pH-Driven Mechanistic Switching from Electron Transfer to Energy Transfer between Ru(bpy)32+ and Ferrocene Derivatives

摘要

The metal-to-ligand charge transfer excited states of [Ru(bpy)3]²⁺ (bpy = 2,2′-bipyridine) may be deactivated via energy transfer or electron transfer with ferrocene derivatives in aqueous conditions. Stern–Volmer quenching analysis revealed that the rate constant for [Ru(bpy)3]²⁺ excited-state quenching depends on solution pH when a ferrocenyl-amidinium derivative (>Fc–am) containing a proton-responsive functionality tethered to the ferrocene center was present. By contrast, the rate constant with which the [Ru(bpy)3]²⁺ excited state is quenched by an analogous ferrocene derivative (ferrocenyltrimethylammonium, >Fc–mam) that lacks a protonic group does not depend on pH. These results show that the presence (or absence) of a readily transferrable proton modulates quenching rate constants in bimolecular events involving [Ru(bpy)3]²⁺ and ferrocene. More surprisingly, transient absorption spectroscopy reveals that the mechanism by which the [Ru(bpy)3]²⁺ excited state is quenched by >Fc–am appears to be modulated by solution proton availability, switching from energy transfer at low pH when >Fc–am is protonated, to electron transfer at high pH when >Fc–am is deprotonated. The mechanistic switching that is observed for this system cannot be aptly explained using a simple driving force dependence argument, suggesting that more subtle factors dictate the pathway by which the [Ru(bpy)3]²⁺ excited state is deactivated by ferrocene in aqueous solutions.