Advances in Photochemistry. Volume 15

ADVANCES IN PHOTOCHEMISTRY Volume 15

ADVANCES IN PHOTOCHEMISTRY Volume 15 Editors

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

GEORGE S. HAMMOND Department of Chemistry, Georgetown University, Washington, D.C.

KLAUS GOLLNICK lnstitut fur Organische Chemie, Universitat Munchen, Munchen, Federal Republic of Germany

A WILEY-INTERSCIENCE PUBLICATION

John Wiley & Sons, Inc. NEW YORK I CHICHESTER I BRISBANE / TORONTO I SINGAPORE

3 1730 00693 3225

Copyright

0 1990 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. Library of Congress Cataloging in Publication Data:

Library of Congress Catalog Card Number: 63-1 3592 ISBN 0-471-63289-9 Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

CONTRIBUTORS

Paul F. Barbara Department of Chemistry University of Minnesota Minneapolis, MN 55455

Wlodzimierz Jarzeba Faculty of Chemistry Jugiellonian University 3 Karasia, 30-060 Krakow Poland

Hans-Dieter Becker Department of Organic Chemistry Chalmers University of Technology and University of Goteborg S-412 Goteborg, Sweden

J. Michael McBride Department of Chemistry Yale University New Haven, CT 06511

Silvia E. Braslavsky Max-Planck-Institut fur Strahlenchemie Stiftstrasse 34-36 D-4330 Miilheim a.d. Ruhr Federal Republic of Germany

Paul D. Maker Scientific Research Laboratory Ford Motor Company Dearborn, MI 48121

Mark D. Hollingsworth Department of Chemistry University of Alberta Edmonton, Alberta Canada T6G 2G2

Hiromi Niki Department of Chemistry and Canadian Center for Research in Atmospheric Chemistry York University North York, Ontario Canada M3J 1P3

Alfred R. Holzwarth Max-Planck-Institut fur Strahlenchemie Stiftstrasse 34-36 D-4330 Mulheim a.d. Ruhr Federal Republic of Germany

Kurt Schaffner Max-Planck-Institiit fur Strahlenchemie Stifstrasse 34-36 D-4330 Mulheim a.d. Ruhr Federal Republic of Germany

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 sense have the articles been simple 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 KLAUSGOLLNICK Davis, California Washington, D.C. Miinchen, Federal Republic of Germany

vii

CONTENTS

Ultrafast Photochemical Intramolecular Charge Transfer and Excited State Solvation PAUL F. BARBARA AND WLODZIMIERZ JARZEBA Atmospheric Reactions Involving Hydrocarbons: FTIR Studies HIROMINIKI AND PAUL D. MAKER Excited State Reactivity and Molecular Topology Relationships in Chromophorically Substituted Anthracenes HANSDIETERBECKER Photophysics and Photochemistry of Phytochrome KURTSCHAFFNER, SILVIAE. BR~SLAVSKY, AND ALFREDR. HOLZWARTH

1

69

139

229

Photochemical Mechanism in Single Crystals: FTIR Studies of Diacyl Peroxides MARKD. HOLLINGSWORTH AND J. MICHAEL MCBRIDE

279

Index

381

Cumulative Index, Volumes 1-15

387

ix

Advances in Photochemistry, Volume15 Edited by David H. Volman, George S Hammond, Klaus Gollnick Copyright © 1990 John Wiley & Sons, Inc.

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE AND EXCITED STATE SOLVATION Paul F. Barbara and Wfodzimierz Jarzeba Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455

CONTENTS I. Introduction A. Overview B. Solvation C. Charge transfer 11. Solvation A. Basic solvatochromism 1. The relationship of C(t) to the solvent coordinate 2. A simple model for C(t) 3. Debye-Onsager model for C(t) and the longitudinal relaxation time z, B. Solvation probes C. Ultrafast fluorescence instrumentation D. Time resolved spectra, solvation dynamics and 2, 1. The practical determination of C(t) 2. Summary of published C(t) measurements 3. An evaluation of the Debye-Onsager model E. Modern theories of solvation 1. Onsager theory for C(t) for non-Debye solvents 2. Onsager inverted snowball F. Solvation dynamics in water 1

2

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

111. Excited state charge transfer

A.

Electronic states and the solvent coordinate 1. A prototype charge transfer molecule: Bianthryl 2. Electronic states in the gas phase 3. Torsional angle 4. The solvent coordinate 5. Vibrational dependence on LE and CT 6. Diabatic charge transfer 7. An adiabatic potential for S , 8. The probability distribution function and spectra B. Theory of dynamics 1. Solvent controlled electron transfer 2. Excited state charge transfer 3. Simulation of the charge transfer dynamics of bianthryls 4. The charge transfer rate constant C. Experiments on 9,9’-bianthryl 1. Time resolved spectra D. ADMA E. DMABN F. Other molecules and related theoretical results IV. Conclusions Acknowledgments References

I. INTRODUCTION

A. Overview The study of the structure and dynamics of electronically excited polar aromatics has been an internationally active area of research for over three decades. Two related phenomena have been at the center of this field, namely: (i) the sensitivity of fluorescence spectra of aromatic solutes to solvation and (ii) the appearance of dual fluorescence bands due to intramolecular adiabatic, excited state charge transfer reactions. The latter process was discovered in 1961 by Lippert et al. [l], who observed dual fluorescence bands for paracyano-N,N-dialkylanilines in polar solvents. Later Grabowski et al. [2] assigned the existence of two fluorescence bands to an excited state torsional (twisting)isomerization, involving a “twisted” intramolecular charge transfer

INTRODUCTION

3

(TICT) mechanism, i.e., both torsional motion and charge transfer occur during the interconversion of the two excited state species. In the intervening years TICT compounds and related examples of excited state charge transfer have been examined from many perspectives, including a fundamental investigation of the molecular interactions responsible for the TICT phenomenon, photodynamic mechanistic studies of the kinetics of TICT, and more recently, the use of TICT (and related dual fluorescence) compounds as model systems for the investigation of ultrafast charge transfer reactions in solution. This last topic is at present an extremely active area of research, and it is emphasized in this review. This paper is concerned with a special type of charge transfer reactions in which both the precursor and product of charge transfer process are emitting species (dual fluorescence).We will not discuss intramolecular charge transfer between weakly coupled donor and acceptor groups of the type that produces nonemitting charge transfer intermediates. Many excellent reviews and well-referenced papers have considered excited state charge transfer with dual fluorescence, and transient solvation. The broad range of chemical systems that exhibit bond torsion “twisting” comcomitant with charge transfer was summarized in 1986 by Rettig [3]. In the same year Kosower and Huppert published a review that emphasized their influential work on the role of solvation dynamics in excited state charge transfer [4]. An extensive and in-depth review on many aspects of the photophysics of internal twisting was published in 1987 by a group of experts in this field i.e., Lippert, Rettig, BonaEiC-Kouteck?, Neisel, and Miehe [ S ] . Recent reviews on excited state charge transfer and related excited state isomerizations have also been published [6-81. The other major subject of this review is the study of excited state solvation of polar aromatics that do not undergo charge transfer. The understanding of excited state solvation was greatly advanced over 30 years ago by the seminal papers of Lippert [9] and Mataga’s group [lo], which were concerned with static (nontime-resolved) fluorescence solvatochromism [9,10]. In 1960s and 1970s, the foundations of the study of transient soluatochromism were pioneered by Bakshiev, Mazurenko, and their coworkers [ll-141. Major progress on transient solvation has been made in just the last few years as a result of new ultrafast spectroscopic methods and the advent of new theoretical approaches. Presently, photodynamic studies on excited state solvation is leading to unprecedented knowledge on the microscopic motion of polar liquids. The recent progress in this area, and its relationship to excited state charge transfer, is extensively reviewed in this article. Very recently, Simon briefly reviewed publications up to 1987 on solvation and some of the recent research on ultrafast spectroscopic measurements on charge transfer [lS].

4

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

The present review article is primarily concerned with the enormous recent theoretical and experimental progress made in the study of excited state dynamics, both solvation and charge transfer. Over 50 new references not previously reviewed can be found in 1987 and 1988. A few new directions have emerged. Ultrafast (femtosecond and subpicosecond) time-resolved fluorescence spectroscopy has allowed for the unprecedented measurements of transient dipolar solvation of excited molecules in common solvents. Coordinated ultrafast experiments on the charge transfer dynamics of a number of molecules has opened the door to careful analysis of the fundamental aspects of the role of molecular motion in excited state charge transfer. At the same time, intense theoretical activity on solvation dynamics and ultrafast charge transfer has made substantial progress in our understanding of these phenomena. Indeed, many important comparisons of theory have been made.

B. Solvation Time-resolved fluorescence spectroscopy of polar fluorescent “probes” that have a dipole moment that depends upon electronic state has recently been used extensively to study microscopic solvation dynamics of a broad range of solvents. Section I1 of this paper deals with the subject in detail. The basic concept is outlined in Figure 1, which shows the dependence of the nonequilibrium free energies ( F g and F,) of solvated ground state and electronically excited probes, respecitvely, as a function of a generalized solvent coordinate. Optical excitation (vertical) of an equilibrated ground state probe produces a nonequilibrium configuration of the solvent about the excited state of the probe. Subsequent relaxation is accompanied by a timedependent fluorescence spectral shift toward lower frequencies, which can be monitored and analyzed to quantify the dynamics of solvation via the empirical solvation dynamics function C(t), which is defined by Eq. (1).

Here v(O), v(t), V ( Q ) represent the frequency of the intensity maximum of the fluorescence spectrum immediately after photon excitation, at some time t after excitation, and at a time sufficiently long to ensure the excited state solvent configuration is at equilibrium. Until recently, due to the lack of sufficiently short time-resolution, C(t) measurements were limited to slowly relaxing and associated viscous solvents, typically at low temperature [16-29). In the last 2 years, the first C(t)

INTRODUCTION

5

I

\ LL

s P

a C w

al

E

LL

~

xgq

xzq

Solvent Polarization, X

Figure 1. Nonequilibrium free energy as a function of instantaneous solvent polarization for the ground electronic state So and the excited state S , of an ideal probe. In this example the equilibrium solvent polarization in S1 ( X t q )is larger than in So ( X i q ) because the dipole moment is larger in S , than So. Reprinted from Ref. 31 with permission, from J . Chem. Phys. 88, 2372 (1988). Copyright 1988, American Physical Society.

measurements of the solvation of ordinary, nonviscous room temperature liquids have been made by state-of-the-art (subpicosecond and femtosecond) fluorescence spectrometers [22, 30-331. Very recently, the first report of a C(t) measurement of the solvation dynamics of water has been published [33]. A few papers have dealt with the potential sources of errors in C(t) measurements, particularly with respect to the properties of the fluorescent probes [22, 23, 311. Theoretical activity on solvation dynamics has also blossomed in the recent past [34-50). Traditionally, solvation dynamics have been described in terms of a simple continuum model [Sl, 521, which treats the solvent as a uniform dielectric medium with exponential dielectric response [52]. The associated dielectric parameters are E ~ and , zD,which are the infinite frequency dielectric constant (approximately n2), the static dielectric constant, and the dielectric relaxation time, respectively. According to the simple continuum model, the microscopic solvation function C(t) should decay exponentially with a time constant that is

6

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

approximately given by

where z, is the so-called longitudinal dielectric relaxation time [1 1- 15, 53561. Considerable progress has been made in going beyond the simple Debye continuum model. Non-Debye relaxation solvents have been considered. Solvents with nonuniform dielectric properties, and translational diffusion have been analyzed. This is discussed in Section 11. Furthermore, models which mimic microscopic solute/solvent structure (such as the linearized mean spherical approximation), but still allow for analytical evaluation have been extensively explored [38,41-431. Finally, detailed molecular dynamics calculations have been made on the solvation of water [57, 58, 711. Analysis of the experimental measurements of transient solvation (primarily C(t)) in terms of contemporary theoretical models has led to several conclusions [lS, 22-26,30-33,411, which are reviewed in detail in Section 11. Continuum treatments are seen to fail in several cases, but are remarkably predictive considering the simplicity of the model. Qualitative features predicted by theories that go beyond the simple continuum model are borne out in experiment, although the agreement is qualitative at best.

