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The ruthenium complexes [Ru(dcbpyH(2))(2)(Cl)(2)] (1), [Ru(dcbpyH(2))(2)(NCS)(2)] (2), (Bu(4)N)(4)[Ru(dcbpy)(2)(NCS)(2)] (3), and (Bu(4)N)(2)[Ru(dcbpyH)(2)(NCS)(2)] (4) were synthesized and characterized by cyclic voltammetry, UV-vis absorption, and emission, IR, Raman, and NMR spectroscopy. The absorption and emission maxima of these complexes red shifted with decreasing pH, and showed pH-dependent excited-state lifetimes. The ground-state pK(a) values were determined by spectrophotometeric methods, and the dissociation of protons was found to occur in two steps (pK(a) = 3 and 1.5). The Ru(II)/(III) couple in the complex (Bu(4)N)(4)[Ru(dcbpy)(2)(NCS)(2)] is shifted ca. 290 mV negatively with regard to that of the complex [Ru(dcbpyH(2))(2)(NCS)(2)] due to the replacement of H(+) by tetrabutylammonium cation. The negative shift for the dcbpy-based reduction potential is even larger, i.e., about 600 mV compared to that of the complex [Ru(dcbpyH(2))(2)(NCS)(2)]. The effect of deprotonation on the performance of these complexes as photosensitizers for nanocrystalline titania was investigated.

A monolayer of a phosphonated triarylamine adsorbed on nanocrystalline TiO2, ZrO2, or Al2O3 film deposited on conducting glass displays reversible electrochemical and electrochromic behavior although the redox potential of the electroactive molecules (0.80 V vs NHE) lies in the forbidden band of the semiconducting or insulating oxides. The mechanism of charge transport was found to involve hole injection from the conducting support followed by lateral electron hopping within the monolayer. The apparent diffusion coefficient ranged from 2.8 × 10(-12) m(2) s(-1) in the neat 1-ethyl-2-methylimidazolium bis(trifluoromethylsulfonyl)imide (EtMeIm(+)Tf2N(-)) to 1.1 × 10(-11) m(2) s(-1) in acetonitrile + 2 M EtMeIm(+)Tf2N(-). A percolation threshold for electronic conductivity was found at a surface coverage corresponding to 50% of a full monolayer.

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).

Forward biasing of transparent nanocrystalline TiO2 (anatase) films in lithium ion-containing organic electrolytes leads to rapid and reversible coloration due to electron accumulation and Li + intercalation in the anatase lattice. Absorption of >90% light throughout the visible and near IR can be switched on and off within a few seconds. The nanocrystalline morphology of the film plays a primary role in enhancing the electrechromic process.Nanocrystalline films of semiconductors are the focus of many recent investigations. ~ These systems are expected to find various important applications. Thus, dye-sensitized nanophase oxide films have shown strikingly high photovoltaic conversion efficiencies. z The transparent nature of these films allows for direct monitoring of electron-transfer processes by spectroscopic means. A new field of electrochemistry for which the term optical electrochemistry has been coined is emerging from these investigations)The films are porous in nature and are distinguished by a high internal surface area. Pores in the nanometer range are present between the oxide particles. These are interconnected and become filled with an electrolyte or a solid hole transmitter. The roughness factor of the film, defined as the ratio between the real and projected surface attains a value of ca. 1000 for a 10 i~m thick film constituted by 15 nm size particles. Due to the large internal surface area, the capacitance of such nanocrystalline films is high, approaching the behavior of supercapacitors.Brief sintering produces efficient electronic contact, not only between the particles and the support, but also between practically all the particles constituting the film. Electronic charges injected into the film from the conducting support are able to percolate through the entire film of nanometer-size particles at high rate. This allows for rapid oxidation and reduction of electroactive species present at the particle surface or in the voids between particles.The investigations presented here take advantage of these intriguing properties of nanocrystalline oxides to achieve rapid electrochromic switching due to reversible ~ithium ion intercalation into TiO2 films. ExperimentalNanocrystalline TiO2 film electrodes were prepared as previously described} Briefly, the electrodes were prepared by spreading a paste of 15 nm size colloidal TiO2 particles on a conducting glass * Electrochemical Society Active Member. ~'~.~.ux'u-3 l I I I I I i 1.5 1.0 0,5 g ~ 0.0 -0.5 ~ -1.0 -1,6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0,2 0.0 E iv] Fig. 1, Cyclic voltammetry (5 mV/s) of a transparent nanocrystalline TiO2 electrode. Charges: Qcathodic ~--0.11 C/cm 2, Qa.oaic = 0.11 C/cm2; film thickness 3.5 p.m; electrode area 1 cm2; electrolyte: 1M LiCIO4 in acetonitrile; Ar purged; reference electrode: Ag/AgCI (saturated KCI in water); counterelectrode was GC; voltage scanned from 0 to -1.50 to 0 V. support (Libbey Owens Ford (LOF), fluorine-doped SnO2 glass, sheet resistance 20 D/D). The colloid was autoclaved at 200~After air drying, the ...