. 17.

R. W. Begland, D. R. Hartter, D. S. Donald, A. Cairncross, W. A. Sheppard, J.

Org. Chem., 39 (1974) 1235, and references therein. Dow Chemical Co., US Patent 3,879,394 (1975) and 4,199,581 (1978). Chem. Abstr., 83, (1975) 133, 397. A. Nakamura, T. Ataka, H. Segawa, Y. Takeuchi, T. Takematsu, Agric. Biol. Chem., 47 (1983) 1555,1561 and 2923. S. Tokita, M. Kojima, N. Kai, K. Kuroki, H. Nishi, H. Tomoda, S. Saito, S. Shiraishi, Nihon Kagaku Kaishi, 1990,219. H. Bredereck, G. Schmotzer, E. Oehler, Justus Liebigs Ann. Chem., 600 (1956) 81. K. Shirai, A. Yanagisawa, H. Takahashi, K. Fukunishi, M. Matsuoka, Dyes and Pigments, Dyes and Pigments, 39 (1998),49.. D. Hou, M. Matsuoka, Dyes Pigments, 22 (1993) 57. D. Hou, A. Oshida, M. Matsuoka, J. Heterocyclic Chem., 30 (1993) 1571. J . Y. Jaung, M. Matsuoka, K. Fukunishi, Dyes Pigments, 31 (1996) 141. J . Y. Jaung, M. Matsuoka, K. Fukunishi, J. Heterocyclic Chem., 34 (1997) 653. H. Shiozaki, A. Oshida, D. F. Hou, M. Matsuoka, Dyes Pigments, 33 (1997) 33. J. Y. Jaung, M. Matsuoka, K. Fukunishi, Dyes Pigments, 34 (1997) 255. J . Y. Jaung, M. Matsuoka, K. Fukunishi, Dyes Pigments, 36 (1998) 395. K. Shirai, M. Matsuoka, J. Society Dyers Colourists, 114 (19981, 368. J. Y. Jaung, M. Matsuoka, K. Fukunishi, Synthesis, (1998) 1347. J. Y. Jaung, M. Matsuoka, K. Fukunishi, Dyes Pigments, 37 (1998) 135. J. Y. Jaung, M. Matsuoka, K. Fukunishi, J. Chem. Research, (1998) (S) 284, (M) 1301.

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18. J. H. Kim, S. R. Shin, M. Matsuoka, K. Fukunishi, Dyes Pigments, 39 (1998) 341. 19. J. Y. Jaung, M. Matsuoka, K. Fukunishi, Dyes Pigments, 40, (1998) 11. 20. J. Y. Jaung, M. Matsuoka, K. Fukunishi, Dyes Pigments, 40 (1998) 73. 21. K. Shirai, J . Y. Jaung, M. Matsuoka, K. Fukunishi, J. SOC.Photographic Sci. and Tech. Japan, 61 (1998) 261. 22. J. Y. Jaung, M. Matsuoka, K. Fukunishi, Dyes,Pigments, 40 (1998) 251. 23. J. Y. Jaung, M. Matsuoka, and K. Fukunishi, unpublished work. 24. J . H. Kim, M. Matsuoka, and K. Fukunishi, Chemistry Letters, (1999) 143. 25. K. Shirai, A. Yanagisawa, H. Takahashi, K. Fukunishi, M. Matsuoka, unpublished work. 26. J. H. Kim, S. R. Shin, M. Matsuoka, K. Fukunishi, Dyes and Pigments, 41 (1999) 183. 27. M. Matsuoka, A. Iwamoto, T. Kitao, J. Heterocyclic Chem., 28, (1991) 1445 and 29, (1992) 439. 28. J. J. P. Stewart, MOPAC ver. 6, ACPE No 455; T. Hirano, Revised MOPAC ver. 6.01, JCPE News Letter, 2 (1991) 26. 29. M. Matsuoka, A. Oshida, A. Mizoguchi, Y. Hattori, A. Nishimura, Nonlinear Optics, 10, (1995) 109. 30. J. H. Kim, M. Matsuoka, K. Fukunishi, Dyes Pigments, 31, (1996) 263. 31. K. Takagi, A. Mizuno, A. Iwamoto, M. Furusho, M. Matsuoka, Dyes Pigments, 36, (1998) 35. 32. H. Shiozaki, M. Matsuoka, J. Mol. Structure (Theochem.),427, (1998) 253. 33. J. H. Kim, M. Matsuoka, K. Fukunishi, Dyes Pigments, 40 (1998) 53. 34. J. Mizuguchi, G. Rihs, H. R. Karfunkel, J. Phys. Chem., 99 (1995) 16217. 35. D. Eastwood, et al., J . Mol. Spectr., 20 (1996) 381. and P. S. Vincett, et al., J . Chem.Phys., 55 (1971) 4131. 36. V. Ramamurty, K. Venkatesan, Chem. Rev., 87, (1987) 433. 37. L. Addadi, M. Lahar, J. Am. Chem. SOC.,100, (1978) 2838. 38. M. Hasegawa, K. Saigo, T. Mori, M. Nohara, H. Nakanishi, J. Am. Chem. SOC.,107, (1985) 2788, and references therein. 39. C. M. Chung, M. Hasegawa, J. Am. Chem. SOC.,113, (1991) 7311. 40. K. Gnanaguru, N. Ramasubbu, K. Venkatesan, V. Ramamurthy, J. Org. Chem., 50, (1985) 2337. 41. C. Hosokawa, H. Higashi, H. Nakamura, T. Kusumoto, Appl. Phys. Lett., 67, (1995) 3853

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9 Natural Dyes

LOIS G. PUNTENER~AND ULRICH SCHLESINGER~ 1

TFL Ledertechnik AG, 4002 Basel, Switzerland Ciba Spezialitatenchemie GmbH, 79630 Grenzach, Germany

2

1. INTRODUCTION AND HISTORICAL OVERVIEW

In the Stone Age powders of coloured minerals (mostly heavy-metal ores) mixed with binders were applied to hair and body to confer magic powers while hunting or to impress others. People started wearing tanned skins, and later also textiles, as protection against the weather. In ancient Egyptian cuneiform texts we find the first descriptions of dyeing clothing and other materials that contain mainly natural organic dyes. These dyes were extremely labour-intensive to produce and apply, and therefore very valuable. In ancient Rome dyes were as valuable as gold. Not surprisingly, given the cultural importance of colours, the techniques of producing and applying dyes were steadily refined. The dyeing recipes themselves were kept secret by the priests and craftsmen and used as monopolistic sources of income. Most of the dyes were fixed with metal salts such as potassium alum, cuprous sulphate, and ferric sulphate to obtain different coloured complex compounds in the form of lakes. Organic mordanting agents such as tannin were also used for fixation. Gallic acid, a tanning agent combines with ferrous sulphate to yield ink black. The Phoenicians were familiar with murex shells for dyeing purple. With the decline of Byzantium the secret of purple dyeing was lost and kermes became the new red until, following the discovery of America, the better and more productive cochineal red of the Mayas replaced kermes. For yellow, the pollen of saffron was widely used in the Arab countries, and in China it was the privilege of the emperors to wear saffron coloured robes. With the advent of world-wide merchant shipping an increasing number of dyes were produced and exchanged. Indigo blue from India, called the ‘king of dyes’, was more brilliant and productive than European dyer’s woad, which subsequently was replaced. Tropical dyewoods like logwood and fustic also became very popular. Until the mid 19th century, textile and leather were dyed with animal and vegetable dyes, plus a few mineral dyes (see recommended literature, H. Schwepe).

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In 1771 Woulfe made the first synthetic dye - picric acid for colouring silk - by oxidation of natural indigo. Perkin synthesized cationic mauve in 1856 and successfully started the first industrial scale dye manufacturing operation. In 1859 Virguin discovered the bluish red coal-tar dye fuchsine. Today, most organic synthetic dyes are basically azo dyes, a family discovered in 1858 by Griess (see recommended literature, H. Zollinger). Azo compounds rarely occur in nature. New substances obviously also entail new, unknown risks. Although most synthetic dyes are classified as safe, consumers are increasingly asking as a matter of principle whether products of natural origin could replace synthetic dyes

ill.

Back to nature - a road to the future? The amount of dyes obtainable from animals and plants varies widely [21,

ranging from ~ 0 . 0 1 %(w/w) in the case of murex shells, to as much as 27% rutin in Persian berries which, in turn, make up only part of the plant’s total weight. In the case of dyewoods such as fustic, roots such as madder and shrubs such as indigo plant, the yields are 0.5 to 5%. It has been estimated that 100 million tondyear of dyer’s plants would be needed to dye only the world‘s annual consumption of fibres. A more realistic long-term source of natural dyes for the future could well involve the use of genetically engineered micro-organisms. Even now indigo 131 yields of a few grams per litre fermentation broth per day are feasible. So far only a very small number of natural colorants has proved commercially viable. Natural colorants are usually a mixture of dyes whose chemistry is often not precisely defined [4]. Since no exact recipe can be formulated for a given shade, it is difficult to reproduce the same shade. Colourants in plants are not designed by nature with transfer to a technical substrate in mind. Therefore, today natural dyes are mainly used for colouring foodstuffs and cosmetics. The very stringent safety and toxicological requirements to be met in dyeing food and cosmetics mean that only selected natural dyes are permitted. To their credit natural dyes display good biodegradability.

1.1 Isotins 1.1.1 Purple

GAS 19201-53-7 C.I. Natural Violet 1 C.I. Constitution 75800

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Purple, also known as purple of the ancients, ‘royal purple’ and ‘Tyrian purple’, occurs as the vat form of 6,6’-dibromo-indigoin a yellowish-green mucus contained in a small sac in the body of marine molluscs of the genus Murex and Purpura. The dye itself forms under the action of air and light via intermediate stages. The word purple is derived from the Latin word ‘‘purpura”the Greek word “porphyra”. Many writers of antiquity regarded the Phoenicians as the pioneers of purple dyeing. Tradition attributes the beginning of this art to the maritime trading city of Tyre in the year 1439 BC, although used murex shells were found on Crete that dated to 1600 BC. Probably the Phoenicians became familiar with purple dyeing in Ugarit on the coast of Syria. Asiatic rulers were the first to wear purple, a luxury subsequently adopted by Alexander the Great. In the early days of Rome chiefly members of the priesthood wore purple cloaks (“pallium”). In the time of the republics these cloaks were reserved for high officials of the state (“purpurati)’).Under Caesar and Augustus the wearing of purple was confined to persons distinguished by office and rank. The most famous factories for the manufacture of purple were in Syracuse and Tarentum. The Byzantine emperors and the Church of Rome have clung longest to costly purple. The church uses purple for the liturgical vestments of its highest dignitaries. In the 12th century there were dyehouses on Saracen Sicily. Also famous are the Purple Codices, exemplified by Ulfila’s Gothic translation of the bible. In 1453 the Turks conquered Byzantium and destroyed the dye factories, putting a sudden end to murex purple because the other dyehouses located around the Mediterranean produced only purple dyeings of inferior quality. In addition, Pope Paul I1 issued a regulation in 1464 requiring cardinals’ robes to be dyed only with kermes. In the 17th century, however, it was found that linen was still being dyed commercially with fluid obtained from molluscs. Murex brandaris was widely available, and dyeing with its fluid became a very simple procedure. In 1909 Friedlander [51 determined the constitution of the principal colouring matter in murex purple, requiring 12000 Murex brandaris molluscs to generate 1.4g of pure 6,6’-dibromo-indigodye.

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1.1.2 Indigo CAS 482-89-3 C.I. Vat Blue 1 C.I. Pigment Blue 66 (2.1. Constitution 73000

Indigo, the ‘Indian colour’, the oldest and most important natural dye, occurs as the colourless precursor product indican in species of Indigofera such as Indigofera tinctoria (indigo plant), a papilionaceous flower indigenous to India. From leaves and stems of the plant the dye precursor, indican, is converted t o the blue dye indigo by a fermentation and oxidation process. Dyer’s woad (Isatis tinctoria), which is also a source of this dye, was long cultivated in Europe. The Roman architect Vitruvius gives the first mention of Indian indigo in a book on architecture in 13 BC. The first reference to the application of indigo by ancient Indian dyers was during the period 640 - 320 BC, though it had probably already been a dyeing colorant in India since the third millennium BC. Indian indigo served the Greeks and Romans principally as a painting pigment, not as a textile dye, for which they preferred the native woad. Marco Polo (1254-1324) in the accounts of his travels provides the first information on the preparation of indigo and on its chief places of production in India. As early as the 12th century indigo, mostly being conveyed from Bagdad to Europe, is mentioned in merchants’ account books and in customs tariffs. Later the indigo trade in was taken over by the Genoese and the Venetians. In the 16th century Portuguese traders carried indigo to Europe in ever increasing quantities, where it was one of the main articles of commerce and became a competitor of the native woad. As a result of huge demand, indigo was cultivated on plantations, often in the colonies of the colonial powers of the time. In 1773 Great Britain imported about 700 tonnes of indigo from North America and in 1896197 indigo production in British India exceeded 9500 tonnes. One of the important importers of indigo was the famous archaeologist Schliemann, who used the fortune he made with indigo to finance his excavation of Troy.

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Woad, a crucifer, was in all probability originally indigenous to steppes in the Caucasus, and spread from there t o Europe. Romans, Greeks, Gauls and Teutons made use of it. A Celtic tribe in Southern Britain tattooed face and body with woad; the Romans called these people ‘picti’ (= painted men) - the later Picts or Scots. Caesar in his account of the Gallic Wars reports that fearful-looking warriors of this tribe “speckled” with woad opposed his landing in Britain. In the Middle Ages woad became the most important dye-yielding plant; it has a lower content of indican, however, than Indigofera tinctoria. The English woad and the German Waid are derived from the West Germanic “waida”, a modified form of “waisda”(= mediaeval Latin “waisdo”). A variant form is the Gothic “wizdila”. Cognates are the Greek “isatis” and the Latin “vitrum”. It was widely cultivated chiefly in France and Germany and brought great prosperity to some parts of these countries. In the 8th century Charlemagne decreed that “waisdo” (woad) be grown on his estates in addition to madder and weld; the labourers had to wear garments dyed blue with it. Centres of woad cultivation in Thuringia (Germany) were Gotha, Langensalza, Erfurt, Arnstadt and Tennstadt, which were simply referred to as ‘woad towns’. In England woad cultivation was carried on in the county of Northamptonshire. The importation of indigo from India, which regulations t o the contrary in the 16th and 17th centuries were powerless to prevent, had the effect of restricting woad cultivation and finally of bringing it to a standstill. Mention should also be made of dyer’s knotweed (Polygonum tinctorium) which is still being grown on a small scale for dyeing purposes in Japan. 1997 was the centennial of synthetic indigo. Based inter alia on the syntheses performed by Professor Heumann of Zurich, it was commercially launched by BASF in 1897. Still today blue jeans are dyed with the ‘king of dyes’. But natural dyes have also been chemically changed. Indigo for example has been sulfonated and used as a direct dye on wool, cotton and silk. It was not a great success, however, because the dyeings had only moderate fastness properties. This variant, indigo carmine, is currently used as a foodstuff dye. Indigo carmine CAS 860-22-0 (free acid form) C.I. Natural Blue 2 C.I. Constitution 75781 or C.I. Acid Blue 74 C.I. Food Blue 1 C.I. Pigment Blue 63 (2.1. Constitution 73015

3 87

S0,Na

1.2 Alizarine

CAS 72-48-0 C.I. Mordant Red 11 (2.1. Pigment Red 83 C.I. Constitution 58000

1.2.1 Madder

C.I. Natural Red 8 (Rubia tinctorum L. and Rubia peregrina L ) C.I. Constitution 75330/75340175350/75370/75410/75420 C.I. Natural Red 16 (Rubia cordifolia L ) C .I. Constitution 75 34017 5 35 01753 7017 5410 Dyes with the anthraquinone basic structure are present in the roots of the Rubiaceae, in species of rhubarb, aloe, lichens, fungi and some species of scale lice. Alizarin is the most important representative of this group. One of the best-known dyer’s plants, native to the Orient, is madder (Rubia tinctorum), the root of which yields alizarine. It has been suggested that the English name madder came from the Latin “materia”. The French “garance” is a transformation of the Late Latin “warentia, warantia, barentia”. The German word for madder, Krapp, originates from the Old High German “krapfo” (= fritter, hook), in reference to the curved prickles on the edges of the leaves and on the angles of the stems. The Arabic name for the whole plant “al-izari” was given to the colouring component, alizarine.

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The madder root has been linked with civilization for thousands of years. The Egyptians dyed with it; Greeks, Romans and Persians were at pains to improve the process. In about the year 800 Charlemagne ordered “warentia” to be grown on his estates. In the eighteen-thirties the French king Louis Philippe decreed that his soldiers wear madder red breeches and headgear. Huge cultivation was started around 1800 at Avignon, in Alsace and in Holland but discontinued following the discovery of how to synthesize Alizarine. A mordant is needed for dyeing with madder. On cotton, the Ca-Al complex of alizarine forms the famous Turkey red or madder red, following a sequence of 17 operations. Turkey red oil, a precursor of modern dyeing auxiliaries, consisting inter ulia of cow dung, rancid olive oil, gall apples and chalk, was also used here. The dye obtained from the madder root has one of the best all-around fastness properties among natural dyes. With iron, alizarine forms a violet complex, with chromium a brown complex. The red colour of the famous cordovan leather (= leather from Cordoba, in Spain) was created by mordanting with alum. Madder lakes were also used as pigments for painting. In the baths of the Roman emperor Titus was a vase painted with a pink pigment; furthermore a number of pigments were discovered in the course of excavations in Pompeii. Madder lake was not found again as a painting material until much later, for example on the marriage certificate of the empress Theophano dating from 972. Nowadays madder is cultivated only on a very small scale. Because natural alizarine contains several accompanying dyes, it can be used to advantage in both watercolour and oil painting. 1.2.2 Kermes

Kermesic acid CAS 18499-92-8 C.I. Natural Red 3 C.I. Constitution 75460

Red dyes obtained from insects were particularly important in earlier centuries alongside purple, indigo and the madder dyes for dyeing textiles, yielding red shades from scarlet to crimson with outstanding fastness properties.

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Kermes (also known as kermes grains) is the dried, egg-containing bodies of female scales of the genus Kermes, of which there are numerous species. The most important species is Kermes vermilio, which infests the kermes oak (Quercus cocciferu), a native of the Mediterranean countries. The colouring constituent is kermesic acid. The word kermes is of oriental origin and means worm; in Sanskrit it has the form “kermi”, in ancient Persian “kerema”. Greeks and Romans believed kermes to be a vegetable product, hence the Greek name, “kokkod’ (berry). In the Middle Ages the name “vermiculum” (small worm) came back into vogue, giving rise to the French “vermeil”. The word kermes entered many European languages as the name of a colour: “karmin” (Ger.), “cramoisi” (Fr.) and “crimson”(UK) are derived from it. In ancient times kermes was used in scarlet dyeing; the word scarlet comes from the Persian “sakirlat”, which means red colour. With a tin mordant kermesic acid gives a scarlet red complex, and with aluminium it forms a shade with a somewhat bluer cast. At the end of the 15th century the tin complex replaced the genuine antique purple because the last mills dyeing purple were destroyed when the Turks occupied Constantinople in 1453. One of the most famous textiles produced in Palermo is the navy blue coronation robe of the Emperors of the Holy Roman Empire, which was made there in 11331134 for the Norman king Roger 11. The hem is dyed red with kermes. The mantle then passed into the possession of the Hohenstaufen emperors. Kermes has been known in the Middle East since the days of Moses; the Hebrew term for it is “tola”, which again means worm. In ancient Greece, dyers used Kermes. In the age of Homer it was an important trading commodity. The Spaniards paid a large proportion of their tributes to Rome with kermes. Into the 19th century the main area of kermes oak cultivation, besides southern France, Spain and Morocco, was the island of Crete. Venice was the port of entry for kermes brought from the Orient and, Marseilles the main port for shipping kermes from southwest Europe. In the 19th century kermes was used in particular for dyeing Turkish hats (fezzes). As a leather dye kermes was used in Morocco, in South Russia and in the Orient for dyeing morocco (goatskin leather). Cochineal heavily curtailed kermes dyeing during the 19th century, kermes disappearing from the market almost entirely towards the end of the century with the advent of the synthetic dyes.

1.2.3 Cochineal Carminic acid CAS 1260-17-9 C.I. Natural Red 4 C.I. Constitution 75470

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Another red dye of insect origin is cochineal, produced from the dried bodies of the females of the Coccus cacti scales which live on a species of cactus (Nopalea coccinellifera, the so-called Nopal) chiefly in Central America and Mexico. Here the colouring component is carminic acid which is present in twelve-fold the concentration of kermesic acid in kermes. After the Spaniards landed in Mexico in 1512 they learned from the Aztecs about a red dye called “nochetzli” in their language, which they used for both dyeing and painting. Subsequent to their conquest of Mexico, the Spaniards recognized the similarity between this dye and kermes, and gave it the name “grana cochinilla” (coccinus = here: scarlet coloured). The French form “cochenille” then passed as “cochineal”into English and unchanged into German. The cochineal dye was carried to Europe and soon started to oust the indigenous kermes as both textile dye and artist’s pigment. The export of cochineal to the Spanish court started around 1523 and after gold and silver cochineal became the most valuable export product of the Spanish colonies in America. In the 17th century the Dutch physicist Drebbel markedly improved the scarlet dyeing of wool by introducing the tin mordant. The shade achieved was seen into the present century as the ‘true scarlet’. In earlier day’s cardinals, kings, judges and the hangman - were entitled to wear robes of this fabulously beautiful red. With aluminium mordant cochineal gives crimson dyeings. The brightest crimson is called “nacarat”, which represents the true peak and magnificence of cochineal. ‘Ammonia cochineal’ yields a shade that is bluer in cast than ordinary cochineal. There also exists a Polish cochineal obtained from the insect Porphyrophora polonica, which lives on the roots of the weed perennial knawel (Scleranthus perennis). This insect used to be collected in Poland, Lithuania, Prussia, Pomerania, the Ukraine and probably also in Turkestan. Collecting started in Eastern Europe traditionally on the feast of John the Baptist, hence the weed‘s alternative name ‘Saint-John’s Wort’. Carmine and carmine lake (dissolved carmine, precipitated with alum, plus additives such as china clay) are still being used today as pigments in painting.

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Carmine lake is also known as Florentine Lake because it was first made in Florence. Carminic acid, produced from cochineal nowadays, colours cosmetics (lipstick, toothpaste, make-up, mouthwash), foodstuffs (confectionery, liqueurs, jams, fruit juices) and galenicals. It also does service as an analytical reagent. 1.2.4 Lac Laccaic acid A. B. C. D and E C.I. Natural Red 25 C.I. Constitution 75450

Another red dye comes from the lac insect (Coccus lacae), which is found on different plants in India, Thailand, Cambodia, the Moluccas and Sumatra. The secretion exuded by these insects is stick or rubber lacquer that gives scarlet shades on tin mordant. The dyes are referred to as ‘lac dyes’. The colouring components of this secretion has been separated into four species (see MerckIndex, p. 910), Dimroth [61 first thought of one compound (see formula above: laccaic acid d) closely related to carminic acid. 1.2.5 Other alizarines

Table 1 Other alizarines from plants

I1 1 I

Chemical name C.I. Constitution & Class Rubierythric acid 75330 Red 6,8,9,10,11,12 Purpuroxanthin 75340 Red 8.16 75350 Red 8,16 Rubiadin Morindaniain 75360 Red 19 Muniistin 75370 Red 8.16 Morindadiol 75380 Red 18

Plant (as example) Chayaroot Rubia cordifolia L Madder (genus p:a 1iu m Morinda umbellata L. Rubia cordifolia L Morinda citrifolia L.

II I I

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Soranjidiol 75390 Chrysophanic acid 75400 Purpurin 75410 Pseudopurpurin 75420 Morindone 75430 Emodin 75440 Quinizarin 58050 Anthrogallol 58200

-----

-----

Red 18 Yellow 23 Red 8,16 Red 8,9,14 Red 19Nellow 13 Green 2Nellow 14 Pigment Violet 12 Mordant Brown 42 Orange 1 Red 13

Morinda citrifolia L. Rhubarb Wild madder Lady’s bedstraw Morinda umbellata L. Alder buckthorn Madder (Al-salt) Genus coprosma, madder Reed creeper Dyer’s woodruff

1.3 Aliphatic polyenes 1.3.1 Carotene Carotene (a,O,rl C.I. Natural Yellow 26 C.I. Natural Brown 5 (2.1. Constitution 75130

The carotenoids are the most import n t group f n tural dyes nd the most widely occurring in the animal and vegetable kingdoms. They are mostly fatsoluble, nitrogen-free yellow t o reddish violet dyes, which contain a long chain of conjugated double bonds, and so constitute aliphatic polyenes. CAS 7235-40-7 @-carotene) C.I. Food Orange 5

393

Chemists and botanists focussed their attention on the carotenoids at an early date. The substances occur in carrots (from which they take their name), tomatoes, egg yoke, yellow pansies and chloroplasts. It is therefore fair to say that every green plant contains carotenoids, which become visible in the green leaves only when these discolour (turning yellow and red) each autumn. They are also widely to be found as provitamins in animals. Carotenoid dyes are also called polyene dyes or lipochromes (fat-soluble). In 1831 Wackenroder isolated carotene from the carrot. Willstadter 171 was the first to obtain it in crystalline form. Karrer, Kuhn, Winterstein, Zechmeister, and coworkers deserve credit for their research on carotenoids. By 1987, 563 different natural carotenoids were known. A detailed summary on the synthesis and use of commercial carotenoids can be found in Ref. [ 8 ] . Of the polyene hydrocarbons, lycopene is contained in tomatoes, and is present along with carotene in red palm oil, rose hips, apricots, butter, etc. Polyene alcohols include cryptoxanthin in yellow Indian corn, egg yolk, butter, mandarine oranges, etc. Of the polyene ketones and hydroxyketones, capsanthin occurs in peppers and canthaxanthin in the edible mushroom Cantharellus cinnabarinus. Polyene carboxylic acids and esters are azafrin in species of Escobedia (South American plants), bixin in Bixa orellana and crocin in Crocus sativus. Viola tricolor contains the carotene epoxide auroxanthin. Vitamin A is an important growth vitamin, which also has a bearing on the faculty of sight.

1.3.2. Saffron (crocidcrocetin) Crocetin CAS 27876-94-4 C.I. Natural Yellow 6,19 C.I. Natural Red 1 C.I. Constitution 75100

HOOC

FH3

FH3

W

r

O

O

H

The crocus-like saffron plant (Crocus sativus) contains the yellow dye crocin (the digentobiose ester of crocetin: (3.1. Natural Yellow 6) in the three thread-like stigmas enclosed within the pistil. The name saffron is derived from the Arabic “za’faran”(= t o be yellow). Originally indigenous to Asia Minor and Persia, saffron has a very long history of cultivation in Egypt, Greece, India, China and elsewhere. After the Arabs in the 10th century took “za’faran” to Andalusia, its cultivation spread in Spain, France and Italy and for a brief period was carried on

394

also in Basel, a centre of the saffron trade. The most important merchants’ guild in Basel proudly called itself ‘zem Saffrant’ (Saffron). Still today saffron is grown on the south-facing slope of the Swiss mountain village of Mund in Upper Valais where pilgrims from Santiago de Compostela (Spain) brought bulbs of Crocus sativus in the 14th century. Saffron is a direct dye for wool, silk and cotton. It can be applied without mordant but also on alum or tin mordant. It gives an orange-yellow shade on unmordanted wool. The plant is mentioned on an Akkadian clay tablet from the 2nd millennium BC, and was also known to the Minoan Civilization on Crete. In Cilicia (Asia Minor) the Krokos grew in the valley out of which rise the ‘Korukos’ foothills; the mountain and area took their name from the plant. The plant itself would appear to have reached Greece from the Orient, as suggested by the name Krokus. Yellow bandages in which Egyptian mummies are wrapped attest that textiles were dyed with saffron in very early times. Babylonian and Persian kings wore saffron-dyed shoes, the Attic maidens fashioned a colourful saffron-dyed robe for Pallas Athena, the daughter of Zeus. When Zeus in the form of a bull approached Europe she happened to be ‘collecting the fragrant hair of the crocus’ - a circumlocution for the tedious job of harvesting the thread-like stigmatas. Saffron is mentioned as a bath additive for a Roman emperor and as an additive in oil for anointing the statues of deities. In the theatre the seats of the distinguished were sprinkled with saffron wine - a reference to its disinfectant action. Saffron was also valued as a remedy, in addition to being used as an artist’s pigment among the Romans. It commonly occurs in recipes for medieval book illumination. In China, saffron yellow robes were reserved for the emperor - the mythical first emperor called himself the Yellow Emperor’ - and the Buddhist monks. But the Chinese dyed with other colouring matters, including yellow gardenia (wongshy) pods, though these contain the same dyes as saffron. The country which people nowadays primarily associate with the production of saffron is Spain. According to the 16th century geographer and historian Hakluyt a pilgrim smuggled saffron to England, hiding a crocus head in his pilgrim’s dress - and thereby risking his life because the Spaniards jealously guarded their wealth. Saffron Walden in Essex became the centre of cultivation in England. Hakluyt’s attempts to export saffron cultivation to other regions, however, came to nought. By the end of the 18th century saffron cultivation had disappeared almost entirely from Northern Europe. Saffron was introduced in Pennsylvania where its production still flourishes today. The method of production made saffron a rare and costly commodity; 200,000 to 300,000 filaments are required to obtain one kilogram of saffron. Accordingly many people tried to adulterate saffron - often with dire consequences. In the 15th century in Germany two dealers were burnt at the stake for that offence. But even

395

today adulterated saffron is still available everywhere, particularly in powder form. In addition to its application as a textile dye, saffron was added to confectionery of many kinds, including liqueurs, perfumes and hair lotions. This practice still persists to some extent today. Saffron also has a reputation as a superlative condiment for certain dishes such as the French fish stew ‘bouillabaisse’and the Spanish ‘paella’.

1.3.3 Bixin CAS 6983-79-5 C.I. Natural Orange 4 C.I. Constitution 75120

HOOC

The annatto tree (Bixa orellana) is a member of the Bixaceae family and contains the red dye, bixin, in the pulp surrounding its seeds. The tree itself is indigenous to the Antilles and tropical South America. Like saffron, annatto is a substantive dye and colours wool, silk and cotton without a mordant, in shades from orange red to yellowish-orange;the light fastness of the dyeings, however, is on the low side. Using alum or tin salts, annato gives yellow and orange red lakes. Bixin is also used to colour foodstuffs, especially dairy products (Edam and Cheshire-cheese, butter), margarine, pastas, pudding mixes, soups, etc. Objects found in ancient Peruvian tombs testify to the age-old cultivation of Bixa orellana. When the Portuguese colonised Brazil they became familiar with the dye extract “urucu” used by the Indians to paint their bodies. The Italian scholar at the court of Charles V, P. Martyr, is the first to mention the plant’s cultivation and processing. Fernandez reports on the bixa tree in his natural history of America in 1526. The Spanish physician Hernandez in his description of Mexican flora states that the ancient Mexicans referred to annatto as “achiotl” and that it served them as an additive in chocolate making. In the 19th century the Antilles and French Guiana produced 500 tons of annatto per year. Brazilian output reached 110 tonnes in 1868. 1.3.4 Canthaxanthin CAS 514-78-3 C.I. Food Orange 8 C.I. Constitution 40850

396

0 The edible mushroom Cantharelles cinnabarinus contains canthaxanthin. 1.3.5 Lycopene CAS 502-65-8 C.I. Natural Yellow 27 C.I. Constitution 75125

Lycopenes is a substance whose chemical name is derived from the scientific word for tomatoes, Lycopersicum, and is contained in tomatoes and marigolds. It is also present in some species of bacteria. Also found in the flowers of the marigold are rubixanthin (C.I. Constitution 75135) and violaxanthin (C.I. Constitution 75138), which is classified as C.I. Natural Yellow 27. 1.3.6 Capsanthin (paprika,red pepper, chilli) Camanthin CAS 465-42-9

397

C.I. Natural Red 34 C.I. Constitution 75133

OH

The pod of the red pepper, Capsicum annuum L., contains a carotenoid dye, capsanthin. Colourless capsaicin is responsible for the hot taste for which the spice is better known. Paprika is valued as an approved foodstuff dye or preferably as a piquant spice. There is a wide range of different varieties, depending on the area of cultivation: Hungary, Mediterranean countries, India or Mexico. Isolation of capsanthrone from different species of paprika is described in Ref. 191. Capsaicin CAS 404-86-4

1.3.7 Other carotenes Azafran is used in Central and South America, especially in Paraguay for colouring fat; C.I. Natural Orange 3, C.I. Constitution 75110, occurs in the root of Scrophulariaceae Escobedia scabrifolia and S.E. linearis. fi-Apo-8cartina1, C.I. Food Orange 6, Constitution 40820, can be found in oranges. C.I. Food Orange 7, C.I. Constitution 40825 is the corresponding ethyl ester. (2.1. Natural Yellow 29 consist of xanthophyll, C.I. Constitution 75136, originally found in egg-yolk and zeaxanthin, C.I. 75137, from yellow corn. It occurs together with the isomeric xanthophyll. 1.3.8 Other aliphatic polyenes Curcumin (turmeric)

398

CAS 458-37-7 C.I. Natural Yellow 3 C.I. Constitution 75300

Turmeric, Curcuma longo, with curcumin as its main dye, finds its main use as a constituent of condiments, giving curry powder and mustard their typical yellow colour. Its chief areas of cultivation are in India, South China and Africa. 1.4 Flavonoids and neoflavanoids 1.4.1 Fustic (morin/maclurin) CAS 480-16-0 C.I. Natural Yellow 8 C.I. Constitution 75660 (morin) CAS 519-34-6 C.I. Natural Yellow 11 C.I. Constitution 75240 (maclurin) C.I. Natural Yellow 9

Morin

'

O

M

\

OH

0

'

Maclurin

oOH OH

H , & ,OH HO

\

HO

\

OH

Morin, maclurin and kaempferol are the colouring constituents of fustic (Maclura tinctoria, Morus tinctoria), known also as dyer's mulberry, yellow wood and Cuba wood, and a member of the Moraceae family. The heartwood of the trunk yields the extract with the colouring constituents. This tree is indigenous to Central America, the Antilles and tropical South America.

399

In the past fustic was chiefly used t o dye wool, and this had to be premordanted. With alum mordant various yellow shades are obtained, tin mordant gives very bright and fast yellow shades, chromed wool dyes light to dark olive yellow, copper sulphate mordant gives an olive shade and iron sulphate a dark olive to blackish brown shade. In silk dyeing fustic did service as a yellowing agent for shading. Full yellow, brown or olive green shades are obtainable on mordanted cotton. It must be said, however, that fustic dyeings are not very fast to light. For printing on cotton use was made of either pure morin or ‘calico yellow’. ‘Calico yellow’ (‘Cuba paste’) is the sulfurous acid ester of morin in paste form. Fustic extract was also applied to dye leather. Fustic extract is used in ‘Patent Fustic’ as a coupling component for leather dyes. Patent Fustic C.I. Mordant Dye C.I. Constitution 20030

1.4.2 Brazilwood (brazilinlbrazilein) CAS 474-07-7 C.I. Natural Red 24 C.I. Constitution 75280

400

BraziIin

Brazilein

The brazilwood or redwood tree (Haematoxylum braziletto), which belongs to the Leguminosae family, contains as principal colouring constituents brazilin and brazilein, the basic structure of which is referred to since 1964 as a neoflavanoid. Related species include pernambuco wood, Nicaragua wood, sappanwood, limawood and peachwood. Originally the rasped heartwood was added straight to the dyebath, then later on extracts were prepared. The brazilwood tree is indigenous to the East Indies, Central and South America and also to Africa. Brazilwood was known in Europe long before the discovery of South America because it was imported as a dyewood from the East Indies. In Spain in the 12th century the grammarian Kimhi or Kimichi reported about dyewoods called “Bresil” or (‘Brasil”,a name claimed t o derive from the Frankish word (‘braza”and meaning fire, glowing coals. Soon after the discovery of South America, large quantities of the valuable redwood were found in the forests along the Amazon. The newly discovered country was given the name (‘Brazil’’(redwood country) - although the Portuguese navigator Cabral, who thought it was an island, had actually named it “Vera Cruz” (True Cross). The colonization of Brazil started with the transportation from Portugal to South America of convicts, whose tasks included collecting redwoods. The soluble brazilwoods dye wool and cotton with the assistance of a mordant (alum, tin chloride, potassium dichromate, copper salts, basic aluminium sulphate, iron salts) to produce red, crimson, bordeaux t o reddish violet, orange red and violet grey shades. The dyeings are not very fast to light. In the Middle Ages brazilwood was firmly established for illuminating books because between their covers its poor light fastness was scarcely evident. A treatise from that period claims it is marvellous on parchment. Redwoods were still being imported and processed in Europe a t the start of the present century.

40 1

''9'

1.4.3 Logwood (haemateidhaematoxylin) CAS 475-25-21517-28-2 (3.1.Natural Black 1,2 (3,4) C.I. Constitution 75290 (75291) (Aluminium, chromium, iron or tin lake) Haematein

H $ & "

Haematoxylin

\

HO

I I 0

HO

/

\

OH

Logwood or campeachy wood is the heartwood of the logwood tree (Haematoxylum campechianum), a further member of the Leguminosae family. As the colouring constituent it contains the leuco compound haematoxylin, which oxidizes t o form the red mordant dye haematein. The tree is native to an area stretching from Mexico to the northern part of South America. The heartwood of this tree is commercially available for dyeing as chips, as powder and as an extract. The Spaniards were the first to come into contact with logwood when they conquered Mexico in 1517-1519. In 1540 they founded the port of Campeche on the West Coast of Yukatan, probably in connection with the logwood forests in that area. The British, who had settled on Jamaica, also started t o show interest in the logwood trade; in the second half of the 16th century logwood reached England where for a time it was forbidden by act of parliament because its dyeings were impermanent. In 1633 large quantities of logwood were carried to Europe, and Barham introduced it in Jamaica in 1794. In France too logwood was at first rejected, only black-dyers being permitted to use it. Around the middle of the 18th century Giros de Gentilly succeeded in fixing the dye on wool by means of a tin mordant. One of the dyeings 'Prune de Monsieur' attained the rank of a fashion shade around 1760.

402

In 1810 Chevreul managed to isolate haematoxylin. Between 1830 and 1840 extracts of logwood were introduced to the market. World production of logwood around 1950 was still as much as 70 000 tondyear. Logwood is one of the natural dyes with the widest range of dyeing applications, depending on the mordant used. It can be used for dyeing different textile substrates such as wool, silk and cotton, for textile printing as well as for leather dyeing. It dyes wool blue with alum mordant, violet with tin mordant, blue black with copper mordant and black with iron mordant. In dyeing morocco (goatskin) leather logwood was once very important for black, grey and violet shades. Black glace kid gloves have such high fastness to rubbing that they can safely be worn together with a white coat. Logwood formerly served also to dye paper, wood, bird feathers, straw, bast fibres and horsehair. In silk dyeing it still has some importance today because in addition to imparting a full black shade it also weights the fibre. From extracts of brazilwood and logwood, colour lakes for use as artists’ pigments can also be obtained with heavy metal salts. 1.4.4 Weld (luteolin) CAS 491-70-3 C.I.Natural Yellow 2 C.I. Constitution 75590

Weld (Reseda luteola) belongs to the family of Resedaceae and as its main colouring constituents it contains luteolin and/or apigenin. The whole plant is used for dyeing purposes. The name “weld” is cognate with the Middle Dutch “wolde”, Modern Dutch “wouw” and has passed into the Romance languages and into German; ultimately it derives from an Indo-European root “uel” -, meaning to pluck. This plant is native to Central and Southern Europe (originally only in the Mediterranean areas) and to Western Asia. Brilliant lemon yellow shades are obtained on wool with alum mordant. Weld was the oldest of the yellow natural dyes and the only one with serviceable light fastness known in antiquity. As a yellow dye for silk, it held out for a long time until the yellow dyes from the exotic dyewoods superseded luteolin. Weld

403

plantations were established in England in the 17th century in Kent and Essex, and also in various sections of France. In Germany the plant was cultivated near Tubingen and Halle/Saale. Weld used to be bound into sheaves like grain crops, and was traded in that form. Colour lakes can also be produced from decoctions of weld by precipitation with different metal ions, but their shade intensity is very low. Weld lakes are mentioned in workshop records of medieval book illuminators such as the Mappa Clavicula and the 14th century Naples codex. 1.4.5 Gamboge (cambogia) and other flavonoids

Gamboge (CAS 9000-25-0) is a greenish yellow resin consisting of 15-20% watersoluble gum and 60430% cambogic acid [CAS 2752-65-01, Morelloflavone is the basic component of the dye used as C.I. Natural Yellow 24. It is obtained by tapping Garcinia morella and pictoria, guttiferous trees that grow in Ceylon, Thailand and Cambodia, and allowing the exudate to dry. The powdered resin is used nowadays as water-colour paint. Table 2 Other flavonoids Chemical name Apigenin Artocarpanon Fukugetin Datiscetin Kaempferol Rhamnocitrin Rhamnetin Xanthorhamnin Isorhamnetin Rhamnazin Quercetin Quercitrin Rutin Gossypetin Butinhutein Citronetin Rottlerin

C.I. Constitution & Name C.I.75580 Yellow 1,2 Yellow 11 C.I.75600 Yellow2 C.I.75630 Yellow 12 C.I.75640 Yellow 10,13 C.I.75650 Green 2, Yellow 13 C.I.75690 Yellow 13 C.I.75695 Green 2, Yellow 13 C.I.75680 Yellow 10 C.I.75700 Yellow 13 C.I.75670 Yellow 10,13, Red 1 C.I.75720 Yellow 10 C.I.75730 Yellow 10 C.I.75750 Yellow 10, Brown 5 C.I.75760 Yellow 28 C.I.75770 C.I.75310 Yellow 25, Orange 2

Typical Plant Roman chamomile Jackfruit Garcinia spicata Bastard hemp Onion Black poplar Common buckthorn Common buckthorn Yellow larkspur Persian buckthorn Red clover Arnica montana Japanese padoga-tree Indian cotton Pallas tree Citrus bergamia Kamala

404

1.5 Porphyrin derivatives

1.5.1 Chlorophyll Chlorophvll A (R=CH& Chlorophvl B (R=CHO) CAS 1406-65-1 C.I. Natural Green 3 C.I. Constitution 75810 C.I. Natural Green 5 (water soluble acid form, obtained by alkaline hydrolysis) C.I. Constitution 75815

Chlorophyll or leaf green is a porphyrin derivative with magnesium as the central atom and is hence a metal complex dye. It is present in the chloroplasts in all green parts of plants as a mixture of blue green chlorophyll a and yellow green chlorophyll b, and constitutes the catalyst for photosynthesis. Chlorophyll c occurs in lacustrine algae and chlorophyll d in red algae. Dry plants are the principal starting materials for the extraction of chlorophyll, mainly nettles and lucerne in Europe, grasses and broccoli in the USA. The name lucerne is derived from the Latin “lucerne” (to gleam), because of the brightness of the beans. The Swiss city of Lucerne has the same name root and means: ‘Shining city’. Commercial chlorophyll is a dark green solution in water, ethanol or oil.

