Prediction of Superconductivity in Porous, Covalent Triazine

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Prediction of Superconductivity in Porous, Covalent Triazine Frameworks Maarten G. Goesten, and Maximilian Amsler ACS Materials Lett., Just Accepted Manuscript • DOI: 10.1021/acsmaterialslett.9b00013 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 27, 2019

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ACS Materials Letters

Prediction of Superconductivity in Porous, Covalent Triazine Frameworks Maarten G. Goesten1* and Maximilian Amsler2*

1: Department of Chemistry and Chemical Biology, Cornell University, 259 East Avenue, Ithaca, NY, USA 2: Laboratory of Atomic and Solid-State Physics, Cornell University, 142 Sciences Drive, Ithaca, NY, USA

ABSTRACT: Conventional superconductivity is a phenomenon where lattice vibrations, phonons, steer electrons through a solid with zero resistance. We theoretically predict such phonon-mediated superconductivity to occur in a Covalent Triazine Framework, CTF-0, when it is intercalated with Li or Na. Porous CTF-0 is computed to possess several anticipated properties of interest, such as low-lying *-bands that can be partially doped, as well as electrostatic anchoring points that improve the stability of the functionalized structures. However, it is the integral porosity that plays a surprisingly crucial role in driving superconductivity, by providing the space for donor atoms to engage in highly localized vibrational modes. These low-frequency rattling phonons couple strongly to the free *-electrons, and they work in concert with in-plane aromatic-ring vibrations of the framework at a higher frequency. We compute a markedly improved critical temperature (Tc) for Li-decorated CTF-0 (6.2 K) over graphite-intercalation compound LiC6 (0.9 K), an improvement that relates directly to the porosity of the material. We also predict a further enhancement of the Tc upon substitution with larger Na (9.1 K), which engages in more localized rattling phonons. This work proposes that CTFs and kindred porous frameworks can be a hub for an exciting new class of materials, for which tunable porosity gives control over superconducting properties.

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INTRODUCTION Propelled by recent breakthroughs, the field of conventional superconductivity is experiencing a surge of recent interest.1-8 While record-breaking critical temperatures (Tc’s) have been realized at extremely high pressures, research on the phenomenon at atmospheric conditions often pursues the functionalization of semiconductors and semimetals by a donor atom.4-8 Some of the most elegant work exploits the use of pore space.9-11 Cradled by the pore, the donor engages in highly localized vibrations of low frequency, sometimes referred to as rattling modes.12,13 Such modes have shown ability to engage in strong electron-phonon coupling, which is the driving force in conventional superconductivity.14,15 The vast body of recent literature on tunable porosity in organic and organic-inorganic networks provides new options here; we are referring to the broadly defined field of Metal- and Covalent Organic Frameworks (MOFs / COFs).16-25 Constructed by adaptable building blocks, the porosity and structural properties can be tailored in these materials, offering a degree of design. In this work, we present a strategy to engineer superconductivity through an effective use of the pore space in a Covalent Triazine Framework, CTF-0. Covalent Triazine Frameworks (CTFs) represent a subclass26,27 of porous frameworks that have recently gained much attention for their ease in tailoring pore shape, and their possibilities toward post-synthetic functionalization.28-38 We will analyze the effects of decorating CTF-0 with Li and Na, the two lightest alkali metals, and describe the performance of the functionalized frameworks as superconductors.

Figure 1. a) The synthesis and structure of CTF-039-42 — the drawing shows a single sheet of CTF-0 and the hydrogens are omitted. The dashed rectangle denotes the 2D unit cell b) Representation of the four rings of a) within a hexagonal 2D lattice c) Visualization of the stacking along a3 as computed from a crystal geometry optimization.


