Advances in Photochemistry, Volume 18

ADVANCES IN PHOTOCHEMISTRY Volume 18 ADVANCES IN PHOTOCHEMISTRY Volume 18 Editors DAVID H. VOLMAN Department of Chemi...

Author: David H. Volman | George S. Hammond | Douglas C. Neckers

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ADVANCES IN PHOTOCHEMISTRY Volume 18

ADVANCES IN PHOTOCHEMISTRY Volume 18 Editors

DAVID H. VOLMAN Department of Chemistry, University of California, Davis, California

GEORGE S. HAMMOND Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio

DOUGLAS C. NECKERS Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio

A WILEY-INTERSCIENCE PUBLICATION

JOHN WILEY & SONS, INC. New York

0

Chichester

0

Brisbane

0

Toronto

Singapore

This text is printed on acid-free paper. Copyright 0 1993 by John Wiley & Sons, Inc. All rights reserved. Published simultaneously in Canada. Reproduction or translation of any part of this work beyond that permitted by Section 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to the Permissions Department, John Wiley & Sons, Inc., 605 Third Avenue, New York, NY 10158-0012. Libmry of Congress Cataloging in Publication Data: Library of Congress Catalog Card Number: 63-13592 ISBN 0-471-59133-5

1 0 9 8 7 6 5 4 3 2 1

CONTRIBUTORS Andre M. Braun Lehrstuhl fur Umweltmesstechnik Engler-Bunte-Institut Universitat Karlsruhe 7500 Karlsruhe, Germany George S. Hammond Center for Photochemical Sciences Bowling Green State University Bowling Green, OH 43403 Graham Hancock Physical Chemistry Laboratory Oxford University South Parks Road Oxford OX1 342,United Kingdom Dwayne E. Heard School of Chemistry Macquarie University Sydney, NSW 2109 Australia Laurent Jakob Lehrstuhl fur Umweltmesstechnik Engler-Bunte-Institut Universitat Karlsruhe 7500 Karlsruhe, Germany

Claudio A. Oller do Nascimento Escola Politechnica da Univesidade de S5o Paulo 01OOO S5o Paulo, SP, Brasil Douglas C. Neckers Center for Photochemical Sciences Bowling Green State University Bowling Green, OH 43403 Esther Oliveros Lehrstuhl fur Umweltmesstechnik Engler-Bunte-Institut Universitat Karlsruhe 7500 Karlsruhe, Germany V. Ramamurthy Central Research and Development Experimental Station The Du Pont Company Wilmington, DE 19880-0328 Oscar M. Valdes-Aguilera Center for Photochemical Sciences Bowling Green State University Bowling Green, OH 43403 Richard G. Weiss Department of Chemistry Georgetown University Washington, D C 20057

PREFACE

Volume 1 of Advances in Photochemistry appeared in 1963. The stated purpose of the series was to explore the frontiers of photochemistry through the medium of chapters written by pioneers who are experts. As editors we have solicited articles from scientists who have strong personal points of view, while encouraging critical discussions and evaluations of existing data. In no sehse have the articles been simply literature surveys, although in some cases they may have also fulfilled that purpose. In the introduction to Volume 1 of the series, the editors noted developments in a brief span of prior years which were important for progress in photochemistry: flash photolysis, nuclear magnetic resonance, and electron spin resonance. Since then two developments have been of prime significance: the emergence of the laser from an esoteric possibility to an important light source; the evolution of computers to microcomputers in common laboratory use for data acquisition. These developments have strongly influenced research on the dynamic behavior of excited state and other transients. With an increased sophistication in experiment and interpretation, photochemists have made substantial progress in achieving the fundamental objective of photochemistry: Elucidation of the detailed history of a molecule which absorbs radiation. The scope of this objective is so broad and the systems to be studied are so many that there is little danger of exhausting the subject. We hope that the series will reflect the frontiers of photochemistry as they develop in the future. DAVIDH. VOLMAN GEORGE S. HAMMOND C . NECKERS DOUGLAS Davis, California Bowling Green, Ohio Bowling Green, Ohio

vii

CONTENTS

Time-Resolved FTIR Emission Studies of Photochemical Reactions GRAHAM HANCOCK AND DWAYNE E. HEARD A Model for the Influence of Organized Media on Photochemical Reactions V. RAMAMURTHY,RICHARDG. WEISS, AND GEORGE s. HAMMOND

1

67

Up-Scaling Photochemical Reactions AND& M. BRAUN,LAURENT JAKOB, ESTHER OLIVEROS, AND CLAUDIO A. OLLER DO NASCIMENTO

235

Photochemistry of the Xanthine Dyes DOUGLAS c.NECKERS AND OSCAR M. VALDES-AGUILERA

315

Index

395

Cumulative Index, Volumes 1-18

402

ix

Advances in Photochemistry, Volume18 Edited by ,David H. Volman, George S. Hammond, Douglas C. Neckers Copyright © 1993 John Wiley & Sons, Inc.

TIME-RESOLVED FTIR EMISSION STUDIES OF PHOTOCHEMICAL REACTIONS Graham Hancock and Dwayne E. Heard Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom

CONTENTS

I. Introduction 11. Fundamentals of Fourier transform spectroscopy 111. A practical guide to time-resolved FTIR emission studies A. Stop-scan (SS) time-resolved Fourier transform spectrometers 1. Historical development 2. The Oxford stop-scan time-resolved FTIR emission spectrometer B. Continuous-scan (CS) time-resolved Fourier transform spectrometers C. Comparison of S S and CS time-resolved FTIR spectrometers IV. Applications A. Internal state distributions of the fragments of molecular photodissociation B. Product state distributions from laser-initiated bimolecular reactions

2 5 10 10 10 12 22 28 31

31 37

Advances in Photochemistry, Volume 18, Edited by David Volman, George S . Hammond, and Douglas C. Neckers ISBN 0-471-59133-5 Copyright 1993 by John Wiley & Sons, Inc.

1

2

V.

G . HANCOCK AND D. E. HEARD

C . Kinetic studies of chemical reactions and energy transfer processes Conclusions Acknowledgments References

INTROD ICTIO

48 57 58 58

i

Fourier transform infrared (FTIR) emission spectroscopy is now the method of choice for observation of weak sources of infrared radiation in a wide variety of applications. The advantages of Fourier transform techniques [11, namely, that all light frequencies are observed simultaneously at the detector (the multiplex or Fellgett advantage [2]), and that the optical throughput is greater than for a dispersive instrument with light-confining slits (Jacquinot advantage [3]), have enabled high-resolution spectra to be obtained routinely in short periods of time with excellent sensitivity. In the field of chemical reaction dynamics, the nascent product state distributions of many bimolecular chemical reactions have been measured by FTIR analysis of the infrared chemiluminescence from vibrationally excited products [4-91. In most of these studies reaction has been initiated by simple mixing of the reagents. The nascent product state distributions are obtained free of the effects of collisional relaxation by extrapolation of the distributions observed at various positions in a flow-tube (measured relaxation [lo, 111) or by carrying out the reactions at low total pressure such that the excited species are completely quenched at the walls before any relaxation occurs (arrested relaxation [lo, 123). Many of the fundamental concepts on which molecular dynamics is based, for example, understanding the partitioning of the available energy into the internal degrees of freedom of the reaction products, or the efficacy of reagent vibrational versus translational energy in overcoming barriers in the potential energy surface, have emerged from such experiments [13,14]. Conventional FTIR instruments, in which the interferometer mirror is translated at a constant velocity, are ideally suited to the analysis of steady state infrared emission. However, time resolution of the infrared emission is required in many applications, such as the measurement of absolute rate constants for the formation or subsequent relaxation of a vibrationally excited species. It is then necessary to follow the intensity of the emission (at a particular wavenumber if state-specific rate constants are required) as a function of time. For continuous-wave experiments, crude time resolution

TIME-RESOLVED FTIR EMISSION STUDIES

3

can be obtained by varying either the distance between the point of mixing of the reagents and the FTIR observation point [lo, 11,151 or the gas flow rate for a fixed mixing-spectrometer separation [161. Analysis of the transformed spectra at different reaction times allows the populations to be monitored with time and, if desired, extrapolated to obtain the initial distribution [lo]. A more desirable approach is to take advantage of the short pulse duration, high energy, and high repetition rates of pulsed laser sources to cause photolytic production of one of the reagents, allowing exact specification of zero time for the reaction. This method is standard for studies of molecular dynamics in which products are observed by laser-induced fluorescence [17], but has been little used for monitoring products by IR emission because it is difficult to obtain good signal-to-noise ratios for spectrally resolved emission from specific vibration-rotation states as a function of time. Interference filters are able to isolate spectrally the emission from one particular molecule (or one vibrational mode within a polyatomic molecule), and in several experiments the temporally resolved filtered emission has been studied under varying conditions of pressure and temperature in order to measure the rates of bimolecular reactions or energy transfer processes [18-22]. Early time-resolved experiments employed circular variable filters with slightly improved but still poor spectral resolution (3060 cm- l ) to measure the vibrational distribution and relaxation of molecular fragments excited by UV photodissociation [23] or translational-tovibrational energy transfer [24]. However, the time dependences at any given wavenumber under these conditions correspond to emission from a wide range of excited levels. Time-resolved experiments using dispersive grating spectrometers to analyse pulsed emission from laser-initiated reactions [25] again only allowed low (11-22 cm-') spectral resolution owing to optical throughput problems. Individual rovibronic spectral features could not be resolved, and vibrational distributions were deconvoluted from the data by spectral fitting routines. The use of FTIR techniques in studies of time-resolved IR emission has been a relatively recent development, and two of the major practitioners in the field have provided excellent reviews of progress up to 1989 [26,27]. This chapter does not attempt a historical survey of the method, but instead describes progress since 1989, suggests possible further areas of study, and most importantly tries to provide the experimentalist with a practical guide to the use of the technique for studying a wide range of photochemical systems. Commercial Fourier transform spectrometers operating at moderate resolution (1 cm-l) require fractions of seconds to complete a scan of the interferometric mirror (scans may only take tens of milliseconds if only low spectral resolution is required). A new strategy must now be used to study the

4

G. HANCOCK AND D. E. HEARD

transient IR emission signals originating from the products of laser-initiated reactions or photofragmentations which typically decay in tens or hundreds of microseconds. A number of early experiments which employed timeresolved FTIR methods are documented [28-361, but during the last six years we have seen extensive development of fast time-resolved FTIR spectrometers to tackle problems in gas-phase reaction dynamics and molecular photodissociation. Two fundamentally different approaches have been employed. The first, referred to as the stop-scan (SS) method, records the entire temporal evolution of the IR emission while the interferometric mirror is held stationary at each of its sampling positions. The key feature is that the complete temporal evolution of every wavenumber in the product emission spectrum is obtained from only one scan of the interferometric mirror [37]. A SS instrument was developed in the authors’ laboratory [37-381 to study the IR vibrational fluorescence of the products of atom-radical reactions and molecular photodissociations, and the modification of a low-cost teaching interferometer for use as a fast time-resolved instrument is described in detail in later sections. Free radicals with low internal energy were generated by IR multiple photon dissociation (IRMPD) [39], and their reactions with atomic partners were studied in a discharge flow system. For example, nascent vibrational distributions in CO(u’) and HF(o’), generated in the exothermic reaction between oxygen (3P) atoms and the monofluorocarbene radical CHF, were measured together with kinetic parameters from the temporal evolution of the emission spectrum [40]. The second approach is referred to as the continuous-scan (CS) method, in which the interferometric mirror is never stationary throughout the duration of the IR transient. The photolysis laser is triggered when the continuously moving mirror reaches each interferogram sampling point, and the subsequent pulse of IR radiation is digitized at some user-defined delay following the laser pulse. It is assumed that the IR signal is constant during digitization (a very short sample-andhold gate width is used) and hence following one sweep of the mirror an interferogram (and therefore a FT spectrum) corresponding to one time point in the transient is obtained. The application of the CS method to reaction dynamics was introduced by Sloan and co-workers in 1985 [41]. Electronically excited O(’D,) was generated from the 248-nm photolysis of ozone, and nascent OH(u’, N ’ ) distributions were measured after reaction with a variety of hydrogen-containing molecules [7,41-451. The same strategy has been successfully applied to time-resolved observations of nascent fragments from the U V photolysis of a variety of molecules by Leone and co-workers [46501. The high resolution of the CS method has allowed emission from polyatomic fragments to be analyzed to obtain vibrational distributions in electronically excited states formed by U V photolysis of suitable precursors [49, SO]. Recent modifications to the technique allow the observation of


5

transient signals which do not conveniently decay before the next sampling position is reached [Sl]. The chapter is set out in the following way. Section I1 contains elements of the theory of Fourier transformations which, rather than being exhaustive (and exhausting), aims to cover the details and limitations of the technique which are of importance for the experimentalist to understand. Section I11 contains descriptions and comparisons of the S S and CS methods and outlines the advantages and pitfalls of each, together with recommendations for their suitability for specific applications. Section IV presents recent results from time-resolved FTIR emission experiments, emphasizing photochemical applications.

11. FUNDAMENTALS OF FOURIER TRANSFORM SPECTROSCOPY Fourier transform methods have revolutionized many fields in physics and chemistry, and applications of the technique are to be found in such diverse areas as radio astronomy [52], nuclear magnetic resonance spectroscopy [53], mass spectroscopy [54], and optical absorption/emission spectroscopy from the far-infrared to the ultraviolet [55-571. These applications are reviewed in several excellent sources [l, 54,581, and this section simply aims to describe the fundamental principles of FTIR spectroscopy. A more theoretical development of Fourier transform techniques is given in several texts [59-611, and the interested reader is referred to these for details. For a conventional dispersive spectrometer operating between 400 and 4000cm-' at a resolution of 1 cm-', only 0.03% of the radiation that enters the instrument reaches the detector at any one time. Such losses can be disastrous if weak emissions are to be observed in the infrared, where detectors are limited by background noise from the black-body surroundings and their own thermal energy. The time taken to record a high signal-to-noise ratio (SNR) spectrum at high spectral resolution becomes prohibitively long. In Fourier transform spectrometers the incident beam of collimated IR radiation is passed into a Michelson interferometer [62,63]. A beamsplitter divides the beam into two equal parts which are reflected by two mutually perpendicular plane mirrors, one of which can be translated along the optical axis. Following recombination at the beansplitter, the two spatially coherent beams interfere to give a stationary pattern of interference fringes. If the radiation is monochromatic this will occur for all values of the translating mirror, but if it is broad band a pattern is only seen in the vicinity of zeropath difference between the two mirrors. If a detector is placed at the center of

6

G. HANCOCK A N D D.E. HEARD

the interference pattern (which consists of circular rings for a monochromatic source) and the path difference between the two beams is precisely varied by translating one mirror, the resulting interferogram has encoded information about the source of radiation. The multiplex or Fellgett advantage resulting from this approach [2] gives a superior SNR of M”’ over a dispersive instrument, where M is the number of resolution elements in a spectrum (defined as the spectral bandwidth divided by the resolution). The same SNR can thus be achieved in a considerably shorter measurement time. The Fellgett advantage is only realized in spectral regions where detectors are background-noise limited, and is lost for detectors which are shot-noise limited (e.g., photomultipliers). The throughput or Jaquinot advantage [3] of an interferometer is typically a factor of 100 or more [64], and is realized for all spectral regions. The signal seen at the detector for a given value of the optical path difference (OPD),given by the symbol 6, is dependent upon the wavelengths, amplitudes, and phases of the components of the radiation. Constructive interference for all components occurs only at 6 = 0, where the maximum signal is observed (often referred to as the centerburst or central maximum). The signal that is seen at the detector as a function of 6, I ( 6 ) for an ideal interferometer, is given by

::j

Z(6) = -

B(V){l + cos 27cV6)dV

where B(S)dS is the intensity of the spectral component in the wavenumber range V to 5 dS. The first term of the expression is constant and is subtracted from the interferogram before computation of the spectrum. The spectrum B(V)is then calculated from an inverse Fourier transformation of the modulated part of Eq. (1):

+

B(V)=

s_a

1(6)cos(2nV6)d6

The cosine Fourier transform given by Eq. (2) is only applicable if the interferogram is perfectly symmetrical about 6 = 0. In practice additional wavenumber-dependent phase shifts are present, owing to beamsplitter characteristics or refraction effects, and cause the interferogram to appear partially asymmetric. The modulated part of Eq. (1) then becomes

:J:-

I(6) = -

B(V)COS(27cV6

+ O(ii))dV

(3)


7

where O(V) is a wavenumber-dependent phase error. The interferogram now contains some sinusoidal character, and a complex Fourier transformation is required:

Z(6) exp{ -2ni56)dd

(4)

yielding a spectrum consisting of real and imaginary parts, representing contributions from the cosine and sine components of the interferogram respectively. The integral expressions (3) and (4)cannot be realized in practice for two reasons: 1. The optical path difference over which the interferogram can be digitized is limited by the dimensions of the interferometer. 2. Digitization of Z(6) can only be performed at finite intervals of 6.

The integrals are thus replaced by summations: B(V)= A6

m= +M m= -M

I(mA6)exp( -2niVmA6)

The interferogram is digitized at a total of 2M + 1 points with a sample interval of A6 and computation now involves evaluation of a sum over the 2M + 1 values of 6 for each value of V in the spectrum. A Fourier transform instrument is able to record an interferogram corresponding to a spectrum with comparable SNR and spectral resolution to a dispersive instrument in a fraction of the time. However the time advantage for very high resolution studies was not realized until the advent of fast digital computers, which, using the “Fast Fourier Transform” algorithm developed by Cooley and Tukey [65], could calculate the Fourier transforms of interferograms with very large numbers of M very quickly. Replacement of Fourier integrals by summations has two important ramifications of practical importance: 1. The resolution of the spectrum is reduced, as no data points are taken in the interferogram beyond ,a,, = iMA6; higher resolution requires greater mirror travel. At the extrema of the mirror travel there is a discontinuity in the value of the interferogram (it suddenly becomes zero), and this is equivalent to multiplying it by a boxcar function D(6) given by

8


One of the fundamental theorems of Fourier transforms states that multiplying two functions in one Fourier domain is equivalent to convoluting the two functions in the other domain [60]. The FT spectrum thus has a lineshape corresponding to the Fourier transformation of D(d), which is the sinc function B(5) = 2d,,,

sin(2~5d,,,) = 26,,, 27158,,

sinc(2~5d,,,)

(7)

where d,,, is the maximum mirror travel. The complex nature of the spectral lineshape [given by Eq. (7)] resulting from Fourier transformation with boxcar truncation has some interesting properties. The full width at half maximum (FWHM, often used as a definition of resolution) is given by 0.6034/6,,,, and is hence inversely proportional to the maximum mirror travel. Lines in the transformed spectrum appear with subsidiary side lobes or “feet,” an inconvenient distortion which may be mistaken for other spectral features or even swamp a genuine weak feature. This problem is circumvented by “apodization” (Greek for removal of the feet) of the interferogram. New improved lineshapes with suppressed feet are obtained by multiplying the interferogram by an apodization function prior to Fourier transformation. The “weighted” interferogram now reaches zero at fd,,, rather more gradually but, as information at the extreme ends of the interferogram is being thrown away, the spectral resolution is degraded in the form of a broader FWHM. Triangular apodization is popular (reduced sidelobes, FWHM = 0.88/6,,,), and gives the same sinc’ lineshape as encountered in a diffraction-limited grating spectrometer. Commercial instruments employ sophisticated apodization functions, optimizing the FWHM and suppression of sidelobes, and are specific to the SNR available [ 5 5 , 5 9 ] . 2. For a finite sampling interval Ad, more than one superposition of cosine/sine waves can give rise to the recorded interferogram. For the transformed spectrum to be unique, the sampling interval Ad must be sufficiently small to detect modulations in the interferogram due to the shortest wavelength present in the spectrum, the so-called Nyquist criterion [66]:

Here V,, is the maximum wavenumber present (Nyquist wavenumber). must be removed by suitable optical filtering or Radiation 5 above V,, additional features and noise will be folded back onto the spectrum. Aliasing, as this phenomenon is called, places constraints on the operation of real


9

interferometers. Experimentally precise sampling at Ad intervals is performed using the sinusoidal interferogram of a monochromatic source. Helium-neon lasers are used for this purpose to provide an accurate reference of the optical path difference, and values of A6 [and hence V,, from Eq. (S)] may be chosen to be 0.5nAH,,,, where n is an integer. Sampling in S S and CS interferometers is discussed in Section 111. If lines in an aliased spectrum do not overlap those in the normal spectrum, more efficient “undersampling” may be used, giving a greater mirror travel and hence enhanced resolution for the same number of data points [67]. A consequence of using a discrete FT is a spectrum with data points equally placed in wavenumber, with a spectral spacing determined by the Nyquist wavenumber and number of interferogram points. It is stressed that the spectral spacing is not equal to the resolution of the instrument, which depends not only upon d,, but also upon the apodization used. As the interferograms contain some asymmetry, data should be recorded on both sides of the centerburst. Collection of a complete double-sided interferogram (whose N data points are real numbers), gives the complex Fourier transform spectrum:

+

forming a Hermitian sequence, that is, B ( N / 2 K ) is the complex conjugate of B(K).The imaginary part would be zero for a symmetric interferogram. The power spectrum is then given by

A disadvantage of calculating the power spectrum is that the spectral information and noise are computed to have positive values (noise is generally randomly positive and negative) due to the squared terms in Eq. (lo), and the height of the baseline may increase above its true value for a noisy spectrum. Recording a single-sided interferogram and performing a cosine Fourier transformation would obviate the need to calculate a power spectrum, and would immediately improve the spectral resolution by a factor of 2 for the same number of data points. However, distortions in the spectrum will result unless the phase error e(5) in Eq. (3) is known for each 5 [SS]. ‘The term e ( i ) can be found by recording a very short double-sided interferogram and calculating

e(v) = tan- lCT,in(v)/T,os(S)I

(11)

Phase correction algorithms can then use O(V) to correct the asymmetry in a

10


much longer single-sided interferogram [68,69]. The discrete nature of interferogram sampling ensures that no single data point will exactly correspond to 6 = 0, generating an additional phase error. Commercial instruments employ a white light reference beam, whose sharp centerburst indicates 6 = 0 precisely, which, together with phase correction packages, allow single-sided data to be taken. In practice, a number of experimental factors degrade the resolution from that of an ideal interferometer. Emission sources are finite in size, and hence the beam entering the interferometer is slightly divergent. A critical angle of divergence for a given resolution and Nyquist wavenumber can be calculated [ S S ] , and for large area sources, an aperture (or Jacquinot stop) may be required to increase the quality of collimation, reducing the throughput advantage. Typically emission occurs only from the region of a laser focus in photochemical applications, and a Jacquinot stop may only be required at very high resolution. Mirror misalignment during a scan can also degrade resolution [ S S ] , and for high resolution work, commercial instruments employ dynamic alignment of the moving mirror (the reference laser forms a two-dimensional image) during a scan. Unapodized resolutions of 0, and hence FTIR emission spectroscopy could not be used to measure spectra in the 2000 cm-’ region for this reaction.

46


A number of studies of the kinetics of the NCO radical have been previously carried out in this laboratory [150-1521, and its reaction with the N O molecule

+

NC0(gZII) NO(X211)-,N,0(X'C')

+ CO(XIC+)

AH:-278 kJmol-' +N2(X1C+)+CO,(X'C;)

(17)

AH: - 643 kJ mol-

(18)

+N2(X'Xl)

+ CO(X'Z+) + O(3P)

AH: - 111 kJ mol-'

(19)

has been recently studied by time-resolved FTIR emission spectroscopy [lOS]. Although the rate constant is well known over a wide temperature range [150-1541, the relative importance of the various reaction channels is still the source of some debate. The reaction is of combustion importance, as it contributes to the removal of N O in the RAPRENO, process, in which the cyclic trimer of cyanuric acid, (HOCN),, is added to combustion flue gases. Reaction (17) is believed to be responsible for the high levels of N z O observed in experimental RAPRENO, reactors [l55], and has been assumed to be the most likely pathway, with the formation of CO, being less favored owing to the 1-3 oxygen migration necessary following formation of the ON-NCO intermediate [1541. However, diode laser absorption measurements have recently shown, surprisingly, that the C 0 2 product dominates, with k , , :k , , : k , , = 0.44:0.33 : 0.23 [156]. Infrared emission experiments can add to this body of data, but care must be taken in the interpretation of the results, as both RAPRENO, reactor and diode laser experiments probe ground states of product molecules, whereas IR emission only refers to vibrationally excited states. Three methods of NCO formation have been used to study IR emission from reactions (17)-(19), namely IRMPD of C,H,NCO, IRMPD of o-CF3C,H,-NCO, and reaction (20)

the C H F being formed from IRMPD of CH,F, as described earlier. Figure 21 shows the spectral region between 1800-2800cm-' measured at a resolution of 9 cm- '. The significant feature, common to all NCO formation methods, is that an initially broad distribution between 1900 and 2300 cm-' relaxes collisionally to produce N 2 0 (001) as the dominant emitting species. Both CO, and CO are also seen but the former, despite having a larger


47

N,0(001-000)

1800

2300

2800

Wavenumberlcm-1 Figure 21. Emission in the 1800-2800cm-' region following the IRMPD of CH,F, (25 mTorr) in the presence of NO (100 mTorr) and Ar (10 Torr). Emission from N,O (001) and C O , (001) are seen at long times following the vibrational cascade of highly excited species; their presence at early times [together with CO(v = l)] was established by cold gas filter experiments. N,O is the dominant emitter, and is believed to originate from the reaction sequence (20), (17).

Einstein A coefficient in its (001-OOO)transition at 2349 cm-' than that for the corresponding band at 2223 cm-' in NzO, appears always to be the minor triatomic product emitting in this range. Although these results are presently preliminary (and their interpretation may need to be revised if, for example, rates of relaxation of the excited products are distorting the partially relaxed vibrational distributions), they seem incompatible with the diode laser observations unless reaction (18) produces CO, substantially in vibrational levels with v 3 = 0. Further experiments are in progress. Atom-radical reactions have recently been investigated using the CS timeresolved instrument (with interleaved sampling) in Leone's laboratory [Sl, 861. Infrared emission from HCl was observed in the chain chlorination of C,H,, initiated by 351-nm photolysis of C1, in the presence of the alkane [86]. Interferograms were recorded at 34-ps time intervals (fixed by the sampling rate of the HeNe crossings) at a resolution of 0.36 cm-' as shown in

48


Figure 12. An initial distribution of HCl(u = 1-3) was obtained, and is believed to originate from the reaction of C1 atoms with the ethyl radical, although several reactions can generate vibrationally excited HC1. Very recently 193-nm ArF excimer laser photolysis of Me1 or acetone and SO, mixtures was employed to generate 0 atoms and CH, radicals, and their reaction was monitored using the product CO(u’) chemiluminescence [S 11.

C. Kinetic Studies of Chemical Reactions and Energy Transfer Processes Measurement of the rates of formation of vibrationally excited molecules and the subsequent relaxation is a natural extension of the time-resolved FTIR technique. No other method exists for measuring the time-dependent population of all the excited rotational-vibrational levels of any emitting species simultaneously. Both CS and S S methods can generate the temporal evolution of a given spectral feature, and observation of the behavior as a function of reagent pressures enables kinetic information to be gained. However, one of the disadvantages of FTIR spectroscopy for kinetic studies is that it is not possible to “sit” on a spectral feature and acquire temporally resolved data for a given number of laser shots. An entire interferogram must be scanned before any spectral information is obtained. To avoid prohibitively long data acquisition times, time-resolved measurements obtained by systematically varying the experimental parameters are usually limited to short scans and hence lower resolution. It must be stressed however that spectra at high resolution are first required to identify unambiguously the emitting species and to measure the degree of internal excitation. An ensemble of vibrationally excited molecules formed, for example, by chemical reaction, will relax via a variety of collisional and radiative processes, the rates of which will depend upon the vibrational level, and will result in strongly wavenumber dependent kinetic behavior in emission. Only the highest vibrational level formed will have a rise which is nofslowed by cascade of population from higher levels. This is clearly illustrated in Figure 22, where the temporal behavior of CO, formed vibrationally excited in the 0 + FCO reaction (16) (see Section 1V.B) is shown for Av, = - 1 emission at four different spectral wavenumbers. The temporal resolution was 3 ,us, and the decays were obtained from sorting the time-resolved spectra, three of which were shown in Figure 19. The shift in emission to higher wavenumbers at longer times is consistent with an initially vibrationally excited set of emitters cascading to lower levels mainly by collisions, with the consequent shift in emission wavenumber being due to anharmonicity. The rising rate is very nonexponential for the higher wavenumbers.


49

Figure 22. Time resolved behavior at four wavenumbers for C 0 2 produced in the reaction sequence (15,16). The peaking of the emission at longer times with increasing wavenumber is characteristic of vibrational relaxation of the emitting CO, molecules. The traces were obtained by sorting time-resolved data such as that shown in Figure 19.

We give the following description of the kinetics of the emission from CO, formed in the reaction sequence (15), (16) as a case history showing the information that can be extracted for IR emission studies, and how, after identification of the emitting species by means of a complete FTIR emission spectrum, the kinetic behavior can be more easily studied by means of narrow band filter experiments. It was first established that the photolysis product CF, and not vibrationally excited precursor CF,HCl, which will undoubtedly be present in higher concentrations (because of the CO, laserinduced photolysis step) is responsible for the observed IR emission. The total CO, chemiluminescence was a linear function of CF2HCl pressure; rising and falling rates were independent of [CF,HCl], but increased markedly with [O] over a wide range of CF,HCl pressures. For the CF, radical to be involved, the IR chemiluminescence intensity should, for a given set of reagent pressures, track the CF, concentration as measured by LIF. Both these quantities were varied by changing the CO, laser fluence, and despite the very nonlinear variation of each of these quantities with CO, laser fluence, they were found to be linearly related [82]. The fluence dependence of the excitation of CF,HCl would be markedly different from that for production of CF,, and this rules it out as a source of the CO, emission. With the time dependence of the emission spectrum thus established by the SS FTIR instrument, kinetic measurements were then carried out using a series of narrow band interference filters, whose transmitted wavenumber ranges

50

G . HANCOCK A N D D. E. HEARD

encompass the spectral features shown in Figure 19. The temporal behavior was identical to that observed at a given wavenumber from FTIR measurements (see Figure 22) but was achieved in a far shorter acquisition time. With a filter centered at 2326 cm-', radiation from the CO2(O,0,1)-+ (O,O,O) transition (among others) was transmitted, and the kinetics of this upper level alone were observed by recording the difference between the emission passed by the filter with and without a cell containing 10 Torr of C 0 2 in the path of the beam. Cold gas filters have often been used to isolate emission from the first vibrationally excited level in kinetic studies using IR emission [157, 1581. In this case C 0 2 absorbs the resonant (0,0,1) + (O,O,O) emission (nonresonant absorption was shown to be negligible at this pressure), and the difference gives the time history of CO,(O,O,l) level. A marked induction time for formation of CO2(O,0,l) was observed. Figure 23 shows the temporal behavior of the emission transmitted by filters centered at 1873 and 2066 cm-' respectively, at a variety of oxygen atom pressures. In both cases the rising and falling rates increase with [O], and the timescale is compressed for the lower wavenumbers. Cascading of population from higher levels complicates the analysis of data such as those of Figure 23b, but this problem is minimized at the lowest wavenumber emission. A simple reaction scheme applied to the data of Figure 23a [82] produced a rate constant for reaction (15) of (2.3k0.2) x lo-'' cm3 molecule-' s-l, in excellent agreement with previous measurements [129,159,160]. The scheme also resulted in a rate constant of (1.8k0.2) x lo-" cm3 molecule-' s-' for quenching of the emitter at this wavenumber, highly vibrationally excited C 0 2 , by 0 atoms, a value almost 100 times faster than that for quenching the CO2(O,0,l) level [161]. Emission near 1873 cm-' is not due to a unique vibrational state of CO,, and has substantial excitation in all three modes. The rapid quenching may involve loss of energy in the lower-frequency v1 and v2 modes, with the magnitudes of their anharmonic couplings to v3 ensuring that the relaxed molecule emits outside the observed range of the filter. Extraction of V-V or V-T rate constants as a function of vibrational level would require significant modelling of the complex cascade processes in the triatomic product. A similar procedure to this was used to measure rate constants for the 0 CHF and NO + NCO reactions [40,108]. Electron bombardment of gas mixtures, although not strictly a photochemical process, has been used in conjunction with time-resolved SS interferometers to obtain rate constants for the vibrational relaxation of highly excited molecules [30, 35,36,71-731. Murphy et al. [35,71] observed the production and relaxation of vibrationally excited N O and the (O,O,l -+ O,O,O) bands of N 2 0 and NO2 following excitation of N 2 / 0 2 mixtures with a pulsed electron gun. The infrared emission created by the electron beam decayed completely between pulses and the complete temporal

+

51


[ O]/m Torr

la)

e \

5

0

0

100

200

Time / microseconds

[ O l l m Torr

0

rn

Time

/

400

microseconds

400

300

Ib)

800

Figure 23. Variation of the intensities and time profiles of the emission from CO, produced in the sequence (15,16) passed through filters centered at 1873 cm-' (a) and at 2066cm-'(b) as a function of oxygen-atom pressure marked on the diagrams in mTorr. [CF,HCl] = 10.4 mTorr, total pressure 4 Torr, 3 p s time resolution. (a) Data were taken over 200 laser shots at 68 J cm-'. (b)Data for 100 laser shots at 59 J cm-'. Note the different timescales for the two sets of traces. Reproduced with permission from Ref. 82.

52


evolution of 50 excitation cycles was recorded and averaged for each interferometric mirror position (or between adjacent sampling positions for a quasi-SS scan). The strongest emission was from NO(u’ = 1-10), generated from the reactions of N(,D) and N(,P) with 0,. The N,O (0,0,1+O,O,O) emission around 2200cm-’ was thought to be due to V-V exchange with excited N, or N,O, or to a three-body process. Time-resolved spectra of the NO(Au’ = - 1) emissions were recorded at 100-ps intervals during and after the beam pulse, with a resolution of 10cm-’. No rotational structure was resolved, and relative vibrational populations as a function of time were obtained from fits to the spectra [71]. The observed temporal histories of the vibrational populations were interpreted in terms of quenching by the reactant gases N,, O,, and the beam-created species 0, and by radiative decay. The master equations which couple together all the quenching processes of all vibrational levels were solved to give vibrational relaxation rates as a function of u’. From time-resolved spectra taken at different 0, pressures, the room temperature rate constants for the processes NO(u’) 0, +NO(u’- 1 ) + 0 , , u’ = 1-7, increased monotonically with 0’. In a related study Caledonia et al. [36] determined the vibrational level dependent quenching rates of CO(u = 1-16) by CO, with a quasi-SS instrument (see Section III.A.1). The CO production mechanism, predominantly dissociative recombination of CO:, was found to produce CO up to u = 19 following electron bombardment of Ar/CO, mixtures. Time-resolved spectra at lOcm-’ resolution were taken at 200-ps intervals, and both CO and CO,(Av, = - 1) features were observed. Quenching of CO occurred primarily by collision with CO,, and spectral fitting was again used to obtain relative vibrational populations as a function of time. At later times a population inversion occurred between u1 = 5 and u1 = 2-4 as a result of a bottleneck in which the quenching of u’ = 5-9 is slower than that of the higher and lower levels. The complex vibrational relaxation was modelled, and including quenching by CO,, V -+ V energy transfer with CO(u = 0), quenching by Ar, and radiative loss in the fundamental and overtone bands. Spectra taken at different CO, pressures enabled the rate constants for CO(u’ = 2-15) quenching by CO, to be extracted from a Stern-Volmer analysis [36], and were in good agreement with previous CW infrared chemiluminescence depletion experiments [162]. Very recently the formation and vibrational relaxation of OH(X211i, u = 1-9) by 0, and CO, [73] and NH(X3X-, u = 1-3) by N,, Ar, and H, [72] has been investigated in electron-irradiated gas mixtures. OH(u + u - 1, u - 2) and NH(u +u - 1) IR emission was observed using a quasi-SS time-resolved FTIR spectrometer, averaging about 100 samples to characterize the IR transient for each of lo00 sampling positions. Timedependent populations are shown for OH(u = 1-6) in Figure 24, taken from

+

53


0.0

0.5

1.5

1.o

Time/(ms)

2.0

Figure 24. Vibrational populations of OH(o = 1-6) following electron beam irradiation of a mixture of Ar (lOTorr), O2 (0.3 Torr), H2 (90mTorr) and an estimated 4 mTorr of 03.Relaxation rate constants were obtained from model calculations fitted to the data and shown by the solid lines. Reproduced with permission from Ref. 13.

Ref. [73]. Note the large number of temporal data points that are generated in this experiment for each vibrational level, enabling accurate fits to a kinetic model to be made. Population data were analyzed using a single-quantum relaxation model, but also considered possible effects of multiquantum relaxation. Derived values for relaxation of OH(u) by both 0, and CO, increase rapidly with u (by a factor of 25 for CO, from u' = 1-4). The NH emission [72] enabled a measurement of the nascent vibrational state distribution of the atom-radical reaction:

-

N(,D)

+ H2(X1Zl)

+ NH(R3Z-,

u)

+ H('S)

(21)

to be made, and the fraction of the available product energy in vibration was found to be (f,) = 0.44.[The NH(u = O)/NH(v = 1) ratio was obtained from a linear surprisal plot.] The evidence suggests that the reaction proceeds by direct H atom abstraction rather than the formation of a long-lived H-N-H intermediate [72].

54


Sloan and co-workers have used their CS time-resolved FTIR spectrometer to determine rates of formation and subsequent V + T and V + V relaxation of OH(u') [45, 1631. In one study vibrationally excited OH in u' = 1-4 was generated by the reaction O('D)

+ H,(X'E:

-+

OH(X21'I,V' = 0-4)

+ H('S)

and time-resolved spectra were taken at high spectral resolution [42]. The vibrational populations in v' = 1-4 were obtained at three time delays for several sets of experimental pressures. Forty V-T and V-V rate constants for the disappearance of vibrationally excited OH in collisions with O 3 and OH were determined by an analytical technique based on a complete solution of the master equation [163]. In a recent study of the O('D) H2S + OH(u') HS reaction [45], time-dependent populations of OH u' = 1-5 were obtained. A numerical integrator was used to model the production of OH and its subsequent vibrational relaxation by H,S and 0, using known or assumed rate constants. The time dependence of the highest accessible vibrational level OH(u' = 5) was calculated, and agreed with the experimental data. Finally, some S S time-resolved FTIR studies in this laboratory of the collisional deactivation of highly vibrationally excited polyatomic molecules are considered. The rates of uni- and termolecular reactions in the gas phase are controlled by the collisional deactivation of highly vibrationally excited reaction complexes, and energy transfer information is often derived from the pressure dependence of thermally or chemically activated unimolecular reactions. The energy transfer efficiencies of various collider gases are often expressed as (AE), the average energy transferred per collision, and have been measured as a function of the total energy within the polyatomic molecule using several methods. Troe and co-workers [164- 1681 prepared highly vibrationally excited molecules by U V laser excitation (one photon absorption to an electronically excited state followed by rapid internal conversion to the ground electronic state), and monitored the collisional energy loss by transient U V absorption spectroscopy (UVA). Barker and coworkers used infrared fluorescence (IRF) to monitor the time-dependent relaxation of excited aromatics [169-1751 following absorption of a single visible or U V photon. The IRF measurements have now been extended to observe excitation of COz by V-V energy transfer from the initially photoexcited molecule [1761, measurements complementary to the timeresolved diode laser studies of Flynn and co-workers [140-142,1771. Both the U V excitation and IRF methods depend critically upon calibration curves to determine the average vibrational energy ( E ) within the molecule. Despite much theoretical and experimental effort by both sets of

+

+


55

workers to demonstrate the consistency of the respective calibration techniques, there remains a discrepancy as to the functional dependence of ( A E ) on ( E ) , although agreement is found for the magnitudes of ( A E ) for a variety of bath gases at specific values of ( E ) . In a more recent development Luther and co-workers selectively detected molecules from a predetermined narrow range of energies within a quasi-continuum of rovibrational states using MPI. The method of “kinetically controlled ionization” uses the strong energy dependence in the kinetics of competitive intramolecular channels to give some energy selectivity in the ionization process [178]. For a given laser wavelength only molecules with a certain range of ( E ) are ionized and hence detected, and the temporal evolution of different total energies is straightforwardly obtained by tuning the laser. Measurements at different pressures of added buffer gas enable energy transfer parameters to be found. The IRF technique so far applied measures the total IR fluorescence within the spectral region corresponding to a given IR active mode, for example, a C-H stretch. The intensity of the emission is related to the average energy content ( E ) via a model calculation: an extension of this technique using the time-resolved FTIR method would be to observe the emission decay in a spectrally resolved manner. Molecules containing different ( E ) will emit at different wavenumbers, and providing some identification of the dependence of the emission spectrum on ( E ) can be found, a measurement of the dependence of ( A E ) on ( E ) can be, in principle, extracted. The method essentially adds another observational parameter to the IRF method applied extensively by Barker and co-workers [169-1751. As an example of the feasibility of this method, we illustrate emission observed during studies designed to create NCO radicals by IRMPD of a precursor C,H,NCO [SS]. Figure 25 shows spectra near 2200 cm-’ taken at early (6 p s ) and late (100 p)times following the irradiation of 70 mTorr pure C6H5NC0, and Figure 26 gives a three-dimensional representation of the temporal behavior of the spectrum, obtained from a single scan of the S S instrument. The spectrum alters markedly with time, with the peak shifting to higher wavenumbers at longer times, indicative of vibrational cascade of a nascent population of vibrational levels. Time-resolved spectra in the presence of 5 Torr Ar were similar, but the emission was much shorter lived. Potential photofragments of the IRMPD of C6H,NC0 were ruled out as the source of the emission on spectroscopic grounds, and on the marked differences between the fluence dependences of this emission and that of the formation of photofragments [SS]. A low-pressure gas phase IR spectrum of C,H,NCO revealed a strong absorption at 2278 cm-’ (-NCO stretch) which coincides well with the emission observed at long times. At early times the fluorescence may originate from both high and low levels within the emitting mode, but is also consistent with low excitation in this mode with significant

2-

I-

=rn -

0 - r--l---r---

2000

I800

2200

Wavenurnber

I

/

cm-I

2400

' I 2600

Figure 25. FTIR emission spectra at two times following the IRMPE of 70mTorr PhNCO. Unapodized FWHM 3.18 c n - Nyquist wavenumber 3950.7 cm-l; the spectrum is corrected for the instrument response function and the maxima of both spectra have been scaled to unity.

