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Ultrathin dual layers of TiO2and Ni have been used to stabilize polycrystalline BiVO4photoanodes against photocorrosion in an aqueous alkaline (pH = 13) electrolyte. Conformal, amorphous TiO2layers were deposited on BiVO4thin films by atomic-layer deposition, with Ni deposited onto the TiO2by sputtering. Under simulated air mass 1.5 illumination, the dual-layer coating extended the lifetime of the BiVO4photoanodes during photoelectrochemical water oxidation from minutes, for bare BiVO4, to hours, for the modified electrodes. X-ray photoelectron spectroscopy showed that these layers imparted chemical stability to the semiconductor/electrolyte interface. Transmission electron microscopy revealed the structure and morphology of the polycrystalline BiVO4film as well as of the thin coating layers. This work demonstrates that protection schemes based on ultrathin corrosion-resistant overlayers can be applied beneficially to polycrystalline photoanode materials under conditions relevant to efficient solar-driven water-splitting systems.

We report the redox properties of Si(111) surfaces functionalized with a mixed monolayer of vinylferrocenyl and methyl moieties that have been characterized using spectroscopic, electrical, and electrochemical techniques. The silicon was functionalized using reaction conditions analogous to those of hydrosilylation, but instead of a H-terminated Si surface, a chlorine-terminated Si precursor surface was used to produce the linked vinyl-modified functional group. The functionalized surfaces were characterized by time-resolved photoconductivity decay, X-ray photoelectron spectroscopy, electrochemical measurements, and photoelectrochemical measurements. The functionalized Si surface was well passivated, exhibited high surface coverage and few remaining reactive Si atop sites, had a very low surface recombination velocity, and displayed little initial surface oxidation. The surface was stable toward atmospheric and electrochemical oxidation. The surface coverage of vinylferrocene (or fluorostyrene) was controllably varied from 0 up to 30% of a monolayer. Interfacial charge transfer to the attached ferrocene group was relatively rapid, and a photovoltage of 0.4 V was generated upon illumination of functionalized n-type silicon surfaces in CH3CN.

The core-level energy shifts observed using X-ray photoelectron spectroscopy (XPS) have been used to determine the band bending at Si(111) surfaces terminated with Si–Br, Si–H, and Si–CH3groups, respectively. The surface termination influenced the band bending, with the Si 2p3/2binding energy affected more by the surface chemistry than by the dopant type. The highest binding energies were measured on Si(111)–Br (whose Fermi level was positioned near the conduction band at the surface), followed by Si(111)–H, followed by Si(111)–CH3(whose Fermi level was positioned near midgap at the surface). Si(111)–CH3surfaces exposed to Br2(g) yielded the lowest binding energies, with the Fermi level positioned between midgap and the valence band. The Fermi level position of Br2(g)-exposed Si(111)–CH3was consistent with the presence of negatively charged bromine-containing ions on such surfaces. The binding energies of all of the species detected on the surface (C, O, Br) shifted with the band bending, illustrating the importance of isolating the effects of band bending when measuring chemical shifts on semiconductor surfaces. The influence of band bending was confirmed by surface photovoltage (SPV) measurements, which showed that the core levels shifted toward their flat-band values upon illumination. Where applicable, the contribution from the X-ray source to the SPV was isolated and quantified. Work functions were measured by ultraviolet photoelectron spectroscopy (UPS), allowing for calculation of the sign and magnitude of the surface dipole in such systems. The values of the surface dipoles were in good agreement with previous measurements as well as with electronegativity considerations. The binding energies of the adventitious carbon signals were affected by band bending as well as by the surface dipole. A model of band bending in which charged surface states are located exterior to the surface dipole is consistent with the XPS and UPS behavior of the chemically functionalized Si(111) surfaces investigated herein.

Ordered structures offer the potential for producing photoanodes with enhanced minority-carrier collection. To evaluate this approach to visible-light-driven oxidation in aqueous electrolytes, porous WO3structures were synthesized by the potentiostatic anodization of W foil. The photoelectrochemical behavior of the porous WO3photoanodes was compared to that of compact WO3films. Relative to planar electrodes, the porous WO3electrodes exhibited a 6-fold increase in photocurrent density, from 0.12 to 0.75 mA cm–2, under 100 mW cm–2of simulated solar illumination. Spectral response measurements indicated that the porous electrodes exhibited internal quantum yields of ∼0.5 throughout most of the region of WO3absorption. The external quantum yield of the porous WO3films was a function of the angle of incidence of the light, increasing from 0.25 at normal incidence to 0.50 at 65° off normal. The porous WO3films showed excellent stability against photodegradation. This work demonstrates that morphological control can improve the internal quantum yield of photoanodes in contact with aqueous electrolytes.

The electronic and photovoltaic properties of junctions between the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and Si(111) surfaces have been investigated for a range of doping types, doping levels, and surface functionalization of the Si. PEDOT–poly(styrenesulfonate) (PSS) formed ohmic, low resistance contacts to H-terminated and CH3-terminated p-type Si(111) surfaces. In contrast, PEDOT formed high barrier height (0.8–1.0 V) contacts to n-Si(111) surfaces, with CH3-terminated n-Si(111)/PEDOT contacts showing slightly higher barrier heights (1.01 eV) than H-terminated n-Si(111)/PEDOT contacts (0.89 V). PEDOT contacts to CH3-terminated and H-terminated n-Si(111) surfaces both produced photovoltages under illumination in accord with the Shockley diode limit based on bulk/recombination diffusion in the semiconductor. Such devices produced solar energy-conversion efficiencies of 5.7% under 100 mW cm–2of simulated air mass 1.5 illumination. The electrical properties of PEDOT contacts to CH3-terminated Si surfaces were significantly more stable in an air ambient than the electrical properties of PEDOT contacts to H-terminated Si surfaces. PEDOT films produced a low resistance, tunnel-barrier type of ohmic contact to n+-Si(111) surfaces. Hence, through various combinations of doping type, doping level, and surface functionalization, the PEDOT/Si contact system offers a wide range of opportunities for integration into monolithic photovoltaic and/or artificial photosynthetic systems.

