Photochemistry and Photophysics of Coordination Compounds

168 S. Campagna et al.
ground state, before excitation and any electron transfer process. The triad
would then be better defined as a D/P–A system, and electron transfer between
D and P + subunits in the intermediate state D/P + –A – can be largely
faster than that reported for the D–P ∗ dyad, and even faster than charge
recombination between C + and A – in the assembly, justifying the high efficiency
of formation of the fully developed charge separation. Ground-state
association of tethered aromatic ligands (with flexible linkages) with Ru(II)
chromophores was also known in a tetranuclear dendrimer [272, 273], further
supporting these conclusions.
The triad 43 involves components similar to those used in the former
discussed systems, but the connecting scheme is different [294]. Here, the
chromophore and the electron donor and acceptor subunits are all linked to
a lysine moiety. The charge-separated state of this triad stores 1.17 eV and
lives for 108 ns (rate constant of charge recombination kCR = 9.26 × 10 6 s –1 )
in acetonitrile, as observed by transient spectroscopy. The efficiency of formation
of the charge-separated state is about 34%. On the basis of detailed
photophysical studies of isolated dyads [294], the authors believe that the only
route leading to fully developed charge separation is reductive quenching of
D–P ∗ –A leading to D + –P – –A, followed by a second electron transfer leading
to D + –P–A – , whereas the route passing through the population of the D–P + –
A – intermediate does not lead to the full charge-separated species, but leads
directly to the ground state. Therefore, the occurrence of oxidative electron
transfer of P* would be the main reason for the efficiency loss in the whole
process. Better results were obtained by modifying the acceptor site of the
triad, as shown in 44 [295]. In this species, the energy stored by the chargeseparated
species is higher (1.54 eV), since the anthraquinone has a less negative
reduction potential than the methyl viologen subunit, and the lifetime
of the charge-separated state is longer (174 ns, kCR = 5.7 × 10 6 s –1 ), but the