Reversed twist effect

If the electrodes of a cell which have been pretreated by rubbing are further coated with a layer of surfactant (e.g., lecithin), the resulting orientation of the molecules of the nematic liquid crystal will be homeotropic. A cell formed in such a way that the directions of initial rubbing are perpendicular to each other transmits light under parallel polars. In an electric field a nematic liquid crystal with negative dielectric anisotropy (e.g., MBBA) must reorient itself into a twist structure, and transmission through the cell decreases to zero (reverse twist effect) [51]. 

Ab initio calculations at the MP2 level of theory of a collection of substituted 1,3-dioxanes, 1,3-oxathianes, and 1,3-dithianes have been employed to study both the position of the conformational equilibria and the validity of the Perlin effect <2005T7349>. The 7h,c coupling constant proved to be a valuable tool in conformational analysis both twist conformers, in addition to the chair and alternative chair forms, could be readily identified simply by comparing experimental Vh,c coupling constants to the corresponding calculated values in the particular forms. In addition, the Perlin and reversed-Perlin effects of the C2-H fragments, c in 1,3-dioxanes,... 

In contrast to what has been proposed by Sinnott (11), this ab initio study has shown the absence of the reverse anomeric effect in 81 (see Section II.I). Therefore, a-glycosides on protonation need not change their conformation toward the twist-boat to place the alkyloxonium group OHRin the pseudoequatorial position prior to a heterolysis of the C(2)—OHMebond. 

This transient effect manifests itself in a direct way in the behaviour of a twisted nematic cell (see 3.4.2). When the external field (assumed to be sufficiently strong) is switched off, the light transmission shows an optical bounce effect , i.e., it does not decrease monotonically but rises again to a peak before decaying to its off value. Calculations have confirmed that the peak in transmission corresponds approximately to a perpendicular alignment of the director in the central portion of the cell. This is caused by fluid motion, which also gives rise to a reverse-twist. ... 

The majority of current commercial liquid crystal displays are based on the twisted nematic electrooptic effect using active matrices to give, for example, complex computer lap-top screens [13]. It is often overlooked that these displays are based on long pitch chiral nematic materials to remove so-called reverse twist, and the original papers [254, 255] referred to the effect as a positive planar cholesteric to nematic phase... 

The fluorescence lifetime of trans-4-[4-(dimethylamino-styryl]-l-methylpyridinium iodide trapped in water-containing AOT-reversed micelles has been found to be markedly influenced by R, implying a significant effect on its excited-state twisting motion [122],... 

These results are compatible with stereochemical predictions derived through orbital symmetry theory, assuming a one-step n2s - -n2a addition. But secondary deuterium kinetic isotope effects on the allene plus allene thermal (2+2) cycloaddition seem to require a two-step mechanism with formation of an intermediate 44>, and as Moore and coworkers fully realized 83> stereoselective formation and reactions of 2,2 -biallylene intermediates will equally well account for the product ratios. In their rationale, two allenes approach and distort through simultaneous conrotatory twistings to give the perpendicular 2,2 -biallylene intermediate, which closes to form products in a disrotatory fashion. The experimentally observed stereochemical selectivity is equally compatible with a reversed order of rotatory motions disrotatory joining of two allenic reactants followed by conrotatory closure to create the 1,2-dimethylenecyclobutane products 83>. 

The colour of ICPs can also be sensitive to temperature. Substituted polythiophenes, in particular, show a marked blue shift of the highest wavelength absorption band when films or solutions are heated [7-9]. These reversible colour changes have been attributed to a twisting of the polymer backbone to a less ordered non-planar conformation. Less dramatic thermochromic effects have also been reported for polyanilines [3,10,11] where circular dichroism studies of... 

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