Tungsten trioxide (WO3) has been investigated as a photoanode for water oxidation reactions in acidic aqueous conditions. Though WO3is not capable of performing unassisted solar-driven water splitting, WO3can in principle be coupled with a low band gap semiconductor, such as Si, to produce a stand-alone, tandem photocathode/photoanode p-Si/n-WO3system for solar fuels production. Junctions between Si and WO3, with and without intervening ohmic contacts, were therefore prepared and investigated in detail. Thin films of n-WO3that were prepared directly on p-Si and n-Si substrates exhibited an onset of photocurrent at a potential consistent with expectations based on the band-edge alignment of these two materials predicted by Andersen theory. However, n-WO3films deposited on Si substrates exhibited much lower anodic photocurrent densities (∼0.02 mA cm–2at 1.0 V vs SCE) than identically prepared n-WO3films that were deposited on fluorine-doped tin oxide (FTO) substrates (0.45 mA cm–2at 1.0 V vs SCE). Deposition of n-WO3onto a thin layer of tin-doped indium oxide (ITO) that had been deposited on a Si substrate yielded anodic photocurrent densities that were comparable to those observed for n-WO3films that had been deposited onto FTO-coated glass. An increased photovoltage was observed when an n-Si/ITO Schottky junction was formed in series with the n-WO3film, relative to when the WO3was deposited directly onto the Si. Hence, inclusion of the ITO layer allowed for tandem photoelectrochemical devices to be prepared using n-WO3and n-Si as the light absorbers.

The photoelectrochemical behavior of n-type ZnO nanowire arrays was compared to the behavior of single crystalline n-ZnO photoelectrodes in contact with either 0.50 M K2SO4(aq) at pH 6.0 or Fe(CN)43–/4–(aq). The use of a thin film of ZnO as a seed layer produced dense nanowire arrays in which the ZnO nanowires were preferentially oriented perpendicular to the substrate. The average diameter of the ZnO nanowires that were produced by two different growth conditions was ∼125 and ∼175 nm, respectively, with a nanowire length of ∼2–4 μm. Under simulated 1 Sun Air Mass 1.5 illumination conditions, the ZnO nanowire arrays exhibited open-circuit potentials, Eoc, and short-circuit photocurrent densities, Jsc, that were very close to the values observed from single crystal n-type ZnO photoanodes in contact with these same electrolytes. Device physics simulations were in accord with the experimentally observed behavior, indicating that, under certain combinations of materials properties and interface recombination velocities, the use of nanostructured light absorbers can provide an approach to efficient photoelectrochemical solar energy-conversion systems.

The photoelectrochemical behavior of methyl-terminated p-type and n-type Si(111) surfaces was determined in contact with a series of one-electron, outer-sphere, redox couples that span >1 V in the Nernstian redox potential, E(A/A–), of the solution. The dependence of the current vs potential data, as well as of the open-circuit photovoltage, Voc, on E(A/A–) was compared to the behavior of H-terminated p-type and n-type Si(111) surfaces in contact with these same electrolytes. For a particular E(A/A–) value, CH3-terminated p-Si(111) electrodes showed lower Vocvalues than H-terminated p-Si(111) electrodes, whereas CH3-terminated n-Si(111) electrodes showed higher Vocvalues than H-terminated n-Si(111) electrodes. Under 100 mW cm–2of ELH-simulated Air Mass 1.5 illumination, n-type H–Si(111) and CH3–Si(111) electrodes both demonstrated nonrectifying behavior with no photovoltage at very negative values of E(A/A–) and produced limiting Vocvalues of >0.5 V at very positive values of E(A/A–). Illuminated p-type H–Si(111) and CH3–Si(111) electrodes produced no photovoltage at positive values of E(A/A–) and produced limiting Vocvalues in excess of 0.5 V at very negative values of E(A/A–). In contact with CH3CN-octamethylferrocene+/0, differential capacitance vs potential experiments yielded a −0.40 V shift in flat-band potential for CH3-terminated n-Si(111) surfaces relative to H-terminated n-Si(111) surfaces. Similarly, in contact with CH3CN-1,1′-dicarbomethoxycobaltocene+/0, the differential capacitance vs potential data indicated a −0.25 V shift in the flat-band potential for CH3-terminated p-Si(111) electrodes relative to H-terminated p-Si(111) electrodes. The observed trends in Vocvs E(A/A–), and the trends in the differential capacitance vs potential data are consistent with a negative shift in the interfacial dipole as a result of methylation of the Si(111) surface. The negative dipole shift is consistent with a body of theoretical and experimental comparisons of the behavior of CH3–Si(111) surfaces vs H–Si(111) surfaces, including density functional theory of the sign and magnitude of the surface dipole, photoemission spectroscopy in ultrahigh vacuum, the electrical behavior of Hg/Si contacts, and the pH dependence of the current–potential behavior of Si electrodes in contact with aqueous electrolytes.

The surface chemistry of CH3–, CD3–, and C10H21–Ge(111) surfaces prepared through a bromination/alkylation method have been investigated by infrared spectroscopy. Well-ordered CH3–Ge(111) surfaces could be prepared only if, prior to bromination, the surface was etched with 6.0 M HCl or with a two-step etch of H2O2(1.5 M)/HF (5.1 M) followed by a short HF (6.0 M) etch. The etching method used to make the Ge precursor surface, and the formation of a bromine-terminated intermediate Ge surface, were of critical importance to obtain clear, unambiguous infrared absorption peaks on the methyl-terminated Ge surfaces. Polarization-dependent absorption peaks observed at 1232 cm–1for CH3–Ge(111) surfaces and at 951 cm–1for CD3–Ge(111) surfaces were assigned to the methyl “umbrella” vibrational mode. A polarization-dependent peak at 2121 cm–1for CD3–Ge(111) surfaces was assigned to the symmetric methyl stretching mode. Polarization-independent absorption peaks at 755 cm–1for CH3–Ge(111) and at 577 cm–1for CD3–Ge(111) were assigned to the methyl rocking mode. These findings provide spectroscopic evidence that the methyl monolayer structure on the alkylated Ge is well-ordered and similar to that on analogous Si(111) surfaces, despite differences in the composition of the precursor surfaces. The X-ray photoelectron spectra of CH3–Ge(111) surfaces, however, were not highly dependent upon the etching method and showed a constant C 1s:Ge 3d ratio, independent of the etching method. The infrared spectra of C10H21–Ge(111) surfaces were also not sensitive to the initial etching method. Hence, while the final packing density of the alkyl groups on the surface was similar for all etch methods studied, not all methods yielded a well-ordered Ge(111)/overlayer interface.

