Photochemistry Volume 14

A Review of the Literature Published Between July 1981 and June 1982

By D. Bryce-Smith

The Royal Society of Chemistry

Copyright © 1983 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-125-8

Contents

Introduction and Review of the Year By D. Bryce-Smith, xiii,
Part I Physical Aspects of Photochemistry,
Chapter 1 Photophysical Processes in Condensed Phases By R. B. Cundall, 3,
Chapter 2 Gas-phase Photoprocesses By M. N. R. Ashfold, 62,
Part II Photochemistry of Inorganic and Organometallic Compounds,
Chapter 1 The Photochemistry of Transition-metal Complexes By A. Cox, 135,
Chapter 2 The Photochemistry of Transition-metal Organometallic Compounds By A. Cox, 158,
Chapter 3 The Photochemistry of the Main Group Elements By A. Cox, 179,
Part III Organic Aspects of Photochemistry,
Chapter 1 Photolysis of Carbonyl Compounds By W. M. Horspool, 191,
Chapter 2 Enone Cycloadditions and Rearrangements: Photoreactions of Dienones and Quinones By W. M. Horspool, 207,
Chapter 3 Photochemistry of Alkenes, Acetylenes, and Related Compounds By W. M. Horspool, 259,
Chapter 4 Photochemistry of Aromatic Compounds By A. Gilbert, 294,
Chapter 5 Photo-reduction and -oxidation By A. Cox, 348,
Chapter 6 Photoreactions of Compounds containing Heteroatoms other than Oxygen By S. T. Reid, 369,
Chapter 7 Photoelimation By S. T. Reid, 417,
Part IV Polymer Photochemistry By N. S. Allen, 451,
Part V Photochemical Aspects of Solar Energy Conversion By A. Harriman, 513,
Author Index, 536,

CHAPTER 1

Photophysical Processes in Condensed Phases

BY R. B. CUNDALL

1 Introduction

The most apparent change in emphasis of the subject of this Chapter is the very active research currently being carried out on heterogeneous systems. This arises from the social need to simulate the action of plants in photosynthesis and utilize solar energy. Other biologically related systems are subjects of particular activity and data obtained from pulsed laser equipment have particular interest. An overall view of the state of contemporary photochemistry is obtained from the published proceedings of the Xth International Conference on Photochemistry.

2 General

Fleming has provided a review of mode-locked lasers with particular emphasis on applications to time-resolved spectroscopy and photochemistry. Tsay and Hochstrasser have also surveyed the use of picosecond spectroscopy in chemistry and biology. Other timely reviews on the use of lasers in photophysics are by Fleming et al. and Crossley. A method for studying picosecond time-resolved luminescence on saturation has been published and suggestions for approaches to the 10–15 s scale have been made, an area which would seem to go well beyond conventional photochemistry.

The availability of lasers has led to extensive use of multiphoton excitation for spectroscopic and photochemical purposes. A general review by Friedrich and description of experimental methods for studying dipole forbidden transitions illustrate this. Assignments of low lying singlet states of naphthalene and acenaphthene and anthracene extend knowledge of these well known molecules. The theory of three-photon excitation has also been given by Friedrich. Multiphoton excitation provides new routes for photochemical reactions, an example of this being the photocyclization of (-)-carvone using rare-gas halide lasers. A 70-fold increase in quantum yield is observed at the highest intensities used.

Site-selection spectroscopy of organic molecules can also be used to control relaxation processes and distinguish closely related molecular structures. Bondy-bey has reviewed the photochemistry and spectroscopy of transients in low-temperature matrices. An analysis of local translation in the vibrational relaxation of matrix-isolated molecules by Shin exemplifies underlying detail.

The whole range of techniques used in photochemistry continue to develop and extend. For example a new filter for the 366nmHg line has been described. An analysis of heavy-atom perturbation of intersystem crossing as a mechanistic tool is given by Morrison and Miller. Quantum yield measurements by means of the well known ferrioxalate actinometer have been re-examined; this paper reviews the methods of actinometry in some detail. Table 1 summarizes quantum yield values determined in this work.

