Twisted chiral

An X-ray analysis 34a) of ( ),( ),( )-l,5,9-cyclododecatriene (38) suggested that this molecule is most comfortable in a twisted chiral conformation of D3 symmetry, and a dynamic NMR study and a force-field calculation Mb) estimated AG = 8.6 kcal/mol for the racemization process between two enantiomeric D3 conformers. A pseudochair conformation of similar D3 symmetry was also suggested35) for 1,5,9-cyclodo-decatriyne (39), following an ab initio STO-3G calculation and photoelectron spectroscopy. 

The twisted, chiral diene 112 undergoes photosensitized [4+2] cycloadditions but only with heteroalkenes (Sch. 25). Napthoquinone 113... 

Figure 7). Osawa s recent force-field calculation study (21) of cage-shaped molecules with ethano bridges affords an excellent demonstration of this twist deformation at the molecular level. In six achiral cage-shaped molecules so far studied, his calculations showed that each molecule assumed a twisted, chiral conformation to minimize torsional and nonbonding strain. Tricyclo[4.2.2.2.2,s]dodecane (5) was shown to be 1.1 kcal/mol more stable in a twisted D2 than in the eclipsed D2h conformation, and his calculation also suggests that perhydrotriquinacene and C16-hexaquinane should assume C3 rather than conformations, contrary to naive pictures obtained by a casual observation of molecular models. 

The linear electro-optic effect in a cholesteric, i.e. a hard-twisted chiral nematic (the helical pitch must be less than the wavelength of visible light) was a very original proposal for using the flexoelectric effect in a new display, shutter or modulator device. The patent application by R.B. Meyer and J.S. Patel dates from 1987 and was granted in 1990. The physics was developed in a series of papers by these authors and later elaborated by others.It is now commonly called the flexoelectro-optic effect. 

Fig. 7.3. The deviation of the optic axis in a cholesteric (hard-twisted chiral nematic, p <, where p is the cholesteric pitch and A is the wavelength of light) when an electric field E is applied perpendicular to the helical axis. The cholesteric geometry allows a fiexoelectric polarization to be induced in the direction of E. The plane containing the director, which is perpendicular to the page in the middle figure and is shown in the lower figure, illustrates the splay-bend distortion and the corresponding polarization that arises. (After Rudquist, inspired by Meyer and Patel.

The twisted structure of 4,5-dimethylphenanthrene (1) contributes to the chirality of the molecule. It was recognized early on that the ability to resolve the two enantiomers could provide supporting evidence for the nonplanarity of the aromatic system [1]. Determinations of the rates of racemization and the activation barriers of twisted chiral polyarenes have been actively pursued. Resolutions of the enantiomers to allow these investigations to proceed were achieved in several cases. In other cases, variable-temperature nuclear magnetic resonance (NMR) experiments were employed to provide insights into the configurational stabilities of the molecules. For practical applications, such as using these twisted compounds... 

Enantiopure thiahelicenes can transfer their molecular chirahty to the whole phase of an achiral liquid crystal phase thus acting as dopant systems. Coupling the analysis of CD spectra with the study of cholesteric meso-phases induced in nematic liquid crystals (LC), a model has been proposed for the transfer of chirality from thiahehcene (Al)-114 (Figure 13) to the whole liquid crystal phase (cholesteric induction) (1996JO2013). The dopant thiahehcene presents a twisted chiral surface, which is homochiral with the induced cholesterics as a consequence of the interaction of its... 

High contrast and uniformity of transmission characteristics at oblique incidence are also beneficial features of the SBE mode which considerable improve the legibility of supertwist displays. Better viewing angles than that for the 90° twist structure seem to be a peculiar feature of highly twisted chiral nematics. Figure 4.23 demonstrates this for a 200° supertwist cell in comparison with the usual 90° twist cell [127]. 

M. Ambrozic and S. Zumer, Axially twisted chiral nematic structures in cylindrical cavities, Phys. Rev. E 59, 4153-4160 (1999). 

The analog of the magnetic intensity H (produced by external currents) would be an external field coupled to the two components of curl n. A local microscopic source of bend [23] is not easy to imagine. On the other hand, chirality is naturally coupled to twist chiral mesogens develop a spontaneous twist in the nematic phase. The cholesteric is thus analogous to a normal metal in a magnetic field. 

This compound has been demonstrated to give highly twisted chiral nematic phases when used as a dopant and shows a helix in-... 

Some of these materials display the potentially chiral ferroelectric SmC phase in addition to a SmA or cholesteric mesophase. Pyzuk [108] found for copper and nickel complexes some blue phase or novel type of amorphous phase between a tightly twisted chiral nematic phase and the isotropic liquid. These ferroelectric metallomesogens are interesting as they can be aligned par-... 

Sugimura A, Luckhurst GR, On-Yang Z (1995) Director deformation of a twisted chiral nematic liquid crystal cell with weak anchraing boundaries. Phys Rev E 52(l) 681-689 Torrent MC, Sagues F, Arias F, San Miguel M (1988) Freedericksz transition in a periodic magnetic field. Phys Rev A 38(5) 2641-2649... 

FIGURE 1 The geometry of the twisted chiral nonatic cell located betweai two planes X3 = 0 and X3 = [4]. 

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