Triiodide Quenching of Ruthenium MLCT Excited State in Solution and on TiO₂ Surfaces: An Alternate Pathway for Charge Recombination

The excited states of [Ru(bpy)2(deeb)](PF6)2, where bpy is 2,2-bipyridine and deeb is 4,4‘-(CO2CH2CH3)2-2,2‘-bipyridine, were found to be efficiently quenched by triiodide (I3-) in acetonitrile and dichloromethane. In dichloromethane, I3- was found to quench the excited states by static and dynamic mechanisms; Stern−Volmer analysis of the time-resolved and steady-state photoluminescence data produced self-consistent estimates for the I3- + Ru(bpy)2(deeb)2+ ⇄ [RuII(bpy)2(deeb)2+,(I3-)]+ equilibrium, K = 51,000 M-1, and the bimolecular quenching rate constant, kq = 4.0 × 1010 M-1 s-1. In acetonitrile, there was no evidence for ion pairing and a dynamic quenching rate constant of kq = 4.7 × 1010 M-1 s-1 was calculated. Comparative studies with Ru(bpy)2(deeb)2+ anchored to mesoporous nanocrystalline TiO2 thin films also showed efficient excited-state dynamic quenching by I3- in both acetonitrile and dichloromethane, kq = 1.8 × 109 and 3.6 × 1010 M-1 s-1, respectively. No reaction products for the excited-state quenching processes were observed by nanosecond transient absorption measurements from 350 to 800 nm under any experimental conditions. X-ray crystallographic, IR, and Raman data gave evidence for interactions between I3- and the bpy and deeb ligands in the solid state.