NMR studies of the ferroelectric SmC* phase

Abstract

In this work an attempt is made to compare and rationalise the structural and dynamic behaviour of some ferroelectric rod-like mesogens studied by our group in recent years, mainly by means of 2H- and 13C-nuclear magnetic resonance (NMR) spectroscopy. This comparison concerning the local orientational order, the average molecular conformation and the chiral smectic C (SmC*) phase structure, with a brief overview on the reorientational dynamic properties, allowed us to identify some common behaviour of these ferroelectric rod-like mesogens when decreasing the temperature from lower ordered phases to and within the SmC* phase. The main results obtained by means of NMR studies on these mesogens are rationalised and discussed in comparison with detailed discrete Fourier transform computations of their molecular conformations (in the limit of isolated molecules) with the purpose of enlightening the role of intermolecular and packing interactions in the ferroelectric phase with respect to the smectic A and nematic phase

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] …”

“…The values of the S zz and D biax obtained from this analysis are typical of ferroelectric liquid crystalline materials. [25,28] At the high magnetic field (16.45 T) different behavior is observed. The SmC* phase unwinds completely as seen from the continuous increase of both the quadrupolar and dipolar splittings by decreasing the temperature (see the Supporting Information).…”

“…We monitored the trend of the tilt angle of the deuterated moiety within the SmA and SmC* phases with an NMR study at high magnetic field. This approach, [27] recently used to investigate standard ferroelectric LCs, [25,28] takes advantage of the ability of high magnetic fields (H) in unwinding the supramolecular heliScheme 1. Molecular structure of the 9 HL-d 2 sample under investigation.…”

Nuclear magnetic resonance: a powerful tool to study liquid crystals

“…showed a behaviour very similar to common ferroelectric smectogens [35]. In particular, we noted an increasing tilt angle in the SmC* phase by decreasing the temperature and a SmA-SmC* transition showing a sudden change in the local order parameter and/or the relative tilt of the deuterated phenyl ring with respect to the phase director.…”