[Ru(bpy)2dppz]2+ light-switch mechanism in protic solvents as studied through temperature-dependent lifetime measurements

Temperature-dependent excited-state lifetime measurements have been performed on four different Ru(II)based dppz compounds in protic and aprotic solvents. This work supports the existence of a dynamic equilibrium between two MLCT states associated with the dppz ligand: one is a bright state with a ligand orbital similar in size to that associated with the (MLCT)-M-3 state of [Ru(bpy)(3)](2+), and the other is a dark phz-like state. Our results are consistent with a light-switch mechanism involving a competition between energetic factors that favor the dark (phz) state and entropic factors that favor the bright (bpy) state. This paper explores the photophysics of these light-switch compounds through a systematic variation of the equilibrium energetics. This is accomplished by (1) varying the dielectric strength of the solvent and (2) making chemical substitutions on the dppz ligand. Observations obtained from all four compounds in six different solvents can be explained using this equilibrium model.