Atomistic insights into liquid crystals of board-like molecules via molecular dynamics simulation.

Abstract

As the temperature decreases, rigid anisotropic molecules that usually incorporate polar groups, aromatic rings or multiple bonds, orient along a common direction, eventually forming liquid-crystalline phases under specific thermodynamic conditions. This study explores the phase behavior and dynamics of board-shaped mesogens with a 1,4,5,8-tetraphenyl-anthraquinone core and four lateral arms forming an oligo(phenyleneethynylene) scaffold. These molecules are promising candidates for forming the elusive biaxial nematic phase. Through atomistic molecular dynamics simulations, we observe the formation of nematic and smectic liquid crystals, in qualitative agreement with experimental observations. To characterize the structure, we compute pair correlation functions along relevant symmetry directions and the nematic order parameter, which indicate a dominant uniaxial ordering with very weak biaxiality. In addition, we analyze the dynamics of our board-shaped mesogens along and perpendicular to the nematic director, revealing an intriguing non-Gaussian behavior and dynamical heterogeneities, with coexisting slow and fast molecules. Building on our recent simulations at the colloidal scale, which demonstrated that monodisperse board-like particles are unable to form biaxial nematics while polydisperse particles can, we hypothesize that a similar behavior may occur at the molecular scale in mixtures of molecules. Although pure-component molecular systems reveal weak biaxiality, our findings suggest that investigating mixtures of the most promising candidates, those molecules that form nematic or smectic phases, could uncover conditions conducive to the formation of biaxial nematic liquid crystals.