Photochemistry Volume 21

A Review of the Literature Published Between July 1988 and June 1989

By D. Bryce-Smith, A. Gilbert

The Royal Society of Chemistry

Copyright © 1990 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-195-1

Contents

PART I PHYSICAL ASPECTS OF PHOTOCHEMISTRY Photophysical Processes in Condensed Phases By R.B. Cundall, 3,
PART II PHOTOCHEMISTRY OF INORGANIC AND ORGANOMETALLIC COMPOUNDS,
Chapter 1 The Photochemistry of Transition-metal Organometallic Compounds By A. Cox, 67,
Chapter 2 The Photochemistry of Transition-metal Complexes By A. Cox, 99,
Chapter 3 The Photochemistry of Compounds of the Main Group By A. COX, 133,
PART III ORGANIC ASPECTS OF PHOTOCHEMISTRY,
Chapter 1 Photolysis of Carbonyl Compounds By W.M. Horspool, 145,
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By W.M. Horspool, 172,
Chapter 3 Photochemistry of Alkenes, Alkynes, and Related Compounds By W.M. Horspool, 239,
Chapter 4 Photochemistry of Aromatic Compounds By A.C. Weedon, 280,
Chapter 5 Photo-reduction and -oxidation By A. Cox, 374,
Chapter 6 Photoreactions of Compounds containing Heteroatoms other than Oxygen By S.T. Reid, 391,
Chapter 7 Photoelimination By S.T. Reid, 446,
PART IV POLYMER PHOTOCHEMISTRY By N.S. Allen, 483,
AUTHOR INDEX, 544,

CHAPTER 1

Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY By R. B. CUNDALL

Photophysical Processes in Condensed Phases

BY R. B. CUNDALL

The review is presented in the same format as in previous years. The emphasis of research in this period has been on the detailed study of particular systems in most of the published work, rather than many extensive accounts of the discovery of new systems or techniques.

1 General

Reports have been published on the proceedings of several conferences dealing with various aspects of photochemistry. An example of special photophysical interest is the collection of papers on excited state processes in condensed matter presented at the International Conference on Luminescence in Beijing in 1987. Another noteworthy example is the group of papers from a discussion meeting on the use of lasers in the life sciences.

Theoretical papers of direct photophysical relevance have been, as in past years, comparatively few and, as remarked in previous surveys, no particular themes command extensive coverage. A translation of a paper by Plotnikov describes results of theoretical studies on the mechanism and rules governing unimolecular processes for formation of free radicals by the unimolecular photodecomposition of organic compounds. Two papers discuss the determination of transition moment directions in molecules of low symmetry by use of polarized fluorescence. This method has been applied to the examination of dyes in polymer films, including pyranine, perylene, and diphenylhexatriene (DPH). It is highly questionable whether the DPH molecule when excited has the high cylindrical symmetry which is usually assumed when this compound is used as a fluorescent probe. Two other papers describe a symmetry based procedure for determining molecular geometry changes following electronic excitation. An extension of the Bader-Pearson concept has been applied to the examination of relaxation pathways for the molecules ethylene, acetone, butadiene, and methyleneimine. Stevens and McKeithan have made a theoretical analysis of static fluorescence quenching in the finite sink approximation and compared their predictions with experimental data. Another paper in this category examines details of the kinetics of partly diffusion controlled reactions in which long distance electronic energy transfer competes with collisional quenching. In many other papers theory is developed for the interpretation of reported experimental data. An intriguing example of wide interest is the chiral discrimination of excited states by measurement of time resolved circular polarised luminescence. The measurement of polarised fluorescence emission from probes near dielectric interfaces is relevant to lipid membrane research but widely interesting in view of possible applications to the study of surfaces generally.

A highly topical review has appeared which deals with magnetic field effects in chemical kinetics and related phenomena. Braslavsky reviews the application of photoacoustics to biology in another timely article.

Descriptions of equipment still maintain fairly extensive literature coverage. Fluorimeters even still merit description in the journals. A simple instrument with a compact inexpensive nitrogen laser source with a good performance could prove useful for teaching of photophysics. In contrast to this, is a report on the design and evaluation of an all purpose dual multichannel detector spectrometer which allows simultaneous observation of absorption and luminescence.

Ultra-low detection limits for an organic dye (IR-140) of about 4.6 x 104 molecules have been achieved by a fluorescence spectrometer using laser diode excitation. Techniques used for imaging have considerable potential for use in observing the photophysics for very small and heterogeneous samples such as biological cells. For example, Jovin and his coworkers have exploited studies of resonance energy transfer and delayed luminescence by use of digital imaging microscopy. This type of experiment offers considerable potential for research in biology and medicine.

