Introduction to Covalent Organic Frameworks: An Advanced Organic

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

Introduction to Covalent Organic Frameworks: An Advanced Organic Chemistry Experiment María Jose ́ Mancheño,*,† Sergio Royuela,†,‡ A. de la Peña,† Mar Ramos,‡ Feĺ ix Zamora,§ and Jose ́ L. Segura† †

Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Madrid 28040, Spain Departamento de Tecnología Química y Ambiental, Universidad Rey Juan Carlos, Madrid 28933, Spain § Departamento de Química Inorgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, Madrid 28049, Spain Downloaded via VOLUNTEER STATE COMMUNITY COLG on July 19, 2019 at 01:38:07 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: In order to introduce the concept of covalent organic frameworks (COFs), an important class of predictable crystalline porous polymers, an integrated laboratory experiment for advanced organic chemistry students is reported. The importance of these kind of polymers and their multiple applications are presented as a part of key concepts in polymer and organic chemistry, bridging these disciplines to materials science. Students carry out the synthesis and characterization of an iminelinked COF and its building blocks. The polymer is formed at room temperature by using solvent-assisted synthesis and mechanochemistry. Students are trained in solid state characterization techniques, including FTIR spectroscopy, thermogravimetric analysis (TGA), 13C NMR crosspolarization−magic angle spinning (CP-MAS) NMR spectroscopy, and powder X-ray diffraction (PXRD), which are essential in the important field of reticular chemistry. Therefore, this lab experience constitutes a practical introduction to COFs. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Collaborative/Cooperative learning, Polymer Chemistry, Organic Chemistry, Materials Science, Solid State Chemistry, NMR Spectroscopy

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INTRODUCTION Polymer chemistry is an important area to be covered in the curriculum for modern chemistry students.1 They are expected to master the fundamental concepts of polymer structure, synthesis, and properties. In fact, the ACS Guidelines for bachelor’s degrees in chemistry call for the curriculum to include polymer chemistry principles and the synthesis of different classes of macromolecules. Thus, implementing a lab experiment covering the comprehensive key concepts of polymer education in-depth is a significant issue to be addressed. Covalent organic frameworks (COFs) constitute a polymeric class of porous crystalline materials that have attracted the interest of the scientific community.2 The reasons are that they can be designed ad hoc, on the basis of the principles of reticular chemistry,3 and different features, such as composition, topology, porosity, and functionalization, can be controlled. This control allows the synthesis of structures with high regularity, enabling the fine-tuning of the network properties. This fact is reflected in the array of structures and applications developed to date, such as gas adsorption and storage, catalysis, sensors, solar energy collectors, optoelectronic devices, and batteries, among others.4 COF networks constitute an excellent opportunity to expand the knowledge base of organic chemistry to include polymer © XXXX American Chemical Society and Division of Chemical Education, Inc.

chemistry and synthesis and polymer characteristics, thereby bridging the gap between organic chemistry and materials science. Students in organic chemistry laboratories often do not have the chance to prepare polymeric materials and control their properties through different reaction conditions or to synthesize the corresponding monomers. With the aim of integrating relevant and modern topics in this polymer synthetic area in an advanced organic course, a multidisciplinary experiment covering the synthesis and characterization of COFs was developed. An advanced laboratory experiment should teach skills such as searching the literature, using proper laboratory techniques, working cooperatively, writing for scientific audiences, and presenting results; thus, these skills are also covered in the experiment. COFs are mostly synthesized using reversible, covalentbond-forming reactions, generally condensations yielding boroxines3 or imines,4a although other reactions have been implemented with success in the preparation of these kind of polymers.5 Although harsh conditions, such as solvothermal reaction conditions,6 are often necessary in order to achieve COFs,2,4 some of them can be obtained by using very smooth Received: October 8, 2018 Revised: May 22, 2019

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DOI: 10.1021/acs.jchemed.8b00810 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Table 1. Topics Addressed and Techniques Studied throughout the Laboratory Experiment Topics Addressed and Techniques Studied Introduction to COFs: fundamentals and applications Vibrational spectroscopy: infrared spectroscopy Powder X-ray diffraction NMR spectroscopy: solid state 13C NMR crosspolarization−magic angle spinning Thermogravimetric analysis Mechanochemical synthesis Organic synthesis: Schlenk techniques Literature searches and scientific communication skills

