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Organic Radical-Linked Covalent Triazine Framework with Paramagnetic Behavior Yi Jiang,†,⊥ Inseon Oh,‡,⊥ Se Hun Joo,∥,⊥ Onur Buyukcakir,† Xiong Chen,† Sun Hwa Lee,† Ming Huang,† Won Kyung Seong,† Jin Hoon Kim,§ Jan-Uwe Rohde,§ Sang Kyu Kwak,*,∥ Jung-Woo Yoo,*,‡ and Rodney S. Ruoff*,†,‡,§,∥ †

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea School of Materials Science and Engineering, §Department of Chemistry, and ∥School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

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‡

S Supporting Information *

ABSTRACT: The production of multifunctional pure organic materials that combine different sizes of pores and a large number of electron spins is highly desirable due to their potential applications as polarizers for dynamic nuclear polarization−nuclear magnetic resonance and as catalysts and magnetic separation media. Here, we report a polychlorotriphenylmethyl radical-linked covalent triazine framework (PTMR-CTF). Two different sizes of micropores were established by N2 sorption and the presence of unpaired electrons (carbon radicals) by electron spin resonance and superconducting quantum interference device−vibrating sample magnetometer analyses. Magnetization measurements demonstrate that this material exhibits spin-half paramagnetism with a spin concentration of ∼2.63 × 1023 spins/mol. We also determined the microscopic origin of the magnetic moments in PTMR-CTF by investigating its spin density and electronic structure using density functional theory calculations. KEYWORDS: different pore size, polychlorotriphenylmethyl radical, electron spins, covalent triazine framework, spin-half paramagnetism, spin concentration

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that have complex pore structures and multiple spins remains challenging and has been little explored.19,20 As an important member of PONs, covalent triazine frameworks (CTFs) typically have high porosity, good thermal and chemical stability, and a high nitrogen content, which enable their use in fields such as gas storage, catalysis, and energy storage.21−26 Recently, Baek et al. reported a πconjugated tetracyanoquinodimethane-based covalent triazine framework (TCNQ-CTF) and showed that it trapped spins formed by the aromatization of cyclohexadiene rings within the network resulting in a plastic magnet.27 However, as the authors mentioned, the concentration of carbon radicals in the TCNQ-CTF was relatively low due to the absence of steric hindrance to protect them, and the material had a low porosity with a Brunauer−Emmett−Teller (BET) surface area of ∼36 m2/g. Moreover, the trapped radicals in the material were randomly located because of its amorphous nature. In this sense, developing a porous and stable pure organic radical-

ightweight porous organic networks (PONs) that have a high surface area and tunable functionalization have shown potential applications in gas adsorption, energy storage, and catalysis.1−11 Producing PONs with different sizes of pores would increase the structural complexity and diversity of the PON family and might lead to interesting properties and applications, so the production of PONs with two or more distinct sizes of pores has recently attracted considerable attention.12,13 Because of the large number of available organic building blocks, PONs have many different structures and properties, which would make it possible to design a material for a specific purpose. Recently, PONs with multiple electron spins in their robust frameworks have emerged because of their potential use as a polarizing agent in dynamic nuclear polarization−nuclear magnetic resonance (DNP−NMR) and as plastic magnets and magnetic separation media.14−17 Wang et al. reported the construction of a covalent organic framework (COF) with embedded organic radicals and demonstrated its performance as a standard polarizing agent in a DNP−NMR experiment, due to its homogeneous distribution and fixed orientation of unpaired electrons.18 However, the production of pure organic radical-linked PONs © XXXX American Chemical Society

Received: December 20, 2018 Accepted: April 23, 2019

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DOI: 10.1021/acsnano.8b09634 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Synthesis of PTM-CTF and PTMR-CTF: (a) Cs2CO3/DMSO, 120 °C, 1 day, and 130 °C, 3 days; (b) n-Bu4NOH/THF, room temperature, 2 days, then p-chloranil, room temperature, 12 h. Both PTM-CTF and PTMR-CTF have two distinct sizes of pores. The circles filled with different colors represent the presence of two sizes of pores.

