Intramolecular Triplet Energy Transfer in Metal Polypyridine Complexes Bearing Ethynylated Aromatic Groups

Abstract

A series of polytopic ligands has been synthesized in which a central aromatic hydrocarbon (1,4-phenylene, 1,4-naphthalene, or 9,10-anthracene) is linked to 2,2‘-bipyridine (bpy) or 2,2‘:6‘,2‘ ‘-terpyridine (terpy) units via ethynylene bonds and the coordination sites have been capped with one or two metal [M = Ru(II) or Os(II)] residues. The phenylene connector has little effect on the photophysical properties of the terminal metal complexes, except in the case of “Ru(terpy)” where the triplet lifetime is prolonged relative to the parent complex. An exceptionally long triplet lifetime (τT = 475 ns) is found for the corresponding binuclear Ru(terpy)-based complex built around the naphthalene-derived connector, although the lowest-energy triplet state is associated with the metal complex. Reversible intramolecular triplet energy transfer occurs between the reactants in the “Ru(bpy)”-containing systems assembled from the naphthalene-based connector, with the lowest-energy triplet state being of naphthalene-like character. The photophysical properties of the corresponding “Os(bpy)” fragments remain relatively unaffected upon replacing phenylene with naphthalene. Fast triplet energy transfer occurs from the Ru(bpy) fragments to the central anthracene unit, the latter lying at much lower energy, but reversible triplet energy transfer is found with the Os(bpy) units linked to anthracene. The various rate constants for electron exchange are considered in terms of current theory to estimate a value for the matrix element (VDA ≈ 8 cm-1). Triplet energy transfer from the Ru(bpy) fragments to anthracene falls well within the Marcus inverted region.