Photochemistry Volume 22

A Review of the Literature Published Between July 1989 and June 1990

By D. Bryce-Smith, A. Gilbert

The Royal Society of Chemistry

Copyright © 1991 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-205-7

Contents

PART I PHYSICAL ASPECTS OF PHOTOCHEMISTRY Photophysical Processes in Condensed Phases By R.B. Cundall, 3,
PART II ORGANIC ASPECTS OF PHOTOCHEMISTRY,
Chapter 1 Photolysis of Carbonyl Compounds By W.M. Horspool, 59,
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By W. M. Horspool, 96,
Chapter 3 Photochemistry of Alkenes, Alkynes, and Related Compounds By W.M. Horspool, 169,
Chapter 4 Photochemistry of Aromatic Compounds By A. C. Weedon, 221,
Chapter 5 Photo-reduction and -oxidation By A. Cox, 295,
Chapter 6 Photoreactions of Compounds Containing Heteroatoms Other than Oxygen By S.T. Reid, 329,
Chapter 7 Photoelimination By S.T. Reid, 375,
PART III POLYMER PHOTOCHEMISTRY By N.S. Allen and M. Edge, 411,
PART IV PHOTOCHEMICAL ASPECTS OF SOLAR ENERGY CONVERSION By A. Cox, 505,
AUTHOR INDEX, 514,

CHAPTER 1

Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

BY R. B. CUNDALL

Photophysical Processes in Condensed Phases

BY R.B. CUNDALL

The divergence between the organic and physical aspects of photochemistry has increased during the past decade. Research in photophysics is now largely occupied with the detailed examination of chemical physics of excited states using the techniques of ultrafast spectroscopy rather than elucidation of reaction mechanisms. As a consequence, most papers are concerned with minutae of specific systems and the general principles and theoretical basis of the subject have not changed greatly over the last few years. For this reason the reviewer has chosen, as in past years, to survey the topic generally rather than make arbitrary choice of some subjects for detailed examination.

The application of techniques in the subpicosecond region is probably the most exciting area during the period of review. The practical application of photophysics to electronics on any great scale although it would appear to have considerable promise for microelectronics is still awaited. The use of fluorescence in imaging seems to be one of the most promising photophysical techniques being exploited. A number of references to this procedure are included in the text.

1 General

Researches on theoretical topics have not been reported very extensively. A few papers are mentioned here and some others at appropriate points later in the article. Weber has re-examined the famous Perrin equation for quantifying the rotational depolarization of fluorescence. The arguments presented in the paper are applied to the temperature dependence of the local motions of tyrosine and tryptophan residues observed in proteins. A unified and comprehensive analysis of diffusion influenced bimolecular quenching of fluorescence by Szabo is also noteworthy for its in-depth treatment of this important topic. Monomer-excimer interaction kinetics have been studied further and a theoretical analysis of energy transfer from both species to an acceptor presented. Fractals are now very much in fashion in all aspects of science and a review by Kopelman includes topics which are specifically of photochemical interest. Systems selected for discussion include naphthalene dimerization, excitation migration, and the role of excitons in doped naphthalene crystals. The consequences of lowered dimensionality on migration phenomena needs to be considered by all photochemists. The most significant effect is a very considerable increase in the efficiency of electronic energy transfer.

Complex kinetic systems with time-dependent rate coefficients, including fluorescence quenching, have been analysed and show considerable variation with system type. The theory of excited state relaxation still offers considerable challenge. In particular this is exemplified by a paper in which the contribution of vibronic coupling to intramolecular radiative and non-radiative singlet state processes of large conjugated organic molecules has been assessed.

Nickel, in a very useful paper, has discussed the elimination of polarization bias effects from the measurement of luminescence properties and transient absorption in isotropic solutions. The theoretical treatment is fully developed and recommendations are given for making reliable observations under a variety of experimental conditions are detailed. Determination of quantitative data from steady state luminescence spectroscopy is by no means as straightforward as many workers assume; this work very convincingly demonstrates otherwise.

A number of books and reviews which describe some of the new experimental techniques have appeared. Topics covered include hole burning spectroscopy, various forms of picosecond and femtosecond spectroscopy, femtosecond coherent spectroscopy, ultrafast time-resolved fluorescence spectroscopy, ultrafast spectroscopy and its application to analytical chemistry, applications of ultrafast laser spectroscopy for the study of biological systems, and luminescence in general. All aspects of photochemical methodology are covered comprehensively in the two-volume handbook edited by Scaiano. Observations of evanescent luminescence is increasingly recognized as a useful procedure for the study of interfaces. Reichert has assessed optical considerations to be taken into account for evanescent detection of absorbance and fluorescence at crystal/solution and polymer/solution interfaces. The technique is becoming referred to as total internal reflection fluorescence spectroscopy (TIRF). A combination of fluorescence detection with two photon excitation gives a very high level of sensitivity. The technique is especially well suited to measurement on small volume samples and provides a very useful probe of microenvironments, surfaces, thin films, micelles, and membranes.

