Phosphorescent platinum dyads with cyclometalated ligands: Synthesis, characterization, and photophysical studies

The synthesis, characterization, and photophysical properties are reported for a series of ((CN)-N-Lambda)Pt dyad complexes with cyclometalated ligands ((CN)-N-Lambda: F = 2-(4',6'-difluorophenyl)pyridyl, tpy = 2-(4'-methylphenyl)pyridyl, thpy = 2-(2-thienyl)pyridyl, and btp = 2-(2-benzothienyl)pyridyl). The dyads are connected with a bridging ligand, sym-tetraacetylethane (tae), that consists of two 2,4-pentadionate units covalently linked at the 3-position. Stepwise synthesis was applied to obtain both homoleptic and heteroleptic dyads that have been characterized by H-1 NMR and elemental analysis. X-ray crystallographic analysis shows a near orthogonal orientation of the two diketonate moieties in the di-tpyPt (84 degrees) and FPt-thpyPt (89 degrees). An investigation of the photophysical properties of the dyads has been carried out. For homodyads, the emission characteristics are governed by the nature of the cyclometalating ligand, allowing the emission to be tuned throughout the visible spectrum. The di-FPt and di-tpyPt dyads show a modest decrease in luminescent lifetimes compared to their mononuclear analogs. The di-FPt complex undergoes efficient self-quenching by an excimer that is presumed to have only pi-pi interactions. The heterodyads exhibit efficient intramolecular triplet energy transfer leading to the luminescence almost exclusively from the lower-energy moiety. The energy transfer is presumed to be mediated through the bridging tae ligand by a Dexter-like mechanism involving a combination of hopping and superexchange processes.