Aspects of Molecular Symmetry for Chiral Nematic Phases

1 Aspects of Molecular Symmetry for Chiral Nematic Phases 

Having defined the different forms of chiral nematogen structure with regard to the positioning of the chiral moiety relative to the liquid-crystalline core unit, it is now necessary to describe further and more precisely the exact nature of this structural relationship. It has been understood for many years that optical properties, such as helical twist sense and the direction of rotation of plane polarised light, depend intimately on the absolute spatial configuration of the chiral centre, the distance the chiral centre is sep- 

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- 

config...tS - parity (m+l)...2 (even, e) rotation of plane polarized light...d twist sense. .LH. 

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