The ferroelectric nematic phase: on the role of lateral alkyloxy chains

The synthesis and characterisation of the 4-({[4'-nitro-(1,1'-biphenyl)-4-yl]oxy}carbonyl)phenyl 2-alkoxy-4-methoxybenzoates are reported: the 1O(m-On)PEPEBNO2 series. These materials include an additional phenyl ring compared to the extensively studied ferroelectric nematogen RM734, added to increase the liquid crystal transition temperatures. All members of the series exhibited both conventional nematic (N) phase and ferroelectric nematic (N-F) phases; T-NI and T-NFN both decrease on increasing n, the length of the lateral alkoxy chain. The properties of this four-ring series are compared with analogous three-ring laterally substituted variants of RM734; the addition of the extra phenyl ring has a significantly more pronounced effect on the value of T-NI than on T-NFN. The increase in T-NI may be attributed to the enhanced structural anisotropy and more favourable intermolecular interactions arising from the insertion of the phenyl ring, whereas the much weaker effect on T-NFN may reflect a change in the shape of the molecule. We also report two materials with three-ring structures, the 4'-nitro-(1,1'-biphenyl)-4-yl 2-alkoxy-4-methoxybenzoates: 1O(m-On)PEBNO2 (n = 1 and 2). The removal of the ester linkage between RM734 and 1O(m-O1)PEPNO2 extinguishes the N-F phase and this appears to be consistent with a model in which the molecules are described in terms of a longitudinal surface charge density wave.

Automated summary

The synthesis and characterisation of a new liquid crystal material with an additional phenyl ring, aimed at increasing the transition temperature, is reported. This four-ring structure exhibits conventional nematic and ferroelectric nematic phases. Compared to similar three-ring structures, the addition of the extra phenyl ring has a significant effect on the transition temperature of the conventional nematic phase. This effect may be attributed to the enhanced structural anisotropy and more favorable intermolecular interactions resulting from the insertion of the phenyl ring.