Photochemistry Volume 29

A Review of the Literature Published Between July 1996 and June 1997

By A. Gilbert

The Royal Society of Chemistry

Copyright © 1998 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-415-3

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, 17,
Part II Organic Aspects of Photochemistry,
Chapter 1 Photolysis of Carbonyl Compounds By William M. Horspool, 71,
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By William M. Horspool, 95,
Chapter 3 Photochemistry of Alkenes, Alkynes and Related Compounds By William M. Horspool, 135,
Chapter 4 Photochemistry of Aromatic Compounds By Alan Cox, 164,
Chapter 5 Photo-reduction and -oxidation By Alan Cox, 204,
Chapter 6 Photoreactions of Compounds Containing Heteroatoms Other than Oxygen By Albert C. Pratt, 239,
Chapter 7 Photoelimination By Ian R. Dunkin, 316,
Part III Polymer Photochemistry By Norman S. Allen, 353,
Part IV Photochemical Aspects of Solar Energy Conversion By Alan Cox, 413,
Part V Artificial Photosynthesis By Anthony Hurriman, 425,
Author Index, 453,

CHAPTER 1

Part I

Physical Aspects of Photochemistry

By Anthony Harriman

Photophysical Processes in Condensed Phases

BY ANTHONY HARRIMAN

1 Introduction

The scope and style of this chapter differs significantly from that used successfully in earlier years, although the literature coverage remains unchanged. More emphasis has been given to experimental methodology and to the advancements made in underlying theoretical concepts. Less consideration is given to the classical distinctions of singlet vs triplet excited states, and instead attention is focussed on the main types of photoprocesses occurring in condensed phases. The current trend in photochemical research is to seek applications for the work and this is reflected throughout the review. There is less coverage of biological systems, except where the biomaterial is simply a target for photoreagents – as with DNA visualization through fluorescent tags, since this subject requires its own chapter. While studies made in homogeneous solution continue to dominate the subject there is a steady migration towards heterogeneous media, with its more demanding analytical platform, and solid-state systems are being actively pursued by the more adventurous research groups. The proliferation of commercial software at affordable prices has ensured that relatively simple photosystems can be examined in great detail, with the inevitable consequence that the conclusions are often overstated. Analytical applications of standard luminescence spectroscopy, such as the routine detection of dissolved cations, have become so numerous that only a few examples can be highlighted herein.

The chapter is organized to cover all important processes leading to deactivation of an excited state in a condensed phase. Special attention has been given to the various fullerenes because of the exceptionally high interest paid to these compounds over the last few years. Other sections consider theoretical concepts, instrumental methods for monitoring photophysical processes, and applications.

2 General Aspects of Photophysical Processes

Various aspects of excited state behaviour have been reviewed or highlighted during the relevant period. The tremendous contribution that flash photolysis has made to our understanding of excited states and free radicals has been exemplified by one of the great pioneers of the field. The general areas of luminescence spectroscopy and chemiluminescence have been reviewed while the photophy-sical properties of benzil have been described in detail The significance of exciplexes as reactive intermediates in polar media has been stressed. Many features of current concern in the field of light-induced electron-transfer reactions have been discussed. Particular attention has been given to energy- and electron-transfer reactions taking place in self-assembled porphyrin complexes. Interest in the emerging field of molecular-scale electronic devices has ensured that the photophysical properties of many novel molecular architectures, such as photoactive rotaxanes and catenates, have been reviewed while the design principles associated with construction of light-activated molecular wires have been outlined. Much attention has been given to the photochemistry of supramolecular entities wherein complex multicomponent conjugates are assembled by non-covalent interactions between simple molecular species. The photoinduced electron transport of macromolecular metal complexes in solution, at solid-liquid interfaces, and in the solid state has been described in a comprehensive report. Some consideration has been given to the design of multi-electron photocatalysts for solar energy conversion.

Several interesting reports have described the photophysical behaviour of conducting or conjugated polymers” and of light-induced energy and electron transfer occurring in polymeric media. A relatively new area of photochemistry concerns trying to better define photophysical processes taking place at liquid-liquid interfaces. Using the technique of second harmonic generation spectroscopy to monitor the course of reaction it has been possible to measure the dynamics of numerous interfacial events. Such diverse processes as photoinduced structural changes, interfacial mass transport, rotational diffusion of molecules held at the interface, interfacial photopolymerization, and energy transfer across the interface have been studied. Photoreactions occurring at the interface between two crystallites have also been reviewed. Some of the photophysical processes associated with organized molecular systems have been addressed using micelles and monomolecular layers to orient the molecules while the photochemistry that takes place between molecules oriented by external electrical fields has been considered. A detailed review of the role of J-aggregates in spectral sensitization of photographic materials has appeared. The application of time-resolved luminescence spectrophotometry to distinguish between static and dynamic quenching processes has been illustrated. A simple methodology is outlined for defining the degree of static quenching in those cases where both static and dynamic quenching processes persist.

