Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 167
the methylene chains connecting the quaternary nitrogens (and, as a consequence,
the reduction potential of the electron acceptor) [283–287]. In
a homogeneous series of experiments performed on such species, however,
the efficiency of charge separation does not change appreciably, remaining
larger than 0.80, although the driving forces and the rate constants of the various
electron transfer steps, as obtained by independent studies performed on
isolated dyads of the type D–P or P–A, were different.
In D–P ∗ –A systems, the fully developed charge-separated state can be obtained,
in principle, by two different routes (excluding direct electron transfer
from D to A): (1) a route initiated by oxidative quenching, that is, the series
of events described by the sequence D–P ∗ –A, D–P + –A – ,D + –P–A – ;and
(2) a route initiated by reductive quenching, described by the sequence D–
P ∗ –A, D + –P – –A, D + –P–A – . Both routes can also take place simultaneously.
The comparison between the photophysical properties of the various triads
and the corresponding isolated dyads of this family of compounds indicated
that the emission decay rates of any D–P–A triad never differed by more
than a factor of two from those of the P–A dyads, although the absolute decay
rate values changed by over a factor of 10 3 (over the whole collection
of compounds). This prompted the authors to attribute the initial quenching
event in all the D–P–A triads of this family to oxidative electron transfer, with
formation of the D–P + –A – intermediate, with the route initiated by reductive
quenching playing a negligible role [288]. However, in all the P–A dyad
systems, it was always impossible to detect the A – radical anion [284], indicating
that back electron transfer in the P–A dyads was faster than the
forward, oxidative electron transfer. This posed some problems in justifying
the efficiency of formation of the fully developed charge-separated state,
where apparently reduction of P + from D in D–P + –A – species efficiently
competes with back electron transfer in the intermediate. In fact, this looks
somewhat puzzling because the reductive electron transfer in D–P ∗ dyads is
reported to be of the order of 10 6 s –1 [289], while oxidative electron transfer
in P ∗ –A dyads ranges from 10 10 to 10 7 s –1 [284, 285, 290–293] and, based
on the circumstances mentioned above, back electron transfer in P + –A – (and
for extension in D–P + –A – ), opposing the formation of the fully developed
charge-separated state, could be even faster. To justify the experimental data,
electron transfer from D to P + in D–P + –A – should be about 1 × 10 10 s –1 or
faster. Therefore, the exceptional properties of these compounds as far as the
efficiency of charge separation is concerned remained largely unexplained.
A recent paper has shed light on the photophysical behavior of these
triads [288]. A series of new experiments, including transient absorption
measurements, emission decay, and a careful examination of the ground-state
absorption spectra of the triads and of various separated dyad components,
suggested that in the D–P–A triads of this family an association between the
tethered phenothiazine electron donor subunit and the Ru(II) chromophore
takes place, in a folded conformation. The association is already present in the