R esearchers are intensively investigating photochemical water splitting as a means of converting solar to chemical energy in the form of fuels. Hydrogen is a key solar fuel because it can be used directly in combustion engines or fuel cells, or combined catalytically with CO 2 to make carbon containing fuels. Different approaches to solar water splitting include semiconductor particles as photocatalysts and photoelectrodes, molecular donor-acceptor systems linked to catalysts for hydrogen and oxygen evolution, and photovoltaic cells coupled directly or indirectly to electrocatalysts.Despite several decades of research, solar hydrogen generation is efficient only in systems that use expensive photovoltaic cells to power water electrolysis. Direct photocatalytic water splitting is a challenging problem because the reaction is thermodynamically uphill. Light absorption results in the formation of energetic charge-separated states in both molecular donor-acceptor systems and semiconductor particles. Unfortunately, energetically favorable charge recombination reactions tend to be much faster than the slow multielectron processes of water oxidation and reduction. Consequently, visible light water splitting has only recently been achieved in semiconductor-based photocatalytic systems and remains an inefficient process.This Account describes our approach to two problems in solar water splitting: the organization of molecules into assemblies that promote long-lived charge separation, and catalysis of the electrolysis reactions, in particular the four-electron oxidation of water. The building blocks of our artificial photosynthetic systems are wide band gap semiconductor particles, photosensitizer and electron relay molecules, and nanoparticle catalysts. We intercalate layered metal oxide semiconductors with metal nanoparticles. These intercalation compounds, when sensitized with [Ru(bpy) 3 ] 2+ derivatives, catalyze the photoproduction of hydrogen from sacrificial electron donors (EDTA 2-) or non-sacrificial donors (I -). Through exfoliation of layered metal oxide semiconductors, we construct multilayer electron donor-acceptor thin films or sensitized colloids in which individual nanosheets mediate light-driven electron transfer reactions. When sensitizer molecules are "wired" to IrO 2 · nH 2 O nanoparticles, a dye-sensitized TiO 2 electrode becomes the photoanode of a water-splitting photoelectrochemical cell.Although this system is an interesting proof-of-concept, the performance of these cells is still poor (∼1% quantum yield) and the dye photodegrades rapidly. We can understand the quantum efficiency and degradation in terms of competing kinetic pathways for water oxidation, back electron transfer, and decomposition of the oxidized dye molecules. Laser flash photolysis experiments allow us to measure these competing rates and, in principle, to improve the performance of the cell by changing the architecture of the electron transfer chain.

Boron-dipyrrin chromophores containing a 5-aryl group with or without internal steric hindrance toward aryl rotation have been synthesized and then characterized via X-ray diffraction, static and time-resolved optical spectroscopy, and theory. Compounds with a 5-phenyl or 5-(4-t-butylphenyl) group show low fluorescence yields (∼0.06) and short excited-singlet-state lifetimes (∼500 ps), and decay primarily (>90%) by nonradiative internal conversion to the ground state. In contrast, sterically hindered analogues having an o-tolyl or mesityl group at the 5-position exhibit high fluorescence yields (∼0.9) and long excited-state lifetimes (∼6 ns). The X-ray structures indicate that the phenyl or 4-tert-butylphenyl ring lies at an angle of ∼60°with respect to the dipyrrin framework whereas the angle is ∼80°for mesityl or o-tolyl groups. The calculated potential energy surface for the phenylsubstituted complex indicates that the excited state has a second, lower energy minimum in which the non-hindered aryl ring rotates closer to the mean plane of the dipyrrin, which itself undergoes some distortion. This relaxed, distorted excited-state conformation has low radiative probability as well as a reduced energy gap from the ground state supporting a favorable vibrational overlap factor for nonradiative deactivation. Such a distorted conformation is energetically inaccessible in a Supporting Information Available: Theoretical analysis of the excited-state surfaces and Franck-Condon-active modes for selected compounds, static absorption and emission spectra, time-resolved absorption and emission spectra, and ORTEP diagrams of the structures. Crystallographic data is available as CIF files. This material is available free of charge via the Internet at http://pubs.acs.org. NIH Public Access

