Design of liquid crystals with ‘de Vries-like’ properties: the effect of an ethynyl spacer in the core structure

ABSTRACT

We investigate the origins of ‘de Vries-like’ liquid crystalline behaviour by introducing an ethynyl spacer in the core of the tricarbosilane-terminated 5-phenylpyrimidine mesogens QL16-6 and QL17-6. The rationale for this structural change is based on the assumption that an ethynyl spacer would create more free volume in the core sub-layer and therefore decrease the orientational order parameter S₂ in the SmA phase. The tricarbosilane-terminated mesogens WL41-5 and WL42-6 with a 5-(phenylethynyl)pyrimidine core in either a normal or inverted orientation were synthesised, and their mesomorphic and ‘de Vries-like’ properties characterised using polarised optical microscopy, differential scanning calorimetry, birefringence measurements, small–angle (SAXS) and 2D X-ray scattering. Reduction factors R derived from SAXS and optical tilt angle measurements suggest that neither WL41-5 (R = 0.49) nor WL42-6 (R = 0.80) exhibit ‘de Vries-like’ properties. The S₂(T) profiles show an increase in orientational order with decreasing temperature and a sharp discontinuity at the SmA-SmC transition, which is consistent with ‘de Vries-like’ behaviour. However, the L_eff(T) profiles suggest an increase in interdigitation that reduces the positive effect of increasing S₂ in compensating for the molecular tilt.

Graphical Abstract

KEYWORDS:

• Liquid crystal
• tricarbosilane
• de Vries-like
• orientational order
• effective length
• interdigitation

Acknowledgements

We thank the Natural Sciences and Engineering Research Council of Canada, the Deutsche Forschungsgemeinschaft (NSF/DFG Materials World Network program DFG Gi 243/6) and the Carl‐Schneider Stiftung Aalen (shared instrumentation grant) for support of this work.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary Material

Supplementary data for this article can be accessed here.

Additional information

Funding

We thank the Natural Sciences and Engineering Research Council of Canada, the Deutsche Forschungsgemeinschaft (NSF/DFG Materials World Network program DFG Gi 243/6) and the Carl‐Schneider Stiftung Aalen (shared instrumentation grant) for support of this work.