The effect of molecular structure on mesophase behaviour of non-symmetric liquid crystal dimers containing mandelic acid and fluorine group

ABSTRACT

A series of non-symmetric dimers with the same chiral core, mandelic acid (MA), have been synthesised by varying the terminal group and the rigidity of the molecules, termed QBBMA-3F3BT, QBMA-3F3BT, QBMA-3F2BT, 3F3BMA-BBCN, 3F2BMA-BBCN and 3F1BMA-BBCN. Chemical structures and liquid crystal (LC) properties of the dimers were characterised by Fourier transform infrared spectroscopy, ¹H NMR, polarised optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermogravimetric analysis (TG). The six dimers displayed different optical activity (in THF). QBBMA-3F3BT and QBMA-3F3BT displayed enantiotropic chiral smectic A (S_mA*) phase. QBMA-3F2BT displayed N phase. The T_m of QBMA-3F3BT is the highest among QBBMA-3F3BT, QBMA-3F3BT and QBMA-3F2BT, which indicated that molecular conformation played an important role on T_m besides molecular weight and molecular rigidity. 3F3BMA-BBCN and 3F2BMA-BBCN displayed chiral nematic (N*), which indicated that the mesogenic arm BBCN is beneficial to the formation of N* phase. The LC range of 3F2BMA-BBCN is wider, which may be due to that the similar length of the two LC arms is advantageous to the interaction between C≡N group and other molecules so as to stabilise the LC phase.

Graphic Abstract

KEYWORDS:

• Liquid crystal dimer
• fluorinated liquid crystal
• mandelic acid
• chiral smectic A
• nematic
• chiral nematic phase

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was financially supported by National Natural Science Foundation of China (2180529). Z. X. Xu acknowledges the financial support from the China Scholarship Council (Grant 201806080123). Fundamental Research Funds for the Central Universities [N180705004].