Designed Synthesis of 3D Covalent Organic Frameworks
三维共价有机框架的设计合成

The chemistry of linking organic molecules together by means of covalent bonds to isolate crystals of discrete zero-dimensional (0D) molecules and 1D chains (polymers) is well established; however, it is undeveloped for 2D and 3D COFs (1). COF structures that contain light elements (B, C, N, and O) should be able to combine the thermodynamic strength of covalent bonds, as those found in diamonds and boron carbides, with the functionality of organic units. Progress in this area has been impeded by longstanding practical and conceptual challenges. First, unlike 0D and 1D systems, the insolubility of 2D and 3D structures precludes the use of stepwise synthesis and makes their isolation in crystalline form very difficult. Second, the number of possible structures that may result from linking specific building-unit geometries into 2D or 3D extended structures is essentially infinite and complicates their synthesis by design.
通过共价键将有机分子连接在一起以分离出离散的零维（0D）分子晶体和一维链（聚合物）的化学原理已相当成熟；然而，对于二维和三维共价有机框架（COFs），这一领域仍处于起步阶段（1）。含有轻元素（B、C、N 和 O）的 COF 结构应能结合共价键的热力学强度，如在金刚石和碳化硼中发现的那些，以及有机单元的功能性。这一领域的进展一直受到长期存在的实际和概念性挑战的阻碍。首先，与 0D 和 1D 系统不同，二维和三维结构的不可溶性排除了逐步合成的方法，使得它们以晶体形式分离极为困难。其次，将特定构筑单元几何形状连接成二维或三维扩展结构可能产生的结构数量几乎是无限的，这使得通过设计进行合成变得复杂。

We recently illustrated how the first challenge could be overcome by judiciously choosing building blocks and using reversible condensation reactions to crystallize 2D COFs in which organic building blocks are linked entirely by strong covalent bonds (2). Here we report how the design principles of reticular chemistry overcome the second challenge (3): Two nets based on the linking of triangular and tetrahedral shapes were selected and targeted for the synthesis of four 3D COFs.
我们最近展示了如何通过审慎选择构建模块并利用可逆缩合反应来克服第一个挑战，从而使有机构建模块完全通过强共价键连接的二维 COFs 结晶（2）。在此，我们报告了网状化学设计原则如何克服第二个挑战（3）：选择了基于三角形和四面体形状连接的两个网络，并将其作为合成四种三维 COFs 的目标。

Self-condensation and co-condensation reactions of the rigid molecular building blocks, the tetrahedral tetra(4-dihydroxyborylphenyl)methane (TBPM), and its silane analog (TBPS), and triangular hexahydroxytriphenylene (HHTP) (Fig. 1, A to C) provided crystalline 3D COFs (termed COF-102, COF-103, COF-105, and COF-108). These COFs are the most porous among organic materials, and a member of this series (COF-108) has the lowest density reported of any crystalline material. Without our a priori knowledge of the expected underlying nets of these COFs, their synthesis by design and the solution of their structures from powder x-ray diffraction (PXRD) data would have been prohibitively difficult.
刚性分子构建单元——四面体四(4-二羟基硼苯基)甲烷(TBPM)及其硅烷类似物(TBPS)，以及三角形六羟基三苯(HHTP)的自缩聚和共缩聚反应(图 1，A 至 C)，生成了结晶的三维共价有机框架(COFs，分别命名为 COF-102、COF-103、COF-105 和 COF-108)。这些 COFs 在有机材料中具有最高的孔隙率，其中一员(COF-108)的密度是所有已知晶体材料中最低的。若无我们对这些 COFs 预期基础网络的先验知识，通过设计合成及从粉末 X 射线衍射(PXRD)数据解析其结构将极为困难。

