Intramolecular charge transfer in pyridinium-substituted Ru-polypyridine complexes

A comparative photophysical study has been carried out on the complexes (bpy)2Ru(MQ+)2 4+ and (bpy)2Ru(bpy-py-Me+)3+ (1 and 2, respectively, where bpy=2,2′-bipyridine, MQ+ is N-methyl-4,4′-bipyridinium and bpy-py-Me+ is 4-(N-methyl-4-pyridyl)-2,2′-bipyridine). In addition, the X-ray crystal structure of 2 is reported. As noted previously by Meyer and co-workers, complex 1 features strong photoluminescence from the Ru→bpy metal-to-ligand charge transfer (MLCT) state at 80 K in an ethanol–methanol glass, but the emission is quenched at the solvent glass-to-fluid transition temperature due to intramolecular ligand-to-ligand charge transfer to produce the Ru→MQ+ MLCT state: [(bpy)(bpy−radical dot)Ru(III)(MQ+)2]4+*→[(bpy)2Ru(III)(MQradical dot)(MQ+)]4+*. The existence of the Ru→MQ+ MLCT state is confirmed in the present study by laser flash photolysis of 1 at 160 K which provides evidence for the reduced monoquat ligand, MQradical dot. The photophysics of the new complex 2 at temperatures ranging from 80 to 300 K is dominated by a the low-lying Ru→bpy-py-Me+ MLCT state. Luminescence is observed from this state in an ethanol–methanol glass at 80 K as well as at temperatures above the solvent glass-to-fluid transition. The photoluminescence of 2 undergoes a large thermally-induced Stokes shift as the temperature is raised through the solvent glass-to-fluid transition region. The large Stokes shift is ascribed to solvent relaxation as well as relaxation with respect to a low-frequency inner sphere mode that consists of rotation around the CC bond between the bpy and N-methylpyridinium rings in the bpy-py-Me+ acceptor ligand. Temperature dependent emission lifetime studies indicate that 2 features a dynamic anti-Stokes shift in the emission at T≤110 K and a dynamic Stokes shift for T>110 K.