INTRAMOLECULAR ELECTRONIC-ENERGY TRANSFER IN RUTHENIUM(II) DIIMINE DONOR PYRENE ACCEPTOR COMPLEXES LINKED BY A SINGLE C-C BOND

The photophysical behavior of [(bpy)(2)Ru(L)](2+) complexes (L = 4-(1 ''-pyrenyl)-2,2'-bipyridine, bpy-pyr; 2-(1'-pyrenyl)-1,10-phenanthroli ne, phen-pyr; and 2-(2'-naphthyl)-1,10-phenanthroline, phen-nap) was i nvestigated in solutions and frozen matrices. The conformation of the linked pyrene differs in the two complexes: The pyrene moiety is confo rmationally constrained to be nearly perpendicular to the phenanthroli ne in the phen-pyr complex while the pyrene in the bpy-pyr complex has much greater flexibility about the C-C bond linking the ligand and th e pyrene. The (MLCT)-M-3 excited state of the Ru(II) diimine complex a nd the (3)(pi-->pi) state of the pyrenyl substituent are nearly isoen ergetic; the (MLCT)-M-3 state is the lowest energy state in the bpy-py r complex, and the pyrene (3)(pi-->pi) state is lower in energy for t he phen-pyr complex. The bpy-pyr complex is unique in that the (MLCT)- M-3 state has a very long lived luminescence (approximately 50 mu s in degassed CH3CN). Luminescence decays for both pyrene containing compl exes can be fit as double exponentials, indicating that the (MLCT)-M-3 and (3)(pi-->pi) states are not in equilibrium. Analysis of decays o btained at several temperatures reveal that energy transfer is slower than relaxation of the (MLCT)-M-3 state but more rapid than decay of t he pyrene localized (3)(pi-->pi) state. The results also suggest that electronic coupling between the two states is weak despite the fact t hat the two chromophores are separated by a single covalent bond.