13C NMR investigations and molecular order of nematogens with biphenyl and bithiophene at terminus

Abstract

ABSTRACT Four rod-like mesogens in which biphenyl and bithiophene units are located at one terminus are synthesised and the occurrence of enantiotropic nematic phase is established by hot-stage optical polarising microscopy and differential scanning calorimetry. For biphenyl based mesogens (BP-1 and BP-2), the mesophase is observed irrespective of number of phenyl rings in the amine moiety. For bithiophene (BT-1), on the other hand, the mesophase is only noted when the amine component has two rings. The mesophase range for BP-1 is found to be 163.7◦C and for BT-1 and BP-2, the values are 84.7◦C and 11.1◦C, respectively. BP-1 and BT-1 are subjected to 13C NMR investigation in nematic phase to determine the local order and accordingly, oriented 1D 13C and 2D SAMPI-4 NMR experiments are accomplished. The remarkable feature of the SAMPI-4 spectrum of BP-1 measured at 145◦C in nematic phase is observation of the 13C-1H dipolar coupling value of 16.67 kHz for terminal carbon (C18). For BT-1, however, the dipolar coupling value for C18 is found to be 8.60 kHz in nematic phase (138◦C). Such large values of 13C-1H dipolar couplings for the terminal ring carbons of the nematogens offer orientation of the rings of the core unit directly.