Handedness inversion

Overcrowded alkenes are a special class of olefins with an inherently helical shape, resulting from extensive steric crowding around the central carbon-carbon double bond (Fig. 5.4b) [30]. Related molecules were first reported by Feringa and coworkers, who used the chiral molecules as chiral dopant in CLC materials [31]. The merit of these molecules as chiral dopants is that the photoisomerization between the two isomers with pseudoenantiomeric relationship often results in the handedness inversion of the cholesteric LCs. 

The handedness inversions induced by a single azobenzene chiral dopant have also been reported due to their dramatic shape change and the subtle interactions with host molecules. Some examples of the dopant are shown in Fig. 5.10. Axially... 

The handedness inversion behaviors upon UV irradiation were observed in both wedge cells and homeotropic cells with the cholesteric (N )—nematic (N)— cholesteric (N ) phase transition sequence as confirmed by the Cano s lines and fingerprint texmres (Fig. 5.12). The mechanism of the helix inversion is proposed... 

Fig. 5.12 Handedness inversion of 0.27 mol % (S, R, S)-17 in 5CB observed in wedge cell (a-c) and homeotropic cell (d-f) from right-handed CLC (a, d) to left-handed CLC (c, f) through transient nematic phase (b, e). Reproduced with permission from [90]. Copyright 2013 John Wiley Sons...

Handedness Inversion with Overcrowed Alkene Dopants... 

Apart from the HTP tuning for phase transition and color change, a binaphthyl-derived diarylethene dopant was demonstrated inducing the handedness inversion behavior by Akagi et al. [119]. (R)-26 with two un-bridged binapthyls can induce a right-handed CLC at its open form, while could be switched to left-handed upon... 

Fig. 5.21 Handedness inversion induced by diarylethene dopant 25. Reproduced with permission from [119] Copyright 2012 American Chemical Society...

UV irradiation (Fig. 5.21). It is believed that the reversible handedness inversion originated from the conformation change of binapthyl moieties as well as the stereospecific intermolecular interactions between the binaphthyl moieties and the surrounding nematic LC molecules. This is the first example of diaryIthene dopant capable of handedness inversion in CLCs although it requires specific LC mixture host and only shows low HTPs. 

M. Mathews, R.S. Zola, S. Hurley, D.-K. Yang, T.J. White, T.J. Banning, Q. Li, Light-driven reversible handedness inversion in self-organized helical superstructures. J. Am. Chem. Soc. 132, 18361-18366 (2010)... 

No matter what kind of molecules is used, the purpose is to efficiently control the properties of CLCs by light. In the following sections, the different types of light-induced behavior in CLCs such as light-induced phase transition, handedness inversion, pitch length change, and the related reflection color control are introduced. 

LIGHT-INDUCED HANDEDNESS INVERSION IN CHOLESTERIC LIQUID CRYSTALS... 

Handedness Inversion in Azobenzene-doped CLCs In 2002, Ruslim and Ichimura reported that the compound 19 (Fig. 5.16) can induce the helix inversion when doping into LC hosts having alkenyl substituents [73]. Upon UV irradiation, the handedness of the cholesteric helix changed from right-handed to left-handed, whieh is most fikely to be the consequence of the competition between different interconvertible rotational species of the chiral alkyl chain relative to the azobenzene core. It is also worth noting here that the handedness inversion was only observed in LC hosts having alkenyl terminal substituents due to the alkenyl-selective interaction... 

The above-described considerations have been confirmed by the different behaviors of films consisting of either an even or an odd number of smectic layers, and by electro-optic switching studies which show the spontaneous formation of macroscopic chiral domains possessing opposite handedness. An important point is that the inversion of the spontaneous polarization, induced by the inversion of an applied field, occurs preferably by the inversion of the tilt direction and not by an inversion of the chiral handedness (inversion of b). 

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