The use of photoacoustic spectroscopy for determining luminescence yields is exemplified by measurements that have been made with sodium salicylate. Fluorescence photobleaching recovery is a technique for the measurement of mobility in complex systems such as membranes, and modulation detection has been shown to be a useful method for analysing such measurements. Kessler and Wilkinson have designed and used a method for diffuse reflectance flash photolysis and employed it to determine triplet-triplet absorptions of aromatic hydrocarbons chemisorbed on γ-alumina. The spectra differ from those of the same hydrocarbons in homogeneous media owing largely to the existence of charge-transfer states of surface-adsorbate systems. Fourier Transform infrared has been used to investigate triplet states in an N2 matrix at 15 K. The use of infrared laser light and excitation of α-diketones for two-photon holographic recording has been described.

The analysis of fluorescence decay kinetics continues to be a challenging topic. To exemplify some of the published work, there is a ‘simplified’ procedure by Hall and Selinger and the design of a picosecond laser with a low jitter photo-multiplier for determination of subnanosecond lifetimes. Some results obtained using the latter are given in Table 2. An example of an analysis of criteria for model evaluation is given in the paper by Ameloot and Hendrickx. Phase modulation fluorometry, the alternative technique to the pulse method, for measuring fluorescence lifetimes provides a quick and easy method for the analysis of multi-exponential fluorescence decays. Scaiano describes an improvement of the time resolution of nanosecond flash photolysis.

It is salutory to recognize that quinine bisulphate dissolved in 1.0N-H2 SO4, the widely used fluorescence standard, shows a two component emission decay. The major component has a decay time of ≈ 20 ns, the minor component has a decay time of about 2 ns.

A correction to absorption spectra measured for strongly fluorescent materials has been listed for N-methylacridone in a variety of solvents. Ilge has published another paper in a sequence dealing with the determination of u.v.–visible extinction coefficients in complex fluorescent and photochemically active systems. Synchronous fluorimetry has been demonstrated as useful for the analysis of polynuclear hydrocarbon mixtures. Fluorescence photoselection has considerable utility for distinguishing overlapping spectra bands in the same systems. The use of a polarizer to selectively pass the fluorescence of one component from mixture after excitation by a polarized (laser) source consider-ably improves the discrimination that can be achieved by this method. The use of a laser to give sequential excitation of fluorescence has been shown to give a high sensitivity detection method for liquid chromatography. A detailed description is given of laser-induced fluorescence detection in liquid chromatography by Folestad et al.. An illustration of the application of fluorescence spectroscopy for kinetics is exemplified in a description of a stopped-flow system. Winefordner and co-workers have developed the technique of ‘negative’ fluorescence peak detection by use of solvents with an added fluorophore. An advantage of this method is that non-fluorescent components can be detected.

A computer method is available for improving the room-temperature phosphorescence detection method by abstracting high substrate luminescence. A plate of some inorganic compound provides an alternative to the widely used filter paper. A micellar mobile phase enhances fluorescence intensity in h.p.l.c. and it is also possible to induce liquid phosphorescence as in thallium/sodium dodecyl sulphate micelles. Analytical figures of merit have been presented for the low-temperature (77 K) phosphorescence of 22 polynuclear aromatic hydrocarbons. The heavy-atom enhancements with a variety of heavy-atom-containing systems are detailed. Sensitized room-temperature phosphorescence in liquid solutions using 1,4-dibromonaphthalene and biacetyl as energy transfer acceptors has also been described. The technique has been used for substituted biphenyls, naphthalenes, and dibenzofurans. A detailed description of a microprocessor-based data acquisition system used for chemiluminescence measurements shows the modern approach to this field.

The reviewer has the impression that theoretical work on photochemical problems has declined and there are few papers to quote within the area covered by this article. A method for analysis of photochemical reactions based on the effects of a randomly modulated light source has been described by Haugen et al. A theory for enhanced photochemistry near rough surfaces has been developed by Nitzan and Brus. Two papers on the photochemistry and spectroscopy of formaldehyde are noteworthy. So also is an analysis of intersystem crossings and heavy-atom effects in aromatic hydrocarbons using the orbital-type selection or propensity rules.

