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The triplet-triplet extinction coefficients of seventeen compounds have been determined in cyclohexane and benzene at room temperature by a method involving energy transfer. Large changes in extinction coefficient at the wavelength maximum &(Am&) were observed upon changing solvent from cyclohexane to benzene. Values of &(Amax) were determined relative to a value of &(Amax) for the ketyl radical ( $zCOH) of 3220 M-' cm-' in water, assuming no change of oscillator strength with solvent. A number of triplet transfer rates, and other reactivities of benzophenone triplet and ketyl radicals, were also estimated.

The technique of time resolved laser flash spectroscopy has been used to study the photophysical and photochemical properties of the lowest triplet of benzophenone in water and acetonitrile. New estimates have been obtained for the triplet extinction coefficient, unimolecuIar and bimolecular rate constant of decay in both solvents, as well as a determination of the quantum yield of ketyl formation in pure water and the rate constants of hydrogen abstraction from various lactams in both solvents. Evidence for hydrogen abstraction from water by a triplet of benzophenone is found. Correlation between rate constants of hydrogen abstraction from lactams by the lowest benzophenone triplet and their ionization potential is discussed. Reversibility of these hydrogen abstractions is considered.

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