Synthesis and properties of substituted benzoxazole-terminated liquid crystals

Abstract

The synthesis and characterisation, and mesomorphic and photophysical properties of a new type of benzoxazole-terminated heterocyclic compounds, 2-(4 -alkoxybiphenyl-4-yl)-benzoxazole derivatives (nB-x), are reported. For better understanding of the structure-property relationships, compounds bearing a variety of polar substituents in the 5-position of the benzoxazole moiety were prepared, including H (nB-H), NO 2 (nB-N), CH 3 (nB-M) and Cl (nB-C). Their phase transition behaviour was investigated by differential scanning calorimetry and polarising optical microscopy. In the case of carbon atoms in the alkoxy chain between 2 and 10 they exhibited enantiotropic smectic mesophases, for which the mesophase ranges were 101-175 • C and 159-182 • C on heating and cooling for nB-N, 5-63 • C and 19-63 • C for nB-H, 81-90 • C and 78-96 • C for nB-M, and 131-132 • C and 131-132 • C for nB-C, respectively. The effect of the length of the alkoxy chain on mesomorphic properties was studied. Compounds nB-N, nB-M and nB-C exhibited a much wider mesophase range either on heating or cooling than the corresponding nB-H series, which indicated that the substituent on the benzoxazole moiety was helpful in increasing the mesophase stability. Substituents with stronger electron-withdrawing properties gave a wider mesomorphic temperature domain, in the order NO 2 > Cl > CH 3 > H, which was similar to the effect of the substituents on the UV absorption bands of nB-x. The opposite effect was found for the corresponding Schiff base liquid crystals (nS-x). With the exception of nB-N, the nB-x series displayed intense photoluminescence emission in methylene chloride solution, with absorption maxima within the range 395-404 nm.

Preparation and properties of lateral monofluoro-substituted benzoxazole-based mesogenic compounds

“…[1] Benzoxazole derivatives have been developed as one kind of mesogenic heterocyclic compounds, including benzoxazole-based mesogenic [2][3][4][5][6][7] and metallomesogenic materials. [8][9][10] Since fluorine is a relatively small atom with a large electronegativity, fluorine atom has been widely introduced into the mesogenic core structure to alter the molecular polarity, and furthermore, to modify mesophase morphology and the physical properties of liquid crystalline materials.…”