An interesting review of some aspects of high pressure photochemistry involving gas–liquid systems has appeared in 1982. The Catalog of Data Compilation on Photochemical and Photophysical Processes in Solution is also a recent and useful reference source.

3 Excited Singlet-state Processes

Lipsky has produced a timely review on the considerable progress made on ionization and excitation processes in non-polar organic liquids. Fluorescence quantum yields for saturated hydrocarbons excited by photon absorption in the liquid phase at energies above and below the photoionization threshold have been determined, and photo fragmentation and photoionization were also measured. A related detailed study of the effects of the excitation of cyclopentane with photons of 7.6, 8.4, 10.0, and 11.6eV (ionization threshold 8.7eV) has also been made. Pulse radiolysis has provided a value of 0.95ns for the lifetime of the fluorescence of liquid cyclohexane. Tabata and his co-workers have made a detailed study of excitation processes in liquid cyclohexane by pulse radiolysis examination of the behaviour of 2,5-diphenyloxazole solutions. Various additives were used to separate different processes and a lifetime of 300 ps assigned to the excited state of cyclohexane. The effect of perfluorocarbons on the ultraviolet absorption and fluorescence of some saturated hydrocarbon liquid has been observed. At low concentrations perfluoro-n-alkanes had no observable effect on the absorption spectrum and only slightly quenched the fluorescence. Perfluorocycloalkanes strongly affected the absorption spectrum and quenched the fluorescence. The change is attributed contact charge-transfer absorption, with the hydrocarbon acting as electron donor. The differences between the fluoro-carbons are considered to be due to different electron affinity values.

A theoretical study of the geometrical relaxation of the excited singlet states of propylene has been reported. This contrasts with the detailed photochemical experiments on cyclopropenes and cyclopentadienes reported by Zimmerman and Kreil.

The fluorescence of NN-diethylmethylamine shows a red shift as solvent polarity is increased and there is also diminution of quantum efficiency and increase in lifetime (Table 3). The spectra are shown in Figure 1. The effects of a change in geometry are examined by studying l-azabicyclo[2.2.2]octane. Formation of a 1:1 exciplex with solvent is excluded and it is concluded that the ether solvent stabilizes a Rydberg or Rydberg-like excited state of the amine.

Research on the simpler aromatic hydrocarbons is less extensive than in previous years. Gustav has analysed the S0-S1 spectra of benzene, naphthalene, and anthracene and has shown that out-of-plane modes sharply increase the rate of the S1(B2u) [right arrow] S0(Alg) transition. CNDO calculations of the second-order vibronic coupling in the 1B2u – 1Ag transition have been made. Two-photon excitation (355 nm) of neat benzene and toluene and picosecond time resolution showed a build-up of transients with maxima at 510 and 560 nm over about 50 ps. Schwarz and Mautner find that in the photoionization region of liquid benzene the fluorescence yield and quenching constants increase from 1900 to 1400 Å and level off from 1400 to 1150 Å. Below 1750 Å the quenching constants of CHCl3, CH3CI, and C2H5C1 are proportional to their reactivities with quasi-free electrons and extrapolate to zero at 7.1 eV which is the estimated ionization potential of liquid benzene. Benzene and its derivatives (phenol, ethoxybenzene, aniline, N-methylaniline, NN-dimethylaniline, and NN-diethylaniline) form inclusion complexes with β-cyclodextrin in aqueous solutions. Fluorescence enhancement occurred for all the derivatives, an effect which is ascribed to the elimination of quenching of the excited state by water molecules. The formation of phenol and cyclopenta-1,3-diene-1-carboxaldehyde in a two-photon ionization (KrF laser light) in oxygenated aqueous solutions of benzene has been reported. Wavelength and solvent effects on the ionic photodissociation of charge-transfer complexes of hexamethyl(Dewar-benzene) have been studied. Thus fumaronitrile and diethyl 1,2-dicyanofumarate form CT complexes which promote photoisomerization to hexamethylbenzene.

