Synthesis of Vinylene-Linked Covalent Organic Frameworks by Monomer-Self-Catalyzed Activation of Knoevenagel Condensation

Reticular chemistry on the basis of thermodynamically controlled linking modes and numerous organic building blocks has constituted versatile crystalline frameworks in molecular-level precision. However, vinylene-linked organic frameworks (COFs) are still quite far from flexible tailoring either in their structures or topologies, due to the lack of monomers with sufficient activities. Herein, we established a strategy to synthesize vinylene-linked COFs via Knoevenagel condensation of a tetratopic monomer 2,2’,6,6’-tetramethyl-4,4’-bipyridine (TMBP) with linear aromatic dialdehydes in a mixed solvent of benzoic anhydride and benzoic acid. Mechanism investigation suggested that the condensation was promoted by a pyridine-self-catalyzed benzoylation upon the cleavage of benzoic anhydride solvent molecules. The layered structures of the resultant COFs were highly crystallized into orthorhombic lattice with vertically aligned AA stacking mode, delivering high surface areas up to 1560 m g. The -extended conjugated skeletons comprising para-bipyridyl units and vinylene linkages endow these COFs with substantial semiconducting properties, releasing visible light-stimulated catalytic activity in water-splitting hydrogen evolution with a rate as high as 3300 μmol g h.


INTRODUCTION
Covalent organic frameworks (COFs) represent an emerged class of high crystalline porous polymers with well-defined periodic structures, which not only allows one to rationally design and tailor their chemical structures or topologies, but also readily manipulate them in various applications on the basis of establishing the essential structure-property relationships and understanding the related mechanism 1-3 . Among them, vinylene-linked organic frameworks (COFs) feature prominent stability and -electron communication, likely helpful for expanding the potential applications of COFs, in particular, in the field of semiconductor [4][5][6] . Thermodynamically controlled Knoevenagel condensation is one of the most powerful tools in the formation of carboncarbon double bond linkage [7][8][9][10] . Unlike those traditional dynamic covalent chemistry, such condensation involves the generation of active intermediates, such as, carbon anions, which is crucial for efficiently boosting chain growth and correcting defect through self-healing processes for a COF [11][12][13][14] . Typically, an available monomer bearing aryl α-carbon atoms, has to be enough electron-deficient for enhancing the α-proton acidity and stabilizing the reactive intermediate, commonly achieved by the introduction of multiple electron-withdrawing moieties, such as cyano group, pyridyl nitrogen [15][16] . However, these modifications normally means a tedious preparation and the utilization of toxic reagent (e.g. NaCN or CuCN), meanwhile, significantly constrains the accessible reaction sites of a monomer, severely limiting its linking modes. Correspondingly, on the basis of very few monomers, vinylenelinked COFs with hexagonal crystal system occupied the major ratio, and several single-pore orthorhombic skeletons were originating from the ditopic monomer 1,4-phenylenediacetonitrile 17 . While, a monomer with more αcarbon atoms (≥ 4) suitable for Knoevenagel condensation could not be found so far. Pyridyl nitrogen-containing molecules are widely used in catalysis, supramolecular chemistry, and organic semiconducting materials, associated with their intrinsic Lewis basicity, substantial physical properties, and self-assembling behaviors 18 . Knoevenagel reaction holds some merits in the preparation of vinylene-linked pyridine-based small molecules or linear conjugated polymers using methyl-substituted pyridine, due to low-cost, environmentally friendly reagents, less unidentified reactive intermediates, and only trans-alkene formation [19][20][21] . Unfortunately, the mechanism of such traditional reaction seems never to be investigated.
Typically, functionalization at nitrogen to generate pyridinium cations, makes pyridine ring favorable for electrophilic and/or nucleophilic substitution. Very recently, we reported a Knoevenagel condensation at the three methyl carbons in one pyridine promoted upon quaternization, leading to the formation of several pyridinium-cored vinylene-linked COFs with high crystallinities and quite large surface areas, suggesting an excellent efficiency in the activation of a multitopic monomer 22 . N-acyl pyridinium has been found as a highly active intermediate formed in the process of pyridine-catalyzed hydrolysis of acid anhydride, or as one type of efficient acylation reagent from the treatment of pyridine with an acyl donor [23][24] . With these reasons in our mind, our effort is to explore a much lower-cost, less-toxic and concise method to construct vinylene-linked COFs by virtue of methyl substituted pyridine-based monomer, and attempt to establish the optimized reaction conditions and corresponding thermodynamically controlled mechanism. Scheme 1. Synthesis and structures of (a) model molecule, (b) COF-DFB and COF-BPDA.
