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Nucleation and Growth of Metastable Polymorphs on Siloxane Monolayer Templates

机译：硅氧烷单层模板上亚稳多晶型的成核与生长

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The development of methods aimed at polymorph screening and effecting phase control is of broad interest to the field of molecular materials. The present work is the most extensive systematic combinatorial study of molecular crystal growth in the presence of modified siloxane templates. 1,3-Bis(m-nitrophenyl)urea (MNPU) is known to crystallize from solution in three polymorphic forms (alpha, beta, delta) as well as a hydrate (gamma), typically concomitantly. Previous growth studies of MNPU on gold-thiol self-assembled monolayers (SAMS) (J. Am. Chem. Soc. 2005, 127, 18321-18327) demonstrated phase selectivity and preferred orientations which could be rationalized on the basis of thermodynamic stabilization at the SAM/crystal interface. Crystallization studies of MNPU on 11 different siloxanes in 3 unique solvents showed that in general siloxane SAMS tended to favor the nucleation and growth of metastable phases (beta, delta, gamma) and completely suppress the growth of the most stable alpha phase. A new epsilon-MNPU phase not previously observed in either solution or template-directed studies was also identified. Differences in the preferred orientations and phase selectivity relative to Au-S SAMS growth studies suggest that kinetic factors play a more pronounced role during interfacial stabilization on siloxane SAMS. This method serves as an effective complement to existing strategies used in the discovery and reliable generation of polymorphs, and may have useful applications in the polymorph screening of other molecular crystal systems.
机译：旨在多晶型物筛选和实现相控制的方法的开发对于分子材料领域具有广泛的兴趣。当前的工作是在改性硅氧烷模板存在下分子晶体生长的最广泛的系统组合研究。已知1,3-双（间硝基苯基）脲（MNPU）通常从溶液中以三种多晶型形式（α，β，δ）和水合物（γ）结晶。 MNPU在金-硫醇自组装单分子膜（SAMS）上的先前生长研究（J. Am。Chem。Soc。2005，127，18321-18327）显示了相选择性和优选取向，可以根据在60℃下的热力学稳定性对其进行合理化SAM /晶体接口。 MNPU在3种独特溶剂中的11种不同硅氧烷上的结晶研究表明，一般而言，硅氧烷SAMS倾向于促进亚稳相（β，δ，γ）的成核和生长，并完全抑制最稳定的α相的生长。还确定了以前在溶液或模板指导研究中未观察到的新的ε-MNPU相。相对于Au-S SAMS生长研究，优选方向和相选择性的差异表明，动力学因素在硅氧烷SAMS的界面稳定过程中起着更为明显的作用。该方法可有效补充现有的多晶型物发现和可靠生成中使用的策略，并且可能在其他分子晶体系统的多晶型物筛选中具有有用的应用。

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