Photochemistry Volume 18

A Review of the Literature Published Between July 1985 and June 1986

By D. Bryce-Smith

The Royal Society of Chemistry

Copyright © 1987 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-165-4

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 Complexes By A. Cox, 57,
Chapter 2 The Photochemistry of Transition-metal Organometallic Compounds By A. Cox, 105,
Chapter 3 The Photochemistry of Compounds of the Main Group Elements By A. Cox, 140,
PART III ORGANIC ASPECTS OF PHOTOCHEMISTRY,
Chapter 1 Photolysis of Carbonyl Compounds By W.M. Horspool, 155,
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By W.M. Horspool, 176,
Chapter 3 Photochemistry of Alkenes, Alkynes, and Related Compounds By W.M. Horspool, 237,
Chapter 4 Photochemistry of Aromatic Compounds By A. Gilbert, 273,
Chapter 5 Photo-reduction and -oxidation By A. Cox, 338,
Chapter 6 Photoreactions of Compounds containing Heteroatoms other than Oxygen By S.T. Reid, 368,
Chapter 7 Photoelimination By S.T. Reid, 420,
PAR IV POLYMER PHOTOCHEMISTRY By N.S. Allen, 457,
PART V PHOTOCHEMICAL ASPECTS OF SOLAR ENERGY By A. Harriman, 523,
AUTHOR INDEX, 555,

CHAPTER 1

Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

Photophysical Processes in Condensed Phases

BY R. B. CUNDALL

The form of this review follows that in previous volumes although it has been decided this year to cite more of the work published on biochemical systems. The applications of luminescence to biological problems is the most active area in condensed phase photophysics and this survey attempts to reflect this situation.

1 General

Theoretical studies in photophysics tend to be diverse and no particular themes are evident at the present time. A somewhat refined paper deals with radiation-molecule interactions from the viewpoint of quantum electrodynamics and a more conventional approach through tunnel effect theory has been used to interpret the α-cleavage of ketones. Solvent influences on excited states and solvatochromism have also been examined theoretically in some detail. A model for time dependent fluorescence dependent solvent shifts is timely in view of the current intensity of experimental effort. Theoretical analysis of the details of quenching processes have involved consideration of long range electrostatic interactions, the role of dielectric constant and ionic strength, and the effects of quenchers on luminescent polyelectrolytes with pendant probes. The theories of fluorescence depolarization in macroscopically ordered universal systems and concentration depolarization in the presence of orientational correlation due to incoherent energy transfer are highly topical in view of many experimental studies which are underway.

Steady progress in instrumental design and data processing is fundamental to research in photophysics. Only selected papers in this area can be quoted. Yip has reviewed picosecond absorption spectroscopy in some detail and treats both methods and applications. Hopkins and Rentzepis discuss biological applications, particularly primary processes in vision, of picosecond laser phenomena. Three papers which deal with measurement of molecular anisotropies using laser techniques, although dealing with gas phase situations are also relevant and applicable to condensed phase photophysics. Details of a high resolution picosecond transient absorption spectrometer utilising dye emission and streak camera have been published and the ultra-high sensitivity of picosecond pump-probe measurements of concentration has been dramatically demonstrated using cresyl violet. Optical multichannel analysis is also very useful in examining short lived transients. The use of a high intensity laser to modulate concentration differences is exploited to increase sensitivity of detection in concentration modulated absorption spectroscopy (COMAS). Using microchannel plate Wehry has published a very comprehensive collection of detectors with time correlated single photon counting coupled with a pico- and sub-picosecond laser makes instrumental function widths of 20 ps seem feasible. A design for a high sensitivity laser fluorimeter detecting as few as 8000 molecules of rhodamine R6G is noteworthy.

