Chemical trends in the optical properties of rocksalt nanoparticles

The nature and magnitude of the optical gaps of rocksalt alkaline earth (MgO, CaO, SrO, MgS, MgSe) and transition metal chalcogenide (CdO, PbS) nanoparticles are studied using time-dependent density functional theory (TD-DFT) calculations on (MX)32 nanoparticles. We demonstrate, just as we previously showed for MgO, that TD-DFT calculations on rocksalt nanoparticles require the use of hybrid exchange–correlation (XC-)functionals with a high percentage of Hartree–Fock like exchange (e.g. BHLYP) or range-separated XC-functionals to circumvent problems related to the description of charge-transfer excitations. Concentrating on the results obtained with TD-BHLYP we show that the optical gap in rocksalt nanoparticles displays a wide range of behavior; ranging from large optical gaps stemming from a localized excitation involving corner atoms in alkaline earth oxides to a delocalized excitation and small optical gaps in the transition metal chalcogenides. Finally, we rationalize this wide range of behaviour in terms of differences in the degree to which the Coulombic interaction between the excited electron and hole is screened in the different nanoparticles, and relate it to the optical dielectric constants of the bulk materials the nanoparticles are made from

Hydrogen Evolution by Polymer Photocatalysts; a Possible Photocatalytic Cycle

We propose, supported by ab-initio calculations, a possible photocatalytic cycle for hydrogen evolution by a prototypical polymer photocatalyst, poly(p-phenylene), in the presence of a sacrificial electron donor. As part of that cycle we also introduce a family of related sites on the polymer that in the absence of a noble metal co-catalyst can facilitate the evolution of molecular hydrogen when the polymer is illuminated. The bottlenecks for hydrogen evolution, electron transfer from the sacrificial electron donor and hydrogen-hydrogen bond formation, are discussed, as well as how they could potentially be improved by tuning the polymer properties and how they change in the presence of a noble-metal co-catalyst

Hydrogen evolution from water using heteroatom substituted fluorene conjugated co-polymers

The photocatalytic performance of fluorene-type polymer photocatalysts for hydrogen production from water in the presence of a sacrificial hole scavenger is significantly improved by the incorporation of heteroatoms into the...</p

Evaluation of the similarity of gene expression data estimated with SAGE and Affymetrix GeneChips

BACKGROUND: Serial Analysis of Gene Expression (SAGE) and microarrays have found awidespread application, but much ambiguity exists regarding the evaluation of these technologies. Cross-platform utilization of gene expression data from the SAGE and microarray technology could reduce the need for duplicate experiments and facilitate a more extensive exchange of data within the research community. This requires a measure for the correspondence of the different gene expression platforms. To date, a number of cross-platform evaluations (including a few studies using SAGE and Affymetrix GeneChips) have been conducted showing a variable, but overall low, concordance. This study evaluates these overall measures and introduces the between-ratio difference as a concordance measure pergene. RESULTS: In this study, gene expression measurements of Unigene clusters represented by both Affymetrix GeneChips HG-U133A and SAGE were compared using two independent RNA samples. After matching of the data sets the final comparison contains a small data set of 1094 unique Unigene clusters, which is unbiased with respect to expression level. Different overall correlation approaches, like Up/Down classification, contingency tables and correlation coefficients were used to compare both platforms. In addition, we introduce a novel approach to compare two platforms based on the calculation of differences between expression ratios observed in each platform for each individual transcript. This approach results in a concordance measure per gene (with statistical probability value), as opposed to the commonly used overall concordance measures between platforms. CONCLUSION: We can conclude that intra-platform correlations are generally good, but that overall agreement between the two platforms is modest. This might be due to the binomially distributed sampling variation in SAGE tag counts, SAGE annotation errors and the intensity variation between probe sets of a single gene in Affymetrix GeneChips. We cannot identify or advice which platform performs better since both have their (dis)-advantages. Therefore it is strongly recommended to perform follow-up studies of interesting genes using additional techniques. The newly introduced between-ratio difference is a filtering-independent measure for between-platform concordance. Moreover, the between-ratio difference per gene can be used to detect transcripts with similar regulation on both platforms

