Charge-Transfer and Non-Charge-Transfer Processes Competing in the Sensitization of Singlet Oxygen: Formation of O_2(~1Σ_g~+), O_2(~1Δ_g), and O_2(~3Σ_g~-) during Oxygen Quenching of Triplet Excited Naphthalene Derivatives

摘要

Both excited singlet states ~1Σ_g~+ and ~1Δ_g and the unexcited triplet ground state ~3Σ_g~- of molecular oxygen are formed with varying rate constants k_T~(1Σ), k_T~(1Δ), and k_T~(3Σ), respectively, during the quenching by O_2 of triplet states T_1 of sufficient energy E_T. The present paper reports these rate constants for a series of nine naphthalene sensitizers of very different oxidation potential, E_(ox) but almost constant E_T. These data complement data for k_T~(1Σ), k_T~(1Δ), and k_T~(3Σ), determined previously for 13 sensitizers of very different E_T. 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(~1Σ_g~+), O_2(~1Δ_g), and O_2(~3Σ_g~-) with varying efficiencies by two different channels, each capable of producing all three product states. One quenching channel originates from excited ~(1,3)(T_1·~3Σ) complexes without chargetransfer character (nCT), which we cannot distinguish from encounter complexes; the other originates from ~1(T_1·~3Σ) and ~3(T_1·~3Σ) exciplexes with partial charge-transfer character (pCT). Rate constants of formation for O_2(~1Σ_g~+), O_2(~1△_g), and O_2(~3Σ_g~-) are controlled by the respective excess energies via an energy gap relation in the nCT channel, whereas they vary with varying free energy of complete electron transfer in the pCT channel. A fast intersystem crossing equilibrium between ~1(T_1·~3Σ) and ~3(T_1·~3Σ) is surprisingly observed only in the nCT but not in the pCT channel.