Bent-core dopant

Stabilization of BP consisting of bent-core N with chiral dopant was also investigated [29-31]. One of the most dramatic results is that BPIII is easily induced by adding a very small amount of chiral dopant such as 1% [29]. The BPIII temperature range was more than 20°. If BSMs show the N phase at room temperature, the BPIII phase over 20° including room temperature is easily realized [30]. Since BSMs have low compared with Kn [25, 32], the effect of elastic constant on the BP stabilization is confirmed [28]. 

Fig. 6 Reciprocal optical pitch as a function of dopant content. Host material is a cholesterol derivative used in the enhanced chirality in N with bent-core molecules [5]...

Spontaneous resolved two chiral domains are formed in equal probability. In other words, enantiomeric excess (ee) is zero. We now ask whether ee can be controlled or not. The answer is yes. Several methods used in bent-shaped mesogenic phases will be introduced. The direct method is of course an addition of chiral dopant. Actually this has been shown to be a viable method [6, 61]. Use of chiral surface is also effective [62], By using polyimide with chiral side chains at both substrate surfaces, imbalance of two chiral domains (10% ee) has been achieved. Another method using macroscopic helical structure was demonstrated by Jakli et al. [63]. They used a nonchiral polymer network, which was formed in the N phase. After the polymerization, N compounds were washed out, then bent-core mesogens were introduced. Because of the polymer helical fibers, bent-core mesogen shows a chiral domain. 

H.-C. Jeong, K.V. Le, M.-J. Gim, S.-T. Hur, S.-W. Choi, F. Araoka, K. Ishikawa, H. Takezoe, Transition between widened BPs by light irradiation using photo-active bent-core liquid crystal with chiral dopant. J. Mater. Chem. 22, 4627-4630 (2012)... 

Many bent-core molecules do not have nematic phases, but may be added to calamitic nematics to tune the flexoelectric behaviour. Several groups have reported measurement results in such guest-host systems ° using either HAN cells or the flexoelectro-optic effect mentioned above. It was found that some BC compounds used as dopants in small (< 10%) concentration may effectively increase ei —es of the host. ° Other, mainly asymmetric, BC compounds had hardly any influence on ei - - es of the host nematic. ... 

In addition to the compounds listed in the tables, several other molecules have also been tested, however, only as a small concentration of dopants in some host nematic material. A linear extrapolation from these data might provide values for the ffexocoefficients of the neat dopant however, the applicability of this procedure to many compounds is questionable (especially for bent-core molecules) therefore these data are not included in the tables. 

It is weU known that E Z photoisomerization of azobenzene-containing LC molecules can lead to a nematic-to-isotropic transition as well as photochromism [167]. It was reported that UV irradiation of a nematic mixture doped by chiral azobenzene bent-core LCs leads to the N -I transition and shift in selective reflection band position of the N phase [177]. These chiral bent-core molecules can induce the N helix upon doping them into a nematic base mixture, and their helical twisting power (HTP) is given by P = UPC where P is the helical pitch length, and C is the concentration of a chiral dopant. The P value for the exclusively E isomer is maximum and decreases with the increase in the ratio of the Z isomer. UV irradiation causes E Z conversion, therefore increasing the helical pitch and shifting the selective reflection band of the N phase. 

Beyond NLO and display applications, bent-core molecules have found many other applications. Azobenzene-containing chiral bent-core molecules are good N dopants... 

Chirality can also be introduced when one or more chiral carbons are incorporated in the molecules, for example in the hydrocarbon terminal chains [67, 68], within the bent-core [69], or by addition of chiral dopants [6, 70], It was noted during the early research that the handedness of the homochiral structures is very sensitive to chiral dopants [6], or even on chiral surfaces [71]. On the other hand, it was observed that banana-smectics made of enantiomeric chiral molecules form synclinic - antiferroelectric [44] and anticlinic ferroelectric [67] domains. This combination of tilt and polar order implies that the phase is racemic, with a rigid alternation of right- and left-handed chiral layers. This shows that the molecular chirality has no or minor effect on deciding about anticlinic or synclinic packing (which is mainly determined by entropic reasons), but it can bias the otherwise degenerate tilt directions. 

Experiments by Binet et al. [72], carried out on chiral bent core materials containing biphenyl (BP) cores with S or R hydrocarbon chains, and on achiral biphenyl core molecules with chiral dopants, reveal the effect of the molecular chirality on the polarization stmcture. In addition the polarization Pb due to the closed packing of the bent-shape molecules, another polarization, P is introduced due to the chiral and tilted molecular structure. It was found [72] that in the antiferroelectric racemic domains at low fields, of the synclinic - racemic... 

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