Photochemistry Volume 30

A Review of the Literature Published Between July 1997 and June 1998

By A. Gilbert

The Royal Society of Chemistry

Copyright © 1999 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-420-7

Contents

Introduction and Review of the Year By Andrew Gilbert, 1,
Part I Physical Aspects of Photochemistry,
Photophysical Processes in Condensed Phases By Anthony Harriman, 13,
Part II Organic Aspects of Photochemistry,
Chapter 1 Photolysis of Carbonyl Compounds By William M. Horspool, 59,
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By William M. Horspool, 78,
Chapter 3 Photochemistry of Alkenes, Alkynes and Related Compounds By William M. Horspool, 119,
Chapter 4 Photochemistry of Aromatic Compounds By Alan Cox, 149,
Chapter 5 Photo-reduction and -oxidation By Alan Cox, 188,
Chapter 6 Photoreactions of Compounds Containing Heteroatoms Other than Oxygen By Albert C. Pratt, 230,
Chapter 7 Photoelimination By Ian R. Dunkin, 296,
Part III Polymer Photochemistry By Norman S. Allen, 331,
Part IV Photochemical Aspects of Solar Energy Conversion By Alan Cox, 389,
Author Index, 398,

CHAPTER 1

Part I

Physical Aspects of Photochemistry

By Anthony Harriman

Photophysical Processes in Condensed Phases

BY ANTHONY HARRIMAN

1 Introduction

The format of this chapter follows that adopted last year. The first section deals with the general aspects of photophysical properties of molecules in condensed phase, with particular emphasis being given to supramolecular systems. This is followed by a review of progress made in the theoretical description of photophysical events and of the kinetic models used to describe photophysical processes taking place in solution. The third section reviews the many different types of photophysical event that might accompany deactivation of an excited state while a separate section is concerned with possible applications for molecular photophysics. The final section describes advances made with instrumentation and data analysis. Regretably, shortage of space prohibits a full listing of all the relevant literature that has appeared during the review period.

2 General Aspects of Photophysical Processes

The complementary features of laser flash photolysis and pulse radiolysis have been reviewed while the application of electron spin polarization techniques to the measurement of molecular photophysics in solution has been highlighted. The fundamental aspects of luminescence and chemiluminescence have been considered in detail and particular attention has been given to the theory of spontaneous emission. A comprehensive review of the dynamics of the fluorescence Stokes shift has appeared and the underlying theory for radiative migration of electronic excitation energy has been considered by special reference to the fluorescence of Rhodamine 101 in ethanol. Attention has been paid to the possibility of extracting a statistical temporal signature of quantum chaos from time-resolved fluorescence decay curves while other studies have concentrated on the implications of gelatin fluorescence for photography. The technique of non-resonant two-photon fluorescence spectroscopy has been reviewed. This is a new field of laser spectroscopy, having particular relevance to the in situ study of biological molecules, that can be readily adapted for examination of two-photon anisotropy of large molecules dispersed in membranes.

The mechanism for light-induced hydrogen atom abstraction by n,π* excited states has been considered while separate studies have addressed the issue of ultrafast proton-transfer reactions. Application of femtosecond spectroscopy to solvation and electron-transfer dynamics continues to be an attractive subject. Recent attention has focussed on the dynamics of geminate recombination in charge-transfer complexes and in geminate radical pairs and of nonergodic reactions. Theoretical models have been presented to account for the selectivity of organic singlet and triplet photoreactions. Other studies have considered the implications of vibronic coupling for light-induced electron-transfer processes occurring in multicomponent supramolecular systems. Marcus theory has been applied to electrochemiluminescence and quantum beats have been reported in the recombination of radical ion pairs, as caused by hyperfine interaction in the radical anions.

Interest has been shown in the development of protecting groups that can be cleaved by photochemical electron-transfer reactions and in the use of polarized light to attain stereocontrol of reactive encounters. The possible discrimination between enantiomers by way of electron-transfer quenching of excited states continues to attract attention, as does the application of fluorescence spectroscopy to monitor chiral resolution of enantiomers. Several new systems have been developed for chiral recognition of targeted substrates. The interconversion of enantiomers has been monitored by time-resolved chiroptical luminescence spectroscopy while a theory has been proposed to account for the special case of circularly polarized fluorescence emanating from chiral nematic liquid crystals. Circularly polarized luminescence has also been reported from rigid complexes of chiral macrocyclic tetranaphthylamides.

