Photochemistry Volume 33

A Review of the Literature Published between July 2000 and June 2001

By A. Gilbert

The Royal Society of Chemistry

Copyright © 2002 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-435-1

Contents

Introduction and Review of the Year By Andrew Gilbert, 1,
Part I Physical Aspects of Photochemistry, 11,
Photophysical Processes in Condensed Phases By Anthony Harriman, 13,
Part II Organic Aspects of Photochemistry, 51,
Chapter 1 Photolysis of Carbonyl Compounds By WilliamM. Horspool, 53,
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By William M. Horspool, 74,
Chapter 3 Photochemistry of Alkenes, Alkynes and Related Compounds By William M. Horspool, 119,
Chapter 4 Photochemistry of Aromatic Compounds By Andrew Gilbert, 155,
Chapter 5 Photo-reduction and -oxidation By Alan Cox, 194,
Chapter 6 Photoreactions of Compounds Containing Heteroatoms Other than Oxygen By Albert C. Pratt, 242,
Chapter 7 Photoelimination By Ian R. Dunkin, 307,
Part III Polymer Photochemistry By Norman S. Allen, 337,
Part IV Photochemical Aspects of Solar Energy Conversion By Alan Co, 405,
Author Index, 415,

CHAPTER 1

Part I Physical Aspects of Photochemistry

By Anthony Harriman

Photophysical Processes in Condensed Phases

BY ANTHONY HARRIMAN

1 Introduction

This review follows the format adopted in recent years, with minor modification according to the volume of work presented in particular areas. It appears that interest in single-molecule photophysics is less than in previous years but that there has been an increase in the number of publications concerning fullerenes. Several research groups are making serious efforts to design molecular-scale photochemical devices and there has been a tremendous upsurge in interest in the synthesis of dendrimers containing multiple chromophores. No attempt has been made to cover all the literature pertaining to the application of luminescent dyes for the detection of solutes in solution and only a few such highlights are given. There has been a progressive increase in the use of quantum chemistry to gain an improved understanding of photophysical processes and it is clear that such approaches, especially quantum dynamics and molecular dynamics simulations, will make major contributions to photophysics research in the near future. Increased interest has also been shown in intramolecular proton-transfer reactions, since the ultrafast instrumentation often needed to follow such processes is now available.

The chapter is organised to cover all important processes leading to the 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 past few years. Other sections consider theoretical concepts, instrumental methods for monitoring photophysical processes and applications. The huge number of journals now in the market place precludes complete coverage of the subject.

2 General Aspects of Photophysical Processes

Various aspects of excited state behaviour have been reviewed or highlighted during the relevant period. Thus, several general reviews of organic photochemistry have appeared and the importance of luminescence spectroscopy has been stressed. The photophysics, photochemistry and optical properties of polyimides have been discussed in terms of charge-transfer effects. Related work has illustrated the importance of ultrafast transient spectroscopy for elucidating the primary photophysical processes inherent to tailor-made organic chromophores in solution. The special effects exerted by intense laser pulses have been highlighted while the use of the phase of the incident light to establish coupling mechanisms has been reviewed. Specifically, this latter work has examined how the phase of a transition dipole matrix element can be measured by the interefence between competing quantum mechanical paths.

The effect of organised media on the photophysical properties of organic molecules has been considered with particular reference to relating dynamics of the probe molecule to the microscopic properties of the host medium. Light harvesting for solar energy conversion, especially with regard to semiconductor-based solar cells, has been reviewed in a comprehensive fashion. An interesting account has been given of n,π* photochemistry for compounds other than aromatic ketones while the effect of ultrasound on the photopinacolisation of benzophenone has been reported. It appears that ultrasound can modify the course of bimolecular processes originating from triplet excited states. Various aspects of photochemical isomerisation have been reviwed, with special attention given to the so-called ‘hula-twist’ mechanism and to isomerisation from the triplet excited state. The more common singlet state induced photo-isomerisation has also been reviewed. The photophysics of phenyl-substituted polyacetylenes, these being important materials for light-emitting polymeric devices, have been subjected to detailed theoretical examination. An important conclusion to emerge from this work is that polyacetylenes display a smaller optical band gap than found for polyenes of the same chain length.

Fluorescence excitation spectra have been reported for some organic radicalsand a new technique, the so-called ‘piston source method’, has been introduced to measure absolute concentrations of singlet molecular oxygen in solution. A review has appeared that covers the basic principles involved in the solvation dynamics of triplet excited states in viscous liquids or glassy solids. It appears that there are many cases where the phosphorescence signals are strongly influenced by local dipolar reorientation dynamics and the mechanisms for such effects have been discussed in detail.

