Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 175
processes require more than one electron to operate: for example, reduction
of H + to H2 is bielectronic, and oxidation of oxygen in water to produce O2 is
a four-electron process. Reduction of CO2 to the high-energy content glucose
species is also a multielectron process. Whereas artificial systems capable of
performing photoinduced charge separation have been reported, species able
to collect, by successive photoinduced processes, more than one single electron
(or hole) in one specific site of their structure are very rare. These species
differ from polymers or dendritic species, which are also able to reversibly
store more than one single electron (or hole) in their structure (in several,
roughly identical, but spatially separated sites), since the accumulated charges
should be located in a single subunit and, at least in principle, could be more
easily delivered simultaneously to a unique substrate.
A breakthrough in this field was the study of the two dinuclear Ru complexes
51 and 52 [312]. These complexes are indeed able to collect two electrons
(and two protons) and four electrons (and four protons), respectively,
within their bridging ligand moieties upon successive light excitation and
in the presence of sacrificial donor species. In a typical (schematized) sequence
of events involving 51: (1) light excitation produces a MLCT state
involving the bpy-like subunit of the bridge; (2) a charge shift takes place
from the bpy-like bridge moiety to the inner, phenazine-like portion of the
bridge, so producing a sort of charge-separated state; (3) the sacrificial donor,
a triethylamino (TEA) species, reduces the Ru(III) center, so restoring the
chromophore; (4) the reduced central moiety of the bridge adds a proton
(originated from irreversible TEA oxidation), so reaching charge neutrality;
and (5) the sequence of events 1–4 is repeated and two electrons and
two protons are collected [312]. However, a recent refinement of the ultrafast
spectroscopic results has evidenced that the product of step 2, initially
identified as a sort of charge-separated state [313], receives a significant contribution
also from a bridge-centered triplet state [314]. The overall process
is perfectly reversible, and 51 is fully restored on leaving molecular oxygen
reaching the complex [312]. For 52, the formerly described sequence of events
is repeated four times, thanks to the presence of the quinone subunits responsible
for the addition of two extra electron/proton couples [312]. All the
various steps of the multielectron processes occurring in 51 have also been
characterized by UV/Vis spectroscopy and each intermediate has a unique