Studies on Synthesis and Substituent Effects of Schiff’s Base Type Liquid Crystals.

A number of banana shape molecules with different substituents, linking group and altering side chains were prepared. Different types of mesophases(B1, B2 and B7 mesogens) were identified by using polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction, and by electro-optical investigations.The effect of lateral substituents such as fluoro, chloro in central core of these molecules may increase the intermolecular attracting forces and thus result in the formation of liquid crystal phase. In addition, bulkier substituents in the banana-shaped molecules may raise intermolecular repulsions, which lead to the disappearance of mesophases. The clearing temperatures of the mesophases increase slightly when the side-chain changes from ether to ester of the same carbon number.

A new homologous series of symmetric dimmers derived from salicylidedeamines as core group has been prepared and investigated for their thermal behavior. A flexible alkylene group -(CH2)n- was applied for the central spacer, and both the length of the spacer (n = 2, 3, 4, 5, 6, 7, 8) and various types of the terminal chains were varied. In these mesogenic dimers, the spacer length has a remarkable influence on the thermal behavior. The dimers connected with an odd carbon number of Cn-alkylene spacer (n = 3, 5, 7) are B1-banana mesogens, and all other dimers connected with even carbon number of Cn-alkylene spacer (n = 2, 4, 6, 8) are smectic C mesogens. In these dimers, the spacer has a significant influence on the thermal behavior. The clearing temperatures of the dimers were slightly decreased with the length of the spacer alkylene.

In order to understand the influence of the linking groups and the substituents at the soft side chain on the liquid crystalline behavior. Octyl 4-(4-halobutoxylideneamino)benzoate, octyl 4-(4-halobutoxylphenylazo)
benzoate and the 4-cyano analogues were prepared and studied. An Imino or azo group were used as linking group, and the H on the end of alkoxy chain was substituted with a halogen (F、Cl、Br、I) or with a pseudohalogen (CN). The liquid crystalline properties of these mesogens were found very sensitive to the nature of the substituents. The transition temperature is lower as an chloro is introduced. The liquid crystallines are smectic A mesogens.

Lastly, the use of microwaves as an energy source to synthesize all aimed products in place of the traditional heating procedure. Superiority in saving time, solvent, and energy could be realized from the comparison of microwave assisted preparation and traditional procedures.