Liquid Crystal Properties of the n-Alkyl-cyanobiphenyl Series from Atomistic Simulations with Ab Initio Derived Force Fields

Abstract

Lengthy molecular dynamics (MD) simulations were performed at constant atmospheric pressure and different temperatures for the series of the 4-n-alkyl-4'-cyanobiphenyls (nCB) with n = 6, 7, and 8. The accurate atomistic force field (Bizzarri, M.; Cacelli, I.; Prampolini, G; Tani, A. J. Phys. Chem. A 2004, 108, 10336), successfully employed to reproduce thermodynamic and transport properties of the 5CB molecule, has here been extended to higher homologues. Nematic and isotropic phases were found for all members of the series, and also, a smectic phase was (tentatively) identified for 8CB at 1 atm and 300 K. Transition temperatures reproduce the experimental values within +/-10 K. Also, structural properties as second and fourth rank orientational order parameters are in good agreement with the corresponding experimental quantities. This means that the well-known odd-even effect, observed for many properties along the nCB series, is well reproduced, despite the narrow range of oscillations, e.g., in clearing temperatures. A detailed analysis of the correlation between molecular properties and odd-even effects is presented.

Comparison of structural properties of some liquid crystals

“…In this study, the shape and inertia tensor anisotropies, ∆L and ∆I , and the biaxialities obtained from molecular dimensions and inertia tensor components, λ L and λ I , were calculated by using Eqs. (3) and (4), respectively, as suggested in previous studies [21,24].…”

“…These include elastic constants [11,12], order parameters [13], flexoelectric coefficients [14,15], transport properties [16][17][18], and static and frequency-dependent dielectric constants [19,20]. Recent lengthy atomistic MD simulations performed to reproduce nematic to isotropic transition temperatures have given satisfactory results for two homolog series, namely phenyl alkyl-4-(4 ′ -cyanobenzelidene) aminocynnamates [21] and 4-n-alkyl-4 ′ -cyanobiphenyls [22][23][24]. Furthermore, these MD simulations have successfully predicted the odd-even effect observed by Gray and Harrison [25].…”

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

“…Nevertheless, the simulated density values represent an improvement on previous modelling which tended to overestimate 1 by about 6 % in the same temperature window. [26,28] …”

“…[31] and by the comprehensive work of the Pisa group in the field. [26,27] During the preparation of this work, a paper has appeared from the same group on cyanobiphenyls, [28] where their fragmentation reconstruction method (FRM) [32] is employed, with a fully atomistic resolution for the cores, thereby obtaining static and dynamic nCB properties with good success. Herein, we opt instead for a parameterization of the force field at united-atom resolution, which has the advantage of reducing the number of centres and of allowing larger integration time steps, increasing the affordable system size and, more importantly, the time window that the computer simulation can span.…”

Computer Simulations of Biaxial Nematics

“…If these optimisations are not sufficient (see e.g. [107,108,109,110]), a long and computationally demanding re-parameterisation procedure is needed [111,110]. We should also notice that atomistic potentials have been successfully used only recently [59] even because no convincing N b had been reported earlier and molecular structures to start with were not available.…”