405

Chlorophyll soluble in fats and oils is used chiefly for dyeing and bleaching oils and soaps, and also for colouring mineral oils, waxes, essential oils and ointments. Water-soluble chlorophyll preparations find application in the foodstuffs industry for colouring confectionery, gelatine products and beverages. In the cosmetics industry they are added to cremes and soaps. 1.5.2 Haemoglobin

Haemoglobin, the red pigment in blood, consists of a protein component and the iron complex of a porphyrin derivative: haemoglobin = globin (protein) + haemochromogen (Fe (11) complex). During the breathing process, oxygen attaches loosely to the iron ion, and the oxyhaemoglobin thus formed surrenders its oxygen in the body and reverts back to haemoglobin. Outside the body the iron converts to the trivalent form. Haemoglobin binds carbon monoxide more strongly than oxygen, its colour weakening to cherry red, hence the pink complexion of victims of carbon monoxide poisoning. One degradation product of haemoglobin is the brown bile pigment bilirubin. Another is green biliverdin, the result of breakdown by oxidation. The first occurs in bile, the second in the liver. 1.6 Tanning agents Vegetable extracts were used to preserve skins as far back as Ancient Egypt. The content of tanning agents in plants is rather difficult to determine because many different substances are involved in the actual tanning process. Besides gallic acid, ellagic acid and the depsides, these substances include the flavonols and the catechins, which leave the leather a yellowish t o reddish brown colour. This form of tanning is also known as bark tanning, and in the bible (Exodus,

406

chapter 25, verse 5 and chapter 26, verse 14) we find reference to rams' skins doubtlessly tanned in this way. The discovery of the Dead Sea Scrolls in the late 1940's, confirmed the vital importance of leather as a substrate upon which documents of religious significance have been preserved [lo]. Vegetable tanning agents are extremely complex mixtures 1111 of low to high molecular aromatic compounds, with several hydroxyl compounds for solubility and binding on skin collagen. Tanning agents used to be classified according to the main cleavage products pyrogallol and pyrocatechin, which form on heating at 180-200°C. Of the numerous other chemical principles for classifying tanning agents, subdivision into hydrolyzable and condensed types should be mentioned. Hydrolyzable tanning agents are high-molecular esters that mostly form gallic acid when hydrolyzed. For instance, tannin (gallotannin) is a compound of gallic acid and glucose. Gallotannin (eallic acid) CAS 149-91-7

HO

OH

The condensed tanning agents are derivatives of flavan, generally types of catechin which are held together via carbon linkages. In simplified terms, the pyrogallol tanning agents represent the hydrolyzable variety and the pyrochatechin tanning agents the condensed variety. Catechin CAS 154-25-4 C.I. Natural Brown 3 C.I. Constitution 75250

PH

6H

407

The most important vegetable tanning agents are derived from barks, woods, leaves and fruits. The chief barks are oak, spruce (Europe), hemlock (USA), and mimosa trees (South Africa, India and South America). In addition, extracts of oak, quebracho (South America), chestnut wood (Mediterranean countries), and tara, extracted from the pods of the tara plant (Casalpina spinosa) are used. A further source is the sumac shrub, one of the many members of the genus Rhus, which forms part of the widely distributed anacardiaceae family. North America sumac is used commercially as a scenting agent for tobacco. Besides gallotannin, sumac contains a high amount of fisetin, which is used as a brown colour. Sumac (fisetin) CAS 528-48-3 C.I. Natural Brown 1,6 C.I. Constitution 75620

Oak apples, formed at the sites where gall wasps have laid their eggs, have a very high tannin content (35-70%)and are used chiefly in Turkey for dyeing cotton and wool. The dyed wool has the colour of camel hair. With iron salts the tannin components become black metal complexes. Black and brown are still the most important colours for leather. Iron complexes of gallic acid derivatives have for centuries been the basis of inks for use on official documents.

1.7 Miscellaneous 1.7.1 Naphtho- and benzoquinone Naphthoquinone and benzoquinone dyes show similar behaviour to the wellknown related anthraquinone dyes. Lawsone CAS 83-72-7 C.I. Natural Orange 6

408

C.I. Constitution 75480

Lawsone occurs in the leaves of species of lawsonia such as Lawsonia inermis L. which belongs to the Lythraceae, originally coming from India. Nowadays the area of cultivation extends from North Africa to China. Henna has also become established in tropical America. Leaf and stem contain the dye precursors in the form of the glycosides hennosides A, B and C; these are converted to lawsone by enzymatic hydrolysis. Henna dye appears red to yellow. It can be classified as an oxidation dye and is applied to wool and silk without prior mordanting. The cultivation of henna can be traced throughout the centuries. The preMohammedan Arabs used henna. In the 12th century there is mention of henna being grown in Morocco, and in the 13th century henna turns up in decrees issued by the Emperor Frederick 11. Persia and Egypt are probably the oldest centres of cultivation. In Persia henna was an important export commodity to all Mohammedan countries, the best quality originating from Khabis. Egypt also cultivated henna on a large scale and exported it in appreciable quantities, shipping out, for example, 3 million henna leaves in 1850. In Arabia and the Mediterranean countries henna is still commonly used to colour hair and nails. Even today brides in some areas still colour their hands with it. Dyeing is performed with the leaves. It was later recognized that the dye is identical to 2-oxy-1, 4-naphthoquinone.

I

Table 3 Other naphthoquinones Chemical name Lapachol Juglone Alkannin Alkannan Shikonin Deoxysantalin

C.I. Constitution & Name C.I.75490 (2.1.75500 C.I.75530 C.I.75520 C.I.75535 C.I.75510

Yellow 16 Brown 7 Red 20 Red 20 Tokyo Violet Red 22,23

Typical Plant Bignoniaceae Walnut tree Onosma Dyer’s bugloss Shikone Sanderswood

I

409

Very useful benzoquinone dyes can be isolated from fungi. Roughly half of the hundred or so known structures of naturally occurring benzoquinone dyes are of fungal origin. As a case in point atromentin is isolated from the fungus Paxillius involtus BATSCH, known as the “brown roll-rim”. This fungus is native to Europe and North America, growing on tree trunks and rotting wood. Atromentin CAS 519-67-5

Another typical benzoquinone is dye is carthamin, (2.1. Constitution 75140. C.I. Natural Red 26 is found in the safflower.

1.7.2 Anthocyans

An interesting class of dyes is the anthocyans, particularly in the blue to red area. The anthocyanidins can be regarded as reduced flavonols, of which they are close physiological relatives. Highly sensitive t o pH conditions, turning red with acid and blue with alkali, these dyes are often used as pH indicators. A plant that has become famous is the Japanese tsuyukusa flower, Commelina communis var. Hortensis Makino, from which ‘Awobamin A’ is obtained and which is used to make the lovely blue awobana paper. Awobamin A (delphidin-3. 5-dielucoside) CAS 17670-06-3 C.I. Natural Blue 3 C.I. Constitution 75190

410

PH HO

A red dye made up of different anthocyans is derived from the seeds of guinea corn, red Durra, several varieties of which occur in China, India, the Near East, North Africa and Southern Europe. The dye has been used not just in China, but more particularly in the Sudan to colour grasses, mats and leather. It has also been patented in Europe under the name Baden Red and is C.I. Natural Red 32. Further dyes of this type, especially in the blue to black area, can be found chiefly in elder and dwarf elderberries, bilberries, blackberries, black currants, grapes, bird cherries and in the fruits of the blackthorn and privet. The dyes physically extracted from the edible fruits are approved for food coloration in the EU. Textiles have also been dyed with dyes of this type, despite their poor fastness to washing and light. Pliny the Elder, for example, reports that in Gaul slaves’ robes were dyed with bilberries In Scotland into the last century, wool was dyed a deep blue with the juice of over-ripe privet berries on an aluminiudiron mordant. The fermented aqueous extract of privet berries gives C.I. Natural Black 5, a grey to black shade on wool with a copper mordant. Elderberries were used for book illumination in the early Middle Ages, and later for colouring playing cards. Caraiurin CAS 491-93-0 C.I. Natural Orange 5 C.I. Constitution 75180

41 1

Another interesting related anthocyan dye is carajurin, found in the leaves of Bignomia chica grown in tropical South America. The leaves are fermented in water until a dark red precipitate settles out. This dye served the Indians not only as body paint but also as an insect repellent. Dragon’s blood contains dracorhodin (C.I. Constitution 75210) and dracorubin (C.I. Constitution 75200) used as C.I. Natural Red 31. 1.7.3 Humin and humic acid (Cassel brown) Cassel brown. Van Dvke brown C.I. Natural Brown 8 C.I. Natural Brown 12

The amorphous, earthy lignite mined in the neighbourhood of Cologne has been used inter alia as a painter’s colour, although because of its high absorbency, oil consumption tends t o be high. The coat dries rather slowly and turns grey under the action of light. Rubens mixed ‘Cassel brown’ pigment with finest yellow ochre and obtained a warm brown that has lasted well in resin varnishes. A paper dye known as ‘Sap Brown’ or ‘Van Dyke Brown’ is obtained by alkaline degradation of lignite rich in humus and humin. It consists of unknown humic acids, which are formed by decomposition of organic matter, particularly dead plants. 1.7.4 Berberine (barberry) CAS 2086-83-1 C.I. Natural Yellow 18 (2.1. Constitution 75160

One of the first naturally occurring cationic dyes t o be identified, berberine occurs chiefly in the stem and roots of the barberry plant, Berberis vulgaris, and is extracted from the roots by boiling in water for one hour. When applied t o wool, silk and cotton it gives lemon-green shades.

412

The Navajo Indians in the Southwest of the USA dyed brilliant yellow shades on buckskin with Berberis fremontii. In the 16th century barberry plants were very popular in England as hedges, rather than as a source of vegetable dyes. At that time scarcely any other plant was used for that purpose in England. 1.7.5 Betanin (red beet) CAS 107-43-7 C.I. Natural Red 33 C.I. Constitution 75840

GI

COOH The juice of the red beet, beta vulgaris, serves chiefly as a dye for foodstuffs. Its chief colouring constituent is bethanidine. Because of its intense red colour it is administered to good effect in folk medicine in the treatment of anaemia and of both liver and kidney disorders. Also, it has been suggested that wine has been successfully adulterated with this natural colour.

1.7.6 Orsellinic acid (orcein, litmus) Orsellinic acid CAS 480-64-8 C.I. Natural Red 28

HO

OH

This important acid-base indicator is produced from lichens. Lichens are differentiated into forms that grow on plants, and those that grow on soil and rocks. The former yields orseille or archil and the second, which commonly flourish on stony coastal fringes, is a source of litmus. The lichens Rocella fuciformis, tintoria, phyopsis and montagnei are found on the French Atlantic coast, in

413

Holland, England and around the Mediterranean, as well as on the Azores, in South Africa (Cape Province), India and New Zealand. The name litmus comes from the Dutch “lakmoed’, (lak = lacquer, moes = pulp). In preparing the colorant, crushed lichen was mixed with ammonia and potash, then allowed to ferment. Afterwards the violet mass is pressed through a sieve and kneaded with gypsum and chalk. Apart from its use as an acid-base indicator, litmus was also used as a food colorant and for bluing laundry, particularly in Holland. Its chief constituent is azolitmin. As in the lichen extract, however, the parent substance is orsellinic acid, which is degraded to orcein. The precursor stage proper probably has a 7-hydroxy-2-phenoxazonestructure. Dyeing with orseille was developed in Italy in the 13th century and long kept secret. Wool dyeing with lichen was practised in Europe into the 19th century. With an aluminum mordant, bright but fugitive red dyeings sensitive to acids and alkalis were obtained. French purple ‘Pourpre franqaise’ gave bluish shades of the utmost clarity. The ammoniacal lichen extract is precipitated with hydrochloric acid, redissolved with ammonia, oxidized by exposure to air, and the purple coloured solution again isolated with acid. The resulting dyeings are fairly fast.

1.7.7Xanthone Euxanthone (Indian Yellow) CAS 529-61-3 C.I. Constitution 75320

OH

0

‘Indian Yellow’, “pioury or piori”, was obtained in the past in Bengal by feeding cows on the leaves of the mango tree, Mangifera indica, and heating their urine t o precipitate the dye as the magnesium or calcium salt. Chemically a xanthone derivative, ‘Indian Yellow’ was an extremely popular pigment for painting miniatures on paper in India at the time of the Mogul Empire. The parent substances include euxanthone.

1.7.8 Naphthalene Shan black Diospyrol CAS 17667-23-1

414

'Shan black' is a famous black for leather and linen, taking its name from the Shan tribe in Burma and Thailand. It is a naphthalene-based compound that is derived from the fresh fruit of the ebony tree. 1.7.9 Riboflavine Riboflavine (vitamin B2) CAS 83-88-5 C.I. Food Yellow 15 C.I. Constitution 50900

HOH,C HO "$OH

Riboflavin, vitamin B2, is found in tomatoes and other vegetables, in whey, and as an enzyme in pig and ox liver. Riboflavin and riboflavin-5'-phosphate are approved in Europe for colouring foodstuffs.

1.8 Pigments Carbon black CAS 7440-44-0 C.I. Food Black 3 (2.1. Constitution 77268:l also

415

(2.1. Pigment Black 6-10 C.I. Constitution 77266177268177267177265 Carbon black is probably one of the most important colorants. It is formed by heating carbon-rich organic substances in the absence of air. Well-washed vegetable black contains 95 % amorphous carbon, the least expensive and deepest black. Not surprisingly, therefore, it has been, and still is, employed for a diversity of colouration purposes. In addition it is used to colour foodstuffs and, because of its high absorptive power, as an adsorbent in the treatment of gastric disorders. Asphalt is the organic black dye C.I. Natural Black 6. It is produced by the action of atmospheric oxygen on high boiling mineral oil. Very similar is C.I. Pigment Black 18, C.I. Constitution 77011, which is a carbonaceous hydrated silicate. Finally, homogenized, finely dispersed natural earth colours are used as body paints and artists' colours. Table 4. Selection of earth colours according t o the Colour Index

1

I

I 1 1 1 I I

C.I. Pigment Yellow 39 Yellow 42.43 Brown6.7 Red101.102 Red 106 Blue29 Blue30 Blue 31 Green 23 Black 14 Black 11 Highly toxic" 2. Synthesis

2.1 Purple

C.I. Constitution Name and Comnosition 77086 Old King's Yellow. regular. as-trisulfide" 77492 Yellow Ochre. hvdrated ferric oxide 77492 Brown Ochre. hvdrated ferric oxide 77535 Indian Red. ferric silicate 77766 Cinnabar. mercuric sulDhide* 77007 LaDis Lazuli. ultramarine S-Al-silicate 77420 Azurite. basic comer carbonate 77437 Egyptian Blue, copper silicate 77009 Green Earth. Al-ME-Fe-silicate 77728 Pyrolusite, manganese dioxide 77499 Manganite, ferroso-ferric oxide

I I I I I I I I

416

Br H

Precursor stage (chromogen) in Murex brandaris: tyrindoxyl sulphate

2.1.1 Formation of purple dyes from the chromogens Marine molluscs of the genera Murex (family: Muricidae) and Purpura (family: Thaididae) are the main sources of natural purple. The purple dyes contained in the secretion from the hypobronchial glands of the molluscs are not present in the finished form but rather as dye precursors. Conversion to the finished form is achieved via hydrolysis by the enzyme purpurase (arylsulphatase) under the simultaneous action of air and light. It was only recently that three working groups successfully characterized the precursors of Purple of the Ancients in the hypobronchial glands of different species of sea snails. The above formula corresponds to the chromogen derived from Murex brandaris (=Bolinus brandaris), a species found in the Mediterranean. From the liquid secreted by this mollusc, Friedlander was able to isolate 6,6’-dibromoindigo, which gives a reddish violet dyeing. The name ‘Purple of the Ancients’ was given to this compound which, strictly speaking, is only one constituent of true ancient purple dyeings. The red component is probably indirubin: Indirubin C.I. Natural Blue 1 C.I. Constitution 75790

2.1.2 Production of purple in ancient times Pliny describes the procedure, though somewhat obscurely, in his ‘Natura Historia’. The snails were either crushed or opened with special instruments. Next

417

the resected hypobronchial gland or the whole creature was encrusted in salt to withdraw the gland contents from the arteries. After three days the liquor formed was heated in a lead vessel with hot air or by injecting steam and kept for ten days at moderate temperature, during which time it concentrated down. In the course of this treatment muscular fibres and what we now know to be the enzyme purpurase separated from the dye precursors, now present as aqueous solutions. The content of dye present in vat form was then determined by carrying out a test dyeing with a small amount of loose wool. Depending on the outcome the liquor was further concentrated or diluted. 2.1.3 Synthesis of 6,6'-dibromoindigo This compound is synthesized by the same methods employed for indigo. The difficulty lies in attaching the bromine atoms at position 6,6' because when indigo is brominated directly positions 5 and 7 are substituted. 5,5'-Dibromoindigo can also be prepared. To obtain the 6,6'-derivative, appropriately substituted starting materials have to be used, and these are not easily synthesized. Numerous attempts have been made to find a simple way to synthesize Purple of the Ancients. Syntheses described in the literature all proceed via 6-bromindoxyl (1) or indolone (2), which is reported to dimerize spontaneously t o 6,6'dibromoindigo. The key compound here - as also in the Baeyer synthesis - is 4bromo-2-nitrobenzaldehyde3, which had to be prepared in several steps.

q""'

BJj--o

BQ - O H

H 1

Br

2

3

A more recent, purportedly simple gram scale synthesis is described in Ref. [12]. The aldehyde 3 is thus obtained in a yield of 92% as follows:

418

aBr aBr HN03+

Br

Br

H

in THF, -105°C

NO2

I

CH,

>

Br

1. - 78”C, 30 min. 2. H2S0, 0°C

3

Aldehyde 3 is condensed with acetone in alkaline solution and the aldol thus produced disproportionates intramolecularly, yielding acetate and the intermediate 2 (indolone), which dimerizes to 6,6’-dibromoindigo. The purple dye was obtained in a yield of 42%. A higher yield was achieved by the Harley-Mason method. Here, the aldehyde 3 is reacted with nitromethane in NaOMe/MeOH, and the nitronate formed is reduced with sodium dithionite. Following several additional steps, indolone 2 is first formed and then 6,6’-dibromoindigoin the form of fine, dark violet crystals in a yield of 66%. Reduction with sodium dithionite produces the water-soluble enolate of the leuco compound, with which the reddish violet colour of Purple of the Ancients formed on cotton after atmospheric oxidation. 2.2 Indigo

FH,OH

H

Precursor: indican

419

2.2.1 Preparation of Indian indigo from Indigofera tinctoria

The indigo plant, containing 0.2 - 0.8 % of the glucoside indican, is cut shortly before flowering, placed in water pits, weighted with boards and left to ferment. Fermentation sets in rapidly, evidenced by foaming and copious evolution of CO,. The indican is split into indoxyl and glucose by an enzyme, indigoemulsin (indoxylase), contained in the indigo leaves. At the same time microorganisms generate hydrogen as a reducing agent and the dye becomes water soluble. After 12-15 hours the process is finished, a yellow liquid (‘Indigo white salt’ or ‘yellow vat’) with blue foam on the surface is formed:

-

-

H

HO

’White Salt’ The yellow solution is run off into so-called beating tanks, where it is contacted with oxygen by churning with paddle wheels or beating with sticks to oxidize the reduced indigo to the dye proper. The dye precipitates as a blue, flaky mass; the clear liquid is discharged, the dye sludge boiled with water, the indigo filtered, pressed, cut into pieces and dried very slowly in well ventilated drying sheds to yield a finely structured, readily vattable product. From 100 kg dried plants, 1.5 2 kg indigo is obtained. 2.2.2 The Heumann / Pfleger syntheses The first successful laboratory synthesis of indigo was performed by von Baeyer 1131 in 1878, starting from phenylacetic acid. Caro later found that o-nitro-phenylpropionic acid formed as an intermediate in the method of synthesis starting with cinnamic acid which had also been developed by von Baeyer [141 could be converted into indigo directly on the fibre with mild reducing agents. The laboratory method of synthesizing indigo, however, was not very successful in large-scale production. However, in 1890 indigo synthesis was given a new impetus by Heumann [15], whose approaches started with phenylglycine-ocarboxylic acid.

FirsWsecond Heumann syntheses:

420 n

BASF and Hoechst developed the second Heumann synthesis further. The starting product, anthranilic acid, is prepared from phthalic acid, and this is followed by Hoffmann degradation. Phthalic acid was obtained by oxidation of naphthalene with fuming sulphuric acid in the presence of mercury as catalyst. 1897 was the year in which indigo was first synthesized. An increase of yield was achieved in 1901 through a discovery made by Pfleger - the use of sodium amide as an additive to the anhydrous alkaline melt of phenylglycine in the first Heumann synthesis. In 1925 BASF had developed the method by which indigo is still mass-produced today; here again phenylglycine is the most important intermediate. The entire process is therefore still based on the Heumann synthesis. Present-day Heumanflfleger synthesis of indigo [161:

A. Phenylglycine nitrile

-0

NH-CH,CN

+CH,O+HCN

850c

B. Hydrolysis O_NHCH,-CN

100°C WNaOH + H,O

a

+H,O

NH-CH,-COOWNa +NH,

42 1

C. Sodium amide Na + NH,

380°C

D. Indoxylate melt NH-CH,-COOWNa

Na NH,

2200c

2 NaNH,

+ I/, H,

Indoxyl-Na-salt

+ 2 NH, + WNaOH

E. Indigo oxidation 2x Indoxyl-Na-salt + 0,

8 % c

Indigo + 2 NaOH

2.2.3 Recent syntheses

Recent syntheses can be exemplified by an article published recently [171. 3-Hindol-3-one 1 was recognized as a suitable, though not directly available, intermediate in the synthesis of indigo; the pertinent synthesis is outlined below.

1 An equally suitable compound, also unavailable, would be the hypothetical product formed by hydrolysis of 1,viz., o-aminophenyl-glyoxal(2):

422

2

The corresponl ing imethyl acetal 3 can, however, be prepared anc. converted to indigo by acid hydrolysis. 1

3 Isatoic anhydride 4 appears to be an attractive precursor because it both activates the carbonyl group and protects the NH group. The anion of dimethyl sulfoxide (dimsyl anion) was chosen as a masked form.

Y d HI

4

Under a nitrogen atmosphere, a dimethyl sulfoxide solution (7%) of 4 was added to a solution of dimsyl-Na (3 equivalents) in the same solvent at room temperature; o-amino-w-methyl-sulfinyl acetophenone 5 was formed in a yield of 82%:

423

5

The Pummerer re-arrangement of 5, achieved by heating the aqueous solution in hydrochloric acid, was accompanied by the separation of methyl mercaptan and the stepwise formation of the desired indigo in the form of a precipitate. The yield of the dye after washing with ethanol and drying was 39%.

6

An increased yield resulted when 5 was acylated in benzene solution at 80°C with acetic anhydride, giving an 86 % yield of 6. Subsequent acid hydrolysis and simultaneous Pummerer re-arrangement produced a 77% yield of indigo. It was also possible to perform the dye formation reaction on fabric or fibres. 2.2.4 Preparation by genetic engineering The very latest methods of producing indigo involve genetic engineering techniques. Representative of numerous references in the literature is a patent to Michigan State University, USA 1181: A method for the production of indigo and indirubin dyes using a recombinant E. coli containing a gene encoding a phenol hydroxylase from Bacillus stearothermophilus is disclosed. The pheA gene of B. stearothermophilus was cloned, sequenced, and expressed in E. coli. Phenol hydroxylase activity was induced by phenol, o-, m- and p-cresol, and 2-chlorophenol. Thus, in a pheAexpressing E. coli culture containing phenol, indigo production was maximal at 24 h and indirubin at 40 h. The pheA and associated pheB genes were localized to the 66-kb plasmid pGGO1.

424

2.3 Alizarine 2.3.1 Alizarine and other colouring components in madder root The colouring components are found in the root of the madder plant (Rubia tinctorurn). Twenty-three anthraquinone derivatives and 50 glycosides have been detected in the root, but not all are important for dyeing. The main dye, alizarine, is present chiefly as ruberythrinic acid (alizarine-2-~-primeveroside). After harvesting, the root can be dried in heated drying chambers (kilns), then separated from the outer skin and the root fibres by pounding, to yield ‘refined‘ madder. The dried root stock is 3 to 6 mm thick, cylindrical, brown outside and red inside. Its surface is covered with a thin cork layer that readily peels off and an underlying dark red or blackish-brown rind, which contains the bulk of the dye. Very little is present in the brick-red wood. During drying and storage of the madder root the glycosides are split by fermentation into the free dyes and sugar. Further degradation takes place during dyeing, as the substrate that has been mordanted with alum and tartar, is placed in the dyebath a t pH 4 - 5 and 80°C. These two factors and the dyeing time employed induce hydrolysis of the residual o-glycosides, so that the dyeing process can be seen as a kind of “dye synthesis”. 2.3.2 Synthesis of alizarine In 1867 Graebe and Liebermann succeeded in determining the constitution of alizarine, and went on to achieve its synthesis one year later, jointly with Car0 [191. They also developed an improved method of synthesis, which is detailed by Fierz-David and Blangey [20]. In this account, alizarine is prepared from the alkaline melt of anthraquinone-2-sodium sulfonate (‘silver salt’) in the presence of an oxidizing agent, usually sodium chlorate. In this reaction not only is the sulfonic acid group replaced by -OH, but a second hydroxyl group is also introduced: NaOH, H,O, NaCIO,

0

+ONa 185”C, 48 h

0

p$ f

u\

0

/

ONa

-

H,O, H,SO,

50%

0

+O \ H

0

425

The reaction mechanism can be described by assuming that after the sulfonic acid group of “silver salt” has been replaced by the hydroxyl group, a +6 charge is generated in the a-position to which the second OH -group can attach. Oxidation to the end-product then takes place:

2.4 Kermesic acid 2.4.1 Production of kermes and kermesic acid Kermes is the dried body of the female insect of the scale species Kermes vermilio, which lives on the kermes oak Quercus cocciferu around the Mediterranean basin and in Asia Minor. A second species, Kermes ilicis, is found on the chestnut or durmast oak Quercus ilex. Kermes vermilio contains 75 - 100% of the anthraquinone dye kermesic acid.

426

The pea-size female insect clings to the leaf stems of the host plant and feeds on its sap. Towards the end of May it lays approximately 2000 red eggs; the mother dies and with its shield-like body protects the eggs against the effects of weather. Before the infant insects emerge from the eggs, the dead mothers are collected together with the eggs and the infants are killed by hot vinegar fumes or immersion in vinegar and afterwards dried. Some are left on the trees for future propagation. To prepare kermesic acid in the laboratory, kermes is extracted with ether and, after a series of steps, the product thus obtained is isolated via the sodium salt as free acid in the form of brick red needle-shaped crystals. 2.4.2 Synthesis of kermesic acid The constitution of kermesic acid was first determined by Dimroth et al. and later by Gadgil [211. A typical synthesis is outlined in Ref. [221: A Diels-Alder reaction between the diene 1 and 2,6-dichloro-1,4-benzoquinone 2, in benzene, yields the naphthoquinone 3 after aromatization of the intermediate addition product:

1

2

3

1:2-Addition of 1,l-dimethoxyethene to naphthoquinone 3 in dimethylformamide gives the anthraquinone 4; the direction of the addition is controlled by the arrangement of the halogen substituents:

421

CO,CH,CH, H,CO H

O 4

Conventional bromination followed by reaction with sodium methoxide in methanol gives the pentamethoxy-anthraquinone 5 . Dealkylation in an AlCl,-NaCl melt finally produces kermesic acid 6 in an overall yield of 25% with respect to 2:

W H,CO

C

O

\

2

C

’

H

-

3

OCH,

OCH, 0

5

W

HO

C

0

\ OH

,

/

H OH

0

6

2.5.Carminic acid (American cochineal) 2.5.1 Preparation of cochineal and carminic acid Cochineal is the dried body of the egg-containing female nopal scale, which lives on cacti of the species Nopalea coccinellifera, also called opuntia or Indian fig; the plant is non-spiney. The principal colouring component is carminic acid; of this dried cochineal contains 9-10%. Following preliminary studies by Dimroth et a l . , Overem and Van der Kerk determined the constitution of carminic acid [23]. The female nopal scale is roughly the size of a ladybird, brownish red in colour, wingless and clings to leaves and stems of the host plant in order to feed on its sap. It lays a thousand eggs, which lie as if in layers under its abdomen. While the young hatch out as larvae, the mother dies. After several moultings, wingless females and winged males emerge. After two t o three months, a second and a third set of offspring develop; then, in Central America the rainy season starts. Male and female insects hide away on

428

the cacti as well as they can. Because gathering scales living in the wild would mean uncertain harvests, host cactus plantations - known as nopalries - for raising the cochineal insect were started some centuries ago. Cochineal insects that have been picked as larvae from the nopal cacti at the onset of the rainy season and kept in nests of hay on opuntia leaves are returned t o the plants. There they are spread out, and after about two months the females are collected shortly before they lay their eggs; the eggs contain the most dye. A few females are left on the cacti. A second and a third harvest - before winter sets in - of the young that have grown t o fully developed females then follow. Young insects are kept to start a fresh population in the spring. The ‘harvesting’, i.e. collecting the gravid females, is done in Mexico with brushes of different degrees of hardness. In the Canary Islands, palm leaf whisks are used to sweep the insects onto sheets of cloth or into wooden bowls. Depending on the later commercial variety the insects are killed with sulphur fumes or in boiling water, then dried in kilns, on metal or by exposure to the sun. Nopal plantations can yield up to 400kg cochineal per hectare. Peru is the most important source of supply at the moment. To produce carminic acid the hot prepared aqueous extracts of commercial cochineal are concentrated to 1 litre, mixed with 400ml of conc. sulfuric acid while being effectively cooled, allowed to crystallise and collected by filtration (yield 50 g k )*

2.6 Saffron 2.6.1.Gatheringthe saffron plant stigmas The fresh stigmas in the flowers of the saffron plant (Crocus sativus) contain the glycoside protocrocin, which readily breaks down into the main dye crocin. Crocin is present as digentiobiose ester, and picrocrocin. The free acid form is called crocetin The stigmas are harvested in the autumn, when the saffron plant’s flowers have fully opened. They are pulled out of the flowers with the ends of the pistils and dried carefully on hair sieves over glowing charcoal or hot ashes. During this drying process the characteristic odour first forms, caused by the decomposition of picrocrocin (‘bitter saffron’) to safranal (dehydrocyclocitral) and glucose. This process takes place slowly during storage of the fresh stigmas. Picrocrocin and safranal are also typical flavours of saffron. The yield per hectare is something like 20kg. To produce 1kg dried saffron (= 5 kg fresh stigmas) requires as many as 20,000 flowers. Besides the main dye crocin, saffron also contains traces of free crocetin, a heptaene dicarboxylic acid, its various esters and a few other carotenoids.

429

2.6.2 Synthesis of crocetin ester Crocin has been discussed in the literature [24], and a total synthesis of alltrans-dimethyl-crocetin reported [251 in the carotenoid series, the basic reaction there being the conversion of synthesised y-bromotiglic acid methyl ester in a Reformatzky reaction with tiglic aldehyde:

dialdehyde

2

+ dialdehyde

BrCH2FHJ02CH3

-

ptoluene sulphonic acid

-2H20

ZnIHg

in THF

- -

Lindlar catalyst

part. hydration

H3C02C

hv

(isomerize)

C02CH3

all-trans dimethy crocetin ester

One publication [261 mainly addresses the constitution of picrocrocin. It indicates that the dye of saffron stems from the middle polyene chain (20 C atoms) and the bitter constituent, picrocrocin, from the terminal ring systems (10 C atoms

430

each), of protocrocin. Finally the bitter constituent forms the odorous substance “safranal” by splitting off the sugar residue. In the crocus stigmas there is 1 mol crocin for 2 mol picrocrocin. The preparation of crocin by extraction from saffron stigmas is also described in Ref. [26].Saffron powder, pre-extracted with petroleum etheddiethyl ether, is boiled with methanol and pressed. The filtrate is diluted with methanol and placed in the refrigerator. Crude crocin crystallizes out after standing for several weeks and is then purified by recrystallization.

2.7 Bixin 2.7.1 Preparation of bixin from the annatto tree The annatto tree Bixa orellana contains up to 13% of the dye bixin, chiefly in the red seed hull. Two isomers can be distinguished, viz., unstable bixin (14-cis-bixin, natural bixin, annatta, monomethyl ester of norbixin, the free dicarboxylic acid), and stable bixin (P-bixin, all-trans-bixin, monomethyl ester of stable norbixin, isobixin). Further carotenoids such as bixein and crocetin are also present in the seed hull. From the sixth year onwards after planting, all the fruit is harvested from the tree. One tree yields 8-24 kg fruit and 1-3 kg seeds. The fruit is opened shortly before ripening, while still green, and the seeds are scraped out with small sticks. After drying in the sun the seeds are traded as “Urucu em gros”, the main form commercially available today. Nowadays the dye is prepared from the seed hull, whereas formerly it was extracted from the fermented fruit pulp. The seeds are covered with hot water and stirred for a prolonged period, and fermentation occurs. After straining, the liquor is left to stand, the extract sinking to the bottom of the vessel. The water is drained off, then the residue is dried, formed into 50 to 100-g rolls or cakes, and wrapped in canna or banana leaves. There used to be various commercial varieties, but currently only Cayenne or Guadalupe annatto is traded in the form of cakes weighing 1to 5 kg. 2.7.2 Syntheses of bixin [271 The industrial synthesis of bixin is based on the oxidation of lycopin, the colorant in tomatoes. The cis-trans-isomerism of carotinoid pigments is reported in Ref. 28. 2.8 Flavonoids and neoflavanoids Flavonoid and neoflavanoid dyes are colours of the mordant class.

43 1

2.8.1 Flavonoids The majority of yellow natural dyes suitable for dyeing purposes belong to the flavonoid class of compounds, which includes the flavones, flavonols, isoflavones, flavanones, chalcones and aurones. The flavonoid dyes contain HO- and CH0,- groups as substituents, the number and position of which give rise to a host of different dyes. The name originates from the Latin “flavud’ (=yellow). Many flavonoids are present in plants as glycosides, which on dyeing are often split into an aglycon dye and sugar. Furthermore they are colourless or have only a weak color. They do not produce a strong shade on a substrate until they have formed a lake with the mordant. Luteolin and fisetin are typical flavone dyes. Fustic (yellow wood yields flavonoles (morin), whereas brazilwood (redwood) and logwood (bluewood) yields neoflavaniodes.

OH

0 FIavone

Flavonole

lsoflavone

Flavanone

Chalcone

Aurone

The basic structure of the flavonoids is the colourless flavone. A synthesis for flavone and flavonol is shown below, which, with appropriately substituted adducts, can also be performed for a large number of flavone derivatives. Biosynthesis of flavone often starts with cinnamic acid analogues 1291.

432

2.8.2 Preparation of weld extract (luteolin) from the dyer’s plant

Weld, Reseda luteola, which belongs to the Resedaceae family, contains the flavonoid colouring constituents luteolin and its various glycosides, traces of apigenin and a further few compounds in all parts except the root. The whole plant is used for dyeing purposes, the seeds being especially rich in dye. Harvesting starts when the stems, leaves and capsules have turned yellowish. The plants are carefully pulled out of the soil, dried and bound into sheaves. Dyeing is performed with an extract of finely chopped weld. In a typical procedure for preparing luteolin from Reseda luteola, 300g weld extract is boiled for several hours with 3L water containing lOOmL conc. HC1 and filtered hot. After several days crude luteolin precipitates, is then stirred into diethyl ether and, after filtration, shaken with dilute NaOH solution. The dye is precipitated from the alkaline medium with dilute HC1 and purified by recrystallization from ethanol.

-

Fries reaction

Q 0 J c H 3

(AICI,)

Phenylacetate

o-Oxy-benzal-acetophenone

= o-oxy-chalkon (yellow)

___2)

OH o-Oxy-acetophenone

Flavanone (colourless)

433

Flavone (colourless)

Flavonol (yellow)

2.8.3 Synthesis of luteolin

A procedure for synthesizing luteolin is described in Ref. 30, and the starting compound for this sequence is 5-bromo-2-hydroxy-4,6-dimethoxyphenyl-~,~dibromo-~-3,4-dimethoxyphenyl-ethyl ketone:

Br, in CS,

190°C

reduced pressure

Br

OCH,

-HBr, -4CH,I

2.8.4 Preparation of fustic extract from the dyer’s plant The dyer’s mulberry Maclura tinctoria (other names: Chlorophora tinctoria, Morus tinctoria), a member of the Moraceae family, contains morin, maclurin and a little kaempferol as dyeing constituents in its heartwood. Formerly, the hard pale yellow wood was sold in blocks weighing 10 - 80 kg; the different types were named after their place of origin. The direct addition of fustic to dyebaths was discontinued in the second half of the nineteenth century; the wood is now used exclusively to produce fustic extract. After rasping, the wood is

434

extracted with water and, the extract is then concentrated by evaporation. The extract is slightly oxidized by atmospheric oxygen, but a t the same time, the shade intensity of the resulting product increases. Extracts of fustic contain chiefly morin and the Na-Ca salt of maclurin. Extracts that have been concentrated under vacuum, and hence not oxidized, are pale yellow; oxidised extracts are brownish yellow to reddish yellow. Fustic extracts are marketed in liquid form, or as solid in cakes containing ca. 17%water, or granulated with a water content of about 5%. Calico yellow is obtained by treating the extract with sodium bisulfite, resulting in the formation of the sulphuric acid ester of morin. This product has been used in calico printing, in combination with chromium acetate. Morin is prepared from the unoxidized fustic extract by treatment with warm hydrochloric acid, dissolving the residue in hot ethanol, followed by fractionated precipitation with water. 2.8.5 Synthesis of morin Morin can be synthesised from chloro-acetophenone-dimethylether and 2,4dimethoxybenzaldehyde [31]. Flavone, the chromogen of the entire group, is colourless, but also occurs in Nature as a colourless coating on species of primula. 2.8.6 Neoflavanoids The term ‘neoflavanoids’, for the basic structure of the dyes brasilin and haematoxylin (logwood dye), has been customary since 1964.

lsoflavane

Neoflavane

435

Dyes of the neoflavanoid class occur as colourless precursor products in the heartwood of tropical trees, notably as brasilin in so-called ‘soluble redwoods’ and as haematoxylin in logwood. Oxidation converts these dyes t o brasilein and haematein. Brasilin is found chiefly in the heartwood of species of Caesalpinia (family Leguminosae). Unlike sandalwood, barwood and camwood, these redwoods contain water-soluble dyes to water, and are therefore called ‘soluble redwoods’. 2.8.7 Preparation of brazilwood extract from the dyer‘s plant The heartwood of Haematoxylum braziletto contains brasilin as precursor of the colouring constituent, and this - as described above - is converted to brasilein by oxidation, e.g. during the extraction process. The heartwood, which was first sold commercially in blocks weighing 10 - 30kg, used to be rasped, and in that form added to the dyebath. Later dyers then used redwood extracts, generated by crushing and grinding the blocks. The resulting powder was extracted with water at 100°C in banks of extractors and the product concentrated to produce a liquid product, or evaporated to dryness to give a powder product.

2.8.8 Preparation of logwood extract from the dyer’s plant Logwood or campeachy wood Haematoxylum campechianum, a member of the Leguminosae family, contains in the heartwood of the trunk 9 - 12% haematoxylin (hydroxybrasilin), part of it combined with a glycoside, which oxidizes to the red mordant dye haematein; both compounds are of the neoflavanoid type. The very hard but easily split heartwood is marketed in large blocks, bluish black on the outside, reddish brown on the inside and smelling slightly of violets. For dyeing purposes the wood is supplied rasped, as a reddish brown powder, and as an extract. Logwood extracts are prepared by boiling fresh logwood with water (French method) or treating with steam under pressure (American method). The product obtained is then concentrated under vacuum. The haematoxylin (glycoside) contained in logwood is gradually decomposed by fermentation processes after the tree has been felled and the wood converted into chips. With chipped wood this process can be accelerated by fermentation or oxidation. In earlier times the dyer leached out the wood after rasping it and dyed his textile materials with the extract. Since the main component of this extract is nondyeing haematoxylin, it has to be converted in the dyebath to haematein lakes by metal salts in a time-consuming process. To shorten this process, dyers explored the idea of converting haematoxylin to haematein in the heartwood itself. Oxidation resulted in woods with a haematein content as high as 50%. Wood prepared in this way is also called finished logwood.

436

Cut logwood is finished by wetting it in piles with water and keeping it a t a temperature of about 30”C, turning it over from time to time. The process of oxidation is completed after 5 - 6 weeks. Oxidation is performed with far greater efficiency in rotating drums, requiring only 48 hours and proceeding much more intensively than in piles. After two weeks the wood has already assumed the characteristic dark red colour. Haematein is also used for staining histological specimens and as an analytical indicator. Unfermented “verum)’ logwood in finely cut form also has pharmaceutical applications.

2.9 Chlorophyll 2.9.1 General One of Nature’s most important organic compounds, chlorophyll 1321 has become an object of intensive development in the past few decades. Staple consumer items, among them toothpaste, chewing gum, and candy, plus potential medical and pharmaceutical uses seem to offer a promising future. In 1906 the first significant research on its purification was reported, Tswett I331 purified chlorophyll for the first time by column chromatography. Willstadter [341, who won the 1915 Nobel prize, isolated chlorophyll a and b in their pure form, and his researches are the basis for today’s commercial production of chlorophyll. With modifications developed by the United States Department of Agriculture (USDA), those solvent extraction techniques are now the foundation of the chlorophyll industry in the United States. 2.9.2 Extraction Basically, the production methods developed by the USDA [351 involve extraction of carotene from dried alfalfa leaf meal with petroleum ether and recovery of carotene by concentration of the extract. Chlorophyll and xanthopyll are then extracted from the meal with acetone. The acetone extract is dissolved in petroleum ether and the major portion of the acetone removed by extraction with water. Xanthophyll is separated from this second extract by extraction with 85% methanoywater. Careful washing with water removes the alcohol from the petroleum ether solution, thus permitting the chlorophyll to be separated by precipitation. Chlorophyll is purified by reprecipitation from acetone/petroleum ether solution. The most popular chlorophyll product, a t present, is the water-soluble chlorophyllin form. American Chlorophyll and other producers use the basic USDA process modified with a saponification step. Thorough saponification of chlorophyll extract with a methanol/KOH solution facilitates separation of the water-soluble

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chlorophyllin from the solvent, usually a hexane/acetone mixture, for further purification and finishing. The water-soluble materials are, strictly speaking, not entirely soluble in water. The extract may be finished, instead, as oil-soluble chlorophyll by removal of plant waxes and oils. Solvent extractions and evaporation complete the finishing of the oil-soluble products.

