Dorothee Nonnenmacher

Tuning ‘de Vries-like’ properties in siloxane- and carbosilane-terminated smectic liquid crystals

Smectic liquid crystals with ‘de Vries-like’ properties are characterized by a maximum layer contraction of ≤1% upon transition from the non-tilted SmA phase to the tilted SmC phase. To expand the current library of ‘de Vries-like’ liquid crystals, we have developed a rational design strategy based on a concept of frustration between two structural elements, one promoting the formation of a SmA phase (chloro-terminated side-chain) and another promoting the formation of a SmC phase (siloxane-terminated side-chain). In this paper, we show that one can tune this apparent frustration—and further improve de Vries-like properties—by substituting the 2-phenylpyrimidine core in our first-generation siloxane-terminated mesogens with one of three cores known to be stronger SmC-promoting elements: 6-phenylpyridazine, 2-phenylpyridine and 2-phenylthiadiazole. We also address a fundamental design flaw of siloxane-terminated mesogens, i.e., the hydrolytic instability of siloxane oligomers, by substituting the siloxane end-group with a chemically inert carbosilane end-group. As a result of this study, we found a carbosilane-terminated 2-phenylthiadiazole mesogen that forms a SmC phase at room temperature with de Vries-like properties that are comparable to those of bona fide de Vries-like liquid crystals.

The effect of lactate unit number in compounds with azo group in the molecular core

A liquid crystalline compound with the azo group in the molecular core and three lactic acid units in the chiral chain (n = 3) was synthesised and its mesomorphic properties established. Spontaneous polarisation and tilt angle were determined in the ferroelectric and antiferroelectric smectic C phases. Dielectric spectroscopy in the frequency range of 1 Hz–10 MHz was performed within the temperature range of smectic phases showing the characteristic modes in each phase. Small-angle X-ray diffraction was performed and temperature dependences of layer spacing value calculated. Comparison with previously studied compounds with one (n = 1) and two lactate units (n = 2) is presented to establish the effect of lactate unit number on the liquid crystalline properties.

Elucidating the smectic A-promoting effect of halogen end-groups in calamitic liquid crystals

Two isometric series of chloro-terminated 5-alkoxy-2-(4-alkoxyphenyl)pyrimidine mesogens QL8-m/n and QL9-m/n (m + n = 16), with the chloro-terminated alkoxy chain tethered to either the pyrimidine ring or the phenyl ring, were synthesized and their liquid crystalline properties characterized by polarized optical microscopy and differential scanning calorimetry. Based on the analysis of mesogenic properties and correlations to electrostatic potential isosurfaces calculated at the B3LYP/6-31G* level, we present evidence suggesting that the SmA-promoting effect of chloro end-groups is not due to strong polar interactions at the layer interfaces, as previously postulated in the literature. Instead, the evidence suggests that the SmA-promoting effect is due to the electron-withdrawing effect of the chloro end-group, which should reduce electrostatic repulsion between alkoxy chains and, consequently, increase van der Waals interactions between aromatic cores in the orthogonal SmA phase.

Antiferroelectric phase in liquid crystalline compounds containing an azo group in their molecular core

Novel lactic acid derivatives containing an azo group in their molecular core have been synthesised and their mesomorphic properties established. Compounds with a short aliphatic non-chiral chain exhibited the SmA* phase and ferroelectric (FE) SmC* phase. The monotropic antiferroelectric (AF) phase was found in the homologues with a longer terminal chain. Spontaneous polarisation and tilt angle were measured for the FE and AF phases. Dielectric spectroscopy in the frequency range 1 Hz–10 MHz was carried out and dielectric properties established within the temperature range of the smectic phases. Two high frequency modes were detected in the AF phase. Small-angle X-ray measurements were conducted, and the temperature dependence of layer spacing values is established and discussed.

Sign Inversion of the Spontaneous Polarization in a “de Vries”‐Type Ferroelectric Liquid Crystal

In contrast to common ferroelectric smectic C* liquid crystals, the siloxane-terminated smectic mesogen E6 is characterized by an unusual temperature variation of the spontaneous polarization. The polarization starts to grow from nearly zero despite the first-order SmA*-SmC* transition, and increases faster than linearly over a large temperature interval while the tilt angle rapidly saturates. To study this behavior in more detail, binary mixtures of different concentrations of E6 in the achiral SmC material C8Cl, which has a similar chemical structure, were investigated. Surprisingly, all mixtures show a temperature dependent polarization sign inversion, which shifts towards the SmC*-SmA* transition with increasing E6 concentration. For the pure E6 the inversion temperature meets the SmA*-SmC* phase transition temperature. In a second binary mixture with E6 and a conventional material C9-2PhP we found out, that the dependence of the inversion temperature on the concentration of E6 changes qualitatively when the nanosegregation is partially destroyed. A molecular theory of the polarization sign inversion in smectics C* with strong polar intermolecular interactions is developed which enables one to explain the concentration dependence of the inversion temperature in both mixtures.