Tuning the excited-state deactivation pathways of dinuclear ruthenium(ii) 2,2′-bipyridine complexes through bridging ligand design

Abstract

A detailed photophysical investigation of two dinuclear ruthenium(II) complexes is reported. The two metallic centers were coordinated to a bis-2,2′-bipyridine bridging ligand, connected either through the para (L_p, D_p) or the meta position (L_m, D_m). The results obtained herein were compared to the prototypical [Ru(bpy)₃]²⁺ parent compound. The formation of dinuclear complexes was accompanied by the expected increase in molar absorption coefficients, i.e. 12 000 M⁻¹ cm⁻¹, 17 000 M⁻¹ cm⁻¹, and 22 000 M⁻¹ cm⁻¹ at the lowest energy MLCT_max transition for [Ru(bpy)₃]²⁺, D_m and D_p respectively. The L_p bridging ligand resulted in a ruthenium(II) dinuclear complex that absorbed more visible light, and had a longer-lived and more delocalized excited-state compared to a complex with the L_m bridging ligand. Variable temperature measurements provided valuable information about activation energies to the uppermost ³MLCT state and the metal-centered (³MC) state, often accompanied by irreversible ligand-loss chemistry. At 298 K, 48% of [Ru(bpy)₃]²⁺* excited-state underwent deactivation through the ³MC state, whereas this deactivation pathway remained practically unpopulated (<0.5%) in both dinuclear complexes.