Li Rebecca L, Yang Anna, Flanders Nathan C, Yeung Michael T, Sheppard Daylan T, Dichtel William R
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208 United States.
J Am Chem Soc. 2021 May 12;143(18):7081-7087. doi: 10.1021/jacs.1c01683. Epub 2021 Apr 28.
Covalent organic frameworks (COFs) generally leverage one or two monomers with specific sizes and shapes to access highly symmetric and periodic polymer networks. Almost all reported COFs employ the minimum sets of monomers needed for the polymerization (usually two, sometimes one) and crystallize in high-symmetry topologies. COFs synthesized from more than two monomers usually employ mixtures with different pendant functionalities to distribute these groups statistically throughout the structure, or monomers with different sizes in ratios targeting lower symmetry topologies. Here, we demonstrate that mixtures of monomers with different lengths generate single-phase, hexagonal two-dimensional covalent organic framework (2D COF) solid solutions at continuously variable feed ratios. X-ray diffraction measurements, Fourier-transform infrared spectroscopy, and Pawley refinement indicate that both monomers distribute randomly within the same lattice, and the lattice parameters continuously increase as more of the larger linker is incorporated. Furthermore, COF solid solutions are accessed directly by polymerizing a mixture of monomers but not via linker exchange from a preformed COF. As strain develops from the lattice accommodating monomers with different sizes, the nonlinear relationship between the monomer incorporation and the COF's lattice parameters suggests that bond-bending of the monomers plays a role in incorporating monomers of different lengths into the solid solutions. Solid solution formation represents a new strategy to design 2D COFs and increase their complexity. Specifically, varying the monomer composition of a given network enables many properties, such as the average pore size, to be continuously tuned between those of corresponding pure COFs.
共价有机框架（COFs）通常利用一种或两种具有特定尺寸和形状的单体来构建高度对称和周期性的聚合物网络。几乎所有已报道的COFs都采用聚合所需的最少单体组合（通常为两种，有时为一种），并以高对称拓扑结构结晶。由两种以上单体合成的COFs通常采用具有不同侧链官能团的混合物，以便将这些基团统计地分布在整个结构中，或者采用不同尺寸的单体以特定比例合成以形成较低对称性的拓扑结构。在这里，我们证明了不同长度的单体混合物在连续可变的进料比下能够生成单相、六方二维共价有机框架（2D COF）固溶体。X射线衍射测量、傅里叶变换红外光谱和 Pawley 精修表明，两种单体在同一晶格中随机分布，并且随着更多较大连接体的加入，晶格参数持续增加。此外，COF固溶体是通过聚合单体混合物直接获得的，而不是通过从预先形成的COF进行连接体交换获得。由于晶格容纳不同尺寸单体时会产生应变，单体掺入与COF晶格参数之间的非线性关系表明，单体的键弯曲在将不同长度的单体掺入固溶体中起作用。固溶体的形成代表了一种设计2D COF并增加其复杂性的新策略。具体而言，改变给定网络的单体组成能够使许多性质，如平均孔径，在相应纯COF的性质之间连续调节。
