Charge-transfer and non-charge-transfer processes competing in the sensitization of singlet oxygen: Formation of O-2((1)Sigma(+)(g)), O-2((1)Delta(g)), and 0(2)((3)Sigma(-)(g)) during oxygen quenching of triplet excited naphthalene derivativest

Both excited singlet states (1)Sigma (+)(g) and (1)Delta (g) and the unexci ted triplet ground state (3)Sigma (-)(g) of molecular oxygen are formed wit h varying rate constants k(T)(1 Sigma), k(T)(1 Delta) and k(T)(3 Sigma) res pectively, during the quenching by O-2 Of triplet states T-1 of sufficient energy ET The present paper reports these rate constants for a series of ni ne naphthalene sensitizers of very different oxidation potential, E-ox but almost constant ET These data complement data for k(T)(1 Sigma) k(T)(1 Delt a), and k(T)(3 Sigma), determined previously for 13 sensitizers of very dif ferent ET. The analysis of the whole set of rate constants reveals that the quenching of triplet states by O-2 results in the formation of O-2((1)Sigm a (+)(g)), O-2((1)Delta (g)), and O-2((3)Sigma (-)(g)) With varying efficie ncies by two different channels, each capable of producing all three produc t states. One quenching channel originates from excited (1,3)(T(1)(.3)Sigma ) complexes without charge-transfer character (nCT), which we cannot distin guish from encounter complexes; the other originates from (1)(T(1)(.3)Sigma ) and (3)(T(1)(.3)Sigma) exciplexes with partial charge-transfer character (pCT). Rate constants of formation for O-2((1)Sigma (+)(g)), O-2((1)Delta ( g)), and O-2((3)Sigma (-)(g)) are controlled by the respective excess energ ies via an energy gap relation in the nCT channel, whereas they vary with v arying free energy of complete electron transfer in the pCT channel. A fast intersystem crossing equilibrium between (1)(T(1)(.3)Sigma) and (3)(T(1)(. 3)Sigma) is surprisingly observed only in the nCT but not in the pCT channe l.