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Abstract

The radiolytic decomposition of diacyl peroxides, azocompounds and bisulphides in aromatic solvents (benzene, toluene) had been examined by us in a number of papers. It has been shown that the decomposition of these solutes is sensitized, i.e. caused by solvent-solute energy transfer. Stern-Volmer constants of the energy transfer for all the aromatic-substituted compounds examined are approximately equal and are found to be ≈ 5 × 10 2 l/mol. Maximum G-values, attained when the energy transfer is total, increase in the order: azo-compounds—bisulphides—peroxides. A comparison has been made of the radiolysis, photolysis, γ- and photoluminescence of three-component solutions with two energy acceptors, one chemically decomposing (benzoyl peroxide), the other a scintillator (PPO or p-terphenyl). The decrease of the quantum yield of sensitized benzoyl peroxide decomposition in the presence of a scintillator, and the quenching of the sensitized scintillator luminescence by peroxide, were explained in terms of a competition between the two energy acceptors as the energy transferred from the solvent. Comparison of the kinetic behaviour of the solutions on γ- and photo-irradiation indicates that energy transfer occurs from the excited molecules of the solvent in the first singlet (fluorescent) state. It has been shown that benzene photosensitized trans-cis-isomerization of stilbene is a result of triplet-triplet energy transfer from the solvent (the Stern-Volmer constant is about 10 4 l/mol). On γ-irradiation of benzene solutions this process is interfered with by the isomerization caused by free radicals produced by the irradiation of the solvent. The benzene-photosensitized o-nitrobenzaldehyde rearrangement into o-nitrozobenzoic acid is caused by triplet-triplet energy transfer at concentrations < 2 × 10 −4 mol/l and by singlet-singlet energy transfer at higher concentrations. This was demonstrated by comparing the concentration dependence of the photosensitized aldehyde rearrangement with that of the benzene fluorescence-quenching by aldehyde. A similar concentration dependence was found for radiolytic rearrangement of the aldehyde in solution. The sensitized reactions described above were used to estimate the G-values of the solvent molecules in fluorescent and triplet excited states. For benzene these G-values were found to be ≈ 4–6 and ≈ 0·4 molecules respectively per 100 eV of energy absorbed.