School of Chemistry - Research Publications
Now showing items 1-12 of 915
Electrical conductivity and defect disorder of tantalum-doped TiO2
The present work reports the electrical properties of polycrystalline Ta‐doped TiO2 (0.39 at.% Ta) determined in situ at elevated temperatures (1173‐1323 K) in the gas phase of controlled oxygen activity (10−12 Pa to 105 Pa). The effect of oxygen activity on the electrical conductivity and thermoelectric power of TiO2 is discussed in terms of defect disorder, including (1) the intrinsic electronic disorder that is governed by electronic compensation in the strongly reducing regime, (2) the extrinsic electronic disorder that is governed by electronic charge compensation in the reducing regime, and (3) the extrinsic ionic disorder that is governed by ionic compensation in the oxidizing regime. It is shown that tantalum ions are incorporated into the titanium sublattice of TiO2 leading to the formation of donor‐type energy levels. The Arrhenius‐type plot of the electrical conductivity data leads to the determination of the formation enthalpy terms. The obtained results are considered in terms of the effect of tantalum and oxygen activity on the defect disorder and the associated key performance‐related properties in the light‐induced partial water oxidation.
Halogen Substitution Effects on N2O Schiff Base Ligands in Unprecedented Abrupt Fe-II Spin Crossover Complexes
(WILEY-V C H VERLAG GMBH, 2017-05-23)
A family of halogen-substituted Schiff base iron(II) complexes, [FeII (qsal-X)2 ], (qsal-X=5-X-N-(8-quinolyl)salicylaldimines)) in which X=F (1), Cl (2), Br (3) or I (4) has been investigated in detail. Compound 1 shows a temperature invariant high spin state, whereas the others all show abrupt spin transitions, at or above room temperature, namely, 295 K (X=I) up to 342 K (X=Br), these being some of the highest T1/2 values obtained, to date, for FeII N/O species. We have recently reported subtle symmetry breaking in [FeII (qsal-Cl)2 ] 2 with two spin transition steps occurring at 308 and 316 K. A photomagnetic study reveals almost full HS conversion of [FeII (qsal-I)2 ] 4 at low temperature (T(LIESST)=54 °K). The halogen substitution effects on the magnetic properties, as well as the crystal packing of the [FeII (qsal-X)2 ] compounds and theoretical calculations, are discussed in depth, giving important knowledge for the design of new spin crossover materials. In comparison to the well known iron(III) analogues, [FeIII (qsal-X)2 ]+ , the two extra π-π and P4AE interactions found in [FeII (qsal-X)2 ] compounds, are believed to be accountable for the spin transitions occurring at ambient temperatures.
Selective Manipulation of Discrete Mannosidase Activities in the Endoplasmic Reticulum by Using Reciprocally Selective Inhibitors
(WILEY-V C H VERLAG GMBH, 2017-06-01)
Within the endoplasmic reticulum, immature glycoproteins are sorted into secretion and degradation pathways through the sequential trimming of mannose residues from Man9 GlcNAc2 to Man5 GlcNAc2 by the combined actions of assorted α-1,2-mannosidases. It has been speculated that specific glycoforms encode signals for secretion and degradation. However, it is unclear whether the specific signal glycoforms are produced by random mannosidase action or are produced regioselectively in a sequenced manner by specific α-1,2-mannosidases. Here, we report the identification of a set of selective mannosidase inhibitors and development of conditions for their use that enable production of distinct pools of Man8 GlcNAc2 isomers from a structurally defined synthetic Man9 GlcNAc2 substrate in an endoplasmic reticulum fraction. Glycan processing analysis with these inhibitors provides the first biochemical evidence for selective production of the signal glycoforms contributing to traffic control in glycoprotein quality control.
Tunable Porous Coordination Polymers for the Capture, Recovery and Storage of Inhalation Anesthetics
The uptake of inhalation anesthetics by three topologically identical frameworks is described. The 3D network materials, which possess square channels of different dimensions, are formed from the relatively simple combination of ZnII centres and dianionic ligands that contain a phenolate and a carboxylate group at opposite ends. All three framework materials are able to adsorb N2O, Xe and isoflurane. Whereas the framework with the widest channels is able to adsorb large quantities of the various guests from the gas phase, the frameworks with the narrower channels have superior binding enthalpies and exhibit higher levels of retention. The use of ligands in which substituents are bound to the aromatic rings of the bridging ligands offers great scope for tuning the adsorption properties of the framework materials.