The open-circuit potentials of p-Si/((MV2+/MV+)(aq)) junctions with Si(111) surfaces functionalized with H−, CH3−, CH2CHCH2−, or mixed CH3−/CH2CHCH2− monolayers have been investigated as the solution pH was changed from 2.5 to 11. The pH sensitivity of the open-circuit potentials, and therefore the band-edge positions, was anticorrelated with the total fraction of Si atop sites that were terminated by Si−C bonds. This behavior is consistent with the hypothesis that the non Si−C terminated atop sites were initially H-terminated and were unstable to oxide growth under aqueous conditions with the oxidation-product inducing a pH-dependent dipole. Metal-semiconductor junctions between Hg and CH3-, CH2CHCH2-, or mixed CH3-/CH2CHCH2-terminated n-Si(111) surfaces formed rectifying Hg/Si Schottky junctions and exhibited mutually similar barrier-heights (∼0.9 V), suggesting similar magnitudes and direction of the surface dipoles on all of these functionalized surfaces.

The response and discrimination performance of an array that consisted of 20 different organothiol-capped Au nanoparticle chemiresistive vapor sensors was evaluated during exposure to 13 different organic vapors. The passivating organothiol ligand library consisted of collections of straight-chain alkanethiols, branched alkanethiols, and aromatic thiols. A fourth collection of sensors was formed from composites of 2-phenylethanethiol-capped Au nanoparticles and nonpolymeric aromatic materials that were coembedded in a sensor film. The organic vapors consisted of six hydrocarbons (n-hexane, n-heptane, n-octane, isooctane, cyclohexane, and toluene), three polar aprotic vapors (chloroform, tetrahydrofuran, and ethyl acetate), and four alcohols (methanol, ethanol, isopropanol, and 1-butanol). Trends in the resistance response of the sensors were consistent with expected trends in sorption due to the properties of the test vapor and the molecular structure of the passivating ligands in the sensor films. Classification algorithms including principal components analysis and Fisher’s linear discriminant were used to evaluate the discrimination performance of an array of such sensors. Each collection of sensors produced accurate classification of most vapors, with misclassification occurring primarily for vapors that had mutually similar polarity. The classification performance for an array that contained all of the sensor collections produced nearly perfect discrimination for all vapors studied. The dependence of the array size (i.e., the number of sensors) and the array chemical diversity on the discrimination performance indicated that, for an array of 20 sensors, an array size of 13 sensors or more produced the maximum discrimination performance.

Au nanoparticles capped with a homologous series of straight chain alkanethiols (containing 4−11 carbons in length) have been investigated as chemiresistive organic vapor sensors. The series of alkanethiols was used to elucidate the mechanisms of vapor detection by such capped nanoparticle chemiresistive films and to highlight the molecular design principles that govern enhanced detection. The thiolated Au nanoparticle chemiresistors demonstrated rapid and reversible responses to a set of test vapors (n-hexane, n-heptane, n-octane, iso-octane, cyclohexane, toluene, ethyl acetate, methanol, ethanol, isopropanol, and 1-butanol) that possessed a variety of analyte physicochemical properties. The resistance sensitivity to nonpolar and aprotic polar vapors systematically increased as the chain length of the capping reagent increased. Decreases in the nanoparticle film resistances, which produced negative values of the differential resistance response, were observed upon exposure of the sensor films to alcohol vapors. The response signals became more negative with higher alcohol vapor concentrations, producing negative values of the sensor sensitivity. Sorption data measured on Au nanoparticle chemiresistor films using a quartz crystal microbalance allowed for the measurement of the partition coefficients of test vapors in the Au nanoparticle films. This measurement assumed that analyte sorption only occurred at the organic interface and not the surface of the Au core. Such an assumption produced partition coefficient values that were independent of the length of the ligand. Furthermore, the value of the partition coefficient was used to obtain the particle-to-particle interfacial effective dielectric constant of films upon exposure to analyte vapors. The values of the dielectric constant upon exposure to alcohol vapors suggested that the observed resistance response changes observed were not significantly influenced by this dielectric change, but rather were primarily influenced by morphological changes and by changes in the interparticle spacing.

The use of Ge in semiconductor electronics has been constrained by the lack of a simple method of passivating the crystal surface. Toward that end, we have explored the utility of chemically bonded hydrocarbon monolayers. Alkylated Ge(111) surfaces have been prepared by addition of 1-alkenes to the H-terminated Ge(111) surface as well as by a two-step halogenation/alkylation procedure. The chemical compositions of the resulting methyl-, ethyl-, and decyl-terminated surfaces have been evaluated using X-ray photoelectron spectroscopy (XPS). Thermal addition of 1-decene produced hydrophobic surfaces with 0.3 ± 0.1 monolayer of Ge oxide detected by XPS, whereas no oxide was observed on the methyl-, ethyl-, or decyl-terminated surfaces that were prepared using the two-step halogenation/alkylation method. Methyl-terminated Ge(111) surfaces prepared by the two-step method displayed a well-resolved C 1s XPS peak at a binding energy of 284 eV, consistent with carbon bonded to a less electronegative element such as Ge. The electronic properties of all of the alkylated surfaces were characterized by measurements of the surface recombination velocity as a function of an externally applied gate voltage. Treatment of HF-etched Ge(111) surfaces with Br2vapor, followed by reaction with alkylmagnesium or alkyllithium reagents, yielded air-stable surfaces that had surface recombination velocities of 100 cm s−1or less under flat-band conditions. The field-dependent surface recombination velocity experiments indicated that, in contact with air, methyl-terminated n-type Ge(111) samples had a negative surface potential approaching 300 mV, in contrast to the oxidized Ge(111) surface, which exhibited a strongly positive surface potential under the same conditions. Mercury contacts to n-type methyl-, ethyl-, or decyl-terminated Ge(111) substrates that were alkylated using the two-step method formed rectifying junctions with barrier heights of 0.6 ± 0.1 eV, whereas no measurable rectification was observed for Hg contacts to p-type Ge(111) substrates that were alkylated by the two-step method, to n-type Ge(111) substrates that were alkylated through addition of 1-decene, or to oxidized n-type Ge(111) samples.