Wehry has published a very comprehensive collection of references to recent work on techniques relevant to molecular fluorescence, phosphorescence, and chemiluminescence. Visser has edited the quite extensive proceedings of a symposium on time resolved fluorescence spectroscopy. Papers deal, amongst other subjects, with multifrequency phase fluorometry, phase resolved fluorescence spectroscopy, time correlated single photon counting using laser excitation, the kinetic interpretation of fluorescence decays, determination of time resolved fluorescence spectra and anisotropy decays using single photon counting and synchronously pumped dye laser excitation, photon time of flight (brought about by use of extremely long silica light pipes) fluorescence spectroscopy, and the application of picosecond spectroscopy to analyse the structure and motions of biopolymers, particularly DNA. Many other publications have dealt with the measurement of fluorescence decay times. A system describing the replacement of a multichannel analyser by ADC and a microcomputer has considerable financial attractions. Short lived fluorescent triphenylmethane dyes may be used to correct for instrumental artifacts in photodetectors in lifetime measurements. On the fly determination of fluorescence lifetimes can be obtained from two point decay measurements and phototube time shifts can be eliminated by phase plane methods. The use of microchannel plate photomultipliers in all aspects of time correlated fluorescence spectroscopy 1s described in a very useful paper by Yamazaki et al. A comparative study of correction methods for the wavelength dependence of instrument response functions in time correlated single photon counting indicates this to be one of the principle difficulties in getting good data fits. A method for carrying out time resolved multiphoton counting has also been developed.

Phase shift and modulation techniques have come into greater Prominence largely due to the feasibility of varying the modulation frequency of the exciting light. An authoritative description of recent developments with illustrative examples of multiexponent decays, decay of anisotropy, and time resolved emission spectra has been given by Lakowicz et al. A new approach to the analysis of heterogeneous emitting systems and time domain fluorescence spectrometry, has been formulated and four components (anthracene derivatives) analysed using two modulation frequencies and three emission wavelengths. All four parameters needed to express systems with two lifetimes have shown to be determinable by the use of modulation techniques.

Global and target analysis of complex decay phenomena and the examination of ground state heterogeneity, anisotropic motions, and excited state reactions has been critically reviewed and a method for the analysis of the complete decay surface for systems undergoing excited state deprotonation proposed. The theory and applications of anisotropy decay associated fluorescence spectra and analysis of rotational heterogeneity have been discussed and applied to the behaviour of all-trans 1,6-diphenyl-1,3,5-hexatriene (DPH) in lipid bilayers. Global deconvolution has been used to generate time emission spectra and applied to 1,2-bis(10-acetoxyanthryl) ethane, intramolecular excimer formation, and analysis of a three component mixture composed of POPOP, anthracene, and diphenylanthracene, Order parameters of ground and excited states have been measured and it has been pointed out that the orientational distributions are not necessarily the same for ground and excited states, as is the case for DPH. The significance of 4th rank orientational order parameters for fluorophores in membranes and their use as a means of distinguishing different models of structure and organization has been made and the much studied DPH system, in particular, has been examined in detail.

The possibility that multicomponent decays obtained by single photon counting which, although appearing to yield a satisfactory fit to a small number of exponential decays may conceal underlying extensive sets of lifetimes with a variety of distributions has been considered in several publications. Faults in equipment can make this situation difficult to distinguish and a method for the recovery such complex distribution has been proposed. The distribution of fluorescence lifetimes of molecules absorbed on surfaces which are consistent with two measured lifetimes can be fitted to either a single Gaussian or bimodal distribution.

A method developed for estimating excited state pK*a for fluorescent compounds from rates of proton transfer to an appropriate donor or acceptor has been applied to tyrosine, β-naphthol, m-methoxycinnamic acid and β-carboline. It has been proposed that β-carboline in 1NH2SO4 is a better fluorescence standard than quinine bisulphate (λmaxem = 450 nm, [??]F = 0.60) with a single exponential lifetime of 22.03 ± 0.12 ns. Measured fluorescence intensity and polarization values are severely affected by absorption and/or scattering in cuvettes and an expression has been derived which relates the observed anisotropy to sample turbidity. Retro-diffracted light in T-format fluorescence spectrometers may be source of error particularly when mode locked laser excitation systems are being used. Two papers deal with characterisation of lateral diffusion in artificial and biological membranes by photobleaching and measurement of the appropriate diffusion constants.

