Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

1

Photophysical Processes in Condensed Phases

BY R. B. CUNDALL AND M. W. JONES

1 Introduction

The pattern of research in the area under review in this Chapter is much the same as in recent years, although there is an emphasis on detailed fluorescence data and their interpretation. The increase in exploitation of fluorescence spectroscopy in biochemical studies is striking and we have not perhaps treated this aspect as comprehensively as the quality of the work merits.

Some of the basic problems have still not been adequately clarified and effort is still being directed towards these.

2 Excited Singlet-state Processes

The dependence of the electronic transition probabilities on the refractive index of the medium has been reviewed and verified experimentally by a comparison of the gas phase and solution phase fluorescence from 9-methylanthracene. Where the absorption is essentially independent of the medium, the radiative rate constant does depend on n2. The spectrum of the secondary radiation emitted from excitonic molecules under resonant two-photon excitation has been investigated theoretically. Both polarization and angular dependence of emission are derived, reflecting directly the relaxation stage of the molecules. The photochemical consequences of exciting molecules containing two or more functional groups have been considered. Observations are reviewed for three interesting bichromophore systems namely, aryl-olefins, keto- and benzoyl-olefins, and aryl-esters. An absorptivity related constant and a quantum efficiency related constant have been derived by a simple graphical method from data obtained in a standard fluorimeter.' Absolute values of absorptivity and quantum efficiency can be obtained by comparison with a fluorophore of known absorptivity and quantum efficiency. The method relies on the approximately hyperbolic relationship between fluorescence and fluorophore concentration. Synchronous fluorescence spectroscopy has been applied to the examination of petroleum products and also to forensic science. The paper reviews the experimental variables present in such an analysis and attempts to provide criteria for satisfactory and reproducible collection of data. A quantum chemical investigation of the electric dipole moments in the ground and excited states and also transition moments of 2,2'-p-phenylenebis (5-phenyloxazole) and 2,2'-p-phenylenebis(4-methyl-5-phenyloxaole) have been carried out. Perhaps surprisingly, it is shown that the directions of these quantities do not necessarily coincide.

A review on the role of molecular photochemistry in polymer-growing processes and in phototransformation of polymers has been given. The correlation with photophysical properties is especially noteworthy. The influences of neighbouring groups and of high local concentrations are discussed in relation to polymer transformation and excimer formation.

Fluorescence has been observed from 20 β,γ-unsaturated ketones in cyclohexane solution at room temperature.

Ab initio and CNDO/S calculations support a mechanism of geometrical relaxation through molecular twisting for the appearance of the long-wavelength fluorescence (Fa) observed in some p-substituted dimethylanilines.

A luminescence study of 96 toxic and hazardous materials has recently been presented. The survey contains a brief literature review, estimated detection limits, and a spectral code for preliminary identification of these materials. Furthermore, the high sensitivity of luminescence plus the specificity afforded by excitation and emission spectra provides a simple and inexpensive approach to the identification of hazardous materials.

Direct fluorimetric analysis of the insecticide carbaryl (1-naphthyl-N -methylcarbamate) and its hydrolysis product 1-naphthol in the range 0-l µml-1 have been made. Spectra, quantum efficiencies, and relative sensitivities are reported for this insecticide. The fluorescence and phosphorescence properties of another pesticide 2-isopropoxy phenyl-N-methyl carbamate (1) have been reported in polar and non-polar solvents at room termperature and 77 K.

For highly purified acetophenone, no evidence for any prompt, short-lived fluorescence emission, which can be distinguished from room-temperature phos-phorescence and the solvent blank, is seen at the 10-6 quantum-yield level. The solvent dependence of the lowest triplet state supports the n-π* triplet assignment in polar solvents and a π [right arrow] π* assignment in non-polar solvents. The interaction of ground- and excited-state m-aminoacetophenone (MAAP) with several aliphatic alcohols in non-polar viscous solvents has been investigated by steady-state and nanosecond fluorescence spectroscopy. Exciplex formation of MAAP with several alcohols is demonstrated.

The photophysical primary processes in solutions of p-substituted dialkylanilines has been investigated by measurements of fluorescence and fluorescence-polarization. Variation of the acceptor substituent in these molecules leads to a reversal of the 1Lb, and 1La type excited states. (See Table 1.)

The fluorescence quantum yields and lifetime of styrene at 298 and 77 K for dilute solutions of styrene and three deuteriated isomers have been measured and are presented in Table 2.

The photochemistry of pentahelicene, acetophenone, and several ethoxycarbonyl- and ethoxycarbonyl-methoxy-substituted cycloheptatrienes has also been investigated with some emphasis on the photophysical aspects.

The mechanisms of hydrogen abstraction of aralkyl thiones to give cyclopentyl thiols has been investigated by fluorescence techniques. The...