V. Grosshenny et al., ELECTRON DELOCALIZATION IN RUTHENIUM(II) AND OSMIUM(II) 2,2'-BIPYRIDYL COMPLEXES FORMED FROM ETHYNYL-BRIDGED DITOPIC LIGANDS, Journal of physical chemistry, 100(44), 1996, pp. 17472-17484
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.