An old dog with new tricks: Schiff bases for liquid crystals materials based on isoxazolines and isoxazoles.

Abstract

Schiff bases are in action again showing an exuberant mesophase range despite their chemical and thermal weakness. Two series of Schiff bases (SB), based on isoxazolines 9 a–f and isoxazoles 10 a–f, are described. 9 a–f were synthesized by a [3+2] cycloaddition 1,3‐dipolar of arylnitrile oxide from p‐nitrobenzaldehyde (1) and alkenes. And, subsequently oxidized to 10 a–f by MnO2. The isoxazolines and isoxazoles thus obtained were reduced to aniline derivatives and condensed with appropriated arylaldehydes. The SBs series isoxazolines 9 a–f presented a narrow mesophase range, while SBs series isoxazoles 10 a–f showed a large mesophase range. Nematic mesophase was observed for SBs with short and non‐polar groups, while SmA and SmC mesophase for SBs with long and more polar terminal groups. The stability of the SBs is also dependent on the clearing temperature. For 10 a–f with high clearing temperature, decomposition induced by heat was observed during the first cycle of heating and in solutions of CDCl3. Series 9 a–f was more resistant to the thermal decomposition.

The use of isoxazoline and isoxazole scaffolding in the design of novel thiourea and amide liquid-crystalline compounds

“…All analogous compounds such as thiourea 18a,b, containing an isoxazole ring, displayed enantiotropic SmA and N mesophases, confirming that the isoxazole ring favors the formation of a stable mesophase. It is interesting to notice that the nematic mesophase appeared only in compounds with the isoxazole ring (18a, 18c and 20), confirming that a longitudinal diffusion is favored for this type of LC compounds [19]. In addition, for amide 19, a second smectic mesophase was identified by POM as the monotropic SmC phase ( Table 1, Figure 2).…”

“…In addition, isoxazolines and isoxazoles have been identified as promising templates in liquid crystal development, mainly due to their structural and electronic features. Isoxazoline and isoxazole cores show strong dipole moments, polarizabilities, anisotropic interactions and geometrical aspects that favor the formation of stable smectic A and nematic mesophases, respectively [19,20]. This work aimed at synthetizing new thioureas and amides by the installation of mesogenic cores derived from isoxazoline and isoxazole scaffolds and 4-n-alkoxybenzoic acid with liquid crystal properties.…”

“…The cycloadducts were subjected to SnCl 2 reduction to yield amines 4, 9 and 13 containing an isoxazoline ring. The amines 3, 8 and 12 containing an isoxazole ring were synthesized in two sequential steps -first a MnO 2 oxidation step transforming the isoxazoline ring to an isoxazole ring followed by SnCl 2 reduction, according to protocols previously published [19,[21][22][23][24].…”

An Optically Isotropic Antiferroelectric Liquid Crystal (OI‐AFLC) Display Mode Operating over a Wide Temperature Range using Ternary Bent‐Core Liquid Crystal Mixtures

“…A second binary mixture formulated by using a bent‐core LC (structurally similar to BC1 ) and 40 wt % of a rod‐like nematic phenylbenzoate LC derivative (mixture II , see Figure S1) lowered the crystallization temperature on cooling even further (phase sequence of that mixture on cooling: Iso 84 °C N 68 °C SmCP A 16 °C Cr), but again failed to display a sufficiently high enough tilt (tilt angle: 30°) . A single‐component bent‐core compound ( III in Figure S1) reported in 2004 formed the desired SmCP A phase over an astonishingly wide temperature interval of 121 °C, including room temperature (22 °C SmCP A , 143 °C Iso on heating), but did not provide the optically isotropic state, as it required fields that were too high for switching and contained (similar to mixture I above) the chemically and thermally comparatively unstable Schiff‐base segments, rendering these materials unsuitable for practical LCD devices.…”

Design of new Schiff base polymers containing thiadiazole rings and study of their liquid crystalline behavior

“…However, research related to the liquid crystalline properties of SBs were started with the discovery of N -(4-methoxybenzylidene)-4-butylaniline and its derivatives (Figure 1) which exhibits a nematic phase at room temperature. 11 -13…”