Phase behavior of bent-core molecules

Abstract

Recently, a new class of smectic liquid crystal phases (SmCP phases) characterized by the spontaneous formation of macroscopic chiral domains from achiral bent-core molecules has been discovered. We have carried out Monte Carlo simulations of a minimal hard spherocylinder dimer model to investigate the role of excluded volume interations in determining the phase behavior of bent-core materials and to probe the molecular origins of polar and chiral symmetry breaking. We present the phase diagram as a function of pressure or density and dimer opening angle ψ. With decreasing ψ, a transition from a nonpolar to a polar smectic phase is observed near ψ = 167• , and the nematic phase becomes thermodynamically unstable for ψ < 135• . No chiral smectic or biaxial nematic phases were found. [5]. In all of these examples, chirality is an intrinsic property built into the chemical structure of the LC molecules. Recently, a new class of smectic LC phases (SmCP phases) characterized by the spontaneous formation of macroscopic chiral layers from achiral molecules has been discovered [6,7]. The molecules comprising these phases have 'bow' or 'banana' shaped cores. The resulting phases exhibit two spontaneous symmetry-breaking instabilities: polar molecular orientational ordering within the layer plane, and molecular tilt, which together produce chiral layers with a handedness that depends on the direction of the tilt relative to the polar axis. Very large second order nonlinear optical (NLO) coefficients have been measured in the ferroelectric state of such materials, bearing some promising applications in NLO devices [8,9].From a theoretical point of view, the relationship of phase behavior to the specific bent-core molecular shape is of fundamental interest. In this paper, we investigate a minimal excluded volume model of bent-core mesogens, focusing on the molecular origin of polar and/or chiral symmetry breaking. Of particular interest is the coupling between polar and chiral symmetry breaking. In all bentcore materials studied to date, polar symmetry breaking is accompanied by chiral symmetry breaking induced by molecular tilt. This empirical fact raises the question whether there is a fundamental connection between polarity and chirality in molecular fluids. Another empirical observation is that bent-core materials exhibiting SmCP phases generally do not exhibit nematic phases, although two exceptions have recently been reported [10,11]. One objective of this study is to establish the molecular shape requirements for the occurence of the nematic phase in bent-core materials. Finally, we explore the possibility of biaxial nematic ordering in bent-core materials, motivated by recent experimental indications [11,12].Hard core models are particularly appealing due to their simplicity and relative ease of computation, both in simulation and theory. In particular, hard spherocylinders have been widely studied as simple models for conventional LCs [13,14]. This model exhibits rich phase behavior including isotropic, nematic, s...

Dipolar interactions, molecular flexibility, and flexoelectricity in bent-core liquid crystals

“…Digits in brackets denote the estimated statistical uncertainty in the last figure based on two standard deviations. (6) 2.93(6) 0.56 (4) observation that these same interactions reduce molecular tilt seems to suggest that there may be another explanation for chirality in banana liquid crystals 5,6,7 . Real bent-core liquid crystals often possess flexible tail groups at the ends of the rigid bent core, and so CBMC simulations of a flexible-rigid-flexible model were performed.…”

“…This is probably due to the tails providing a lubricating barrier between the smectic layers that serves to decorrelate the order within neighboring layers. Hence, the addition of molecular flexibility is likely to mitigate against the type of entropic 'sawtooth' mechanisms that have been found to stabilize antiferroelectric ordering in hard-particle bent-core models 6 . The addition of the flexible tails was also seen to give rise to significant spatial fluctuations in the smectic layers.…”

“…One popular explanation involves the long-range dipole-dipole interaction 4 , while recent theoretical work has identified a central role for dispersion interactions 5 . Other possible explanations include entropic 'free-volume' mechanisms 6 in which an antiferroelectric ordering of the smectic-layer polarizations affords more room for layer fluctuations, and mechanisms in which the molecules themselves spontaneously select chiral molecular conformations 7 .…”

“…One of the most simple bentcore molecular models is a dimer made up of two hard spherocylinders 6,8 . This system exhibits isotropic, nematic, smectic, and crystalline phases, but no tilted phases.…”

Optical Simulation of Viewing Angle Property of Biaxial Nematic Bent-Core Liquid Crystal

“…The BLC medium showed various new physical properties and suggested important clues for understanding the underlying physics in the condensed matter [1][2][3][4]. The BLC also has a great potential of new electro-optical applications [5][6][7][8][9][10][11][12][13].…”