A. El-ghayoury et al., Intercompartmental electron exchange in geometrically-constrained Ru-Os triads built around diethynylated aryl hydrocarbons, J PHYS CH A, 104(33), 2000, pp. 7906-7915
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 butadi
ynylene residue bearing a central aromatic nucleus. The aromatic groups are
1,4-phenylene, 1,4-naphthalene, and 9,10-anthracene, and they exert a mark
ed influence on the nature of intramolecular triplet energy-transfer proces
ses involving the terminals. The phenylene unit facilitates long-range ener
gy transfer from the "Ru(bpy)" fragment (bpy = 2,2'-bipyridine) to the corr
esponding "Os(bpy)" unit. Electron exchange in this system takes place via
superexchange interactions with the central phenylene group acting as media
tor. Replacing phenylene with naphthalene decreases the triplet energy of t
he 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 invo
lves two discrete steps, forming the naphthalene-like triplet as areal inte
rmediate, both of which are fast. The triplet energy of the anthracene-deri
ved connector is lower than that of the Os(bpy) fragment, and this unit act
s as an energy sink for photons absorbed by the terminal metal complexes. H
owever, slow energy leakage occurs from the anthracene-like triplet to the
Os(bpy) unit, stabilizing the latter triplet state, and providing a means f
or achieving energy transfer along the molecular axis. The various kinetic
results are discussed in terms of intercompartmental energy transfer.