A Photosensitizer Bearing an Asymmetric “Push-Pull” Bipyridine Ligand Enhances Excited State Electron Transfer

Abstract

Herein we report RuII photosensitizers that bear a single “push-pull” ligand designed to polarize charge on a single pyridine ring of 2,2′-bipyridine (bpy) in the metal-to-ligand charge-transfer (MLCT) excited state for enhanced excited state electron transfer. The photosensitizers have the general structure [Ru(dtb)2(4-CO2H,4′-R-bpy)]2+, where R is –H (RuHcb), −CF3 (RuFcb), and −CH3 (RuMcb) and dtb is 4,4′-(tert-butyl)2-bpy. These photosensitizers were synthesized, characterized spectroscopically and electrochemically, and contrasted with [Ru(dtb)2(dcb)]2+ (RuDcb) that bears the more common dicarboxylic acid ligand, Dcb = 4,4′-(CO2H)2-bipyridine. When anchored to mesoporous TiO2 thin films in a 0.1 M LiClO4/CH3CN electrolyte, RuMcb|TiO2 displayed the largest excited state electron injection yield ϕinj = 0.90 and, generally, ϕinj decreased as R became more electron withdrawing. RuDcb|TiO2 displayed a significantly lower ϕinj = 0.80. Transient absorption spectra recorded on ps and ns time scales and spectroelectrochemical data revealed that ∼70% of the total ϕinj for RuMcb occurred on a sub ps time scale that could not be kinetically resolved, only 20% occurred on this same time scale for RuDcb. Quantum chemical simulations were utilized to characterize the charge transfer excited states, the surface binding, and the interfacial electron transfer kinetics. Apart from RuFcb, the data reveal that the push–pull ligand had little impact on the ground state and a significant one on the MLCT excited state, polarizing charge on a sub-ps time scale for more efficient interfacial electron transfer. More broadly, the data reveals that push–pull ligands can be optimized for excited state electron transfer reactivity.