Intramolecular Electronic Energy Transfer in Ruthenium(II) Diimine Donor/Pyrene Acceptor Complexes Linked by a Single C−C Bond

Abstract

The photophysical behavior of [(bpy)2Ru(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 3MLCT excited state of the Ru(II) diimine complex and the 3(π → π*) state of the pyrenyl substituent are nearly isoenergetic; the 3MLCT 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 3MLCT state has a very long lived luminescence (approximately 50 μs in degassed CH3CN). Luminescence decays for both pyrene containing complexes can be fit as double exponentials, indicating that the 3MLCT 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 3MLCT 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.