Nanoscrolls Formed from Two-Dimensional Covalent Organic

Subscriber access provided by IDAHO STATE UNIV

Article

Nanoscrolls Formed from Two-Dimensional Covalent Organic Frameworks Haoyuan Li, and Jean-Luc Brédas Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00186 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Nanoscrolls Formed from Two-Dimensional Covalent Organic Frameworks

Haoyuan Li and Jean-Luc Brédas*

School of Chemistry and Biochemistry Center for Organic Photonics and Electronics (COPE) Georgia Institute of Technology Atlanta, Georgia 30332-0400

E-mail: [email protected]

ACS Paragon Plus Environment

1

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT:

Page 2 of 31

Two-dimensional (2D) covalent organic frameworks (COFs) represent an emerging

class of nanomaterials with building blocks precisely connected in-plane through covalent bonds. Gaining insights into their structure and stabilities is critical to both their preparation and applications. Here, via atomistic molecular mechanics simulations and free-energy calculations, we investigate 2D COFs both under vacuum conditions and in solution, taking representative boronate ester-based and imine-based COFs as examples. Rather than remaining flat, single-layer 2D COF sheets whose at least the length is larger than ~15-~20 nm are found to preferably form nanoscrolls. These nanoscrolls display a finite number of configurations and represent open structures due to the large pores present in the 2D sheets; this feature distinguishes them from nanoscrolls formed by dense 2D materials such as graphene. Density functional theory calculations indicate that the intra-sheet interactions in the nanoscrolls make their optical and electrical properties different from those of stand-alone sheets. The formation of such scroll-like structures can pave the way to extended spiral growth of 2D polymer networks and porous nanotubes.


2

Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction

The precise assembly of organic building blocks to form well-ordered, nanometer-scale materials as two-dimensional covalent organic framework (2D COF)1-20 layers, while critical to their successful applications, remains a challenging task. A major issue is that the monomer sequence does not always follow the desired path and the resulting product therefore lacks long-range order.21 In recent years, increasing interest has been given to obtaining few-layer, especially singlelayer 2D COFs.22-29 Such 2D sheets could be potentially realized via either direct synthesis of 2D monolayers (in solution or on a substrate) or exfoliation of monolayers from high-quality crystals such as those recently fabricated through modifications of solvents.30,31 Regardless of which approach will eventually be preferable to form high-quality 2D monolayers, an understanding of what types of nanostructures are actually stable is becoming very valuable. In spite of the common description of 2D COFs as flat sheets, strictly 2D structures do not exist in nature as atoms within 2D sheets keep moving in the out-of-plane directions.32-34 Importantly, sheets could further form secondary structures in three-dimensional (3D) space. Therefore, gaining knowledge of the stabilities of the 2D COF nanostructures can provide critical information, for instance, to experiments designed to exfoliate monolayers from stacked 2D sheets.

Recently, we performed molecular dynamics (MD) simulations to investigate the intrinsic atomic motions within single sheets of a prototypical boronate ester-based COF, COF-5 (see its chemical structure in Figure 1a).34 We found that large degrees of out-of-plane deformations are present; the limitations in the time scales accessible via MD simulations prevented us from addressing the possibilities of formation of other nanostructures of interest. For instance, it has been reported that


3


Page 4 of 31

2D materials such as graphene, MoS2, or hexagonal boron nitride (h-BN) can form nanoscrolls.35-39 These nanostructures can even possess superior thermal stabilities, catalytic activities, and tunable magnetic properties as compared to single-layer sheets. Also, they have been suggested as precursors in the formation of nanotubes.35-39

Figure 1. Illustration of the chemical structures of (a) COF-5 and (b) the TAPB-PDA-COF. Red arrows highlight those dihedral angles around which there can be relatively easy rotation.

To shed light on the potential formation of specific 2D COF nanostructures, we apply here atomistic molecular mechanics (MM) simulations, free-energy calculations, and density functional theory (DFT) calculations to investigate the properties of representative COFs, including COF-51 and the imine-based 1,3,5-tris(4-aminophenyl)benzene (TAPB)-terephthalaldehyde (PDA)-COF 40-42

depicted in Figure 1b. Importantly, in both vacuum and solution conditions, single-layer


4

Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sheets of COF-5 and TAPB-PDA-COF with at least their length larger than ~15 nm or 20 nm, respectively, are found to preferably form nanoscrolls instead of remaining flat sheets. These nanoscrolls are open structures due to the large pores present within the 2D sheets. The number of (metastable and stable) nanoscrolls that can be formed is found to be finite and related to the size of the sheet. These features distinguish these structures from other nanoscrolls that have been identified, such as those formed from graphene.

2. Computational methodologies

The molecular mechanics and molecular dynamics (MD) simulations were carried out with the OPLS-AA force field43 using the GROMACS package44. This force field has been shown to reproduce very well the results obtained at the density functional theory level (see Ref 45 and Sections 2 and 5 of the Supporting Information, SI).

