A buffered tris(2,2′-bipyridyl)ruthenium complex-colloidal IrO 2 system was studied as a photocatalyst for the production of O 2 from water. Phosphate buffer, which has historically been used to control the pH in this system, accelerates the decomposition of the photosensitizer and inhibits O 2 evolution, whereas sodium hexafluorosilicate (Na 2 SiF 6 )-base solutions are ideal buffers for the reaction. Na 2 SiF 6 -containing buffers poise the solution under visible light irradiation at ca. pH 5, preventing the pH drop that accompanies oxidation of water in unbuffered solutions. Decomposition of the photosensitizer is not kinetically competitive with oxygen evolution in these buffers. In particular, the Na 2 SiF 6 -NaHCO 3 buffer greatly improves the turnover number of the photosensitizer, relative to previously used phosphate buffers, without any decrease in activity. Photocatalytic reactions studied under various conditions suggest that adsorbed carbonate or bicarbonate on the surface of the colloidal IrO 2 particles contributes to the increased turnover number of the photosensitizer.

Sequential adsorption of polyanions and polycations was used to make a five-component energy/electron-transfer cascade, which mimics some of the functions of natural photosynthetic assemblies. The photon antenna part of the system consists of coumarin- and fluorescein-derivatized poly(allylamine hydrochloride) (Coum-PAH and Fl-PAH), palladium(II)tetrakis(4-N,N,N-trimethylanilinium) porphyrin (PdTAPP4+) or palladium(II)tetrakis(4-sulfonatophenyl) porphyrin (PdTSPP4-) layers, interleaved with anionic Zr(HPO4)2·H2O (α-ZrP) sheets. α-ZrP or HTiNbO5 sheets separate the porphyrin electron donor from a polyviologen electron acceptor layer. Layer-by-layer growth of these thin film assemblies was characterized by atomic force microscopy (AFM) and ellipsometry on planar supports, and by elemental analysis, surface area measurements, and transmission electron microscopy high on surface area silica supports. UV−vis absorption and steady-state emission spectroscopies showed that the overall energy/electron-transfer reaction (Coum → Fl → PdTSPP4- → viologen) occurs with approximate quantum yields of 0.47 and 0.61 for systems containing α-ZrP and HTiNbO5 sheets, respectively. Transient diffuse reflectance spectroscopy established that a porphyrin−viologen charge separated state is formed in the reaction, and that it has an exceptionally long-lived component (τ ≈ 900 μs) with the HTiNbO5 spacer. It is inferrred that the semiconducting HTiNbO5 sheets play an active role in relaying the electron from photoexcited PdTSPP4- to the viologen electron acceptor.

A cationic polymer containing tris(4,4′-dialkyl-2,2′-bipyridyl)ruthenium groups linked by aliphatic spacers was studied as a photosensitizer for the catalytic oxidation of water in the presence of colloidal IrO 2 . The polymer-colloidal IrO 2 system photocatalytically reduced persulfate, a sacrificial electron acceptor, and oxidized water to O 2 and H + in solutions that were buffered at pH 6 by Na 2 SiF 6 and NaHCO 3 . The quantum efficiency for O 2 evolution and turnover number with respect to the Ru complex in the polymer reached 25% and 100, respectively. The polymer gradually aggregated in the Na 2 SiF 6 -NaHCO 3 buffer during the reaction, and this aggregation gradually decreased the photocatalytic activity of the system. Heterogeneous photosystems composed of this polymer and colloidal IrO 2 were also prepared using 70 nm diameter SiO 2 particles as supports. Photocatalysts made by the sequential loading of colloidal IrO 2 and the photosensitizer polymer onto SiO 2 particles at ca. pH 6 had much lower photocatalytic activity than did the unsupported system, presumably because there was little physical contact between the polymer and colloidal IrO 2 particles under these conditions. The most efficient heterogeneous photocatalyst was obtained by the adsorption of a mixture of the polymer and colloidal IrO 2 onto SiO 2 in Na 2 SiF 6 -NaHCO 3 solution. This composite had a high activity, comparable to that of the polymer-colloidal IrO 2 system. Transmission electron microscopy showed that the colloidal IrO 2 particles were covered with the polymer, which had aggregated in the solution. This result indicates that the polymerIrO 2 aggregates retain their activity when immobilized on a support that might be used to organize overall water splitting systems. IntroductionThe goals of photocatalytic decomposition of water are to construct catalytic systems that split water into H 2 and O 2 under visible-light irradiation and to produce efficient photoconversion systems and devices for storing solar energy. Two basic approaches to these problems have emerged. One is to utilize wide band gap inorganic semiconductors as particles, single-crystal electrodes, or thin films. This approach has been investigated since the discovery of the Honda-Fujishima effect, 1 and there have been several reports of photocatalytic overall water splitting under UV irradiation. 2-5 Another strategy is to use photosensitized systems that are responsive to visible light. 6-8 In a previous paper, we reported light-