The most important aspect of contemporary instrumental development in photophysics is concerned with methods for use in ultra-short time resolution spectroscopy. Lin and Fain have applied the theory of two dimensional spectroscopy to real time femtosecond transition state spectroscopy. This is a method which is now applicable to dense condensed phases. Ultrafast emission spectroscopy in the UV is obtained by time gated up-conversions. Measurements using full width ps pulses gives a minimum of 300 fs full width half maximum response function. From measurements of Raman to fluorescence intensity ratios it is possible to estimate ultra-short excited state lifetimes. Values have been obtained for crystal violet (1.2 ps), malachite green (0.8 ps), and β-carotene (0.15 ps). A gallium arsenide detector for ps photon counting has a wide dynamic range and is reported to be particularly useful for long wavelengths as demonstrated between 766-772 nm.

A diffuse reflectance laser photolysis study of the ps dynamics of excited states in organic microcystals has been reported for the first time. This is certain to be a fruitful field of investigation.

A picosecond flash photolysis apparatus with high absorption resolution using fibre optics in the probing pathway has been used to measure the S1 state absorption of erythrosin and rose bengal. An image intensifier coupled with a linear position-sensitive detector has been used to record absorption and emission spectra extending from the ns to ms time regimes. Johnson et al have also described a ps time-resolved spectrometer with a remarkable expanded delay range which covers nine decades of time (ps to ms). This, it is claimed, will be particularly useful for the examination of complex systems such as proteins. The first detection of singlet oxygen luminescence using pulse radiolysis with a germanium diode detector offers considerable promise.

Techniques for the analysis of fluorescence decay data continue to be refined. Beechem has produced a second generation global analysis program for recovery of fluorescence decay kinetics from complex inhomogeneous systems and another program for decay data resolution has also been published by Van Stan and Medhage. A powerful global analysis of fluorescence depolarisation data has been applied to rhodamine 6G, rhodamine B, eosin Y, rose bengal, and merocyanine 540. The procedure used has been very thoroughly assessed in this work. The effects of reabsorption of emitted light on the deconvolution of nonexponential emission have also been examined and the theoretical predictions checked with the dyes, rhodamine B and cresyl violet, over the concentration range 10-7 to 10-3 molar. Analysis of the time dependent fluorescence anisotropy of perylene has considerable importance in view of its extensive use as a model system. A powerful general method for analyzing all processes in anisotropy decay of this molecule uses a compartmental analysis of data in various solvents. The time correlated single photon counting fluorescence decay technique has been extended for measurements with emission up to 930 nm wavelength using a spark source excitation. Data from the new system are exemplified by results for rose bengal using 555 nmexcitation and 580 nm emission with an H2 filled lamp.

An analysis of spectrally resolved kinetic data and time resolved spectra by linear regression is shown to be applicable to the results of flash photolysis experiments.

Lakowicz et al have reviewed the principles and biochemical applications of frequency domain spectroscopy. Phase resolution is claimed to be advantageous for the analysis of two component systems by fluorescence lifetime selectivity in the study excitation-emission matrices.

Time resolved fluorescence provides a very powerful experimental method for experiments relevant to molecular theories of solvation and Maroncelli and Fleming have used coumarin 153 as an example of a suitable probe system. An analytical formulation of the rate of such time dependent solvation effects deals particularly with the details of non-radiative relaxation processes.

A very important experimental achievement during the past year has been described in a report of the first observation of femtosecond microscope solvation dynamics in aqueous solution. A lifetime of 0.16 ps has been reported for this process in the case of the dye 7-(dimethylamino)coumarin-4-acetate.

Abe has made a theoretical treatment of solvent induced shifts of fluorescence spectra. The model is the same as that used for the analysis of absorption shifts and applies to both nonpolar and polar solvents.

It is interesting to note the apparent advantages of a phase nulling technique in a comparison of time and frequency domain methods for rejecting fluorescence from Raman spectra. A successful combination of fluoxescence probing and ultrasonic absorption methods has been used to study the self association of 1-butanol in aqueous solution. Self association of the alcohol occurs above 0.9 M.

A critical assessment of fluorescence polarization measurements with a FACS IV cell sorter is useful in view of the widespread application of such techniques in cell biology.

The published plenary and invited lectures at the 12th International Symposium On Photochemistry held in Bologna included the IUPAC recommendations on terminology for use in photochemistry and reference material for use as fluorescence standards. The latter has also been reproduced as another paper entitled “Reference materials for fluorescence measurements”. The IUPAC recommendations on chemical actinometry deals with suitable systems, procedures and general considerations. These papers are obviously of special interest to all photochemists.

QYFIT is algorithm for the simultaneous determination of photochemical quantum yields and product spectra. The cited paper gives results for tests carried out on several systems. Thermal lensing measurements can be used to measure absolute quantum yields for fluorescence by using fully quenched samples as reference. Using this technique it is found that fluorescein in 0.1M NaOH at concentrations below 10-5 M has a quantum yield of 0.92 [+ or -] 0.03.

There is increasing interest in the use of ultrasound as an alternative to light for producing electronic excitation. Luminescence and chemical reaction have been studied in the complex behaviour of ultrasonicated luminol and iodide solutions. Another technique examined in some detail is photochemical hole burning observed by one and two photon absorption in the particuar case of the anthracene-tetracene photoadduct.