To evaluate the potential-energy profiles of the nanoscrolls, the initial nanoscroll structures are generated by rolling the flat sheets at radius intervals of 0.1 nm. The atomic structures are then relaxed using the conjugate gradient algorithm.46 The convergence criteria are carefully checked to ensure that reliable results are obtained (see Figures S1 and S2 in the SI); a convergence threshold of 2 kJ mol-1 nm-1 is used. The size of the simulation box is set in such a way that it is significantly larger than the largest dimension of the 2D sheet. Long-range electrostatic interactions are treated with the smooth particle-mesh Ewald (PME) method.47


5


Page 6 of 31

To perform free-energy calculations, we consider periodic sheets; they are infinitely extended in the armchair direction for rolling in the zigzag direction and infinitely extended in the zigzag direction for rolling in the armchair direction. The initial structures of the nanoscrolls are solvated with a 4:1 dioxane/mesitylene mixture21,41 using the PACKMOL package48. Energy minimization is then carried out, followed by a 5-ns equilibration under NPT conditions, with a time step of 1 fs. A harmonic potential is applied at the two specified parts of the nanoscrolls (see Section 3.2 for details), which pulls them at a speed of 0.1-0.13 nm/ns to allow the nanoscrolls to flatten over a time period of ~100 ns. A series of sampling windows are then constructed from the trajectory at intervals of 0.1 nm. In each window, a 1-ns equilibrium simulation is carried out, followed by a 10-20 ns MD simulation for sampling, with a harmonic potential that restrains the distance between the two specified parts. The Nose-Hoover thermostat 49-51 is used to keep the temperature at 298 K, with a weak coupling factor of 0.5 ps. The Parrinello-Rahman barostat52 is used with the pressure set to 1 bar (semi-isotropic control). Finally, the weighted histogram analysis method53,54 is applied to construct the free-energy profiles. The statistical errors are estimated using a bootstrap analysis (see Section 4 of the SI).54

Density functional theory calculations at the ωb97xd/6-31g(d,p) level55,56 were carried out with the Gaussian 09 package (Rev. D.01)57. The optimal range-separation parameter ω for COF-5 and the TAPB-PDA COF are evaluated to be 0.15 and 0.18 bohr-1, respectively; the ω optimization follows the tuning procedure58.


6

Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3. Results and Discussion

Bending a 2D COF sheet to lead to nanoscroll formation is clearly expected to induce mechanical strains; thus, it is of interest to learn the exact nature of these strains. On the other hand, once a sheet curves and meets itself, the intramolecular interactions taking place over the overlapping (stacked) areas contribute to stabilize its structure. Whether nanoscroll formation is favorable or not will therefore depend on the relative strength of intra-sheet stacking interactions with respect to the mechanical strain within the sheet.

3.1 Simulations in vacuum COF-5 and the TAPB-PDA COF were first simulated in vacuum in order to address the intrinsic trends. We consider nanoscrolls formed by rolling in both zigzag and armchair directions. We first discuss rolling in the zigzag direction. Figure 2 illustrates a typical COF-5 sheet (with length of 22 nm [20 nm] in the zigzag [armchair] direction) and the nanoscroll it forms in the zigzag direction. When the sheet interacts with itself, the initial intramolecular stacking distance d, see Figure 2b, is 3.4 Å for COF-5; this value is similar to the interlayer separation reported for stacked (flat) sheets.1 For the TAPB-PDA COF, the corresponding value is 3.5 Å41.


7


Page 8 of 31

Figure 2. Structures of (a) the (x = 22 nm, y= 20 nm) COF-5 sheet and (b) the nanoscroll formed by rolling the sheet in the zigzag direction. The zigzag and armchair directions correspond to x and y, respectively.

Figure 3a shows a COF-5 sheet with a dimension of 10 nm (3 hexagons) in the zigzag direction. The potential energy of the nanoscroll structure with respect to the flat sheet during the rolling in the zigzag direction is recorded as a function of the outer radius R2, as shown in Figure 3b. A negative energy thus points to the nanoscroll structure being more stable compared to flat sheets. For a COF-5 sheet of this limited size, there occur just two minima, one corresponding to a metastable structure and the other to a stable structure. From the outset, this result suggests that there is only a limited number of nanoscrolls that can be formed from 2D COF sheets. For the sake of ease of discussion, we label these two nanoscroll structures as COF-5-z3a and COF-5-z3b, respectively (where “z” stands for zigzag; “3”, the number of hexagons in the zigzag direction; “a” and “b”, the first and second nanoscrolls formed from the flat sheet, respectively).


8

Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Nanoscroll COF-5-z3b, which has an outer radius of 1.2 nm, is more stable than a flat sheet by 91 kcal mol-1; this value amounts to 4.5 kcal mol-1nm-1 per unit of length in the armchair direction. Both nanoscrolls correspond to open structures, see Figure 3c, which originates in the large pores present in the 2D COF sheet. These two characteristics, large pore sizes and finite number of metastable/stable structures, do in fact distinguish 2D COF nanoscrolls from nanoscrolls formed from “dense” 2D materials such as graphene, MoS2, or h-BN.35-39

Figure 3. (a) Structure of a COF-5 sheet with a length of 10 nm in the zigzag direction; (b) potential energy per unit of length in the armchair direction as a function of the outer radius upon rolling in the zigzag direction; (c) nanoscroll structures COF-5-z3a and COF-5-z3b; red and blue mark the overlapped parts.


