Orientational Order of Two Fluoro- and Isothiocyanate-Substituted Nematogens by Combination of ¹³C NMR Spectroscopy and DFT Calculations

Abstract

Orientational order properties of two nematogens containing a fluoro- and isothiocyanate-substituted biphenyl moiety have been investigated by means of (13)C NMR spectroscopy. (13)C NMR spectra acquired on static samples under high-power (1)H-decoupling allowed both (13)C chemical shift anisotropies and (13)C-(19)F couplings to be measured. These data were used to determine the local principal order parameter and biaxiality for the different rigid fragments of the molecules. To this aim, advanced DFT methods for the calculation of geometrical parameters and chemical shift tensors were used. The orientational order parameters obtained by NMR have been critically compared with those obtained by dielectric spectroscopy. Trends of order parameters with temperature have been analyzed in terms of both mean field theory and the empirical Haller equation.

Orientational ordering studies of fluorinated thermotropic liquid crystals by NMR spectroscopy

“…F‐LCs containing fluorinated biphenyl or terphenyl aromatic moieties were also investigated by several research groups, exploiting either 13 C or 19 F NMR spectroscopies . In particular, the Jim Emsley's group studied two nematogens mono‐fluorinated on a biphenyl moiety (I35 and I52 in Table ), differing only in the lateral chain length, and a nematogen with a terphenyl core containing the 2,2,′‐difluorobiphenyl group (Table ) .…”

“…13 C NMR spectroscopy was also recently applied by Calucci et al . to investigate the orientational order of two fluroro‐ and isothiocyanate nematogens containing the biphenyl fragment, differing only for the position of the fluorine atom (ORTHO and META in Table ).…”

“…In recent years, comparative investigations of orientational order have been undertaken on calamitic F‐LCs with wide nematic ranges by employing different NMR techniques and optical and/or dielectric anisotropy measurements …”

“…F-LCs containing fluorinated biphenyl or terphenyl aromatic moieties were also investigated by several research groups, exploiting either 13 C or 19 F NMR spectroscopies. [35][36][37][38] In particular, the Jim Emsley's group studied two nematogens mono-fluorinated on a biphenyl moiety (I35 and I52 in Table 1), [35] differing only in the lateral chain length, and a nematogen with a terphenyl core containing the 2,2,′-difluorobiphenyl group (Table 1). [36] 13 C- 19 F splittings between all the aromatic carbons and the fluorine(s) were detected in proton-decoupled 13 C NMR static spectra for all mesogens; in the case of I35 and I52, couplings were also recorded between the aliphatic carbons and the fluorine nucleus.…”

“…For the 2,2′-difluorobiphenyl fragment of the mesogen investigated by Castiglione et al, [36] the probability distribution for the inter-ring angle was found to be peaked at 47.5°and 122.5°, with an integrated normalized probability that the inter-ring angle is less than 90°of 0.43. 13 C NMR spectroscopy was also recently applied by Calucci et al [38] to investigate the orientational order of two fluroroand isothiocyanate nematogens containing the biphenyl fragment, differing only for the position of the fluorine atom (ORTHO and META in Table 1). In this work, proton-decoupled 13 C direct excitation spectra were acquired on static samples at different temperatures in the N phase.…”