Cornelis A. van Walree

The determination of first hyperpolarizabilities β using hyper-Rayleigh scattering : a caveat

Abstract A comparison of first hyperpolarizabilities β determined by hyper-Rayleigh scattering (HRS; β HRS ) and electric field-induced second-harmonic generation (EFISH; β EFISH ) for a series of donor-acceptor substitutetd π-conjugated organic compounds shows a marked discrepancy between β HRS and β EFISH values for NLO-chromophores which fluoresce in the frequency-doubled wavelength region (532 nm). For MONS and DANS it is unequivocally established that two-photon absorption (TPA) at the fundamental wavelength (1064 nm) occurs followed by Stokes and anti-Stokes fluorescence around the second-harmonic wavelength. Hence, hyper-Rayleigh scattering at 1064 nm is not suited to determining the β of these fluorescent NLO-chromophores.

The styrene-maleic acid copolymer: : a versatile tool in membrane research

A new and promising tool in membrane research is the detergent-free solubilization of membrane proteins by styrene–maleic acid copolymers (SMAs). These amphipathic molecules are able to solubilize lipid bilayers in the form of nanodiscs that are bounded by the polymer. Thus, membrane proteins can be directly extracted from cells in a water-soluble form while conserving a patch of native membrane around them. In this review article, we briefly discuss current methods of membrane protein solubilization and stabilization. We then zoom in on SMAs, describe their physico-chemical properties, and discuss their membrane-solubilizing effect. This is followed by an overview of studies in which SMA has been used to isolate and investigate membrane proteins. Finally, potential future applications of the methodology are discussed for structural and functional studies on membrane proteins in a near-native environment and for characterizing protein–lipid and protein–protein interactions.

Molecular three-terminal devices: fabrication and measurements

Incorporation of a third, gate electrode in the device geometry of molecular junctions necessary to identify the transport mechanism. At present, the most popular technique fabricate three-terminal molecular devices makes use of electromigration. Although it statistical process, we show that control over the gap resistance can be obtained. A detailed analysis of the current-voltage characteristics of gaps without molecules, however, shows that they reveal features that can mistakenly be attributed to molecular transport. This observation raises questions about which gaps with molecules can be disregarded which not. We show that electrical characteristics can be controlled by the rational design of the molecular bridge and that vibrational modes probed by electrical transport are potential interest as molecular fingerprints.

Alkyl sulphonic acid surface-functionalised silica as heterogeneous acid catalyst in the solvent-free liquid-phase addition of acetic acid to camphene

In search for heterogeneous Bronsted acid catalysts as alternatives for corrosive liquid acid catalysts, silica’s were surfacefunctionalised with linear alkyl chains containing thiol and thioacetic ester end-groups, which were subsequently converted into sulphonic acids by oxidation with H2O2/HOAc. FT-IR, CP/MAS 13 C NMR, XPS, TGA and acid–base titrations established the formation of a surface-bound monolayer of alkyl sulphonic acid groups. The catalytic properties of the surface-functionalised silica’s were evaluated in the solvent-free liquid-phase hydro-acyloxyaddition of acetic acid (1) to camphene (2), yielding the pine-fragrance isobornyl acetate ( 3). Reaction rates (normalised on the concentration of sulphonic acid groups) were identical to those found using the homogeneous CH3SO3H reference catalyst. The equal rates suggest that a similar reaction mechanism is operative. The reaction rate with the solid surface-functionalised silica catalysts increased by two orders of magnitude upon addition of a small amount of water. In the presence of water the alcohol analogue of 3, i.e. isoborneol (15), is an important intermediate. Generation of homogeneous alkyl sulphonic acids via hydrolysis followed by leaching from the silica surface does not occur; the formation of 3 stopped after removal of the solid catalyst from the reaction mixture by filtration. Thus, the alkyl sulphonic acid functionalised silica’s act as proper heterogeneous Bronsted acid catalysts.

