Synthesis and mesomorphic properties of new compounds exhibiting TGBA and TGBC liquid crystalline phases

Abstract

We synthesised a series of rod-like mesogens with a (S)-2-methylbutyl-(S)-lactate unit in the chiral chain that exhibited extremely wide temperature ranges in the TGBA and TGBC* phases. TGB phases were identified, based on typical textures in confined samples and in free-standing films, by Grandjean-Cano texture and by NMR studies on a deuterium-labelled isotopomer. A sufficiently high electric or magnetic field transformed the TGBA and TGBC* phases into their respective SmA and SmC* phases, the TGB structures being restored within some 20-30 minutes. Therefore values of the spontaneous polarisation and spontaneous tilt angle, when measured under a sufficiently high field, gave evidence of the properties of the SmC* phase. Temperature dependencies of relaxation frequency, dielectric strength, selective reflection and layer spacing showed anomalies at a certain temperature within the TGBC* phase range. Also, changes in textures, as well as in 2H-NMR spectra, occurred at this same temperature. These results suggest the existence of two TGBC* phases

Twist Grain Boundary Liquid‐Crystalline Phases under the Effect of the Magnetic Field: A Complete2H and13C NMR Study

“…The mesophasic behaviour of HZL 7/* was reported in earlier work [15] where different techniques, including DSC, textures observed by optical microscope in different conditions and dielectric measurements, were employed. Herein, a detailed NMR study on the stability of TGB phases under the effect of the NMR magnetic field is presented.…”

“…The transition between the TGBA* and TGBC 1 * is known to be second-order, as found by optical investigations. [15] In our study, a sudden increase of the linewidth occurs at the transition between the unwound TGBA* and the unwound TGBC 1 * (from 650 Hz to the maximum value of 2500 Hz). There is, however, no discontinuity in the linewidth at the TGBC 1 *-TGBC 2 * transition.…”

“…Zero field corresponds to DSC and optical measurements. [15] Below 85 8C, the 2 H NMR spectra change drastically. The appearance of a distributed spectrum, with different features when decreasing the temperature across the temperature transitions of TGBA*-TGBC 1 * and TGBC 1 *-TGBC 2 *, implies that at this magnetic field the supramolecular structure of the TGBC* is neither unwound nor destroyed.…”

“…The values for the tilt angle obtained from NMR are in perfect agreement with those obtained by X-ray and optical methods (in particular, the saturation value of tilt angle was found to be 35 degrees). [15] In fact, even by applying the electric field in order to measure the tilt angle and the spontaneous polarization, the TGBC*-structures are unwound. The fact that the evaluated tilt angle is the same within experimental error under both the electric and the magnetic fields further supports our analysis.…”

“…Moreover, only few papers are known on structural studies of the TGBA phase by NMR. [14,15,16] While the electrical response of these complex phases has been investigated, showing that the structure of the TGB phases can be unwound or strongly distorted by the electric field, [17,18] no studies of the effect of external magnetic fields have been reported. However, such studies on chiral smectic C (SmC*) phases, [19] show the strong influence of the magnetic field: the helical structure of the SmC* phase can indeed be completely or partially unwound [20] by external magnetic fields in the range of 4-20 Tesla.…”