Comparison of structural properties of some liquid crystals

Turk J Phys (2017) 41: 124 – 132 Turkish Journal of Physics http://journals.tubitak.gov.tr/physics/ c TÜBİTAK ⃝ doi:10.3906/fiz-1610-6 Research Article Comparison of structural properties of some liquid crystals E. Mine ÇAPAR∗ Department of Physics, Faculty of Science, Ege University, İzmir, Turkey Received: 04.10.2016 • Accepted/Published Online: 27.12.2016 • Final Version: 18.04.2017 Abstract:Molecular dynamics simulations were performed for three liquid crystalline molecules, 4-cyano-4 ′ -octyloxybiphenyl (8OCB), n-p-cyanobenzylidene-p-octyloxyaniline (CBOOA), and p-n-hexyloxybenzylidene-p ′ -aminobenzonitrile (HBAB). Simulation data were obtained for three liquid crystals in the nematic and isotropic phases to analyze their structural properties. The imine spacer group in the Schiff bases induces a stepped core structure in which the linearity is maintained. The distributions of the angles between some defined vectors imply that the core segments of CBOOA and HBAB are more floppy than the 8OCB core. The biaxialities, molecular dimensions, and molecular anisotropies were calculated. The results were compared for three mesogens in the nematic and isotropic phases. Key words: Liquid crystals, Schiff base, molecular dynamics simulation, structural properties 1. Introduction Liquid crystals (LCs) have been employed in a range of important electrooptic devices including today’s mobile phones, laptops, and flat panel televisions [1–3]. Many biological materials give structural features that are closely related to those of liquid crystals [4]. Therefore, mesophases have attracted special scientific attention that can be ascribed to a need for understanding of biological systems and to many technical applications. A combination of theoretical aspects of statistical mechanics, computer simulations, and experiments provides a useful approach to establish a relationship between liquid-crystalline properties and molecular structure [5–8]. Over the last decade, with the increase of computational power, molecular dynamics (MD) simulations have been used to study mesophases in more detail. Recently, a number of MD simulation studies of LCs have been carried out with not only less complex molecular models [9] but also to obtain additional information on a molecular level with very complex atomistic models [10]. Atomistic simulations can yield a wealth of information about liquid crystal phases. These include elastic constants [11,12], order parameters [13], flexoelectric coefficients [14,15], transport properties [16–18], 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 [22-24]. Furthermore, these MD simulations have successfully predicted the odd-even effect observed by Gray and Harrison [25]. These encouraging results motivated us to investigate structural properties of cyano compounds different from the previously investigated ones by means of atomistic MD simulations. There has been considerable interest in the strongly polar cyanobiphenyl-based liquid crystals with the ∗ Correspondence: 124 ÇAPAR/Turk J Phys observation of the twisted nematic effect by Schadt and Helfrich [26]. Schiff bases and alkoxy biphenyls with cyano substituents along their long molecular axis are commonly employed in twisted nematic displays with low threshold voltages. To widen the nematic range, their eutectic mixtures are used [27,28]. The other reason why many reports are still concerned with these types of materials is reentrant behavior in these LCs discovered by Cladis [29]. The reentrant nematic phase of 4-cyano-4 ′ -octyloxybiphenyl (8OCB) was observed at high pressure [30]. In certain binary mixtures of 8OCB, n-p-cyanobenzylidene-p-octyloxyaniline (CBOOA) and p-n-hexyloxybenzylidene-p ′ -aminobenzonitrile (HBAB) exhibit the reentrant nematic phase [31]. The cyanobiphenyl and two Schiff bases with alkoxy end groups chosen in this work were three typical liquid crystals, 8OCB, CBOOA, and HBAB. In this study, atomistic MD simulations for 8OCB, CBOOA, and HBAB in the nematic and isotropic phases were performed. In the following, the results of simulations on 8OCB, CBOOA, and HBAB are reported and their structural properties are compared. 2. Materials and methods 2.1. Molecular dynamics simulation Atomistic MD calculations on three samples each consisting of 8OCB, CBOOA, or HBAB molecules were carried out by using GROMACS (version 4.5.5) software [32]. In each system modeled as in Figure 1, the aromatic hydrogens were explicitly considered, while the CH 2 and CH 3 groups were treated as single interaction centers. Figure 1. Assignment of the molecular models for the three molecules with the numbering of five atoms: (a) 4-cyano4 ′ -octyloxybiphenyl (8OCB), (b) n-p-cyanobenzylidene-p-octyloxyaniline (CBOOA), (c) p-n-hexyloxybenzylidene-p ′ aminobenzonitrile (HBAB). The bonding and nonbonding interactions of mesogens were based on the OPLS force field [33,34] using some additional parameters taken from previous studies [21,35]. The fractional atomic charges were determined 125 ÇAPAR/Turk J Phys by using the CHELPG [36] scheme following the Hartree–Fock geometry optimization with a 6-31G** basis set. The charges were calculated with the GAUSSIAN 03 program package [37]. Bond stretching was explicitly considered and a time step of 0.5 fs was used for the integration. Nonbonding interactions were truncated by a cutoff radius of 1 nm. Periodic boundary conditions were applied in all three dimensions. The long-range electrostatic interactions were evaluated with the particle-mesh Ewald method [38]. Initially, each simulation was started from a cubic lattice of 512 molecules oriented with perfect order at a low density. The orientations of the molecules were altered in order to prevent a net dipole moment from occurring. The systems at low density were compressed by high pressure (P = 50 atm) at the initial temperature of 250 K. The configurations for which the system density almost equaled 1 g/cm 3 were chosen as initial configurations for equilibration, and then each system was equilibrated at atmospheric pressure by slowly increasing the initial temperature to 346 K, 373 K, and 368 K for nematic 8OCB, CBOOA, and HBAB, respectively, while the temperatures were increased to 500 K for isotropic 8OCB and to 530 K for isotropic CBOOA and HBAB. In the NPT ensemble, a v-rescale thermostat [39] and Berendsen barostat [40] were used. The molecular long axis was found by the diagonalization of the inertial tensor: Iab = N ∑ ( ) mi ri2 δab − ria rib , (1) i=1 where ri and mi are the positions relative to the molecular center of mass and the masses of the atoms, respectively. The diagonal tensor components are averag (...truncated)