Long-Lived Photoinduced Charge Separation and Redox-Type Photochromism on Mesoporous Oxide Films Sensitized by Molecular Dyads

The photoinduced charge separation in three different assemblies composed of an electron donor D and a chromophore sensitizer S adsorbed on nanocrystalline TiO2 films (D−S|TiO2) was investigated. In all of the systems, the sensitizer was a ruthenium(II) bis-terpyridine complex anchored to the semiconductor surface by a phosphonate group. In two of the assemblies, the donor was a 4-(N,N-di-p-anisylamino) phenyl group linked to the 4' position of the terpyridine, either directly (dyad D1−S) or via a benzyl ether interlocking group (dyad D2−S). In the third system, the sensitizer and the donor (3-(4-(N,N-di-p-anisylamino)phenoxy)-propyl-1-phosphonate) were coadsorbed on the surface ((D3+S)|TiO2). Laser flash photolysis showed that the photoinduced charge separation process follows the sequence D−S*|TiO2 D−S+|(e-)TiO2 D+−S|(e-)TiO2 D−S|TiO2 Resonance Raman spectroscopy indicates that in the excited assemblies D2−S*|TiO2 and (D3+S*)|TiO2, one electron is promoted from the metal center to the terpyridine ligand linked to the semiconductor, whereas in the system D1−S*|TiO2 the excited electron is located on the ligand linked to the donor. The quantum yield of charge separation (steps 1 and 2) was found to be close to unity for the two former assemblies but only 60% for the latter one. In all three cases, the electron injection was very fast (<1 ns), and the hole transfer to the donor was fast (10−20 ns). The half-lifetime of the charge separated state (step 3) was 3 μs for (D3++S)|(e-)TiO2, as in the model system S+|(e-)TiO2; it was 30 μs in D1+−S|(e-)TiO2 and 300 μs in D2+−S|(e-)TiO2. Electrodes made of any of the surface-confined dyads on conducting glass display a strong redox-type photochromism. When a positive potential (+0.5 V vs NHE) is applied to the electrode, charge recombination (step 3) is blocked. As a result, the visible absorption spectrum of the electrode changes, due to the appearance of the absorption feature of the oxidized donor (λmax = 730 nm). Return to the reduced state is achieved by electron injection through the conduction band of the TiO2 under forward bias (−0.5 V). None of the assemblies D1−S|TiO2 and D2−S|TiO2 gave better photovoltaic performances than the model system S|TiO2. This was attributed in the first case to the low injection efficiency and, in the second case, to an additional short-circuiting pathway constituted by the charge percolation inside the molecular monolayer and to the underlying conducting glass, as previously observed with monolayers of the donor D3 (Bonhôte, P.; Gogniat, E.; Tingry, S.; Barbé, C.; Vlachopoulos, N.; Lenzmann, F.; Comte, P.; Grätzel, M. J. Phys. Chem. B 1998, 102, 1498−1507).