Phase behaviour of ester-linked cyanobiphenyl dimers and fluorinated analogues: the direct isotropic to twist-bend nematic phase transition

Abstract

ABSTRACT Typically, the twist-bend nematic (NTB) phase appears below the temperature of the conventional nematic (N) phase. The direct formation of the NTB phase from the isotropic (Iso) phase is significantly rare. In this study, we discovered that certain ester-linked bent-shaped dimers exhibit a direct Iso–NTB phase transition. We synthesised two homologous series of ester-linked cyanobiphenyl-based dimers (COOn) and their fluorinated analogues (F-COOn) containing different numbers of carbon atoms (n) in the alkyl spacer chains (n = 3, 5, 7, 9, 11, and 13). The phase transitions of these dimers were investigated using polarised optical microscopy and differential scanning calorimetry. The long-spacer COOn (n = 7, 9, 11, and 13) exhibited the usual Iso–N–NTB phase sequence, whereas the short-spacer COOn (n = 3 and 5) showed the direct Iso–NTB phase transition. Lateral fluorination significantly reduced the phase-transition temperatures for F-COOn compared to COOn. The short-spacer F-COOn (n = 3 and 5) did not exhibit any liquid crystal phases. The F-COOn with longer spacers (n = 9, 11, and 13) displayed the Iso–N–NTB sequence, whereas F-COO7 showed the direct Iso–NTB transition. Additionally, we analysed a single-crystal structure of F-COO7 to investigate its molecular conformation and superimposition.