Temperature And Solvent-Dependent Photoluminescence Quenching In [Ru(Bpy)(2)(Bpy-Cc-Aq)](2+)

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 (MLCT)-M-3 state and a charge-separated state that lies higher in energy relative to the (MLCT)-M-3 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 (MLCT)-M-3 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.