C. Charge Transfer Much of the recent activity in the study of the excited state charge transfer reactions stems from the impact of these studies on the understanding of the role of solvent dynamics in charge transfer reactions of many types, including examples in biology, inorganic chemistry, and organic chemistry. There is a growing appreciation that fast electron transfer reactions are not well characterized by traditional models (such as Marcus-Hush theory or related nonadiabatic theories), which are based on quasi-equilibrium approximations for the population of thermally excited states of the solute/solvent systems [59-611. Contemporary theories go beyond and treat solvation dynamics in detail. In Section 111 we review many recent papers in this field [62-73,136-1421. A key result is that the rate of a charge transfer reactions should be a function of the microscopic dynamics of the specific solvent. In fact, in the case of very small intrinsic charge transfer activation barrier, the rate is predicted to be roughly equal to the rate of solvation (i.e., z'; for a solvent with a single relaxation (rD) time). This result was first derived over 20 years ago by

SOLVATION

7

Mozumder for the neutralization of an isolated ion pair in polar media [74]. The predictions are considerably more complex for solvents with a distribution of relaxation times and barrier energies that are comparable or larger than the available thermal energy. Intramolecular vibrational effects can also be important. Recent progress in evaluating the new theoretical models by comparing predictions to ultrafast transient fluorescence data [4, 30,75-881 on excited state charge transfer of a variety of molecules is also reviewed in Section 111. The first observation correlating k,, with t;' was made by Huppert and Kosower [4]. Many of the theoretical predictions of new models are indeed observed experimentally, supporting the need for the new theoretical models. Unfortunately, many aspects of the experiments remain poorly defined, so the comparison to theory is often qualitative at best. In other cases, the complexity of the solvation dynamics (nonexponential)and the complexity of the solute solvent interactions make it difficult to make an unambiguous interpretation of the ultrafast data. A number of potential new directions for future experiments on charge transfer that could circumvent the present problems are discussed in Section 111.

11. SOLVATION

A. Basic Solvatochromism The photodynamics of polar fluorescent molecules, has been the subject of a number of theoretical studies. The earliest studies were from Bakshiev and Mazurenko [12- 141. Contemporary treatments that take advantage of recent theoretical advances have been published by a number of groups [53561. The formalism of Van der Zwan and Hynes [54] is particularly convenient and we employ it here, as we review the derivation of some of the basic equations of dynamic solvatochromism. To begin with we consider a fluorescent probe molecule that has ideal properties: it is not polarizable, it does not undergo excited state chemical reactions of any sort (including charge transfer) and it does not interact with the solvent by specific interactions such as hydrogen bonding. Furthermore, we assume that the ground state dipole moment of the probe pr is in the same direction as the excited state dipole moment p e . Assuming further that linear response theory for the solute/solvent system applies, the dependence of the nonequilibrium free energies of the system (in the ground F , and excited F, states) are portrayed in Figure 1 as a function of the electrical polarization of the solvent (see below). In a transient fluores-

8

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

cence experiment the probe/solvent system is in an equilibrium configuration Xi9 of the ground state prior to photoexcitation. Immediately after FranckCondon (vertical) excitation, only the electronic polarization of the solvent has adjusted to the new enlarged dipole p e . Subsequently, the solvent reorients, and the Franck-Condon (vertical)emission energy shifts to lower frequency (larger wavelength). As stated above, the quantity that is typically used to characterize the evolution of the fluorescence spectrum is C(t)which is defined in Eq. (1). In the remaining part of Section 1I.A we review the formal relationship of C(t) to fundamental quantities in the statistical mechanical description of solvation. The derivation we review is adopted from the work of Van der Zwan and Hynes. A useful result of the derivation is that a physical basis for the solvent coordinate in Figure 1 is established [54]. The reader is referred to papers by Bagchi et al. [53], and Sumi and Marcus [54] for related treatments. 1. The Relationship of C(t) to the Solvent Coordinate. The fluorescence frequency of a solvating probe molecule whose spectrum is dominated by solvent interactions (see below) can be expressed by the following equation h ~ ( t=) AU(t) = F,(t) - F,(t)

(3)

Here F e ( t ) and F,(t) are the time-dependent nonequilibrium Helmholtz free energies of the e and g states, respectively. The energy difference AU(t)can be replaced by a free energy difference due to the fact that the entropy is unchanged in a Franck-Condon transition [Sl]. Free energies in Eq. (3) can be represented [54] by a sum of an equilibrium value Feq and an additional contribution related to nonequilibrium orientational polarization in the solvent. Thus for the free energy in the excited state F,(t) we have

The first term in Eq. (4), Feq, is given by

where Uz is the gas phase electronic energy of the excited state. The second term $Bop + B,,)p: is the equilibrium solvation energy according to linear response theory. The excited state dipole moment is represented by pc, and B,, and Bop are the orientational and optical components of the “force constant” of solvation, respectively.

SOLVATION

9

The physical significance of these variables is apparent when they are evaluated in the Onsager cavity description of solvation, which treats the solute as a sphere (which we will assume here is unpolarizable) of radius a. The solvent is modeled as a uniform dielectric medium with a static dielectric constant E~ and an optical dielectric constant E , ~ . The following relationships apply in the Onsager cavity description B

2 & -1 a3 2Eop+ 1

=-= OP

In simple models the orientational component is associated with the orientations of the dipole moments of the solvent, but in general other nuclear displacements may play a role. The optical component Bop is associated with the instantaneous electronic response. copis often assumed to be equal to the square of the refractive index n. The nonequilibrium term in Eq. (4)which is responsible for the time evolution of the fluorescence frequency can be expressed as

where p(t) is one possible representation of the solvent coordinate (i.e., orientational displacement of the solvent). p(t) is denoted by the term “instantaneous effective dipole moment.” It is a hypothetical dipole moment that “would be in equilibrium with the actual instantaneous orientational polarization of the solvent” [54]. It is interesting to note that the nonequilibrium free energy term is analogous to the equilibrium term if p, - p(t) is replaced by pe. To further develop the notion of a solvent coordinate associated with Figure 1 and the solvent time-dependent reorientation, it is useful to introduce the dimensionless solvent coordinate z(t), which is defined by

Figure 2 shows the dependence of the nonequilibrium free energy of the excited state F,(z(t)) and ground state F,(z(t)). The expressions for the two curves in Figure 2, Eqs. (10) and (11) follow from the derivation outlined

10

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

2

Figure 2. Schematic illustration of nonequilibrium free energies F in the ground and excited electronic states versus the solvent coordinate. Absorption and prompt fluorescence are illustrated, as is relaxed fluorescence. From Ref. 54 with permission, from J . Phys. Chem 89, 4181 (1985).Copyright 1985, American Chemical Society.

above, with an analogous treatment for the ground state.

+

F,(z(t)) = F:q

+

F,(z(t)) = F:

+5 (Ap)’[~(t)]~ 2

(Ap)’[l - ~(t)]’

Here A p = p, - pS. The solvent coordinate z(t)is dimensionless; it is zero when p(t) is equal to pLsand it is unity when p(t) is equal to p e . This representation of the solvent coordinate leads to a compact form for some of the key equations of solvation and electron transfer (see Section 1II.B) in solution. An expression that relates z(t) and C(t)can easily be derived by recalling the definition of C(t) (Eq. (l)),the result for h d t ) (Eq. (3)) and Eqs. (10) and (1 1). C(t)= 1 - z(t)

(12)

SOLVATION

11

Thus it becomes clear that C(t)in fact measures the evolution of the solvent coordinate as it evolves from its initial (Franck-Condon) nonequilibrium displacement in the excited state, to its final equilibrium value, i.e., C(00) = 0 = 1 - z(o3). A related view of the physical basis of the solvent coordinate is the wellknown reaction field R, which is the field located on the solute dipole due to the dipole induced solvent polarization. For example, the reaction field for the equilibrated excited state dipole R:q is given by

We can also consider a time-dependent reaction field (Eqs. (14) and (15)) which includes an orientational term R,, and an instantaneous (optical) term Ro, *

Thus the optical part of the reaction field responds instantaneously to the change of the molecular dipole moment when the probe is optically excited. In contrast, the orientational component relaxes as z(t) evolves. In summary, in this subsection we have attempted to give a physical basis of C(t)in terms of different definitions of the solvent coordinate. We have not yet described how the time evolution of C(t)is related to dynamical properties of the medium, which is the subject of the next two subsections and Section ILE of this review. 2. A simple model for C(t). In this subsection we explore the relationship of C(t) to dynamic properties of the solvent, in terms of the Onsager cavity description, following the work in the literature on this subject [12- 14, 53571. Theories that go beyond the Onsager model are described in Sections 1I.E and 1I.D. The Onsager cavity description of solvation treats the solvent as a dielectric continuum. The dielectric dynamics of the solvent is typically characterized by the frequency-dependent complex dielectric constant 2(w). The measurement of 2(o) for a neat solvent is conventionally called a dielectric dispersion measurement. Several authors have discussed how C(t) can be calculated from the Onsager cavity model. Briefly, we need to consider the time-dependent reaction field, which was related above (Eqs. (12) and (15)) to C(t). For simplicity we consider the case of a probe with pg = 0 and p e # 0. If the probe

12

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

is excited at t = 0, then it can be shown that

rr R(t) = ,ue J dt’r(t’) ~

0

where r(t) is the so-called pulse response function of the medium above the cavity. It is the inverse Laplace transform of the frequency-dependent response, ?(a)that is defined by

2 ;(a)- 1 a3 2i(w) E,

30) = -

+

E,

+2 3

where E, is the dielectric constant of the cavity which is often assumed to be negligible ( E , x 0). Thus by combining Eqs. (12), (15)-(17), a procedure exists for calculating C(t) i j the dielectric response ?(w) of the medium is known. 3. Debye-Onsager Model for C(t) and the Longitudinal Relaxation Time TI. The physical meaning of the relationship described in the previous subsection becomes apparent when we consider the popular special case of the Onsager cavity model that arises if we assume that the solvent’s dielectric properties are well described by a Debye form. qw) = E,

Em + 16s+-iwr,

Here zD is the so-called Debye dielectric relaxation time. One could view tD as a phenomenological time constant which applies to dielectric relaxation measurements, or alternatively for simple causes, involving dielectric relaxation of weakly interacting dipoles, zD is related to the reorientation time constant of the solvent dipole in the laboratory frame. Following the analysis of the previous section it has been shown that C(t) is given by Eq. (19) for a Debye solvent C(t) = exp( - t/zF)

where rF is given by

where z, is the longitudinal relaxation time defined by Eq. (2).

(19)

SOLVATION

13

According to Eq. (19), tIis the time scale for excited state solvation for a Debye solvent. In fact, it is the time scale for both excited state and ground solvation of dipolar solutes and ionic solutes. t l also plays a role in a broad range of reactive (Section 111) and nonreactive charge transfer processes in solution. It is clearly worthwhile to establish a physical picture for this important variable. The solvation time zI is considerably shorter than tD for many solvents. For example for water E , = 4.84, E,, = 79.2 and tD = 8 . 7 2 ~ s[33]. Thus in water tI= 0.59~s.Why is the time scale for solvation of a dipole so much shorter than t,? Why are there apparently two characteristic times (tland T ~ ) for a dielectric medium? Friedman [SS] suggested two simple thought experiments to resolve the paradox of two times. The relevant theory of dielectrics was described in the 1940s by Frohlich [89]. The first thought experiment corresponds to dielectric measurements. It involves applying a voltage to a capacitor containing a dielectric medium at t = 0, and then holding the voltage constant at t > 0. The dependent variable is the time-dependent current which decays as dielectric relaxation of the medium occurs. From the current, the characteristic relaxation time of the time-dependent displacement (D(t)))field can be calculated. The time is t,. This is essentially a time domain analog of 2(0) dielectric measurements. The second thought experiment resembles transient solvation. At t = 0, a certain amount of charge is put on the capacitor plates. This charge jump (D field jump) is analogous to the photon induced change of the dipole moment in the fluorescence solvation experiment. Subsequently (t > 0), the decay of the voltage on the capacitor due to dielectric relaxation of the medium is measured. Note the capacitor in this experiment is not connected to an external power supply for t > 0. The characteristic relaxation time for the decay of the voltage (and electric field E) is tI. According to Friedman’s analogy, t, is much shorter than tD because much less charge is carried in the second experiment (charge jump) than the first experiment (field jump), because the power supply was working during the first experiment while it was disconnected in the second experiment. In other words, in the first experiment,as the dielectric medium relaxes (creating a polarization which tends to lower the electric field E), the power supply compensates in order to keep the voltage (and E ) constant by changing the charge on the capacitor. In the second experiment, as the polarization develops, the E field decreases, so that the relaxation is sensitive to the history (correlations)of the dielectric medium. A microscopic model for coupling of a small number of dipoles that emphasizes the dipolar correlation picture of r1 has been developed by Berne et al. [90]. Section 1II.D describes experiments to test the applicability of the t, concept to real measurements of solvation dynamics.