The application of time resolved resonance Raman spectroscopy for studying transients has been reviewed and the properties of bacteriorhodopsin and haemoglobin dynamics are specifically discusseds. This technique allows photophysical and photochemical processes in the subpicosecond regime to be elucidated and characterised. Circularly polarized luminescence spectroscopy, a technique which is limited to a small number of laboratories, has also been comprehensively reviewed. The application of Eu(III) and Tb(III) complexes and photon counting methods demonstrates a method with considerable promise for time resolved luminescence probe studies of biochemical structures. Braslavsky has extensively reviewed the application of photoacoustic and photothermal methods as means of studying radiationless deactivation processes in systems of biological interest. The application of 30 fluorescence to the needs of forensic science. in particular the identification of gasoline, have also been surveyed by Siege1. Another useful review which as been published deals with bioanalytical applications of fluorescence quenching.

A number of interesting observations using luminescence spectroscopy for the observation of new effects have been reported. S1 [right arrow] S0 fluorescence has been observed to follow Sn [left arrow] T1 absorption, apparently for the first time. Shock compression of condensed media can be produced by intense laser light illumination. A red shift of up to 800 cm-1 has been observed with bands in the anthracene fluorescence spectrum at pressures up to 10 kbar and the change of viscosity of glycerol with pressures up to 19 kbar estimated from crystal violet fluorescence polarization. A study of the effects of pH, substrate, heavy atoms, etc. on use of solid state room temperature fluorescence and phosphorescence for analysis of mixtures has also been reported. Time resolution has been used to suppress the effects of fluorescence in Raman spectroscopy. Conversely, Raman background in laser induced fluorescence has been reduced by detection of the second harmonic: in fact the background is so reduced that amounts as low as 210 fluorophore molecules can be detected in the probe volume. Time resolved determination of terbium as binary and ternary complexes has also been described. An interesting design for an annular photochemical reactor shows that some of the more conventional photochemical techniques are still being improved also.

A comprehensive atlas of fluorescence spectra and lifetimes of dyes attached to proteins will prove invaluable to users of biological probes. Fluorescence is proving to be useful phenomenon in cytogenetics. Perturbations of dye fluorescence by variations in DNA composition and interdye energy transfer can be used to study chromosome structure, replication and repair. This is obviously of considerable value to biologists. Fluorescence microscopy is becoming an extremely useful tool in cell biology especially in view of the extensive range of probes which have become available. A very comprehensive review on fluorescence digital imaging microscopy indicates the power and promising future of this technique. Amongst other papers particular attention is drawn to a design for a fluorescence microscopy system with picosecond time resolution and the observation of fluorescence microscopy in three dimensions or microtomoscopy

2 Singlet State Processes

There is a trend away from the study of particular molecules, such as benzene, which have been regarded as being examples of specific chromophore properties which are essential for a complete understanding of behaviour in more complex situations. Simple molecules are however still important subjects for research. The lifetimes of 1Δgo2 molecules in rare gas matrices have been measured and found to relate to the refractive index of the medium. Emission from 02 (1Σg+) has been detected for the first time by sensitization from the benzophenone triplet at 1.93 µm with a very short lifetime. The same state has been detected by decomposition of 11,4-dimethylnaphthalene endoperoxide and by direct excitation of ground state 3O2 to the S2 state. The photochemistry of selected C2 and C3 compounds in low temperature matrices has been reviewed by Perutz and four wavemixing has been used to study interactions and reorientation of carbon disulphide in organic liquids. An unusual study is that reported on the photolysis of S8 in cyclopentane to give S3 and S4.

The fluorescence decay of alkanes is accelerated by xenon which influences both intersystem crossing and internal conversion processes. The fluorescence lifetime of cyclohexane in solid liquid, and vapour phases has been measured by VUV synchrotron radiation. Synchrotron radiation has also been used to measure lifetimes of a number of paraffins and alicyclic compounds. S2 [right arrow] S0 emission has been seen in all-trans – 1,3,5,7,9,11,13-tetradeca-heptaene in solution at room temperature and at 77 K in glassese. In this case the S2 [right arrow] S0 transition is [PROGRAM LISTING NOT REPRODUCIBLE IN ASCII] and that for S2 [right arrow] S0 transition is [PROGRAM LISTING NOT REPRODUCIBLE IN ASCII]. Picosecond time resolved absorption spectra of liquid CC14 and alkyl chlorides has been measured by 266 nm muliphoton laser photolysis. Photoionisation is involved. The singlet and triplet excited states of nitrosamines have been extensively examined and the mechanism of photolysis of nitromethane in ethyl ether solution studied.