of bulk plastic magnets. Here, we report the production of a PTM radical-linked CTF (PTMR-CTF) structure by the polycondensation of trialdehyde 1 and diamidine salt 2 in relatively gentle conditions (120 °C, no strong acid), followed by deprotonation to generate PTM anions and further oxidation to produce PTM radicals within the framework at room temperature (Figure 1). Characterization shows that PTMR-CTF is a pure organic network material with two different sizes of micropores and proves the formation of carbon radicals. Preliminary magnetic measurements suggest that the organic radicals in the network produce a paramagnetic carbon structure with a spin concentration of ∼2.63 × 1023 spins/mol for each molecular unit. The saturation magnetic moment of PTMR-CTF was estimated to be ∼1.82 emu/g, ∼23 times that of TCNQ-CTF.27 We also revealed the microscopic origin of the magnetic moments in PTMR-CTF

linked CTF with the radicals protected by large steric hindrance is needed. Polychlorotriphenylmethyl (PTM) radicals with a propeller-like conformation are well-known for their high stability because of the existence of three bulk polychlorophenyl groups.20,28 Their D3 symmetry and high stability make PTM radicals ideal building blocks to produce highly stable and porous organic radical-linked CTFs. Wu et al. reported a PTM radical-linked graphdiyne-type framework produced by the Glaser−Hay coupling reaction and mainly demonstrated its application in the oxygen reduction reaction.20 However, magnetic PTM radical-linked framework materials constructed by the other reactions remain highly desirable due to their potential application as a future DNP− NMR polarizing agent in NMR measurements, and the microscopic origin of their magnetic moments deserves careful investigation to give a hint for the future design and realization B

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Figure 2. Structural characterization of PTM-CTF and PTMR-CTF. (a) FT-IR spectra of trialdehyde 1, diamidine salt 2, PTM-CTF, and PTMR-CTF. (b) Solid-state 13C NMR spectra of bulk PTM-CTF and PTMR-CTF. Asterisks denote spinning side bands. (c) UV−vis spectra of PTM-CTF and PTMR-CTF. (d) Survey XPS spectrum of PTMR-CTF.

between PTM-CTF and PTMR-CTF; that of PTMR-CTF exhibits an intense absorption peak at ∼595 nm, whereas PTM-CTF does not. This is due to the formation of PTM radicals and thus a π-conjugated structure. X-ray photoelectron spectroscopy (XPS) was performed to investigate the surface chemical composition of PTMR-CTF. The core levels of C 1s, N 1s, Cl 2s, Cl 2p, and O 1s were observed in the survey XPS spectrum (Figure 2d). We attribute the O 1s peak to moisture trapped in the pores of PTMR-CTF. For the high-resolution C 1s XPS spectrum (Figure 3a), the peak with a binding energy of 284.6 eV is assigned to sp2 carbons (C−C•− and −CC−), whereas the peak at 286.2 eV is attributed to the carbons bonded with N atoms in s-triazine rings (−CN−) or chlorine atoms ( C−Cl). The peak for the nitrogen atoms bonded to carbons (−NC−) in s-triazine rings was also observed at 398.8 eV (Figure 3b). These results are very similar to the XPS results of reported CTF structures and thus indicate the formation of CTF.23,26,29 The high-resolution Cl 2p XPS spectrum shows two peaks (Figure S5), with binding energies of ∼200.5 and ∼202.1 eV, which correspond to the Cl 2p3/2 and 2p1/2 levels, respectively. This result suggests the existence of PTM building blocks. N2 sorption measurements at 77 K were conducted to explore the porosity of PTM-CTF and PTMR-CTF powders (Figure 3c,d). The sharp uptake of N2 at low relative pressures (P/P0 < 0.1) for both PTM-CTF and PTMR-CTF indicates a microporous structure. The BET surface areas for PTM-CTF

by investigating its spin density and electronic structure using density functional theory (DFT) calculations. These results provide a hint for the future design and realization of bulk plastic magnets, and this material might be used as a future DNP−NMR polarizing agent in NMR measurements.