',

1

2000

I

I

2200

I

Wavenurnber/cin-l

1 24w

Figure 26. Three-dimensional representation of the time evolution of the PhNCO fluorescence following IRMPE, showing a spectral shift to higher wavenumbers at longer times as a result of vibrational relaxation. Spectral and temporal resolution of 3.18 cm-' and 3 p s respectively. 56


57

cross-anharmonic couplings with other excited modes causing substantial shifts of the emission to lower wavenumbers. No emission was observed from other modes, which are all weak in absorption compared to the -NCO stretch at 2278 cm-'. The temporal decay of each wavenumber (decaying more rapidly at lower wavenumbers) was found to follow single exponential behavior, except near 2280 cm- '. The interpretation of these data to extract collisional quenching parameters is not straightforward, as it requires a correlation between emission wavenumber and ( E ) . This would be straightforward if the emission was purely from a cascading ensemble of molecules excited in only one mode, the vibrational anharmonicity then allowing identification of emission at a given wavenumber as originating from a molecule with given ( E ) . This is undoubtedly not the case for the C6H,NC0 molecule, and thus the precise correlation of ( E ) with wavenumber is not unambiguous. Future ways forward can be suggested. Nascent IR spectra should be taken following monochromatic UV excitation at a variety of irradiation frequencies (i.e., values of ( E ) ) of a molecule which undergoes rapid internal conversion. If these spectra differ sufficiently, then their absolute intensities and distributions could be used to fit the collisionally evolving spectra taken as a function of time following monochromatic UV excitation. To this end we have recently managed to resolve spectrally the nascent emission in the 3000cm-' region following the 266-nm excitation of azulene, a molecule whose collisional properties have been previously investigated by the (unresolved) IRF method [169-1731. Although collisional evolution of the spectrum was observed, it has so far proved impossible to measure spectra from excitation at other (longer) wavelengths, the signals being extremely weak: a stronger emitter is needed. Another use for infrared spectra as a measure of ( E ) values would be in attempts to deconvolute spectra such as those shown in Figures 25 and 26, observed following infrared multiple photon excitation (IRMPE), to obtain a measure of the distribution of energies in such systems. Little is known of the population distribution of molecules formed by IRMPE [39, 112, 179, 1801, and even qualitative information in this area would be most useful.

V. CONCLUSIONS This review has attempted to highlight recent progress in the development and use of time-resolved FTIR emission in three areas of chemical kinetics and dynamics, namely, product state distributions in photodissociation processes, product state distributions in chemical reactions, and rate processes involving the formation and loss of the internally excited species

58


responsible for the emission. Applications in these areas will clearly increase, and that is the only safe prediction of future uses for the SS and CS instruments described in Section 111. We also hope that the method will be applied more to one of the most important and difficult areas in chemical kinetics-quantitative measurement of quantum yields of product channels in reactive processes, particularly those of combustion and aeronomic importance. The following caveat applies: emission fails to measure population in the lowest vibrational level, and for relative quantum yields this must be estimated, for example, by information-theoretic analysis of vibrationally excited populations, and extrapolation of these to v = 0. The limitations of the method are clear. We finally come full circle. The greatest advance influencing the practical use of interferometers has undoubtedly been the development of computers able to perform the Fourier transforms rapidly and efficiently. Computers rely on integrated circuits, now almost all of which are produced by processes involving the gas phase plasma etching of semiconductor surfaces. Plasmas glow, and infrared emission is readily seen [181-1841. Modulation of the plasma current can be used in conjunction with time-resolved measurements to separate the emission which arises from species produced and destroyed during the plasma current cycle from the blackbody background, and emission from species such as CO,, CO, HF and highly excited atoms has been seen with the SS instrument [184]. Emission on or near the etched surface also occurs [185], and we hope that studies of this may aid in understanding the complex etching processes which are used in the production of the fast Fourier transform integrated circuit.

ACKNOWLEDGMENTS This work would not have been possible without the initial support of the United States Army European Research Office and, more recently, the SERC. We thank our colleague Richard Wayne for introducing us to the benefits of interferometry, and to the other members of our group who have labored with an often cantankerous but generally rewarding interferometer to generate the results presented here.

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A MODEL FOR THE INFLUENCE OF ORGANIZED MEDIA ON PHOTOCHEMICAL REACTIONS V. Ramamurthy Central Research and Development, Experimental Station, The Du Pont Company, Wilmington, Delaware Richard G. Weiss and George S. Hammond Department of Chemistry, Georgetown University, Washington, DC

CONTENTS I. Introduction 11. A brief introduction to organized media A. Organic inclusion hosts B. Silica, clay, and zeolite surfaces C. Micelles, monolayers, and LB films D. Liquid crystals 111. The concept of reaction cavity A. Reaction cavity defined by boundary, size, and shape B. Concept of free volume: Stiff and flexible reaction cavities C. Active and passive reaction cavities D. Microheterogeneity in organized media E. Limitations and conclusions

69 70 70 78 83 86 88 91 96 97 100 103

Advances in Photochemistry, Volume 18, Edited by David Volman, George S. Hammond, and Douglas C . Neckers ISBN 0-471-59133-5 Copyright 1993 by John Wiley & Sons, Inc.

67

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V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

IV. Reaction cavity model applied to examples A. Consequences of an enclosure B. Reaction cavity and free volume C. Location and directionality of free volume D. Magnitude of free volume V. Reaction cavities as “templates” highlighted with examples A. Reaction cavities with “active walls” as templates: Photodimerization B. Reaction cavities with “passive walls” as templates VI. Anisotropy (microheterogeneity) in organized media VII. Norrish I1 photochemistry of ketones in media affording reaction cavities with little or no constraints A. Photophysical characteristics of ketones B. Photoreactions of simple ketones other than Norrish I1 processes C. Norrish I1 processes 1. Medium effects on Norrish I1 reactivity and photoproduct selectivity VIII. Norrish I1 reactions in organized media A. Neat crystalline phases 1. Reaction cavities of alkanones in neat solid phases 2. Reaction cavities of alkanophenones in neat solid and liquid-crystalline phases B. Reaction cavities with very stiff walls and preformed shapes and sizes: Silica gel and zeolites C. Reaction cavities with some wall flexibility: Solid inclusion complexes D. Reaction cavities with strong external medium influences: Cyclodextrin complexes and their aqueous solutions E. Reaction cavities with walls of variable flexibility and strong external medium influences: Aqueous microheterogeneous complexes F. Reaction cavities with walls of variable flexibility 1. Polymer matrices 2. Thermotropic liquid crystalline and solid matrices IX. Perspectives on future research Acknowledgment References

105 105 109 117 126 132 133 150 153 162 163 164 165 169 171 171 173 177 186 195 200 204 210 210 212 220 22 1 22 1

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

69

I. INTRODUCTION Two striking biological phenomena which serve as a source of inspiration to photochemists and which illustrate what can be achieved in a confined environment are photosynthesis and visual signal transduction. In the former, the medium (protein) is able to organize a number of active elements so as to predispose them to a desired physical process; in the latter, the medium (rhodopsin) restricts the rotational mobility on certain parts of a single molecule, retinal. Being inspired by and having realized the complexity of natural systems, chemists have utilized a number of organized media to study the photochemical behavior of molecules [l-lo]. Examples of organized media which have been investigated include molecular crystals, inclusion complexes (both in the solid and solution states), liquid crystals, micelles and related assemblies such as vesicles, microemulsions, and membranes, monolayers, Langmuir-Blodgett (LB) films, surfaces (silica, clay, and zeolites) and more recently natural systems such as proteins and DNA [l 1691. These media have been referred to in the literature under various names-organized, confined, constrained, ordered, restricted, microheterogeneous, nonhomogeneous and anisotropic media and nanoscopic reactors. However, most of these media fit only some of the categories listed above. Photochemical and photophysical studies carried out in these media can be classified under the following categories: Influence of media on the photochemistry and photophysics of molecules. Photochemistry and photophysics as a tool to understand the media. Influence of photochemistry and photophysics on the media. Application studies directed toward devices, solar energy, and biological implications. Progress has been made in each category and the results have been the object of a large number of recent reviews and’monographs [l-691. This chapter provides an overview of the activities carried out under the first category and, almost as an inescapable consequence, aspects of the second category are provided. It is not intended to be a summary of the activities in the field. Recent reviews and monographs should be consulted for extensive coverage of the literature [l-691. Kuhn, in his book entitled The Structure of Scientijic Revolutions, points out that in order for science to progress, two types of scientific activities are required [70]. He states, “It is necessary for normal science to be to a large extent uncritical. If all scientists were critical of all parts of the framework in

70


which they worked all of the time then no detailed work would every get done.” At the same time, “If all scientists were and remained normal scientists then a particular science would become trapped in a single paradigm and would never progress beyond it.” With these words of wisdom in mind, this chapter attempts to provide a minimal conceptual model for the manner by which organized media influence the photochemical and the photophysical behavior of organic molecules incorporated within them. Concomitantly, we take heed of the words of Chalmers [71] that “a paradigm will always be sufficiently imprecise and open-ended to leave plenty of work to be done.” Our presentation has benefited from the attempts of others to develop unified descriptions for photochemical and photophysical phenomena in organized media [13,72]. A conceptual model which is the centerpiece of this chapter is developed in Section 111. This is preceded (Section 11) by a brief introduction to various organized media. The validity and generality of the model is examined by two approaches. In the first (Sections IV-VI), selected photochemical reactions belonging to various classes and chromophores are presented as supporting examples. In the second (Sections VII and VIII), a critical reevaluation of the results reported on Norrish I1 reactions in a number of organized media is made on the basis of the model. However, although we examined the literature examples on the basis of our model, we often have deviated from the initial explanations offered by the authors.

11. A BRIEF INTRODUCTION TO ORGANIZED MEDIA To appreciate the model discussed in this chapter it is essential to have some knowledge of the characteristics of various organized media. A large number of monographs are available on this topic [73-911 and readers should consult them for detailed understanding. For the sake of completeness, we provide below a very brief summary of the structures and characteristics of the relevant media.

A.

Organic Inclusion Hosts

An inclusion compound is composed of two or more distinct molecules held together by noncovalent forces in a definable structural relationship. Hosts can contain cavities that are rigid or that are developed by reorganization of the hosts during the process of complexation. Inclusion compounds may be subclassified as (1) the true clathrate type in which the guest molecules are

71


imprisoned in discrete cavities or cages; (2) the channel type in which the guest molecules are accommodated in continuous canals running through the crystal; or (3) the layer type in which the guest component is situated between bands of the host structure. Urea, deoxycholic acids, tris-ortho-thymotide, Dianin's compound, perhydrotriphenylene, and cyclodextrin are a few examples of organic hosts. Clays and zeolites are a few of the well-known inorganic host systems. The urea inclusion compounds generally crystallize in long, hexagonal prisms or occasionally as hexagonal plates. X-ray structural analysis of the urea-n-hydrocarbon complex shows clearly the existence of a central channel the diameter of which is about 5.3 A (Figure 1). The arrangement of thiourea molecules in rhombohedra1 crystals is similar to that of urea in urea inclusion compounds where the channel size is about 6.1 A. Urea and thiourea channel-type inclusion complexes are generally stabilized by van der Waals forces between the host and the guest and by hydrogen bonding between the host molecules. Deoxycholic acid (DCA), apocholic acid (ACA), and cholic acid (CA) form channel-type inclusion compounds with a wide variety of organic molecules. Of these DCA has been extensively investigated.

HO*'

CA

DCA

ACA

Deoxycholic acid complexes can be grouped into three crystal forms: orthorhombic, tetragonal, and hexagonal. In the more common orthorhombic structures, a two-dimensional bilayer motif with axial dimensions of b 13.6 A and c 7.2 A is observed. These bilayers contain grooves parallel to the c axis which induce DCA to form channel inclusion complexes (Figure 2). These channels have a variable size and shape depending on the mutual positions of two adjacent bilayers along the b axis and this attribute accounts for the ability of the DCA host lattice to accommodate guest molecules of very different dimensions. The dimension (between 2.6 x 6.0 8, and 5 x 7 A) and the length of the channel vary depending on the size of the guest. Perhydrotriphenylene (PHTP) is a chiral molecule. The equatorial isomer of PHTP gives rise to a wide variety of inclusion compounds with different kinds of molecules, ranging from those with a nearly spherical or planar

-

-

-

-

(4

(b)

Figure 1. (a) End-view cross section of the urea-hydrocarbon complex. (b)View of the hexagonal PHTP inclusion compound in the ab plane. PHTP inclusion compounds are composed of infinite stacks of host molecules, repeating at about 4.78 A, parallel to the molecular threefold axes. The regular packing of the stacks gives rise to parallel channels. Channel cross section in both cases is 5 A.

-

(4

(d 1

Figure 2. Deoxycholic acid packing illustrating the flexible size of the channel cross section. Packing viewed along the c axis: (a) no guest, (b) phenanthrene, (c) norbornadiene, and (d)acetone as guests. The channels have variable size and shape depending on the mutual positions along the b axis of two adjacent bilayers. (Reproduced with permission from E. Giglio in Inclusion Compounds, Vol. 2, J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, Eds., Academic Press, New York, 1984, p. 215.) 72


73

shape to linear ones. All the investigated adducts have a channel-like structure with the PHTP molecules arranged in infinite stacks whose axes are parallel to the threefold axis of the molecule (Figure 1). The diameter of the channel is about 5 A and it is slightly flexible. Guest molecules are held within the hydrophobic channels through van der Waals interactions.

Dianin’s compound, (1)

PHTP

The general crystal structure of the Dianin’s compound (1) lattice consists of hexamers of Dianin’s compound held together by a ring of hydrogen bonds involving the phenolic hydroxy groups. The monomeric units of the hexamers form chains of hourglass-shaped cages. As illustrated in Figure 3, the upper half of each cage consists of three molecules of 1 from one hexamer, and the lower half is three molecules of 1 from another hexamer. The cage is held together at the roof and floor by the hydrogen bond network and at the waist by van der Waals forces between molecules. These cages are quite large, as

H:.

R

RY+(, R‘

(4

(b)

(4

Figure 3. The shape and size of the cavity/channel of Dianin’s compound. (a) The cage formed by two sets of hexamers. (b)The top or bottom half of the cage, hexamers are held together by hydrogen bonds. (c) Schematic illustration of the cage/channel. The stacking is along the c axis. (Reproduced with permission from D. D. MacNicol in Inclusion Compounds, Vol. 2, J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, Eds., Academic Press, New York, 1984, p. 1.)

74

V. RAMAMURTHY, R. G. WEISS AND G. S . HAMMOND

shown in Figure 3, and can accommodate even bulky molecules. Each one of these hourglass-shaped cages is connected on both the top and the bottom to yet another cage and thus this arrangement gives rise to a column consisting of bulged channels. Every column is ideally infinite in extent, runs parallel to the c axis, and is surrounded by (but not interconnected with) six other identical columns related by a threefold screw axis. Tris-ortho-thymotide (TOT) forms two types of crystalline inclusion complexes: in one, the guest molecules are enclosed in discrete closed cavities, in the other, the guest molecules are accommodated in continuous linear channels running through the crystal along a crystallographic axis. Cage-type complexes are formed with guests of length less than -9 A and channel-type structures are formed with long chain-like molecules. The cage in the former type of structure is comprised of eight TOT molecules related pairwise about a crystallographic twofold axis. The average diameter of the ellipsoid cage is about 1 2 A (Figure4). These cages deform to a limited extent to accommodate molecules of different dimensions.

Channel-type structures are formed in the case of trans-stilbene and benzene as guests. In these cases at least two independent channels are

Figure 4. Stereoview of the cage in the TOT complex. [Reproduced with permission from R. Gerdil, Topics Curr. Chem. 140, 71 (1987).]


75

present, one along the a and the other along the b axis, as illustrated in Figure 5. The channel along the a axis is of fairly uniform cross section, whereas the one along the b axis presents a succession of bulges and constrictions. The TOT cages and channels lack any specific binding sites and the host-guest complexation therefore is essentially controlled by van der Waals’s forces. Cyclodextrins (CD), one of the most commonly used host systems, possess hydrophobic cavities that are able to include, in aqueous solution, a variety of organic molecules. Internal diameters and depths of cyclohexaamylose (aCD), cycloheptaamylose (p-CD), and cyclooctaamylose (y-CD) provide cavities of different sizes (Figure 6). The oligosaccharide ring forms a torus with the primary hydroxyl groups of the glucose residues lying on the narrow end of the torus. The secondary glucopyranose hydroxyl groups are located on the wider end. Inclusion complexes of known ratio can be precipitated from aqueous solutions of C D when an excess of guest is added. Such precipitates contain the guest accommodated within the cavities of CD. In addition to the local structure, the global structure of the solid is determined by how these individual complexes are arranged in the solid state. Based on

(b)

Figure 5. Stereoview of the two channels present in TOT-benzene complex. (a)View along the a axis and (b)viewed down the b axis. One channel runs along the a axis and the other along the b axis. [Reproduced with permission from R. Gerdil, Topics Curr. Chem. 140, 71 (1987))

V. RAMAMURTHY, R. G . WEISS AND G . S. HAMMOND

76

G6

ci,

d

P Y

5.6 6.0

4.2 5.6

0.0 10.8

7.0

0.0

6.0

12.0

7.8

0-5

7.8

Figure 6. Shape and structure of cyclodextrin cavity.

the overall appearance, these are described as cage- or channel-type structures (Figure 7). 1,1,6,6-Tetraphenylhexa-2,4-dyne1,6-diol(2) and 1,6-bis(o-chlorophenyl)1,6-diphenyl-2,4-dyne-1,6-diol (3) function as hosts to a number of organic

2

3

molecules. One of the advantages of using these hosts is the flexible packing they provide. When the size of the guest molecule is on the order of the size of the host molecule, the host molecules accommodate themselves to form a channel. When the guest molecule is smaller than the host, the latter forms a cage with a cross section enclosing the guest molecules in an antiparallel fashion. Two such structures are shown in Figure 8. In both of these structures, hydrogen bonding between the host and the guest plays an important role in guest inclusion. The ability of chiral host 3 to form channel inclusion complexes has been utilized to resolve optical isomers and to conduct asymmetric photochemical transformations.

a

C

Figure 7. Schematic representation of the packing arrangement in cyclodextrin complexes: (a) channel type, (b) cage or herringbone type, and (c) brick type. (Reproduced with permission from W. Sanger in Inclusion Compounds, Vol. 2, J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, Eds., Academic Press, New York, 1984, p. 231.)

Figure 8. A stereoscopic view of the packing in the inclusion compound of 1,1,6,6tetraphenylhexa-2,4-dyne-1,6-diol,2, with (a) chalcone and (b) 9-anthraldehyde. 9Anthraldehyde gives cage-type and chalcone gives channel-type structures. [Reproduced with permission from F. Toda, Topics Curr. Chem. 140,43 (1987).] 77

78

V. RAMAMURTHY. R. G. WEISS AND G. S. HAMMOND

B. Silica, Clay, and Zeolite Surfaces Silica gel and porous silica are rigid three-dimensional networks of silica particles. They are constituted from basic tetrahedral SiO, units. Their porous, sponge-like structure results in large surface areas (100-600 m2 g-') which correspond primarily to areas on the internal pore walls. Pore sizes can range from micropores of < 20 A to macropores of > 2000 A, although values from 20 to 150 A are typical. Porous glass (commonly called porous Vycor) is also structurally similar to the other porous silicas but has a composition of -97% SiOz and -3% B,O,. The surface of porous silica consists of a network of siloxane (Si-0-Si) and silanol (Si-OH) linkages and physically adsorbed water molecules. There are generally 4-5 silanols in each square nanometer of area and these may be isolated, geminal, or vicinal (hydrogen bonded to water or to each other), as illustrated in Figure 9. Surface adsorption can occur via dispersion forces arising from induced dipole interactions, induction forces, and chargetransfer interactions such as hydrogen bonding. Adsorption of most polarizable organic molecules is generally accepted to involve interaction with the surface hydroxyls. It has been suggested that the vicinal silanols provide active sites for adsorption. Adsorbed molecules will usually interact with several surface silanols because of their relatively large size.

H

D

E

Figure 9. Schematic representation of active groups on silica surfaces: (A) isolated silanols; (B) siloxane bonds; (C) geminal silanols; (D) hydrogen-bonded silanols; ( E ) hydrogen-bonded water. (Reproduced with permission from L. Johnston in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991, p. 359.)


79

The term clay generally refers to sheet aluminosilicates with particle sizes in the micron range. These consist of layered structures, some of which are capable of expanding and intercalating a variety of guest molecules (swelling clays are also known as smectites). Clays are composed of two distinct types of layers consisting of [SiOJ tetrahedra and [M(O,OH),] octahedra with M being, in general, A13+, MgZ+,or Clays of principal interest result when one octahedral layer is sandwiched between two tetrahedral layers (Figure 10). In addition to substitution in the octahedral layers, the tetrahedral layers may also be partially substituted by ions of lower valency. Substitution of this type produces a negative charge on the layers which is balanced by interlamellar exchangeable cations. Two of the most commonly used clays are montmorillonite and hectorite (whose synthetic form is known

Al (with some Mg replacement)

Figure 10. Schematic illustration of the structure of a montmorillonite like clay. Two layers of silica-oxygen tetrahedra are joined together by an octahedrally coordinated aluminium-oxygen/hydroxide layer to give an individual clay sheet. (Reproduced with permission from W. Jones in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991, p. 387.)

80


as laponite). In montmorillonite, octahedral substitution consists of A13 and MgZ', whereas in laponite it is primarily Mg2+ (with the charge compensating cation Li'). Clays swell in the presence of water as a result of the increased uptake of interlayer water. To a considerable extent the interlayer water may be replaced by other (neutral) guest molecules (Figure 11). The +

- - - _ _

-

-

-

-

I

ONE, LAYER HYDRATE

I

TWO.LAYER HYDRATE

(4

-I

THREE.LAYER HYDRATE

a

& \

wm (b)

Figure 11. (a) For certain sheet silicates (e.g., montmorillonite), several layers of guest may be intercalated in a gallery region. The spacings illustrate the values obtained with water as the guest molecules. (b) Idealized diagram illustrating the three possible arrangements of intercalated n-hexylamine molecules inside a sheet silicate. (Reproduced with permission from W. Jones in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991, p. 387.)


81

hydrated nature of the interlayer region tends to make the uptake of nonpolar molecules more difficult. However, once an organic “surface” is generated the sorption properties will change. Pyrene, for example, is adsorbed little by a clay until detergent molecules are also sorbed. Zeolites may be regarded as open structures of silica in which aluminum has been substituted in a fraction of the tetrahedral sites. The frameworks thus obtained contain pores, channels, and cages. Substitution of trivalent aluminum ions for a fraction of the tetravalent silicon ions at lattice positions results in a network that bears a net negative charge which must be compensated for by other counter ions. Such ions are mobile and may occupy various exchange sites depending on their radius, charge, and degree of hydration. They can be replaced, to varying degrees, by exchange with other cations. If zeolite water is removed, many other organic and inorganic molecules can be accommodated in the intracrystalline cavities. The topological structure of X- and Y-type zeolites (also known as faujasites) consists of an interconnected three-dimensional network of relatively large spherical cavities termed supercages (diameter of about 13 A; Figure 12). Each supercage is connected tetrahedrally to four other superCATION LOCATION INSIDE FAUJASITE CAGES

Figure 12. Supercage structure, cation location (I, 11,111or 1, 2, 3 ) within X- and Ytype zeolites. Bottom portion shows the reduction in available space (relative) within the supercage as the cation size increases.

82


cages through 8-A windows or pores. The interior of zeolites X and Y also contains, in addition to supercages, smaller sodalite cages. The windows to the sodalite cages are too small to allow organic molecules access. Chargecompensating cations present in the internal structure occupy three different positions (Figure 12) in the zeolites X and Y. Only cations of sites I1 and I11 are expected to be readily accessible to the adsorbed organic molecule. Among the medium-pore sized zeolites, perhaps the most studied are the pentasil zeolites, ZSM-5 and ZSM-11 (Figure 13). These zeolites also have three-dimensional pore structures; a major difference between the pentasil pore structures and the faujasites described above is that the pentasil pores do not link cage structures as such. Instead, the pentasils are composed of two intersecting channel systems. For ZSM-5, one system consists of straight channels with a free diameter of about 5.4 x 5.6 8, and the other consists of sinusoidal channels with a free diameter of about 5.1 x 5.5 di. For ZSM-11, both are straight channels with dimensions of about 5.3 x 5.4 8,. The volume at the intersections of these channels is estimated to be 370A3 for a free

/

5.1 x 5.5

A

(b)

Figure 13. Channel structure of ZSM-5. Note the presence of two types of channels, dg-zag and straight. (A)Viewed through the straight channel. (B) Schematic drawings of both zig-zag and straight channels; channel dimensions are noted.

83


diameter of about 8.9 A. Other zeolites of interest for photochemical studies include the LZ-L, mordenite, offretite, omega, and beta (Table 1).

C. Micelles, Monolayers, and LB films The term micelle denotes an assembly formed by aggregation of surfactant molecules, consisting of long hydrocarbon chains and ionic or nonionic polar head group, in water. Hydrophobic repulsion between the hydrocarbon chain and water is responsible for the aggregation of surfactant molecules in water. There is a concentration value (critical micelle concentration, CMC) below which aggregation of surfactant molecules is absent or occurs among a few molecules (premicellar aggregation) and above which association leads to TABLE 1 Size of Pore Openings and Dimensionality of the Pore System for Selected Medium- and Large-Pore Molecular Sieves'

Molecular Sieve Name Faujasite (X and Y type) Omega Linde Type L Mordenite Offretite ZSM-34 ZSM-11 ZSM-5 Theta-1 4A

Pore (Window) Size

(A)

Channel/Cage Size

-

Three-dimensional channel with a cage (d 12A) Two noninterconnected 7.5 (3.4 x 5.6) channels Single channel with a lobe 7.1 (d 7.5A) 7.0 x 6.7 and (2.6 x 5.7) Two interconnected channels Two interconnected 6.7 and (3.6 x 4.9) channels Two interconnected 6.7 and (3.6 x 4.9) channels Two interconnected 5.3 x 5.4 channels 5.3 x 5.6 and 5.1 x 5.5 Two interconnected channels Single channel 4.4 x 5.5 4.2 Three-dimensional channel with a cage (d 12A) 7.4

-

-

"W. M. Meier, and D. H . Olson, in Atlas of Zeolite Structure Qpes, 2nd ed., Butterworths, Cambridge, 1987; D. W. Breck, Zeolite Molecular Sieoes: Structure, Chemistry, and Use, Wiley, New York, 1974.

84

V. RAMAMURTHY, R. G. WEBS A N D G. S. HAMMOND

micelles. The number of detergent molecules that aggregate to form micelles is called the aggregation number. Structure, aggregation number, and CMC are all dependent on the nature of the hydrocarbon chain, head groups, counterions, temperature, and added electrolytes. The currently accepted model of an ionic micelle consists of a core where the radius approximates the length of the fully extended alkyl chain of the detergents (15-30 A), surrounded by the Stern layer containing water, head groups, and more than half of the counter ions and Gouy-Chapman layer, extending to several hundred angstroms, containing water and remaining counter ions (Figure 14). Micelles are dynamic in nature and exist in equilibrium with monomers and other forms of aggregates. Organic substrates, owing to their hydrophobicity, tend to be solubilized inside micelles which provide a restricted hydrophobic space in an aqueous environment. The site of solubilization may be either the micellar interior or the micellar surface. Surfactants commonly used to form micelles are sodium dodecyl sulfate (SDS),cetyl trimethylammonium chloride (CTAC), cetyl trimethylammonium bromide (CTAB), and Triton X-100. Some amphiphilic molecules such as oleic acid and hexadecyl alcohol containing an alkyl chain and a polar head group form monolayers on the surface of water. The polar head groups of these molecules are attracted to and are in contact with water while their hydrocarbon tails protrude above it (Figure 15). The term monolayer implies the presence of a uniform monomolecular film on the surface of water. Monolayer films can be classified as gaseous, liquid, or solid depending upon the degree of compression and the effective area per molecule. Clearly the liquid phase of a monolayer film and, more so, the solid represent constrained environments for individual molecules of amphiphiles. Monolayers, just like micelles, are dynamic species.

ebon 'tor

exta: r i o r

-+-+-+*+Micellar c o r e Gouy-Olaprnan layer

k

r

n layer

Figure 14. A cartoon representation of the structure of a micelle.


85

X. Deposition

Y . Deposition

Z - Deposition

(b) Figure 15. ( a ) A schematic representation of organic amphiphilic molecules at airwater interface. (b) Schematic representation of the various deposition modes for monolayer films and the resulting L-B assemblies. (Reproduced with permission from H. Kuhn, D. Mobius, and H. Bucher, Physical Methods of Chemistry, Vol. I, Part IIIB, A. Weissberger and B. W. Rossiter, Eds., Wiley, New York, 1972, p. 577.)

Stepwise transfer of a monolayer film to a rigid support results in a Langmuir-Blodgett (L-B) assembly (Figure 15). Transfer can be accomplished only for those monolayer films that compress to a tightly packed solid phase. Although it is frequently assumed that the transfer of a monolayer film from the air-water interface to a rigid support is accomplished with minimal reorganization of the molecular arrangement of the monolayer film, this is certainly not always the case. In L-B assemblies, molecules are held in a rigid and well-defined geometry. Since the monolayer deposition occurs in steps, the interlayer distances may be controlled by varying the number of spacer layers, the surfactant chain length within spacer layers, or the mode of deposition. One of the commonly used spacers is arachidic acid.

86

V. RAMAMURTHY, R. G . WEISS A N D G. S. HAMMOND

D. Liquid Crystals Liquid crystals, as the name implies, are condensed phases in which molecules are neither isotropically oriented with respect to one another nor packed with as high a degree of order as crystals: they can be made to flow like liquids but retain some of the intermolecular and intramolecular order of crystals (i.e., they are mesomorphic). Two basic types of liquid crystals are known: lyotropic, which are usually formed by surfactants in the presence of a second component, frequently water, and thermotropic, which are formed by organic molecules. The thermotropic liquid-crystalline phases are emphasized here; they exist within well-defined ranges of temperature, pressure, and composition. Outside these bounds, the phase may be isotropic (at higher temperatures), crystalline (at lower temperatures), or another type of liquid crystal. Liquid-crystalline phases may be thermodynamically stable (enantiotropic) or unstable (monotropic). Because of their thermodynamic instability, the period during which monotropic phases retain their mesomorphic properties cannot be predicted accurately. For this reason it is advantageous to perform photochemical reactions in enantiotropic liquid crystals. Thermotropic liquid-crystalline phases can be grouped according to their microscopic organization into four major classes: nematics, smectics, cholesterics, and discotics (Figure 16). Molecules forming nematic, smectic, and cholesteric phases are typically rod-like. They usually consist of at least one rigid group and at least one flexible chain at a molecular extremity. Plate-like molecules (or molecular aggregates that adopt this shape) are found in discotic phases that include a rigid central core from which several flexible chains emanate. The structures and acronyms for some molecules that form liquid crystalline phases are shown below.

BS

n-Butylstearate (hexatic B phase)

BCCN

(stnectic and nematic phases) OR I

Cholesteryl chloride (cholesteric phase)

I

OR

(discotic phase)


87

The directors (long molecular axes) of the constituent molecules in nematic phases are parallel to one another on average. This is the only order present in nematic liquid crystals, which are the most fluid type of liquid-crystalline phase. Molecules that form cholesteric phases must be optically active or contain an optically active dopant. As the phase name implies, the constituent molecules are frequently steroids and most commonly are cholesteric esters or halides. A conceptual model of the cholesteric phase includes “layers” of molecules in nematic-like positions, each layer being twisted slightly with respect to the ones above and below it. When the phase consists only of optically active molecules, the angle of twist between “layers” is typically less than one degree. Several subclasses of discotic phases exist. In all, the molecular planes of the constituent molecules are parallel. However, the discs can pack in nematic-like arrangements (N,) or in columns that are internally ordered (Do) or disordered (Dd)and may be stacked vertically,

nematic

cholesteric

columnar discotic

Srnecljc A structure

Snit.Lltc C structure

Figure 16. Idealized cartoon representation of the molecular shapes and orientations of the major liquid-crystalline phase types. (Reproduced with permission from R. G. Weiss in Photochemistr>y in organized and constrained media, V. Ramamurthy, ed., VCH, New York, 1991, p. 603).

88

V. RAMAMURTHY, R. G.WEISS AND G. S. HAMMOND

tilted, or twisted. In smectic phases, the constituent molecules are arranged in layers with their long axes parallel to one another. A number of subclassifications of smectics are known. Each differs from the others based on the packing arrangement of molecules within a layer (hexagonal, orthorhombic, nematic, etc.) and the angle of tilt between the long molecular axes and the layer plane (Figure 16). Two of the most common ones are smectic A and smectic B types. In these, the long molecular axes of the constituent molecules are in a perpendicular orientation with respect to the layer plane.

111. THE CONCEPT OF REACTION CAVITY We believe that the manner in which organized media, in general, control or modify the reaction course followed by organic molecules included in them can be viewed on the basis of a simple qualitative model. One can envision reactions taking place in an organized medium as occurring within enclosed surroundings. This enclosed space we call the “reaction cavity.” Semantically “enclosed” suggests that there is a physical boundary around the guest reactants, and in this sense it may be a misleading term. One should think of a reaction cavity as the space in which excited state reactant molecules and their preproduct intermediates are confined (restricted or mobilized) during their lifetimes. Unlike isotropic solution media where molecules have limited but equal mobility and conformational flexibility in all dimensions, in an organized medium their mobility and flexibility is restricted or constrained in one if not all three dimensions. The dimension or dimensions along which translational and conformational changes of the reactant molecules are constrained determine the ability of the reaction cavity to influence reactivity. Terms such as “restriction” and “confinement” are integral parts of the description of a reaction cavity. This description is elaborated below with an idealized model shown in Figure 17. Imagine a molecule tightly enclosed within a cube (model 10). Under such conditions, its translational mobility is restricted in all three dimensions. The extent of restrictions experienced by the molecule will decrease as the walls of the enclosure are removed one at a time, eventually reaching a situation where there is no restriction to motion in any direction (i.e., the gas phase; model 1). However, other cases can be conceived for a reaction cavity which do not enforce spatial restrictions upon the shape changes suffered by a guest molecule as it proceeds to products. These correspond to various sit ations in isotropic solutions with low viscosities. \ We term all models in Figure 17 except the first as “reaction cavities” even


Model

89

Equivalent organized spaces

1 No Enclosure

2

Single plane

3

Two parallel planes

4.

Two perpendicular planes

5

Three adjacent sides oDen cubes

6.

Three adjacent sides open cubes

7.

Two adjacent sides open cubes

8

Two opposite sides open cubes

9.

One side oDen cube

Cavitands

10

Closed cube

Crystals, Zeolite cages, TOT cages

Silica surface air-water interface Clays, Liquid crystals

Zeolite, Urea channels, Cyclodextrin Liquid crystals

Figure 17. An idealized representation of a cubic model. Restriction imposed on a ball trapped within a cube with different faces lacking. In models 1-10 various faces of the cube are absent. Such an absence increases the freedom the trapped ball experiences as one moves from model 10 to 2. The similarity between this situation and a guest within a restricted space is highlighted.

though the dimensionality of their inhibition of guest motions differ. Although it is difficult to find examples for all the models, several can be related to the reaction cavities afforded by the organized media discussed here; they are as noted. Reaction cavities of some media, like micelles and liquid crystals, may be categorized more precisely as model 2 or 10 depending upon the nature of the motions required to effect the reaction in question. Thus, to which class a medium belongs will be a function of the guest molecule and the nature of the reaction it undergoes. According to this model, the restrictionJconfineVentJinhibitionof motion experienced by the guest depends on the particulat case and may be usefully represented in terms

90


of angstroms (one dimension inhibited), square angstroms (two dimensions inhibited) or cubic angstroms (three dimensions inhibited). The term “reaction cavity” was originally used by Cohen to describe reactions in crystals [13]. He identified the reaction cavity as the space occupied by the reacting partners in crystals and used this model to provide a deeper understanding of the topochemical control of their reactions. Selectivity seen for reactions in crystals, according to this model, arises because of lattice restraints on the motions of the atoms in reactant molecules within the reaction cavity. In other words, severe distortion of the reaction cavity will not be tolerated and only reactions that proceed without much distortion of the cavity are allowed in a crystal (Figure 18). Crystals possess timeindependent structures; the atoms that form the walls of the reaction cavity are fairly rigid and exhibit only limited motions (e.g., lattice vibrational modes) during the time periods necessary to convert excited state molecules to their photoproducts. Therefore, in the Cohen model, the space required to accommodate the displacement of reactant atoms from their original positions during a chemical reaction must be built largely into the reaction cavity. Packing of polyatomic molecules in crystals leaves some distances between neighboring nonbonded atoms greater than the sum of their van der Waals radii. This creates a certain amount of free volume which may be so disposed as to allow the atomic motions required to effect a reaction. In the usual case, a reaction product will also place some stress on the host crystal, as is evidenced crystals such as those studied by Schmidt and Cohen, which are usually reduced to powders as the reaction progresses [l]. Can we extend the reaction cavity concept, which emphasizes the shape

REACTANTS

TRANSITION STATE

---

PRODUCTS

Figure 18. The reaction cavity model as presented by Cohen. Reaction cavity before and after the reaction is shown as full lines. Transition state requirements for a reaction are shown as broken lines. Case I represents a favorable and case I1 an unfavorable reaction. [Reproduced with permission from M. D. Cohen, Angew. Chem. Int. Ed. Eng. 14, 386 (1975).]


91

changes that occur as the reactant guest transforms itself to the product, so that we can understand and predict the photobehavior of guest molecules included in organized media in general? We believe that such an extension should be possible with some limitations. The concept of the reaction cavity will serve well as a vehicle for the discussion of results obtained in media in which organized structures of hosts have significant effects on the photochemical response to excitation of guests. The cavities may change their size and shape over time periods which may or may not be important depending on the transition times involved in the reactions. Reaction cavities will also vary in the stiffness and chemical activity of their walls. Finally, the relative sizes and functionalities of the guest molecule and its reaction cavity can influence the model of Figure 17 into which a system will fall.

A. Reaction Cavity Defined by Boundary, Size, and Shape The boundaries of the enclosed space (reaction cavity) in an organized medium may be physically realistic as in the case of some zeolites or may be more nebulous and transitory as in the case of liquid crystals. Although the presence of an enclosure comprised of a network of atoms or molecules around reactant molecules is common in all condensed media, the size, shape, and nature of the reaction cavity differs significantly from one medium to the other. Such factors, we believe, are responsible in large part for the different influences exerted by organized media on the course of reactions of included guest molecules. The size of the reaction cavity may vary from being very small (as in crystals, cyclodextrins, etc.) to being potentially very large (as in micelles) or unbounded in at least one dimension (as with surfaces of silica). The exact size of the enclosure can be estimated easily for systems such as crystals and some inclusion complexes where the guest molecules are totally immobilized within a single site (model 10). Unlike these systems, wherein individual reaction cavities are physically separated and the migration of reactant molecules from one reaction cavity to any other is arrested, the regions in which potential reactant molecules are accommodated on surfaces of silica, between the layers of clay, or within zeolites are interconnected and migration from one region to the other may not be inhibited (models 2-9). When guest molecules are able to explore more space during their transformation to products than is available in the cavity in which they are accommodated at the time of excitation (initial reaction cavity),their behavior may depend upon the “effective space” explored. The effective reaction cavity, the space explored, will depend on the lifetime of the excited state, the nature of the mobility, and the structure of the guest molecule and the intermediate(s) derived therefrom. The initial and effective reaction cavity

I

92


sizes and shapes for molecules included within spaces corresponding to model 10 are expected to be the same if the volume of the guest molecule and its cavity are similar; this may not be so in micelles, for instance, which provide a potential volume for reaction which exceeds the size of most organic molecules. However, when the molecules are included in a cavity/cage/surface which is continuous through interconnections, such as those corresponding to models 2-9, the initial and effective reaction cavity sizes and shapes need not be the same. Since the effective reaction cavity size depends on the structure and on the lifetime of the species involved, these sizes may be different for the excited state of guest molecules and the intermediates derived from them. Also, two intermediates with different structures and lifetimes may have different effective reaction cavity sizes. Consider a guest molecule within the supercages of a faujasite. In the absence of mobility, the initial enclosure size would be the size of the supercage (i.e., -800A3). Since the supercages in X and Y zeolites are interconnected, the guest molecules may migrate from one cage to the other. Assuming that the guest is able to scan n cages within its lifetime, the effective enclosure size would be n x 800 A3, where the value of n depends upon the diffusion rates and the lifetimes of the migrating transients (e.g., the electronically excited state of guest molecules and reaction intermediates derived from them). For instance, a molecule with a long excited state lifetime and a high diffusion rate might travel through a number of supercages giving it a very large effective reaction cavity. This is illustrated in Figure 19. Consider another example of a molecule adsorbed in a pore of diameter 100 A on a silica surface. The “enclosure” in which the molecule resides may be thought of as a cylinder with lo4 A2 cross-sectional area and undefined height. Our model would indicate that the volume of the cavity is potentially infinite since there is no boundary in one dimension. However, many molecules adsorbed on silica exhibit photochemical behavior far different from that found in isotropic liquids [31,46,50]. In fact, molecules adsorbed on silica are not able to explore the full extent of their reaction cavity since they are tethered to the surface of the pore through strong interactions; translational motion in the open dimension is restricted. For the same reason, we cannot assume that an electronically excited guest molecule can freely explore even lo4 A2 crosssectional area. However, we can assert that diffusion and changes in molecular structure in these two dimensions are definitely limited. An illustration of the different space explored by two radicals generated from the same reactant is provided by the photobehavior of l-phenyl-3-(0tolyl)propan-2-one (4) adsorbed onto zeolite ZSM-5 [42]. ZSM-5 is a narrow pore zeolite which is known to admit p-xylene but not o-xylene. On this basis, 4 is expected to adsorb only on the outer surface of ZSM-5 (Figure 20). Upon photolysis, of the two radicals produced, benzyl (A) and 0-xylyl (B), the latter

-

-

-

-


6 a

b

c

o* a

b

I

c

d

e

93

a, b, c and d correspond

to different interconnected reaction cavities.

hu

d

e

Excited guest, confined to cvaity 'a' upon excitation, may travel between cages

'a' and Id'.

Excited guest reactant

h*

a

b

0 Reactant

c

I

d

e

t ran s fo r ins to products wlien located in cage Id'.

U Product

a, b, c, d, e correspond to the cavity volumes of various cages. Initial Reaction Cavity = a Effective Reaction Cavity = a+b+c+d Final Reaction Cavity = d

Figure 19. An illustration of three possible reaction cavities as the reaction occursinitial, effective, and final reaction cavities.

is too bulky to enter the channels of ZSM-5. Therefore, the effective space explored is expected to be different for the two radicals A and B; radical A will remain essentially on the outer surface and radical B can also explore the interior of the zeolite. The actual difference is reflected in the product

4

94

V. RAMAMURTHY, R. G. WEISS A N D G. S. HAMMOND

-

I -co

L

AA

LL-

A -

-A

suc

Figure 20. Different spaces explored by two radicals of different sizes, generated from the same molecule. Because of its large size, radical A stays on the surface of ZSM-5 whereas radical B enters the channels. Products formed are controlled by the different

spaces explored by the two radicals.

distribution of the photolysis. While a statistical mixture of AA, AB, and BB is obtained in solution, only AA and BB are formed in ZSM-5. Our concept of a reaction cavity in organized media has been considerably modified from that of Cohen [13]. It requires the inclusion of more factors to be used effectively, but it provides a base for discussion of a myriad of reaction environments. It is very important to note that the exact size and shape of a reaction cavity (initial, effective, and Jinal) that control the excited state behavior of guest reactants will depend on the particular reaction as well as on the guest and intermediate(s) themselves. Whether the information regarding the space explored (effective reaction cavity) by the excited molecule will be registered in the distribution or stereochemistry of the products will depend on the nature of the mechanism involved in the product formation. In some cases, explorations over a larger space by excited state species and their intermediates may not be germane to the distribution and types of products formed. In certain cases, especially those that involve the probability of encounters, all of the space excited molecules and their intermediates explore before they yield final products may be important. In cases for which the distribution of specific product types is being probed, only the site in which


95

decay of the final intermediates to products occurs may define the reaction cavity. If this reaction cavity is different from the initial one, the space within which the final reaction occurs we call final reaction cavity. An illustrative example of this phenomenon is provided by the photobehavior of a-alkyl dibenzylketones, 5, included in the supercages of faujasites (Scheme 1; for details see Section 1V.D)[92,93]. Irradiation of these ketones results in a-cleavage (in addition to y-hydrogen abstraction). Subsequent decarbonylation will yield the geminate radicals A and B within a supercage. Recombination of radicals A and A can yield only coupling products (AA), whereas recombination of A and B or B and B can yield both coupling (AA, AB, and BB) and disproportionation products, toluene, and olefins (Scheme 1).The ratio of total products (coupling and disproportionation products) obtained by encounters of A and A, A and B, and B and B will depend upon the total space explored by A and B. That is, the effective reaction cavity for encounter of radical pairs is a function of the total volume explored by the two radicals from the moment of their creation until they combine. O n the other hand, the ratio of coupling to disproportionation products from each of the A and B and B and B encounter partners will not depend on the total volume explored by the two radicals. The specific nature of the products from those encounters will depend only upon the nature of the enclosure which contains them at the moment when they establish ultimate contact and collapse to even electron species (final reaction cavity). It is clear from the example given above that the cage effect, which is a measure of products from the combination of A and A, B and B, and A and B, provides information regarding the space the individual radicals A and B explored from the time of their inception to the time of their destruction, whereas the individual products obtained from the combination of either A and B or B and B do not carry the memory of their path into the final products.