In planning the synthesis, we chose the tetrahedral building blocks (Fig. 1, A and B) and the triangular unit (Fig. 1C), because they are rigid and unlikely to deform during the assembly reaction. Dehydration reactions of these units produce triangular B₃O₃ rings and C₂O₂B rings (Fig. 1, D and E). Based on these building blocks, we envisioned two kinds of reactions in which either of the tetrahedral blocks (Fig. 1, A and B) undergoes self-condensation or co-condensation with the triangular unit (Fig. 1C) to give COF structures based on nets with both tetrahedral and triangular nodes (Fig. 1, D and E).
在规划合成时，我们选择了四面体构建单元（图 1A 和 B）和三角形单元（图 1C），因为它们刚性强，在组装反应过程中不易变形。这些单元的脱水反应生成三角形 B ₃ O ₃ 环和 C ₂ O ₂ B 环（图 1D 和 E）。基于这些构建单元，我们设想了两种反应类型：四面体块（图 1A 和 B）中的任一单元进行自缩合或与三角形单元（图 1C）共缩合，从而生成基于含有四面体和三角形节点的网络的 COF 结构（图 1D 和 E）。

In principle, there are an infinite number of possible nets that may result from linking tetrahedra with triangles. However, our analysis of previous assembly reactions suggests that the most symmetric nets are the most likely to result in an unbiased system and that those with just one kind of link will be preferred and are thus the best to target (3). In the present case of linking tetrahedral and triangular building blocks, the only known nets meeting the above criteria are those with symbols ctn and bor (Fig. 1, F and G) (4). The nodes of the nets are thus replaced by the molecular building units with tetrahedral and triangular shapes (Fig. 1, H and I). The use of rigid, planar triangular units, such as B₃O₃ rings, requires that rotational freedom exist at the tetrahedral nodes for the 3D structures ctn and bor to form.
原则上，通过将四面体与三角形连接，可以产生无数种可能的网络结构。然而，我们对以往组装反应的分析表明，最具对称性的网络最有可能形成无偏系统，而那些仅由一种连接方式构成的网络将被优先选择，因此是最佳目标（3）。在当前连接四面体和三角形构建单元的情况下，唯一已知符合上述标准的网络是具有符号 ctn 和 bor 的结构（图 1，F 和 G）（4）。因此，网络的节点被替换为具有四面体和三角形形状的分子构建单元（图 1，H 和 I）。使用刚性平面三角形单元，如 B ₃ O ₃ 环，要求四面体节点处存在旋转自由度，以形成 ctn 和 bor 的三维结构。

We then used Cerius² software to draw the “blueprints” for synthesis of COFs based on ctn and bor nets by fitting molecular building blocks (Fig. 1, A and B) on the tetrahedral nodes and by fitting the triangular unit and the B₃O₃ ring (Fig. 1, C and D) on the triangular nodes of these nets adhering to their respective cubic space group symmetries: $I\overline{4}3d$ (ctn) and $P\overline{4}3m$ (bor). Energy minimization by means of force-field calculations was performed to produce the models in which all bond lengths and angles were found to have chemically reasonable values (5).
随后，我们利用 Cerius ² 软件绘制了基于 ctn 和 bor 网络的 COFs 合成“蓝图”，通过将分子构建块（图 1A 和 B）拟合到四面体节点上，并将三角形单元和 B ₃ O ₃ 环（图 1C 和 D）拟合到这些网络的三角形节点上，遵循各自立方空间群对称性： $I\overline{4}3d$ （ctn）和 $P\overline{4}3m$ （bor）。通过力场计算进行能量最小化，生成的模型中所有键长和键角均被发现具有化学上合理的数值（5）。

