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 RuII complex, the electron residing in an extended π*-orbital that partially encompasses the bridge. In marked contrast, insertion of a PtII 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 PtII centre decreases the magnitude of the electronic coupling matrix element (HDA < 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 (HDA≈ 12 cm–1). The PtII 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 PtII centre) gives a slightly slower rate of intramolecular triplet energy transfer. The importance of Coulombic energy transfer in these systems is discussed.