Electron Delocalization in Ruthenium(II) and Osmium(II) 2,2‘-Bipyridyl Complexes Formed from Ethynyl-Bridged Ditopic Ligands

Abstract

Photophysical and electrochemical properties have been recorded for a series of mono- and binuclear ruthenium(II) and osmium(II) 2,2‘-bipyridyl complexes that contain an ethynyl-bridged ditopic ligand. In particular, the electrochemical properties are indicative of electron delocalization over an extended π*-orbital in the π-radical anions. The site of attachment of the ethynyl substituent to the 2,2‘-bipyridyl ring 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. This effect is rationalized in terms of electron delocalization over part of the ditopic ligand within the triplet state and its significance depends markedly on the triplet energy of the complex in question. The lowest-energy triplet mixes to some extent with an upper-lying triplet that is more strongly coupled to the ground state. According to the nature of the metal complex, this higher-energy triplet might originate from (i) charge transfer from metal center to parent ligand, (ii) a π,π* state localized on the ditopic ligand, or (iii) a metal-centered excited state. For the OsII complexes at 77 K electron delocalization over an extended π*-orbital is accompanied by a reduction in the amount of nuclear displacement between triplet and ground states and by a smaller vibronic coupling matrix element relative to the parent complex. These two factors combine, within the framework of the energy-gap law, to decrease the rate at which electronic energy can be dissipated among medium-frequency vibrational (i.e., −CC− and −CN−) modes. This realization permits a quantitative explanation of the measured rate constants for nonradiative decay of the triplet excited states of these ethynyl-substituted metal complexes.