Dean Goodwin

Quenching of singlet and triplet excited aromatic hydrocarbons by sulphides: the amine and sulphide enhanced photo-induced degradation of chloro- and cyanoaromatic hydrocarbons

Abstract Sulphides have been shown to be capable of quenching the excited singlet and triplet states of several aromatic hydrocarbons and their derivatives. The quenching is proposed to involve an electron transfer mechanism. The use of sulphides and amines to generate aromatic hydrocarbon radical cations from the excited states of these compounds has been utilised in order to carry out reductive decyanation reactions of cyanoaromatic hydrocarbons.

The amine assisted photo-dehalogenation of halo-aromatic hydrocarbons

Abstract The rate constants for quenching the triplet states of 4-chlorobiphenyl, 1-chloronaphthalene and 1-methylanpthalene have been determined by use of the technique of laser flash photolysis. Evidence in favour of the quenching occurring by a charge transfer process, is presented.

The role of electron transfer processes in the photoinduced decarboxylation reaction of α-oxo-carboxylic acids

Irradiation of pyruvic acid in the presence of electron acceptors gives rise to enhanced carbon dioxide production, and in the case of methylviologen, this leads to the formation of reduced methylviologen, which provides further evidence that the photodecarboxylation of pyruvic acid proceeds via an electron transfer process. The addition of pyridine to benzene solutions of pyruvic acid enhances decarboxylation. The effect of added triplet quenchers on the decarboxylation of a variety of α-oxo-carboxylic acids is reported. In some cases this led to enhanced carbon dioxide yields, e.g. this occurs on the addition of various aromatic hydrocarbons to benzene solutions of pyruvic acid. The fact that the quenchers can enhance or ‘sensitise’ carbon dioxide production from α-oxo-carboxylic acids is explained on the basis of an electron transfer mechanism.

A solvent isotope effect upon exciplex photochemistry

Abstract For several naphthylalkylamines, change of solvent from methanol to deuterated methanol (CH3OD) leads to an increase in quantum yield of intramolecular exciplex fluorescence, an increase in exciplex fluorescence lifetime and an increase in naphthalene triplet yield. The solvent effect is explained by the relative abilities of the two solvents to form hydrogen (deuterium) bonds and to protonate (deuterate).

The mechanism of the norrish type II reaction of α-keto-acids and esters

Abstract Methyl-α-keto-octanoate undergoes a Type II reaction to give pent-1-ene and photophysical measurements show that this and the Type II reactions of α-keto-acids occur, contrary to previous claims, from the excited singlet state.

The mechanism of the direct photo-oxidative decarboxylation of α-oxo-carboxylic esters

Abstract A variety of α-oxo-carboxylic esters are shown to undergo direct photo-oxidation, in both polar and non-polar solvents, to produce a higher yield of carbon dioxide than under degassed conditions. In the case of n-hexyl pyruvate, hexaldehyde is also a product of the reaction. Evidence is presented which indicates that singlet oxygen plays little, if any, part in these decarboxylations. A mechanism involving electron transfer from the excited α-oxo-carboxylic ester to ground state oxygen, leading to per-acid intermediates, is proposed as being in competition with the Type II process for fragmentation of these esters.

The direct photo-oxidative decarboxylation of α-oxo-carboxylic acids

Abstract α-Oxo-carboxylic acids undergo photo-oxidative decarboxylation from both C-1 and C-2 positions, and reaction does not involve singlet oxygen: a mechanism involving an electron transfer reaction is postulated.

Factors affecting dye sensitised photo-oxidation reactions; the effect of oxygen concentration and its use to probe the mechanism of decarboxylation of α-oxo-carboxylic acids and esters

Abstract The dye sensitised photo-oxidation of α-oxo-carboxylic acids and esters leads to the production of carbon dioxide. The yield of carbon dioxide is dependent upon the oxygen concentration. This, together with the fact that carbon dioxide evolution can be observed in the absence of oxygen, is in accord with the view that the decarboxylation reactions involve an initial dye-substrate interaction rather than a singlet oxygen mediated reaction.

Problems associated with distinguishing between singlet oxygen and electron transfer photooxygenation reactions.

In the introduction a review is given of the various methods available for probing the mechanism of photooxygenation reactions. To illustrate the methodology, some new results on the photooxygenation of alpha-ketocarboxylic acids and esters is given in which it is shown that these compounds sensitise singlet oxygen formation but are relatively unreactive to this oxidising species. Alternative mechanistic schemes are proposed.

The photophysics of some intramolecular ternary complexes formed between aryl and amino groups

Abstract The fluorescence properties of (3)–(6) in solvents of varying polarity have been examined and the conclusion reached that in non-polar solvents they exhibit fluorescence (low quantum yield) from an intramolecular ternary complex. In polar solvents very little fluorescence is observable. Although each compound exhibits extensive intramolecular fluorescence quenching, the quantum yield of localised triplet naphthalene in non-polar solvents remains relatively high indicating that intersystem crossing occurs in the non-relaxed exciplex.