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 sta tes by molecular oxygen was studied by time-resolved O2(1DELTA(g)) pho sphorescence 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 d uroquinone, respectively. The k(q)T values are practically constant wh en the viscosity of solution increases from 0.3 to 4.2 cP, although th e diffusion-controlled rate constant (k(d)) decreases by more than a f actor of 2. For these carbonyls, the usual observation that k(q)T appr oximately 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 r ange 0.31-0.35 for benzophenone and 0.79-0.95 for duroquinone but incr eases in alcohols, to 0.43-0.60 and 0.87-1.02. Apparently, specific so lvation of triplet carbonyls by alcohols results in a decrease in the deactivation rate constant of the triplet encounter complex to the gro und state [3(S...O2) --> (S(O)\3O2) --> S(O) + O-3(2)] and an increas e in the contribution of the energy-transfer reaction.