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RESULTS AND DISCUSSION CTF-0 We start with a description of CTF-0, the material we are looking to functionalize. CTF-0 has a highly stable, layered structure that polymerizes from a single monomer (1,3,5-tricyanobenzene) via a triazination reaction (Figure 1a).43 One could stress the similarity with graphite (commonly doped to render superconductors) by looking at a single sheet of both materials: the 2D hexagonal structure of graphene is well known, and we can think about at a single sheet of CTF-0 as consisting of hexagons too, in this case ‘super hexagons’ that are built by three C6H3 and three C3N3 units (Figure 1b). But let us mainly underline the structural differences to graphite, which are the following: i) CTF-0 contains ‘pyridinic’ nitrogen sites that can be used to anchor metal ions and complexes by strong chemical bonds,44 ii) the super hexagons in CTF-0 provide porosity within the sheets and thus, additional space (the closest H-H distance between C6H3 rings is just under 3Å) iii) CTF-0 sheets do not stack with the same regularity along the (vertical) a3 axis, as we will see in the next section. The Structure and Bands of CTF-0 The result of an interplay between dispersion and steric factors, stacking arrangements for porous aromatic frameworks come in a rich variety.45 For CTF-0, there is a caveat, as X-Ray Diffraction data suggests there to be a degree of stacking disorder.43 The (fully crystalline) structure that we will find through optimization should therefore be considered as a representative model. But regarding CTF-0’s well-defined porosity and local chemical environment (as characterized by physisorption and solid-state NMR, respectively),43 we do expect our representation to be sound in depicting the electronic and vibrational properties of the material. The detailed description of our calculations can be found in the Computational Methods section at the end of this work. The crystal geometries of CTF-0 were probed by starting the optimization from various stacking dispositions. An inclined structure in the P1 space group was found to be energetically preferred (Figure 1c), where the stacking offset is maximized and the distance between the sheets is 3.2 Å; 0.2 Å down from that found in graphite. CTF-0 is a semiconductor for which we compute an indirect band gap of 2.6 eV at the PBE level and 3.5 eV at the HSE06 one (Supplementary Information (SI) Figure S1). We employ the


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PBE functional in the main text from here onwards. Figure 2 shows the calculated electron and phonon band structures of CTF-0 along a path in the Brillouin Zone. To the right, the corresponding density of states (DOS) plots are shown, with atom-resolved contributions. The band structures bear no surprises. Along the -Z segment, which corresponds to the direction of stacking, the energy of both the electrons and phonons is mostly flat. And the electronic bands run most steeply in the energy-domain of -bonding (< -2 eV).

Figure 2. Electronic (top) and phonon (bottom) band structures of CTF-0. The electronic  and * bands, and those of the nitrogen lone-pair (NLP) combinations are designated by arrows. The band electronic energies are referenced against the valence band maximum, which is set at zero. The electronic and phonon DOS are shown to the right, with the atom-resolved contributions (lines) to the total DOS (area plot).

For the electrons, two energy regions of main interest lie just below the valence band maximum, and some three eV above it. Close to that maximum, we find two almost perfectly flat bands, a sign of electron localization. They contain some contribution from the carbons, but are best characterized as nitrogen lone-pair (NLP) combinations. If orbital-controlled bonding is to


