All-atom simulations of bent liquid crystal dimers: the twist-bend nematic phase and insights into conformational chirality

The liquid crystal dimer 1,7-bis-4-(4 '-cyanobiphenyl)heptane (CB7CB) is known to exhibit a nematic-nematic phase transition, with the lower temperature phase identified as the twist-bend nematic (N-TB) phase. Despite the achiral nature of the mesogen, the N-TB phase demonstrates emergent chirality through the spontaneous formation of a helical structure. We present extensive molecular dynamics simulations of CB7CB using an all-atom force field. The N-TB phase is observed in this model and, upon heating, shows phase transitions into the nematic (N) and isotropic phases. The simulated N-TB phase returns a pitch of 8.35 nm and a conical tilt angle of 29 degrees. Analysis of the bend angle between the mesogenic units reveals an average angle of 127 degrees, which is invariant to the simulated phase. We have calculated distributions of the chirality order parameter, chi, for the ensemble of conformers in the N-TB and N phases. These distributions elucidate that CB7CB is statistically achiral but can adopt chiral conformers with no preference for a specific handedness. Furthermore, there is no change in the extent of conformational chirality between the N-TB and N phases. Using single-molecule stochastic dynamics simulations in the gas phase, we study the dimer series CBnCB (where n = 6, 7, 8 or 9) and CBX(CH2)(5)YCB (where X/Y = CH2, O or S) in terms of the bend angle and conformational chirality. We confirm that the bent molecular shape determines the ability of a dimer to exhibit the N-TB phase rather than its potential to assume chiral conformers; as |chi|(max) increases with the spacer length, but the even-membered dimers have a linear shape in contrast to the bent nature of dimers with spacers of odd parity. For CBX(CH2)(5)YCB, it is found that |chi|(max) increases as the bend angle of the dimer decreases, while the flexibility of the dimers remains unchanged through the series.