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'-bipyridyl (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 heterodiads. In each case, quantitative intramolecular triplet energy transfer takes place along the molecular axis from the Ru-based terminal to its Os-based counterpart. Energy transfer, which is believed to involve through-bond electron exchange, is extremely fast and only slightly dependent on the geometry of the bridging ditopic ligand. Evaluation of vibronic overlap integrals, 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, and it appears that electronic coupling within the triplet manifold is both modest and insensitive to the site at which the bridge is connected to the metal complexes. There exists a shallow relationship between the rate of electron exchange and the energy gap between triplets localized on donor and bridge, suggesting that this latter species participates in the energy-transfer process as a virtual state. Triplet energy transfer can also be considered in terms of a simultaneous two-electron, two-site exchange involving both LUMOs and HOMOs of the bridging ligand, with electron transfer through the LUMO being promoted by selective charge injection into the ditopic ligand under illumination. In this case, the products of the atomic orbital coefficients that describe coupling into and out of the bridge at the tripler level control the rate of energy transfer.