9


Page 10 of 31

Figure 4 displays the corresponding structures for a TAPB-PDA COF sheet with 3 hexagons in the zigzag direction as well as the evolution of the potential energy during the formation of nanoscrolls in the zigzag direction. As in the COF-5 case, there are two nanoscroll structures that can be identified, labeled here as TAPB-PDA-z3a and TAPB-PDA-z3b. In contrast to the COF5 situation, both nanoscrolls are found here to be more stable than the flat sheet. In particular, nanoscroll TAPB-PDA-z3b is 301 kcal mol-1 lower in energy, which amounts to 12 kcal mol1nm-1

per unit of length in the armchair direction.

Figure 4. (a) Structure of a TAPB-PDA COF sheet with a length of 13 nm in the zigzag direction; (b) potential energy per unit of length in the armchair direction as a function of the outer radius upon rolling in the zigzag direction; (c) nanoscroll structures TAPB-PDA-z3a and TAPB-PDAz3b; red and blue mark the overlapped parts.


10

Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As the 2D sheet bends, the stress within the sheet increases the potential energy. Interestingly, this stress is found to be predominantly associated with torsions, while the contributions from variations in bonds and angles are minimal (see Figures S3 and S4 in the SI). For COF-5, these torsional strains occur mainly at the linkages between 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and 1,4-phenylene bis(boronic acid) (PBBA) units of the extended polymer networks, where rotations are more feasible compared to the more rigid (fused) benzene ring(s) found in the monomer units; in the case of TAPB-PDA COF, the torsional strains are distributed throughout the structure (Figure 5). Interestingly, at a local scale, some areas in the TAPB-PDA COF nanoscrolls are found to have reduced dihedral torsions compared to the flat sheet. The reason is that, in contrast to COF-5, dihedral torsions are in fact present in a flat sheet of the TAPB-PDA COF; upon nanoscroll formation and thus stacking, reconfiguration of the local morphologies leads to a relaxation of these torsions.


11


Page 12 of 31

Figure 5. Distribution of the strain energies (relative to the flat sheet) associated with torsions for (a) COF-5-z3a, (b) COF-5-z3b, (c) TAPB-PDA-z3a, and (d) TAPB-PDA-z3b.

The stability of the nanoscrolls originate in the van der Waals interactions appearing in the overlapped areas, which compensates for the negative impact of torsional strain. However, the large pore size present in the 2D COF sheets implies that strong intra-sheet interactions can only be achieved at specific radii. To characterize the nanoscroll formation, it is useful to introduce two parameters, α and γ. We take α to represent the ratio of interacting monomer units (using a centerof-mass distance of [2×3.4 Å] 6.8 Å for COF-5 and [2×3.5 Å] 7.0 Å for the TAPB-PDA COF as the criterion) with respect to the total number of monomer units; parameter γ is used to quantify the coverage ratio (that takes account of the two sides of the sheet).


12

Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Structures COF-5-z3a (α=19%, γ=9%) and TAPB-PDA-z3a (α=19%, γ=9%) correspond to the situation where the two edges of the sheet start interacting. Reducing the radii of COF-5-z3a and TAPB-PDA-z3a first leads to an increased potential energy, which is due to a combination of reduced intra-sheet overlap due to lattice mismatch (see Figures S5 and S6 in SI) and increased mechanical strain from sheet bending. However, when further reducing the radii to approach structures COF-5-z3b (α=67%, γ=33%) and TAPB-PDA-z3b (α=78%, γ=39%), which possess much larger degrees of intra-sheet overlap, the potential energy markedly decreases. We stress that nanoscrolls with radii smaller than COF-5-z3b and TAPB-PDA-z3b are not stable due to the substantially enhanced in-plane strain.

We now turn to the impact of increasing the size of the sheets. It results in two important evolutions: (i) The number of stable nanoscroll structures increases. (ii) The nanoscrolls become more stable. For a COF-5 sheet with a length of 16 nm in the zigzag direction (i.e., 5 hexagons) (Figure 6a), there exist four nanoscrolls (COF-5-z5a, COF-5-z5b, COF-5-z5c, COF-5-z5d, see Figure 6b) that are each more stable than the flat sheet. The most stable nanoscroll, COF-5-z5c, is 608 kcal mol-1 (30 kcal mol-1nm-1 per unit of length in the armchair direction) lower in energy vs. the flat sheet. For a TAPB-PDA COF sheet with 5 hexagons in the zigzag direction (Figure 7a), there also exists four stable nanoscrolls (TAPB-PDA-z5a, TAPB-PDA-z5b, TAPB-PDAz5c, and TAPB-PDA-z5d) (Figure 7b); the most stable nanoscroll, TAPB-PDA-z5c, is 744 kcal mol-1 (29 kcal mol-1nm-1 per unit of length in the armchair direction) lower in energy vs. the flat sheet. These features are consistent with the increased interactions stemming from more extended overlapping areas and reduced dihedral strains (compare Figures 5b, 5d, 6d, and 7d). We expect


13


Page 14 of 31

that the stability of the nanoscrolls will keep increasing in the case of longer sheets, as indicated by further analyses, which are depicted in Figure S7.