Scheme 1.

%

V. RAMAMURTHY, R. G . WEISS AND G. S. HAMMOND

In this example, we have assumed that the cages in which the two radicals are generated and finally collapse to products have the same size and shape. In some media, the shape and size of individual sites may differ (see Section D) so that predictions concerning the excited state behavior of guest molecules require knowledge only of the characteristics of the final reaction cavity. Thus, we envision three possible types of reaction cavities (Figure 19): the initial reaction cavity, defined by the space in which the excited states of reacting molecules are generated; the effective reaction cavity, which encompasses the space the excited states and the intermediates explore from the time of their inception to the moment of their final product formation, and the jinal reaction cavity, which includes only the space in which the product determining steps occur. Whether they are different depends upon the nature of the guest molecule, its photochemical reactions, and the medium in which the transformation occurs.

B. Concept of Free Volume: Stiff and Flexible Reaction Cavities The concept of free volume has been introduced into the reaction cavity model to accommodate the shape changes that occur as the reactants transform themselves to products. Free volume exists in all condensed media, with the possible exception of perfect crystals of monatomic solids. Normally, there is more free volume in a liquid than in the crystalline state of the same substance, as evidenced by the decrease in density of most solids on melting (with ice as a familiar exception). In isotropic liquids, free volume is highly mobile because it wanders about by virtue of motions (translation, rotation, vibration, internal rotation, etc.) of the molecules that constitute the liquid. In organized media, the free volume may be essentially immobile, as in crystals, or have mobility ranging from that of crystals to that of isotropic liquids. The motions of potentially reactive molecules will be subject to gross one-, two-, or three-dimensional constraints as discussed above (see also Figure 17), to the fluctuating availability of appropriately disposed free volume and to the flexibility (response to stress) of the constraining structures. As indicated in Figures 1-7, there exists a well-defined free space in a number of host systems where the guest molecules can be accommodated. In the case of inclusion complexes, depending on the size of the guest molecule, there will be some free volume within the host channel/cage/cavity. Since surfaces of silica, the interplanar regions of clay, and the interiors of zeolites, possess timeindependent structures like crystalline materials (i.e., their relaxation times are much longer than the periods necessary to transform a reactant molecule to its products), the free volume needed to accommodate shape changes which occur during the course of a reaction must be present intrinsically within the fixed structure. Reaction cavities of such media possess stif walls.


97

Distinct differences exist between the rigidly organized structures discussed above and media such as micelles, microemulsions, molecular aggregates, and liquid crystals. In the latter, guest reaction cavities may contain minimal intrinsic free volume at the time of photoexcitation. However, since the molecules that constitute these organized assemblies are mobile, the reaction cavity can respond to shape changes as the reaction occurs. How much and how readily each medium accommodates shape changes is an important determinant of the selectivity obtained. In our nomenclature, these media possess Jlexible reaction cavities. How easily a medium responds to shape changes that occur during the course of a reaction depends on the microviscosity of the medium and on the extent of cooperative motions involving guest and host molecules. For a guest molecule to react in the environment provided by a restrictive host, the medium must make necessary adjustments within the time frame determined by the rate-limiting spatial decay paths leading to the preproduct transition state@). If the host medium cannot respond in a “timely” fashion, no reaction will occur. The time scale for common photochemical and photophysical processes spans many orders of to 10 s. The dynamics of significant magnitude, from approximately relative movement of the various hosts also span a very wide time range. Thus the boundaries of the enclosures in organized media may be of two types: they may be stty (i.e., none of the guest molecules can diffuse out and the walls do not bend), as in the case of crystals and some inclusion complexes, or Jlexible (i.e., some of the guest molecules may exit the cavity and the walls of the cavity are sufficiently mobile to allow considerable internal motion of the enclosed molecules), as in the case of micelles and liquid crystals. In these two extremes, free volume needed for a reaction is intrinsic (built into the reaction cavity) and latent (can be provided on demand).

C. Active and Passive Reaction Cavities In discussions to this point, no significant interaction between a guest and its medium has been considered. This is probably the case in the reaction cavity model of Cohen [13] as well, since product selectivity was attributed mainly to the presence or absence of free volume within the cavity. The analogy of guests in hosts to balls in boxes is very deficient, but is really not different from the situation in the kinds of crystal systems which first inspired the Cohen nomenclature. Interatomic attraction and repulsion was important in analyzing those systems and was even critical to the “crystal engineering” used to assemble some of the systems used in the studies by Schmidt and his co-workers [l, 48,891. In addition to being stiff or flexible, cavity walls must

98


be characterized as active or passive (i.e., are the wall interactions with guests attractive, repulsive, or benign?) (Figure 21). When the interaction between a guest molecule and a wall of the cavity is attractive or repulsive, the cavity wall is termed active and it may serve as a template for the guest as it proceeds to products; when there is no significant interaction it is considered to be passive. There will always be a net attractive action between guest and host if the system is at equilibrium, or the guest would not be there! However, different parts of a guest molecule may be strongly bound to some parts of the host and other parts of the two may be repulsive. When the walls of the cavity are passive, predictions concerning the excited state behavior of guest molecules can be made on the basis of size, shape, and flexibility of the reaction cavity. Active walls require possible wall-guest interactions to be considered also. The walls of some active cavities bear functional groups which are inhomogeneously distributed and which interact noncovalently with specific functional groups of guest molecules and their intermediates. If sufficiently attractive or repulsive, the interactions will influence the locations or conformations of guest molecules in the cavity. Additionally, the interactions must persist for times at least comparable to those required for the reaction to occur if they are to have a discernible influence on the course of the transformations. Although it is easy to recognize the presence of specific binding or repulsion between the ground state guest molecules and host framework, it is important to note that new potentially stronger or weaker interactions may develop between the walls and functional groups created in intermediates during the course of a reaction. Interactions may vary from weak van der Waal’s forces to hydrogen bonds to strong electrostatic forces

Active Walls

Passive Walls

Figure 21. A cartoon representation of active and passive walls of a reaction cavity. Active wall is characterized by inter guest-wall interactions. Such is absent in the case of passive walls.

99


between charged centers. For example, a number of hosts capable of forming inclusion complexes (such as cyclodextrin, urea, and deoxycholic acid) possess functional groups which can form hydrogen bonds with guest molecules. Silica surfaces possess silanol groups which may orient the adsorbed molecules through hydrogen bonding. Surfaces of clays and zeolites often carry a large number of cations that can interact electrostatically with guests. Micellar, monolayer, and related assemblies have interfaces at which guest molecules can be oriented through hydrophobic-hydrophilic interactions. Most reaction cavity walls may in fact be active to some extent. As an illustration of the phenomena involved, consider the photocycloaddition of fumaronitrile to 5-X adamantanone, 6 (Scheme 2; X = F, C1, Br, OH, Ph, or t-Bu) [94]. In isotropic solvents, different quantities of adducts to the two carbonyl faces are formed. When 6 is complexed by p-CD, the intrinsically more reactive face of the carbonyl group becomes more hindered toward attack by fumaronitrile than the less reactive one. As a result of this attractive interaction (NB, hydrogen bonding between the carbonyl oxygen and a hydroxyl on the p-CD torus), the distribution of photoadducts is reversed. In this example, p-CD serves the function of a reaction cavity with “active” walls ke., a template). N

X

6

5 - X . Anti

5-X-AD X= F. CI.

N

Br

5-

(favored in solution)

x - Syn

(favored in p-CD)

\

\

m-

-&-0

I1

X

In Solution

In p-CD Solution

Scheme 2.

100


D. Microheterogeneity in Organized Media In most noncrystalline phases, guest molecules reside in a distribution of reaction cavity types. Fluid solutions allow reacting molecules to experience an average microenvironment by virtue of fast relaxation time of the solvent and/or high mobility of the reactant molecules. However, in organized and very viscous media, solvent relaxation and rates of guest diffusion may be slower than the time period of a photoreaction, leading to reaction occurring in a variety of reaction cavities (“sites”).On the other hand, even if there is no migration of molecules between sites, one could imagine a situation in which all reacting molecules experience identical environments because all sites are equivalent. However, this is not expected to be the case in most organized media. Even perfect crystals have at least two types of reaction cavities corresponding to molecules in the interior and on the surface. Normally, crystals have defects wherein the molecules may react still differently. Diffusion of molecules among these site types is not expected to occur as rapidly as single molecules are transformed to photoproducts; from a mechanistic point of view at least three different reaction cavity types should be expected in crystalline media. By contrast, interconversion between molecules present at interfacial and interior sites of a micelle may be comparable to the time scale of a photoreaction. Generally, the relationship between the rates of hopping by molecules among various sites and the rates of photoreactions will determine the importance of multiple sites on the transformation of single molecules and, therefore, the selectivity of their product distribution. Scheme 3 summarizes this problem with a minimum number of sites and competing processes. In this scheme, two sites, square-well type (X) and spherical-well type (Y), are available for the residence of reactant molecules (A). For the sake of convenience, molecules residing at sites X and Y are labeled A, and A,. Excitation of these molecules gives rise to A; and A t . Photoreactivity of molecules excited in each site will be identical if they equilibrate between X and Y before becoming photoproducts. In media with time-independent structures, such as crystals, equilibration requires diffusion of molecules of A; in media with time-dependent structures, such as micelles and liquid crystals, equilibration can be accomplished via fluctuations in the microstructure of the reaction cavities as well as translational motion of A (Scheme 4). An additional mechanism for site selective reactions or equilibration of A; and A t molecules can be achieved via energy migration (e.g., energy hopping, exciton migration, or Forster energy transfer). Three limiting dynamic situations can be envisioned in Scheme 3: (1) the rate of excited state reaction is slower than the rate of interconversion between A; and A? (equilibrium is established between A; and A t before

lhv k*,,

1 op’

-

lhu

-uScheme 3.

(I) Translational of guest from site X to site Y

(Ii) Fluctuationsof the media (Time dependent structures-shapes of sites change with ttme)

lJ-(J--QJlJx

*J-f

Y

-

(Hi) Energy transfer between molecules present at sites X and Y

Scheme 4.

101

102

V. RAMAMURTHY, R. G. WEISS A N D G. S . H A M M O N D

decay; kfy and k& >> k;, and k;,); (2) the rate of excited state reaction is faster than the rate of interconversion between A$ and A;: (3) the rate of interconversion between the Af and A: and the rates of reaction (decay) of excited guest molecules at these sites are comparable (assume for instance, k;, >> ktx and kfY >> k;, and k:,), Mechanism 1 corresponds to the situation in a fluid isotropic solution where a single effective reactive cavity should exist. It should also apply to micellar solution. Under such conditions single exponential decay would be expected for the guest excited states and the photoreactivity would be predictable on the basis of a single effective reaction cavity. In mechanism 2 there should be two kinetically distinct A* in two noninterconverting sites. Double exponential decay will be expected for the excited state of A. The quantum efficiency of product formation and the product distribution may depend upon the percent conversion. An elegant example of mechanism 2 is available from the studies of Pokkuluri et al. in crystalline media (Scheme 5) [95]. The ratio of products 8,9, and 10 from 7 depends on the crystal size (i.e., the ratio of the number of molecules present in the bulk to those on the surface). According to mechanism 3, even if the concentration of A in one of the two sites (Y) is much smaller than the amount in X sites, appreciable

r-.

CH3

L

i

-

* hv

7

i

CH3

via s1

CH3

i E

El

E

9

i

-A.

E=C02CH3

CH3

i

E

& 8

CH3

Scheme 5.

10


103

photoreaction from A, may occur through A$ -,A t energy transfer as outlined in Scheme 4 or if the quantum yield of reaction from A? is much larger than that from A;. An example in which mechanism 3 operates is the photodimerization of 9-cyanoanthracene in the crystalline state presented above (Scheme 6) [96]. On a statistical basis, many more molecules within the crystalline bulk phase are expected to be excited than those at defect sites. However, the reaction cavities capable of supporting reaction are specific to defect sites. Efficient photodimerization is believed to occur owing to exciton migration from the inert bulk sites to the defect sites.

Adjocent molecules in bulk

@P NC

i

8 CN

hr,

CN

9 cyono onthroceno Rote of r e a c t i o n Vs. Rote of energy tronsfer

1

Adjocent niolecules in structural fault

Mirror symmetric dimer not formed

Scheme 6.

Terms such as microheterogeneous, anisotropic, and nonhomogeneous media used to describe a number of organized media derive from the unique features described above (multiple sites) they exhibit.

E. Limitations and Conclusions Thus far, attention has been focused on the guest molecules in their ground states. This is so because it is relatively easy to predict and visualize the geometry and orientation of molecules within reaction cavities based on attractive and repulsive interactions between ground state guest molecules and the host structure. However, electronic excitation frequently lead to changes in molecular geometry and polarizability [97]. For example, it is well known that formaldehyde becomes pyramidal upon excitation and the C-0

104

V. RAMAMURTHY, R. G. WEISS A N D G. S. HAMMOND

group dipole moment is greatly reduced. Such differences in geometry and polarizability between the ground and the reactive excited state are expected to have subtle consequences on the interaction between the excited guest molecules and their neighbors. Even more complications arise when intermediates intervene between the excited states of reactants and their photoproducts. Clearly, consideration of interactions of reaction cavity with the ground state, excited states, and reaction intermediates of a guest at the atomic level will be required to achieve reliable predictability concerning the specific influence of aniostropic environments on the course of photoreactions. Additionally, an understanding of the response of the medium during transformation of a guest molecule will also be necessary. Both are lacking presently in all but the most elementary systems. The following points with respect to our model emerge from the presentation given above: (1) A reaction cavity is an enclosure that reduces the mobility of reactant molecules in at least one dimension and provides a boundary which reactant molecules may not cross without overcoming an energy barrier. (2) The sizes and shapes of the reaction cavities among organized media may vary. (3) Free volume within a reaction cavity is an important parameter; it is the shape, size, location, and directionality of free volume and the dynamics of their change that control in large part the extent to which the medium influences a photoreaction. (4) When the atoms/molecules that constitute the walls of the reaction cavity are stationary (i.e., possess time-independent positions on the time scale of the guest reaction), the free volume necessary to convert a guest molecule to its photoproducts must be built into the reaction cavity. On the other hand, in systems where the walls are flexible, the free volume may become available during the course of a reaction. For these media, the free volume content in a reaction cavity cannot be readily represented by static molecular models, since it is a property of the macro medium and exhibits structural fluctuations. ( 5 ) The reaction cavity may contain specific functional groups or atoms which may strongly interact attractively or repulsively with either a guest molecule and/or the transition state and/or the intermediate that is formed as the guest proceeds to products. Such specific interactions may lead to unique product selectivity and either enhance or decrease the quantum yields for reactions. (6) Unlike in fluid isotropic solutions, there may be more than one type of reaction cavity in which guest molecules reside in an organized medium. If interchange of molecules among different types of reaction cavities is slow on the time scale of excited state processes, prediction of quantum yields and product distributions requires detailed structural and dynamic knowledge of the system, especially the nature and relative abundance of the different reaction cavities.


105

IV. REACTION CAVITY MODEL APPLIED TO EXAMPLES In this section several photoreactions from the literature are examined in terms of the reaction cavity concepts outlined above. Examples have been so chosen that only the particular aspect highlighted is the major influencing factor. However, in certain cases there may be more than one factor responsible for the changes observed. To establish the generality of the proposed model, examples have been chosen from a number of different organized media. In this section features relating to enclosure and free volume are discussed.

A. Consequences of an Enclosure When reacting molecule(s) are confined within a small enclosure, both uni and bimolecular reactions would be expected to be affected (in comparison to behavior in isotropic solution or in vapor phase). It is easy to visualize that the probability of collisions between two molecules or fragments, generated from a molecule, will be higher when the space within which these are enclosed is small. On this basis one would predict that a common characteristic of organized media would be to provide cage effects that are larger than those commonly observed in solvents of high fluidity. Also the efficiency of bimolecular reactions would be enhanced when more than one molecule is confined within a single enclosure. Both of these aspects have received attention. Cage effects in organized media have been extensively investigated by Turro and co-workers using the photobehavior of parent and unsymmetrically substituted dibenzylketone (DBK) as a model [98-1201. Photolysis of DBK has been so thoroughly investigated that its behavior can be used as a test to explore the existence and nature of the enclosure in an organized medium. Irradiation of DBK in fluid solutions leads to the formation of benzyl radicals following a-cleavage and decarbonylation processes. The termination process of these radicals is generally the uncorrelated (random) coupling to yield 1,2-diphenylethane. Photolysis of unsymmetrically substituted derivatives (represented as ACOB) results in the formation of three radical coupling products, AA, AB, and BB, in a ratio of 1:2:1 (Scheme 7). If the volume of the space in which the radical fragments A and B are generated is too large, separated radical pairs will tend to lose their correlation and yield noncage products. In contrast, when separation of the radicals is impeded by confining environments, the radical recombination probability is modified by

106

*


&”’+ /

-

Me

hv

Me

{*TB]

Me

’

/

Me

12

+

11

Me

AA

AB

BB

Scheme 7.

favoring the preferential formation of geminate products of the AB type. The cage effect, defined as Cage effect =

+ + +

AB -(AA BB) (AA AB BB)

is a measure of restriction and is calculated from the yields of diary1 ethanes AA, AB, and BB. It has also been observed that recombination of the primary radical pair can sometimes occur before decarbonylation (or separation), giving rise to rearranged ketone photoproducts (ortho and para isomers) and recovered starting material (Scheme 7). Recombination of the primary radical pair generates 1-phenyl-ortho-methyl acetophenone (11) and l-phenyl-paramethyl acetophenone (12) by ortho and para coupling, respectively, in the case of DBK. Thus, in using unsymmetrical DBK as a probe one measures the yields of diphenylethanes and rearranged products and, in certain cases, the I3C isotope enrichment factor (which is not the subject of discussion here). The cage effects measured in various media are compiled in Table 2. Results clearly show that all of the organized media listed in the table have a cage effect larger than is observed in benzene (-0%). Also note that the magnitudes of the cage effect and the yields of the rearrangement product depend on the medium (probably a reflection of the differences in the characteristics of the reaction cavity in various media). The importance of the size of the enclosure (reaction cavity) on a reaction course has been a subject of investigation in several laboratories. On the basis of the proposed mechanistic scheme for DBK fragmentation and on the basis of the “effective reaction cavity” model presented in Section 111, the following predictions can be made: (a) a relationship will exist between the cage effect and the reaction cavity size; (b) the cage effects observed for singlet and triplet benzyl radical pairs will be different (assuming very similar diffusion rates)

107


TABLE 2 Representative Examples of the Product Distribution from Photolysis of Dibenzyl Ketones in Various Environments‘

Percentage Yield ~~

Media Benzene

Crystals Silica surface (22 A) Zeolite Na X p-Cyclodextrin complex Aqueous complex Solid complex HDTCI-Micelles Liquid crystals CCI/CNd,Chol‘ CCI/CN, Isof

~

Reactant

DPE

pMeDBK a-MeDBK pMeDBK DBK-d,

100 100

100 55

DBK a-MeDBK DBK r-MeDBK DBK p-MeDBK

19 40 100 98 98 100

p-MeDBK

-

100 100

RP

C.Eb

-

0

100

80 60 2

-2

-

22 22

-

80 100 96 33‘ 59’ 18 14

“Adopted from Ref. 70. bCage effect (%). ‘Cage effect measured by quenching of the escape radicals with CuCI,. dCholesteryl chloride/cholesteryl nonanoate, 35/65, wjw. ‘Chloesteric phase. ’hotropic phase.

since the lifetimes can be very different. Indeed these predictions have been verified. A relationship between the cage effect and the pore size on silica surfaces has been established by Turro and co-workers with the photobehavior of DBK-d, and unsymmetrical DBK as probes [101-1041. An inverse relationship between the cage effect and the pore size has been observed at all coverages as summarized in Table 3. de Mayo et al. estimated the cage effects for singlet and triplet benzyl radical pairs generated independently by photolysis of benzyl phenyl acetate, 13a, and dibenzyl ketone 13b, respectively on the surface of silica (Scheme 8) [121, 1221. This study presumes that the excited ketone crosses rapidly to the triplet state and that the excited ester does not. The cage effect measured by monitoring the yields of diarylethanes (AA, AB, and BB) from the singlet pair was approximately twice that from the triplet radical pair (Table 4).This is consistent with the postulate that the lifetime difference between the singlet and the triplet radical pair gives rise to a difference in size of the “effective” reaction cavity. The

108


t

AA

+

AB

t

BB

OCH,

Scheme 8.

longer lifetime of the triplet allows the radical pair to explore more space and thus favor noncorrelated products. Studies in micellar media provide some elegant examples of the influence of the reaction cavity size on reactions carried out in micelles. One can alter the micellar size by changing the length of the constituent detergent molecules, the aggregation number, and the concentration of the detergent and by the addition of salts. Turro and Weed, utilizing p-methyl DBK as the probe, showed that the measured cage effect depends linearly on the chain length of the detergent that forms the micelle [123]. Further studies from the laboratories of Turro established that the exit rate of the benzyl radical pair from micelles depends inversely on the size of micelles [ 117, 1181. Exit rates of TABLE 3 Cage Effects for Triplet 4-MethylbenzylBenzyl Radical Pairs on Silicas with Various Pore Sizes at Room Temperature [102,461 Pore Size

Coverage

Cage

22

16 3 1 25 4 1

18 23 28 6 14 21

(4

95

(%I

(%I

109


TABLE 4 Cage Effects for Singlet and Triplet Benzyl Radical Pairs Produced from Dibenzyl Ketones and Benzyl Phenylacetates on Silica at 20°C (see Scheme 8) 1121, 1221

Coverage

Radical Pair

Precursor

(%I

Cage Reaction (%) ~

4-CH,-C,jH,CH,/ 4-CH3O-C,jH,CH,

Triplet

4-CH3-C,jH,CH,/ 4-CH30-C,jH,CHZ

1 10 50

25 23 21

Singlet

1 10 50

51 39 32

benzyl radicals from micelles of C,,, Cl2, and C , , sulfates were determined to be 2.7,1.8, and 1.2 x lo6 s-’, respectively. Quantitative parameters, such as radical exit rate, rate of coupling of radical pairs within the cage, and fluorescence quenching rate, have been measured for a number of systems and correlated with micellar size. Evans, Scaiano, and Ingold showed that both the rate of cage reaction and the exit rate of the triplet radical pair derived from photoreduction of butyrophenone by cl-tocopherol depend on the micellar size [124]. Both decrease with the increasing size of the micelle. de Schryver and co-workers have shown that the rate of fluorescence quenching of 1-methylpyrene by N-methyl-N-decylaniline and alkylpyridinium chloride (both quencher and quenchee being present within a micelle) is micellar size dependent [125]. The rate decreases with the increasing size of the micelle.

B. Reaction Cavity and Free Volume Over the last decade, a large number of examples (in the crystalline state) have corroborated the reaction cavity model due to Cohen and have brought out elegantly the need to have free volume within reaction cavities for the occurrence of solid state reactions. Even quantitative correlations have been attempted. Scheffer, Trotter, and co-workers have examined free volumes within reaction cavities to gain insight into the mechanism of intramolecular photorearrangements of enones [126,1273. They have shown that “the course of a solid state reaction is influenced profoundly by certain specific

110


steric interactions which develop between the reacting molecules and their stationary lattice neighbors.” This approach has been termed steric compression control. Gavezzotti has applied theoretical analyses in terms of empty and filled volume spaces in the crystal to understand the photodecomposition of a number of peroxides and azo compounds originally investigated by McBride [66, 128, 1291. He concluded that “a prerequisite for a crystal reactivity is the availability of free volume around the reaction site.” Ramamurthy, Venkatesan, and co-workers have utilized similar ideas to understand the photobehavior of a number of olefins which do not follow Schmidt’s original postulates [ 1301. In this context, lattice energy calculations involving van der Waals attractive and repulsive terms were performed for incremental changes in the orientation of reactive pair along the dimerization coordinate. Zimmerman and Zurcaw [131,1321 have utilized molecular volume calculations to understand the solid state reactivity of a number of systems potentially capable of undergoing di-n-methane rearrangement. In this approach quantitative prediction of solid state reactivity was based on the superimposability of the molecular structures of the product(s) or intermediate(s) into the reaction cavity of the reactant in the crystal lattice. The studies mentioned above serve as a basis for extension of the free volume concept to several other media considered in this chapter. In this as well as in the following sections free volume includes both typesobvious and concealed corresponding to stiff and flexible reaction cavities. We illustrate in this section with a number of examples how the presence or absence of free volume within a reaction cavity determines the feasibility of a reaction in organized media. Presence of free volume alone may not be sufficient to effect a reaction within a reaction cavity. Its location and its directionality (presence of free volume in the critical dimension) are extremely important, as revealed by a number of examples discussed in Section D. Zimmerman and Zuraw [131,132] have illustrated with a number of examples that the product selectivity in the solid state can be predicted on the basis of whether intermediate(s) and/or resultant product(s) from a reaction will fit within the reaction cavity in which reactant molecules are accommodated. In other words, intermediates whose sizes are larger than the reaction cavity will not be favored-the larger the reaction cavity the poorer the selectivity. One example from their studies is discussed below. Tetraphenyldicyanotriene, 14, upon irradiation in solution yields products resulting from di-n-methane rearrangement (Scheme 9). Irradiation in the solid state, surprisingly, gave only a tri-n-methane rearrangement product, and none of the solution products were formed. This unique preference has been rationalized on the basis of molecular volumes of the reactive intermediates involved in the di-n-methane and tri-n-methane rearrangements (Table 5). As reflected in the values of A V and AS (Table 5), diradical intermediates 15,16,

t

A

Ph

rh

Ph

14 -

Ph

CN

Ph

Ph

CN

Ph, CN

Ph

Ph Ph Ph

t "

1 1 5

dc:

Ph

CN

Ph

Ph

hv

ph& CN

CN Ph

gph CN

CN

Ph

Crystal)CN

Ph

Ph

Ph

CN Ph

Ph

p

CN

18 -

14

:

Ph

L

Ph

Ph

h

CN

Scheme 9.

TABLE 5 Calculated Overlap Parameters for the Diradical Intermediates During the di-n Methane and tri-n-Methane Rearrangements of 14 (see Scheme 9)" Diradical Intermediates 15 16

17 18

AMb 0.78 1.87 0.53 0.24

A V'

ASd

15 26 15 6

5 10 2 1

(%I

(%I

"Adopted from Refs. 131 and 132. *Average straight line atomic movement from reactant to the intermediate diradical. 'Volume of superimposed species not in common with reactant. dOverlap of the inserted diradical species with neighboring molecules.

111

112


and 17 are too large to fit within the reaction cavity of crystalline tetraphenyldicyanotriene.On the other hand, diradical intermediate from tri.n-methane rearrangement, 18, is small enough to fit within the reaction cavity. A comparison of the photobehavior of two crystals, 7-methoxycoumarin (19) and methyl m-bromo-cinnamate (20), exemplifies the importance of the need for the existence of free volume close to the reaction site [133-1351. Neither of these molecules is topologically oriented for efficient reaction in the crystal, meaning that the reactive pairs need to undergo some intermolecular realignment before they can dimerize. COOMe

Me0

Br

19

20

In spite of the unfavorable topochemical arrangement (the reactive double bonds are rotated by 65" with respect to each other and the center-to-center double-bond distance is 3.83 A, see Figure 22), photodimerization occurs in crystals of 19 to give the syn head-tail isomer. On the other hand, 20 which also has nonideal topological arrangement of double bonds in the crystalline state (the distance between the centers of adjacent double bonds is 3.93 .$; the double bonds are rotated and make an angle of 28" when projected down the

Figure 22. Packing arrangement of 7-methoxycoumarin in the crystals-a unit cell representation. Note the two reaction centers (darkened circles) are twisted with respect to each other.

113


line joining the centers of the bonds) does not undergo photochemical dimerization in the crystalline state. This difference in behavior has been rationalized on the basis of lattice energy calculations. The lattice energy increase upon reorientation of the reactive pair of 19 in the crystal lattice was estimated to be roughly of the same order of magnitude as for many photoreactive crystals with favorably oriented pairs. On the other hand, the lattice energy increase necessary to align the molecules parallel to each other in a geometry suitable for dimerization is enormous in the case of 20. Such a large increase in the lattice energy indicates that the cavity wall will resist the reorganization required for the photodimerization of 20. To rephrase this conclusion in terms of the reaction cavity model, the reaction cavity occupied by a pair of 7-methoxycoumarin molecules contains a large amount of free volume than the ones present in the crystals of methyl rn-bromo-cinnamate. Because of the larger free volume, the reaction cavities in 19 crystals are tolerant toward large motions executed by a pair of 19 molecules during dimerization. The importance of free volume within the reaction cavities in the case of inclusion complexes has also been shown by several examples. Lahav, Leiserowitz et al. have observed that irradiation of the inclusion complexes of acetophenones with deoxycholic acid yields an addition product enantiomerically pure in each case (Scheme 10) [136]. Supported by a detailed

a : X=H

b:X=F

Scheme 10.

crystallographic study, they have proposed a mechanism in which a net rotation of 180" of the C-C bond specifically along one direction connecting phenyl and carbonyl carbon atoms precedes the final addition product (Scheme 11).Such a specific rotation is shown to occur only under conditions wherein short contacts between the host framework and the rotating guest molecules do not exist. In other words, where there is void volume around the reaction center the rotation in a specific direction is favored. In the propiophenone-DCA complex where there is minimal free volume near the reaction center, both optical isomers were obtained (Scheme 12). They are

114


1

-c-

Scheme 11.

Scheme 12.

proposed to arise from guests which add to the host steroid without the rotation of the C-C bond connecting phenyl and carbonyl carbon. The importance of free volume and the influence on a reaction of variation in the size, shape, and mobility of free volume in various organized media will be illustrated in the following paragraphs by examining the geometric isomerization of olefins conducted in various media. In fact, as pointed out in Section I, the primary photochemical process of visual pigments is geometric isomerization of a specific double bond in retinyl polyene. This occurs within the cavity defined by the protein opsin to which the polyene is bound. The rapidity (less than one picosecond) and the specificity with which the isomerization occurs suggest that the reaction cavity in these protein helices probably has some free volume surrounding the Cll-C,, bond of retinyl chromophore. Although the photoisomerization of retinal has not been


115

investigated in a wide variety of organized media, geometric isomerization of the derivatives of cinnamic acids, stilbenes, and stilbazoles has been examined. The ease with which the isomerization occurs is a reflection of the medium’s accommodation to shape changes that occur during the transposition of the groups attached to the double bond. Photolysis of alkyl cinnamates and cinnamic acids (21) has been carried out in aqueous media, in micelles, in liquid crystalline and crystalline media. In the crystalline state, trans-cinnamic acid has been reported not to isomerize to the cis isomer. Dimerization does occur. However, there are a few examples of cis-cinnamic acids and cis- 1,2-di(1-naphthy1)ethylenes undergoing photochemical isomerization to the trans forms in the crystalline state [I 1,137, 1381. Although the exact mechanism by which such an isomerization occurs is debatable, free volume near the C-C double bond has been identified in the crystal structures of the isomerizable compounds. A recent report concerning P-nitrostyrene (22) also highlights the importance of free volume near the C-C double bond [139]. Irradiation of trans-22 crystals gives an anomalous dimer which is suggested to result via the cis isomer. Geometric isomerization of alkyl trans-cinnamates readily occurs in the micellar phase of sodium dodecyl sulfate, in the aqueous phase (where cinnamates are proposed to exist as molecular clusters) and in the liquid crystalline smectic phase of n-butyl-stearate [140-1421. This is not surprising considering the fact that the walls of the reaction cavities of these media are fairly mobile and the volume demand during isomerization of these cinnamates is not excessively high. However, when the volume demand during isomerization is high and when the medium is highly ordered, isomerization is inhibited as in the case with 5a-cholestan-3/.?-yl transcinnamate (23) [143]. Upon irradiation of 23 in its crystalline and in cholesteric liquid crystalline phases, no isomerization was detected although this process occurs in isotropic media. Although the reported lack of cis isomer may be due to its very low concentration at the small conversion of trans which were affected, the propensity to dimerize is clearly greater in the ordered phases. The photochemistry and photophysics of trans-stilbene derivatives (24) have been utilized by Whitten and co-workers to understand the relaxation characteristics of media such as micelles, monolayers, and LB films [144, 1451. For example, the CD,,, to cis for stilbene derivatives show the following trend: solvent system methylcyclohexane > SDS micelle >> multilayer assemblies (with arachidic acid). In fact, no isomerization is observed in multilayer assemblies. This is the trend expected on the basis of how readily the media can respond to stilbene shape changes during isomerization process. The importance of free volume has been demonstrated further in L-B

116


L f P 23 21

22

24

assemblies by Whitten and co-workers [146-1481 through investigation of the isomerization of thioindigo dyes 25 and 26 in L-B assemblies. In solution, the reversible photoisomerization of both thioindigo isomers is well known. Irradiation of cis-thioindigo in L-B assemblies leads to rapid, irreversible cis to trans isomerization. On the other hand, no reaction was observed when trans-thioindigo L-B assemblies were irradiated over prolonged periods. The fact that cis-thioindigo isomerizes and trans-thioindigo does not was attributed to rotational constraints imposed by the rigid L-B assembly and the larger volume occupied by the cis isomer (from the isotherm data it was inferred that the cis-thioindigo required a larger area per molecule than the trans isomer). Studies by Nishiyama and Fujihara [1491 utilizing azobenzene derivative (27) as isomerizable chromophores have demonstrated the importance of reaction cavity free volume in L-B films. The L-B films of amphiphilic derivative 4-octyl-4’-(3-carboxytrimethyleneoxy)-azobenzene (27) upon irradiation was found to be stable, no geometric isomerization of the azobennzene moiety occurred. This compound forms L-B films with water soluble polyallylamine 28 at an air-water interface. Reversible cis-trans photoisomerization occurs in the film containing 28. The reversible photoisomerization reaction in polyion complexed films is thought to occur because of the increased area per molecule provided in the film. The cross sections of molecule 27 in the pure film and in film containing 28 were estimated to be 0.28 and 0.39nm2. Such an increased area per molecule

117

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS 0

R)pJ$JR'

R'

/

0

0

R 0

25

W U ~ SR = n-C6H130; R = H b trans R = H; R = n-C4H9 tram R = H; R = r u t - m y 1

27

28

guarantees a large reaction cavity, as illustrated in Figure 23. The surface area per molecule was found to be controlled by varying the size of the monomer unit chosen in the ionic polymer.

C. Location and Directionality of Free Volume Consideration of overall free volume within a reaction cavity may not always help in understanding or predicting the photobehavior of guest reactant

(4

(b)

Figure 23. (a) Schematic representation of an anionic surfactant azobenzene derivative monolayer film at the air-water interface. (b) Schematic representation of the stable monolayer film formed from the polyion complex of anionic surfactant azobenzene derivatives with a cationic polymer. Note the difference in free volume around the reactant chromophores in the two monolayers.

118

V. RAMAMURTHY. R. G. WEISS A N D G. S. H A M M O N D

molecules in organized media (Figure 24). We show below that preference for one reaction among the many possible ones is directly related to the presence of free volume near one part of the molecule and absence of it near the other. Further, directionality of free volume within a reaction cavity is an important parameter.

-

(I) Overaii free volume is large, but not in the required place

Reactant

’

Free Volume

Product

(ii) Large free volume is in the wrong place

(iii) Directionaiity of free volume

Reaction is along this axis

Figure 24. A relationship between free volume and the feasibility of a reaction in an organized media. Filled areas correspond to the shapes and sizes of reactants and products. Note the shape changes between the reactant and the product. Free volume around a reaction center is represented as unfilled regions. In all three cases shown here the total free volume present is much larger than needed for a reaction to occur, but it is not present at the correct location. Importance of location and directionality of free volume highlighted.

119


An illustrative example is available from the studies of Scheffer et al. on enones [126,127]. Enones of general structure 29, when irradiated as neat solids, can undergo either a or /J hydrogen abstraction to yield products shown in Scheme 13. Scheffer et al. noted that irradiation of 29 in the solid state gave exclusively products arising from a-hydrogen abstraction. The preference for a-hydrogen over p- has no topochemical origin. Based on lattice energy calculations, they showed that pyramidalization of the pcarbon following hydrogen abstraction is not favored by the nearest neighbors. On the other hand, similar pyramidalization of the a-carbon does not pose any steric compression and is expected to cause no increase of lattice energy (Figure 25). The steric compression between the nearest neighbors and the reactive center arises because of the absence of free volume. The selectivity obtained is therefore the result of the presence of free volume near the a-site and absence of it near the p-site.

/ * \ Path A hu

29 -

Path C

Me R4

R3

Me

Scheme 13.

Pokkuluri et al. have recently addressed the question of the directionality of free volume in their studies on the solid state di-x-methane rearrangement of methyl 2-benzoyl-1,4-dihydrol,4-ethenonaphthalene-3-carboxylate, 30 [1501. The title compound gives two different sets of products upon solution and solid state irradiation, both resulting from di-x-methane process involving 1,3-diradical intermediate (Scheme 14). Strikingly, the solid state products derive via less stable 1,3-diradical intermediates, 31 and 32 whereas the

120

V. RAMAMURTHY, R. G. WEISS A N D G. S. H A M M O N D

Methyl Group from Neighboring Molecule

(4

(b)

Figure 25. (a) Steric compression accompanying pyramidalization at C,. Note the interaction between methyl groups of adjacent molecules as the reaction occurs. Substituents have been omitted. (b)Stereoview of the same phenomenon in a molecule

carrying OAc group at y position. [Reproduced with permission from S. Ariel, S. Askari, S.V. Evans, C . Hwang, J. Jay, J. R. Scheffer, J. Trotter, L. Walsh, and Y. F. Wong, Tetrahedron 43, 1253 (1987).] solution products, as expected, derive via the most stable intermediates, 33 and 34. No understanding of the preference for 31 and 32 over 33 and 34 was offered by the estimated molecular volumes, as per the methods of Zimmerman and Zuraw [131,132], of the potential intermediates. Molecular volumes of reactive intermediates 31 and 32 differed only by 2 A3. This approach suggests that the reaction cavity is large enough to accommodate either one of these intermediates. However, lattice energy calculations revealed that motions required for the formation of more stable 1,3-diradical

-

121


-b

B E

0 II

B=-C-Ph

34

-C

32 -

-d

\--

0 II

E= - C - OMe

Scheme 14.

intermediates was not favored by the lattice whereas formation of the less stable intermediates did not result in lattice energy increase. This suggests that the free volume, the presence of which was indicated by the calculations of Zimmerman and Zuraw, is located specifically in certain regions around the reactant molecule and that dictates the course of reaction. Location of free volume has also been shown to be important in bimolecular reactions [130,1341. Irradiation of crystalline 7-chlorocoumarin yields a single dimer (syn-head to head). The packing arrangement shown in Figure 26 reveals that there are two potentially reactive pairs of 7chlorocoumarin molecules in a unit cell. One pair being translationally related, has a center-to-center distance of 4.54 8, (favored to yield syn headto-head dimer). The other pair, being centrosymmetrically related, has a center-to-center distance of 4.12 A (favored to yield anti head-to-tail dimer). In spite of a favorable arrangement, the latter does not react; dimer is

122


a :4 . 4 5 4

p

I91.21’

i

b

=

30.488

c

:5 . 6 8 4

2 :4

Viewed down a - a x i s

Figure 26. Packing arrangement of 7-chlorocoumarin in the crystalline state. Note the presence of two pairs of reactive 7-chlorocoumarin. In one pair the molecules are related by a centrosymmetry and in the other by a mirror symmetry. On this diagram mirror symmetric molecules appear one on top of another. Centrosymmetric molecules appear to be displaced with respect to the other.

obtained only from the translationally related pair. Lattice energy calculations reveal that the relative increase in lattice energy were the centrosymmetrically related pair to start reacting, would be much higher than when the translationally related pair reacts. The calculations given above indirectly point out the presence of free volume near the translationally related pair and its absence near the centrosymmetrically related pair. Another set of examples consists of enantioselective photocyclizations reported by Toda, Kaftory, and co-workers [56-58, 151-1541. Upon excitation, a-tropolone alkyl ethers and pyridones undergo intramolecular cyclization to the corresponding bicyclic products by disrotatory ring closure. Depending on the direction of disrotation, two optical isomers may be formed. Racemic products are obtained in solution, but irradiation of crystalline inclusion complexes of a-tropolone alky ethers and pyridones with 1,6-bis (2-chlorophenyl)-l,6-diphenylhexa-2,4-diyne-1,6-diol (3) gave cyclic products of 100% optical purity (Scheme 15). The X-ray structure of the 1 : 1 a-tropolone ethyl ether complex with 3 shows that the guest molecule is held by hydrogen bonds to two host molecules as illustrated in Scheme 16 (note the reaction cavity walls are “active” and the interaction between the walls and the reactant molecules is not the determining factor for selectivity). On the basis of crystal packing, Kaftory reasons that the enantiomeric control results from the chiral environment provided by the host and from the differences in short contacts that develop between the alkoxy group of the


123

0

-

OMe

hu

OMe

ee

=

100%

'Me

Scheme 15.

(lS,5R)-(-)

(1R.55)- ( + I

Scheme 16.

guest and the aryl group of the host during the two directions of rotation. In other words, presence of free volume on one face of the molecule allows the rotation in that direction and absence of it on the other face forbids the rotation along that side. On the basis of the crystal structure of the 1 : 1 complex of 4-methoxy-1-methylpyridonewith 3, Fujiwara et al. have reached similar conclusions in the case of pyridones [lSS]. These two examples

124


abundantly make it clear that it is important to note the absence of free volume in some places as it is to note its availability in others. Irradiation of the cr-cyclodextrin complexes of rn-alkoxyphenyl ally1 ethers (35) gave only a single ortho isomer in each case, although in solution each ether gave two ortho and one para isomers via photo-Claisen rearrangements (Schemes 17 and 18) [156,157]. This can be attributed to the difference in free volume available within the cavity near the two ortho centers and to the difference in accessibility of the radical fragments to them. In support of this conclusion, irradiation of the /J-cyclodextrin complexes of 35 results in no

q2

hlJ +

OR

Scheme 17.

