Differences in Photophysical Properties and Photochemistry of Ru(II)-Terpyridine Complexes of CH 3 CN 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 3 MLCT state in 1a - 11a , such that it does not require the population of a 3 LF 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 3 MLCT states in both series. Arrhenius analysis of the temperature dependence of 3 MLCT lifetimes shows that the energy barrier ( E a ) to thermally populating a 3 LF state from a lower energy 3 MLCT state is significantly higher in the pyridine than in the CH 3 CN 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 3 MLCT 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.