Photoelectrochemical water splitting directly converts solar energy to chemical energy stored in hydrogen, a high energy density fuel. Although water splitting using semiconductor photoelectrodes has been studied for more than 40 years, it has only recently been demonstrated using dye-sensitized electrodes. The quantum yield for water splitting in these dye-based systems has, so far, been very low because the charge recombination reaction is faster than the catalytic four-electron oxidation of water to oxygen. We show here that the quantum yield is more than doubled by incorporating an electron transfer mediator that is mimetic of the tyrosine-histidine mediator in Photosystem II. The mediator molecule is covalently bound to the water oxidation catalyst, a colloidal iridium oxide particle, and is coadsorbed onto a porous titanium dioxide electrode with a Ruthenium polypyridyl sensitizer. As in the natural photosynthetic system, this molecule mediates electron transfer between a relatively slow metal oxide catalyst that oxidizes water on the millisecond timescale and a dye molecule that is oxidized in a fast light-induced electron transfer reaction. The presence of the mediator molecule in the system results in photoelectrochemical water splitting with an internal quantum efficiency of approximately 2.3% using blue light.artificial photosynthesis | photoelectrochemistry T he design of biomimetic systems for artificial photosynthesis is of fundamental interest in the study of light-driven electron and proton transfer reactions. It also represents a potential route to the efficient conversion of solar energy to energy stored in fuel. System modeling has shown that it should be possible, using complementary dye molecules that absorb in the infrared and the visible, to construct artificial Z-schemes that split water with over 10% energy conversion efficiency (1, 2). It is simpler in many ways to design small molecules with the proper photoredox properties than it is to find a set of semiconductors that can be coupled for visible light water splitting. Nevertheless, the molecular approach has so far lagged behind the semiconductor-based approach where high efficiencies have been realized with expensive materials (3-7).A ubiquitous problem in molecular artificial photosynthesis is back electron transfer, which rapidly thermalizes the energy stored by light-induced charge separation in donor-acceptor pairs. Recently, our group and several others have studied this problem in dye-sensitized solar cells where a molecular dye and a porous TiO 2 electrode act as the donor-acceptor dyad (8-13). The dye is covalently coupled to a colloidal or molecular water oxidation catalyst. Fast back electron transfer, relative to the rate of water oxidation, results in low quantum yields for water splitting in these systems.It is well known that charge-separation lifetimes in molecular donor-acceptor systems can be increased dramatically by adding secondary electron donors or acceptors to form triads, tetrads, and more complex supermolecu...

Luminescent perovskite nanosheets were prepared by exfoliation of single- or double-layered perovskite oxides, K2Ln2Ti3O10, KLnNb2O7, and RbLnTa2O7 (Ln: lanthanide ion). The thickness of the individual nanosheets corresponded to those of the perovskite block in the parent layered compounds. Intense red and green emissions were observed in aqueous solutions with Gd1.4Eu0.6Ti3O10- and La0.7Tb0.3Ta2O7-nanosheets, respectively, under UV illumination with energies greater than the corresponding host oxide band gap. The coincidence of the excitation spectrum and the band gap absorbance indicates that the visible emission results from energy transfer within the nanosheet. The red emission intensity of the Gd1.4Eu0.6Ti3O10-nanosheets was much stronger than that of the La0.90Eu0.05Nb2O7-nanosheets reported previously. The strong emission intensity is a result of a two-step energy transfer cascade within the nanosheet from the Ti-O network to Gd(3+) and then to Eu(3+). The emission intensities of the Gd1.4Eu0.6Ti3O10- and La0.7Tb0.3Ta2O7-nanosheets can be modulated by applying a magnetic field (1.3-1.4 T), which brings about a change in orientation of the nanosheets in solution. The emission intensities increased when the excitation light and the magnetic field directions were perpendicular to each other, and they decreased when the excitation and magnetic field were collinear and mutually perpendicular to the direction of detection of the emitted light.