(p(3s

a

I

+

OR

&QH

OR

OR

OR

8-co

OR QLCYD

R = Methyl

62

30

0

100

Propyl

51

49

5

95

Hoxyl

24

76

12

88

Oclyl

10

90

12

a8

Dodecyl

25

75

36

64

Scheme 18.

OR


125

significant product selectivity (Scheme 18). The P-cyclodextrin cavity, being larger, probably provides adequate free volume around the two ortho centers for easy attack by the ally1 radical. The tight fit necessary for achieving selectivity in P-cyclodextrin can apparently be provided simply by increasing the space filling capacity of the substrate (e.g., by adding a long alkyl chain, Scheme 19).

R = CH3

R = octyl

R = dodecyl

Scheme 19.

Elegant illustration of the need for free volume along a specific direction/dimension is provided by studies on the photochemical geometric isomerization of stilbenes included in tris-onho-thymotide (TOT) and with in zeolites [ 158,1591.Presence of free volume along the perpendicular axis is essential for geometric isomerization of stilbene caged in a small reaction cavity since such an isomerization process involves shape changes only along the axis perpendicular to the long molecular axis. The photochemistry of stilbene and methyl cinnamate in TOT has been subjected to extensive investigation by Arad-Yellin et al. [1581. Although isomerization of cisstilbene to trans-stilbene occurs readily in the TOT matrix, the trans to cis isomerization does not take place. The difference in behavior has been attributed to, among other factors, the amount of void volume present near the central double bond in these two isomers when they are enclosed within the channels formed by TOT molecules. On the basis of X-ray structures and computer calculations, the authors have estimated the void volume near the olefinic double bond to be 56 and 103 A3 for trans and cis stilbene complexes, respectively. Although there is very large void volume within the stilbeneTOT cages (826 for trans and 887 A3 for cis) apparently it is not present where it is required. Sharp difference in void volume near the isomerizable double bond (56 vs. 103 A3) is probably responsible for the difference in behavior between the trans and the cis isomers. Similar studies in zeolites having different sizes of channels/cages also reveal the importance of the directionality of free volume within the reaction cavity [159]. Both the photochemical and the photophysical behavior of trans-stilbene and longer all trans-a,o-diphenyl polyenes critically depend on the zeolite in which they are included. In pentasil zeolites (ZSM-5, -8, and

126


-ll), wherein these polyenes are tightly held with no free volume near the double bonds, no photochemical isomerization occurs. In the faujasites, which contain larger chambers, the photoisomerization does take place. Such a dramatic difference in influence on the rotational motions of the guest by the zeolites is also reflected in the photophysics of these molecules. In the extremely confining space of the pentasil channels, all of the polyenes examined exhibit enhanced fluorescence and singlet lifetimes which are significantly longer than in fluid solution or in the supercages of faujasites. It is important to note that in the channels of ZSM-5, -8, and -1 1 there is a large amount of free volume along the long axis of the polyenes, but that does not help the isomerization since for this process to occur, as pointed out earlier, the free volume is needed along the axis perpendicular to the molecular long axis (Figure 27).

D. Magnitude of Free Volume Consider reactant molecules or intermediates being caged within a reaction cavity with limited free volume. A preference might be envisioned in the reactions these reactant molecules or intermediates undergo, if the competing reactions require different amounts of free volume for shape changes that take

:Fh\9 \ /

1 1,

n

Figure 27. Inclusion of trans-stilbene in X type and in ZSM-5 zeolites. Required free volume for geometric isomerization is present in supercages of X zeolite and such is absent in ZSM channels. Extensive free volume in ZSM-5 channel is present along the molecular axis, but that is of no use for the reaction to occur.

127


place during the conversion of the reactant to products. Indeed several such examples are available in the literature. An a-alkylbenzyl radical pair, as discussed in Section III.A, can undergo both coupling and disproportionation processes (Scheme 1). In solution, coupling is predominant; on surface of silica or within zeolites or micelles, disproportionation also occurs. Within the supercages of X and Y zeolites, two benzylic radicals generated from a-alkyldibenzyl ketones (5)react by both coupling and disproportionation processes, the latter yielding the olefins (Table 6 ) [92,93,160]. The ability of the disproportionation reaction, a minor process in solution, to become the major process within faujasite supercages and on silica surfaces is attributed to an inhibition of certain specific motions of the radicals in the restricted environments (Scheme 20). Relatively larger overall motion would be required to bring the two benzylic radical fragments together for head-to-head coupling than to move an alky group of one of the benzylic radical fragments so that one of its methylene hydrogens is in a suitable position for abstraction by the other benzylic carbon radical. A model developed on the assumption that the radical pair would prefer the pathway of “least volume” when the free volume around it is small, would predict that there should be an inverse linear relationship between the reaction free volume and the yield of the disproportionation products. Indeed this has been observed. Free volume can be controlled by altering the cations (Li, Na, K, Rb, and Cs) present within the supercages of X and Y zeolites (Table 6). The yield of disproportionation products indeed increases with the TABLE 6 Product Distribution Upon Photolysis of a-Alkyl Dibenzylketones in Zeolites Yields of Olefin, Coupling Product (AB) and Rearrangement Product 7 (see Scheme l)n,b

5b

5a Zeolite

Ole

AB‘

7

Ole

AB

7

Na-X K-X Rb-X

16.8 26.6 50.1 55.9

18.2 18.9 12.5 10.2

62.5 54.5 37.4 33.6

26.3 36.3 50.6 64.7

23.1 28.5 23.8 20.8

50.5 34.6 25.3 14.4

cs-x

Supercage Free Volume

(A3) 852 800 770 132

-

“Adopted from Ref. 93. benzene 5 gave AA ( - 20%), AB (-4079, BB ( - 20%), and type I1 products (DBK, 5% and cyclobutanol, 5%). Neither rearrangement product 7 nor disproportionation product olefin were obtained. ‘In zeolites no AA and BB were obtained.

-

128


Scheme 20.

decrease in the size of the supercage free volume (i.e., with the increase in the size of the cation). Values of k,/k, of 0.055 and 0.28 (k, is the rate constant for the disproportionation process and k, is that for the coupling process) were measured for cumyl radicals (36)in benzene and on silica, respectively (Scheme 21) [1611. Similarly, increases in the yields of disproportionation products have been observed for a-methylbenzyl radical pairs at low temperature on porous glass and several types of silica [loll. For example, k,/k, ratios of 0.054 and 1.33were obtained in pentane and on porous glass at


129

36

Scheme 21.

- 77°C. The increasing order of preference for disproportionation follows the same increasing trend in restriction experienced by the guests/intermediates and the available free volume within the reaction cavity: isotropic solution, silica surface, and zeolite, the last being the medium providing very high restriction and smallest free volume. Another example of the impact of the size of free volume within the supercages of faujasites on the course of a reaction is provided by the pathways undertaken by the primary triplet radical pairs, 40, generated by the a-cleavage of the a-alkyl dibenzylketones (9,a-alkyl deoxybenzoins (37), and benzoin ethers (38) [160, 162, 1631. In zeolites, owing to the cage effect, the primary triplet radical pair primarily undergoes cage recombination to yield a rearrangement product, 39, in addition to the starting ketone (Scheme 22). Examination of the results obtained with a number of a-alkyl dibenzylketones and benzoin ethers and a-alkyl deoxybenzoins reveals that although the rearrangement takes place in all X and Y zeolites, its yield is dependent on the cation and decreases as the cation size is increased from Li’ to Cs’. As the available free volume inside the supercage is decreased by the increase in the size of the cation, the translational and rotational motions required for the rearrangement process become increasingly hindered (Scheme 23). Under these conditions, the competing path, coupling to yield the starting ketone, a process which require “less volume and less motion,” dominate. This pathway eventually leads to products resulting via type I1 reaction. The pathways followed by radical pairs, 42 (disproportionation or coupling with rearrangement) resulting from the type I cleavage of 2phenylcycloalkanones (41) are influenced by cyclodextrin complexation [164]. The product ratio depends both on the size of the cyclic ketone and on

+

130

+

131


Scheme 23.

the diameter of the CD cavity, as illustrated in Scheme 24. The relationship between the sizes of the guest and the a-, p-, and y-CD cavity points out that the reaction selectivity depends on the tightness of guest inclusion (Scheme 25, the tighter the fit, the smaller the free volume).

0

41

C

E

= 10 Methanol a-CD complex p-CD complex y-CD complex

E/C 0.12 3.4

II

1.3 0.5 Scheme 24.

132


-

Closed chain form (Tight complex)

Biradical separation (Loose complex)

42

\

p-coupling

lntremolecular hydrogen transfer

Enal

para-Cyclophane

Scheme 25.

V. REACTION CAVITIES AS “TEMPLATES” HIGHLIGHTED WITH EXAMPLES The strategy by which reactant molecules are predisposed toward a particular reaction coordinate we call a template efect. In this approach the products are prepared from the “mold” or “structural pattern” imposed on the reactant molecule(s) by their reaction cavity. The template efect involves orientation and maintenance of reactant molecules in a particular arrangement (relative orientation in the case of bimolecular reactions and conformation in the case of unimolecular reactions) till the reaction is completed. Orientation of guest reactant molecules can be achieved in a predictable manner when the walls of the reaction cavity are active, i.e., they contain functional groups which can interact specifically with specific parts of potential reactant molecules. If the reaction cavity is “stiff’ and small (e.g., in crystals), even weak wall-guest interactions may be sufficient to reduce the mobility of guest molecules and, thus, maintain their preorganization. On the other hand, if the reaction cavity is “flexible” and large (e.g., in micelles), the preorientation may be disturbed upon excitation and stronger wall-guest interactions may be needed to maintain the orientation throughout the transformation of reactants to products.

133


A. Reaction Cavities with “Active Walls” as Templates: Photodimerization In this section we explore a number of photodimerizations in organized media in which the results can be interpreted on the basis of control by template effects. Crystals. Photodimerization in the crystalline state has been known for over a century. Based on the pioneering crystallographic and photochemical investigations on cinnamic acids and a large number of its derivatives (which exhibit a rich variety of polymorphic forms and photochemical reactivity patterns), Schmidt and co-workers formulated an important set of “topochemical rules” connecting the crystal structures of the reactants and the configurations of the products (Figure 28) [l]. In almost all cases, the stereoand regiochemistry of product dimers can be predicted on the basis of molecular orientation in the reactive crystal. This clearly suggests that once the reacting pair is preoriented, it dimerizes upon excitation to a predictable dimer. Mobility of molecules in the crystalline state is highly restricted since the reaction cavities are small and stiff. Orientation of the reacting pair is achieved through intermolecular interactions. Crystal packing is controlled by a large number of repulsive and attractive forces of which some are not often considered important by chemists utilizing isotropic solvents as reaction media. These include C-H...O, X...X (where X is a halogen), S...S, S...X, and C=O...X interactions [87-891. Examples of [2+ 23 photodimerizations reported in the literature may COOH

1 ’ coon

d - form Doub!: bond s e p a r a t i o n : 3.6 -4.1.A N r a r r a l nrighbour relation; Centric

p-

f o r m (3.9-4.1.A;

Tranalation I

’

coow ~ - T R U X I L L I C ACID

coow J3-TRUXINIC

ACID

Figure 28. Topochemical rules are based on the photochemical behavior of cinnamic acids in three crystallographic modifications.

134


provide the impression that it is fairly straightforward to orient molecules in crystals. This is certainly not true. In the absence of an a priori recipe several empirical strategies under the title of “crystal engineering” have been employed [48,89]. One of the crowning achievements in the field, the asymmetric photodimerizations of chiral and achiral crystals, have been achieved by this strategy (Scheme26) [20,21]. A number of reviews and a monograph of a collection of selected articles by Schmidt should be consulted for details concerning photodimerization in crystalline media [l, 15, 48, 49, 601.

X

S,single crystal

AX

R, single crystal

SOLID STATE CHIRAL SYNTHESIS

Scheme 26.

Inclusion Complexes. Within the reaction cavities of inclusion complexes, prealignment can be achieved through specific and well-defined (geometrydependent) interactions between the host and the guest. One of the illustrative hosts of this type are the diacetylene diols 2 and 3 that form crystalline inclusion complexes with a variety of small molecules [165]. In both, hydrogen bonding plays an important role in orientating the guest with

135


respect to its host [166]. In essence, the host acts as a template to preorient the reactive guest molecules. Irradiation of powdered complexes of benzylidene acetophenone 43 included in the nonchiral2 yields a single photoproduct, syn head-tail dimer 44 (Scheme 27) [167,168]. Irradiation of 43 in solution gives a mixture of cis Solution

7 Solid State

Polymer

Complex mixture

Complex with 2

Bz

43 Ar

Scheme 27.

and trans isomers of 43 and a polymer, and irradiation of 43 in pure crystals produces a mixture of stereoisomeric photodimers in low yields. Host 3 brings pairs of 43 molecules close to each other (3.86 A) and orients them in a head-tail fashion, as shown in Figure 8. This is distinctly different from the situation in pure crystals of benzylidene acetophenone wherein the potentially reactive molecules are farther apart (> 4.8 A). Other examples of this effect include [4+ 41 dimerization of 9-anthraldehyde and of 2-pyridone [1661. In these two cases, also, hydrogen bonding between the host and the guest is responsible for orientation of the reactive pair in the proper geometry for dimerization. One should not assume that any olefin can be made to dimerize by including it within the crystalline matrices of diols 2 and 3. It has been found that inclusion complexes of coumarin, 7-methoxycoumarin, and 7methylcoumarin in chiral3 form the mirror image symmetric syn head-head dimers [169]. However, there is a large number of coumarins which fails to dimerize as complexes of 2. Similar observations have been made in the case of benzylidene acetophenone derivatives. Although some of them dimerize when included in 3, many others are inert. In the absence of crystal structures, no correlations between reactivity and molecular structure and the disposition of molecules in the complexes can be made. Clearly, the templates can deactivate as well as activate their guests. Tamaki et al. were the first to explore the utility of the cavities of CD’s in effecting regio- and stereoselective dimerization of guest molecules (Scheme 28) [170-1721. The CDs in aqueous solutions may be thought of as affording “active” reaction cavities which inhibit guest motion rigorously in two dimensions, but only moderately along the third (defined by the axis of C D symmetry) owing to the hydrophobicity of the guests and the mobility of

136


p - cyclodextrin

y - cyclodextrin

46

47

48

49 -

Scheme 28.

the capping water molecules. Upper and lower rims of CD’s carry a large number of hydroxyl groups which can interact as templates with guest molecules and hold them in a specific geometry. Photodimerization of 2anthracenesulfonate (45) in aqueous solution produces four photodimers: anti-head-tail(46), syn-head-tail(47), anti-head-head (48), and syn-head-head (49) in the ratio 1 :0.8 :0.4: 1.1. Photolysis of a 2 :2 complex of 45 with p-CD yields exclusively 46. By contrast, photodimerization of 45 included in y-CD (2: 1 complex) gives almost the same relative yields of the photodimers as those in the host-free solution. Also a l-anthracenesulfonate (50) forms only 1 : 1 complexes with p-CD and 2 : 1 complexes with y-CD but regioselective photodimerization was not observed in either case. Although interactions between the p-CD cavity and 45 must be responsible for the photodimer selectivity, no comprehensive guiding principle has emerged which allows one to predict the orientation of guest molecules within a CD cavity, or which explains fully the quantitative changes in photoproduct yields which accompanies complexation. These templates can operate in a myriad of ways.


137

so; I

50

45

Surfaces. Preference in orientation of the substrates on the surfaces of silica, due to cavity wall-guest interactions, may also lead to some regio- and stereoselectivity. Examples of photoreactions of steriodal enones provided by de Mayo and co-workers illustrate the principles involved [173-1751. In these examples, the surface serves as a template when guest molecules are adsorbed on it. Preferential adsorption from the less-hindered face of the steriodal enones, 51, in Figure 29 should force addition of an olefin (from gas or solution phase) (ii) In solution: (more hindered) p face

-

I

J

Preferred

a-face (less hindered)

(ii) Silica surface: Preferred

I

Silica surface J

Figure 29. A schematic illustration of the face preference for addition of olefins to a steroidal enone in solution and on silica surface. In solution the less-hindered CI face is preferred. On silica surface the molecule adsorbs from the less-hindered face exposing the more-hindered face for attack by an olefin.

138

V. RAMAMURTHY, R. G . WEISS A N D G . S. H A M M O N D

to occur from the more-hindered face of the enone. Selectivity should be large when the template surface prefers to bind one face of the molecule and when the adsorbed molecules do not undergo motions that would permit the reagent to attack both faces with equal facility. In the examples shown in Scheme 29 a preference for the attack from the more-hindered side is commonly observed, although mixtures of products are typical. Adsorption on the silica surface leads not only to reduced attack from the less-hindered aside, but also to an increase in the barrier to inversion of the 1,4-diradical required for the formation of the trans-fused 4a,5p adduct. The preference for reaction on the more-hindered side is consistent with adsorption of the lesshindered side to the template surface. The primary mode of binding interaction is surely hydrogen bonding of surface hydroxyls to the oxygen atom of the carbonyl group. However, if that was the only attachment, the steroidal molecule would be free to assume an orientation approximately normal to the average surface plane. The stereochemical result implies that there is also attractive interaction between the surface and at least one point of the molecule, probably by polarization of the x system of the conjugated enone system. The results suggest a methodology for further study of the nature of the interactions of adsorbed species on solid template surfaces. O n the surfaces of clay, much stronger electrostatic interactions are

__*

0

CH 2=CH2

51

La 5a

LP 5p

MeOH

-78°C

82

12

6

SiO2

-78°C

42

50

-

~ a 5 p

OCOEt

LU 5d MeOH 5iO2

-78'C

-70'C

53 32

Scheme 29.

4p58

La5p

41 57

6 10

139


utilized to orient a photoactive species. The spatially controlled photodimerization of stilbazolium cations 52 inside saponite and montmorillonite clays has been reported: UV irradiation of the intercalate resulted in the generation of the syn head-tail as the dominant dimer (Scheme 30) [1761791. Selective dimer formation has been rationalized on the basis of an Py+

Ar

L-\ PY+

hdPyrex clay

+y+

Ar

+

A+y

+

Ar

PY+

PY+

52 b A r = e C H 3

c A r = e C N

Scheme 30.

arrangement shown in Figure 30, wherein unsaturated molecules are packed alternately in an antiparallel alignment. Such an arrangement is facilitated by the interaction between the stilbazolium cations and the anionic framework

Figure 30. Schematic illustration of the packing of stilbazolium cations inside a magnesium-rich saponite clay. Stereoselectivedimerization takes place to give the syn head-to-tail dimer. [Reproduced with permission from K . Takagi, H. Usami, H. Fukaya, and Y. Sawaki, J . Chem. Soc., Chem. Commun. 1174 (1989).]

140


of the clay. Among the stilbazoles investigated in saponite clay, the p cyanophenyl derivative gave a considerable amount of syn head-head dimer along with syn head-tail dimer. This is suggested to be the result of competition between the two types of interactions, the cyano group with sodium ions and pyridinium cation and framework anions. In any case it is important to note that strong ionic interactions with clay surface may result different template effects for the reacting olefins.

Micelles and LB Films. In micellar media, especially for photodimerization of amphiphilic molecules, the reaction cavity is primarily the interface ik., the poorly defined region where surfactant head groups and water molecules mingle most; and not the micelle itself. The ability of micelles to solubilize substrates and to orient them at the interfaces can often lead to regioselective photocycloadditions. de Mayo et al. have shown that micellar preorientation has a profound influence on the photochemical dimerization of 3-alkylcyclopentenones (53) [1803. Efficient dimerization in potassium dodecanoate micelles was observed for 3-n-butyl- and 3-n-decylcyclopentenones with a reversal in regiochemistry compared with that in organic solvents (Scheme 31). The almost exclusive formation of head-head dimers in micellar solution was attributed to orientation of cyclopentenone with its carbonyl oxygen at the interface and the remaining hydrophobic portion oriented

9% Benzene

91 OIo

50% Methanol

50% R-nCbHg

53

A

\

KDC Micelle ~

@Ao+ R

2 Yo Cyclohoxane

' Scheme 31.

0

4:l

1:19

R R 98%


141

toward the more hydrophobic interior of a micelle. Similar template like orientation of isophorone (Scheme 31) and coumarins (Scheme 32) in micellar solution can be invoked to explain the high regioselectivities of both photodimerizations [181-1861.

Syn-HH

(0.02 M I

Anti-HH

Solvent

Product

Methanol

Anti-HH

Benzene

Anti H H

Figure 47. Plot of the E/C product ratios from irradiation of neat 8ld versus

temperature. Arrows indicate phase transitions.

ratios from irradiations in the nematic, smectic A, smectic B, or isotropic phases. The larger E/C ratios in the monotropic temperature ranges of the smectic phases can be attributed to rapid phase transitions initiated by small amounts of photoproducts and subsequent solid phase reactions [278]. On this basis, the reaction cavities in the mesophases must be larger than the van der Waals volume of the part of the BR which is central to reaction and/or have very flexible walls. We suspect both in spite of the fact that 'H NMR studies with 81 deuteriated at the methylene carbons alpha to the carbonyl groups indicate that this region of the ground state molecule in the liquidcrystalline phases is quite constrained conformationally. Even if some interdigitation of the alkyl chains were present in the anisotropic phases of these molecules, the volume they would occupy does not equal the volume potentially available if the cyclohexylphenyl core groups of 81 limit the intermolecular distances; these phases are well ordered macroscopically, but some parts of the molecules must be much less organized. The solid phases of 81 are also well ordered macroscopically and their higher E/C ratios require that the hydroxy- 1P-biradical be in rather inflexible reaction cages with little excess free volume. Hydrogen bonding to neighboring ketone molecules may be partially responsible for the high photoproduct ratios found upon collapse of the biradicals in the solid phases, but the size, shape, and flexibility of the reaction cavity are clearly the more important factors. The highest E/C ratios observed in the second solid phase of 81a

181


exceed 100, indicating that the ability of its polymethylene chains to bend is severely compromised. On the other hand, the quantum yields are not inconsequential so that it must be possible for a reasonable fraction of the molecules to reach a conformation in which y-hydrogen abstraction can occur during the triplet lifetime of 81. Since “melting” of the alkyl chains of 81 is probably a key factor in converting a solid to a liquid crystal, it is not unreasonable to assume that the reaction cavities might differ in the two phases. Another important difference between them is that molecules of the crystal are translationally fixed within a lattice while they are free to diffuse anisotropically in the liquid crystal and, in some cases, cooperatively with their neighbors. Thus, during its lifetime, a BR in liquid-crystalline 81 may migrate to places where the free volume is greatest or wait until adequate free volume diffuses to it before decaying to products. As mentioned previously, the concept of a “reaction cavity” as a discrete, definable entity becomes tenuous in this case. Again, Scheffer and co-workers have provided the most extensive and intensive studies concerning the Norrish I1 reactions of alkyl aryl ketones in neat crystals [49,260,276]. Their analyses, which assume that the reaction cavity is very stiff (i.e., that ground state ketone molecules surrounding a hydroxy-1,4-biradical intermediate remain in almost the same conformations and positions as in the pristine crystal) and which do not consider intermolecular hydrogen bonding between the carbonyl oxygen of neighboring ketone molecules and the hydroxyl proton of a BR, have provided important insights into how neat solvent order influences Norrish I1 processes. For example, they find that initial hydrogen abstraction by the triplet states of crystalline cr-cyclohexyl para-substituted acetophenones (82a-c) occurs from boat-like transition states [279]. The fate of the so-formed triplet BRs is similar to that experienced by them in acetonitrile solutions and the photoproduct ratios are constant over a wide temperature range. Although there is a somewhat greater preference for fragmentation (83) in the solidstate irradiations, the 84/85 ratio from each of the three ketones is virtually invariant in its solid and in acetonitrile solution (Scheme 43). Crystal structures show that all three ketones adopt very similar conformations in the crystal which should favor the abstraction of an equatorial y-hydrogen atom of a cyclohexyl ring on the basis of the C=O...H distance ( 2.6 A versus 3.8 A for axial y-hydrogen abstraction) and orientation (see Newman projection of 82 and its biradical in Figure 48). The biradicals, once formed, are poorly shaped to provide any of the Norrish I1 products. Since extensive motions are required to bring the 1,4-biradicals to preproduct conformations [279], the reaction cavity must be flexible and capable of either rearranging or adding an appreciable amount of free volume during the short triplet biradical lifetimes. An intriguing possibility is that the elusive singlet

-

N

182

Ar

52

L

ArCOCH3

83 a, Ar

-

V. RAMAMURTHY, R. G. WEISS A N D G. S . HAMMOND

= p-tolyl;

b, Ar

Ar

";o""'U3 84

= p-chlorophenyl;

HA

c, Ar

55

= p-methoxyphenyl.

Scheme 43.

biradicals [280] are formed (via intersystem crossing of the triplets) long before they collapse to the products. The fragmentation route of hydroxy-1,4-biradicals could be effectively removed as a viable possibility by replacing the cyclohexyl rings of 82 with a 1 -adamantyl group; the l-adamantene which would be formed violates

%Ax

X

Figure 48. Newman projections along the equatorial carbon-carbon bond of 82, its biradical, and its cyclization product (Copied with permission [279].)


183

Bredt's rule. In this way, 1-adamantyl p-methoxyacetophenone 86a was forced to yield only cyclobutanols 87a and 88a as photoproducts 12811. Whereas (benzene) solution phase irradiations of 86a yielded a 2.6 ratio of 87a/88a, the solid state photoproduct ratio, 0.5, favors the more sterically hindered cyclobutanol. X-Ray diffraction studies predict a chair-like yhydrogen abstraction pathway for 86a (in contrast to the boat-like transition states of 82) in which the C=O...H, distance is 2.67 A. Other abstractable hydrogens (HJ are at least 0.3 A farther from the carbonyl oxygen (Scheme 44). If 1-BR has a conformation which mimics that of the ketone, its least motion pathway favors formation of the more sterically hindered cyclobutanol 88a.

86 a ) Ar b ) Ar

88

=

pmethoxyphenyl p-chlorophenvl

87

Scheme 44.

What is as noteworthy as the excess of 88a from the solid-state irradiation of 86a is the presence of any 87a at all. Its formation requires either that one of the more distant y-hydrogen atoms be abstracted also or that the reaction cavity suffer some dramatic changes in shape and size while the hydroxy-1,4biradical is present. Based upon the lattice properties of 86a, the latter seems the more probable scenario; without the movement of groups on adjacent

184


ketone molecules or extensive molecular motion of the 1,4-biradical within a unit cell, no 87a should be possible. Rather phase-insensitive Norrish I1 photoproduct ratios are reported from irradiation of p-chloroacetophenones with a-cyclobutyl, a-cyclopentyl, acycloheptyl, a-cyclooctyl, and a-norbonyl groups [282]. In each case, the E/C and cyclobutanol photoproduct ratios are nearly the same in neat crystals as measured in benzene or acetonitrile solutions. On this basis, we conclude that the reaction cavity plays a passive role in directing the shape changes of these hydroxy-1,4-biradicals. As long as the initial ketone conformation within the cavity permits y-hydrogen abstraction (and these ketones may be able to explore many conformations even within their triplet excited state lifetime), the cavity free volume and flexibility allow intramolecular constraints to mandate product yields. These studies demonstrate that the reaction cavity provided by a neat crystal may or may not remain passive and stiff during the transformation of a ketone to its Norrish I1 photoproducts. Significant flexibility of the reaction cavity can accompany the transformations. In fact, Norrish I1 photoproducts form in crystals where the C==O...H, distance exceeds the sum of the van der Waals radii of 0 and H (2.7A) [282]. (This observation is somewhat deceptive since it is known that the C-0 bond length of ketones is longer in n, IT*states than in the ground state and the carbon atom may not be planar [282].) On the other hand, as witnessed by the lack of reactivity of the solid sym-alkanones and the almost complete stereospecificity exhibited by Norrish I1 rearrangements of the dimorphs of 77 (n = 12), the reaction cavity of neat crystals can provide rigidity and shape which make the motions necessary for some photoproducts to form so energetically disfavored that they cannot compete with others. Examples of this extreme stereoselectivity and specificity have been found by Scheffer and Trotter in crystals of a-adamantyl p-chloroacetophenone (86b), also [282]. Figure 49 shows ORTEP-like drawings of the conformations in the dimorph plate and dimorph needle crystals of 86b as obtained upon recrystallization from different solvents. A major difference between the two is the cant of the aryl ring (and the carbonyl group) with respect to the adamantyl moiety. The selectivity afforded upon irradiation of the plate dimorph of 86b can be traced to the cant of the aryl rings and the specific disposition of the carbonyl group in the molecule (Table 10). Since one of the ortho aromatic hydrogen atoms is between an adamantyl methylene and the methylene alpha to the carbonyl group, the ring is impeded intramolecularly from rotating without prior motions of the bulky adamantyl group. Although formation of 87b is quite feasible from a BR of 86b in this conformation, 88b is not. We conjecture that the cavities of the biradicals in the plate and needle dimorphs

185


x

h

needles

P I atG

Figure 49. Structures of 86 in its dimorphic crystals. (Copied with permission [282].)

are equally flexible and contain about the same free volume. The difference between them lies in the initial disposition of the groups and the degree to which specific motions can occur within the defined shapes of the two cavities. Thus, the ortho hydrogen atom on the aromatic ring in the needle dimorph of 86b is far removed from the impeding atoms and its biradical has greater conformational flexibility. In fact, the solution phase cyclobutanol ratios indicate that the hydroxy-l,4-biradical from the needle dimorph is able to equilibrate its conformations over the same space as it requires in a fluid isotropic solvent. This conclusion must be tempered, however, by the observation that photoreaction occurs preferentially near crystal surfaces and the photoproducts are not isomorphous with 86b [282c]; the Norrish I1 reactions of needle dimorphs may not be topochemically controlled. The substitution of a methyl group at carbon 3 of the adamantyl group of 86b (designated 89)lowers the molecular symmetry and makes conceivable the formation of three cis and three trans cyclobutanol photoproducts. Of these, only two cis (90 and 91) and two trans (92and 93)isomers could be TABLE 10 Requisite Geometric Factors and Norrish I1 Product Ratios from 86b I2821

86b Phase

44

Plates Needles Benzene solution

2.53 2.78

Abstraction Geometry 43 62

92 77

Chair Chair

87b/88b

9911

74/26 73/27

186


detected after irradiation of 89 in solution or its neat crystalline state (Eq. 10) [281b].

90

92

i)H

& J

91

hr

Al

(1 H

nr

93

Presumably, the same steric factors that discourage formation of the other two isomers in solution are operative in the solid state. In addition, the reaction cavity of the solid places clear conformational constraints upon which y-hydrogen atom can be abstracted: 91 and 93 have a common BR precursor; 90 and 92 emanate from a different BR conformer. The control is selective, but not specific. Unlike 86b, 89 crystallizes in only one form. However, it is chiral [281b,284]. The major isomer of the cyclobutanol mixture, 90, has been isolated from irradiation of a single crystal of 89 and shown to have a > 80% enantiomeric excess. Inversion twinning in the crystals, disruption of the crystalline lattice due to the presence of photoproducts [281b], possible preferential reaction at surface and defect sites, and comparable activation energies for abstraction of enantiotopic y-hydrogen atoms on different methylenes of the adamantyl group in the undisturbed lattice may explain the lack of total enantiomeric purity. Again, even in this well-controlled system, the results point to a reaction cavity which is not totally rigid or which contains adequate free volume to permit rather large conformational changes of molecules during the course of the reaction. In subsequent sections we shall see that in extreme cases, the control provided by reaction cavities in foreign solid hosts can be as great or greater than those provided by the neat crystals.

B. Reaction Cavities with Very Stiff Walls and Preformed Shapes and Sizes: Silica Gel and Zeolites Silica gel surfaces (Figure 9) offer an anisotropic environment to adsorbed ketone molecules in which motions are restricted (in a crude sense) to two dimensions. Under the best of circumstances, a distribution of site sizes and


187

activities is present so that the photochemical parameters associated with the Norrish I1 reactions of adsorbed ketones are best described by expressions much more complex than Eqs. 6-8. De Mayo and Ramnath [285] have reported that irradiation of ketone (94) adsorbed on dehydrated silica (heated at 200°C under vacuum) and in methanolic solutions at 25°C results only in Norrish I1 isolated photoproducts (Eq. 11) whose E/C ratio is 2.5-3. Thus, the silica surface under these conditions affords reaction cavities which allow 94 to react as though it were in a fluid isotropic solution.

-

94 -

95 -

96

Using silica of a different source, Turro [46] again found that the E/C ratios from irradiation of adsorbed alkanophenones 97 (n = 4,7,10) at room temperature (3.7-3.9) are close to the values obtained from irradiations in tbutyl alcohol (4.2-6). Consistent with the expected reduction in mobility of adsorbed molecules on silica surfaces at lower temperatures, only cyclization products were isolated from irradiations of 97 on silica at - 125°C. At these

97 very low temperatures, the distribution of ground-state ketone conformers may be nearly static and only those initially bent to allow y-hydrogen abstraction after electronic excitation may be able to react. Under conditions of very slow shape changes, the favored photoproducts from the biradicals are cyclobutanols and they should form more quickly than in solution. On these bases, we conclude that there is no compelling evidence for specific control over Norrish 11 photoreactions of ketones in rigid reaction cavities on silica surfaces. However, this does not mean that the rates of conformational changes or the distribution of conformers of 97 and its biradical intermediates cannot be altered by adsorption onto a silica surface. Evidence for this is found in diffuse-reflectance flash photolytic studies of valerophenone (97, n = 4) adsorbed onto silica (average pore diameter = 255 A) [286]. Whereas in fluid solution the triplet lifetime of molecules like 97 is < 5 ns

