Effect of alkyl chain and linking units on mesophase transitions and molecular order of rod-like thiophene mesogens: 13C NMR investigation

Thermotropic liquid crystals with π-conjugated cores are increasingly witnessed due to their promising optoelectronic and optophotonic properties. As π-conjugated mesogens are often realized by direct linking of the rings of the core, the mesophase transitions are usually modulated by inserting the required alkyl/alkoxy chains either at a terminal or lateral location. In this work, five mesogens in which thiophene and two phenyl rings are common in the core unit with terminal alkyl/alkoxy chains are investigated to probe the influence of (a) a terminal alkyl chain on the mesophase characteristics, (b) the linking units on the mesophase range and (c) the disparity of thiophene orientational constraints. The mesophase properties reveal that whenever the alkyl chain is located at one end of the molecule, polymesomorphism is observed. The XRD studies reveal layer ordering typical of smectic mesophases with lower temperature and higher order phases, namely SmB and CrE phases. A detailed 13C NMR study of the mesogens in the liquid crystalline phase indicates that the order parameters are governed by the nature of the substitution of thiophene and its linkage with two phenyl rings. Importantly, the 13C NMR data of the terminal chains in the liquid crystalline phase are compared with the recently reported thiophene mesogen in which the n-hexyl chain is located at the lateral position and completely different orientation characteristics were found for the terminal versus lateral chains.