Learning Outcomes Students will be able to understand what COFs are, how to obtain them, and what their main properties and applications are. Students will be able to assign the main functional group bands of the synthesized monomers and polymers and report the corresponding data. Students will be able to distinguish between amorphous and crystalline materials and calculate cell parameters of hexagonal lattice COFs. They will be able to report the corresponding data as well. Students will learn how to choose the most adequate techniques to characterize the obtained monomers and polymers. They will also be able to apply the techniques to elucidate structures by interpreting the data. They will be able to report the data properly. Students will be able to assess the thermal stabilities of the obtained polymers. Students will be able to understand and apply mechanochemical methods. Students will be able to understand and use organic transformations to synthesize COFs and also apply air-free manipulation techniques. Students will be able to search an online database in order to find original research papers and analyze the raw information in the papers to find the synthetic information. They will be able to write a scientific report and communicate their results in an oral presentation.

reaction conditions, such as room temperature.7−9 This is dependent upon the reactivity of the starting materials and the empirical conditions utilized, but in all cases, dynamic reversibility of the linkage is a key feature.2 The experiment reported herein consists of the formation of a covalent organic framework by two different methods, as well as the synthetic preparation of the building blocks necessary to construct it. The synthesis of the COF is achieved at room temperature (RT) by solvent-assisted synthesis and mechanochemistry,9 a nontraditional method in an organic chemistry laboratory that refers to chemical reactivity conducted by mechanical force or sudden frictional heating.10 Throughout the experiment, students are introduced to the concept and properties of COFs in context, review classical organic reaction mechanisms and procedures, use Schlenk techniques, and are initiated in the use of mechanochemical reactions as mentioned above. First, in order to familiarize students with COFs, an introductory lecture is part of the syllabus. The fundamental concepts of the characterization techniques to be used (Table 1) are covered.11,12 Moreover, within the experimental exercise of the lab, analysis with powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), and nonconventional solidstate 13C NMR cross-polarization−magic angle spinning (CPMAS NMR) spectroscopy is part of the syllabus, which helps students learn and interpret these techniques. For instance, PXRD allows the distinction between amorphous and crystalline materials and covers topics such as unit cells and lattice parameters. Although liquid NMR spectroscopy of simple molecules is an integral part of the introductory organic chemistry curriculum, solid state CP-MAS NMR spectroscopy of polymers is not usually studied by chemistry students, even though it is currently a fundamental tool in the characterization of many materials.12 In this laboratory experiment, students can carry out both NMR spectroscopic experiments (liquid and solid) and compare the differences between the results, developing interpretation skills. The experiment was implemented and developed in the last two academic years, as a part of the final degree projects of chemistry students in the Faculty of Chemistry of Universidad Complutense de Madrid. The students were divided into two groups of eight students. In each group, the students worked in pairs and then shared their results with the whole group in the discussion sessions. A short test was included as a part of the evaluation in the final session. Finally, the students presented

their work as scientific reports and as PowerPoint presentations to a panel of academic staff.

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EXPERIMENT This laboratory experiment was designed for an advanced undergraduate organic chemistry laboratory course. It was performed in five separate 3.5 h lab periods. Prior to performing the experiment, students attended a 60 min prelaboratory session, in which the instructor presented an introduction about COFs and the project, focusing on the nonconventional techniques that they were going to use. Students were asked to read general information about covalent organic frameworks and the techniques to be used.2,4,11,12 An understanding of the theory behind the experiments is required, so questions about the type of materials to synthesize, mechanisms of the different synthetic reactions, alternative methods to carry out the synthesis, and methods of characterization were posed by the instructor before the students started the synthesis. Students were prompted to use literature searches in order to answer these questions (Supporting Information, Instructor Notes, p S3). The use of SciFinder13 was mandatory for this purpose; thus, a learning outcome about literature searching was incorporated.14 After the introductory sessions, students worked in pairs to prepare the building blocks (Scheme 1) to synthesize COF-1 (Scheme 2). 1,3,5-Benzenetricarbaldehyde (BTCA)6,15 and 1,3,5-tris(p-aminophenyl)benzene (TAPB)16 were obtained by two-step methods. In this way, students mastered synthetic Scheme 1. Synthesis of 1,3,5-Tris(p-aminophenyl)benzene (TAPB) and 1,3,5-Benzenetricarbaldehyde (BTCA)