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play any role, these are the bands we expect to participate in binding the donor atom. Directly below, there are two bands which do contain some dispersion – their energy-variation with respect to the wave vectors spans about 1 eV. They are -combinations, mainly located on the C6 ring. Their * equivalents, around 3 eV, form the two lowest conduction bands. These are the bands we seek to fill partially by doping. The phonon bands contain a distinct region with hydrogen modes at ~ 95 THz. Next down in frequency, roughly between 25 and 50 THz, we find a broad spectrum of in-plane phonon vibrations involving carbon, nitrogen and hydrogen. Below that frequency window, the out-ofplane vibrations make up the bands, with those of the C3N3 ring at the bottom. The structure of CTF-0-Li and CTF-0-N In our theoretical set-up, we introduce two Li or Na atoms per unit cell; we insert two valence s electrons to fill the set of two * bands half-way. This also corresponds to one donor atom per one aromatic ring, which facilitates a comparison between Li-doped CTF-0 (CTF-0-Li) and the wellknown, graphite-based superconductor LiC6. We compute a large thermodynamic stabilization for both Li (-1.33 eV/Li) and Na (-1.00 eV/Na) upon their introduction in the framework, taking the optimized CTF-0 and bcc metal lattices as reference. In good agreement with earlier reported cohesion energies in CTF-1,46 these values are considerably lower than the corresponding graphite intercalation energies (-0.22 eV/Li for LiC6 and a positive value for NaC6),47,48 but higher than those for nonporous, layered carbon nitrides.49 A representation of the optimized crystal geometries for CTF-0-Li and CTF-0-Na is given in Figure 3. In CTF-0-Li, the direction of inclined stacking changes somewhat with respect to CTF-0, with a smaller offset along a2. The two Li ions assume different modes of attachment to the framework. One finds itself held by a single nitrogen from one sheet, whilst being centered on top of a C6 ring that belongs to the next sheet; this can be described as 6-coordination. The other Li bridges two nitrogens, between two sheets. The computation of instantaneous interaction energies between the Li’s and the framework suggests that the second mode of coordination holds the Li a little stronger (0.3 eV difference). Overall, the Li-N bond lengths are comparable to those found for the NLi6 octahedra in Li3N at ambient pressure,50 and the Li-C6 distance is essentially identical to that in LiC6.51


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Whilst Li is clearly an intercalant, residing between sheets in both modes of coordination, we can also observe an effect of the pore. It pulls the ion to its void upon Li-intercalation, and helps the sheets decrease their spacing between, to 3.1 Å. This effect marks another stark difference with graphite-based LiC6, for which the spacing gains over 0.3 Å during the same process.52

Figure 3. Graphical representation of CTF-0-Li (left) and CTF-0-Na (right), with close-ups of the two local coordination environments. For the purpose of clarity, only two donor atoms are drawn in the structural representation.

Space helps the stabilization of CTF-0-Na too. The intercalation of Na does lead to some increase in the stacking with respect to bare CTF-0, but only by 0.1 Å, to 3.3 Å. Meanwhile, the modus of stacking now becomes more symmetric, with a zero offset along a2 (i.e. the angle between a2 and a3 is 90o). Electronegativity, which scales inversely with ionic radius, appears to be a driving force here; the arrangement increases the number of nitrogens in the coordination sphere of the larger and more electropositive cation (2 and 4 for Na versus 1 and 2 for Li). One Na is held by two nitrogens from a single sheet, whilst engaging in 6-coordination to a C6 ring from the next one. The other Na is held by a 6-connection to a C3N3 ring from one side, and by a single nitrogen from the other. The instantaneous interaction between Na and the framework is slightly stronger for the first mode of coordination, by 0.15 eV. If taking the ionic radii as yardstick (a difference of 0.26 Å between Li+ and Na+), the Na-N bonds in CTF-0-Na are comparatively long when compared to Li-N in CTF-0-Li. This suggests that the vibrational normal modes for Na are softer, something we will shortly return to.


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The electronic bands of CTF-0-Li and CTF-0-Na Figure 4 shows the electronic band structures of the two functionalized frameworks. For both materials, the Fermi level EF is placed halfway the *-band region, and at the onset of a peak in the DOS, which implies an appreciable population of free electrons. Note how the Li/Na states contribute negligibly little to the occupied bands below (and at) EF for both materials; they get involved at much higher energies. This signals that the alkali atoms act as trademark electron donors, injecting their valence s electrons into the *-system without contributing themselves to the free electron states. Yet, they still play an important role in the superconducting mechanism, as we will see. We also remark how the NLP bands, now located just below -3 eV, have obtained some energy-dispersion, a result from them binding the donor. In line with the proportionally shorter NLi bonds, this is more pronounced for CTF-0-Li.

