Temperature-Induced Switching of the Mechanism for Intramolecular Energy Transfer in a 2,2':6',2'-Terpyridine-Based Ru(II)-Os(II) Trinuclear Array

摘要

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 at metal" approach,and the Ru(II)-Ru(II) separation is ca.50 A.Energy transfer occurs with high efficiency from the Ru(II) units to the Os(II) center at all temperatures.Forster-type energy transfer prevails in a glassy matrix at very low temperature,but this is augmented by Dexter-type electron exchange at higher temperatures.This latter process,which is weakly activated,involves long-range superexchange interactions between 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.The CT 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 the geometry of the linker,especially in a glassy matrix,such that low-temperature electron exchange occurs across a particular torsion angle of 37deg.The probability of triplet energy transfer depends on temperature but always exceeds 75%.