Intramolecular triplet energy transfer in metal polypyridine complexes bearing ethynylated aromatic groups

A series of polytopic ligands has been synthesized in which a central,aroma tic hydrocarbon (1, 4-phenylene, 1,4-naphthalene, or 9,10-anthracene) is li nked 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 tw o metal [M = Ru(II) or Os(II)] residues. The phenylene connector has little effect on the photophysical properties of the terminal metal complexes, ex cept in the case of "Ru(terpy)" where the tripler lifetime is prolonged rel ative to the parent complex. An exceptionally long tripler lifetime (tau(T) = 475 ns) is found fur the corresponding binuclear Ru(terpy)-based complex built around the naphthalene-derived connector, although the lowest-energy tripler state is associated with the metal complex. Reversible intramolecu lar triplet energy transfer occurs between the reactants in the "Ru(bpy)"-c ontaining systems assembled from the naphthalene-based connector, with the lowest-energy tripler state being of naphthalene-like character. The photop hysical propel-ties of the corresponding "Os(bpy)" fragments remain relativ ely unaffected upon replacing phenylene with naphthalene. Fast tripler ener gy transfer occurs from the Ru(bpy) fragments to the central anthracene uni t, the latter lying at much lower energy, but reversible triplet energy tra nsfer is found with the Os(bpy) units linked to anthracene. The various rat e constants for electron exchange are considered in terms of current theory to estimate a value for the matrix element (V-DA approximate to 8 cm(-1)). Tripler energy transfer from the Ru(bpy) fragments to anthracene falls wel l within the Marcus inverted region.