Charge-Transfer and Non-Charge-Transfer Processes Competing in the Sensitization of Singlet Oxygen:  Formation of O₂(¹Σg⁺), O₂(¹Δg), and O₂(³Σg⁻) during Oxygen Quenching of Triplet Excited Naphthalene Derivatives

Both excited singlet states 1Σg+ and 1Δg and the unexcited triplet ground state 3Σg- of molecular oxygen are formed with varying rate constants kT1Σ, kT1Δ, and kT3Σ, respectively, during the quenching by O2 of triplet states T1 of sufficient energy ET. The present paper reports these rate constants for a series of nine naphthalene sensitizers of very different oxidation potential, Eox but almost constant ET. These data complement data for kT1Σ, kT1Δ, and kT3Σ, 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 O2 results in the formation of O2(1Σg+), O2(1Δg), and O2(3Σg-) with varying efficiencies by two different channels, each capable of producing all three product states. One quenching channel originates from excited 1,3(T1·3Σ) complexes without charge-transfer character (nCT), which we cannot distinguish from encounter complexes; the other originates from 1(T1·3Σ) and 3(T1·3Σ) exciplexes with partial charge-transfer character (pCT). Rate constants of formation for O2(1Σg+), O2(1Δg), and O2(3Σg-) 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(T1·3Σ) and 3(T1·3Σ) is surprisingly observed only in the nCT but not in the pCT channel.