Multiple-component covalent organic frameworks

Covalent organic frameworks are a class of crystalline porous polymers that integrate molecular building blocks into periodic structures and are usually synthesized using two-component [1+1] condensation systems comprised of one knot and one linker. Here we report a general strategy based on multiple-component [1+2] and [1+3] condensation systems that enable the use of one knot and two or three linker units for the synthesis of hexagonal and tetragonal multiple-component covalent organic frameworks. Unlike two-component systems, multiple-component covalent organic frameworks feature asymmetric tiling of organic units into anisotropic skeletons and unusually shaped pores. This strategy not only expands the structural complexity of skeletons and pores but also greatly enhances their structural diversity. This synthetic platform is also widely applicable to multiple-component electron donor–acceptor systems, which lead to electronic properties that are not simply linear summations of those of the conventional [1+1] counterparts.

COFs are typically synthesized via topologically directed [1 þ 1] condensation reactions between a knot component and another linker unit. As a result, for example, only a maximum of 10 different COFs can be synthesized from a library of one knot and 10 linkers. Under this conventional [1 þ 1] design scheme, development of new COFs is largely dependent on the exploration of new knot and linkers, which is however, tedious and unproductive. Here we report a general multiple-component (MC) strategy that allows for the use of more than two components for the topological design and practical synthesis of MC-COFs, which are formed in a single phase with permanent porosity and high crystallinity, irrespective of their components and topologies. Notably, this MC strategy is exceptionally effective at increasing the structural diversity of COFs, and a collection of one C 3 -symmetric vertex and 10 C 2 -symmetric linkers can generate 210 new hexagonal MC-COFs according to the law of combinatorics 42 . To demonstrate the effectiveness of various combinations, 53 MC-COFs were synthesized by condensing one knot with two or three linkers to produce hexagonal MC-COFs and two linkers to prepare tetragonal MC-COFs. Furthermore, unlike conventional [1 þ 1] COFs, which undergo symmetric tiling and produce regular polygon pores, MC-COFs considerably enhanced complexity in both skeletons and pores by creating sequenced anisotropic tiling and unusually shaped yet ordered pores. Interestingly, this MC strategy is also applicable to the synthesis of multiplecomponent electron donor-acceptor COFs in which sequenced donor and acceptor p-arrays trigger strong electronic correlations among the latticed p-components. As a result, MC-COFs exhibit greatly enhanced electronic properties that are not simple linear summation of these of the conventional [1 þ 1] two-component COFs. Therefore, the multiple-component COFs provide a new platform that considerably expands the designability of structures and functions of porous organic materials.

Results
Design principle of MC-COFs. Recently, COFs with complicated lattices and porous structures have been developed by using several different approaches ( Supplementary Fig. 1). The development of the C 2 þ C 2 topology diagram using one knot and two linkers enables the synthesis of imine-linked kagome-type COFs with triple pores 43 . Two examples of such COFs were demonstrated although their crystallinity and porosity are quite low; SIOC-COF-1 has the surface area of 478 m 2 g -1 with the total pore volume of 0.30 cm 3 g -1 and SIOC-COF-2 has much low surface area of 46 m 2 g -1 and decreased pore volume of only 0.09 cm 3 g -1 . This condensation reaction is interesting for creating triple pores, but the low porosity and limited crystallinity suggest that the tetraphenylethene knot-based reaction systems are likely very sensitive to the length of the linkers while the reason for the extremely low porosity remains unclear. On the other hand, the use of a bifunctional linker of 4-formylphenyl boronic acid allows for the synthesis of COFs with two different boronate and imine linkages in the skeletons that are not available for conventional [1 þ 1] based COFs 6,7 . The exploration of desymmetric knot for the [1 þ 1] condensation reaction leads to the synthesis of COFs that consist of co-existed two different crystalline structures 44 . The desymmetric strategy thus focuses on the exploration of [1 þ 1] combination, while the desymmetric knot is the key building block. The desymmetric approach was exemplified for the hexagonal dual-pore COFs, but it did not show its capability of synthesising tetragonal COFs. These approaches are interesting as specific cases of complicated COFs and demonstrate that COFs are capable of complicated structural formation. Herein, we report the multiple-component (MC) [1 þ 2] and [1 þ 3] strategies based on the general topology diagrams of the C 3 þ C 2 and C 4 þ C 2 schemes for the synthesis of hexagonal and tetragonal COFs. We highlight that these MC-COFs cannot be predicted and synthesized by using the above three approaches ( Supplementary Fig. 1).