14

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

B. Solvation Probes The analysis of the transient fluorescence spectra of polar molecules in polar solvents that was outlined in Section LA assumes that the specific probe molecule has certain ideal properties. The probe should not be strongly polarizable. Probe/solvent interactions involving specific effects, such as hydrogen-bonding should be avoided because specific solute/solvent effects may lead to photophysically discrete probe/solvent complexes. Discrete probe/solvent interactions are inconsistent with the continuum picture inherent in the theoretical formalism. Probes should not possess low lying, upper excited states which could interact with the first-excited state during the solvation processes. In addition, the probe should not possess more than one thermally accessible isomer of the excited state. The spectral shape of the emission band should be dominated by probe/solvent interactions as opposed to vibronic activity in intramolecular modes. Rigid molecules with large A p = p e - pg come closest to having a probe/solvent dominated spectrum [23]. Excited state (adiabatic) chemical reactions, such as charge transfer (Section 111), cis/truns isomerization [6-8, 91-93], and proton transfer [S] complicate the C(t)analysis, by producing additional emission bands and by leading to a time-dependent pe. In addition, useful dynamic solvation probes should have the following properties: 1. They should be soluble in a broad range of solvents 2. They should not have very rapid excited state radiationless decay mechanisms which would compete with fluorescence 3. They should have large radiative rate constants (kOJ so that emission can be observed on the femtosecond time scale.

There are a number of empirical tests for the near ideality of potential probes. Ideal probes should exhibit a solvent dependence of the fluorescence Stokes-shift (hi,,, - hi,,) that is in account with the Lippert-Mataga equation [9, lo].

In other words, a plot of the Stokes shift versus [(cO - 1)/(2&,+ l)] - [ ( E , - 1)/(2~, + l)] should be linear. Alternatively, a plot of the Stokes shift versus empirical measures of the solvent polarity, such as E , [30] or TL* may be a more realistic test of the ideality of a specific probe [22,23]. Again, a

SOLVATION

15

linear plot represents ideal behavior. In contrast, nonideal effects such as excited state charge transfer can lead to a plot of the type just mentioned that is significantly curved [23]. For ideal probes the shape of the fluorescence spectrum should be structureless and smooth without vibronic features in a range of solvents of different polarity. Another important observable is the radiative rate constant (Eq. (22)) where 4 is the absolute quantum yield and zobs is the fluorescence lifetime. This measure of the spectroscopic properties of the probe should not be strongly dependent on solvent polarity. A strong dependence would suggest that the electronic character of the excited state is strongly solvent polarity dependent, a situation expected for molecules capable of excited state charge transfer [23,30,82], a nonideal effect. A variety of probe molecules have been used in studies of microscopic solvation dynamics, such as: 1-aminonaphthalene [29,94], 4-aminonaphthalimide [23,95,96], coumarins 102 [30, 311, 152 [32], 153 [22,97], 311 [31], 343 [98], 7-dimethylamino-4-coumarin-acetic acid (DMACAA) [33], MPQB [24] and rhodamine 6G [26] (see Figure 3). In many solvents, many of these probes exhibit the empirical behavior expected for ideal probes. Figure 4 is an example of the usual trend observed for the absorption maxima (hvabs), emission maxima (hv,,) and the Stokes shift (ha,- hv,,) for a polar probe, in this case 4-aminophthalimide. The solvents employed in Figure 4 are all nonhydrogen bond donating solvents. Hydrogen bond donating solvents have a large specific interaction with 4-aminophthalimide [23,96]. The probes C311, C152, and DMACAA lack the bridging alkyl groups found in C102, C153, and C343. Coumarins without bridging alkyl groups have been reported to undergo excited state charge transfer associated with twisting motion about the single bond that connects the amino group to the aromatic ring [99]. However, linear Stokes shift versus solvent polarity plots are observed for these compounds. Furthermore, the fluorescence spectrum is not obviously due to two emitting species (the initial excited state and a charge transfer form). For example see absorption and emission spectra for C152 in Figure 5. The degree of the excited state charge transfer character for the emitting state of these probes must be very small, perhaps negligible for the use of these compounds as solvation probes. An additional group of molecules that have been used as transient solvation probes actually rely on charge transfer to produce the necessary dipole moment charge for solvation measurements. Examples here are 4-(9anthry1)-N,N-dimethylaniline (ADMA) [23,75] and bis(4-

16

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

m I-AN

H ,C\ N I CH 3

4-AP

0

cc$Lo

C311

Go

c102

C153

p

-

0

"

"

(CH312

c343

DMACAA

Figure 3. Molecular structures for many of the probes that have been used in excited state solvation measurements.

17

SOLVATION L

28

-

24

-

20

,*

6.4

6.0

5.6

-,

;/

-

w

5.2

*

I

30

L

40

50

to),

Figure 4. Absorption maxima (e), static emission maxima and Stokes shift (V) for 4AP as a function of solvent E,(30). Vertical bars indicate the uncertainty in the measurements. The lines are linear least-squares fits to the data. From Ref. 23 with permission, from J . Chem. Phys. 86, 3187 (1987). Copyright 1987, American Physical Society.

dimethylaminopheny1)sulfone (DMAPS) [15,251. These molecules are dual fluorescent in nonpolar solvents due to emission from the initial FranckCondon state and from a charge transferred form (see Figure 6 and Section 111). In Figure 6, R denotes reactant (the initial excited state) and P denotes product of the charge transfer state. Normally, charge transfer would be an unwanted effect, but in these compounds the charge transfer time is much faster than the average solvation dynamics and the ratio of concentrations of the charge transfer state to the initial state rapidly becomes >> 1. One must be careful when using probes of this nonideal character to measure early time solvation dynamics because electron transfer may not be more rapid than the fastest components of solvation; thus inaccurate results will be obtained (see Section 1II.C).

300

400

500

600

WAVELENGTH(nm1 Figure 5. Electronic absorption of coumarin 152 in methanol (---), and emission spectra in ethyl acetate (----), propylene carbonate (.-.-.-.-), and methanol (.---.-- ). The fluorescence spectrum red shifts with increasing solvent polarity; there also appears to be an additional red shift in the presence of hydrogen bonding. The absorption spectrum does not change significantly with change of solvent. From Ref. 32 with permission, from J . Chem. Phys. 90,153 (1989). Copyright 1989, American Physical Society.

Figure 6. Nonequilibrium free energy as a function of both internal coordinates and instantaneous solvent polarization for the ground electronic state So (F,) and the excited state SI ( F J . From Ref. 23 with permission, from J. Chem. Phys. 86,3183 (1987). Copyright 1987, American Institute of Physics. 18

Solvent Polarization and Internal Coordinates

SOLVATION

19

C. Ultrafast Fluorescence Instrumentation The investigation of solvation dynamics in typical solvents at room temperature requires time-resolved fluorescence measurements with picosecond to femtosecond resolution depending on the solvent. There are several emission techniques available in the picosecond time range. Time correlated single photon counting [1001 offers high sensitivity and time resolution near 20 ps. Streak cameras [1001 can measure fluorescence transients with time resolution close to 1 ps. Resolution on the order of picoseconds can be obtained using laser spectrometers with optical Kerr shutter [loll. But, only the fluorescence upconversion technique has been demonstrated to have sufficiently short time resolution to study C(t) for common, nonviscous liquids. Castner, Maroncelli, and Fleming used fluorescence upconversion with subpicosecond resolution to study the dynamics of a visible light absorbing probe, LDS-750 [22]. Our group used ultraviolet, subpicosecond resolved fluorescence upconversion to study the solvation dynamics of the nearly ideal coumarin probes [30,31]. Recently, we made the first femtosecond resolved solvation dynamics measurements [32, 33, 97, 98, 1021 using an improved version of our apparatus. The fluorescence upconversion apparatus recently built in our laboratory [lo21 can measure fluorescence following ultraviolet excitation with time resolution near 50fs. In this section we describe this apparatus in order to demonstrate the principles of the fluorescence upconversion experiment. Figure 7 shows the schematic of the fluorescence upconversion technique. Fluorescence at frequency wf, following excitation with an ultrafast, ultraviolet laser pulse is mixed with another laser pulse at frequency w , . The two beams are focused into a nonlinear optical crystal where light at the sum frequency ws = wfl w1 is generated. The intensity of the generated light is proportional to the fluorescence intensity. Changing the time delay between the fluorescence and gating laser pulse with optical delay stage, we can optically gate different parts of the fluorescence transient so we can record

+

m2

n

w

Sample

fl

Nonlinear crystal

Figure 7. Schematic of the fluorescence upconversion apparatus. The notation is defined in the text. From Ref. 6 with permission, from J . Irnag. Sci . 33, 53 (1989). Copyright Society for Imaging Science and Technology.

20

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

fluorescence intensity versus time. In our apparatus the excitation pulse is the second harmonic of an amplified ultrafast dye laser and the probe pulse is the fundamental of the same laser. Time resolution in this method is limited by the width of the laser pulses used for excitation and for optical gating, and by optical effects that produce time broadening [102,103], such as the frequency dependence of the speed of the fluorescence light as it travels through the apparatus. The laser source in our spectrometer is an amplified femtosecond dye laser with a much larger repetition rate than many of the existing amplified laser systems used for femtosecond spectroscopy. The amplification is necessary to improve the signal intensity which actually depends on roughly the third power of the laser intensity. The large repetition rate helps average over pulse-to-pulse fluctuations of the laser. The instrument consists of three sections-a synchronously pumped linear cavity femtosecond dye laser, a dye laser amplifier which amplifies individual pulses several thousand times, and the upconversion system where the fluorescence intensity versus time is measured using sum frequency generation. Figure 8 shows the schematic of the femtosecond linear cavity dye laser, built following the design of Dawson et al. [104]. The laser is synchronously pumped at 76 MHz by a mode locked frequency doubled, Nd :YAG laser. With the dye combination styryl 8 as a gain dye and HITCI as a saturable absorber, the laser produces 70 fs pulses (FWHM) at 792 nm. The average output power is 25mW which corresponds to energy 0.3 pJ/pulse. To amplify the dye laser pulses we use a copper vapor laser (CVL) pumped amplifier. The design of the amplifier based on the approach of Knox et al. [lo51 is shown in Figure 9. The amplifying medium is a dye jet pumped by CVL at 8kHz with 15ns pulses and average power 20 W. The CVL pulses are synchronized with dye laser train. The pumping laser beam is 3 . 7 5 c m R.C.

5 c m R.C.

HlTCl 790

nm

PUMP B E A M

500

-

700 mW. 80 p8

Figure 8. Schematic of the femtosecond linear cavity dye laser (see [lo21 and [104]).