In this work, we present the preparation of two vinylene-linked COFs with orthorhombic reticular topology by condensation of D2h-symmetric 2,2',6,6'-tetramethyl-4,4'-bipyridine (TMBP) as a tetratopic monomer with the ditopic linear aromatic dialdehydes in benzoic anhydride and benzoic acid. The reaction mechanism manifested that the pyridine units in monomer TMBP enable catalyzing their benzoylation via the cleavage of benzoic anhydride solvent molecules, thus helpfully activating the aryl α-carbon atoms for Knoevenagel reaction. The resulting COFs with high crystallinities and surface areas, were considerably characterized. Their -extended conjugated skeletons endow them with excellent semiconducting properties and corresponding application, such as, visible light catalysis of water-splitting hydrogen evolution.

RESULTS AND DISCUSSION
Traditionally, in order to activate the methyl groups of pyridine ring for Knoevenagel condensation, incorporation of the electronwithdrawing substituents or quaternization at pyridine ring is necessary. Here, our attempt is to take use of the Lewis basicity of pyridine moiety for promoting the Knoevenagel reaction of a newly designed pyridine-based monomer 2,2',6,6'-tetramethyl-4,4'bipyridine (TMBP), bearing four pyridylmethyl terminal groups. Some previous works reported that methyl-substituted pyridine derivatives were facile to conduct Knoevenagel reaction with aromatic aldehydes in a mixture solvent of acetic anhydride and acetic acid at ca. 135 o C with moderate yield (< 60%) 25 . Considering the relatively weak reversibility of such carbon-carbon double bond formation reaction, we decided to raise the reaction temperature in a mixed solvent of benzoic anhydride and benzoic acid for the preparation of model compound and COFs 26 .
The key monomer TMBP was prepared via a facile one-step Yamamoto coupling in high yield 27 . The Knoevenagel condensation between TMBP and benzaldehyde was performed in a mixture of benzoic acid and benzoic anhydride under 180 ℃, affording the model molecule 2,2',6,6'-tetrastyryl-4,4'-bipyridine in a yield of 85% (synthesis detailed in the Supporting Information). The structures of these new compounds were unambiguously characterized by NMR, MALDI-TOF MS and single crystal X-ray diffraction analyses. The packing diagram of the model molecule showed a -dimer arrangement of the neighbouring molecules with a distance of 3.43 Å, manifesting the distinct - interaction. By using the condition of the model reaction, TMBP was copolymerized with 1,4-diformylbenzene (DFB) or 4,4biphenyldialdehyde (BPDA), respectively, to synthesize COFs. In a typical protocol, TMBP, DFB, benzoic anhydride and benzoic acid in a molar ratio of 1:2:2:0.2 was sealed in glass ampoule and heated at 180 °C for 3 days (Scheme 1), leading to the generation of large amount of red-brown precipitates. After washed with base solution and organic solvents (methanol, THF and DCM), the resultant crude product was converted to yellow powder in 90 % isolated yield (termed COF-DFB). Similarly, another COF (COF-BPDA) were derived from a larger aldehyde monomer (BPDA) as yellow powders in 94 % yield.
It is well-known that, acylation or alkylation at the pyridyl nitrogen always leads to nucleophilic attack on the pyridine ring due to the strengthened electron deficiency 18,28 . Alkylation at the pyridyl nitrogen has been verified to efficiently promote a Knoevenagel condensation at the pyridylmethyl carbon atoms 22,29 .
