Synthesis and Mesomorphic Properties of New Fluorinated Schiff Base Liquid Crystals

Two new compounds, 4-alkanoyloxybenzylidene-4′-fluoroaniline and 4-fluorobenzylidene-4′-n-alkanoyloxyaniline comprising a terminal fluorosubstituent were studied. The fluoro substituent contributes to the molecular polarizability, thus affecting intermolecular interactions and hence resulting in smectic mesomorphism. The mesomorphic properties were studied using differential scanning calorimetry and polarizing optical microscopy techniques. The mesomorphic properties of compounds studied are strongly dependent on the orientation of the imine (CH=N) linkage. The former Schiff base exhibited smectic A phase whereas the latter compound did not display any mesophase. Reversed imine linkage has caused depression of mesomorphic property in the compound studied (4-fluorobenzylidene-4′-n-alkanoyloxyaniline). The mesomorphic properties of the present compounds were compared with other structurally related series to establish the chemical structure-mesomorphic property relationship.

Synthesis and Phase Transition Behaviours of New Chalcone Derivatives

A series of new chalcone derivatives with a general formula of C11H27COOC6H4COCH=CHC6H4X where X=F, Cl, Br, and NO2were well synthesized and crystallized from organic solution. The physical properties as well as the chemical formulations of these compounds were determined by spectroscopic techniques (FTIR, and1H and13C NMR). Differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) techniques were employed to study their transition temperatures and mesophase characteristics. DSC thermograms of compounds with fluoro and nitro substituents displayed direct isotropization and recrystallization during heating and cooling processes. Chloro and bromo analogues exhibited Cr1-to-Cr2transition within the crystal phase region. It was also found that enone linkage showed fewer tendencies to exhibit mesomorphic property compared to imine linkage. However, when enone linkage is combined with other central linkages and additional phenyl rings, it turns conducive to mesomorphism.

Synthesis and Mesomorphic Property of 2,5-Aryl-1,3,4-Oxadiazole/Thiadiazole Derivatives Bearing a Terminal Thiophene Unit

The synthesis and characterization of a series of heterocyclic compounds 2a-2d based on 1,3,4-oxadiazole/ thiadiazole, furan and thiophene units are reported. The thermal behaviors of these compounds were investigated by means of differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and thermogravimetric analysis (TGA). The 1,3,4-thiadiazole derivative 2b exhibited enantiotropic smectic C and A mesophases with wide temperature range and good thermal stability, while all of the other compounds did not show liquid crystalline behaviors. The results indicate that 1,3,4-thiadiazole-based compounds are more beneficial to form stable mesophases than the corresponding 1,3,4-oxadiazole analogues, and the compounds with a central furan unit as the rigid core may result in the loss of the meosgenic behaviors due to the non-linear molecular structure.