Oriented Covalent Organic Framework Film Synthesis from Azomethine Compounds

Strategies enabling solution processing of covalent organic framework (COF) thin films will become increasingly important as these versatile materials are integrated into a wide range of electronic and optical devices. This work highlights an approach to yield thin film synthesis of TAPA‐PDA (TAPA: tris(4‐aminophenyl)amine, PDA: terephthalaldehyde) and TAPB‐PDA (TAPB: 1,3,5‐tris(4‐aminophenyl)benzene) imine COFs using azomethine compounds which can be drop cast onto a variety of substrates. High crystalline COF films are shown to form on various electronically‐relevant substrates. Grazing incidence wide angle X‐ray scattering characterization reveals COF films with a preferred horizontal orientation in the case of TAPA‐PDA COF and a more mixed/vertical orientation in the TAPB‐PDA COF film regardless of the substrate. As this exciting class of crystalline organic materials becomes more relevant for various device applications, solution processing techniques will be vital to take advantage of the properties of thin film COFs.

Strategies enabling solution processing of covalent organic framework (COF) thin films will become increasingly important as these versatile materials are integrated into a wide range of electronic and optical devices. This work highlights an approach to yield thin film synthesis of TAPA-PDA (TAPA: tris(4-aminophenyl)amine, PDA: terephthalaldehyde) and TAPB-PDA (TAPB: 1,3,5-tris(4-aminophenyl)benzene) imine COFs using azomethine compounds which can be drop cast onto a variety of substrates. High crystalline COF films are shown to form on various electronically-relevant substrates. Grazing incidence wide angle X-ray scattering characterization reveals COF films with a preferred horizontal orientation in the case of TAPA-PDA COF and a more mixed/vertical orientation in the TAPB-PDA COF film regardless of the substrate. As this exciting class of crystalline organic materials becomes more relevant for various device applications, solution processing techniques will be vital to take advantage of the properties of thin film COFs.

Introduction
Covalent organic frameworks (COFs) are a class of crystalline organic materials with attractive properties such as high porosity, tunable pore-size, and designable framework functionalities. As a result, COFs have been reported to have various applications in gas sorption/separation, catalysis, sensing, electronic, and opto-electronic applications. [1][2][3][4][5] For the practical www.advmatinterfaces.de pyrolytic graphite, this method yields a COF film with a random orientation of pores on substrates. Ideally, COF thin films that can be solution-processed would facilitate a major step forward in creating thin films incorporating methods relevant for electronic device fabrication, such as spin-coating and 3D printing.
In conventional solvothermal synthesis, amorphous imine polymers were shown to initially form and subsequently rearrange to crystalline COFs as a result of the reversible iminebond formation reaction. [13,14] Based on a similar proof of concept, the transformation of amorphous imine polymers to crystalline COFs either by using more reactive aldehyde monomers or the formation of more stable β-ketoenamine or imide COFs was reported. [15][16][17][18][19] The amorphous to crystalline approach was also employed recently to construct stand-alone COF membranes, where amine and aldehyde starting materials were allowed to react to form the pristine membranes followed by heating the amorphous membrane in solvent vapors to yield crystalline COF membrane. [19,20] Even though this demonstrates the feasibility of large-area COF membrane preparation, it lacks details about the pore channel arrangement as well as the generality of substrate scope which are important for devices. [20] In this work, we demonstrate COF film synthesis from polyimines, also referred to as azomethine (AM) compounds, through a two-step procedure ( Figure 1A). The first step involves the ex-situ synthesis of soluble AM compounds allowing solvent-processed fabrication of AM films on various substrates. After deposition, the AM film was subsequently converted to a crystalline COF film, facilitated by imine bond rearrangement within the network. The ability to perform solvent-processed AM film fabrication allows for the preparation of large-area films and facilitates the homogeneous distribution of starting materials for the synthesis of a crystalline oriented COF films. This method was demonstrated on two different imine COFs: TAPA-PDA (TAPA: tris(4-aminophenyl)amine, PDA: terephthalaldehyde) and TAPB-PDA (TAPB: 1,3,5-tris(4aminophenyl)benzene) COFs ( Figure 1B) with various substrates including glass, indium tin oxide (ITO), strontium titanate (STO), sapphire, mica, graphene, and transition metal dichalchogenides (TMDs). This is the first report for the usage of AM compounds which are soluble in common organic solvents as precursors for COF film synthesis.