Synthesis of the COFs was carried out by suspending either TBPM or TBPS in mesitylene/dioxane. The suspensions were placed in partially evacuated (150 mtorr) Pyrex tubes, which were sealed and heated (85°C) for 4 days to give white crystalline COF-102 and COF-103 in 63 and 73% yields, respectively. Similarly, co-condensation of TBPM or TBPS with HHTP (3:4 molar ratio) produced green crystalline solids of COF-105 (58% yield) and COF-108 (55% yield) (6). The colors of COF-105 and COF-108 likely arise from the possible inclusion of a small amount of highly colored oxidized HHTP in their pores. The use of dioxane and mesitylene in their respective ratios was necessary to control the solubility of the starting materials and to maximize crystallinity of the products.
通过将 TBPM 或 TBPS 悬浮于间二甲苯/二氧六环中进行 COFs 的合成。将悬浮液置于部分抽真空（150 mtorr）的派热克斯玻璃管中，密封后加热（85°C）4 天，分别以 63%和 73%的产率得到白色晶体 COF-102 和 COF-103。类似地，TBPM 或 TBPS 与 HHTP（3:4 摩尔比）共缩聚，分别以 58%和 55%的产率生成绿色晶体 COF-105 和 COF-108。COF-105 和 COF-108 的颜色可能源于其孔隙中可能包含的少量高色度氧化 HHTP。在其相应比例中使用二氧六环和间二甲苯是必要的，以控制起始材料的溶解度并最大化产物的结晶度。

To show that the products of synthesis are indeed covalently linked into the designed structures, we studied the materials by PXRD, spectroscopy, microscopy, elemental microanalysis, and gas adsorption (6). A comparison of PXRD patterns of modeled COFs to those observed for the products of synthesis (Fig. 2, A to D) reveals that they are indeed the expected COFs with ctn or bor type. The observed PXRD patterns display narrow line widths and low signal-to-noise ratios indicative of the high crystallinity of COFs. A marked degree of correspondence between peak positions and intensities is also observed, substantiating that the H, B, C, and O atomic composition and positions in the respective modeled unit cells are correct. The PXRD data of the COFs could also be indexed, yielding unit cell parameters nearly identical to those calculated from Cerius² (table S5).
为了证明合成产物确实共价连接成设计的结构，我们通过粉末 X 射线衍射（PXRD）、光谱学、显微镜、元素微分析和气体吸附（6）对材料进行了研究。将模拟的 COFs 的 PXRD 图谱与合成产物（图 2，A 至 D）的观察结果进行比较，揭示它们确实是预期的 ctn 或 bor 型 COFs。观察到的 PXRD 图谱显示出窄的线宽和低信噪比，表明 COFs 具有高结晶度。峰位和强度的显著对应关系也得到了观察，证实了各自模拟单元细胞中 H、B、C 和 O 原子组成及位置的正确性。COFs 的 PXRD 数据也能被索引，得到的单元细胞参数与从 Cerius ² 计算出的结果几乎相同（表 S5）。

To further verify the unit cell parameters, PXRD patterns were subjected to model-biased Le Bail full pattern decomposition to extract the structure factor amplitudes from the x-ray data. For this procedure to be successful and yield acceptable reliability factors, a close correspondence in peak position and intensity between the model and the experimental data is required. All peaks undergo some broadening because COF crystallites have micrometer dimensions (7). After accounting for line broadening in the initial stages of Le Bail extractions, fitting of the experimental profiles readily converged with refinement of the unit cell parameter, a. Refinements for all structures led, again, to values nearly identical to those calculated from Cerius² (table S5). Too few peaks were available to perform full Rietveld refinement of atomic positions and thermal parameters. Nonetheless, a near equivalence and low uncertainty [estimated SD (table S5)] between calculated and refined cell parameters, in addition to the facile and proper fit of the refined profiles [as indicated by statistically acceptable residual factors (table S6)], support the assertion that the COF structures are indeed those identified through modeling (Fig. 2; atomic coordinates in tables S1 to S4).
为进一步验证晶胞参数，对 PXRD 图谱进行了模型偏置的 Le Bail 全谱分解，以从 X 射线数据中提取结构因子振幅。为确保此过程成功并获得可接受的可靠性因子，模型与实验数据在峰位和强度上需高度一致。所有峰均因 COF 晶体微米级尺寸（7）而有所展宽。在 Le Bail 提取的初始阶段考虑线展宽后，实验轮廓拟合迅速收敛于晶胞参数 a 的精修。所有结构的精修结果再次与 Cerius ² 计算值（表 S5）几乎相同。由于可用峰数量有限，无法进行原子位置和热参数的完整 Rietveld 精修。 然而，计算与精修的晶胞参数之间近乎一致且不确定性较低[估计的标准偏差（表 S5）]，加之精修轮廓的拟合简便且恰当[由统计上可接受的残差因子（表 S6）所指示]，均支持了 COF 结构确实如模型所识别的这一论断（图 2；原子坐标见表 S1 至 S4）。