Fourier transform infrared (FTIR) spectroscopy was used to investigate C2H5−Si(111) surfaces prepared using a chlorination/alkylation method. After alkylation, in addition to ethyl groups, such surfaces showed the presence of hydrogen bonded to atop silicon surface atoms. Systematic isotopic substitution of protic solvents and reagents with their fully or partially deuterated counterparts revealed the origin of the surface-bound hydrogen on the C2H5−Si(111) surfaces. The presence or absence of the Si−H stretch at 2080 cm−1and the Si−D stretch at ∼1510 cm−1, respectively, indicated that the hydrogen originated from the methyl group of the ethyl Grignard reagent.

The electron injection dynamics of dye-sensitized TiO 2-based solar cells have been investigated to determine the effects of replacing the I 3−/I −redox system by non-redox-active supporting electrolytes. TiO 2films were sensitized with Ru(dcbpy) 2(NCS) 2, where dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine (the “N3” dye), and placed in contact with either M(ClO 4) or M(I 3−/I −) solutions (M = Li +or (n-C 4H 9) 4N +); cells that contained I 3−/I −were fully functional solar cells whose steady-state photocurrents were directly measured. In (n-C 4H 9) 4N +-containing solutions, significant differences were observed between the measured kinetics when ClO 4−was replaced by the redox-active I 3−/I −system. In particular, a ps time scale loss of the metal-to-ligand charge-transfer excited-state of the N3 dye, associated with electron injection, that was observed in cells containing either LiClO 4or [(n-C 4H 9) 4N]ClO 4was absent in fully functional solar cells that contained [(n-C 4H 9) 4N]I/I 2. These results underscore the importance of performing kinetics measurements on this class of solar cells under operational conditions if one is to obtain reliable correlations between the dynamics data and the steady-state performance metrics of the solar cell devices.

A series of chromophoric salts has been prepared in which electron-rich 4-(dimethylamino)phenyl groups are connected via polyenyl chains to electron-accepting N-methylpyridinium or 3-methylbenzothiazolium units. These compounds have been characterized by using various techniques, including electronic absorption spectroscopy and cyclic voltammetry. Single-crystal X-ray structures have been determined for several salts, all of which crystallize centrosymmetrically. Molecular quadratic nonlinear optical (NLO) responses have been determined using femtosecond hyper-Rayleigh scattering (HRS) at 1300 and 800 nm and via Stark (electroabsorption) spectroscopic studies on the intense, visible ? intramolecular charge-transfer (ICT) bands. Large red shifts in the ICT transitions on replacing a pyridinium with a benzothiazolium unit indicate that the latter acts as a more effective electron acceptor. Both HRS and Stark measurements show that the static first hyperpolarizability 0increases with polyene chain extension in both types of chromophore, and the benzothiazolium salts have larger NLO responses than their pyridinium analogues. The results of time-dependent density functional theory calculations using a polarizable solvent continuum model agree with the observation that 0increases with chain lengthening, but the observed superiority of the benzothiazolium acceptor is not predicted either in the ICT energies or 0values. Coupled perturbed Hartree-Fock and semiempirical INDO/S calculations similarly fail to reproduce this principal conclusion from the experimental studies.

Chemically sensitive resistors have been fabricated from composites of carbon black and low volatility, nonpolymeric, organic molecules such as propyl gallate, lauric acid, and dioctyl phthalate. Sorption of organic vapors into the nonconductive phase of such composites produced rapid and reversible changes in the relative differential resistance response of the sensing films. Arrays of these sensors, in which each sensing film was comprised of carbon black and a chemically distinct nonpolymeric organic molecule or blend of organic molecules, produced characteristic response patterns upon exposure to a series of different organic test vapors. The use of nonpolymeric sorption phases allowed fabrication of sensors having a high density of randomly oriented functional groups and provided excellent discrimination between analytes. By comparison to carbon black−polymer composite vapor sensors and sensor arrays, such sensors provided comparable detection limits as well as enhanced clustering and enhanced resolution ability between test analytes.

AbstractMartial Gueroult (1891–1976) and Victor Goldschmidt (1914–1981) are two major figures in French history of philosophy during the second half of the last century. The latter has often been described as one of the former's “disciples”, on the basis of their common opposition to the “geneticist” approach in the study of past philosophers, and their common support for a “structuralist” one, which was an influential paradigm in various fields of French thought at the time of their activity. A detailed study of their methodological principles and their concrete practice shows that their relationship was much more complicated.

E-government websites represent legal information predominantly in textual form. Given the potential of the new Information and Communication Technologies (ICT) to visualise information, this is astonishing. This paper adopts an innovative, multidisciplinary approach and a case study to demonstrate that mind mapping legal information would enhance its representation for the greater benefit of e-government users.