Figure 4. Electronic band structures of CTF-0-Li (left) and CTF-0-Na (right). The DOS is shown to the right, with the atom-resolved contributions (lines) to the total DOS (area plot). The Fermi energy EF is set to zero, and represented by a dashed line.

The electron-phonon coupling and superconductivity We apply the Allen-Dynes formalism for the computation of the critical temperature Tc (below which the material is in the superconducting state):53 𝜔𝑙𝑜𝑔

1.04(1 + 𝜆)

𝑇𝑐 = 1.20exp [ ― 𝜆 ― 𝜇 ∗ (1 + 0.62𝜆) ]

(1)


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Here, μ* represents a renormalized Coulomb repulsion, which is set to 0.10. The electron-phonon coupling constant λ is defined by integration over the full frequency domain of the Eliashberg electron-phonon spectral function a2F(ω), where ω denotes the phonon frequency. ∞𝑑𝜔 2 𝜔 𝑎 𝐹(𝜔)

(2)

𝜆 ≡ ∫0

The function a2F(ω) is effectively a phonon DOS, where the phonons are weighted by their strength of coupling to electrons. The ωlog in (1) is the logarithmic average frequency, defined as: 2

∞ln (𝜔)𝑑𝜔 2 𝑎 𝐹(𝜔) 𝜔

𝜔𝑙𝑜𝑔 ≡ exp [ 𝜆 ∫0

(3)

]

These equations predict that the coupling of both low- and high-frequency coupling phonons is important to establishing a high Tc.54 In (1), the Tc goes up with an increase in λ, for which the lowfrequency modes are weighted stronger, as ω is in the denominator in the integrand in (2). The ωlog term however, scaling linearly with the Tc, becomes larger in the presence of high-frequency coupling modes. With this physical framework in mind, we will make sense of the computed physical parameters in Table 1, through an analysis of the computed phonon spectra, the Eliashberg spectral function a2F(ω) and the cumulative integration trend of λ over the frequency domain ω: λ(ω). The relevant plots are shown in Figure 5. Table 1. The computed electron-phonon coupling constant, logarithmic average frequency and critical temperature for CTF-0-Li and CTF-0-Na.

λ CTF-0-Li

ωlog / K TC / K

0.48 591

6.2

CTF-0-Na 0.57 458

9.1

The trends of λ(ω) — the green curves to the right of the phonon DOS in Figure 5 — show two regions where λ(ω) increases steeply in ω for both materials, marked by arrows. These are


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frequency windows where phonons couple strongly to the *-electrons at EF. One starts at 0 THz, the second appears around 35 THz. The calculated Tc of CTF-0-Na is higher than that of CTF-0Li by some 3 K, and this difference is driven by a larger electron-phonon coupling λ. When comparing λ(ω) for both materials, one can see that the larger λ for CTF-0-Na stems mainly from a stronger coupling in the designated frequency window that starts from 0 THz (i.e. λ(ω) climbs faster in ω for CTF-0-Na at these frequencies). The atom-resolved contributions to the phonon DOS show that this region of phonons is controlled by vibrations of the Li/Na donor atoms.55 And an inspection of the eigenmodes suggests that these vibrations are best described as rattling modes with a large excursion. The presence of the donor atom also suppresses the out-of-plane vibrations of the C3N3 rings (SI), which, in line with the shorter Li-N equilibrium bond lengths, is more substantial for CTF-0-Li than it is for CTF-0-Na. Na’s softer displacement potential and bigger mass lead to rattling modes that are more localized for CTF-0-Na, and we observe a sharper Na-resolved DOS maximum. Ergo, the Na ion gives rise to rattling phonons of a lower average frequency, and this leads to an increase in λ.

Figure 5. Phonon band structures with, going from left-to-right; the phonon DOS with atom-resolved contributions; a2F

(blue) and λ(ω) (orange). The arrows indicate regions of strong electron-phonon coupling.