The covalent linking of building blocks through expected six-membered B₃O₃ boroxine or five-membered C₂O₂B boronate ester rings in the COFs was assessed with Fourier transform infrared (FTIR) and multiple-quantum magic angle spinning nuclear magnetic resonance (MQ MAS NMR) spectroscopies. FTIR spectra of all COFs contain strongly attenuated bands arising from boronic acid hydroxyl groups indicative of successful condensation of the reactants (figs. S14 to S17). COFs prepared from self-condensation reactions all exhibit the diagnostic band at 710 cm^–1 for the out-of-plane deformation mode of boroxine rings. Co-condensed COF-105 and COF-108 products have strong C-O stretching bands at 1245 cm^–1 (COF-105) and 1253 cm^–1 (COF-108), signals that are distinctive for boronate ester five-membered rings (6).
通过傅里叶变换红外光谱（FTIR）和多量子魔角旋转核磁共振（MQ MAS NMR）光谱学，评估了在共价有机框架（COFs）中通过预期的六元 B ₃ O ₃ 硼氧环或五元 C ₂ O ₂ B 硼酸酯环连接构建单元的情况。所有 COFs 的 FTIR 光谱均包含来自硼酸羟基的强烈减弱谱带，表明反应物成功缩合（图 S14 至 S17）。由自缩合反应制备的 COFs 均在 710 cm ^–1 处显示出诊断性谱带，对应于硼氧环的平面外变形模式。共缩合产物 COF-105 和 COF-108 在 1245 cm ^–1 （COF-105）和 1253 cm ^–1 （COF-108）处具有强烈的 C-O 伸缩谱带，这些信号是硼酸酯五元环的特征（6）。

These FTIR spectroscopy data are fingerprints for the expected boron-containing rings; however, solid-state ¹¹B MQ MAS NMR spectroscopy is highly sensitive to the immediate bonding environment of boron. Any differences in B-C and B-O distances and/or angles will result in a notable change in the line shape and intensity of the spectra. The acquired ¹¹B MQ MAS NMR spectra for evacuated COFs were compared to those of molecular model compounds and starting materials (Fig. 2, A to D, right insets). The spectra of all of the COFs are coincident to those of the model compounds and are different from the starting materials. Thus, the boron-containing units in all the COFs have not only formed but are well-formed B₃O₃ and C₂O₂B rings. Additionally, data from ¹³C and ²⁹Si MQ MAS NMR experiments show the presence of the expected number and environment of each type of respective nucleus, further substantiating the structural assignments (figs. S18 to S38).
这些 FTIR 光谱数据是预期含硼环的特征指纹；然而，固态 ¹¹ B MQ MAS NMR 光谱对硼的直接键合环境极为敏感。B-C 和 B-O 距离及/或角度的任何差异都会导致谱线形状和强度的显著变化。所获得的真空 COFs 的 ¹¹ B MQ MAS NMR 光谱与分子模型化合物及起始材料的光谱进行了比较（图 2，A 至 D，右侧插图）。所有 COFs 的光谱均与模型化合物一致，且不同于起始材料。因此，所有 COFs 中的含硼单元不仅已形成，而且形成了结构良好的 B ₃ O ₃ 和 C ₂ O ₂ B 环。此外， ¹³ C 和 ²⁹ Si MQ MAS NMR 实验数据表明，每种核素预期的数量和环境均存在，进一步证实了结构归属（图 S18 至 S38）。

