SOLVENT EFFECTS ON SINGLET OXYGEN YIELD FROM N,PI-ASTERISK AND PI,PI-ASTERISK TRIPLET CARBONYL-COMPOUNDS

Quenching of benzophenone (n,pi*) and duroquinone (pi,pi*) triplet states by molecular oxygen was studied by time-resolved O2(1DELTA(g)) phosphorescence and nanosecond laser photolysis in 12 solvents. With the exception of benzophenone in acetonitrile and CCl4 and of duroquinone in acetonitrile, the quenching rate constant (k(q)T) is independent of the polarity or viscosity of solvents and lies in the range (2.1-2.55) x 10(9) M-1 s-1 and (1.3-1.7) x 10(9) M-1 s-1 for benzophenone and duroquinone, respectively. The k(q)T values are practically constant when the viscosity of solution increases from 0.3 to 4.2 cP, although the diffusion-controlled rate constant (k(d)) decreases by more than a factor of 2. For these carbonyls, the usual observation that k(q)T approximately 1/9k(d) for quenching by the energy-transfer mechanism does not hold. The efficiency of O2(1DELTA(g)) formation (S(DELTA)) is also independent of viscosity or polarity of the solvent and lies in the range 0.31-0.35 for benzophenone and 0.79-0.95 for duroquinone but increases in alcohols, to 0.43-0.60 and 0.87-1.02. Apparently, specific solvation of triplet carbonyls by alcohols results in a decrease in the deactivation rate constant of the triplet encounter complex to the ground state [3(S*...O2) --> (S(O)\3O2) --> S(O) + O-3(2)] and an increase in the contribution of the energy-transfer reaction.