Potassium hexaniobate nanoscrolls (NS-K4Nb6O17) formed by exfoliation of lamellar K4Nb6O17 were studied as building blocks for visible-light-driven H2 production (λ > 420 nm) from water using tris(2,2′-bipyridyl)ruthenium(II) chloride (Ru(bpy)3 2+) as a sensitizer and ethylenediaminetetraacetic acid (EDTA) as an electron donor. The surface of NS-K4Nb6O17 is negatively charged at pH 3−11, enabling cationic Ru(bpy)3 2+ molecules to be efficiently adsorbed onto the surface, allowing for rapid excited-state electron and subsequent H2 evolution without any chemical bond linkage between the sensitizer and the oxide surface. The rate of visible light H2 production in the nanoscroll-based system is 10 times higher than that of similarly sensitized K4Nb6O17. The difference can be primarily attributed to the strong adsorption of Ru(bpy)3 2+ in the case of the nanoscrolls. The maximum photocatalytic reactivity is found over a narrow range of pH and Pt-loading. This study highlights the utility of single-crystalline oxide nanosheets as components of photosystems for visible-light-driven H2 production from water.

Dicarboxylic acid ligands (malonate, succinate, and butylmalonate) stabilize 2 nm diameter IrO2 particles synthesized by hydrolysis of aqueous IrCl(6)2- solutions. Analogous monodentate (acetate) and tridentate (citrate) carboxylate ligands, as well as phosphonate and diphosphonate ligands, are less effective as stabilizers and lead to different degrees of nanoparticle aggregation, as evidenced by transmission electron microscopy. Succinate-stabilized 2 nm IrO2 particles are good catalysts for water photo-oxidation in persulfate/sensitizer solutions. Ruthenium tris(2,2'-bipyridyl) sensitizers containing malonate and succinate groups in the 4,4'-positions are also good stabilizers of 2 nm diameter IrO2 colloids. The excited-state emission of these bound succinate-terminated sensitizer molecules is efficiently quenched on a time scale of approximately 30 ns, most likely by electron transfer to Ir(IV). In 1 M persulfate solutions in pH 5.8 Na2SiF6/NaHCO3 buffer solutions, the excited-state of the bound sensitizer is quenched oxidatively on the time scale of approximately 9 ns. Electron transfer from Ir(IV) to Ru(III) occurs with a first-order rate constant of 8x10(2) s(-1), and oxygen is evolved. The turnover number for oxygen evolution under these conditions was approximately 150. The sensitizer-IrO2 diad is thus a functional catalyst for photo-oxidation of water, and may be a useful building block for overall visible light water splitting systems.

The measurements were performed with a Brookhaven Instruments Zeta PALS at 298 K. R-HCa 2 Nb 3 O 10 and NS-H 4 Nb 6 O 17 powders were dispersed in pure water with concentrations of ca. 0.1 and 0.01 g•L -1 , respectively. In order to measure the pH dependence of the zeta-potential, the pH was adjusted by addition of aqueous HCl or sodium hydroxide (NaOH) solutions.

To examine the effects of molecular structure on charge storage in self-assembled monolayers (SAMs), a family of redox-active molecules has been prepared wherein each molecule bears a tether composed of a tripodal linker with three protected thiol groups for surface attachment. The redox-active molecules include ferrocene, zinc porphyrin, ferrocene-zinc porphyrin, magnesium phthalocyanine, and triple-decker lanthanide sandwich coordination compounds. The tripodal tether is based on a tris[4-(S-acetylthiomethyl)phenyl]-derivatized methane. Each redox-active unit is linked to the methane vertex by a 4,4'-diphenylethyne unit. The electrochemical characteristics of each compound were examined in solution and in SAMs on Au. Redox-kinetic measurements were also performed on the SAMs (with the exception of the magnesium phthalocyanine) to probe (1) the rate of electron transfer in the presence of an applied potential and (2) the rate of charge dissipation after the applied potential is disconnected. The electrochemical studies of the SAMs indicate that the tripodal tether provides a more robust anchor to the Au surface than does a tether with a single site of attachment. However, the electron-transfer and charge-dissipation characteristics of the two tethers are generally similar. These results suggest that the tripodal tether offers superior stability characteristics without sacrificing electrochemical performance.