Synthesis And Characterization Of Aromatic And Acetylenic Derivatives

As liquid crystal technology develops quickly, one has more and more strict requirements for liquid crystals. LCD response time is an important aspect. Natures of liquid crystalline threshold and low viscosity would be necessary for making fast response LCD. A new class of liquid crystals of aromatic and acetylenic derivatives with large thresholds, low viscosities, high birefringence and clear points would enhance the response speed of LCD and would possibly be used in new liquid crystal formulations of TN, STN and TFT. In our experiments, diphenylacetylene and triphenyldiacetylene were designed as the mesogenic unite of the liquid crystalline molecules, and alkoxyl or esterified alkoxyl as the flexible end groups, which would influence the temperature range of the mesogenic phase. We investigated the affection of the end groups on the mesogenic phase temperature by changing the chain length of the flexible end group.The novel aromatic and acetylenic derivatives were synthesized via Sonagashira reaction by using Pd(PPh3)2Cl2 and copper(I) iodide as the catalyst and co-catalyst. The synthesis of R-C6H4-≡- C6H4-OR'was fulfilled in the mixed solvent of triethylamine and tetrahydrofurane. Here, R= H , O(CH2)3OH, O(CH2)3OCOC3H3,OCH3; R'= (CH2)3OCOC3H3, (CH2)3OH, (CH2)6OH, (CH2)6OCOC3H3 . For synthesis of the compound R1-C6H4-≡- C6H4 -≡-C6H4-OR2, PdCl2 was used as the catalyst, instead of Pd(PPh3)2Cl2; pyridine as solvent, instead of triethylamine and tetrahydrofurane. R1= OCH3 ; R2 = (CH2)3OH,O(CH2)3OCOC3H3 . The purities and chemical structures of the as-synthesized compounds were examined and characterized by using high-pressure liquid chromatography, IR and 1H NMR. The measurements of phase transition temperatures and enthalpies of the liquid crystallinecompounds were carried out using differential scanning calorimenty (DSC).The relationship between molecular structure and mesogenic natures was discussed. The as-synthesized aromatic and acetylenic derivatives showed respectively thermotropic mesophase in the temperature range from 53.9 to 63.9°C, 58.0 to 81.8°C, 127.5 to 161.4°C and147.7 to162.5°C. However, the compound with triphenyldiacetylene as the mesogenic unite, OCH3 and O(CH2)3OCCH=CH2 as the end groups showed a more narrow range of mesophase temperature of 132.0 to 150.0°C , but a higher thermal stability.