Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 149
polyacetylenic backbone. For the phenyl-containing bridge system, the triplet
state of the bridge is higher in energy than both the Ru(II) and Os(II) MLCT
levels. Energy transfer takes place directly from the Ru(II) chromophore
to the Os(II) one via a superexchange-assisted Dexter mechanism. In the
naphthyl-bridged Ru–Os species, the triplet state of the bridge is intermediate
in energy between donor and acceptor levels: the energy transfer from the
Ru(II) subunit to the Os(II) one occurs in a stepwise manner, first to the central
bis(alkyl)naphthalene unit of the bridge and then to the Os(II) site. In
the anthryl-bridged species, the bis(alkyl)anthracene triplet is lower in energy
than both Ru- and Os-based MLCT states, and the bridge plays the role
of an energy trap [230].
The last Ru–Os compound discussed above has some similarity with the
Ru–anthracene–Os species 22 [231, 232]. In this species, missing the ethynyl
groups, the anthracene triplet lies in between the Ru donor and Os acceptor
energy transfer subunits, so the behavior of the bridge is similar to that of
the naphthyl-bridged species mentioned above. However, in air-equilibrated
solution the energy transfer rate constant significantly decreases with increasing
irradiation time. This effect is due to the formation of singlet oxygen by
bimolecular energy transfer from the Os(II) excited state. The singlet oxygen
reacts with the anthracene unit to give a peroxide species which cannot behave
as the intermediate “station” for energy transfer, so that the overall process
is significantly slowed down. This complex was called a “self-poisoning”
species [231, 232].