RESULTS AND DISCUSSION As shown in Figure 1, the polycondensation of trialdehyde 1 and diamidine salt 2 in the presence of Cs2CO3 at 120 °C yields the nonconjugated polymer PTM-CTF by a previously reported method.29 The PTM anion polymer generated by the deprotonation of PTM-CTF can be converted to PTMR-CTF by oxidation with p-chloranil at room temperature. The formation of PTM-CTF and PTMR-CTF was confirmed by Fourier transform infrared (FT-IR) spectroscopy, solid-state 13 C nuclear magnetic resonance (NMR), and ultraviolet− visible (UV−vis) spectroscopy. As shown in Figure 2a (also see Figure S4 for the FT-IR spectral range of 3800 to 400 cm−1), the disappearance of the otherwise intense aldehyde band at ∼1708 cm−1 is indicative of the polycondensation, whereas the appearance of two distinct additional peaks at ∼1517 and 1355 cm−1 suggests the formation of s-triazines. These peaks are consistent with the reported CTF structures.23,26,29 The solidstate 13C NMR peaks of PTM-CTF and PTMR-CTF (Figure 2b) can all be assigned to the carbon signals of the proposed structures. The characteristic signal of s-triazine carbons was observed at 172 ppm, indicating the formation of s-triazine rings.26 UV−vis spectra (Figure 2c) show a distinct difference C

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Figure 3. High-resolution C 1s and N 1s XPS, N2 sorption analysis, and PXRD patterns of PTM-CTF and PTMR-CTF. (a,b) High-resolution C 1s and N 1s XPS spectra of PTMR-CTF. (c) N2 adsorption and desorption isotherms of PTM-CTF and PTMR-CTF. (d) Pore size distributions of PTM-CTF and PTMR-CTF. (e) Experimental and simulated PXRD profiles of PTM-CTF and PTMR-CTF. The unit cell parameters for PTM-CTF are a = 34.76 Å, b = 34.76 Å, c = 7.70 Å, α = 90°, β = 90°, and γ = 120°, and those of PTMR-CTF are a = 34.72 Å, b = 34.72 Å, c = 7.70 Å, α = 90°, β = 90°, and γ = 120°. (f) Space-filling model of the PTMR-CTF structure.

and PTMR-CTF were determined to be 448 and 209 m2 g−1, whereas their total pore volumes were calculated to be 0.27 and 0.15 cm3 g−1, respectively. The pore size distribution of PTM-CTF calculated by a nonlocal DFT model shows two major peaks centered at ∼1.1 and 1.3 nm, whereas that of PTMR-CTF has two major peaks at ∼1.1 and 1.2 nm, suggesting the formations of two different sizes of micropores.

The crystallinity of PTM-CTF and PTMR-CTF was studied by powder X-ray diffraction (PXRD). The PXRD patterns of both PTM-CTF and PTMR-CTF show a relatively broad peak at 5.9°, which corresponds to the (200) planes (Figure 3e,f). The poor crystallinity observed for PTM-CTF and PTMR-CTF has also been reported for other CTF structures.23,29,30 D

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Figure 4. Magnetic analysis of PTMR-CTF powder. (a) Electron spin resonance spectrum (X-band) of a solid sample of PTMR-CTF at 295 K. (b) Field-dependent magnetization of the PTM-CTF and PTMR-CTF measured at 2 K. (c) Temperature dependence of magnetization obtained with an applied field of B = 500 Oe. (d) Plot of χ−1 versus T. (e) Field-dependent magnetization of PTMR-CTF measured with decreasing temperature from 300 to 2 K. (f) Magnetization as a function of μBB/kBT at various temperatures. The green line represents a fit to the Brillouin function with J = 1/2 assuming g = 2.

