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

Abstract

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-NX) 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.