21

SOLVATION

Figure 9. Design of the dye laser amplifier. Ultrafast laser pulses are amplified roughly 10,000 times by seven passes through a dye jet pumped by a copper vapor laser.

focused to a 1 mm spot size in a 1 mm thick dye jet of ethylene glycol/styryl 8 solution. The ultrafast dye laser pulses are amplified several thousand times passing seven times through the dye jet. The energy of the output pulses is usually in the range 2-3 pJ. Figure 10 shows the schematic of the fluorescence upconversion section. The amplified femtosecond laser pulses with a repetition rate 8 kHz and frequency w1 are used to generate second harmonic (396 nm) in a ' l mm KDP crystal with conversion efficiency of ~ 2 0 %The . second harmonic (wz)and residual fundamental (wl) laser beams are separated using a dielectric mirror. The laser beam at w 2 passes through a variable optical delay stage and excites a sample, which flows in 0.5 mm thick cell. The residual fundamental at w1 is used as the gate beam for upconversion. The fluorescence from the sample is collected and collimated with a microscope objective. It is focused and combined linearly or nonlinearly with the residual fundamental into a KDP crystal. The KDP crystal is angle tuned to phase match the sum frequency The light at the sum frequency is separated from generation, ws = wf, ol. residual fluorescence, second harmonic, and fundamental using a prism or a spatial filter, for the linear and nonlinear geometries, respectively. The signal is measured with a photomultiplier, read in and stored by a microcomputer. A plot of the fluorescence intensity versus time is obtained scanning the optical path length for ultraviolet excitation beam. A time response function of the apparatus can be measured by upconversion of the excitation beam. The width of such measured instrument response function is 280fs (FWHM). Comparing this result with the width of the autocorrelation function of the dye laser llOfs we observe -17Ofs broadening of the instrument response function due to group velocity

+

N

22

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

input

Upconversion

E

Sample

..

BS

KDP

Pol

Figure 10. Schematic of the upconversion: A/2, halfwave plate; BS,dichroic beamsplitter; POI, polarizer; M, monochromator; P, photomultiplier; and KDP, potassium dihydrogen phosphate, nonlinear optical crystal.

dispersion and group velocity mismatch 11021 in this system. Therefore, this apparatus with different nonlinear crystals (0.1mm BBO) and reflective optics for the fluorescence collection can still have two times better resolution. Figure 11 shows the instrument response function and fluorescence transients measured for DMACAA in water [97], for which femtosecond measurement was critical (see below for a full discussion of the probe in water.) Some ultrafast measurements presented in Section I11 were obtained with 200 fs instrument response function. The better time resolution of the instrument was obtained by replacing the fluorescence collecting microscope objective by a quartz lens. The sensitivity of the apparatus is cut in half due to a less well-focused fluorescence spot.

D. Time Resolved Spectra, Solvation Dynamics and TI 1. The Practical Determination of C(t). The time-dependent fluorescence Stokes shift of the spectrum should manifest itself as (i) a rapid decay in the fluorescence intensity on the blue edge of the fluorescence spectrum, (ii) a

23

SOLVATION

i -1.0

time

(PSI

-

4.0

Figure 11. Fluorescence transients of 7-(dimethylamino)coumarin-4-acetateion in water recorded at 445 nm (upper), 483 nm (middle), and 509 nm (lower).The solid line through the points is a fit of the data to a multiexponential decay. The peak near zero time in the upper panel is instrument response function (280fs fwhm). From Ref. 33 with permission, from J . Phys. Chem. 93, 7040 (1988). Copyright 1988, American Chemical Society.

rapid rise in the intensity at the red edge of the spectrum, and (iii) no fast transient at wavelengths near maximum of the spectrum (assuming that total shift is small). Figure 11 shows just this result for DMACAA in water [97]. The recorded wavelengths are 445 nm, 483 nm, and 509 nm at the blue edge, center, and red edge of the fluorescence spectrum, respectively. Note that the decay of population of the excited state of the molecule (roughly 1 ns) is not evident on this time scale.

24

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

Time-resolved fluorescence spectra have usually been measured by recording several transients at different wavelengths, a technique which is denoted by spectral reconstruction. The intensity from each of the transients is adjusted to correct for the wavelength-dependent sensitivity of the apparatus by setting the time integral of the transient intensity equal to the intensity from the static fluorescence spectrum. The corrected transients can then be used to reconstruct fluorescence spectra at different times after excitation. Usually, to obtain better time resolution, deconvoluted multiexponential functions are used for reconstruction instead of the experimental transients (see Figure 11). Following a procedure of Maroncelli and Fleming [22], the reconstructed time-resolved fluorescence spectra are fitted to log-normal line shape function [22,106]. Figure 12 shows examples of time-resolved reconstructed spectra and log-normal fits for DMACAA in water. The Stokes shift correlation function C(t) can be now calculated (Eq. (1)) using spectra maxima or their first moments from the log-normal function. The calculated function C(t)for DMACAA in water is shown in Figure 13. Unfortunately, the procedure just described to determine C(t) can consume many hours of spectrometer time, since several transients must be acquired and processed. Recently, an alternative timesaving procedure for measuring C(t)was developed [23,31]. The procedure, which is approximate, requires a single emission transient and certain photophysical data on the probe. It is based on a simple photodynamic model, in which it is assumed that the spectrum of the probe is a simple function of a single solvent parameter, X, denoted as the solvent polarization. During the solvation process, X is time-dependent, such that C(t) = [X(t) - X(co))]/ CX(0) - X(w)l. The instantaneous emission intensity at a certain detection wavelength ;Iis given [23] by

. Figure 12. Reconstructed fluorescence spectra of 7-(dimethylamino)-coumarin-4acetate ion 0.1 and Ips after excitation. The solid line represents the best fit of the log normal distribution function to the data. From Ref. 33 with permission, from J . Phys. Chem. 93, 7040 (1988). Copyright 1988, American Chemical Society.

.-c>. 03 c

Q)

c

w

1

1

1

1

1

1

1

25

SOLVATION

0.0

time (ps)

5.0

Figure 13. The upper section of the figure is a superposition of an experimentally determined C(t) function for the fluorescing probe 7-(dimethylamino)coumarin-4acetate ion in the water and biexponential fit of C(t).The lower section in the figure portrays the difference between the experimental C(t) and the biexponential fit on a scale that is expanded by a factor of 3 in the Y direction. From Ref. 33 with permission, from J . Phys. Chem. 93,7040 (1988). Copyright 1988, American Chemical

Society.

where C is an instrument constant, k&(X)is a fluorescence radiative rate constant (in the general case solvent polarity dependent), g ( X , A) is a spectral shape function that describes the (X) (polarization) dependent shape of the fluorescence spectrum and S&) is the population of the excited state e. The , in Eq. polarity dependence of the spectral density function k:, [ X ( t ) ] g [ X ( t )A] (23) can be estimated empirically from the static fluorescence spectroscopy and excited state population lifetimes of the chosen probe molecule in various quickly relaxing solvents [23] of known solvent polarity. Such measurements give us values of kF,[X(t = 00)]g[A,X ( t = co)] where the equilibrium excited state solvent polarity, X ( t = 00) varies from solvent to solvent. To estimate an empirical E,(30) scale [lo71 or A* scale the solvent dependent X ( t = a), [lo81 can be used. Alternatively, the solvent dependence of X ( t = co) can be estimated from the equilibrated fluorescence maximum hv,, of the probe in each solvent. Thus the dependence of the photophysical properties of the probe, on its fluorescence maximum hv,,, can be established. This is demonstrated in Figure 14. The usefulness of the curves in Figure 14 is clear if one considers Eq. (23) and the fact that in the subpicosecond solvation experiments, the excited state population S, can be assumed to be a constant during the solvation process. It follows that the curves in Figure 10 represent how the fluorescence intensity at different wavelengths should change as the emission maximum

26

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

0

390 nm

a 400 nm

4 1 0 nm

2.3

~~

2.4

Wavenumber, 104Cm-’

2.5

Figure 14. The spectral density function versus the frequency that corresponds to the fluorescence maximum in various solvents for coumarin 311. The points for each wavelength correspond to different solvents. Reprinted from Ref. 31 with permission, from J . Chem. Phys. 88, 2372 (1988). Copyright 1988, American Physical Society.

shifts, e.g., during the transient solvation process. The curve for 400nm is particularly important. At this emission wavelength, the change in intensity is directly proportional to the frequency of the fluorescence maximum. It follows from Eqs. (1) and (23) and Figure 14 that the measured fluorescence intensity at 400 nm is directly proportional to correlation function C(t) for this probe! In summary, to obtain C(t), one merely has to measure the fluorescence transient at the wavelength where the intensity is linearly dependent on the fluorescence maximum. This variation of the single wavelength method should be denoted by the linear wavelength method. The linear wavelength method has been used to study the solvation dynamics of numerous solvents employing several coumarin probes [31, 32, 97,981. The results are in reasonable agreement with those obtained through the spectral reconstruction method. The linear wavelength method is experimentally much easier than spectral reconstruction, but it also has some disadvantages. For some probes a linear wavelength may not exist. In such a case, a more complex variation of the single wavelength method can still be used [23].

27

SOLVATION

2. Summary of Published C(r) Measurements. Transient solvation dynamics of a variety of excited state probe molecules have been studied in the last few years. Table 1 summarizes the measurements that have been made at room temperature. Table 2 gives a brief summary of the low temperature results. For each entry in Table 1, the measured C(t) was fit by either a single exponential function [exp( - t/z,)] or a biexponential function [A, exp( - t / z J + A, exp(-t/z,)]. For example where a single exponential fit was satisfactory, a dash is found in the zt column. On the other hand, when an observed C(t) was better fit by a biexponential form, the best fit times r 1 and T, and their relative amplitudes (as a percentage) are listed in Table 1. The average relaxation time ( r s ) is the zero moment of the fitted C(t)(i.e., the amplitude weighted average of the two times). For some measurements, the biexponential fit was only a slight improvement over the single exponential form. In these cases two entries are given for the same measurement. There is reasonably good agreement between the measurements made by spectral reconstruction and the linear wavelength method. This supports the validity of this method which takes considerably less instrument time (see above). Some of the measurements in Table 1 were made with 500 fs resolution. Other results (from [32,97,98]) were made with 50 fs time resolution. The shorter time resolution measurements tend to exhibit short components (

-c

0,

1.1 0.8 0.5

0.2

0

J-o.

I

-0.4 1

0

.

.

.

1

.

0.4

.

.

1

3. . 0.0

Log

'

1

'

"

1.2

1

'

I.6

(Eo / E l )

Figure 15. Average response times (z)/zL versus E&,. The numbered points are experimental data from various groups denoted according to solvent and probe molecule: 1 = alcohols/Cul53; 2 = ethanol/DMAPS; 3 = methanol, n-butanol/LDS750; 4 = NMP/Cul53; 5 = PC/Cu153; 6 = PC/CulO2; 7 = nitriles/Cul02, Cu311; 8 = acetates/Cul02, Cu311; 9 = dimethylsulfoxide/LDS-750.The solid curve shows the MSA result for parameters p = 1 and E , = 5. The dashed line represents the continuum prediction. From Ref. 44 with permission, from J . Chern. Phys. 89, 879 (1988). Copyright 1988, American Physical Society.

32

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

It is always much smaller than zD. Furthermore, there is an apparent trend that as the ratio of E,JE, increases the deviation from the Debye model becomes more severe (the solvent propylene carbonate [ 5 ] at low temperature shows an especially big deviation from r,). This trend is consistent with theories that go beyond the continuum model (see Section 1I.E). It is interesting to consider how the new, 50 fs measurements affect the observation that (z,) is often much longer than z1 for very polar solvents. Looking at Table 1, one can see the -5Ofs results give shorter (zs) values which are much closer to r, than the previous subpicosecond results would have indicated. A similar conclusion can be made for propylene carbonate at lower temperatures. Previous C(t) measurements [22] using time-correlated single photon counting gave longer (z,) values than measurements of C(t) with better time resolution [97]. Nevertheless, it would be a mistake to assume that the newer, better resolved measurements necessarily contradict or diminish the trends that have been observed between q and (z,), because, in most cases, the dielectric measurements i ( w ) have themselves been made with limited time resolution (actually in most cases frequency resolution). We assume that if many of the liquids of interest, such as propylene carbonate, were studied by higher frequency 2(w) measurement techniques, new, high frequency components would be discovered which would account at least partially for the short time scale dynamics we see in the solvation C ( t ) data. Indeed, the apparent observation of a single Debye time is inconsistent with theories of liquids that take into account dipole-dipole interactions (see Kivelson [109]). Furthermore, some of the liquids studied have extraordinarily large apparent infinite frequency dielectric constants E , (e.g., E , = 10 for propylene carbonate at lower temperatures [22]). A very large E , value can be a harbinger of unresolved, high frequency relaxation components [l lo]. 7,.

