Synthesis of mixed tail triphenylene discotic liquid crystals: Molecular symmetry and oxygen-atom effect on the stabilization of columnar mesophases

Small change in chemical structure of discotic liquid crystals can cause big difference in their mesomorphism. Replacing of the alkoxy peripheral chains of triphenylene by oxygen-atom containing ester chains would result in novel mesomorphism. A series of mixed tail triphenylenes containing propoxyacetyloxy and alkoxy, abbreviated as C18H6(OCnH2n+1)(3)(OCOCH2OC3H7)(3), n=4-8, and hexa(propyloxyacetyloxy)triphenylene, C18H6(OCOCH2-OC3H7)(6) were synthesized. Thermal gravimetry analysis (TGA) of three discogens showed that they had good thermal stability till 350 degrees C. The mesomorphism was investigated through differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The preliminary X-ray diffraction (XRD) results of one compound showed that it exhibited ordered hexagonal columnar (Col(ho)) mesophase. These mixed tail triphenylene derivatives possessed much stable Col(ho) mesophase and wider mesophase ranges than their hexaalkoxytriphenylene C18H6(OR)(6) and hexaalkanoyloxytriphenylene C18H6(OCOR')(6) analogues. The asymmetrical compounds 2,6,11-trialkoxy-3,7,10-tri(2-propyloxyacetyloxy)triphenylenes with n=5-8 displayed higher clearing points and wider temperature ranges than their symmetrical isomers 2,6,10-trialkoxy-3,7,11-tri(2-propyloxyacetyloxy)triphenylenes, while C18H6(OCOCH2OC3H7)(6) had the highest clearing point due to the beta-oxygen-atom effect.