Non-symmetric dimers: effects of varying the mesogenic linking unit and terminal substituent

Abstract

Six new non-symmetric dimers belonging to the family of compounds the (R,S)-(benzylidene-4ʹ-substituted-aniline)-2ʺ-methylbutyl-4ʹʹʹ-(4ʹʹʹʹ-phenyloxy) benzoate-oxy) hexanoates have been prepared and their thermal behaviour characterised. The terminal substituents on the 4-benzylideneaniline fragment are H, CH₃, C₂H₅, Cl, Br and I. All six dimers exhibit an enantiotropic nematic phase, and the efficiency of the terminal substituent in enhancing the nematic–isotropic transition temperature is largely consistent with that established for conventional low molar mass liquid crystals. This trend is discussed in terms of the size of the terminal substituent and its effect on the shape of the molecule. The halogen-substituted and ethyl-substituted dimers also exhibit smectic behaviour. For the halogen-substituted dimers, the driving force for smectic phase formation is considered to be, at least in part, the specific anisotropic interaction between the unlike mesogenic units. In contrast, for the ethyl-substituted dimers, the driving force is thought to be the enhanced molecular inhomogeneity arising from the longer terminal chains. The properties of these new dimers are broadly similar to those of the corresponding dimers in which the ester linkage between the spacer and mesogenic unit is an ether link. This strongly suggests that the ether- and ester-linked materials have similar average molecular shapes.

Keywords:

• liquid crystal dimers
• non-symmetric dimers
• intercalated smectic phases
• terminal substituents
• spacer linking unit

Additional information

Funding

One of us (G.-Y. Yeap) would like to thank Universiti Sains Malaysia for providing the research facilities and financial support under RU Research Grant [Account No. 203/PKIMIA/6711192] and PRGS Postgraduate Research Grant [Account No. 1001/KIMIA/835010].