Photochemistry and Photophysics of Coordination Compounds
Photochemistry and Photophysics of Coordination Compounds
Photochemistry and Photophysics of Coordination Compounds
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194 S. Campagna et al.<br />
1. To increase the absorption properties <strong>of</strong> the (multicomponent) sensitizer,<br />
by using systems featuring the antenna effect, with the energy trap <strong>of</strong> the<br />
antenna being the Ru(II) unit directly connected to the semiconductor.<br />
One example is the trinuclear Ru(II) species 71 showninFig.20;in71,the<br />
light absorbed by the peripheral Ru(II) chromophores is transferred quantitatively<br />
to the central Ru(II) chromophore, from which electron injection<br />
takes place [420]. Experiments on this complex adsorbed on polycrystalline<br />
TiO2 gave an overall conversion efficiency <strong>of</strong> ca. 7% withturnover<br />
numbers <strong>of</strong> at least 1 × 10 6 . The antenna effect is expected to be <strong>of</strong> relevance<br />
for applications requiring very thin TiO2 layers.<br />
2. To spatially separate the injected electron <strong>and</strong> the hole on the sensitizer,<br />
so decreasing losses due to charge recombination. A system designed for<br />
this aim (72) is shown in Fig. 21, where the possible electron transfer steps<br />
are indicated [421]. In 72, the MLCT state <strong>of</strong> the Ru(II) unit is rapidly<br />
quenched by the Rh(III) species (step k1 in Fig. 21), followed by injection<br />
<strong>of</strong> the electron onto the semiconductor conductance b<strong>and</strong> from the reduced<br />
Rh unit (step k2, in competition with step k4). The recombination<br />
process (k5) is slow because <strong>of</strong> a very weak electronic coupling. To reach<br />
thesameaim,thespecies73 shown in Fig. 22 has been designed [422].<br />
Here, the first step is the electron injection from the Ru unit. Then, electron<br />
transfer from the phenothiazine unit to the oxidized Ru unit takes<br />
place, resulting in a charge-separated species which decays to the ground<br />
state with a rate <strong>of</strong> 3.6 × 10 3 s –1 (lifetime, 0.3 ms). The dyad <strong>and</strong> model<br />
molecules were also tested in solar cells, with iodide as an electron donor.<br />
While the observed IPCE was <strong>of</strong> the order <strong>of</strong> 45% for both systems, the<br />
open circuit photovoltage was higher for the dyad by 100 mV. The effect<br />
was more pronounced in the absence <strong>of</strong> iodide with Voc = 180 mV. Applying<br />
the measured interfacial electron transfer rates to the diode equation<br />
Fig. 20 Structural formula <strong>of</strong> a branched antenna system <strong>and</strong> schematization <strong>of</strong> energy<br />
transfer <strong>and</strong> charge injection in TiO2. Complex charge is omitted for clarity