Orientational Order of Difluorinated Liquid Crystals: A Comparative ¹³C-NMR, Optical, and Dielectric Study in Nematic and Smectic B Phases

Abstract

Structural and orientational order properties of 3Cy2CyBF2 and of 5CyCy2BF2 have been investigated by means of (13)C-NMR, optical, and dielectric spectroscopy methods. In the case of NMR, order parameters have been independently obtained from the analysis of either (13)C-(19)F dipolar couplings or (13)C chemical shift anisotropies, both measured from (13)C-{(1)H} NMR static spectra. The assignment of the (13)C resonances has been carried out thanks to the comparison with solution state spectra and DFT calculations, and the relevant geometrical parameters and (13)C chemical shift tensors needed to derive orientational order parameters have been calculated by DFT methods. In the analysis of (13)C-(19)F dipolar couplings, empirical corrections for vibrations and anisotropic scalar couplings have been included. Dielectric measurements have been performed over a broad frequency range for two orientations of the nematic director with respect to the measuring field. At low frequencies (static case) a positive dielectric anisotropy has been determined, which has enabled the calculation of the order parameters according to a well-tested procedure. At high frequencies the dielectric anisotropy changes its sign, a property which can be useful in designing a dual addressing display. The nematic order parameter determined from optical, dielectric, and NMR methods have been compared: their trends with temperature are very similar, apart from some slight shifts, and were analyzed by Haller and Chirtoc models. The differences among the results obtained by the four methods have been discussed in detail, also with reference to the assumptions and approximations used in each case, and to the results recently reported for similar fluorinated nematogens. The presence of a non-negligible order biaxiality has been related to the presence of a CH2CH2 bridging group, linking one cyclohexylic unit with either the other cyclohexyl or the phenyl ring.

Mesomorphic and dielectric properties of fluorosubstituted isothiocyanates with different bridging groups

“…According to the Maier and Meier equations (15), the dielectric anisotropy is proportional to the order parameter divided by temperature: Á" , S(T)/T (other factors are only slightly dependent upon T). It has been shown that the S-values obtained in this way agree well with those determined by the nuclear magnetic resonance (NMR) and optical anisotropy methods (16,17). The order parameters calculated in this way are presented in Figure 9.…”

Orientational ordering studies of fluorinated thermotropic liquid crystals by NMR spectroscopy

“…Fluorinated nematogens containing mono-fluorinated or difluorinated phenyl rings and cyclohexane in the core, structurally resembling those studied by Magnuson et al, were more recently investigated by Catalano et al [33] (3CyHeBF and 3CyCyBF2 in Table 1, with 3CyCyBF2 corresponding to 3CCP F F of 32) and Geppi et al [34] (3Cy2CyBF2 and 5CyCy2BF2 in Table 1) by 13 C NMR spectroscopy. In all cases, the use of solid state techniques and, in particular, the SPINAL-64 decoupling scheme, allowed well resolved spectra to be obtained, from which accurate 13 C-19 F splittings and anisotropic chemical shifts were measured for all the carbons of the rigid aromatic moiety, the assignment being performed on the basis of observed splittings and of the trends of signals with temperature ( Fig.…”

“…Fluorinated nematogens containing mono‐fluorinated or di‐fluorinated phenyl rings and cyclohexane in the core, structurally resembling those studied by Magnuson et al ., were more recently investigated by Catalano et al . (3CyHeBF and 3CyCyBF2 in Table , with 3CyCyBF2 corresponding to 3CCP F F of ) and Geppi et al . (3Cy2CyBF2 and 5CyCy2BF2 in Table ) by 13 C NMR spectroscopy.…”

“…(b) Experimental (symbols) and calculated (lines) splittings for each 13 C– 19 F pair of 3Cy2CyBF2 versus temperature. Adapted from .…”

“…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 …”