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Scheme 2. Synthesis of COF-1

tetrahydrofuran to yield COF-1A as a yellow solid. Then, the solid was dried under vacuum. (b) Mechanochemical method: COF-1B was synthesized by mixing TAPB and BTCA in solid form (1:1 molar ratio) together with p-toluenesulfonic acid, using an Agatha mortar and continuous grinding for 5 min. After water was added and grinding continued for an additional 15 min, the resulting solid was washed with methanol and tetrahydrofuran to yield COF-1B as a yellow powder. Then, the solid was dried under vacuum. Characterization

All COFs were characterized using FTIR spectroscopy, 13C CP-MAS NMR spectroscopy, PXRD, and TGA. FTIR spectroscopy, together with PXRD, is an essential tool in the characterization of COFs. The starting materials and products of the linkage reaction possess different characteristic IR stretches; thus, formation of the network was first verified by FTIR spectroscopy. Students collected the PXRD patterns of their samples to assess crystallinity. In general, for the COF’s characterization, a structural model of the COF network that takes into account the topology of the building blocks is first performed using a software package such as Materials Studio.17 From this model, a simulated PXRD pattern can be generated, and the Bragg positions compared to the experimental data. Therefore, the packing mode can be ascertained. In this case, the theoretical profile of the PXRD patterns for the different packing modes was provided to the students (Figure 1),18 and they were required to determine the packing mode and the main parameters of the unit cell. Specifically, students used Bragg’s

methodology, including Schlenk techniques, purification and characterization by FTIR, and solution 1H NMR spectroscopy. Planning sessions, detailed experimental conditions, safety precautions, hazards, and experimental data are provided in the Supporting Information (see the Instructor Notes). Synthesis of COFs

Formation of COF-1 (Scheme 2) was performed by two different methods. Each pair of students synthesized COF-1 following these two different routes. (a) Solvent-assisted method:7 COF-1A was synthesized by adding acetic acid to a DMSO solution of TAPB and BTCA (1:1 molar ratio) under manual shaking at room temperature. After 1 min, a yellow gel was formed, which was filtered and repeatedly washed with methanol and

Figure 1. (a) View of the slipped AA stacking crystal structure of COF-1 (left) and simulated and experimental PXRD patterns for solvothermal synthesis of COF-1 (right). Adapted with permission from ref 18. Copyright 2018 The Royal Society of Chemistry. (b) Packing modes. (c) Comparison of X-ray diffraction patterns of a set of three samples of COF-1 obtained by the students. One sample is amorphous (red); thus, it cannot be considered a COF. C




Figure 2. FTIR-ATR spectra of (a) 1,3,5-benzenetricarbaldehyde (BTCA), (b) 1,3,5-tris(p-aminophenyl)benzene (TAPB), and (c) COF-1A (solvent-assisted method at RT).

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RESULTS AND DISCUSSION The lab experiment began with the synthesis and purification of the building blocks needed to prepare the COFs. The Supporting Information provides the data to help characterize of all of them, as well as the FTIR and PXRD data of the COFs obtained by the different procedures tested. The reported data for this COF, obtained at room temperature by a solventassisted reaction7 and solvothermal conditions, are also included for comparison.18 Once the starting materials were obtained, COF-1A was prepared in variable yields from 80 to 96%, following the solvent-assisted method. Because porosity is an essential property of COFs,2,4 they are usually thermally activated before full characterization. Thus, the materials were dried under vacuum to remove the solvent from the cavities. Then, all COFs were characterized using FTIR spectroscopy, 13C CPMAS NMR spectroscopy, PXRD, and TGA. The FTIR spectrum of COF-1A (Figure 2) shows clearly the presence of both imine CN and C−CN−C stretching bands at 1623 and 1137 cm−1, respectively (Figure 2c). Students were encouraged to examine and distinguish the significant features in comparison with the starting materials. Another feature highlighted for the students’ training was to observe the lack of absorption bands in the range of 3211− 3433 cm−1, characteristic of the amine group (Figure 2b). This helped students to understand that these data were consistent with the formation of the imine framework. Then, a solid-state 13C CP-MAS NMR spectrum of COF-1A could be obtained, and the data could be interpreted. Because of difficulties associated with the use of such equipment, the reported 13C CP-MAS NMR spectrum7 (see the Supporting Information, Instructor Notes, p S14) was handed to the students by the instructor, together with those of the starting materials. In COF-1, imine bonds are shown around 157 ppm, and the rest of the aromatic carbons appear in the usual range. Students could check the similarity of the chemical shift for these moieties with those reported in conventional solution 13 C NMR spectra. They could also observe the difference in spectral resolution with other samples run in solid state, where broad bands are obtained in comparison with narrow peaks for

