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 <i>n</i> = 0–7. The central aromatic ring is either singly or doubly fluorinated at positions <i>ortho</i> to the terminal 3,4,5-trifluorophenyl to give the series D(<i>n</i>)IO-F13 and D(<i>n</i>)IO-F23. All homologues except D(<i>n</i>)IO-F13 and D(0,1,2)IO-F23 show conventional paraelectric nematic phases, and homologues of F13 with <i>n</i> = 1–4 and F23 with <i>n</i> = 1–5 exhibit the ferroelectric nematic phase, N_F, including rare enantiotropic examples (<i>n</i> = 1–2 and <i>n</i> = 1–3, respectively). The antiferroelectric N_X phase is also observed in homologues <i>n</i> = 2–5 and <i>n</i> = 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 T_NI and T_SmA-N but enhances T_NF- and T_NX- with respect to their F13 counterparts and this is interpreted in terms of changes to molecular shape anisotropy and enhanced intermolecular interactions. Extension of <i>n</i> leads to rapid destabilisation of the N_F and N_X phases, though the effect is not as pronounced in the F23 series such that N_F persists for longer <i>n</i>.