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 3 MLCT excited state of the Ru(II) diimine complex and the 3 (π → π*) state of the pyrenyl substituent are nearly isoenergetic; the 3 MLCT state is the lowest energy state in the bpy-pyr complex, and the pyrene 3 (π → π*) state is lower in energy for the phen- pyr complex. The bpy-pyr complex is unique in that the 3 MLCT state has a very long lived luminescence (approximately 50 μs in degassed CH 3 CN). Luminescence decays for both pyrene containing complexes can be fit as double exponentials, indicating that the 3 MLCT and 3 (π → π*) states are not in equilibrium. Analysis of decays obtained at several temperatures reveal that energy transfer is slower than relaxation of the 3 MLCT state but more rapid than decay of the pyrene localized 3 (π → π*) 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.