Tuning the electronics of bis(tridentate)ruthenium(II) complexes with long-lived excited states: Modifications to the ligand skeleton beyond classical electron donor or electron withdrawing group decorations

A series of homoleptic bis(tridentate) [Ru(L)₂]²⁺ (1, 3) and heteroleptic [Ru(L)(dqp)]²⁺ complexes (2, 4) [L = dqxp (1, 2) or dNinp (3, 4); dqxp = 2,6-di(quinoxalin-5-yl)pyridine, dNinp = 2,6-di(N-7-azaindol-1-yl)pyridine, dqp = 2,6-di(quinolin-8-yl)pyridine] was prepared and in the case of 2 and 4 structurally characterized. The presence of dqxp and dNinp in 1-4 result in anodically shifted oxidation potentials of the Ru^3+/2+ couple compared to that of the archetypical [Ru(dqp) ₂]²⁺ (5), most pronounced for [Ru(dqxp)₂] ²⁺ (1) with a shift of +470 mV. These experimental findings are corroborated by DFT calculations, which show contributions to the complexes HOMOs by the polypyridine ligands, thereby stabilizing the HOMOs and impeding electron extraction. Complex 3 exhibits an unusual electronic absorption spectrum with its lowest energy maximum at 382 nm. TD-DFT calculations suggest that this high-energy transition is caused by a localization of the LUMO on the central pyridine fragments of the dNinp ligands in 3, leaving the lateral azaindole units merely spectator fragments. The opposite is the case in 1, where the LUMO experiences large stabilization by the lateral quinoxalines. Owing to the differences in LUMO energies, the complexes reduction potentials differ by about 900 mV [E_1/2(1^2+/1+) = -1.17 V, E _c,p(3^2+/1+) = -2.06 V vs Fc^+/0]. As complexes 1-4 exhibit similar excited state energies of around 1.80 V, the variations of the lateral heterocycles allow the tuning of the complexes excited state oxidation strengths over a range of 900 mV. Complex 1 is the strongest excited state oxidant of the series, exceeding even [Ru(bpy)₃]²⁺ by more than 200 mV. At room temperature, complex 3 is nonemissive, whereas complexes 1, 2, and 4 exhibit excited state lifetimes of 255, 120, and 1570 ns, respectively. The excited state lifetimes are thus somewhat shortened compared to that of 5 (3000 ns) but still acceptable to qualify the complexes as photosensitizers in light-induced charge-transfer schemes, especially for those that require high oxidative power.