Photophysics of Re(I) and Ru(II) diimine complexes covalently linked to pyrene: Contributions from intra-ligand charge transfer states

The photophysical properties of Ru(II) and Re(I) polypyridyl complexes including a bis-bipyridyl pyrene ligand are presented. The complexes {[(bpy)(2)Ru](2)bpb}(4+) and [(CO)(3)ReCl(bpb)] (bpy = 2,2'-bipyridine, bpb = 1,6-bis-(4-(2,2'-bipyrid-yl)-pyrene) were designed with the intent of examining intramolecular energy migration between MLCT states localized on the metal complexes and pyrene-localized states. Absorption spectroscopy of both complexes containing the bpb ligand reveals that in addition to the MLCT and the pyrene-centered transitions, a new absorption band is observed near 400 nm for both complexes. Absorption spectral data for the Re(I) complex strongly suggest the presence of a pyrene(pi) to bpy(pi*) intraligand charge transfer (ILCT) transition. Emission spectra at room temperature and at 77 K are almost identical for the Ru(II) and Re(I) complexes containing the bpb ligand. The (MLCT)-M-3 emission of related bipyridyl compounds lacking the pyrene is observed at higher energy than for the pyrene-containing complexes, {[(bpy)(2)Ru](2)bpb}(4+) and [(CO3ReCl(bpb)], The Ru(II) complex emits at room temperature with a remarkably long lifetime (130 mus in degassed DMSO). This emission is also strongly sensitive to oxygen and Is almost entirely quenched in an aerated solution. In addition, excited-state absorption spectra exhibit features not consistent with (MLCT)-M-3 or states of the parent chromophores. The combined characteristics suggest the emission arises from either or (ILCT)-I-3 states or a state with mixed parentage.