The photophysical properties of tetrapyrrolic pigments continue to attract attention and increased interest has been given to deactivation of the upper-lying excited singlet states. The underlying mechanisms whereby light-emitting polymeric devices operate have been reviewed and the role of electron-transfer reactions in photoinitiation of polymers has been examined. Considerable attention has been given to the photophysics of transition metal complexes, especially with respect to metal-to-ligand, charge-transfer excited states. A direct obsevation has been made of the charge-transfer-to-solvent reactive mode in the photoexcited alkali metal anion Na-. A theoretical evaluation has been made of photoluminescence from semiconductors.

The intramolecular magnetic interactions between two nitrosyl nitroxide radicals separated by a thiophene residue have been probed and compared with the corresponding phenylene-linked compound. Closely-related systems have also considered the photoswitching of intramolecular magnetic interactions in radical-substituted chromophores. Photoinduced spin states have been reported for compounds known to undergo a light-induced phase change and the mechanisms for such magnetic interactions have been considered. The potential for generating molecular-scale magnets has been highlighted.

The use of confined environments, such as zeolites and clays, has received considerable attention while many aspects of bimolecular photochemistry occurring in crystals have been reviewed. A general kinetic model has been proposed to account for the optically and thermally stimulated luminescence observed with samples of pure quartz. There is continued interest in using luminescent compounds to detect analytes in solution. Similar attention has been given to the analytical applications of chemiluminescence. The design of liquid membranes bearing light switches has been highlighted. In such systems, a liquid membrane is used to separate two different solutes, usually dissolved in aqueous solution. Selective transport across the membrane is facilitated by doping the membrane with a light-activated carrier molecule. The general technique of sonoluminescence has been reviewed, especially with regard to single-bubble sonoluminescence.

A light-drived moleular rotor, capable of unidirectional rotation, has been described. Other interesting molecular-scale photochemical devices have been constructed from catenanes and rotaxanes while a fluorescent probe has been reported to mimic the functions of a simple logic gate. Ways to control the helix content of short peptides by photochemical means have been reviewedwhilst the design of ‘off-on’ luminescent systems has received much attention.A reversible molecular shuttle has been produced where translational motion is controlled by hydrogen bonds. Related molecular switching events have been described A review has covered the application of near-field fluorescence imaging to the detection of single pigment molecules using an open-ended probe.

Recent years have seen a major initiative made into placing a large number of chromophores in close proximity, primarily to build models for the natural light-harvesting complexes. A variety of approaches have been advocated and the effects of spatial crowding on the photophysical properties of the chromophores have been documented. Thus, the fluorescence properties have been described for nano-sized star-like molecules, organic-based dendrimers,and doughnut-like assemblies. A dendrimer has been described that hosts 32 dansyl groups and optically active dendrimers have been synthesised that are capable of modest levels of entioselective fluorescence sensing. Other dendrimers have been reported to display ‘off-on’ luminescence switching effects in the presence of certain solutes. Photophysical probes for organised assemblies have been described while artifical light-harvesting arrays have been assembled by way of non-covalent associations. The photophysical properties of large aggregates of tetrapyrrolic pigments have been reported and the fluorescence behaviour of other nano-sized aggregates has been recorded.

Parallel to the studies devoted to the preparation of photoactive dendrimers, there has been a concerted effort to construct linear molecular-scale wires for future use in molecular opto-electronic devices. Thus, ultrafast energy transfer has been observed to take place in long molecular wires formed from zinc porphyrins. Related meso,meso-linked porphyrin arrays have been described and the use of ethynylene bridges to couple together porphyrin-based chromophores has been highlighted. Related porphyrin-based arrays have been formed by fusing adjacent porphyrins at the pyrrole positions. Many different dendrimers have been reported to contain photoactive transition metal-based chromophores. The emission properties of a molecular rectangle have been described.

Great interest has also been shown in the design of novel light-emitting polymeric devices and the photophysical properties of appropriate model compounds and oligomers have been measured. In particular, the importance of interchain exchange effects has been stressed while the significance of triplet excited states has been considered. The luminescence properties of highly-conjugated oligomers have been reported with a view to better establishing the mechanism for light emission from the corresponding polymeric devices.

3 Theoretical and Kinetic Considerations

Theory has always been an integral part of photophysical investigations and the current availability of cheap but powerful computers has greatly aided the detailed examination of experimental data. There is a growing use of quantum chemical calculations to interpret decay kinetics and to explore how the solvent enters into photophysical processes. Experimental verification has been provided for the theoretical prediction of a kinetic transition in a reversible binding reaction driven by the difference in effective lifetimes of bound and unbound species. A hopping model has been proposed to account for thermally stimulated luminescence in disordered organic molecules. The model is based on the premise that such emission arises from radiative recombination of long-lived geminate pairs of charge carriers. A theoretical model has been presented that allows determination of the donor-acceptor distribution functions in Forster-type energy transfer. Unlike previous approaches to this problem, the new model makes no a priori assumptions about the nature of the distribution and it is reported that the method has particular application to measuring the acceptor distribution in luminescence sensing protocols. The possible role of inversion symmetry in intramolecular vibrational relaxation has been considered and the dynamics of vibrational motion in electron donor-acceptor complexes has been addressed by ultrafast transient spectroscopy.

