Differences in Photophysical Properties and Photochemistry of Ru(II)-Terpyridine Complexes of CH3CN and Pyridine

A series of 22 Ru(II) complexes of the type [Ru(tpy)(L)(L')](n+), where tpy is the tridentate ligand 2,2';6,2 ''-terpyridine, L represents bidentate ligands with varying electron-donating ability, and L' is acetonitrile (1a-11a) or pyridine (1b-11b), were investigated. The dissociation of acetonitrile occurs from the (MLCT)-M-3 state in 1a-11a, such that it does not require the population of a (LF)-L-3 state. Electrochemistry and spectroscopic data demonstrate that the ground states of these series do not differ significantly. Franck-Condon line-shape analysis of the 77 K emission data shows no significant differences between the emitting (MLCT)-M-3 states in both series. Arrhenius analysis of the temperature dependence of (MLCT)-M-3 lifetimes shows that the energy barrier (E-a) to thermally populating a (LF)-L-3 state from a lower energy (MLCT)-M-3 state is significantly higher in the pyridine than in the CH3CN series, consistent with the photostability of complexes 1b-11b, which do not undergo pyridine photodissociation under our experimental conditions. Importantly, these results demonstrate that ligand photodissociation of pyridine in 1b-11b does not take place directly from the (MLCT)-M-3 state, as is the case for 1a-11a. These findings have potential impact on the rational design of complexes for a number of applications, including photochemotherapy, dye-sensitized solar cells, and photocatalysis.