Law to relate the 2θ angles of the (100) and (001) reflections to the pore sizes and interlayer spacing distances of their COF structures. In COF-1, formation of imine linkages between the trigonal triamine and trialdehyde linkers results in the formation of an extended hexagonal lattice, which stacks in an eclipsed fashion, resulting in a series of one-dimensional channels along the stacking direction, as displayed in Figure 1a. Comparison of the PXRD pattern data obtained for COF-1 using both methods and comparison with the reported data under solvothermal conditions18 also allowed students to observe how the synthetic conditions affect the crystallinity of the COF.19 Students also ran TGA experiments20 to verify solvent desorption from the cavities in the prepared materials and to check the thermal stability of the framework.

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HAZARDS

All reagents should be handled in a well ventilated hood; students should wear gloves, safety goggles, and lab coats, avoiding contact of the reagents with skin. All the reactions must be carried out under the strict supervision of the instructor. Caution: LiAlH4 powder,21 in contact with water, releases flammable gases which may ignite spontaneously. Pd(C) may cause an allergic skin reaction. The hydrazine/ Pd(C) mixture releases hydrogen, which is highly flammable, so the reaction must be carefully performed in a very well ventilated hood. Trifluoromethanesulfonic acid causes severe skin burns and eye damage. Hydrazine hydrate is toxic if inhaled or swallowed or if it comes into contact with skin. Pyridinium chlorochromate may cause an allergic skin reaction. Tetrahydrofuran, toluene, and hexanes are highly flammable. Methylene chloride and chloroform-d are very hazardous if ingested or inhaled or in the case of eye contact (irritant). 1,3,5-Tris(p-aminophenyl)benzene causes skin and eye irritation. 1,3,5-Benzenetricarbaldehyde causes skin and eye irritation. It may cause respiratory irritation. COF-1 may cause skin and eye irritation. It is slightly hazardous in the case of inhalation. Please, see the Supporting Information for detailed and complete data about all the chemicals used and products synthesized throughout the experiment. D




set and the information obtained by them and provided by the instructor. Thus, scientific communication skills were also implemented as a part of the training. Finally, a requirement of the course was to provide a short PowerPoint presentation to a panel of academic staff. Both the write-up and presentation provided real-life opportunities for students to share their findings and learning in a typical scientific manner. The lab experiment was reviewed by anonymous evaluation. The results showed that the students approved of the introduction to these new techniques and concepts as a part of their academic formation: 100% of the students responded affirmatively to statements such as, “This project provided me relevant information that otherwise I would not see in any course of the Chemistry curriculum,” and “This project allows me to implement my knowledge about: solid state techniques; organic synthesis and new methods of synthesis.” Nevertheless, they found as the main drawback difficulty in fully understanding the reading assignments in the syllabus. Other students encountered difficulties in learning and applying new concepts, especially PXRD analysis. It is important to note that for many students, this lab was their first formal experience with synthetic macromolecules or networks. Thus, it was challenging for them to use these solid state techniques.