-

E. Modern Theories of Solvation A number of theoretical models for solvation dynamics that go beyond the simple Debye-Onsager model have recently been developed. The simplest is an extension of Onsager model to include solvents with a non-Debye like E(w). In other regards that approach still assumes the solvent is a uniform dielectric continuum and the probe can be represented by a spherical cavity. Newer theories allow for nonspherical probes [46], a nonuniform dielectric medium C4.51, a structured solvent represented by the mean spherical approximation [38-431, and other approaches (see below). Some of these are discussed in this section. Attempts are made where possible to emphasize the comparison between theory and experiment.

33

SOLVATION

1. Onsager Theory for C(t) for Non-Debye Solvents. Generally solvents have more complex dielectric responses than described by the Debye equation (Eq. (18)). To obtain the time dependence of the reaction field R from Eqs. (12, (15), (16) and (7) an appropriate model for dielectric behavior of a specific liquid should be employed. One of the most common dielectric relaxation is given by the Debye-type form, which is applicable to normal alcohols.

If there is no overlap between different regimes, j , then the pulse response function r(t) and accordingly C(t)decays as a sum of n exponents [22,53]. C(t) =

1 uj exp(-t/zFj) n

j=l

with TFj =

+ +

2Emj

E,

2EOj

E,

7Dj

and relative amplitudes

uj x

Eoj - E m j

(2Em j

+ ~c ) ( ~ Ej o+ ~

c )

(27)

Bagchi et al. have derived analogous equations for a solvent with two Debye times associated with two overlapping dispersion regimes [53]. These results allow a test of the Onsager cavity model for a uniform dielectric continuum solvent with a dielectric response that is well modeled by Eq. (24). Our group recently tested this model for methanol. In this case, both high frequency S(w) data (see Barthel et al. [lll]) and short time resolution C(t) data [32] exist. The experimental C(t) curve (solid line) and the predicted C(t) curve (dashed dot line) are in remarkable agreement (Figure 16), as are the predicted and observed (zs) values. The agreement is also excellent for npropanol [97]. In fact, good agreement between the continuum model and C(t) for alcohols has been noted in several cases. The dielectric dispersion for some solvents is poorly modeled by a multiple Debye form. Alternative, E(w) distributions such as the Davidson-Cole equation or the Cole-Cole equation are often more appropriate. The Davidson-Cole equation for S(o)is S,(o)

= Em

+ (1 +-iwto)p' Eo

Em

0 A,, the slope is predicted to decrease. In the limit of AJA, .+ 03, solvent diffusion does not contribute to the reaction process at all and the reaction rate ( w 7,) will become independent of change in 7,. From the correlation observed in Figure 32, they conclude that fluctuations in the intramolecular modes of the molecule make the dominant contribution to the reaction process. Su and Simon have reported similar

Figure 32. The kinetic data for the intramolecular charge transfer reaction of DMAPS in alcohol solutions, k,z, is plotted as a function of the solvent relaxation k e q . These data span the temperature range from -50°C to +3o"C. The solid line corresponds to the case where T~ = q, the expected result for a solvent controlled chemical reaction. The solvents plotted are ethanol (+), propanol (*), butanol( x ), pentanol (O),and hexanol(0). From Ref. 87 with permission from Chem. Phys. Lett., in press.

CONCLUSIONS

61

results for the intramolecular charge transfer reactions of DMABN and DEABN in low temperature alcohol solutions [SO]. Another interesting class of molecules are stilbene derivatives with charge donating groups. These compounds offer the opportunity to explore the role of polar solvation dynamics (dielectric friction) in cisltrans isomerization. Interesting papers on this subject have been published by Waldeck et a]. [I451 and Rulliere et al. [146], Other well-studied polar excited state isomerization examples include pinacyanol, l,l'-diethyL4,4'-cyanine, and crystal violet, which have been studied by Sundstrom, Gilbro and their coworkers [148]; and Ben-Amotz and Harris [148] and others who are referenced in these papers [148,149]. One of the most important class of excited state charge transfer molecules is the alkyloaminonaphthalenesulfonates,which have been employed by Kosower and Huppert to make some of the early observations of the role of solvation dynamics in fast charge transfer reactions. As stated previously compounds in this class have been reviewed recently [4]. The reader is also referred to the innovative nonphotochemical electron transfer studies of Weaver et al. [147]. These authors have been exploring dynamical solvent effects on ground state self-exchange kinetics for organometallic compounds. This work has explored many aspects of solvent control on intermediate barrier electron transfer reactions, including the effect on a distribution of solvation times. The experimental C(t) data on various solvents have been incorporated into the theoretical modeling of the ground state electron transfer reactions studied by Weaver et al. [147]. One of the most important new areas of theory of charge transfer reactions is direct molecular simulations, which allows for an unprecedented, molecular level view of solvent motion during reactions in this class. One of the important themes for research of this type is to ascertain the validity at a molecular level of the linear response theory estimates of solvent interactions that are inherent in Marcus theory and related approaches. In addition, the importance of dynamic solvent effects on charge transfer kinetics is being examined. Recent papers on this subject have been published by Warshel [71], Hynes [141] and Bader and Chandler [137, 138).

IV. CONCLUSIONS An international effort over the last decade on the photophysics of polar aromatic molecules has led to a detailed, and in some cases microscopic, understanding of these systems. One area, the study of the excited state solvation of nearly ideal probe molecules has advanced to the point where

62

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

measurements of this type are leading to valuable data on the motion of complex solutions. The impact of this work goes far beyond photophysical interests alone. In particular, important new theoretical research on the dynamics of liquids has been strongly influenced by the availability of the dynamic solvation data. Photodynamic studies on excited state charge transfer has advanced to the point where some of the simpler prototype molecules are now well understood. The charge transfer dynamic measurements have also been centrally important in the great progress being made on understanding of the solvent’s role in photochemical and nonphotoinduced fast charge transfer processes in solution. Future prospects on the study of intramolecular photoinduced charge transfer and solvation is bright and will undoubtedly involve many new challenges as researchers examine molecular systems with more and more rich photochemical behavior.

ACKNOWLEDGMENTS We would like to thank following individuals for stimulating and useful discussions, and/or important preprints of papers, relating to this review: B. Bagchi, B. J. Berne, W. Baumann, B. Brunshwig, E. W. Castner, D. Chandler, R. I. Cukier, M. Fayer, R. W. Fessenden, G. R. Fleming, T. Fonseca, H. L. Friedman, D. Huppert, R. M. Hochstrasser, J. T. Hynes, J. Jortner, D. Kelly, J. Klafter, M. Kreevoy, R. A. Marcus, M. Maroncelli, N. Mataga, J. Michl, S. Mukamel, M. 0. Newton, W. Rettig, C. Rulliere, J. D. Simon, H. P. Trommsdorff, D. Waldeck, A. Warshel, M. Weaver, and P. T. Wolynes. Our contribution would not have been possible without our co-workers: A. M. Brearley, V. Nagarajan, A. E. Johnson, M. A. Kahlow, T. J. Kang, G. C. Walker, and others who collaborated on this research. Our research discussed in this review was generously supported by the National Science Foundation. PFB was a visiting Assistant Professor at the University of Grenoble I for some of the time during which this review was written, and would like to thank the CNRS for support.

REFERENCES 1. E. Lippert, W. Liider, and H. Boos, in Advances in Molecular Spectroscopy, A. Mangini, Ed., Pergamon Press, Oxford, 1962, p. 443. 2. Z . R. Grabowski, K. Rotkiewicz, A. Siemiarczuk, D. J. Cowley, and W. Baumann, Nouv. J . Chirn. 3, 443 (1979).

REFERENCES

63

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Advances in Photochemistry, Volume15 Edited by David H. Volman, George S Hammond, Klaus Gollnick Copyright © 1990 John Wiley & Sons, Inc.

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS: LONG PATH-FTIR STUDIES Hiromi Niki Department of Chemistry and Centre for Atmospheric Chemistry, York University, North York, Ontario, Canada M3J 1P3

Paul D. Maker Scientific Research Laboratory, Ford Motor Company, Dearborn, Michigan 48121, U.S.A.

CONTENTS I. Introduction 11. Experimental methods A. Preparation of reactive species B. Measurement of rate constants C. Determination of reaction mechanism 111. Atmospheric oxidation of hydrocarbons A. Methane 1. CH,O, reaction 2. HCHO oxidation 3. Kinetics and mechanism for HO CO 4. Kinetics and mechanism for HO CH,OOH B. Ethane and higher alkanes 1. C,H,02 reaction 2. CH,CHO oxidation and CH,C(O)O, reaction 3. R 0 2 (R 2 C,) reaction

+ +

69

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ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

C . Alkenes and alkynes 1. HO-initiated reaction 2. 0,-initiated reaction 3. NO,-initiated reaction D. Aromatics 1. HO-initiated oxidation 2. Unsaturated 1,4-dicarbonyl reaction Acknowledgments References

I. INTRODUCTION In recent years it has been well established that a large variety of volatile organic and inorganic compounds introduced into the atmosphere from both natural and anthropogenic sources undergo chemical transformations induced either by direct photochemical action of the solar ultraviolet (UV) radiation or by reactive photochemical products [1-71. Notably, recent advances in our understanding of the relevant chemical reactions have led to major progress in the development of highly sophisticated, comprehensive computer kinetic models of urban, regional and global atmospheres [8-121. These advances can be attributed largely to recent improvements in experimental methods for obtaining laboratory data on reaction kinetics and mechanisms associated with atmospherically important chemical systems. In this regard, Fourier Transform Infrared Spectroscopy (FTIR) has emerged in a relatively short period as a uniquely powerful analytical tool for both laboratory and field studies of atmospheric chemistry [13-18]. Laboratory studies of atmospheric chemistry demand analytical methods and instrumentation having extremely high performance capabilities, i.e., unambiguous, universal, multicomponent in situ quantitative analysis with high detection sensitivity, and data acquisition speed fast enough to resolve concentration-time profiles of reactants and products. There are two primary requirements for such laboratory studies; firstly, since reactions of atmospheric interest are often sensitive to the pressure of diluent gases and molecular oxygen [6], it is imperative to study them in the presence of atmospheric pressures of air; secondly, many of the atmospherically important molecular products are physically and/or chemically unstable and are difficult to sample and gas-handle for chemical analysis. These two requirements demand the use of an in situ detection method operative under atmospheric conditions. Among various optical detection methods, mid-IR (approx. 500-4000 cm- ') absorption spectroscopy of molecular vibrational-

INTRODUCTION

71

rotational transitions offers the most nearly universal finger-printing capability. UV/VIS spectroscopy is highly useful for selective, sensitive detection, but is limited to those compounds possessing strong, discrete vibronic transitions in that region. Also, note that vacuum UV spectroscopy will suffer from interference from the strong absorption by atmospheric pressures of O,, and that p-wave and far-IR spectroscopies are not compatible with in situ analysis of high pressure samples because of overwhelming pressure broadening of rotational lines. With its inherent capabilities as mentioned above, IR spectroscopy has served atmospheric chemists for many years as a general analytical tool. For instance, it played a key role in the unique identification of O3 and in the discovery of peroxyacetyl nitrate (PAN, CH,C(O)OONO,) over 20 years ago [19,201. However, conventional scanning monochromators do not realize the full potential of the IR method for such applications. Their deficiency arises from the severe limitations imposed by the “trade-off between instrumental resolution and detection sensitivity. Namely, numerous molecules of interest here exhibit rotation-vibrational fine structure of GO.1 cm- spectral width in ambient air. A correspondingly high resolving power is required to record their absorption spectra faithfully and thereby maximize the information content of the spectra for the purpose of quantitative spectroscopic characterization and chemical analysis [131. It has been recognized for some time that these shortcomings are greatly mitigated by use of the Fourier transform-Michelson interferometric method, by virtue of its high optical throughput and its multiplexing advantage [21]. These advantages as well as others (e.g., accuracy of frequency measurement, ease of data manipulation because of its digitized form) have been amply demonstrated by its applications to astrophysical observations and high resolution spectroscopy [21]. However, its full application to routine laboratory analysis of trace gases in the air was not feasible prior to the recent arrival of commercial rapid scan Michelson interferometers with sensitive IR detectors and powerful mini-computers for performing the data acquisition, processing and analysis [13, 18,221. The foregoing considerations dictate the need for a photochemical reactor that provides a clean, controllable chemical environment and simultaneously a long IR absorption path. Several such facilities have recently been described [23-26). That used in our laboratory [18] is quite simple, suited to rapid change of the chemical mixture, and to the conduction and recording of many experiments in a single day. Others are much more complex. That shown in Figure 1 illustrates a fully implemented facility designed specifically for a long path-FTIR instrument system, providing carefully controlled photolysis illumination, temperature control, sample mixing, and diluent-air preconditioning. While the FTIR instrument system as a whole is still evolving, it

Figure 1. Schematic diagram of a reactor designed for long path-length interforometricabsorption measurements upon a photochemically induced reaction system (Courtesy of K. H. Becker).

