Photochemistry Volume 15

A Review of the Literature Published Between July 1982 and June 1983

By D. Bryce-Smith

The Royal Society of Chemistry

Copyright © 1984 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-135-7

Contents

Introduction and Review of the Year By D. Bryce-Smith, xiii,
Part I Physical Aspects of Photochemistry,
Chapter 1 Photophysical Processes on Condensed Phases By R. B. Cundall, 3,
Chapter 2 Gas-phase Photoprocesses By M. N. R. Ashfold, 55,
Part II Photochemistry of Inorganic and Organometallic Compounds,
Chapter 1 The Photochemistry of Transition-metal Complexes By A. Cox, 141,
Chapter 2 The Photochemistry of Transition-metal Organometallic Compounds By A. Cox, 170,
Chapter 3 The Photochemistry of Compounds of the Main Group Elements By A. Cox, 188,
Part III Organic Aspects of Photochemistry,
Chapter 1 Photolysis of Csrbonyl Compounds By W. M. Horspool, 199,
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreactions of Cyclohexadienones and Quinones By W. M. Horspool, 212,
Chapter 3 Photochemistry of Olefins, Acetylenes, and Related Compounds By W. M. Horspool, 260,
Chapter 4 Photochemistry of Aromatic Compounds By A. Gilbert, 291,
Chapter 5 Photo-reduction and -oxidation By A. Cox, 342,
Chapter 6 Photoreactions of Compounds containing Heteroatoms other than Oxygen By S. T. Reid, 363,
Chapter 7 Photoelimination By S. T. Reid, 406,
Part IV Polymer Photochemistry By N. S. Allen, 439,
Part V Photochemical Aspects of Solar Energy Conversion By A. Harriman, 495,
Errata, 517,
Author Index, 519,

CHAPTER 1

Photophysical Processes in Condensed Phases

BY R. B. CUNDALL

1 Introduction

The format of this Review is the same as in previous years, but there is once again evidence of increasing interest in biological and biochemical systems. The physical aspects of photochemistry are becoming particularly relevant to photobiology, and this trend is enhanced by realization that sunlight has wider ranging pathological influences than previously thought. Phototherapy is also attracting increasing attention and its exploitation requires detailed studies of photochemistry in some very complex systems.

2 General

The past year has not produced many theoretical papers on any special topics. Irreversible thermodynamics has been applied to fluorescence spectroscopy, especially to photon counting techniques. A series of papers by Grigolini deals with radiationless decay processes by generalization of the Mori theory. A more conventional approach has been made by Medvedev in his paper on the Franck–Condon Principle and selection rules for radiationless transitions. Kanamaru has appraised the choice of sets which can be used to describe radiationless transitions.

Porter has analysed photochemical systems in terms of chemical potential. This is particularly important for reactions to be used for the conversion and storage of energy.

Two papers have appeared on the determination of excited dipole moments by the rate of solvent-shifts of absorption and fluorescence spectra, as well as one dealing with the relationship between solvatochromic shifts and local polarity of solvent mixtures. Fleming and his co-workers have developed a theory to explain the effects of viscosity on electronic relaxation processes in solution, and predictions from theory are compared with experimental data for triphenyl-methane dyes. Zimmerman has reviewed some of the contemporary approaches to mechanistic organic photochemistry. The use of lasers makes the study of non-linear processes possible and this area of research is now a vigorous one. The theory of co-operative two-photon absorption using quantum electrodynamics has been developed by Andrews and Harlow and expressions derived for the rate of co-operative absorption for transitions forbidden in the absence of any such interaction. Two-photon absorption cross-sections of naphthalene and diphenylbutadiene have been analysed making allowance for thermal lensing:this study provides accurate measurements of one- and two-photon absorption. Theory and experiment for two-photon spectroscopy have been compared by Holneicher and Dick. An overall view of non-linear photochemistry of organic, inorganic, and organometallic systems, and multiphoton ionization (MPI), classified into ionization followed by fragmentation (class A) or fragmentation followed by ionization (class B), has been presented by Gedanken et al. The subject of non-linear optics in the vacuum u.v. (VUV) has been reviewed by Wallace. Illustrative experimental work has been reported on chromium organometallics and methyl iodide. The presence of strong fields has also been examined.

Krüger has reviewed techniques for the study of fast reactions in solution, including photochemical methods. More specific reports involve the application of graphical rank analysis to flash photolysis and the distortion of laser flash photolysis absorption measurements by lack of overlap of the laser beam and analysed volume. Spectral and temporal properties of single picosecond pulses from dye lasers and dephasing effects of fluorescence are topics also examined. The effect of fluctuation in the pulse duration of synchronously pumped mode-locked pulse trains in excite-and-probe measurements has been discussed. The influence of the shape of the exciting laser pulse on fluorescence saturation in the analysis of trace organic substances has been studied, as has the low-temperature spectroscopy of organic molecules in solids by photochemical hole burning. The use of Raman spectra to observe relaxation processes has been outlined and should be applicable to excited-state properties. Diffuse reflectance flash photolysis has been used for study of transients in polycrystalline samples. Solid-state photochemistry can also be followed by time-resolved photochemistry, as reported by Kamat and Fox for studies on TiO2–MeCN.

