Electro‐optical switches based on polymer and dendrimer filled nematics

Two‐phase systems which can be switched electrically between a light scattering and a transparent state can be prepared based on either a dispersion of well‐defined sub‐micron sized crosslinked polymeric particles in a liquid crystal (LC) matrix (Polymer Filled Nematics, PFN's) or on a dispersion of palmitoyl‐functionalised poly(propylene imine) dendrimers in an LC (Dendrimer Filled Nematics). The present paper describes the preparation of both systems and their properties. The PFN's can be electrically switched between a scattering and a transparent state by an appropriate choice of materials and refractive indices. The preparation of the disperse polymeric phase is separated from the preparation of the polymer/LC blend, which enables control over the morphology of the system. Rather surprisingly, it is found that, due to the rheological properties of the PFN blends, stable films of the blends can be simply produced by conventional coating processes. The dendrimer‐stabilised nematics require extremely low switching voltages to switch from a scattering to a highly transparent state. Moreover, the switching process is totally reversible and hysteresis effects appear to be absent.     

Enantioselective room temperature phosphorescence detection of non-phosphorescent analytes based on interaction with beta-cyclodextrin/1-bromonaphthalene complexes

Menthol (MT) induces strong room temperature phosphorescence (RTP) of 1-bromonaphthalene (1BrN) in aqueous β-cyclodextrin (β-CD) suspensions, even under non-deoxygenated conditions. Interestingly, (−)-MT and (+)-MT enantiomers give rise to different phosphorescence intensities, the difference being 19 ± 3%. It is argued that the signal can be mainly ascribed to the formation of ternary complexes β-CD/1BrN/MT which show different RTP lifetimes, i.e. 4.28 ± 0.06 and 3.71 ± 0.06 ms for (−)-MT and (+)-MT, respectively. Most probably, the stereochemical structure of (−)-MT provides a better protection of 1BrN against quenching by oxygen than (+)-MT. This interpretation is in line with the observation that under deoxygenated conditions the phosphorescence intensity difference for the two complexes becomes very small, i.e. only about 4%.The lifetime difference under aerated conditions enables the direct determination of the MT stereochemistry. For mixtures, in view of the 0.06 ms uncertainty in the lifetime, enantiomeric purity can be determined down to 10%. Furthermore, in the case of MT the concentration of the least abundant enantiomer should be at least 3 × 10⁻⁴ M, since otherwise complex dissociation would obscure the lifetime difference.     

Pre-organization induced synthesis of a crossed alkene-bridged nisin Z DE-ring mimic by ring-closing metathesis

An alkene-bridged mimic of the complex DE-bisthioether-ring system of the antibiotic nisin was prepared in one step from the linear precursor.     

Thermochemical behavior of mixtures ofN,N-dimethylformamide with dimethylsulfoxide,acetonitrile, andN-methylformamide: Volumes and heat capacities

Densities and molar heat capacities have been measured for mixtures ofN,N-dimethylformamide with dimethylsulfoxide, acetonitrile, andN-methylformamide at 25°C over the complete mole fraction range. From these data the apparent molar volumes and heat capacities have been calculated for both components. These quantities, as a function of the mole fraction, deviate very little from their molar values, indicating that the mixtures can be regarded as almost ideal.     

High-speed imaging of an ultrasound-driven bubble in contact with a wall: “Narcissus” effect and resolved acoustic streaming

We report microscopic observations of the primary flow oscillation of an acoustically driven bubble in contact with a wall, captured with the ultra high-speed camera Brandaris 128 (Chin et al. 2003). The driving frequency is up to 200 kHz, and the imaging frequency is up to 25 MHz. The details of the bubble motion during an ultrasound cycle are thus resolved, showing a combination of two modes of oscillations: a radius oscillation and a translation oscillation, perpendicular to the wall. This motion is interpreted using the theory of acoustic images to account for the presence of the wall. We conclude that the bubble is subjected to a periodic succession of attractive and repulsive forces, exerted by its own image. Fast-framing recordings of a tracer particle embedded in the liquid around the particle are performed. They fully resolve the acoustic streaming flow induced by the bubble oscillations. This non-linear secondary flow appears as a tiny drift of the particle position cycle after cycle, on top of the primary back and forth oscillation. The high oscillation frequency accounts for a fast average particle velocity, with characteristic timescales in the millisecond range at the lengthscale of the bubble. The features of the bubble motion being resolved, we can apply the acoustic streaming theory near a wall, which provides predictions in agreement with the observed streaming velocity.     

Natural Abundance 13C-NMR. of Reduced Isoalloxazine

When an isoalloxazine derivative ( 1 ) was reduced ( 2 ) its ¹³C-NMR. resonances were all shifted upfield and the largest shifts were observed for C (4a), C(6) and C(8) but downfield shifts for C(2), C(4) and C(10a). The results and their biochemical implications are shortly discussed.     

Substitution Effects on the Photophysical Characteristics of the Salicylic Anion

Spectral and photophysical properties of substituted salicylic anions are investigated by steady-state and time-resolved fluorescence spectroscopy for a variety of electron donating substituents at the positions para to the hydroxyl and carboxylic groups. Next to the usual excited-state intramolecular proton transfer in aqueous solution, an excited-state intermolecular proton transfer is found to be responsible for the dual emission observed in the case of 5-aminosalicylic anions.