Triplet energy transfer within closely spaced positional isomers of Ru/Os polypyridine-based heterodiads

A series of ditopic ligands has been synthesized in which terminal 2,2'-bip yridyl (bpy) groups are connected via an ethynylene group through different sites on the pyridine ring. These terminals have been capped with [Ru(bpy) (2)](2+) and [Os(bpy)(2)](2+) metallo-fragments to form photoactive heterod iads. In each case, quantitative intramolecular triplet energy transfer tak es place along the molecular axis from the Ru-based terminal to its Os-base d counterpart. Energy transfer, which is believed to involve through-bond e lectron exchange, is extremely fast and only slightly dependent on the geom etry of the bridging ditopic ligand. Evaluation of vibronic overlap integra ls, using Dexter-type formulism, or factors for the Franck-Condon weighted density of states, using Meyer's approach, allows estimation of the matrix elements for electron exchange. The two methods give comparable results, an d it appears that electronic coupling within the triplet manifold is both m odest and insensitive to the site at which the bridge is connected to the m etal complexes. There exists a shallow relationship between the rate of ele ctron exchange and the energy gap between triplets localized on donor and b ridge, suggesting that this latter species participates in the energy-trans fer process as a virtual state. Triplet energy transfer can also be conside red in terms of a simultaneous two-electron, two-site exchange involving bo th LUMOs and HOMOs of the bridging ligand, with electron transfer through t he LUMO being promoted by selective charge injection into the ditopic ligan d under illumination. In this case, the products of the atomic orbital coef ficients that describe coupling into and out of the bridge at the tripler l evel control the rate of energy transfer.