Towards in Silico Liquid Crystals. Realistic Transition Temperatures and Physical Properties for n‐Cyanobiphenyls via Molecular Dynamics Simulations

Abstract

We study the important n-cyanobiphenyl (with n= 4-8) series of mesogens, using modelling and molecular dynamics simulations. We are able to obtain spontaneously ordered nematics upon cooling isotropic samples of 250 molecules. By using the united-atom force field developed herein, we show that the experimental isotropic-nematic transition temperatures are reproduced within 4 K, allowing a molecular-level interpretation of the odd-even effect along the series. Other properties, like densities, orientational order parameters and NMR residual dipolar couplings are also reproduced well, demonstrating the feasibility of predictive in silico modelling of nematics from the molecular structure.

Efficient analysis of highly complex nuclear magnetic resonance spectra of flexible solutes in ordered liquids by using molecular dynamics

“…36,37 The time window is also adequate for observing possible fluctuation of the 5CB nematic order parameter, as shown in Ref. 27. We have already validated this simulation protocol by studying the ordering effect of 5CB on simple rigid oblate and prolate solutes and comparing the MD predictions with experimental results and mean field theories.…”

“…34,35 The 5CB solvent molecules have been instead described at the united atoms level of detail, using a force field that reproduces the experimental nematic-isotropic transition temperature and a number of other observables for nCB (n = 4 − 8). 27 The Lennard-Jones non-bonded interactions were evaluated using a cutoff of 12 Å, while long-range electrostatic ones were calculated with the particle mesh Ewald method and a mesh size of 1.2 Å. NPT MD simulations were run at atmospheric pressure and scanning the temperature interval T = 285-310 K with steps of 5 K. We used a timestep of 1 fs, a typical MD equilibration time ranging from 30 to 50 ns and a production time of 30 ns for all temperatures, a time long enough to exceed, for these low molecular mass solutes, their typical reorientational and conformational relaxation times. 36,37 The time window is also adequate for observing possible fluctuation of the 5CB nematic order parameter, as shown in Ref.…”

“…[27][28][29] It is therefore tempting to make a detailed comparison between the temperature-dependent results that can be obtained from a liquid-crystal NMR experiment to date, and the predictions on the same physical system that can be made employing state-of-the-art atomistic simulation methods. In order to make such a comparison, we choose the benchmark system of n-pentane in the nematic phase of 5CB, studied as a function of temperature.…”

Cost-Effective Force Field Tailored for Solid-Phase Simulations of OLED Materials

“…The torsional parameters V n , n, γ n were borrowed from the AMBER95 dataset for what concerns the rigid ones, e.g. torsions involving carbon atoms belonging to the same aromatic ring, while soft, anharmonic torsions were here parametrized with quantum chemistry calculations 29,41,44 at PBE0-D3(BJ)/def2-TZVP level. In practice, only three types of torsion were reworked: phenyl-phenyl, phenylcarbazole, and the improper torsion involving the out-of-plane vibration of sp 2 nitrogen; the phenyl-nitrogen torsion in TPA being modeled with standard parameters, as it is dominated by steric repulsion which confers it a propeller-like structure.…”

Phase Organization of Mesogen-Decorated Spherical Nanoparticles

“…While simulations at space resolutions going from the mesoscopic scale, with positions arbitrarily fixed on lattices [16], to the molecular scale with off-lattice coarse grained [17] or even down to atomistic [18] level of detail exist for many mesogenic systems of various complexity [19-23], we are not aware of any modelling and simulation studies trying to understand the phases and the ordering process in these exotic systems which, on the other hand, have many promising applications, e.g. in the field of optical metamaterials and more generally of novel functional materials with hybrid properties [24][25][26][27].…”