As shown in Figure 4a, an intense electron spin resonance signal was observed for PTMR-CTF with a g value of 2.002, indicating the presence of carbon radicals.20,28 To further confirm the presence of the stable radicals in PTMR-CTF, we also analyzed PTM-CTF and PTMR-CTF powders using a superconducting quantum interference device−vibrating sample magnetometer (SQUID−VSM). Figure 4b shows the measured magnetization (M) versus applied field (B) curves for both powder samples at 2 K. These results show the substantial increase of the magnetization in PTMR-CTF compared to that

of PTM-CTF, indicating that the stable spins of PTMR-CTF are due to the PTM radicals. To further investigate the magnetic behavior in PTMR-CTF, we measured its temperature-dependent magnetization (Figure 4c). The magnetization steeply increased with decreasing temperature, which is a typical behavior of paramagnetic materials. The χ−1 versus T curve shown in Figure 4d is a straight line that follows Curie’s law. Figure 4e shows the magnetization (M) as a function of the applied magnetic field (B) measured at various temperatures. The magnetization at 2 K shows a substantial E

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Figure 5. Calculated spin density of PTMR-CTF and magnified views of the PTMR building block. The isosurface of the spin density is represented with an isovalue of 0.009 electrons/Å3. The yellow and green isosurfaces indicate the spin density for α-spin and β-spin electrons, respectively.

increase and saturation behavior at high magnetic field, also revealing a typical paramagnetic feature. The field-dependent magnetization measured at T = 2, 3, 5, and 10 K are shown in Figure 4f. We note that the linear diamagnetic background and ferromagnetic impurity signals have been subtracted from all data (Figure S6). Using the scaling parameter μBB/kBT, all magnetization curves measured at different temperatures were excellently fitted to a single Brillouin function as follows: ÄÅ É ij 2J + 1 yz ij z yzÑÑÑÑ ÅÅÅ 2J + 1 1 cothjjj M = NgJμB ÅÅÅ ·z zzz − cothjjj zzzÑÑÑ ÅÅ 2J k 2J { 2J k 2J {ÑÑÑÖ ÅÇ

which describes the unpaired electron distribution in PTMRCTF. Most of the unpaired electrons in the PTMR-CTF are localized on the PTMR building blocks, whereas the spin population on the CTF building blocks is negligible. In particular, the unpaired electron in the PTMR building blocks mainly resides on the central methyl carbon atom (αC) (0.67 μB) and is partially delocalized in the ortho- and parapositioned carbon atoms (0.07 μB) of the three chlorinated phenyl rings bonded to the αC radical center due to the πconjugated nature. We note that the spin density of each atom has the same two-lobed shape as p orbitals. The calculated spin distribution of the PTMR building blocks in PTMR-CTF is almost the same as that of the molecular PTMR derivatives (Figure S10). The magnetic state of each molecule is ∼0.2 eV more stable than its nonmagnetic counterpart, and each molecule in the magnetic state has an unpaired electron (1.0 μB), which is mainly localized on αC (∼0.67 μB) and partially delocalized over the phenyl rings bonded to the αC. This class of radical is known to be highly stable and long-lived due to the spin delocalization through πconjugation as well as the protection of the αC radical center by the large steric hindrance of the three chlorinated phenyl rings.28 In addition, the degrees of spin delocalization and spin localization on αC are closely associated with the structural nonplanarity, where the three phenyl rings are twisted with respect to the central sp2 carbon atom plane.31 Observing the sustainability of the PTMR’s spin distribution and the structure of the PTMR-CTF, we believe that the magnetic moments in PTMR-CTF are caused by the unpaired electrons of the PTMR. The spin magnetic moments in PTMR-CTF are confirmed by its calculated electronic structure. The calculated band structure and projected density of states of PTMR-CTF show spin-polarized singly occupied/unoccupied molecular orbitals (SOMOs/SUMOs) with a SOMO−SUMO energy gap of 0.83 eV, regardless of the magnetic ordering (Figure S11). The SOMO and SUMO band edges are relatively flat, indicative of highly localized electronic states. From the observed charge densities of the SOMOs and SUMOs, it is easy to show that these orbitals are primarily localized at the PTMR, which suggests that the unpaired electrons are highly localized within

