Liquid crystal and photophysical properties of laterally fluorinated azo-ester materials

ABSTRACT

Some laterally fluorinated three-benzene-ring molecular core of azo-ester compounds bearing 2-methylbutoxy unit as the terminal chain were prepared to investigate the effects of terminal substituents on the liquid crystal properties where different opposite terminal groups are introduced. The opposite terminal side substituent, R, is a polar group that alternatively changes between the electron-donating group (OCH₃ or OC₄H₉) and electron-withdrawing group (Br or NO₂), including the terminally unsubstituted compound. The structures of all synthesised compounds were confirmed by Fourier transform infrared spectroscopy, ¹H and ¹³C nuclear magnetic resonance spectroscopy, and elemental analysis. Their mesomorphic properties were studied by differential scanning calorimetry, polarised optical microscopy, and small- and wide-angle X-ray scattering. Mesomorphic studies revealed that all derivatives were liquid crystalline materials, except for the unsubstituted terminal derivative (F1). When the electron-donating group is attached to the terminal phenyl ester moiety, the analogues (F2 and F3) were enantiotropically nematogenic. Terminal substitution with the electron-withdrawing bromine atom (F4) induced the nematic-to-nematic (N-N_X) phase transition upon cooling, while the nitro group (F5) exhibited an enantiotropic nematic phase. The terminally nitro-substituted analogue F5 showed the highest bathochromic shift UV-vis absorption bands due to the stronger electron-withdrawing inductive effect of NO₂ terminal group.

Graphical abstract

KEYWORDS:

• azo-ester
• lateral fluoro-substitution
• electron-withdrawing substituent
• electron-donating substituent
• mesomorphic

Acknowledgments

Salwa J. Kamal would like to thank Imam Abdulrahman Bin Faisal University and the Saudi Arabian Cultural Mission for their invaluable support.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

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Funding

This work was supported by the Universiti Malaya Research University Grant-Faculty Program [GPF060B-2018].