Photochemistry Volume 23

A Review of the Literature Published Between July 1990 and June 1991

By D. Bryce-Smith, A. Gilbert

The Royal Society of Chemistry

Copyright © 1992 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-215-6

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, 53,
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By W.M. Horspool, 84,
Chapter 3 Photochemistry of Alkenes, Alkynes, and Related Compounds By W.M. Horspool, 141,
Chapter 4 Photochemistry of Aromatic Compounds By A.C. Weedon, 189,
Chapter 5 Photo-reduction and -oxidation By A. Cox, 282,
Chapter 6 Photoreactions of Compounds Containing Heteroatoms Other than Oxygen By S.T. Reid, 320,
Chapter 7 Photoelimination By S.T. Reid, 369,
PART III POLYMER PHOTOCHEMISTRY By N.S. Allen and M. Edge, 403,
PART IV PHOTOCHEMICAL ASPECTS OF SOLAR ENERGY CONVERSION By A. Cox, 471,
PART V ADSORBATE PHOTOCHEMISTRY By S.R. Meech, 481,
AUTHOR INDEX, 522,

CHAPTER 1

Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

By R. B. CUNDALL

Photophysical Processes in Condensed Phases

BY R. B. CUNDALL

The description of the subject follows the same pattern as in previous years. Most of the experimental work which is cited has used some form of laser technique and an increasing proportion of this involves the use of extremely short duration pulses. Very few investigations nowadays use lamps except as components of steady state luminescence spectrometers. It is curious to note the way that problems of study in science follow fashions. This is apparent in photophysics. What criteria dictate selection of these fashions would make an interesting study in the organisation and development of at least one area of science.

Industrial developments arising from research in photophysics are again not very evident, except in field of radiation curing, and papers on theoretical topics seem to have declined in number during the year. No striking new developments in technique have emerged during the year but existing methodology has been applied to gain more precise data on selected systems. Femtosecond techniques are now well established and surely at least one final frontier must have been effectively achieved.

1 General

During the year a number of specialized monographs of relevance to photophysics have appeared. Two of general interest deal with applications of time resolved optical spectroscopy and luminescence techniques in chemical and biochemical analysis.

In view of the exciting research now in progress in the femtosecond time regime the review of Mukamel which goes deeply into relevant theory is of general interest. A collection of some twenty six papers on various facets of ultrafast spectroscopy of chemical and biological processes is extremely useful. All aspects, including experimental techniques and methodology are examined by acknowledged experts. Fleming and Wolynes have reviewed recently developed knowledge of chemical dynamics in solution in a very concise and authoritative account.

Theoretical papers on effects directly observable in the very short time regime are notable in this years collection. The theory of femtosecond pump-probe spectroscopy of ultrafast internal conversion processes in polyatomic molecules has been developed using the behaviour of the excited pyrazine molecule as an example. The solvation dynamics for an ion pair in a polar solvent can now be examined by the time dependence of fluorescence and by direct observation of photoinduced charge transfer. A theoretical treatment developed for understanding solvation structure and the interpretation of time resolved Stokes shifts in non-Debye solvents has been compared with experimental data obtained with systems in a variety of solvents. The decay time of the fluorescence anisotropy of excited aniline is about 1 ps and femtosecond techniques have been used to elucidate the ultrafast rotational dynamics involved. A comparison of subpicosecond, subnanosecond, and steady state studies of diffusion influenced fluorescence quenching has provided the basis for a considerable improvement upon the classical models for the quenching process which are usually used. A truly fundamental study of the energy transfer processes which occur during relaxation of vibrationally hot molecules generated by internal conversion in solution has made on S0-S1 transitions in vibrationally hot azulene. A comprehensive theoretical analysis is applied to experimental data. Okamoto and Yoshihara provide a theoretical treatment of prolonged time resolution of resonance coherent anti-Stokes Raman scattering and the effect of ultrafast rearrangement on the electronically excited state transition dipole rotation. The profile of radiation scattering is affected since the latter process is very fast. Picosecond Stokes shift studies involving measurement of the time dependence of band shapes and the integrated intensity provide information on solvent friction at the microscopic level. This paper is part of a collection on the subject of solvent friction in a dedicated issue of Chemical Physics. Subpicosecond relaxation of the solvation of nonpolar electronic states has been examined by the transient hole burning technique for dimethyl-s-tetrazine in several solvents. Various details of the short time scale changes in interactions operating in these solvents have been studied. The validity of gas phase vibrational relaxation models, probably accepted intuitively by most photochemists, for reactions in the liquid state have been critically examined and correlated with the density dependence of bound electronic state lifetimes. The “optical” Kerr effect induced by nanosecond laser pulses is another rapid photophysical effect measured and analysed for various fluids.

