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

Abstract

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).

Interdye Hole Transport Accelerates Recombination in Dye Sensitized Mesoporous Films

“…11 In our study correlation between hole transport and hole lifetime is observed for samples fabricated on glass substrates or including a TiO2 compact layer covering the FTO contact indicating that hole transfer to electrons in FTO does not contribute to observed trend in recombination kinetics.…”

“…The results presented evidence for systems showing longer lifetimes due to reaction limitations as opposed to electron transport limitation. 8,9,10,11 Thus, tuning surface properties 12,13 and modifying the dye chemical structure 9 have been emphasized as routes to minimise electron-dye recombination. On the other hand, other reports showed unchanged recombination dynamics upon change in dye chemical structure, suggesting that diffusion limited encounter between electrons and holes represents the rate limiting process in some systems.…”

Organometallic Materials for Electroluminescent and Photovoltaic Devices

“…The observed suppression of the carriers recombination is attributed to an increase in the physical separation between the dye cation and the metal oxide surface. Bonhôte and coworkers have studied the charge recombination process in a series of Ru dyes connected with triphenylamine (Bonhôte et al 1999). The lifetime of injected electrons in TiO 2 is enhanced by a significant factor of 100 times after the incorporation of those units in their model system (without I − /I 3 − redox couples so that the carrier recombination mostly occurs between the oxidized dye and the injected electrons).…”

Nanochromics: old materials, new structures and architectures for high performance devices

“…The derivatization of nanocrystalline films with different types of molecular monolayers led to, in addition to solar cells and electrochromic devices, to elaborate sensors, 20 photoelectrocatalytic, 21 electroluminescent 22 and photoelectrochromic devices. 23 The common principle of efficiency in these devices lies on fast interfacial electron transfer between the nanocrystalline material and the adsorbed modifier as well as on the high surface area of the substrate that amplifies optical phenomena by two or three orders of magnitude. The general principle of the electrochromism of a molecular monolayer adsorbed on TiO 2 is depicted in Figure 2, electrons are injected from the conducting substrate into the conduction band of the semiconductor and from there, reduce the adsorbed organic chromophore.…”