Photochemistry Volume 27

A Review of the Literature Published Between July 1994 and June 1995

By A. Gilbert

The Royal Society of Chemistry

Copyright © 1996 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-405-4

Contents

Introduction and Review of the Year By Andrew Gilbert, 1,
Part I Physical Aspects of Photochemistry,
Photophysical Processes in Condensed Phases By Robert B. Cundall, 15,
Part II Organic Aspects of Photochemistry,
Chapter 1 Photolysis of Carbonyl Compounds By William M. Horspoo, 63,
Chapter 2 Enone Cycloadditions and Rearrangements; Photoreactions of Dienones and Quinones By William M. Horspool, 80,
Chapter 3 Photochemistry of Alkenes, Alkynes, and Related Compounds By William M. Horspool, 129,
Chapter 4 Photo-reduction and -oxidation By Alan Cox, 161,
Chapter 5 Photoreactions of Compounds containing Heteroatoms other than Oxygen By Stewart T. Reid and Andrew Gilbert, 216,
Chapter 6 Photoelimination By Stewart T. Reid and William M. Horspool, 262,
Part III Polymer Photochemistry By Norman S. Allen, 303,
Part IV Photochemical Aspects of Solar Energy Conversion By Alan Cox, 363,
Author Index, 371,

CHAPTER 1

Part I

Physical Aspects of Photochemistry

By Robert B. Cundall

Photophysical Processes in Condensed Phases

BY R.B. CUNDALL

1 Introduction

Research in condensed phase photophysics continues to be actively concerned with the elucidation of the details of the elementary processes capable of study by ultrafast time resolution techniques. Charge transfer effects are again of major interest and, in particular the diversity of photochemical processes involving the fullerenes continues to widen further.

2 General

The timely appearance of a volume dealing with ultrafast processes in chemistry and photobiology in a series dealing with “Chemistry for the 21st Century” is an authoritative survey for all photochemists. Other reviews of this area include one on femtochemistry by Thorne and Beddard and another on vibrational relaxation dynamics in solution. The latter is concerned mainly with smaller molecules but there is an analysis of effects in proteins and surface adsorbates. Many aspects of photophysics are covered in papers presented at a discussion meeting on molecular spectroscopy and molecular dynamics held in 1994. The January issue of Applied Spectroscopy in 1995 contains twelve papers dealing with several aspects of molecular fluorescence.

Reviews on the photophysical and photochemical properties of the fullerenes and excited states and electron transfer reactions of fullerenes, and several papers published from the proceedings of a symposium on fullerenes give a perspective of progress in this topical area.

A number of papers with a theoretical emphasis deal with the details of ultrafast processes. Wynne and Hochstrasser develop the theory of ultrafast vibrational spectroscopy and in particular analyse the effect of vibrational modes which couple to bring about photochemically induced reactions: a subject of considerable interest. A closely related experimental study has been reported on energy flow from excited solute molecules to the solvent probed by fs IR spectroscopy for the specific cases of malachite green and haem protein solutions where the response times are a few ps. Fleming and his coworkers have used fs wavepacket spectroscopy to examine the influence of temperature, wavelength, and pulse duration. The pulses used in this work are effectively shorter in duration than the vibrational period of the excited molecules. Electronic coherence in pseudo two colour pump probe spectroscopy provides a method for extracting information on electronic coherence dynamics. This technique is particularly useful in situations where there are overlapping bands. The dynamics of chemical processes in polar solvents and the solvation dynamics and dielectric response in the glass forming solvent, 2-methyltetrahydrofuran, occurring over time ranges from ps to seconds are reports on systems of wide interest. The polarity response of pyrene fluorescence through vibronic coupling is well known and the effect has been analysed in detail by Karpovich and Blanchard. Although the treatment presented involves diatomic molecules the Brownian oscillator model for solvation effects in light emission and its relationship to electron transfer processes is a useful contribution to knowledge of primary processes. A good correlation between solvatochromic Stokes shift data and values of excited state dipole moments has also been achieved. Ultrafast time resolved spontaneous and coherent Raman spectroscopy are techniques for studying the structure and dynamics of photogenerated transients which have been reviewed. Attention is given in this paper to ring opening in paraphenylene, photobiologically related polyenes, and the photophysical properties of stilbene and related compounds. A technique for the direct detection of zwitter-ionic excited states has also been described.

The fluorescence spectra and values of the integrated emission have been used to study preferential solvation in supercritical fluid mixtures. Pyrene has been used to estimate the degrees of selective solvation which occur in CO2/CHFz3 supercritical mixtures. Molecular effects in organic solvents are important in affecting excimer fluorescence as shown by hydrogen bond interaction effects examined by NMR. Unusual environmental effects have been studied in observations made on the excited state solvation dynamics of tetraalkylammonium salts in the solid and molten states. Solvation processes are the same in both situations but are slower in the solid. Another paper deals with the effect of intermolecular vibronic coupling on nonradiative transitions and the optical absorption of dimers.

