Effect of the Magnetic Field on the Supramolecular Structure of Chiral Smectic C Phases: ²H NMR Studies

Abstract

We present a theoretical and experimental (2)H NMR study of the effect of external magnetic fields on the supramolecular organization of chiral smectic liquid-crystalline mesophases, such as SmC* and re-entrant SmC*. Three experimental cases in which the supramolecular helical structure of the smectic C* phase is unwound by a magnetic field (H), parallel to the helical axes of this phase, are discussed in detail. Unwinding of the helical structure is described by using a theoretical model based on the Landau-de Gennes theory, which allows us to explain the transition temperatures among the SmA, SmC*, and uSmC* phases. The energy-density behavior in the vicinity of the transitions and the value of the critical magnetic field H(C) for unwinding the helical structure are discussed by applying this model to three ferroelectric smectogens (MBHB, 11EB1M7, ZLL7/*), which are studied by (2)H NMR spectroscopy at different magnetic fields (from 2.4 to 9.4 Tesla). Furthermore, the tilt angle of the three smectogens in the SmC* phase has been directly evaluated, for the first time, by comparing the quadrupolar splittings at different magnetic fields. In one case, (2)H NMR angular measurements are used to obtain the tilt angle in the re-entrant smectic C phase.

From the SmA to the Hexatic, Including the SmC*, SmC*A and SmC*re Phases: A 2H NMR Relaxation Study

“…[9][10][11][12][13] On the basis of previous results, [22] the order parameter S was fixed in the range 0.68-0.65, the para axes of the two fragments are almost parallel, the angles between the CÀD bonds and the relative para axes are fixed to 608, and the molecular tilt angle is in the range 12.0-18.58. [22][23][24] Even though, in the SmC* phase, the experimental data are affected by larger errors (see Figure 5) than in the SmA phase (in particular at the lowest frequency), the fitting is quite good and the best fitting parameters, also reported in Table 2, are in agreement with those found in the SmC* phase of similar FLC systems. [11-13, 49, 50] Concerning the diffusion motions, if no sensitive changes are observed for the spinning and internal biphenyl ] for the smectogen ZLL7/* in the SmA, uSmC*, and SmC* phases.…”

Direct Measure of the Tilt Angle in de Vries‐Type Liquid Crystals through NMR Spectroscopy

“…In fact, other than for standard SmA phases, [25,[27][28][29] in the present case, the values of the S zz , defined in the local frame of the deuterated phenyl ring, are quite different at the two magnetic fields. Taking into account the known de Vries character [7][8][9] of 9 HL, we can explain this behavior by assuming the presence of a tilt angle of the para axis of the labeled phenyl ring (n p in Scheme 1) with respect to the magnetic field also in the SmA phase.…”

“…Taking into account the known de Vries character [7][8][9] of 9 HL, we can explain this behavior by assuming the presence of a tilt angle of the para axis of the labeled phenyl ring (n p in Scheme 1) with respect to the magnetic field also in the SmA phase. Indeed, our previous studies of ferroelectric phases based on 2 H NMR spectroscopy [25,[27][28][29][30] suggest that the possibility to use a very high magnetic field, H (as in the present case, namely at H = 16.45 T) ensures the complete alignment of the molecular director with respect to H both in the SmA and, clearly, in the SmC*, which is, in fact, completely unwound. The direct comparison between the order parameter obtained at the high field, S hf zz (H = 16.45 T) and that obtained at the lower magnetic field S lf zz (H = 7.05 T) allows us to have a direct measure of the tilt angle (q) of the deuterated phenyl ring, through Equation (3): [27] …”

“…[27,29,30] Herein we report the results which we believe could be of help in understanding more about the molecular arrangements within the de Vries SmA phases.…”

Nuclear magnetic resonance: a powerful tool to study liquid crystals

“…In the SmC* phase, the 9HL behaviour at increasing magnetic field strength is similar to common rod-like ferroelectrics [37]. Below a critical value of the magnetic field, Hc, the helical structure is preserved, the helical axis of the SmC* domains is parallel to the magnetic field, and the molecules are tilted.…”