Figure 6. (a) Structure of a COF-5 sheet with a length of 16 nm in the zigzag direction; (b) potential energy per unit of length in the armchair direction as a function of the outer radius upon rolling in the zigzag direction; (c) nanoscroll structures COF-5-z5a and COF-5-z5c, with red and blue marking the overlapped parts; (d) distribution of the strain energies (relative to the flat sheet) associated with torsions for COF-5-z5c. The COF-5-z5b and COF-5-z5d structures are shown in Figures S8 and S9, respectively.


14

Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. (a) Structure of a TAPB-PDA COF sheet with a length of 20 nm in the zigzag direction; (b) potential energy per unit of length in the armchair direction as a function of the outer radius upon rolling in the zigzag direction; (c) nanoscroll structures TAPB-PDA-z5a and TAPB-PDAz5c, with red and blue marking the overlapped parts; (d) distribution of the strain energies (relative to the flat sheet) associated with torsions for TAPB-PDA-z5c. The COF-5-z5b and COF-5-z5d structures are shown in Figures S10 and S11, respectively.

The results for rolling in the armchair direction give similar trends. For a COF-5 sheet with a length of 10 nm in the armchair direction (i.e., 3 hexagons, see Figure 8a), there also exist two nanoscroll structures, labeled COF-5-a3a and COF-5-a3b (Figure 8bc). COF-5-a3a is metastable and corresponds to the situation where the two edges first overlap, while COF-5-a3b is more stable


15


Page 16 of 31

than the flat sheet (4 kcal mol-1nm-1 per unit of length in the zigzag direction). In comparison, nanoscrolls from the TAPB-PDA COF with 3 hexagons in the armchair direction (Figure 8d) can be much more stable (16 kcal mol-1nm-1 per unit of length in the zigzag direction); this can be attributed to the large coverage α, for instance, 90% for TAPB-PDA-a3b (Figure 8ef). When the size of the sheet increases, the nanoscrolls further stabilize (Figures S16 and S17). For the sheets with 5 hexagons in the armchair direction, nanoscroll formation can lead to stabilizations of 24 and 27 kcal mol-1 nm-1 per unit of length in the zigzag direction for COF-5 (COF-5-a5b) and the TAPB-PDA COF (TAPB-PDA-a5c), respectively.

After having considered the intrinsic trends of nanoscroll formation in vacuum, we now turn to a discussion of more realistic solution conditions.


16

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. (a) Structure of a COF-5 sheet with a length of 10 nm in the armchair direction; (b) potential energy per unit of length in the zigzag direction as a function of the outer radius upon rolling in the armchair direction; (c) nanoscroll structures COF-5-a3a and COF-5-a3b, where red and blue mark the overlapped parts. (d) Structure of a TAPB-PDA COF sheet with a length of 12 nm in the armchair direction; (e) potential energy per unit of length in the zigzag direction as a function of the outer radius upon rolling in the armchair direction; (f) nanoscroll structures TAPBPDA-a3a and TAPB-PDA-a3b.


17


Page 18 of 31

3.2 Simulations in solution It is critical to extend our simulations and include experimentally more relevant factors such as the presence of a solvent, which is usually involved in the fabrication of 2D COFs. From a potential energy standpoint, interactions of a sheet with the solvent molecules will compete with intra-sheet stacking. In order to evaluate the impact, we performed free-energy calculations on COF-5 and TAPB-PDA nanoscrolls in explicit solution conditions. The solvent is taken as a 4:1 dioxane/mesitylene mixture, as it is commonly used in the syntheses of COF-5 and the TAPBPDA COF.21,41 Since the computational resources required by considering large, extended twodimensional polymer networks would be prohibitive, we choose to model sheets with periodic boundary conditions in either the armchair direction (for rolling in the zigzag direction) or the zigzag direction (for rolling in the armchair direction). Two nanoscrolls of COF-5 and one nanoscroll of the TAPB-PDA COF were considered, corresponding to structures COF-5-z5b, COF-5-a5b, and TAPB-PDA-a5b. The simulations were carried out at 298 K and 1 bar.

Here, we start from nanoscrolls and Figure 9 shows the calculated free-energy profiles during this unrolling, as a function of the distance between the parts marked in blue and red in the Figure. While a comparison can be made to the potential-energy profiles in Figures 6b, S16, and S17, it must be borne in mind that here the roles of solvation, finite temperature, and entropy are all included. In each profile, two main minima can be seen: One corresponds to the nanoscroll structure and the other one, to the flat sheet (we note that the sharp energy increase at large distances is due to induced in-plane strains from stretching).