conventional solution spectra (see Figures S7 and S8, Supporting Information, Instructor Notes). The structure of a polymer is also defined in terms of crystallinity. This might also be thought of as the degree of order or regularity in how the molecules are packed together. Crystalline structures are generally very ordered, which is what gives them strength and rigidity. Thus, powder X-ray diffraction was used to assess the crystallinity of the polymer structure and determine the mode of packing and interlayer distance. In PXRD measurements of materials, crystalline domains produce sharp reflections that correspond to groups of planes within a structure containing identical spacing, whereas amorphous zones produce a wide and diffused halo. Students confirmed the structures of their COFs by matching all Bragg reflection positions with the reported Bragg peaks for COF-1.7 For the solvent-assisted method (COF-1A), the peaks (d-spacing: 15.55, 8.94, 7.71, 5.75, 4.53, and 3.54) are consistent with the (100), (110), (200), (210), (220), and (001) diffraction planes, respectively, of a hexagonal unit cell with cell parameters of a = b = 17.9 ± 0.3 Å and c = 3.54 ± 0.03 Å. Similar results have been reported for the solvothermal synthesis of the same COF19 (Figure 1). Thus, COF-1A shows a laminar structure with sheets stacked in an AA fashion (see Supporting Information, Instructor Notes, pp S5 and S16).7,18 Likewise, COF-1B, prepared by a mechanochemical reaction, was also characterized. Yields for COF-1B (60− 80%) obtained by the students were lower than those obtained with the solvent-assisted method. COF-1B displayed similar spectroscopic characteristics, with its FTIR spectrum showing the same bands as those for COF-1A. The exact match of the FTIR and solid-state NMR spectra indicated that the COFs obtained by the two methods had the same structure. COF-1B also exhibited a comparable PXRD pattern to that of COF-1A. 2θ peaks values at 5.8, 9.9, 11.5, and 25.2°, correspond to the (100), (110), (200), and (001) diffraction planes, respectively, of a hexagonal unit cell with cell parameters of a = b = 17.7 Å and c = 3.54 Å. PXRD relative peak intensities and full-width at half-maximum (fwhm) analysis of both COFs allowed for discussions about crystallinity (see Supporting Information, Instructor Notes, p S6). The fwhm is sensitive to the variation in microstructure, so increases in stacking faults and structural disorder widen the XRD peaks.19 Students compared the data obtained in both PXRD patterns in terms of number, relative intensities, and fwhm values of all diffraction peaks (Table S2, Instructor Notes, p S17). The differences encountered between both methods were reported by the students. Significantly, the first peak is relatively less intense for the mechanochemically synthesized COF-1B (see the comparison of the diffraction patterns in Figure 1c). This could be due to random displacement of the 2D layers (i.e., exfoliation) affecting the distributions of eclipsed pores. As a result, the reflection corresponding to the (100) plane becomes weak.9 After data analysis it was concluded that solvent-assisted synthesis at RT yielded a more ordered structure than the mechanochemical method. Finally, by TGA,20 students could verify solvent desorption from the cavities in the prepared materials. Both polymers exhibited quite high stability as no decomposition was observed up to 500 °C (Supporting Information, Instructor Notes, p S15). After finishing the experiments, students compared their results and took a short test related to the topics reviewed with the instructor. They wrote a scientific report based on the data

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SUMMARY This experiment provides an excellent opportunity to introduce students to the novel concept of covalent organic frameworks, a type of crystalline polymer with multiple applications. Syntheses of the polymers are conducted by either a solvent-assisted method or mechanochemistry, showing students useful chemical techniques such as solid state techniques. It integrates advanced organic synthesis as well as several methods for structural determination in liquid and solid state, including FTIR and NMR spectroscopies, PXRD, and TGA. During the lab sessions, students are encouraged to interpret experimental results, comparing them with reported data. Thus, they become confident with interpreting literature procedures; using Schlenk techniques; purifying mixtures by flash chromatography; and analyzing NMR spectra and TGA or PXRD data. The final discussion session helped them to consolidate their knowledge. The scientific report and oral presentation familiarized them with communication methods used by the scientific community. This approach can give them an idea of the problems associated with the design and characterization of COFs, exposing students to many of the fundamental aspects of reticular chemistry from a multidisciplinary point of view, relating organic synthesis to other disciplines.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00810. Instructor Notes, including laboratory experiment setup and pedagogy, experimental procedures, example spectra, results of learning outcomes after the laboratory experience, and example results of a final test (PDF, DOC) Notes for Students, including laboratory experiment setup, prelab questions, experimental procedures with synthesis instructions, guiding questions, evaluation E



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criteria, and final grade component weightings (PDF, DOC)