Model hairy nanoparticles of highly cross-linked polystyrene (PS) cores and linear polybutadiene (PBd) brushes were synthesized. The PS cores were obtained using conventional microemulsion polymerization. The linear PBd brushes were synthesized using surface initiated living anionic polymerization. The PS core nanoparticles of 44 ( 12 nm in diameter (volume weight average) were first lithinated using a combination of butyllithium and N,N,N′,N′-tetramethylethylenediamine. Butadiene was subsequently polymerized at the surface of the particles. Two series of samples were synthesized with different degrees of polymerization (N) and grafting densities (σ). In the first series, while σ was constant at ∼0.14 chains/nm 2 (∼500 chains per particle), N changed from 37, 74, 139, to 198. Accordingly, the Mw of brushes changed from 2.0, 4.0, 7.5, to 10.7 kg/mol. In the second series, the weight ratio between PS and PBd was fixed at PS/PBd = 5/2 (σN was constant in this case) and σ varied from 0.15 to 0.08 chains/nm 2 and lower. On the basis of the atomic force microscope (AFM) results, it was found that these hairy particles appeared to obtain better dispersion in PBd matrix as N of grafted chains was increased in the first series. More pronounced changes were observed, such as severe aggregation of particles, when σ decreased from relatively high to low with fixed total weight of brushes in the second series. The results were explained using theoretical calculations for grafted planes in contact with a chemically identical melt. In the system of grafted particles with short brushes (N < P, P is the degree of polymerization of the melt) studied in this paper, it was realized that the parameter σ N can represent the surface characteristics of the particles and be correlated to the dispersion of particles. A higher σ N value corresponds to better dispersion. The properties of cured rubber samples containing hairy particles were also examined using a rubber process analyzer (RPA) and a dynamic mechanical analyzer (DMA). RPA results are consistent with the morphological changes observed with AFM in terms of the "Payne effect". DMA results show that the glass transition temperatures, associated with the core and brushes, shift when the morphology changes. The AFM, RPA, and DMA results prove the strong correlation among the nanostructure of brushes, the microscopic dispersion of particles, and macroscopic properties of nanocomposites.

Three neutral cyclophanes were synthesized, and their association with indole, an aromatic pi-donor, was studied. The cyclophanes were designed to contain a rigid, hydrophobic binding cavity with 1,4,5,8-naphthalenetetracarboxylic diimide or 1,5-dinitronaphthalene as the pi-acceptor. Two of the cyclophanes also contain a (S)-(valine-leucine-alanine) tripeptide unit to provide chiral hydrogen bonding interactions with guest molecules. Despite the fact that these cyclophanes contain a hydrophobic binding cavity of appropriate dimensions, their association with indole is very weak. In the case of cyclophanes derived from 1,5-dinitronaphthalene, steric interactions force the nitro groups out of the plane of the naphthalene ring, diminishing their effectiveness as pi-acceptors. A simple UV--visible titrimetric method, using N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) as a pi-donor, was used to rank the pi-acceptor strength of these and other aromatic units. These titrations show that 1,4,5,8-naphthalenetetracarboxylic diimide and 1,5-dinitronaphthalene derivatives are weaker pi-acceptors than viologens, which make good pi-acceptor cyclophanes. Methyl viologen is in turn a weaker pi-acceptor than anthaquinone disulfonate, suggesting that the latter may serve as a useful building block for pi-accepting cyclophane hosts.