where z = gμBJB/kBT, g is the g-factor, J is the total angular momentum number, kB is the Boltzmann constant, and N is the number of spins. At all temperatures, the Brillouin function provides good fits for J = 1/2 (Figure S7). To determine the saturation magnetization of PTMR-CTF, we plotted the M versus B curve using the parameters obtained from the Brillouin function fit (Figure S8) and obtained a value of ∼1.82 emu/g, which is ∼23 times that (7.98 × 10−2 emu/g) reported for TCNQ-CTF.27 This result means that the paramagnetism of PTMR-CTF is greatly increased due to its higher spin concentration compared to that of TCNQ-CTF. The calculated molar magnetization of PTMR-CTF was 2435 emu/mol of the repeating unit of PTMR-CTF (C64H24Cl12N9, molar mass = 1338 g/mol). Given that the corresponding molar magnetization of S = 1/2 spin, in principle, is μB × 6.023 × 1023 = 5586 emu/mol, we concluded that ∼43.6% of the PTMs in PTM-CTF were converted to carbon radicals, and the spin concentration was calculated to be ∼2.63 × 1023 spins/mol. To test the stability of PTM radicals in PTMRCTF, we measured the temperature-dependent magnetization of the PTMR-CTF sample after it had been stored under a nitrogen atmosphere for 1 and 3 months, whereby only a slight degradation was observed (Figure S9), suggesting the relative high stability of PTM radicals in PTMR-CTF. In order to understand the microscopic origin of the magnetic moments in PTMR-CTF and give a hint for the future design and realization of plastic magnets, we determined its spin density and electronic structure using DFT calculations (see computational details in the Supporting Information). Figure 5 shows the isosurface of the spin density (i.e., the charge density difference between α-spin and β-spin electrons), F

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μB/mol. Theoretical calculations also determined the microscopic origin of the magnetic moments in PTMR-CTF by investigating its spin density and electronic structure. We expect that this work will push forward possible applications of carbon radical-linked porous polymeric framework materials in DNP−NMR, catalysis, and perhaps others.

the PTMR building blocks rather than fully delocalized over the in-plane π-conjugated PTMR-CTF. To further elucidate the magnetic ordering of the PTMRCTF, we examined two different spin configurations: ferromagnetic (FM) and antiferromagnetic (AFM) ordering. As shown in Figure S12, the energy difference between the FM and AFM spin ordering was negligible (i.e., EFM‑AFM is ∼0 meV). This indicates that the magnetic interaction between the nearest localized spins, 19.8 Å apart, is relatively weak and results in the paramagnetism of PTMR-CTF. Considering the relatively poor crystallinity of PTMR-CTF, the hexagonal skeleton, as shown in Figure 6, is likely closer to

EXPERIMENTAL SECTION Methods. A Cary series UV−vis spectrophotometer (Agilent Technologies) was used to record the UV−vis spectra of PTM-CTF and PTMR-CTF powders. XPS data were collected using an ESCALAB 250Xi XPS. Thermogravimetric analysis was conducted under N2 by heating to 900 °C at a rate of 5 °C min−1 on a TA Instrument Q 500 analyzer. Nitrogen sorption analysis was carried out using a surface area and porosity analyzer (Micromeritics ASAP2020). The samples were degassed under vacuum at 100 °C for 6 h before sorption measurements. XRD was performed on a Rigaku SmartLab powder X-ray diffractometer. Magnetization analysis was performed on a SQUID−VSM. Solid-state NMR was conducted on a Bruker AVANCE III spectrometer (500 MHz). The EPR spectrum of a solid sample was obtained on a Bruker EMXplus 9.5/12 spectrometer at 295 K under nonsaturating conditions. Measurement parameters were as follows: X-band microwave frequency, 9.39 GHz; modulation frequency, 100 kHz; modulation amplitude, 1 G; microwave power, 0.633 mW. Synthesis of PTM-CTF. Trialdehyde 1 (194.0 mg, 0.2 mmol), terephthalamidine dihydrochloride 2 (140.4 mg, 0.6 mmol), and cesium carbonate (430.0 mg, 1.3 mmol) were added to a solution of DMSO (30.0 mL) in a 50 mL glass pressure vessel. The mixture was heated at 120 °C for 24 h and then at 130 °C for 72 h. After being cooled to room temperature, the mixture was poured into an aqueous HCl solution (1 M, 200 mL), and the resulting precipitate was collected by filtration, washed with water (3 × 10 mL), acetone (3 × 10 mL), and THF (3 × 10 mL), and dried at 80 °C under vacuum for 24 h to yield PTM-CTF as a red power (219.2 mg, 82% yield). Synthesis of PTMR-CTF. A 50 mL flask was filled with PTM-CTF (219.0 mg), tetra-n-butylammonium hydroxide 30-hydrate (320.0 mg, 0.4 mmol), and THF (20 mL) under a nitrogen atmosphere in the dark. After being stirred at room temperature for 2 days, p-chloranil (197.1 mg, 0.8 mmol) was added, and the mixture continued to be stirred for 12 h. The precipitate was collected by filtration and washed with excess THF and DCM, and after being dried in a vacuum at room temperature for 24 h, a dark red powder was obtained (196.6 mg, 90%).