EXPERIMENTAL METHODS

73

already fulfills virtually all of the performance requirements for studies of atmospheric reactions, and to a high degree. In this chapter, recent laboratory results are highlighted to illustrate the many important advances in the understanding of atmospheric transformations of hydrocarbons achieved through use of the FTIR method.

11. EXPERIMENTAL METHODS Effective utilization of a long path-FTIR spectroscopic facility for studies of atmospheric reactions requires appropriate design of chemical systems to match the analytical capabilities of the instrument (e.g., data acquisition speed and detection sensitivity), and characterization of the large reactors frequently used (e.g., photochemical versus thermal reactor, photolysis wavelength, homogeneity of irradiation, mixing time of reactants in atmospheric pressures of air, diffusion time to reactor surfaces, and various associated heterogeneous processes). Experimental parameters such as initial reactant concentrations, decay rate of reactants and extent of conversion of reactants to primary and secondary products all need to be carefully controlled and fine-tuned together with the optimization of instrumental performance. Some of the experimental techniques commonly employed with the long path-FTIR method for generating reactive species and for obtaining kinetic and mechanistic data are described below.

A. Generation of Free Radical Reactants Atmospheric molecules such as O,, O,, NO and NOz are inherently reactive because of the “free radical” nature of their electronic structures. In addition, there are literally hundreds of free radical species produced in the atmosphere via either photochemical or dark reactions of various hydrocarbons [1,2,27]. Clearly, an important prerequisite to laboratory studies of atmospheric chemistry is the ability to generate key free radical species in a “clean” fashion. Some representative techniques for generating the major free radical reactants, i.e., HO, HOO, R, RO and ROO (R = alkyl or other organic group), in combination with a long path IR absorption cell-chemical reactor are described below. Many of the atmospheric reactions leading to the formation of HO and HOO radicals have been adopted for laboratory FTIR-based studies. Namely, HO radicals are produced photochemically in the atmosphere from 0, in the presence of water vapor, and also from atmospheric products such

74

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

as H,02 and HONO [1,2]: 0 3

+ hv( 1 s are generally applicable to this experimental technique. In particular, there are a large number of reactions involving molecular species which are difficult to monitor by other analytical techniques but are well suited for the FTIR method. Although free radicals are not generally detectable by this technique when reactions are carried out under simulated atmospheric conditions, several indirect methods can be used for their kinetic studies. For instance, large RO radicals (R = 2 C4 alkyl group) undergo unimolecular dissociation and isomerization in competition with bimolecular reactions with O2 [2,27]. It is important to accurately determine the relative rather than absolute values of these uni- and bimolecular rate constants. Ratios of these rate constants can be determined from the corresponding product yields. For free radical reactions involving hydrocarbons, the bimolecular rate constants can be derived from measurements of their decay rates relative to those of a reference compound for which the rate constant has been well established: -d[RH]/dt

= fk~”[x][RH]

-d[Ref]/dt

= kRcf[X][Ref]

EXPERIMENTAL METHODS

77

where [RH], [Ref] and [XI are the concentrations of the reactant hydrocarbon, reference compound and a free radical reactant, respectively, and kR, and kRef are the corresponding bimolecular rate constants. When the decay rates of both RH and Ref are measured in a chemical system containing X, the standard integrated relative rate expression yield the rate constant ratio kRH/kRef:

where tl and t 2 are the two reaction times at which both [RH] and [Ref] are measured. An underlying assumption for this method is that both RH and Ref are removed solely by the free radical X. In cases where X is HO or NO,, most of the chemical systems used are well characterized and the experimental conditions can be optimized to test and ensure the applicability of this assumption. Also, commonly encountered first-order loss processes such as heterogeneous or photolytic decay can be accurately taken into account as correction factors to the above expression [30]. For bimolecular reactions involving two molecular reactants, e.g., 0, alkenes, concentration-time profiles of both reactants can often be recorded simultaneously, and the data can be treated using the integrated second-order rate expressions to derive the corresponding rate constants:

+

and

where [0lelto and [Ole],, are the alkene concentrations at reaction times kale and k,, are the apparent rate constants derived from the decay rates of the olefin and 0 3 ,respectively. These expressions are required in order to treat the kinetic data obtained with mixtures containing approximately equimolar concentrations of 0, and the reactant alkene. Such experimental conditions can be utilized very effectively by the FTIR technique to obtain both kinetic and mechanistic information from the same data set. More commonly, for the purpose of kinetic measurements alone, large values of the reactant mixing ratios, i.e., 10-100, are selected to reduce these cumbersome integrals to simple pseudo firstt = to and t,, respectively, and

78

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

order kinetics:

where [O,],, and [Olelsv are the concentrations of the larger components averaged over the reaction time period (t, - tl), and are very close to their initial concentrations because of their small fractional consumption. Often, the values of ko, and kale thus determined are not identical due to the occurrence of secondary reactions consuming either the reactant alkene in the former or 0, in the latter case. In such cases, the smallest k,, and kale values determined can be tentatively assigned to the bimolecular rate constant for the primary reaction step [35]. Clearly, a complete mechanistic knowledge of the reaction system is required to confirm the validity of such kinetic measurements.

C. Determination of Reaction Mechanism The long path-FTIR spectroscopic method has a great advantage over more selective detection methods for carrying out thorough product analyses to obtain mechanistic information. Simultaneous in situ measurements of all the molecular reactants and products, and thereby the determination of a complete material balance, can be made with this technique. In many cases, individual species appearing in composite product spectra can be uniquely identified and quantitatively analyzed using a computer-based library search of suitable reference spectra recorded beforehand [13,141. In addition to gaseous species, aerosols arising from the condensation of less volatile molecular products have also been observed in the form of the corresponding liquid or solid-phase absorption spectra [36]. Furthermore, a large number of “new” or “as-yet unidentified products have been detected by the FTIR method in complex atmospheric reactions. Since the authenticated pure samples required for recording standard spectra are often not available, tentative identification of unknown species has to be based on kinetic and spectroscopic evidence. Through systematic product studies starting from relatively simple chemical systems and increasing the degree of complexity in a step-wise fashion, there is a good chance for gaining a better understanding of atmospheric reactions involving hydrocarbons as the primary reactants. Some important mechanistic considerations for designing experiments are described below.

EXPERIMENTAL METHODS

79

In the atmospheric free radical reactions involving hydrocarbon species, molecular products of interest are formed via either radical chain propagation or termination steps. Chain Propagation

HCO + 0, + HOO'

+ CO RO' + 0, + HOO' + aldehydes and ketones Chain Termination

+ NO -,RON02 ( b C 4 alkyl nitrates) HOO' + HOO' 0, + H 2 0 z ROO' + HOO' + O2 + ROOH (alkyl peroxides) ROO'

-P

RC(0)OO' RC(0)OO'

+ NOz -+

RC(O)OONO,

(peroxyacyl nitrates)

+ HOO' + 0, + RC(0)OOH

(peroxy acids)

With the FTIR spectroscopic method, these free radical reactions cannot be studied individually under completely isolated conditions since competing side reactions and also secondary reactions involving the molecular products must be taken into account. These mechanistic complications can be greatly reduced by appropriate selection of the method of free radical generation, as described in Section 1I.B. In general, to minimize the occurrence of secondary reactions, the conversion of the molecular reactants, and consequently the product yields, have to be kept as small as is permissible in order to obtain accurate concentration measurements. Also, the reaction time required for such chemical analysis must be kept as short as possible to minimize photochemical and heterogeneous losses of labile products. To optimally utilize the product data for mechanistic interpretations, consideration should be given to the data analysis methods. Although in some instances conventional steady-state kinetic analyses can be performed in order to derive analytical expressions for the temporal relationships among the various observed products, these treatments are not generally applicable to complex chemical systems. Recent progress in numerical analysis methods and the ready accessibility of powerful minicomputers have made it possible to routinely carry out numerical simulations of the experiments to preselect appropriate experimental parameters as well as to evaluate the sensitivity and validity of a reaction mechanism against data bases. It should be noted in this regard that computational techniques dealing with hundreds of reaction steps and chemical species are being

80

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

employed for the development and testing of detailed chemical mechanisms for use in atmospheric models [37-481. Many, but not all, of the experimental results described in the following section involve a relatively small number of reaction steps and species, and are designed to uniquely characterize as many relevant elementary reactions as possible in a given chemical system.

111. ATMOSPHERIC OXIDATION OF HYDROCARBONS Analogous to high temperature combustion processes, atmospheric oxidation of hydrocarbon compounds ultimately leads to the formation of CO, and H 2 0 via free radical reactions involving a large variety of oxygenated intermediate products. Some of the representative products and their free radical precursors are illustrated schematically in Figure 2. As stated in the preceding section, many of the molecular products formed from individual hydrocarbons are still poorly characterized, primarily because of the difficulties associated with their detection. Nevertheless, numerous significant findings have been made in recent years largely as a result of advances in FTIR spectroscopic instrumentation. Some of the reactions described below are not important in the atmosphere per se but serve as reference reactions whose knowledge is essential for a better understanding of the atmospheric chemistry of hydrocarbons.

Figure 2. Schematic illustration showing the atmospheric transformation undergone by hydrocarbons.

81

ATMOSPHERlC OXIDATION OF HYDROCARBONS

A. Methane The chemical fate of CH, in the troposphere is governed solely by the reaction with HO radicals [1,301.The HO-radical initiated oxidation of CH, involves the CH,OO radical as the precursor for oxygenated products such as CH,OOH and HCHO, i.e., R = CH, in Figure 2. Both of these products can, in turn, undergo photodissociation and/or reaction with HO radicals to eventually yield CO and CO,. Illustrated in Figure 3 are the IR absorbance spectra of some of the CH,-derived products in the frequency region of 6003300 cm - I recorded at 0.1 cm - resolution in the presence of 760 Torr of air. All these compounds are seen to exhibit sharp, distinct vibrational-rotational bands which can be used for unique identification.

1745.8

1800

1033.4

1700

I100

1000

(C) CH300H

3601.3 3595.5

A-

2963.8

1 3 3 2 37 . 9 821.I

(D) C H 3 0 0 C H 3

3000

103I

2000

1000

I / A ( c m-' 1

Figure 3. IR absorbance spectra of CH,-derived products: (A) HCHO, (B) CH,OH; (C) CH,OOH; and (D) CH,OOCH,.

82

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

1. CH,OO Reaction

+

CH,OO CH,OO. Reactions of the C H 3 0 0 radicals in air under NO, free conditions have been studied using the long path-FTIR method by Kan, Calvert, and Shaw (KCS) and by the authors' group (NMSB) both in the photooxidation of CH3N2CH3(KCS, NMSB) and in the C1-atom initiated oxidation of CH, (NMSB) [49, SO]. These product studies have provided evidence for the occurrence of the following three reaction channels, suggested earlier by Heicklen and co-workers, for the self-reaction for the CH30, radicals [Sl]:

+ 0, + HCHO + CH30H + 0, CH302CH3 + 0 2

2CH300' -+ (adduct) + 2CH,O'

+

(la) (1b)

(W

followed by the two dominant secondary reactions

+ 0, + HCHO + HOO' C H 3 0 0 ' + HOO' CH,OOH + O2 CH30'

-.