A theoretical analysis carried further by Molski develops relations among steady state, time and frequency domain fluorescence quenching rates using a statistical approach based on the principles of non-equilibrium thermodynamics. A distinction between differing competing kinetic models of irreversible two-state excited state processes with a quencher can be made by global analysis of the fluorescence decay surface. A theoretical analysis has been made of fluorescence and absorption in molecular complexes with two chromophores and non-parallel absorption and emission transition dipole moments. Time dependence of optical polarization is re-examined with regard to the predictions of the established Perrin model. The effects of light intensity on diffusion influenced fluorescence quenching have been examined for the case of a model using a hard sphere liquid. Diffusional limitations and consequent effects in liquid phase photocatalysis are extensively surveyed.

Nonlinearity effects can arise from the strong interaction of vibrations and thermal fluctuations and induce a development of limit cycles in fluorescence depolarization.

Other papers dealing with time resolved fluorescence studies include a theoretical examination of fluorescence anisotropy arising from a coupled chromophore pair where there is an influence of nuclear motion and energy transfer, the effect of added quencher on bichromophoric emission, the analysis of intermolecular two state excited state processes, a new method for the analysis of multi-component exponential decays, and the analysis of anisotropy decays in terms of the correlation time distributions measured by frequency domain spectroscopy. Multidimensional least squares resolution has been used for the study of intermediates in photochemical reactions, in particular the procedure has been used to study upper triplet states of benzophenone’ and energy transfer from the benzophenone triplet to biacetyl. Quenching has been applied to the control of excited state lifetimes and orientation in an investigation which has been reported with extensive theory. The evolution of rates and yields of second order photoinitiated reactions where there is a Gaussian distribution of excited states in spaces has been developed and applied to the phosphoresence and delayed fluorescence arising from anthracene and benzophenone triplet systems. Fisz has put forward an algorithm which can be used for analysing ps fluorescence in two excited state systems in solution where there is the possibility of both electron and proton transfer.

Thermal effects have an established role in experimental photophysics. Papers have appeared on temperature lensing and grating processes in aqueous solution and have been used to investigate fast dynamics and energy transfer effects, the photothermal grating calorimetry technique for investigating the kinetics of free radical chain reactions exemplified by the photochlorination of chloroform, and the measurement of thermal energy recovery by thermal lens spectroscopy in fluorescence quenching measurements.

Measurements of the energy content and quantum yield of formation of transients by photoacoustic techniques has been discussed and the application of photothermal beam deflection calorimetry has been compared with those of the related methods of photoacoustic calorimetry and thermal lensing. Both enthalpies and rates can be determined by this last method. The evaluation of errors, distribution, and confidence limits in data analysis in pulsed laser time resolved photoacoustic measurements has also been usefully discussed. A useful technique for the simultaneous acquisition of photoacoustic and fluorescence data on powdered samples at high sensitivity is of considerable utility for many systems which would have in the past been difficult to study. Fluorescence thermometry has been used to study the blue tail emission of the dye N,N’-bis(2,5-di-tert.butylphenyl)-3,4,9, 10-perylenecarboxim ide.

There are a number of reports on the development of physicochemical techniques dependent upon luminescence. Phase resolved evanescent wave induced fluorescence has ps resolving power and is likely to be very useful for interfacial studies. Variable angle ultraviolet totai internal reflection fluorescence spectroscopy has been used for polymer films containing pyrene probes and has proved suitable for providing depth profiles. Spectrofluorimetric hydrodynamic voltametry is another method which has been used for investigating reactions at solid/liquid interfaces using fluorescent probes.

Experimental innovations which have been reported during the year include microsecond time resolved measurements of semiconductors using correlational analysis, an ultrahigh sensitive single photon detector using a Si avalanche photodiode for measuring ultraweak bioluminescence, a dual-channel phosphorimeter for studying the anisotropy of antigen-antibody complexes, an interferometer system which outperforms grating systems is used to measure 1O2 luminescence, a lifetime synchronous system for time resolved spectra by phase modulation involves a new format, and an integrated instrument for light scattering and time resolved fluorescence measurements. A powered fourth harmonic cavity improves the stability of a synchrotron radiation source for time resolved fluorescence measurement. A high frequency modulated light source for phase fluorometry has been found to be more convenient than the conventionally used sources such as Pockels cells. A method for prolonging the effective pathlength in laser kinetic spectroscopy has been exemplified by measurements made on pyrene. A new fibre optic based high pressure cell for fluorescence is useful for measurements in supercritical solvents. A new system designed for studying thermoluminescence has been used to record glow peaks from photosynthetic material.

