¹³C NMR Studies and Orientational Order of Mesogens With Thiophene and Dimethylamino in Terminal Position

Three thiophene-based spacer-containing mesogens with a rigid core made up of three phenyl rings and a terminal dimethylamino group are synthesized and revealed to exhibit enantiotropic nematic mesophase. In these mesogens, thiophene is connected to the rest of the molecule at the 2-position, 3-position, and 2,5-positions, respectively, to yield monomers and a dimer. The influence of the electron-releasing dimethylamino group on the mesophase transition temperatures is carried out by comparing the structurally similar mesogens reported in the literature in which dimethylamino is replaced by alkoxy chains. Further, the high melting and clearing temperatures observed for the mesogens are accredited to enhancement in the molecular anisotropic polarizability owing to the presence of the electron-releasing dimethylamino group at the terminus. The ¹³C NMR studies carried out in solution and nematic mesophase enabled us to find structural and orientational information. The 2D separated local field (SLF) measurements of the mesogens in the nematic phase offered ¹³C-¹H dipolar couplings and by making use of them the order parameters (S_ZZ) of thiophene and phenyl rings are computed. The disparity in the order parameters of thiophene in monomers and dimer is discussed. The study revealed that the orientation of the methyl of the terminal dimethylamino group follows a different trend in the nematic mesophase compared with solution and is ascribed to dissimilar orientational constraints in the dimer in contrast to monomers.