Effortless Extraction of Structural and Orientational Information from C-H Dipolar Couplings for Thiophene Mesogens.

Five molecular mesogens containing phenyl rings and thiophene are subjected to a detailed C NMR investigation. The first mesogen contains only phenyl rings, while the other four have thiophene with substitution at position 2 or 3. Two of these also have a spacer inserted between the thiophene and the rest of the core unit. The mesophase properties evaluated by complementary techniques reveal an enantiotropic nematic phase for all the cases and smectic C as well as Crystal J for a few mesogens. In addition to solution C NMR, the samples were studied using 1D and 2D solid-state C NMR experiments in the liquid crystalline phase. The chemical shifts and C-H dipolar couplings obtained in the mesophase provided cutting-edge information about the molecular structure and orientation of the thiophene mesogens. Accordingly, dramatic differences in these parameters are noted for the mesogens, and consequently, the identification of 2- and 3-substituted thiophene mesogens is accomplished by a simple visual examination of the 2D spectra. Furthermore, for mesogens with a spacer between thiophene and the rest of the core, C chemical shifts and C-H dipolar couplings showed remarkable variation, which was directly reflected in the order parameters. For instance, the order parameter (S) of thiophene in 2- and 3-substituted mesogens in which the spacer is absent is ∼0.63 whereas for those with spacer, it is reduced to ∼0.14-0.18. In comparison, the mesogen in which the core unit is made up of phenyl rings alone that is used to benchmark the characteristics of thiophene ordering showed an order parameter of ∼0.85. The study unambiguously demonstrates the supremacy of C NMR in extracting the structural and orientational information on mesogens in which thiophene is a constituent of the core unit.