The photodynamics of ethylene have been explored by ab initio quantum chemical calculations of the conical interesction. It is reported that the twisted geometry of ethylene corresponds to a saddle point, rather than being a local minimum. Other reports have shown the value of the conical intersection method while a theoretical analysis has been made of the absorption spectra and dynamics of photosynthetic reaction centres. This latter work is based on a microscopic exciton-vibrational model that includes temperature effects and that takes into account the inherent inhomogeneity of the reaction centre complex. An approximate analytical solution has been provided for photochromic and photorefractive gratings observed with certain materials.

A configuration-interaction description has been given for intersystem crossing and spin-orbit coupling in conjugated polymers. An analytical routine has been described for non-linear least-squares fitting of fluorescence quenching data and a quantum dynamics approach has been applied to analyse fractional wave packet behaviour in random phase fluorescence interferometry. Theoretical studies have been used to probe the photophysics and structure of adenine, various aromatic amino acids and 7-azaindole. Semi-empirical AM1 calculations have been used to calculate potential energy surfaces relating to isomerisation of unsymmetrical carbocyanine dyes. It was found that the isomerisation potential surface was highly dependent on chain length and on the nature and position of the terminal groups. The results of this study also indicated the importance of steric hindrance around the isomerising bond. Related studies have addressed the triplet potential energy surface for hexa-triene. Twisting around the C-7-C-6 and C-4-C-7 bonds in coumaric acid has been studied by ab initio MO calculations while related calculations have been applied to the problem of photochromiticity in substituted dithienylethenes and to the photoreactivity of fulgides.

A theoretical study has considered the mechanism of energy transfer in metal cation-containing cryptates and separate work has focussed on the nature of the Kekule vibration in styrene for the S1 state. An ab initio study has considered the mechanism for photoisomerisation of acrylic acid and has shown the importance of the triplet state as a reactive intermediate. Theoretical investigations have explored the spectroscopic properties of charge-transfer complexes and have described anharmonic effects in electron-transfer processes. A computational study has considered the factors that govern the triplet state reactivity of 1,4-pentanone while other studies have examined how the fluorescence properties of highly conjugated organic molecules are affected by changes in molecular geometry. The reaction pathway for electrocyclic reactions has been studied by ab initio multistate, second-order perturbation theory.

The ground- and excited-state structures of intramolecular donor-acceptor complexes have been examined by DFT calculations while large-scale confor-mational exchange has been studied by molecular dynamics simulations. The role of molecular symmetry in intersystem crossing processes perturbed by an external magnetic field has been considered and a dynamical theory has been proposed to account for time-resolved fluorescence spectroscopy. Propagator calculations have been described for the electronic spectra of photochromic spiro-oxazines. The ultrafast energy- and electron-transfer reactions occurring in bacterial photosynthetic systems have been explained in terms of a microscopic model and contributions of short-distance donor-quencher pairs in intermolecular fluorescence quenching have been considered. Incorporating such effects into conventional Rehm-Weller quenching expressions is reported to explain the discrepancies between theory and experiment. The special case of reversible intramolecular energy transfer has been treated in terms of integral encounter theory.

Certain aspects of photochemical ring-opening reactions have been subjected to theoretical examination. The question of non-Arrhenius temperature dependencies in electron-transfer processes has received further study. Theoretical studies have also addressed the low-lying excited singlet states in styrene,reversible intermolecular photochemical processes, Franck-Condon factors in polyaromatics, and proton-transfer reactions. This latter study used a semi-classical molecular dynamics simulation to construct the relevant potential energy surfaces. Proton transfer was found to be greatly affected by isotopic substitution and to be coupled to internal vibrational modes. Potential energy surfaces have also been constructed for light-induced metal-ligand bond weakening.

4 Photophysical Processes in Liquid or Solid Media

4.1 Detection of Single Molecules. – The most elegant photophysical processes are undoubtedly those attributed to single or isolated molecules and this type of spectroscopic investigation has been popular for a number of years. The review period has seen little progress in this area, however, and most research has been devoted to looking at isolated molecules on inert surfaces. A technique has been introduced, based on near-field fluorescence imaging, that allows detection of single molecules with a spatial resolution of about 10 nm. This high resolution is attributed to the onset of non-radiative energy transfer from the fluorescent molecule to the coated metal of the probe. A theoretical investigation has been made for single-molecule fluorescence detection on thin metallic layers using a classical electrodynamics approach. A correlation has been made between the fluorescence intermittency and spectral diffusion for single semiconductor quantum dots. A new algorithm has been described for single-molecule identification by time-correlated, single-photon counting techniques.

(Continues…)Excerpted from Photochemistry Volume 33 by A. Gilbert. Copyright © 2002 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.