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°. Analysis of the bend angle between the mesogenic units reveals an average angle of 127°, which is invariant to the simulated phase. We have calculated distributions of the chirality order parameter, χ , 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 CB n CB (where n = 6, 7, 8 or 9) and CBX(CH 2 ) 5 YCB (where X/Y = CH 2 , 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 | χ | 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(CH 2 ) 5 YCB, it is found that | χ | max increases as the bend angle of the dimer decreases, while the flexibility of the dimers remains unchanged through the series.