Six new complex salts trans-RuIIClpdma2LPF6npdma 1,2-phenylenebisdimethylarsine; L (E,E,E)-1,6-bis4-pyridylhexa-1,3,5-triene bph, n 1, 5; L N-methyl-4-(E)-2-4-pyridylethenylpyridinium Mebpe, n 2, 7; L N-methyl-4-(E,E)-4-4-pyridylbuta-1,3-dienylpyridinium Mebpb, n 2, 8; L N-methyl-4-(E,E,E)-6-4-pyridylhexa-1,3,5-trienylpyridinium Mebph, n 2, 9; L bis4-pyridylacetylene bpa, n 1, 10; L N-methyl-4-2-4-pyridylethynylpyridinium Mebpa, n 2, 11 have been prepared. The electronic absorption spectra of 5and 7–11display intense, visible metal-to-ligand charge-transfer MLCT bands, with λmaxvalues in the range 434–492 nm in acetonitrile. Cyclic voltammetric studies reveal reversible RuIIIIIwaves with E12values in the range 1.06–1.15 V vs.Ag–AgCl, together with irreversible L-based reduction processes. Along with a number of previously reported related compounds B. J. Coe et al., J. Chem. Soc., Dalton Trans., 1996, 3917; 1997, 591; 2000, 797, salts 5and 7–11have been investigated by using Stark electroabsorption spectroscopy in butyronitrile glasses at 77 K. These studies have afforded dipole moment changes Δμ12for the MLCT transitions which have been used to calculate molecular static first hyperpolarizabilities β0according to the two-state equation β0 3Δμ12μ122Emax2μ12 transition dipole moment, Emax MLCT energy. MLCT absorption and electrochemical data show that a trans-RuIIClpdma2centre is considerably less electron-rich than a RuIINH352unit. Although the β0responses of the pdma complexes are only a little smaller than those of their RuIINH352analogues, this result is partly attributable to unexpected changes in the relative μ12values on freezing. Thus, substantial increases in μ12for the arsine compounds act to partially offset the β0-decreasing influence of their higher Emaxvalues when compared with the analogous pentaammine species. Single crystal X-ray structures have been obtained for the salts 1·2.5MeCN, 4·2MeCN, 7and 11, but only 1·2.5MeCN adopts a non-centrosymmetric space group Fdd2 such as may show bulk NLO effects.

Four series of dyes with dimethylamino electron donor groups and N‐R‐pyridinium (R = methyl Me, phenyl Ph, 2,4‐dinitrophenyl 2,4‐DNPh, or 2‐pyrimidyl 2‐Pym) electron acceptors are studied as their hexafluorophosphate salts. The intramolecular charge‐transfer (ICT) energies (Emax) of these compounds decrease within each of the series in the order R = Me > Ph > 2,4‐DNPh > 2‐Pym, as the electron‐accepting ability of the pyridinium ring increases. Hyper‐Rayleigh scattering with femtosecond 1300 or 800 nm lasers yields fluorescence‐free first hyperpolarizabilities β, and static first hyperpolarizabilities β0[H] are obtained via the two‐state model. Dipole moment changes Δμ12for the ICT transitions obtained from Stark spectroscopy afford β0[S] values by using β0= 3Δμ12(μ12)2/2(Emax)2(μ12= transition dipole moment). The β0[S] data show that the combination of pyridyl N‐arylation with conjugation extension affords large increases in β0. The β0[H] data generally agree with this conclusion, but resonance effects may explain some apparent anomalies. X‐ray structural studies on various salts reveal that the use of tosylate anions is not a generally applicable approach to engineering noncentrosymmetric structures of pyridinium salts. However, trans‐N‐phenyl‐4‐(4‐dimethylaminophenyl‐4‐buta‐1,3‐dienyl)pyridinium hexafluorophosphate adopts the polar space group Cc, and shows a very large powder second harmonic generation efficiency from a 1907 nm laser, which is similar to that of the well‐studied material trans‐4′‐(dimethylamino)‐N‐methyl‐4‐stilbazolium tosylate (DAST).

Seven new complex salts trans-RuIINCSNH34LAPF62LAN-methyl-4,4′-bipyridinium MeQ1, N-methyl-2,7-diazapyrenium Medap2, N-methyl-4-trans-2-4-pyridylethenylpyridinium Mebpe3, N-phenyl-4,4′-bipyridinium PhQ4, N-phenyl-4-trans-2-4-pyridylethenylpyridinium Phbpe5, N-4-acetylphenyl-4,4′-bipyridinium 4-AcPhQ6, or N-2-pyrimidyl-4,4′-bipyridinium 2-PymQ7 have been prepared. The electronic absorption spectra of 1–7display intense, visible metal-to-ligand charge-transfer MLCT bands, the energies Emax of which decrease in the order 2> 1> 3> 4> 6> 5> 7, as the electron-accepting ability of LAincreases. The Emaxvalues generally correlate with cyclic voltammetric and 1H NMR data. Comparisons of the MLCT absorption and electrochemical data for 1–7show that a trans-RuIINCSNH34centre is a stronger electron donor than analogous groups containing only neutral ligands. Molecular first hyperpolarizabilities βhave been measured by using the hyper-Rayleigh scattering technique with acetonitrile solutions and a 1064 nm laser, and static first hyperpolarizabilities β0H were obtained by application of the two-state model. Stark electroabsorption spectroscopic studies in butyronitrile glasses at 77 K have afforded dipole moment changes Δμ12for the MLCT transitions which have been used to calculate β0S values according to the two-state equation β0 3Δμ12μ122Emax2μ12 transition dipole moment. Both the β0H and β0S data confirm that the increased electron donor ability of a trans-RuIINCSNH34centre affords enhanced β0values, when compared with related complexes containing the neutral transligands NH3, 4-dimethylaminopyridine, N-methylimidazole mim or pyridine. However, the magnitude of the observed β0increase with respect to the analogous mim complexes varies over a range of ca. 25–120, depending upon LA.

In the Robin and Day classification, mixed-valence systems are characterized as Class I, II or III depending on the strength of the electronic interaction between the oxidized and reduced sites, ranging from essentially zero (Class I), to moderate (Class II), to very strong electronic coupling (Class III). The properties of Class I systems are essentially those of the separate sites. Class II systems possess new optical and electronic properties in addition to those of the separate sites. However, the interaction between the sites is sufficiently weak that Class II systems are valence trapped or charge localized and can the be described by a double-well potential. In Class III systems the interaction of the donor and acceptor sites is so great that two separate minima are no longer discernible and the energy surface features a single minimum. The electron is delocalized and the system has its own unique properties.The Robin and Day classification has enjoyed considerable success and most of the redox systems studied to date are readily assigned to Class II. However the situation becomes much more complicated when the system shows borderline Class II/III behavior. Such “almost delocalized” mixed-valence systems are difficult to characterize. In this article spectral band shapes and intensities are calculated utilizing increasingly complex models including two to four states. Free-energy surfaces are constructed for harmonic diabatic surfaces and characterized as a function of increasing electronic coupling to simulate the Class II to III transition. The properties of the charge-transfer absorption bands predicted for borderline mixed-valence systems are compared with experimental data. The treatment is restricted to symmetrical (ΔG0 = 0) systems.

