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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

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