(6) Côté, A. P.; Benin, A. I.; Ockwig, N. W.; O’Keeffe, M.; Matzger, A. J.; Yaghi, O. M. Porous, Crystalline, Covalent Organic Frameworks. Science 2005, 310 (5751), 1166−1170. (7) De la Peña Ruigómez, A.; Rodríguez-San-Miguel, D.; Stylianou, K. C.; Cavallini, M.; Gentili, D.; Liscio, F.; Milita, S.; Roscioni, O. M.; Ruíz-González, M. L.; Carbonell, C.; Maspoch, D.; Mas-Ballesté, R.; Segura, J. L.; Zamora, F. Direct On-Surface Patterning of a Crystalline Laminar Covalent Organic Framework Synthesized at Room Temperature. Chem. - Eur. J. 2015, 21 (30), 10666−10670. (8) Matsumoto, M.; Dasari, R. R.; Ji, W.; Feriante, C. H.; Parker, T. C.; Marder, S. R.; Dichtel, W. R. Rapid, Low Temperature Formation of Imine-Linked Covalent Organic Frameworks Catalyzed by Metal Triflates. J. Am. Chem. Soc. 2017, 139 (14), 4999−5002. (9) Biswal, B. P.; Chandra, S.; Kandambeth, S.; Lukose, B.; Heine, T.; Banerjee, R. Mechanochemical Synthesis of Chemically Stable Isoreticular Covalent Organic Frameworks. J. Am. Chem. Soc. 2013, 135 (14), 5328−5331. (10) (a) Zhang, P.; Dai, S. Mechanochemical Synthesis of Porous Organic Materials. J. Mater. Chem. A 2017, 5 (31), 16118−16127. (b) For an educational mechanochemical synthesis experiment, see Wixtrom, A.; Buhler, J.; Abdel-Fattah, T. Mechanochemical Synthesis of Two Polymorphs of the Tetrathiafulvalene-Chloranil Charge Transfer Salt: An Experiment for Organic Chemistry. J. Chem. Educ. 2014, 91 (8), 1232−1235. (11) (a) Pinto, A. H. Designing and Teaching a Course about Characterization Techniques for Solid State Materials in an Undergraduate Institution. J. Chem. Educ. 2018, 95 (10), 1717− 1723. (b) Wriedt, M.; Sculley, J.; Aulakh, D.; Zhou, H.-C. Using Modern Solid-State Analytical Tools for Investigations of an Advanced Carbon Capture Material: Experiments for the Inorganic Chemistry Laboratory. J. Chem. Educ. 2016, 93 (12), 2068−2073. (c) Crane, J.; Anderson, K.; Conway, S. Hydrothermal Synthesis and Characterization of a Metal−Organic Framework by Thermogravimetric Analysis, Powder X-ray Diffraction, and Infrared Spectroscopy: An Integrative Inorganic Chemistry Experiment. J. Chem. Educ. 2015, 92 (2), 373−377. (12) Kinnun, J. J.; Leftin, A.; Brown, M. F. Solid-State NMR Spectroscopy for the Physical Chemistry Laboratory. J. Chem. Educ. 2013, 90 (1), 123−128. (13) Scifindern. CAS. https://www.cas.org/products/scifinder-n (accessed May 2019). (14) (a) Gawalt, E. S.; Adams, B. A Chemical Information Literacy Program for First-Year Students. J. Chem. Educ. 2011, 88 (4), 402− 407. (b) Jensen, D.; Narske, R.; Ghinazzi, C. Beyond Chemical Literature: Developing Skills for Chemical Research Literacy. J. Chem. Educ. 2010, 87 (7), 700−702. (15) Kaur, N.; Delcros, J.-G.; Imran, J.; Khaled, A.; Chehtane, M.; Tschammer, N.; Martin, B.; Phanstiel, O., IV A Comparison of Chloroambucil- and Xylene-Containing Polyamines Leads to Improved Ligands for Accessing the Polyamine Transport System. J. Med. Chem. 2008, 51 (5), 1393−1401. (16) (a) Kathiresan, M.; Walder, L.; Ye, F.; Reuter, H. Viologenbased benzylic dendrimers: selective synthesis of 3,5-bis(hydroxymethyl) benzylbromide and conformational analysis of the corresponding viologen dendrimer subunit. Tetrahedron Lett. 2010, 51 (16), 2188−2192. (b) Fourmigué, M.; Johannsen, I.; Boubekeur, K.; Nelson, C.; Batail, P. Tetrathiafulvalene- and dithiafulvene-substituted mesitylenes, new π-donor molecules with 3-fold symmetry and the formation of an unprecedented new class of electroactive polymers. J. Am. Chem. Soc. 1993, 115 (9), 3752−3759. (17) Materials Studio; Accelrys Software Inc.: San Diego, CA, 2017. (18) Dong, J.; Wang, Y.; Liu, G.; Cheng, Y.; Zhao, D. Isoreticular covalent organic frameworks for hydrocarbon uptake and separation: the important role of monomer planarity. CrystEngComm 2017, 19 (33), 4899−4904. (19) A careful analysis of fwhm peaks related to defects in crystallinity can be found in Slater, B.; Wang, Z.; Jiang, S.; Hill, M. R.; Ladewig, B. P. Missing Linker Defects in a Homochiral Metal−

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

María José Mancheño: 0000-0003-1491-6362 Sergio Royuela: 0000-0003-3113-1831 Félix Zamora: 0000-0001-7529-5120 José L. Segura: 0000-0002-3360-1019 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by MINECO (MAT201677608-C3-1-P and MAT2016-77608-C3-2-P). REFERENCES

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Introduction to Covalent Organic Frameworks: An Advanced Organic

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