Photophysical Properties and Photochemical Behaviour of Ruthenium(II) complexes containing the 2,2′‐bipyridine and 4,4′‐diphenyl‐2,2′‐bipyridine ligands

The temperature dependence of the emission lifetime of the series of complexes Ru(bpy) n (4,4′‐dpb)   2+ 3−(bpy = 2,2′bipyridine, 4,4′‐dpb = 4,4′‐diphenyl‐2,2′‐bipyridine) has been studied in propionitrile/butyronitrile (4:5 v/v ) solutions in the range 90–293 K. The obtained photophysical parameters show that the energy separation between the metal‐to‐ligand charge tranfer ( 3 MLCT) emitting level and the photoreactive metal‐centered ( 3 MC) level changes across the series ( ΔE = 3960, 4100, 4300, and 4700 cm −1 for Ru(bpy)   2+ 3 ), Ru(bpy)2(4,4′‐dpb) 2+ , Ru(bpy)(4,4′‐dpb)   2+ 2 , and Ru(4,4′‐dpb)   2+ 3 , respectively, where ΔE is the energy separation between the minimum of the 3 MLCT potential curve and 3 MLCT – 3 MC crossing point. Comparison between spectral and electrochemical data indicated that the changes in ΔE are due to stabilization of the MLCT levels in complexes containing 4,4′‐dpb with respect to Ru(bpy) 2+ 3 . The photochemical data for the same complexes (as I − salts) have been obtained in CH 2 Cl 2 in the presence of 0.01 M Cl − upon irradiation at 462 nm. The complexes containing 4,4′‐dpb are more photostable than Ru(bpy)   2+ 3 . Comparison between the data for thermal population of the 3 MC photoreactive state and those for photochemistry indicated that the overall photochemical process is governed by ( i ) a thermal redistribution between the emitting and photoreactive excited states, and ( ii ) mechanistic factors, likely related to the size of the detaching ligand.