Tuning the mesogenic properties of 5-alkoxy-2-(4-alkoxyphenyl)pyrimidine liquid crystals: the effect of a phenoxy end-group in two sterically equivalent series

Abstract

Two sterically equivalent series of phenoxy-terminated 5-alkoxy-2-(4-alkoxyphenyl)pyrimidine liquid crystals were synthesised, and their mesogenic properties were characterised by polarised optical microscopy and differential scanning calorimetry (DSC). The phenoxy end-group causes a significant increase in melting point and inhibits – at least partially – the mesomorphism of these materials relative to the parent isomers; in most cases, the broad enantiotropic SmC phase formed by the parent isomers is suppressed by the addition of the phenoxy end-group. However, detailed analyses by small-angle X-ray scattering and monodomain 2D X-ray scattering suggest that these compounds form a SmA phase with a partially intercalated bilayer structure in which the phenoxy end-groups are nanosegregated. Such an intercalated bilayer structure might enable the tuning of smectogenic properties by appropriate substitution of the phenoxy end-groups.

Keywords:

• smectic liquid crystal
• sterically equivalent series
• nanosegregation
• phenoxy end-group
• orientational order parameter
• effective length

Acknowledgements

We thank the Natural Sciences and Engineering Research Council of Canada (Discovery and CREATE grants), the Deutsche Forchungsgemeinschaft (NSF/DFG Materials World Network program DFG Gi 243/6), Westgrid and Compute Canada for supporting this work.

Notes

1. Recent 2D X-ray scattering analyses of smectic monodomains have shown that ‘de Vries-like’ organosiloxane liquid crystals such as QL1-7 exhibit unusually large molecular tilt fluctuations, and that the layer contraction due to molecular tilts at the SmA-SmC transition is almost fully compensated by an increase in orientational order as the tilt fluctuations decrease with decreasing temperature.[40]

2. The degree of ‘de Vries-like’ behaviour in a smectic liquid crystal may be quantified on a scale of 0 (perfect ‘de Vries’) to 1 (conventional SmA-SmC transition) at a given temperature T below T_AC by the reduction factor R according to the equation:

where δ(T) is the tilt angle required to give the observed layer contraction d_C(T)/d_AC assuming a model of hard spherocylinders in which the layer contraction scales with the cosine of the tilt angle, and θ_opt is the observed tilt angle measured by polarised optical microscopy.[50]. In the present case, the reduction factor for QL11-10/6 at T – T_AC = −10 K is ~1, assuming an experimental error of ±0.1 Å for the SAXS measurements.

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