A series of 20 ruthenium(II) complex salts, known to exhibit large molecular static first hyperpolarizabilities β0[HRS] from hyper-Rayleigh scattering measurements on acetonitrile solutions at 298 K (Coe, B. J.; et al. Inorg. Chem. 1997, 36, 3284; 1998, 37, 3391; J. Chem. Soc., Dalton Trans. 1999, 3617), have been studied by using electroabsorption spectroscopy in butyronitrile glasses at 77 K. The salts are of the form trans-[Ru^II(NH3)4(L^D)(L^A)][PF6]3 [L^D = NH3 and L^A = N-methyl-4,4‘-bipyridinium (MeQ⁺) (1), N-phenyl-4,4‘-bipyridinium (PhQ⁺) (2), N-(4-acetylphenyl)-4,4‘-bipyridinium (4-AcPhQ⁺) (3), N-(2,4-dinitrophenyl)-4,4‘-bipyridinium (2,4-DNPhQ⁺) (4), N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Mebpe⁺) (5), N-phenyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium (Phbpe⁺) (6), or N-methyl-2,7-diazapyrenium (Medap⁺) (7); L^D = 1-methylimidazole and L^A = MeQ⁺ (8), PhQ⁺ (9), 4-AcPhQ⁺ (10), N-(2-pyrimidyl)-4,4‘-bipyridinium (2-PymQ⁺) (11), Mebpe⁺ (12), or Phbpe⁺ (13); L^D = 4-(dimethylamino)pyridine and L^A = MeQ⁺ (14), PhQ⁺ (15) or 4-AcPhQ⁺ (16); L^D = pyridine and L^A = 2-PymQ⁺ (17), Mebpe⁺ (18) or Phbpe⁺ (19); L^D = 4-(dimethylamino)benzonitrile and L^A = MeQ⁺ (20)]. The electroabsorption spectra afford dipole moment changes Δμ12 for the visible metal-to-ligand charge-transfer (MLCT) excitations of the dipolar complexes. The transition dipole moments μ12 cover a range of ca. 4−7 D and generally increase with the electron accepting strength of L^A, most notably on replacing an N-methyl with a N-phenyl substituent. The Δμ12 values are large (ca. 14−21 D) and generally increase with the size of L^A. Comparison of experimental and calculated diabatic dipole moment changes Δμab suggests that the orbital(s) that receive the MLCT electron are delocalized only over the first two aryl rings. ZINDO calculations on the pentaammine complexes do not accurately reproduce the MLCT energies or β0[HRS] values, but they can predict the dipole properties with reasonable accuracy and also indicate that the low lying π* orbitals span only the first two rings of L^A. β0[Stark] values of 1−20 calculated according to the two-state model by using β0 = 3Δμ(μ12)²/(Emax)² (Emax = MLCT energy maximum) are mostly in good agreement with β0[HRS]. The β0[Stark] values are the first meaningful first hyperpolarizabilities for 18 and 20. The electroabsorption results confirm the unusually large magnitudes of β0 in 1−20 and also that N-arylation enhances β0, the latter effect being most significant in the 4,4‘-bipyridinium-based complexes. Increases in β0 are generally associated with decreases in Emax and increases in both μ12 and Δμ12, and insertion of a trans-CH&dbd;CH bridge into the 4,4‘-bipyridinium unit of L^A enhances β0 by ca. 35−50%.

The relaxation dynamics and product distribution resulting from the decay of high lying excited states generated via sequential two-photon capture by [Ru(bpy)3]²⁺ or electron capture by [Ru(bpy)3]³⁺ have been investigated by flash photolysis and pulse radiolysis techniques. In comparison to the decay dynamics for monophotonic excitation, dramatically different relaxation dynamics have been observed. High-power flash excitation yields both the lowest lying metal-to-ligand charge transfer (³MLCT) state and a new transient photoproduct associated with nonradiative decay through the photodissociative metal-centered (³dd) excited state/s. The photoproduct is postulated to be [Ru^II(bpy)2(η¹-bpy)]²⁺ where the pendant pyridine has rotated to yield a transient that is stabilized by a π−bonded or a three-centered Ru−C−H agostic interaction.

The Mi-2 protein is the central component of the recently isolated NuRD nucleosome remodelling and histone deacetylase complex. Although the NuRD complex has been the subject of extensive biochemical analyses, little is known about its biological function. Here we show that the two C. elegans Mi-2 homologues, LET-418 and CHD-3, play essential roles during development. The two proteins possess both shared and unique functions during vulval cell fate determination, including antagonism of the Ras signalling pathway required for vulval cell fate induction and the proper execution of the 2 degrees cell fate of vulval precursor cells, a process under the control of LIN-12 Notch signalling.

The Mi-2 protein is the central component of the recently isolated NuRD nucleosome remodelling and histone deacetylase complex. Although the NuRD complex has been the subject of extensive biochemical analyses, little is known about its biological function. Here we show that the two C. elegans Mi-2 homologues, LET-418 and CHD-3, play essential roles during development. The two proteins possess both shared and unique functions during vulval cell fate determination, including antagonism of the Ras signalling pathway required for vulval cell fate induction and the proper execution of the 2° cell fate of vulval precursor cells, a process under the control of LIN-12 Notch signalling.

The role of cobalt and iron phthalocyanines in catalytic CO2 reduction has been studied. Chemical, photochemical, and radiolytic reductions of the metal phthalocyanines (Pc) have been carried out in organic solvents, and reduction of their tetrasulfonated derivatives (TSPc) in aqueous solutions. Co^IIPc and Fe^IIPc are readily reduced to [Co^IPc]^- and [Fe^IPc]^-, which do not react with CO2. Reduction of [Co^IPc]^- yields a product which is characterized as the radical anion, [Co^IPc^•-]^2-, on the basis of its absorption spectra in the visible and IR regions. This species is stable under dry anaerobic conditions and reacts rapidly with CO2. Catalytic formation of CO and formate is confirmed by photochemical experiments in DMF and acetonitrile solutions containing triethylamine (TEA) as a reductive quencher. The photochemical yields are greatly enhanced by the addition of p-terphenyl (TP). The radical anion, TP^•-, formed from the reductive quenching of the singlet excited state with TEA, reduces the phthalocyanines very rapidly. The rate constants for reduction of Co^IIPc, [Co^IPc]^-, and [Fe^IPc]^- by TP^•-, determined by pulse radiolysis in DMF solutions, are nearly diffusion-controlled. The mono-reduced species formed from [Co^IPc]^- is unstable under the pulse radiolysis conditions but is longer-lived under the flash photolysis conditions. The interaction of this species with CO2 is either too weak or too slow to detect in the current experiments, where a competing reaction with protons predominates.