The use of microscopy in the study of luminescence properties is a technology which has developed rapidly. The probing of the fluorescence of individual molecules is now well established and the use of confocal microscopy for this purpose has been reviewed. The lifetime of the fluorescence of single molecules measured by near field optical microscopy is another methodology which has been examined. Fluorescence detection of magnetic resonance of single molecules has been used to observe the interaction of single triplet spin with 13C nuclear spin in the case of pentacene in p-terphenyl. In another investigation of single pentacene molecules in p-terphenyl at 1.7K the technique for imaging light from the luminescence of a single molecule is described in detail. The design and construction of a scanning near-field optical microscopy used to fluorescence image single molecules of rhodamine 6G has also been published. The spatial resolution achieved is about 160nm. The dye IR140 has been used as a model compound in designing a system for the laser induced fluorescence detection of single molecules flowing through capillaries. Raman and fluorescence spectra can be obtained from single optically trapped microdroplets in emulsions (1-bromonaphthalene containing dimethyl yellow) held in focus by the field produced by a highly focussed beam. Fluorimetric analysis of 6nl (225µm diameter) droplets containing rhodamine 6G has been demonstrated. A fluorescence microscope has also been used to image the effects of electron transfer taking place between dye molecules in polymeric media. Three-dimensional microspectrometry has also been deployed in the study of time dependent fluorescence depolarization.

Sensitive fluorescence detection in capillary electrophoresis has been achieved using laser diodes and multiplex dyes. Wavelength resolution using sulforhoda-mine and fluorescein using a multichannel laser and independent fluorescence detectors has also been used for this purpose. The limits of detection and resolution for on the fly fluorescence lifetime detection of polycyclic aromatic hydrocarbons by HPLC have also been assessed.

Fluorescence detected circular dichroism has been used to determine relative amounts of enantiomers in binaphthyl compounds. A ns transient absorption spectrophotometer with dye laser probing and computer control has a performance which has been demonstrated in the case of benzophenone in methylcyclohexane.

Selective laser photolysis of organic molecules in complex matrices has been used to eliminate undesirable molecules which are toxic, carcinogenic, etc. Several examples of this interesting procedure are given. Another applied investigation is a detailed study of fluorescence lifetimes which has been made on crude oil samples. A comparison with the spectra of the components and the role of energy transfer, etc. have been examined. A fluorescence system for the examination of mixed solids, such as rock specimens, has also been reported.

Another paper in the extensive series dealing with industrial photochemistry has appeared which in this case deals with chemical, transport, and refractive index effects in space resolved laser photopolymerization. This is applicable to stereophotolithography and surface resolved photopolymerization.

Jameson and Sawyer have extensively surveyed the use of fluorescence anisotropy to the study of biomolecular interactions in Volume 246 of Methods in Enzymology. Other articles in this volume deal with fluorescence resonance energy transfer, time resolved fluorescence anisotropy, and optically detected magnetic resonance of photoexcited triplet states. Phillips has surveyed the now very wide use of fluorescence lifetimes in studies of biological systems. Nonlinear Stern-Volmer plots can be utilized to study fluorescence quenching in some antioxidant systems.

3 Singlet State Processes

The photochemical properties of the fullerenes are being very actively studied. An extensive examination of the effect of 15 solvents on the electronic spectrum of C60 and an investigation of the nonlinear optical responses of C60 in solution showing 3rd order optical nonlinearity and 4 wave mixing at 532nm allowing singlet-singlet absorption at 522nm are two such examples. In toluene/ ethanol (10:1) at room temperature and 140K (where there is essentially no molecular rotation of the excited states) the fluorescence is unpolarized. This is claimed to be the first observation of such behaviour. The rotational dynamics of C60 and C70 investigated by ps fluorescence anisotropy decay in toluene as a function of time show no anisotropy which could be measured. Interpretations offered involve the presence of Jahn-Teller active modes or an inverse viscosity effect. A transient grating experiment shows that rotational reorientation dynamics of C60 in various solvents also indicates these processes occur within a few ps. Two papers on solid states of fullerenes deal with luminescence line narrowing in the low temperature glassy phase of solid C60 and on the crystalline state of C60 at 1.2K single crystals which shows two components arising from different trap levels. The unusual photoluminescence of C70 and 13C70 in glassy methylcyclohexane (fluorescence and IR phosphorescence) has been compared with the data from toluene solutions. A comparative study of the fluorescence of four substituted C60 fullerenes shows that symmetry breaking increases the yield fluorescence over that for C60 although the lifetimes of the different excited states are all much the same (1.2-1.4ns).

A few reports have appeared on some aliphatic compounds. The excited state of liquid nitromethane involved in UV photolysis dissociates in 1.1 [+ or -] 0.3ps. The yield is 0.24 [+ or -] 0.05 and a nonradiative transition to the ground state occurs. Four papers of a theoretical nature are concerned with an analysis of the (11Bu [left arrow] 1Ag absorption and resonance Raman spectral profiles of trans-1,3,5-hexatriene, excited state reaction pathways for 2A1 and 1B2 states of s-cis-buta-1,3-diene, cascade through the singlet manifold of the hexa-1,5-dienes, and Franck-Condon modelling of the So [right arrow] S2 transition of different conformers of octotetraene.

The absorption spectra of neat benzene and in concentrated solution in n-hexane between 170 and 220nm are in good agreement with earlier work and have been analysed theoretically. The ultrafast internal conversion in the benzene cation and excited states and reactivity of fulvene are allied theoretical studies.

(Continues…)Excerpted from Photochemistry Volume 27 by A. Gilbert. Copyright © 1996 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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