R. Stephen Davidson

The Photochemistry of Aryl Halides and Related Compounds

Publisher Summary This chapter discusses the photochemistry of aryl halides and related compounds. Due to the widespread use of substituted aryl halides as pesticides, electrical insulators, moth proofing agents, etc. and the environmental problems which the use of these compounds pose, it seems worthwhile reconsidering the photochemistry of these compounds. Furthermore, the photoinduced decomposition of aryl halides is used extensively in synthesis, and much of this latter work has been recently reviewed. It is well established that on photolysis, many aryl halides undergo homolysis to generate aryl radicals and halogen atoms. The question as to whether the excited singlet or triplet state or indeed both excited states are responsible for reaction, is of considerable importance. Thus, if reaction occurs from the excited singlet state, the homolytic process may well be in competition with an electrocyclic reaction. The precise mechanism for photoinduced homolysis of aryl-halogen compounds is dependent upon both the nature of the halogen substituent and the aryl group.

Mechanistic aspects of the triethylamine assisted photo-induced dehalogenation of halogeno-aromatic compounds

Abstract Criteria for the intervention of radical ions in the photoreactions of aromatic halides with triethylamine is given and where appropriate evidence for the intermediary of such species is presented.

The mechanism of the photo-induced decarboxylatton of pyruvic acid in solution

Abstract It is proposed that photo-induced decarboxylation of pyruvic acid occurs via electron transfer from an excited to a ground-state molecule. Quantum yields of triplet production at 295 K in deoxygenated benzene, acetonitrile and water are 0.65, 0.88 and 0.22. The triplet state of naphthalene is quenched by pyruvic acid and it is suggested that this occurs via electron transfer.

Visible-light-induced photodissolution of α-Fe2O3 powder in the presence of chloride anions

Haematite iron oxide (α-Fe2O3), an n-type semiconductor, has been shown to corrode in the presence of chloride anion in aqueous acidic media when in powder suspension. The rate of corrosion was enhanced by a factor of ten upon band-gap excitation using visible light. Surface adsorption and ease of complex formation with solution species are shown to be a major factor in the dissolution process. A mechanism for dissolution is proposed and implications for iron oxide powder photocatalysis are assessed.

Examples of competition between energy transfer and excited complex formation in the reactions of benzenoid compounds with tertiary amines

Abstract Although many 2-phenylethylamines exhibit fluorescence due to intramolecular exciplex formation in cyclohexane, the excited singlet state of benzene, at low concentrations, docs not form fluorescent cxciplexes with tertiary amines. Instead, sensitised amine fluorescence may be observed. Exciplex formation between benzene and tertiary amines is observable when the benzene concentration is high.

Intramolecular quenching of the excited singlet state of a naphthyl group by halogeno substituents

Abstract The halogeno group in a number of naphthyl alkyl halides has been shown to quench the fluorescence of the naphthyl group. The efficiency of quenching is dependent on the length of the chain and the nature of the hologeno group. By comparison of triplet yields in fluid solution and a rigid matrix it is shown that the magnitude of triplet production is also dependent upon the orientation of the halogeno group with respect to the aromatic nucleus.

Factors affecting dye-sensitised photo-oxygenation reactions

The kinetics of dye-sensitised oxidation reactions are considered. Utilisation of rate of oxygen absorption measurements to provide information on light intensity and the quantum yield of triplet formation of the dye, is discussed. The way in which solvent isotope effects, sensitiser–substrate triplet–triplet energy transfer processes, and quenching of the sensitiser excited singlet state by the substrate affect the kinetics is considered. Quantitative measurements on the quenching of the excited singlet states of Methylene Blue and Rose Bengal by anions, β-carotene, and 2,5-diphenylisobenzofuran are reported. A comprehensive rate expression is obtained for oxidation reactions in which the substrate interacts with the excited singlet and triplet states of the sensitiser, and with singlet oxygen.

The application of some polymeric type-I photoinitiators based on α-hydroxymethylbenzoin and α-hydroxymethylbenzoin methyl ether

Abstract Some polymeric photoinitiators based on α-hydroxymethylbenzoin and its methyl ether are used to initiate the polymerization of methyl methacrylate in solution and the crosslinking of an epoxyacrylate formulation in thin films. In general the polymeric photoinitiators perform better than corresponding low molecular weight model compounds. Moreover, it is found that polymeric photoinitiators based on α-hydroxymethylbenzoin methyl ether perform better than those based on α-hydroxymethylbenzoin. Explanations for these observations are advanced.

Fluorescent excimer formation by α,ω-diaminoalkanes and related compounds

The fluorescence properties of a number of α,ω-diaminoalkanes have been studied in which the amino groups include diethylamino, pyrrolidino, piperidino, and morpholino residues. Intramolecular excimer formation was usually observed when di- and tri-methylene chains separated the amino groups. α,ω-Dimorpholinoalkanes having chain lengths of varying size between the morpholino groups, exhibited fluorescent excimer formation. Owing to the high concentrations employed it could not be ascertained whether the emission was the result of inter- or intra-molecular complex formation. In all cases the excimer fluorescence was considerably red shifted by a change of solvent from cyclohexane to tetrahydrofuran. This solvent dependency leads to the conclusion that amine excimers have some charge-transfer character.

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.