Charge transfer induced quenching of triplet sensitizers by ground state oxygen and of singlet oxygen by ground state sensitizers: A common deactivation channel

Abstract

The photosensitized production of singlet molecular oxygen O₂(¹Δ_g) during oxygen quenching of excited triplet states, T₁, and the deactivation of O₂(¹Δ_g) by the sensitizer have been investigated for a set of biphenyl derivatives, in carbon tetrachloride, dichloromethane and acetonitrile. The rate constants of the quenching of T₁ by ground state oxygen, O₂(³Σ_g⁻), leading to O₂(³Σ_g⁻) and of O₂(¹Δ_g) quenching by ground state sensitizers, S₀, have been subjected to a common analysis. It is shown that the rate constants for both processes are described by one Marcus-type plot in each solvent. This indicates that both processes proceed ia exciplexes (T₁³Σ) and (S₀¹Δ) of the same partial charge transfer (pCT) from sensitizer to O₂ . These findings are in accordance with results obtained previously with a series of naphthalene derivatives. Further comparative analysis leads to a relation for the estimation of the average charge transfer character of the (T₁³Σ) and (S₀¹Δ) pCT complexes, which is consistent with the existing concept of the reorganization energy. The average charge transfer character varies only slightly between the series of biphenyl and naphthalene derivatives, and is shown to increase in each case with solvent polarity. The overall reorganization energies could be separated into constant intramolecular and solvent-dependent outer reorganization contributions. The small but significant differences in the reorganization energy indicate that the structure of pCT complexes varies between these two series.