A ps absorption-emission spectrometer design which uses both pump-probe and streak camera measurement with a single-mode locked Nd-YAG laser has been described in detail. The theory of non-stationary time-dependent emission measurement and its application to ultrafast processes has been exemplified by analysis of data on the fs time-resolved emission from dye molecules in water. The power of this experimental technique is exemplified by the determination of the evolution of the vibronic absorption spectrum in molecules following excitation with a 6 fs optical pulse. Nile blue A and malachite green are two specific systems which have been followed after a delay of up to 120 fs over 1.5 ps. It has been demonstrated by this type of experiment that conformational changes of excited states can be followed with a time resolution of 10 ps. Solvent cage effects and intra- molecular twisting are specific processes that can be observed. This is very well illustrated by published measurements of the ultrafast vibrational changes which occur in four co-ordinated nickel (II) porphyrins after excitation using fs time-resolved spectroscopy. The steady state condition of the initial excited state is reached in about 20 ps. The authoritative feature article by Bagchi and Fleming reviews both theoretical and practical aspects of ps and sub-ps laser spectroscopy in considerable detail. The dependence excited state relaxation on viscosity, polarity, temperature, and wavelength are experimental variables examined by these authors.

Environmental influences are basic to the whole field of photochemistry and the depth of understanding of these effects is a good measure of progress in photophysical research. Perfluoroalkanes provide unique solvents of special interest because of their slight influence on excited states. Spectral, photophysical, and photochemical studies on these interesting systems have been very comprehensively reviewed by Maciejewski. A general relationship between the time dependence of the energy gap between electronic states and fluorescence Stokes shift has been interpreted using models of solvation dynamics by Chandra and Bagchi. An improved method of measuring the magnitude of dipole moments of molecules in excited states and also their direction from solvatochromic shifts is likely to be a very useful extension of presently available methods. Other related papers published during the year have dealt with influence of electric fields on the fluorescence decay of polar molecules and solvent effects on the magnetic field perturbation of photogenerated radical ion pairs.

Photoacoustic calorimetry has been used as method for the measurement of reaction volumes of excited states. Partial volumes of excited states obtained thereby can be used for estimating structural changes which occur on excitation. A dual laser beam flash photolysis technique in which fluorescence signals from both the sample and reference, separated in time, can be recorded has been employed in magnetic field effect studies.

Time-resolved luminescence measurement techniques for the measurement of fluorescence lifetimes continue to be re-appraised and improved. For example, measurement of submicrosecond time scales using a high-speed photomultiplier gating circuit has been described, the turn-off and turn-on times are 60 and 40 ns. Optimization of signal to noise ratios in time-filtered fluorescence detection can allow satisfactory subnanosecond lifetime measurement to be achieved.

Data treatment of luminescence decay using pulse excitation is still undergoing active development. Probably the most significant papers consider the maximum entropy method, a technique similar to that used in resolving of astronomical images, which has been fully described and tested by Ware and his group. The procedure is compared with results achieved with the well established exponential series method for recovering a distribution of lifetimes from fluorescence lifetime data. It is clearly demonstrated that the maximum entropy method is well adapted to dealing with situations in which a distribution of decay times is expected and must be determined.

Libertini and Small have applied the older method of moments to determine the decay lifetime distribution. A related paper by Beechem describes a second generation global analysis program for the recovery of complex inhomogeneous fluorescence decay kinetics, A contribution from the Belgian group also describes a systematic study of global analyses of multiexponential fluorescence decay surfaces using reference convolution. A comparatively simple deconvolution procedure has been designed for application to the analysis of stretched exponential decay functions.

It is pointed out that single photon counting lifetime measurements on long-lived samples (hundreds of ns up to microseconds) are complicated by the long tails in the emissions found with both H2 and D2 lamps. On the practical side, equipment for measuring longer luminescence lifetimes from 100 µs up to 1s using low-cost analog input interfaces has been described which can be employed as an accessory for use with standard spectrofluorimeters.

Powerful data handling capacity is involved in a general global analysis program which allows simultaneous analysis of single photon timing data for complete determination of activation energies, frequency factors, and quenching rate constants. This has been applied to analysis of data from tryptophan photophysics as a foundation for the general treatment of protein luminescence. The “long time” approximation for the intensity decay equation in diffusion limited fluorescence quenching has been critically re-examined and the importance of the usually neglected terms in the theory applied to the analysis has been indicated. Wang and Harris describe a method for the quantitative estimation of component amplitudes in multiexponential data obtained from time-resolved fluorescence spectroscopy. A design of apparatus which uses time correlated and single photon counting with alternate recording of excitation and emission minimises troublesome lamp instability effects and consequently simplifies and improves the analysis of data.

