Intramolecular charge transfer in pyridinium-substituted Ru-polypyridine complexes

A comparative photophysical study has been carried out on the complexes (bp y)(2)Ru(MQ(+))(2)(4+) and (bpy)(2)Ru(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 c rystal structure of 2 is reported. As noted previously by Meyer and co-work ers, complex 1 features strong photoluminescence from the Ru --> bpy metal- to-ligand charge transfer (MLCT) state at 80 K in an ethanol - methanol gla ss, but the emission is quenched at the solvent glass-to-fluid transition t emperature due to intramolecular ligand-to-ligand charge transfer to produc e the Ru --> MQ(+) MLCT state: [(bpy)(bpy(-.))Ru(III)(MQ(+))(2)](4 + *) --> [(bpy)(2)Ru(III)(MQ(.))(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, MQ(.). T he 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. Luminesce nce is observed from this state in an ethanol-methanol glass at 80 K as wel l as at temperatures above the solvent glass-to-fluid transition. The photo luminescence of 2 undergoes a large thermally-induced Stokes shift as the t emperature is raised through the solvent glass-to-fluid transition region. The large Stokes shift is ascribed to solvent relaxation as well as relaxat ion with respect to a low-frequency inner sphere mode that consists of rota tion around the C-C bond between the bpy and N-methylpyridinium rings in th e bpy-py-Me+ acceptor ligand. Temperature dependent emission lifetime studi es indicate that 2 features a dynamic anti-Stokes shift in the emission at T less than or equal to 110 K and a dynamic Stokes shift for T > 110 K. (C) 2000 Elsevier Science S.A. All rights reserved.