ELECTRON DELOCALIZATION IN RUTHENIUM(II) AND OSMIUM(II) 2,2'-BIPYRIDYL COMPLEXES FORMED FROM ETHYNYL-BRIDGED DITOPIC LIGANDS

Photophysical and electrochemical properties have been recorded for a series of mono- and binuclear ruthenium-(II) and osmium(II) 2,2'-bipyr idyl complexes that contain an ethynyl-bridged ditopic ligand, In part icular, the electrochemical properties are indicative of electron delo calization over an extended pi-orbital in the pi-radical anions, The site of attachment of the ethynyl substituent to the 2,2'-bipyridyl ri ng affects the various properties, especially absorption and emission spectral maxima. In most cases, the rates of nonradiative deactivation of the lowest-energy triplet excited states are slower than expected for a corresponding complex not possessing a conjugated substituent, T his effect is rationalized in terms of electron delocalization over pa rt of the ditopic ligand within the triplet stale and its significance depends markedly on the triplet energy of the complex in question, Th e lowest-energy triplet mixes to some extent with an upper-lying tripl et that is more strongly coupled to the ground state. According to the nature of the metal complex, this higher-energy triplet might origina te from (i) charge transfer from metal center to parent ligand, (ii) a ,pi,pi state localized on the ditopic ligand, or (iii) a metal-center ed excited state. For the Os-II complexes at 77 K electron delocalizat ion over an extended pi-orbital is accompanied by a reduction in the amount of nuclear displacement between triplet and ground states and b y a smaller vibronic coupling matrix element relative to the parent co mplex, These two factors combine, within the framework of the energy-g ap law, to decrease the rate at which electronic energy can be dissipa ted among medium-frequency vibrational (i.e., -C=C- and -C=N-) modes, This realization permits a quantitative explanation of the measured ra te constants for nonradiative decay of the triplet excited states of t hese ethynyl-substituted metal complexes.