Considerable attention has been given to the study of those systems in which light is used to engineer a conformational change in a large molecule. Such effects are important in certain biological systems and the effect of solvent on the conformational equilibrium of previtamin D has been described. Related studies have been devoted to following the α-helix-to-coil transformation for a series of photochromic polypeptides. Several artificial phototropic systems have been developed” in which light is used to drive a largescale conformational change. The mechanism and dynamics of the transformation have been followed for the photoejection of a guest from a macrocyclic host and of its subsequent reentry into the cavity. The photochemistry of many supramolecular assemblies has been considered in terms of energy or electron transfer between the reactive subunits. A strategy has been proposed for the construction of extended 2D and 3D arrays that display long-range magnetic ordering and for which there are interesting photophysical properties. During the past year there has been a tremendous upsurge in interest in the design of fluorescent sensors for the recognition of target molecules in solution and the field has been comprehensively reviewed. Because of its great biomedical significance, attention has focussed on the detection of nitric oxide by fluorescence techniques. The development of photozymes for water purification has been reviewed.

Attempts to better understand natural photosynthesis and to construct artificial models continue to be important areas of modern photochemistry. The photophysical properties of certain bacteriochlorophylls and carotenes have been reported while the mechanisms of electron and energy transfer in natural photosynthetic systems have been reviewed. Particular attention has been given to the design of artificial photosystems that mimic the manganese-containing enzyme of PS2 that is responsible for water oxidation in green plants. Most of these photosystems use a powerful oxidant to photo-oxidize a simple manganese complex but they lack the ability to store oxidizing equivalents that is an inherent feature of the natural organism. Progress is being made in this area, however, and many of the important spectroscopic features of the natural manganese cluster can now be duplicated in model systems.

3 Theoretical and Kinetic Considerations

One of the great strengths of photochemistry is the close interplay between experimental work and theoretical analysis and there has been a constant evolution of the theoretical framework for many types of photochemical processes. A model has been proposed for the specific case where proton transfer is coupled to light-induced electron transfer in a polar solvent, with proton transfer being sequential or concerted to the electron-transfer event. Semi-classical and quantum mechanical treatments have been given for nonadiabatic photodissociation dynamics, with particular reference to interference effects. A complete thermodynamic classification has been presented that describes orientational relaxation in both excited and ground states following Franck-Condon transitions. The photophysical properties of styrene and indene have been explained in terms of a combined molecular mechanics and valence bond methodology, wherein the geometry of the excited state is fully optimized before calculation of the various potential energy surfaces. Quantum mechanical approaches, based on the LCAO MO SCF method, have also been used to calculate the photophysical properties of complex heteroaromatic compounds.

Electron-transfer processes continue to play a central role in molecular photo-physics and there remains much fundamental information to be learned before we have a complete understanding of the mechanism of such reactions. Although it has been known for many years that intramolecular charge recombination can lead to population of both the ground state and the triplet excited state of the chromophore little is known about the mechanism of this latter process. It has now been found that the two recombination steps possess quite disparate attenuation factors for through-bond electron tunnelling. An evaluation has been made of the electronic coupling matrix elements for electron exchange in mixed-valence complexes while an expression has been derived for the dependence of the rate of self-exchange on the extent of electron delocalization in the corresponding mixed-valence complex. Path integral calculations have been used to follow the charge-transfer events that follow from laser excitation of these mixed-valence complexes. The calculations indicate that the strong electronic interaction between the metal centres gives rise to very fast oscillations in the electronic state population as the wave function oscillates coherently between donor and acceptor. It is suggested that the Fermi golden rule might not be applicable to such systems. Other theoretical evaluations of charge-transfer effects in solution and in weakly coupled conjugated polymers have been described. Detailed information about the nuclear and solvent re-organization energies that accompany electron transfer is critical for a deeper insight into the reaction mechanism and evaluation of these parameters is an important subject. Replacing the solvent bath with a protein scaffold, as in photosynthetic bacterial reaction centres, introduces additional complications for understanding the dynamics of electron-transfer events. Attention has been given to modelling the non-linear dynamics of photosynthetic reaction centres and to estimating the importance of protein relaxation dynamics on the rate of electron transfer from the bacteriochlorophyll special pair.

Increasing attention is being paid to the possibility of calculating the fluorescence properties of large molecules, especially for those cases where charge-transfer interactions are likely to be important. Calculations have also been made to gauge the significance of incorporating vibronic transitions into the expression for ultrafast solvation of excited state dye molecules in polar solvents. Experimental studies have addressed the mechanisms by which rapid re-orientation and solvation of dye molecules occur in organized media and in polar solution. Particular attention has been given to seeking a better understanding of the transient dynamics of the solvatochromic shift in binary solvents. The role of the excitation lifetime in controlling the initial distribution of electron-transfer products has been considered.