188


-

[287], it is found to be 310 ns for the adsorbed valerophenone. Furthermore, the triplet biradical of valerophenone, whose lifetime in methanol is 100 ns [287,288], cannot be detected upon irradiation of the adsorbed species due either to an enormous decrease in the lifetime or, more likely since the photoproduct ratios are like those found in isotropic solutions [286], to its very low concentration. Zeolites are somewhat like silica in their surface characteristics. Ketones and hydroxy-1,4-biradicals have very polar groups which can interact favorably with metal cations located along zeolite walls. The potential effect of the metal ions on the position of the reacting ketones is twofold. First, the cations may force a ketone molecule into a conformation or a site which it would normally not occupy based solely upon free-volume considerations. Second, the diffusion coefficient of a ketone or a hydroxy-l,4-biradical is probably much more than an order of magnitude smaller than that of benzene [289] so that the residence time of a ketone and its Norrish I1 intermediates in a zeolite site with at least one metal ion is expected to be closer to 100ns than to 1 ns. These numbers should be compared with the lifetimes of excited states and BR triplets derived from ketone guests before interpreting the influence of zeolite reaction cavities on the course of the Norrish I1 reactions of guest molecules. Since the biradical lifetimes are usually tens of nanoseconds and triplet excited state lifetimes may vary from < 5 ns to hundreds of nanoseconds, it seems safe to assume that a ketone guest molecule will be able to sample no more than one or two cages, cavities, or channels in a zeolite during the period that separates the moment of electronic excitation and eventual photoproduct formation. Presumably, the time spent in transit is much shorter than the time spent at preferred guest sites. In spite of this, the normally transitory positions of a guest molecule can become the primary ones of intermediates and products whose shapes or polarities differ significantly from those of their precursor. Thus, although the sites at which guest molecules reside in zeolites may be well defined (see Figures 12 and 13), the location at which they react may be less clear in cases that allow initial cation-carbonyl interactions to be interrupted. In the absence of specific evidence for such changes, we will assume that the reactive ketones remain within their original cage and, possibly, explore a portion of a neighboring channel if a long alkyl chain is present. Since the diameter of a phenyl ring is 6 A, aromatic ketones can enter sites within a zeolite whose constrictions at all points from the surface are no smaller than that dimension (Table 1). A clear indication that zeolites can control the Norrish I1 selectivities of aromatic ketones (97) has been provided by Turro and Wan [290] and by Ramamurthy and co-workers [291]. They find extremely diverse photoproduct yields for different members of the 97 series in zeolites whose pore sizes and shapes differ (Table 11). The absence of selectivity from irradiations in

-

~~~

---

~

4 8 8 8 8 8 7 6 5.5 5.5 5.5 -7.5-8.0

(4

"R.J. Ward, J . Catal. 10, 34 (1968).

~

Benzene solution Na-A Li-X Na-X K-X Rb-X cs-x Na-Mordenite Silicalite Na-ZSM-5 Na-ZSM-8 Na-ZSM-11 Na-Zeolite$

Zeolite

Pore Diameter

0.6 0.95 1.33 1.48 1.69

(8)"

Cation Radius

873 852 800 770 732

(A3)

Cage Free Volume

73 82 56 0.54

3.9 2.7 3.3 1.9 2.3

6.2

n =3 EJC

E/C

2.6 2.2 2.6 2.6 1.1 1.6 3.2 2.3

tJc

n=7

3.0 1.9 2.7 2.3 1.6 1.3 1.6 1.1 1.9 1.1 1.9 1.6 1.9 1.3 2.2 2.5 > 50 > 50 > 100 > l o o > 100 >loo > 100 >loo 1.3 0.62

n =4 EJC

0.68

> 100 > 100 > 100

0.6 0.6 0.5 0.7 1.2

1.2

EJC

> 100 > 100 > 100 0.31

0.4 1.4 1.5 1.9 1.7

2.7

2.4 1.4 1.4 1.1 4.2 6.7

EJC

2.1 0.8 1.3 2.2 5.1

2.4

tJc

n=13

tJc

n=ll

TABLE 11 Dependence of Photoproduct Ratios from 97 on the Nature of Zeolite Hosts 1290, 2911

0.48

> 100 > 100 > 100

1.8 1.3 2.0 3.4 4.9

2.7

2.7 2.5 2.7 4.3 6.2 6.8

t/c

EJC

n=17

190


Na-A may be explained easily as a consequence of the aromatic rings of 97 being unable to enter the pores. Photoreactions are forced to occur at the surface in a two-dimensionally restrictive environment reminiscent of that provided by silica. The absence of selectivity upon irradiation of 97 in the NaZeolite-/l or in Na-mordenite follows from the relatively large size of their pores and channels which impose few conformational restrictions on BRs in the two dimensions that affect the ease of c-BR formation. Whereas a small increase (over the values in isotropic media) is found in the E/C and t/c ratios from the longest 97 in the X faujasites with the largest metal ions, no discernible differences could be found for any of the 97 in any of the Y faujasites. Apparently, all of the ketone molecules can be accommodated into Y supercages which impose few constrictions to the internal motions BRs. When the number of space-filling large ions per cage is increased, the hydroxy-1,4-biradicals with the longest tails sense the restrictions imposed by the limited volume. Those restrictions, somewhat surprisingly, appear to favor the BR conformations which are most extended, implying that the long alkyl tails of 97 (n = 11,13,17), especially, are forced from one cage into a neighboring one. In this way, the reaction cavity may be thought of as offering constraints in two dimensions, but not in the third (along which the alkyl chains lie). Selectivity is apparent, however, from irradiations conducted in silicalite or the Na-pentads (ZSM-5). The interior of the silicalite is rather hydrophobic and should provide a nonpolar environment to reacting ketones. More importantly, its relatively small pores can provide snug fits to aromatic rings in the channels. Adequate mobility must exist to permit the bent ketone conformations necessary for y-hydrogen abstraction, but the dearth of cyclobutanol photoproducts indicates that the stiff walls of the reaction cavity repel attempts by the hydroxy-1,4-biradicals to adopt cisoid conformations whose cross-sectional areas are larger than that of the pores. As shown in Scheme 45, the very stiff walls of the narrow channels prohibit the BR precursors to the cyclobutanols from forming, but allow even the longest 97 to be fully extended [291]. It should be noted in this regard that the circumference of a c-BR is larger than that of the corresponding cyclobutanol since the carbon atoms bearing the odd electrons are within the sum of their covalent radii in the product; in the BR, the radical centers can be no closer than the sum of their van der Waals radii. Additional spectroscopic information which supports these conclusions is available [292]. For instance, the diffuse reflectance absorption maxima of valerophenone (97, n = 4) in Li-X and Cs-X zeolites are shifted only slightly from each other and both are similar to the spectrum obtained in methanol solution (Figure 50). Although the absorption spectrum of valerophenone in the less polar ZSM-5 (Si/Al -490) also resembles that in hexane, the

191


R=OH

-

Scheme 45.

spectrum in ZSM-5 (%/A1 24) is much more like the one in methanol. Both spectra in the ZSM-5 zeolites exhibit broadened n -+ TC* transitions which may be due to the aromatic rings being in a variety of site types. The corresponding phosphorescence spectra at 77 K also exhibit intensities, shapes, and wavelength maxima that depend upon the zeolite host [292]. Transient absorption spectra from valerophenone in Na-ZSM-5 and CsZSM-5 could be assigned to triplet-triplet transitions; no spectra which can be attributed to the hydroxy-1,4-biradicals were detected. Furthermore, the decay of the transient signals could not be fit to either a single or double exponential expression, and samples prepared under apparently identical conditions exhibited half-lives that varied by a factor of 2 [292]. All of these spectroscopic observations suggest that the valerophenone molecules reside in a distribution of sites within a zeolite and migration among them under the experimental conditions is slow. Thus, the Norrish I1 photoproduct ratios must be interpreted in terms of not only the relative populations of alkanophenones at each site type, but also the quantum efficiencies of each and the conformational preferences of the intermediate BR in each environment. In fact, the rates of y-hydrogen abstraction in 97 must be attenuated severely in the restrictive channels of zeolites. Even in Na-X and Na-Y, where virtually no photoproduct selectivity from valerophenone is observed, its 1 ,us (vs. 5 ns in fluid isotropic solvents), The triplet state half-life is narrower channels of Na-ZSM-5 increase the triplet half-life to 15 p ! Narrow zeolite channels that normally either exclude or provide a very tight fit for alkanophenones due to the much larger cross-sectional area of an

-

N

-

1.2 -

1

Nanometers

(4

'\

Y

ZSM-5 (SiAI-24)

ZSM-5 (SVAI-490) ZSM-5 (SiAI-490)

n

a

0.0 220

260

300 Nanometers

340

(4 Figure 50. Absorption spectra of valerophenone in zeolites and isotropic solutions [2923. 192

193


aromatic ring include n-alkanones. On the other hand, n-alkanones are unable to fill as well the supercages of X and Y zeolites, allowing much greater conformational motions and much lower selectivity in the Norrish I1 processes. These expectations have been borne out by experiment [293]. For instance, 2-tridecanone gives rise to E/C ratios in Na-X (1.0) and Na-Y (0.9) zeolites which are very similar to those found from valerophenone in the same media. However, whereas only fragmentation products were detected from valerophenone irradiated in ZSM-5 and ZSM-11, the E/C ratios in these zeolites with narrower channels were 4.3-4.5 from 2-tridecanone. Clearly, the channel cross sections of these zeolites (>28A2) can accommodate the c-BR conformations necessary to form cyclobutanols. Closer examination of the cyclobutanol t/c ratios from homologous and isomeric n-alkanones in the same zeolites reveals some interesting trends (Table 12). The t/c ratios from all of the alkanones in the Na-X and Na-Y zeolites are as expected from reaction in a large or flexible reaction cavity which is very polar (like that provided by an alcohol solution). The t/c ratios in the ZSM zeolites indicate that selectivity depends upon the total ketone length and the position of the odd electron centers of the BR along the chain. Ratios of 60 or more are reported from 4-alkanones with 9 or more carbon atoms; the t/c ratios from 4-octanone in ZSM-5 and ZSM-11, 15 and 18 respectively, mark 8 carbon atom chains as being critical to a c-BR occupying TABLE 12 The t/c Ratios from Irradiation of Alkanones in Zeolites and in Hexane Solution 12931 Alkanones 4-Nonanone 4-Undecanone 4-Dodecanone 4-Tridecanone 4-Tetradecanone 4-Decanone 3-Decanone 2-Decanone 3-Octanone 4-Octanone 2-Octanone 2-Heptanone 3-Heptanone 2-Hexanone

Hexane

NaX

NaY

ZSM-5

ZSM-11

1.8 1.8 1.7 1.7 1.7 1.8 1.8 1.5 1.8 1.8 1.4 1.7 1.5 1.5

0.6 0.4 0.7 0.7 0.8 0.4 0.4 0.6 0.7 0.7 0.8 0.8 0.6 0.8

1.3 0.7 0.9 1.1 1.1 0.7 0.9 1.0 0.8 1.3 1.2 0.9 0.9 1.3

60 60 65 70 72 60 16 6.0 20 15 8.0 3.8 2.8 2.4

60 60 70 68 66 60 14 6.5 18 18 7.1 4.1 2.6 2.7

194


fully the cross-sectional area of a ZSM zeolite. Furthermore, the t/c ratios from 2-, 3-, and 4-decanone in the ZSM illustrate the shape differences that accrue in the c-BRs from isomeric alkanones. As shown in the cartoon of Figure 51, the family of c-BR, conformational precursors of a cis cyclobutanol is bulkier and not amenable to accommodation by a 28 A' cross section when 4-decanone is the ketone since both R, and R, are of equal length and are projected in directions which are unlikely to alleviate the steric crowding that ensues. The fact that the reaction cavity allows virtually unlimited expansion along a channel length is of no use to a biradical whose width along any axis is greater than the diameter of the channel.

t-BR

#

C

- BR,

C - BR,

Figure 51. Cartoon representations of hydroxy-1,4-biradicals from 2- and symalkanones. The methylene chains (solid lines) are assumed to be extended and the sizes of the atoms between the odd-electron centers and the hydroxy groups are depicted as circles and filled dots, respectively.

195


C. Reaction Cavities with Some Wall Flexibility: Solid Inclusion Complexes Comparatively, the walls of a reaction cavity of an inclusion complex are less rigid but more variegated than those of a zeolite. Depending upon the constituent molecules of the host lattice, the guest molecules may experience an environment which is tolerant or intolerant of the motions that lead from an initial ketone conformation to its Norrish I1 photoproducts and which either can direct those motions via selective attractive (NB, hydrogen bonding) and/or repulsive (steric) interactions. The specificity of the reaction cavity is dependent upon the structure of the host molecule, the mode of guest inclusion, and the mode of crystallization of the host. Several interesting examples of solid inclusion complexes with ketone guests which undergo the Norrish I1 reactions have been examined. They illustrate the breadth of reaction cavity types and resultant selectivities that can be expected in such systems. For instance, 5-nonanone has been included into channels of solid urea crystals and irradiated [294]. The conformation associated with a yhydrogen being within an abstractable distance from a carbonyl oxygen atom is possible, but, as was shown in Figure 50 for alkanophenones in zeolites, some motions of the BRs necessary to form cis-cyclobutanols, especially, will be inhibited. Other conformational changes may be strongly directed by hydrogen bonding of the hydroxy group to the urea. In a 5-nonanone/urea complex whose molar ratio suggests that the channels of the host are nearly filled with guest molecules [294], it was found that the time required to effect photoconversion is similar to that required in solution; consistent with 5-8, diameter channels (see Figure 1) attenuating only slightly the motions necessary for the initial y-hydrogen abstraction, the total quantum efficiency in the complex does not appear to be reduced appreciably. However, the photoproduct ratios clearly indicate that the fate of the hydroxy-1,4-biradicals is controlled by the urea channels. Only Norrish I1 photoproducts could be isolated, but they did not include the cis cyclobutanol (Eq. 12).

b

complex

bII \\ T

')' '

-4TH u

Since the volume of a methyl group is slightly larger than that of a hydroxyl, the lack of the cis cyclobutanol may result from an inability of its precursor


1%

(which is larger in diameter than the cis cyclobutanol), c-BR,, to fit within a 5-A diameter channel or to specific BR-cavity wall interactions (i.e., template effects). The E/C ratio from irradiation of 5-nonanone in methanol was measured to be about 3; in the complex, the ratio was about 1.5. Since E/C ratios from irradiations of alkanones in fluid isotropic media are somewhat smaller when the environment is more polar, the lower ratio in the urea complex indicates that local polarity effects alone cannot explain the course of this photoreaction. A similar study of Norrish I1 reactions has been conducted on complexes of aryl ketones in Dianin’s compound 1 [295], a nonpolar host whose channels are effectively truncated at each 11 A of length by a 2.8-A constriction from 6 hydrogen-bonding hydroxyl groups (see Figure 3) [296]. Table 13 summarizes the results from complexes with ketones expected to undergo primarily the Norrish I1 reactions [297]. As befits the rather large (and mostly) nonpolar reaction cavities, the E/C and t/c ratios in Table 13 provide evidence for relatively little control by the channels of Dianin’s compound over the fate BRs. Even in the most selective case from 5-methylTABLE 13 Photoproduct Ratios from Alkanophenones in Benzene and Complexes of Dianin’s Compound (2971 Guest Ketone 0

II

(PhCR) R= -(CH2)3H

+CHZ),H +CH,),H +CHZ)CH(CH3)2 -CH2CH(CH3)2 -CH,-cyclohexyl

-CH,-cyclopent yl

Medium“ Benzene complex (5-7) Benzene complex (5-7) Benzene complex (18-22) Benzene complex (9) Benzene complex (20-26) Benzene complex (6) Benzene complex (9)

“Host/guest ratio in parentheses. bOnly traces of cyclobutanols detected.

E/C

8.0 8.4 4.0 3.7 4.9 5.3 7.3 27.6 7.3 6.1 0.5 0.9 12.7 Largeb

t/c

2.5 1.6 3.4 2.8

2.2 1.9


197

I-phenylpentanone, the E/C ratio from Dianin’s complex is only a factor of 3 more than the value observed in benzene. Additionally, the relative reactivities of the ketones in benzene and in the complexes are reported to be similar. The constancy of quantum yields indicates that even the bulkiest ketone in Table 13 has little trouble in finding appropriate conformations during its triplet lifetime to permit y-hydrogen abstraction and for the hydroxyl,4biradicals to adopt all of the preproduct conformations that they would in fluid isotropic media. The walls of the channels in Dianin’s complex are probably more flexible than those in urea complexes (owing to the saturated nature of several bonds) and the potentially active hydroxyl groups of 1 are networked with each other and in a location which makes it improbable that the hydroxyl group of a BR could reach them. Even if it could, the arrangement shown in Figure 3 for the hydroxyl groups of 1 should inhibit their acting as a template for guest reactions. In essence, the channels appear analogous to the “supercages” of a zeolite, but have walls that are less rigid. However, it should be possible to observe selectivity from irradiation of ketone molecules complexed to Dianin’s compound if the shape of the guest and its intermediates match more closely the size and shape the host channel [298]. Several attempts have been made to engineer host molecules for clathrate formation [298]. Among the most successful of these efforts for the investigation of the photochemical reactions of guest molecules has been the studies of Toda and co-workers with 1,1,6,6-tetraarylhexa-2,4-diyne-1,6-diols (2and 3)1561. Pertinent guest molecules in the complexes are N,N-dialkyl-aoxoamides (98) whose photochemistry in the neat solids and solutions has been investigated by Aoyama and co-workers [299] and by Ackerman and Johansson [300]. For instance, irradiation of 98a in either phase yields the two cyclization products shown in Eq. 13, including both diastereomers of each.

198

V. RAMAMURTHY, R.G. WEISS A N D G. S. H A M M O N D

Although all the products can be rationalized on the basis of y-hydrogen abstraction followed by cyclization or rearrangement-cyclization of 1,4biradical intermediates, the mechanism has been shown to involve analogous zwitterionic intermediates [299b, 3011. Although they are not strictly Norrish I1 reactions, transformations of 98 will be considered so for the purposes of discussion since the photoproducts and the mechanisms of their formation are very similar to those expected of Norrish I1 processes. The clathrate inclusion complex of 98a with 2 is somewhat disordered; two conformations of 98a exist but in both, there are hydrogen bonds to only one of the two carbonyl groups on a molecule of 98a [166]. As shown in Figure 52, the major conformer of 98a in its clathrate complex with 2 is nearly the same as in the neat crystal. However, only 99a was obtained as a photoproduct from irradiation of the clathrate. This result can be explained readily since the carbonyl group alpha to the nitrogen atom is hydrogen bonded to 2, making the tautomerization from 101 to 102 unfeasible. No hydrogen bond donors exist in the neat crystal and tautomerization of the intermediates can occur. Both butyrophenone and valerophenone form 1 :2 host :guest complexes with 2 [302]. Although crystal structures to reveal the mode of guest incorporation are unavailable, it is likely that the two hydroxyl groups of each molecule 2 are occupied by hydrogen bonds to the carbonyl groups of two guest molecules. Given the similarity between the structures of the guests and the stoichiometries of their complexes, it is surprising that the butyrophenone complex is reported to be nonreactive while the one with valerophenone produces E/C and t/c ratios similar to those found from isotropic media [302]. Since the more voluminous guest is able to attain the requisite conformation for y-hydrogen abstraction and its BR is able to explore a wide variety of preproduct conformations during its triplet lifetime

3

(4 (b) Figure 52. ORTEP molecular structures of 98a (a) in its neat crystal and (b)the major conformer in its clathrate complex with 2. (Copied with permission [166].)

199


in the reaction cavity of 2, one might expect that butyrophenone would also be photoreactive. Given the similarity between the electronic and structural natures of the two ketones, we suggest that the role of the reaction cavity of 2 in the inhibiting Norrish I1 reactions of butyrophenone merits reexamination. Complexes of 98a-d have been prepared with optically active 3, also [56, 152, 154, 3031. Their irradiation yields only 99 as photoproducts, as before, but in many cases they have a large enantiomeric excess. The data are summarized in Table 14. Presumably, the same hydrogen bonding between 2 and 98a which was cited as suppressing the formation of 99 is responsible in the clathrates of 3 for stabilizing the 101 intermediates so that they may equilibrate between their diastereomeric conformers which precede formation of the cis and trans p-lactam photoproducts [1521. Enantioselectivity is highest in the complex of 98a (and perhaps comparable to that of 98b), but is clearly very low with 98c. A reasonable interpretation of this result is that the reaction cage is enlarged by the presence of two guests and that they may reside in a twinned crystal. Single crystal X-ray structure determination would be very helpful in elucidating the reasons for the lack of appreciable chiral induction from 98c. Figures 53 and 54 show the structure of the 31981 complex as it exists in the unit cell [154,303]. Unlike the complexes with 98a-q the 98d complex has both hydroxyl groups of one 3 hydrogen bonded to both carbonyl groups of one molecule of 98d. As a result, the diyne backbone is curved (Figure 53) [154,303]. There is no reason to believe that the walls of the reaction cavity experienced by 98d or by transients, lOld and 102d derived from it, in optically active 3 complexes are any more rigid or contain less free volume than do the other complexes. The enantiomeric purity of the product must result from specific attractive host-guest interactions retained along the

TABLE 14 Photoproduct Ratios from 98 in Complexes with Optically Active 3 1152, 154, 3031 Enantiomeric Excess (%)

98

Stoichiometry (Host : Guest)

translcis 99

cis

43/57 22/78 51/49

62.5 55.8 11.2

trans ~

a b c d

1 : 1 (+one molecule of benzene) 1:l 1:2 1:l

"Purity unknown; [a],-48.7"

-

_

95 a

0 100

_

_

200


Figure 53. Structures of 3 and 98d in their solid complex. (Copied with permission ~561.)

reaction coordinate to 99,the crystalline matrix of 3 supplies a cavity which is better described as a well-defined template than a rigid cage whose active walls preclude the motions necessary to form both enantiomers of 99 or even 100.

D. Reaction Cavities with Strong External Medium Influences: Cyclodextrin Complexes and Their Aqueous Solutions Solid inclusion complexes of photoreactive ketones with cyclodextrins as hosts (Figure 6 ) provide interesting examples of how a fairly stiff, somewhat heterogeneous (in terms of polarity, size, and guest orientation) reaction

H'

(-)-rn

Figure 54. Conversion of the solid 98d/3 complex to (-)-99d. (Copied with permission ~561.1

201


cavity can influence the Norrish I1 processes. Single crystal X-ray diffraction studies to obtain details of a ketone guest's structure and orientation within a C D torus have been unsuccessful owing to disorder of the complexes. Reddy et al. have examined the influence of p-CD on a series of alkanophenones 97 and 103 for which Norrish I reactions in solution do not

103 103

R

R'

H H H o-Me o-Me m-Me m-Me p-Me p-Me

compete with Norrish I1 processes [304]. All of the complexes had 1 :1 stoichiometries as solids; aqueous solutions were prepared by dissolving the preformed complexes in water. For the most part, E/C ratios were measured. They and K , , the dissociation constants of the complexes in solution, are included in Table 15. In the solid p-CD complexes where escape from the torus is not possible during the period between excitation of 97 or 103 and formation of its photoproducts, all of the E/C ratios resemble those from the nonpolar solvent benzene more than the polar solvent t-butyl alcohol. This is in spite of the very high polarity estimated for the interior of a p-CD. Evidence that polarity is not the dominant product-determining factor in these systems is found in the t/c ratios from 97 ( n = 4) and 97 ( n = 13): whereas P-CD/97 ( n = 4)yields a mixture of trans and cis cyclobutanols whose ratio is like that found from benzene, the P-CD/97 (n = 13) complex yields only the trans isomer; benzene solutions of 97 ( n = 13) produce both cis and trans cyclobutanols. If 97 ( n = 13) (and other 97 with long alkyl chains) is held as shown in Figure 55, with the alkyl chain helping the aromatic ring to fill the torus, the motions of the derived hydroxy-1,4-biradical may be expected to be limited sterically and by hydrogen-bonding interactions with rimhydroxyl groups of the CD. It would appear, however, that this form of N

103 a b c d e f g h i

3 4 7 9 13

R H H H o-Me o-Me m-Me m-Me p-Me p-Me

97 n =

Alkanophenone

~~

~

1.9 0.8 1.6 3.5 3.2 1.9 3.1 2.6 2.5

6.5 3.0 1.2 2.5 1.6

Benzene

~~~~

2.9 1.1 2.1 5.1 4.9 3.4 4.7 4.7 3.6

8.5 4.2 2.5 3.3 2.9

t-BuOH

-

~-

1.7 0.4 1.1 2.2 0.8 1.3 0.7 2.2 1.6

3.5 2.7 0.8 0.7 0.4

fi-CD Solid

-

-

~-

~-

3.6 1.0 2.7 2.3 2.1 2.5 2.1 2.5 2.4

3.8 3.8 1.8 1.6 1.4

8-CD Aqueous

~

d

(0.52)

(4.81)

(2.07)

M) ,

-

(1.1) (0.77) (0.6) (0.17)

(103 ~

_ _

TABLE 15 E/C Photoproduct Ratios from 97 and 103 in Various Media, Including Aqueous and Solid fl-CD Complexes 13041


203

(trans)

Figure 55. Representation of possible modes of inclusion of 97 (n = large), its c-BRs, and the corresponding photoproducts in a B-CD torus [304].

complexation would favor formation of the cis cyclobutanol isomer instead of the observed trans. If the rim hydroxyl groups act as a template, other factors must override their influence. The data in Table 15 reveal that the values of E/C ratios from the solid pC D complexes are always lower than from benzene solutions. The degree to which the cis cyclization products are favored over the trans increases from 103a to 103e as expected from the model in Scheme 46. Also, the longer homologue of each ortho-, meta-, and para-methylated 103 yields the greater amount of cyclobutanols when irradiated in solid p-CD complexes. Thus, Scheme 46 explains qualitatively the photoproduct ratios from solid p-CD. In the aqueous p-CD complexes, the E/C ratios remain generally lower than those found in t-butyl alcohol. A specific exception to this is the three ethers 103a-c whose relative yields of fragmentation products in the p-CD solutions are as high as, or higher than, in t-butyl alcohol. If the ether oxygens permit the BRs to protrude farther into the aqueous medium than the BRs from 97, the higher E/C ratios are to be expected. The hydrophobic shielding nature of the alkyl chains is again demonstrated by the E/C ratios of 97. As the tails increase in length, the degree to which the molecules are bound to pC D increases (& decreases) and the E/C ratios decrease; the longer alkyl chains may shield the BRs somewhat from water, making their local environment appear less polar, or force a smaller fraction of the reaction to occur in the aqueous bulk. The progression of Kd from 103e to 103f and to 103h is as anticipated. The para isomer is bound most strongly since its methyl group does not inhibit

204


the aromatic ring from entering the torus. However, the effect of greater binding (and probably deeper penetration into the torus) of the para isomer is not seen in the E/C ratios. It should be remembered that the Norrish I1 product ratios depend more upon the lability and location in a torus of the hydroxy-1,4-biradicals (i.e., the final reaction cavity) than of the ketone precursors (i.e., the initial reaction cavity). These results suggest again that the biradicals probably migrate somewhat from their initial positions, when yhydrogen abstraction occurs, to locations which may be quite different from those experienced by the ketones. Moreover, in both the solid complexes and in aqueous solutions, the effective reaction cavities cannot be defined exclusively by considering the shape of the torus; water molecules near the rims, the hydroxyl groups at the rims of the host, and the guests themselves will all contribute to the definition of the free volume, the shape, and the polarity of the reaction cavity in these systems. Clearly, the nature of the reaction cavity provided by a cyclodextrin in solution (where movement of the guest into and out from the torus are facile and where water molecules can vitiate selective hydrogen bonding interactions between the hydroxyl groups on the C D rims and functional groups of the guest) and in the solid state are very different for several mechanistically important reasons. Furthermore, escape from either C D rim by a guest ketone is thwarted in a solid complex by the proximity of other species (unless the mode of crystallization forms channels as shown in Figure 7a and neighboring C D molecules are free of guests); the inhibition of dissociation of a complex in solution is largely due to the necessity to bring water molecules into a torus as the guest leaves and for the (usually) nonpolar guest molecule to be in a highly polar, aqueous environment outside of the cage. Thus, depending upon the binding constant for a complex in solution, a significant fraction of ketone guest molecules may exist outside the torus in the aqueous bulk and react there. This requires that complicating factors not included in Eqs. 6-8 be considered when discussing microheterogeneous reaction cavities like those of C D toruses in solution.

E. Reaction Cavities with Walls of Variable Flexibility and Strong External Medium Influences: Aqueous Microheterogeneous Complexes Some of the complexities mentioned above have been encountered by Hui et al. who examined the Norrish I1 reactivity of two benzoyl carboxylic acids 104a and 104b in aqueous solutions of carboxymethylamylose (CMA) [305]. Like a C D torus, CMA can form “pockets” where guest molecules like 104 may reside. The CMA can adopt helical conformations whose cross-sectional


205

areas may vary within limits to accommodate the size of the guest molecules. The factors reponsible for ketone complexation in a CD torus should operate as well with CMA with some modification because the latter contains carboxyl groups (0.12 per glucose residue) in addition to the hydroxyl moieties. By determining the fraction of 104a and 104b complexed by the relatively

104

a)n=3; R = H b) n = 10; R = H

c)n=14; R=C%

hydrophobic pockets of aqueous CMA and correlating those numbers with the quantum yields for the Norrish I1 reactions, it was possible to extract the relative @,I values for totally bound (0.14) and totally free x 1) guest molecules. This reduction of reactivity of the complexed molecules cannot be attributed to polarity changes alone since the @,I values of molecules like 104 in aromatic or aliphatic hydrocarbons are 0.25-0.40. Thus, the reaction cavities experienced by excited 104 are envisioned to be rather small and stiff in order to restrict in some way the motions that lead to products. Since it is likely that hydroxyl groups of the CMA project from the active cavity walls, they may also direct the course and efficiency of the reaction. Unfortunately, the distribution of photoproducts from the study is unavailable. In an investigation which compares the fragmentation component of 011 for 104c in a variety of media, Winkle et al. found that monolayer assemblies (Figure 15) of 104c and an equimolar amount of arachidic acid are essentially nonreactive [306]. In t-butyl alcohol, the relative fragmentation quantum yield is reported to be near unity (Table 16). In each of the microheterogeneous assemblies examined, the carboxylate groups of 104c are anchored at or near the aqueous/surfactant interface. The reactive benzoyl group then is located at positions which are determined by the order and polarity of the surfactant assembly. Between the two extreme values from the assemblies and t-butyl alcohol, (fragmentation) varies according to polarity and the type of microheterogeneous solvent aggregate. The stiffness of the polymethylene chains of the surfactant molecules and the availability of water molecules in the vicinity of the reactive aroyl group and the BRs derived from it appear to act in tandem to moderate mI1:the stiffer the alkyl chains, the more closely packed and better aligned they will be; the more

-

206


TABLE 16 Relative Quantum Yields for Fragmentation of 104 in Various Media 1305, 3061 -

Medium

104 a or b C C C

C C

C

@,I

CMA/H,O/NaCl t-Butyl alcohol Sodium dodecylsulfate (SDS) micelles/H,O Cetyltrimethylammoniumchloride (CTAC) micelles/H,O

(Fragmentation)’

l~,,,ax(nm)

0.14

Benzene

Dioctadecyldimethylammonium chloride (DODAC) vesicles/H,O Arachidic acid assemblies < O.OOlb

1.02 0.81

255

0.72

256

0.27 0.22

252

257

”Relative to @,I (fragmentation) of butyrophenone in t-butyl alcohol. ’@ for disappearance of 104c was 60.06 (no Type I), suggesting some intermolecular reactions.

closely packed the chains, the more difficult water penetration and bending necessary for y-hydrogen abstraction will be. Since alldecreases as solvent polarity decreases and as the reaction cavity becomes smaller and its walls become stiffer, the two factors act synergistically. Clear evidence for the existence of more than one type of ketone reaction cage in SDS micelles (see Figure 14), perhaps the most flexible and least “organized” of the microheterogeneous media in Table 16, was obtained upon laser flash photolyses of 104c [306]. In the presence of 5.4 x M 2,5dimethyl-2,4-hexadiene, an efficient (diffusion-controlled) quencher of excited triplet states of 104c which is not soluble outside the micelles, the intensity of the BR transient absorption was reduced by 55% but its lifetime was virtually unchanged. Addition of 1.4 x M Eu3+,a diffusion-controlled quencher of 104c triplets which is only water soluble, resulted in a 59% depletion of the biradical signal and a small decrease in its lifetime. These results indicate that during the excited triplet lifetime of 104c ( 15 ns), the reactive groups are either in a distribution of sites which are available to the hydrophobic tails of the SDS micellar molecules or the Eu3+ ions at the Stern layer (i.e., the aqueous interface), or they migrate rapidly between the two. In fact, a distribution of final reaction cavities in micelles may lie between the hydrophobic interior and the aqueous exterior since the biradicals from alkanophenones have lifetimes which are long enough to allow them to explore large effective reaction cavities [307]. The highly flexible nature of the micellar reaction cavities has been N


207

demonstrated by Turro, Liu, and Chow [308] who measured the E/C and t/c product ratios and CD,, from octanophenone 97 (n = 7) and valerophenone 97 (n = 4) in hexadecyltrimethylammonium chloride (HTAC) micelles. In contrast to the efficient quenching of the Norrish I1 reactions of 104c in anionic SDS micelles by Eu3+ [306], 0.005 M of the same ion caused no discernable quenching of the Norrish I1 reactions of valerophenone in cationic HTAC micelles. The same concentration of Eu3+ in water without surfactant molecules suppresses more than one-third of the Norrish I1 product formation. The difference between the influence of Eu3 + on ketone photoreactions in the two micelles is easily explained: since the cationic head groups of HTAC repel the metal ions electrostatically, they shield even those ketone triplets which reside near the Stern layer; the head groups of SDS attract Eu3 +,making their local concentration higher than in the bulk. Pertinent data concerning the photoreactions are summarized in Table 17. Quantum yields are for loss of starting ketone and are relative to irradiation of valerophenone in t-butyl alcohol. Both the values of CD,I(rel)and t/c from the micelles resemble more closely those from the t-butyl alcohol solution than from the nonpolar benzene. Coupled with the data of Winkle et al. [306], these results indicate that the alkanophenones reside primarily within the hydrophobic interiors of the micelles, but that the BRs migrate to waterenriched environments at or near the nebulous micellar surfaces. As such, even the template effect on the BRs is nonselective and probably allows all of the conformational changes which occur in a polar isotropic environment. In another set of surfactant systems comprised of 50% potassium stearate (KS) and water, 50% potassium palmitate (KP) and water, and 50% 1 : 1 KS/l-octadecanol (KSO) and water, the Norrish I1 reactions of the homologous series of 2- and sym-alkanones (2-105 and s-105, respectively, with n as the total number of carbon atoms in the alkanone) have been investigated [309].

TABLE 17 Relative Quantum Yields and t/c Ratios from 97 in Isotropic and Micellar Media I3081 PhCO(CH,),H 97

n=4 n=7

Medium

@11(rel)

t/c

t-Butyl alcohol Benzene HTAC micelles t-Butyl alcohol Benzene HTAC micelles

1.00 0.33 1.06 1.oo 0.29 0.71

1.5 3.6 1.9 1.1 4.7 1.2

208

V. RAMAMURTHY, R. G. WEISS A N D G. S.HAMMOND

In the gel phases of these systems [310], the surfactant molecules are in extended conformations and packed hexagonally in bilayers. A cut-away cartoon representation of the layers is presented in Figure 56: the KS and KP gel layers are nearly completely interdigitated; the KSO gel layers are without interdigitation, and KS and 1-octadecanol molecules alternate within a hexagonal layer. All of the detected photoproducts from s-105 irradiated in the gel phases

2-105 (n = m+2)

S-105 (n = 2m

+ 1)

can be rationalized as being of Norrish I1 origin. About 10% of the photoproducts from 2-105 are from Norrish I reactions. It is likely that CI-

'M!' Figure 56. Stylized representation of the molecular order in layers of the hexatic B and solid phases of BS and gel phases of KS, KP, and KSO. The dark circles and the straight lines represent the carboxyl(ate) groups and the methylene chains of the

molecules, respectively. The dark triangles represent hydroxyl groups.


209

cleavage occurs to some extent in the s-105 systems, but that the radical pairs combine before cage escape allows them to yield Norrish I products [311]. 2H NMR studies on a-deuteriated alkanones in the gel phase indicate that the guest molecules substitute for a KP or KS molecule within a bilayer with varying degrees of disturbance to the local environment [309]. Those 105 whose lengths match that of the polymethylene portion of the surfactant molecules appear to fit best into a gel matrix; the s-105 whose length approaches twice that of a KS molecule, s-105 (n = 35), also seems to fit well. The model described above of incorporation places the relatively polar carbonyl groups of 2-105 molecules at or near an aqueous interface, but allows the carbonyl groups of s-105 which are no longer than a KP or KS molecule to be buried in the hydrophobic portion of a gel layer. As expected from this model, the t/c ratios from 2-105 (n = 13) to 2-105 ( n = 21) are always significantly less than 1 when they are irradiated in KS or KP gels at 25°C; in KSO gels at 38"C, the t/c ratios from 2-105 (n = 13, 15, and 17) are near unity and the E/C ratios vary between 5.0 and 6.9 (vs. 0.9 to 2.6 in KP and KS gels) [309]. Even in a polar solvent like t-butyl alcohol, the t/c ratios are greater than 1. This suggests that the reaction cavity experienced by BRs from 2-105 in the gel phases (F in Figure 57) is even more polar and probably dominated by the presence of disordered water molecules at a layer boundary (except, perhaps, in the case of the KSO gels). Presumably, migration of molecules in the gel layers is much slower than in micelles, and the initial and final reaction cavities are almost the same. The photochemical behavior of the s-105 in the gel phases is very different. All of the homologues whose length are no more than 2 carbon atoms longer than the polymethylene portion of a surfactant host yield values for the E/C ratios which are like those from fluid solutions; however, the corresponding

R

D

Figure 57. Stylized representation of the possible solubilization sites for 2- and s-105 in KS and KP gels.

210


t/c ratios and the quadrupolar splitting parameter (Av90) from the 2H NMR spectra are maximized for the s-105 homologue which is expected to fit best into a gel matrix. As the length of the s-105 becomes significantly longer than that of a surfactant molecule, the values of the t/c ratio become very small, as though the carbonyl groups (and the corresponding hydroxy- 1,Cbiradicals) are no longer shielded in the hydrophobic middle of a bilayer. Pertinent t/c and Av90 data are shown in Figures 58 and 59 for the KS and KSO gels. Based upon these data, the longer s-105 appear to bend into a hairpin conformation which places the carbonyl groups at the water interface of a layer and buries the flanking polymethylene chains within it (D in Figure 57). Thus, the reaction cavity in the gel systems are fairly uniform for each solute, but can change enormously depending upon the length of the alkanone homologue. As with the CD complexes, the guest molecules can also exert a large influence on the nature of their reaction cavity by influencing its local anisotropy, flexibility, and polarity. Even under the most favorable conditions, the reaction cavities afforded by the gel matrices are not very restrictive when compared to those clathrate and zeolite complexes in which guests occupy nearly fully the channel cross sections; the walls of the gel reaction cavities give every evidence of being relatively flexible and maleable.

F. Reaction Cavities with Walls of Variable Flexibility 1. Polymer Matrices. An excellent example of systems in which there is potentially a wide distribution of reaction cavity volumes and for which

Figure 58. The t/c cyclobutanol ratios (0) and

'H NMR Avg0 values ( 0 )versus chain length for s-105 in KS gels [S0/50 (w/w)H,O/KS] at 38°C.


211

12 10

a Ic 6 4

2 17

25

33

cartxn c h k length

Figure 59. The t/c cyclobutanol ratios (0,38°C) and ’H NMR Avg0 values (0,70°C) versus chain length for s-105 in KSO gels [S0/50 (w/w) H,O/KSO].

translational diffusion should be relatively slow during the excited state lifetime of a guest molecule is Norrish I1 reactions in amorphous polymer matrices. However, as pointed out by Heskins and Guillet [312], rates of diffusion of small guest molecules in a polymer matrix, although several orders of magnitude lower than when in a low viscosity solvent, may be severely underestimated by the Debye equation [313] which predicts an inverse relationship between macroscopic viscosities and microscopic selfdiffusion rate constants. Equally important is the discussion of diffusion of free volume in polymers. It, too, may migrate through a polymer matrix allowing electronically excited ketones and hydroxy- 1,4-biradicals to explore temporarily conformations not normally permitted by their environment. In these cases, the efficiency and selectivity of Norrish I1 processes of guest molecules will be dominated by “solvent” motions [314]. In polymers with units capable of undergoing the Norrish I1 processes, migration of excitation energy from sectors of the polymer which are crystalline or lacking in free volume adequate to allow reaction may skew the photochemical results so that they do not reflect the distribution of reaction cavities in the polymer; those chromophores at sites with the greatest free volume and flexibility will react preferentially and may even use excitation energy originally delivered elsewhere in the sample. Many of these concepts have been discussed in some detail especially by Guillet [315]. Unfortunately, it has been assumed in many of the studies on the irradiation of polymer-doped or polymer-based ketones that cyclization processes are the fate of only a small fraction of the BRs. Although this may

212


be true in some instances, it is not generally the case. The lack of analyses for cyclobutanols in the polymer studies compromises their utility. 2. Thermotropic Liquid Crystalline and Solid Matrices. A related thermotropic liquid-crystalline medium, the hexatic B (meso)phase of n-butyl stearate (BS) [316], consists of layers like those of the gel phases with the exception that the “head group” of each molecule is a carbobutoxy group instead of a carboxylate anion and the hydrophobic layers are in a continuous assembly (Figure 16). The solid phases of BS are also packed in layers with individual molecules in extended conformations and normal to the layer planes. The orthorhombic (instead of hexagonal) arrangement in the solid is related to the inability of individual molecules to rotate about their long axes. The magnitudes of the largest of the E/C values from the 2-105 (n = 17) and 2-105 (n = 18) in BS at 0°C are very close to the E/C ratios found upon irradiation of neat 2-105 ketones in their layered solid phases [272]. Again, the t/c ratios are about the same as in the isotropic phase of BS [272]. Detailed investigations of the Norrish I1 reactions of 2-105 in the meso and solid phases of BS show no exceptional values for t/c ratios but do indicate values of E/C ratios which are dependent on the ketone chain length [268]. The maximum ratios are found when the 2-105 have n between 17 and 19 (Table 18). This is significantly shorter than a BS molecule (with 22 carbon atoms and one oxygen atom in its main chain). The model which has been proposed substitutes a 2-105 molecule for a BS molecule in a layer [317] (analogously to the mode shown in E or F of Figure 57). When n = 17-19, the BRs from 2-105 are situated in such a way that they can donate hydrogen bonds to the carboxylate groups of neighboring BS molecules. Longer BR homologues cannot hydrogen bond without extending beyond a layer boundary or making a kinked conformation; shorter homologues are free to hydrogen bond, but they introduce greater disorder in their somewhat flexible reaction cavity by not occupying the total volume allocated to a stearoyl group. The Norrish I1 reactions of the s-105 homologues vary in a somewhat different fashion [268]. Of the ketones examined, both the E/C and t/c product ratios are largest for the same homologues, n = 19 and 21 (Table 18); the product ratios from n = 23 and n = 25 could not be measured. Even with this limitation, it is clear that the Norrish I1 reactions of s-105 and 2-105 in BS ordered phases are sensitive to different guest length considerations. Apparently, this is due to the inability of the BRs from the s-105 homologues to hydrogen bond to the carboxyl groups of BS molecules when they are extended in the layers. A second consequence of the inability of the biradicals from s-105 to hydrogen bond to solvent molecules can be seen in the


213

TABLE 18 Photoproduct Ratios From 105 in the Isotropic, Hexatic, and Solid Phases of BS 12681 2-105

n 15 17 18 19 20 21

S-105

T("C)"

E/C

t/c

30 20 0 30 20 0 30 20 0 30 20 0 30 20 0 30 20 0

3.6 11.7 20.5 3.7 15.1 40.0 3.7 14.8 42.3 4.1 16.5 29.4 3.9 10.4 18.5

1.3 1.3 1.6 1.5 1.9 2.0 1.1 1.6 1.7 1.3 2.4 2.5 1.0 1.5 1.5

E/C

t/c

1.9 2.3 3.6 2.2 3.2 10.0

2.4 2.8 3.5 2.2 10.8 6.8

1.8 6.2 15.9

13.3 4.2

2.0 5.5 > 15

2.5 14.1 6.0

"30°C,isotropic; 20°C, hexatic B; O T , second solid phase.

magnitudes of the E/C ratios which, at their largest, are less than one-half of the maximum values from 2-105. The dependence of the magnitude of the Norrish I1 photoproduct ratios on the total ketone length and the position of the BR within the ketone in the ordered phases of BS has been examined in some detail using and p-alkyl alkanophenones (106) [3 181 and alkanophenones (97) [319]. No difference

106

between the E/C ratios from 97 (n = 4) or 97 (n = 10) in the hexatic B phase and isotropic melt phases of BS could be discerned. However, the longer 97

214

V. RAMAMURTHY. R. G. WEISS AND G. S . HAMMOND

(n = 17, 19, 21) exhibit high E/C ratios due to their ability to fill more efficiently the volume allocated by the cylindrical reaction cavity (Table 19). As shown in Figure 60, the temperature profile of the E/C ratio from 97 (n = 10) remains isotropic-like in the hexatic B phase whereas that of 97 TABLE 19 E/C Photoproduct Ratios from 97 in the Isotropic and Hexatic B Phases of BS" I3191

4

10 17 19

21

30 20 30 20 30 20 30 20 30 20

3.2 3.3 1.7 2.0 3.0 21 19 15

1.3 9.5

"30"C, isotropic; 20°C, hexatic B.

Figure 60. Temperature dependence on the ratio of elimination to cyclization and 97 ( n = 19) (A) in n-butyl products for photolysis of ketones 97 (n = 10) (0) stearate.


215

(n = 19) responds to changes in the anisotropy and flexibility of the reaction cavity around it in a very sensitive fashion. As noted before with the s-105 [268], the rise in E/C values in the solid phases of BS is not continuous. The discontinuity corresponds roughly to the temperature reported for the transition between the first and second solid phases of BS [316]. The dependence of the photoproduct ratios on the location of the oddelectron centers along a polymethylene chain of a BR derived from 106 has been examined also in the ordered phases of BS [318]. Starting with m = 5 and allowing n to vary from 11 to 18, it was established that the largest E/C and smallest t/c values were observed when n = 16; the onset of these excursions occurs at the isotropic-hexatic B transition temperature of BS. Since this homologue is longer than a BS molecule, the cylindrical reaction cavity must be somewhat distorted. The bend imposed on BRs by the aromatic ring along a chain (Figure 61) must also contribute to the distortion and to the optimal 106 length being longer than a BS molecule; when the aromatic ring is at a molecular terminus of 97, the largest E/C ratios are observed when the total molecular length is at least four carbon atoms shorter than the optimal 106. Apparently, the 97 are able to fit into a lessdistorted cylinder than 106 of the same length, but one which places the oddelectron BR centers in a more flexible environment near a layer (cylinder) end. Starting with 106 for which m n = 21, the relative wall stiffness and

+

\

hw

hw

hv

hw

BS Figure 61. Representation of extended transoid hydroxy-1,4-biradicalsfrom selected 97 and 106 (m n = 21) and their orientations in a BS layer according to the solubilization model. Note the approximate depths of the hydroxyl groups of the biradicals and their relationship to the carboxyl of BS. The approximate layer

+

boundaries are shown as shaded areas.

216


constraints imposed by various portions of the distorted cylindrical reaction cavities in hexatic B and solid phases of BS were explored [318]. As shown in Figure 62, extraordinarily high E/C ratios result from irradiation of 106 (m,n = 3,18) in the anisotropic BS phases. Both values are similar in magnitude to the ratios found from irradiation of alkanophenones in ZSM type zeolites. Although both media may be thought of as having cylindrical (channel-like) reaction cavities, those from the zeolites have very rigid walls and uncapped ends; those from BS have somewhat flexible walls which can both expand and compress the cross section of BRs, but offer little room for longitudinal expansion. As a result, the BS reaction cavities provide the greatest influence on the fate of the BRs from 97, 105, and 106 homologues which have very specific lengths. Furthermore, Figure 60 suggests that only a few of the p-alkyl alkanophenone isomers with m n = 21 can be arranged in the BS reaction cavities so that the hydroxyl group of their BRs will be hydrogen-bonded to a BS carboxyl group. Of these homologues, only the ones with the shorter m (e.g., m, n = 3,18) will provide, the two radical centers with relatively stiff walls. The isomers with longer m (e.g., m, n = 15,6) must be arranged in the reaction cavity so that the odd-electron centers are adjacent to a very flexible and easily distorted section of the reaction cavity. Other isomers which allow the biradical centers to reside at the most ordered middle of a BS layer (e.g., m, n = 10,ll) and, thereby, experience the stiffest walls of the reaction cavity do not permit hydroxyl-carboxyl hydrogen bonding. Thus, for the 106 with m + n = 21, the most restrictive reaction cage is one that promotes hydrogen bonding [318] and places the radical centers nearer a layer middle; its walls

+

0

--

100.0

0 80.0--

--

0 0

0

60.0 EIC

40.0--

0 0

0

m

Figure 62. E/C ratios from 2% (w/w)106 (m + n = 21) in the solid (lO°C, O),hexatic B (20°C, a), and isotropic (30°C, 0)phases of BS. n = 21 - m on the abscissa.


217

act as a template while affording minimal flexibility to the locus of reaction. Evidence of the existence of the purported hydrogen-bonding interactions was provided with 107, which mimics the structures of the BRs from 106, in the ordered phases of BS [318].

107

The general model which emerges for the reaction cavity of BS is a cylinder whose dimensions are approximately the same as the van der Waals size of an extended BS molecule and whose wall stiffness is influenced by the size and shape of the reacting guest molecules [317]. At one point along the length of the cylinder is a cross-sectional segment of much higher polarity than the rest, where the carboxyl groups reside and which is capable of acting as a hydrogen-bond acceptor. Thus, the walls of the reaction cavity in hexatic or solid phase BS may be either active or passive depending upon the location of the hydroxyl group in a BR from an alkanone or alkanophenone. Since cis cyclobutanols can form in low yields even in the most ordered solid BS phases, some flexibility in even the stiffest reaction cavities must remain (or, perhaps, the cyclobutanols are produced at defect sites?). The lifetime of BR generated from 97 (n = 19) was found to be 64 k 5 and 70 A 5 ns in the isotropic and hexatic B phases of BS [319]. The lack of influence on the lifetime of the biradical by the hexatic phase when the E/C ratios are clearly affected is at first puzzling. However, it can be cited as evidence that the T -,S rate is independent of the conformation in which a BR is held [263]. Note that the BR from 97 (n = 21) as shown in Figure 60 has its hydroxyl group far removed from the cross-sectional segment of the BS-provided reaction cavity cylinder which is quite polar. In any case, the long lifetime of the BR found in hexatic BS and its near equivalence to that in the isotropic phase indicate that the various biradical conformers have equilibrated in the cylindrical reaction cavity prior to collapsing to products. Removal of the carboxyl group from a BS molecule leaves heneicosane (C21) which exhibits no liquid-crystalline phase, but does form two layered solid phases in which the C21 molecules are fully extended with the molecular axes normal to the layer planes [320]. In the higher temperature solid (Phase 11),the molecules are hexagonally packed within and execute hindered rotations about their long axis. In the lower temperature solid (Phase I), the rotational motions are damped and the molecules are orthorhombically packed within a layer. Gauche defects in the preferred all-transoid conformations occur more frequently near layer ends than near the middle of layers

218

V. RAMAMURTHY, R. G. WEISS AND G.S. HAMMOND

[271]. Thus, the ordered phases of BS and C21 are quite analogous, except that C21 lacks a polar carboxyl group along its stiffer molecular backbone. Exploration of the modes of solubilization of 2- and s-105 in the solid phases of C21 has led to many unexpected and complicating observations [321]. First and foremost among these is that only guest alkanone molecules whose lengths are very near that of C21 can be incorporated isomorphously into its solid phases. Thus, C21 is a much more demanding host matrix than BS; the dimensions and free volume content of the reaction cavities it provides have a very narrow distribution and their walls are probably less flexible than those in many neat crystals. Using differential scanning calorimetry and 'HNMR spectroscopy, it was possible to ascertain that at least 1 wt% of 105 with it = 18-22 is solubilized into the solid phases of C21. Representative results from irradiations of these solid solutions are collected in Table 20. They and other results from irradiations in the solid phases of eicosane (C20) [321] indicate that the highest photoproduct ratios are observed when the alkanone is one carbon atom shorter than the alkane host. As in many other ordered systems, the t/c ratios are probably mechanistically meaningless when the E/C ratios are very large since a very small fraction of the BRs cyclize and those that do may

TABLE 20 Photoproduct Ratios from 105 in the Isotropic and Solid Phases of C21 13211 S-105"

2-105"

n 18 20 21 22

45 35 25 45 35 25 45 35 25 45 35 25

E/C

t/c

4.0 3.9 14 4.9 18 165 2.4 6.0 46 4.0 8.4 29

1.5 4.6 2.4 1.6 4.7 18 1.0 2.5 3.6 1.7 2.8 4.0

E/C

t/c

1.8 3.5 >69

2.5 25 0.9

"Error limits, which are large in the case of high ratios, have been deleted for the sake of clarity. b45"C,isotropic; 3 5 T , Phase 11; 25°C Phase I.