Considering that pyridine can act as Lewis base for catalyzing hydrolysis of acetic anhydride through the formation of the Nacetyl pyridinium ion intermediate 23 , under our circumstance, the proposed mechanism could begin with the benzoylation of methylpyridine moieties by benzoic anhydride to give the benzoyl pyridinium salt. The electron-deficient character of pyridine ring was significantly increased by the benzoylation, thus favorable for the enhancement of C-H acidity on the pyridyl methyl. Meanwhile, the generated conjugated base benzoate also could capture the protons at the methyl groups, leading to the formation of enamine intermediate. Subsequently, electrophilic attack of the protonated aldehyde in the presence of benzoic acid, to the above   The crystallinity of COF-DFB was examined by powder X-ray diffraction (PXRD) measurements. Theoretically, the condensation of a D2h monomer and a C2 monomer can give rise to two possible 2D COF structures, that is, an orthorhombic system that bears only a single-pore framework and a hexagonal system that possesses two different kinds of pores with Kagome lattice 30 . Indexing of the PXRD pattern of COF-DFB identified the space group Cmmm (No. 65), indicating an orthorhombic lattice. The diffraction peaks at 5.81°, 11.67° and 25.90° could be assigned to (110), (220) and (001) facets, respectively (Figure 1a). The experimental PXRD pattern of COF-DFB was in accordance with simulated pattern of a orthorhombic lattice with AA stacking mode, whereas the staggered AB stacking model and Kagome lattices exhibit significant difference from the experimental PXRD data ( Figures  S21 and S22). The first dominant diffraction peak at 5.81° corresponds to the d-spacing of 1.52 nm, which was in good agreement with the calculated diameter of the rhombic pore (1.4 nm, Figure S18)  Fourier transform infrared (FT-IR) spectra of COF-DFB and COF-BPDA were analyzed by comparison with the monomer and the model molecule (Figure 2a and Figure S2). The disappearance of C-H vibration (2916 cm -1 ), attenuation of C=O stretching band (1694 cm -1 ) and appearance of new intensive vinyl (C=C) bands at 961 and 1631 cm -1 , confirmed the high polycondensation degree of COFs. These observations were further confirmed by the complementary Raman spectra analysis (Figure 2c), which showed the characteristic Raman signals at 1630 cm -1 for the C=C stretch vibration. The formation of vinyl-linked COFs was further verified by 13 C cross-polarization magic angle spinning (CP-MAS) solidstate NMR spectroscopy, showing good match with the 13 C NMR spectra of the model molecule (Figure 2b and Figure S3). The permanent porosity of COF-DFB and COF-BPDA was assessed by N2 sorption measurements at 77 K (Figure 2d). Both COFs show the reversible type I isotherms with rapid N2 uptakes at the low relative pressure range P/P0 < 0.05, characteristic of microporous materials. Brunauer-Emmett-Teller (BET) surface areas calculated from N2 adsorption isotherms revealed that the COF-DFB and COF-BPDA exhibited high specific surface areas of 1104 m 2 g -1 and 1560 m 2 g -1 , respectively. The pore size distribution analysis of COF-DFB and COF-BPDA based on the quenched solid density functional theory (QSDFT) indicates one main distribution centered at 1.40 nm and 1.70 nm, respectively, which is in consistent with the pore diameter based on the Conolly surface in their eclipsed (AA) layer stacking mode (Figures S18 and S19). The morphologies and microstructures of the two COFs were studied by scanning electron microscopy (SEM) and Highresolution transmission electron microscopy (HRTEM). The SEM images show that COF-DFB and COF-BPDA are composed of layered blocks (Figures 3a and 3c). HRTEM images exhibit longrange ordered microstructures of the two COFs. The lattice fringes was clearly visualized in Figures 3b and 3d, suggesting the high crystallinities of these samples. One-dimensional channels with uniform distance of 1.40 nm and 1.70 nm for COF-DFB and COF-BPDA was clearly visualized along the [001] direction, corresponding to the d-spacing of (110) facet. Fast Fourier transform (FFT) of the raw HRTEM images generates a tetragonal arrangement of four white diffraction spots, corresponding to (110) facet (low right of Figures 3b and 3d). A rhombic porous structure can be observed on the HRTEM image of COF-BPDA, further indicating the formation of orthorhombic lattice, which is in accordance with the PXRD analysis ( Figure S7).
