Molecular dynamics simulations of nematic phases formed by cyano-biphenyl dimers

Abstract

Molecular dynamics simulations of selected members of the cyano-biphenyl series of dimers (CBnCB) have been set up using atomistic detail interactions among intermolecular pairs of united atoms and allowing fully for the flexibility of the spacer chain. Detailed results are presented for the CB7CB dimers, showing an isotropic fluid phase and two nematic phases. The positional and orientational correlation functions extracted from the simulations are used to elucidate the structure of the low-temperature nematic phase. Polar molecular ordering is clearly identified along a direction undergoing helical twisting at right angles to the helical axis, with a constant pitch of about of 8nm. The local ordering of the various molecular segments is calculated and found to be in excellent agreement with experimental NMR measurements. Key findings of the simulation are shown to be correctly predicted by the theoretical model of the polar-twisted nematic (NPT) phase [A.G. Vanakaras, D.J. Photinos, Soft Matter. 12 (2016) 2208-2220]. The complete failure of the usual twist bend model (NTB) to account for these findings is demonstrated. 2 direction, represented by a unit vector n, the nematic phase "director". The uniaxiality of the phase implies full rotational symmetry (i.e. invariance of all the physical properties) about the director whilst the a-polarity of NU implies the physical indistinguishability of the directions n and -n. The lowest free energy state of the bulk NU phase corresponds to spatially uniform director field. Mild deformations of the director field, preserving the local symmetry of the phase and the degree of molecular ordering, give rise to elastic behavior of the bulk NU; three types of such director-field deformation are possible: bend, splay and twist. [1] Despite theoretical predictions suggesting the possibility of various deviations from the NU phase, corresponding to biaxial nematic phases (NB) [2], polar nematic phases, [3,4] uniaxial [5] (NPU) or biaxial[6] (NPB), spontaneous twist-bend (NTB) nematics [7], etc, the NU phase remained until recently the only stable thermotropic nematic phase that could be produced experimentally. Thus, unlike the rich polymorphism found in the smectic phases or in the columnar phases of LCs [8,9], all known thermotropic nematics belonged to the NU phase and its chiral counterpart (N*). Chiral nematic phases were known to be produced only by chiral mesogens. Such phases usually show spontaneous twist of the director n, with helical pitch often in the optical wavelength regime. Close to the isotropic phase (I) transition temperature, the helical structures may show three-dimensional topological order, leading to the blue phases (BPs).The above landscape of the nematic phase, comprising only liquid crystals of the NU phase, presently termed as "common nematics", and the chiral N* (also known as cholesteric), has changed significantly, and somehow surprisingly, as a result of novel experimental findings in the last 15 years: (i) The long predicted biaxial nematic phase ...

Distinct twist-bend nematic phase behaviors associated with the ester-linkage direction of thioether-linked liquid crystal dimers

“…On the other hand, another model, named the polar-twisted nematic (N PT ) phase, for the heliconical mesophase identified as the N TB phase was proposed, which is under debate. [15][16][17] Macroscopic chirality of the N TB phase was verified using circular dichroism spectroscopy. 18 Several techniques, including electro-optical measurements, 19 freeze-fracture transmission electron microscopy, 20,21 and resonant soft, hard, and tender X-ray scattering measurements, [22][23][24][25][26][27] suggest that the helical pitch of an N TB phase formed by several molecules is B10 nm, which is significantly shorter than the pitch of a typical chiral N phase or cholesteric phase exhibiting a helicoidal helix (generally hundreds of nanometers).…”

Probing molecular ordering in the nematic phases of para-linked bimesogen dimers through NMR studies of flexible prochiral solutes

“…The latter has been successful in accounting for the main structural feature of the Nx phase, namely short-pitch modulation of the orientational ordering in the presence of complete positional disorder together with fundamental features of the molecular interactions as revealed by NMR spectroscopy. [25,26] A distinguishing NMR signature of the Nx phase, which has been used to clearly identify the N-NX phase transition is the doubling of spectral lines associated with pairs of prochiral sites (enantiotopic discrimination). [2,4,7,8] Such pairs show coincident residual dipolar and quadrupolar NMR spectral lines in the N phase.…”

Thioether-linked azobenzene-based liquid crystal dimers exhibiting the twist-bend nematic phase over a wide temperature range

“…Electro-optical analysis [55], transmission electron microscopy [56,57], and soft, tender, and hard resonant X-ray scattering [34,[58][59][60][61] confirmed that the helical pitch of the NTB phase is considerably short, typically the length of several molecules or 10-20 nm. On the other hand, another heliconical model for the Nx phase, identified as the NTB phase, was proposed, and it was named the polar twisted nematic (NPT) phase [62][63][64][65][66]. It has been suggested that the identification of the NTB phase to date is inadequate: the Nx phase identified as the NTB phase is different from the true NTB phase predicted originally by Meyer.…”