| Abstract
| - The synthesis and photophysical properties of a linear 2,2‘:6‘,2‘ ‘-terpyridine-based trinuclear Ru(II)−Os(II) nanometer-sized array are described. This array comprises two bis(2,2‘:6‘,2‘ ‘-terpyridine)ruthenium(II) terminals connected via alkoxy-strapped 4,4‘-diethynylated biphenylene units to a central bis(2,2‘:6‘,2‘ ‘-terpyridine) osmium(II) core. The mixed-metal linear array was prepared using the “synthesis atmetal” approach, and the Ru(II)−Ru(II) separation is ca. 50 Å. Energy transfer occurs with high efficiencyfrom the Ru(II) units to the Os(II) center at all temperatures. Förster-type energy transfer prevails in aglassy matrix at very low temperature, but this is augmented by Dexter-type electron exchange at highertemperatures. This latter process, which is weakly activated, involves long-range superexchange interactionsbetween the metal centers. In fluid solution, a strongly activated process provides for fast energy transfer.Here, a charge-transfer (CT) state localized on the bridge is populated as an intermediate species. TheCT triplet does not undergo direct charge recombination to form the ground state but transfers energy,possibly via a second CT state, to the Os(II)-based acceptor. The short tethering strap constrains thegeometry of the linker, especially in a glassy matrix, such that low-temperature electron exchange occursacross a particular torsion angle of 37°. The probability of triplet energy transfer depends on temperaturebut always exceeds 75%.
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