Figure 6. Calculated spin densities of one hexagonal pore of PTMR-CTF with ferromagnetic (left) and antiferromagnetic (right) orderings. The isosurface of the spin density is represented with an isovalue of 0.009 electrons/Å3. The yellow and green isosurfaces indicate the spin densities for α-spin and β-spin electrons, respectively. Carbon, chlorine, hydrogen, and nitrogen atoms are light gray, red, white, and blue, respectively.

the actual structure of the synthesized PTMR-CTF instead of an extended 2D form. We thus did DFT calculations for one hexagonal pore in an isolated manner (Figure 6). The calculated magnetic property of the isolated hexagonal pore was the same as that of the 2D extended form. Most of the unpaired electrons (i.e., spins) in the hexagonal pore are localized on the PTMR building blocks, whereas the spin population on the CTF building blocks is negligible. The unpaired electron mainly resides on the αC of the polychlorotriphenylmethyl radical and is partially delocalized in the ortho- and para-carbon atoms of the three chlorinated phenyl rings bonded to the αC of the polychlorotriphenylmethyl radical. The calculated energy difference between the FM and AFM spin ordering was negligible (i.e., EFM‑AFM is ∼ 0 meV), indicating that the magnetic interaction between the nearest localized spins is weak in the isolated hexagonal pore (i.e., resulting in paramagnetism).

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b09634. Computational details, synthesis of trialdehyde 1, additional XPS spectra, calculation results, ICP-mass result (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: rsruoff@ibs.re.kr or ruoffl[email protected].

CONCLUSION We produced a pure organic PTMR-CTF with two different sizes of micropores. The radicals within this framework were protected by the bulky polychlorophenyl groups and showed high stability. SQUID measurements showed that PTMR-CTF is paramagnetic due to the relatively long distance between neighboring carbon radicals. The radicals in PTMR-CTF were characterized as spins of S = 1/2 moment, and the saturation magnetic moment of PTMR-CTF was estimated to be ∼0.436

ORCID

Yi Jiang: 0000-0003-1080-5884 Se Hun Joo: 0000-0003-4507-150X Onur Buyukcakir: 0000-0003-4626-8232 Ming Huang: 0000-0002-9188-4619 Jan-Uwe Rohde: 0000-0002-0691-6699 Sang Kyu Kwak: 0000-0002-0332-1534 G

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Jung-Woo Yoo: 0000-0001-7038-4001 Author Contributions ⊥