Lanthanide-doped CaS and SrS luminescent nanocrystals : A single-source precursor approach for doping

The incorporation of dopants with optical or magnetic functionalities into colloidal nanocrystals (NCs) has been a longstanding challenge for nanomaterial research. A deeper understanding of the doping kinetics will aid a better control of the doping process. In particular, alkaline-earth sulfides are an important class of host materials for a range of luminescent dopants, including transition-metal and lanthanide ions. Their nanocrystalline analogues have many potential applications. However, the lack of synthetic methodologies hampers their development. Here we introduce a single-source precursor approach that successfully leads to Ce(3+)- and Eu(2+)-doped CaS and SrS luminescent NCs with diameters of ∼10 nm and with luminescent properties similar to those of the bulk analogues. The characteristic absorption and luminescence of Ce(3+) and Eu(2+) depend on the local coordination and are applied to probe dopant ion internalization. We demonstrate that controlling the reactivity of the precursors is crucial for achieving effective doping. By designing the chemical structure of the dopant precursor to vary the reactivity relative to that of the host precursor, the doping efficiency can be controlled. In addition, we have applied a growth doping strategy to further improve internalization of the dopants. Finally, we demonstrate nucleation doping as an alternative method to achieve lanthanide NC doping for dopant and host precursors with strongly different reactivities. The single-source precursor approaches proposed here allow for a flexible design of synthesis strategies and have the potential to be widely applicable to the doping of colloidal chalcogenide NCs with transition-metal and lanthanide dopant ions.

Intramolecular excimer and exciplex emission of 1,4-dipyrenyl substituted cyclohexasilane

Intramolecular excimer emission is observed for cis-1,4-di(1-pyrenyl)decamethylcyclohexasilane in nonpolar solvents. Time-resolved fluorescence spectroscopy and kinetic modelling indicate that the driving force of excimer formation is very small, and that the process is governed by the flexibility of the silicon ring. In the polar solvent acetonitrile, photoinduced electron transfer occurs, with the cyclohexasilane ring acting as electron donor and the pyrenyl group as electron acceptor. 2002 Elsevier Science B.V. All rights reserved.

Transmission of electronic effects via a SiMe2 spacer in 4-mono- and 4,4′-disubstituted diphenyldimethylsilanes: 29Si and 13C NMR spectroscopy and PM3 semi-empirical calculations

Abstract Some 29Si and 13C NMR studies were performed on series of 4-substituted phenyltrimethylsilanes and 4-substituted and 4,4′-disubstituted diphenyldimethylsilanes. Within each series linear relationships are found between the chemical shifts and (a) the Hammett substituent constants σp and (b) the PM3-calculated total atomic charges. It is inferred that the SiMe2 σ-type spacer weakly mediates substituent effects in the ground state via a bond polarization mechanism. A comparison with available data for 4-substituted and 4,4′-disubstituted diphenylmethanes shows that the SiMe2 group is a better transmitter of electronic effects than the CH2 group.

Twinning by merohedry in cyclohexanone oxime: a revised structure

The crystal structure of cyclohexanone oxime, C(6)H(11)NO, was reported as severely disordered in the trigonal non-centrosymmetric space group P3 [Olivato, Ribeiro, Zukerman-Schpector & Bombieri (2001). Acta Cryst. B57, 705-713]. Re-investigation of the crystal structure as twinned by merohedry in the trigonal centrosymmetric space group P-3, with a twofold rotation about [001] as twin law, resulted in a well ordered structure and low R values. The asymmetric unit contains three independent molecules, existing as a hydrogen-bonded trimer, having an R(3)(3)(9) graph set.

Second-order nonlinear optical properties of stilbene,benzylideneaniline and azobenzene derivatives. The effect of π-bridgenitrogen insertion on the first hyperpolarizability

The second-order nonlinear optical (NLO) and electronic properties of stilbenes (CC bridge), benzylideneanilines (CN and NC bridges) and azobenzenes (NN bridge) containing either an N,N-dimethylamino donor and/or a nitro acceptor were investigated using EFISH, UV spectroscopy, cyclic voltammetry and PPP/SCF calculations. It appeared that although first hyperpolarizabilities of the ethylene and azo bridged donor–acceptor compounds are of comparable magnitude, substitution of one carbon by a nitrogen atom reduces the NLO activity. Differences in hyperpolarizabilities were rationalized with the aid of a two-level model, which revealed that they find their origin in the redox activity of the nitrogen-containing bridges.

Pseudomerohedrally twinned crystal structure of 2,3-diphenylbuta-1,3-diene.

The title compound, C16H14, is twinned by reticular pseudomerohedry of twin index 2. The primitive monoclinic cell of the single crystal can be transformed into a B-centred pseudo-orthorhombic supercell with a fourfold volume. The twofold twin operation about the reciprocal a* axis of the primitive monoclinic cell is co-directional with the approximate C2 axis of the molecule and a symmetry element of the orthorhombic supercell. A tentative twin domain model is proposed.