Electronic to vibrational energy conversion and charge transfer contributions during quenching by molecular oxygen of electronically excited triplet states

Abstract

The bimolecular rate constants k_q^T for quenching of some substituted naphthalene triplet states by molecular oxygen (O₂(³Σ_g⁻)) in acetonitrile and cyclohexane and the efficiencies, f_Δ^T, with which singlet oxygen (O₂^*(¹Δ_g)) is thereby produced are reported for naphthalene derivatives with a wider range of oxidation potentials than those previously measured. The magnitude of k_q^T and f_Δ^T are inversely correlated, and both parameters exhibit pronounced sensitivity to the oxidation potential (E_M^OX) of the naphthalene derivative and to the solvent polarity. The modified charge transfer mediated mechanism of quenching based on singlet and triplet channels for oxygen quenching is invoked to discuss these results. In cyclohexane the maximum value for f_Δ^T of one is observed for compounds with high oxidation potentials indicating no contribution from the triplet channel whilst in acetonitrile the limit for f_Δ^T of 0.25 expected when singlet and triplet channels give equal contributions, when spin statistics is taken into account, is observed for 2,6-dimethoxynaphthalene, which is the derivative with the lowest oxidation potential. These results are combined with those previously reported by ourselves in cyclohexane in order to examine the dependence of the quenching rate constants due to energy transfer on the energy gap (E_T–E_1Σ). This allows the quenching rate constants due to energy transfer in cyclohexane to be separated into contributions with and without charge transfer assistance. The latter contribution shows a smooth dependence on (E_T–E_1Σ) and the same dependence on the electronic energy, which has to be converted into vibrational energy, probably applies when the solvent is acetonitrile since making this assumption results in similar dependences of the charge transfer contributions in both singlet and triplet channels in acetonitrile. The free energy of activation for charge transfer assisted quenching is shown to have a linear dependence on the free energy change for full charge transfer but the indications are that quenching is via singlet and triplet charge transfer complexes with only partial charge transfer character being 12.5% and 17% in acetonitrile and cyclohexane respectively.