Apparent Scarcity of Low-Density Polymorphs of Inorganic Solids

For most inorganic solids, very few dense polymorphs and no low-density polymorphs are observed. Taking a wide range of tetrahedrally-coordinated binary solids (e.g., ZnO, GaN) as a prototypical system, we show that the apparent scarcity of low- density polymorphs is not due to significant structural or energetic limitations. Using databases of periodic networks as sources of novel crystal structures, followed by ab initio energy minimization, we predict a dense spectrum of low-density low-energy polymorphs. The diverse range of materials considered indicates that this is likely to be a general phenomenon

Controlling Visible Light-Driven Photoconductivity in Self-Assembled Perylene Bisimide Structures

Alanine-functionalized perylene bisimides (PBI-A) are promising photoconductive materials. PBI-A self-assembles at high concentrations (mM) into highly ordered wormlike structures that are suitable for charge transport. However, we previously reported that the photoconductive properties of dried films of PBI-A did not correlate with the electronic absorption spectra as activity was only observed under UV light. Using transient absorption spectroscopy, we now demonstrate that charge separation can occur within these PBI-A structures in water under visible light. The lack of charge separation in the films is shown by DFT calculations to be due to a large ion-pair energy in the dried samples which is due to both the low dielectric environment and the change in the site of hole-localization upon drying. However, visible light photoconductivity can be induced in dried PBI-A films through the addition of methanol vapor, a suitable electron donor. The extension of PBI-A film activity into the visible region demonstrates that this class of self-assembled PBI-A structures may be of use in a heterojunction system when coupled to a suitable electron donor

Color Differences Highlight Concomitant Polymorphism of Chalcones

The meta- and para-nitro isomers of (E)-3′-dimethylamino-nitrochalcone (Gm8m and Gm8p) are shown to exhibit concomitant color polymorphism, with Gm8m appearing as yellow (P2_{1}/c) or orange (P1̅) crystals and Gm8p appearing as red (P2_{1}/n) or black (P2_{1}/c) crystals. Each of the polymorphs was characterized optically via UV–vis spectroscopy, and their thermal behavior was characterized via differential scanning calorimetry and low-temperature powder X-ray diffraction. To assess the effect of molecular configuration and crystal packing on the colors of crystals of the different polymorphs, time dependent density functional theory (ωB97x) calculations were carried out on isolated molecules, dimers, stacks, and small clusters cut from the crystal structures of the four polymorphs. The calculated color comes from several excitations and is affected by conformation and most intermolecular contacts within the crystal, with the color differences between polymorphs mainly being due to the differences in the π–π stacking. The visual differences between these related polymorphic systems make them particularly useful for studying polymorph behavior such as phase transitions and concomitant polymorph growth

Photocatalytic proton reduction by a computationally identified, molecular hydrogen-bonded framework

We show that a hydrogen-bonded framework, TBAP-α, with extended π-stacked pyrene columns has a sacrificial photocatalytic hydrogen production rate of up to 3108 μmol g^{−1} h^{−1}. This is the highest activity reported for a molecular organic crystal. By comparison, a chemically-identical but amorphous sample of TBAP was 20–200 times less active, depending on the reaction conditions, showing unambiguously that crystal packing in molecular crystals can dictate photocatalytic activity. Crystal structure prediction (CSP) was used to predict the solid-state structure of TBAP and other functionalised, conformationally-flexible pyrene derivatives. Specifically, we show that energy–structure–function (ESF) maps can be used to identify molecules such as TBAP that are likely to form extended π-stacked columns in the solid state. This opens up a methodology for the a priori computational design of molecular organic photocatalysts and other energy-relevant materials, such as organic electronics

Water oxidation with cobalt-loaded linear conjugated polymer photocatalysts

The first examples of linear conjugated organic polymer photocatalysts that produce oxygen from water after loading with cobalt and in the presence of an electron scavenger are reported. The oxygen evolution rates, which are higher than for related organic materials, can be rationalized by a combination of the thermodynamic driving force for water oxidation, the light absorption of the polymer, and the aqueous dispersibility of the relatively hydrophilic polymer particles. We also used transient absorption spectroscopy to study the best performing system and we found that fast oxidative quenching of the exciton occurs (picoseconds) in the presence of an electron scavenger, minimizing recombination