Molecular orientational distribution function of a chiral de Vries smectic liquid crystal from birefringence measurements

Abstract

An alternative method for determining the orientational distribution function and the order parameter from the electric field-induced birefringence measurements of a chiral liquid crystal compound in its Smectic A* is being introduced. A chiral mesogen based on a 5-phenyl-pyrimidine benzoate core terminated by a trisiloxane group on one side and the chiral alkyloxy chain on its opposite side is designed and synthesized to exhibit the “de Vries” smectic characteristics. The compound exhibits first order Smectic A*–Smectic C* phase transition, evidenced by the results of differential scanning calorimetry. The material is being investigated by electro-optical experiment in its smectic phases. We present a model that incorporates the generalised Langevin-Debye model which includes the Maier-Saupe effective mean-field potential term in order to explain the change in birefringence with the electric field. A good agreement between the experimental results and the predictions from the model leads to the determination of the molecular orientational distribution function in Smectic A phase. Furthermore, the temperature dependency of the Saupe orientational order parameter ⟨P2⟩ is obtained using the parameters of the model. Based on the experimental and theoretical results, we show that de Vries Smectic A* phase exhibits a broad volcano-like tilt angle distribution with the two maxima occurring at finite tilt angles closer to the Smectic A*–Smectic C* transition temperature, and a sugarloaf-like distribution occurs in the tilt for temperatures close to the Isotropic—Smectic A* phase transition.

Fluorenone imidazolium salts as novel de Vries materials

“…[26][27][28][29] De Vries materials have created much interest recently, because ferroelectric displays based on de Vries materials might overcome limitations of current nematic LC displays such as limited resolution and low optical efficiency. [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] One of the characteristic features of de Vries materials is their tilting transition without defect generation. These materials show no or only very low layer contraction upon the SmA to SmC phase transition, enabling ferroelectric displays without zig-zag defects caused by the chevron-like arrangement of the molecules in the mesophase.…”

“…These materials show no or only very low layer contraction upon the SmA to SmC phase transition, enabling ferroelectric displays without zig-zag defects caused by the chevron-like arrangement of the molecules in the mesophase. With regard to the design of calamitic liquid crystals displaying SmA and SmC phases and de Vries behaviour most previous work has focused on 5-phenylpyrimidines, 39,41,43,[48][49][50][51][52][53] 2phenylpyrimidines, [54][55][56][57][58] biphenyls, 40,44,[59][60][61][62][63][64] thiadiazoles, 54,65 aroylhydrazones 66 and bent-core mesogens. 37,[67][68][69][70][71][72][73][74] Lemieux and Ivanov have reported independently for non-charged liquid crystals 3-5 that a rigid uorenone core promotes formation of the SmC phase.…”

Stereochemical Rules Govern the Soft Self‐Assembly of Achiral Compounds: Understanding the Heliconical Liquid‐Crystalline Phases of Bent‐Core Mesogens

“…However, the linear increase of d from 1/6 to 1/14 (Figure b) suggests that the tilt should be almost identical in these smectic phases. The average birefringence of the planar sample does not significantly change at the de Vries to SmC a P A transition (Figure b–e), though an increase of birefringence is usually observed at the de Vries SmA to SmC s transition of rod‐like molecules . In contrast, the de Vries SmA to anticlinic SmC a transition is associated with a decrease of birefringence .…”