A useful review of the state of the art of picosecond spectroscopy has been made by Rentzepis, and an illustration of progress in technique is provided by a description of picosecond performance of streak cameras in following exciplex formation. Picosecond transient Raman spectroscopy of trans-stilbene excited to the S1 state has been successfully used to study photoisomerization, while picosecond resonance Raman spectroscopy has provided evidence for a triplet state of oxyhaemoglobin. A new development has been the use of time-resolved photoacoustic spectroscopy in the picosecond regime to study intersystem crossing of the benzophenone triplet.

The increasing use and availability of lasers has led to advances in specialized applications in spectroscopy. Three-wave mixing under fully resonant conditions had been theoretically analyzed, and picosecond time-resolved absorption spectroscopy has been used to study localization of electrons in polar fluids. The role of vibrational processes for electron transfer to solvent clusters has also been reported. The strong field-effects of sub-picosecond laser spectroscopy on photophysical and excited-state dynamics has also been analysed by Kenney-Wallace and co-workers; the i.r. absorbance properties of the same system have also been reported earlier. Measurements of degenerate four-wave mixing in liquids, used to study the picosecond orientational dynamics of molecules, have been presented for CS2 in alkane solutions. Supersonic beams provide evidence of van der Waals molecules and the influence of such species on the photo-dynamics of excited states; this is exemplified by work on trimethylamine-noble gas molecules.

The sub-picosecond flash is now being exploited for the study of very fast time-effects. An example of the practical generation of such pulses has been reported by Yasa and Amer, and the reviews of Shank and his co-workers give detailed state-of-the-art presentations together with examples of systems being studied. The applications to chemistry are limited so far, as these reviews indicate, but such techniques open up new types of relaxation processes for examination. A particularly interesting example of a picosecond pulse study is the acceleration of the trans [right arrow] cis isomerization of stilbene, which can be induced by use of two consecutive pulses. Picosecond pulse radiolysis is a closely related technique and has been applied to study solute excitation in liquid hydrocarbons. The several processes involved in excited-state formation can be identified by systematic kinetic investigations.

Although it is not possible to survey systematically developments of the many techniques used in photophysical research, some publications of interest will be mentioned. Electron excitation can be used to excite molecular fluorescence, a technique which has the advantage of inducing excitation with selection rules different from those applying to optical methods. Laser excitation can be used to extend the detection limits of dyes by fluorescence to attogram levels; this is especially significant for flow cytometry.

Fluorescence spectroscopy is being increasingly used, and equipment and techniques continue to develop and improve. Microprocessors are well-adapted to this area, as exemplified in a design for a spectrofluorimeter published by Gratton and Limkeman. Correction of excitation spectra has been reviewed in detail by Melhuish, who also surveys the problems in data collection brought about by the nature of the sample. Many interesting systems other than homogeneous solutions require examination but many precautions need to be taken. The radiation pattern of fluorescence from molecules embedded in small particles is one such system. In view of biophysical applications, the analysis of the fluorescence depolarization by anisotropic rotational diffusion of a luminophore attached to a carrier molecule is timely. A number of dye-labelled systems are used to illustrate the theoretical arguments. The theory of time-resolved polarization spectroscopy has been adapted to systems in which there are a number of states having differing rotational diffusion behaviour: tryptophan and coronene are examples of systems analysed.

The technique of synchronous luminescence spectroscopy and its applications to analysis of mixtures has been assessed by Vo-Dinh. A theoretical paper examines the use of time-resolved luminescence in the study of lipid aggregate symmetry; this is applicable to viscoelastic properties, field-oriented liquid crystalline phases, micellar solutions, and membranes.

The analysis of fluorescence decay data remains an active area. Phase-plane and Guggenheim methods are compared by Bacon and Demas. The phase-plane deconvolution method is shown to allow rapid determination of lifetimes when there are single decays and also multiple decays and scattered light contributions. Balter and Lakowicz have, in a series of papers, analysed several aspects of the theory of the phase-modulation method. These involve the analysis of multiexponential decays, detection of reversibility of an excited-state reaction (2-naphthol), theory of a two-state model, application to a number of biochemically interesting probes and biopolymers as well as a detailed examination of tryptophan, and the use of a differential wavelength deconvolution. The use of isochronal reference standards has been proposed as a means of reducing errors in phase and modulation fluorescence lifetime experiments.

The advantages and disadvantages of quencher addition as a means of separation of decay data in multicomponent mixtures have been described. Inner filter effects need to be carefully considered as a source of error. The effects of the mutual diffusion coefficient and collision distance on the lifetime of electronically excited states are very significant; the difficulties in interpretation can be circumvented by the use of continuous excitation. Experiment and theory have been compared for a number of anthraquinones. A generalization of the Smoluchowski theory of diffusion-controlled reactions is used to explain the positive curvature frequently seen in Stern–Volmer plots. The calculations depend on the fluorescence lifetime, encounter radius, diffusion constants of the fluorophore and quencher, and intrinsic reactivity.

