Photochemistry and Photophysics of Coordination Compounds

146 S. Campagna et al.
complexes (that is, the acceptor state of the energy transfer process) has the
promoted electron on the non-cyclometallating ligand, i.e., on the bridging
ligand for 10 and on the terminal ligand for 11. The lowest MLCT excited state
of the non-cyclometallated Ru(II) subunit (that is, the donor state of the energy
transfer process) has the promoted electron on the bridging ligand in
both cases. The energy transfer process, in a Dexter mechanism, is equivalent
to simultaneous electron–hole transfer between the molecular components.
The hole transfer process is the same (metal-to-metal) for both isomers. The
electron transfer, on the other hand, is different for the two isomers, taking
place between the two halves of the bridging ligand for 10 and from the
bridging ligand of one unit to the terminal ligand of the other unit in the
case of 11. The exchange electronic coupling is clearly expected to be higher
in the former than in the latter case. Interestingly, in both cases the energy
transfer processes also have a slower component of about 40 ps, which was
tentatively assigned to roughly isoenergetic electron hopping between terminal
and bridging ligands in the non-cyclometallated Ru chromophore, in
agreement with reported rate constants for isoenergetic electron hopping in
Ru(II) polypyridine complexes, as discussed in Sect. 4.2. This study clearly
highlights the peculiar intricacies of intramolecular energy transfer in inorganic
dyads involving MLCT excited states.
Oligophenylene bridges have also been employed for studying photoinduced
electron transfer from Ru(II) chromophores to Rh(III) subunits. In
this type of multicomponent species, electron transfer from the Ru-based
MLCT state to the Rh(III) component takes place, followed by charge recombination.
The compounds 12–17 are an interesting series of homologous
systems [220, 221]. The rate constants of the photoinduced electron transfer
processes reported confirm the distance dependence of the process, as well as
the effect of the twist angle between adjacent spacers [106, 213, 222, 224, 225]
on the electronic coupling (and, therefore, on the electron transfer kinetics)
across the spacer.