The second region of strong-coupling phonons around 35 THz is important as well. The phonons here consist of the in-plane vibrational modes from the aromatic rings present in the framework. These vibrations were already present for bare CTF-0, but they now couple to the free *-electrons in CTF-0-Li and CTF-0-Na. The significance of these phonons is reflected by the a2F(ω), which takes up its largest value in this frequency window. And the way it tees off is qualitatively similar for both functionalized materials, although there is a somewhat larger spectral density at lower frequencies


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for CTF-0-Na. With the a2F(ω) pulling its weight toward these higher frequencies, it increases ωlog and props up the Tc. In such, the contribution of both low and high-frequency phonons is important to the superconductivity. The donor atom does not only fill the *-bands partially to create metallicity, it helps in mediating superconductivity by creating rattling phonons that couple strongly to the *electrons. Furthermore, the framework itself provides strong-coupling modes of a higher frequency, and these support a high value for ωlog. Thus, when Li is substituted by Na, λ increases because the coupling rattling phonons are of a lower average frequency. This by itself would not necessarily present a viable strategy to increase the Tc, as it also invokes an unfavorable decrease in ωlog. The combination with the inplane modes at around 35 THz is vital. They keep a decrease of ωlog in check, and the net result is an increase in Tc of about 3 K. CONCLUSION We have theoretically described a new pair of superconductors, created through the donor intercalation of CTF-0 by Li or Na. The framework’s porosity, an attribute that has drawn much attention from the catalysis community, was shown to play a pivotal role in the electron-phonon coupling. It leads to rattling modes of donor atoms, which couple strongly to free electrons. This feature draws comparison to a broad range of clathrate compounds, for which high-pressure synthesis is applied to construct materials containing cages with rattling guest atoms (references 9-13 direct to a fraction of the existing literature on this subject). We computed large intercalation energies for both CTF-0-Li and CTF-0-Na, and foresee a wider variety of donor atoms and comparably porous frameworks to lead to stable superconductors. The premeditated selection of appropriate building blocks allows for vast control over pore size and shape in many organic and inorganic-organic materials. On that account, we believe that the exploitation of well-defined pore space in such materials may give rise to a new class of superconductors. We remark that the optimization of superconducting properties through a variation of the degree of doping may not be trivial from a priori considerations. For instance, we calculated the electron-phonon interactions for CTF-0 that is decorated by just one Li per unit cell. The trends for a2F(ω) and λ(ω) (see: SI) are very similar to those of our CTF-0-Li above


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(which contains two Li’s per unit cell), and even though it is a little lower, the predicted Tc of 5.4 K lies in the same regime. COMPUTATIONAL METHODS GGA-PBE Kohn-Sham Density Functional Theory was used throughout. The pw.x module of Quantum Espresso 6.0 (QE) was employed for variable-cell nuclear relaxation on a 4x4x8 electron-momentum mesh.56,57 We used the Rappe-Rabe-Kaxiras-Joannopoulos ultrasoft scalarrelativistic pseudopotentials,58 with a plane-wave cutoff energy of 100 Ry. The crystal geometries of CTF-0, CTF-0-Li and CTF-0-Na were probed by starting local optimizations from various stacking dispositions. Tight convergence thresholds (1E-6 Ry/bohr) were applied for the forces. The electronic band structure and DOS calculations on the optimized structures were performed with the ADF BAND package,59 at the ZORA-PBE/DZP level of theory. In these calculations, we have expanded all wave functions through an all-electron Slater-type orbital (STO) basis set, and the DZP basis set size was converged with respect to the band structure. Atomic orbital contributions to the DOS were obtained from the STO basis set (and not from projections). For semiconductor CTF-0, the band structure was also computed at the ZORAHSE06/DZP level. This plot is shown in the SI, next to the coordinates for the special points in reciprocal space. The electron-phonon interaction calculations were done with the ph.x module of QE, and converged with a 60 Ry cut-off for CTF-0-Li and a 40 Ry cut-off for CTF-0-Na. The dynamical matrices were computed on a 2x2x4 phonon-momentum mesh within density functional perturbation theory (DFPT). A 6x6x12 electron-momentum mesh was used for the DFPT phonon calculations. The electron-phonon interaction was computed on a 12x12x24 electron-momentum mesh. The phonons of pristine CTF-0 converged at 60 Ry. The force constants from the phonon calculations were computed with the q2r.x module of QE, and were fed into the Phonopy60 package to obtain the atom-resolved contributions and further post-processing. The computation of the Tc via the Allen-Dynes formula was done with the lambda.x module of QE, and we increased the integration mesh in the code (variable ‘nex’) to converge the value for ωlog.