Transients in the vapour and solution phases of hexafluorobenzene have been characterised as Sn states and the precursor of the Dewar isomer. Laser flash photolysis has used to study nucleophilic substitution of 4-chloro- and 4-fluoroanisole in aqueous solution. The photophysical behaviour of phenol and anisole in various solvents involves deactivation of S1 via the triplet manifold, although the nature of the final state is solvent dependent. Solvent effects on the fluorescence spectra of aminobenzenes, picosecond studies on excited state formation in p-diethylanilineg, and the effect of intramolecular rotation in p-cyano-N,N-dialkylaniline have also been reported. Red shifts in the fluorescence spectra of two pyrrylium ion derivatives have been assigned to fast solvent relaxation processes.

The S1, state of biphenyl in solution has been observed by both absorption and by Raman spectra. The cation radical is formed by stepwise two photon excitation. Pulse radiolysis has been used to follow intramolecular excimer formation in stereoisomeric diphenyltridecanes which are model systems for polystyrene. The S1 state of naphthalene has also been observed by time resolved resonance Raman spectroscopy and has a broad absorption around 425 nm97. Other short time studies on the S2 state of azulene have demonstrated single vibrational level fluorescence and this has allowed the dynamics of vibrational energy relaxation to be studied. The same investigators have also examined the transient Raman spectra of various substituted stilbenes. Fluorescence lifetimes of trans-stilbene and its structurally rigid derivatives, the tetrahydrochrysenes, have been measured and compared. The effect of moderately high pressure on the absorption and fluorescence spectra of DPH, 1.8-diphenyloctatetrene, and 4,4′-dicyano-DPH show the separate effects on the emission from both the number of DPH derivatives has also been published. S2 state properties of fixed 8-cyanoheptafulvenes, in particular values of θF and τF have been measured. Anthracene excimers in pyridine matrices and in liquid anthracene have examined as well as mixed excimers formed by anthracene and phenanthrene. S1 [right arrow] Sn and T1 [right arrow] Tn absorption spectra of anthracene derivatives have bee: determined by laser flash photolysis. The fluorescence anisotropy of 1,9-dimethylanthracene has been studied over a wide range of temperature and viscosity using synchrotron radiation for excitation. Solvent effects on electronically excited states of 9,9′-bianthryl show evidence for solvent exciplexes which may, in strongly polar solvent, be precursors of ions E-1-(9-anthryl)-2-(10-methyl-9-anthryllethene gives excimer emission which correlates well with predictions which can be made from the crystal structure. The excited state properties of trans-(9-anthryl)ethylenes show the effects of geometric distortion about the single bond. The conformations of 9-cyanoanthracene excimers have studied in the crystalline state as a function of pressure. Torsional dynamics of 9-carbonyl substituted anthracenes after the S0 [right arrow] S1 (π, π* transition show a change from a nearly perpendicular to a coplanar form. The fluorescence properties of meso-substituted amidoanthracenes have been examined also. Pyrene excimers still attract considerable interest, for example, a picosecond time resolved study has been made of excimer formation in single crystals. The correct emission spectrum of fluoranthrene has been obtained by use of purified samples. The emission at 350 nm, which has been reported previously, is probably an impurity, acephenanthrene. Solvent effects on the pure radiative lifetime of ovalene have also been observed. Trace amounts of polunuclear aromatic compounds (PNA) can be detected by the use of anthracene as sensitizer. The rotational diffusion of p-terphenyl and p-quarterphenyl in solution has been examined as a function of viscosity and temperature by time resolved fluorescence depolarization. The details of processes involved have been distinguished.

(Continues…)Excerpted from Photochemistry Volume 18 by D. Bryce-Smith. Copyright © 1987 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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