The effect of a lateral alkyloxy chain on the ferroelectric nematic phase.

The synthesis and characterisation of two series of low molar mass liquid crystals, the 4-[(4-nitrophenoxy)carbonyl]phenyl 2-alkoxy-4-methoxybenzoates (series 5- m ) and the 4-[(3-fluoro-4-nitrophenoxy)carbonyl]phenyl 2-alkoxy-4-methoxybenzoates (series 6- m ) are reported in order to explore the effects of a lateral alkyloxy chain on the formation and stability of the recently discovered ferroelectric nematic phase. In both series m , the number of carbon atoms in the lateral chain, is varied from one to nine. The two series differ by the addition of a fluorine substituent in the 6- m series. 5-1 is the extensively studied ferroelectric nematogen RM734. All the members of the 5- m series exhibited both a conventional nematic, N, and ferroelectric nematic, N F , phase, whereas all the members of the 6- m series exhibit a direct N F -I transition with the exception of 6-1 that also exhibits a N phase. The replacement of a hydrogen atom by a fluorine atom reduces the nematic-isotropic transition temperature, T NI , whereas the ferroelectric nematic-nematic, or isotropic, transition temperature, T N F N/I , increases. This is interpreted in terms of the reduced structural anisotropy associated with the larger fluorine atom whereas the increase in the stability of the N F phase reflects changes in polarity and polarizability. The dependence of T NI and T N F N/I on m in both series is similar, and these initially decrease on increasing m but converge to limiting values on further increasing m . This suggests that the lateral alkyloxy chain may adopt conformations in which it lies along the major axis of the mesogenic unit.