Orientational order parameters of a de Vries–type ferroelectric liquid crystal obtained by polarized Raman spectroscopy and x-ray diffraction

Abstract

The orientational order parameters (P{2}) and (P{4}) of the ferroelectric, de Vries-type liquid crystal 9HL have been determined in the SmA and SmC phases by means of polarized Raman spectroscopy, and in the SmA phase using x-ray diffraction. Quantum density functional theory predicts Raman spectra for 9HL that are in good agreement with the observations and indicates that the strong Raman band probed in the experiment corresponds to the uniaxial, coupled vibration of the three phenyl rings along the molecular long axis. The magnitudes of the orientational order parameters obtained in the Raman and x-ray experiments differ dramatically from each other, a discrepancy that is resolved by considering that the two techniques probe the orientational distributions of different molecular axes. We have developed a systematic procedure in which we calculate the angle between these axes and rescale the orientational order parameters obtained from x-ray scattering with results that are then in good agreement with the Raman data. At least in the case of 9HL, the results obtained by both techniques support a "sugar loaf" orientational distribution in the SmA phase with no qualitative difference to conventional smectics A. The role of individual molecular fragments in promoting de Vries-type behavior is considered.

Conformational Properties and Orientational Order of a de Vries Liquid Crystal Investigated through NMR Spectroscopy

“…The occurrence of a conformational change is also supported by the trends of the measured 13 C chemical shift d h i of the alpha methylenoxy carbons (31) and (34) in the chiral lateral chain (region of 40 Ä 75 ppm) at the SmA-SmC* transition, as reported in Figure 4. [5,15,32] This change in temperature-dependence of d h i, opposite for the two carbon sites is an indication of a different configuration of the lactate lateral chain in the SmA and SmC* phases, similarly to what has been previously observed in two smectogens having a lactate chain as the 9HL.…”

“…[47-50, 52, 54] Figure 6 reports the fitted order parameters vs temperature in the SmA phase, for the three aromatic moieties. [34,37,38] In the case of 9HL, the measured 1 H and 2 H longitudinal and transverse relaxation times (not reported here) confirm that the dynamic properties of its aromatic core are similar to common smectogens, and this is also in agreement with Raman [32] and Dielectric Spectroscopy results. The minimum and maximum values of S zz arẽ 0.66 (at higher temperature) and~0.83 (at lower temperature), respectively.…”

“…This NMR study [5] showed also that the smA phase of the investigated de Vries smectogen has a quite high orientational order, typical of regular smectic phases; this result was also confirmed by X-ray and Raman techniques on the same smectogen. [32] In the present work, we show that a combination of different NMR techniques [33][34][35][36][37][38][39][40] can help in understanding basic molecular and conformational properties within the whole de Vries SmA mesophase and at the SmA-SmC* transition, thus adding fundamental information to discriminate among the theoretical models reported in the literature. In particular, we focus our attention to the representative de Vries smectogen 9HL, a (S)-hexyl-lactate derivative, which consists of a rigid core of three benzoate moieties and a lactate unit, and two linear alkyl chains of 6 and 9 carbon atoms respectively (see Figure 1).…”

Reliability of Orientational Order Parameters Determined from Two‐dimensional X‐ray Diffraction Patterns: A Simulation Study

“…S 2 can be obtained by calculating the moments of of the ODF [Eq. (8)]: [8] C. [10] ChemPhysChem 2016, 17,1568 -1572 www.chemphyschem.org…”

“…[1,2] In practice, there are severale xperimental ways to determine S 2 ,f or instance by NMR, by measuring diamagnetica nisotropy or optical birefringence and by X-ray diffraction. [3][4][5][6][7][8] All these methods require simplifications and assumptionsw hich could influencet he result in aw ay that mayn ot be obvious apriori.…”