Photophysical processes can be used to probe the local structure of organized systems that are difficult to characterize by conventional analytical protocols. Photophysical probes provide valuable structural information for monolayers assembled on metal supports and for polymeric composites. In the latter case, it has been possible to determine the level of thermal residual strain and to monitor architectural deformations. Fluorescence from single molecules continues to attract great attention’ while the dynamics of photodissociative processes have been reviewed, with emphasis given to experimental techniques. There is growing interest in the development of fluorescence-based molecular sensors and of photoswitchable systems. These photosystems involve the design of elaborate molecular receptors for binding adventitious guests in such a way that the complexation event causes a change in the photophysical properties of the host.

3 Theoretical and Kinetic Considerations

A comprehensive discussion has been made of the many factors that can control or modulate the rate of light-induced electron transfer in fluid solution. Particular emphasis has been given to the role of the solvent structure and to mutual donor-acceptor diffusion rates. It is shown that hydrodynamic effects can be very important at short separation distances, with the theory appearing to work especially well in viscous solvents. A related treatment extends the work to consider the rate of charge recombination under similar experimental conditions. It has proved necessary to introduce the concept of a time-dependent dielectric constant to properly account for the time-varying Coulomb potential that characterizes charge recombination between oppositely-charged ions. One- and two-dimensional diffusion processes have been monitored for dye molecules entering into an intense laser beam. A treatise has been presented that accounts for fluorescence quenching taking place at the diffusion-controlled limit with a time-dependent rate c0nstant. An explicit form of the rate constant is derived, together with examples of how to determine the dynamics of bimolecular quenching processes using combined time-resolved and steady-state measurements. A kinetic model has been described for the special case in which a gaseous or liquid reactant interacts with a polymer-bound fluor0phore. The model provides a quantitative description of the quenching behaviour and seems applicable to many such systems. Polymer-bound materials often possess chromophores existing in different environments that differ markedly in their ability to react with added substrates. The same situation abounds in biological media, especially high molecular weight proteins. It is interesting to note, therefore, that a protocol has been invoked to separate the emitting species according to their relative accessibility to quenchers. A comparison of diffusion models relevant to fast time-scales has been made while the distance-dependence for bimolecular fluorescence quenching has been considered. A relationship between the rates of diffusion and triplet-triplet annihilation in liquid solutions has been expressed. It is proposed that triplet-triplet annihilation occurs at the diffusion-controlled rate limit via formation of a weak exciplex-type intermediate.

A theoretical description has been given to account for the kinetics of free radical initiated photopolymerization in solution, using Marcus theory as its basis. The effect of temperature on the yield of charge-separated redox products has been analysed in terms of the dynamics of charge recombination. It is concluded that only the inverted region of a Marcus-type rate vs energy gap plot is seen and not the bell-shaped curve expected for genuine Marcus behaviour. A compilation has been made of the factors affecting adiabaticity in bimolecular electron-transfer reactions, with special reference to the difference between aromatic and aliphatic amines as electron donors. Since the HOMOS are different for these two types of donor, the disparate electron-transfer properties are ascribable to differencies in the size of the electronic coupling matrix elements. Diffusion-limited reactions occurring in one-dimensional systems have been reviewed. A related study considers luminescence quenching taking place by way of a hopping mechanism and uses a Monte Carlo approach to analyse the relationship between luminescence yield and donor concentration. Kinetic models have been presented for bimolecular photochemical reactions occurring on solid surfaces and for energy transfer between ions in solid-state materials.

Considerable attention has been given to quantifying microscopic diffusion of probe molecules or of solvent molecules close to the probe. The effect of rotational diffusion on fluorescence spectra has been considered in terms of the excited state dipole moment. The rotational dynamics of Coumarin-153 have been measured in a wide range of solvents and related to the structure of the solvent. The importance of friction and dielectric-friction is discussed, together with the concept of solute-solvent coupling. In most cases studied, the rotational diffusional behaviour of the probe molecule differs markedly from that expected on the basis of hydrodynamical theory. Calculations of the time-dependent Stokes shift have been reported. Solvation dynamics have been measured for probe molecules dispersed in reverse micelles and, on the basis of time-dependent Stokes shift measurements, it is suggested that individual water molecules within the aqueous pool undergo slow (i.e. nanosecond) rotation. The dynamics of solvation in non-dipolar solvents, such as benzene or 1,4-dioxane, have been described, together with time-dependent Stokes shift measurements. At present, there is no simple theory for modelling such solvation processes, unlike the case with polar solvents.