Techniques and equipment used to obtain data in this type of experiment are discussed and described in quite a number of papers published during the year. The generation and properties of ultrashort pulses has been clearly presented in an article published in American Scientist. An apparatus for carrying out pump-probe broad band spectroscopy by transient absorption in the subpicosecond region has been described in detail. This equipment has been used to observe the photodissociation of bis-(p-aminophenyl) disulphide and also to show a biexponential frequency shift arising from solvent relaxation of the photo-generated p-aminophenyl thiyl radicals. Another picosecond time resolved absorption spectrometer system using a streak camera has been reported by Japanese workers. Okamoto and Yoshihara have used a femtosecond time resolved CARS system for measuring Raman scattering under various conditions of polarization. The problem of optimizing the sensitivity of detection when there is ground state depletion and photodestruction are particularly acute with direct laser excitation; this problem has been discussed in some detail by Mathies et al.

The measurement of fluorescence decay is an extremely important aspect of experimental photochemistry and attention needs to be given by all those who publish data to a working party recommendation on the data processing methods regarded as acceptable for analysis of fluorescence decay data. Inevitably the report gives a consensus view and it is likely to require early revision as new or modified techniques are introduced.

The use of an internal quantum counter which provides a standard for making lifetime measurements by the pulse method is recommended by Pelletier et al. A Monte Carlo convolution method proposed for simulation and analysis of fluorescence decay data has been compared with other established methods. The theory of a priori analyses of fluorescence decay surfaces of excited state processes improves on some of the difficulties encountered in the use of the important, and now widely used, global method. An analysis of the effect of distributions of lifetimes, in the relaxation of excited states by fluorescence decays, which must exist for molecules in nonuniform media, shows for both uniform and Gaussian distributions that the decay profiles for such systems must be non-exponential. A method for fluorescence data reduction provides another example of work on this particular methodology. A powerful combination of different data analysis methods has been applied to the resolution of heterogeneous fluorescence by making use of indirect excitation decay together with spectral and principal factor analysis. Fisz has considered problems involved in interpreting the fluorescence depolarization of fluorophores in solutions and ordered systems.

Experimental equipment reports in this area of photophysics include a description of a picosecond single photon timing measurement made with a proximity type microchannel plate photomultiplier together with global analysis of data which uses reference convolution. It employed a laser source and the performance demonstrated with data obtained for several compounds. A particularly interesting paper presents evidence for the measurement of high quality fluorescence decay times and also lifetime imaging which can be obtained using an elliptical scan streak camera recently developed by Hamatsu Photonics. 1.09 ps channel-1 resolution is shown in the paper but improved devices for better resolution are said to be possible. This technology should be very useful for imaging.

A new review publication has appeared which sets out specifically to cover the technique and applications of multidimensional luminescence.