AM synthesis
AM compounds are imine-based organic compounds which, similar to COF, can be formed by the condensation reaction between amines and aldehydes. This includes monomer pairs such as TAPA with PDA, TAPB with PDA, and TFPA (tris(4-formylphenyl)amine) with DAB (1,4-diaminobenzene).

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Using appropriate conditions this can lead to materials with utility in resistive switching memory devices, [21] solid support for CO 2 to fuel conversion, [22] and negative electrodes for flexible thin film batteries. [23] Most of these reported polyazomethines (PAMs) have very low solubility in common organic solvents, limiting their usage in solution-processed thin film fabrication methods. In order for us to obtain soluble AM compounds, we performed an extensive study to identify the optimum reaction conditions (Table S1, Supporting Information) by changing solvents and stoichiometric ratios between amine (TAPA) and aldehyde (PDA) components. Optimization was performed on these substrates due to their availability and desirable structural and electronic properties. [24] Initial attempts with methanol as a solvent and TAPA:PDA ratio of 1:1.5 yielded a solid polymer with poor solubility due to the high molecular weight of the product. As a result, the ratio of PDA was subsequently increased to three equivalents to lower the molecular weight and increase the product's solubility. A further study revealed that toluene is a more suitable solvent than MeOH since the low molecular weight AM compounds have enhanced solubility in toluene. The optimized reaction condition includes heating TAPA:PDA (1:3 ratio) in toluene at 100 °C for 4 h. After completion, the filtrate was removed from the precipitate and evaporation of solvent gave desirable TAPA-PDA AM mixture with good solubility in chloroform, THF, DMF, and DCM. The precipitate contains higher molecular weight and rigid hyperbranched PAMs and therefore has low solubility in common organic solvents (further details can be found in Figure S2, Supporting Information). The composition of the filtrate was analyzed by Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC), and thermogravimetric analysis (TGA). Specifically, FTIR measurements revealed the presence of CN bonds (1615 cm −1 ) and a high relative intensity peak corresponded to CO bonds (1684 cm −1 ) in the filtrate ( Figure S2, Supporting Information), suggesting that it is a mixture of TAPA-PDA AMs and excess PDA which is in good agreement with NMR analysis ( Figure S9, Supporting Information). TGA was performed to quantify the amount of excess PDA in the mixture. Since PDA degrades at 165 °C, this temperature was used to identify the excess amount of PDA in the material obtained from the filtrate. Figure S5 (Supporting Information) shows the TGA traces of this filtrate and at 165 °C there is a 26% mass loss that can be attributed to the PDA and the remaining 74% is the TAPA-PDA AM. The composition of TAPA-PDA AMs was analyzed by GPC in THF as the eluent and showed a mixture of AM compounds with number average molecular weights (M n ) of 3.3 kDa and the polydispersity (PDI) of 2.73 indicating the presence of both high and low molecular weight AMs ( Figure S7 and Table S4, Supporting Information). Based on NMR analysis, the general formula for the TAPA-PDA AM mixture can be nominally calculated as (C 18 H 12 N 4 ) 1.7 (C 8 H 6 O) 3.1 (C 8 H 6 ) . This filtrate was carried to the next step without further purification and is designated as TAPA-PDA precursor mixture.

COF Synthesis from AM Compounds
Based on the fact that imine bonds can undergo rearrangement in the presence of an acidic catalyst, we treated the TAPA-PDA