2.9.3 Synthesis The total synthesis of chlorophyll was successfully performed by the Nobel laureate Woodward. This extremely complicated synthesis that requires four simple pyrrole derivatives is described in Ref. 36. 2.10 Haemoglobin Haemoglobin (Hb) or ferrohaemoglobin is the major component of red blood cells. It transports oxygen from the lungs to body tissues and facilitates the return transport of carbon dioxide. Mammalian haemoglobins have molecular weights of about 64,500. They are composed of four peptide chains called globins, q. v., each of which is bound t o a heme. Normal human haemoglobin consists of a pair of identical chains. Iron is coordinated to four pyrrole nitrogens of protoporphyrin IX, and to an imidazole nitrogen of a histidine residue from the globin side of porphyrin. The sixth coordination position is available for binding with oxygen and other small molecules. The pigment is called oxyhaemoglobin, Hb02, in the oxygenated form and carboxyhaemoglobin, HbCO, when the oxygen is displaced by carbon monoxide. In the lungs HbCO binds oxygen reversibly, with iron remaining in the ferrous state. Autoxidation is prevented by the neighboring hydrophobic groups of the globin residue. When the iron in haemoglobin is oxidized from the ferrous to the ferric state the compound is called methaemoglobin, q. v., and is accompanied by a loss of oxygen-binding capacity. Research workers who played a major role in elucidating the constitution of haemoglobin and synthesizing it include Nencki, Piloty, Kuster and Fischer, who received the Nobel Prize; structure studies are reported in Ref. 37. Haemoglobin is usually prepared by separating the red blood corpuscles from the lighter plasma by centrifuging; the plasma is siphoned off, and on adding ether t o the blood corpuscle paste, the cells burst. After further centrifugation to remove the ruptured cell envelopes, a clear red solution of the protein is obtained [381. Haemoglobin has not been used in dyeing. 2.11 Tanning agents [391 Today, tanning extracta are produced by several processes, e.g.: - disintegration of dried tanning material - extraction of the tanning substance with water a t 80-130 "C

438 -

purification and drying to obtain a powder of 60-80 % pure tannin.

The aromatic synthetic tanning agents (syntans) are based on a very simple principle: polynuclear phenols are sulfonated to increase solubility and then condensed with aliphatic carbonyl compounds. They exist mostly in the form of sodium salts. By varying the starting material and procedures it is possible to manufacture diverse syntans having specific properties without copying any natural tanning species. Determination of the tanning agent content is highly empirical but very expedient, and consists solely in ascertaining the absorption capacity on so-called hide powder.

2.12 Naphthoquinone 2.12.1. Preparation of lawsone from henna Lawsone, the dyeing component of henna in the henna plant Lawsonia inermis, occurs in amounts up to 1%in the plant’s leaves and stems. As mentioned earlier in the section dealing with the history of natural dyes, lawsone is not contained as such in the plant, but is formed by enzymatic hydrolysis of the hennosides (glycosides)A, B and C and autoxidation of aglucon. 2.12.2 Syntheses of lawsone A procedure for synthesizing lawsone is reported in Ref. 40:

wl \

SOCIL -H,O

2 CH,N, (diazomethge)

*\ o

0

-N,

0

Triketo-indan

Ninhydrin

0 Lawsone

2-Methoxy-naphthoquinone

(2-Hydroxy-l,4-naphthoquinone)

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3. Dye Application 3.1 General We have collected a few typical recipes from different sources without making allowances for ecological and toxicological factors [41]. Some of these recipes are taken from historical sources or are based on old traditions. Any duplication of these recipes should therefore be attempted only under the supervision of an experienced chemist who is aware of the latest ecological, toxicological and safety requirements. This includes also the dye syntheses and isolations mentioned in the previous sections. These requirements must be consistently observed even if attention is not explicitly drawn to them. Furthermore the recipes are purely for guidance and must be adapted to local conditions and equipment. As many natural dyes can be applied to different substrates, the relevant cross-references should be noted.

3.2 Wool and silk 3.2.1 Mordanting of wool and silk Generally speaking, heavy metal salts are used for the pre-treatment or mordanting of wool. The following amounts are customary [421: 6-25% alum: AlK(SO,), 12 H,O 3-4% copper (11)sulphate: CuSO, 5 H,O 10-15% iron (11)sulphate: FeSO, 7 H,O The percentages are based on the dry weight of the substrate to be dyed. Tin (11)chloride SnC1, is also used, but the resulting light fastness is on the low side compared with that achieved with the other metal mordants. Chromium in the form of chromium (VI) (carcinogenic &Cr207),which is reduced t o chromium (111)by wool, has been in use up to the present time. Titanium salts, which could not be manufactured until around the turn of the present century, are also suitable as metal mordants. Mixtures of metal salts can be used for intermediate shades. Tartrate (0.3-0.7%)is used as complexing agent, particularly with salts of aluminium and iron. The metal salts, thoroughly dissolved in water, are applied cold t o the substrate to be dyed. Good liquor circulation is important, but should not be so vigorous as t o overstress the wool or silk. For wool the bath temperature is raised slowly to the boil and held there for 10-60 min. After cooling, the goods are rinsed. Silk is mordanted cold for longer (up to 72h). For bright shades a single rinse is recommended.

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3.2.2 Saffron yellow Crushed stigmas (lg) are boiled gently in 500g of water for 30 min and filtered. The purpose of boiling is t o improve the yield and fastness of the dye. After filtration, the solution is diluted with water and the wool, which has been premordanted with alum, is dyed just below the boil. To improve the colour value the goods are left for a prolonged period in the cooling dyebath. Silk can be dyed in a similar way but at a lower temperature. A pale or warm dyeing, unfortunately not very fast t o light, is obtained, depending on the quality of the saffron. Typically) l g saffron dye gives a pale yellow color on 200g wool, while 4g give deep yellow. 3.2.3 Madder Madder roots are used either fresh or dried and ground. The slower fermentation of the dried roots is claimed to intensify the colour. A bright red is obtained by dyeing alum-premordanted wool with 30% ground madder root. The madder is left to stand for 12h in water then the mordanted wool is saturated with the dye extract and treated for 1-2 h at about 70°C. Afterwards the wool is left for a further 12 h in the slowly cooling bath. If dyeing is performed a t the boil, the light fastness increases from about 4-5 (blue scale) and the shade deepens. A chromium mordant is claimed to give an even deeper shade with a light fastness rating of 6. But the deepest shade and the best light fastness are obtained on wool premordanted with iron (11) sulphate. The violet to black shade can have a light fastness of 7-8 if dyeing is done at the boil. Adding the iron (11) sulphate direct to the madder solution produces a brown dyeing. Unmordanted wool is dyed a deep pink with a light fastness of only 2. 3.2.4 Dyeing with gallnuts To dye lkg wool requires 500g - lkg gallnuts crushed in a mortar. The crushed nuts are steeped for about 24h in water, then filtered. The wool is added to the extract solution, boiled gently for about l h , cooled and then left to stand in the dyebath for a further 12h. After washing and drying) the wool is a camel hair brown with a light fastness of 7, disproving that lightfast dyeings can only be achieved with the use of mordants. The shade deepening effect is undisputed. Pretreatment with alum leaves wool a greenish shade, which subsequently darkens. Dyeings after-treated with iron sulfate turn dark violet to grey.

44 1

3.3 Cotton [431 3.3.1 Mordanting cotton Cotton is generally mordanted with tannin prepared from gallnuts and sumac, particularly for dyeing red and brown shades. Depending on the shade depth envisaged and on the quality of the tannin, the mordant is made up with 10 - 30% tannin and, as a rule, the cotton is steeped in the mordant solution for 12 h (overnight) at room temperature, squeezed off and dried without rinsing. The cotton is then treated with alum by steeping for 10-20 min in a solution of about 2% alum. After rinsing and drying, the cotton is a yellowish colour and can be dyed with suitable vegetable extracts. 3.3.2 Madder, alizarine red Cotton can be dyed with madder in almost the same way as wool. With the advent of synthetically produced alizarine, alizarine red, or “Turkey Red” as it is also called, became the most important mordant dye for cotton. The addition of Turkey red oil (sulphated castor oil) gives a bright scarlet that possesses outstanding wet and light fastness properties. As a general rule, aluminum acetate is now used instead of alum. Chromium produces a dull bordeaux color, iron a red brown, copper a yellowish brown and tin an orange color. There are numerous processes; by and large, a distinction is made between what is known as the “old red process”, which yields the fastest and most beautiful dyeings, and the ‘(newred process”. Generally speaking, the “new red process” is only a simplified version of the more complicated and ingenious old red process. The boiled (bucked) yarn is impregnated at 60-70°C with a 10-20%solution of Turkey red oil (50%)to which a small amount of ammonia has been added. After drying, the oiled cotton yarn is steeped for about 24 h in a basified solution of aluminium sulphate (about 3-4% Al,(SO,)&queezed, and rinsed. The dye liquor contains 1-2% alizarine, ca. 1% tannin, a small amount of prepared chalk (calcium carbonate), and very soft water. The goods are predyed cold, and the bath is then slowly brought to the boil and kept at that temperature for 30-60 min. The goods were then often washed under pressure with soda and soap in specially designed tanks t o improve the fastness properties and beauty of the red dyeing. In the “old red process”, oiling was performed in several steps, followed by leaching with potassium carbonate, mordanting with sumac extract, and then dyeing and brightening.

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3.3.3 Indigo Natural crude indigo usually contains by-products such as indigo gluten, red indigo and a brown dye. Boiling crude indigo in water eliminates the brown dye kills any microorganisms present. Boiled crude indigo gives a brighter shade than that of unboiled indigo. Pale shades are obtainable with ca. 0.75% indigo and dark shades with ca. 3%, depending on the quality of the indigo. For application, indigo is reduced (vatted). Nowadays, after reduction with NaOH and hydrosulfite, the reduced form is applied in the presence of common salt or Glauber’s salt a t 40-60°C. For log indigo, about log NaOH, 20g sodium hydrosulfite, and 10-15L of water are required. Following application, indigo is then oxidized with, for example, 1-2%hydrogen peroxide and soaped thoroughly at the boil to optimize development of the shade. Because indigo does not exhaust very well, it is applied in several stages, i.e. after a certain time the dyeing process is interrupted, the goods are overdyed with the reduced vat, re-oxidized, and this sequence is repeated until the desired shade has been achieved. Indigo can be used as a vat dye for wool. Because wool is sensitive to strong alkali and is yellowed a t high temperatures, dyeing need to be performed for a shorter time with mild alkali at a lower temperature and in the presence of a protective colloid such as gluten (gelatine). Soaping must be done below the boil, and the finished dyeing must be thoroughly scoured Traditionally, indigo used to be mixed with organic matter such as fragments of plants, cow dung, and urine to assist fermentation. At the same time these additives gave dyeing the reputation of being a “disreputable trade”. Dyers were only allowed to practise their trade outside their village or town, and preferably as far downstream as possible. Almost every dyer had his own secret method; for example, ashes were used instead of NaOH. They were extracted by boiling and the resulting clarified, alkaline and slightly yellowish solution was used. Reduction was brought about with glucose prepared from cereals or other starchcontaining plants by fermentation. It took days for the indigo to ferment. To vat one tablespoon of indigo required for example 2kg ash, 400g barley, 300g garlic and 1OL water. Often urine (urea) was also added to the vat. Another and older artisan’s method that was also used on an industrial scale involved reduction with iron (11) hydroxide. For log indigo, 20g iron (11) sulphate and 30g quicklime (calcium oxide) are required. Reduction is performed at about 50°C and after 4-6h the sediment is filtered off and the solution is used for dyeing.

3.3.4 Khaki shades with tannin The famous khaki shades seen on the uniforms and tents of the colonial powers were noted for good fastness to weathering and washing. In principle the operation is as follows: the cotton fabric is nip or slop padded with 1-3% tannin and 1-2% soap, squeezed well, and sometimes dried. It is then taken through a boiling

443

mordanting liquor and fixed. The soap is converted into an insoluble metal salt and makes the cotton water-repellent. Repeating the padding and fixing operations several times intensifies the desired effect. In the boiling liquor, which is slightly acidified with acetic acid, a greenish khaki is obtained with about 5% potassium dichromate and about 5% sodium bisulfite as reducing agent. Copper sulphate produces a somewhat yellowish khaki. Sumac, and particularly quebracho, gives a deep cutch shade.

3.4 Leather [44] 3.4.1 Dyeing Textiles are through-dyed by batchwise exhaustion. Leather is much thicker and therefore much more difficult and complicated to dye through. Because the bright colours were often very expensive they were only applied to the surface by brushing. Artisans used a variety of brush-dyeing techniques. A coat of warm water was generally brushed on first. [Note: it is wrong to try and achieve the desired shade with one or two brush coatings.] The concentration of the dye liquor ranges from 1to 10gL dye, depending on depth of shade. The temperature is generally 30-40°C. After each coat has been brushed on it is allowed to absorb before the next is applied. Wetting and levelling agents are commonly used. For full-grain leather with a very tight grain, alcohol (spirit) and lactic acid are included in the first coat. The brush dyeing technique is rarely employed nowadays, and has been largely superseded by exhaust dyeing in the drum. Currently spray guns or rollers are used for surface dyeing on an industrial scale, the dye being applied and fixed by means of a binder. Currently, only specialist colorists elect brush dyeing to correct and restore old leather objects. Penetrative, or through dyeing, of leather is now carried out in large rotating drums at temperatures of 50-60°C. 3.4.2. Treatment with metal salts The oldest method of dyeing vegetable tanned leather involved treating it with solutions of metal salts that formed complex compounds with tanning agents. Iron salts yielded a grey to black shade, copper salts a dark brown shade, titanium salts orange shades, and dichromate deep brown shades. The leather was steeped in metal salt solutions, but more frequently the solutions were applied to the leather by means of brushes. Brushing was favoured particularly for large pieces of leather, which as a rule had only t o be dyed on one side and would not stand up well to an additional bath treatment. However, such treatments with metal salts can result in problems. Hydrolytic mineral acids can be formed or, as is the case with oxygen-rich dichromate, uncontrollable oxidation

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can occur. It must not be forgotten that vegetable tanned leather can easily yellow on contact with air, and in the course of time, acquire a reddish brown colour. A method of avoiding acid damage was used in connection with iron black: the concentrations had to be no higher than 5 g L iron vitriol (FeSO, 7H20) or 10gL Fe(OAc), 4H20, otherwise grain damage could occur in storage, especially with iron vitriol, which forms free sulphuric acid. Consequently only light grey and grey shades were produced. The so-called “Beer black”, was based on the same dyeing principle. Old nails and iron turnings were added to fermenting beer to form the corresponding iron salts. The resulting organic iron complexes were somewhat less corrosive and could be used to yield slightly deeper shades.

3.4.3 Dyewoods for leather Following the introduction of synthetic dyes into leather dyeing, the natural dyes derived from dyewoods were the ones that held out longest against the new competitors. The most important was logwood, followed by brazilwood and fustic extract. To some extent these extracts also have a tanning action and were therefore used, in particular, for vegetable tanned leather. By using strongly coloured dyewoods it was also possible t o cut down on the amount of metal salt, thereby reducing the risk of chemical damage associated with such salts. Genuine old, red Russian leather was first brushed with a 5% solution of alum, then brushed several times with brazilwood extract solutions until the desired depth of shade had been achieved. The switch from dyewood extracts to aniline dyes a t first posed problems in terms of affinity and fixation. On leathers tanned with strongly anionic vegetable substances the anionic dyes did not fix well and the synthetic cationic dyes had very poor light fastness. It is, therefore, not surprising that the natural mordant colour of the dyewood extracts was able to survive for so long on certain types of strongly anionically tanned leather. The synthetic aniline dyes only became established with the advance of tanning with basified chromium (111) sulphate, which left the leather slightly cationic in character. Logwood extract is pre-eminent among the various dyewoods. Dyeing recipes can be extremely complicated, depending on the type of leather, and have to be adapted t o the tanning and finishing method for the leather in question. Leather dyeing therefore often calls for more dexterity and skill than dyeing textiles. This can be exemplified with respect to the dyeing of strong leather with logwood black. Saddle, fancy and upholstery leathers are still commonly produced using vegetable tanning agents usually thicker and more robust than shoe uppers; they often have to be heavily stuffed to ensure that they stay supple. Dry harness leather (less than 5% water content) for example is usually dip stuffed, which

445

means it is immersed in a hot (65-85 "C) grease mixture until the required grease content of 25-30% has been reached. This high grease content makes the leather more resistant to alkaline animal sweat, but, at the same time absorption of brush-applied aniline dyes becomes even more difficult. For an attractive, well-dyed black, three t o four coats with an ammoniacal logwood solution containing 1OOgL logwood extract and about 25mLL conc. ammonia should be given with intermediate drying. In between, a solution of 1OgL iron (11) acetate is applied. The dyeing can be shaded with fustic, brazilwood or tannin in the first solution, or copper sulphate or alum in the second. Frequently, thickening agents, weak reducing agents, weak complexing agents or stabilizers such as sugar, gelatine and organic acid are added. The temperature of the solution should not be higher than 35 "C, otherwise the grease is dissolved out. 3.4.4 Cross-comparisonwith textiles Textiles (wool, silk, and cotton) are usually dyed with dyewood extract after double mordanting, and in principle the dyeing procedure is the same as for leather. Mordanting with alum in the usual way is followed by treatment with a combined irodcopper sulphate mordant. One kg textile material premordanted with alum is impregnated for l h with 100-120g iron sulphate and 10-30g copper sulphate in 51OL water a t 40-60 "C, then rinsed and dried. The mordanted dye goods are next boiled gently for about an hour in 5-1OL of a logwood-tannin bath containing about lOOg logwood and 100 g tannin extract powder. The first dyeing gives a blue. For a black, the iron-copper mordant has to be repeated and the goods have to be boiled again with logwood-tannin until a deep black is obtained. In principle, fustic and brazilwood extracts can be dyed in a similar way to that described above. The shades listed below can be produced by laking are shown in Table 4. Table 4.

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Logwood

Titanium potassium oxalate Potassium alum Copper sulphate Iron sulphate I Potassium dichromate I Titanium potassium oxalate

Dark brown Violet Blue black Green black I Dark brown I Light brown

3.5 Paper and wood dyeing 1451 To produce coloured papers, aqueous suspensions of finely ground mineral colours are blended with the pulp in a beater. A n important paper dye for a long time was “sap” or Vandyke Brown, which was obtained by alkaline decomposition of Cassel Brown. It was used t o dye paper, more particularly wrapping paper, and wood. Cassel brown (500 parts) is boiled with 50 parts potassium carbonate in 1000 parts water for several hours until completely dissolved. The coloured substance formed is essentially a mixture of humic acids. Wrapping paper consists principally of ground wood and waste paper, with and without size. Ground wood is made up of fine ligneous fibres and still contains all the constituents of wood such as cellulose and lignin. Because paper comprises mostly cellulose, the relevant dyeing rules are similar to those for dyeing cotton in the cold. For a wood stain 27 parts shellac and 16 parts borax are heated with 270 parts water until homogeneous and, depending on the desired shade depth, added to the dye solution and filtered.

3.6 Cosmetics 3.6.1 Hair dyeing with henna [46] Henna is currently back in use as a hair dye. In the methods customary in the past henna leaves, alone or together with powdered indigo leaves, commercially available under the name “reng”, were applied as a paste (cataplasm dyeing). Here the blue colour of the indigo neutralizes the red colour of the natural henna. The paste was made up with hot water, moist heat being particularly important for hair dyeing because it develops and fixes the colour. Nowadays special heaters are used to steam the hair. Henna alone dyes the hair red; ‘reng’ is often being pressed into service again for hair toning. Blond, brown t o deep black dyeings are obtained, depending on contact time and amount applied.

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For dyeing hair, 1cup of pulverized henna leaves is mixed with one egg yolk and 2 teaspoons of olive oil. Strongly brewed tea is added slowly until the paste is readily spreadable, then it is left to stand for one day. The hair is shampooed and partly dried, then the paste is applied evenly by means of a brush. If the paste has dried too much overnight, it can be diluted with tea. After application of the paste, the hair should be kept at about 30°C under a plastic hood. In the sunny Mediterranean countries the hair is usually warmed and dried in the sun. Next the hair must be thoroughly washed. Together with dyeing, the oil and egg yolks, which nourish the hair, impart a magnificent sheen. The hair becomes carrot red, mahogany red, red brown or black with a soft red reflection, depending on the base colour of the hair, the quality of the henna, and variations in the recipe.

3.6.2 Lipstick with cochineal Cochineal is nowadays one of the dyes approved within the EU for use in foodstuffs. It received a phenomenal boost once lipstick came on the scene. Lipstick consists of a base mixture, 1-20% dye and some perfume. The base mixture is generally a complex mixture of different waxes and oils, which are melted together. Its basic ingredients are up to 15% beeswax, up to 15% paraffin oil, 1020% cresin, ca. 5% cetyl alcohol, ca. 15% castor oil, 10-30%paraffin wax and up to 50% hydrogenated oils and fats. First the dye is dissolved in the soluble part of the base mixture, then the remaining molten components are stirred in. After slight cooling the mass is ground in an ointment mill before pouring into the final lipstick mould. Because of its solubility cochineal was not particularly kiss proof. Nowadays the free dye acids, more particularly monoazo dyes, which are more wash proof and kiss proof, are more commonly used. 3.7 Painters’ and artists’ colours 3.7.1 Binders Linseed oil is probably the best known binder. A level 0.Olmm thick coating of linseed oil dries to form a tenaciously adhering, durable film. As a rule it is not easy to mix the powdered colouring matter with the liquid binder. Even after prolonged mixing of the fine powder, unwet powder can still be present and gives rise to unwanted lumps on brushing. It that not every binder can be used for all kinds of colouring matter because sometimes decomposition reactions may occur. In the past, the main colouring agents were earth colours, charcoal and, later, mineral colours. To some extent, these have now been replaced by synthetic organic coloured lakes. Natural colours were not often used. But as a result of the

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present “back to Nature” trend the natural colours have been making something of a comeback. For use in watercolour paints, pigments have t o be thoroughly dispersed and ground (particle size 0.25pm), although a slight amount of binder or thickener, such as gum arabic mixed Ca-Ma, K-salt of arabic acid, is often added. The first earth colours in cave paintings were fixed with sour milk and lye prepared from ashes. At a very early stage slaked lime and potash waterglass (potassium silicate=&Si30, + &Si409) was used, occasionally also in mixtures. Sodium silicate is unsuitable because it effloresces on drying. Since both binders are strongly alkaline, the pigments have to be suitably resistant. On plaster, natural stone and brickwork, as well as on glass and zinc plate, waterglass colours form a very hard dull coating that is lightfast, washable and weather resistant. Milk contains the albuminous substance casein. It is believed that the Romans stirred milk into the mortar used to build the temple to Minerva in Elis. For mural paintings, fresh milk casein and slaked lime were mixed with aqueous suspensions of the colours. A wide variety of natural oils such as linseed oil, poppyseed oil and nut oil, and resins from different trees, were also used for colour fixation. The tempera technique reached its peak during the early Renaissance. Its roots reach back to antiquity and it is still in use today. The word tempera comes from the Italian “temperare” = to temper (as of metals) and presumably refers to the fact that the aqueous and oily binders are mixed together, or more precisely, emulsified. Egg white, casein or soap are used as emulsifiers. The basic recipe for genuine tempera consists of about 20% eggs (yolks and whites), about 40% linseed oil and about 40% water. A fine emulsion is prepared from these ingredients and a slight amount of salicylic acid is now added as a preservative. 3.7.2 Selection of pigments and colours [471 In addition to organic carbon blacks, various other kinds of inorganic earth colour pigments can be used. Good results have been achieved with the following selection of natural organic pigmented colours:

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Table 5 Organic pigments for artist's colour

Colour Yellow Red Blue Green Brown Black

Sources Gamboge Madder roots Indigo Spinach Catechu-acacia Logwood

Lake Alum-lake Al-lake

-a -b

Alum-lake (flavone) Fe-lake

"Insoluble Chlorophyll

b

3.8 Ink Iron gallate ink, which has been known for centuries, is prepared by dissolving 23.4g tannin, 7.7g gallic acid crystals, 30g green iron (11) sulphate crystals, log gum arabic, 7g conc. HC1 and l g carbolic acid in 1L of water. This ink takes a little time to develop its deep black colour, and does so by absorbing oxygen, the iron becoming trivalent. The iron-gallic acid complex then forms a dark black compound, which lasts for centuries. For that reason important state documents are still signed with iron gallate ink. Gum arabic inhibits premature flocculation of the complex, and carbolic or salicylic acid prevents mould fungus. Hydrochloric acid, however, prevents premature oxidation. As this ink flows rather pale or watery from the pen, a blue aniline dye is often blended with it and bleaches out in course of time.

3.9 Dyes for foodstuffs 3.9.1 General Approval of dyes for use in foodstuffs is decided almost exclusively from toxicological and pharmacological data. Also very important is chemical resistance, exemplified by stability t o heat for confectionery. Many foodstuffs are sensitive to shifts in pH and change their colour accordingly. As the foodstuffs and their colours are broken down in the body, the degradation products have to be closely studied. All these limitations have created a situation in which the use of dyes for foodstuffs is regulated by law in almost all countries.

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3.9.2 Cooking (chlorophyll, saffron, paprika) Chlorophyll is the most important dye we consume on a daily basis, in terms of quantity. It is present in all vegetables. The chemical structure of many natural edible dyes has still not been elucidated. Spices such as saffron and paprika often also provide colouring. Saffron gives paella not just a strong aroma but also a bitterlsweet taste. If a pinch of saffron is added t o a cup of lukewarm water, the filaments dissolve and colour the water yellow. If this fails to occur, the saffron can be assumed to be either impure or old. Paprika also is available in different qualities. Indians refer t o ground dried red chillies as red chilli powder. American chilli powder is a spice mixture, which includes ground cumin seeds. Italian green hot peppers can be very mild in flavour, and the Mexicanjalapefio is extremely hot. A typical paella can be prepared as follows. For 6 persons heat lOOml olive oil, sear and put aside 6 pieces of chicken and about 250g cubed pork. Then, in the same oil, sear and braise onions, garlic and paprika. Now add 250g peeled tomatoes and 250g sliced chorizo sausage. After 15 min at a gentle boil add the seared meat together with 6 cleaned mussels. Keep boiling gently until the mussels open. At this point add 500g boiled long-grain rice, 600ml chicken broth and the steeped saffron. Bring the whole to a gentle boil and after 10-20min add lOOg green beans cut into short pieces, then lOOg peas and 6 prawns. On completion of boiling, remove the heat, garnish with 6 large shrimp, leave to stand for a further 3-5 min with the cover on, then serve. 3.9.3 European Union foodstuff colour list The EU list of approved vegetable or animal colouring matters for foodstuffs includes the following: Table 5 EU Foodstuff colour list [481

I No.

I Name

I C.I. No.

I Use

45 1

4. Fastness and shades

Natural colouring matters will, in principle, produce all required shades. As a rule, however, they are rather dull because generally they can only be used in the form of the corresponding metal salts. The wet fastness properties depend very much on the application methods, which, in common with the preparation of the dyes, can be complicated and lengthy. The same is also true of the light fastness, which can be very good. As a rule of thumb, the deeper the shade has been dyed or painted, the better its light fastness. Table 6 . Light fastness

Brown Black

Brazilwood (Cu) Cochineal (Cu, Fe) Brazilwood (Fe) Logwood (Fe) I Tannins (Fe)

I

Adequate Very good Moderate I Good I Very good

452

5. Safety, toxicology and ecology People are exposed to dyes during their manufacture and application, and when wearing textiles or other substrates coloured with them. The risks associated with manufacture can be minimized by optimized working conditions and safe equipment. This is particularly necessary with mordanting baths, which should be discharged to the drains only after cleaning and removal of metal. With dyes manufactured by present-day technology, the risk of migration into the skin as a result of, say, perspiration is not very high where textiles and leather are concerned if the dyes are properly fixed. The risk could be higher with coloured articles of daily use (pots, glasses, etc.) which come into contact with beverages and food. It is very high in the case of cosmetics and colouring matter in foodstuffs. It has often been alleged that the natural colorants in foodstuffs have been studied too little, or a t any rate, less than synthetic colorants. As natural products generally speaking, they are conceded advantages over the products developed by the chemical industry. But natural products can also cause allergies and lead to contact hypersensitization (cochineal, cis-bixin, beta-carotene, etc.). In relation to the large number of persons who come into daily contact with dyes, allergenic effects are an infrequent occurrence. The allergenic potential of the natural fibres cotton and wool is rated markedly higher. Botanists and gardeners are aware that many plants recommended for dyeing are poisonous. Natural substances are by no means harmless t o humans and the environment. At the same time, however, many plants also have a curative medicinal action. Phytotherapy demonstrates that plants can be used to dye, paint and heal. A point worth discussing is whether the natural earth colours are more suitable for colouring substrates than synthetic colours, which are brighter. 6. Use of natural dyes today

At present, several thousand tonnes of paprika and curcuma extracts are being produced and consumed. The annual crop of cis-bixin in Central and South America is something like 500-1000ton, referred to the seeds of the annatto or roucou tree. Dyewood extracts are also traded in sizeable quantities, though these are far short of the 1950 level of 30,000ton. This is also true for cochineal, of which 1,000ton were exported in 1935. Many natural dyes including indigo and carotenes, especially vitamin A, are presently produced synthetically. Though the natural dyes play no predominant role in large-scale industrial production, they are finding increasing use in arts and crafts, and for therapeutic purposes. The old dyeing techniques are being rediscovered. In this connection, it

453

can be noted that the cupola of the Rudolf Steiner centre was painted using natural colours. Whether this is a phenomenon of the times or the way back to Nature is open t o discussion. Having regard t o the energy crises in the recent past, however, it is wise to study possible alternatives. We are all aware that, following the man-made shortage of the natural product mineral oil, even this source of energy will not flow forever. Self-renewing raw materials could be a replacement. With today’s technologies, however, it is impossible to ensure the estimated requirement of lOOmil todyear of dyer’s plants. New thinking, along the lines of, say, fermentation or even genetic engineering, is needed.

7. ACKNOWLEDGEMENTS We wish to express our gratitude to the many unnamed helpers who assisted us in our work.

8. LITERATURE 8.1 Recommended literature and software program

-Naturfarbstoffe, lecture by R. Wizinger, 1953, Institut fur Farbenchemie der Universitat Base1 -Handbuch der Naturfarbstoffe-Vorkommen, Venvendung, Nachweis. H. Schweppe, ecomed Verlagsgesellschaft, 1992, ISBN 3-609-65130-X -Farbepflanzen, Pflanzenfarben, L.Roth, K.Kormann and H. Schweppe, ecomed Verlagsgesellschaft, 1992, ISBN 3-609-65490 -Colour Index International, SDC/AATCC, CD-ROM, third edition 1995 -Merk Index, twelfth edition, Merck and Co., Inc. 1996 -STN-Express 4.0, American Chemical Society 1996 -Chemie Lexikon “Hermann Rompp”, fourth and later editions, Verlagsbuchhandlung Stuttgart -The Function of Natural Colorants, H-D. Martin 1995 Chimia No. 3, 45 ISSN 0009-4293 -Color Chemistry, H. Zollinger, ,second edition, 1991, VCH, ISBN 1-56081-149-8 8.2 Literature cited

1 Dyes for textiles and leather, Environmental aspects, 1991, Ciba-Geigy Ltd. 2 U. Sewekow, Melliand, 1988,271. 3 D. Ensley, Chimia, 1994,491.

454

4 5 6 7 8 9 10 11 12 13 14 15 16

17 18 19 20 21 22

23 24 25 26 27 28 29 30 31 32 33 34 35

36 37 38 39 40 41

G. Eck, PCT, EP86-00309. P. Friedlander, Ber. 1909,765. 0. Dimroth and S. Goldschmidt, Ann. 1913,62. R. Willstatter and Mieg, Ann. 1907 1. J. Paust, Chimia, 1994,494. J.Deli et al., Chimia 1995, 69. R. Higham, JSLTC, 1996,46. 0. Schmidt, Angew. Chem., 1956,103. G. Voss and H. Gerlach, Ber. 1989, 1199. A. Bayer und A. Emmerling, Ber. 1870, 3, 514. A. Bayer, Ber. 1880, 13,2254. K. Heumann et al., DRP 54626, 1890 and DRP 56273, 1890. H. Schmidt, Chemie in unserer Zeit 1997,3, 121. J. Gosteli, Helvetica Chimica Acta, Fasc. 6 (1977) 1980. P. Oriel and I.C. Kim, US 95546829. H. Car0 et al., Ber. 1870,359 see also USP 95465 and USP 153536. H.E. Fierz-David and L. Blangey, Farbenchemie, 1952, Springer Verlag Wien. D. Gadgil et al., Tetrahedron Letters, 1986,2223. D.. Cameron et a., J. Chem. SOC.,Chem. Commun., 1978,688. C. Overeem and G. van der Kerk, Rec. Trav. Chim., 1964, 1023. T Reichstein, Angew. Chem., 1962, 887. Inhoffen et al., Ann., 1953, 580. R. Kuhn und A. Winterstein, Ber. 1934, 344. P. Karrer and E. Jucker, Carotenoids, e.g. Chapter X W l Bixin, Elsevier Publishing, 1950, NY. L. Zechmeister, Ch. org. Naturst. 1960, 223. H-D. Jakubke und H. Jesschkeit, Biochemie, 1976,228 Brockhaus Verlag Leipzig. W. Hutchins and T. Wheeler, J. Chem. SOC.1939, 91. von Kostanecki et al., Ber., 1906 4014. T. Goodwin, Chemistry and Biochemistry of Plants, Vol 1, 1976, Academic Press, NY. Tswett, Ber. Deutsch. Bot. Ges. 24, 1906,316, 385. R. Willstatter and Stoll, Investigatings on Chlorophyll (transleted by Schertz and Merz) 1928, Lancaster. Melvin A.Judah et al., Industrial and Engineering Chemistry, 1954, 2262. R.B. Woodward et al, J.Am. Chem. SOC.1960,3800. M. Perutz et al., Nature 1963,633 and 1968, 131. Drabkin J. Biol. Chem. 1946 703. K. Farber, Gerbmittel, Gerbung, Nachgerben, Bibliothek des Leders, Band 3. 1985, Umschau Verlag. B. Eistert und Muller Ber. 1959, 2071. B. Glover, JSDC, 1998,4.

455

42 J. Dean, Natural Dyeing, 1994, Search Press. See also G. Schneider, Fbben mit Naturfarben, 1979, Otto Maier Verlag Ravensburg. 43 FarbrezeptbuchBand III, 1934, J.R.Geigy AG. 44 Handbuch der Gerbereichemie und Lederfabrikation, dritter Band, 1961, Springer Verlag Wien. 45 O.Unzeitig, Chemisch technische Rezepte,l949. Hartleben Verlag , Wien. 46 S. Faber, Das neue Rezeptbuch der Naturkosmetik, 1983/1987Goldmann Verlag, Munchen. 47 G. Meier, Pflanzenfarben fur den Maler, 1979, Domach. 48 European Parliament and Council Directive 94/36/EC of June 1994.

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10 Synthetic Dyes for Human Hair JOHN F. CORBETT Bristol-Myers Squibb Worldwide Beauty Care, 2 Blachley Rd., Stamford, CT 06922, U.S.A. 1. INTRODUCTION

Throughout recorded history, men and women have had the urge to change the natural color of their hair or to restore the color when, with age, it became grey. Before the advent of synthetic organic chemistry, their desire for change had to be fulfilled by the creative use of natural products. It is well established that the ancient Egyptians employed henna (2-hydroxy-1,4-naphthoquinone), derived from the plant Lawsonia ineris, to produce an orangered tone on their hair. It is interesting to note that this technique is still practiced today among the devotees of natural products. Roman women, noting the blond tresses of slaves brought back from Northern Europe, tried to emulate them by applying lye and exposing their hair to the sun. Throughout Europe, grey hair was colored by combing it, on a daily basis, with lead combs dipped in vinegar. In the nineteenth century, this approach was commercialized by selling products that comprised an aqueous solution of lead acetate containing a little suspended sulfur. Daily application results in a gradual buildup of lead sulfides and oxides on the hair thus "restoring" the color. Such products are still available today, the best known being the Grecian Formula. In the eighteenth and early nineteenth centuries, the practice of giving fashion colors to hair and wigs was a major business. Talc or starch was mixed with inorganic pigments to produce a variety of pastel shades. The volume was so significant that the French treasury derived a considerable income from taxes on such products. With the advent of synthetic textile dyes in the second half of the nineteenth century, it was natural that attempts would be made t o use them for coloring human hair. Such an approach was far from satisfactory due t o the fact that the molecular size of these dyes was relatively large and that human hair has a thick cuticle layer - six to ten scales as opposed to two to three in wool, and further, due

457

t o the constraint that an on-head process had to be performed at ambient temperatures. In 1863, Hofmann [l]observed that the colorless p-phenylenediamine produced color on a variety of substrates when exposed t o oxidizing agents, including air. Twenty years later, Monnet [2] patented a process for coloring human hair based on Hofmann’s observation. Thus, 1883 can be considered the birth of what has become a worldwide multi-billion dollar industry. Hair colorants based on the oxidation of para-diamines are variously referred to as permanent, para dyes, or oxidation dyes. We shall use the latter term in this chapter to better distinguish them from products that employ pre-formed dyes, which are applied directly to the hair without the need of an oxidant. Today, oxidation dyes represent about 80% of the hair colorant market while the non-oxidative segment, based on synthetic organic dyes, represent the remainder other than a very low percentage which comprises the natural dyes and the lead acetate products. 2. OXIDATION HAIR DYES

2.1. Historical development The 1883 Monnet patent mentioned above offered a process for coloring human hair based on the oxidation of p-phenylenediamine or the related 2,5diaminotoluene. These two materials have remained the most important precursors to the present day. p-Phenylenediamine was used in the United States, England and Japan while 2,5-diaminotoluene was employed in France and Germany due to a ban on the use of p-phenylenediamine in hair dyes which was enacted due to the level of allergic reactions observed in the first twenty years of its use. These bans have since been rescinded. Nevertheless there are occasional reports of allergic reactions with products based on either of these para-diamines and the search for less allergenic alternatives continues. During the period 1888-97 Erdmann and Erdmann [3] patented the use of a number of para-diamines and 4-aminophenols for use in dyeing human hair. 4,4‘-diaminodiphenylamine, pThese included 4-aminodiphenylamine, aminophenol, 4-amin0-3-methylpheno1, 4-amino-2-methylphenol and N-methyl-paminophenol. In addition they also introduced the use of hydrogen peroxide as the oxidant. The above para-diamines and 4-aminophenols, and their ortho-isomers, all produce color when oxidized in-situ on the hair. They are known collectively as primary intermediates or, by analogy with photographic chemistry, developers. The use of this latter term is confusing because, in the hairdressing trade, the

458

term "developer" is used in reference to the hydrogen peroxide or other oxidant. For this reason, "primary intermediate" will be used in this chapter. The use of primary intermediates as the sole dye precursors severely limits the range of shades available. It was soon discovered that this could be overcome by adding t o the composition various materials which do not themselves produce significant color effects on oxidation but which, when present in a mixture of primary intermediates and an oxidant, markedly modify the resulting color. These additional materials, which are mainly m-diamines, 3-aminophenols, mdihydroxybenzes and certain monohydric phenols, are referred to as color modifiers or couplers. The more important classes of couplers are listed in Table 1, together with the color they give when present during the oxidation of pphenylenediamines or 4-aminophenols. Table 1.The Role of Couplers in the Coloring Process.

Couder

Color on Hair With ouru-diamines

m-Phenylenediamine 4-substituted 3-aminophenol 6-substituted Resorcinol 2 -methyl l-Naphthol Thymol

Bluish-brown-black Blue-blue-violet Warm-browdmagenta Magenta Greenish-brown Golden-brown Blue-violet Blue-violet

With mumaminophenols Reddish brown Red Red-brown Orange-red Light brown Light brown Red Red

With the various combinations of primary intermediates and couplers it is possible to produce a full range of natural looking shades including drab and warm browns and blondes, auburn and chestnut and black, together with various fashion shades such as platinum blond, burgundy and aubergine. Of course, the color produced is also a function of the hair's natural color and that must be taken into account when selecting the shade to be used for a desired end result. The versatility of oxidation dyes is further enhanced by the ability of hydrogen peroxide to lighten the hair's natural color that is due to melanin. Under the conditions of oxidative dyeing, which generally has an on-head hydrogen peroxide concentration of 3-6% at a pH of 9.5, the hair can be lighted from medium brown to light blond or dark brown to dark blonde.

459

The alkaline pH required in oxidation dye products generally is obtained by the use of ammonium hydroxide in the dye lotion. This is the best alkalizer for enhancing penetration of the dye precursors into the cortex of the hair fiber and also the most effective at promoting bleaching of melanin by hydrogen peroxide. An unfortunate effect of the partial bleaching of melanin is that the lightening does not progress on tone. Thus, even on drab brown hair there is marked reddening of the color as the bleaching proceeds. While this can be used beneficially if the desired final color is a warm or auburn shade, it must be compensated for if neutral or ash shades are desired. Herein lies the importance of the couplers that give blue, or greenish or purple tones with the para-diamines. Lower levels of bleaching are achieved if monoethanolamine is employed as an alkalizer while the use of sodium carbonate or aminomethylpropanol can almost eliminate melanin bleaching while still allowing an effective color development. This is particularly useful for men's hair colorants where warm tones are generally considered undesirable. Oxidation dyes that do not bleach melanin are known by a variety of generic names including "no-lift tints", "demi-permanent", "long lasting semi-permanent" and, simply "semi-permanent", This last leads to some confusion since it is also the accepted generic for non-oxidative colorants using nitro-, azo- and anthraquinone dyes. To sum up, the essential ingredients for color production in oxidation hair dyes are primary intermediates, couplers, and an alkalizing agent, which are premixed as the color lotion or cream, and hydrogen peroxide as the developer or oxidant which is packaged separately and is mixed with the lotion or cream immediately before application to the hair. In general, any colored molecules formed in the dyebath do not penetrate into the hair. 2.2 The chemistry of oxidation dyeing During the first sixty years of the commercial practice of oxidation dyeing there was little effort directed towards a detailed understanding of the chemical reactions involved. Formulation of products was an empirical art and few if any companies performed research into the synthesis of new and improved precursors. The materials used were essentially commodity chemicals for which there was another industrial use, and it was from among these that companies sought additional materials. It was correctly believed that the color forming reactions involved the formation of indo-dyes but incorrectly conjectured that most of these underwent rapid intramolecular cyclization to form phenazines or phenoxazines.