In order to establish the phase purity and synthetic reproducibility of the COF materials, multiple samples were exhaustively imaged with scanning electron microscopy (SEM). The SEM images of COF-102 and COF-103 revealed agglomerated and nonagglomerated 1- to 2-μm-diameter spheres, respectively (figs. S39 and S40). This morphology is likely caused by a polar hydroxylated (-OH) surface that causes spherical crystal growth to minimize interfacial surface energy with the relatively nonpolar solvent media. The SEM images recorded for COF-105 and COF-108 revealed 5-μm platelets and 3- to 4-μm irregular spheres, respectively (figs. S41 and S42). For each of the COFs, only one morphology was observed, negating the presence of impurity phases. Furthermore, C and H elemental microanalysis confirmed that the composition of each COF corresponded to formulations predicted from modeling (6).
为了确定 COF 材料的相纯度和合成可重复性，使用扫描电子显微镜（SEM）对多个样品进行了全面成像。COF-102 和 COF-103 的 SEM 图像分别显示了 1 至 2 微米直径的聚集和非聚集球体（图 S39 和 S40）。这种形态可能是由于极性的羟基化（-OH）表面导致球形晶体生长，以最小化与相对非极性溶剂介质的界面表面能。COF-105 和 COF-108 的 SEM 图像分别显示了 5 微米板状和 3 至 4 微米不规则球体（图 S41 和 S42）。对于每种 COF，仅观察到一种形态，排除了杂质相的存在。此外，碳和氢元素的微分析证实了每种 COF 的组成与模型预测的配方相符（6）。

The derived structures for COF-102, COF-105, and COF-108 are shown in Fig. 3 (COF-103 has a tetrahedral Si replacing C, and its structure is virtually identical to that of COF-102). COF-102 (Fig. 3A), COF-103, and COF-105 (Fig. 3B) are based on ctn, and COF-108 (Fig. 3C) is based on bor. It is hard to assesswhy one of the two structure types would be preferred over the other. However, it is a notable confirmation of our original thesis that we find one or the other of the two structures. The only notable differences between the two structure types are that bor is about 15% less dense than ctn (compare the densities of COF-105 and COF-108) and has larger pores. The three-coordinated vertices in both structures are constrained to be planar with threefold symmetry, but the point symmetry at the tetrahedral site in ctn is only a subgroup $(\overline{4}=S_4)$ of that at the tetrahedral site in bor $(\overline{4}m2=D_{2d})$ ; this difference gives ctn less constraints and potentially makes it a more strain-free structure than bor.
COF-102、COF-105 和 COF-108 的衍生结构如图 3 所示（COF-103 以四面体硅替代碳，其结构与 COF-102 几乎相同）。COF-102（图 3A）、COF-103 和 COF-105（图 3B）基于 ctn 结构，而 COF-108（图 3C）基于 bor 结构。难以评估为何其中一种结构类型会优于另一种。然而，我们发现其中一种结构的事实，有力地证实了我们最初的论点。两种结构类型之间唯一显著的差异在于，bor 的密度比 ctn 低约 15%（比较 COF-105 和 COF-108 的密度），且孔径更大。两种结构中的三配位顶点均被限制为具有三重对称性的平面，但 ctn 中四面体位点的点对称性仅为 bor 中四面体位点对称性的一个子群 $(\overline{4}=S_4)$ ；这一差异使得 ctn 受到的限制较少，可能使其成为比 bor 更无应变的结构。

