Molecular structure and the ferroelectric nematic phase: the effect of terminal chain and fluorination pattern

Synthesis and characterisation of mesogenic behaviour are reported for two homologous series of molecules based on DIO ([3-fluoro-4-(3,4,5-trifluorophenyl)phenyl] 2,6-difluoro-4-(5-propyl-1,3-dioxan-2-yl)benzoate). In both series, the terminal alkyl chain is extended from n = 0–7. The central aromatic ring is either singly or doubly fluorinated at positions ortho to the terminal 3,4,5-trifluorophenyl to give the series D(n)IO-F13 and D(n)IO-F23. All homologues except D(n)IO-F13 and D(0,1,2)IO-F23 show conventional paraelectric nematic phases, and homologues of F13 with n = 1–4 and F23 with n = 1–5 exhibit the ferroelectric nematic phase, NF, including rare enantiotropic examples (n = 1–2 and n = 1–3, respectively). The antiferroelectric NX phase is also observed in homologues n = 2–5 and n = 3–5, respectively, and is enantiotropic in all cases for the latter. Longest homologues show conventional SmA phases below the N phase. The additional F atom in the F23 materials acts to reduce TNI and TSmA-N but enhances TNF- and TNX- with respect to their F13 counterparts and this is interpreted in terms of changes to molecular shape anisotropy and enhanced intermolecular interactions. Extension of n leads to rapid destabilisation of the NF and NX phases, though the effect is not as pronounced in the F23 series such that NF persists for longer n.