460

XH

x

1

2

Figure 1.Oxidation of primary intermediate 1. The first step in color formation is the oxidation of the primary intermediate (1) to give p-benzoquinondiimines (2; X=NH) from para-diamines or p-benzoquinonemonomines (2; X=O),from p-aminophenols. This reaction, which is shown in Figure 1,increases in rate with increasing pH over the range 6-10. In general, p-aminophenols are more readily oxidized than para-diamines and observation suggests that the oxidation of all primary intermediates by hydrogen peroxide is much faster inside the hair fiber than it is in the aqueous dyebath. Presumably, this is due to catalytic decomposition of the hydrogen peroxide to give species that are much more reactive than the hydrogen peroxide anion H O i which is the predominant active species in aqueous solution. X

NH, X= NH, or OH 3 Figure 2. Product of a self-coupling reaction. In the absence of couplers, the oxidation of p-phenylenediamine and of 4aminophenol leads to trinuclear species having the structure shown in Figure 2, which shows Bandrowski’s Base (3; X = NH2) from p-phenylenediamine and the analog (3; X = OH) from 4-aminophenol. Kinetic studies have shown that at pH 7-10 the reactive imine species is the conjugate acid of p-benzoquionediimine but the neutral form of pbenzoquinonemonoimine. These are both strong electrophiles and react readily

46 1

with unoxidized primary intermediates to give an easily oxidized leuco compound whose oxidation product adds another molecule of primary intermediate. The further oxidation of the product of addition gives the trinuclear dye 3. The primary intermediates are not strong nucleophiles as compared to the generally employed couplers. In the presence of an adequate amount of a coupler, reaction with the coupler occurs preferentially and there is almost no self-coupling i.e. no 3 is formed. The reaction scheme for the oxidation of mixtures of p-phenylenediamine and meta-di-substituted couplers containing no substituents para to the electron donating groups is shown in Figure 3. The reactive imine (4) attacks the coupler (5) para to one of its substituents to give a leuco compound (6) which is readily oxidized to an indo-dye (7), which then adds a molecule of the p-diamine to give a trinuclear leuco dye, which after oxidation gives a trinuclear indo-dye (8). These dyes have been isolated and their structures confirmed. Comparison of their color during experiments in aqueous solution and the color formed on hair strongly suggests that the same species are formed in the hair dyeing process [4]. In the case of resorcinol (5; A= OH), the trinuclear dye is green, but it has been shown that a considerable amount of brown polymeric material is also formed. The case of m-aminophenol ( 5 ; A=NH2, B=OH) the initially formed indoaniline (7; A=NH2, B=O) is magenta and the tricuclear dye 8 is brown, while for mphenylenediamine ( 5 ;A=B=NH2)the corresponding 7 is blue-violet and 8 is almost black. Analogous reactions occur when mixtures of p-aminophenol and unblocked meta-couplers are oxidized [5]. These unblocked couplers play an important role in producing background color for most drab shades. Where more distinct tonalities are desired, additional couplers having a blocking substituent para to one of the electron donor groups are added to the formulation. These blocked couplers react as shown in Figure 4 to give binuclear indo dyes that have much brighter colors than do the trinuclear species. Thus the 2-aminoindamines (X, X = NH) are blue to blue violet, the 2-aminoindoanilines (X; X=O) are magenta and the 2-aminoindophenols (11;X=O) are bright orange [4,5]. Although the dyes (11;X = NH) are red they have found little utility due to the extreme pH sensitivity of the color and poor light stability.

462

Z=NH or 0

7

Trinuclear Leuco-dye A and B are NH2or OH

H2N 8

Figure 3. Formation of trinuclear dyes with unblocked couplers.

4

9

9

10

R=alkyl or alkoxy X=NH or OH

11

Figure 4. Formation of a binuclear dye with blocked couplers.

463

Good red tones can be achieved using a mixture of a p-diamine, a paminophenol, and 5-amino-2-methylphenol (9; R = Me, X=O), the mixture of the magenta (10)and orange (11)being red. However, this was first patented in 1965 was used for 161, and prior to that the non-oxidative 2-nitro-p-phenylenediamine red tones. It should be mentioned that some N-mono and N,N-disubstituted pphenylenediamines are used in hair dyeing and have been shown to undergo analogous reactions [71. Thus the chemistry of oxidation dyeing can be described as a complex series of competing, concurrent and sequential oxidation, electrophilic substitution and 1,4quinonoid addition reactions giving rise to strongly colored indo dyes. 2.3 Commercial dye palettes As mentioned previously, the early oxidation colorants were based on readily available commodity chemicals and made extensive use of certain red and yellow nitro-dyes in warm or golden shades. Table 2 lists the more important materials used by most manufacturers until the late 1960's or early 1970's when a number of proprietary systems claiming improved stability or more benign toxicological properties were introduced. It can be seen that the list contains a number of preformed yellow and red dyes to overcome the absence of good yellow and red-forming couplers. In 1965, Schwarzkopf [6] introduced the use of 5-amino-2-methylphenol in conjunction with para-diamines andor p-aminophenols for the production of bright orange to red to burgundy shades, specifying thirteen primary intermediate materials including all the commonly used para-diamines and p-aminophenol and its 2- and 3-methyl derivatives. In 1989, Wella obtained a selection patent for pure red shades using a mixture of p-phenylenediamine or 2,5-diaminotoluene with 4-amino-3-methylphenol and 5amino-2-methylphenol [8]. L'Oreal later obtained a patent for the use of the same two aminophenols with most other p-phenylenediamines [91! L'Oreal also patented the N-2-hydroxyethyl derivative of 5-amino-2-methylphenol as being superior to the parent compound [lo]. They have used this and the parent compound in their products. In 1975 Clairol introduced the use of 2-methylresorcinol as a means of obtaining shades with gold tones, while Wella [lll pioneered the use of the "self-coupling"2amino-5-methylphenol for a similar purpose. Prior to this some manufacturers had used o-aminophenol, which oxidizes to the orange yellow 2-aminophenoxazineor aminonitrophenols for this %one, or had resorted to 4-nitro-o-phenylenediamine tonal quality.

464

Table 2. Oxidation Dye Precursors Used in 1960. p-Phenylenediamine 2,5-Diaminotoluene 2-Chloro-p-phenylenediamine 4-Aminodiphen ylamine 4,4’-Diaminodiphenylamine 4-Aminophenol N-Methyl-p-aminophenol 2-Aminophenol m-Phenylenediamine 2,4-Diaminoanisole 2,4-Diaminophenetole m-Aminophenol 3,3’-Dihydroxydiphenylamine Resorcinol Catechol

1,5-Dihydroxynaphthalene

Pyrogallol 2-Nitro-p-phenylenediamine 4-Nitro-o-phenylenediamine 2-Amino-4-nitrophenol 2-Amino-5-nitrophenol Picramic acid

Until 1970 the preferred blue couplers had been 2,4-diaminotoluene and 2,4diaminoanisole. Concerns about their toxicological properties led manufacturers to replace them with newly developed m-diamines or a new blue forming pdiamine/coupler combination. L’Oreal patented and used a close analog, i.e., 2,4-diaminophenoxyethanol1121, Wella chose 2-amino-4-(2’-hydroxyethylamino)anisole[ 131 and Henkel 1,3-bis-(2,4diaminophenoxy)-propane [ 141 and, for some specialty shades, Clairol used 4,6bis(2-hydroxyethoxy)-1,3-diaminobenzene[15]. All of these are meta-diamine couplers. For most of its products, Clairol changed away from meta-diamines in favor of a mixture of N,N-bis(2-hydroxyethyl)-p-phenylenediamineand 1-naphthol which gives a blue indoaniline dye. Table 3 shows the palettes used by four major companies in 1998 which, but for the continued use of p-phenylenediamine, 2,5-diaminotoluene, resorcinol and maminophenol, show considerable differences from the palette in Table 2.

465

Table 3. Dye Materials in Commercial Palettes in 1998.

ComDanv+

Precursor

L'O p-Phenylenediamine 2,5-Diaminotoluene 2-Chloro-p-phenylenediamine

X

N,N-bis(2-hydroxyethy1)PPD Bis-diamine" 4-Aminophenol 4-Amino-3-methylphenol 2,4-Diaminophenoxyethanol 2-Amino-4-(2-hydroxyethylamimoanisole 1,3-bis(2,4-Diaminophenoxy)propane 4,6-bis(2-hydroxyethoxy)-1,3diaminobenzene m-Phenylenediamine 3-Aminophenol N,N-Dimethy-m-aminophenol 5-Amino-2-methylphenol 5-(2-Hydroxyethylamino)-2methylphenol 6-Hydroxybenzomorpholine 6-Hydroxyindole Resorcinol 4-Chlororesorcinol 2-Methylresorcinol 2-Aminophenol 1-Naphthol 2-Amino-5-methylphenol 2-Methyl-1-naphthol N-Methyl-p-aminophenol Phenylmethylpyrazolone 3,4-Methylenedioxyphenol

X

c1

We

Re

He§

X X

X

X X

X

X

X

X

X

X

X X X

X X

X

X

X

X

X

X

X

X

X X

X

X

X

X

X X X X

X X

X

X

X

X X

X X

X X

X

X

X

X

X

X

X

X

X

466

Notations for Table 3: Companies L'O = L'Oreal; C1 = Clairol; We - Wella; Re = Revlon; He = HenkeVSchwarzkopf "2-Hydroxy-1,3-bis(N1-2-hydroxyethyl-4-aminophenylamino)propane

+

$Additional dyes include 2-aminomethy1-4-aminopheno1, 1,5 - and 2,7-naphthalen2-amino-3-hydroxyediol, tetraaminopyrimidine 5-amino-6-chloro-2-methylphenol, pyridine and 2,6-dihydroxy-3,4-dimethylpyridine. Recent innovations in oxidation dye precursors include the use of 2hydroxyethyl-p-phenylenediamine,claimed to be much less allergenic than the parent diamine or its 2-methyl derivative [16]. Although a primary intermediate, it has been used commercially but the lack of an economic synthetic procedure for its manufacture has hampered further development. A bis-para-diamine, 2-hydroxy-l,3-bis(N'-2-hydroxyethyl-4-aminophenylamino~propane is claimed to give dyeings having improved wash fastness. However, the continued use of p--phenylenediamine in conjunction with this new primary intermediate suggests that the latter alone does not have good dyeing properties presumably due to its molecular size. A number of pyridine and pyridine derivatives have been used in commercial oxidation dyes, particularly by Henkel in Europe. As a primary intermediate 2,4,5,6-tetra-aminopyrimidinehas been used, while examples of couplers include 3,4-dimethyl-2,6-dihydroxypyridine (a resorcinol analog) and 2,6-diaminopyridine (a meta-diamine). Additionally 2-amino-3-hydroxypyridine,an analog of oaminophenol, has been used in commercial oxidation dyes.

Air oxidation dyes Although it had been recognized that the auto-oxidation of 1,2,4-trisubstituted benzenes having three amino andor hydroxy groups is facile, these have seen only minor commercial exploitation. The problems include difficulty in handling such easily oxidized raw materials, slow diffusion of oxygen from the ambient air into the hair, and the inability to reuse a partially used container of product. An opportunity was exploited in a frequent use gradual hair colorant for men which employed 1,2,4-trihydroxybenzene,2,4-diaminophenol, N,N-bis(2-hydroxyethyl)-p-phenylenediamine and 4-aminophenol as the major precursors packaged as a multi-application mousse in an aerosol can. The handling of solid 1,2,4trihydroxybenzene during manufacture is overcome by preparing it from its stable triacetate by transesterification in ethanol containing a catalytic amount of 2.4

461

sulfuric acid. The resultant mixture is employed directly in formulating the product. Another air oxidation system involves the use of 5,6-dihydroxyindole as the precursor and can be claimed to simulate the natural coloring process, since oxidation of the indole produces a melanin pigment. Naturally, only a single tonality can be obtained and the product has been useful only as a gradual hair color for men. A similar product, based on 5,6-dihydroxyindoline has also been marketed.

3. NON-OXIDATIVE HAIR DYES 3.1 Historical development There were many early attempts to employ textile dyes in hair coloring products. These met with little success for a variety of reasons. In particular, the acid dyes for wool gave only poor dye uptake when employed on human hair at ambient temperature, while the basic dyes for synthetics gave rapid but uneven dyeings and resulted in unacceptable skin staining. The first successful products were marketed around 1950 and were based for the most part on nitrobenzene derivatives containing amino andor hydroxy groups. As mentioned earlier, nitroaminophenols and nitrophenylenediamines had been used for some time as toners in oxidation dyes. However these are yellow to red and unsuited as the sole dyes in products for use on grey hair. Thus, the early semi-permanent colorants were used for adding gold and red tones to blonde t o brown hair.

Table 4.Dyes Used in Early Semi-permanent Hair Colorants.

Dye

Color

2-Nitro-p-phenylenediamine 4-Nitro-o-phenylenediamine 2-Amino-4-nitrophenol 4-Amino-3-nitrophenol Picramic acid N-Methyl-isopicramic acid

Orange-red Orange-yellow Yellow Orange Red-orange Magenta

468

The main dyes used in these products are listed in Table 4, among which the magenta N-methylisopicramic acid is the bluest dye. The launch of semi-permanent direct dyeing products for grey hair had to await the development of bluer low molecular weight dyes. These became available in the 1950’s [181 due to an observation that N-substitution of 2-nitro-pphenylenediamine produced successive bathochromic shifts of the absorption as shown in Table 5. It can be seen that the effect of the substituents is additive and independent of the native of the substituent. An exception t o this is that a second methyl group on the 4-amino group has no additional effect; however a second substituent which is other than methyl (e.g. ethyl or 2-hydroxyethyl) shifts the absorption to around 535 nm, which is acceptable for a blue-violet dye. Table 5 The Effect of N-Substitution on Color of 2-Nitro-p-phenylenediamine. Substituents*

kmax+

63.

N1

“I

N4

(nm)

(nm)

H Me he H H Me he Me H Me Me he

H H H Me he Me he he he he Me he

H H H H H H H H he he Me he

47 1 497 495 492 490 515 513 520 510 536 480 536

-_

___

26 24 21 19 44 42 39 39 65 9 65

*he = 2-hydroxyethyl; Me = methyl ‘spectra in 95% aqueous ethanol From Table 5 it can be seen that each N-substituent shifts the absorption maximum by about 23 nm. Thus while the parent compound absorbs at 471 nm,

469

the mono-substituted derivatives absorb at about 495 nm., the di-substituted at about 500-520 and the tri-substituted a t about 535 nm. To add a second substituent on the l-amino group is counterproductive and results in an hypsochromic shift and a decrease in intensity due t o steric hindrance with the nitro chrompohore [171. Similar effects are seen for the N-substitution of nitroaminophenols, while 0substitution has little effect on the color compared with the neutral phenol. In selecting which structures to use one, has to consider the effect of the substituents on solubility and hydrophobicity. The more hydrophobic dyes tend to give better dye uptake but only if their solubility in the aqueous dye base is sufficient to allow a concentration of about 2 4 % to be employed. Table 6 lists the commercially used nitro-dyes together with their INCI HC- names where such names have been assigned; the other dyes are known solely by their chemical name. Table 6 Commercially used Nitro-dyes* 2-Nitro-p-phenylenediamines

N' Ph He

DhP He 3-hp He Me 4-Nitro-o-phenylenediamines N1-he 4-Nitro-mphenylenediamine N3-he-541

N4

He He DhP DhP He He He He Me

N4

Other

he

he he Et dhP

541 5-C1 5-C1

INCI Name HC Red No. 1 HC Red No. 3 HC Red No. 7 HC Red No. 13 HC Red No 10 HC Red No. 11 HC Blue No. 2 HC Violet No. 2 HC Blue No. 12 HC Blue No. 6 HC Yellow No. 5 HC Yellow No. 10

470

4-hino-3nitrophenols

Parent N-3-hp N-he

2-hino-5nitrophenol

HC Yellow No. 4

0,N-bis-he


N-Me-0-he N-2-aminoethyl-0-Me


0-dhp


6-chloro N-he-6-nitro

2-Nitroanilines

N-substituent He He He DhP

Other group 4-chloro 4-Me 4-CF3

N-Chydroxyphenyl

He He

4-CN 4NHzCO

HC Yellow No. 12 HC Yellow No. 2 HC Yellow No. 6 HC Orange No. 1 HC Yellow No. 14 HC Yellow No. 15

*he = 2-hydroxyethyl; dhp = 2,3-dihydroxypropyl; 3-hp = 3-hydroxypropyl; Me = methyl; E t = ethyl

In addition to the nitro dyes, it has been found beneficial to add certain azo and anthraquinone dyes to products intended for grey coverage. These larger molecules do not readily penetrate the proximal area of the hair fibers but do diffuse into the more damaged and porous distal portions from where they are less

47 1

easily shampooed out than are the smaller nitro-dyes. This results in a more even color from root to tip and better on-shade fading during shampooing. The preferred anthraquinone dyes include C.I. Disperse Blue 3 (a mixture of 1,4bis-methylamino and 1,4-bis(2-hydroxyethylamino)-anthraquinone and 1methylamino-4-(2-hydroxyethylamino)-anthraquinone)and 1,4-diaminoanthraquinone (C.I. Disperse Violet 1).The most effective dye of this class is 1,4,5,8tetraminoanthraquinone (C.I. Disperse Blue 1). However, this is no longer used due to its reported carcinogenicity to the rat bladder when fed at high doses. The orange azo-dye C.I. Disperse Black 9 (4-amino-4'-bis-2-hydroxyethylaminoazobenzene) and its 2-methyl analog (HC Yellow No. 7) are used as are two higher molecular weight nitro dyes - 2-nitro-4'-hydroxydiphenylamine(HC Orange No. 1) and 4-amino-2-nitrodiphenylamine (HC Red No. 1).

3.2 Synthesis of nitro-dyes The first blue nitro dye to be used commercially was N',N4,N4-tris(2hydroxyethy1)-p-phenylenediamine(also known as HC Blue No. 2) and was made by direct hydroxyethylation of 2-nitro-p-phenylenediamine with epichlorohydrin. The commercial product contained about 5% of the mauve N4,N4-bis(2hydroxyethy1)-p-phenylenediamine(HC Red No. 13) which somewhat detracted from its blueness. The discovery of an economic high-yield process for the manufacture of 4-flUOrO3-nitroaniline by nitration of 4-fluoroaniline in an anhydrous medium (fuming nitric acid in oleum) [19] made possible the manufacture of pure nitro dyes of this type. Thus HC Blue No. 2, as well as HC Blue No. 1, its N1-methyl analog, and N1-(2hydroxyethy1)-p-phenylenediamine(HC Red No. 3) can readily be made from 4flouro-3-nitroaniline as shown in Figure 5. HC Blue No. 1 was found t o have better dyeing properties than the more hydrophilic HC Blue No. 2 and was used for some years until it was found to be hepatocarcinogenic in mice, and the industry reverted to H C Blue No. 2 which had no carcinogenic properties. Nucleophilic substitution is used in the manufacture of 4-hydroxy-2' nitrodiphenylamine (HC Orange No. 1) and of 2-(2-hydroxyethylamino)nitrobenzene (HC Yellow No. 2) which are obtained from 2-chloronitrobenzene by reaction with p-aminophenol and monoethanolamine respectively. An interesting nucleophilic substitution which can be used to synthesize 2-(2hydroxyethylamino)-5-nitrophenol(HC Yellow No. 11) involves reaction of 3,4methylenedioxynitrobenzene with monoethanolamine (Figure 6) in the absence of additional solvents. However the yield is only 59% due to the concomitant formation of 2-[2-hydroxy-3-N-(2-hydroxyethyl)-amino-6-nitrobenzy1oxy1-ethylamine, but the two products are easily separated [201.

412

qNo2 qNo2 o-""' /f i"' qNo2 NHC,H,

NH2

NHCH,CH,OH

F

MEA

NH2

HC Red 3

HC Red 1

NH2

F

($"02*NH3

N(CH,CH,OH),

NHR

RNHL

N(CH,CH,OH),

N(CH,CH,OH), HC Blue 1 (FISH3)

HC Red 13

HC Blue 2 (R=CH,CH,OH)

Figure 5. Synthesis of nitro dyes from fluoronitroaniline.

NH2CH2CH20H HOCH,CH,NH,

Figure 6 . An unusual nucleophilic substitution reaction.

413

Direct N- or 0- alkylation or hydroxyalkylation of nitro-phenylenediamines or nitroamino phenols or anisoles is also used in the manufacture of nitro dyes. N4-(2-Hydroxyethyl)-N',N4-dimethyl-2-nitro-p-phenylenediamine, which would be expected to be more hydrophobic than HC Blue No. 2, is synthesized from N1,N4-dimethyl-2-nitro-p-phenylenediamine reaction with bromohydrin in an alkaline medium [211. Hydroxyethylation of 2-amino-5-nitrophenol with ethylene oxide gives a good yield of the 0,N-bishydroxyethyl derivative (HC Yellow No. 4). with "bromohydrin" in alkaline Reaction of 3-N-methylamino-4-nitrophenol dimethylformamide is used to manufacture the corresponding 2-hydroxyethylether [221. Reaction with glycidol similarly gives the 2,3-dihydroxy-propylether (Figure 7). Both these dyes are used commercially. Reaction with glycidol is also used to N-dihydroxypropylate the 4-amino group of N1,N4-dimethyl-p-phenylenediamine to give a blue dye (HC Blue No. 6 ) which is said to increase the solubility of other blue nitro dyes having a mixture of Nmethyl and N-hydroxyethyl groups [23]. An alternative method of introducing 2-hydroxyethyl groups involves the use of 2-chlorethylchloroformate [24]. Thus reaction of 4-amino-3-nitrophenol with this reagent gives the corresponding 4-(2-hydroxyethylamino)-3-nitrophenolby the action of 10N potassium hydroxide a t room temperature (Figure 8). This reaction t o yield the O,N-bis(2can also be performed on 4-amino-3-nitrophenoxyethanol hydroxyethyl)-4-amino-3-nitrophenol (HC Orange No. 2), [251. OH

OCH2CHOHCH20H

NO2

NO2

I

VHCH,

NHCH,

I

YHCH,

k

CH, CH2CHOHCH20H

474

Figure 7. Use of glycidol for 0- and N-dihydroxypropylation.

NHC0,CH2CH,CI KOH NH2

NHCO,CH,CH,CI

NHCH2CH,0H

Figure 8. N-Hydroxylation with 2-chloroethyl chloroformate (CECF). Chloroethyl chloroformate is more selective than chlorohydrin, in that reaction with 2-nitro-p-phenylenediamine gives a good yield of the N',N4-bis-carbamate which can be hydrolyzed t o N',N4-bis(2-hydroxyethyl)-2-nitro-p-phenylenediamine (Figure 5) which is a red-violet dye [26]. Reaction of this dye with alkylating agents can yield additional N4-alkylated derivatives such as N4-ethyl-N1,N4-bis(2hydroxyethyl)-2-nitro-p-phenylenediamine (HC Blue No. 12). Selective reduction of 2,4-dinitroaniline derivatives is an excellent route to some (Figure 9). nitro-dyes, particularly the derivatives of 4-nitro-o-phenylenediamine Reduction with sodium sulfide of 2,4-dinitro-2-hydroxyethylaniline, which is readily obtained by reaction of 2,4-dinitrochlorobenzenewith monoethanolamine, (HC Yellow gives a good yield of "-2- hydroxyethyl-4-nitro-p-phenylene-diamine No. 5) which is actually an orangeyellow dye [27]. Reduction under acid conditions with hydrogen and a platinum catalyst gives mainly the isomeric HC Red No. 3 [28]. However the product also contains some HC Yellow No.5 and is thus less desirable that the pure compound obtained from fluoronitroaniline as described above.

475

Nitration of phenylenediamines or aminophenols provides another useful route to certain nitro-dyes. Thus 2-nitro-p-phenylenediamineis manufactured by nitration of 'the N,N'-diacetyl derivative of p-phenylenediamine. NHCH,CH20H

u

HC Yellow 5

NH2 HC Red 3

Figure 9. Selective reduction of a dinitrobenzene. Nitration of N1,N4-diacetyl-N1-methyl-p-phenylenediamine occurs exclusively in the 3-position and hydrolysis of the product yields N1-methyl-3-nitro-pphenylenediamine. The fact that nitration does not occur ortho to an N-acetyl-Nmethylamino group renders this otherwise attractive route impractical for the manufacture of the useful intermediate N1,N4-dimethyl-2-nitro-p-phenylenediamine. N-Methyl-isopicramic acid can be obtained by nitration of 4-hydroxy-Nmethylaniline with nitric acid in sulfuric acid, provided that the reaction mixture is kept cold [291.

3.3 Other non-oxidativehair colorants Other products for hair coloring comprise colored setting lotions, color refresher shampoos and temporary colorants which wash out when the hair is next shampooed. For the most part these products employ dyes which have not been

476

specifically designed for hair coloring, such as the U S . certified FD&C and D&C colors and assorted acid or basic dyes. One group of dyes that is specifically marketed for hair coloring under the Arianor trade name comprises a group of basic dyes that have been found to be useful in color refresher shampoos and conditions. Specifically these are (2.1. Basic Brown 16, C.I. Basic Brown 17, C.I. Basic Red 76, (2.1. Basic Yellow 57 and (2.1. Basic Blue 99. Among the acid dyes that have been found useful are C.I. Acid Red 33, C.I. Acid Blue 9, (2.1. Acid Violet 43, C.I. Acid Orange 7, C.I. Acid Yellow 3 and Ponceau SX (C.I. Food Red or C.I. FD&C Red No. 4). In an anionic shampoo base these dyes can deliver a pleasing highlighting effect on pigmented hair which withstands two to three shampooings. If applied under a heating cap a more intense and relatively long lasting color is obtained. Another approach t o the use of acid dyes as a temporary hair colorant involves mixing a solution of the dyes with a solution of a cationic surfactant such as stearalkonium chloride so as to produce a very fine dispersion of the insoluble cationiclanionic complex. The product is applied to the hair, combed through, styled and allowed to dry. This is probably the most effective temporary product for covering grey hair but has the disadvantage of leaving a ”coated” feel to the hair, 4. References

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

A.W. Hoffman, Jahr. Chem. 42 (1863). P. Monnet, F.P. No. 158 558 (1883) H. Erdmann and E. Erdmann, D.R.P. No. 92 006; 98,431; 47,349; 80,814; 51,073 J. F. Corbett, J. SOC.Cosmet. Chem. 24 (1973) 103. K.C. Brown and J.F. Corbett, ibid., 30 (1979) 191. R. Blanke, US Patent No. 3 210 252 (1965). K.C. Brown and J.F. Corbett, J. SOC.Cosmet. Chem., 37 (1986) 1. Wella, US Patent No. 4 883 656 (1989). L’Oreal, US Patent No. 5 500 021 (1996). L’Oreal, US Patent No. 4 065 255 (1977). Wella, EP Patent No. 8039 (1980). L’Oreal, US Patent No. 4,125 367 (1978). Wella, personal communication. Henkel, US Patent No. 4,314,809 (1982). Clairol, US Patent No. 4,566,876 (1986). Wella, GB Patent No. 2,239,265 (1991). J.F. Corbett, J. SOC. Cosmet. Chem., 35 297 (1984). Unilever, GB Patent No. 741,334 (1955).

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19 20 21 22 23 24 25 26 27 28 29

Clairol, GB Patent No. 1,206,491 (1970). L’Oreal, GB Patent No. 2,104,895 (1983). L’Oreal, US Patent No. 3,632,292 (1972). L’Oreal, US Patent No. 4,417,896 1983). L’Oreal, US Patent No. 4,601,726 (1986). L’Oreal, US Patent No. 4,125,601 1978). L’Oreal, US Patent No. 4 555 247 1985). Wella, US Patent No. 4 900 869 (1990). Unilever, GB Patent No. 707 618 (1952). Clairol, US Patent No. 3 168 442 (1965). Gillette, GB Patent No. 1 172 635 (1965).

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11 Leather Dyes ALOIS G. PUNTENER TFL Ledertechnik AG, 4002 Basel, Switzerland

1. INTRODUCTION Modern leather dyes with their high functional value have a long history. Many of their properties are taken so much for granted that the consumer accepts as a matter of course such quality features [ll as high light fastness and good hation, and scarcely notices them any more. At the first International Leather Congress in 1897 in London, W. H. Perkin, inventor of the first industrially produced aniline dye, Mauve, presided over some of the meetings [21 and doubtlessly discussed the use of the new synthetic dyes on leather. From then onwards, the use of synthetic organic dyes for leather increased steadily and currently they are used predominantly in the coloration of this natural substrate. Leather dyers select dyes on the basis of hue, application and fastness properties as well as for their tinctorial strength-cost factor. Most of these dyes are chosen from the huge textile dye ranges. Only a minority of the dyes used was designed only for leather, this includes modern fur dyes, which are entirely wool dyes.

2. GENERAL ASPECTS 2.1 Tanning Leather manufacture today is subject to economic conditions [31 such as high raw material prices and stringent environmental regulations. The available savings potential has therefore to be optimized, necessitating measures such as product mix optimization, efficient process technologies [4], and selection of efficient chemicals and optimal dyes. Tanning and dyeing are inseparably interlinked in leather processing. Before dyeing, a skin or hide may acquire a characteristic colour, depending on how it has been cleaned, depilated and tanned by the so-called beamhouse processes [5]. Every tannage is an irreversible stabilization of the hide. This is achieved by crosslinking of the animal skin collagen without modifying the natural fibre texture. Bovines are the major source of raw material for leathermaking. A high proportion of lambskins and sheepskins is used for fur skin production, but most are converted to pelt like bovine hide, which is freed of hair and then tanned into leather.

479

The process of tanning is the heart of leathermaking [61. Crosslinks are introduced into the collagen fibre structure by bifhctional or polyhnctional agents in order to raise the denaturation point to around 100°C and stabilize the natural organic material. Tanning gives leather its own colour to a certain degree, which will influence the final shade and its brilliance. Technically the most important process is chrome tanning with trivalent chromium compounds. Much evidence has been presented that chromium and other metal tanning salts like aluminium, zirconium, titanium, iron and others react with the carboxyl groups of collagen to form multinuclear complexes, mainly cationic but also anionic. Chrome tanned leather is greenish blue and traded in the wet blue state or dried and slightly retanned as crust leather. For tanning today use is also commonly made of small covalently bonded organic molecules for special effects. Aldehydes, mostly glutaraldehyde, isocyanates, epoxides, quinone, reactive halogens and others react primarily with the amino groups of collagen. This type of leather is mostly white to slightly yellowish, and called wet-white. In the past the term wet-white was also used for aluminium tanned leather. Today aluminium wet white pretanned leather mostly contains a polymeric retanning agent. Tanning with organic macromolecules is based on natural and synthetic polyphenols. Many of the vegetable-tanning agents are water-soluble components of shredded wood, bark, leaves and roots. Vegetable tanned leather is yellowish to reddish brown and turns a markedly darker shade on exposure to light. The principal attraction between collagen and these macromolecules arises from hydrogen bonding and dipole interactions. The addition of synthetic tanning agents, called syntans, makes the leather whiter. Today, pure vegetable-tanned leather is confined mostly to shoe soles, but in the past it was also used for technical applications such as drive belts. Output of this heavy leather has long been on the decline, mainly as a result of a continuing change to other shoe soling materials and replacement of technical leather. Vegetable tanning agents are still used for retanning and for special items. A completely vegetable tanned end product is being promoted as bioleather in keeping with a fashion trend. As yet, however, no general definition or classification of bioleather exists.

2.2 Dyeing Colour is one of the key attributes by which the consumer judges the quality of leather goods. High-grade leathers nowadays are through dyed so that they have the same shade on the cut edge and on the back (flesh side) as on the more compact grain side. The dyes are applied not to the skin (collagen)but to the tanned leather, which differs according to the tanning process. Dyeing leather [71 is a challenging task if current market requirements are to be met. The need to meet criteria such as cost-effectiveness flexibility fastness standards

480

levelness and coverage of defects reproducibility therefore often causes a great deal of upheaval in the life of a dyehouse manager [8l. Furthermore, because leather is of heterogeneous animal origin it does not have a definite composition. It also has inherent grain characteristics, which change within animal species according to breed, age and nutritional status of the animals. All this makes it difficult to attain uniformity of shade throughout the piece and within a batch. Therefore dyeing agents play an important role in improving levelness, penetration and fixation of dyeing 191. Besides shade, gloss is also an essential attribute, and special products like cationic fat liquors are used. Silicones on the other hand give a very appropriate silky sheen often seen as a two-way (often called writing) effect on buffed leather. Also most finishing processes provide a glossy appearance. The chemical industry makes strenuous efforts to convert the theory of leather dyeing into practice through extensive research into application technology [ 101. However, we are all too oRen confronted with surprises in dyeing. 2.3 The isoelectric point The isoelectric point is a key criterion not only for selection of dyes and dyeing methods but also for the whole leather manufacturing process [lll.Pelts have an isoelectric point of approximately 5. Chrome leather has an isoelectric point of 4.56.5, depending on the chrome masking process at the end of the tanning step, and that of vegetable tanned leather is 3.2-4. The magnitude of the isoelectric point depends on the tanning and retanning process. Retanning and filling agents are used for subsequent treatments to give the leather special effects similar or different in character to those imparted by the fat liquoring process. Deposition of fatty substances in the interfibrillar spaces gives leather softness and water-repellent properties [121. Retanning, dyeing and fat liquoring are performed in the so-called wet end and the process sequence can differ, depending on the desired end-product or on the chemicals and dyes used. Increasingly tanners are using so-called compact processing methods in which retanning, dyeing and fat liquoring are carried out in the same float to save time [13]. This does, however, mean a much more complicated application formula. The isoelectric point changes with the order in which the chemicals are added. Changes of pH during dyeing can also influence the absorption behaviour, penetration and fixation, irrespective of the isoelectric point.

2.4 Finishing In most cases the surface or grain of the leather has to be protected. Materials are used for closing and sealing the porous surface against humidity and any kind of dust. The application of wax and proteins imparts a glossy appearance. Casein finishes have been used for centuries. About 100 years ago pigmented nitrocellose finishes were introduced and the first polymer dispersions came into use in the

48 1

nineteen-thirties. Spraying is the most common method for aqueous and solvent containing finishing floats. It is applied by means of compressed air. Today, finishes are applied to cover leather raw hides and skin defects so making the leather more uniform and suitable for the manufacture of the end-products. "he finish consists basically of three coats 1141. The base coating equalizes the absorbing capacity of the leather surface, covers skin defects and acts as an adhesion promoter between leather and the subsequent coats. "he final colour of leather is achieved by mixing and shading inorganic or organic pigments, which are incorporated in a binder film system, mostly in the middle coats, also called pigment coats. Pigment coat binders are polymer dispersions, which form films on drying. They can be solvent, solvent-water or water based polyacrylates,polybutadienes or polyurethanes [ E l . The final top coat provides protection against soiling, moisture and damage caused by scratching, besides ensuring a matt or gloss effect. Best quality leather, known as aniline leather, receives at most only a very thin transparent coat. Semianiline leather has only a small amount of base or pigment coat applied to the topcoat in order to maintain the breathability of the material. Leathers with a large number of grain defects require a finish giving stronger cover with appropriate amounts of pigments, resins and auxiliaries. Foam finishing of leather was therefore introduced in the 1990s [161. Virtually all polymer dispersions with good mechanical stability containing a foam-producing expanding agent can be used. These include fatty acid soaps, fatty alcohol sulfates, alkyl aryl sulfonates and sulfosuccinates. Foam based coats are mostly applied by the roll coating process. Surface defects can be concealed with a small amount of substance, which make this process more economically attractive. 2.5 Origin and u s e of leather

Hides and skins, the raw material for leather production, are traded either preserved with salt, or dried or fresh (green hides) from the slaughterhouse [171. "he term hide is used for the outer covering of animals with a body surface area of more than 1m' and the term skin for that of smaller animals. According to the statistics of The Food and Agriculture Organisation of the United Nations (FAO) (average 1992-1994),the bovine population of 1,435 million head yielded approximately 5.1 million tons of wet salted raw hides [MI. The average unit weight was estimated at 18.3kg. The size of raw hide is 1.5-6.0m' and the thickness is 3-12mm. Bovine hides are the largest source of raw material for the leather industry and occur as a by-product in the slaughtering of domesticated animals that are kept for meat and milk production. The properties of the raw hides are of the greatest importance both for the leather making process itself and for the production of all types of leather goods. The trade statistics differentiate bovine leather into heavy and light leather. Today, heavy bovine leather production, mostly for shoe sole and technical products, which has been in long-term decline as a result of continuing change to other shoe soling materials, amounts to only about 400,000ton.

482

The sustained expansion of tanning capacity in developing countries is reflected in the growth of output of light bovine leather, reaching 9,550 million square feet (ca. 887 million square metres). The FA0 has estimated that approximately 67%of this leather are converted into 4,200 million pairs of shoes with leather uppers. The remaining light bovine leather is used for garments, furniture and travel goods, including handbags. Sheep, goats and pigs are the second most important sources of leather making material. A global livestock population of approximately 1,100 million sheep and 600 million goats yielded approximately 577,000ton of raw skins on a dry basis with average unit weight of 0.75-0.72kg per raw hide. This raw material was converted into ca. 3,900 million square feet (ca. 362 million square metres) of leather. It is used in a vast variety of lighter leather items like garments, bookbinding, gloves and lining leathers. Large proportions of sheepskins are used for fur skin production. Sheepskins with a tighter texture are used for shoe uppers. Overall, goatskins are of much greater firmness than sheepskins. The best goatskins are used for high quality shoe uppers. The world pig population is around 840 million and only a few countries use their skins for manufacturing shoe uppers, garments or handbags. Large amounts of pigskin are used for the production of gelatin; accordingly pig leather does not account for much more then 3% of total leather production. Other sources like horses, farmed crocodiles or ostriches as well as fishes or deer, kangaroos and other wild living animals are of much less commercial importance. The ultimate value of a hide or skin depends on the demand for meat, which increased 1-2% in the past year. However, we should not overlook current challenges such as the cost of animal feeding stuffs, mad cow disease (BSE) [191 and changes in eating habits. 2.6 Demand for leather dyes Leather dye demand statistics are very difficult to come by. If in the year 2000 approximately 6 million ton of hides and skin are used, this will create 1.5 million ton of leather, assuming an average yield of 25%. Depending on the current fashion, leather requires 0 5 5 % (for deep black as much as 8-10%)dye. Taking an average of 2.5%this would mean 30-50 thousand ton of dyes for leather. Estimates for textile dyes lie at around 1million ton. Black, grey and brown shades are still the most important leather colours for shoes and to a lesser extent also for upholstery and garments. The market share of these dominant colours is 60-90%, depending on the user segment. This may have traditional reasons. Brown is the natural colour of vegetable tanned leather, and black the simplest colour produced in the past by the reaction of vegetable tanning agents with ferric salts or ferric oxide. 2.7 Classificationof leather dyes in drum application Dyes for leather are selected according to the application process. In the wet end process, dyeing is normally conducted in wooden drums. Flawless dyeing in these vessels depends on good mixing of float and leather. Bars and pegs prevent the

483

leather from rolling up at the bottom of the drum. Other techniques, such as the classical brush dyeing, have mostly been replaced by the more efficient immersion dyeing or finishing roll coater, spraying and printing machines. The requirements for the various end products are as follows: 1)Surface (top) dyeing for levelling and correcting defects on the grain side: - high molecular weight anionic direct dyes - 1:2 metal complex dyes with fair solubility 2) Semi-penetrated dyeing to reduce the obtrusiveness of damaged patches: - medium molecular weight anionic acid or direct dyes - 1:2 metal complex dyes with high solubility 3) Through dyed leather to avoid edges on the goods: - low molecular weight acid dyes - 1:lmetal complex dyes - soluble sulphur dyes

Special dyes are used for different purposes, cases in point being selected reactive dyes for high fastness to washing, pigments to improve light fastness, cationic dyes to overdye anionic dyeings and deepen the shade on the surface. Natural dyes have gradually lost importance. However, they dye the grain side and flesh side evenly without accentuating the defects of leather. This may be the reason why chemical companies still use fustic extract as a precursor for synthetic leather dyes. 2.8 Classificationof leather dyes in finishing In the finishing process, homogenized, finely dispersed pigment pastes are used in the coats. Spraying by means of compressed air is the most common application method. Other methods are padding, brushing, sponging or curtain coating. The most frequently used pigments are: Inorganic pigments: titanium dioxide iron oxide carbon blacks Organic pigments: water and solvent insoluble metal complex and other dyes for good fastness qualities Solvent dyes: metal complex and other water or solvent-soluble dyes for brilliant shades Natural earth colours: with the exception of iron oxide ores these dyes are rarely applied. They give rather dull shades.

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2.9 Classification of fur dyes Oxidation dyes have been used in the past for fur skins [201. These dyes are mainly aromatic oxiamines, oxidiamines and aminophenols, which oxidize on the substrate or develop into dye. Today their only application is for special farm fur hair dyeing, and they will therefore not be covered in this review. Acid dyes are outstandingly suited to dyeing the wool of sheepskins, though the fact that the operation cannot be carried out above 60°C introduces an element of difficulty. In earlier times the practice used to involve treating (chlorinating) the wool with NaOC1, then dyeing it a level shade with anionic dyes. Nowadays socalled carriers are used. "hey help to ensure extremely brilliant, tinctorially strong dyeings. Black fur shades are problematical to obtain, because such dyes have a large molecule and often behave like direct dyes. They shade the leather but not the wool component. Therefore dye mixtures are used to dye the wool, especially brown dyes mixed with blue and shaded with yellow or red. 3. DYEING LEATHER 3.1 Terminology The tanner's terminology distinguishes between dyeing that is carried out by exhaustion, and pigmentation that is done in the course of finishing. Exhaust dyeing, as mentioned earlier, is performed in the drum with water-soluble dyes. The dyes used are mostly selected from ranges designed for textiles. The Colour Index is almost exclusively a listing of textile dyes with dyes for leather appearing only in a supplementary chapter and classified mostly as acid, direct, and mordant dyes [211. In point of fact, the ranges of dyes for wool, and particularly cotton, are the richest sources of dyes for leather, and in this connection the chemical relationship between wool, polyamide and leather must not be overlooked. Surprisingly, however, the types of dyes used for cotton, with which it is chemically scarcely related, are also extremely popular on leather, especially anionic substantive cotton dyes. The wool, polyamide and cotton dyes in question have been fully described in several textbooks and manuals [22], so we can confine our attention more to the selection criteria and to current trends in leather dyeing. Consequently, fur dyeing theory is not discussed in detail, as it is basically a wool dyeing procedure at low temperature. For coloration as part of finishing, the tanner can also fall back on the familiar classes of pigment colours. These too are sufficiently well known and no detailed discussion of them is necessary. 3.2 Requirements in leather dyeing The relevant trends and selection criteria can only be understood by reference to the distinctiveness of leather dyeing. As mentioned earlier, a batch of leather,

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unlike one of textiles, invariably consists of structurally dissimilar individual pieces, which are not uniformly dyeable. A further difference between leather and textile dyeing is that leather is not an evenly two-dimensional material but a spatial structure of varying thickness (ca. 0.4 - 4 mm). The main difference,however, is probably the third characteristic: viz., in leather dyeing the colorants are applied to two surfaces of the same piece, the grain side with lower dye affinity, and the flesh side with higher dye affinity. The dissimilarities referred to can be accentuated or weakened by tanning agents and auxiliaries. All these phenomena combine to make leather dyeing one of the most demanding dyeing techniques and serve to explain why tanneries often entrust it to specialists. A very important distinction between leather and textiles is the temperature at which dyeing is performed. Because leather shrinks at high temperature and withstands 100°C or higher for only a very short time, the dyeing temperature has to be monitored very closely. In bulk production it is customary to dye leather at temperatures up to 60 "C. Dye selection and classification used to be largely empirical and was based primarily on specific tannages and on the requirements to be met by the leather end product envisaged. Drawing on this fund of knowledge, supplemented by scientific findings and conclusions, the dye-manufacturing industry creates essentially tailor-made ranges. By and large, the principle of grouping together dyes with identical dyeing characteristics and marketing them as specialized ranges has paid handsome dividends.