Our idea is based on the following polygon geometric transformation mechanism: The regular hexagon (C 6 ) and tetragon (C 4 ) have three and two pairs of same-sized parallel sides, respectively. From the perspective of polygon geometry, stretching or shrinking along one pair of parallel sides can produce C 2 -symmetric hexagons and tetragons while retaining original 120°and 90°angles (Fig. 1b). This geometric transformation makes it possible to develop the C 2 -symmetric hexagonal and tetragonal pores through the topology design of COFs by developing multiple-component reaction systems. On the basis of the above idea, our concept is to integrate two or three different linkers into the frameworks while keeping one-knot structure. Owing to the high reversibility of the boronate-linkage reaction, the 2D polygons are capable of quick structural self-healing. The disordered 2D polygon layers if any formed are difficult to induce effective p-p interactions that are essential for the formation of layered crystalline frameworks; such unstable disordered polygons would decompose and finally leaves ordered 2D layers in which linker units are statistically balanced and are integrated into an ordered lattice. For the synthetic reactions, we utilized the conventional solvothermal conditions that are similar to those for the synthesis of [1 þ 1] boronate-linked COFs. The 10 different linkers were selected as they have similar solubility and reactivity under the solvothermal conditions. Figure 1a shows the conventional hexagonal and tetragonal topologies of covalent organic frameworks (COFs) and their design schemes based on the two-component [1 þ 1] copolymerisation of a C 3 or C 4 -symmetric knot and a C 2 -symmetric linker 1,2 . The C 3 -symmetric triphenylene (TP) and C 4 -symmetric nickel phthalocyanine (NiPc) units are representative knots used for the synthesis of hexagonal and tetragonal COFs 1,2,45,46 , respectively. The crystalline ordered structures of COFs are error-checked and repaired via self-healing through reversible covalent bonding reactions 1,2,47,48 . Figure 1b shows the multiple-component (MC) [1 þ 2] or [1 þ 3] strategy that we developed for the synthesis of MC-COFs, using one knot and three different linkers to showcase their diversities and transitions in their network lattice and pore size and shape. In the hexagonal topology (Fig. 1c), each TP knot was connected to three linker units arranged with intervals of 120°. For the three-component systems, this geometry required the stoichiometric ratio of the two linkers to be 1:2 or 2:1 for the formation of closed hexagons and extended lattice structures. We developed two different [1 þ 2] three-component copolymerisation systems in which the molar ratio of the two linker units was 1:2 or 2:1 (Fig. 1c). We further varied all three linkers and explored [1 þ 3] copolymerisation systems to achieve four-component hexagonal MC-COFs. We applied this synthetic strategy to create tetragonal MC-COFs in which each knot was connected to two sets of the linker units (Fig. 1c). To demonstrate the rational design of MC-COFs, we synthesized 10 different linkers (Fig. 2a) for [1 þ 2] or [1 þ 3] copolymerization with TP and NiPc (Fig. 1c). Notably, these hexagonal and tetragonal MC-COFs ( Fig. 2b-d) were obtained as crystalline porous materials in a single phase that featured the asymmetric tiling of organic units and specifically shaped polygon channels. Their lattice and porous structures were distinct from those of the conventional [1 þ 1] two-component COFs (Fig. 1a) and were characterized using various analytical methods (Supplementary Figs 2-193; Supplementary Tables 1-18).
Three-component [1 þ 2] MC-COFs. We conducted threecomponent [1 þ 2] copolymerisations by using the shortest linker (E 1 ) and a long linker (E 7 ) at a molar ratio of 1:2 or 2:1 as the linkers to condense with 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene (TP) as the knots (Fig. 2a,b). These two sets of [1 þ 2] three-component reactions were performed in a mixed solvent consisting of mesitylene and dioxane under solvothermal conditions and yielded two MC-COFs ( Fig. 3 23 . In control experiments, we measured the PXRD patterns of simple mixtures of COF-5 and TP-COF at weight ratios of 1:2 and 2:1. These mixtures showed different PXRD peaks ( Supplementary Fig. 6). The Pawley-refined patterns ( Fig. 4a,b, black curves) confirmed the PXRD peak assignments because the differences from the observed PXRD pattern were negligible ( Fig. 4a,b, blue curves). Structural simulations using self-consistent charge density-functional tight-binding (SCC-DFTB) method 49 with starting structures created by AuToGraFS and preoptimized using a topology-preserving force field were used to optimize the monolayer and were further extended to layered frameworks with different stacking modes. In both MC-COFs, the slipped AA stacking modes were the most stable structures among the various stacking modes, including eclipsed AA and staggered AB ( Fig. 4a Hexagonal COF Tetragonal COF Streching of regular polygons AA stacking mode to reconstruct the crystal structures of the two MC-COFs and the resulting PXRD patterns (Fig. 4a,b, green curves) were in agreement with the experimentally observed profiles. MC-COF-TP-E 1 1 E 7 2 assumed a space group of P2 with a ¼ 33 (  for atomic coordinates see Supplementary Table 12), and MC-COF-TP-E 1 2 E 7 1 adopted a P2 space group with different lattice parameter of a ¼ 33 Table 13).