18

Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


An important message from Figure 9 is that, when in solution, nanoscrolls of COF-5 and TAPBPDA remain more stable than single-layer, flat sheets. The nanoscrolls of COF-5-z5b, COF-5a5b, and TAPB-PDA-a5b in 4:1 dioxane/mesitylene are more stable than the flat sheet by 3.2, 5.9, and 1.4 kcal mol-1nm-1, respectively; these values amount to energies of ca. 30-120 kcal mol1

for sheets having a dimension of 20 nm in the orthogonal direction. However, when comparing

to the situation in vacuum, the energy differences between the stable nanoscrolls and the flat sheets are found to reduce significantly, by about 86%, 76%, and 94% for COF-5-z5b, COF-5-a5b, and TAPB-PDA-a5b, respectively. This reduction is of course in line with the role expected to be played by solvation. Compared to the TAPB-PDA COF, nanoscrolls of COF-5 show a smaller reduction in stability when solvated, which suggests weaker interactions of COF-5 with the solvent molecules, in comparison to the bulk phase; this is consistent with the poor solubility of COF-5 monomers.59


19


Page 20 of 31

Figure 9. (a) Free energy per unit of length in the armchair direction for the unrolling of nanoscroll COF-5-z5b in the zigzag direction; (b) free energy per unit of length in the zigzag direction for the unrolling of nanoscroll COF-5-a5b in the armchair direction; (c) and free energy per unit of width in the zigzag direction for the unrolling of nanoscroll TAPB-PDA-a5b in the armchair direction. The x-axis corresponds to the center-of-mass distance between the red and blue parts. Simulations are in solution conditions (where the solvent is taken as a 4:1 dioxane/mesitylene mixture) at 298 K. The total number of atoms in (a), (b), and (c) are 320,632, 143,256, and 274,376, respectively. The total sampling time is 4 μs.


20

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Interestingly, in the case of the TAPB-PDA COF, the energy barrier corresponding to nanoscroll formation is much smaller than for COF-5 and amounts to only ~0.2 kcal mol-1 nm-1 (Figure 9c). One reason is related to the intrinsic small sheet-bending barrier for the TAPB-PDA COF. Indeed, the energy barrier for nanoscroll formation in vacuum for TAPB-PDA-a5b is only 1.3 kcal mol-1 nm-1, which is three times as small as that of COF-5-a5b, 4 kcal mol-1 nm-1 (see Figures S16 and S17). Another reason can be found in the fact that strictly 2D sheets are unstable at finite temperature. We previously studied the out-of-plane deformations of COF-5 sheets;34 here, we extended that investigation to the TAPB-PDA COF and found that the out-of-plane deformations of a macrocycle are even larger (130% larger than in COF-5, see the movie included in the SI); this increased flexibility comes from both the larger number of dihedral angles around which rotations are easy (Figure 1b) and the decrease in torsion barriers (see Ref 45 and Section 5 of the SI). Such motions can thus destabilize the flat sheet structure and lead to a reduction in the sheetbending barrier.

3.3 Implications of the stability of 2D COF nanoscrolls Our results demonstrate that stand-alone 2D COF sheets with at least a length larger than ~15-~20 nm tend to form nanoscrolls in both vacuum and solution conditions. Such a small size is comparable to the in-plane dimensions found in routinely synthesized COFs.21 Very recently, scroll-like structures of 2D COFs (although having much larger radii than those studied here and lacking nm-sized pores) have been reported;60 our simulations are thus consistent with these observations.


21


Page 22 of 31

One important implication of our simulations is that nanoscrolls can be expected to form when exfoliating monolayers from stacked sheets in solution. In addition, when monomers are still present, further expansion of these nanoscrolls is in principle possible. In fact, the growth of nanoscrolls is expected to be faster than the lateral expansion of single-layer sheets when considering the strong templating effect that has been identified for monomer addition.45,61 While we did not explicitly explore this possibility in the present work, it is also likely that nanotubes can be formed from nanoscrolls, due to the reversible nature of the condensation reactions.

Considering that 2D COFs have proven useful in device applications,62-70 we have carried out longrange corrected DFT calculations in order to shed light into the potential optical and electrical properties of the nanoscrolls investigated here and the differences with respect to the flat sheets. For COF-5, in both flat sheets and nanoscrolls, the highest occupied molecular orbitals (HOMOs) are mainly localized on HHTP units, while the lowest unoccupied molecular orbitals (LUMOs) are localized on PBBA units. For the TAPB-PDA COF, they are localized on TAPB units and PDA units, respectively. While in the flat sheets the upper occupied MOs and lower unoccupied MOs are closely spaced energy-wise, they become energetically more separated in the nanoscrolls. In the latter, as a result of the stacking effect, the ionization potential (IP) and electron affinity (EA) values are defined by molecular orbitals that are localized within the overlapped areas and the transport gap (i.e., IP minus EA) reduces by up to 0.5 eV. More details can be found in Section 2 of the SI.