3.3 Dyeing theory Efforts have long been made to establish a theory of leather dyeing but the processes that occur during the drum dyeing of leather in an aqueous medium are often obscure [231. Even if the same processing procedure and the same chemicals are used, differences in depth and levelness of shade may occur from batch to batch [241. Textile dyeing models are therefore often adopted, perhaps too uncritically at times. For the binding of anionic dyes there are three conceivable options: 1)salt formation between dye anions, particularly of the sulfonic acid group, and the basic groups of the collagen; %)bindingof the dye ions on the tanning agents embedded in the leather. For example, combinations of the cationic chrome tanning agent with the anionic dye groups, and reactions of anionic synthetic or vegetable tanning agents with the cationic groups of the dye; 3) Secondary valency binding forces, viz., dipole-dipoleinteractions, hydrogen bonding, and dispersion forces, come into play. Attempts have been made to form primary valence bonds with collagen using reactive dyes [25l.

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Dyes are often assumed to have a tanning action, and are selected on that basis. The tanning action of the natural wood dyes (fustic, brazilwood and logwood) is well known. Also the tanning behaviour of synthetic dyes is discussed [26] but often overlooked and difficult to interpret. Practical knowledge of leather dyeing is generally very rudimentary, primarily because wool or polyamide dyeing is seen as the model. This rather empirical approach and the technology developed by Otto in the 1950s still serves as the basis of leather dyeing [271. It thus becomes clear that modern science still views leather dyeing as an art, and that leather production benefits from scientific discoveries in other areas. The converse tends to be the exception at the present time. Nevertheless we must not forget that a huge amount of literature can be found on practical aspects of leather dyeing. A series of semi-empirical models have been tested, including thermodynamics of leather dyeing with hide powder 1281. Simplified models permitted an evaluation of the relative importance of each theory in the dyeing process. This means that attention must be given to diffusion behaviour in order to understand compatibility [291. The pore model and the free volume model could deserve particular attention as a measurement of dyeing and diffusion through leather. However these models are never discussed on leather as such, but they can possibly lead to a better understanding of leather dyeing. According to the pore model, the leather substrate is seen as a more or less rigid body with a labyrinth filled with twisted pores. These pores are full of water and dissolved dyes from the dyebath. Dye diffusion into the leather interior is assumed to take place through these water filled pores. The free volume model postulates dye diffusion through the holes in the noncrystalline areas of the substrate by dynamic fluctuation. These non-crystalline areas have a melting point referred to as the glass transition temperature. Some experiments with textile fibres indicate that the pore model applies up to this temperature, and above it the free volume model should be used 1301. Dyeing assistants can be used which lower the glass transition temperature; these compounds are conventionally called carriers because they improve the dyeability of the substrate. The use of carriers is common in through dyeing, as small molecules diffuse more easily than aggregated dyes. It has been reported that up to 90 molecules can cluster to form an aggregate. Scientific investigation is necessary to interpret these two models. In research into the dyeing processes two factors play a key role: dyeing kinetics and dyeing equilibrium [311. Typical dyeing kinetics are shown in Fig. 1,plotting the uptake of dye from the dyebath to the leather surface during the dyeing process. The dyeing process is a distribution of a dye or dyes between the dyebath and the leather substrate. The distribution process is called adsorption, if the dyes are retained by the leather surface. If the dye enters into the interior of the leather structure, the process is termed absorption, sorption or diffusion within the structure.

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15min

30min

45min

60min

75min

90min

Figure 1.Plot of dye uptake on leather. Dyeing equilibrium is the balance of absorption and desorption aRer infinite time. At constant temperature, the relation between the concentration of dye in the dyebath and that fixed on the leather is called the isotherm. As the dye concentration increases, the saturation point beyond which the shade strength does not increase is approached at the surface. The dyeing curves are mostly classified as Langmuir isotherms, as depicted by Fig. 2. In order to examine the diffusion of the through dyeing it is expedient to focus on the kinetics. The kinetic behaviour of a dye in the course of its transfer from the dyebath into the leather comprises at least four stages: 1)diffusion from the dyebath to the leather surface 2) diffusion through the waterlleather boundary layer 3) adsorption on the leather surface 4)diffusion into the leather matrix.

For the kinetics of the overall dyeing process stages 1, 3, and 4 are important. Stage 1, liquor circulation, is normally sufficient in a leather dyeing process and can therefore mostly be neglected. Diffusion through the boundary layer (stage 2) can also be ignored. It is more of theoretical interest because it is not a decisive step in a standard leather dyeing process. Adsorption and diffision are the most important factors. The question now arises as to which stages are rate determining in the dyeing process. Experiments normally show that diffusion is faster than the preceding adsorption stages. In

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practice, this means that after exhaustion is completed the diffusion stops. "he dye is attracted to the leather fibres and further diffusion is almost precluded. Prolongation of the dyeing time will not produce full penetration on a surface dyed leather. Nevertheless, by insufficient dye fixation, desorption (co-called bleeding) may negate this expectation.

--

Colour strength

% Dye

Figure 2. Langmuir isotherm. The theory of dye diffusion is extremely complex. However, the purely mathematical description of diffusion was established more than 140 years ago, with the aid of Fick's law. Formula 1.Ficks first law:

where, dddt is number of particles that diffuse through the cross-section in the x direction, D is the proportionality constant, and dddx is the concentration gradient. Fick's first law characterizes the diffusion process under steady-state conditions. This is the case for the diffusion of a dye solution through a membrane into another medium, provided that the concentrations of the two media are different and kept constant. The diffusion of a mobile particle can be regarded as a "random walk"; but this condition is only present at the very beginning of the dyeing process.

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During dyeing, the system changes to a non-steady state, since the concentration changes. Therefore Fick’s second law needs to be applied. Formula 2. Ficks second law:

-=D7 dc d2c dt

dx

where c (concentration)is a function of space (x) and time (t). Fick’s second law can also be formulated for two and three-dimensionaldiffusion. However for practical reasons, the diffusion coefficient (D) is the most important factor for the leather dyer. The magnitude of a leather dye diffusion coefficient is not known, but may be 10~12-107cm2/sec, as occurs with textile substrates [321. Experience shows measurement of the diffusion Coefficient to be very difficult [33]. The diffusion coefficient can be determined from concentration profiles through the substrate. However, due to the complexities of the polymeric structure of leather, and the differences between the more open flesh side and the more compact grain side, reliable measurements are very difficult. Often the practice of using differently tanned hides or a hide powder is used for measurement of affinity (adsorption) and diffusion [341. In practice, empirical methods are often used, by comparing the dyeing rate of one dye relative to another [35]. Tanners know that they can make several process variations to improve through dyeing, including the following: 1)increased mechanical action 2) longer running time 3) addition of electrolytes 4) low float 5) lower temperature 6) low molecular weight dyes 7) high pH (> the iso-electricpoint) 8) use of special auxiliaries.

At first glance, it would seem that some of these parameters may not fit the purely theoretical assumptions, which might surprise both scientists and pragmatic leather dyers. As discussed earlier, the dyer is confronted not with pure collagen but rather with tanned material, i.e. three-dimensionalcrosslinked helical structures. Initially, therefore, the interpretation of leather dyeing will have to be restricted to semi-empiricalmodels. The theoretical and semi-empiricalkinetic and thermodynamic deductions were mostly conclusive in themselves, but they proved difficult to correlate with the multitude of variable bulk working requirements. The influence of the isoelectric point and “outwitting” it, primarily with dyeing auxiliaries, is frequently overlooked. But the influence of the retanning agents, filling agents and fat liquors has also to be taken into account as well as process

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parameters [361. A further factor to consider is that leather is not always dyed right through, but often only part way (penetrative dyeing). 4. TRENDS OVER THE LAST 100 YEARS IN LEATHER DYEING

Replacement of the natural dyes by the synthetic aniline dyes was favored by the following factors: 1)the synthetic dyes were more cost-effective,besides being easier to manufacture and apply; 2) fashion called for brighter, more brilliant shades, which are only obtainable with synthetic dyes; 3) in the course of time it also became possible to achieve better in-use properties with synthetic dyes than with natural dyes. 4.1 The first synthetic cationic leather dyes Introduction of the new synthetic dyes was not a simple matter. The presence of W.H. Perkin at the first International Leather Congress undoubtedly ensured the breakthrough in the use of synthetic dyes for leather. It was perhaps also essential, because unwittingly and indirectly, the synthetic dyes caused substantial damage when incorrectly applied. Bookbinding leather was produced with vegetable tanning agents. Around 1830 a phenomenon known as ”red decay” became noticeable on leather bindings. With age the leather became hard and brittle though its surface remained more or less intact. Far worse was the red decay that started in 1860. The leather dried out completely and turned to dust when lightly rubbed. This phenomenon was first recognized in England, where the “Royal Society of Arts” appointed an investigating committee comprising 20 members, librarians, bookbinders, leather producers and chemists [371. As the emergence of red decay happened to coincide almost exactly with the introduction of the novel synthetic dyes, it seemed logical to seek the cause in the leather dyeing. Afker W.H. Perkin launched Mauvein, also known as Aniline Purple, on the market in 1856, a series of further basic dyes were discovered in rapid succession. The most important of these were Fuchsin, Methylene Violet, Safranine, Phosphine and Bismarck Brown. To enhance shade brilliance, sulfuric acid was added before and after dyeing. The investigation showed that this sulfuric acid treatment alone and in conjunction with unsuitable tanning agents and insufficient fat liquoring resulted in premature destruction of the leather. Since sulfuric acid remains in situ in leather, dye fixation must be carrying out with readily removable organic acids like formic acid. Consequently the basic dyes were not to blame but even so their success was slow in coming because their light fastness was considered too low. At the present time basic dyes are used almost exclusively for overdyeing, e.g. to achieve deep black shades.

49 1

4.2.The first anionic dyes The cationic dyes were characterized by a previously unknown brightness, and this was also sought with anionic dyes. Nicholson sulfonated the basic dyes (1862) and in that way created the first amphoteric leather dyes, Acid Fuchsine and Sea Blue, which are still being used today. In the same year Griess discovered the azo dyes. In 1876 Roussin produced the first acid dye, (3.1.Acid Orange 7. This dye is still in widespread use today for leather dyeing. The light fastness properties of this dye type were not very good initially, but steadily improved as future developments occurred. The first anionic leather dyes, mostly belonging to the azo dye class, were very problematical at first, particularly in terms of uptake and fixation, on leathers tanned with strongly anionic vegetable tanning agents. Full surface-dyed shades were feasible only at fairly low pH. The tanning agents block the basic groups of the collagen. The anionic dyes became predominant only with the spread of chrome tanning [381. Chrome leather binds more anionic dye than vegetable tanned leather. The more basic chromium sulfate a leather substrate contains, the more anionic dye it is capable of binding. This is exploited technically to obtain exceptionally deep shades. 4.3 Classification of the azo dyes The rough classification of leather dyes into surface, penetrative and throughdyeing mentioned earlier can be simplified for anionic azo dyes by the number of azo groups as discussed in section 6.2.8. Converting aromatic amines to diazonium compounds and coupling the latter with an appropriate aromatic species produces azo dyes. The synthesis is outlined in Fig. 22 of section 9.3.2. Generally, monoazo dyes penetrate polyamide and protein substrates well. The Colour Index assigns these dyes to the acid dyes group, which are applied to wool, silk and nylon fibers. They are not substantive to cellulosic fibres. Acid dyes have a relatively low molecular weight, 1-3 sulfonic acid groups, and react with leather preferentially in an acid medium, i.e. below the isoelectric point. Polyazo dyes belong to the surface dyeing class where leather is concerned. They are classified as substantive cotton dyes and listed as direct dyes in the Colour Index. They have not only a high molecular weight, but also fewer water-soluble sulfonate groups in relation to their molecular weight and therefore have a marked tendency to undergo aggregation because of their relatively linear structures. These substantive dyes are bound more quickly and strongly on leather and are, therefore, normally not capable of through-dyeingleather. Direct dyes do not need salt in the dyebath for exhaustion on leather. Dipoledipole interactions and hydrogen bond formation between these dyes and leather appear to be stronger than the ionic bond between leather and monoazo dyes. This difference is also apparent in the wet fastness properties. Typical acid dyes can sometimes bleed out again up to 50%, as against only about 1% for substantive dyes. The best pH for dyeing is the isoelectric point or even slightly higher, to promote partial penetration.

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The ideal dyes for leather lie intermediate between mono and polyazo types. First they dye the leather surface a sufficiently deep shade and, second, they penetrate far enough into the leather to make scratches less easily visible. The transition from surface dye to through dye is not well defined. The dyes between these two extremes can act as through or surface dyes, depending on the substrate and method of application. They include the weakly acid to neutral dyeing wool dyes and preferably the dyes for polyamide fibers. It is inappropriate to conclude from this, however, that good leather dyes must also be good polyamide dyes. In most cases the Colour Index classification can be adopted, with the acid dyes assigned to the penetrating dyes and the direct dyes to the surface dyes. In actual fact there ought to be an intermediate class, viz. leather dyes. Consequently there has been no lack of effort by dye producers and research institutes to classify leather dyes more precisely by means of different methods. These methods include standardized uptake curves, bath exhaustion, penetration power into gelatin and tanned skin powder, flow rates on defined chromatogram systems, etc [39]. 4.4 Acid dyes These are, with the exception of some metal complexes, low-molecular-weight wool dyes that usually have monoazo or simple anthraquinone systems. They exhaust from a strongly acid to more neutral bath. The relatively small molecules create exceptionally bright shades, with the exception of some premetallized dyes. Given their small size, they diffise rapidly into the interior of the leather without fast binding. They are therefore classified as penetrating dyes. In addition acid dyes are also excellent fur dyes. To avoid leather damage the dyeing pH is not lowered to the point where the number of protonated amino groups is sufficient to completely bind the dye anions. The fixation and the shade depth attainable with acid dyes are therefore oRen on the low side. The natural shade brilliance of the acid dyes is not always seen when the dyes are applied alone. Surprisingly, however, a brightening effect can be obtained by adding direct dyes.

4.5 Special azo dyes for leather The triumphal progress of anionic dyes in leather dyeing back in the 1930s was accompanied by the first targeted syntheses of leather dyes, focussing on black and especially all possible varieties of brown. There were probably three major reasons for this: 1)browns are the most important shades for leather other than blacks; homogeneous brown dyes are unfortunately difficult to synthesize;

2) brown shades are oRen much easier to dye with dye combinations on textiles than on leather. Therefore ,while a single brown dye may be enough in a range for textiles, leather dye ranges offer a broad swathe of brown shades;

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3) finally, however, the readily synthesized polyazo brown dyes have given good result particularly on leather though they could scarcely meet textile requirements in terms of stability and fastness properties. This was particularly the case with azo dyes having resorcinol or diaminobenzene as coupling component. 4.5.1 Coupling on dihydroxybenzenederivatives The most important coupling component in this series is 1,3-dihydroxybenzene (resorcinol). Multiple azo coupling is feasible on resorcinol at three points, depending on reaction conditions. This means that homogeneous dyes are extremely difficult to synthesize on a bulk scale. As a rule dye mixtures are formed, in which the components differ in terms of the coupling position [401. The anion with a double negative charge has a 10,000-fold higher rate of coupling than the single anion, so that even extremely weakly electrophilic diazonium components can be induced to react with resorcinol in an alkaline medium. With equivalent amounts of resorcinol and diazonium component, the 4monoazo compound is obtained, together with about 10% 2,4-disazo compound. With excess diazo component resorcinol is also substituted in the 2,4-positions, at pH 5-8, but in the 4,6-positions in a strongly alkaline medium. Generally speaking, 4-substituted monoazo dye is obtained as the main component below pH 7. By pH 8-9 mostly the 2,kdisazo dye forms and above pH 9-10 the 4,6-disazo product forms, with a slight amount of trisazo dye generated as secondary component. Besides the intended higher molecular polyazo dyes, small molecular coloured monoazo dyes can also be formed, depending on reaction conditions. Scaling up from laboratory synthesis to bulk production and transfer from one production plant to another is a major chemical and engineering challenge, because under different reaction conditions the composition of the components, and hence shade, fastness properties and application behaviour, all change. By-products in the form of monoazo dyes can have a decidedly beneficial effect in leather dyeing, similar to that of the acid “brightening”dyes. Because of their lower tendency to undergo aggregation, the small molecular dyes give brighter shades. Probably they act not only as ‘brightening” dyes but also as a kind of dispersant, reducing the aggregation tendency of the polymeric dyes. In this way, the monoazo dyes not only yield hller shades but also penetrate better into the leather. Many of these dyes are therefore also included in the “penetrating”dye class. The drawback of low molecular dyes being more easily washed out is less serious with leather than with textiles. Leather is not laundered and the soluble, low molecular components are concentrated more in the interior of the material and scarcely migrate. However, for very high-class leather like automotive seats, many of these dyes do not pass the migration test and are no longer used. The low-molecular resorcinol dyes also act as indicators and change colour very markedly, depending on pH. This property makes them less obtrusive on leather. Because of their medium molecular weight and strong intermolecular forces of attraction, resorcinol dyes are so well fixed on leather that their indicator properties are scarcely noticeable, in contrast to the case with textiles. This

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explains why leather dyes proper are not necessarily suitable for textiles, particularly polyamides. A large number of brown shades can be synthesized, by varying the azo component. Although the major of research on resorcinol dyes was actually completed in the 1930s, an increasing numbers of patent applications were filed into the 1980s [41]. The more recent work was conducted to enhance the scope of available structures, to optimize synthetic procedures, and to meet new fastness requirements. Resorcinol dyes are still used today as leather dyes, though rarely encountered for textile coloration.

4.5.2 Coupling on diaminobenzenes Like resorcinol, 173-diaminobenzeneis widely used as a coupling component. In acidic media, it forms predominately the 2,kdisazo dye and in alkaline media the 4,6-disazo compound forms, along with the 4-monoazo and the 2,4,6-trisazo compound. The diaminobenzenesare more readily oxidized than resorcinols. The presence of free amino groups intensifies the basic character of the dyes generated from these couplers, which is advantageous with leather, enabling the dyes to form an additional linkage with the normally anionic substrate. The increased substrate interaction results in deeper dyeings because more dye is bound, especially at the leather surface. Bismarck Brown is a typical cationic azo or solvent azo dye. The title diamines and the corresponding l-amino-3hydroxybenzene analog are frequently encountered residues, particularly in black dyes. 4.5.3. Coupling on other hydroxybenzenes The results achieved with resorcinol have led researchers to examine the utility of certain resorcinol derivatives and high molecular phenols as coupling components for leather dyes. The couplers studied include trihydroxybenzenes such as phloroglucine and pyrogallol, along with synthetic and vegetable tanning agents, and dyewood and lignin derivatives. While this work afforded dyes with good shade levelness the tinctorial strength was often unsatisfactory, varying widely with the diazo component, coupling component, and coupling method employed. Furthermore, the light fastness was generally unsatisfactory. Currently, only a few dyes coupled on dyewood extracts, which are analogous to Patent Fustin, are commercially available [421. 4.6 Direct dyes The direct or substantive dyes, which were designed for cotton, generally have a molecular weight in excess of 800, and elongated, planar structures. Their water solubility is less than that of the acid dyes. Hard water or acid can speed up their natural tendency to undergo aggregation; this tendency has a number of disadvantages. The aggregated dye exhausts very rapidly onto the damaged areas of the grain side of leather and onto the larger surface of the flesh side. These parts of the leather are, therefore, frequently dyed a duller, deeper shade than the

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others. Under unfavorable dyeing conditions, direct dyes can also flocculate out of solution and smear the leather surface, a phenomenon also referred to as bronzing. Direct dyes have a decisive advantage over acid dyes. Although they fix at a faster rate and, as a consequence, tend to give more surface dye, tanners can produce wetfast, penetrated, and even through-dyeings with them. Desirable results are achieved with the aid of special techniques, e.g. by using a short float, low temperature, fat liquors andor dyeing auxiliaries, particularly when dyeing at above the isoelectric point. Since dye fixation takes place below the isoelectric point, the initial dye application step must be followed by an acid treatment, preferably with formic acid or, if necessary, with acetic acid. By examining the constitution of the direct dyes that are most suitable for leather, the following features emerge: 1) long-chain dye molecules predominate 2) sulfonate groups are present at the end of the molecule 3) a balanced ratio of double bonds to solubilizing groups exists 4)use of meta-substituted azo coupling components is prevalent 5 ) insulation groups are present.

These empirical observations have been used to enhance the technical properties of anionic polyazo leather dyes [431. While the development of direct dyes was essentially completed in the 1930s, it was reactivated in the 1960s and 1970s, when it was found that benzidine, an important direct dye intermediate at the time, was carcinogenic [441. Although no potential hazard was perceived in dyeing mills and consumer circles, this finding was quite relevant to workers in the dye industry. In view of this concern, the leading dye producers withdrew benzidine-based dyes preemptively to protect their workers, the dyers and the consumers. Studies towards alternative dyes were undertaken, and this work led eventually to many of today’s most important black dyes for leather [45].

4.7 Metal complexes and mordant dyes 4.7.1 Wooddyes Before the synthetic dyes started to dominate the coloration of leather, extracts of dyewoods, chiefly logwood, redwood and fustic were used. These dyewoods exhibit a tanning action, dye the flesh and grain sides of leather equally well, and have no tendency to accentuate grain defects. These favorable properties arise from the fact that the dyewoods contain low molecular, less aggregating constituents. All wood dyes are so-called mordant dyes because they form stable complexes with metals such as iron, copper, chromium, and titanium. The complex compounds of the dyewoods are sparingly soluble lakes. 4.7.2 Mordant dyes The use of both synthetic mordant dyes and dyewoods has long passed its zenith. This is also true of the synthetic mordant dyes still listed as leather dyes in the

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Colour Index. The operations involved in the post-metallization step are too involved, and modern tanneries prefer premetallized dyes.

4.7.3 Monoazo metal complex dyes

Currently, tanneries use 1:l and 1:2 metal complexes of azo dyes that have dyemetal interactions shown in Figures 3 , 4 and 5.

Figure 3. An ortho,ortho’-dihydroxyazometal complex

Figure 4. An ortho,ortho’-hydroxyaminoazo metal complex

Figure 5 . An ortho,ortho’-carboxyhydroxyazometal complex The 1:l complexes contain one metal ion (Met) and one dye ligand. The 1:l complexes with a trivalent, 6 coordinated chromium ion contain further ligands (e.g. water), depending on the method of syntheses. The general structure is shown in Figure 6.

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Figure 6. A 1:lazo dye-metal complex In contrast, 1:2 complexes contain one metal ion and two dye ligands. This system is characterized by five or six-membered rings, with the metal in the center of the octahedral structure shown in Figure 7. Metals are the trivalent, 6 coordinated chromium, cobalt and iron ions. Since copper is divalent and 4 coordinated, it does not give 1:2 complexes.

Figure 7. Potential structures of a 1:2 metal complex - right angle (I or) planar 0.

The structures of the chromium and cobalt complexes are largely known. Depending on the arrangements of the dye ligands and the involvement of the p l or (32 nitrogen atom in the azo group, isomers can arise in certain complexes 1461. The iron and copper azo complexes have so far been little studied, probably because they exist as a wide variety of transitional structures and are difficult to isolate as pure 1:l or as 1:2 complexes. Metallization causes a bathochromic shift in the shades of the metal free azo dyes [471. While the dyes lose some of their brilliance, the wet and light fastness properties are markedly enhanced. In view of the demand for increased light and wet fastness, over the last few years research into new leather dyes has tended to focus on metal complexes rather than their unmetallized counterparts. Today, principles underlying the structure and synthesis of metal complex dyes are well known [481.

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4.7.4 1:l Chromium complexes To give level dyeings on wool, the 1:lchromium complexes have to be applied in the presence of sulfuric acid; but such conditions are not possible with leather. They exhibit the usual behaviour of low molecular weight dyes, viz., good penetration and weak shades. Surprisingly their fixation and hence their wet fastness properties are outstanding, which may be a consequence of the amphoteric character of these dyes. A salt-like bond forms between sulfonic acid groups of the dye and the free amino end-groups of the leather or with the cationic chrome tanning agent. Additionally, a salt bond forms between the cationic chromium of the dye and the anionic groups of tanning agent and collagen. An exchange of ligands between the water molecule in the chromium complex and the collagen or the anionic tanning agent is also possible. To prevent the dye from reverting to the betaine structure - the isoelectric point lies at about pH 3 - it is frequently recommended to fix the dye with acetic acid rather than with formic acid. This is believed to provide good wet fastness. 4.7.5 1:2 Cobalt and chromium complexes Since the 1:l chromium complexes can easily be converted to 1:2 complexes with complexable azo dyes, it is also possible to produce unsymmetrical chromium complexes. This opens up an extraordinary number of combination options for producing brown and olive shades, along with navy blue, grey and a variety of black shades, enormously extending the shade spectrum of chromium complexes. Yellow, red, blue and black are also obtainable with symmetrical complexes. 1:l Cobalt complexes are almost impossible to isolate because they convert too rapidly to the 1:2 complex. Since the bathochromic shade shift accompanying cobalt complex formation is not as marked as with chromium, the cobalt complexes are chiefly encountered in the yellow, light brown, orange and bluish grey shade areas. The light fastness of the cobalt complexes is often somewhat better than that of the corresponding chromium complexes. 4.7.6 Copper dyes Both the 1:l copper azo complexes that were often used in the past, and the so called copper formazan dyes that are still occasionally encountered in cotton dyeing are now rarely employed in the leather sector. Only the copper phthalocyanines, important in the fashion shades of turquoise and green, are still prominent. Their light fastness is outstanding on leather. 4.8 Special metal complexes for leather 4.8.1 Disazo complexes Disazo 1:2 complexes containing iron, chromium and cobalt are outstanding as leather dyes. The leading disazo dyes are made from resorcinol derivatives and various arylamines, with the latter including bi-couplable hydroxyaminonaphthalenes.

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The 1:2 leather complex dyes proper are often resorcinol (and amino variant) complexes, usually disazo or trisazo dye types, which were developed in the nineteen-thirties exclusively for leather [49] in common with the brown polyazo dyes. They already contained the essential sulfonic acid groups, though these dyes oRen appeared unsuitable for wool. The same also applies to the disazo dyes derived from hydroxyaminonaphthalenes. 4.8.2 Iron complexes Soluble iron complexes are extremely unstable and therefore hardly suitable for textile dyeing. On leather, however, they have given very good results. Homogeneous iron complexes are almost impossible to isolate. When applied on leather, however, at about pH 5 under the usual conditions they give attractive deep brown, dark brown and also black shades [50]. Surprisingly, once the dyes have exhausted onto the leather they are extremely stable to acid. This is true of all 1:2 metal complexes, though if applied at too low a pH (below 3.5) they can revert to the unmetallized starting form or the 1:l complex, possibly causing serious dyeing problems. In addition, iron complexes can be decomposed by sequestrants [51].

4.8.3 Sparingly soluble metal complexes The advent of the synthetic polyamide fibres brought a further field of application for the insoluble 1:2 metal complexes. A kind of disperse dye for the highest requirements on polyamide was developed by fine dispersion of these complexes [52}.Replacement of the cation (usually sodium or potassium) by organic amines gave solvent-soluble dyes. These dyes were outstandingly suitable colours in the finishing process. In the 1950s, when the first range of wool dyes based on an anionic 1:2 metal complex appeared on the market, a new development from which leather also ultimately benefited. Dyes with sulfonamide groups proved especially useful for dyeing wool from an aqueous solution[531. On leather, these types of dye had the reputation of being purely surface dyes. They were therefore only used for top dyeings.

5. NEW DEVELOPMENT TRENDS IN LEATHER DYEING The dyes and drum dyeing systems introduced over the past two decades are the h i t s of widely differing paths of development. The use of liquid dyes and dyeing with computer recipe prediction are clearly on the advance, with soluble sulfur dyes being increasingly recommended for through-dyeings on leather. Drum pigmentation is a new and promising application technique. The use of reactive dyes for improved wet fastness is frequently discussed, but such dyes are used only for special cases.

500

Judging trends often involves very personal considerations, and while the author cannot claim to be unbiased, it is hope that the views presented in this section will stimulate discussion and new research. 5.1 Computer recipe prediction Modern colour theory teaches that all shades can be matched with the three primary colours: yellow, red and blue [541. The technique involved in doing this is known to dyers and printers as trichromatic dyeing or printing and is used on a wide scale in the paper and the textile industries. For leather dyeing only ideally compatible dyes are suitable for a trichromatic system with computer or colour triangle recipe prediction. The advent of this new dyeing procedure affords the leather dyer new scope for rationalisation [551. Besides fastness properties and price, the dyeing behaviour of a dye is crucially important. The compatibility of dyes in particular is a matter of special concern to the leather dyer, first and foremost for trichromatic dyeing. Specialists long regarded this procedure as impractical for leather, allegedly owing to poor dye compatibility and affinity differences between flesh and grain sides. The traditional recommendation for dye combinations has been to choose as the main dye one as close in colour as possible to the customer’s sample and then to shade it. That of course meant that the dyer had to have a large selection of dyes available. Consistent application of the new dyeing principle yields economic advantages that are not to be underestimated. An extensive palette of shades can be covered with just a few dyes, with the bonus that the dyer does not have to be familiar with too many types of dye and their properties. This reduces not only the laboratory costs for recipe prediction and the time for scaling up from laboratory to bulk production, but also the costs for dye storage. A n extra bonus is the possible use of computer-controlled dispensing equipment whereby the probability of errors and inaccuracies occurring can be greatly limited. Today’s requirements for trichromatic dyes are good fastness properties, high colour strength, good levelness, homogeneity, low sensitivity to salt and simple dyeing [561. When selecting dyes for a leather trichromatic system, the overriding consideration is the compatibility of the component dyes. This compatibility depends on their chemical structure, method of synthesis and formulation and takes into consideration not just the molecular weight but also the molecular volume and the hydrophilidhydrophobic balance. If the dye components are not compatible minute changes in substrate, dyeing method or dye correction cause large shade variations. In the scientific investigation of compatible dyes [57], equal dyeing kinetics and dyeing equilibrium should be checked. On the kinetics side, increased attention should be paid to looking for synchronized exhaustion and diffusion of the dyes into the interior of the leather to ensure ideal dyebath exhaustion of the trichromatic system. For computer recipe prediction, it is essential that exhaustion rate and saturation points are very well balanced, otherwise, the compatibility of the dyes is impaired and the computer colour prediction equipment is overstretched.

50 1

Depending on the retannages and leather types, different shades will be obtained and correction will become too sophisticated for the mathematics of the computer program. The secret for compatible dyes is equality not in one criterion but in as many as possible. It is important for the tanner to know what range of shades (in colour space terms) a trichromatic dye combination can cover at a given shade depth. As we know, the absorption curve of a dye changes with increasing concentration. This phenomenon can also be demonstrated for the trichromatic dyes: the deeper the dyeing, the brighter or more saturated the dye. If the concentration is too high, however, unsuitable dyes become duller again. That means that the dyes bronze and are deposited only on the leather surface without being linked to the leather substrate by chemical and physical bonds. To avoid this situation the trichromatic dyes have to be readily soluble and must not be affected by salt or hard water [581. This is another of the most important selection criteria of our trichromatic dye range. The dyes we need for dyeing leather are not pure spectral colours. A yellow dye, for example, contains small amounts of a green and a red dye and can have a sharp or broad absorption maximum. In a mixture with a blue dye, the red portion of the yellow dye dulls the resulting green colour. The result is a shade perceived as olive. Conversely, the green shade turns out brighter with increasing amounts of green component in the two starting dyes, added to which a steep absorption maximum is required by the initial dyes. This phenomenon is called subtractive colour mixing and is customary in dyeing. Every dye addition means that less light is reflected and the dyeing becomes darker but also more intense. To produce dark shades it is therefore better to mix dull dyes with a broad absorption maximum rather than bright dyes with a narrow absorption maximum. The explanation for this is that dull shades already contain some of the complementary colour and therefore appear stronger. As a case in point, a brown dye contains orange as its main component and a small proportion of blue as a complementary colour. Consequently to darken this brown dye less blue is needed than if the starting dye was a bright orange. For economic reasons colours with broad absorption like 1:2 metal complex dyes are used for standard shades, and for bright shades dyes with intense absorption like aciddirect dyes are the most suitable. The basic stock for a leather dyer should comprise a yellow, a red and a blue dye rounded off with a black and possibly a brown dye. Alongside these there is a small selection of brilliant dyes for the bright and less common fashion shades. Besides for self-shades they may be used for brightening brown and dark combination shades. Parameters must be considered not just in isolation but also as a coordinated and well-balanced combination. Modern technology enables dyeing recipes to be worked out rapidly and accurately by means of a computer [591. The recipes suggested by the computer only need checking [60]. By suitable programming it is also possible to select the recipe that is the ideal mix in terms of price and dye combination. On

502

top of that shade variations under different illumination can also be calculated and an optimum dye combination selected.

5.2 Trichromatic dye systems It was not until the 1960s that a range of wool dyes with a sulfonic acid group in the unsymmetrical chromium complex became available, and another ten years later the symmetrical disulfonated dyes were commercialized. The introduction of sulfonic acid groups into the 1:2 complexes improved the dyes’ hydrophilic properties to the point that they could also readily dye leather and fLll advantage be taken of their good light fastness. 1:2 Complexes containing two sulfonic acid groups have to be applied to wool in the presence of auxiliaries before they can achieve adequate levelness. On leather, however, excellent levelness is obtainable with 1:2 metal complexes containing two sulfonic acid groups. Leather dye research was re-activated and the simple monoazo ligands were weighted with additional azo couplings to optimize them for leather. Finally, in the nineteen-eighties, the first trichromatic dyeing was achieved on leather with simple 1:2 metal complex dyes, making possible computer recipe prediction and control. Shade consistency is a prerequisite for reproducibility and an absolute must for computer recipe prediction. Shade consistency can be ascertained by the ABC test in which dye mixtures are applied on standard chrome leather in the presence of syntans having either a marked or a slight blocking or shade deepening action. In this test A is the standard chrome leather, B and C and others are the same leather retanned with different types of syntan. Shade consistency [611 means that all dyeings have the same shade - but may vary in intensity, whether:

- produced by different methods, - applied on different leathers, and - pre-treatment or after-treatment employed. If the above conditions are met, an established dye recipe will tolerate small variations and minor differences in retanning and fat liquoring without any change in shade. Shade matching becomes much easier. Before a trichromatic range of dyes is created, various characteristics have to be laid down and a structurallactivity programme is drawn up for each key characteristic with a view to optimizing the dye structure. The various basic structures are then improved by chemical substitution to give the best possible chemical and physical agreement between the key characteristics. Serial tests have shown that 1:2 metal complex dyes (Metal = Me/2 )with two sulfonic acid groups best match the requirement profile [621. As a rule the choice is:

503

Figure 7. Yellow, an arylazo-acetoacetanilide system

Figure 8. Red, an arylazo-pyrazolonesystem

Figure 9. Blue, an arylazo-naphthol system. Nevertheless, even within this dye type, the basic modules have widely differing structures. Implementation of this new trichromatic dyeing philosophy has already made good progress in tanneries and, as shown in these examples, is continuing to make steady headway. For the tanner this future-oriented research means a further significant step towards rationalization and cost saving.

5.3 Sulfur dyes

Sulfur dyes are water insoluble, macromolecular, coloured compounds formed by treating aromatic amines, aminophenols, with sulphur andor sodium polysulfide.

