二维共价有机框架材料的局限性是因为它们通常以粉晶的形式分离，这使其结构，电子和光学特性变得复杂。该研究表明通过将相关COF颗粒制备为胶体悬浮液，可以将层状2D-COFs合成为单晶。该课题组将这种方法从硼酸和邻苯二酚的缩合扩展到聚硼酸的脱水三聚，所得的与环硼氧烷相连的胶体是作为单晶颗粒获得的下一类2D-COFs，有望成为一种提升2D-COFs材料光电性能的重要方案。

Figure 1. A) Previous work demonstrating stabilization of 2D boronate-ester COFs as colloidal suspensions. B) This work reports the colloidal stabilization of 2D and 3D boroxine-linked COFs, expanding the topological and chemical versatility of this strategy The self-condensation of polyboronic acids in the presence of electron-donating solvents produces colloidal boroxine-linked COFs. This approach is also amenable to the synthesis of a 3D framework, further extending the generality of this polymerization method.

这些COF颗粒的良好分散性和可分离性使其能够使用低剂量高分辨率透射电子显微镜（HR-TEM）和同步加速器X射线衍射（SXRD）进行结构分析。由于COF胶体与它们各自的单体相比具有的高荧光性，包括报道过COFs粉末，但这些实验通常由于粉末样品的高度散射性以及与晶界处物种的发射或固态捕获的杂质而变得复杂。此研究中定义明确的COF胶体可以进行复杂而丰富的光谱表征，包括激发发射矩阵荧光光谱（EEMS）。EEMS显示，COF的增强发射源于荧光的2D分层结构。同时此研究预计这些结果将极大地影响COF合成的方法，并有助于理解2D聚合物和其他框架材料中的空间电子分布。

Figure 2. A) Synthesis of boroxine-linked Ph-COF particles through solvothermal condensation of PBBA (10 mM). B) Photograph comparing the transparent monomer solution to typically isolated Ph-COF powders28 and dispersed Ph-COF crystallites synthesized in the presence of electron-donating solvents. C) Dynamic light scattering (DLS) Z-average size and polydispersity index (PDI) of Ph-COF synthesized with variable initial PBBA concentrations.

Figure 3. A) Monomer structures and optical images of structurally diverse COF nanoparticle suspensions. B) X-ray diffraction (grazing-incidence of drop-cast crystallites: TMPh-COF or in solvo wide-angle X-ray scattering: Ph-COF, BPh-COF, DBD-COF, Py-COF) of boroxine-linked COF nanocrystals.

Figure 4. Orange: Ph-COF; Yellow: BPh-COF; Green: DBD-COF; Blue: Py-COF; Red: TMPh-COF. A-E) Low-resolution TEM images of COF particles. F-J) High-resolution TEM images of COF particles. Inset; lattice fringes of COF particles. K-O) FFTs of COF particles.

Figure 5. Orange: Ph-COF; Yellow: BPh-COF; Green: DBD-COF; Blue: Py-COF. Photographs of COF monomer solutions and their resultant COF nanocrystals A) under natural light and B) under UV irradiation.

Figure 6. A) EEMS of DBD-COF. Inset; optical image of DPBBA monomer (left) and DBD-COF (Right) under UV irradiation B) EEMS of Py-COF. Inset; optical image of PyBA monomer (left) and Py-COF (Right) under UV irradiation. C) Emission of DBD-COF compared to the DPBBA monomer at the same concentration. D) Normalized absorption and emission intensity of Py-COF compared to the PyBA monomer showing a redshift of the absorption spectra and blue shift and broadening of the emission spectra. E) Normalized emission of Py-COF under two excitation wavelengths showing the emergence of an excimer emission (430 nm).

此项研究中展示了在存在供电子溶剂的情况下发生了环硼氧烷连接的骨架的胶体稳定作用，SXRD和HR-TEM提供了对这些材料的层间排列的深入了解，并证实了它们的单晶性质。此外，稳定COF作为胶体悬浮液能够利用EEMS探测这些材料的固有光学性质。这些材料的高质量光谱学将光学特征的起源分配给π激基复合物的形成，这表明在COF结构中紧密结合的生色团通过空间电子联接。同时该课题组预计未来的胶体COF将被证明是研究2D聚合物系统中新兴的光电现象的通用平台，例如三重态上转换，单重态光氧化还原过程.由受控COF聚合获得的高度结晶的材料最终将有助于将2D聚合物并入相关的光电器件中。

本文的第一作者为Austin M. Evans（ Northwestern University，USA），通讯作者为William R. Dichtel（ Northwestern University，USA）。该研究发表在JACS上，“Emissive Single-Crystalline Boroxine-Linked Colloidal Covalent Organic Frameworks ”，DOI：10.1021/jacs.9b08815。