13C NMR investigations and order parameters of rod-like molecules with terminal phenyl and thiophene rings in mesogenic core

Rod-like mesogens with the rigid core constituted by a combination of phenyl and thiophene moieties reveal interesting orientational ordering behavior. In this article, four rod-like mesogens constructed with a terminal alkoxy chain on only one side of the mesogenic core with the other end terminated with either a phenyl or thiophene ring have been investigated using 13C NMR spectroscopy. The mesogen with the terphenyl moiety in the core exhibited polymesomorphism with nematic, smectic C, and smectic I mesophases with very high clearing temperatures. The replacement of terphenyl with terthiophene moiety resulted in suppression of polymesomorphism, with only nematic mesophase being observed. Mesogens with single terminal thiophene connected to two phenyl rings in the core also showed only the nematic mesophase for both the possibilities of linking the core to the thiophene ring at the 2-and 3-positions. One-and two-dimensional 13C NMR spectra have been obtained in the mesophases from which the alignment induced chemical shifts of the ring carbons and 13C -1H dipolar couplings, as well as the local order parameters, have been obtained. The considerable difference in 13C chemical shifts and the 13C -1H dipolar couplings of thiophene ring with change in position of the linking unit is attributed to the difference in the order parameters of the thiophene moiety between the two cases. The data obtained on the ordering of phenyl and thiophene rings in the core units offered information into the effect of replacing the phenyl ring with the thiophene ring. So also, the change of position of the link to the thiophene moiety provided important insights into the orientational behavior in the liquid crystalline phase and on the molecular structure.