Y.J., I.O., and S.H.J. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by IBS-R019-D1 and the National Research Foundation of Korea (NRF) grants funded by the Korea government (2017M3A7B4049172 and 2017R1A2B4008286). Computational resources were used from CMCM and UNIST-HPC. J.-U.R. acknowledges support from the UNIST Research Fund (1.130085.01). REFERENCES (1) Cooper, A. I. Conjugated Microporous Polymers. Adv. Mater. 2009, 21, 1291−1295. (2) Diercks, C. S.; Yaghi, O. M. The Atom, the Molecule, and the Covalent Organic Framework. Science 2017, 355, No. eaal1585. (3) Xu, Y.; Jin, S.; Xu, H.; Nagai, A.; Jiang, D. Conjugated Microporous Polymers: Design, Synthesis and Application. Chem. Soc. Rev. 2013, 42, 8012−8031. (4) Bae, S. Y.; Kim, D.; Shin, D.; Mahmood, J.; Jeon, I. Y.; Jung, S. M.; Shin, S. H.; Kim, S. J.; Park, N.; Lah, M. S.; Baek, J. B. Forming a Three-Dimensional Porous Organic Network via Solid-State Explosion of Organic Single Crystals. Nat. Commun. 2017, 8, 1599. (5) Beyeh, N. K.; Nonappa; Liljestrom, V.; Mikkila, J.; Korpi, A.; Bochicchio, D.; Pavan, G. M.; Ikkala, O.; Ras, R. H. A.; Kostiainen, M. A. Crystalline Cyclophane-Protein Cage Frameworks. ACS Nano 2018, 12, 8029−8036. (6) Byun, J.; Patel, H. A.; Thirion, D.; Yavuz, C. T. Charge-Specific Size-Dependent Separation of Water-Soluble Organic Molecules by Fluorinated Nanoporous Networks. Nat. Commun. 2016, 7, 13377. (7) Guo, J.; Xu, Y.; Jin, S.; Chen, L.; Kaji, T.; Honsho, Y.; Addicoat, M. A.; Kim, J.; Saeki, A.; Ihee, H.; Seki, S.; Irle, S.; Hiramoto, M.; Gao, J.; Jiang, D. Conjugated Organic Framework with ThreeDimensionally Ordered Stable Structure and Delocalized π Clouds. Nat. Commun. 2013, 4, 2736. (8) Lee, J.; Buyukcakir, O.; Kwon, T. W.; Coskun, A. Energy BandGap Engineering of Conjugated Microporous Polymers via AcidityDependent in Situ Cyclization. J. Am. Chem. Soc. 2018, 140, 10937− 10940. (9) Mahmood, J.; Lee, E. K.; Jung, M.; Shin, D.; Jeon, I. Y.; Jung, S. M.; Choi, H. J.; Seo, J. M.; Bae, S. Y.; Sohn, S. D.; Park, N.; Oh, J. H.; Shin, H. J.; Baek, J. B. Nitrogenated Holey Two-Dimensional Structures. Nat. Commun. 2015, 6, 6486. (10) Servalli, M.; Celebi, K.; Payamyar, P.; Zheng, L.; Polozij, M.; Lowe, B.; Kuc, A.; Schwarz, T.; Thorwarth, K.; Borgschulte, A.; Heine, T.; Zenobi, R.; Schluter, A. D. Photochemical Creation of Covalent Organic 2D Monolayer Objects in Defined Shapes via A Lithographic 2D Polymerization. ACS Nano 2018, 12, 11294−11306. (11) Biswal, B. P.; Becker, D.; Chandrasekhar, N.; Seenath, J. S.; Paasch, S.; Machill, S.; Hennersdorf, F.; Brunner, E.; Weigand, J. J.; Berger, R.; Feng, X. Exploration of Thiazolo[5,4-d]thiazole Linkages in Conjugated Porous Organic Polymers for Chemoselective Molecular Sieving. Chem. - Eur. J. 2018, 24, 10868−10875. (12) Pang, Z. F.; Xu, S. Q.; Zhou, T. Y.; Liang, R. R.; Zhan, T. G.; Zhao, X. Construction of Covalent Organic Frameworks Bearing Three Different Kinds of Pores through the Heterostructural Mixed Linker Strategy. J. Am. Chem. Soc. 2016, 138, 4710−4713. (13) Zhou, T. Y.; Xu, S. Q.; Wen, Q.; Pang, Z. F.; Zhao, X. One-Step Construction of Two Different Kinds of Pores in a 2D Covalent Organic Framework. J. Am. Chem. Soc. 2014, 136, 15885−15888. (14) Liu, G.; Levien, M.; Karschin, N.; Parigi, G.; Luchinat, C.; Bennati, M. One-Thousand-Fold Enhancement of High Field Liquid Nuclear Magnetic Resonance Signals at Room Temperature. Nat. Chem. 2017, 9, 676−680. H

DOI: 10.1021/acsnano.8b09634 ACS Nano XXXX, XXX, XXX−XXX