It is also of interest to consider the pore sizes. In the COFs with the ctn structure, the center of the largest cavity in COF-102, COF-103, and COF-105 is 5.66, 5.98, and 10.37 Å, respectively, from the nearest atoms (H). If we allow for a van der Waals radius of 1.2 Å for H, spheres of diameter 8.9, 9.6, and 18.3 Å, respectively, are available in these three COFs. However, the pores in these materials are far from spherical, and we expect the effective pore size to be somewhat larger. COF-108 has two cavities, and the atoms closest to the center are C atoms at 9.34 and 15.46 Å. If we allow for a van der Waals radius of 1.7 Å for C, these cavities can accommodate spheres of 15.2 and 29.6 Å, respectively. It may be seen that the larger pores are well above the lower limit (20 Å) for the material to be described as mesoporous, and COF-108 is a rare example of a fully crystalline mesoporous material.
考虑孔径大小同样颇具意义。在具有 ctn 结构的 COFs 中，COF-102、COF-103 和 COF-105 的最大空腔中心分别距离最近的原子（H）5.66、5.98 和 10.37 Å。若考虑氢的范德华半径为 1.2 Å，则这三个 COFs 中可容纳直径分别为 8.9、9.6 和 18.3 Å的球体。然而，这些材料的孔隙远非球形，我们预期有效孔径会稍大一些。COF-108 含有两个空腔，中心最近的原子是距离 9.34 和 15.46 Å的碳原子。若碳的范德华半径为 1.7 Å，这两个空腔分别可容纳直径为 15.2 和 29.6 Å的球体。由此可见，较大孔隙远超介孔材料下限（20 Å），COF-108 作为全晶介孔材料的罕见实例，实属难得。

An important feature of 3D COFs is the full accessibility from within the pores to all the edges and faces of the molecular units used to construct the framework. A previous study found that maximizing the number of edges arising from aromatic rings in a porous material increases the number of adsorption sites and surface area (8). Porous zeolites, carbons, and metal-organic frameworks (MOFs) all contain latent edges in their structures; however, the structures of COFs contain no latent edges, and the entire framework is a surface replete with binding sites for gas adsorption. The structures also have extraordinarily low densities: COF-102 (0.41 g cm^–3), COF-103 (0.38gcm^–3), COF-105 (0.18 g cm^–3), and COF-108 (0.17 g cm^–3). The last two values are markedly lower than those of highly porous MOFs such as MOF-5(0.59gcm^–3) (9) and MOF-177 (0.42 g cm^–3) (8) and are the lowest density crystals known (10) [compare these values with the density of diamonds (3.50 g cm^–3)].
三维 COFs 的一个重要特征是其孔道内部对构建框架所用分子单元的所有边缘和面的完全可达性。先前的一项研究发现，在多孔材料中最大化芳香环产生的边缘数量可以增加吸附位点和表面积（8）。多孔沸石、碳材料和金属有机框架（MOFs）在其结构中均含有潜在的边缘；然而，COFs 的结构中不含潜在边缘，整个框架就是一个充满气体吸附位点的表面。这些结构还具有极低的密度：COF-102（0.41 g cm ^–3 ），COF-103（0.38 g cm ^–3 ），COF-105（0.18 g cm ^–3 ），以及 COF-108（0.17 g cm ^–3 ）。最后两个数值明显低于高孔隙率的 MOFs 如 MOF-5（0.59 g cm ^–3 ）（9）和 MOF-177（0.42 g cm ^–3 ）（8），并且是已知密度最低的晶体（10）[与钻石的密度（3.50 g cm ^–3 ）相比]。