(2) (3)

The rate of reaction (2) is significant in ambient air, even though its rate constant is only 1.5 x 10-"cm3 molecule-'s-' at 298K [5,6]. Also, the self-reaction of the radical product H 0 2 [6], i.e., 2H0, --* H,02 + 0,, could be kept negligible in these experiments, so that the relative importance of the three primary reaction channels could be derived from the product distribution

[HCHoI/[CH30Hl

= (2klo

[CH300H]/[CH,0H]

[CH30,CH31/[CH30Hl

+ klb)/klb

= 2kl,/kl, = klc/klb

Based on measurements of these product ratios, KCS [49] reported the relative rate constants k , , / k , b = 0.75 k 0.10 and k l c / k l b < 0.14. The corresponding values reported by NMSB [SO] were 0.55 L 0.07 (0) and 60.13, respectively. In both studies, a weak, structureless IR absorption band centered at approximately 1030cm-' in the C-0 stretching region was detected but not positively identified. Some portion of this band overlapped with one of the bands of CH,0,CH3 (cf. Figure 3), from which the indicated upper limit values for k l c / k l b were derived.

ATMOSPHERIC OXIDATION OF HYDROCARBONS

83

Prior to these two FTIR studies, Nangia and Benson advanced thermochemical arguments in favor of an alternative H-atom transfer mechanism to account for then available experimental results [52]: 2CH300’ -,CH,OOH

+ kH,OO’

(4)

followed by

CH,OO* + CH,OO.

-+

HCHO + CH,O’

+ 0,

(5)

where CH,OO was assumed to be a zwitterion originally postulated by Criegee as an intermediate in the O3 + C2H, reaction [53]. However, this scheme does not yield CH30H as a primary product, contrary to the observation. Also, experiments designed to examine the presence of CH,OO were negative [49, SO]. Another important implication of the results of these product studies is that many of the previously measured second-order rate constants for the CH302decay should be greater than the true value of k, by as much as 30% depending on the extent of the secondary reactions (2) and (3) in these kinetic measurements [6,54]. Also, reaction (1) presumably has a slightly negative activation energy [S, 61, but the temperature effect on the primary reaction channels remains unknown at present. CH,O, + NO,@ = 1-2). The kinetics and mechanism for the two crucial radical propagation steps in the atmospheric oxidation of CH,, i.e., CH,OO + NO -+ CH,O + NO, and CH,O + 0,-+ HCHO + HOO have been well established by other experimental techniques [.5,63, and these reactions are frequently employed for HO-radical generation in FTIR-based studies. Another potentially important atmospheric reaction of CH,OO is that with NO,. The first positive identification of peroxynitric acid, formed via HOO + NO, + HOONO,, made by the authors’ group using the FTIR method, suggested the probable occurrence of its higher homologues, i.e., RO, NO, -+ ROONO, [SS]. The peroxy nitrates with R = alkyl or haloalkyl group up to C, have been identified in a series of studies from this laboratory [56,57]. For comparative purposes, the characteristic absorption bands of RNO,, RONO, RONO, and ROONOz for R =C,H, are illustrated in Figure 4. As can be seen in this figure, the ROONO, spectra resemble, in general, those of RONO, whose major “nitrate (ON0,)-type” bands occur at around 850 (N-0 stretching), 1290 (symmetric NO, stretching) and 1670 cm-’ (asymmetric NO, stretching), However, the corresponding frequencies for “peroxynitrate (-OON0,)-type” bands are seen to be shifted to 800, 1300,

+

84

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

R-NOz

I576

k-(& \ /

R-ON0

1391 1363

\ /

,166 I

R-ONO,

kJ,

R-OONOZ ' 7I/

I

2000

I

I

1000

I / X (ern-')

Figure 4. IR absorbance spectra of RNO,, RONO, RONO,, and ROONO,

(R = CZH5).

and 1720 cm- '. Both the intensities and shapes as well as frequencies of these bands are distinguishable for the peroxyalkyl nitrates containing small R groups ( S C J but become virtually identical for larger R groups. These peroxyalkyl nitrates are thermochemically unstable at room temperature and readily undergo dissociation back to their radical precursors [27], thus, these compounds have to be produced at a detectable level in the shortest possible reaction time. The main radical-generation scheme employed for their observation involved the C1-atom initiated oxidation of the corresponding hydrocarbon [56]. However, in the case of CH,, its concentration had to be adjusted upward by a factor of lo00 relative to those of

85

ATMOSPHERIC OXIDATION OF HYDROCARBONS

other hydrocarbons because of its low reactivity toward C1 atoms [6]. This resulted in nearly total attenuation of the incident radiation from the TR source in the frequency regions corresponding to the two major bands of CH, at 1300 and 3000cm-’. Thus, the presence of a high concentration of CH, prevented the reliable analysis of one of the “peroxynitrate-type”bands occurring in the vicinity of 1300cm- To minimize such spectral interferences, an alternative scheme was also used whereby the C H 3 0 0 radicals were produced from CH,OOH by C1 CH,OOH + CH,OO HC1 [56]. When CH,OONO, was prepared in this manner in the presence of excess NO,, its observed lifetime became longer than 30min after many repeated runs, presumably due to conditioning of the reactor walls. However, to date there have been no reported experimental data on the thermal dissociation of CH,OONO,. In the case of C,H,OONO,, Edney, Spence, and Hanst used the photolysis of C1,-C,H,-NO,-air mixtures for its generation and, by the FTTR method, determined its dissociative lifetime to be 0.5s at 300K ~581.

’.

+

+

2. HCHO Oxidation

HO + HCHO. Despite the well-recognized, critical role of the HO + HCHO reaction in atmospheric chemistry [1, 113, considerable uncertainty existed until recently concerning both the rate constant and the mechanism operative under tropospheric conditions. Namely, of the two exothermic reaction channels (6a) and (6b), HO

+ HCHO -+ H e 0 + H,O + HC(0)OH + H

(64 (6b)

the H-atom abstraction channel was commonly assumed to be the dominant path, although the possible occurrence of the H-atom displacement channel with a rate constant ratio k6b/k6a < 2 had been reported in the literature [59]. Since reaction (6b) proceeds via a chemically activated adduct, pressure dependence in the rate constant was also expected. In fact, an earlier determination of k(H0 HCHO) (i.e., k,, in 700Torr of air made by the authors’ group) gave a value approximately 50% higher than those obtained directly by Atkinson and Pitts [60] and Stief et al. [61] using the flashphotolysis-resonance fluorescence technique in the presence of diluent Ar at

4 4 0 nm

69a

82

+

83

Q Q

84

I:.'

'

I

.-.', :;

P I

& cC

- hr

85

86

TABLE 14 Angle (degrees) Between Planes in 9-Styrylanthracenes [[lo51 1051 Compound EthyleneIAnthracene 68a 69a 86

78.8 65.5 81.9

Ethylene/Phenyl 7.0 6.8 39.1

AnthraceneIPhenyl 76.8 12.4 84.4

179

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

In the cis-isomer 68a, the ethylene system is twisted out of the plane of the anthracene by 78.8", but the trans-isomer 69a also deviates from molecular planarity considerably, namely by as much as 65.5". cis- 1-(9-Anthryl)-2-phenyIethylene 68a, in contrast to both cis-stilbene 38a, is fluorescent (4= 0.16) at room temperand cis- 1,2-di-9-anthrylethylene ature in cyclohexane. In its rather structureless shape and Stokes shift of 4000 cm- ',the emission spectrum closely resembles that of the trans-isomer 69a, whose emission (4 = 0.46) is characterized by a Stokes shift of 4600 cm- (see Figure 12). For the structurally rigid cis-9-styrylanthracene 86, the emission (4 = 0.50) is structured, and is associated with a Stokes shift of only 850 cm ' (see Figure 14). In acetonitrile, the fluorescence properties of 86 are virtually the same as in cyclohexane. By contrast, the fluorescence quantum yield of cis-9-styrylanthracene 68a is markedly affected by solvent polarity, decreasing from 0.16 in cyclohexane to 0.04 in acetonitrile. Timeresolved fluorescence measurements have revealed that photoexcitation of cis-9-styrylanthracene 68a not only leads to emission from the locally excited state but also to emission from electronically excited trans-isomer 69a. The formation of singlet excited trans-isomer 69a by photoexcitation of 68a has been rationalized in terms of an adiabatic photoreaction. Polar solvents and polar substituents, as in cis-9-styrylanthracenes 68i (R = formyl) and 68j

'

~

300

400

500

800 nm

Figure 14. Absorption and emission spectra of cis-9-styrylanthracene68a (-) and rigid styrylanthracene 86 (----) in cyclohexane. (Reprinted with permission from Ref. 90).

180

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

(R = p-phenylsulfonyl) have been found to enhance the proposed adiabatic isomerization [90,93]. The hypothetical existence of two distinct conformers of cis-9-styrylanthracenes has also been invoked in order to explain the solvent-dependent fluorescence properties of cis-9-styrylanthracenes C92-J. The effects of substitution and solvent polarity on the fluorescence properties of trans-9-styrylanthracenes 69a-k in terms of Stokes shift and fluorescence quantum yields have been summarized in Table 15. The fluorescence quantum yields in cyclohexane solution generally are about 0.5, exceptions with lower quantum yields (0.27) being the N,N-dimethylamino and nitro derivatives. For nonpolar substituted trans-9-styrylanthracenesin acetonitrile solution, the quantum yields are of the same order of magnitude as in cyclohexane. By contrast, the fluorescence quantum yields for trans-9styrylanthracenes substituted by polar groups are drastically reduced in acetonitrile, as would be expected for bichromophoric excited state species of polar character (cf. Section 1II.B).

87a.e

88

89

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

181

TABLE 15 Fluorescence Properties of 9-Styrylanthracenes 68/69 in Cyclohexane and Acetonitrile 190, 951

Compound

68a 68i 68j 69a 69b 69c 69d 69e 69f 69g 69h 69i 69j 69k

R H Formyi 4-Phen ylsulfonyl H N,N-Dimethy lamino Methoxy Methyl Phenyl Chloro Cyano Benzoyl Formyl 4-Phen ylsulfon yl Nitro

Stokes Shifts (cm- I ) in Cyclohexanel Acetonitrile

Fluorescence Quantum Yields in Cyclohexanel Acetonitrile 0.1610.040 0.3 3/0.005 0.3 1/0.002 0.4610.45 0.28/0.0076 0.49/0.0044 0.4610.38 0.5410.3 5

0.4710.39 0.48/0.024 0.41/0.10 0.5110.045 0.51/0.026 0.27/nonfl.

The effects of limited molecular flexibility and increasing deviation from coplanarity of the anthracene and ethylene n-systems on the radiative properties have been assessed in a series of symmetrically 2,2-substituted 1-(9anthry1)ethylenes 87 [63]. As for structurally rigid 9-anthrylethylenes 88 and 89, for which rotation about the anthryl-ethylene single bond is not possible, and in which the ethylene double bond has been forced to be coplanar with the anthracene x-system, their fluorescence quantum yields in cyclohexane are exceptionally high, i.e., 0.94 and 0.96, respectively, and the Stokes shifts are less than 200cm-' (see Figure 15). For nonplanar 9-vinylanthracene 87a and its dimethyl derivative 87b,whose ethylene double bond may be twisted out of the plane of the anthracene by about 60", the quantum yield is 0.63, and the Stokes shifts are around 1OOOcm-' (see Table 16). The fluorescence properties of 2,2-diaryl-substituted l-(9-anthryl)ethylenes 87c-e differ markedly from those of 87a,b by a decrease in the quantum yields of emission, and by the loss of vibrational fine structure of the emission spectra, which is associated with a dramatic increase of the Stokes shifts. For the 2,2-diphenyl derivative 87c in cyclohexane solution, the quantum yield is 0.29, and the Stokes shift is 5600cm-'. For 9-anthrylethylene 87e, in which the formal conjugation has been extended by a terminal methylene group, the quantum yield in cyclohexane is as low as

Figure 15. Absorption (-) and emission spectra (----) of 9-anthryl-substituted ethylenes in cyclohexane. (Reprinted with permission from Ref. 63. Copyright 1983 American Chemical Society). TABLE 16 Absorption and Emission Spectral Data For 9-Anthrylethylenes 87a-e in

G ? X

87

(nm)

(nm)

(cm-’)

C,HI2

CH,CN

a b

386 386 388 388 390

405

1200 900 5600 8100 10700

0.63 0.64 0.29 0.0003 0.015

0.72 0.50 0.00033 lo7

> 2 x 106 0.025 0.28 573 273

3 x 105 0.003 0.34 584 273

2x104 0.003 0.52 607 213

>lo6

0.013 0.54 573 273

-

0.07 608 210

"In toluene. "In cyclohexane.

