Mechanism of quenching by oxygen of the excited states of ruthenium(ii) complexes in aqueous media. Solvent isotope effect and photosensitized generation of singlet oxygen, O2(1Δg), by [Ru(diimine)(CN)4]2− complex ions

Abstract

In this study we report on the photophysical properties of some [RuL(CN)₄]²⁻ complex ions where L = 2,2′-bipyridine (bpy), 5,5′-dimethyl-2,2′-bipyridine (dmb), 1,10-phenanthroline (phen), 1-ethyl-2-(2-pyridyl)benzimidazole (pbe), 2,2′:6′,2‴-terpyridine (tpy) and [RuL₃]²⁺ where L = bpy or phen. Measurements were carried out in H₂O and D₂O. The effect of the deuterium isotope effect on the lifetime of these complexes is discussed. It has also been found that the presence of cyano groups has a pronounced effect on the lifetime of the excited metal-to-ligand charge transfer (³MLCT) of these complexes. Quenching of the ³MLCT states by oxygen is reported in H₂O and D₂O. The rate constants, k_q, for quenching of the ³MLCT states of these ruthenium complex ions by molecular oxygen are in the range (2.55 to 7.01) × 10⁹ M⁻¹ s⁻¹ in H₂O and (3.38 to 5.69) × 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_Δ^T, is reported in D₂O and found to be in the range 0.29–0.52. The rate constants, k_q^Δ, for quenching of singlet oxygen by ground state sensitizers in D₂O is also reported and found to be in the range (0.15 to 3.46) × 10⁷ M⁻¹ s⁻¹. The rate constants and the efficiency of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a non-charge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of ¹(T₁³Σ) and ³(T₁³Σ) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O₂(¹Δ_g), and O₂(³Σ⁻_g). The balance between CT and nCT deactivation channels which is described by the relative contribution p_CT of CT induced deactivation is discussed. The kinetic model proposed for the quenching of π–π* triplet states by oxygen can also be applied to the quenching of ³MLCT states by oxygen.