Twist sense, chiral nematics

The twist sense of a chiral nematic phase can be determined, relatively easily, by observing contact preparations in the microscope [19]. A standard material of known twist sense is allowed to make contact with a chiral nematic of unknown twist direction and then the area of contact between the two compounds is observed in the microscope. If the two materials have the same twist sense, then the contact region will exhibit a chiral nematic (cholesteric) phase, but if... 

Subsequent experimentation by contact studies with other chiral systems (/ )- and (S)-, of differing parity revealed the following helical twist sense relationships, entries 1 to 4 in Table 1. These results form the basis of a set of empirical rules termed the Gray and McDonnell rules [32] and are particularly useful in predicting the properties of a chiral nematic phase of a given com-... 

As seen from Fig. 96b, the switching process is unambiguous as regards the motion of n and P (sterically bound to n) on the upper side of the chevron, P rotates counterclockwise, on the lower side it rotates clockwise when we switch from 1 to 2 everything turns around in the reverse switching direction. This explains why there are no twist and antitwist domains like the ones observed in twisted nematics prior to the time when chiral dopants were added in order to promote a certain twist sense. 

Recently, the importance of the structure of chiral metal complexes on the handedness of the mesophases induced in a nematic LC was exemplified [114]. The chiral metal complexes 10 and 11—in which the alkyl substituents are aligned almost perpendicularly to the C2 axis in the former and parallel in the latter—show very different induction phenomena. Not only are the induced helicities in the nematic LC of opposite sense for the two compounds, but the helical twisting power of 10 is much higher than that of 11. The reason for these differences is the way in which the molecules are incorporated into the host nematic phase and exert their force upon it to create the twist between the layers. 

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