Time-resolved spectroscopy is finding increasing use in chemical analysis to judge from some papers in analytically-oriented journals. For example, McGown describes how fluorescent components can be separated by differences in lifetimes, a technique which allows the elimination of interfering signals from unwanted luminescence signals. Multicomponent fluorescence analyses can be carried out by use of a fibre optic probe. Processing of data is aided by a canonical correlation technique for rank estimation of excitation-emission matrices which allows determination of the number of components.

Phase shift fluorimetry, the other important method for measuring fluorescent lifetimes, also continues to be developed and improved. The effects of inaccurate reference lifetimes on the interpretation of frequency domain fluorescence data can be removed or minimized by a least squares analysis method. The direct collection of multi-frequency data for obtaining fluorescence lifetimes can be achieved by the use of digital parallel acquisition in frequency domain fluorimetry. Frequency domain lifetime measurement has been used for on-line fluorescence lifetime detection of eluents in chromatography. An unusual use of frequency domain measurement which has been reported is for the examination of photon migration in living tissue. Photons in the region between 620 and 740 nm were detected in both the human arm and finger.

The correction of systematic phase errors in frequency domain spectroscopy can be achieved by use of a fluorophore of known lifetime as standard. It has been pointed out that a simple scattering solution can be used as standard. This ingenious suggestion very conveniently dispenses with the need for a fluorescent standard with a previously-determined lifetime value. Phase noise, another troublesome factor encountered in frequency domain fluorimetry, can be eliminated by use of a differential method.

Diode lasers are Finding use as light sources for the generation of molecular fluorescence and subsequent time analysis. The high sensitivity obtainable by use of such sources is exemplified by detection of concentrations as low as 5 x 10-12 M dye. A visible semiconductor laser fluorimeter system has reportedly been used for the detection of as low as 4 x 10-12 M rhodamine 800. This is a very useful development for the measurement on dyes which emit in the deep red and require excitation at comparatively long wavelengths.

Modern imaging techniques are beginning to be increasingly exploited in applications of photophysics. For example, a video-fluorimeter has been used to image tissue metabolism. Reduced nicotinamide adenine dinucleotide (NADH) fluorescence was used to acquire digital metabolic images using emission collected during mitochondrial oxidative metabolism by this non-invasive technique. An image analyzer has similarly been used for data acquisition from biological systems in studies of phototropism.

Optical fibres are useful accessories in a variety of photophysical measurements. The current status and prospects for their further use in chemical analysis are assessed in an article by Norris. Luminescence spectroscopy and evanescent wave devices are topics highlighted in this review. By way of example of this developing technology a device described for luminescence examination of inorganic semiconductors may be cited.

pectroscopic techniques based on the optical microscope are being used with increasing success in photophysics. Microscopic fluorescence decay measurements have been made on both thin liquid films and droplets of concentrated dye solutions. Illustrative data are given for rhodamine B in 20 µm films. A luminescence lifetime microscope spectrometer based on time-correlated single photon counting with an avalanche diode detector has measured luminescence decay times of 10 [+ or -] 2 ps upwards on GaAs surfaces. A versatile set up for observing the effect of pressure and temperature (up to 11 GPa and 77-400 K) using a fluorescence microscope has also been described.

A new cell design and improved analysis algorithm for photo-acoustic calorimetry has improved the sensitivity which can be achieved by this method by a factor of about 10; also the use of thicker samples allows use of bigger exciting laser pulses with an increase in detectable signal. A time-resolved photoacoustic method described by Terazima and Azumi can be used for direct measurement of inter-system crossing yields. A combination of laser flash and optoacoustic calorimeter has been used to study relaxation in two laser and two photon excited species. Diphenylmethyl and 1-naphthylmethyl radicals are two systems which have been studied by this interesting technique for study of reactive intermediates.

More esoteric experimental procedures which can be applied to complex molecules include, for example, picosecond time-resolved circular dichroism. A spectrometer for this purpose has been described and used to observe carbon monoxide elimination from excited carbon monoxymyoglobulin. Fluorescence detected CD has also been used to observe riboflavin in capillary electro-phoresis.

Liquid rare gases have been used as unfamiliar solvents which are well suited for the application of time-resolved infra-red spectroscopy to the direct measurement of an intermolecular C-H oxidative additive reaction of alkenes.

The methylene-blue sensitized photo-oxidation of mesodiphenyl-helianthrene has been proposed as a convenient new chemical actinometer for the extended wavelength range 610-670 nm. Chelation enhanced fluorescence can be used to detect non-metal ions such as carboxylate, sulphate, and phosphate groups. Anthrylpolyarnines can be employed as effective probes for this purpose.

(Continues…)Excerpted from Photochemistry Volume 22 by D. Bryce-Smith, A. Gilbert. Copyright © 1991 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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