Temperature and solvent-dependent photoluminescence quenching in [Ru(bpy)2(bpy-cc-AQ)]2+

Abstract

I have herein investigated the solvent-dependent photoluminescence quenching mechanism of [Ru(bpy)₂(bpy-cc-AQ)]²⁺ using variable temperature emission spectroscopies. The photophysics of this complex are dominated by an excited-state thermal equilibrium between a photoluminescent ³MLCT state and a charge-separated state that lies higher in energy relative to the ³MLCT state in low polarity solvents and approximately isoenergetic in high polarity solvents. Furthermore, an unusual photoluminescence temperature-dependence in high polarity solvents is shown to arise from competition between enthalpic factors favouring the charge-separated state and entropic factors favouring the photoluminescent ³MLCT state, analogous to the molecular light-switch effect of [Ru(bpy)₂(dppz)]²⁺. The solvent-dependent photoluminescence quenching of [Ru(bpy)₂(bpy-cc-AQ)]²⁺ is attributed to two key solvent-dependent factors: (1) the excited-state equilibrium position and (2) the rate of charge-recombination from the charge-separated state.