At this stage, it is useful to discuss in more detail how nanoscrolls form, since, while they are thermodynamically more stable than flat sheets, there can exist energy barriers along their


22

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


formation pathways, especially for the more rigid COFs such as COF-5. Thus, an activation process can be needed to lead to nanoscrolls from flat sheets or to switch among nanoscroll structures with different diameters. Indeed, our MD simulations show that single-layer sheets do not spontaneously form nanoscrolls at least on time scales on the order of 100 ns.34 In fact, the formation of nanoscrolls in other 2D materials has also been found to require stimuli such as sonication, plasma treatment, freeze-drying, or the presence of additives.35-39 In order to gain insight into how nanoscrolls form from 2D COF sheets, we further carried out MD simulations on a system consisting of a nanoscroll structure lying on top of a flat sheet. The presence of a nanoscroll in the starting configuration is meant to trigger the rolling of the flat sheet, and represents a likely situation where multiple nanostructures co-exist. In this instance, we find that, for both COF-5 and the TAPB-PDA COF, the flat sheet very easily evolves into a nanoscroll in the early stage of the simulation (less than 330 ps) at a time when the temperature of the system is still being raised towards room temperature at a speed of 0.06 K/ps from 0 K (see the movie included in the SI); this confirms the catalytic effect played by the existing nanoscroll.

We have also examined the stabilities of nanoscrolls formed from few-layer COF sheets. We have applied similar methodologies (in vacuum condition) on two-layer COF-5 and TAPB-PDA stacks (see Section 3 of the SI). Many of the features of nanoscroll formation from single-layer 2D COF sheets are observed in this case as well: There exist multiple minima in the potential-energy profiles, which correspond to a limited number of metastable/stable nanoscrolls. Also, the potential energies of the nanoscrolls with respect to the flat sheets reduce when the size of the sheets increases. The main difference compared to the case of single-layer 2D COF sheets is that a larger minimum size is required in order to form stable nanoscrolls: For rolling in the zigzag direction,


23


Page 24 of 31

calculations in vacuum suggest that the size of the two-layer stack in this direction needs to be larger than 22 nm (to be compared to a value of 10 nm for monolayers) for COF-5 and larger than 20 nm (vs. 13 nm for monolayers) for the TAPB-PDA COF. This is consistent with the fact that the flat two-layer stack already has a significant coverage ratio γ of 50% (calculated for both layers, with respect to a γ value of 0% for monolayers); a stable nanoscroll thus needs to reach an even larger γ value while maintaining a relatively small torsional strain, which is easier to achieve for larger sheets. We note that bending multi-layer 2D COF stacks leads to lattice mismatch among the layers, which will have to be compensated by the van der Waals interactions from the additional overlapping areas in the nanoscrolls in order for them to be stable. Overall, stacked 2D COF sheets have increased rigidity and do not form nanoscrolls as easily as monolayers.

The results we have obtained on COF-5 and the TAPB-PDA COF provide insight into nanoscroll formation in other types of 2D COFs as well. Since the tendency to form nanoscrolls depends on the relative strengths of interlayer interactions and the rigidity of the sheets, we expect easier nanoscroll formation in the case of 2D COFs with: (i) extended π-conjugated moieties (that are highly polarizable); and (ii) more dihedral angles (either within the monomer units or at their connections) around which rotations can occur with small torsion barriers. In addition, the applied experimental conditions can also play a role: Vacuum processing, as in the case of freeze-drying experiments,71 is expected to strongly favor nanoscroll formation as can be inferred from the differences in the calculated energy profiles. In the case of solutions, the strength of the intermolecular interactions with the solvent molecules will affect the relative stabilities between flat sheets and nanoscrolls. We note that this also implies that the formation of nanoscrolls can be potentially tuned by modifying the nature of the solvent.


24

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4. Conclusions

In summary, atomistic molecular mechanics simulations and free-energy calculations show that single-layer COF-5 [TAPB-PDA COF] sheets with at least a length larger than ~15 nm [~20 nm] form stable nanoscrolls in both vacuum and solution conditions. One important implication of these results is that, when exfoliating monolayers from stacked sheets in solution, mild experimental conditions (e.g., avoiding sonication) should be used in order to retain the (metastable) flat structure of the 2D sheet. On the other hand, when targeting nanoscrolls, external stimuli (e.g., sonication) should be used to facilitate sheet bending.

These nanoscrolls are systems of interest for further investigations. They can only exist in a finite number of configurations (from a 2D sheet with a fixed size) and represent open structures stemming from the large pores present in the 2D sheets. These features distinguish them from the nanoscrolls formed by “dense” 2D materials such as graphene, MoS2, or hexagonal boron nitride (h-BN). The COF nanoscrolls are expected to have modified growth dynamics and different optical and electrical properties compared to single-layer sheets. However, the formation of nanoscrolls from flat sheets or the switching from a stable nanoscroll structure to another one can require activation. Finally, it is worth noting that nanoscroll formation can provide a strategy for the helical extension of 2D COFs or for the realization of porous nanotubes.

ASSOCIATED CONTENT Supporting Information. Additional results for nanoscroll formations of single-layer COF-5 and TAPB-PDA COF sheets; DFT calculations on the nanoscrolls; nanoscrolls formed from two-layer


25


Page 26 of 31

COF-5 and TAPB-PDA stacks; additional results of free-energy calculations; benchmarks of the OPLS-AA force field; movies showing nanoscroll formation and motions of COF-5 and TAPBPDA COF macrocycles. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

ACKNOWLEDGMENTS This work was supported by the Army Research Office under Grant No. W911NF-17-1-0339. We are grateful to Dr. William R. Dichtel for a critical reading of our manuscript.