219

react in cavities formed at defect sites. Regardless, the E/C ratios show that the motional restrictions experienced by alkanones in the C21 solid phases can be exceedingly high in spite of the passive walls of the reaction cavities (i.e., in the absence of a group which can act as a hydrogen-bond acceptor for the BRs). Comparison between the results from irradiations of common alkanones in C21 and BS suggest strongly that the degree of flexibility and the amount of free volume in a reaction cavity are more important factors in directing Norrish I1 transformations than are template effects. Other comparisons of this sort should be made before a definitive conclusion is reached. Regardless, the fit between the ketone and the cylindrical reaction cavity of the C21 phase must be very tight if its influence on photoproduct ratios is to be manifested. Apparently, the stiffness of the reaction cavity walls in Phase I, even near a layer end, is sufficient to inhibit selective motions of the BRs of 2105. However, the reaction cavities provided by Phase I1 have walls which are somewhat more flexible as witnessed by the relatively large fraction of transcyclobutanol produced from s-105 (n = 21) and the low E/C and t/c ratios from 2-105 (n = 21).

An extreme example of the flexibility which can be incorporated into a cylindrical reaction cavity is given by the two smectic G (or J) phases [275] of perfluorodecyldecane (FlOH10). In these smectic phases, the long axes of individual FlOHlO molecules are tilted with respect to the layer planes and are packed pseudohexagonally within a layer. An interesting characteristic of FlOHlO is that its two parts are incompatible both in terms of size and solubility: the molar volumes of hexane and perfluorohexane are 131 and 201 cm3, respectively, at 25°C [322]; although the dielectric constants of hexane (1.89) and perfluorohexane (1.69) are similar, that of perfluoropropylpropane (5.99) is much larger [322] and indicates the predictable polarization along the chain. Thus, in its isotropic phase, FlOHlO may also segregate itself into regions that are respectively methylene and difluoromethylene enriched, although the two halves of the molecule behave very differently than they do as a combined unit. Even in the smectic phases, NMR evidence suggests that the polymethylene portions of the FlOHlO molecules retain a great deal of flexibility [273,322]. This assertion is borne out by the small deviations from fluid-isotropiclike values of the photoproduct ratios found in the Norrish I1 reactions of the two alkyl perfluoroalkyl ketones 76 (with m,n = 7,8 and 9,lO) in the (macroscopically) highly ordered smectic phases of FlOHlO [273,274]. As

220


TABLE 21 Protoproduct Ratios from Neat Melted and Solid 76 and Their Isotropic and Srnectic Phase Solutions in FlOHlO [273,274] 76

T("C)

m, n

=

m, n

= 9,

7, 8

10

65 48 26 25 1 65 48 26 50 26

Phase" Isotropic FlOHlO Smectic I1 FlOHlO Smectic I FlOHlO Neat isotropic Neat solid Isotropic FlOHlO Smectic I1 FlOHlO Smectic I FlOHlO Neat isotropic Neat solid

E/C

t/c

1.9 1.4 1.5 3.7 10.9 1.5 0.7 1.0 3.5 31

3.5 9.4 6.2 3.4 3.9 2.7 4.5 12.2 3.4 18

"Smectic I1 and I are the higher and lower temperature smectic phase of FlOH10, respectively.

seen in Table 21, only the t/c ratios from the smectic phases are elevated with respect to the isotropic phase values. The reaction cavity afforded by smectic FlOHlO to the BRs from the two partially fluorinated ketones does influence the conformational preferences which determine the fragmentation and trans cyclobutanol products, but modestly retards formation of the cis cyclobutanols and their precursor, c-BR, , due to its large cross-sectional area. By contrast, in the neat solid phases of the two ketones where carbonyl groups on neighboring molecules may function as hydrogen bond acceptors for BR intermediates, the E/C ratios are high; they increase relative to the isotropic phase values more than the t/c ratios [273]. Since the exact packing arrangement of each of the neat ketones in its solid phase is unknown, it is not possible to provide more than speculative explanations at this time. However, it is clear that the free volume of the reaction cavities and/or flexibility of their walls in neat solid 76 are much greater than that in the neat crystalline phases of the analogous s-105 which are virtually photoinert.

IX. PERSPECTIVES ON FUTURE RESEARCH The last decade or so has witnessed a great deal of activity to explore the influences of anisotropic environments on photochemical reactions. Results from this research have led to a rudimentary understanding of how photochemical processes may be directed by media in individual cases, but


221

the general principles which underlie the field do not seem to have been collected into a cohesive model. Possible reasons for this include the interdisciplinary nature of the research and the disparate ways in which the participating researchers have approached the problem and the interpretation of their data. To provide a common basis for discussion, we have attempted in this chapter to provide a workable, rudimentary model which includes the more important identifiable parameters of anisotropic media responsible for directing photochemical processes. In spite of the many examples cited during its development and the many more which have not been, it is clear that the model requires a great deal more experimental testing and refinement if it is to become a paradigm. We hope that the model will be tested, criticized, and refined in the future. The words of Chalmers [69], that “a paradigm will always be sufficiently imprecise and open-ended to leave plenty of work to be done,” have not been forgotten.

ACKNOWLEDGMENTS The authors gratefully acknowledge the intellectual and experimental contributions of their colleagues whose names appear in the references. Without their efforts, the authors’ portion of this chapter could not have been written. We thank Dave Eaton, Bob Liu, John Scheffer, Nick Turro, K. Venkatesan and C. L. Khetrapal for useful discussions and helpful comments. The National Science Foundation is also thanked for its support of the research of RGW related to this chapter.

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273. P. M. Vilalta, P b D . Thesis, Georgetown University, Washington, DC, 1992. 274. P. M. Vilalta, G. S . Hammond, and R. G. Weiss, Photochem. Photobiol. 54,563 (1991). 275. (a) W. Mahler, D. Guillon, and A. Skoulios, Mol. Cryst. Liq. Cryst. Lett. 2, 111 (1985);(b) C. Viney, T. P. Russell, L. E. Depero, and R. J. Twieg, Mol. Cryst. Liq. Cryst. 168,63 (1989). 276. J. R. Scheffer,in Organic Solid State Chemistry,G. R. Desiraju, Ed., Elsevier, New York, 1987, Chapter 1. 277. (a) T. J. Lewis, S . J. Rettig, J. R. Scheffer,J. Trotter, and F. Wireko, J . Am. Chem. SOC.112,3679 (1990);(b) T. J. Lewis, S . J. Rettig, J. R. Scheffer, and J. Trotter, J . Am. Chem. SOC. 113,8180 (1991). 278. (a)I. Furman and R. G. Weiss, J . Am. Chem. SOC.114,1381 (1992).(b) I. Furman, R. J. Butcher, R. M. Catchings, and R. G. Weiss, J . Am. Chem. SOC.,114,6023 (1992). 279. S. Ariel, V. Ramamurthy, J. R. Scheffer, and J. Trotter, J . Am. Chem. SOC.105, 6959 (1983). 280. L. J. Johnston and J. C. Scaiano, Chem. Rev. 89, 521 (1989). 281. (a) S . Evans, N. Omkaram, J. R. Scheffer, and J. Trotter, Tetrahedron Lett. 26, 5903 (1985); (b) S. V. Evans and J. Trotter, Acta Crystallogr. B45, 5 0 0 (1989). 282. (a) J. R. Scheffer, J. Trotter, N. Omkaram, S . V. Evans, and S . Ariel, Mol. Cryst. Liq. Cryst. 134, 169 (1986); (b) S. Ariel, S. V. Evans, M. Garcia-Garibay, B. R. Harkness, N. Omkaram, J. R. Scheffer, and J. Trotter, J . Am. Chem. SOC.110, 5591 (1988); (c) S . V. Evans and J. Trotter, Acta Crystallogr. B45, 159 (1989). 283. See for instance: V. T. Jones and J. B. Coon, J . Mol. Spectrosc. 31, 137 (1969). 284. S . V. Evans, M. Garcia-Garibay, N. Omkaram, J. R. Scheffer, J. Trotter, and F. Wireko, J . Am. Chem. SOC.108,5648 (1986). 285. P. de Mayo and N. Ramnath, Can. J . Chem. 64,1293 (1986). 286. N. J. Turro, I. R. Gould, M. B. Zimmt, and C.-C. Cheng, Chem. Phys. Lett. 119, 484 (1985). 287. J. C. Scaiano, Acc. Chem. Res. 15,252 (1982). 288. R. A. Caldwell, T. Majima, and C. Pac, J . Am. Chem. SOC.104,629 (1982). 289. B. Zibrowius, J. Caro, and H. Pfeifer, J . Chem. SOC.Faraday Trans. 184,2347 (1988). 290. N. J. Turro and P. Wan, Tetrahedron Lett. 25, 3655 (1984). 291. V. Ramamurthy, D. R. Corbin, and D. F. Eaton, J . Chem. SOC.,Chem. Commun. 1213 (1989). 292. V. Ramamurthy, D. R.Corbin, and L. J. Johnston, J . Am. Chem. SOC.114,3870 (1992). 293. V. Ramamurthy and D. R. Sanderson, Tetrahedron Lett. 33, 2757 (1992). 294. H. L. Casal, P. de Mayo, J. F. Miranda, and J. C. Scaiano, J . Am. Chem. SOC.105, 5155 (1983). 295. A. P. Dianin, J . Russ. Phys.-Chem. SOC.46, 1310 (1914).


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296. (a) D. D. MacNicol, J. J. McKendrick, and D. R. Wilson, Chem. Soc. Rev. 7 , 65 (1978); (b) J. L. Flippen, J. Karle, and I. L. Karle, J . Am. Chem. Soc. 92, 3749 (1970). 297. P. C. Goswami, P. de Mayo, N. Ramnath, G. Bernard, N. Omkaram, J. R. Scheffer, and Y.-F. Wong, Can. J . Chem. 63, 2719 (1985). 298. (a) I. Goldberg, Topics Curr. Chem. 149, 1 (1988);(b) Topics in Current Chemistry, Vol. 140, E. Weber, Ed., Springer-Verlag, Berlin, 1987. 299. (a) H. Aoyama, T. Hasegawa, and Y. Omote, J . Am. Chem. SOC.101,5343(1979); (b) H. Aoyama, M. Sakamoto, K. Kuwahara, K. Yoshida, and Y. Omote, J . Am. Chem. Soc. 105, 1958 (1983). 300. B. Ackerman and N. G. Johansson, Tetrahedron Lett. 5, 371 (1969). 301. C. A. Chesta and D. G. Whitten, J . Am. Chem. SOC. 114, 2188 (1992). 302. J. Vincens, Mol. Cryst. Liq.Cryst. 187, 115 (1990). 303. M. Kaftory, F. Toda, K. Tanaka, and M. Yagi, Mol. Cryst. Liq. Cryst. 186, 167 (1990). 304. G. D. Reddy, B. Jayasree, and V. Ramamurthy, J. Org. Chem. 52, 3107 (1987). 305. Y. Hui, J. R. Winkle, and D. G. Whitten, J . Phys. Chem. 87, 23 (1983). 306. J. R. Winkle, P. R. Worsham, K. S . Schanze, and D. G. Whitten, J . Am. Chem. SOC.105, 3951 (1983). 307. J. C. Scaiano and J. C. Selwyn, Photochern. Photobiol. 34, 29 (1981). 308. N. J. Turro, K.-C. Liu, and M.-F. Chow, Photochern. Photobiol. 26, 413 (1977). 309. R. L. Treanor and R. G. Weiss, J . Am. Chem. SOC. 110, 2170 (1988). 310. J. M. Vincent and A. Skoulios, Acta Crystallogr. 20, 432, 441, 447 (1966). 311. D. A. Hrovat, J. H. Liu, N. J. Turro, and R. G. Weiss, J . Am. Chem. Soc. 106,5291 (1984). 312. M. Heskins and J. E. Guillet, Macromolecules 3, 224 (1970). 313. (a) P. Dbeye, 7kans. Electrochem. Soc. 82, 265 (1942); (b) A. D. Osborne and G. Porter, Proc. Roy. SOC.(London) 284A, 9 (1965). 314. H. A. Kramers, Physica 7 , 284 (1940). 315. (a) J. Guillet, Polymer Photophysics and Photochemistry, Cambridge University Press, Cambridge, 1985; (b) J. Guillet, Adu. Photochem. 14, 91 (1988). 316. (a) D. Krishnamurti, K. S . Krishnamurthy, and R. Shashidhar, Mol. Cryst. Liq. Cryst. 8, 339 (1969); (b) K. S . Krishnamurthy and D. Krishnamurti, Mol. Cryst. Liq. Cryst. 6,407 (1970); (c) J. S. Dryden, J . Chem. Phys. 26, 604 (1957);(d) K. S . Krishnamurthy, Mol. Cryst. Liq.Cryst. 132, 255 (1986). 317. R. G. Weiss, R. L. Treanor, and A. Nunez, Pure Appl. Chem. 60, 999 (1988). 318. Z. He and R. G. Weiss, J . Am. Chem. SOC. 112, 5535 (1990). 319. D. A. Hrovat, J. H. Liu, N. J. Turro, and R. G. Weiss, J . Am. Chem. SOC.106,7033 (1984). 320. (a) M. G. Broadhurst, J . Res. ( N B S )66A, 241 (1962) and references cited therein; (b) M. Maroncelli, S . Ping Qi, R. G. Snyder, and H. L. Strauss, J . Am. Chem. Soc. 104, 6237 (1982).

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321. (a) A. Nunez and R. G . Weiss, J. Am. Chem. SOC. 109,6215 (1987); (b) A. Nunez and R. G. Weiss, Boll. SOC. Chilena Quim. 35, 3 (1990); (c) A. Nunez, G. S. Hammond, and R. G. Weiss, J . Am. Chem. Soc., in press. 322. W. Mahler, private communication of unpublished results.


UP-SCALING PHOTOCHEMICAL REACTIONS Andre M. Braun, Laurent Jakob, and Esther Oliveros Lehrstuhl fur Umweltmesstechnik, Engler-Bunte-Institut, Universitat Karlsruhe, 7500 Karlsruhe, Germany Claudio A. OIler do Nascimento Escola Politecnica da Universidade de SBo Paulo 01000 SBo Paulo, SP, Brasil

CONTENTS

I. Introduction 11. Parameters for qualitative reactor design A. Reaction system 1. Gas phase reactions 2. Liquid phase reactions 3. Solid-liquid heterogeneous reaction systems 4. Gas-solid heterogeneous reaction mixtures 5. Liquid-liquid microheterogeneous reaction systems 6. Reaction mechanism and reactor design 7. Reaction system and safety requirements B. Light sources 1. Point sources 2. Extended light sources 3. Excimer lamps 4. Safety requirements

236 239 239 239 239 243 243 244 245 248 25 1 255 256 259 262

Advances in Photochemistry, Volume 18, Edited by David Volman, George S. Hammond, and Douglas C. Neckers ISBN 0-471-59133-5 Copyright 1993 by John Wiley & Sons, Inc.

235

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111. Concepts of qualitative reactor design A. Reactor design and light source B. Light distribution IV. Qualitative up-scaling rules and parameters A. Rate of production B. Sensitized oxidations C. Photocatalyzed reactions V. Mathematical modelling of photochemical reactors A. Radiation models 1. Incidence models 2. Emission models B. Coupling of mass and light energy balances VT. Modelling and optimization of pilot results A. Optimal experimental design 1. Fundamentals 2. Example of application: TiO, photocatalyzed oxidative degradation B. Neural networks 1. Fundamentals 2. Example of application: Large-scale actinometry VI. Conclusions Acknowledgments References

264 264 268 272 272 274 277 282 282 283 286 29 1 29 1 292 292 295 302 302 304 305 307 307

I. INTRODUCTION Industrial preparative photochemistry has been ill-famed for many decades [l], and successful processes and production units are kept out of publicity, because invested know-how is of considerable value. It is, hence, quite difficult to collect and review data on preparative photochemical technology and to discuss the evolution in this field [2,3]. For a few years, interest in photochemical reactions of potential preparative interest has however been revived. Reasons may be found for instance in: Constraints due to regulations concerning environmental protection, such as limitations in the choice of solvents, waste disposal, and work-up procedures.

UP-SCALING PHOTOCHEMICAL REACTIONS

0

237

The evolution of teaching programs in academia and, therefore in a larger awareness of research chemists and engineers for the potential of photochemical processes. The development of photochemical reactor engineering.

Rapidly outgrowing the number of photochemical reactions of potential interest for industrial production, photochemical oxidative degradation processes will most probably find applications in the chemical treatment of polluted surface and ground waters, as well as of (industrial) waste waters containing nonbiodegradable or highly bactericidal pollutants [4]. In implementing these photochemical or photocatalyzed methods of oxidative degradation, the same or similar concepts of photochemical reactor engineering are used as for preparative purposes. This chapter does not deal with the technical developments in copying, isotope separation, microelectronics, medicine, nonlinear optics, photography, sculpturing, and surface coating, domains that are still neglected by a large part of the chemical industry and, certainly, by the majority of academic teachers. General views on the different fields of applied photochemistry have been published for the first two “Schools of Industrial Photochemistry” which were held in 1988 and 1990 at the ENSIC-INPL in Nancy [S, 6 ) . The Chapter does, however, address problems linked to the technical development of photochemical processes with a potential interest in chemical synthesis or large-scale photolysis. It describes therefore the different approaches of photochemical engineering which comprise the fundamentals and the developments of methods and procedures used to implement and optimize large-scale photochemical reactions. The technical development of photochemical reactions involves specific problems due to the necessity to activate by electronic excitation one of the substrates present in a reaction mixture. Thus in addition to the rather wellmastered mass and energy balances in conventional process development, optimal light distribution must be achieved, a condition difficult to satisfy when taking into account synthetically meaningful substrate concentrations. Difficulties encountered in up-scaling photochemical reactions are then mainly due to the consequences of the inhomogeneity between irradiated and nonirradiated reaction volumes which are created by the reaction systems particular absorption characteristics and by the light distribution of the chosen light source. Even in a well-mixed reactor, where a thermal reaction would proceed in all parts of the reactor, a photochemical reaction will only take place within the volume where light is absorbed, and no means of mixing will be able to move electronically excited states within their lifetime into the nonirradiated volume. The selection of an appropriate light source and the design of a reaction specific photochemical reactor are, hence, the major tasks

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A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER D O NASCIMENTO

IPhotochemical Engineering

Figure 1. Interactive domains in photochemical engineering (see also [7]). of photochemical engineering and require experience in mechanistic and analytical chemistry, chemical engineering, and photochemical technology (Figure 1). Among the few academic and industrial groups working in the field of photochemical engineering, two different strategic lines of research and development can be distinguished:

1. A pragmatic but mostly qualitative approach is using a check list of concepts, such as a. Specific excitation b. Spatial separation between light source and reaction mixture c. optimal gas saturation d. Convergence of light source and reactor geometry e. Fixed exitance* during up-scaling and optimizing a concept related reactor module in accord with the absorption parameters and the kinetics of the photochemical reaction. 2. A more quantitative approach is investing in a. A precise description of light distribution within the reactor volume and, hence, in the calculation of the irradiated volume and its shape. b. A quantitative combination of this spatially defined energy uptake with ideal classical mass and energy system balances which in turn may then yield reaction specific reactor geometries and operational limits for optimal reaction conditions. Although the first approach has been surprisingly successful in most of the recent industrial development projects, accurate models of light distribution *Exitance: power radiated in all directions per unit area of an extense light source [2,3].


239

and propagation would be definitely needed to overcome some of the disadvantages of the thumb-rule designed reactor modules, where professional experience is the main asset.

11. PARAMETERS FOR QUALITATIVE REACTOR DESIGN A. Reaction System The reaction system is one of the most important parts of choosing a basic module for the design of a photochemical reactor. This might be explained by enumerating some of the corresponding parameters and describing their impact on reactor geometry and operational conditions: Physical state of substrates, intermediates, and products Absorption characteristics of substrates and products Reaction mechanism Thermodynamic and kinetic parameters and chemical yield Quantum yield Safety requirements.

1. Gas Phase Reactions. Substrates in the gas phase show low absorption cross sections, and reactor geometries providing long optical paths are usually required for efficient operation (e.g., tubular photochemical reactor designed for irradiation by a laser beam; see Figure 2). However, annular reactor geometries with light sources placed in their axis have proven to be extremely useful for reactions exhibiting high quantum yields, as for example in photochlorinations of partially fluorinated hydrocarbons of low molecular weight (Figure 3). Gas phase reactions yielding liquid or solid products might be realized in vertically mounted tubular reactors operating at wall temperatures that provoke product condensation. The tube may be fitted with a device to rinse off the reaction products (see Figure 7). 2. Liquid Phase Reactions. Photochlorination of low molecular weight hydrocarbons, such as methane or ethane, can be achieved in liquid phase using pressurized reactors or an inert solvent (e.g., CC1,) [9,10]. Photolysis of chlorine saturated, liquid phase substrates is the most common procedure reported in research and patent literature [2,3]. Absorption cross sections of saturated chlorine solutions allow optical pathlengths of a few centimeters. Organic radicals generated by hydrogen abstraction (Eqs. 1 and 2) react with molecular chlorine (Eq. 3) at high efficiency, and secondary reactions, such as

jacket

Figure 2. Diagram of a tubular photochemical reactor irradiated by a laser beam [2, 3, 71 (dimensions in mm). Coolant outlet

$?

Product

t 1. f

CH3

- CHFz-

CH, - CHF2

I

C'2

source Coolant

Figure 3. Conceptual diagram of an annular photochemical reactor for the production of 1-chloro-l,l-difluoroethanein the gas phase with introduction of 1,ldifluoroethane at different levels [2, 31. 240


241

dismutation reactions (Eq. 4), are almost negligible. Under those conditions, immersion-type reactors provide most economic operation units, although the lamp wells must be periodically cleaned from polymerized secondary products.

c1, 42C1'

-

+ RH, --+ HR' + Clz

C1'

2HR'

(1)

+ HCl HRCl + C1' HR'

R+RH,

A similar mechanism has been proposed for photonitrosylations, with the difference that the reaction of the alkyl radical with NOCl (Eq. 5 ) is not competitive (absence of chain reaction) and that the rate of reaction 6 cannot prevent dismutation (Eq. 4) and subsequent radical polymerization of unsaturated hydrocarbons (e.g., cyclohexene in the case of caprolactam synthesis). HR'

+ NOCl

HR' + N O '

__+

HRNO HRNO

+ C1'

(5)

(6)

Photonitrosylations show an extremely high tendency for filming, and the difficulties in implementing the caprolactam synthesis on an industrial scale are also linked to reactor designs not taking into account the competitive secondary reactions leading to polymerized material. Liquid phase photochemical reactions may show in most cases filming effects [2,3]. This is due to usually high-absorption cross sections of the substrates dissolved up to concentrations of preparative importance, simple calculations on the basis of Beer-Lambert's law showing that optical paths might be as short as a fraction of a millimeter (Figure 4) [2,3]. Under these conditions, high local concentrations of electronically excited molecules and intermediates are produced within the irradiated reaction volume and cannot be transfered into nonirradiated zones within the time frame of diffusion controlled primary reactions. For optimal reactor operation, extremely inhomogeneous distributions of reactive states within the reaction volume must be avoided, a task which can be achieved by either reducing the absorbance of the reaction mixture by diminishing the concentration of the light-absorbing substrate, by reducing the radiant power, or by changing the light distribution in modifying the design of the photochemical reactor. Extremely inhomogeneous conditions are found in oxidative degradation processes induced by vacuum ultraviolet (VUV) irradiation of aqueous reaction systems. In fact, the absorption cross section of water for an almost monochromatic excitation at 172 nm (Xe excimer lamp, vide infra) being very

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A. M. BRAUN, L.JAKOB, E. OLIVEROS AND c. A. OLLER DO NASCIMENTO

0

0.004

0.008

0.012

0.016

0.02

1 [cml

Figure 4. Relative number of photons transmitted as a function of distance 1 traversed by the light (Beer-Lambert law); E of reactant at the wavelength of irradiation: lo4 ~.mol-'.crn-'; concentration of reactant: (I) c = mo1.L-'; (2) c = lo-' rno1.L-I [2,3].

efficient [ll], light is totally absorbed within an optical path of less than approximately m [121. Combining this very small irradiated reaction volume with a high irradiance leads in accord with Eq. (7) to large local HO' radical concentrations. Under conditions of residual (industrial) waters containing more than lo3 ppm of total organic carbon (TOC), subsequent hydrogen abstraction (Eq. 8) leads to organic radical concentrations which will almost instantaneously deplete the irradiated volume of dissolved oxygen (Eq. 9). This latter reaction is, however, of primary importance, not only for initiating some thermal chain reaction of oxidative degradation, but also for preventing the deposit of polymerized material at the lamps surface.

+ HO' + H,O + RH' hv

H,O + H'

+ RH, RH' + 0, -+

HO'

RHO,' +

-

(7)

(8) (9)

Assuming chemical inertness of sensitizers, immersion-type reactors apply practically without restriction to sensitized reactions, and by varying the concentration of the sensitizer, radiant power and irradiated volume of the reaction system can be controlled rather easily. Secondary reactions usually occur with low efficiencies, for example, in sensitized oxidations they may include electron transfer reactions [13] and singlet oxygen reactions with and photolysis of the sensitizer molecule, the latter due to insufficient light stability. Changes of the absorption spectrum and, hence, of the absorbed photon rate might also occur as a consequence of the changing composition of the reaction mixture in the course of the reaction. It is for instance well


243

known that role bengal solutions decolorize in the presence of Brernsted or Lewis acids. To avoid purification of the reaction product from the (colored) sensitizer or its oxidation and photolysis products, the use of insoluble sensitizers has been proposed, in particular for sensitized oxidations [141. Whereas sensitizers adsorbed on solid supports, such as ion exchange resins, silica, or alumina [l5], show considerable leading rates [16] and must be discarded, potential application might be found for sensitizers that are chemically bound to inert surfaces [14-171. However, a loss of efficiency of at least 50% has to be taken into account when comparing overall quantum yields with those determined in homogeneous reaction systems [171.

3. Solid-Liquid Heterogeneous Reaction Systems. For solid-liquid heterogeneous reaction systems, immersion-type reactors provide good technical and economical results. Transition to fluidized bed reactors would be continuous, but high contents of solid particles is severely limiting reactor dimensions. For Ti0,-photocatalyzed oxidative degradation, multilamp immersion-type photoreactors have been used to offset the problem of limited light penetration due to diffusion of light (vide infra) [l2, 181. However, alternative reactor geometries may be developed using fixed photocatalysts [191 and consequently taking into account smaller efficiencies.

4. Gas-Solid Heterogeneous Reaction Mixtures. Gas-solid heterogeneous reaction mixtures may be advantageously irradiated in annular (immersiontype) photochemical reactors. Again, the content of solid particles is limiting the size and the productivity of the reactor system. This is of particular importance when the solid support is used to specifically adsorb substrates or products of the photochemical reaction; the first to enhance specificity of radical substitution reactions [20], the latter to reach better photostability and to ensure optimal purity. Particular ingenious reactor geometries have been developed for the photochlorination of PVC [2,3]. By exploiting the fact that PVC particles swell in liquid chlorine, the reaction has been carried out at temperatures below the condensation temperature of Cl,. Starting from experiments with tubular photochemical reactors, where light sources were installed at the outside wall of the reactor to ensure efficient mixing, a continuous process has been developed in which the stirrer was replaced by a helical stirring blade (Figure 5) [21]. This production unit may be relatively easily optimized in varying the principal parameters, such as number, mounting places and electrical power of a given type of light sources, reactor diameter, feeding speed, and particle charge.

244

A. M. BRAUN,L. JAKOB, E. OLIVEROS AND c. A. OLLER DO NASCIMENTO

Porous plate

CPVC

Figure 5. Tubular reactor equipped with a helical agitator for the photochlorination of PVC in liquid chlorine [2,3,21].

5. Liquid-Liquid Microheterogeneous Reaction Systems. Several authors have shown that liquid-liquid microheterogeneous reaction systems may be advantageous for overall chemical yield and positional and stereochemical specificity of photochemical reactions [22]. Ionic interphases may for instance assist in differentiating between reactive intermediates and thus enhance reaction specificity and chemical yield. Sensitized oxidations may often include singlet oxygen as well as superoxide or hydroperoxide reactions depending on the redox potentials of ground and excited states of the sensitizing molecule, as well as of the substrate to be oxidized. In fact, besides energy transfer to oxygen (Eq. lo), electron transfer to oxygen (Eq. 11) or to singlet oxygen (Eq. 12) may occur ~ 3 1 : 3Sens 3Sens

+ 0, + 0,

'O,+RH, '0,

+R

__+

Sens

+ '0,

Sens+'

+ 0;'

0; + R H l '

-

R0, ++

(10) (11) +

(12) (13)

In contrast to singlet oxygen, charged intermediates cannot penetrate ionic liquid-liquid interphases (ionic micelles or microemulsions) owing to charge repulsion or attraction [23]. Sensitizer and substrate solubilization in different phases may therefore be of interest for an improvement of chemical yield.


245

Charged interphases may also be exploited to create high local concentrations of electron acceptors which affect the rate of electron transfer reactions confined within these restricted reaction volumes and diminish considerably the efficiency of the corresponding back-transfer [24]. These results have been primarily applied in photochemical conversion projects [22,25], but technically more interesting applications may be found in their use for the development of new specific analytical procedures (e.g., optical or photoelectrochemical probes). High local concentrations are also of considerable interest in the optimization of photochemical dimerization reactions [22], as the rate of bimolecular reactions between excited and ground state molecules confined in an extremely restricted reaction volume (microreactor) will be considerably enhanced. In addition, spatial gradients of polarity may lead to preferential structures of the solvated substrate and, hence, to the synthesis of specific isomers [24,22,26]. Similar selectivities have been found when monomolecular photochemical or photoinduced reactions [2,3] are made via inclusion complexes [27,28]. Micelles are of no practical interest as far as preparative chemistry and photochemistry is concerned, their capacity of substrate solubilization being far too restricted. On the other hand, microemulsions can dissolve rather large concentrations of starting materials and provide, because of their transparency, an excellent environment for photochemical reactions on a preparative scale [26, 28, 291. However, pseudo-ternary phase diagrams of clearly defined microemulsions are certainly altered by adding rather high concentrations of substrate or product, and care must be taken that microheterogeneity is not lost during the course of the reaction owing either to overloading or incompatibility between solubilizates and environment. Secondary reactions with the components of the environment or changes in temperature which may reduce the microheterogeneous area in the pseudoternary diagram are additional constraints for the use of microemulsions that may cause severe turbidity of the reaction mixture, aggregation, or precipitation of its components. Technical development of such preparative procedures appears in general to be rather difficult, and, as in the case of liquid phase homogeneous reaction mixtures, the use of immersion-type photochemical reactors cannot be recommended.

6. Reaction Mechanism and Reactor Design. The impact of reaction mechanism and rates of competitive steps of reactions have already been mentioned in order to explain the phenomenon of filming (polymerization of secondary products at the irradiated reactor surface). Even without filming, photochemical reactions might come to a halt, if secondary reactions counteract with high efficiency. The fact that the first step of the propagation in photobrominations with Br, is reversible (Eq. 15 [2,3]) has for a long time

246

A. M. BRAUN, L. JAKOB, E. OLIVEROS A N D C. A. OLLER DO NASCIMENTO

been neglected, treating corresponding photochlorinations and photobrominations without differentiation. Br,

Br' HR'

hv

+ RH,+===HBr + Br,

2Br'

+ HR'

+ HRBr

+ Br'

(16)

In batch procedures, liquid phase photobrominations halt in general at conversions between 50 and 70%, the concentration of HBr reaching a level where the back reaction (Eq. -15) becomes more efficient than the productforming step (Eq. 16). Particular reaction conditions and, hence, particular reactor geometries and/or accessories are then required to remove continuously the HBr produced. Photobrominations are preferentially developed at boiling point temperatures, as has been beautifully demonstrated in the industrial synthesis of 1-bromo-diethylcarbonate (Eq. 17) [30]. Br,

-

hv + CH,-CH2-OCOO-CH,-CH, CHBr + CH,-CHBr-OCOO-CH,-CH,

(17)

Photochemical reactors designed for this purpose may either be linked to (flash) distillation columns or be part of the distillation column itself, the latter being an advantageous solution for continuous production units. Bromination of allylic positions cannot be achieved specifically by using elementary bromine, unless electrophilic addition to the z-bond (Eq. 18) is unfavorable because the substituents have a high negative inductive effect. Efficiency of electrophilic addition of Br, may also be diminished by steric effects. In addition to these secondary thermal reactions, the specificity of photochemical brominations of allylic positions using Br, will also suffer from the competing (radical) addition of Br' to the double bond (Eq. 19) [31].

Br'

+

Rl

qR3 -R')d,R3 Br,

Ri

*R3

Br

Br

+

Br'

247


Rather specific thermal or photochemical bromination of allylic positions is, however, possible by using N-bromosuccinimide as brominating agent. Both procedures produce, however, variable quantities of Br, depending on reaction conditions [32]. In thermal procedures, the concentration of intermediate Br2, and, hence, the importance of secondary addition products, can be controlled by the relative quantity of radical initiator (e.g., AlBN) and by reaction temperature. The appearance of addition products in a photochemical procedure would be evidence for the mechanism proposed by Adam et al. [2, 3, 331 which includes the intermediate production of Br, (Eq. 21).

Br,

+

+

Br'

In the photochemical procedure, addition product can be minimized in keeping the relative NBS concentration as small as possible. In addition, substrate concentrations should be optimized with regard to the exitance of the chosen light source to avoid secondary recombination reactions. Under these conditions 4-bromomethyl-5-methyl-1,3-dioxol-2-one can be prepared with only minor impurities (bromine addition and multiple allylic bromination reactions (Eq.23)) [34].

Br2

+

)=( OKo 0

B

hv

r

w

OKo 0

WBr B

+

OKo 0

+

r

x

B

r

O K 0 0

(23)

248


7. Reaction System and Safety Requirements. For most photochemical reactions, both chemical yield and quantum yield of product formation depend primarily on substrate concentration, reaction environment, temperature, mixing, and specific excitation. In addition, the rate of the photochemical reaction is strongly affected by the incident photon rate and the absorption cross section of the reaction mixture. In designing and up-scaling photochemical reactors, combinations of some of those parameters have to be taken into account to comply with safety requirements. Leaving aside those safety rules that concern light sources and their operation, decisions on reactor geometry and accessories to be mounted are also affected by experimental and calculated results concerning, for example, exothermicity as a function of incident photon rate and substrate concentration, reactive gas consumption and pressure regulation, reaction volume, temperature, and inert gas saturation. As an example of safety requirements linked to the reaction system, the problem of oxygen present in photohalogenation may be mentioned. Except in the very few cases of applied photochemical oxychlorination [2,3], the presence of oxygen should be eliminated in photohalogenations. The presence of oxygen may be difficult to avoid in photochlorination units where production rate is primarily controlled by chlorine feed and consequently operating conditions may include periods during which less pressure is created in parts of the feeding installation. In fact, organic radicals generated by hydrogen abstraction (Eqs. 2 or 15) react efficiently with oxygen (Eq. 9) to form peroxyl radicals and subsequent products. This situation is exploited for the synthesis of trichloroacetyl chloride from 1,1,1,2-tetrachloro-ethane [Eqs. 2, 9 (R = CC1,-CC1) and 241,

2 CC13-CHC100'

+ C1,

2 CCl3-COCl

+ 2 HCl + 0

2

(24)

from pentachloro-ethane [35], as well as from tetrachloro-ethylene [36]. The overall reaction scheme of the synthesis of trifluoro acetylchloride from 1,ldichloro-2,2,2-trifluoro-ethane [37] includes a similar sequence of reactions as postulated in Ref. 35. All procedures produce or may produce phosgene as a secondary product either by radical fragmentation of the peroxyl radical CCl,-CHC100' (Eq. (25), CC13-CHC100'

+ COCl,

+ HCl + CO + C1'

by reaction of carbon monoxide with chlorine (Eq. 26),

co + c1,

+ coc1,


by fragmentation of CCl,-CCl,OO'

cc1,-cc1,00'

-

249

(Eq. (27), 2COC1,

+ C1'

(27)

or light-induced fragmentation of the same intermediate radical (Eq. 28).

CC~,-CC~,OO*

4CC~,O'+ COC~,

The last reaction has also been postulated in the course of the trifluoro acetylchloride synthesis [38] and is of general importance, as there is no qualitative restriction of this rearrangement to a-trihalogenomethyl groups. Trichloromethyl-chloroformate has been proposed as a relatively safe phosgene source for small-scale diisocyanate and polyurethane production units. Phosgene is produced upon fragmentation of this compound on a hot iron contact. ClCOOCH,

+ 3c1, 4ClCOOCCl, + 3HC1

(29)

Trichloromethyl-chloroformate is produced by exhaustive photochlorination of methyl-chloroformate (Eq. 29) [39], and, although so far never observed, phosgene could be an additional hazard in case of an equipment breakdown. In up-scaling the preparative procedure, care has to be taken that A compact production unit (Figure 6) can be installed in an area isolated from general ventilation system. This area is equipped with a fire-extinguishing installation (CO,). Cooling water circuits can be controlled from an outside stand. The lamp (Figure 6: 16) is mounted in a double jacket which would withstand mechanical strain due to explosion of the light source (e.g., upon rupture of the inner tube of protection). Addition of gaseous chlorine involves a drying device (Figure 6: 1) and buffer volume (Figure 6: 2) and can be controlled from an outside stand. Exiting gaseous HCl is checked for unreacted C1, (Figure 6: 5 ) before neutralization [pressure variations between neutralization tank (Figure 6: 6) and active carbon filter (Figure 6: 9) are balanced in an intermediate buffer system (Figure 6: 8)]. The operator has at his disposal a personal gas mask (active carbon filter) as well as an operator's mask (compressed air) to be used in case of product spills.

Cl,

!i

cIcooccI,

CICOOC%

electricity

H,O

20% NaOH

P

.

*

drain

*

waste

Figure 6. Block drawing of the pilot installation for the production of trichloromethyl chloroformate by exhaustive photochlorination [39]; 1: Dryer for gaseous C1, (H,SO, conc.). 2 Safety tank. 3: Thermoregulated immersion-type photochemical reactor. 4: Raschig column. 5: C1, detection system (1,2,4-trichlorobenzene).6: Neutralization tank (20% NaOH). 7: Reservoir of 20% NaOH. 8: Buffer to atmospheric pressure (20% NaOH). 9: Active carbon filter. 10: Reservoir of crude trichloromethyl chloroformate. 11: Buffer to normal atmosphere via CaCl, filter and direct entry for trichloromethyl chloroformate to be distilled. 12: Distillation flask with Vigreux column. 13: Exit to vacuum pump. 14: Solid NaOH filter before pump. 15: Cooling water alarm linked to power supply of the light source. 16: Medium pressure mercury arc. 17: Heater for distillation apparatus. 18: Magnetic stirrers. I,: thermometer; I,: manometer. 250


251

More recent developments favor bis(trichloromethy1)-carbonate, which is produced by exhaustive photochlorination of dimethyl carbonate. Singlet oxygen reactions with unsaturated hydrocarbons are rather specific and lead to endoperoxides (by 2 + 4 cycloaddition from 1,Cdienes) or hydroperoxides (by ene reaction from acyclic allylic systems) [2,3]. Whereas for ene reactions protic solvents may be used, such a reaction medium could lead in the case of endoperoxides to substitution reactions and thus direct the synthesis to partially oxidized products rather than cis-diols (Eq. 30). The hazard of explosion of intermediate hydroperoxides is usually avoided by in situ reduction by Na,SO,, NaHSO,, or reducing solvents like methanol [2,3]. In apolar and aprotic solvents, thiourea may be used for the same purpose, but reduction may be less efficient, and peroxide intermediates may accumulate. Under these conditions, the reaction should be carried out at low temperature with a workup procedure including a progressive temperature increase, after the reducing agent has been added.

A

OOH

OH

RO

0

B. Light Sources Besides the system parameters mentioned above, emission characteristics and geometry of the light source are decisive for the design of a photochemical

252

A. M. BRAUN, L. JAKOB, E. OLIVEROS A N D C. A. OLLER D O NASCIMENTO

reactor. In fact, light sources with defined emission characteristics exist either as point sources or as extended light sources. For instance,

0

Low-pressure mercury and sodium arcs, as well as fluorescent tubes [2,3] of technical importance are necessarily extended light sources because their mode of operation is incompatible with compression of the source to a point without alteration of their emission spectra. Technical limits of exitance also impose minima for the dimensions of medium-pressure light sources (length of arc, length and diameter of bulb), for example, medium-pressure (doped) mercury arcs, highpressure sodium vapor lamps, and incandescent lamps.

If the size of the production unit requires higher radiant power than can be provided, for technical reasons, by one lamp, clusters of light sources may be installed, which, consequently will alter the diameter or the height of the inner core of, for example, an annular photochemical reactor. However, following the check list of concepts (vide supra), optimal reaction conditions will in most cases limit the size of the photochemical reactor, and the planned rate of production may require several reactor units installed in a parallel mode (batch process) or in series (continuous process). Basically, the light source is chosen for its emission characteristics in order to achieve optimal selectivity of excitation. Spectral selectivity may be enhanced by using special glasses for the lamp well. On the other hand, aqueous solutions of transition metal salts [40] fulfill reasonably well their double task as IR and selective UV-VIS filters. They operate on a closed circuit including a heat exchanger; these installations are normally less expensive than glass filters in investment and maintenance. This is particularly true with respect to colored glasses, the light stability of which is in most cases not satisfactory. In a second step, several options on light sources must be evaluated with respect to their impact on production rate and experimental conditions imposed by the reaction system. Many technical applications require high radiant power, which cannot be furnished by an operationally reasonable number of lamps having otherwise optimal emission characteristics. Examples of this situation are mostly found in applications of.the 254-nm line, where a number of low-pressure mercury lamps may be replaced by one medium-pressure mercury arc. This substitution represents a compromise where spectral selectivity and energy wasting (VIS and IR radiation) is traded against a compact production unit which is less expensive (number of reactors, quartz, safety requirements) and easier to operate (number of reactors, space, and overview). Absorption conditions of the reaction system or spectral selectivity of


253

excitation imposed by the reaction mechanism may call for spatial separation of consecutive steps of chemical transformation. The photochlorination of a substrate absorbing in the same spectral region as the dissolved elemental chlorine is a typical example. Excitation of C1, (Eq. 1) under these conditions is far from being efficient, since the substrate to be chlorinated is acting as an inner filter. On the other hand, direct excitation of the substrate should be avoided as it leads to secondary products, both upon direct excitation and subsequent to chlorination. This rather difficult task in photochemical engineering may be solved with a reactor configuration as shown in Figure 7 Gas

I

Gas introduction tube

Cooling coil

Central tube

Reactant liquid (or solution)

4

t Light beam

Figure 7. Falling film tubular photochemical reactor, for reactions requiring excitation of a reactive gas [2,3].