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The elastic constants were computed with the ElaStic code,61 using carefully selected polynomial fits on the energies of 41 strained configurations along each of the 21 distortions to obtain the derivatives. ASSOCIATED CONTENT A Supporting Information file with the HSE06 band structure of CTF-0, the rattling phonon displacements for CTF-0-Li and CTF-0-Na, the superconducting parameters for CTF-0-Li with one Li in the unit cell, the elastic constants and the crystal coordinates of all structures. AUTHOR INFORMATION Corresponding author: *[email protected] *[email protected] ORCID Maarten G. Goesten: 0000-0003-1296-0255 Maximilian Amsler: 0000-0001-8350-2476 ACKNOWLEDGEMENTS MGG acknowledges the support from the Rubicon research program with project number 019.161BT.031, which is (partly) financed by the Netherlands Organisation for Scientific Research (NWO). MA acknowledges the support from the Novartis Universität Basel Excellence Scholarship for Life Sciences and the Swiss National Science Foundation (project No. P300P2-158407, P300P2-174475). The computational resources from the Cartesius Supercomputer, the Swiss National Supercomputing Center in Lugano (projects s700 and s861), the Extreme Science and Engineering Discovery Environment (XSEDE) (which is supported by National Science Foundation grant number OCI-1053575), the Bridges system at the Pittsburgh Supercomputing Center (PSC) (which is supported by NSF award number ACI-1445606), the Quest high performance computing facility at Northwestern University, and the National Energy Research Scientific Computing Center (DOE: DE-AC02-05CH11231), are all gratefully acknowledged.


