DEFINITION OF THE NATURE OF KETONE TRIPLET-STATES ON THE BASIS OF SINGLET OXYGEN GENERATION EFFICIENCY

The efficiency of singlet oxygen generation (S(DELTA)) was measured for acetophenone, p-aminobenzophenone, and Michler's ketone by the time-resolved O2(1DELTA(g)) phosphorescence method in various solvents. The S(DELTA) value for acetophenone is 0.3-0.4 in benzene, cyclohexane, acetonitrile, and alcohols (practically independent of solvent); the lowest triplet state of acetophenone is therefore probably pure n,pi* in all solvents. The values for Michler's ketone and p-aminobenzophenone are 0.24-0.4 in all solvents investigated (except in ethanol for Michler's ketone) and are also typical for ketones with n,pi* triplet states. For Michler's ketone in ethanol, S(DELTA) = 0.65; the triplet state therefore probably has substantial CT character. Amino-substituted benzophenones do not undergo photoreduction in alcohols, which is explained partly by a low quantum yield of triplet and mainly by the considerable negative charge at the carbonyl oxygen because of intramolecular charge transfer, especially in alcohols. As a result, the oxygen atom in the 3n,pi*state is considerably less electrophilic than in benzophenone. Internal conversion is the main deactivation channel for the excited singlet state of Michler's ketone and p-aminobenzophenone and probably occurs by fast back electron transfer with deactivation to the ground state; the rate of this process increases strongly with decreasing energy of the CT state.