REFERENCES (1) Cote, 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, 1166. (2) Furukawa, H.; Yaghi, O. M., Storage of Hydrogen, Methane, and Carbon Dioxide in Highly Porous Covalent Organic Frameworks for Clean Energy Applications, J. Am. Chem. Soc. 2009, 131, 8875. (3) Feng, X.; Ding, X. S.; Jiang, D. L., Covalent Organic Frameworks, Chem. Soc. Rev. 2012, 41, 6010. (4) Wu, D. C.; Xu, F.; Sun, B.; Fu, R. W.; He, H. K.; Matyjaszewski, K., Design and Preparation of Porous Polymers, Chem. Rev. 2012, 112, 3959. (5) Colson, J. W.; Dichtel, W. R., Rationally Synthesized Two-Dimensional Polymers, Nature Chem. 2013, 5, 453. (6) Ding, S. Y.; Wang, W., Covalent Organic Frameworks (COFs): From Design to Applications, Chem. Soc. Rev. 2013, 42, 548. (7) Zhang, Y. B.; Su, J.; Furukawa, H.; Yun, Y. F.; Gandara, F.; Duong, A.; Zou, X. D.; Yaghi, O. M., Single-Crystal Structure of a Covalent Organic Framework, J. Am. Chem. Soc. 2013, 135, 16336. (8) Bertrand, G. H. V.; Michaelis, V. K.; Ong, T.-C.; Griffin, R. G.; Dincă, M., ThiopheneBased Covalent Organic Frameworks, Proc. Natl. Acad. Sci. 2013, 110, 4923. (9) Xu, H.; Gao, J.; Jiang, D., Stable, Crystalline, Porous, Covalent Organic Frameworks as a Platform for Chiral Organocatalysts, Nature Chem. 2015, 7, 905. (10) Waller, P. J.; Gandara, F.; Yaghi, O. M., Chemistry of Covalent Organic Frameworks, Acc. Chem. Res. 2015, 48, 3053.


26

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(11) Huang, N.; Wang, P.; Jiang, D., Covalent Organic Frameworks: A Materials Platform for Structural and Functional Designs, Nat. Rev. Mater. 2016, 1, 16068. (12) Ascherl, L.; Sick, T.; Margraf, J. T.; Lapidus, S. H.; Calik, M.; Hettstedt, C.; Karaghiosoff, K.; Döblinger, M.; Clark, T.; Chapman, K. W.; Auras, F.; Bein, T., Molecular Docking Sites Designed for the Generation of Highly Crystalline Covalent Organic Frameworks, Nature Chem. 2016, 8, 310. (13) Baldwin, L. A.; Crowe, J. W.; Pyles, D. A.; McGrier, P. L., Metalation of a Mesoporous Three-Dimensional Covalent Organic Framework, J. Am. Chem. Soc. 2016, 138, 15134. (14) Zhao, Y.; Guo, L.; Gándara, F.; Ma, Y.; Liu, Z.; Zhu, C.; Lyu, H.; Trickett, C. A.; Kapustin, E. A.; Terasaki, O.; Yaghi, O. M., A Synthetic Route for Crystals of Woven Structures, Uniform Nanocrystals, and Thin Films of Imine Covalent Organic Frameworks, J. Am. Chem. Soc. 2017, 139, 13166. (15) Karak, S.; Kandambeth, S.; Biswal, B. P.; Sasmal, H. S.; Kumar, S.; Pachfule, P.; Banerjee, R., Constructing Ultraporous Covalent Organic Frameworks in Seconds via an Organic Terracotta Process, J. Am. Chem. Soc. 2017, 139, 1856. (16) Ascherl, L.; Evans, E. W.; Hennemann, M.; Di Nuzzo, D.; Hufnagel, A. G.; Beetz, M.; Friend, R. H.; Clark, T.; Bein, T.; Auras, F., Solvatochromic Covalent Organic Frameworks, Nat. Commun. 2018, 9, 3802. (17) Beuerle, F.; Gole, B., Covalent Organic Frameworks and Cage Compounds: Design and Applications of Polymeric and Discrete Organic Scaffolds, Angew. Chem., Int. Ed. 2018, 57, 4850. (18) Zhang, B.; Wei, M.; Mao, H.; Pei, X.; Alshmimri, S. A.; Reimer, J. A.; Yaghi, O. M., Crystalline Dioxin-Linked Covalent Organic Frameworks from Irreversible Reactions, J. Am. Chem. Soc. 2018, 140, 12715. (19) Lohse, M. S.; Bein, T., Covalent Organic Frameworks: Structures, Synthesis, and Applications, Adv. Funct. Mater. 2018, 28, 1705553. (20) Halder, A.; Ghosh, M.; Khayum M, A.; Bera, S.; Addicoat, M.; Sasmal, H. S.; Karak, S.; Kurungot, S.; Banerjee, R., Interlayer Hydrogen-Bonded Covalent Organic Frameworks as HighPerformance Supercapacitors, J. Am. Chem. Soc. 2018, 140, 10941. (21) Smith, B. J.; Dichtel, W. R., Mechanistic Studies of Two-Dimensional Covalent Organic Frameworks Rapidly Polymerized from Initially Homogenous Conditions, J. Am. Chem. Soc. 2014, 136, 8783. (22) Bunck, D. N.; Dichtel, W. R., Bulk Synthesis of Exfoliated Two-Dimensional Polymers Using Hydrazone-Linked Covalent Organic Frameworks, J. Am. Chem. Soc. 2013, 135, 14952. (23) Liu, X.-H.; Guan, C.-Z.; Ding, S.-Y.; Wang, W.; Yan, H.-J.; Wang, D.; Wan, L.-J., OnSurface Synthesis of Single-Layered Two-Dimensional Covalent Organic Frameworks via Solid– Vapor Interface Reactions, J. Am. Chem. Soc. 2013, 135, 10470. (24) Xu, L.; Zhou, X.; Tian, W. Q.; Gao, T.; Zhang, Y. F.; Lei, S.; Liu, Z. F., Surface-Confined Single-Layer Covalent Organic Framework on Single-Layer Graphene Grown on Copper Foil, Angew. Chem., Int. Ed. 2014, 53, 9564. (25) Steiner, C.; Gebhardt, J.; Ammon, M.; Yang, Z.; Heidenreich, A.; Hammer, N.; Görling, A.; Kivala, M.; Maier, S., Hierarchical On-Surface Synthesis and Electronic Structure of Carbonyl-Functionalized One- and Two-Dimensional Covalent Nanoarchitectures, Nat. Commun. 2017, 8, 14765. (26) Liu, J.; Zan, W.; Li, K.; Yang, Y.; Bu, F.; Xu, Y., Solution Synthesis of Semiconducting Two-Dimensional Polymer via Trimerization of Carbonitrile, J. Am. Chem. Soc. 2017, 139, 11666.


