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The fluorescence of 7-aminocoumarins is quenched by a variety of organic electron donors or acceptors in acetonitrile. In general, donors with half-wave oxidation potentials less positive than 1.0 V vs. SCE and acceptors with reduction potentials less negative than -1.5 V vs. SCE are candidates for diffusion limited quenching of coumarin singlet states. Profiles of quenching rates are consistent with calculated free energies for electron transfer between excited coumarins and donors or acceptors. In flash photolysis experiments electron transfer for several dyes and quenchers (e.g., methyl viologen) is demonstrated. Relatively low yields of net electron transfer are consistently obtained due to inefficient ionic photodissociation via singlet quenching or a low yield of more photoactive coumarin triplets. Electrochemical properties of the coumarins have been investigated by cyclic voltammetry with the indications of reversible oxidation and irreversible reduction as important processes.The 7-aminocoumarins (e.g., 1-5) constitute an important class of organic dyes which lase1 2 34and which in some circumstances may act as photosensitizers.3,4 These 6 structures are also related to the furocoumarins which have received much attention due to their photobiological properties.5 In previous papers in this series, photophysical and photochemical properties for coumarin dyes have(1) On sabbatical leave from Ithaca College, Ithaca, NY.(2) Drexhage, K. D. "Dye Lasers"; Schafer, F. P., Ed.

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The reduction of benzophenone by hydrogen donor molecules such as tetralin and dihydroanthracene to give diphenylmethane was investigated in the temperature range 300-400 °C. Several lines of evidence indicate that the reaction occurs in three distinct stages. The first stage is a radical process which gives benzhydrol. In the second stage, this intermediate undergoes an SN reaction to produce water and bis(diphenylmethyl) ether. The ether disproportionates in a readily initiated, free radical chain reaction to give diphenylmethane and benzophenone.

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