Intercompartmental electron exchange in geometrically-constrained Ru-Os triads built around diethynylated aryl hydrocarbons

A set of molecular triads has been synthesized in which terminal ruthenium(II) and osmium(II) tris(2,2'-bipyridyl) fragments are separated by a butadiynylene residue bearing a central aromatic nucleus. The aromatic groups are 1,4-phenylene, 1,4-naphthalene, and 9,10-anthracene, and they exert a marked influence on the nature of intramolecular triplet energy-transfer processes involving the terminals. The phenylene unit facilitates long-range energy transfer from the "Ru(bpy)" fragment (bpy = 2,2'-bipyridine) to the corresponding "Os(bpy)" unit. Electron exchange in this system takes place via superexchange interactions with the central phenylene group acting as mediator. Replacing phenylene with naphthalene decreases the triplet energy of the connector such that the naphthalene-like triplet lies at slightly lower energy than the Ru(bpy) fragment but well above the tripler state localized on the Os(bpy) unit. Triplet energy transfer along the molecular axis involves two discrete steps, forming the naphthalene-like triplet as areal intermediate, both of which are fast. The triplet energy of the anthracene-derived connector is lower than that of the Os(bpy) fragment, and this unit acts as an energy sink for photons absorbed by the terminal metal complexes. However, slow energy leakage occurs from the anthracene-like triplet to the Os(bpy) unit, stabilizing the latter triplet state, and providing a means for achieving energy transfer along the molecular axis. The various kinetic results are discussed in terms of intercompartmental energy transfer.