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azomethine mixture under acidic conditions to facilitate conversion to crystalline TAPA-PDA COF (Figure 2A). Specifically, TAPA-PDA precursor mixture obtained from Step 1 was treated with dioxane, mesitylene, and acetic acid 10.5 M at 70 °C overnight. FTIR measurements confirm the presence of CN imine bond (1615 cm −1 ) in the product ( Figure S2, Supporting Information). The depletion of CO signal in the product compared with those of the starting materials indicated that excess PDA does not inhibit the conversion to COF and can successfully be removed after the reaction. Powder X-ray diffraction (PXRD) analysis confirmed the crystallinity of the azomethine-derived TAPA-PDA COF. The fact that the diffraction pattern of TAPA-PDA COF made from AM matches that of the monomer-derived COF ( Figure 2B) indicates the two COFs have the same crystal structure. The surface area and porosity of the COF were evaluated from N 2 gas sorption measurement at 77K, revealing an uptake of 225 cm 3 g −1 at 1 bar, 77K ( Figure 2C). Calculations from the N 2 isotherm gave a Brunauer-Emmet-Teller (BET) surface area of 584 m 2 g −1 , similar to the BET surface area of TAPA-PDA COF made from monomers (648 m 2 g −1 ). [24,25] Notably, elemental analysis confirms the C, H, and N compositions of TAPA-PDA COF made from AM and from monomers are the same (Table S2, Supporting Information). All of these data indicated that the presence of excess PDA does not affect the COF formation reaction and that TAPA-PDA COFs with high crystallinity and porosity can be made from AM compounds.

COF Film Fabrication
To prepare COF films, the azomethine TAPA-PDA precursor mixture was initially dissolved in chloroform and drop-cast onto a glass substrate. After drying, the film was wetted with a solution of dioxane, mesitylene, acetic acid 10.5 M and baked in the capped vial at 70 °C overnight with the aforementioned solution at the bottom of the vial. During the reaction, a Teflon holder was used to lift the film off the base of the vial (Figure 3A). PXRD analysis confirmed the complete conversion to crystalline COF as the obtained pattern matched the TAPA-PDA COF powder ( Figure 3B) diffraction pattern. Further optimization studies showed that the COF can be formed at 70 or 90 °C and the presence of excess aldehyde together with a dioxane/mesitylene/acetic acid solution are essential for the formation of crystalline COF (Table S3, Supporting Information).
To demonstrate that the method can also be used to generate films of other imine-based COF directly on the substrate, we extend this study to a benzene, rather than an amine, centered vertex. TAPB-PDA COF can be made from the condensation reaction between TAPB and PDA and is a well-studied COF with applications in modern separation and filtration technologies. [26,27] A TAPB-PDA COF precursor mixture was prepared by reacting TAPB and PDA (1:3 equivalent) in toluene at 100 °C (See the Supporting Information for further characterization including FTIR, TGA, GPC, and NMR, Figures S3, S6, S8, and S10, Supporting Information), which revealed that the mixture contained a larger percentage of excess PDA (64%) and its AM compounds has a M n of 1.4 kDa and PDI of 1.08 (Table S5, Supporting Information). Despite having slightly lower solubility as compared with TAPA-PDA precursor mixture, the TAPB-PDA precursor mixture is also soluble in chloroform and THF. Subjecting the TAPB-PDA precursor mixture to dioxane/mesitylene/acetic acid with heat results in the formation of crystalline TAPB-PDA COF powder with identical PXRD pattern to that of monomer-derived TAPB-PDA COF ( Figure 3C). The

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drop-cast precursor film was successfully converted to TAPB-PDA COF film under our solvothermal conditions and crystallinity was confirmed by PXRD measurements ( Figure 3C). As we previously reported, TAPA-PDA COF undergoes a reversible phase transition after solvent removal. [24] However, the TAPB-PDA COF is more stable toward activation conditions as evidenced by PXRD patterns of obtained films ( Figure 3C).

