Optical and x-ray evidence of the "de Vries" Sm-A*-Sm-C* transition in a non-layer-shrinkage ferroelectric liquid crystal with very weak interlayer tilt correlation.
TL;DR: A non-layer-shrinkage fluorinated ferroelectric liquid crystal compound, 8422[2F3], has been characterized by means of optical, x-ray, and calorimetric methods and there is strong evidence that the second-order Sm-A*-Sm-C* transition is well described by the diffuse cone model of de Vries.
Abstract: A non-layer-shrinkage fluorinated ferroelectric liquid crystal compound, 8422[2F3], has been characterized by means of optical, x-ray, and calorimetric methods. The orientational distribution within macroscopic volumes, determined through wide-angle x-ray scattering and birefringence measurements, was found to be identical in the $\mathrm{Sm}\ensuremath{-}{A}^{*}$ and helical $\mathrm{Sm}\ensuremath{-}{C}^{*}$ phases. Together with the absence of layer shrinkage, this constitutes strong evidence that the second-order $\mathrm{Sm}\ensuremath{-}{A}^{*}--\mathrm{Sm}\ensuremath{-}{C}^{*}$ transition in this material is well described by the diffuse cone model of de Vries. The absolute values of the layer spacing show that the molecules aggregate to antiparallel pairs. The molecular interaction across the layer boundaries will then occur only between fluorine atoms, leading to unusually weak interlayer tilt direction correlation. This explains the experimental observations of a very easily disturbed $\mathrm{Sm}\ensuremath{-}{C}^{*}$ helix and a peculiar surface-stabilized texture. Tilt angle and birefringence values as a function of field and temperature have been evaluated in the $\mathrm{Sm}\ensuremath{-}{A}^{*}$ and $\mathrm{Sm}\ensuremath{-}{C}^{*}$ phases and the results corroborate the conclusions from the x-ray investigations.
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TL;DR: The second- and fourth-order apparent orientational order parameters of the core part of the molecule P2 and P4, have been measured by polarized vibrational Raman spectroscopy for a homogeneously aligned ferroelectric smectic liquid crystal with three dimethyl siloxane groups in the achiral terminal chain and provide evidence for the existence of the de Vries Sm-A phase.
Abstract: The second- and fourth-order apparent orientational order parameters of the core part of the molecule ${⟨{P}_{2}⟩}_{\mathrm{app}}$ and ${⟨{P}_{4}⟩}_{\mathrm{app}}$, have been measured by polarized vibrational Raman spectroscopy for a homogeneously aligned ferroelectric smectic liquid crystal with three dimethyl siloxane groups in the achiral terminal chain, which shows de Vries--type phenomena, i.e., very large electroclinic effect in the smectic-$A$ $(\mathrm{Sm}\text{\ensuremath{-}}A)$ phase and a negligible layer contraction at the phase transition between the $\mathrm{Sm}\text{\ensuremath{-}}A$ and $\mathrm{Sm}\text{\ensuremath{-}}{C}^{*}$ phases. The apparent orientational order parameters of the rigid core part of the molecule are extremely small both with and without the external electric field in $\mathrm{Sm}\text{\ensuremath{-}}A$. These results provide evidence for the existence of the de Vries $\mathrm{Sm}\text{\ensuremath{-}}A$ phase, where the local molecular director is tilted at a large angle.
31 citations
TL;DR: In this paper, the atropisomeric compound (R)-2,2′,6,6′-tetramethyl-3,3′-dinitro-4,4′-bis[(4-nonyloxybenzoyl)oxy]biphenyl ((R)-1) was doped in the achiral liquid crystal hosts 2-(4-butoxyphenyl)-5-octyloxypyrimidine (2-PhP) and 4-(4)-heptyl[1,1′-
Abstract: The atropisomeric compound (R)-2,2′,6,6′-tetramethyl-3,3′-dinitro-4,4′-bis[(4-nonyloxybenzoyl)oxy]biphenyl ((R)-1) was doped in the achiral liquid crystal hosts 2-(4-butoxyphenyl)-5-octyloxypyrimidine (2-PhP) and 4-(4′-heptyl[1,1′-biphen]-4-yl)-1-hexylcyclohexanecarbonitrile (NCB76), and electroclinic coefficients ec were measured as a function of the dopant mole fraction x1 in the chiral SmA* phase at T
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TC = +5 K. The extrapolated ec values of 3.07 and 2.28 deg µm V−1 are comparable to some of the highest ec values reported for neat SmA* materials. The electroclinic coefficient of a 4 mol% mixture of (R)-1 in 2-PhP is amplified by achiral 2-phenylpyrimidine additives (5 mol%) that are longer than 2-PhP; in the best case, ec is amplified by a factor of 3.2 with 5-(tetradecyloxy)-2-(4-(tetradecyloxy)phenyl)pyrimidine (3g), which is almost twice as long as 2-PhP. However, no amplification is observed in a 4 mol% mixture of (R)-1 in NCB76 using the same series of additives. A correlation between ec values and the temperature range of the SmA* phase suggests that the amplification of ec with increasing length of the additive 3 in the (R)-1/2-PhP mixture is due primarily to a decrease in the tilt susceptibility coefficient α as the second-order SmA*–SmC* phase transition moves away from the tricritical point. Measurements of smectic layer spacing as a function of T
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TC by small-angle X-ray scattering are consistent with this explanation. The results show that the variation in reduced layer spacing dA/dC with T
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TC for the pure host 2-PhP fits to a square-root law, which indicates that the second-order SmA–C transition is nearly tricritical. On the other hand, the corresponding variation in dA/dC with T
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TC for a 5 mol% mixture of 3g in 2-PhP fits to a linear relation, which indicates that the second-order SmA–C transition approaches typical mean-field behavior.
