Intramolecular triplet energy transfer in alkyne-bridged Ru-Os multinuclear complexes: Switching between dipole-dipole and electron-exchange mechanisms

Intramolecular triplet energy transfer occurs from a ruthenium(II) centre to an osmium(II) centre in heteropolynuclear polypyridine complexes at room temperature in acetonitrile solution. When the metal complexes are linked together by way of a butadiynyl bridge (polypyridine = 2,2':6,6 ''-terpyridine), energy transfer is extremely fast and takes place via Dexter-type (through-bond) electron exchange. This process is facilitated by selective electron donation to the bridging ligand in the MLCT triplet state of the Ru-II complex, the electron residing in an extended pi*-orbital that partially encompasses the bridge. In marked contrast, insertion of a Pt-II centre into the butadiynyl bridge (polypyridine = 2,2'-bipyridine) serves to inhibit through-bond electron exchange so that triplet energy transfer occurs slowly. It is considered that the Pt-II centre decreases the magnitude of the electronic coupling matrix element (H-DA < 0.2 cm(-1)) for electron exchange, in part because of the high energy of its LUMO and HOMO states, relative to the butadiynyl bridge (H-DA approximate to 12 cm(-1)). The Pt-II centre, which donates charge to the bridging ligand, also induces localisation of the promoted electron at an unsubstituted 2,2'-bipyridyl ligand in the triplet excited state. The corresponding cis isomer (with respect to the Pt-II centre) gives a slightly slower rate of intramolecular triplet energy transfer. The importance of Coulombic energy transfer in these systems is discussed.