Photoacoustic measurements are making an increasing contribution to photochemical experiments. Photoionization after two-photon absorption has been studied and used as a detector for liquid chromatography. Photoacoustic spectroscopy has been used to determine the spectra of Methylene Blue and Malachite Green adsorbed on silica. The photophysics of pyrene under conditions where photoionization (N2 laser) is possible have also been examined using this technique. Photoacoustic effects have been used to measure excited singlet and triplet absorption spectra as well as to determine excited-state lifetimes. Excited singlet spectra of diphenylhexatriene and dibenzanthracene in cyclohexane solution, Rhodamine 6G in ethanol, and the triplet state of dibenzanthracene have been characterized. The sensitivity of the method is considered to be about 40 times greater than that of conventional laser pump flash lamp probe techniques. The lowest triplet state of a free base porphyrin is reported and considered to show the use of a technique for measuring photophysical properties unobtainable by optical methods and providing a means of measuring intersystem crossing yields. A paper by Hey and Gollnick on Eosin shows how the rate of radiationless transition can be measured by optoacoustic methods. A useful description of experimental technique as well as theory is provided.

The debate on the application, or otherwise, of a refractive index connection to the radiative rate constant in fluorescence lifetime measurements continues. Phillips and his co-workers have used an integrating sphere to show that in single- and mixed-solvent systems there is an n2 dependence on the radiative rate constant for some anthracene chemiluminesce activators, although it is admitted that the problem associated with solvent refractive index is still not completely clarified. A comparison of exciplex formation and decomposition kinetics in the vapour phase and in non-polar solution has been made for 1-cyanonaphthalene and triethylamine. This is the first time that such a comparison has been attempted.

Actinometry is indispensable to photochemistry and the examination of likely systems continues. The quantum yield of ferrous ion formation from ferri-oxalate on irradiation at 313 nm has a value of 1.24 in the temperature range 5–80 °C. At temperatures above ambient the samples should be deoxygenatd to eliminate reoxidation of Fe2+ by O2. Brauer and his group have put forward a number of new interesting systems. Heterocoerdianthrone (HCD) and its endoperoxide (HCDPO) are useful in the visible (400–580 nm) range ([empty set]HCD = 0.016 [+ or -] 0.001). The highly reversible, thermally stable photochromic system consisting of HCDPO and its parent compounds HCD and O2 provides an advantageous liquid-phase actinometer for the region 248–334nm. meso-Diphenylhelianthrene is also a convenient actinometer for the 475–610 nm region (using self-sensitized photo-oxidation to the endoperoxide). The yield for this system is wavelength-independent. Methods have been reported for measuring photon absorption when polychromatic light is used and for quantum yields of reactions involving intermolecular proton transfer when there is absorption saturation., Expressions for calculating quantum yields of reactions from absorbance measurements at different times in systems where the reactant is the only light-absorbing species have also been reported.

Hamai and Hirayama have remeasured the fluorescence quantum yield for 9,10-diphenylanthracene, using ferrioxalate actinometry, and found it to be 0.90 ( [+ or -] 4%). The emission is measured by almost completely surrounding the sample with actinometer solution. Luminescence from the uranyl ion excited by pulsed dye-lasers can be used for its detection and assay. The effects of added salts and pH on the system have been examined.

Hurtubise has surveyed solid-surface, micelle-stabilized, and solution-sensitized room-temperature phosphorescences and their application to analytical chemistry. The use of phosphorescence detection with micellar chromatography in h.p.l.c. is described by Weinberger et al.

Circular polarization of luminescence is not extensively used and only a limited amount of work has been published. Schippers and Dekkers have used circular polarization of luminescence as a probe for intramolecular 1(n, π*) energy transfer in meso-diketones.

3 Excited Singlet-state Processes

Non-radiative decay originating from S, states has been investigated by studying the fluorescence lifetime as a function of temperature for cis- and trans-decalin and alkyl-substituted cyclohexanes. An activated Arrhenius-type process is assigned to internal conversion and a temperature-independent process attributed to intersystem crossing. Perfluorodecalin has been used to scavenge electrons and reduce the efficiency of geminate ion recombination fluorescence produced by 165 nm light in alicyclic hydrocarbons. Perfluoro-n-hexane has also been used to quench fluorescence of saturated hydrocarbons above and below the ionization threshold. The results obtained are related to product formation by the process of geminate ion recombination. Excimer emission from naphthalene is used to follow processes in X-irradiated saturated hydrocarbon glasses. The photo-physical properties of liquid alkanes excited by two-photon excitation from a nitrogen laser have been examined and energy transfer to benzene solute measured. The data obtained are consistent with a diffusion process with an interaction radius of 1 nm. The formation of fluorescent excited states of cis- and trans-decalin by high-energy radiation is studied in some detail with evidence of 85% efficiency for excited-state formation by electron-hole recombination in cis-decalin. Warman has found that solvent cations may be involved in laser flash photoionization of aromatic compounds in alkane solvents.

(Continues…)Excerpted from Photochemistry Volume 15 by D. Bryce-Smith. Copyright © 1984 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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