The temperature dependence of the emission lifetime of the series of complexes Ru(bpy),(4,4'-dpb): ?
(bpy = 2,2'-bipyridine, 4,4'-dpb = 4,4'-diphenyL2,2'-bipyridine) has been studied in propionitrile/butyronitrile (4:5 u/u) solutions in the range 90-293 K. The obtained photophysical parameters show that the energy separation between the metal-to-ligand charge transfer ('MLCT) emitting level and the photoreactive metal-centered ('MC) level changes across the series ( d E = 3960, 4100, 4300, and 4700 cm-' for Ru(bpy)?, Ru(bpy),(4,4'-dpb)'+, Ru(bpy)(4,4'-dpb)P, and Ru(4,4'-dpb)P, respectively, where d E is the energy separation between the minimum of the 3MLCT potential curve and 'MLCT -'MC crossing point). Comparison between spectral and electrochemical data indicates that the changes in d E are due to stabilization of the MLCT levels in complexes containing 4,4'-dpb with respect to Ru(bpy)?. The photochemical data for the same complexes (as Isalts) have been obtained in CH,Cl, in the presence of 0 . 0 1 ~ C1-upon irradiation at 462 nm. The complexes containing 4,4'-dpb are more photostable than Ru(bpy)?. Comparison between the data for thermal population of the 3MC photoreactive state and those for photochemistry indicates that the overall photochemical process is governed by i) thermal redistribution between the emitting and photoreactive excited states, and ii) mechanistic factors, likely related to the size of the detaching ligand.