Partial charge transfer contribution to the solvent isotope effect and photosensitized generation of singlet oxygen, O2(1Δg), by substituted ruthenium(ii) bipyridyl complexes in aqueous media

Abstract

The efficiency of singlet oxygen photosensitized by some ruthenium(II) bipyridyl complex ions in aqueous media is reported in this study. Measurements were carried out in H₂O and D₂O. The effect of the deuterium isotope on the lifetime of ³MLCT excited states of these complexes is studied in H₂O and D₂O. The deuterium isotope effect was discussed in terms of the vibronic coupling to the solvent in addition to the charge transfer to the solvent mechanism due to their dependence on the oxidation potential of the sensitizer. Quenching rate constants, k_q, for quenching of the ³MLCT states of these ruthenium complex ions by molecular oxygen were found to be in the range of (2.08–3.84) × 10⁹ M⁻¹ s⁻¹ in H₂O and (1.69–3.48) × 10⁹ M⁻¹ s⁻¹ in D₂O. The efficiency of singlet oxygen, O₂(¹Δ_g), production as a result of the ³MLCT quenching by oxygen, f^T_Δ, is reported in D₂O and found to be in the range 0.25–0.56. It has been found that the lifetime of the excited state is longer in D₂O, τ^D₀, than in H₂O, τ^H₀, which was related to partial charge transfer to the solvent in addition to the vibronic coupling mechanism. Mechanisms by which the excited states of these ruthenium complexes are quenched by molecular oxygen that shows the competition between charge transfer, non-charge transfer deactivation channels or energy transfer assisted charge transfer deactivation mechanisms are reported.