The alternative technique to the pulse method for lifetime measurements is the phase and modulation procedure recently reviewed by Bright, Betts, and Litwiler. Exploitation of the modulation frequency dependence of the decay kinetics has considerably increased the power of the method. In a eulogy upon the technique the resolution of multicomponent emissions has been strikingly demonstrated. A fluorescence lifetime resolution of spectra in the frequency domain has been achieved by multiway analysis. This clearly provides a valuable means of resolving different fluorescent components in mixtures. Data analysis by the maximum entropy method, already successfully used in the pulse technique, can be applied to frequency domain fluorometry and shown to be a suitable method for recovery of fluorescence lifetime distributions. Morgan and Murray have published details of the design of a phase/quadrature correlator for determining fluorescence decay times by single photon counting. This has been used in a system with a fluorescence microscope and shown to be applicable to the generation of time resolved images. Equipment for phase modulation fluorometry using frequency doubled pulse laser diode light source modulated at frequencies up to 2000 MHz has also been fully described.

Time resolved evanescent wave induced fluorescence spectroscopy is a powerful method for the investigation of dye molecules at interfaces. This technique has been used on studies on the popular photosensitizer aluminium phthalocyanine tetra/sulphonate absorbed at fused silica/methanol interfaces. 2nd harmonic detection of sinusoidally modulated two photon excited fluorescence can also be used to obtain luminescence spectra.

A notable experimental achievement is the detection of single molecules of rhodamine 6G by the means of a system involving repetitive laser pulsing together with time gated discrimination of fluorescence photons. In electrically levitated microdroplets as few as 12 molecules of rhodamine 6G have been detected in glycerol-water solution. An advantage claimed for this unusual system is that lifetimes can be measured with reduced impurity effects and also minimization of the Raman solvent background signal. A gated photomultiplier circuit which can be applied to the determination of phosphorescence lifetimes is also very useful.

A comprehensive review of hole burning spectroscopy of species in glasses has been prepared by Haarer and Silbey. Electric field effects on the spectral holes for perylene in Shpol’skii matrices in n-heptane provide a useful extension of this technique. The limitations of the thermal lens method for the measurement of fluorescence yields have been examined and discussed. It is pointed out that the method is not always accurate, especially when used with species, such as dyes, which have high triplet state yields.

Imaging systems have been briefly mentioned earlier. Other examples of publications of this field of photophysical interest include accounts of a laser microfluorometer with intensified photodiode array detector and scan stage useful for imaging cytometry of cellular specimens and a luminescence imaging system using bioluminescence. Fluorescence energy transfer has been employed in eluant detection during capillary electrophoresis at attomole levels. A photodiode array has also been employed as a fluorescence detector for HPLC. A remote photoacoustic measurement technique for aqueous solutions using an optical fibre is shown to be capable of considerable spatial resolution. Laser induced optoacoustical effects combined with near IR emission have been successfully used to determine intersystem crossing yields of porphyrins. An unusual application of photophysics in measurement is a report on the use of the quenching of platinum octaethylporphyrin phosphorescence by oxygen for luminescence activated barometry in wind tunnels. This may encourage other similar developments. The study of radicals in the condensed phase by fluorescence detected magnetic resonance is a procedure which is discussed in a paper by Werst and Trifunac.

A review of the putative technology of molecular computing discusses the possibilities for the high density storage of information by means of spectral hole burning. Another approach through photophysics towards the technology of microelectronics is exploitation of light directed chemical synthesis of solid state chemistry of materials with photolabile protecting groups. Light directed chemical synthesis with peptides has successfully produced a 1024 array.

Re-examination of what are generally considered to be well established theories are needed for extension of photophysics into new areas. For example, the theory of concentration quenching of fluorescence in 1, 2 and 3 dimensional media has been examined by Sienicki and the time dependence of decay and fluorescence quenching in a one dimensional lattice analysed by Dudkiewicz and Twardowski.

A stimulating paper deals with a revision of the familiar, and widely used, monomer-excimer kinetics by treating such systems as examples of reactions with time dependent rate constants. The simple mathematical formulations usually employed in systems where excimers are involved are shown to be inadequate. No doubt future efforts will be directed to rectifying the situation. Strong transient effects arising from nonstationary diffusion which occur during excimer formation through reactions with time dependent rate coefficients have been used as a scheme to test different models used in convolution kinetics. Time dependent excimer formation rate constants have also been considered in another detailed study of the reversible monomer-excimer kinetics.

