City Research Online

Abstract

The processes leading to the formation and decay of excimers and exciplexes are briefly outlined. Excited complexes can be considered as potential sensitisers of singlet oxygen on the basis of their ability to decay to yield localised triplet excited states.

Fluorescence and laser flash photolysis studies have shown that amines and sulphides can quench the excited states of polycyclic aromatic hydrocarbons as well as their chloro- and cyano-derivatives. This quenching is proposed to occur via an electron transfer process. The ability of amines and sulphides to enhance the photo-reductive dehalogenation of halo-aromatics and decyanation of cyano-aromatics can be rationalised on the basis of electron transfer reactions between these compounds.

The photophysical properties of intramolecular exciplex forming systems were found to be sensitive to the isotopic composition of protic, but not aprotic, solvents.

The most commonly encountered photo-oxidation processes are reviewed.

Evidence for the involvement of electron transfer processes in the direct photo-decarboxylation of a-keto-carboxylic acids and esters has been obtained from the observed effects, upon these reactions, of added electron acceptors. a-Keto-carboxylic esters are found to undergo photo-oxidative decarboxylation. The effects of changes in oxygen concentration upon the dye- sensitised decarboxylation of a-keto-carboxylic acids and esters are indicative of a mechanism involving electron transfer processes rather than a singlet oxygen mediated reaction.

The observed effects of the nature and concentration of the sensitising dye upon the magnitude of kinetic solvent isotope effects upon the photo-oxidation of alkenes are attributed to quenching of singlet oxygen by the dyes.

An example of how the sensitivity of photophysical parameters to the isotopic composition of the solvent can result in kinetic solvent isotope effects being almost impossible to interpret is provided by the direct photo-oxidation of 1,3- diphenyl-2-pyrazoline.

Kinetic solvent isotope effects, despite their drawbacks, have proved useful in determining the role of singlet excited states in the production of singlet oxygen by anthracene compounds.

Both intramolecular and intermolecular excited complex forming systems are found to sensitise the photo-oxidation of alkenes and sulphides. On the basis of kinetic solvent isotope effects it is proposed that these reactions occur, at least m part, via a singlet oxygen mediated process.

Titanium dioxide, a well-known semiconductor, has been shown to sensitise the photo-oxidation of sulphides via an electron transfer mechanism. As a consequence of the ability of the intermediate radical ions to undergo a variety of reactions the yields of oxidation products were often low.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science Q Science > QC Physics Q Science > QD Chemistry
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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