Scope of Substrates
Versatile deposition processes that integrate COF thin films onto a variety of substrates will become increasingly important in future device structures featuring COFs. Depending on the application of interest, electrically conductive (ITO and graphene), semiconducting (TMDs) or insulating (glass and mica) substrates may be necessary for optimal performance and device integration. PXRD analysis confirms that TAPA-PDA COF films can be synthesized on a wide range of substrates with high crystallinity (Figure 4A and Figure S19, and S20, Supporting Information (for graphene and TMDs)). Once subjected to activation condition, however, TAPA-PDA films on glass, mica, ITO, STO, and sapphire cracked and ruptured, presumably due to internal film stresses during the phase change after solvent removal and weak interaction with these substrates. By switching to graphene and TMDs, which offer stronger π-π and van der Waals interactions, we were able to obtain activated TAPA-PDA COF films (Figures S19 and S20, Supporting Information). [8,28,29] For substrates with weak interaction such as glass, the TAPA-PDA COF film easily detached in solvents such as acetone or methanol, which may offer a route to generate standalone COF films. TAPB-PDA COF films, on the other hand, do not undergo conformational phase changes upon drying. This in turn allows for facile activation to generate dry, and highly crystalline films on diverse substrates including glass, ITO, graphene, and TMDs ( Figure 4B). SEM images of TAPA-PDA COF and TAPB-PDA COF films on graphene revealed that the surface of TAPA-PDA COF film is smoother ( Figure 4D,E). Thickness measurements using a contact profilometer indicated that the TAPA-PDA film had a thickness of 0.47 µm while the TAPB-PDA film was 1.23 µm thick. Raman studies confirmed the quality of graphene and WSe 2 , which are covered by COFs, is maintained following the conversion of AM compounds to COFs ( Figure 4C and Figure S12, Supporting Information). As this method is a solution-processing approach, it allows for the synthesis of large-area COF films (up to 2 cm 2 in this work) which is only limited by the size of the substrate. Additionally, the films of several COFs, in this case, TAPA-PDA and TAPB-PDA COFs, can be concurrently fabricated on the same substrate ( Figure S22, Supporting

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Information) with high crystallinity ( Figure S23 and S24, Supporting Information) using the AM approach reported here. Thus, the method is extremely versatile toward the integration of exciting crystalline organic materials onto a variety of electronically-relevant substrates.

Orientation Studies
For COF film applications in electronic devices and filtration/separation, a high degree of ordering and alignment of COF layers can enhance performance. A COF film fabrication method that can deliver this property is therefore greatly desirable. In this work, the arrangement of COF films was studied using grazing incidence wide angle X-ray scattering (GIWAXS) techniques ( Figure 5A, and Figure S25-33, Supporting Information). Scattering patterns of activated TAPA-PDA COF film on glass, WSe 2 , MoSe 2 , and graphene revealed pronounced orientation, seen in the localized azimuthal intensity distribution ( Figure 5A, C). Results demonstrate that the COF layers arrange parallel to the solid support as indicated by the concentration of in-plane scattered intensity near q z = 0, which is commonly reported among other COF films. [29][30][31][32][33][34] It is worth mentioning that TAPA-PDA COF films prepared by traditional solvothermal synthesis were reported to have a random arrangement. [24] In contrast, TAPB-PDA COF films on glass, ITO, WSe 2 , MoSe 2 , and graphene exhibit primarily out-of-plane arrangement of COF layers over solid supports as indicated by the concentration of scattered intensity near q xy = 0. This perpendicular arrangement is very rare among reported COF films and can only be achieved by modification of the solid supports which subsequently direct the growth of COFs. [35] The TAPB-PDA COF films were reported to exhibit either random or parallel orientation with respect to the substrates depending on the method of synthesis. [9,36] The differences in orientation of TAPA-PDA COF and TAPB-PDA COF films are in good agreement with the SEM and thickness studies where TAPA-PDA COF is thinner and has a smoother surface. Moreover, the scattering intensity of TAPB-PDA COF films ( Figure 5A) suggests there is another minor component in preferential alignment at 30 degrees with respect to the substrate, indicating that TAPB-PDA COF orientation of films can be further optimized, if this secondary structure is not inherent to the films.