The low densities coupled with the maximized fraction of surface sites in 3D COFs naturally impart their exceptional porosities, which were shown in gas adsorption studies on evacuated samples of COF-102 and COF-103. Samples of “as synthesized” COF-102 and COF-103 were immersed in anhydrous tetrahydrofuran to remove solvent and starting materials included in the pores during synthesis and were then placed under dynamic vacuum conditions (10^–5 torr) for 12 hours at 60°C to completely evacuate the pores (6). Thermogravimetric analysis confirmed that all of the guests were removed from the pores and revealed the thermal stability of all COFs beyond 450°C (figs. S43 and S46). Argon isotherms for COF-102 and COF-103 were recorded at 87 K from 0 to 760 torr (Fig. 4, A and B). COF-102 and COF-103 exhibit a classic type I isotherm characterized by a sharp uptake at the low-pressure region between P/P_o =1 ×10^–5 to 1× 10^–2, where P is gas pressure and P_o is saturation pressure. The apparent surface areas calculated from the Brunauer-Emmett-Teller (BET) model were 3472 and 4210 m² g^–1 for COF-102 and COF-103, respectively. The pore volume determined from the Dubinin-Radushkevich equation afforded values of 1.35 cm³ g^–1 (COF-102) and 1.66 cm³ g^–1 (COF-103). The BET surface areas of COFs exceed porous carbons (2400 m² g^–1) (11), silicates (1300 m² g^–1) (12), recently reported 2D COFs (1590 m² g^–1) (2), polymers of intrinsic microporosity (1064 m² g^–1) (13), and polymer resins (2090 m² g^–1) (14) and are comparable to some of the highest surface areas of MOFs [MOF-177 (4500 m² g^–1) (8) and MIL-101 (4100 m² g^–1) (15) (MIL, Matérial Institut Lavoisier)]. Calculation of pore size obtained from appropriately fitting density functional theory (DFT) models to the isotherms (figs. S48 and S52) yielded pore size distributions of COF-102 (11.5 Å) (Fig. 4A, inset) and COF-103 (12.5 Å) (Fig. 4B, inset) (16). Narrow distributions are obtained and are centered at values close to the pore diameters obtained from the crystal structures.
低密度与三维 COFs 中最大化表面位点比例相结合，自然赋予其卓越的多孔性，这在 COF-102 和 COF-103 的真空样品气体吸附研究中得到了展示。“合成态”的 COF-102 和 COF-103 样品被浸入无水四氢呋喃中，以去除合成过程中孔隙内残留的溶剂和起始材料，随后在 60°C 下置于动态真空条件（10 ^–5 托）中 12 小时，以彻底排空孔隙（6）。热重分析证实所有客体物质已从孔隙中移除，并揭示了所有 COFs 在 450°C 以上的热稳定性（图 S43 和 S46）。COF-102 和 COF-103 的氩等温线在 87K 下从 0 至 760 托记录（图 4A 和 B）。COF-102 和 COF-103 呈现出典型的 I 型等温线，其特征在于在 P/P _o = 1×10 ^–5 至 1×10 ^–2 的低压区域出现急剧吸附，其中 P 为气体压力，P _o 为饱和压力。根据 Brunauer-Emmett-Teller（BET）模型计算的表观表面积分别为 3472 和 4210 m ² g ^–1 ，对应于 COF-102 和 COF-103。 由 Dubinin-Radushkevich 方程确定的孔体积分别为 1.35 cm³/g（COF-102）和 1.66 cm³/g（COF-103）。COFs 的 BET 比表面积超过了多孔碳（2400 m²/g）（11）、硅酸盐（1300 m²/g）（12）、最近报道的二维 COFs（1590 m²/g）（2）、本征微孔高分子（1064 m²/g）（13）和聚合物树脂（2090 m²/g）（14），并与一些最高比表面积的 MOFs 相当，如 MOF-177（4500 m²/g）（8）和 MIL-101（4100 m²/g）（15）（MIL，Matérial Institut Lavoisier）。通过将密度泛函理论（DFT）模型适当拟合到等温线（图 S48 和 S52）计算得到的孔径分布显示，COF-102 的孔径为 11.5 Å（图 4A，插图），COF-103 的孔径为 12.5 Å（图 4B，插图）（16）。所得分布较窄，且中心值接近从晶体结构获得的孔径。

At the outset of this study, crystallization of 3D COFs (such as cross-linked polymers) was believed to be difficult, if not impossible, to achieve for both thermodynamic and kinetic reasons. This report demonstrates that this challenge can be met by striking a balance between these two competing factors and that the principles of reticular chemistry provide the basis for design and structure solution of the resulting materials.
在本研究之初，人们普遍认为三维共价有机框架（如交联聚合物）的结晶化由于热力学和动力学原因，即便不是完全不可能，也是极其困难的。本报告表明，通过在这两个相互竞争的因素之间找到平衡，这一挑战是可以克服的，并且网状化学原理为这些材料的结构设计和解析提供了基础。