30 citations
TL;DR: The synthesis and the mesomorphic properties of novel imidazolium salts with mesogenic 2-phenylpyrimidine or 2-alkyl pyrimidinecarboxylic acid central cores are reported in this paper.
Abstract: The synthesis and the mesomorphic properties of novel imidazolium salts with mesogenic 2-phenylpyrimidine or 2-alkylpyrimidinecarboxylic acid central cores are reported. The mesogenic units are connected to the imidazolium head groups via an alkoxy spacer. In order to adjust the mesomorphic properties the length of the alkoxy spacer and the terminal alkyl group, the counter ion, the substitution pattern of the imidazolium head group and the molecular geometry (linear vs. bent) are varied and the corresponding compounds were investigated in detail using differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray scattering (WAXS, SAXS). Whereas SmA phases with monolayer orientation were observed for imidazolium salts with short N-substituents (R = CH3, C4H9) at the imidazolium head group, the corresponding derivatives with longer N-substituents (R = C12H25, C12H25OC6H4) displayed SmA phases with bilayer orientation irrespective of a linear or bent geometry. For two derivatives, p-5(10,8) and p-5(12,8), a SmC phase was observed. Indeed, bending of the mesogenic core led to a shift of the mesophases towards lower temperatures. Several of the meta-2-phenylpyrimidine derivatives as well as 2-pyrimidine carboxylates displayed melting points below 50 °C. For 2-pyrimidine carboxylates replacement of a bromide anion by triflate resulted in a further decrease of the melting transition close to ambient temperature.
29 citations
TL;DR: The results of optical texture, birefringence, and the tilt angle suggest that a part of the Sm-A* phase is of de Vries type, since an increase in the tilt angles with decreasing temperature results in a reduction in the value of the bireFringence in the smectic-A-Sm-C* phase.
Abstract: Mixtures of different compositions of an antiferroelectric liquid crystal compound that exhibits direct smectic-A*(Sm-A*)-smectic-C*(A) (Sm-C*(A)) transition with a ferroelectric liquid crystal compound that exhibits Sm-A*-smectic-C*(Sm-C*) transition are studied using electro-optics and dielectric spectroscopy. The results of optical texture, birefringence, and the tilt angle suggest that a part of the Sm-A* phase is of de Vries type, since an increase in the tilt angle with decreasing temperature results in a reduction in the value of the birefringence in the Sm-A* phase, whereas the birefringence at Sm-A* to Sm-C* transition goes up by 12.7%. The soft mode relaxation strength, the Landau coefficient of the temperature dependent term, and the other related parameters of the de Vries-type Sm-A-Sm-C*(A) and Sm-A*-Sm-C* transitions are determined using the Landau theory of the second-order phase transition. For the Sm-A*-Sm-C* transition, we find that the soft mode relaxation strength decreases, the Landau coefficient increases, and the Curie-Weiss temperature range decreases with an increased ferroelectric composition in the mixture. These observations can be explained by assuming that with increased ferroelectric composition in the mixture, the layer shrinkage at the de Vries Sm-A*-Sm-C* transition increases. On comparing the results of de Vries-type Sm-A* to Sm-C*(A) and Sm-C* transitions, we find that the soft mode dielectric strength and the other related Landau parameters of the de Vries Sm-A* phase are of the same order of magnitude for transitions from Sm-A* to Sm-C* and to Sm-C*(A) except for the composition of the mixture where both Sm-C* and Sm-C*(A) transitions are stable and the phase diagram shows phase sequence Sm-A* to Sm-C* to Sm-C*(A).
28 citations
TL;DR: In this article, it was shown that the electron-withdrawing effect of the chloro end-group is not due to strong polar interactions at the layer interfaces, as previously postulated in the literature.
Abstract: 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.
26 citations