The cell parameters of MC-COF-TP-E 1 1 E 7 2 were larger than those of MC-COF-TP-E 1 2 E 7 1 , because the former contained much longer E 7 linkers in its lattice. Moreover, the difference in the a and b values observed for the two MC-COFs was in good agreement with the asymmetric MC tiling of the lattice because the lengths of two sets of parallel linker pairs were different from that of another set of linker pairs (Fig. 3b,c). These space groups and lattice parameters were also different from those of the [ AB stacking modes could not reproduce the PXRD patterns ( Fig. 4a,b, purple curves).
To quantitatively determine the ratio of the two linkers in the MC-COFs, we hydrolysed the MC-COF samples with HCl and measured their 1 H nuclear magnetic resonance (NMR) spectra. Resonances with the predicted coupling patterns were observed in the expected regions for each of the linkers' unique protons.
By integrating the resonance peak intensities, the E 1 and E 7 linkers were present in ratios of 1:2 for MC-COF-  (Supplementary Fig. 53). These proton integrations quantitatively confirmed the lattice components of these MC-COFs. Field emission scanning electron microscopy revealed that MC-COF-TP-E 1 1 E 7 2 and MC-COF-TP-E 1 2 E 7 1 exhibited completely different morphologies (that is, belts and flakes, respectively) ( Supplementary Fig. 127). The belts were as large as several micrometres and the flakes were extended to several hundred nanometres. High-resolution transmission electron microscopy was used to visualize their order structures ( Supplementary  Figs 148 and 149). These observations again confirmed that the resulting MC-COFs were obtained as single phases.

Discussion
We further explored this MC strategy by using a NiPc knot and two linkers for the construction of tetragonal MC-COFs (Figs 1b  and 2d). For example, MC-COF-NiPc-E 1 E 7 (Fig. 7a-d) consisted of E 1 and E 7 linkers that were parallel matched to form an oblong polygon lattice. The compositions, linkage, crystalline structures, morphology and porosity were unambiguously determined using various analytic methods (Fig. 7e- Fig. 30). The negligible difference between the Pawley-refined PXRD pattern (Fig. 7e, black and blue curves) and the experimentally observed profile supported the peak assignments. The slipped AA stacking mode (Fig. 7c,d) was the most stable structure that reproduced the PXRD pattern (green curve) and adopted a P222 space group with (for  atomic coordinates see Supplementary Table 17). This difference between the a and b values indicated asymmetric tiling of the two linkers in the tetragonal lattice. The staggered AB stacking mode gives rise to a PXRD pattern (Fig. 7e, purple curve) that is different from that of experimentally observed one. MC-COF-NiPc-E 1 E 7 was highly porous, with a BET surface area of 672 m 2 g -1 and included one kind of mesopore with a pore size of 2.6 nm (Fig. 7f,g; Supplementary Table 1). The conventional [1 þ 1] strategy generated 10 hexagonal and 10 tetragonal COFs for TP and NiPc knots combined with 10 linkers (Figs 1a and 2a). In contrast, the three-component [1 þ 2] and four-component [1 þ 3] systems yielded 90 and 120 hexagonal COFs, respectively, whereas the [1 þ 2] tetragonal strategy yielded 45 different COFs. Therefore, the MC strategy greatly enhanced the number of COF structures from 20 to 255. Among these structures, we randomly choose 53 combinations and prepared 53 different COFs (Fig. 2b-d). Notably, the MC strategy was compatible with various linkers with lengths from 7 to 22 Å, structures ranging from simple arenes to heterocycles, and large p-systems that can be predesigned with electron-donating and accepting functions (Fig. 2).
In addition to considerably enhanced structural diversity, the MC strategy had two profound effects on the structural development of COFs. (1) This strategy provides a method for preparing tailor-made, specially shaped pores that are difficult to achieve with other porous materials and might have applications in shape-selective separation and catalysis [24][25][26][27][28][29]44,51 .   (Fig. 8a-e). These MC-COFs triggered charge transfer from the TP knots to the E 2 linkers, while the lattice tiling patterns tuned the charge-transferring capability, as evidenced by the different degrees of red-shifting of the band in the near infrared region of the electronic absorption spectra ( Supplementary Figs 190-193).
