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

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 thepara(L-p,D-p) or themetaposition (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 theL(m)bridging ligand. Variable temperature measurements provided valuable information about activation energies to the uppermost(3) MLCT state and the metal-centered ((MC)-M-3) state, often accompanied by irreversible ligand-loss chemistry. At 298 K, 48% of [Ru(bpy)(3)](2+)* excited-state underwent deactivation through the(3)MC state, whereas this deactivation pathway remained practically unpopulated (<0.5%) in both dinuclear complexes.