Koo B T, Heden R F, Clancy P
Robert F. Smith School of Chemical & Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA.
Phys Chem Chem Phys. 2017 Apr 12;19(15):9745-9754. doi: 10.1039/c6cp08449d.
We establish a theoretical foundation for understanding the nucleation and growth of 2D covalent organic frameworks (COFs) from solution. This foundation should make it easier to realize some of the unique properties of COFs in targeted applications by allowing us to understand how processing variables such as solvent choice and linkage chemistry lead to larger crystalline domains. We use free energy techniques to map out the reaction mechanisms and activation energies of three fundamental reactions that are responsible for the early stages of 2D COF nucleation for a prototypical and commonly used 2D boronate ester material, COF-5, in water and methanol solvents. We show that the presence of water and methanol greatly catalyzes the boronate ester formation reactions, lowering the activation energy barrier by about 10 kcal mol relative to an uncatalyzed reaction pathway. This is in good agreement with experimental observations by Smith and Dichtel (JACS 2014). Our crystallization studies also conclusively eliminate certain proposed mechanisms of growth, such as polymerization of large sheets followed by stacking, while strengthening the case for templated polymerization as a likely growth mechanism for COF crystals.
我们为理解二维共价有机框架（COF）从溶液中的成核和生长建立了理论基础。通过让我们了解诸如溶剂选择和连接化学等加工变量如何导致更大的结晶域，这一基础应有助于在目标应用中更轻松地实现COF的一些独特性能。我们使用自由能技术来描绘出三种基本反应的反应机理和活化能，这三种反应是二维COF在水和甲醇溶剂中形成原型且常用的二维硼酸酯材料COF-5成核早期阶段的原因。我们表明，相对于未催化的反应途径，水和甲醇的存在极大地催化了硼酸酯形成反应，将活化能垒降低了约10千卡/摩尔。这与史密斯和迪特尔（《美国化学会志》，2014年）的实验观察结果高度一致。我们的结晶研究也最终排除了某些提出的生长机制，例如大片层聚合后堆叠，同时强化了模板聚合作为COF晶体可能的生长机制的情况。