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To further investigate the orientation of COF layers, the Hermans orientation factor of TAPA-PDA COF and TAPB-PDA COF films were calculated. [37,38] First, the azimuthal averages at the strongest Bragg peaks of TAPA-PDA COF films (q = 0.23 Å −1 ) and TAPB-PDA COF films (q = 0.20 Å −1 ) were deduced from the 2D scattering patterns ( Figure 5C and Figure S34, and S35, Supporting Information) which correspond to the (100) Bragg diffraction peaks of both COFs. The results confirm orthogonal orientation since TAPA-PDA COF has a maximum intensity at an azimuthal angle of 0° and 180° (in-plane), while TAPB-PDA intensity reaches a maximum at 90° angle (out-of-plane). The azimuthal data for TAPB-PDA COF contains two sets of peaks: the main domain peak at 90° and a minor component at ≈30° and ≈150°. This suggests that while the majority of the COF film exhibits vertical alignment, there is another preferred orientation within the film at 30° with the substrate. At this point, it's unclear whether this is an inherent feature of the packing of TAPB-PDA COF or is due to incomplete processing (island formation of a minor component). Based on the definition of the Hermans orientation factor [39] and assuming the substrate is the reference direction, an orientation factor of 1 is synonymous with a perfect alignment parallel, or in-plane, with the substrate, while a value of −0.5 is perfect alignment perpendicular, or out-of-plane, with the substrate. A value of 0 represents no alignment (random). Orientation factors were calculated for TAPA-PDA COF films at the main azimuthal peaks (0, 180°) on different substrates ( Figure 5D, see orientation calculation section in Supporting Information). TAPA-PDA COF films have orientation factor ranging from 0.41 (graphene) to 0.54 (WSe 2 ) (Table S7, (Table S8, Supporting Information). The orientation factor of TAPA-PDA COF films is close to 0.5 (broader distribution of orientation angles), signifying in-plane alignment of the COF film to all substrates explored in this study. On the other hand, orientation factors of TAPB-PDA COF films are closer to −0.5 (ideal perpendicular), in agreement with highquality alignment orthogonal to the substrates.
While atomic force microscopy images illustrate that the nanoscale structures of the two COFs are comparable ( Figure S18, Supporting Information), microscale morphologies of the two films was also investigated using scanning electron microscopy (SEM). The SEM images obtained indicate that TAPA-PDA COF crystallites assemble with well-ordered flat crystallites oriented in plane with the substrate (Figure S14, Supporting Information). In contrast, TAPB-PDA COF assembles in a more randomly oriented microstructure with somewhat 1-D fiber characteristics ( Figure S16, Supporting Information), a feature that has been documented in other systems. [40][41][42] The mechanism behind this microstructural orientation change requires further investigation but is expected to be a consequence of several competing factors. It is hypothesized that variations in the compositions of the precursor mixtures lead to the formation of COFs with different morphologies. TAPA-PDA precursor mixtures contain more concentrated higher molecular weight AMs, which may allow abundant TAPA units to react and form 2-D sheets ( Figure S17B, Supporting Information). TAPB-PDA precursor mixtures, on the other hand, have low molecular weight AMs species in relatively dilute concentration (36%). The dilution of TAPB, in this case, may hinder expansion into 2-D sheets leading to the formation of materials with some 1-D fiber character ( Figure S17A, Supporting Information). [43] The differing orientations of these films open up the possibility to implement various applications where orientation control is critical. In electronic devices such as field effect transistors and electrical/electrochemical sensors, control of orientation is extremely important for device design since many of the properties of COF films are anisotropic. [44] While most electronic devices harnessing COFs require horizontal orientation (i.e., transistors), vertically oriented films may provide significant benefits for devices where the conductive mechanism is out-of-plane rather than in-plane (such as ambipolar COF devices). [45] Control of horizontal and vertical orientation is also important for sensor applications that require high surface-to-volume ratio without sacrificing semiconducting electronic performance. Finally, control of COF crystal orientation can also be very important for gas storage and has been shown to improve hydrogen separation using the vertical orientation of COF-LZU1 membranes. [35]

Conclusion
This work introduces a two-step process that facilitates solutionprocessed fabrication of COF films from soluble azomethine precursor mixtures. The approach consists of the synthesis and casting of soluble AM mixtures to form precursor films followed by conversion to COF films after exposure to solvent vapors. The method was demonstrated on two different iminebased COFs: TAPA-PDA and TAPB-PDA COF and numerous substrates. Investigation of COF films on various solid supports also revealed that graphene and TMDs offer stronger interaction with COFs and facilitate strong attachment of COF films. Furthermore, the two COF studied in this work exhibit orientation differences in their films with respect to the substrates. While TAPA-PDA COF films arranged parallel to the substrate, TAPB-PDA COF films demonstrated perpendicular orientation, which is unique for COF films and membranes and offers unique opportunities for applications that require tuning of directionality in electronic and optical properties. Overall, this method allowed for the synthesis of large-area COF films with oriented crystalline domains. Further development of this methodology may allow for the generation of highly homogenous COF films on various substrates for electronic casting and printing techniques.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.