504

These dyes were developed as long ago as 1893 by R. Vidal but they only became attractive for leather with the introduction of water-soluble groups. They are eminently suitable for through-dyeing leather and because of the recent upsurge in demand for through-dyed leather this class of dyes has experienced a kind of renaissance. The physical form - fine disperse insoluble, leuco or watersoluble - plays a crucial role. "he conditions of manufacture are also important. Today, the number of commercially available sulphur dyes is small but their production volume is large. C.I. Sulfur Black 1with all conceivable variations may be the biggest synthetic dye. In spite of numerous advantages, ecological problems regarding sulfur by-products have to be taken into account by the manufacture of these dyes and by tanneries applying them on leather. All shades can be produced with the water-soluble dyes containing thiosulfate groups. However, these dyes seem to react with the leather substrate more slowly than their sulfonate group-containing counterparts, which give penetrated dyeings with a weak surface shade. 5.4 Drum pigmentation A number of tanners have recently evinced growing interest in drum pigmentation [63], chiefly because it can reduce the accentuation of defects caused by grain damage including pinholes, mite attack, fungal infections, dung-induced burns, curry-comb scratches, wire scratches and damage caused by warble flies. Pinholes are a particular problem because at the sorting stage it is difficult for the tanner to tell whether a skin is damaged. The extent of the damage is only fully apparent aRer dyeing, when the pinholes are lighter in shade than the rest of the leather. In view of the increasing incidence of pinholes, a commission was set up in Germany in the 1990s to investigate the phenomenon. It was found that this damage can be better covered with pigments. A few dye-producing firms then optimized drum pigmentation in different ways. It is an open question whether this method will catch on because although the results achieved are surprisingly good it still seems very difficult to apply. Drum pigmentation competes with the wellestablished technique of pigmentation in finishing. A n argument in favor of drum pigmentation is the fact that in keeping with today's '?back to nature" trend, aniline and semi-aniline leathers, without a top finish, are increasingly in demand. Drum application of pigments has been practiced for years for white leathers. From the technical standpoint dispersed pigments are preferable to titanium dioxide in powder form. Decisive for the whiteness and the handle is the question where and when the pigment is to be applied. The highest whiteness and the pleasantest handle are obtained when the pigment is added before fat liquoring. If the pigment is added before retanning, the whiteness is inadequate and if it is added aRer fat liquoring, the handle is too harsh. The use of black pigments in combination with dyes has also given good results. Bright shades are often produced by a separate addition of anionic dyes and pigments. Dyeing method and selection of dyes are crucial for the quality of the leather. As the pinholes are coloured darker by the pigments and lighter by the

505

anionic dyes, the two effects have to be coordinated. This can be greatly simplified by computer recipe prediction, which works out in advance the respective amounts of pigment and anionic, water-soluble dye. In addition, the affinity of the colours too has to be optimally coordinated by the retanning agents and auxiliaries. The suggestion for upholstery leather is first to through-dye with a mixture of pigment and anionic dyes, or with anionic dyes alone, and then to fat liquor. The defective parts are prepared with a chrome syntan in such a way that the dyeings can be ideally levelled. After pigment coloration the leather is again treated with anionic trichromatic dyes to apply the top dyeing. A final treatment with cationic auxiliaries further improves fixation. The dyeing performance of the pigments in the drum pigmentation of leather is largely influenced by the size of the particles and the dispersant [641. Organic micropigments should have a particle size less than 0.1 pm for good tinctorial strength, high transparency, moderate covering power and improved levelling. With these micropigments leather can be dyed through. Particles of about 0.5 pm in size, which characterize many inorganic pigments, are less suitable because they have low tinctorial strength, low transparency, and their covering power is too strong. Dyeing auxiliaries are essential if good results are to be obtained. Pigments respond very markedly to natural unlevelness such as that of loosely structured areas. In drum pigmentation the transparency of the leather surface, which is otherwise lost by finishing, is largely retained. An outstanding advantage of the pigments is their unequalled high light fastness. The often criticized drawback of poor wet and dry rubbing fastness can also often be overcome by a judicious selection of pigments and auxiliaries. And last but not least, the selection of dye dispersants is crucial. But pigments are almost invariably applied in conjunction with anionic dyes. Pigment dyeings can easily be overdyed with anionic dyes. The dyeing procedure is to work out the pigment dyeing and the dyeing with 1:2 metal complex dyes tone-in-tone. The pigment dyes are used for through dyeing. Of the anionic dyes about 66%are used for through dyeing and the remaining 33% for the top dyeing. Before the final addition of the anionic top dyeing the actual and set values of the half-finisheddyeing can be compared and any correction required carried out. 5.5 Reactive dyes

Although reactive dyes have the potential to provide the best fastness to wet treatments, that potential has only been realized in some small segments. In 1951, the first reactive dyes were designed for cotton and it was assumed that a covalent bond is formed between dye and fibre during the dyeing process. Reactive dyes are mostly azo dyes which contain one or more groups capable of forming covalent bonds between a carbon atom of the dye and an oxygen or amino group of the substrate. On leather reactive dyes attach to the amino group of lysine and hydroxy-lysine moiety of collagen. When more then one reactive group reacts, a tanning effect occurs. However, the reaction of the electrophilic group of reactive dyes with water (hydrolysis) competes with the fixation reaction of forming a

506

covalent bond between the dye and the substrate. The hydrolyzed dye cannot react with the fibre, but will be absorbed like an anionic dye [651. The affinity of reactive dyes has to be adjusted to the conditions of application. A high ratio of fixation to hydrolysis is therefore an important requisite for high fixation and hence for practical usefulness. In addition, the wash fastness of reactive dyed leather depends on the stability of the dye-fibre linkage. In the patent literature more then 200 different reactive groups have been mentioned. In the 1970s, reactive dyes with 30 special reactive systems were produced. The fixation rate of the first reactive cotton dyes was not very high, however, and unfixed dyes could be rinsed out. An essential aspect of the first reactive dyes for cellulose fibres is that after the reactive dye has been sorbed from a neutral dyebath into the fibre, the pH value has to be increased for dye-fibre bond formation. If the pH is too high, however, it is unsuitable for leather. The fixation rate of the first reactive dyes was less than 50%. Leather absorbed the non-covalently bound dye like a conventional anionic dye. Unlike on cotton it could not be washed off, so no wet fastness improvement could be achieved. The bi-reactive dyes for cotton and wool are certainly even more suitable. But the restricted availability of the free amino and hydroxyl groups in leather is still a problem, so that the desired good wet fastness can only be achieved with specially selected reactive dyes and optimized dyeing conditions. Today, reactive dyes are only used for a few selected high quality leather goods, which require good wet fastness, e.g. gloves and sports shoes. 5.6 Liquid dyes

For the past 20 years the preference in leather dye formulation has been for liquid systems, especially those incorporating polar solvents. These liquid dyes can then be further diluted with either water or organic solvents. Metal complex dyes with limited water solubility exemplify such liquid dye products, which are typically used for spray dyeing, curtain coating and printing [661. They are also the preferred dyes for immersion dyeing. Recently these products have been made with a higher water content [671, but still with a considerable solvent component, and they have also been recommended for drum dyeing. Depending on the type of solvent used there is often a noticeable contamination of both wastewater and air. Non-volatile solvents in particular can persist for a long time in the leather and consequently can have a deleterious effect. Therefore, research was focussed on the true water based liquid formulation. With their variety of hues and fastness properties, powder dyes form the largest part of the leather dye range. As a result, there are many occasions on which liquid dyes must be used with powder dyes, which can oRen be a problem in practice. This explains the popularity of the conventional liquid dye types for those blacks for which no shading is necessary as well as for the trichromatic systems which have a wide range of applications with a small number of dyes. The fact that some leather dyes have very high solubility has made it somewhat easier to create aqueous liquid dye formulations. With this aim in mind it was

507

found necessary with some of the dyes to use special procedures to remove common salt, which is a by-product of the dye synthesis. This can be achieved by reverse osmosis or isolating dyes at low pH. A change of cation may also help to improve solubility. The liquid dyes should be measured by weight. Changes in density with temperature make volumetric measurements inaccurate and therefore unacceptable.Nevertheless the liquid dyes still offer numerous advantages. The benefits include ease of dissolution. A good solution is essential for the correct dyeing results. If the dye is not properly dissolved, or has poor solubility, problems can arise. Stable predissolved liquid commercial products are ideal, eliminating the problems at their source, especially with the large batches that are often dyed today. Conditions of high humidity, which are prevalent in laboratories and dye houses, are very bad for powder products. These are often somewhat hygroscopic and can absorb moisture. This may lead to weighing errors and consequently to formulation and recipe errors in laboratory trial dyeings as well as in plant production. Hygroscopicity can also lead to the formation of lumps if drums of powder dye are left standing open for long periods. These lumps can be difficult and unpleasant to dissolve as standardization or anti-dust agents often turn gluey when moist. Liquid dyes have a constant water content, they are not hygroscopic and any possible evaporation of the water is minimized if the drums or containers are closed properly. The concentration of powder dyes is standardized with inorganic salts and soluble organic compounds. Under certain circumstances these components can increase the hygroscopic behaviour of the dyes. Apart from this drawback such salts are also discharged into the effluent. Liquid dye containers are easier to empty completely and easier to rinse free from any residual dye than powder dye drums. Even with the use of low dusting dyes or dust-free dyes in the dye house it is difficult to totally avoid creating dust when weighing large quantities of dyes. Sudden changes in air circulation can easily lead to contamination of other dye samples. The formation of dye dust in the dye house has also a negative connotation for those working there. By contrast, the handling of liquid dyes, even in large amounts, is without question cleaner and therefore more hygienic. The concept of liquid dyes offers considerable advantages in the use of those computer controlled dosing systems that already exist in some tanneries. Here the liquid tanning agents and chemicals are taken directly from containers and tanks. These dosing systems are very efficient and have proved themselves in practice, especially in the case of liquid pretanning and retanning agents as well as for fat liquors. While for liquid chemicals a degree of weighing accuracy of * 50 - lOOg is satisfactory in the plant, for liquid dyes a precision of * 5g should be aimed for. This precision can now be achieved and it opens the way to the use of computer controlled dyeing processes. These systems can almost exclude the possibility of

508

human error, simplify handling when weighing large amounts, and make an important contribution to improving the quality of work. On the basis of space requirements and for cost reasons it is only possible to justify a small number of tanks for a dosing system. Some of these will be necessary for tanning agents and chemicals and only a few tanks will be available for dyes. Using dosing systems yields improvements in work efficiency, which should increase the ability of the dyer to meet his customers’ orders in time. Use of computer controlled dosing and better weighing conditions can give an improvement in the working conditions and in safety. It is possible to work more rationally and economically while at the same time having working conditions which are less costly, better ecologically and more hygienic as well. In finishing, liquid dyes are often used to correct the shade. Depending on the application coats, water or solvent soluble dyes are applied. These provide a natural cover, avoiding or reducing heavy finishing and the resulting plastic appearance. A more semi-aniline leather type can be achieved then with heavily pigmented coats. 6. NOMENCLATURE AND STRUCTURE OF LEATHER DYES 6.1 Availability of leather dyes Around 1300 leather dyes are listed in the Colour Index, with more then 900 Acid and more then 200 direct dyes. However, an even larger number are suitable and actually used for leather. Conversely not all the dyes listed are manufactured and commercially available nowadays. The chemical structure of many dyes recorded in the Colour Index (C.1.) has not been declared. 31% of leather browns and 14%of leather blacks structures are not disclosed. The C.I. name refers to the colorant only and not to the commercial preparation, purity of the main component, balance of synthesis related by-products, shading elements and their diluents, presence of dispersants and other chemicals. As a consequence they can differ in technical properties, concentration and eco-toxicologicalparameters [68]. 6.2 Structural formulas of leather dyes Leather dyes are found in the Colour Index among:

C.I. Acid dyes C.I. Direct dyes C.I. Mordant dyes C.I. Reactive dyes C.I. Solvent dyes C.I. Solubilised sulfur dyes The C.I. Natural and C.I. Basic dyes are chiefly of historical interest, being little used today. The (3.1.Oxidation bases for fur are of only minor interest. Fluorescent brighteners are not used in drum application but sometimes to improve the brilliance of the hair side of fur and occasionally for finishing. Finally, the inorganic

509

and organic C.I. Pigments mainly encountered in finishing should not be overlooked. The most important leather dyes identified by typical chemical nomenclature are nitroso and nitro, azo, stilbene, triarylmethane, xanthene, quinoline, azine, oxazine, thiazine, sulphur and phthalocyanine. The C.I. Constitution number is given in this paper, as a guide to references for synthesis in the Colour Index. Any duplication of these syntheses should be attempted only under the supervision of an experienced chemist who is aware of the latest ecological, toxicological and safety requirements. These requirements must be constantly observed even if attention is not explicitly drawn to them. Many dye synthesis recipes have been given by Fierz-David [691, and mainly azo dyes are reported in Ref [701. A hrther point to note is that dye chemists often write the free sulfonic acid form, even if they have obtained a salt. The reason is not idleness but the dyes are often not completely neutral or may not have a definite salt. This commonly arises when potassium or sodium salts are involved and therefore the free sulfonic acid formula is frequently used as a general term. A sulfonate salt does not influence the colour as such, but water solubility is influenced. Consequently, for pigments the cation is always given in the chemical formula. 6.2.1 Natural wood dyes Dyewood extracts can be considered as mordant dyes. Typical shades obtained by laking with potassium alum are listed below.

Table 1 Lake of dyewood [711 Fustic Yellow Brazilwood Red Logwood Violet

(yellowwood) (redwood) (bluewood)

Fustic C.I. Natural Yellow 8, 11 C.I. Constitution 75660, 75240

510

Morin

'

O

I

M

I

Maclurin

o OH

H

OH

OH

H HO

0

o \

HO

d \

Brazilwood Natural Red 24 C.I. Constitution 75280

Brazilin

Brazilein

HO

HO

HO

OH

Logwood Natural Black 1,2, (3,4 Al, Cr, Fe or Sn lakes) C.I. Constitution 75290, (75291)

HO

0

OH

511

Haematein

Haematoxylin

YH

Ho%H

HO

HO\

9H

0 HO

OH

6.2.2 Azine and oxazine dyes Azines are predominantly of historical interest. The first commercially used synthetic dye, Mauveine, belongs to this group. A large number of azines were produced in the early decades of industrial dye manufacture. One of the most important for leather was Safianine T. The synthesis was an ‘all-in’ operation involving a sequence of oxidation reactions. Mauveine for example was prepared by oxidizing aniline containing o and p-toluidines with potassium dichromate in cold dilute sulfuric acid solution. Yellow, red, brown, blue and black shades can be obtained with these dyes.

Mauveine Basic Dye C.I. Constitution 50245

Safianine T C.I. Basic Red 2 C.I. Constitution 50240

512

Some bright blue direct dyes with good light fastness are sulfonated dioxazine compounds, mostly synthesized by condensation of chloranil with amines followed by ring formation and sulfonation. C.I. Direct Blue 106 C.I. Constitution 51300

6.2.3 Nitro and nitroso dyes Nitro dyes comprise o and p-nitro phenoldnaphthols and amines and are mostly

yellow, orange or yellowish red brown. The oldest synthetic nitro dye is picric acid, discovered by Woulfe in 1771 involving an oxidation reaction on natural indigo. It dyes leather from an acid dyebath, giving greenish yellow shades with poor fastness to light and washing. Nitro dyes are prepared by the action of nitric acid on aromatic hydroxy or amine components. Some of them have to be handled carefully a s they can act as explosives, like picric acid. Picric acid C.I. Constitution 10305

513

NO*

Nitroso dyes contain nitroso groups in the o-position to a hydroxyl group and are prepared by the reaction of nitrous acid with mostly 2-naphthol and resorcinol derivatives. Nitroso dyes are not of much interest for dyeing, only their metal complexes being of any importance. They behave as mordant dyes and form chelate complexes. Iron 1:3 complexes have a dull green shade with very high light fastness. C.I. Acid Green 1 C.I. Pigment Green 12 (barium salt) C.I. Constitution 10020

L

J

6.2.4 Triarylmethane (triarylmethine)dyes Triarylmethane dyes give red, violet, blue and green shades on leather. Light fastness is unfortunately mostly poor. Industrial production follows a standard principle, but with various modifications:condensation of an arylaldehyde with two molecules of substituted arylamines or phenols in the presence of acid or condensation of formaldehyde with three molecules of the same amine or phenol structure but in the presence of oxidizing agents. The oldest representative is fixhsine or magenta. The central carbon is sp2 hybridized and therefore the name triarylmethine proved a better chemical description. It also demonstrates the correlation between these dyes and the polymethine species.

514

Fuchsine C.I. Basic Violet 14 C.I. Solvent Fkd 41 (free base) C.I. Pigment Violet 4 (phosphotungstomolybdicacid salt) C.I. Constitution 42510 YH3

+

Variations of these triamino derivatives are pararosaniline (without methyl groups) and new hchsine (with three methyl groups, supplied by ortho-toluidine), both of which were important ruby-red leather dyes. Highly methylated pararosaniline gives Methyl Violet, discovered by Lauth in 1861. The pure hexamethylated variation is called Crystal Violet. Arylation gives a greenish blue like the triphenylated “aniline blue”, which is insoluble in water. The weak cationic character can be changed by sulfonation. Monosulfonated ‘alkaline blue” dye is soluble in hot water and is used as an acid dye for wool and silk and printing dye. Disulfonated “ sea blue” is used as a leather dye and for bluing paper and washed textiles. Finally, the trisulfonatd dye is used as ink blue. The most important diaminotriphenylmethane is Malachite Green, produced by condensation of benzaldehyde with N, N, -dimethylaniline, Table 2 Triphenylmethine dyes

515

Ink blue Malachite green

I Acid Blue 93 1 Basic Green 4

I 42780 I 42000

6.2.5 Acridine and xanthene dyes The synthesis of acridines and xanthenes is in principle similar to that of triarylmethanes, condensing mta-diaminoaryls or meta-aminophenols with aldehydes, cyclizing, and subsequently oxidizing. The dyes are basic and have been used in the past for leather dyeing, giving yellow, red, orange, violet and brown shades. Light fastness is mostly poor but the shades of xanthenes are remarkably bright, and some of their solutions are strongly fluorescent. Acid dyes can be obtained by sulfonation. Phospine (acridine) C.I. Basic Orange 15 C.I. Constitution 146045

+

C.I. Acid Violet 30 (sulfonatedxanthene) C.I. Constitution 45186

so,COOH

6.2.6 Polymethine (quinoline)and stilbene dyes

516

A chain of methine groups characterizes these dyes, viz., a system of conjugated double bonds. The best known of these dyes are the natural carotenoids, which are scarcely used as leather dyes. This is also true for the methine and polymethine dyes listed in the Colour Index. For leather the quinoline chromophore, prepared by condensation of quinoline derivatives with phthalic anhydride or similar substances, is of some interest. The mainly yellow or red compounds are sulfonated and yield valuable acid dyes for leather. C.I. Acid Yellow 2 C.I. Constitution 47010

The stilbene dyes contain ethylene groups and are synthesized by condensation of 4-nitrotoluene-2-sulphonicacid in aqueous alkaline solutions with other aromatic amines, often in the presence of reducing agents like glucose. Walther obtained the first direct dyes by self-condensation.So-called Sun Yellow is a mixture of different components, depending on the concentration of sodium hydroxide, the temperature, and the duration of the reaction. Oxidation of the intermediate Sun Yellow, then reduction with iron and hydrochloric acid gives 4,4’ diamino-stilbene-2,2’-disulphonic acid, which is used for fluorescent whitening agents and azo dyes. The shades are mostly yellow to red. Sun Yellow (one of the main components) C.I. Direct Yellow 11 (3.1.Constitution 40000

517

6.2.7 Anthraquinone dyes In the past, natural alizarine was obtained as a glycoside from the madder root and has been linked with civilization for thousands of years. Mordanting with alum created the red colour of the famous cordovan leather (Spain). However the main colour of the anthraquinone dye class is blue. Graebe and Liebermann elucidated the structure in 1868. Alizarine was prepared by heating 1,2 dibromoanthraquinone with alkali. Anthraquinone as such can be synthesized by oxidation of anthracene or by condensation of phthalic anhydride with benzene. Anthraquinone dyes are the classical source for vat dyes discovered by Bohn in 1901. The German word for vat days is Indanthren, an acronym of 'indigo from anthracene'. Bohn's intention was to synthesize the classical vat dye indigo from 2aminoanthraquinone instead of aniline. Vat dyes are not applied to leather. "he alkali used for the reduction of the vat dyes would destroy the leather substrate. In 1869 Caro succeeded in performing the sulfonation step, and the anthraquinone dyes could be used to colour leather. The sulfo groups can be present in the anthraquinone orland in the substituent. Alizarine C.I. Natural Red 6,8,10,11,12 C.I. Constitution 75330 C.I. Mordant Red 11 C.I. Pigment Red 83 (ofken aluminium complex) C.I. Constitution 58000

C.I. Acid Blue 25 C.I. Constitution 62055

*

518

S03Na

\

0

HN /

C.I. Acid Blue 27 C.I. Constitution 61530

CH3 6.2.8 Azo dyes

The characteristic feature of the azo dyes is the presence of the azo group -N=N-. These dyes are manufactured by treating a primary arylamine with nitrous acid in an aqueous media to obtain the diazonium salt, which is then coupled without isolation, with an arylamine, a hydroxyaryl or a keto compound capable of enolization. More than one thousand azo compounds have been described and they include the most important synthetic dyes for leather and textiles. Almost all shades can be obtained with them, as a general rule [721 (see Table 3).

Yellow Orange, red, blue Brown, black

Aryl-N=N-alkylheterocycleor phenyl-N=N-phenyl Aryl-N=N-aryl Aryl-N=N-aryl-N=N-aryland polyazo

519

Azo dyes with naphthalene moieties give deeper shades then those with phenyl residues. Typical leather dyes are shown below including stilbene, thiazole and developed dyes: C.I. Acid Yellow 11 C.I. Constitution 18820

C.I. Direct Yellow 18 C.I. Constitution 13930

C.I. Direct Yellow 12 C.I. Constitution 24895

520

Orange I1 C.I. Acid Orange 7 C.I. Pigment Orange 17 (barium salt), 17:l (alum salt) C.I. Solvent Orange 49 (amine salt) C.I. Constitution 15510

C.I. Acid Orange 12 C.I. Pigment Orange 18 (calcium salt) C.I. Constitution 15970

\

S0,Na

C.I. Acid Red 4 C.I. Constitution 14710

o0:+ N

\

S0,Na

C. I. Acid Blue 92 C.I. Constitution 13390

/

521

Bismarck Brown I (m-phenylendiamine) C.I. Basic Brown 1 C.I. Solvent Brown 41 (free base) C.I. Constitution 21000

Analogues Bismarck Brown I1 (toluene-2,4-diamine) C.I. Basic Brown 4,Solvent Brown 12, C.I. Pigment Brown 3 (phosphotungstomolybdicacid salt) C.I. Constitution 21010 Acid Brown 123 C.I. Constitution 35030

522

Patent Fustin C.I. Mordant dye C.I. Constitution 20030

HO

%H

C.I.Direct Black 80

C.I. Constitution 31600

HO

-

OH

523

8

Ho

\ /

N\\

S0,Na

S03Na

NaO,S/

Other syntheses of leather azo dyes are based on oxidation (mostly with hypochlorite) of arylamines. C.I. Direct Yellow 28 C.I. Constitution 19555

Reductive condensation of aromatic nitro groups, mostly with glucose, yields dyes containing azo or azoxy groups, such as Sun Yellow (see stilbene dyes). Developed dyes are a special class of azo dyes. They have a free aromatic amino group and, aRer dyeing, can be diazotized and coupled with a developer, usually 2naphthol. This procedure essentially completes the dye synthesis on leather. In the past this method was very popular with undeveloped benzidine dyes such as C.I. Direct Blue 2 to form a deep black. Today, benzidine has been almost entirely abandoned in dye synthesis, in view of its toxicologicalproblems.

524

C.I. Direct Blue 2 (navy blue, developed black) C.I. Constitution 22590

S0,Na 6.2.9 Azo mordant and metal-complexeddyes The most important metal-complexed dyes are chromium, iron and cobalt azo dyes. Nickel complexes are scarcely used nowadays, but copper complexed azo dyes can occasionally be found among reactive dyes. Non reactive copper complexed leather dyes are rare. Of the 1:lazo metal complexes developed in the 1920s, almost the only ones left are those containing chromium, which are fairly easy to prepare in the pure state. They are obtained by reacting in aqueous solution equivalent amounts of chromium (111) salts with the corresponding sulfonated monoazo dyes under pressure at fairly high temperature (120-130 "C). The 1:l chromium complexes arising from the acid chroming solution in the betaine form are insoluble in water and hardly suitable for dyeing leather. The betaine complexes formed are therefore converted into water-soluble dye sulfonates with a neutral hydroxy complex by treatment with caustic soda solution. For the 1:2 metal complex dyes cobalt (and iron) is used in addition to chromium. Formation of the 1:2 complex proceeds via the 1:l intermediate stage which, however, can be isolated practically only in the case of the chromium complex (see above) by shifting the equilibrium using high temperature and a slight excess of chroming agent. Therefore, it is possible to produce asymmetrical 1:2 chromium complexes. The reaction conditions depend very closely on the solubility of the starting dyes. Dyes difficult to chrome are often prepared with the 1:2 auxiliary complex of salicylic acid. Depending on the molar ratio of auxiliary complex to dye, the 1:2 dye complex forms, accompanied by complete development of salicylic acid, or alternatively, in the presence of an excess of the auxiliary complex the colorless salicylic acid forms a mixed complex with the dye. These types of dye yield the 1:2 complexes with the brightest shades but they are no longer used for leather. To produce 1:2 cobalt (111)complexes it is usually enough to disperse the coupling components in gently boiling water and add acid-binding agents such as sodium acetate. Readily oxidizable cobalt (11) sulfate or chloride is usually added as the metal salt, with or without oxidizing agent.

525

For iron 1:2 complexes in general, the dye is stirred with iron (111)chloride for a few hours in water at 80 - 100°C and pH from neutral to weakly acid. Copper (11) azo complexes are similarly synthesized, but they are sometimes prepared just by grinding the unmetallized dye with copper sulfate to form the 1:lcomplex. The shades of metal-complexed dyes are relatively dull by comparison with those of the non-metallized acid dyes. They are use chiefly to produce a variety of deep shades. Some typical examples are shown below. 1:2 Cr complex C.I. Acid Yellow 121 (disperse form without sulfonic acid groups) (2.1. Solvent Yellow 2 1 C.I. Constitution 18690

HO

1:2 Co complex C.I. Acid Yellow 151 (sulfonamide group, top dyeing) C.I. Constitution 13906

1:lor 1:2 Cr complex C.I. Acid Red 186 C.I. Constitution 18810

526

OH

' Y

Na03S-@N,h% -

\ / 1:2 Cr complex C.I. Acid Blue 193 C.I. Constitution 15707

NaO,S

-N S0,Na

H3C

$%( ,N

\ /

Cu complex C.I. Acid Violet 62 C.I. Constitution 14646 ,OH

1:2 Cr complex (2.1. Acid Brown 143 C.I. Constitution 20260

\ /

-

\ /

527

bH

Fe complex C.I. Acid Brown 216 C.I. Constitution 34906

Na0,S

Cr complex C.I. Acid Black 82 c.1. Constitution 20265

S0,Na

528

6.2.10 Other metal complex dyes Other metal complex dyes are produced with meta-hydroxy-hydroxy,hydroxy-oxy, oxy-nitroso, hydroxy-carbonyl and similar groups (see wood, alizarine and nitroso dyes). The best light fastness and brilliant blue and green (turquoise) shades are obtained with copper phthalocyanine dyes. Phthalocyanines are synthesized by heating phthalodinitrile with copper chloride. C.I. Direct Blue 86 C.I. Constitution 74180

I--

1

6.2.11 Sulfur dyes The sulfur groups can be classified in three groups: the classical water insoluble, leuco, and solubilized sulfur dyes. Aromatic amines or aminophenols are sulfurized

5 29

either by dry heating with sulfur or sodium polysulfide by the so-called bake process or in water or under pressure as a solvent reflux reaction. Most sulfur dye structures are unknown. Many sulfur dyes contain benzothiazole groups. Almost the whole range of shades can be obtained. The dye manufactured in the largest volume could well be C.I. Sulfur Black 1, which is used as a penetrating dye on leather. The water-insoluble forms of these dyes can be treated with various reducing agents, to yield the water-soluble leuco sulfur dyes. "he latter are fairly unstable. The more stable water solubilized sulfur dyes can be obtained by the action of sodium sulfite or bisulfite on the parent sulfur dyes to produce functional thiosulfonic acid groups. Only the starting chemicals or intermediates are shown below. C.I. Sulfur Black 1 (refluxed sodium polysulfide) C.I. Leuco Sulfur Black 1 (2.1. Constitution 53 185 (presumably a phenothiazonethianthrene derivative)

C.I. Sulfur Red 6 ((refluxedsodium polysulfide)) C.I. Leuco Sulfur Red 6 C.I. Constitution 53720

C.I. Sulfur Green 3 (sodium polysulfide refluxed in presence of copper sulfate) C.I. Leuco Sulfur Green 3 C:I: Constitution 53570 C.I. Solubilized Sulfur Green 3 (thiosulfonic acid of 53570) C.I. Constitution 53573

530

Analogues C.I. Sulfur Green 2 (prepared analogously to Green 3, omitting the copper sulfate) C.I. Leuco Sulfur Green 2 C.I. Constitution 53571 (2.1. Solubilized Sulfur Green 2 (thiosulfonicacid of 53571) C.I. Constitution 53572 Hybrid sulfur dyes structures are rather new: e.g., C.I. Condensed Sulfur Orange 2 [731

The chromophore of this dye, however, is not representative of sulfur dyes. "he thiosulfate group is the only classifier. Very recent research and development have produced a true azo-sulfur hybrid [741 and also cationized sulfur [751 dyes for leather. These dyes will reportedly be put on the market soon [761. 6.2.12 Reactive dyes

Reactive dyes are colored compounds that contain groups capable of forming covalent bonds between dye and substrate. Approximately 80 to 90% of reactive dyes are azo dyes. The other chromogenic classes are anthraquinones, dioxazines, phthalocyanines and some 1:l copper azo complexes. The characteristic structural features of a reactive leather dye are as follows: -water solubilizing group(s) -chromogen

53 1

-bridge link (often amino) between chromogen and electrophilic group -electrophilic group with a -nucleofugicleaving substituent The trichlorotriazine molecule was the first reactive compound that was found to be able to form a reactive bridge between dye and substrate. One chlorine atom is substituted with the amino group of the dye and the other two chlorine atoms can react bifunctionallywith the substrate to form covalent bonds. Reaction with water is also possible and hydrolyzed dyes cannot form a covalent bond. Another reactive component for leather is the sulfatoethylsulfonylgroup. C-I. Reactive Yellow 4 C.I. Constitution 13190

NaO,S*

C.I. Reactive Blue 4 C.I. Constitution 61205

532

1:1Copper complex C. I. Reactive Red 6 C.I. Constitution 17965

6.2.13 Powder, liquid, and solvent dyes including pigment dyes Water-soluble, solvent-soluble or pigment dyes for drum dyeing or for finishing can be obtained, depending on the hydrophobidhydrophilic character of the chromogen and its form in isolation. The typical and best-suited substituent for synthesizing readily water-soluble dyes is the sulfonic acid group. An aqueous form can only be launched successfully if the dye solution is free of salt. Sometimes the aqueous reaction solution can be used as such. However, this gives a formulation with a high tendency to precipitation. Better results can be achieved by removing salt by reverse osmosis, ultra-filtration or by a standard filtration of dyes in the free acid form. ARer filtration the dye should be neutralized to a pH of 6-9 with sodium hydroxide to form the sodium salt and improve solubility. The lithium cation may provide even better water solubility. For the normal powder form the water-soluble dye is isolated with sodium chloride or sulfate. Potassium chloride is oRen used to start precipitation. A further option is the use of a spray dryer for the reaction solution, but impurities will remain. Solvent dyes can be made using ammonia as cation, or special tetraalkylamines. Many solvent dyes contain no sulfonic acid groups, only hydroxy or amine groups. Alcohols or glycols or mostly used as solvents. Solvent dyes are often applied in finishing to improve brilliance of shades. Nowadays, water based finish recipes, which are combinations of solvent with water, are commonly preferred. The sulfonate group with a calcium or barium cation gives the classical organic pigment obtainable with most azo dyes. In addition we find many pre-metallized mordant dyes without sulfonic acid groups in them as pigment or solvent dyes. Cationic dyes are most soluble in water as chloride, sulfate or nitrate salts. There are used as solvent dyes in free base form. Phosphotungstomolybdic acid salt is often used to obtain pigments. Furthermore, anionic dyes are frequently overdyed with cationic dyes to form insoluble double dye salts.

533

Examples of organic pigments and solvent dyes are shown below. It should not to be forgotten that carbon black, brown inorganic iron oxides and white titanium dioxide are the pigments most commonly used in leather finishing. The physical form and crystallographic structure of pigment particles are of primary importance for their application and properties. Physical operation has to be carried out after synthesis. Pigments are ground to ll5000 mm, and depending on the crystallographic form different coloristic and application properties are obtained from the same chemical structure. Finally, all pigments must be dispersed and completely embedded in auxiliaries to avoid instability in application. C.I. Pigment White 6 (2.1. Constitution 77891 This 730,pigment exists in three forms: anatase, rutile and brookite. The most popular are anatase, which gives a creamy white, and rutile for bluish white. Pigment Black 6,7,8,9,10 C.I. Constitution 77265-77268 Carbon black is available in different qualities. Pigment Brown 6,7(Yellow 42,43) C.I. Constitution 77492 FeO(0H) nH,O, occurs as goethite. This pigment is the main constituent of various ochres and umbers (containing manganese). It is a typical earth color, one of the oldest pigments known. 6.2.14 Fluorescent brighteners Fluorescent brighteners are not used in drum application, very seldom in finishing, and only occasionally for brightening the wool of sheepskins. About 80 % of all commercially produced fluorescent brighteners belong to the stilbene group. The most important stilbene brighteners used to be the acylation products of 4,4' diamino-stilbene-2,2-disulphonicacid. With the introduction of triazine stilbene for detergents and whitening cotton, nylon, wool and paper the universal brightener was launched. Today, this type is available in more then 20 commercial variations with different substituents on the triazine ring.

C.I. Fluorescent Brightener 32 C.I. Constitution 40620

534 /

S0,Na

Na0,S'

'Y

OH

7. PRACTICAL ASPECTS OF DYEING AND DYEING AUXILIARIES 7.1 Carriers In the leather dyeing literature the term carrier (familiar from polyester dyeing) is not very common, occurring only in connection with fur dyeing. However, carriers affect the magnitude of the diffusion coefficients of dyeing processes and this is very o h n discussed in leather research. There is almost no leather dyeing process known which is not influenced by carrier effects (diffusioncontrolled) or retarder effects (desorptioncontrolled). The typical carrier, whose effect is to lower the glass transition point, is alien to leather dyeing as such. But recent research shows that leather can be influenced by fat as measured by differential scanning calorimetry analyses [771. This may explain why fat liquors are o h n used in order to improve dye penetration. For coloring fur, which is basically a low temperature wool dyeing procedure [781 so-called carriers are used to give uniform, deep shades with metal complex, disperse but mainly acid dyes of low molecular weight. Alkyl phosphates are often used in combination with nonionic emulsifiers.

[ Alkyl-OkP=O Figure 10. General structure for a carrier employed in leather dyeing. The effect of these auxiliaries is very difficult to explain. Perhaps they form a weak complex with the dye, are surface-active andor reduce the glass transition point, so that binding occurs on the protein at sites not normally available at low temperature. Many details remain to be clarified.

7.2 Change of pH in the dyebath Raising the pH in the dyebath above the isoelectric point will change the ratio of dissociated groups, making the leather more anionic. This allows easier

535

penetration of anionic dyes, since there are fewer interactions between the cationic groups of the chrome-tanned collagen and the anionic dye. It is difficult for a chemical or physical bond to form at this stage. On the contrary, the equal charged substrate and dyes may repel each other. Therefore, increased mechanical action will improve conventional distribution to the fiber surfaces and, more importantly, even into the fiber pores. In accordance with the pore model, longer running time will allow complete diasion, but for fixation it is necessary to reduce the pH below the isoelectric point to enable the cationic charge of the collagen to increase and activate additional ionic binding points. However the acid should not be too strong if the anionic charge, especially of the direct dyes, is to be maintained. 7.3 Influence of anionic auxiliaries Raising the pH is not always a practical solution because a high pH can adversely affect the character of the resultant substrate, e.g. reduced grain tightness. Therefore, the use of an anionic auxiliary is recommended. Naphthalenesulfonic acid-formaldehyde condensation products are best known for this purpose.

Na0,S

S0,Na

Figure 11.Naphthalenesulfonic acid-formaldehydecondensate.

This auxiliary is a competitor for anionic dyes and produces two effects by reaction with the cationic chromed fibers. First, it decreases the cationic charge of the leather fibers, allowing better penetration of the anionic dyes, and second, it lowers the isoelectric point of the leather fibers, permitting through-dyeing at a lower pH. It also has a dispersing effect on the undissolved dyes. Many anionic syntans function in this manner by exploiting the so-called bleaching effect used for dyeing pastel shades. Best effects are achieved by adding the anionic auxiliaries together with the dyes or a few minutes earlier. 7.4 Influence of electrolytes The answer to this question could well be extremely interesting because adding salt reduces the rate of dye uptake on wool and speeds it up on cotton. A n anionic dye with sulfonate groups (SO,-) is reliably dissociated by very diluted solution. The dye will react very quickly with the cationic surface of chrome tanned leather fibers. The uptake curve is very steep and the color strength very marked, especially at higher temperatures (Figure 12), and with long floats and with saltfree dyes. Experience indicates that this is accurate for a typical leather dye with two sulfonate groups and a molecular weight of approximately 800 to 1200.

536

If salts are added, the dissociation balance decreases the activity of the dye ion and increases the amount of undissociated dye, according to the mass balance law. The uptake curve will become less steep and the saturation point will be reached later than with dissociated dyes [791. A high amount of undissociated dyes will be adsorbed and then penetrate into the leather fiber far more easily without reaction. A low temperature will furthermore increase the percentage of undissociated dyes and also help to promote a through-dyeing. But if the amount of salts is too high and the float too short, the dye will aggregate and the adsorption will be faster than the diffusion process. Overdyeing will occur and the unfixed dye will precipitate on the leather surface causing bronzing. This may happen with short dyeing times, less float and less mechanical action. Bronzing can be reduced by prolonging dyeing time, enabling the unfixed dye to be detached from the surface and then penetrate better into the interior of the leather. Strongly anionic auxiliaries and long floats are a help here.

3.5 T

Colour strength

3

2.5 2 1.5 1 0.5

0

0.5

1

1.5

2

-without salt - -with salt 1

2.5

3

YODye

Figure 12. Influence of salt on dye uptake However, aggregation of dyes in the dye float is quite normal, and not only dimeric and trimeric but up to ninety times aggregated dye are found in dyebath and this should not occur in the leather substrate [80]. The use of so-called organophilic agents or disaggregating agents to overcome excessive aggregation is often recommended. These are chemicals such as benzyl alcohol or urea and diethylformamide.They lead to disaggregation of the dyes, allowing better binding forces and easier access of dyes. Besides chemicals, a higher dyebath temperature reduces aggregation through entropy. Dyeing at high temperature therefore gives better fixation and more brilliance.

531

Experiments show two effects using disaggregating agents and high temperature. First, they minimize the tendency towards overdyeing, lowering the rate of bronzing by reducing excess aggregation. Second, they often induce increasing uptake with dyes that have poor exhaustion. In general, disaggregation has the effect of making the ionic charge more dominant and fixation is improved, but levelling and through-dyeing need additional attention.

7.5 Auxiliaries which react with the dyes A similar, but sometimes more marked improvement of penetration and fixation is achieved with auxiliaries which neutralize the ionic charge of the dyes. These chemicals are mostly polyoxyethylene derivatives of fatty alcohols, which are much more effective for penetration and levelling than salt or disaggregating agents are [811.

Figure 13. Polyoxyethylenederivatives of fatty alcohols. Polyoxyethylenes are considered nonionic, even though they possess a partial cationic charge, on account of their polar ether linkages. Their behavior in the dyebath is based on their ability to combine with anionic dyes. As a result, different rates of dye uptake become more uniform, because the variations in reactivity of different dyes are reduced. The same retarder effect as with electrolytes is obtained, preventing the dyes from being fixed too rapidly. These types of auxiliaries promote improved uniformity of dyeing on the leather surface, as well as enhanced penetration on thinner leathers. On thicker leathers, the diffusion may be somewhat slowed, because inside the leather the dyes lose their link with the weak cationic auxiliaries. Diffusion will now be controlled by interaction between the dyes and the leather fiber. It has to be realized that the attraction between dye and leather is stronger than the link between dye and polyoxyethylene. Afterwards the released auxiliary can be washed out to some extent [821.

7.6 Amphoteric auxiliaries Peptide hydrolyzate, e.g. from lime fleshing, belongs in this category and is often utilized in modern retanning agents [831. Mixtures of sulfonated ethoxylated fatty alcohols and ethoxylated fatty amines also are frequently used. In contrast to nonionic auxiliaries, they can promote a more intense shade on the surface, given it slightly stronger cationic properties As a result, penetration is reduced but retarding ensures outstanding levelness.

538

R = alkyl, aryl

Figure 14. Chemical structure of the peptide group in protein substrates.

7.7 Top dyeings Some selected cationic auxiliaries can boost shade intensity. The amount of product and the timing of the addition should be in line with the affinity of the substrate. Cationic resins, polyurethane, cationic fat liquors and retanning with chromium or aluminum salts have been used for this purpose. However, too strong cationic auxiliaries cause dye precipitation, and form a layer on the leather surface which can weaken the adhesion of finishes and give poor rub fastness. As the affinity of the substrate and the intensity of the shade are very closely interlinked, the method of application is crucially important to the final result. An improvement in shade intensity is only successful if the properties such as rubbing fastness or color brightness are not weakened. It has to be kept in mind that cationic auxiliaries hinder penetration and therefore the dye tends to be fixed on the leather surface. A balanced cationic product with a nonionic chain (e.g. polyquaternary amine (Figure 15)with ethylene oxide) has been found to overcome this disadvantage.

Figure 15. General structure of a polyquaternary amine auxiliary. These auxiliaries are applied aRer a first dyeing for penetration and fixation with an organic acid. After the cationic treatment there follows the second dyeing in a fresh bath, avoiding precipitation with the dyes. 8. FASTNESS PROPERTIES AND HOW TO IMPROVE AND TEST THEM

8.1 Wet fastness properties First, dyes should be fixed by acidification. As mentioned previously, an organic acid should be used. There is also a limit to the amount of acid, if too much chrome

539

is not to be stripped from the leather into the float. Second, wet fastness can be However, the chemistry involved is improved by the use of a cationic product [84]. completely different from that of the shade intensifying products. The cationic equivalence is much greater with a wet fastness improver, which has the function of neutralizing the anionic dye with a large cation. The dye has to lose its solubility to be better fixed. Mostly a quaternary amine in combined with a cationic amino resin, e.g. melamine formaldehyde compounds (Figure 16). NH-CH,-OH

I

Figurel6. Chemical structure of a melamine-formaldehydecondensation products.

A combination of waterproofing agent with a cationic fixing agent often achieves even better results, simply because the leather will not be fully wetted and therefore the dye cannot be stripped. 8.2 Water repellents (waterproofing) Water repellents have long been used to proof leather. Typical products are neutral oils, chromium and zirconium soaps and silicone compounds B51. Fluorochemicals are still fairly new on the scene. Fluoro-olefins are resistant to heat and to chemical attack by virtue of the small interatomic spacing between fluorine and carbon. The most prominent representative is polytetrafluoroethylene (PTFE, Figure 17) known as Teflon. (-CF,-C F,-)n

Figure 17. PTFE structure. Fluorochemicals are mostly applied in the form of fluorocarbon resin or polymers like polyacrylic fluoroalkyl ester. When correctly applied to leather, mostly as spray, fluorochemicals alter the surface properties in a different manner to conventional water repellents. Besides repelling water, fluorochemicals also repel oil. Physically speaking, fluorochemicals lower the surface energy of the leather treated with them. This energy is reduced to a level at which the surface is wetted by neither water nor oil and thus has ideal soil-repellentproperties. The repellent performance of the treated leather is good even aRer washing or dry cleaning. However in long term the repellency is sometimes difficult to maintain with fluorochemicals.

540

8.3 Light fastness When leather is exposed to sunlight, it absorbs light energy. Ultra-violet radiation is very energetic and is sufficient to destroy a dye molecule. Also the fibers within the leather itself can be degraded by sunlight. To slow down the degradation, the tanner must be very careful in the selection of the dye, syntan, tanning agent, fat liquor and auxiliaries [%I. Besides visible light (400-700nm),solar radiation comprises infrared (IR>7OOnm) and ultraviolet (W-A 315-400nm, W - B 280-315nm). This global radiation is not constant. It depends on the time of day, season, geographical location and weather conditions. In Central Europe, summer sunlight contains some W - B and a great deal of W - A rays, whereas in winter the W - B light is considerably reduced. Normal window glass filters out radiation below 315nm, i.e. W - B rays are eliminated. The strength of the sun’s radiation outside the atmosphere is 1320 W/m2(solar constants) and in Central Europe on a bright summer day about 650 J/cm2. Measurements of the yearly exposure dosage in the spectral interval from 300 to 800nm in BaseVSwitzerland, Central Europe average 333’000 J/cm’. This is considerably different from the dry and sunny Phoenix, Arizona region with 825’000 J/m2.However, if we expose dyed leather in the Texas Gulf Coast area with the same light dosage as in Phoenix it may fade faster due to higher air humidity. In addition, at higher altitudes, the W intensity increases. Mountain sports items can, therefore, be problematical, since this can lead to unexpectedly rapid fading and unpredictable shade change. Thus natural and artificial sources of radiation have to be precisely defined. Degradation of dyes in daylight is largely triggered by the W component. The visible component of light plays a smaller role. In addition, the temperature, both ambient and direct IR radiation, and the relative moisture in the air are of importance. Another factor to be noted is that on leather, not only the dyes but also the tanning agents fade, the fat liquors and other finishing agents are subject to shade change, e.g. yellowing, on exposure to light and the leather can thus be spoiled. Though anionic dyes generally have better light fastness than cationic dyes, they usually fail to achieve the same light fastness on leather as on cotton, wool or polyamide fibers. This gave rise to the theory that the different water contents leather about 15 %, wool about 8 % and polyamide least with about 3 % - were the explanation for this phenomenon 1871, Water content undoubtedly does have an influence but it has to be pointed out that in the case of polyamide the light fastness can depend very much on the stabilizers used, and wool contains sulfide links that can act as radical scavengers. These are not present in leather. An appropriate measure, therefore, is to apply W absorber, radical scavengers and antioxidants to the surface of leather. In addition, dry or moist heat and in particular the present of impurities, speed up the fading as well the ageing of leather. The chemistry of ageing can be closely linked to W radiation. The physical effects initiate dye-bleaching and leather-

54 1

destroying chemical reactions, as the leather and its tanning agents, fat liquors, dyes and finishing agents react with oxygen. This chemical process of W radiation and ageing, known as autooxidation (sometimes also autoreduction) is initiated by the energy-rich radiation from the sun, by intense heat, but also by mechanical action, and catalyzed by chemicals or impurities, which act as radical initiators. Chemical compounds are split off in the process and so-called free radicals are formed. These are extremely reactive and combine with atmospheric oxygen in a primary step towards the creation of peroxide radicals Figure 18).These peroxide radicals hrther react with the organic constituents of leather. In this secondary process the chemicals on the surface, e.g. fat liquors, finishing agents and the dyes are exposed to attack by oxygen. The tanned collagen and the tanning agent itself can also be destroyed. R" + 0, = R-0-0" Figure 18. Peroxide radical When very high light fastness is required, additional special light stabilizing agents can be employed. The best option is to use a mixture of a W-absorber together with a radical scavenger type product. They are applied mainly by spraying and dipping. The improvement can be very significant. These additives can interrupt and terminate the reactions at all stages of autooxidation. These products should be fixed on the surface in order to give the best possible protection. Also they should be applied at a pH close to neutral otherwise the light stabilizing effect can be considerably reduced. W absorbers for technical applications must absorb below 420nm, and those for incorporation in cosmetics must absorb at below 320nm because human skin has a well-defined sensitivity maximum at 297nm. Therefore, an ideal light stabilizer for leather should have high absorption over the W range of 290 - 400nm, and should be very lightstable. Generally, benzotriazole types (Figure 19) come closest to these requirements. W absorbers and thermostabilizers intervene actively in the ageing and U V degradation process. UV absorbers physically de-activate UV energy. For example 2-2'-hydroxyphenyl-benzotriazoles absorb the incident energy via hydrogen bond formation and transform it into harmless infrared radiation of longer wavelength.

