DINUCLEAR RUTHENIUM(II) AND/OR OSMIUM(II) COMPLEXES OF A NONSYMMETRICAL BIS-CHELATING QUATERPYRIDINE LIGAND - SYNTHESIS, ELECTROCHEMICAL-BEHAVIOR, ABSORPTION-SPECTRA, LUMINESCENCE PROPERTIES AND INTERCOMPONENT ENERGY-TRANSFER

We have synthesized the hexafluorophosphate salts of the mono- and di-nuclear compounds Ru-LL'. Os-LL', Ru-LL'-Ru, Os-LL'-Os, Ru-LL'-Os and Os-LL'-Ru, where Ru and Os are {Ru(bipy)(2))(2+) and {Os(bipy)(2)}(2+) fragments (bipy = 2,2'-bipyridine), and LL' is the bis-chelating 2.2':3',2 '':6 '',2 triple prime-quaterpyridine bridging ligand with inequivalent bipy-type binding sites (L and L'). The compound [(bipy)(2)Os(LL')Ru(bipy)(2)][PF6](4);3MeCN has been crystallographically characterized, the co-ordination environment about the Os-II centre is essentially identical to that of [Os(bipy)(3)](2+), but the co-ordination environment about the Ru-II centre is somewhat distorted with one particularly long Ru-N bond, due to the inherent sterically hindered nature of the C site of the bridging ligand. Electrochemical studies show that a given metal ion is slightly easier to oxidize when in the L co-ordination site, but the difference in the properties of the two moieties of the LL' bridging ligand is much smaller than difference in the properties of Ru-II and Os-II, so that in the mixed-metal complexes Ru-LL'-Os and Os-LL'-Ru the metal easier to oxidize is always Os and the luminescent moiety is always the Os-based one. The lowest energy (luminescent) level in the homodinuclear compounds is located on the L co-ordination site. In Ru-LL'-Os and Os-LL'-Ru electronic energy transfer from the higher energy (Ru-based) to the lower energy (Os-based) moiety is very fast and 100% efficient.