Photoexcitation of lepidopterene 118 (L; Y = H in cyclohexane solution results in cycloreversion and gives the electronically excited product E* whose deactivation to ground state is characterized by the structureless emission around 600nm (see Figure 32). The quantum yield of the E* emission is 0.58 (0.80),while that of the emission from the locally excited state L* is only 0.005 (0.016). (The lower quantum yield data have been reported by

L*

hv] ihv'

1

hv'

hvl

ihv

/

t I

I

I

I

I

I

c _ L

I

L

E

i

* I l /

I

I

I

I

Figure 32. Absorption (-) and triple emission spectra (----) of lepidopterene in cyclohexane. (Reprinted with permission from Ref. 33).

210

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

TABLE 27 Adiabatic Photolytic Cycloreversion of Lepidopterene in Methylcyclohexane/Isopentane upon Excitation at 274 nm. Temperature Dependent Fluorescence Quantum Yields and Lifetimes 11341" Quantum Yields of Emission from Temperature (K) 295 250 203 170

I55

130

Lifetimes (ns)

L*

E*

L*

0.016 0.033 0.096 0.18 0.21 0.22

0.80 0.75 0.50 0.17 0.076 0.015

2 6 15 16 19

E* 27 26 26 26 25

'Reprinted with permission. Copyright 1984 American Chemical Society.

Becker et al. [133] for cyclohexane solutions, while the higher values in parentheses refer to measurements in methylcyclohexane/isopentane, reported by Ferguson et al. [134]; cf. Tables 26-28). The highly efficient spontaneous regeneration of L subsequent to the deactivation of E* leads to the conclusion that the geometry of the photolytically generated exciplex E* is such as to have the exocyclic ethylene double bond and the anthracene moiety aligned at a distance favorable for bond formation, i.e., the exciplex E* geometry closely resembles the geometry of L*. The photolytic cycloreversion of lepidopterene to give E* occurs at room temperature with a quantum yield of 90%. Upon lowering the temperature, TABLE 28 Adiabatic Photolytic Cycloreversion of Dimethyllepidopterene (DM-L) in Methylcyclohexane/Isopentane: Temperature Dependent Fluorescence Quantum Yields and Lifetimes 11341"

Temperature (K) 295 250 203 170

Quantum Yields of Emission from

Lifetimes (ns)

DM-L*

DM-E*

DM-A*

DM-L*

DM-E*

0.033 0.08 1 0.2 1 0.26

0.80 0.62 0.21 0.047

0.01 0.0004

2 4 12 14

24 24 24 24

'Reprinted with permission. Copyright 1984 American Chemical Society.

211

ADJABATIC PHOTOCHEMICAL CYCLOREVERSIONS

the quantum yield for the E* emission decreases while that for the emission from the locally excited state L* increases (see Table 27). On the other hand, the efficiency of adiabatic conversion of L* into E* increases upon excitation at shorter wavelengths. Thus, the quantum yield of photolytic cycloreversion at 230 nm remains close to unity down to a temperature of 90 K, suggesting that cycloreversion from higher vibrational levels can compete efficiently with usually fast internal conversion [134]. Photoexcitation of lepidopterene in solution also gives rise to a structured emission of low intensity around 400 nm. This emission is attributable to the deactivation of the locally excited state of the E* rotamer A*, formed mainly by inadvertent direct excitation of the ground state cycloreversion product 114 [131]. The absorption and emission spectra of 114 are typical of the anthracene chromophore (see Figure 33). Selective excitation of 114, experimentally possible because of the suitable ground state &]/[A] equilibrium ratio, gives rise to locally excited A*, which in cyclohexane solution at room temperature has a fluorescence quantum yield of 0.84 [131]. The adiabatic conversion of A* into E* is difficult to detect because it proceeds at 298K

I

A W

U

z 4

m a

-m >

m 0

1

c

m

z

4

W

c

z

I

W U

z

yl

m U W

a

0 3 2

U.

i

I

350

Figure 33. Absorption (-) A in cyclohexane.

400

450

500

550 nm

and emission spectra (----) of the cycloreversion product

212

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

OICHLORO-L

TETRACHLORO-L

120

121

PERI-TETRACHLORO-L

122

with a quantum yield of only 0.005, and is associated with an activation energy of 10.3kcal/mol [133]. For dichlorolepidopterene 120 and tetrachlorolepidopterene 121, whose equilibrium ratios (see Table 29) are also such as to make selective excitation of the corresponding cycloreversion products A possible, the activation energies for the adiabatic conversion of A* into E* are 5.7 and 4.0 kcal/mol, respectively, and the formation of E* by excitation of A is easily detectable (see Figure 34). Concerning the molecular geometry of the cycloreversion product 114, molecular mechanics calculations ideally arrive at a conformation "A60" for the 1,2,2-substituted ethane which is characterized by a torsion angle of 60" about the ethane bond which links the anthracene and the 1,l-diarylethylene chromophores [135]. Single-crystal X-ray diffraction analysis of the structurally related 10-(9-anthryImethyl)anthronelOla (see Section IV) suggests that the actual molecular geometry of 114 is governed by intramolecular hydrogen atom repulsions, and that "A60 assumes an energetically favorable conformation by twists (of 46" and 6" in the case of the carbonyl derivative TABLE 29 Ground State and Excited State Properties of Chloro-Lepidopterenes at 298 K

CLI/CAI" L*4Fb E*&(Aexc 280nm)b E*&(Aerc 230 nm)b E*A,, (nm)b "In toluene. cyclohexane.

Dichloro-L 120

Tetrachloro-L

peri-Tetrachloro-L

121

122

63 ,275 K are discussed in terms of the working hypothesis shown in Figure 14, with mention of critical and uncertain points in this scheme whenever appropriate. Studies at relatively low temperatures have led to a similarly complex reaction picture, although the transformation may not always proceed to completion under such conditions [8b]. Low-temperature data have served to identify spectral properties of intermediates, such as the absorption coefficients which have been calculated by Eilfeld and Rudiger [l 113 from measurements in the temperature range 108-273 K with 124-kDa P, in buffer solution containing 66% glycerol.

A. Primary Photoreactions and the First Thermal Steps Flash photolysis with nanosecond laser excitation, monitoring absorption [112- 1141 and optoacoustic signals [82,90,115] of the transients, showed that in the primary reaction of P:, two photoproducts are formed, presumably simultaneously in parallel pathways (see below and [136] for a caveat), with a total quantum yield of 2 0.5: 38% of I+oodecaying with a

7

THE LIGHT-INDUCED P,+ P,, TRANSFORMATION

P:.*

200 @S

Z+E

isomerization of 15,16 C=C

251

\

JPf'

A

conformation relaxations of chromophore protein solvent assisted

Lifetimes: 275 K

Figure 14. Working hypothesis of the mechanism of the P, -+ P,, transformation. The number of steps between I;, and P,, (---) is still unknown, and the thermal restitution of P, from the intermediates is not included. For critical evaluations see in particular Sections IILA, C, and E.

lifetime of ~ i ( 7 0 0 x ) 21 ps, and 62% of I;oo decaying with tZ(700)x 2 0 0 p at 275 K in ethylene glycol-free buffer solution (Table 3). The transient absorption maximum at 695nm, which is also observed in vivo at low temperature [116], had initially been termed "lumi-R and was recognized in the first flash photolytic studies of 64-kDa P, at room temperature by Linschitz [117,118] to arise from two components (for subsequent investigations of 124-kDa P, and of the degraded forms see also [ l l l , 1191271). The energy released as heat in the course of the nonradiative decay of P: to the ground state and detected as a pressure wave by laser-induced optoacoustic spectroscopy (LIOAS) exhibits positive deviations (i.e., 0:> 1; cf. Eq. (1)) from the values which were calculated on the basis of the absorption spectrum of P, alone (Figure 15) [90,115]. This indicates that already within the 15-11s duration of the excitation flash, one or several intermediates must have been formed. These in turn, within the same interval, may again absorb light from an intense laser flash and (at least in part) dissipate heat upon their return to the ground state of the same species (internal conversion) and/or to P, (photochemical back reaction). The formation of primary photoproducts within the nanosecond flash duration was of course to be expected in view of the much shorter lifetimes of the photochromic fluorescence decay compo-

I

I

1.6 -

d

1.4

-

1.2

-

I

-I

1

-

-

.....I

0.8

0.6

- Figure 15. Optoacoustic action spec-

-

-

-

I

I

I

600

650

700

trum of the heat release (a, A-A) from the P,* (64 kDa) and Iyoo states, relative to the light energy absorbed by P, (calibration with CuCI, (.--)). (Spec-

trum after Figure 5 in Jabben et a]. ~901.)

Wavelength (nm)

TABLE 3 Lifetimes, Decay Activation Parameters and Distribution of the Primary Photoproducts I+ooand I:oo of Free (64, 114/118 and 124kDa) and LiposomeBound P, Phytochrome From Oat

Iio0 of free P, at 275 K"

Parameter 'l(700)

@s)

Ea. 1

(kJ x mol-')

64 kDa ~1131

114/118 kDa [121]

21 f 4

15

60+2

logA1

16.2 0.4

*2(700) (ps)

230+30

Ea.2

(kJ x mol- ')

217

63+3

log A ,

15.7 f0.3

at/@, +%)e

0.38f0.04

0.38

124kDa [ll3, 1141

21 +_3 25 3d 54f2 54 f2d 15.1 k0.3 14.9 f0.2d 200f20 240 f 20d 58f 1 58fld 14.7f0.2 14.6f0.1d 0.38f0.02 0.20 f0.02d

+

Iio0 from liposome-bound P, (124 kDa) at 273 K" SYL b

DOL'

[128]

c w

19k2

22k2

59f 10

50f2

16.0+ 1.7

14.3f0.3

226 rfr 23

196f 20

58f3

54+3

14.7f0.5

14.1 f0.2

0.43f0.01

0.43f0.01

data were obtained also for SYL-boundb P, "Potassium phosphate buffer solution. Similar in tris(hydroxymethy1)aminomethane buffer solution. bUnilamellar liposomes of 500-600 A diameter from soybean lecithin. 'Unilamellar lyposomes of .500-600 A diameter from dioleyl lecithin. dPotassium phosphate buffer solution with 20% (v/v) ethylene glycol. 'cf. Eq. (2).

252

THE LIGHT-INDUCED P, + P,, TRANSFORMATION

253

nents P:* and Pf*. The deviations of the a values show a positive maximum around 695 nm characteristic of absorbing I,,,-type intermediates. This was independently confirmed by the observation that during the flash the transient absorption at 695 nm increased by approx. 25%, whereas the P, absorption at 660 nm was bleached during the same period by a comparable percentage [82]. Furthermore, the optoacoustic results also imply that the dynamic photoequilibrium P,esI$Go is established within the 15 ns period (cf. Figure 16), since the thermal forward reaction of I;,, (-+I;,) is considerably slower than the photochemical back reaction to P, at sufficiently high laser intensity [125]. A similar conclusion has been drawn from in vivo experiments with nanosecond laser flashes [34,126,129- 1321. Under natural conditions the equilibrium P, k I: