弯曲液晶二聚体的全原子模拟：扭曲弯曲向列相和对构象手性的洞察,Soft Matter

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弯曲液晶二聚体的全原子模拟：扭曲弯曲向列相和对构象手性的洞察
Soft Matter ( IF 2.9 ) Pub Date : 2022-03-25 , DOI: 10.1039/d2sm00291d
Gary Yu ₁ , Mark Richard Wilson ₁

Affiliation

已知液晶二聚体 1,7-bis-4-(4'-cyanobiphenyl)heptane (CB7CB) 表现出向列-向列相变，低温相被确定为扭曲弯曲向列 (N _TB ) 相. 尽管介晶具有非手性性质，但 N _TB相通过螺旋结构的自发形成表现出新兴的手性。我们使用全原子力场对 CB7CB 进行了广泛的分子动力学模拟。在该模型中观察到N _TB相，并且在加热时显示相转变为向列相 (N) 和各向同性相。模拟的 N _TB相位返回 8.35 nm 的间距和 29° 的锥形倾斜角。对介晶单元之间弯曲角度的分析显示平均角度为 127°，这对于模拟相位是不变的。我们已经计算了 N _TB和 N 相中构象异构体集合的手性顺序参数χ的分布。这些分布说明 CB7CB 在统计上是非手性的，但可以采用手性构象异构体，而不偏好特定的手性。_{此外，N TB}和 N 相之间的构象手性程度没有变化。使用气相中的单分子随机动力学模拟，我们研究了二聚体系列 CB n CB（其中n = 6、7、8 或 9）和 CBX（CH₂ )₅ YCB (其中X/Y = CH₂、O或S)在弯曲角和构象手性方面。我们证实，弯曲的分子形状决定了二聚体表现出 N_TB相的能力，而不是其假设手性构象异构体的潜力；作为 | χ | _max随着间隔长度的增加而增加，但偶数二聚体具有线性形状，这与具有奇数奇偶校验间隔的二聚体的弯曲性质形成对比。对于CBX(CH₂ )₅ YCB，发现| χ | _max随着二聚体弯曲角的减小而增加，而二聚体的柔韧性在整个系列中保持不变。

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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 CBnCB (where n = 6, 7, 8 or 9) and CBX(CH₂)₅YCB (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 |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(CH₂)₅YCB, 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.

更新日期：2022-03-25

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