Intramolecular excimer formation in various dinaphthylalkanes in which the naphthyl groups are separated by three carbon atoms has been determined by steady-state and time-resolved fluorescence methods. Excimer formation is controlled by relaxation processes. A conformational analysis has also been made.

The fluorescence lifetimes of anthracene and 9-methylanthracene have been measured over the range 20 — 50 °C in a variety of hydrocarbon solvents. The changes are due to dynamic effects of the solvent which are correlated with viscosity.

Excimer formation in crystalline anthracene derivatives has been studied in some detail during the past year. 9-t-Butylanthracene has a low fluorescence yield that is increased markedly in highly viscous media and no triplet is observed. This is attributed to the formation 9-t-butyl-9,10-(Dewar anthracene). Photoreactions involving intramolecular exciplex formation of anthracene derivatives (lepidopterenes) has been studied by emission spectroscopy. The enthalpy change for dissociation of the crystalline anthracene photodimer has been measured. Complications in the mechanism of the photodissociation of the 9-methylanthracene photodimer in polar solvents have been revealed. Above 220 K the dissociation occurs from a non-relaxed singlet state whereas below 200 K an intermediate excimer is involved. Around 77 K the triplet state is the most important intermediate. Corrected spectra for anthracene and tetracene crystals show that the short wavelength fluorescence is blue shifted. Pressure and temperature both induce modifications of the luminescence from tetracene singlet crystals. Quantum beats in the decay of pulse-excited ‘prompt’ fluorescence in tetracene crystals have been measured as a function of magnetic field. A thorough study of the emissions from pyrene crystals and the dynamics of excimer formation in perylene crystals have been investigated by time-resolved fluorescence. A vibrational analysis of highly resolved absorption, fluorescence, and phosphorescence spectra of perdeuterotriphenylene in polycrystalline n-heptane matrix at 77 K has been published.

Modulation spectroscopy is a technique that has been used to assign transitions for the 1S1 state of pyrene and the pyrene excimer. Fluorescence spectra of pyrene have been run in 62 solvents of widely different polarity. The intensity ratio of the first (0-0 band) and third bands in the vibronic fine structure are very sensitive to hydrogen bonding and correlations with Winstein’s Y values and Dimroth’s ET values were examined (Table 4). A new scale, the so-called Py scale for relating the fluorescence to solvent type, is put forward. Environmental effects on the kinetics of intramolecular excimer formation in dipyrenylalkanes have been discussed. The corrected fluorescence excitation and emission spectra of benzo[a]pyrene and its 9- and 3-hydroxy derivatives in 19 solvents and in interaction with DNA have been determined. The absorption, fluorescence, and phosphorescence spectra, fluorescence yields, and decay times of fluoranthene and its four benzo analogues (benzo[a], –[b], –[j], and –[k]) have been measured in heptane. Only the benzo[k] fluorescence arises from a 1La [right arrow] 1A transition and the 1S states of fluoranthene and benzo[b] fluoranthene are not quenched by oxygen in a diffusion-controlled process.

The picosecond time evolution of the fluorescence spectrum of 3,4,9, 10-dibenzopyrene in hexane at 300 K showed the presence of two short-lived transients in the region 380 — 480 nm. One with a strong ‘hot-band’ was pulse limited and assigned to the 1S2 state. The second was weaker and survived the excitation pulse. From these evolved the structured spectrum characteristic of the normal S1 [right arrow] S0 fluorescence. In an extension of this work, fluorescence from higher excited singlet states has been observed. It is interesting to find that these upper states relax electronically before significant vibrational relaxation occurs, and excess of vibration energy increased the rate of internal conversion. Approximate values of the lifetimes of nearly pure electronic states are listed in Table 5.

Pulse flurometry has been clearly demonstrated as a means of investigating rotational diffusion. The fluorescence decay and decay of emission anisotropy of perylene and 9-aminoacridine the rotational dynamics of perylene are consistent with the model of a disc with a slipping boundary condition. The ratio of diffusion coefficients about axis perpendicular and parallel to the plane of the disk is found to be 10 [+ or -] 1.

(Continues…)Excerpted from Photochemistry Volume 14 by D. Bryce-Smith. Copyright © 1983 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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