SENSITIZATION OF O-2(1-DELTA-G) BY N-METHYLACRIDONE - SOLVENT AND O-2-CONCENTRATION DEPENDENCE AND O-16/O-18 ISOTOPE EXCHANGE EXPERIMENTS

The quantum yield of singlet oxygen O2(1DELTA(g)) formation by the ketone N-methylacridone (NMA) varies strongly with solvent: 0.05 (methanol) less-than-or-equal-to Q(DELTA)NMA less-than-or-equal-to 0.29 (tetrachloromethane). The analysis of the solvent and O2-concentration-dependent data on NMA fluorescence demonstrates that O2(1DELTA(g)) is only formed in quenching of the ketone's triplet state by 02 with an efficiency of S(DELTA) = 0.29 being independent of solvent polarity. Thus the solvent effect on the quantum yield of intersystem crossing determines the solvent dependence of Q(DELTA)NMA. The small value of S(DELTA) could lead to the expectation that the T1 state of NMA is an npi* state. However, NMA definitely has a T1 (pipi*) state. Therefore a strict classification of values of S(DELTA) corresponding to T1(npi*) ketones (S(DELTA) almost-equal-to 0.3) and T1(pipi*) ketones (0.7 less than or similar to S(DELTA) less than or similar to 1) does not exist. Results of photochemical O-16/O-18 isotope exchange experiments exclude the participation of a trioxetane in the diabatic deactivation route to the ground state ketone and O2(3SIGMA(g)-)).