Notably, these MC-COFs exhibited ohmic-type conducting profiles but different currents (Fig. 8f). An enhancement of nearly 180,000% was observed for MC-COF-TP-E 2 2 E 3 1 (red) compared with the conventional [1 þ 1] counterparts (that is, In summary, we have developed a general strategy for the design and synthesis of crystalline porous COFs that enable the integration of multiple components into lattice structures with sequenced alignment. The multiple-component COFs greatly expand the structural complexity via asymmetric tiling of building blocks, providing a new platform for constructing anisotropic p-columnar arrays and unconventionally shaped pores. At the same time, this strategy considerably increases the structural diversity of COFs while retaining high crystallinity and porosity. We envisage that the MC-COFs constitute an important step towards various unprecedented molecular systems for functional exploration with enhanced structural complexity and diversity that are hardly available for conventional COFs architectures and other porous materials.  Conductivity measurement. MC-COFs samples (5 mg) were dispersed in 2 ml anhydrous dichloromethane and sonicated for 10 min. The highly dispersed MC-COFs solution was dropped on the center of conducting electrodes as a film. The measurement was conducted on MC-COF films between 10 mm platinum electrodes at 25°C in Ar using a two-probe method with a subfemtoamp sourcemeter (Keithley 6,430). I-V curves were recorded at bias voltages from À 10 to 10 V.
Structural characterization. 1 H NMR spectra were recorded on JEOL models JNM-LA400 NMR spectrometers, where chemical shifts (d in p.p.m.) were determined with a residual proton of the solvent as standard. Solid-state 13 C NMR spectra were recorded on JEOL model 920 MHz NMR spectrometer with a magnetic field of 21.62 Tesla. The frequency of the rotors was 15 kHz. For solidstate cross-polarization magic angle spinning 13 C nuclear magnetic resonance ( 13 C CP/MAS NMR), cross polarization with polarization inversion 1,808 scans ( 13 C CPPI) and cross polarization with non-quaternary suppression ( 13 C CPNQS) were performed with a delay time of 5 s. Ultraviolet-vis-infrared diffuse reflectance spectrum (Kubelka-Munk spectrum) was recorded on a JASCO model V-670 spectrometer equipped with integration sphere model IJN-727. Field-emission scanning electron microscopy was performed on a JEOL model JSM-6700 operating at an accelerating voltage of 5.0 kV. The sample was prepared by drop-casting a supersonicated solvents suspension onto mica substrate and then coated with gold. High-resolution transmission electron microscopy images were obtained on a JEOL model JEM-3200 microscopy. The sample was prepared by drop-casting a supersonicated tetrahydrofuran suspension of the COFs onto a copper grid. PXRD data were recorded on a Rigaku model RINT Ultima III diffractometer by depositing powder on glass substrate, from 2y ¼ 1.5°up to 30°w ith 0.02°increment. Elemental analysis was performed on a Yanako CORDER MT-6 elemental analyser. Thermogravimetric analysis measurements were performed on a Mettler-Toledo model TGA/SDTA851 under N 2 , by heating to 800°C at a rate of 10°C min -1 with samples held in aluminium pans. Nitrogen sorption isotherms were measured at 77 K with Micromeritics Instrument Corporation model 3Flex surface characterization analyser. Before measurement, the samples were degassed in vacuum at 120°C for more than 10 h. By using the non-local density functional theory (NLDFT) model, the pore volume was derived from the sorption curve.
Computational calculations. The structures of COFs were calculated using the density-functional tight-binding (DFTB þ ) method including Lennard-Jones dispersion. The calculations were carried out with the DFTB þ program package version 1.2. DFTB is an approximate density functional theory method based on the tight-binding approach and utilizes an optimized minimal LCAO Slater-type all-valence basis set in combination with a two-center approximation for Hamiltonian matrix elements. The Coulombic interaction between partial atomic charges was determined using the self-consistent charge (SCC) formalism. Lennard-Jones type dispersion was employed in all calculations to describe van der Waals (vdW) and p-stacking interactions with starting structures created by AuToGraFS 53 and preoptimized using a topology-preserving force field 54 were used to optimize the monolayer and were further extended to layered frameworks with different stacking modes. The lattice dimensions were optimized simultaneously with the geometry. Standard DFTB parameters for X-Y element pair (X, Y ¼ C, O, H and N) interactions were employed from the mio-0-1 set10. The accessible surface areas were calculated from the Monte Carlo integration technique using a nitrogen-size probe molecule (diameter ¼ 3.68 Å) roll over the framework surface with a grid interval of 0.25 Å. The X-ray diffraction pattern simulation was performed in a software package for crystal determination from PXRD pattern, implemented in MS modeling version 4.4 (Accelrys Inc.). We performed Pawley refinement to optimize the lattice parameters iteratively until the RP and RWP values converge. The pseudo-Voigt profile function was used for whole profile fitting and Berrar-Baldinozzi function was used for asymmetry correction during the refinement processes.
Data availability. The data that support the findings of this study are available from the corresponding author on request.