A physiochemical approach to analytical problems is well illustrated by the examination of the use of a positive charged cation-ionophore complex that attracts a negative species which is then detected by fluorescence. The luminescent detection of latent fingerprints by laser excitation using suitable reagents is an interesting system described by Menzel .

The use of photophysical effects in a variety of technical applications, particularly optoelectronics, continues to increase. For example, Becker and co-workers have reported optical changes in cholesteric liquid crystals mediated by guest species under laser excitation. The effect is particularly dependent upon the pitch of the ordered molecules in the liquid crystal medium.

2 Singlet Processes

A calculation of adiabatic and vertical excited state energies of S1 states of saturated amines has been made by MNDO and AM1 methods. This quantum mechanical work closely relates to several experimental studies previously reported. Excited states of cyclohexane still merit attention. The fluorescence lifetime in a low temperature argon matrix has been measured using synchrotron radiation excitation. The source of excited states of cyclohexane arising from the effects of radiolysis has been examined by a detailed study of the influence of a range of electron scavengers.

Excited state structure and ring opening dynamics of 1,3-cyclohexadiene have been studied in two papers. In one resonance Raman spectroscopy shows that allowed crossover from the 1B2 state to the 1A1 state is responsible for the excited state ring-opening reaction. A direct ring opening also observed by ps time resolved UV resonance Raman shows that single bond isomerization occurs within a tine of 20 ps after crossover to the ground state.

Experimental and theoretical studies of excimer formation have been reported in 1-azobicycloalkanes. A three electron model being used to interpret the binding energies involved in this process.

Carotenoid fluorescence in CS2 solution shows that the transition involves a very fast nonradiative relaxation probably through an initial 1Bu [right arrow] 21Ag transition. The carbon-carbon stretching Raman lines of β-carotene isomers in the S1 state have been detected by pump probe resonance Raman spectroscopy.

In the case of benzene photophysics only one paper, a valence isomerization of the hexamethyl Dewar benzene cation studied by pulse radiolysis, has come to notice.

Inversion of the 11Bu and 21Ag levels of the excited states of diphenylhexatriene occurs in the polar solvent carbon disulphide. In the vapour and a hydrocarbon matrix the 21Ag state is lowest in energy. Picosecond spectroscopy has been reported on the electronically excited singlet states of biphenylene.

Deuterium isotope effects on the S1 radiationless decay in phenol and on the intermolecular vibration frequency have been observed in the phenol-water complex. Fluorescent lifetimes have also been reported for methyl cinnamate and indenoate.

Thermal history and concentration effects on the Shpolskii Fluorescence spectra of acenaphthalene in hexane have been investigated in detail. Fluorescence spectra of 1,6-naphthalenediol associated with cyclodextrins show the presence of several different states depending upon the local environment.

Anthracene photophysics has proved much less popular this year. The effect of pressure on 9,10-dibromoanthracene brings about a phase transition which causes a reduction of excimer stability in the molecular solid. A picosecond time study shows a highly selective rotational isomerization of 2-vinylanthracene with two clearly identifiable rotamers. An investigation of the low temperature multiple fluorescence of 2-acetylanthracene shows a strong solvent dependence and that impurity and aggregation effects also have a strong influence. Testa has examined the luminescence and photochemistry of 9-nitroanthracene at 4.2K. The “red edge effect” has been used as a tool for investigating the anomalous fluorescence of 9,9′-bianthryl in rigid polymer matrices. Intramolecular hydrogen bonding has been examined by a comparison of free jet and Shpolskii matrix spectra of 1-aminoanthraquinone.

A detailed research has been reported on the excited state protonation and photophysical properties of azaphenanthrene. The presence of cations increases the rates of protonation.

The resonance Raman spectra of the S1, T1, and S0 states of pyrene in solution have been observed by exploitation of the fluorescence rejection technique. The correlation between rate constants for reactions in viscous media with the viscosity, for diffusion controlled reactions has beenn examined by measurements of pyrene fluorescence. Free volume theory gives good agreement with experiment. Intermolecular pyrene excimer formation has been studied with polystyrene molecules labelled only at one end. Excimer formation in pyrene single crystals shows that a precursor involving moderate electron phonon coupling is involved in partial self trapping. Intramolecular excimer formation of the diastereoisomers of bis[1-(2-pyrenyl) ethyl] ether as revealed by ps time resolved absorption spectra shows intramolecular excimer build up occurs in a third the time required for rotation of the main molecular backbone. An unusual but technically important study is that on the kinetics of pyrene excimer formation occurring with cholesteryl 3-(1-pyrenyl) propanate in cholesteric mesophases.

A three state analysis of the time resolved fluorescence of 1,3-di(N-carbozoyl) propane shows that in toluene there are two monomeric and one excimer species.

(Continues…)Excerpted from Photochemistry Volume 21 by D. Bryce-Smith, A. Gilbert. Copyright © 1990 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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