A mathematical model for the measurement of pseudo first order rate constants in laser flash photolysis has been put forward. Another data treatment provides a method for determining quantum yields of reactions of the type

A(+hv) = B(+hv,Δ)

where only the spectrum of A is known, provides an extension of an earlier proposal for the analysis of such systems by Fischer.

A chemical actinometer for light in the long wavelength range 670 to 795 nm uses 1,1′,3,3,3′,3′-hexaethylindotricarbocyanine iodide sensitized photooxygenation of mesodiphenylhelianthrene in CHCl3. Benzophenone-benzhydrol functions as an actinometric system applicable to practical reactors where there is complex dilution during operation. This is similar to the situation which applies in a typical preparative organic photoreactor system. At 254 nm the photolysis of aqueous solutions potassium peroxydisulphate in the presence of t-butanol is recommended as a simple and convenient actinometer system. Colorimetric characterization of Magarai QS label (a commercial γ-irradiation detector) can be conveniently used also for measurement of the intensity of UV radiation at levels such as those encountered under industrial conditions.

Recommendations on experimental methods for the determination and the compilation of molar absorption coefficients for transient species in solution have been proposed by Bonneau, Carmichael, and Hug. Note should be taken of the proposals put forward.

Structural transformations in N-isopropylcarbazole crystals have been monitored by pressure induced luminescence. The effect observed is related to the general phenomenon of piezoluminescence.

Chemical and biological microstructures have been probed by means of arrays of excitable donor and acceptors whose spacing are measured by means of energy transfer. The structures can be determined from the measured spread of separation distances. Intracellular sensitization of fluorescence has been applied to biological systems which are studied by a combination of microfluorimetry and fluorescence spectroscopy.

A topical area of research in chemical kinetics is the detailed study of chemical oscillators. A recent contribution from photochemistry is the study involving the wavelength dependent photoinhibition of oscillations in the reactions of phenol and aniline substrates.

A useful suggestion for general photochemistry is the addition of pieces of solid CO2 to remove O2, which acts as a fluorescence quencher, from small samples of solution.

Two more papers in their unique and extensive series on industrial photochemistry have been produced by the Nancy group. One is a Monte Carlo modelling of light curing as applied to photolithography. The other deals with macroscopic transport effects on the performance of photochemical reactors.

A multi-author monograph on the chemistry of isolated species contains a number of articles of photochemical interest.

2. Singlet State Processes

Study of the photochemistry of 1,1-dichloroethane in xenon matrices shows that the nature of the environment affects crossing of potential energy surfaces and also hydrogen bonding geometries. Out of plane deformation of the molecules in the S1 state strongly influences the rate of internal conversion of s-trans-butadiene. This accounts for the absence of fluorescence in this molecule.

Carotenoids are still highly topical systems for research. Both S1 [right arrow] S0 and S2 [right arrow] S0 electronic relaxation process in carotenoids with 7 to 11 conjugated double bonds have been subjected to very comprehensive study. The lifetime of the S2 state of β-carotene in CS2, measured by a femtosecond absorption method, is found to be 200-250 fs at room temperature. Fs time resolved CARS from β-carotene in solution shows the occurrence of ultra-high frequency (11THz) beating phenomena and sub-ps vibrational relaxation. The same technique has been used to observe solvent effects on the ag C=C stretching mode in the 21Ag excited state of β-carotene and two derivatives. A similar study has been made with several derivatives of β-carotene.

Buckminsterfullerene (C60) has not escaped the attention of photochemists. Determination of luminescence and absorption spectra of C60 films has made some excited state assignments possible for this molecule. Some work has also been reported on the triplet state properties.

(Continues…)Excerpted from Photochemistry Volume 23 by D. Bryce-Smith, A. Gilbert. Copyright © 1992 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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