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-phenanthroline, phen-pyr; and 2-(2'-naphthyl)-1,10-phenanthroline, phen-nap) was investigated in solutions and frozen matrices. The conformation of the linked pyrene differs in the two complexes: The pyrene moiety is conformationally constrained to be nearly perpendicular to the phenanthroline 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 the pyrene. The (MLCT)-M-3 excited state of the Ru(II) diimine complex and the (3)(pi-->pi*) state of the pyrenyl substituent are nearly isoenergetic; the (MLCT)-M-3 state is the lowest energy state in the bpy-pyr complex, and the pyrene (3)(pi-->pi*) state is lower in energy for the 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 complexes can be fit as double exponentials, indicating that the (MLCT)-M-3 and (3)(pi-->pi*) states are not in equilibrium. Analysis of decays obtained at several temperatures reveal that energy transfer is slower than relaxation of the (MLCT)-M-3 state but more rapid than decay of the pyrene localized (3)(pi-->pi*) state. The results also suggest that electronic coupling between the two states is weak despite the fact that the two chromophores are separated by a single covalent bond.