Several studies have been concerned with improving our existing theoretical understanding of light-induced electron-transfer reactions. Although most attention has been given to reaction in fluid solution, electron transfer occurring in frozen media has been considered in some detail. Frozen glasses present very different properties to those provided by the same solvent in the liquid state, being characterized by a lower dielectric constant, higher refractive index, smaller solvent reorganization energy, and restricted solvation of polar species. The importance of vibrational dynamics for light-induced electron-transfer processes has been considered in light of time-resolved infrared spectral measurements made with fast temporal resolution. Determination of inner-sphere’ and solvent reorganization energies is an important operation in the electron-transfer field and often limits the reliability of calculations. In particular, a simple and realistic methodology for calculating solvent reorganization energies would be a valuable addition to the subject since the outdated solvent dielectric continuum theory requires considerable patching in order to obtain viable numbers. The extent of electronic interaction between donor and acceptor pairs has concerned numerous research groups, especially in connection with covalently-linked molecular dyads. In several such cases, the rates of through-bond electron exchange have been measured for the same donor-acceptor pair separated by different spacer groups and the results discussed in terms of the superexchange model. The conformation of the bridge, as well as the energy of localized HOMOS and LUMOs, is of particular importance in controlling the degree of through-bond interactions while flexible bridges also permit through-space electronic coupling. The relationship between the rate of electron transfer and the thermodynamic driving force for the process has received further study.

Distinguishing between through-space and through-bond electron-transfer processes can be a hazardous task but this distinction has been studied systematically with a series of donor-acceptor dyads of differing orientation. Charge separation and subsequent recombination can show different orientational behaviour to such an extent that in dyads possessing an oblique geometry these processes could take place via disparate routes. Other molecular dyads have been designed in order to separate electron transfer occurring at direct orbital overlap from superexchange-mediated electron transfer. In most photoactive dyads, charge recombination involves both reverse electron transfer to restore the initial ground state and intersystem crossing to form the corresponding triplet ion pair. The relative significance of these two processes depends on the molecular system, especially the lifetime of the radical pair, and the temperature. A careful study of this competition has now been made for charge recombination in rigid molecular dyads. It is shown that the spin multiplicity of the original localized excited state exerts a strong influence on the recombination pathway. Several photosystems have involved coupled electron-proton transfer events.

Photophysical processes involved in the formation of twisted intramolecular charge-transfer (ICT) states continue to be of considerable interest and to receive intense investigation. It has been found that the efficiency for formation of the charge-transfer excited state can depend on the excitation wavelength, increasing with decreasing excitation energy. Such systems are often characterized by dual fluorescence, emission arising from both the charge-transfer state and the locally-excited (or Franck-Condon) state, that shows a marked solvent dependence. A stochastic model has been introduced to account for the fluorescence behaviour of an ICT system in glycerol. An approximate description of the Stokes shift for ICT states has been given and related to the dynamic properties of the surrounding polar solvent. Restricting the conformational mobility of the molecule affects its ability to attain the appropriate geometry to form an ICT state while the conformation of the spacer used to separate donor and acceptor units controls the extent of electronic coupling along the molecular axis. A comprehensive description of radiative and nonradiative decay processes in donor-acceptor complexes has been given. The special case of intramolecular charge transfer in betaines has been considered in terms of the large change in molecular dipole moment expected to accompany the charge-shift reaction. Such systems use an organic bridge to spatially isolate a positively-charged electron acceptor from its complementary negatively-charged electron donor. Light-induced, intramolecular charge transfer should generate a neutral species from a giant dipole and might have interesting applications for nonlinear optical spectroscopy or for second harmonic generation.

The extent to which coupling to symmetric and anti-symmetric modes affects the spectral properties of intervalence charge-transfer complexes has been considered as a refinement to conventional Hush theory in the strong-coupling limit. Exciton trapping in crystals of weak donor-acceptor complexes has been reported and the effect of intermolecular coupling on spontaneous emission has received attention. The use of resonance Raman spectroscopy to determine nuclear reorganization energies for ground-state donor-acceptor complexes has been reviewed. Monte Carlo methods for real-time path integration have been evaluated while a theoretical treatment of simple photoisomerization processes has been given.

(Continues…)Excerpted from Photochemistry Volume 29 by A. Gilbert. Copyright © 1998 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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