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Synthetic Control of the Pore Dimension and Surface Area in Conjugated Microporous Polymer and Copolymer Networks. Jiang, J.-X.; Su, F.; Trewin, A.; Wood, C. D.; Niu, H.; Jones, J. T. A.; Khimyak, Y. Z.; Cooper, A. I. Journal of the American Chemical Society 2008, 130, 7710-7720. Molecular Organic Crystals: From Barely Porous to Really Porous. Cooper, A. I. Angew. Chem. Int. Ed. Engl. 2012, 51, 7892-7894. Constructing Monocrystalline Covalent Organic Networks by Polymerization. Beaudoin, D.; Maris, T.; Wuest, J. D. Nat Chem 2013, 5, 830-834. We are semantically loose on the nomenclature of porous organic frameworks. A more detailed classification can be found in reference 27. Nanoporous Polymers: Bridging the Gap between Molecular and Solid Catalysts? Rose, M. ChemCatChem 2014, 6, 1166-1182. Porous, Covalent Triazine-Based Frameworks Prepared by Ionothermal Synthesis. Kuhn, P.; Antonietti, M.; Thomas, A. Angewandte Chemie International Edition 2008, 47, 3450-3453. Solid Catalysts for the Selective Low-temperature Oxidation of Methane to Methanol. Palkovits, R.; Antonietti, M.; Kuhn, P.; Thomas, A.; Schüth, F. Angew. Chem. Int. Ed. Engl. 2009, 48, 6909-6912. Covalent Triazine Framework as Catalytic Support for Liquid Phase Reaction. Chan-Thaw, C. E.; Villa, A.; Katekomol, P.; Su, D.; Thomas, A.; Prati, L. Nano Lett. 2010, 10, 537-541 Triazine-Based Polymers as Nanostructured Supports for the Liquid-Phase Oxidation of Alcohols. Chan-Thaw, C. E.; Villa, A.; Prati, L.; Thomas, A. Chemistry: a European Journal 2011, 17, 10521057. Rational Extension of the Family of Layered, Covalent, Triazine-Based Frameworks with Regular Porosity. Bojdys, M. J.; Jeromenok, J.; Thomas, A.; Antonietti, M. Adv. Mater. Weinheim 2010, 22, 2202-2205. An Energy Storage Principle Using Bipolar Porous Polymeric Frameworks. Sakaushi, K.; Nickerl, G.; Wisser, F. M.; Nishio-Hamane, D.; Hosono, E.; Zhou, H.; Kaskel, S.; Eckert, J. Angew. Chem. Int. Ed. Engl. 2012, 51, 7850-7854. A Perfluorinated Covalent Triazine-Based Framework for Highly Selective and Water-tolerant CO2 Capture. Zhao, Y.; Yao, K. X.; Teng, B.; Zhang, T.; Han, Y. Energy & Environmental Science 2013, 6, 3684-3692. Covalent-Organic Frameworks: Potential Host Materials for Sulfur Impregnation in Lithium-Sulfur Batteries. Liao, H.; Ding, H.; Li, B.; Ai, X.; Wang, C. J. Mater. Chem. A 2014, 2, 8854-8588. Platinum-Modified Covalent Triazine Frameworks Hybridized with Carbon Nanoparticles as Methanol-Tolerant Oxygen Reduction Electrocatalysts. Kamiya, K.; Kamai, R.; Hashimoto, K.; Nakanishi, S. Nat Commun 2014, 5, 1-6. Efficient Production of Hydrogen from Formic Acid Using a Covalent Triazine Framework Supported Molecular Catalyst. Bavykina, A. V.; Goesten, M. G.; Kapteijn, F.; Makkee, M.; Gascon, J. ChemSusChem 2015, 8, 809-812. Carbon- and Nitrogen-Based Porous Solids: A Recently Emerging Class of Materials. Sakaushi, K.; Antonietti, M. Bulletin of the Chemical Society of Japan 2015, 88, 386-398. This is the original and to our knowledge most common synthesis protocol. Alternative methods have been developed, all with their own advantages – we are directing the reader to references 40-42. Porous, Fluorescent, Covalent Triazine-Based Frameworks via Room-Temperature and MicrowaveAssisted Synthesis. Ren, S.; Bojdys, M. J.; Dawson, R.; Laybourn, A.; Khimyak, Y. Z.; Adams, D. J.; Cooper, A. I. Adv. Mater. Weinheim 2012, 24, 2357-2361. Elemental-Sulfur-Mediated Facile Synthesis of a Covalent Triazine Framework for HighPerformance Lithium-Sulfur Batteries. Talapaneni, S. N.; Hwang, T. H.; Je, S. H.; Buyukcakir, O.; Choi, J. W.; Coskun, A. Angewandte Chemie 2016, 128, 3158-3163. Covalent Triazine Frameworks via a Low-Temperature Polycondensation Approach. Wang, K.; Yang, L.-M.; Wang, X.; Guo, L.; Cheng, G.; Zhang, C.; Jin, S.; Tan, B.; Cooper, A. Angew. Chem. Int. Ed. Engl. 2017, 56, 14149-14153. Covalent Triazine Frameworks Prepared from 1,3,5-Tricyanobenzene. Katekomol, P.; Roeser, J.; Bojdys, M.; Weber, J.; Thomas, A. Chem. Mater. 2013, 25, 1542-1548. “Strong” is never objective when applied to chemical bonding. It this case, we consider the bonding strong in comparison with the eta-6 coordination mode which is found in GICs. The Structure of Layered Covalent-Organic Frameworks. Lukose, B.; Kuc, A.; Heine, T. Chemistry: A European Journal. 2011, 17, 2388-2392.


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Prediction of Superconductivity in Porous, Covalent Triazine

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