Figure 19. General chemical structure of benzotriazole stabilizers.

542

The extent of photostabilization achieved with such compounds depends on the nature of the substituents attached. The essential substituent in benzotriazoles is the ortho-hydroxyl group in the aryl ring, which is capable of forming an intramolecular hydrogen bond. This bond involves a nitrogen atom of the heterocyclic ring. Besides W absorbers, antioxidants are also used. The primary types, mainly sterically hindered phenols (Figure 20), react chiefly with the peroxide radicals and prevent further reaction.

A Figure 20. General structure of hindered phenol stabilizers. The secondary antioxidants react with the final peroxides and for that reason they are also called peroxide destroyers. Secondary antioxidants are particularly useful in combinations with primary antioxidants. New on the scene are the multifunctional antioxidants with a special molecular structure enabling them to function as both primary and secondary antioxidants. !These multifunctional compounds include the new so-called Hindered Amine Light Stabilizers (HALS).Their mechanism of action is a mixture of radical scavenging, peroxide destruction and energy transfer reactions.

.VR R

Figure 21. HALS chemical structure. The use of these stabilizers has given extremely good results with leather, among other effects improving light fastness by up to 2 points on the blue scale. The crucial factors here are correct choice, judicious combination and correct application.

543

8.4 International standard methods for determining the color fastness of leather Tests on leather using physical and chemical methods resulted from a demand for more accurate information on the performance of leather in use. However, before 1946, no coordinated attempt had been made to establish standard fastness tests for dyeings on leather. Dye manufacturers published very little fastness data, with most of the data based on textile testing procedures. Wherever possible test methods were so drafkd as to conform to the current IS0 (International Organization for Standardization) recommendation for textiles. A typical example of the development of leather test standards is the determination of light fastness. In 1792, the French scientist, Dufay, set up a light fastness standard: for the purpose of comparison, a wool dyeing that began to fade after 12 days' exposure to sunlight was exposed to light together with the dyeing to be tested. It was not until 1913, however, that the German Fastness Commission produced the first blue scale and recommended exposure to light in daylight behind glass in a special testing cabinet. In 1961, in Cheltenham (UK), the test using artificial light was included in the IS0 T38/SCl.Commission as a "tentative test". These tests are now collated in the IS0 105-B series. Later on light fastness determination on leather was defined by the IUF Commission (International Union (of Leather Technologists and Chemists' Societies) Fastness) and published as IUF 401 (daylight) and IUF 402 (xenon lamp) specifications. In practice, light fastness on leather, textiles and paper is determined by almost identical procedures. The Mth Congress of the International Union of Leather Technologists and Chemists Societies (IULTCS) held in Lyons in 1965 initiated the discussion, comparison and adoption of international test methods. These were called IUF Methods. Cooperation has continued since and the number of methods is increasing year by year. Tests are developed by local committees in the member countries, and the IULTCS sponsors the methods for international use. The present standards are all published in the Journal of the Society of Leather Technologists and Chemists (JSLTC). The following IUF standard color fastness testing methods currently exist (some as drafts) besides some national and other standards [881:

Table 4 IUF Standards

I IUF 105 IUF 120 IUF 131 IUF 132 IUF 142 IUF 151

Numbering code for the standard methods and standards for methods of testing General principles of color fastness testing of leather Grey standard scale for assessing change in color Grey standard scale for assessing staining Artificial ageing of leather Preparation of storable standard chrome grain leather for dyeing

I

544

IUF 662 Color fastness of leather to accelerated ageing IUF 670 Test for adhesion of finish to leather 9. TRENDS IN HEALTH AND ENVIRONMENTAL REGULATION There appears to be growing concern regarding the impact of leather and dyes on the environment and the health of consumers. The general public and the authorities are paying increasing attention to these areas, and additional regulations are being developed accordingly [891. Although these hazards are not substantiated by any established findings, the media seize on them and often paint a very alarmist picture in their reporting. Fortunately, the real picture is not all black, there are also some positive aspects [go]. 9.1 Life cycle and risk assessments Life cycle and risk assessments are important tools to investigate the ecological and toxicological impact of processes and products. Life cycle and risk assessments address different but similar issues. Life cycle assessments (LCAs) examine the ecological consequences, or what is known as the "environmental burden" of producing and using goods [911. Any systematic approach to shifting the life cycle of leather towards a lower environmental impact should cover all the relevant aspects including the air, soil,

545

water, energy and resources involved for all parties. The cycle starts with the farmer and slaughterhouses, and moves on to the tanners, and dye and chemical manufacturers, then on to the leather goods manufacturers and retailers, and finally ends with disposal. All relevant emissions and resource consumption should be studied. Risk assessment (RA) studies, like ”Risk Assessment of Leather Dyes”, focus on toxicity and the impact on human beings and the environment [92]. The integrated approach of such a study needs to consider the starting chemicals, the method and control of synthesis including purification, the packaging, the means of transport and storage, performance in dyeing, impact on the consumer and finally behavior on ultimate disposal. 9.2. Risk assessment Three different issues that are often not clearly differentiated have to be addressed: workplace, consumer and non-consumer risk. 9.2.1 Workplace risk Human exposure to leather dyes occurs primarily at dye manufacturing plants, within tannery dyehouses and to some lesser extent at the leather goods producers. Today, there is a great deal of data available on the toxicological aspects of dyes. In these respects, dye manufacture is no different from other industrial scale production such as the manufacture of pharmaceuticals, detergents, “natural” or “synthetic”chemicals, or even tanneries. Analysis of the available data on these dyes provides adequate evidence that no major hazard is to be feared. An examination of more than 4400 organic dyes by analyzing the Material Safety Data Sheets has confirmed that most of them have an LD5o > 2000 mg/kg [931. Only a minority of commercially available dyes must be classified as “harmful" according to EC Guidelines. 9.2.2 Consumer risk Direct contact between dyed leather and the skin of the consumer represents a veG small part of leather usage. It occurs with some watchband and sandal leathers, but the bulk of leather goods do not normally involve direct skin contact. In contrast, most textiles are designed for direct skin contact. Dyes can also be inhaled mostly in the form of dust but this happens, if at all, during dye manufacturing and application process. Possible uptake through the skin or via ingestion is extremely slight, but may occur if the dye is not properly fixed. But if leather dyes have a high level of fastness and they are properly fixed, there is little risk for the consumer of dye uptake via skin or mouth, and the risk of allergies is very small. 9.2.3 Non-consumerrisk A prerequisite for a colorant to enter the environment is its solubility. This may affect the so-called non-consumer. In order to study the effects on aquatic

546

organisms, aquatic toxicity is tested using zebra fish and daphnia magna. Growth inhibition on alga is also measured. There is a considerable body of evidence that synthetic colorants in general, and water-soluble dyes in particular, are unlikely to be bio-accumulative. On this basis it may be predicted that long-term chronic effects on aquatic organisms are very unlikely to result from continuous exposure. Where sewage sludge is concerned, and in particular when it is used as an agricultural fertilizer, the possible effects of sludge contaminants on the soil must be considered. According to different studies, it seems unlikely that a calculation of the theoretical concentration of a dye in sewage sludge will lead to any concern about use as an agricultural fertilizer.

9.3. Regulations and laws 9.3.1 Registration of new leather dyes In order to control production and marketing of chemicals including leather dyes almost every nation implemented and enforced stringent laws to control production and uses [941. Before being released to the trade, new leather dyes have to be tested for ecological and toxicological properties, as dyes are chemicals foreign to the human body. The principles of risk assessment of new substances, like leather dyes, are laid down in European Union Commission Directive 93/67/EEC and include: hazard identification, dose-response assessment, exposure assessment for the environmental compartments, and risk characterization. In the USA the closely similar EPA (US.Environmental Protection Agency) regulation has to be complied with. The test criteria are specified in official regulations and followed by the manufacturer as part of his responsibility towards the user. Costs of registration of a new leather dye can lie between 50,000 and 100,000 US$ [951. A testing program for existing products is also under discussion. 9.3.2 Banned amines Recently, the German Consumer Goods Act (Bedarfsgegenstandeverordnung), known as “the German Ban on use of certain azo compounds”,was passed by the parliament in Germany [961. The 5thamendment was approved in November 1996, providing further clarification by listing examples of articles falling under this ordinance, mostly clothing, bedding sheets, towels etc. The German Health Ministry has indicated that the term clothing is to be understood in a broad sense, including underwear and outer garments, sportswear, head coverage, gloves, shoes, belts and more. The German ban concerns itself with the prohibition of the manufacture, marketing and import of consumer goods dyed with those azo dyes which, by cleavage of the azo bond(s), can liberate certain toxic listed aromatic amines. It does include leather as a consumer material and has intensified the discussion about the potential risk of dyed leather goods. Similar restriction applies in the Netherlands, in Austria and is pending in France.

547

A common misconception is that these regulations apply to all azo dyes and that every azo dye is harmful. This is far from true. Azo dyes are the world's most commonly used dyes, and their use in textile and leather dyeing is only one of many applications. Many food dyes are also members of the azo dye class. It should be noted that the German ban affects only a very small number of the azo dyes manufactured today. While the German Consumer Goods Act does not cite any specific azo dyes, it does list 20 amines that could form any of the specified 20 aromatic amines through cleavage of one or more azo bonds. The 20 aromatic amines forbidden in the Consumer Goods Act amendment are the same as those in the 1994 issue of the German MAK I11 A1 and A2 lists (MAK = maximum workplace concentration). Presumably it is planned that the list of amines in the Consumer Goods regulation will be amended as necessary to comply with the current MAK I11 A1 and A2 lists. The German MAK Commission is responsible for regulating the use of substances in the workplace, especially those that could be hazardous to health. Not only does the Commission set maximum exposure levels for the workplace, but they also publish lists of known and suspected carcinogenic substances. The MAK I11 A1 list contains compounds that are known to be human carcinogens, whereas the MAK I11 A2 list contains substances that have been shown to be carcinogenic in animal studies. Both lists are updated annually. Table 5 Banned amines (aromatic amines listed in the Consumer Goods Act in Germany) Name CAS No. 4-Aminodiphen yl 92-67-1 92-87-5 Benzidine 95-69-2 4-Chloro-o-toluidine 91-59-8 2-Naphthylamine 97-56-3 ortho-Aminoazotoluene 99-55-8 5-Nitro-Ztoluidine 106-47-8 4-Chloroaniline 615-05-4 2,4-Diaminoanisole 101-77-9 4,4-Diaminodiphenylmethane 91-94-1 3,3'-Dichlorobenzidine 119-90-4 3,3'-Dimethoxybenzidine (0-dianisidine) 119-93-7 3,3'-Dimethylbenzidine (0-tolidine) 3,3'-Dimethyl-4,4'-diaminodiphenylmethane 838-88-0 120-71-8 para-Cresidine 101-14-4 4,4'-Methylene-bis(2-chloroaniline) 101-80-4 4,4'-Oxydianiline 139-65-1 4,l'-Thiodianiline 95-53-4 ortho-Toluidine 95-80-7 2,4-Toluylenediamine 2,4,5-Trimethylaniline 137-17-7

MAK I11 A1 I11 A1 I11 A1 I11 A1 I11A2 I11 A2 I11 A2 I11 A2 I11 A2 I11 A2 I11 A2 I11 A2 I11 A2 I11 A2 I11 A2 I11 A2 I11 A2 I11 A2 I11 A2 I11 A2

548

Even though the above 20 aromatic amines themselves are known carcinogens, there is still considerable doubt as to whether there is a toxic risk to consumers from skin contact with dyed goods [97,98]. However, it is clearly sensible in situations where human health is involved, to take the cautious approach and reduce possible exposure to these above amines. Research has indicated that some dyes can break down in the body and release small quantities of amines. To protect consumers from any potential danger the worst case scenario has been assumed. A dye must not be harmful, even if is completely absorbed into the body and broken down into its individual constituents. Carcinogenic compounds such as benzidine, therefore, should clearly not be used. Azo dyes are compounds containing at least one of the -N=N- chromophore groups, which are linked to sp2-hybridized carbon atoms. To produce this azo chromophore; synthesis begins with an aromatic amine 1 which is converted into a diazonium ion 2, see Figure 22. This normally occurs in an aqueous solution in the presence of nitrite and mineral acid. The diazonium ion is a fairly weak electrophilic reagent and reacts (so-called azo coupling) with aromatic (Ar) species 3 that contain electron donor substituents (D) like hydroxy or amino groups (whereas R = H or other substituents). Logically these species are called the coupling components, whereas the starting amine is called the diazo component. Under certain reaction conditions the azo group can be cleaved and two amines can be formed (Figure 22). One of the amines is the original diazo component (11, and the second (5) is the coupling component with an additional amino substituent. It is this splitting of the azo group that is referred to in the German Consumer Goods Act. The official German analytical methods are published in the official collection of test procedure under Section 35 of the Food and Consumer Goods Act. For leather is DIN 53316 (DIN = German Industry Norm '=Standard'), using dithionite. According to the DIN method, use of forbidden azo dyes is confirmed if one of the listed amines is found with a concentration in excess of 30 mgkg (=30ppm, 0.003 %). Special rules apply to the amines 2-naphthylamine (mostly found in textile dyes) and 4-aminodiphenyl (mostly found in leather dyes), as testing cannot confirm their presence unequivocally. The German Ordinance permits the use of other validated and comparable test methods that are recognized in other member states of the European Union. Many of the harsher test conditions employed by some laboratories tend to produce false positive results, unlike the weakly acidic citrate-buffered DIN method. The frequent present of 4-aminodiphenyl will be explained with references to figure 23 and 24. This amine is not used in commercial leather dyes [991. In 1993 a relatively simple thin layer chromatography (TLC) analytical test was suggested for dyes based on benzidine and benzidine moieties. This method is based on the reductive cleavage of the azo compound by sodium dithionite. Since then, this basic analytical scheme has been modified to allow for the extraction of the dye from the leather or textile item involved and the detection apparatus used is more sophisticated. Several alterations have been proposed concerning the

549

reaction conditions and the detection of the amines, but reduction with dithionite has remained the most common method of splitting azo dyes into amines.

1

4

1

reduction

1

5

Figure 22. Azo dye formation and reductive-cleavage. However, in 1993 the original analytical method was developed effectively for the qualitative analysis of benzidine only [lOO]. The German Consumer Goods Act amendment states first simply that the above mentioned 20 amines must not be detectable in the defined consumer goods. Treatment of aromatic azo compounds with sodium dithionite results in the reductive cleavage of the nitrogen-nitrogen double bond. However, as shown in Figure 23, a side reaction can occur if one of the aromatic substituents is an unsubstituted phenyl ring, such as for example with amine 10. In this case, the MAK I11 A1 listed amine, 4-aminobiphenyl, can be formed in trace amounts as a by-product of the normal cleavage of the azo bond which gives the amine 11 and aniline (12).

550

10

Ar'-NH2 11

o

-N 12

(Main products)

H

2 +

m

N

-H

2

13 (By-product)

Figure 23. Side reaction associated with reductive-cleavage of azo dyes. The detection of 4-aminodiphenyl 13 might be explained by the formation of phenyl radicals, as shown in the reaction scheme in Figure 24. In contrast to aromatic azo compounds 10,which are thermally quite stable, the corresponding aromatic hydrazo compounds 6 are known to form aryl radicals 7 when heated [loll. If the aromatic residue is an unsubstituted phenyl ring, a phenyl radical would be formed, which can then react with the aniline formed through the normal reduction pathway, see Figure 24. A direct attack of the phenyl radical on the electron-rich azo compound could also be possible. Therefore the formation of phenyl radicals under the strong reductive conditions is a logical explanation for the presence of trace amounts of 4-aminobiphenyl in the analytical process. On the other hand, in uivo the comparatively mild physiological conditions mean that a reaction like that outlined above would be extremely unlikely [1021. These examples of false positive results have been observed in the development of the currently available official methods for leather. 2-Naphthylamine was found to be a side product of dyes based on Tobias acid was, when the textile method for cellulose and protein fibers was developed. Since then, products of other side reactions have been detected, but the concentration of the resulting amines found is mostly smaller than 30ppm [103].With procedures other than the current official analytical methods, false positive results are even more likely to occur. It must, however, be acknowledged that these official methods are very much better than the early test procedures. It is already clear that there are several possible interferences and side reactions with the dithionite reduction when it is used for analyzing finished leather samples down to low ppm levels. On the other hand the

55 1

dithionite system is the traditional and most convenient available method for cleavage of the azo group.

Ar’-NH,

+O

J N

J. H

,

11

13 Figure 24. Formation of 4-aminobiphenyl during the reduction of an azo dye. It should be stated that azo dyes are the major class of commercial synthetic dyes for leather, textile, paper, organic pigments and food. Today, only a few dyes are produced that fall under the German decree and alternatives are available for those dyes. It is clearly unlikely to be representative of the in vivo reaction which may take place in people wearing dyed leather as only a items such as watch straps are worn in direct contact with the skin. 9.4. Pathways of leather dyes into the environment Despite the negligible acute toxicity of dyes and the numerous efforts undertaken to avoid or reduce risks, dye manufacturers and tanneries are confronted with effluents, wastes and contaminated containers or packaging material that require carefully thought out disposal [104]. The primary route by which dye enters the environment from dye manufacturers and tanneries is through the production of wastewater, and also through the disposal of sludge containing dyes precipitated from the effluent by flocculation. Estimates are 1to 5% of the world production of leather dyes enter the wastewater

552

streams. Regarding waste regulation the dye manufacturer must fulfil the same requirement as the tanneries. In tanneries the exhaustion of a good dye is 96 to 99% and therefore the problem is less relevant than with other chemicals. In tanneries, which in many cases produce large quantities of waste, only approximately 1%of waste arises from the dyeing process. "he initial environmental concentration may be calculated for a day's operation of a tannery dyehouse using the following equation for wastewater emissions [105]: Wl.W2.-

E = Wi = Wz = F = P =

100 - F 100

100 - P -= mass of dye released (kg/day)

100

emission per day (kg/day) mass of dyed goods per day (todday) mass of dye used per mass of raw hides (kg/ton) degree of fixation of dyes (%) degree of elimination

Elimination of dyes may occur through adsorption on sediments and suspended particles with subsequent removal from the wastewater by settling or filtration. The results can be quite significant, depending on the nature of the dye molecule. Realistic parameters for a tannery producing dyed grain leather with an output of approximately 400 - 600 hides per day are given in Table 7 [106]. Table 7 Wastewater from dyed grain leather

Dyes %Fixation %Dyeing Dyed grain leather Dyes used %Elimination

Sulfur 92 2 lOton 200kg 70

Acid 96 2 lOton 200kg 70

Direct 98 2 lOton 200kg 95

Metal Complex 99 2 lOton 200kg 90

Realistic parameters for a tannery dyeing suede leather with an output of 400 600 hides per day are given in Table 8. In this case we have to consider that exhaustion is normally higher for suede than for grain leather: However, more dye is often needed and therefore fixation levels are somewhat lower.

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Table 8 Wastewater from dyed suede leather

Dyes %Fixation %Dyeing Dyed suede leather Dyes used %Elimination

Sulfur 90 4 lOton 400kg

70

Acid 94 4 lOton 400kg 70

Direct 96 4 lOton 400kg 95

Metal Complex 98 4 lOton 400kg 90

The volumes of effluent and the concentration of ingredients vary to a large degree depending on plant capacity, tanning technology, dyeing and waste water management. To produce lOton of leather, approximately 40ton of raw hides are used and up to an estimated 2,000m3water [1071. Today, modern tanneries have reduced this amount dramatically down to around 1000m3 [1081. Applying the above equation for total environmental concentration gives an amount of 10 - O.lppm dyes in the wastewater for an average tannery. Such a concentration is well below any of concern for ecotoxicity and would hardly be visible in a well-mixed effluent. 9.5 Disposal of dyed leather goods Leather is used to make consumer goods that sooner or later end up as household refuse. The consumer has a responsibility not to use the simplest method of disposal - unfortunately still in common practice - that is, just throwing refuse away. Campaigns to collect worn and unwanted goods and sporadic recycling initiatives do somewhat improve the situation. However the best solution would be to make new material or products from recovered leather. The second best way to dispose of used leather is by incineration, because leather provides considerable amounts of energy. The simplest method would be to return the used natural product leather to nature, e.g. to compost it. We are therefore studying this solution carefully. Some initial progress has been achieved. We have ascertained that soluble dyes and some organic pigments degrade under given biological conditions. This also applies to metal complex dyes. Only copper complex dyes in high concentrations can slow down the degradation of organic substances to minerals and humic acids, since copper salts have a fungicidal effect [log]. Various studies have shown that dyes are degradable, and degradation of colorants in the environment is likely to be a natural process. Degradation in the soil or air is comparable to the process that takes place in water. There is an increasing body of circumstantial evidence that the small fraction of colorants entering the water or soil does not significantly harm the environment.

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The open literature does not appear to contain data that would indicate an accumulation of dyes in terrestrial and aquatic food chains [110]. 10. ACKNOWLEDGEMENTS The author wishes express his gratitude to the many unnamed helpers who assisted in writing this chapter.

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Colorants for Non-Textile Applications H.S.Freeman and A.T. Peters (Editors) 2000 Elsevier Science B.V.

12 Structural Colors: Nano-optics in the Biological World MOHAN SRINIVASARAO Fiber and Polymer Science Program, North Carolina State University, Raleigh, NC 27695-8301

1. INTRODUCTION Brilliant, iridescent colors found on the bodies and wings of many birds, butterflies and moths are produced by structural variations and have been the subject of study for centuries. Newton was the first to suggest that such brilliant iridescent colors in birds and insects might be due to the presence of thin film structures, as he had examined the color-producing properties of such thin films. In his "Treatise on Opticks" [London, 17041, Newton stated the following: "...The finely coloured Feathers of some birds, and particularly those of Peacocks Tails, do in the very same part of the Feather appear of several Colours in several positions of the Eye, after the very same manner that thin plates were found to do .... and therefore arise from the thinness of the transparent parts of the Feathers; that is, from the slenderness of the very fine Hairs, or Capillamenta, which grow out of the sides of the grosser lateral branches or fibres of those Feathers." With the development of the wave theory of light, it became clear that interference phenomena played a key role in the color of bird feathers and insects. It was recognized that such colors arise from physical effects such as interference or diffraction as opposed to colors that are normally arising from the presence of chromophores that absorb or emit light. Such brilliant colors have been described as metallic colors due to the saturation or purity of the color produced. Common examples of substrates exhibiting physical colors are certain butterfly wings [l], color of Indigo snake skin [2], hummingbird feathers [3,4], arthropod cuticles, all due to selective reflection of color from the solidified cholesteric phase of chitin [5], gemstones like opal [6,7], and crystals such as potassium chlorate [8]. While the origins of such colors are well understood, the properties of color and color specification have not received much attention. It is worthwhile at this point to ask the question "what is color?". To answer this question, one can turn to The Oxford English Dictionary, which states: "The particular colour of a body depends upon the molecular constitution of its surfaces, as determining the character and number of light vibrations it reflects. Subjectively, colour may be viewed as the particular sensation produced by the stimulation of the optic nerve by particular light vibrations----. This sensation can be produced by other means, such as pressure on the eye-back or an electric current". In order for us to see or perceive an object is color, purely from a physical standpoint, three things are essential: a light source, an illuminated object, and an eye (and brain) to

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perceive the color. To specify color according to a universal standard, the eye is replaced by a photodetector for making quantitative measurements of the light that would have reached the eye. However, color specification depends on spectral color matching functions (spectral tristimulus values) provided by panels of human observers. This review will mainly deal with the variety of colors produced in the biological world, focusing on a variety of interesting substrates that include various butterfly wings and bird feathers (which are due to interference), optically active beetles (scarab beetles), Hercules beetle (use of color for camouflage), moths which produce color both by interference and diffraction, interference filters in the eyes of butterflies, and cholesteric liquid crystals. Iridescent color has also been discovered in the fossils of a well preserved fossil site, Burgess Shale, [9,10] in the Canadian rockies. Issues related to color perception in the biological world will also be discussed. This is important because the color space that the insects use is more substantial than ours, since many of these animals can have as many as five chromophores [ 111, and hence are pentachromatic, as opposed to our trichromatic visual system. The implications of such a large color space will also be discussed towards the end of this chapter. A unifying theme for this diverse set of materials is the nature of color generation. Since we will be interested in specifying color generated by physical means, it becomes necessary to review the language of color science. There are three main attributes that need to be considered for specifying color: hue, saturation, and in the case of non-self-luminous objects, lightness or brightness. Knowing these, it is possible to predict (approximately) the appearance of colors to an average observer. Of course, we will run into some difficulty due to the fact that the colors that we humans "see" are not necessarily the same as detected by certain animals when they view those same colors!

2. CIE COLOR SPACE CIE is an acronym for the International Commission on Illumination or Commission Znternationale de 1'Eclairage. The color notation bearing this name was accepted in 1931, based on 2" observer data, and provides an international language for the science of colorimetry [12]. The system developed by CIE is psychophysical, a description of the nature of the response of "average" observers to an isolated color stimulus on which perception is based. The system is based on two premises: the Young-Helmholz concept that all colors can be matched by additive mixing of appropriate amounts of three primary lights (with the restriction that none of the primary lights can be matched by mixtures of the other two) and Grassman's laws for additive color mixtures. One of the Grassman's laws states that the luminance of any additive mixture of lights is the sum of the luminances of each of them, regardless of the spectral power distributions. Additive mixing of lights occurs only in the eyes of the observer where the same area of the retina in the eye is stimulated, in contrast to subtractive mixing which occurs in colorants, due to selective absorption, transmission or scattering of light. A physical description of the light reaching the eye of an observer, together with the measured additivity of color mixture, provides the basis for a numerical description of perceived color [ 121. The perception of isolated color is a psychological phenomenon, while being threedimensional in nature [12]. The attributes that describe color in three-dimensional space are the

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quantities that specify the color that in the case of isolated colors are hue, brightness, and saturation. Hue is that quality we often describe by the words red, yellow, blue, green, etc. Brightness is a quality of color that can be classified as equivalent in lightness to some member of a series of gray samples ranging from white to black. Saturation represents the extent to which a given color differs from a gray of the same brightness. As we will see, hue and saturation can be represented by the chromaticity coordinates x, y on the chromaticity diagram, while the brightness (or lightness) is given the third dimension as shown in Figure 1. When the lightness or brightness (given by Y , the luminous reflectance) is 1, the only color that can be perceived is the color described as white. As brightness decreases more colors are possible, as can be seen clearly from Figure 1. The limits of attainable color at each Y value defines the limits of possible colors, the MacAdam limit. The term "isolated color" is used to denote the perception of color from a uniformly colored area, say of a painting that is not influenced by the colors that surround the painting. Colors are often influenced by their surroundings, leading to a psychological phenomenon known as simultaneous contrast, which artists use to achieve specific color effects. An example of an isolated color is provided by a green railway signal glowing from a distance at night in the absence of other lights. It can, however, be argued that the darkness acts as a border for the light source, thus providing a time-varying input that allows for the perception of color. It has been argued that a truly isolated color cannot be perceived in the absence of a time-varying or spatially-varying signal to the eyes. One group described a rather simple experiment to illustrate this point. Consider the two halves of a table tennis ball placed over the open dyes which is illuminated with a uniformly colored light. At first the color is perceived, but it fades in a matter of a few seconds, reappearing on closing and reopening the dyes, only to fade once again. The perception of isolated color by an observer depends on the nature of the light that enters the eye and is best described by the spectral power distribution. Thus, if the relative spectral power distribution [P(h),where h is the wavelength] of the light source is known and if the relative spectral reflectance [(R(h))or transmittance] of the object is also known, the relative spectral distribution of the light entering the eye can be computed rather easily, simply by multiplying the spectral power distribution of the light source with the spectral reflectance. An analytical description of the response of a human observer to color can be described in terms of the relative amounts of three primary colors that must be mixed additively to match each wavelength of the visible spectrum. This can be done for all lights of all wavelengths if the human observer is sometimes allowed to add one of the primary lights to the light to be matched. The task for an observer is to color match light of each wavelength illuminating a half circle with appropriate amounts of light from three different primary sources focused on the other half of the circle. The amounts of each primary required to match each wavelength are called the observer color-matching functions and there are three, i.e. one for each primary at each single wavelength. Such measurements having a field of view of 2" have been made leading to the color-matching functions plotted in Figure 3, defining the 1931 2" observer. The primaries are designated X for "red", Y for "green" and Z for "blue", and the color matching functions are designated as X , L , and i , respectively.

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Figure 1. 1931 CIE (x, y, Y) color space (with MacAdam limits) for a non-self-luminous object illuminated by a CIE standard light source. (Reproduced from Ref. 12, with permission from Springer-Verlag, New York.)

n

Match Sample

Test Wavelength

U

Figure 2. Illustration of the method for generating the color matching functions.

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Wavelength

Figure 3. Color matching functions of 1931 CIE standard 2" observer. The triangles, filled circles and filled diamonds represent X , L , and Z , respectively [4]. The color perceived by an observer is then represented by the integral of the product of the color-matching functions of the standard observer, the relative spectral reflectance of the object viewed, and the relative spectral power distribution [13]. This process is illustrated schematically in Figure 4.

Wavelength 'pectrum Of Light Source

Wavelength Reflectivity of the object

Standard Observer Functions

Tristimulus Values

Figure 4. Procedure for calculating the tristimulus (X, Y, Z) values. The magnitude of the integrated products is called the tristimulus value and is described by the following set of equations:

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X = k 1R(h)H(h) ~ ( h dh ) Y = k R(h)H(h) y(h) dh Z = k 1R(h)H(h)z(h)dh

1

where k is a normalizing factor, which depends on the number of wavelengths used and dl is the wavelength interval. The product of the color matching functions and the spectral power distribution of the (standard) light sources are published as tables in many books on color science [ 141. Hence in order to obtain the tristimulus values one needs to make a measurement of the reflectance (or the transmittance) of the object that one is interested in. When each of the tristimulus values for a measured color is divided by the sum of the three, a fraction attributable to each primary is obtained and since these need to add up to 1, two fractions are sufficient to describe the "chromaticity" of the object. These fractions are known as the chromaticity coordinates and are designated by x, y, and z: X

= X+Y

X

+z

y=-

Y X t Y tZ

z

=

z

X + Y +Z

In principle any two of the three chromaticity coordinates, invariably x and y, may be plotted in rectangular coordinates for comparison. Plotting the pure spectrum colors in this way gives the limit locus, called the spectrum locus, inside which all colors must fall. Such a chromaticity diagram is shown in Figure 5. The chromaticity diagram can be used to demonstrate the linearity of additive mixing of colors. Consider two colored lights that are mixed to produce a third color. The x, y values for the mixture color will lie on a line joining the chromaticity coordinates of the two colors that were mixed. When three colored lights are mixed additively, all the colors that can be produced using those three colors must lie within the boundaries of the triangle connecting the x, y coordinates of the individual colors. This triangle defines the color gamut that is available using these three colored lights. When calculating the chromaticity coordinates for a given color, the closer the value to the spectrum locus the more saturated the color. It will be shown later that colors produced by cholesteric liquid crystals are saturated colors and lie close to the spectrum locus. The above discussion of quantitative description of color is applicable to animal life that has trichromatic visual system, like the standard human observers. In the case of animals such as butterflies, it is hard to describe what "color vision" means. Our own experience of this colorful world tells us that to have color vision is to see colors. The challenge now becomes one of extending and translating this operational definition to other animals. 3. GENERAL METHODS OF COLOR PRODUCTION

In this section, the various methods of color generation that uses physical optics will be described. This includes interference, diffraction, dispersive refraction, scattering, and combinations of these methods. While examples of all methods except dispersive refraction will be presented, the reader is referred to a book by Nassau [ 151 for examples of the latter.

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0.9 0.8 0.7 0.6

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0.3 0.2 0.1 n W

0

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X Figure 5. Various colors are represented in the CIE chromaticity diagram. The various colors are abbreviated as follows: pB: Purplish blue, B: Blue, gB: Greenish blue, BG: Blue green, G: Green, yG: Yellowish green, YG: Yellow Green, gY Greenish yellow, Y: Yellow, yo: Yellowish Orange, 0: Orange, OPk: Orange pink, rO: Reddish orange, Pk: Pink, R: Red, pR: Purplish red, pPk: Purplish pink, RP: Red purple, rP: Reddish purple, P: Purple, bP: Bluish Purple. 3.1. Colors due to interference Interference colors are observed in a variety of situations involving thin films. A very common example is the color of soap film seen in sunlight [16,17].

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Here, the author will deal primarily with coherent light, as the theory of interference from an incoherent source is not well understood. Although the light source under consideration might be incoherent in comparison to a laser light source, however, the coherence length of such incoherent sources are on the order of a few microns. We shall be interested in reflection of light producing colors only from films that are a few microns thick, and therefore one can use the theory of interference developed for coherent sources. This color arises when sunlight reflected from the top of the film interferes with light reflected from the bottom surface of the film. In this case, the two beams of light can be viewed as two independent coherent sources, the two virtual images of the real light source (Figure 6 ) . One sees interference colors when the film thickness is on the order of the wavelength of visible light. Therefore, changes in the color observed can be effected by changing either the film thickness or the viewing angle. Fresnel equations provide the theoretical foundation necessary for understanding the interference phenomena that is observed from thin film structures. A detailed discussion of optical effects from submicron structures has been published by Pfaff and Reynders [18].

Film

Figure 6. Interference produced by reflection at a liquid (soap film) air interface. R1 and R2 are the reflectivities with a1 the incident angle, a'1 the reflected angle and a2 the angle of refraction. When reflections from the two surfaces add in phase or constructively interfere, a large net reflection is created that the observer "sees" as certain colors. Constructive interference occurs when the Fresnel reflection coefficient is positive for a light beam being reflected from a soap film (liquid) at the liquid-air interface and is negative for reflection from the air-liquid interface. A negative reflection coefficient can simply be thought of as a beam undergoing a 180" phase shift between the incident and reflected light waves. Therefore, the 180" phase shift suffered at the air-liquid interface together with the 180" phase shift suffered in a round-trip through the h/4 layer leads to perfect constructive interference for all the reflected waves. The optical path difference experienced by the two rays that interfere constructively is simply equal to the extra distance the light beams had to travel in the medium. Based on Figure 6, this

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is 23dcoscu2, where n2 is the refractive index of the film and a2 is the angle of refraction. In addition to the optical path difference of ~ ? ~ C O S Q , ,there will be a path difference of Y2due to the phase difference 71 that occurs at the air-film interface whenever an incident light beam is reflected by a medium of higher refractive index than the initial medium. Thus the effective path difference between the two rays is:

2n,dcosa2 +

+%

Consequently, if 2%dcosa, = nA , where n is an integer, the two rays will interfere constructively and give an intensity maximum [17]. On the other hand, the following equation applies if one has destructive interference which results in zero intensity:

Since we have assumed the amplitudes, A, of the two beams to be equal, the amplitude of the reflected wave will be given by, A,, where A, = A + Ae" , with the phase difference, 6, given by 6=2R(24dcosa, +%). The total reflected intensity is given by I, = A,A,*, and is equal to /z

g. This equation can be rewritten in terms of the reflectivity R, to have the form

4A2cos2 below.

where I, is the incident intensity. One can, of course, rather easily eliminate the angle of refraction from the above formula to show the dependence of intensity on the incident angle. Quantifying the above discussion leads to the task of deriving equations for the reflected and transmitted light intensity. Fresnel equations predict the amplitude (r) of the reflected light from thin film structures [17,19] and can be written as:

rs

ts

=[ =[ [

n, cos a, - n2 cos a2 nl cos a, + n2 cos a,

2q cos a,

qCOSa,+n,cosa2

1

= n, cos a, - % cos a L ]

+

n,cos a, n2 cos a,

(4)

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t =[

2 q cos a, n,cosa, +n,cosa,

I

(7)

where nl, n2 are the refractive indices of the two media in which light propagates, rs and rp are the amplitudes of reflection for s- and p-polarization of the incoming light beam, and ts and tp are the corresponding amplitudes for transmitted light. S- and P- polarizations are defined with respect to the plane of incidence of the light; S-polarization implies the polarization is perpendicular to the plane of incidence, while P-polarization implies the polarization is parallel to the plane of incidence, where the plane of incidence is defined as that plane which contains both the incident and reflected light beams. The reflected intensities (and transmitted intensities) for both type polarizations are related to the amplitudes in a simple manner, and can be written as:

R, = (r,>’ and R, = (rp>” 2

2

T, = at, and T, = atp where

a is defined as a =z!!

, with a1 and a 2 being the incident and refracted angles.

The propagation of the light waves can be traced simply be realizing that the beams obey Snell’s law, nlsinal = n2sina2, and that the incident angle is equal to the reflected angle. In the case of normal incidence, a, = a, = 0, and we have

The results for S-polarization are of course indistinguishable from those for P-polarization at normal incidence. When using unpolarized light, the intensity is the sum of the intensities of the two polarized components. The reflected intensity for normal incidence is then given by the well-known Equation 8 [20]. 2

R = [ E ) Equations 4-7 relate the amount or fraction of incident light reflected or transmitted as a function of (i) the angle of incidence a l , (ii) the angle of refraction a 2 , and (iii) the refractive indices. One can eliminate the angle of refraction using the laws of refraction and generate

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equations as a function of incidence angle. The desire to have equations as a function of incidence angles is motivated by the fact that the colors of a butterfly wing is angle dependent. The Fresnel equations for the two polarizations can be written as shown below [19].

These equations reduce to the reflectivities when the incident angle is equal to zero, which is gratifying. In the above discussion only two waves, the most important, have been taken into account. In principle there will be an infinite number of waves producing interference which can be represented by R1, R2, R3, ...etc, and similarly for transmitted waves, TI,Tz, T3, etc Figure 71. In assessing the different reflections giving rise to interference and contributing to the total intensity, one needs to consider the third and higher order waves. In order to do this effectively it becomes necessary to introduce reflection factors r and r', and transmission factors t and t'. The ratio of the reflected to the incident amplitude of the wave in air when reflected by the liquid film surface is denoted r and r' is the reflection factor for an incident wave from the liquid film. t and t' are defined in analogous way for transmission. If A is the amplitude of the incident wave and d the phase difference between successive rays emerging from the film, then the total reflected amplitude is obtained by adding the contributions from each reflected ray. This sum can be written as shown below [ 191:

rA +tt'r' Ae* + t t ' r ' 3 A e 2 ' * + , . , . , . + t t r r ' ( Z pAe"p-"* -3' +... The reflection coefficients, as already described, are related by r = -r' . As the optical sense of color perceived is dependent on the reflected intensity, one can easily write an expression for the reflected light intensity. The intensity of the reflected light is given by I, = A,A,* and can be written as Equation 11.

I, = -I'

4Rsin';

6

s

(1- R)* +4Rsin22

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(A’XA’)’

where I‘ = is the intensity of incident light. In order to get the angular dependence one can easily eliminate the angle of refraction and write the optical path difference in terms of the incident angle. The maximal reflectance can then be easily expressed by Equation 12.

This equation clearly shows that the reflectance is shifted to shorter wavelengths with increasing angle of incidence, which is consistent with all interference colors, including those due to the wings of the butterflies.

Figure 7. Interference produced by reflection (and transmission) due to multiple reflections of a single incident wave.

3.2. Colors due to diffraction When a propagating light wave encounters an obstruction having dimensions similar to its wavelength, part of the energy of the incident beam is scattered. If the obstruction is periodic, or for that matter periodic variation of any parameter that affects the propagation of a light beam (or wave), light is scattered into various discrete directions or diffracted orders. A structure that functions in this fashion is referred to as a “diffraction grating”. Each of the diffracted orders that have been diffracted by the grating have a direction and the amount of this deviation depends, among other things, on the periodicity of the grating and its relation to the wavelength. In this way a grating disperses a variety of wavelengths to form a spectrum. Therefore, the grating appears to function as a prism, but in many respects it does so better and far more conveniently. There are many examples in nature where the interaction of light with matter is used to produce brilliant colors for a variety of purposes, which include colorful displays, for warning predators and for courtship. In the examples we will consider, the periodicity often approaches that of the wavelength of light, and therefore such gratings produce

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color. This chapter will cover gratings in which the periodicity has dimensions equal to or smaller than the wavelength of light, giving rise to "zero-order diffraction grating". In its simplest form a diffraction grating consists of a large number of parallel grooves drawn on a sheet of transparent material such as glass or a polymer film. The light beam passing through a diffraction grating consisting of a set grooves is analogous to the case of double-slit interference. Figure 8 schematically depicts diffraction from a series of slits of width s, and separated by a distance d. The diffraction angle is given by the grating equation that can be defined by equation 13. d sin 6 = rnA , where rn = 0, 1, 2,.....

(13)

where m = 0, 1, 2 ,..... Here m is the order of the diffracted light beam passing through the diffraction grating. When light is incident at some arbitrary angle a, the grating equation can be written as shown in Equation 1 4 [21]: d (sin 6 - sin a ) = mA

(14)

Figure 8. Schematic diagram of diffraction grating having slit width s and slit separation d. Taking into account the finite widths of the slits, the intensity distribution due to diffraction can be determined by Equation 15 [21].

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I , = I,

0)

sin'('ssin

(15)

(fssinB)'

Considering the case of multiple slits, which is necessary when one is considering diffraction from a grating, the intensity distribution due to interference can be expressed by Equation 16. sin2( N

d sin 0)

I@ = I, sin'( ;dsin

8)'

The total intensity distribution of the diffracted beams is given by the product of the contribution due to diffraction and that due to interference, and can be expressed by Equation 17 [20,21] sin2(:ssin I , = I,

(f

B)sin2(N d:

s sin 0)' sin'(

sins)

f d sin 0)'

(,"

1

It should be noted that the intensity is proportional to sin2 ~ - d s i n 8 , so that the intensity in the principal maxima are proportional to the square of the number of slits, N. As the number of slits increases, the principal maxima become increases, and narrower, thus separating the light source into its component spectrum. Colors produced by fine gratings will lie on the spectrum locus of the chromaticity diagram shown in Figure 1. As might be anticipated, the principal use of diffraction gratings is in spectroscopy where it generates the spectrum of a light source. The zeroth order retains the composite color of the source. When the spacing becomes comparable to the wavelength of light, the transmitted light (zeroth order diffraction) becomes a function of wavelength. This has implications for color production in biological systems, where one often finds that submicron structures are responsible for the color. In such cases reflection gratings will be described as opposed to transmission gratings.

