Charge-transfer and non-charge-transfer processes competing in the sensitization of singlet oxygen: Formation of O<inf>2</inf>(¹∑<inf>g</inf>⁺), O<inf>2</inf>(¹Δ<inf>g</inf>), and O<inf>2</inf>(³∑<inf>g</inf>^-) during oxygen quenching of triplet excited naphthalene derivatives

Abstract

Both excited singlet states1∑g+and1Δgand the unexcited triplet ground state3∑g-of molecular oxygen are formed with varying rate constants kT1∑, kT1Δ, and kT3∑, respectively, during the quenching by O2of triplet states T1of sufficient energy ET. The present paper reports these rate constants for a series of nine naphthalene sensitizers of very different oxidation potential, Eoxbut almost constant ET. These data complement data for kT1∑, kT1Δ, and kT3∑, determined previously for 13 sensitizers of very different ET. The analysis of the whole set of rate constants reveals that the quenching of triplet states by O2results in the formation of O2(1∑g+), O2(1Δg), and O2(3∑g-) with varying efficiencies by two different channels, each capable of producing all three product states. One quenching channel originates from excited1,3(T1·3∑) complexes without chargetransfer character (nCT), which we cannot distinguish from encounter complexes; the other originates from1(T1·3∑) and3(T1·3∑) exciplexes with partial charge-transfer character (pCT). Rate constants of formation for O2(1∑g+), O2(1Δg), and O2(3∑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 between1(T1·3∑) and3(T1·3∑) is surprisingly observed only in the nCT but not in the pCT channel. © 2001 American Chemical Society.