To contribute to the development of the transcription map of human chromosome 21 (HC21), we isolated a new transcript, C21orf5 (chromosome 21 open reading frame 5), encoding a predicted 2298-amino-acid protein. Analysis of the genomic DNA sequence revealed that C21orf5 consists of 37 exons that extend over 130 kb and maps between the CBR3 (carbonyl reductase 3) and the KIAA0136 genes. Northern blot analyses showed a ubiquitously expressed RNA species of 8.5 kb. RNA in situhybridization on brain sections of normal human embryos revealed a strong labeling in restricted areas of the cerebral cortex. In silicoanalysis of the deduced C21orf5 protein revealed several highly probable transmembrane segments but no known protein domains or homology with known proteins. However, there were significant homologies to several hypothetical Caenorhabditis elegansproteins and Drosophila melanogastergenomic sequences. To investigate the function of C21orf5, we isolated the cDNA of the C. elegansortholog and performed double-stranded RNA-mediated genetic interference experiments. The major phenotype observed in the progeny of injected animals was embryonic lethality. Most of the tissues of the embryo failed to undergo proper patterning during gastrulation, and morphogenesis did not occur; thus we termed the ortholog pad-1,for patterning defective 1. These results indicated that pad-1is essential for the development and the survival of C. elegans.This study provides the first example of the use of C. elegansas a model to study the function of genes on human chromosome 21 that might be involved in Down syndrome.

The Caenorhabditis elegans lin-39, mab-5 and egl-5 Hox genes specify cell fates along the anterior-posterior body axis of the nematode during postembryonic development, but little is known about Hox gene functions during embryogenesis. Here, we show that the C. elegans labial-like gene ceh-13 is expressed in cells of many different tissues and lineages and that the rostral boundary of its expression domain is anterior to those of the other Hox genes. By transposon-mediated mutagenesis, we isolated a zygotic recessive ceh-13 loss-of-function allele, sw1, that exhibits an embryonic sublethal phenotype. Lineage analyses and immunostainings revealed defects in the organization of the anterior lateral epidermis and anterior body wall muscle cells. The epidermal and mesodermal identity of these cells, however, is correctly specified. ceh-13(sw1) mutant embryos also show fusion and adhesion defects in ectodermal cells. This suggests that ceh-13 plays a role in the anterior organization of the C. elegans embryo and is involved in the regulation of cell affinities.

The outer-sphere electron-transfer reactions between diastereomers of Ru(menbpy)3^•+ (menbpy = 4,4‘-di{(1R,2S,5R)-(−)-menthoxycarbonyl}-2,2‘-bipyridine) and enantiomers of Co(acac)3 and Co(edta)^- have been studied by pulse radiolysis. Δ-Ru(menbpy)3^•+ rapidly reduces Co(acac)3 in 85% EtOH/H2O (1 mM NaH2PO4) with second-order rate constants of (2.1 ± 0.1) × 10⁷ and (7.8 ± 0.2) × 10⁶ M^-1 s^-1 for the Δ- and Λ-Co(acac)3 enantiomers, respectively, and an enantioselectivity factor (ket^Δ/ket^Λ) of 2.7. Λ-Ru(menbpy)3^•+ preferentially reduces Λ-Co(acac)3 with an enantioselectivity factor (ket^Δ/ket^Λ) of 0.8. Activation volume data (ΔV^‡) suggest that the association between the Δ−Δ isomers in the encounter complex allows closer interaction of the metal centers than between the other isomer combinations. The value of ket^Δ/ket^Λ for the reaction of Δ- and Λ-Co(edta)^- with Δ-Ru(menbpy)3^•+ is 1.2. Electron-transfer reactions of seven racemic Ru(L)3^•+ (L = substituted phenanthroline) complexes with Co(acac)3 were also studied and gave rate constants of ≈1.5 × 10⁹ M^-1 s^-1. The quenching of photoexcited *Ru(menbpy)3²⁺ by Co(acac)3 and Co(edta)^- exhibits small stereoselectivity: For Co(acac)3 in 95 and 85% EtOH/H2O the enantioselectivity factor is 1.2 and 1.1, respectively, barely outside the experimental error. For all other cases the selectivity was unity within the experimental error of the measurement. The quenching rate constants were ≈1 × 10⁸ and 1.1 × 10⁹ M^-1 s^-1 for Co(acac)3 and Co(edta)^-, respectively. Quenching reactions of seven racemic ruthenium(II) phenanthroline complexes with Co(acac)3 were also studied and found to be faster than those of Ru(menbpy)3²⁺ by only a factor of ≈3 despite an increase in the driving force of ≈0.5 eV for electron-transfer quenching. The quenching of *Ru(menbpy)3²⁺ by Co(acac)3 is dominated by an energy-transfer mechanism. This conclusion is supported by the magnitude of the quenching rate constants compared with the rate constants for reduction by Ru(menbpy)3^•+, the effect of driving-force changes on the quenching rate constant, the low quantum yield of Co(II) products observed in the CW photolysis, and the lack of long-lived products observed in the flash photolysis experiments. The factors responsible for the selectivity exhibited in the CW photolysis studies of Ru(menbpy)3²⁺ with Co(acac)3 are discussed.