A strong cis-effect in an imidazole/imidazolium substituted alkene
(John Wiley & Sons Ltd., 2017-07-10)
We report the first example of an alkene with two carbon‐bound substituents (imidazole and imidazolium rings) where the Z‐isomer has a greater thermodynamic stability than the E‐isomer which persists in both the gas phase and in solution. Theoretical calculations, solution fluorescence spectroscopy and gas‐phase ion mobility mass spectrometry studies confirm the preference for the Z‐isomer, the stability of which is traced to a non‐covalent interaction between the imidazole lone pair and the imidazolium ring.
Photo- and Electronically Switchable Spin-Crossover Iron(II) Metal-Organic Frameworks Based on a Tetrathiafulvalene Ligand
(WILEY-V C H VERLAG GMBH, 2017-05-08)
A major challenge is the development of multifunctional metal-organic frameworks (MOFs), wherein magnetic and electronic functionality can be controlled simultaneously. Herein, we rationally construct two 3D MOFs by introducing the redox active ligand tetra(4-pyridyl)tetrathiafulvalene (TTF(py)4 ) and spin-crossover FeII centers. The materials exhibit redox activity, in addition to thermally and photo-induced spin crossover (SCO). A crystal-to-crystal transformation induced by I2 doping has also been observed and the resulting intercalated structure determined. The conductivity could be significantly enhanced (up to 3 orders of magnitude) by modulating the electronic state of the framework via oxidative doping; SCO behavior was also modified and the photo-magnetic behavior was switched off. This work provides a new strategy to tune the spin state and conductivity of framework materials through guest-induced redox-state switching.
A QCM-D and SAXS Study of the Interaction of Functionalised Lyotropic Liquid Crystalline Lipid Nanoparticles with siRNA
Biophysical studies were undertaken to investigate the binding and release of short interfering ribonucleic acid (siRNA) from lyotropic liquid crystalline lipid nanoparticles (LNPs) by using a quartz crystal microbalance (QCM). These carriers are based on phytantriol (Phy) and the cationic lipid DOTAP (1,2‐dioleoyloxy‐3‐(trimethylammonium)propane). The nonlamellar phase LNPs were tethered to the surface of the QCM chip for analysis based on biotin–neutravidin binding, which enabled the controlled deposition of siRNA–LNP complexes with different lipid/siRNA charge ratios on a QCM‐D crystal sensor. The binding and release of biomolecules such as siRNA from LNPs was demonstrated to be reliably characterised by this technique. Essential physicochemical parameters of the cationic LNP/siRNA lipoplexes—such as particle size, lyotropic phase behaviour, cytotoxicity, gene silencing and uptake efficiency—were also assessed. The SAXS data show that when the pH was lowered to 5.5 the structure of the lipoplexes did not change, thus indicating that the acidic conditions of the endosome were not a significant factor in the release of siRNA from the cationic lipidic carriers.
Guest-induced Assembly of Bis(thiosemicarbazonato) Zinc(II) Coordination Nanotubes
A ZnII complex of the dianionic tetradentate ligand formed by deprotonation of glyoxal‐bis(4‐phenyl‐3‐thiosemicarbazone) (H2gtsp) is a [3+3] trinuclear triangular prism. Recrystallization of this complex in the presence of either CO2, CS2, or CH3CN leads to the formation of [4+4] open‐ended charge‐neutral tetranuclear coordination nanotubes, approximately 2 nm in length and with internal dimensions large enough to accommodate linear guest molecules, which serve to template their formation. Upon removal of the templating molecules the nanotubes demonstrated reversible sorption of CO2 with an isosteric enthalpy of sorption of 28 kJ mol−1 at low loading.
Selectivity Effects in Bimetallic Catalysis: Role of the Metal Sites in the Decomposition of Formic Acid into H-2 and CO2 by the Coinage Metal Binuclear Complexes [dppmMM(H)](+)
Design of new bimetallic catalysts requires an understanding of how cooperative effects of the metal sites influences reactivity. Here we show how switching one or both of the silver atoms in binuclear silver hydride cations, [dppmAg2(H)]+ (dppm=1,1‐Bis(diphenylphosphino)‐methane), with all combinations of copper and/or gold maintains selective dehydrogenation of formic acid, enhancing reactivity by up to 2 orders of magnitude. This is a key step in the selective, catalyzed extrusion of carbon dioxide from formic acid, HO2CH, with important applications in hydrogen storage and in situ generation of H2. Decarboxylation of [dppmMM′(O2CH)]+ through collision induced dissociation regenerates [dppmMM′(H)]+. DFT calculations provide insights into these cooperative effects. The copper homobinuclear catalyst performs best overall.