The electronic structures of the two COFs were systematically investigated by ultraviolet/visible diffuse reflectance spectroscopy (UV/vis DRS), photoluminescence (PL) spectroscopy and ultraviolet photoelectron spectroscopy (UPS). As shown in Figure  4a, COF-DFB shows an absorption maxima at ca. 475 nm, and COF-BPDA exhibits a relatively broad absorption band with a tail covering 450 ~ 700 nm. Accordingly, the optical band gaps of the COFs were determined from Kubelka-Munk function as 2.5 eV and 2.3 eV for COF-DFB and COF-BPDA, respectively (Figure 4a inset). The energy of valence band maximum (EVB) of the three COFs relative to vacuum level were evaluated by UPS analyses ( Figure S9). The EVB values of COF-DFB and COF-BPDA were calculated by subtracting the UPS width from excitation energy (He I, 21.2 eV), which were −6.4 and −6.3 eV, respectively. Coupled with their optical band gaps, the energy of conduction band minimum (ECB) of COF-DFB and COF-BPDA were determined as −3.9 and −4.0 eV, respectively. The ECB of both COFs are positioned higher than the reduction potential of proton (EredH + / H2 = −4.44 eV) (Figure 4c), allowing for reducing proton to hydrogen once the COFs were excited by light with photon energy larger than their band gaps. The steady-state photoluminescence (PL) spectra of the two COFs were measured. Upon excitation at 365 nm, the emission maxima of COF-BPDA appeared at λ em = 605 nm, distinctly red-shifted by 30 nm over that of λem = 575 nm for COF-DFB (Figure 4b), attributable to the larger π-extended structure of COF-BPDA. Additionally, the PL decay curves of two COFs are well fitted with the lifetimes with two exponential components ( Figure S9). The average lifetimes of COF-DFB and COF-BPDA were estimated to be 7.08 and 7.56 ns, respectively. The longer lifetime of COF-BPDA indicates the suppressed radiative recombination of photogenerated excitons, associated with its extended π-conjugated backbone. As a proof of concept, we studied the visible light photocatalytic H2 evolution activities of COF-DFB and COF-BPDA from water reduction. Both COF samples can be easily dispersed in water, which is related to their pyridine-rich backbones that strongly interacted with water. The photocatalysis experiments were performed by dispersing 20.0 mg of COF powder into 0.1 M aqueous ascorbic acid solution, in which ascorbic acid was applied as a sacrificial electron donor, and 3 wt % platinum was in situ photo-deposited on the COF. Upon visible light illumination, COF-BPDA showed an average hydrogen evolution rate (HER) of 3229.01 μmol h -1 g -1 , nearly 1.54 times faster than that of COF-DFB (2099.60 μmol h -1 g -1 ) (Figure 4d). This indicated that the photocatalytic H2 generation performance of COF-DFB and COF-BPDA were mainly influenced by their intrinsic semiconducting properties. COF-BPDA with longer conjugation length and narrower band gap delivered higher activity as compared to COF-DFB.

CONCLUSIONS
In conclusion, we have established an efficient synthetic strategy that can combine a D2h symmetrical building block 2,2',6,6'tetramethyl-4,4'-bipyridine with C2 symmetrical aromatic dialdehyde linkers to construct vinylene-linked COFs through Knoevenagel condensation in a mixed solvent of benzoic anhydride/benzoic acid. The possible reaction mechanism involves the self-catalyzed benzoylation of the used pyridine-based monomer, which subsequently promoted the Knoevenagel condensation at the pyridylmethyl carbon atoms of this monomer. The as-synthesized COFs were highly crystallized in orthorhombic lattices, releasing relatively large surface areas. The fully πconjugated structures give rise to abundant semiconducting properties of the COFs, allowing for efficiently visible lightcatalyzed hydrogen evolution from water reduction. This strategy paves a low-cost, environmentally friendly pathway to enlarge the regimes of vinylene-linked COFs with complex geometric and topological structures, as well as intriguing properties.

EXPERIMENTAL SECTION
All experimental procedures are provided in the Supporting Information.