254


[2], where a narrow diameter light beam traverses the reactor from bottom to top and excites gaseous Cl,, which is introduced at the top of the reactor in the region of the central tube. The generated chlorine atoms propagate in turn a chain reaction at the gas-liquid interface of the falling film (liquid substrate, substrate solution) which is produced by an overflow at the top of the central tube. Another means of selectively exciting a gas phase initiator (e.g., chlorine) could be conceived in developing the idea of a partitioned parallel plate reactor [41]. The reactor volume is divided into two separate compartments by a grid (e.g., of Teflon or titanium), and the reactant gas is introduced into the irradiation compartment (A, Figure 8) at a pressure sufficient to prevent the liquid contained in compartment B from leaking across the grid. Several authors gave, in fact, evidence that chlorination in the liquid phase (B, Figure 8) takes place after chlorine atoms are generated in A and pass through the grid separating the different reactor compartments 142-441. The grid may be designed in such a way as to prevent electronic excitation in compartment B. In addition to specific excitation, this design may also be of potential use for the differentiation between radicals of different reactivity. Lucas has proposed an annular reactor with concentric irradiation and reaction zones for an improvement of the photochemical synthesis of cyclohexanone oxime, caprolactam precursor, (Eqs. 31-33) [45]. The design would prevent filming [by radical polymerization of unsaturated hydrocarbons (Eq. 4)] and reduce chlorinated secondary products (Eq. 3), as the

Gas

Products

I

I

Gas

Reactant liquid (or solution)

faces and two compartments [2,3,41]. A : Region of gas excitation. B : Region of radial reaction with a liquid (or gaseous) substrate.


255

probability of recombination of chlorine atoms (Eq. 34) in the reaction compartment would be considerably reduced.

However, radical generation and transfer efficiency and/or chain length of the radical reaction must be sufficiently high to ensure reaction rates of technical importance. Radical transfer efficiency is highly dependent on the thickness of compartment A, irradiance, and flux of added reactant gases. By varying the irradiance in an experimental design, keeping the geometry of the reactor module and working conditions constant, Tournier et al. obtained similar results for the photochlorination of toluene as in chlorine-saturated liquid phase conditions [43]. On the other hand, Lucas [46] and Richard and Lenzi [44] reported increasing reaction rates in maintaining irradiance but increasing the flux of the reactant gases by decreasing the thickness of the initiation compartment. A general verification of Lucas' theoretical model [46] seems timely, even if there is no immediate industrial potential, given the generally low specificity of radical reactions and the technical obstacles at the present state of experience. 1. Point Sources. Point sources are only applied in preparative photochemistry, when excitation is required in a restricted area (particular angular distribution of light intensity due to the shape of the light source or the optics applied [2,3]). This concerns then in most cases the generation of light beams which may be used for gas phase reactions in reactor geometries providing long optical paths (Figure 2). Usually, excitation in a narrow spectral (band pass or interference filters) or spatial domain (Figure 7) also calls for a beam geometry. Narrow band emission light sources may also be built using the absorption and emission characteristics of fluorescent dyes. Actinometry of polychromatic light using fluorescent substances makes use of a photophysical process (fluorescence) whose quantum yield is in most cases independent of the wavelength of the incident radiation within the spectral domain examined [2,3]. A common fluorescent substance often used is Rhodamine B, and Amrein et al. have constructed an integrating actinometer using a combination of this fluorescent dye and a photodiode as a detector system [47]. Since the incident photons of different wavelengths are completely absorbed by the fluorescent substance, a narrow-band fluorescing dye can in

256

A. M. BRAUN, L. JAKOB, E. OLIVEROS A N D C. A. OLLER DO NASCIMENTO

fact be used for the construction of a fairly monochromatic light source. Such a set-up has been devised by AndrC and Viriot [48] using a mercury arc immersed into a solution of a fluorescent substance. Under conditions of total reflection of the emitted light at the outer wall of an annular reactor, a relatively high photon rate can be measured at the surface of an optical window, to which the emitted light is directed (Figure 9). The efficiency of light harvesting by this “optical fiber effect” is approximately 100% [49]. However, limited photostability of the fluorescent dye, temperature effects on fluorescence emission, and the already mentioned disadvantages of external light sources and attached parallel plate photochemical reactors will most probably restrict the use of such “fluorescent relays” to laboratory investigations. Recent developments make use of laser excitation where, besides excellent monochromacy, high irradiance may be used for changing the course of a photochemical reaction [SO]. Extremely high irradiances would cause, however, rapid filming of liquid reaction systems in tubular reactors (e.g., Figure 2), and preparative application became only possible after spatial separation between irradiated reactor surface and reaction system had been achieved [Sl]. Filming may also be avoided by using pulsed light sources, as enough time between pulses may be allowed in order to change the irradiated reaction mixture in a highly turbulent system (e.g., a jet-injection photochemical reactor [2]). 2. Extended Light Sources. As mentioned above, extended light sources emit, with the exception of low-pressure mercury and sodium arcs, polychromatic light, which can be restricted to some spectral areas by liquid filter solutions or special glass tubes. It is, for instance, advantageous to use a potassium chromate filter in combination with a medium pressure mercury arc for dye-sensitized oxidations, the products of which absorb at wavelengths longer than 320nm. Optimal results are usually obtained, if a highpressure sodium vapor lamp is used, however up-scaling of the latter fluorescent emission

I

optical wlndow

I

photochemical reactor ercury low pressure arc fluorescent dye solution

Figure 9. “Fluorescent relay” photochemical reactor [49].


257

procedure is very difficult, as sodium lamps of high electrical power are not available commercially and therefore large multilamp reactors have to be built. The use of doped medium-pressure mercury arcs [2,3] is another means of concentrating the emitted light to a more or less restricted spectral area. Doped mercury arcs with up to tens of kilowatts of electrical power are now available. Extended light sources may be installed around a tubular reactor or in the axis of an annular irradiated reaction volume. In the first case, an annular (or coaxial) radiation field focalized on the axis of the tubular reactor is created (Figure lo), and, in reaction mixtures of very low absorbance, irradiance as a function of the radius of the cylindrical reactor shows highest values in the axis of the reactor (positiue geometry of irradiation, Figure 11 [2,3]). Calculated values approach those obtained by actinometry, if an empirical relation taking into account a fraction of the reflected light is used (Eq. 35): E(r) = r R M[lO-EC('R-r) + 0.5 x 10-&C(rR+*)] r

(35)

where E(r) = irradiance in the interior of a cylindrical reactor at a distance r from its axis r R = radius of a cylindrical reactor M = radiant exitance of the light source used, assuming that the irradiance at the reactor surface is equivalent to the exitance of the lamp in a given sector E = molar absorption coefficient c = concentration of the dissolved substrate

(4

(b)

Figure 10. Cross section of a cylindrical reactor with a coaxial radiation field: (a)cross section perpendicular to the axis of the reactor; (b)cross section along the axis of the reactor (rR = reactor radius) [2, 31.

258


Reactor axis

Figure 11. Relative irradiance along the diameter of a cylindrical photochemical reactor surrounded with many tubular light sources; transmittance ( T )of the reaction mixture through 1 cm: (1) T = 0.7; ( 2 ) T = 0.65 [ 2 , 3, 521.

Cylindrical photochemical reactors placed in coaxial radiation fields are often used for carrying out photochlorination reactions, but their most important application remains in the area of water sterilization [52-541. We use the same reactor geometry for photochlorinations because, given the high quantum yields of these chain reactions, irradiance must not be optimized to achieve good productivity. These reactors may then be equipped with fluorescent tubes for which no external cooling and no water filters are needed. Since a positive irradiation geometry cannot be successfully exploited in applications where high substrate concentrations and absorbances are used, corresponding reactor geometries should be avoided, although investment costs might be lower than for more complicated reactor designs. In addition, operational difficulties might arise from the control and maintenance of lamp clusters, corresponding mirrors, and cooling and IR-filter installations (e.g., when medium-pressure mercury arcs are used). The extended light source may also be placed at the axis of a reactor composed of two coaxial cylindrical tubes (Figure 12). The emitted radiant power is absorbed by the reaction system contained in the annular reactor volume. Irradiance diminishes in a filled reactor with increasing radius (Eq. 36), this geometry is called the negative geometry of irradiation [2,3].


259

Linear light source

Exterior tube of reactor

I Interior tube of reactor

Figure 12. Cross section of an ideal annular photochemical reactor along the plane perpendicular to its axis (symbols, see Eq. 36) [2, 31.

where ER = irradiance at the outer reactor wall E, = irradiance at the inner reactor wall ri = reactor inner tube radius rR = reactor outer tube radius, 1, = r R - ri = thickness of the annular reactor

This geometry of irradiation makes the most efficient use of the light emitted by an extended light source. In fact, this geometry is used in all immersiontype photochemical reactors, and most industrial photochemical production units are based on this design.

3. Excimer Lamps. Recently, the group of extended light sources could be opened to the new class of excimer lamps. These new light sources are unique for their relatively good monochromacy of the emitted light. Even more important is that for the first time in photochemical engineering, the geometry of a light source can be adapted to optimal reaction (and reactor) conditions. Excimer formation in certain gases and vapor mixtures is known from modern UV lasers; typical examples are excited complexes between rare gas

260


atoms (excimers) [55] and between rare gas and halogen atoms (exciplexes) [56]. Unlike the excitation technique used in excimer lasers, dielectric-barrier discharge, typically applied in ozone generators, is used for excimer formation in these new light sources [57]. In a simple configuration, the lamp consists of a hermetically sealed annular quartz cell containing the gas and of electrodes located at the inner and outer wall, outside the cell and, hence, separated from the discharge plasma. The lamp may be used as any other extended light source, if the outer electrode is made of a wire mesh or a transparent conductive layer (Figure 13), allowing the emitted light to radiated into an annular reaction volume. In applying this configuration to problems of preparative photochemistry and large-scale photolysis, we discovered that direct contact between the outer electrode and the reaction mixture can be avoided by taking the latter as part of the dielectric [12]. The outer electrode may then be placed outside an annular reactor of limited thickness (Eq. 36) depending on the chemical properties of the reaction system (Figure 14) [12,58,59]. The simple configuration principle of the excimer lamp combined with the possibility of including the reaction system among the dielectrics yield unprecedented freedom in light source and reactor design. For instance, an ideally irradiated tubular reactor with positive irradiation geometry (Figure 15a) may be built with combined negative and positive irradiation geometries, where the reaction system is between two discharge layers (Figure 15b) [60]. Early experiments with planar electrodes (Figure 16a)lead to “car battery” assemblies of radiating walls, where the reaction mixture is circulating in between the lamps (Figure 16b) [60-621.

hV

Figure 13. Cross section of a cylindrical excimer lamp along the plane perpendicular to its axis; see also [57].

Quartz

Reaction mixture Figure 14. Cross section along the plane perpendicular to the axis of an annular reactor with a cylindrical excimer lamp mounted in its axis [12, 58, 591.

(4

' Reaction mixture

Quartz

'

Reaction mixture

(b)

Figure 15. Cross section along the plane perpendicular to the axis of a photochemical reactor using (a) an annular excimer lamp of coaxial (positive) irradiation geometry and (6) of a combination of cylindrical and annular excimer lamps (see also [60].

261

262


Reaction mixture

/ \

Figure 16. Cross section along the plane perpendicular to (a) the planar electrodes of a flat excimer lamp and (b) a corresponding multilamp assembly (see also [60-621.

The excimer light sources are driven with frequencies of about 50 kHz, and voltage amplitudes of 5-10 kV are required to ignite the discharge. Under those conditions, bursts of light pulses with a repetition frequency of about 100 kHz are produced which provide this light source, in addition to its largely variable geometry, with unique advantages-narrow spectral bandwidth of emission, short pulse duration, high peak intensity, and high repetition frequency. Table 1 shows the potential wavelengths of emission (see also [57]), of which some have already been used in projects of metal vapor deposition [63], photopolymerization [64], oxidative degradation of organic materials [l2, 58, 591, and preparative photochemistry [65]. The narrow bandwidth emission is shown for the Xea excimer lamp (172 f 12nm, Figure 17). The monochromacy of the light source is a great advantage in many preparative applications [2, 3, 661 and facilitates radiant power and irradiance measurements and calculations [2, 31 in up-scaling projects. Eximer light sources may require water cooling for optimal operation. Their life times have not yet been determined, but excimer lamps (see Table 1) have been operational for more than 3000 hours with radiant efficiencies of approximately 6%. 4. Safety Requirements. Besides the potential danger of radiation for eyes (UV/VIS) and skin (UV), high-pressure light sources require special pre-

TABLE 1 Potential Peak Wavelengths of Excimer Light Sources‘ Peak Wavelength (nm)

Gas

126 146 152 165 172 175 188 193 207 222 249 253 259 283 289 308 334 342 351 443 503 558

Art Kr T

FT

ArBrf Xer ArClT KrJT ArFT KrBrt KrClr KrFT XeJT Clt XeBrt Br; XeClT HgT

Jt

XeFz HgJT HgBrz HgC1;

References

12, 58, 59

59

63, 64 65

“See also Ref. 57.

1 ,o

Intensity [arbi!raw unils]

0,5

-

0100

‘

150

1

200

250

300

nm

Figure 17. Emission spectra of a Xe excimer lamp operated at 15OW of electrical power [12] (see also [57]). 263

264

A. M. BRAUN, L. JAKOB, E. OLIVEROS AND c. A. OLLER DO NASCIMENTO

caution against the risk of explosion. Lamps should be mounted in accord with the manufacturers instructions and mechanical strain as well as contact with cold surfaces must be avoided. Safety requirements also include protection against risks of electrocution, in particular in applications requiring cooling water (excimer lamps). Special care in designing photochemical production units at all levels must also be applied to avoid contact of explosive gas mixtures with the heated lamp.

111. CONCEPTS OF QUALITATIVE REACTOR DESIGN A. Reactor Design and Light Source As indicated earlier, positioning a light source to obtain a negative geometry of irradiation provides the most economical result in most cases of industrial preparative photochemistry and photolysis. Extended light sources of a given length and diameter, contained in a corresponding protection and filtering/cooling system, may be immersed into a reaction mixture of minimum volume required to absorb entirely the emitted light. However, as many negative experiences demonstrated over a period of about 50 years, this simple design cannot cope with the consequences of a strongly inhomogeneous distribution of activated species and is in general a complete failure. To maintain the negative geometry of irradiation, we must ensure filming prevention: 1. During photonitrosylation of cyclohexane (caprolactam synthesis, Eqs. 31-33 [2,3]), the formation of deposits on the cooling jacket of the medium-pressure or doped mercury arc could apparently be prevented by introducing a liquid-liquid phase barrier. Intermittent additions of concentrated sulfuric acid along the wall of the cooling jacket [67] create a strongly hydrophilic phase within the hydrocarbon bulk solution. This prevents penetration by relatively apolar intermediates and products from which precursors of the polymeric deposits originate. 2. Deposits at the lamp jacket can also be prevented in bubble photochemical reactors, where small rising gas bubbles of nearly uniform size and distribution create strong turbulences [ 2 , 3, 18, 681. This reactor is usually conceived as an annular photochemical reactor of relative small thickness (IR, Eq. 36). In principle, its use with reactive gas is rather limited, as the turbulence provoked by the bubbles must be maintained over the entire height of the reactor [69,70]. Bubble characteristics may also change as a function of the flux of the substrate solution.


265

3. The concept of spatial separation between light source and liquid reaction mixture is by its simplicity highly advantageous in technical development work. In general, it can be achieved by generating thin layers of substrate solution on a surface placed parallel to and at optimal distance from the surface of the lamp jacket. In early proposals, photochemical reactors comprised moving parts, such as rotating reactor walls on the inner surface on which the liquid to be irradiated could be spread [2,3]. None of these designs found technical application because of severe deficiencies due to material strain, leaks of liquids and gases, and difficulties in placing such reactors into large batch or continuously operating production units.

Nevertheless, subsequent reactor designs reflect clearly some of the ideas pioneered in earlier constructions. The falling film photochemical reactor first published by Tarkoy and Campana (Figure 18) [2, 3, 711 may be seen as an important step in the technical development of the rotating reactor originally designed by the photochemistry unit of the Max Planck Institut at Mulheim [72]. The falling film is generated in the later design by dispersion of the liquid reaction mixture at the top of an inverted immersion-type photochemical reactor and covers the entire inside surface of the outer reactor wall. A modified version of this design is one of our basic reactor modules [2,3]. It is mainly chosen for the development of photochemical processes in which reaction mixtures of high-absorption cross sections are used. The design is also advantageous in cases where pronounced photoreactivity of compounds produced in a single pass might generate secondary products and, consequently, diminish chemical yield. The short residence time limits such secondary reactions, and very low ratios of reactor to reservoir volumes (vide infra) ensure almost instantaneous high dilution of such photoreactive compounds at the elevated flow rates required. For processes requiring medium- or high-pressure arcs, the bottom-up mounting of the light source is leading to up-scaling limitations due to problems of heat dissipation. Other means of cooling or an improved design of the lamp jacket would be needed if technical development were to require reactor units of more than 5 kW of electrical power. Another module adapted to photochemical processes working inversely at low absorption cross sections and at rather limited flow rates is pictured in Figure 19 [2,3]. Again, this design can be seen as the subsequent development of an earlier design by de Meijere et al. [73,74] which, because of its complicated construction, became inflexible and in most cases inefficient. The later design works at higher ratios of reactor to reservoir volumes (vide infra), the thickness of the annular reactor volume depending on the absorption cross section of the substrate solution. This has been achieved by substituting

266

A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO Liquid distributor

Light source

Falling film

Coolant

Reaction mixture

i

1

Reaction mixture

Cleaning solvent for cooling jacket

Figure 18. Annular falling film photochemical reactor [2, 3, 711.

the original double wall with a more efficient serpentine thermoregulating system. In addition, the new design exhibits an exchangeable inner reactor tube and may, hence, accommodate light sources of different diameter. Most important is the entry point for the substrate solution, which is located at the bottom of the reactor, thus ensuring a better mixing of the reaction system. A falling film is created as the reaction mixture is flowing over the rim of the inner reactor tube, its thickness being limited by the spatial separation between this tube and the lamp jacket. This design is complementary to the one shown above (Figure 18) and has found successful application in

267

UP-SCALING PHOTOCHEMICAL REACTIONS Inert gas (around lamp) /

d i gaseous reactant

Liquid level

Falling film -

Coolant

I

Reaction mixture

Reaction mixture

Figure 19. Annular falling film photochemical reactor [2,

31.

photoinitiated reactions (photohalogenations) and sensitized dimerizations and oxidations. In general, these are processes where low absorption cross sections are the rule, and secondary photochemical reactions are practically nonexistent. So far, no technical limits in up-scaling this reactor module have been encountered. In summary, spatial separation of reaction mixtures from the lamp jacket is one of the most efficient means of preventing filming (see also earlier comments on laser photolysis). In addition, as shown in Figure 7, the same principles of design may be used for solving problems created by inner filters. Experience with falling film photochemical reactors lead us to the

268


application of the concept of convergence of light distribution and reactor geometries. Traditional reactor design placed the entire reaction volume of a batch operation into the photochemical reactor, although absorbance calculations could show that in most cases only an extremely small volume fraction would be irradiated (vide supra). In contrast, modern reactor design aims at reactor geometries adapted to the size of the light source and would limit their size in accord with the spatial distribution of the emitted light which might be either measured or calculated for a given reaction system. This concept allows for: Batch processes using a number of photochemical reactors of optimal size in a parallel arrangement with a central unit in which optimal reaction conditions (temperature, gas saturation, mixing, etc.) can be maintained and classical operations of product separation can be performed (Figure 20). Such a production unit also permits maintenance of photochemical reactors without interruption or strong disturbance of an ongoing production or photolysis process. Continuous processes if photochemical (and thermal) reactors are installed in series. Such an arrangement could be useful in the production of previtamin D (PD), where tachysterol (T) produced as a secondary product (Figure 21) may be recovered by a subsequent triplet sensitized cis-trans isomerization [2, 3, 75-77].

B. Light Distribution The spatial distribution of the emitted light within a certain reactor geometry may be directly determined by radiometric measurements. For this purpose,

Figure 20. Batch process design using several photochemical reactors (hv) in a parallel arrangement linked to a central reservoir (R) [18].

269


T

Figure 21. Principal photochemical reactions of the previtamin D synthesis (for a complete scheme of photochemical and thermal isomerizations, see [2, 31).

a light-sensitive probe is placed at different locations within the reactor, its position being always defined by x, y, and z axes, the origin of which may be placed at the center of the light source. Probes may be directive or consist of an integrating sphere (Figure 22). The first are mainly used to determine

(4

(b)

Figure 22. Directed (a) and spatially integrating (b) probes for the measurement of irradiances at any point within a photochemical reactor [Z, 3, 6, 78-80].

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A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A OLLER DO NASCIMENTO

irradiances (of point sources) in the function of distance and attenuating or amplifying effects by transparent materials and mirrors. For such tasks, optical fibers with defined apertures or small bolometers placed behind a focusing cone (Figure 22a) have been used [2,3]. As reflection and refraction of light must be taken into account in a photochemical reactor irrespective of its geometry, the incident photon rate at a given point in the reactor originates from any direction. It is thus best measured by an integrating sphere attached to an optical fiber, which in turn is connected to an appropriate diode, bolometer, or photomultiplier (Figure 22b) [2, 3, 6, 78SO]. The irradiance measured by such an integrating device is related to the radiance of a light source by

where Ee,sph= irradiance measured by a spherical probe Le = radiance of the light source R = solid angle By definition, for a given wavelength, the photon (Gp,J and radiometric (GeJ spectral quantities are related by Eq. (38),

Since in industrial photochemistry mostly polychromatic light sources are used, photon quantities are relatively difficult to calculate and require knowledge of the spectral distribution of the radiometric quantity measured. Assuming on the other hand that the radiometric measurements do not need to be corrected for the spectral response of the probe, the photon irradiance at a given point within the reactor volume would then be given by Eqs. (39) and (40), respectively.


271

where ,Il, 2, are the spectral limits of (specific) excitation.

Under conditions equivalent to spectrophotometric measurements and for a solution of a light-absorbing compound of known c Aand concentration, the thickness of the system traversed by light ( I ) is given by the Beer-Lambert law (Eq. 41):

where

T = internal transmittance (or transmission factor) P = measured (transmitted) radiant flux Po = incident radiant flux cA = molar absorption coefficient of the light-absorbing substance at wavelength 2 1 = thickness of the solution traversed by light c = concentration of the light-absorbing substance In taking into account the concept of convergence of light distribution and reactor geometries, the thickness 1, (Eq. 36) of an irradiated solution in a negative irradiation geometry should not exceed considerably the limit defining total absorption. However, under conditions where irradiation of a given unit of volume is arriving from many directions, the outer diameter of a photochemical reactor is difficult to calculate and empirical measurements are by far more efficient [2, 3, 6, 78-80]. Tournier et al. [78] measured the irradiance with a spherical probe within an immersion-type photochemical reactor at different distances from the surface of the light source and attempted to correlate the results to different emission models of extended light sources (vide infra). In fact, none of the chosen models would fit the experimental results, the best fits being obtained by introducing a correction factor into an equation similar to Eq. (36):

272


where k is a correction factor, which is found to be 1.14 under the experimental conditions described in [78].

IV. QUALITATIVE UP-SCALING RULES AND PARAMETERS

A. Rate of Production The principal aim of an up-scaling project is to increase production volume, and thus the production rate, of a given preparative procedure without major changes of previously determined reaction conditions. In our qualitative approach to industrial photochemistry, optimal conditions would have clearly been determined on a laboratory scale by using a specific reactor module. Following the qualitative concepts of photochemical reactor design, the volume of the photochemical reactor is reduced as much as possible, ideally to the limits of the irradiated reaction volume, which itself depends on the geometry of the light source, the spatial distribution of the emitted light, and the geometry of the chosen reactor module. It should satisfy the condition of total absorption of the emitted light at the wavelengths of (specific)excitation. For a given unit (light source and reactor module), the irradiated reaction volume is best determined experimentally (vide supra). Evaluations of model calculations have also been published for simple reactor geometries (spectroscopic cell and immersion-type geometries) and where changes of reaction parameters in the course of the ongoing transformation need not be taken into account [78]. The concept of convergence of light distribution and reactor geometries would facilitate such calculations, as mixing between irradiated and nonirradiated reaction volumes within the photochemical reactor does not practically enter into consideration. The rate of a photochemical reaction can be defined as

where S = substrate (I-s = quantum yield of substrate consumption Pa = absorbed photon rate = Po(l- 1 0 - A ~ ) Po = incident photon rate A , = substrate absorbance

(44)


273

and it was shown above that calculation of Pa in accord with the BeerLambert law (Eq. 44) can only serve as an estimate, since the incident photon irradiance arriving at a given point of the irradiated reaction volume cannot be calculated precisely [78]. Up-scaling of the light source and, hence, of a given reactor geometry is therefore easier if the radiant power but not the exitance of the (same kind of) light source is varied. Under otherwise unchanged conditions, the irradiance and consequently the rate of reaction per unit of irradiated reaction volume would remain unchanged. In the case of batch processes, where multiple passes of the reaction mixture are needed to reach the required percentage of transformation, the rate of production is then increased by enlarging the irradiated reaction volume, and the up-scaling factor corresponds to the factor by which the radiant power of the light source at the wavelengths of (specific) excitation has been increased. Variations may show toward the end of a given photochemical reaction, where the rate of photochemical transformation slows down owing to lower absorbance (Eq. 44). Within the technical limits implied (limited irradiated reaction volume) and given the required rates of production at an industrial level, production units usually comprise a number of photochemical reactors mounted in parallel (Figure 20). If the number of photochemical reactors is reduced (e.g., for technical or economical reasons), a lower rate of production results. During an up-scaling procedure, it might become necessary for technical reasons (availability of light sources) to vary the exitance of the lamp. Since the rate of production is linearily related to the absorbed photon rate (Eq. 43), the up-scaling factor remains predictable within the conditions given above. However, if the exitance is increased, higher rates of photochemical reaction per unit of irradiated reaction volume may result in an accumulation of intermediates or products which may lead by subsequent photochemical or thermal reactions to disproportionate increase in the yield of secondary products. In this case, the geometry of the reactor would have to be changed. For continuous processes, all substrate molecules entering the irradiated reaction volume should be transformed during the residence time. This residence time is defined as the time after which a reaction volume corresponding to the irradiated reaction volume has been completely replaced by fresh input material. With the exception of some chain reactions and applications of UV irradiation to sterilization purposes, radiant power is usually not high enough to complete the photochemical reaction in one pass. In any case, conditions of very high radiant power might imply considerable energy wasting, since the absorbed photon rate could decrease rapidly as the substrate molecules react during residence time. Continuous production units may be conceived advantageously as a number of photochemical reactors mounted in series, each adapted to a given range within the variation

274

A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C . A. OLLER DO NASCIMENTO

of reaction parameters that results from the progressing level of transformation. Under these conditions, best results have been achieved in sequential up-scaling, which as far as the photochemical transformation is concerned, does not present any additional problems to those already evoked for batch processes.

B. Sensitized Oxidations In sensitized or photocatalyzed reactions, conditions of total or constant absorbance can easily be controlled by the concentration of the sensitizer or photocatalyst added. In addition, experience has shown that the concept of spatial separation between the light source and the reaction mixture is in general not required. Dragoco uses immersion-type annular reactor geometries [2, 3, 69, 701 for the production of (-)-rose oxide by rose bengal sensitized oxidation of ( -)-citronello1 (Eqs. 45-48, Figure 23). RB 'RB

3RB

+ O2

4'RB __+

__+

(45)

3RB

(46)

RB

(47)

+ '0,

On the other hand, sensitized oxidation of highly concentrated sensitizersubstrate mixtures have been successfully developed in falling film reactors (for example, the synthesis of 2-hydroxy-5H-furanone (Eq. 48 [82, 831) [12]. hv QCHO

Sens. i 0, I H,O

w

HO

For the sensitized oxidation, the rate of production of the oxidized substrate, that is, the rate of the chemical reaction of singlet oxygen, depends on the efficiencies of singlet oxygen production and singlet oxygen reaction (Eqs. 49 and 50).

where AO, = product of singlet oxygen reaction DA= quantum yield of singlet oxygen production 4r= efficiency of singlet oxygen reaction


275

k, = rate constant of singlet oxygen reaction (Eq. 51) singlet oxygen acceptor (substrate) k, = rate constant of physical deactivation of singlet oxygen by A (Eq. 5% kd = rate constant of physical singlet oxygen deactivation by the solvent (Eq. 53) A

=

lo2+ A*

lo, + A 4A + 0 10,

hv

(51)

'402

40

I

2

2

(52)

(53)

Sens./O,

4 O H

Figure 23. Synthesis of (-)-rose oxide by sensitized oxidation of (-)-citronello1 [2, 3, 811.

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A. M. BRAUN, L. JAKOB, E.OLIVEROS AND c. A. OLLER DO NASCIMENTO

Therefore, technical means are required to keep oxygen concentration at its highest level [2,3,69]. Up-scaling limits are known for cases where irradiated reaction volumes have been practically depleted of oxygen by applying too high radiant powers, mostly in combination with very high concentrations of sensitizer. Work at high acceptor concentrations is advantageous (Eq. 50). The rate of reaction following pseudo-zero order kinetics for approximately 90% of transformation, deviation from optimal conditions during up-scaling is easily detectable. In most cases, work-up procedures are most economical at levels of transformation close to loo%, but long irradiation times might be necessary due to depletion of the substrate toward the end of the reaction. The efficiency of the chemical reaction of singlet oxygen depending primarily on k , (Eq. 53), the choice of a solvent exhibiting weak interaction with the excited oxygen species, is advantageous. However, most sensitized oxidation reactions terminate by a sequence of thermal reactions of the intermediate product A 0 2 in which the solvent may play a decisive role as far as the specificity and thus the chemical yield of the sensitized oxidation are concerned. The intermediate endoperoxide of furfuryl aldehyde (Eq. 48) reacts preferentially by homolysis of the 0-0 bond in apolar solvents, yielding a complex mixture of products due to subsequent radical reactions (Eq. 54).

In water, however, the endoperoxide is exclusively transformed by S,2 reaction with the solvent into the corresponding ring-opened hydroperoxide (Eq. 5 9 , which then undergoes intramolecular fragmentation to the desired product [12]. Limitations in the choice of the solvent may in turn restrict the choice of the sensitizer for reasons of solubility and photochemical (and thermal) stability. As a result, experimental conditions for an optimal chemical yield may call for a sensitizer exhibiting a relatively small Q,. As indicated above, considerable efforts in research and development have been deployed into the development of insoluble singlet oxygen sensitizers. Although, rose bengal chemically bound to chloromethylated copolymer of styrene and divinylbenzene [14,841 (and commercialized as “Sensitox”) has shown excellent results as far as its light stability in solid matrices is concerned [ 8 5 ] , the sensitizer is suffering, in addition to the drawbacks of the


+

H20

271

+ HCOOH

chromophore (e.g., pH dependence of the absorption spectrum and reactivity in acid solutions), from a relatively labile chemical fixation (solvolysis) and a severe loss of efficiency due to the solvent-dependent large particle diameter [16,17]. The technical and economical advantages of a workup procedure involving a heterogeneous reaction system are nevertheless very substantial, and more work in this area might be triggered with research on spectral sensitization in photocatalysis.

C. Photocatalyzed Reactions Applications of photocatalyzed reactions are actually focused on the oxidative degradation of organic material dissolved or suspended in aqueous systems [86]. For practical reasons, TiO, is taken most frequently as a photocatalyst, its absorption spectrum showing a strong band gap edge at about 360nm with an exponential tail dropping to zero at about 400nm. These absorption characteristics might prove extremely advantageous for the chemical treatment of industrial waste water, where other photochemical degradation processes (e.g., H20,/UV or 03/UV) are severely limited by the UV absorption characteristics of the substrates to be degraded. In addition, TiO,, absorbing at the onset of the sea level solar spectrum, industrial waste water treatment, or chemical treatment of polluted surface waters driven by solar light, has initiated some interest in research and development [S7]. Research on spectral sensitization (sensitizer-semiconductor combinations) [SS] and catalyzed photoelectrochemical reactions (semiconductor-catalyst

278


combinations) [89] has shown very promising results in the domain of photochemical solar energy conversion. In relation to degradation of organic material in aqueous systems, two mechanisms of photocatalyzed oxidation have found experimental confirmation so far. Promotion of an electron from the valence to the conduction band upon electronic excitation leads to charge separation (Eq. 56), the negative charge being readily transferred to, for example, oxygen dissolved in the aqueous system (Eq. 57). Depending on the pH of the system, superoxide anion and its protonated form (Eq. 58) rapidly dismutate to yield H 2 0 2(Eqs. 59 and 60), the decomposition of which is in turn catalyzed by TiO, surfaces (Eq. 61). hv

TiO, + TiO,(e-

+ h')

+ h') + 0, + TiO,(h+)+O;' O;'+H,O+ * HO; + H,O O;*+HO; + HO; + 0, HO; + H,O++ H,02 + H,O

TiO,(e-

HO;

+ H,O,

O;'+HO'+H,O

(56)

(57) (58) (59) (60) (61)

The valence band edge of TiO, exhibits in aqueous suspensions a potential of more than +2V vs. SCE and is capable of oxidizing hydroxide anions or water molecules adsorbed to the semiconductors surface (Eq. 62 and 63) producing hydroxyl radicals [90]. Hydroxyl radicals, when desorbed from the surface, may react with organic substrate [91] by hydrogen abstraction (Eq. 64) or by electron transfer (Eq. 65), thus initiating oxidative degradation which leads to CO, (mineralization). TiO,(h')

+ HO-

+ H,O HO' + RH,

TiO,(h+)

HO'

+ RH,

+ HO' + TiO, + H O ' + H 3 0 + H,O + H,O + RH' HO- + RH: --+ TiO,

(62)

-

(63) (64) (65)

A second mechanism proposes electron transfer between the photogenerated hole (h') and an adsorbed organic substrate (Eq. 66) [92].

TiO,(h+)

+ RH,

--+

TiO,

+ RH:

(66)


279

Because of the highly positive valence potential of TiO,(h+), electron transfer from some of the halogenated (chlorinated) pollutants (e.g., tetrachloroethylene) might be possible, thus initiating the degradation of compounds which cannot be oxidized by hydrogen abstraction. However, under the realistic conditions of waste or surface water treatment, reaction 66 seems of lesser importance. This is due to the high dilution of the pollutants and their usually rather weak adsorption to the semiconductor surface with respect to water or hydroxide ions. Photocatalyzed degradation may therefore be classified among those mineralization procedures in which oxidation is initiated by hydrogen abstraction and/or electron transfer reactions by hydroxyl radicals generated as intermediates [93]. As stated earlier, the concept of spatial separation between light source and reaction mixture does, in general, not apply to photocatalyzed processes. However, similar to the results obtained from a series of experiments in the domain of sensitized oxidations using insoluble sensitizers, particle size and mobility are among the decisive parameters as far as quantum efficiency of TiOz photocatalyzed oxidative degradation is concerned. Degussa P-25 TiO, is a powder of relatively small particle size distribution (diameter: 50 I 20nm). This particle size offers optimal conditions for electronic excitation in heterogeneous (solid-liquid) phase [17], but seems counterproductive in up-scaling procedures owing to technical difficulties in separating and recycling the photocatalyst. On the other hand, reactors utilizing fixed photocatalysts [19, 94-96] are, similar to those constructed for singlet oxygen generation, of distinctly lower efficiency or, when conceived as irradiated coated fibers [97], difficult to install even at a pilot level. Precipitation of the photocatalyst can be provoked, in particular in largescale degradation processes, by taking advantage of the increasing ionic strength of the irradiated system [l2]. In designing modules of mono- or multilamp immersion-type photochemical reactors, again the concept of convergence of light distribution and reactor geometries is followed, and knowledge of light penetration in a suspension of optimal photocatalyst concentration is therefore essential. Optimal thickness of annular irradiated reaction volume is best determined by a spherical probe under conditions where only absorption by the photocatalyst has to be taken into account [12, 78, 98, 991. The radiant power P = f ( r ) within the limits of ri and rR,respectively, has been simulated by the Monte Carlo method on the basis of

where coefficient K depends on the nature and the concentration of the suspended photocatalyst.

280


In contrast to the investigations made by Andre et al. [79, 98, 991, these evaluations should be made without regard to any absorption due to dissolved or suspended organic pollutants. However, absorption of the suspended photocatalyst should be taken into account, as described by Yokata et al. in the case of suspended sand particles [loo]. Efficient mineralization requires efficient trapping by oxygen of free organic radical intermediates, the resulting peroxyl radicals and (hydro)peroxides initiating themselves chains of thermal oxidation reactions. Consequently, optimal saturation of the reaction system with oxygen or air is a necessity, and on a pilot level, air or oxygen bubbles may be injected, which serve at the same time as a means of maintaining high concentrations of photocatalyst suspended [121. Model calculations of three-phase heterogeneous reaction systems have been published in the domain of photocatalyzed water decomposition [1011, and design studies for oxidative degradation reactions and fluidized bed conditions are known mainly from the collaborative effort between the groups of Yue and Rizzuti [e.g., 1021. As these investigations concern particular reactor geometries, a factor of K = 1.2 has been chosen for our module of a multilamp immersion-type reactor (Figure 24) in order to take into account additional reflection and transparence by gas bubbles of different sizes [l2]. The resulting 1, is large enough to provide an optimal absorption cross section under conditions of high levels of mineralization. Even though an increasing number of publications now deal with quantum efficiencies of photosensitized or photocatalyzed reactions in heterogeneous media, the determination of quantum yields under conditions where light is absorbed by solid surfaces remains, for mostly technical reasons, impossible. Good approximations of the absorbed radiant power are possible with, for example, equipment like that described by Iatridis et al. [102], but a rectangular photochemical reactor would certainly not be the best choice for further technical development. In addition, in the potential applications of the photocatalyzed oxidative degradation, one does not know precisely the composition of the mixture of organic compounds dissolved or suspended in water. In general, under those conditions, only the efficiency of total mineralization, that is, the production of CO,, has a real meaning [lZ, 93, 1031. It seems therefore reasonable to determine the degree of mineralization from the results of TOC (total organic carbon) or DOC (dissolved organic carbon) analyses performed on samples of the reaction mixture at different times of irradiation. In parallel, energy consumption of the light source(s) might be measured, permitting us to determine an “energy efficiency” of mineralization in ppmC.kWh-’ or mg C.L-’.kWh-’. Maintaining equal photocatalytic and thermal parameters, rates of mineralization of different types of organic compounds vary considerably [e.g., 1031. Conse-


air

281

n

Figure 24. Scheme of multilamp immersion-type photochemical installation for the photocatalyzed oxidative degradation of industrial waste water [121. A: Bypass circuit. B: Reactor circuit. 1: Gas-liquid mixture and injection. 2: Reservoir. 3: Pump (ceramics).4: Water pump. 5: Heating circuit. 6: Cooling circuit. hv: Medium pressure mercury lamps (Pyrex).T: Thermometers.

quently, the performances of different modules of photochemical reactors can only be compared if the same model compound or mixture of substrates is used. In addition, the ratio of irradiated to total reaction volumes varying between different experimental designs, such “energy efficiencies” of mineralization must be corrected for the total reaction volumes implied. The resulting volume-corrected efficiencies (mgC * kWh - ’) are in contrast to what was proposed earlier (ppmC.kWh-’.L-’ or mgC.kWh-’.L-2 c93, 1043). At present, only results of the mineralization of industrial waste water containing relative high concentrations (ca. 5000 ppm C) of mainly nitroxylenes [12] and of phenol and salicylic acid as model compounds at initial concentrations of approximately 100 ppmC [ 1041 are available. Volumecorrected “energy efficiencies” of 142 and 820 mgCekWh-’ for the waste water and the aqueous solution of phenol, respectively, would indicate that the tubular photochemical reactor with negative irradiation geometry used by Matthews and McEvoy is more efficient by a factor of 5.8 than the multilamp module shown in Figure 24. However, the two experiments cannot be compared, since the rate of TiO, photocatalyzed mineralization of phenol

282


is certainly higher than that of nitro aromatics, and Matthews et al. used a low-pressure mercury arc (Aexc:254 nm), reporting that excitation at higher photon energy is increasing overall mineralization efficiency by a factor of approximately 2 [1041. Interesting comparisons of reactor module designs must, hence, be postponed, until a set of standard model compounds and mixtures fits the interests of all research units working in this field. Up-scaling a photochemical reactor module for photocatalyzed oxidative degradation follows the rules indicated above for batch operations of photochemical and sensitized reactions, the volume-corrected “energy efficiency” providing the means to keep the decisive parameters in an optimal range (vide infra). However, a main concern for the technical development of any light-induced oxidative degradation technique, and of the Ti0,photocatalyzed oxidative degradation in particular, is the limited number of photons available (limited radiant power). This results in a rate of mineralization which is too small to cope with large volumes (> lo3m 3 . d - l ) of weakly contaminated surface waters as well as with restricted volumes (ca. 10, m3.d-’) of industrial waste water of high TOC. The process is, however, accelerated upon addition and electronic excitation of H,O, [SO, 1051, and research on combined thermal and photochemical catalysis shows quite interesting results [106].

V. MATHEMATICAL MODELLING OF PHOTOCHEMICAL REACTORS The mathematical modelling of chemical reactors is based on the coupled heat and mass system balances and on the kinetics of the chemical reaction [7]. Its progress is linked to the evolution of digital computers and powerful mathematical methods. Modelling of photochemical reactors presents an additional complication, as the light energy balance has to be taken into account. This new coupling between mass and light energy balances, the latter depending on the concentrations of reactants and products, incident light distribution, and photon concentration, presents additional difficulties which are not yet resolved.

A. Radiation Models Calculations taking into account the strong coupling between the radiant energy conservation equation and the momentum, mass, and heat balances call for a model describing this interaction as a function of space and time.


283

Such radiation models have been in permanent development over the last 30 years, and the published results may be classified in two main categories: incidence models which may be characterized by mathematical models assuming the existence of a given radiant energy distribution in the vicinity of the reactor, and emission models in which lamp characteristics, reaction, and flow processes are taken into account. 1. Incidence Models. First attempts to describe the radiant power distribution in photochemical reactors can be summarized under the heading of the RI model (radial incident model, Figure 25a). This model is based on the hypothesis of a radial radiation field [2, 31, that is, that all the light striking the reactor wall will be directed radially inward. Corresponding profiles of radiant power or of irradiance are strongly dependent on the radius of the cylindrical reactor (Eq. 68).

where qA= photon flux density or photon irradiance ( E , [2, 31) qA,w= photon flux density or photon irradiance at the wall R , = radius of a cylindrical photochemical reactor (see Figure 25) r = radial coordinate p = attenuation coefficient

Among the first to use the RI model were Gaertner and Kent, who studied the

(4 (b) (4 Figure 25. Characteristics of two-dimensional incidence models: (a) radial, (b) partially diffuse, (c) diffuse [lo71 (see also [2, 31).

