Orlandi Silvia, Muccioli Luca, Ricci Matteo, Berardi Roberto, Zannoni Claudio
Dipartimento di Chimica Fisica ed Inorganica and INSTM, Università di Bologna, Bologna, Italy.
Chem Cent J. 2007 Jun 8;1:15. doi: 10.1186/1752-153X-1-15.
The relation between the structure of discotic molecules and columnar properties, a crucial point for the realization of new advanced materials, is still largely unknown. A paradigmatic case is that hexa-alkyl-thio substituted triphenylenes present mesogenic behavior while the corresponding azatriphenylenes, similar in shape and chemical structure, but with a different core charge distribution, do not form any liquid crystalline mesophase. This study is aimed at investigating, with the help of computer simulations techniques, the effects on phase behaviour of changes of the charge distribution in the discotic core.
We described the shape and the pair, dispersive and electrostatic, interactions of hexa alkyl triphenylenes by uniaxial Gay-Berne discs with embedded point charges. Gay-Berne parameters were deduced by fitting the dispersive energies obtained from an atomistic molecular dynamics simulation of a small sample of hexa-octyl-thio triphenylene molecules in columnar phase, while a genetic algorithm was used to get a minimal set of point charges that properly reproduces the ab anitio electrostatic potential. We performed Monte Carlo simulations of three molecular models: the pure Gay-Berne disc, used as a reference, the Gay-Berne disc with hexa-thio triphenylene point charges, the Gay-Berne disc with hexa-thio azatriphenylene point charges. The phase diagram of the pure model evidences a rich polymorphism, with isotropic, columnar and crystalline phases at low pressure, and the appearance of nematic phase at higher pressure.
We found that the intermolecular electrostatic potential among the cores is fundamental in sta-bilizing/destabilizing columnar phases; in particular the triphenylene charge distribution stabilizes the columnar structure, while the azatriphenylene distribution suppresses its formation in favor of the nematic phase. We believe the present model could be successfully employed as the basis for coarse-grained level simulations of a wider class of triphenylene derivatives.
盘状分子的结构与柱状性质之间的关系是实现新型先进材料的关键，但目前仍知之甚少。一个典型的例子是，六烷基硫代取代的三亚苯呈现介晶行为，而相应的氮杂三亚苯，其形状和化学结构相似，但核心电荷分布不同，却不形成任何液晶中间相。本研究旨在借助计算机模拟技术，研究盘状核心中电荷分布变化对相行为的影响。
我们用嵌入点电荷的单轴 Gay-Berne 圆盘描述了六烷基三亚苯的形状以及成对的、色散和静电相互作用。通过拟合从柱状相中一小部分六辛基硫代三亚苯分子的原子分子动力学模拟获得的色散能来推导 Gay-Berne 参数，同时使用遗传算法获得一组最小的点电荷，以恰当地再现从头算静电势。我们对三个分子模型进行了蒙特卡罗模拟：用作参考的纯 Gay-Berne 圆盘、带有六硫代三亚苯点电荷的 Gay-Berne 圆盘、带有六硫代氮杂三亚苯点电荷的 Gay-Berne 圆盘。纯模型的相图显示出丰富的多态性，在低压下有各向同性、柱状和结晶相，在高压下出现向列相。
我们发现核心之间的分子间静电势对于柱状相的稳定/不稳定至关重要；特别是三亚苯电荷分布稳定了柱状结构，而氮杂三亚苯分布抑制了其形成，有利于向列相。我们相信本模型可成功用作更广泛类别的三亚苯衍生物粗粒度模拟的基础。