1Epidemiological and experimental data have shown that homocysteine may provoke vascular lesions and that moderate homocysteinaemia may constitute an independent risk factor for vascular disease. It is now documented that homocysteine damages human endothelial cells in culture, possibly by producing hydrogen peroxide in an oxygen‐dependent reaction.2In this study, we have examined the direct effect of this sulphur amino acid on pancreatic vascular resistance. Experiments were performed on the vascular bed of the rat isolated pancreas perfused at constant pressure; thus, any change in pancreatic vascular resistance resulted in a change in the flow rate. D,L‐Homocysteine perfused for one hour at three different concentrations (200 μM, 2 mM, 20 mM) did not induce any significant change in the flow rate per se. Homocysteine infusion for 30 min at a concentration of 200 μMor 2 mMabolished the endothelium‐dependent vasodilatation induced by acetylcholine (0.05 μM), but did not modify adenosine (1.5 μM)‐induced vasodilatation.3The effect of D,L‐homocysteine (200 μMor 2 mM) cannot be ascribed to a direct antimuscarinic effect since 30 min pretreatment of rat ileum with these concentrations did not significantly change the contractile effect of increasing concentrations of acetylcholine (0.015–15 μM).4Preincubation of human umbilical vein endothelial cells with D,L‐homocysteine (0.2–5.0 mM) had no significant effect on overall cell number or viability during 18 h of incubation; the endothelial cells exposed to concentrations up to 5 mMexhibited a spindle‐shaped, whirled pattern. This pattern was reversed 48 h after the removal of homocysteine. A cytotoxic effect was seen after 18 h incubation in 10 mMD,L‐homocysteine.5In conclusion, an acute infusion of homocysteine altered acetylcholine endothelium‐induced vasodilatation, whereas the adenosine vasodilatator effect was insensitive to the deleterious action of homocysteine in vitro.

Comparative biophysical and biochemical analyses were performed on purified preparations of hepatitis B antigen (HBs Ag) subtypes adw and ayw, including isoelectric pH evaluations, analysis of the different morphological forms, molecular weight determinations, and analysis of the polypeptides by polyacrylamide gel electrophoresis, Both HBs Ag-positive plasma and purified HBs Ag were analyzed by electrofocusing in a sucrose ampholyte gradient. Four distinct populations of HBs Ag with a pH range of 4.5 plus or minus 0.1 to 5.4 plus or minus 0.1 for unfractionated plasma samples and 3.9 plus or minus 0.05 to 4.9 plus or minus 0.05 for purified samples were detected in both adw and ayw preparations. Electron microscopic studies of each population of purified HBs Ag revealed 19- to 27-nm spheres in each fraction. Purified material labeled with 125I by the chloramine-T method behaved as one major population with an isoelectric pH value of 3.9 plus or minus 0.1. Purified adw preparations revealed a major population with a molecular weight of 3.7 times 10-6 and a second one of 4.6 times 10-6. Purified preparations of ayw contained one population with a molecular weight of 4.6 times 10-6. Polyacrylamide gel electrophoretic analysis of purified HBs Ag revealed nine polypeptides for ayw and seven for adw particles. These studies indicate that purified preparations of HBs Ag are heterogeneous and that distinct differences can be detected between the two subtypes.

Clusters of homeobox-containing HOM-C/hox genes determine the morphology of animal body plans and body parts and are thought to mediate positional information. Here, we describe the onset of embryonic expression of ceh-13, the Caenorhabditis elegans orthologue of the Drosophila labial gene, which is the earliest gene of the C. elegans Hox gene cluster to be activated in C. elegans development. At the beginning of gastrulation, ceh-13 is asymmetrically expressed in posterior daughters of anteroposterior divisions, first in the posterior daughter of the intestinal precursor cell E and then in all posterior daughters of the AB descendants ABxxx. In this paper, we present evidence that supports position-independent activation of ceh-13 during early C. elegans embryogenesis, which integrates cell fate determinants and cell polarity cues. Our findings imply that mechanisms other than cell-extrinsic anteroposterior positional signals play an important role in the activation and regulation of the C. elegans Hox gene ceh-13.

Tannic acid mordanting during fixation of isolated vesicles from skeletal muscle enhanced the resolution of the images. Isolated triadic junctions displayed two characteristic features not previously described: (a) a clear gap separated terminal cisternae from transverse tubules; (b) this gap was bridged by a separating array of structures which resembled the "feet" of intact muscle. When the triad was broken in a French press and subsequently reassembled by joining the two organelles, a similar gap was seen but the structure of the feet was less well defined. When the membrane of the triad was extracted by Triton X-100, the junctional region was retained and a similar gap between the two organelles could be discerned. The terminal cisternae characteristically displayed a thickening of the cytoplasmic leaflet of the membrane in select areas in which electron-dense material was apposed on the luminal leaflet. This thickened membrane was not observed in longitudinal reticulum or in terminal cisternae regions distal to the electron-dense matter. This thickened leaflet was not invariably associated with the junction, and some junctional regions did not display discernible thickening of the membrane. When the triad was treated with KCl, the electron-dense aggregate was dispersed and the thickened leaflet of the terminal cisternae dissipated, whereas the triadic junctional region with its feet remained unchanged. KCl treatment caused dissolution of three proteins of Mr = 77,000, 43,000, and 38,000. Treatment of Triton-resistant vesicles with KCl caused the loss of electron-dense aggregate but did not otherwise influence the appearance of the junction. A good degree of correlation both qualitatively and in quantitative parameters between the isolated vesicles and the intact muscle was observed.

A monoclonal antibody has been developed against the putative junctional protein or spanning protein (SP) from skeletal muscle triads. By immuno-affinity chromatography, we have purified this protein. The native protein has a molecular mass of 630-800 kD, as determined by gel filtration and rate zonal centrifugation. Within the limits of the methods used, the basic unit of the SP appears to be a dimer. In electron micrographs, it is shown to exhibit a circular profile with a diameter of approximately 100 A. In thin section analysis, the protein is frequently observed as parallel tracks of electron-dense particles bordering a translucent core. We suggest that the basic unit of the junctional structure is a dimer of 300-kD subunits and that four such entities constitute the intact SP. The purified protein has been used to develop polyclonal antibodies. By immunoelectron microscopy using immunogold probes, the SP has been localized to the junctional gap of the triad. By attaching the SP to an affinity resin, three proteins have been identified as forming associations with the SP. The Mrs of the proteins are 150, 62, and 38 kD; the 62-kD protein is calsequestrin.