3.3. Colors due to scattering Many of the blues found in nature, like the blue feathers of the bird bluejay, is due to light scattering rather than colorants. Consequently, it is useful to discuss how scattering can produce the "perception" of a blue color. In 1871, Strutt wrote "It is now, I believe, generally admitted that the light which we receive from the clear sky is due in one way or another to small suspended particles which divert light from its regular course. On this point the experiments of Tyndall with precipitated clouds seem quite decisive" [22]. It was generally believed that these particles were composed of water or ice. However, Lord Rayleigh was somewhat skeptical of the nature of small particles in the atmosphere responsible for the blue sky. He wrote in 1871, " If it were at all probable that the particles are all of one kind, it seems

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to me that a strong case might be made out for common salt. Be that as it may, the optical phenomena can give us no clue". After an interval of 28 years Lord Rayleigh wrote, " I think that even in the absence of foreign particles we should still have a blue sky". This implied that the air molecules themselves were sufficient to provide the blue sky. The arguments that Lord Rayleigh put forward in 1871 based purely on dimensional analysis, and make an interesting introduction to the discussion presented next [23]. Consider a particle that is small compared to the wavelength of light in the path of the illuminating beam. The scattering from such a particle is proportional to its volume (V). This means that the elementary oscillators into which the particle may be subdivided, scatter waves that are in phase with one another, primarily due to the small size of the particles, when illuminated by a light beam. The total scattered electric field (E,) is therefore proportional to the particle's volume. Since Es is due to the excitation beam with amplitude Ei, Es must be proportional to Ei. Since the particle is small, the scattered field must drop with distance from the particle in a manner that conserves energy. To see how this might occur, consider the particle to be a sphere of radius r, with a surface area of 4nr2. The total energy scattered across this spherical surface must then be independent of r. This will be the case if the irradiance or the radiant flux density decreases as Since the irradiance is proportional to E:, E~~must

x.

x2.

therefore be proportional to Dimensional considerations will quickly make it obvious that the scattered field must be inversely proportional to the square of the wavelength 1, the only relevant quantity with dimensions of length, and can be written as Equation 18:

E, =

KE,V ii2r

where K is a dimensionless constant. However, the scattered irradiance or the intensity is proportional to the square of the scattered field and can be written conveniently as Equation 19:

Is

=

K21.V2

where 1, is the incident light intensity. While constant K is certainly dimensionless, it does depend on the refractive index of the particle. This can be ignored as long as there are no adjacent absorption bands. Equation 19 is often referred to as "Rayeleigh scattering". Equation 19 also shows the now famous relation, I, which is usually cited for the "blue" sky.

-&.,

However, knowing that the wavelength of violet is shorter than blue, the question of why the sky is not violet is never asked by any student. In retrospect, it is obvious why the sky does not "appear" violet, but rather is blue!. What is often omitted in most textbooks that deal with scattering is that the perception of a particular color is determined not only by the optical properties of the medium being observed, but also by the way we see things [23]. As mentioned earlier, the perception of a color is determined by the product of the amount of light reaching the detector, the illuminant, and the

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spectral response of the eye. So, the sky blue that is perceived by the brain is a product of the solar spectrum (Figure 9), light scattering from the molecules in the atmosphere (Figure 10) and spectral sensitivity of the eye (Figure 11). Even though violet is scattered more than blue, the eye is less sensitive to violet, and the solar spectrum is somewhat depleted in violet. This leads to a combined signal processed by the brain to yield the sensation we call blue.

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Wavelength

Figure 9. The spectrum of sunlight outside the atmosphere [ 141.

400

450

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550

600

650

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A. Figure 10. Wavelength dependence of scattered intensity by molecules in the atmosphere.

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x

.e c)

>

350

400

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h Figure 11. Spectral sensitivity of the human eye [20]. Many of the non-iridescent blues in the animal kingdom are a result of scattering and the greens of parrot feathers are due to a combination of natural color bodies and the blue resulting from scattering. The barbules of the parrot feathers have a yellow colorant and this when combined with the blue from scattering produces a green color.

4. COLOR GENERATION ON WINGS 4.1. Butterfly wings It was pointed out earlier that some of the wings of butterflies and the cuticles of beetles produce rather remarkable colors using arrays of precisely fabricated structures, providing a striking example of pattern formation in biological systems. These elaborate architectures lead to structural colors that are seen in various insects and birds. Most of the colors are produced by either thin-film interference or diffraction or, as in the case of some beetles, by selective reflection of light. In the case of thin-film interference (which is known as thin-film reflectors) coloration is due to alternating layers of high and low refractive index materials. Such assemblies are usually referred to as Bragg-reflectors in the physics literature and have been considered for use in optical limiting and switching applications, using so-called "photonic band gap" (PBG) crystals [24]. The more than 100,000species of Lepidoptera can be identified solely by the color pattern of its wings. Such magnificent colors (and patterns) are even more remarkable when one considers how they are produced [25-281. The lepidopteran wing is made of scales that are quite small and form a layer over the wing membrane. Each scale is about lOOpm long and 50pm wide. The scales cover the membrane completely and when viewed under a microscope appear to overlap like roof tiles. When one looks at the wing it becomes evident that on a given patch of wing there are typically two and sometimes three types of scales, which alternate positions on roof tiles like arrangement. The larger "cover" scales and the smaller "ground" scales are arranged in an alternating fashion. In most cases, the cover scales tend to be architecturally more elaborate. The density of the scales varies from about 200 to 500 scales

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per square millimeter. The scales making up the color pattern are quite delicate, and anyone who has handled a butterfly by its wings would have noticed that the scales rub off easily along with the color pattern. At high magnification it becomes evident that the color pattern is a result of finely tiled mosaic, with each tile of the pattern being made up of a single wing scale with a unique color. This is schematically shown in Figure 12, where the butterfly and its wing structure are shown at increasing magnification [29]. The entire color pattern is then made up of single colored tiles, often comprising between three to five colors, and the unique variations in color and hue are created by varying the number and density of the different colored scales on the wings. However, even in the case of colors produced by structural variations, one often finds that there are pigments involved, as the membrane of the wing usually contains pigments, either melanins or pterins. The primary purpose of these pigments is to accentuate the color produced by structural variations [29]. The spectacular "metallic" color of the tropical Morpho butterfly has been attributed to interference brought about by the elaborate structural features on the wings of these animals. Depending on the species, colors arise from structural variations that produce constructive interference. These structures frequently contain pigments, usually a dark melanin, that absorb light that is not reflected, producing particularly bright colors. The elaborate structures may also function as a way of regulating the body temperature of these animals [30]. The work of Mason [31-341 remains one of the most complete accounts of the origin and diversity of color in insects. Among the most common colors observed is white, and Mason pointed out that white is observed when a colorless cuticle has many small, irregular surfaces that reflect light. He demonstrated that white is a structural color by immersing a portion of the wing in xylene (which almost matches the refractive index of the cuticle), producing a pale brown color. Upon evaporating the solvent, the original wing color reappears. Similar results were obtained with a variety of beetle scales, and demonstrated the very close similarity between iridescent scales and thin film colors as regards their optical properties [33]. Quantitative optical studies by Rayleigh and by Memtt also emphasize this resemblance. Others employed electron microscopy [35-391 to arrive at the same conclusions. Virtually all iridescent colors, and most of the blues and greens, are due to structural variations on the wing. Ghiradella [40] surveyed the diversity of the structures that produce colors from interaction of light with matter that is periodic in nature. At least six distinct variations were categorized that produce color, and it was noted that three different structural components of a scale can be modified and elaborated into self-assembled arrays repeating themselves to produce the brilliant colors. The structures were the ridges, the flats between the ridges, and the interior lumen. Figure 13 shows the different possible structural variations of a generic wing scale. The diversity and complexity of the fine structure makes the butterfly wing scale among the most complicated extracellular structures manufactured by a single cell [25]. Color Palette 1 (page 604), a light micrograph of the scales of two different butterfly wings, Omithoptera priamus with the green scales, and Necuria duellona with the blue scales, demonstrates the precision with which the individual tiles are placed on the wing membrane. The individual wing scales are clearly seen and it is apparent that each scale is a monochrome color. Figure 14 is the reflectance spectrum of an individual wing scale measured with a Zeiss Microspectrophotometer, with a measurement spot size of about 20pm. It is quite clear that the

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spectrum reflects the blue or green color that is perceived by the observer. Color Palette 2 (page 604) shows a part of the wing scale of Papilio daedalus at high magnification which reflects in the blue part of the visible spectrum, but the ultrastructure giving rise to the blue is quite different in appearance in comparison to the scales of Necyria duellona. Even though the reflectance at the peak is not as high as one might expect the dominant wavelength in the reflectance spectrum is in the blue.

Figure 12. The structure that gives rise to the colors of Morpho rhetenor, a South American butterfly, is depicted at increasing magnification [29].

Figure 13. Structural variations that give rise to the colors of butterfly wings. In the center is a schematic cut-away view of a scale fragment showing the upper and lower layers, ridges, crossribs, ridge-lamelle, and microribs. A. Ridges that produce thin-film reflectors giving rise to the colors. B. The flats between the ridges may have an elaboration that give rise to color due to scattering. C. The lamelle/microrib system now becomes the structure producing color. D. A structure where the microribs fill the space and are the structural elements E. The flats may be filled with plates and pores pattern. F. The interior of the scale may be filled with bodylamelle that now become the elements of a thin-film reflector G. The scales may be filled with a crystalline lattice that produce diffraction colors and behave as zero-order gratings {[reproduced with permission from Wiley-Liss Inc.[40]},

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Wavelength (nm) Figure 14. Reflectance spectrum of individual wing scales from the butterfly wings shown in Color Palette 1 (p. 604). It is interesting to note that the melanin-containing ground scales are visible against the bright green or the blue color of the individual scales (see Color Palette 1, page 604). A phenomenon known as assimilation or the Bezold Spreading Effect [41,42] can occur when such intricate patterns are present. Assimilation occurs when a background and an interlaced pattern of color fail to oppose each other, but seem to blend together. An example is shown in Color Palette 3 (page 605, where four different colors or hues that have black or white crosshatched patterns superimposed on the colors. Even though the background hue is the same within each hue, it appears different when superimposed with black or white cross-hatches [41]. While the phenomenon of assimilation is not well understood, it is clear that it cannot be

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explained by the scattering of light from one region of the image to another. When the crosshatched pattern is fine relative to the diameters of the individual receptors, additive mixing of light occurs, but the cross-hatches themselves are not visible. On the other hand, if the elements making up the hatching are small relative to the postreceptoral elements that sum inputs from the cones, assimilation can occur. So by varying the viewing distance, thus varying the relative size of the pattern, allows one to observe mixing of light or assimilation. Each of the individual wing scales produces structural colors, and examination of these individual scales has revealed that there are three parts of a scale-the ridges, the flats between the ridges, and the interior lumen. Each of these may be modified to produce changes in scale color. The ridge-lamellae form stacks, with as many as a pile of 5 to 10 deep per stack, depending on the species of the butterfly and the reflectivity of the wing scale. These ridgelamellae then act as thin-films, and together with the air spaces between the lamellae, they form a quarter-wave interference filter that meets the condition for constructive interference [40]. It is usually necessary to have multiple layers of thin films to produce brilliant colors (see equations 11 and 12), so that the reflectivity can approach unity, for a given wavelength. Although the reflectivity of an individual scale does not reach unity, it can be high enough to produce the sensation of a particular color. It is the dominant wavelength of reflection that matters for a particular color to be perceived, rather than a reflectance that reaches the maximum attainable reflectivity. For example, the blue of the sky cannot exceed a purity of about 42%. This means that there are other wavelengths in the observed spectrum of sky, though the dominant wavelength is blue (476nm), the sky “appears” blue“ to the observer [23]. Figure 15A is a light micrograph which shows the transparent cover scales on top of the iridescent scales. Figure 15B is an electron micrograph of the wing structure of a Morpho butterfly, supporting the shingles (or the wing scales), about lOOpm in length. Figure 15C shows the Morpho wing scale (glass scales) at higher magnification, revealing the structure responsible for color generation. The structure is generated using chitin, a biopolyrner, and air as the building materials, with the chitin held apart by vertical spacers. A dramatic example which shows that the color is a structural color can be demonstrated as shown in Color Palette 4 (page 605), where the top of the figure shows several blue scales and the bottom figure shows the same set of scales in an acetone medium. Acetone replaces the air in the ridge-lamallee, thereby increasing the optical path that the light beam has to travel, thus shifting the color of the blue wing scale to the green part of the spectrum (see equations 11 & 12). On evaporation of acetone the original blue color and reflectance spectra return (Figure 16). As can be seen from Figures 15B and C, the thin-film structure comes in the form of stripes. Between the stripes one can see the color of scales of the body or, as is the case sometimes with transparent scales, the color of the scales underneath. Consequently, in this case one can see the two colors, a structural color from the ridges and a pigment-based color from the scales underneath or from a neighboring ground scale, This essentially will form a color pattern containing dark stripes, due to the pigment-based colors. In principle, this can also lead to assimilation, as discussed above.

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Figure 15. A. Wing scales of a Morpho (Morpho menelaus) butterfly at low magnification, where both the cover scale and the ground scales are clearly visible (left). B. An electron micrograph of several wing scales (upper right). C.Electron micrograph showing the ridges and lamellae responsible for the brilliant blue color of the wings (lower right).

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Figure 16. Reflectance spectrum of an individual wing scale shown in Figure 15A.

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In the examples of the blue Morpho butterflies, the deep blue comes from the melanized ground scales while the transparent iridescent glass cover scales provides the shhmery appearance. There are several examples of such interaction between pigment-based and iridescent color [MI. The base structure is also made of chitin but contains some melanin which absorbs the light that is transmitted through the thin-film structure responsible for the iridescent color, and at grazing incidence the wing looks almost a dull shade of dark gray, characteristic of melanin. In attempting to understand the basis for color formation, models based on thin-film structures have been constructed by several authors [3, 43-46], all of which assume that the complex structure can be modeled using plane parallel sheets of chitin sepbated by air to simplify the structure considerably. Theses simplified models have been effective in predicting the trends in the reflectance data obtained experimentally [ 1,45461. Structural color can also reside in the flats between the ridges that have been the focus of this discussion so far. One of the modifications that will give rise to the colors is due to microribs that extend across from one ridge to the next. One can conceive of other modifications of the flats between the ridges. It is not uncommon to find regularly spaced crossribs that might produce colors due to diffraction. The scales of the moth Trichoplusia orichulcea are an example of such diffraction grating structures. Diffraction of incident light by the structure shown in Figure 17 is responsible for the metallic yellow, and the specular reflection and polarization properties of the scattereddiffracted light [47]. In some butterflies the network of crossribs or microribs are transformed into a set of periodic pores (diameter of 200-400pm), which produce a blue color due to scattering, often referred to as Tyndall blue [48].

Figure 17. (a) An S E M micrograph showing a top view of part of a scale from the moth Trichoplusia orichalcea. The length of the marker is 1.6pm (b) oblique section of a wing scale. The length of the marker in (b) is 2pm [47].

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The origin of structural colors can also reside within the lumen of the scale [40]. It is known that in at least three lepidopteran families, the Lycaenidae, the Papilionidae, and the Uranidae, the quarter-wave plates that are responsible for the interference, and therefore iridescent, colors lie within the body of the scale [1,40]. The body-lamellae generally seem to occupy the lumen of the scale, rendering it opaque and highly reflective, thus hiding any of the scales beneath it. These lepidopteran also possess another kind of structural color producing element, a three dimensionally ordered lattice of air pockets embedded in a soft polymer matrix [1,40], producing a sparkling green color. Such submicron structures, also known as zero-order diffraction gratings, have transmittance that is a function of wavelength. They are called zeroorder gratings due to higher orders becoming evanescent, and are non-propagating beams. Many of the structures that have been discussed in the preceding pages are schematically represented by Figure 13, demonstrating how the various modifications of a generic wing scale can yield a multitude of structures through self-assembly. While this process has been the subject of many studies, scientists are still far from duplicating the precision with which a single cell produces the multitude of structures that give rise to the beautiful colors and patterns on the ever so fragile wings of butterflies. One is then faced with the issue of how such structures are produced, a topic that has also been studied for a long time. For useful information on this subject, the interested reader is referred to a review by Ghiradella [40] and a book by Nijhout [25]. An example of color produced by diffraction can be found on the wing scales of Lycaenidae (Lepidoptera). The Green Hairstreak, Cullophrys rubi, displays a very uniform green iridescence over the whole of the underside of its wing. Morris [49] examined the structure responsible for the color using a light microscope and found that each scale was composed of a mosaic of irregular polygonal grains. He also used an electron microscope to study the structure in detail and found the lamellae within each of the grains to be perforated with a high degree of ordering. Upon studying a number of grains, Morris came to the conclusion that the structure could be represented as a cubic network of perforations, as depicted in Figure 18. The lattice parameter or constant was estimated to be 0.257 f 0.025pm, with individual grains having a diameter of 5.4pm. The structure is schematically shown in Figure 18. Figure 19 provides a comparison of the experimental reflectance curve for this substrate, and a theoretical curve calculated using Equation 20. $o

= (sin x I x>2

20

where = 2 z A -Ao), with t the thickness of the walls of the structure shown in Figure 18. It is clear that the shape of the calculated and the experimental curves agree quite well. It should be pointed out that the structures of Cullophiys rubi are in a range of size that allows the diffraction to be limited to zero-order diffraction. All other orders become evanescent, thus are non-propagating waves and do not contribute to the observed reflectance. However, in recent years, interest in sub-wavelength structures has increased, due to the fact that many of these structures have rather interesting optical properties [50-601.

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Figure 18. Model of the wing scale of Cullophrys mbi, a cubic network B. Unit cell that forms the cubic network [49].

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Figure 19. Plots of the reflectance. (A) from the green iridescent scales; (B) theoretical reflectance for the iridescent scale [Redrawn with data from ref. 491.

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In particular, submicron structures having surface relief features have been constructed. They function as good antireflection coatings and as an example, Figure 20 shows the protrusions on the cornea of a moth, discovered by Bernard and Miller [61], who noted its function in antireflection. Subsequently, the optical properties of the so-called "moth eye" antireflection surfaces have been studied extensively [ 5 8 ] . Consider a one-dimensional surface relief profile with a period p , the duty cycle g, and a profile height h. If the grating is composed of two materials of refractive index nl, n2, for normal incidence no light will be diffracted into transmitted (and reflected) higher orders, when n 4 n l (and Un2). Such a condition implies for the wing of Callophrys rubi, the submicron structure giving rise to zero-order diffraction will fall below 0.3ym, as is the case for the wing. The wing's transmission is simply given by Equation 21 [49]

@)= c o s 2 [ n ( n - l ) a / i l ] where n is the refractive index, a is the depth of the relief structure and r ( A ) is the wavelength dependent transmittance. The color that is reflected (or transmitted) is determined by (n -1)a. It would be interesting to study the reflectance and transmittance of many of the sub-wavelength structures that are found in nature. For example, from Figure 15C it is obvious that the dimensions of air pockets forming the ridge-lamellae are on the order of 0.1-0.2ym. Most of the models that have been used to calculate the reflectance of such structures assume that each structure can be simplified to a stack of thin plates alternating in refractive index. The effects of zero-order diffraction have hardly been taken into account. It should be pointed out that in characterizing the optical properties of these intricate structures, interaction of light with subwavelength structures will play a very important role. 5. EYESHINE OF BUTTERFLIES

The eyes of many insects contain structures that are comparable in dimensions to the wavelength of light, and therefore it seems relevant to provide a few examples of such structures. Many of the optical effects that have been discussed in this chapter are also present in the eyes of various insects. Much of the summary that follows arises from the work of Bernard and Miller [61]. The compound eye of an insect consists of many little eyes, called ommatidia, which are close-packed on the surface of the insect's head. An individual ommatidium views only a small part of the scene; typically a few degrees centered about the axis of the ommatidium. Each ommatidium possess the optics necessary for detection and processing of the light that is received by the eye, which includes a photodetector composed of about eight retinular cells [61, 621. The output (due to excitation by light) from the retinular cells is processed by the neural part of the visual system. The butterfly eyes are sensitive to wavelengths ranging from about 0.3ym to 0.65ym, which includes the near ultraviolet, 0.3ym to 0.4ym. This is due to the presence of more than three photoreceptors, some of which are sensitive in the ultraviolet

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region of the electromagnetic spectnun. It is known that these insects can have as many as five photopigmentdphotoreceptors [111.

Figure 20. An electron micrograph of the front surface of a sulfur butterfly cornea [61]. Figure 21 represents the optical elements making up the ommatidium of a typical nocturnal insect, where the section shown is perpendicular the eye's surface. The optical characteristics of the eye determines the kind of signal delivered to the neural part of the visual system. The refractive index of both the tract and rhabdom being greater than the surrounding medium, makes the crystalline tract and rhabdom h c t i o n as optical waveguides. The light carrying the information propagates down the tract and delivers information into the rhabdom, the photodetector. The crystalline tract can function as a lossless waveguide or as a lossy waveguide, depending upon the interaction with the secondary pigment. When the secondary pigments are activated by light, they move down the tract, making the tract lossy, due to evanescent waves being siphoned away fiom the waveguide, thus being converted to propagating waves outside the tract. This implies that less information about the image is sent to the photodetectors thus compromising the daytime vision of the insects. A phenomenon known as the eyeshine is visible at night, when moths or butterflies attracted to a light source. In moths eyeshine (brightly glowing eyes) is caused by a reflecting layer or an interference filter, the tapetum, that is part of the tracheole bush (Figure 21). Another example of eyeshine that is most f e is the reflection fiom a cat's eye viewed in head lights. The butterfly also has a tapewhich apparently is unusual for a diurnal (daytime) animal.

586 Air

Figuie 21. Schematic of the optical components of a typical insect ommatidium. The left side depicts the eye in the light-adapted state, while the right side shows the dark-adapted state [61].

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Figure 22. Schematic diagram of the distribution of colors in eyeshine over the compound eye of the monarch butterfly [reprinted with permission fiom Science, 621.

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Eyeshine from a monarch butterfly is schematicallyrepresented in Figure 22, and results from the fact that most butterfly eyes have a colored mirror located at the bottom of each rhabdom [61]. This is shown in an electron micrograph (Figure 23) of a nearly longitudinal section of several ommatidia from a butterfly eye. Rhabdomes and the tapetal filters are the key features in this figure. It is remarkable that the butterfly tapetum is periodic in refractive index due to the periodic set of cytoplasmic plates that alternate with air space. This produces, successively, a quarter-wave plate, an interference filter, and a reflected band of visible light that propagates up the rhabdom and out the ommatidium where it is observed as the colored eyeshine. Wavelengths in the filter band pass propagates down the filter stack only to be absorbed in the basal pigment. An unusual aspect of the butterfly filter system is that each rhabdom's mirror has its own filter characteristics, which could be entirely different from that of the neighboring rhabdomeres, as is shown in Color Palette 5 (page 606). It is clear that there are a variety of interference a t e r sets in the butterfly visual system. The micrographs were obtained [61] by illuminating a living eye with white light and observing the eyeshine from the direction of illumination. These are visible if the directions of illumination and observation are different by only a few degrees.

Figure 23. An electron micrograph of a section of parallel to the ommatidial axis fiom a Buckeye butterfly eye. The periodicity of 0.23pm of the cytoplasmic plate leads to constructive interference and eyeshine [61].

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6. BIRD FEATHERS Experimental evidence for the iridescent colors of bird feathers primarily came from studies involving thin film colors that were devoid of colored pigments that shift the reflectance maximum or the color to shorter wavelengths with increasing angle of incidence, and that change color towards the red end of the spectrum on immersing the feathers in a medium with a higher refractive index than air. Based on these observations, investigations such as those of Rayleigh [63] and Mason [31], refuted earlier suggestions by Michelson [64] that such colors are "surface colors" similar to those seen from reflection metallic surfaces. While the physical origin of color in feathers was recognized at an early point, it was not until the early 1960's that the structural basis for the color of bird feathers was determined. The work of Greenwalt stands as an outstanding example of the study of iridescent color of bird feathers, in particular Hummingbird feathers [4,65]. Figure 24 is a light micrograph of a typical iridescent platelet mosaic of hummingbird feather from Heliangelus Viola,blue gorget. The micrograph shows that elliptical platelets, about 23pm long and 1-1.5pm wide, packed into a beautiful mosaic, almost like a tiled floor. The platelets are clustered into cells that are separated by diagonal lines crossing the width of structures known as barbules. The platelets are embedded in a dark matrix and it is apparent that the platelets are responsible for the observed color. By measuring the reflectivity as a function of incident angle, it was found that the refractive index of various feathers varied from about 1.85 for red feather to 1.5 for blue feathers. Such variations can be accomplished either by producing materials with different refractive indices, for every color that nature wishes to produce, or by making the interference films responsible for the brilliant colors from two different materials, one of high and the other of low refractive index, varying the average index simply by varying the composition of the two substances. It is rather ingenious of nature to produce a myriad of colors by varying the proportion of two substances, air and keratin. This is demonstrated in Figure 25, which is an electron micrograph of an iridescent feather; shown is a section that consists of tiny air pockets embedded in keratin with as many as eight layers forming the walls of a platelet. This structure is responsible for the thin film interference and the color of hummingbird feathers. The measured reflectance curves agree with the calculated curves. Details of the optical model can be found elsewhere [65].

7. COLOR SPECIFICATIONAND COLOR VISION On examining the kinds of structures responsible for colors on butterfly wings, it is clear that precise control of the way wing structure is created is the key element to produce the color we perceive. The purpose of such elaborate display of patterns and colors are several fold, including warning, camouflage, courtship, species recognition, and perhaps thermoregulation of body temperature [66-69, 1,401. In previous sections of this chapter, little has been mentioned about how to specify color that is displayed by butterfly wings. It is apparent to the human observer who views the color of the wings of butterflies in the Morpho family, and in particular, Morpho Meanalus, that it appears to be an intense blue and that the color seen shifts from blue to violet as one increases the angle of viewing. This is characteristic of most wings that possess

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color due to thin-film interference. While it is often said that butterfly wings reflect a pure blue color,, there has been little quantification of color specification to support this statement.

Figure 24. Typical mosaic of platelets of hummingbird feathers (Heliangelus Viola, blue gorget) [after ref. 65, reprinted with permission from OSA].

Figure 25. Cross section of iridescent feather surface of Clytolaema Ruricauda, (red gorget) produced at high magnification [65, reprinted with permission from OSA].

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Figure 26 is a plot of the chromaticity coordinates of a Morpho wing, the coordinates computed for three different illuminants. Each of the illuminants has a different spectral power density, giving rise to the perception of different colors for the same butterfly wing under different illuminants. The calculations were performed in accordance with the procedure outlined in the earlier sections, employing the product of the standard observer function, together with the spectral reflectivity of the wing and the spectral power density of the incident light source used to observe the specimen. It is clear from this figure that the purity of the blue color is not very high, with purity of a color being defined as the ratio of its distance from the illuminant point to the total distance of the illuminant point to the spectrum locus. It is clear, however, that the calculated chromaticity coordinates for daylight (approximating to illuminant C or D65) falls within the range of blue colors. One of the reasons that the blue color of the Morpho wings appear quite pure has to do with the fact that the melanin present in the body scales absorb light that is not reflected and hence the blue appears pure. The blue of the Morpho wing appears pure for a reason similar to the "pure" blue sky, in that the blue of the sky is viewed against a dark background.

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Figure 26. CIE coordinates for a Morpho wing with three different illuminants, Illuminant A, Cool White Fluorescence (CWF) and D65. 7.1. Polarization colors Birefringent colors are produced by the interaction of polarized light with anisotropic materials. Since they are not seen by the naked eye under normal light, they are not studied with the same approach as standard reflection and transmission colors. However, it is the

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author's intent to suggest that since polarization colors have characteristic transmission wavelengths and are viewed by the human eye with a specific light source, they can be described with the simple tools of color science. Thus, a connection between the Michel-Levy color chart and the CIE chromaticity diagram can be established [70,71]. It is well known that light transmitted through a birefringent medium under crossed polarizers often appears colored. The observed transmission is a function of the birefringence and thickness of the medium in question. Birefringence (An) is usually defined as the difference in the refractive indices of polarized light parallel and perpendicular to the objects optical axis. The product of the birefringence and the thickness (t) gives the retardation, often denoted by r (=t An). If a birefringent medium is oriented under a microscope such that its optical axis is at a 45' angle with respect to the polarizer (and the analyzer), the intensity of transmitted light is given by equation 22 [20]:

z=sin2S/2 where 5 = 2 m h I A , and h the wavelength of the light beam. The Michel-Levy color chart is a reference for comparing birefringence, thickness, and retardation of a given medium to its expected polarization colors. Every 550nm of retardance corresponds to a different order, with each order ending in red. It can be observed that the first order colors are mostly dull grays, the second to fourth orders give a brilliant array of all hues, and orders five and above become increasingly less saturated until almost full achromaticity is observed around the tenth order. Many birefringent mediums correspond to the first order, due mostly to the fact that their thickness is very small, thus forcing retardation to also be small. Since first order colors are mostly whites, blacks, and grays, it is often desirable to view the same medium in second order colors. This can be accomplished with the aid of a magenta plate, which simply increases the retardation of a medium by 550 nm. Therefore, first order colors appear to be of the second order, and thus more brilliant and identifiable. Using the transmission equation for the birefringent medium defined above, the Spectral Transmission for an object under a microscope can be calculated. The light source in most polarizing microscopes is a halogen lamp, so a general Spectral Power Distribution for a Halogen light source can be assumed. Finally, the CIE 1931 Standard Observer Functions can be used as the color detector. Therefore, all the components needed for the chromaticity diagram are supplied, and it seems feasible that the calculation of a color point for a given retardation will match well with its location on the Michel-Levy color chart. Tungsten-Halagen Lamp (400 to 700 nm at 20 nm increments), the 1931 standard observer functions, and the product of the latter values were used for computing the color points. For retardations ranging from 550 to 2200 nm, tristimulus values and chromaticity coordinates were calculated for every 25 nm of retardation. As mentioned before, 550 to 2200 nm corresponds to the second to fourth orders of the Michel-Levy chart, and these are of the greatest interest since they represent the most brilliant colors. Colors of the first order or greater than the fourth order are more or less achromatic and tend to occupy the central area of the chromaticity diagram (see Figure 27). The retardances were increased by 25nm to obtain the chromaticity coordinates.

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Second order colors, ranging from 550 to 1100 nm, sample every hue rather well with a very smooth and slowly inflecting curve. The relative brilliance of the purples and blues, and the extreme saturation of the yellows are readily apparent. It is remarkable that the second order yellows almost reach the physical limit of the spectrum locus, as this degree of chromaticity is much greater than can be attained by an R,G,B color monitor! The third order polarization colors also sample all hues and have a smooth curve, but note that the blues and yellows of this order are far less saturated than those of the second order. However, the third order has a much stronger representation of greens than the second order. Finally, the fourth order begins to take the shape of very slender curve, oscillating between red and green; it has absolutely no trace of magentas, blues, or cyans, and has a very small degree of saturation. Such is the trend with the higher orders of the Michel-Levy chart, which decompose to dull reds and greens, and eventually lose color completely.

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Figure 27. Polarization colors of a birefringent object under crossed polarizers illuminated by a tungsten light source, as seen by the position on the CIE chromaticity diagram. Comparing a Michel-Levy chart and the calculated color points on the chromaticity diagram in the other, it caq be concluded that the colors match almost perfectly in hue and saturation for a given retardation. This comparison is designed to show a relationship between the abstract polarization colors of birefringent media and the basic calculations of color science. A word of caution is essential at this point. The calculation of the colors due to polarization was conducted using the basic equation I = sin26 I 2 . This assumes that the birefringence, An, is independent of wavelength and that dispersion is not important. Since the opposite is true for many materials, one needs to be cautious of how the calculations are conducted. The plot shown in Figure 27 was developed to demonstrate that one can easily interpret the Michel-Levy charts that are so commonly used.

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The color points plotted for increasing retardance correspond almost perfectly to the color points for interference light from soap films at increasing thickness (Figure 28) [72]. Like birefringent colors, the colors produced by a soap film encompass the entire range of hues, and become less saturated as thickness increases. Also, soap film colors can attain extreme saturation of certain blues and yellows, like the polarization colors discussed above. Although this may seem like a peculiar comparison, the reason these two very different sources of color have such similar color points on the chromaticity diagram lies in the way in which they produce color. Both produce colors by constructive interference of light. If a birefringent sample is oriented at a 45" angle with respect to the polarizer and analyzer of a microscope, the only factors affecting the production of color are thickness and birefringence, or change in refractive index. As earlier defined, the product of these two parameters is retardance. At any given retardance, the color produced is the s u m of all the wavelengths of light that interfere constructively. For instance, violet light, with a wavelength of 400 nm, would destructively interfere at retardations of 0,400 nm, 800 nm, etc. Thus, constructive interference would occur at 200 nm, 600 nm, 1000 nm, etc. Therefore, at a retardance of 600 nm, blue light experiences a maximum, whereas red light, with a wavelength of around 690 nm, approaches destructive interference. The result is a polarization color of bright blue, which is correctly displayed by the corresponding point on the chromaticity diagram. 0.9

0.8 0.7

0.8 0.5

Y 0.4

0.3 02

0.1

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

X

Figure 28. CIE plot 'for colors due to interference by soap films [reprinted with permission from Wiley, 721.

594

Soap films produce interference colors in much the same fashion. As light is incident at the film's surface, part of the light is specularly reflected, while the rest is refracted into the film. The refracted light is then reflected at the back surface of the film. Assuming that the film has a refractive index greater than that of air, and that the surface under the film has a refractive index greater than that of the film, reflection at both the front and the back of the film will experience a reversal of electric field. This causes a half-wave change in its cycle. If the distance traveled by the refracted light (2t) multiplied by the refractive index of the film (n) equals any multiple of a certain wavelength of visible light, the corresponding color is produced by constructive interference. That is, if 2nt = m (400 nm), blue light is seen, along with any other colors that are constructively produced. Of course, as the angle of incident light with respect to the surface of the film increases, the thickness (t) of the film also increases, so greater wavelengths of light are seen. Like polarization colors, as film thickness gets very large, many different colors of light are produced, until complete achromaticity is attained. There are some fundamental differences between polarization colors and soap film interference colors. For instance, soap film colors can be seen in normal light, while polarization colors can only be seen with a polarizing microscope. In turn, this is due to the fact that soap films have only one refractive index and light is recombined with simple interference. On the other hand, polarization colors require an anisotropic medium with a change in refractive index, and the interference is more complicated as Ordinary and Extraordinary Rays experience a phase shift and are then recombined in the plane of the analyzer. However, a key similarity of these two kinds of colors is that they basically depend only on refractive index and thickness. Most importantly, these two kinds of colors share the interesting characteristic that neither requires light absorption to produce color.

8. COLOR PROPERTIES OF CHOLESTERIC LIQUID CRYSTALS 8.1. Color by selective reflection Of the many thousands of compounds that have been synthesized, a significant fraction exhibit a liquid crystalline phase [73].Liquid crystals constitute a state of matter that has order in between the isotropic liquid and crystalline solid. These are fluids phases but possess molecular order leading to some unusual physical and optical properties. Depending on the nature of molecular ordering, these phases can be classified as nematic, cholesteric, or smectic phases [73]. In the case of a cholesteric liquid crystal (CLC), the rodlike molecules that comprise this fluid phase have long range orientational order and form a layered structure. In each successive layer the direction of the long axis is rotated by an angle of 10-20 arc min. This gives rise to a helical arrangement of the rodlike molecules. The spacing between layers differing by 360"is called the pitch (p ), of the helical structure. Cholesteric liquid crystals (also known as chiral nematics), whose helical pitch is in the visible region, selectively reflect light with a peak at ,A = 2np, where n is the average refractive index [73]. Selective reflection occurs when the optic axis of the liquid crystal is parallel to the bounding surfaces, known as the Grandjean texture. Such an alignment orients the helical axis normal to the bounding plates. As a result of the periodicity in molecular orientation, reflections from all layers separated by p/2 interfere constructively, giving rise to a

595

reflection band of wavelength that is relatively narrow and steep. The color so produced appears highly saturated and metallic in nature. In our studies, the CLCs were formed by adding appropriate amounts of an optically active material to a commercial nematic liquid crystal [73]. Nematic liquid crystals have long range orientational order, with the molecules oriented essentially parallel to one another and the addition of an optically active dopant produces the cholesteric phase. Appropriate amounts of CB15 (a chiral dopant) was added to E48 (a commercial nematic fluid) to obtain the cholesteric phase. Well aligned cells, with planar alignment (the helical axis normal to the glass surfaces), about 3pm thick were prepared for reflectance measurements. Reflectance measurements were made for materials with pitch covering the entire visible regime. An example is shown in Figure 29 for four different concentrations of the chiral dopant.

40 30 20 10 0

400

450

500

550 Wavelength

600

650

700

Figure 29. Reflectance curves for cholesteric liquid crystals with four different pitches. Note that the reflectivities are close to the maximum theoretical limit of 0.5. The theoretical maximum is 50%, due to the fact that a CLC selectively reflects light of the same handedness [73]. Unpolarized light can be considered as a superposition of left-circularly and rightcircularly polarized light, and therefore light of the same handedness (-50%) is reflected while the rest is transmitted. It should also be emphasized that there is very little absorption loss. It is this ability of selective reflection that produces the additive color mixing properties for CLCs, while producing a color gamut greater than those attainable with dyes, inks and

596

pigments. Figure 30 demonstrates the additive color properties where two CLCs have been sandwiched to obtain the observed reflectance curve. It is clear that the reflection bands of both samples are present in the reflectance spectrum. In this case, light that does not fall in the reflection band of the top sample is transmitted only to be reflected at the sample in the bottom. Thus, the reflectance spectrum contains bands that are reflected from both the samples, and the eye perceives these bands as the color of an additive mixture, as in the case of colored lights. From the discussion of additive color mixing properties, the x, y values for the mixed color must lie on the line joining the two colors. Consequently, it is clear from Figure 31 that the x, y values lie on the line joining the two individual colors, demonstrating additive mixing in a quantitative fashion. It is also clear that the color gamut produced by CLCs is larger than that obtainable by other conventional means, and that the colors produced by CLCs are quite pure since many of the points in Figure 35 are very close to the spectrum locus. 4ut , . . . , . . , .

400

450

I

500

.”’,

I

.... .... I

I

550

600

650

. . . .

700

Wavelength (nm)

Figure 30. Reflectance spectrum for two cholesteric liquid crystals placed on top of each other. So far I have described how CLCs reflect narrow bands of visible light based on the length scale of the pitch in comparison to visible light. One of the problems is that the maximum reflectance is only -50%, but one can use an optical illusion to get almost 100% reflectance as has been demonstrated by Makow [74, 751. The illusion requires using a half wave plate that is sandwiched between the liquid crystal samples of the same handedness. It is known that scarab beetles use this basic mechanism to reflect almost 100% of the incident light of a given wavelength band in the visible region of the spectrum [ 5 ] .

597

t 0.8L*****

f

0.6

8

Y

.:.1 I

"

I

'*

0.4

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 X

Figure 31. Plot of the CLCs on showing the wide color gamut that can be produced using cholesteric liquid crystals. The squares are due the CLCs.

9. APPLICATION OF CLCs TO OPTICAL LIMITING It is possible to use the color properties of cholesteric liquid crystals to our advantage for applications in optical limiting. Since is clear that the reflectance is dependent on the pitch of the CLC, if one were to photochemically change the pitch of the CLC, the reflected wavelength will shift as well. Such a change can achieved by having certain dye molecules dissolved in the CLC, which on irradiation with light of the appropriate wavelength will change the pitch of the CLC. The dye can be so chosen that its absorption peak lies outside of the reflectance band of the starting cholesteric material. Azobenzene is an example of a dye molecule that undergoes a reversible trans-cis isomerization upon irradiation. It has been demonstrated more than 25 years ago that irradiation with 313nm light creates a trans-cis isomerization and that the reflectance spectrum of the CLC can shift by as much as lOOnm [76]. The time dependence of that process was not studied. One can choose dye molecules that would have similar effects at a more convenient wavelength for irradiation.

10. COLOR DUE TO VIBRATIONAL TRANSITIONS 10.1. Is water blue? If so, why? When one is asked the question, why is water blue, one finds a variety of answers ranging from light scattering to blue sky reflection to coloration due to impurities like Cu2+. However, most of those popular answers are incorrect and it will become apparent that water has an intrinsic blue color. This intrinsic color is readily seen in the Caribbean and Mediterranean

598

seas, and of course in mountain lakes in the Canadian Rockies, as well as in photographs of the icebergs in Alaska. The blue color of water comes from the selective absorption of the red part of the spectrum and therefore, the transmitted light is blue in color. This color of water has implications for life under the depths of the ocean floors [77]. Although the focus of much of this chapter has been on color due to physical structures and not colors due to absorption, water occupies a special place in the discussion of color even though its color results from absorption. This is due to the fact that the selective absorption in the red part of the visible spectrum promotes transitions to high overtones leading to highly excited vibrations. Water is a particularly unique example of a substance possessing color as a consequence of vibrational transitions of the molecule. In most of the cases presented so far, color production is mainly due to Rayleigh scattering, interference, diffraction, or refraction, but in each of these cases, the photons primarily interact with electrons. Water is essentially the only known substance in which color is due to vibrational transitions. Braun and Smirnov [78] measured the absorption spectrum of water shown in Figure 32, where the range covered includes the visible to the near infrared. The experiments were carried out using a 10-cm quartz cell filled with water. The weak absorption bands below 700nm contribute to the color of water. Even though the absorbance is low, the total absorption can be significant if the path length becomes large, say on the order of 3m. In this case, light exiting the end of tube filled with 3m of water will look blue, due to absorption in the red part of the spectrum. This absorption comes from a weak absorption peak centered at around 760nm and the two weaker bands at 660 and 605nm. The fact that the bands have a vibrational origin is demonstrated by the spectrum of D20 in the same cell (10-cm path). The 760nm band is now shifted to approximately lOOOnm in D20. If one calculates the loss of red part of the visible spectrum based on the absorbance data, the transmission through a 3-meter water filled tube will be around 44%, significant enough to be observed as a loss in the red, thus making the light that is transmitted to "look" blue. The vibrational modes that are responsible for these transitions are shown in Figure 33, where the various modes for a molecule of water are shown. The OH symmetric ( ~ 1 and ) the antisymmetric (v3) vibrational modes are at high enough energy that a higher overtone (V1+3V3) occurs at 698nm, just at the red edge of the visible spectrum. Knowing that the intrinsic color of water is blue and that it is due to the vibrational transitions, the question of "why then when one looks at a body of water, and not through it, does it appear blue in color?" is a relevant one to ask. The answer to this question is given in detail by Bohren in his book, Clouds in a Glass of Beer. The answer is not a simple one. It is in fact true that both the intrinsic color of water and the reflected incident light make a contribution to the observed color [79]. In order for the intrinsic color of water to be observed incident light must be scattered back, and of course this scattering can in fact shift the frequency of light. Also this scattering is angle dependent and hence is not as simple as one might initially anticipate.

599 0.20-

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