Quenching of ketone triplet states by silanes

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Abstract

Triplet state quenching of the ketones benzophenone, p-methylbenzophenone, p-phenylbenzophenone and benzil by silanes containing an SiH bond was investigated in a 9:1 (v/v) mixture of acetonitrile and acetone. It was found that both the n,π* and n,π*−π,π* triplet states are quenched if their energies are above 280 kJ mol−1. Quenching rate constants kH were determined using steady state phosphorescence quenching and the time-resolved measurement of the triplet—triplet absorption. From an analysis of the behaviour of the triplet—triplet absorption of benzophenone as a function of time at variable silane concentrations, a reaction scheme was derived which includes the combination reactions of diphenyl ketyl, solvent and silyl radicals and the addition of silyl radical to the ground state of benzophenone. kH values obtained in this way agree with the phosphorescence quenching data and are of the order of 3 × 106 to 3.4 × 108 dm3 mol−1 s−1. Furthermore, it was found that the log kH values depend on the magnitude of the wavenumber of the SiH vibration of the silanes.

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