Given the widespread interest in the photophysics of supramolecular assemblies wherein numerous identical chromophores are built into a large array it is surprising to find that little concern has been shown about the possibility of fast energy migration amongst the chromophores. This subject has now been examined and an expression has been formulated for the temporal dependence of the distribution of the excited states of structurally identical chromophores in bichromophoric molecules. The expression describes the temporal dependence of emission anisotropy in cases where dissipative iriteractions between the chromophores can take place, A combined quantum mechanical and classical description of non-adiabatic photoprocesses, such as internal conversion, has been given. The model might be useful for looking at interconversion between high-lying excited states. Advances in kinetic theory, with particular reference to fluorescence quenching in fluid solution, have been reported while the advantages of monitoring fluorescence quenching by way of stimulated emission have been stressed. A theoretical expression has been given for resonance energy transfer occurring in dense dispersive media and separate calculations have been concerned with the possibility of energy transfer taking place via higher-energy excited states. Limitations to the analysis of fluorescence quenching techniques have been raised while a kinetic model has been presented for the photomodulated transport of species across liquid membranes.

Quenching of excited singlet and triplet states by molecular oxygen has been examined for systems where the second-excited triplet states lies close in energy to the first-excited singlet state. Other studies have reported anomalously high rates of bimolecular fluorescence quenching that could be ascribed to the effects of static quenching. A kinetic model has been proposed for the slow photo-tautomerization of free-base porphyrins. Numerous studies have considered the kinetics of fluorescence quenching in terms of structural or environmental effects, such as temperature, solvent, electrolyte composition, or viscosity. Additional studies have addressed the issue of non-diffusional formation of excimers and the relaxation behaviour of excited-state complexes. The influence of secondary structure on the decay properties of fluorescent donor-acceptor labelled peptides has been considered while the fluorescence of chlorophyll in concentrated solution has been re-examined. Particular attention has been given to the kinetics of triplet-triplet annihilation in fluid solution and to the various factors that influence the rate of reverse electron transfer in charge-transfer complexes or radical ion pairs. Diffusional processes available to transient species formed during photochemical processes, especially hydrogen-atom abstraction reactions, have been reviewed and it has been shown that the diffusion coefficients depend markedly on the nature of the surrounding solvent.

Photophysical processes involved in the formation of twisted intramolecular charge-transfer (ICT) states continue to be of considerable interest and to receive intense investigation. Theoretical studies have addressed the structure of the ICT state in amino-substituted benzenes while the dual fluorescence of dialkylaminobenzonitrile has been examined in different solvents and as a function of temperature. Coupling between the close-lying S1 and S2 energy levels in dimethylaminobenzonitrile has been detected by picosecond emission anisotropy and related to internal twisting of the amino group. Numerous derivatives of dimethyaminobenzonitrile have been studied in order to determine the structural requisites for dual fluorescence. Similar ICT state formation occurs in diphenylamino-substituted biphenyl and in N,N-dimethyladenosine while triple fluorescence has been found for certain benzonitriles substituted with tetraazacyclotetradecyl rings. The dynamics of ICT state formation have been monitored for amino-substituted oxadiazoles and ketones while the effects of pressure-tuning of the solvent relaxation time on the rate of internal rotation of the ICT state have been measured. At high viscosity, it appears that the rate of ICT state formation greatly exceeds the solvent relaxation time. The importance of hydrogen bonding in controlling ICT state formation has been stressed while the involvement of the excited triplet state has been noted for ICT states formed from 4-amino-N-methylphthalimide.

Ultrafast intramolecular charge transfer occurs in the excited singlet state of trans-4-dimethylamino-4′-cyanostilbene, followed by trans-to-cis isomerization by way of a highly polar ICT state. The activation barrier for isomerization increases with increasing solvent viscosity in nonpolar solvents but the specific effect of viscosity cannot be separated from polarity effects in alcohols or polar nitriles. Competing photoisomerization also occurs in 4-dialkylamino-9-styryl-acridines. Several reports have addressed the photoprocesses occurring in the push-pull polyenes where both one- and two-photon effects have been observed and where complex formation has been noted at high concentration. The absence of dual fluorescence from 4-dimethylaminophenylacetylene has been attributed to a large energy gap between S1 and S2 levels but ICT state formation has been detected for N,N-dimethylaminophenyl-4′ -cyanophenyl-acetylene by virtue of picosecond laser spectroscopy measurements. 9,9′-Di-anthrylmethanol shows complicated fluorescence behaviour that depends markedly on temperature and solvent polarity due to the co-existence of local π,π*, excimer, and ICT states. The results have been considered in terms of different molecular conformations for which the degree of orbital overlap between the aryl rings can vary over a wide range. Internal conversion in 1-aminonaphthalene derivatives has been linked to twisting of the amino group, even in nonpolar solvents, and the activation energy has been measured. In polar solvents, ICT state formation begins to compete with internal conversion but there are interesting effects caused by pre-twisting of the donor group. Formation of an ICT state has also been invoked to explain the photophysical properties of 4-(1H-pyrrol-1-yl)benzoic acid.

(Continues…)Excerpted from Photochemistry Volume 30 by A. Gilbert. Copyright © 1999 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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