2-Octyl thiophene based three ring mesogens: solid state ¹³C NMR and XRD investigations

Abstract

2-Octyl thiophene based three-ring mesogens namely 4-n-alkoxyphenyl 4-(5-n-octyl-2-thienyl)benzoates are synthesized by employing palladium acetate based direct arylation. The alkoxy terminal is varied with even carbons from C2 to C14 and enantiotropic polymesomorphism is noticed for all the homologs. Accordingly, phase sequence consisting of nematic, smectic A, smectic C and smectic B is seen for mesogens with terminal chains C6, C8, C10 and C12 on cooling the isotropic phase. For mesogens with C2, C4, C8 and C10 terminal alkoxy chains, the mesophase assignment from hot-stage optical microscopy and differential scanning calorimetry is further confirmed by variable temperature powder X-ray diffraction measurements. The appearance of smectic B phase is established by noticing sharp and intense peaks in both small-angle and wide-angle regions. For a representative mesogen, i.e. T10, high-resolution solid-state (13)C NMR investigations are carried out in all the phases, viz. nematic, smectic A, smectic C and smectic B phases. The orientational order parameters calculated from (13)C-(1)H dipolar couplings from 2D SAMPI-4 experiments are found to be 0.44, 0.67, 0.73 and 0.79 in nematic, smectic A, smectic C and smectic B mesophases for the center phenyl ring respectively. Remarkably, the thiophene order parameter in all mesophases is found to be higher than that of phenyl rings and is explained by considering the molecular shape, which has a terminal bend. Further, the mesogens are found to be photoemissive in chloroform solution with an emission band at ∼410 nm.

13C NMR investigations of molecular order of rod‐like, bent‐core, and thiophene mesogens

“…Significant effort has been made to investigate thiophene‐based mesogens by our group using 13 C NMR measurements . It was well recognized that the replacement of phenyl ring with thiophene in mesogen results in decrease of melting point, increase of optical anisotropy, reduction in viscosity, promotion of negative dielectric anisotropy, and fast switching times .…”

“…The usefulness of the 2D-SLF spectroscopy for providing information on the molecular order and thereby on the dynamics and the topology of mesogens has been demonstrated. The combination of 13 C chemical shifts and 13 C─ 1 H dipolar [51] MN-2 SmA Veeraprakash et al [83] MN-3 SmC, SmB Veeraprakash et al [83] MN-4 Nematic, SmA, SmC Veeraprakash et al [82] MN-5 SmA Pratap et al [64] MN-6 SmA Pratap et al [64] MN-7 Nematic Santhosh Kumar Reddy et al [60] MN-8 SmC, SmB Santhosh Kumar Reddy et al [60] Abbreviations: SmA, smectic A; SmB, smectic B; SmC, smectic C.…”

“…Significant effort has been made to investigate thiophenebased mesogens by our group using 13 C NMR measurements. [47,51,58,[60][61][62][63][64][82][83][84][85][86] It was well recognized that the replacement of phenyl ring with thiophene in mesogen results in decrease of melting point, increase of optical anisotropy, reduction in viscosity, promotion of negative dielectric anisotropy, and fast switching times. [87,88] Samulski et al showed that replacement of phenyl ring with thiophene would bring profound changes in molecular shape.…”

Mesomorphism, polymerization, and chirality induction in α -cyanostilbene-functionalized diacetylene-assembled films: Photo-triggered Z /E isomerization

“…The XRD profile of ( Z )‐CNBE‐DA in nematic phase at 120 °C was demonstrated in Figure (b), with a sharp and intense reflection at 1.75°, corresponding to layer spacing of 5.05 nm. The broad hump in the wide‐angle region suggested the absence of in‐plane positional order in LC phase . Transmission electron microscopy (TEM) images for the ( Z )‐CNBE‐DA assemblies in crystal state showed highly ordered lamellar structure (Supporting Information Fig.…”

Intramolecular charge transfer interactions and molecular order of rod like mesogens

“…16 For ring III also 13 C NMR data in liquid crystalline phase is available from the literature. 35 By making use of 13 C-1 H dipolar couplings and employing the eqn (1) mentioned below, the order parameter of the phenyl rings of the core unit is determined. The method adopted for extracting the dipolar couplings from experimentally determined dipolar oscillation frequency and their use for determining the order parameter is briey mentioned.…”