284

A. M. BRAUN, L.JAKOB, E. OLIVEROSAND c. A. OLLER DO NASCIMENTO

uranyl nitrate sensitized photolysis of oxalic acid (Eq. 89) in a tubular flow reactor [l08], but the pioneering work of Hill and Felder [l09], Cassano and Smith [llO], Jacob and Dranoff [lll], and Santarelli and Smith El121 should also be mentioned. These authors made important contributions to today’s experience as far as applicability and limitations of this model are concerned. The RI model in particular has been tested in an ideal elliptical photochemical reactor (Figure 26), where the cylindrical reactor tube and the lamp considered as a linear light source are located at the foci of an elliptical reflector [2, 3, 1131. In actual practice, any tubular light source will have a finite diameter and will not behave as a true line source. Radiation from an extended light source will emanate from points displaced from the lamp’s axis, causing the lamp to appear rather like a diffuse light source. In addition, imperfections in the

Elliptical reflector

Light source

Cylindrical reactor

I

‘V

Light rays

Figure 26. Diagram of a photochemical apparatus with an elliptical reflector [2, 3, 1131.


285

reflector surface will result in a more or less strong diffusion of the light, which is assumed to be radially emitted. The DI model (diffuse incident model, Figure 25c) is thought to take into account the abovementioned inadequacies. The model in which profiles of radiant power or of irradiance are independent of the radius of the cylindrical reactor was originally proposed by Huff and Walker [114] and has been tested by Jacob and Dranoff [ll 11 using sensor equipment. Their results show that radius-independent radiant power or irradiance distribution can only be found for radii of less than 0.5in. in their particular equipment (Figure 27). In the RI model, all incident rays intersect at the center axis of the reactor tube, and Eq. 68 produces an infinite value of irradiance as r 0. The DI model, on the other hand, proposes parallel layers of rays which are wider than the diameter of the tubular reactor and which ‘traverse the reactor perpendicularly to its axis from all directions with equal probability. The calculated results of both models are far from reality, as found in industrial size photochemical reactors. Matsuura and Smith [1071 proposed an intermediate model (PDI model, partially diffuse model, Figure 25b) in which parallel layers of rays are assumed, and the width of each is smaller than the diameter of the tubular reactor. These two-dimensional bands form by themselves radial arrangements, the center ray of each band intersecting the

Reactor axis Figure 27. Profile of radiant power in a tubular reactor [ill].

286


axis of the reactor tube. Based on this hypothesis, the profile of radiant power or irradiance within a tubular reactor can be calculated by developing Eq. 69:

where

p = sin-' p1 = sin-' R, R,

= halfwidth

(?)

(2)

M) of the dye. Thus the yellow-green fluorescence gives way to a dull orange solution at high concentrations. Transport of solutes across a membrane is accompanied by a change in volume. Thus Chen et al. [34] and

322

D. C. NECKERS AND 0. M. VALDES-AGUILERA

Kim et al. [35] have studied the kinetics of transport across membranes by measuring the variation in emission intensity of the entrapped fluorophore. Polarization of fluorescein fluorescence provides useful information on the rotational mobility of the fluorophore. Binding to a macromolecule decreases mobility of the fluorophore and it is possible to differentiate between bound and free fluorophore by measuring the depolarization of fluorescence that results. The fluorophore is thus used as a probe for macromolecular binding. This is the basis for Fluorescence Depolarization Immunoassay, a diagnostic technique which, along with other optical methods, is rapidly replacing Radioimmunoassay in clinical applications. In the technique the difference in the polarization of fluoresced light of a fluorescein analogue functionalized with an antitoxin for the drug is quantitated as a function of the concentration of drug in serum. This can lead to a direct measure of the amount of drug in the serum at picomolar concentrations. Steady-state measurements of the fluorescence anisotropy of fluorescein derivatives form the basis of a sensitive analytical technique also used to detect and quantitate proteins [36], steroids [37-391, therapeutic drugs, and narcotics [40-421. In a different approach, the anisotropy of the fluorescein conjugate is measured as a function of the medium viscosity to determine the segmental mobility of the chains that cover the binding site [43-451. Conformational changes of a protein to which a fluorophore is bound alter the microenvironment of the probe and can be followed by measuring the changes in the fluorescence. Thus spectral shifts, changes in the emission intensity and singlet lifetimes have all been employed to study the conformational changes produced as a consequence of protein-protein association [46], association of receptors to cells [47], ligand binding [48-521, and ternary complex formation [53]. Fluorescein-labeled proteins are also used to measure the translational mobility of proteins and lipids by the Fluorescence Recovery After Photobleaching technique [54-591. The uniformly labeled fluorescent sample is flashed with an intense light source to bleach a spot, thus producing a concentration gradient. The rate of recovery of fluorescence in that bleached area is measured and used to calculate the diffusion coefficient of the probe dye into the bleached zone. Such diffusion coefficient measurements have been used to determine the association constants of proteins in cells [60], to measure the exchange of tubulin between the cytoplasm and the microtubules [61,62], to study the polymerization-depolymerization process of actin [63651 and to monitor the changes that occur upon cell maturation [66,67]. Fluorescein is an energy acceptor for chromophores such as naphthalene and anthracene and acts as energy donor toward Eosin and Rhodamine, so derivatives have been used for singlet-singlet energy transfer studies. According to Forster’s theory [68] the rate constant for energy transfer increases

PHOTOCHEMISTRY OF THE XANTHENE DYES

323

with the overlap between the donor emission and acceptor absorption spectra, and decreases with the sixth power of the spatial separation of the donor and acceptor. Thus, provided that energy transfer occurs by the Forster’s mechanism, the distance between donor and acceptor can be estimated. Recent studies using fluorescein derivatives include the determination of intramolecular distance between binding sites [69-761 and between active sites and the surface of the membrane [77,78], study of the intercalation of ethidium in RNA [79], and use of energy transfer for determination of human lactoferrin [SO]. The kinetics of fusion of vesicles [Sl], polymerization of myosin, and the rate of exchange between filaments [82,83] have been followed in situ by monitoring the efficiency of energy transfer as a function of time. The changes in energy transfer efficiency observed upon addition of electrolytes to a solution of histone H4 have been attributed to an electrolyteinduced conformation change of the enzyme [84]. Energy transfer measurements have been performed to study aggregation of proteins [85-871 and the formation of polymeric clusters of protein complexes in membranes [88]. By measuring the efficiency of energy transfer as a function of protein concentration, with appropriate control experiments, Podgorski et al. [89,90] obtained the equilibrium constants for association of spectrin with band 4.1 and band 2.1 proteins in aqueous solution.

111. EOSIN A. Background

Eosin, tetrabromofluorescein, derives its name from EOC (eos-Gr., dawn). It has been used since the 19th century as a histological stain, being employed to observe certain components of the leucocytes (Eosinophiles), and as a reagent for the spectrophotometric determination of silver [9l]. Its photochemistry is of particular interest because its maximum absorption is near the lLemissions of the Ar(+) laser. The ionization of Eosin in solvent mixtures of differing compositions demonstrates, as do the other xanthenes, the effect of hydrogen bonding (see Figure 2) [92]. Several derivatives of Eosin have been prepared and employed to study biological systems. Their main applications are as singlet energy acceptors and as triplet probes [93-971 to measure the rotational mobility of virus particles [98] and proteins in membranes and in solution. Examples of proteins studied using Eosin derivatives include myosin [99,100], band 3 protein [lOl, 1021, pyruvate dehydrogenase [103,104], and Sarcoplasmic

324

D.C. NECKERS AND 0. M. VALDES-AGUILERA

Reticulum ATPase [l05, 1061. Owing to the long-lived nature of the triplet state, Eosin derivatives are suitable to study protein dynamics in the microsecond-millisecond range. Rotational correlation times are obtained by monitoring the time-dependent anisotropy of the probe’s phosphorescence [107-1121 and/or the recovery of the ground state absorption [1131181 or fluorescence [119-1221. The decay of the anisotropy allows determination of the mobility of the protein chain that cover the binding site and the rotational diffusion of the protein, the latter being a function of the size and shape of the protein, the viscosity of the medium, and the temperature. Eosin triplet is efficiently quenched by oxygen and its lifetime depends on the access of oxygen and other quenchers to a given binding site. Therefore, Eosin triplet lifetime is sensitive to both the proximity of the dye to the bulk medium and to the flexibility of the protein chains that cover the binding site wherein quenchers might reside. It is then possible, by measuring the triplet lifetime, to discriminate between binding sites at the protein surface, or within a flexible protein, and those inside a rigid protein matrix [123,124]. Eosin generates singlet oxygen upon irradiation with visible light. Its ability to act as a photodynamic sensitizer has been known for many years [125]. Irradiation of Eosin in the presence of oxygen is found to deactivate viruses [126] and yeasts [127] and to damage the photosystem I1 of leaf tissue [128,129]. Eosin covalently immobilized in polystyrene beads has been used to remove E. coli from drinking water [130]. The localization of the sensitizer has a significant effect on the photodynamic activity. Eosin is scarcely taken up by cells, and Pooler and Girotti [131] report that Eosin isothiocyanate is 50-100 times more efficient for inducing photohemolysis of human erythrocytes. The spectral properties and quantum yields for singlet oxygen production are identical for both compounds. Therefore, they attribute the difference in photodynamic activity to the ability of the isothiocyanate derivative to bind covalently to band 3 protein. Singlet oxygen produced by the triplet state of the sensitizer is commonly assigned as the exclusive toxic intermediate during photodynamic inactivation. However, Kepka and Grossweiner [132] present evidence that photoinactivation of lysozyme occurs by both singlet oxygen-dependent and independent mechanisms. Electron transfer to Eosin triplet from tryptophan and other amino acids has been studied in detail [133-1351, charge separation being evidenced by the appearance of semireduced Eosin. Furthermore, fluorescence measurements and flash photolysis of the lysozyme-Eosin complex show that both the singlet and triplet states of Eosin are quenched by complex formation with the enzyme [136]. Koloczec and Vanderkooi [137] studied the structure flexibility of


325

rhodanese in aqueous solution by measuring the phosphorescence quenching of the Eosin label. Using quenchers of different sizes, iodide ions, thiosulfate, methylvinyl ketone, and the spin label reagent 2,2,6,6-tetramethyl-lpiperidinoloxy (TEMPO), they were able to determine the amplitude of the motion for the protein conformational changes. Determination of translational diffusion rates of proteins requires measurements at longer timescales, one-tenth of a second to several minutes. Eosin derivatives are also commonly used to measure translational diffusion coefficients using the Fluorescence Recovery After Photobleaching technique [138-1411. Eosin emission characteristics depend strongly on the solvent. Specifically transfer from aqueous solution to a nonaqueous solvent shifts the emission of Eosin toward longer wavelengths and increases the emission intensity. Wang and Cheung [142] have used the fluorescence enhancement of the Eosin label to study the association of troponin I with troponin C. Similarly Skou and Esmann [143] and Helmich de Jong et al. [144] have used Eosin itself as a fluorescent probe to study the conformational changes of enzymes involved in ionic transport. The photochemistry of Eosin under both reductive and oxidative conditions has been studied by several groups [145-1511. Photoreduction by amines such as tribenzylamine ( R = CH,$, R” = 6) produces two leuco analogues, the dihydro derivative, and the cross-coupled product formed from the amine radical and the dye radical anion (2) [152]. In addition, debromination of Eosin is reported during photobleaching with amines and phenols. The reader however is referred to the extensive studies of Rose Bengal dehalogenation by Paczkowski and Neckers [1531. Radiolysis of Eosin in methanol shows that debromination is a consequence of the photochemical decomposition of semireduced Eosin [1541.

br

I r

2

Photosensitized electron injection in colloidal TiOz has been reported by Moser and Gratzel as part of a scheme to photoreduce water [155]. Moser et al. [156] and Rosetti and Brus [157] saw the formation of semioxidized Eosin by monitoring its absorption in the visible and by Raman spectroscopy. No oxidation of the triplet of Eosin adsorbed in TiO, is observed, the lifetime

326


being the same as in pure water. Electron injection occurs exclusively from the excited singlet with a rate constant of 9.5 x lo8 s - l at pH 3 and 25°C.

B. Eosin as a Photoinitiator Electron transfer to and from the xanthenes and other dyes has been employed for a number of years to initiate polymerization of a variety of monomers in solution. The excited state of Eosin serves both as an oxidizing agent [l5S] and a reducing agent [159]. Though generally used for acrylate polymerization [1601, it has also been used-with ascorbic acid-for vinyl acetate [1611, with triethanolamine and fumed silica for unsaturated polyesters [162,163], alone with acrylonitrile [164], in combination with UV initiators (where it is claimed to sensitize in the visible) and aromatic sulfides for acrylates [165], pressure-sensitive adhesives [166], and acrylonitrile [167]. In combination with methylene blue, refractive index patterns have been recorded by means of a He-Ne laser into thin PMMA films with styrene as the polymerizable monomer [168]. In color imaging systems Eosin is used as a green (530 nm) sensitive charged, photoconductive toner [169], as well as in Fourier imaging systems [ 1701. Reports on the kinetics and mechanisms of photopolymerization are numerous as well as controversial. One of the issues of substantial disagreement is the effect of oxygen on the rate of the reaction. For example, Chaberek and Allen [171] found oxygen to be an inhibitor at any level, whereas Yang and Oster [172,173] report the polymerization will not proceed in carefully degassed solutions and a small amount of oxygen is required for polymer formation. At the core of the discussion is whether the initiating radicals are produced in the photochemical step from the dye (D; (Eq. 1) or whether the photochemical reaction forms a product, such as the leuco dye (DH,), that reacts with oxygen to generate the initiating radicals (Eqs. 2 and 3).

D D

+ reducing agent + hv-

+ reducing agent + hvDH,

+ 0,-

radicals

(1)

leuco dye (DH,)

(2)

radicals

(3)

Oster [174] proposed the second hypothesis to explain his results on the photopolymerization of acrylonitrile in aqueous solution, buffered at pH 7.0, and sensitized by xanthene dyes and riboflavin using ascorbic acid as the reducing agent. Whereas the monomer is efficiently polymerized when the solution is illuminated in the presence of oxygen, irradiation in its absence leads to photoreduction of the dye to its leuco form but no polymer is formed. Therefore, the author suggests that the leuco dye reacts with atmospheric


327

oxygen to generate hydroxyl radicals that can initiate polymerization (Eqs. 4 and 5).

+ 302-

DH, OH'

DH'

+ OH'

+ monomer --+polymer

(4) (5)

Evidence supporting this mechanism is presented for the case of acrylamide polymerization sensitized by riboflavin, but not for the case of fluorescein and its halogenated derivatives. Irradiation with a millisecond flash in the presence of air leads to polymer formation after an induction period of one hour. In contrast, when the irradiation is carried out with degassed solutions, polymerization starts only after the sample is exposed to atmospheric oxygen. Pemberton and Johnson [175,176] have performed a thorough study of the photopolymerization of vinyl acetate in aqueous methanol sensitized by ethyl Eosin and ascorbic acid. In agreement with Oster's results, these authors find that polymer is not formed in the absence of oxygen. In addition, dye photobleaching without polymer formation is observed when water is excluded. Polymer formation starts after an induction period that depends on the light intensity and ascorbic acid concentration. During this period the dye is completely bleached at a rate that varies linearly with light intensity. The rate of polymerization, measured after the induction period, is independent of light intensity and dye concentration, indicating that the initiating species is not produced in the photochemical step. They detect hydrogen peroxide as a product formed during dye photobleaching and propose that the initiating radicals are generated by reaction between hydrogen peroxide and excess ascorbic acid. Control experiments show the system hydrogen peroxideascorbic acid is an efficient initiator of vinyl acetate polymerization. In contrast to the proposal by Oster that radicals are produced by oxidation of the leuco dye, Pemberton and Johnson propose that hydrogen peroxide is formed by oxidation of the semireduced Eosin radical, as shown in Eqs. (6)-(10) (AH, is ascorbic acid; AH' is the semioxidized ascorbic acid radical), and that the initiating radicals are produced by thermal reaction D

+ AH, + hv-

DH'

DH'

DH,

+ AH'---+

DH'+O,-HO;+D HO; DH'D

+

2HO;-

+ AH'

+ dehydroascorbic acid

+ H202 H 2 0 2+ 0,

(6) (7) (8) (9)

(10)

328


between hydrogen peroxide and ascorbic acid. Our examination of the reported values for the induction period reveals that its reciprocal is proportional to the square root of the light intensity (Figure 3). This intensity dependence, combined with the known mechanism of bleaching for the xanthenes [177], provides support for the suggestion that the initiating radicals are generated as shown above rather than by oxidation of the leuco dye. In particular, if Eqs. (8)-(10) are replaced by Eq. (ll), the rate of DH, generation and the reciprocal of the induction period are predicted to increase linearly with light intensity. DH,

+ 02+

radicals

(11)

A more conventional mechanism appears to be operative in the photopolymerization of ethyl acrylate 11781 and methyl methacrylate [179] in aqueous solution, sensitized by fluorescein and Erythrosin, respectively. Ascorbic acid is the reducing agent in both cases and it is observed that the reaction does not proceed in the absence of buffer, usually phosphate buffer pH 6 . Polymer formation starts after an induction period but its dependence on light intensity and ascorbic acid concentration has not been determined. The rate of photopolymerization is proportional to the monomer concentration and to the square root of the light intensity, dye, and ascorbic acid concentration. The authors report the order with respect to the monomer as 3/2. However, from our analysis of the data for fluorescein, which are more

I

.oe-2 8.0e-3 -

4.0e-3 6.0e-3

0.0

0.2

0.4

0.6

(Relative I)

0.8

1:0

0.5

Figure 3. Variation of the reciprocal of the induction period with light intensity for the polymerization of vinyl acetate in aqueous methanol. Data from D. R. Pemberton and A. F. Johnson, Polymer 25, 536 (1984).


329

extensive than for erythrosin, we conclude that the rate of photopolymerization increases linearly with the monomer concentration. These results indicate that the initiating radicals are generated in the photochemical step and that the termination reaction is bimolecular. The lack of polymerization observed in the absence of buffer is explained by postulating the formation of a complex between the buffer and ascorbic acid. The complex reacts with the excited state of the dye to produce the radicals (Eqs. 12 and 13).

+ buffer t-l(AH,-buffer) R' + DH' + buffer D(T) + (AH,-buffer)AH,

(12) (13)

However, no independent evidence is presented for the existence of this complex. In our view a simpler explanation for the lack of reaction in unbuffered solution is that the nondissociated form of ascorbic acid is a chain terminator. At the typical concentrations employed, 1-10 mM, more than 90% of ascorbic acid (pK = 4.1) is not dissociated when dissolved in pure water. At pH 6.0 only 1% is present as the protonated form. Furthermore, in their study of the photopolymerization of methyl methacrylate initiated by acriflavine-ascorbic acid, Lenka and Mohanty [180] report the rate of polymerization reaches a maximum when the ascorbic acid concentration is approximately 10 mM. The decrease in polymerization rate at higher concentrations suggests ascorbic acid participates in chain transfer and/or termination reactions. Photoreduction of Eosin, Erythrosin, and Rose Bengal with amines has been employed for a number of years to initiate polymerization of monomers in the absence of solvent. Examples of recent applications include the production of volume holograms [l8l], color images on plain paper [182], printing plates [183-1851, and, in the presence of suitable peroxides, manufacture of coatings for glass fibers [186]. Photooxidation of Eosin with periodate ion has been used to initiate the polymerization of acrylonitrile in aqueous solution [1871. Addition of acrylonitrile to a periodate solution shifts the absorption maximum from 220 to 280nm. This spectral change is interpreted as being due to complex formation between the monomer and oxidizing agent. The rate of photopolymerization increases linearly with the absorbed light intensity and monomer concentration. The observed intensity dependence indicates the main chain terminator is not produced photochemically. Polymer is not formed when the concentration of periodate ion is lower than 0.5 mM and the rate of polymerization is independent of its concentration for higher values.

330


The mechanism suggested by the authors is shown in Scheme 1.

-

+ HJO; ?==?.complex D + hv D(S) D(T) OH-M' + D" + H2104 + H 2 0 Complex + D(T)M

M;+M-M; Mi-l Mi

+ M+

+ complex-

Mi polymer

+ HJO; + H 2 0

Scheme 1.

The complex formed between the monomer and oxidizing agent acts both as the source of initiating radicals, by reacting with the triplet of the dye, and as a chain terminator. In both cases the net effect is transfer of a hydroxyl group. The monomer concentration employed is typically 300 times higher than periodate ion and the concentration of free periodate ion is too low to participate in the termination reaction. The IR spectrum of the isolated polymer shows a strong absorption at the O H stretching frequency (3600 cm- I), providing support for the proposed mechanism. Photopolymerization of multifunctional monomers occurs rapidly under UV irradiation, even in the presence of oxygen, to form highly crosslinked insoluble polymers. These reactions have found many industrial applications [1881, including the production of fast drying protective coatings, printing inks, and adhesives. Lasers have played an important role in the continuing search for faster polymerizing systems and in the use of photopolymerization in the fields of microelectronics, holographic recording, and the reproduction of three-dimensional objects. The high intensity of the laser allows completion of the photopolymerization in the millisecond timescale and the small beam diameter makes it possible to write on photosensitive plates with high spatial resolution. Extension of laser initiated polymerization to the visible range depends on the existence of sensitizers which absorb at the wavelengths of available lasers as well as a chemical reaction able to produce free radicals from the sensitizing dye. One such system, sensitive to the wavelengths of the Ar' laser, is the photoreduction of Eosin. The photopolymerization of pentaerythritol triacrylate sensitized by Eosin and several amines has been studied by Chesneau and Fouassier [189]. Samples containing the dye, amine, and UV initiator which consumes oxygen to produce a tack free surface are irradiated in thin films w was detected with superoxide dismutase and an oxygen electrode in water in the presence of sulfite. Rodgers and Lee later confirmed

331


this result quantitatively [259] in water where 25% superoxide was observed. Superoxide formation may also require something other than direct electron transfer from Rose Bengal triplet, though this is surely not the case in the time-resolved measurements. Under the conditions used in the time-resolved experiments, Rose Bengal triplet produces 75% singlee bond conversion at the end of the exposure is approximately 25%. Examples of UV photoinitiators employed as additives are alkyl ethers of benzoin, benzophenone, thioxanthones, and the oxime ester (PDO, 3). Of course most of these UV photoinitiators decrease the rate of photopolymerization sensitized by Eosin and N-methyldiethanolamine (MDEA) likely by serving as radical traps. The exception is PDO which, at a concentration of 3%, increases the photospeed by a factor of 2.

0-C -OC,H, II

0

3

The photospeed increases linearly with incident light intensity (Figure 4). The linear relationship between the photospeed and the incident light intensity is explained by considering that in this viscous monomer the main termination mechanism is radical occlusion instead of bimolecular reaction between macroradicals. We will comment more extensively on the intensity 1.1

0.83

0.55 028 0.0 0

50

100

150 2M) I (mWlcm2)

250

300

Figure 4. Intensity dependence of the photospeed. Eosin (OD=0.3 at 488 nm), 10% PDO, 5% MDEA, pentaerythritol triacrylate. Data from E. Chesneau and J. P. Fouassier, Angew. Makromol. Chem. 135, 41 (1985).

332


dependence when we describe our photopolymerization results on a similar system. The effect of the structure of the coinitiator is studied using 3% PDO and 5% electron donor. The measured photospeed increases in the order NN,N dimethylbenzylisopropylamine < dibenzylamine < triethylamine amide < N-benzylethanolamine < MDEA. However, a reactivity order for the electron donors can not be inferred from these data because the molar concentration is not the same for the different coinitiators. As shown for MDEA in Table 2, the variation of the photospeed with the amine concentration is not simple. The photospeed increases with increasing amine concentration up to 5%, higher concentrations resulting in lower polymerization rates. At the high MDEA concentration employed, quenching of Eosin singlet state becomes important, and part of the observed inhibition is certainly due to nonreactive quenching of the singlet. Amines are known to quench the fluorescence of xanthene dyes with an efficiency that increases with decreasing ionization potential of the amine [190]. However, the rate of photoreduction of Eosin [191] and Rose Bengal [192] by amines in solution starts decreasing at amine concentrations high enough to quench more than 10% of the fluorescence intensity. Therefore, although the dependence of quenching efficiency on the ionization potential suggests the dye singlet is quenched by a charge transfer reaction, it appears that radicals are not generated because of incomplete charge transfer or the singlet radical pair (Dye'- Am") decays preferentially by black electron transfer to the original reactants. Our work on the photopolymerization of multifunctional monomers has been concentrated on the development of faster initiators to be used in the

-

TABLE 2 Effect of MDEA Concentration on Photospeed" MDEA

(%I

Relative Photospeed

1

0.33

5

1.00 0.50 0.33

3

10 15

0.67

"Eosin (OD=O.lS at 488nm); 10% PDO, pentaerythritol triacrylate; I,,,= 488 nm, I=2S W cm-', Ar* laser. Data from E. Chesneau and J. P. Fouassier, Angew. Makrornol. Chem. 13541 (1985).


333

reproduction of three-dimensional objects with the recently developed technology of stereolithography. Stereolithography is a new technology linking the power of computer graphics to the rapid formation of a solid, shaped object. Patented originally by Chuck Hull [193] and assigned to 3D Systems, stereolithography converts CAD/CAM/CAE generated solid or surface model data to a three-dimensional real part synthesized, via photopolymerization, from a photosensitive monomer such as an acrylate. Cured by a laser beam directed across an x,y surface, a liquid monomer mix is converted to a solid plastic object, point by point, layer by layer, true as allowed by the photopolymer in all three dimensions x, y , and z (Figure 5). Stereolithography is simple in concept and it provides great economies for the design lab as well as for the modeling process. It also provides previously unrecognized challenges for the polymer photochemist, for it is entirely a laser-initiated technology, and the polymerization reactions take place to depths below a finitely thin surface layer. Initiator systems used to initiate photopolymerization with UV light generate radicals mostly by homolytic dissociation. Ketone acetals (e.g., Irgacure 651) absorb in the mid-range UV, dissociating to a benzoyl radical which initiates acrylate polymerization. The benzoyl group remains as part of the formed polymer (Eqs. 14 and 15).

-

monomer

elevator

~~

STEREOLITHOGRAPHY APPARATUS (SLA)

Figure 5. Diagram of the stereolithography apparatus.

334

D. i. 7;ZCKERS AND 0. M. VALDES-AGUILERA

Ultraviolet lasers such as the He-Cd laser used in 3D Systems' SLA 1 and SLA 250 have a number of drawbacks. The power available is somewhat limited and the lifetime of the laser tube tends to be uneconomical. Since high power, stable visible lasers are available and convenient, targeting stereolithographic systems to them is an obvious goal. In recent years visible photoinitiators for the formation of polymers via a radical chain reaction have also been developed. These absorb light which is blue, green, or red and also cause the polymerization of polyolacrylates, in some instances, such as encapsulated systems, with speed which is near photographic. Some of these photoinitiators provide the photochemical backbone of the nonsilver, near-photographic speed, imaging processes such as the CycolorTMprocesses invented by the Mead Corporation. CycolorTM initiators are cyanine dye, borate ion salts (4)-so-called (+,-) ion pair

4

initiators-which utilize single electron transfer to the excited singlet state of the light-absorbing dye from the gegen or partner ion as the initial step. Thus the cyanine dye, in its excited state, accepts an electron from the partner borate, producing a neutral boranyl radical (Ar,BR'), which subsequently fragments, producing an alkyl radical (R') which is the real polymerization initiator. Other initiators active in the visible have been invented by us. These are xanthene dye, onium salt (-,+), ion pair photoinitiators (e.g. 5, a xanthene bis iodonium salt), which function by oxidation of the excited state of the dye rather than by reduction.

5


335

As part of our interest in visible light photoinitiators, we have been particularly interested in developing systems which are active at the emissions of convenient lasers such as the argon ion laser (488 and 514 nm) or the helium neon laser (632 nm) and which also allow the formation of thick films. In the process of studying photoinitiating systems based on Eosin and other dyes, it became apparent that a previously unrecognized component of the photoinitiation process, namely the photobleaching of the dye initiator, has an extraordinary impact on the overall polymerization process when lasers of moderate intensity are used as the initiating radiation source. The photobleaching of the dye initiator increases the laser penetration depth. As a consequence, polymer formation is not constrained to a thin film near the surface and the reaction can occur to a depth which is controllable by the dye concentration, irradiation time, and laser power (Figure 6). In stereolithography the positions of polymerization x and y are controlled by a mirrored scanner which reflects the laser onto the surface of the to-be-polymerized monomer at a point x, y and the z dimension is determined by the position of the elevator, as shown in Figure 5. During the polymerization z, the depth to which reaction occurs, is held constant through the use of a UV photoinitiator that bleaches either not at all or very slowly, relative to the rate of polymerization. Our use of bleachable photoinitiators to carry out polymerization at depth opens the possibility of controlling the vertical dimension photochemically rather than mechanically. We have used the photoreduction of Eosin by triethanolamine to sensitize the polymerization of multifunctional acrylates to demonstrate the principle. Irradiation is carried out at 514 nm with an Ar+ laser having a beam diameter of 1.4 mm. The volume of sample irradiated is a small fraction of the total, simulating the conditions found in stereolithography. Because of bleaching of the photoinitiator, the irradiation generates

polymerized solid object

Figure 6. Polymerization at depth.

336


a spike the dimensions of which depend on the irradiation time and the laser power (Figure 7). As a measure of the photospeed we determine the length of the formed spike or, if the strength of the polymer permits, we isolate the spike and estimate the photospeed gravimetrically. These methods are an extension to thick samples of the sensitometric techniques used to evaluate the photospeed in thin films [194]. A typical composition contains Eosin or Eosin lactone (Eosin lactone is neutralized by the amine to form the bisammonium salt of Eosin in situ) at a concentration of 1.3 x M, triethanolamine (0.12 M), trimethylolpropane triacrylate, and 5% vinyl pyrrolidone as reactive diluent. Table 3 summarizes our results obtained with Eosin lactone using different exposures and incident laser intensities. These results indicate that the rate of growth of the spike increases linearly with the incident intensity. The data show that the polymerization depth depends on the total incident energy and not on the rate at which the energy is delivered. The system is said to obey reciprocity since irradiation with a high intensity for a short period of time produces the same effect as irradiation with a low intensity for a long time, provided the total energy delivered is the same in both cases. From Table 3 we calculate the average polymerization depth per unit exposure, that is, the sensitivity, at 0.30 k 0.03 cm3 J-'. Decreasing the concentration of triethanolamine to 0.06 M changes the sensitivity to 0.46 cm3 J- Fluorescence measurements in ethyl acetate/20% methanol indicate triethanolamine quenches the emission intensity of Eosin with a Stern-Volmer constant equal to 2.2 M - '. From this value we conclude that the lower sensitivity at the higher amine concentration is probably due to quenching of the Eosin singlet. As we discussed above, this quenching does not produce free radicals and the net effect is to decrease the quantum yield of intersystem crossing.

Figure 7. Schematic representation of spike formation upon laser irradiation.


337

TABLE 3 Average Depth of Polymerization"

I (w cm-') 1.62 3.25 3.25 6.50 16.2 16.2

E (J cm-2)

Polymerization Depth (cm)

0.20 0.21 0.41 0.81 1.08 2.03

0.060 0.066 0.125 0.206 0.360 0.540

"Photopolymerization of trimethylolpropane triacrylate/5% vinyl pyrrolidone. M, triethanolamine 0.12 M; li,,=514nm; Ar' laser, Eosin lactone 1.3 x beam diameter = 1.4 mm.

Using Eosin instead of the lactone as the reactant increases the sensitivity by approximately 40%. Because of the high triethanolamine concentration employed (0.12 M), it is unlikely that the lower sensitivity in the former case is due to incomplete neutralization of the lactone. More likely it is due to quenching of the Eosin singlet and/or triplet by the ammonium ion. Fluorescence studies with several Rose Bengal derivatives in nonpolar solvents indicate that ammonium ions quench the singlet of Rose Bengal by a combination of hydrogen bonding and tight ion pair formation [195]. We, as well as Chesneau and Fouassier, find that the photospeed increases linearly with light intensity. From this observation one can conclude that chain termination reaction is not the usual interaction between two macroradicals. We have measured the initial rate of photopolymerization using thin foil calorimetry and find a linear relationship between the rate of photopolymerization at low conversions (less than 15%) and the absorbed light intensity. Using the same monomer but with a different photoinitiator (to be discussed in detail later) we observe an order of one half with respect to light intensity both by thin foil calorimetry and by measuring the polymer spike. Therefore we conclude that the linear dependence observed for the Eosintriethanolamine system is real and not an artifact of the technique employed to determine the photospeed. It has been known for some time that UV photopolymerization of multifunctional monomers does not obey the classical rate expression, R, proportional to Z0.5, but follows an approximately first-order relationship [196,1971. These results have been explained by postulating that, in these viscous monomers, radical occlusion competes with bimolecular termination.

338

D. C. NECKERS A N D 0 . M. VALDES-AGUILERA

Scheme 2 presents a general mechanism and the intensity dependence derived for the limiting cases. Initiator + hv +radicals R'+M+

initiation

Mi

M;+M---+M;

propagation

Mi + M i -----+ polymer M n ' j inert

bimolecular termination radical occlusion

Scheme 2.

Intensity dependence: (a) Termination exclusively by bimolecular reaction: R , is proportional to 10.5

(b) Termination exclusively by radical occlusion: R, is proportional to I . When both types of termination occur simultaneously, the observed order with respect to light intensity depends on the relative contributions of radical occlusion and bimolecular reaction to the termination process. An example of this condition is reported by Bellobono and co-workers for the case of diallyloxydiethylene dicarbonate [1981. They measure the intensity dependence of the rate of photopolymerization at different conversions and find that the apparent order with respect to light intensity increases with the conversion at which it is determined. At the initial stage of the reaction the photopolymerization follows the classical rate law, indicating that bimolecular termination is predominant at low conversion. The dependence of the reaction order on the percent of double bonds reacted is indicative of increasing contribution of radical occlusion to the overall termination at the higher conversions. Recent work by Decker and Moussa [199,200] on the photopolymerization of acrylic polyurethane resins in the presence of monoand diacrylate diluents indicates bimolecular termination reactions predominate at all conversions in formulations containing equal parts of resin and diluent. Postpolymerization effects give an estimate of five seconds for the lifetime of the macroradicals in these monomer systems. It is reasonable to expect that in a viscous monomer such as trimethylolpropane triacrylate (q = 65 cp), bimolecular termination reactions proceed more slowly than in monofunctional monomers. However, considering the long lifetime observed for the polymer radicals in these monomers, caution must be exercised in the interpretation of the linear intensity dependence. Long-lived radicals are more likely to terminate by chain transfer and


339

inhibition even at relatively low concentrations of transfer agents. Chen [201,202] has presented evidence that the dimers of methylene blue are the main chain terminators when this dye is used, with triethanolamine, to sensitize the photopolymerization of acrylamide in aqueous solution and it is possible that a similar degradative chain transfer process is occurring with Eosin under our conditions. Eosin dimerizes in water and in organic solvents, the equilibrium constant being equal to 91 M-' in glycerol [203]. For the dye concentration we have M, the concentration of dimer is calculated employed in our work, 1.3 x M. Using a 5-sec lifetime estimate for the macroradicals, the to be 1.5 x rate constant for the reaction shown in Eq. (16) needs to be 1.3 x lo5 M-' S-' to compete with the reaction in Eq. (17) and would be the predominant mode of termination if the rate constant equals 7 x lo5 M-'s-'. Since the rate con-

MI, + (D)2+inert polymer MI, + Mk-

inert polymer

stant for a diffusion controlled reaction in the monomer is 1.7 x 108M-'s-', we may have a situation in which the aggregates are present at a concentration high enough to dominate the termination process, but too low to be detected spectroscopically. Flash photolysis of Eosin in the presence of amines in aqueous solution has been performed by Kasche and Linsqvist [204] and by Rizzuto and Spikes [205]. The photooxidation of phenol and derivatives of phenol has been studied by Zwicker and Grossweiner [206,207] and by Chrysochoos and Grossweiner [208]. The rate constants for quenching of Eosin triplet by ferricyanide ion, p-phenylenediamine, and phenol are 2.5 x lo8, 1 x lo1', and 1.5 x 105 M-' s-' respectively. In the absence of oxidizing or reducing agents the triplet of Eosin decays by mixed first- and second-order processes. The triplet lifetime depends on the energy of the flash and the concentration of Eosin ground state. Scheme 3 has been proposed to account for the observations.

-

D(T)-

D(T) + D

D

k,

2D

k2

D(T) + D +D"

D(T)+ D(T) +2D D(T)+ D(T)

D(T)+Am-

D"

+ D'-

k, k4

+ D'-

D'- +Am'+

Scheme 3.

k5 k6

D.C. NECKERS AND 0. M. VALDES-AGUILERA

340

D represents Eosin ground state, D(T) the dye in its triplet state (A, = 540 nm, E,,, = 4.4 x lo4 M-' cm- '), D" semioxidized Eosin radical (A,, = 462 nm, E,,, = 6 x lo4 M-' cm-' ), D' - semireduced Eosin radical (A,,, = 405 nm, E,,, = 4 x lo4 M-' cm-' ), Am represents amine. The rate constants obtained by Kasche and Lindqvist in aqueous solution, with Am = p-phenylenediamine as donor, are summarized in Table 4. We have performed laser flash photolysis experiments in ethyl acetate/20% methanol under conditions in which triplet-triplet annihilation is not important. We achieve these conditions by using Eosin concentrations higher M and low pulse energies in order to obtain clean exponential than 1 x decays for the triplet. Determination of the triplet lifetime at different Eosin and amine concentrations allows us to obtain the rate constants collected in Table 5. A summary of our observations is as follows: 1. In the absence of amine both D" and D'- appear simultaneously with, the decay of the triplet. The absorptions due to the radicals decay to zero within one millisecond. 2. In the presence of triethanolamine at concentrations higher than 2 x lo-' M only the semireduced Eosin radical is formed and its absorption decays in a few minutes. M is used to quench the triplet only 3. When triphenylamine 3 x the semireduced Eosin radical is formed and its absorption decays to zero within one millisecond.

-

TABLE 4 Triplet Decay Rate Constants for Eosin in Aqueous Solution" Rate Constant

kl k*

Value

k3

540 s - l 3 . 0 10' ~ M-'s-' 7.0 x lo7 M-'s-'

k4

1.1 x 109 ~

k5

k6

- 1 s - 1

1.0 x lo8 M-'s-' 1 . 0 10" ~ M-'s-'

"See Scheme 3 for reactions. Am represents p phenylene diamine. Data from V. Kasche and L. Lindqvist, Photochem. Photobiol. 4, 923 (1965).


341

TABLE 5 Triplet Decay Rate Constants for Eosin in Ethyl Acetate/20% Methanol" Rate Constant kl

k2

Value 3.4 x 1 0 3 s - 1

6.0 x 107

+k 3

6.ox 105 2.3 x 107

k6 k6

~ - 1 s - 1 ~ - 1 s - l b ~ - 1 s - 1 ~

"See Scheme 3 for reactions. 'For Am = triethanolamine. 'For Am = triphenylamine.

The stability of the semireduced Eosin radical produced by reduction with triethanolamine is similar to that reported for the radical in aqueous solution [209]. Our results indicate that the lifetime is substantially shorter when it is generated by reduction with triphenylamine. We attribute this difference to the ability of the triethanolamine radical cation to form the a-amino radical by loss of a proton, as shown below, thus preventing the reaction of the radical by reverse electron transfer (Eq. 18).

+'N(CH,CH,OH),

I

CH,CH,OH

-1

N(CH,CH,OH), CHCH,OH

+H+

(18)

It has been suggested in the literature that the a-amino radical is the species that initiates polymerization [210]. This view is supported by our observation that, in spite of the relatively high quenching rate constant of Eosin triplet by triphenylamine (Table 5), the system Eosin-triphenylamine does not sensitize the photopolymerization of multifunctional acrylates. Thus, it is necessary that the amine contains a hydrogen at the a-carbon to be released as a proton after oxidation of the amine by the dye triplet. This deprotonation prevents the back electron transfer and forms a carbon radical that is sufficiently long-lived to be captured by the monomer. We have also used the Eosin-triethanolamine system to design a twophoton system based on visible laser-induced photopolymerization followed by UV-induced crosslinking as a means of building a three-dimensional network structure aimed at three-dimensional imaging. The key to our design, shown in Scheme 4 below, is incorporation of an acrylate monomer

342


with UV sensitive pendant group as a comonomer and/or as a reactive diluent along with di- and trifunctional acrylate monomer compositions.

/3D UV laser

Object

Polyfunctional UV sensitive Visible laser Polymer with UV / + acrylate P acrylate Polymerization sensitive pendant groups

Scheme 4.

Acrylate derivatives of various oximes have been selected for this purpose for the following reasons: (1) A wide range of oxime acrylates having desired absorption maxima in the UV range may be prepared by a one-step procedure from corresponding oximes. (2) Polymers bearing 0-acyloxyimino groups are reported by Tanaka et al. [211] and Hong et al. [212] and by Ghogare and Kumar [213,214] to be UV-sensitive and participate in photocrosslinking or photodegradation. (3) 0-acetyl oxime esters have been used as photoinitiators, but the photopolymerizability of acrylates with pendant oxime groups has not been studied in detail. Photopolymerization studies involving these nitrogen-containing monomers is of interest. (4) Selection of monofunctional, low-viscosity UV sensitive monomers as reactive diluents introduces a new formulation latitude in image forming. The acyl oximes introduced by Delzenne et al. [215] in 1970 have proven to be highly effective in photogenerating free radicals (quantum yields of radicals are of the order of 90%). Delzenne et al. and more recently Li, Liang, and Reiser [216] have investigated the photofragmentation of linear polymers containing acyloxime moieties in the main chain and in the crosslinks, respectively. Their respective goals were to use the acyloximes as a means of photodepolymerization and eventually to provide the basis of new positive photoresists. Preliminary work by Tanaka et al. and Hong et al. shows that acyl oxime pendant groups in the side chain of linear polymers undergo photocrosslinking or photodegradation depending on the structure and physical state of the polymer and nature of oxime. Part of their success is based on the fact that primary bond scission is followed in these systems by a rapid second fragmentation step (Scheme 5). This puts the final radicals at least 4.5 A apart outside the primary cage. As a result the radical pairs do not recombine. In looking for reactive structures that could serve as photolabile side groups, this was an important consideration because cage recombination is an important factor in viscous media such as those used in the polymerization of polyfunctional acrylates.

343


I Scbeme 5.

Another important consideration was the fact that the absorption maxima of oxime acrylate monomers could be tuned to the desired wavelength by the selection of desired ketone or aldehyde chromophore from whence the oxime was originally synthesized. In our design, UV photocrosslinking followed dye-sensitized visible polymerization which formed a soft gel. It is very important to be able to choose the absorption window for the crosslinking reaction to prevent interfering absorption, if any, from the residual dye initiator. Acyl oximes as part of the backbone of a photodegrading polymer chain have been studied by Smets [217]. Table 6 lists the absorption characteristics of the series of UV-sensitive monomers investigated [218]. Aliphatic oxime acrylates show absorption maxima below 230 nm, while benzophenone oxime acrylate shows an absorption maximum at 252nm. It is important to note that while benzophenone has absorption at 360nm owing to the n-z* transition, the corresponding oxime has no strong absorption above 300 nm. Pyrene-lcarboxaldehyde oxime acrylate (POA) and 9-phenanthrenecarboxaldehyde TABLE 6 Spectral Characteristics of Oxime Acrylates *ma,

Compound

(nm) 364 290 316 280 252 228

Pyrene oxime acrylate (POA) Phenanthrene oxime acrylate (PhOA) Benzophenone oxime acrylate (BOA) 2,3 Butanedione monooxime acrylate (BMA) Cyclohexanone oxime acrylate (COA)

I

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