posted on 2023-12-08, 13:35authored byBathini Veeraprakash, Malireddygari Kesava Reddy, Bibhuti B. Das, Nitin P. Lobo, Krishna V. Ramanathan, Tanneru Narasimhaswamy
Five molecular mesogens containing phenyl rings and thiophene
are
subjected to a detailed 13C 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 13C NMR, the samples were studied using 1D and 2D solid-state 13C NMR experiments in the liquid crystalline phase. The chemical
shifts and 13C–1H 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, 13C chemical shifts and 13C–1H dipolar couplings showed remarkable
variation, which was directly reflected in the order parameters. For
instance, the order parameter (Szz) 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 13C NMR in extracting the structural and orientational
information on mesogens in which thiophene is a constituent of the
core unit.