27


Page 28 of 31

(27) Wang, S.; Wang, Q. Y.; Shao, P. P.; Han, Y. Z.; Gao, X.; Ma, L.; Yuan, S.; Ma, X. J.; Zhou, J. W.; Feng, X.; Wang, B., Exfoliation of Covalent Organic Frameworks into Few-Layer Redox-Active Nanosheets as Cathode Materials for Lithium-Ion Batteries, J. Am. Chem. Soc. 2017, 139, 4258. (28) Li, Y.; Zhang, M.; Guo, X.; Wen, R.; Li, X.; Li, X.; Li, S.; Ma, L., Growth of High-Quality Covalent Organic Framework Nanosheets at the Interface of Two Miscible Organic Solvents, Nanoscale Horiz. 2018, 3, 205. (29) Chen, C.; Joshi, T.; Li, H. F.; Chavez, A. D.; Pedramrazi, Z.; Liu, P.-N.; Li, H.; Dichtel, W. R.; Brédas, J.-L.; Crommie, M. F., Local Electronic Structure of a Single-Layer PorphyrinContaining Covalent Organic Framework, ACS Nano 2018, 12, 385. (30) Ma, T.; Kapustin, E. A.; Yin, S. X.; Liang, L.; Zhou, Z.; Niu, J.; Li, L.-H.; Wang, Y.; Su, J.; Li, J.; Wang, X.; Wang, W. D.; Wang, W.; Sun, J.; Yaghi, O. M., Single-Crystal X-Ray Diffraction Structures of Covalent Organic Frameworks, Science 2018, 361, 48. (31) Evans, A. M.; Parent, L. R.; Flanders, N. C.; Bisbey, R. P.; Vitaku, E.; Kirschner, M. S.; Schaller, R. D.; Chen, L. X.; Gianneschi, N. C.; Dichtel, W. R., Seeded Growth of Single-Crystal Two-Dimensional Covalent Organic Frameworks, Science 2018, 361, 52. (32) Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S., The Structure of Suspended Graphene Sheets, Nature 2007, 446, 60. (33) Fasolino, A.; Los, J. H.; Katsnelson, M. I., Intrinsic Ripples in Graphene, Nat. Mater. 2007, 6, 858. (34) Li, H. Y.; Brédas, J.-L., Large Out-of-Plane Deformations of Two-Dimensional Covalent Organic Framework (COF) Sheets, J. Phys. Chem. Lett. 2018, 9, 4215. (35) Viculis, L. M.; Mack, J. J.; Kaner, R. B., A Chemical Route to Carbon Nanoscrolls, Science 2003, 299, 1361. (36) Meng, J. L.; Wang, G. L.; Li, X. M.; Lu, X. B.; Zhang, J.; Yu, H.; Chen, W.; Du, L. J.; Liao, M. Z.; Zhao, J.; Chen, P.; Zhu, J. Q.; Bai, X. D.; Shi, D. X.; Zhang, G. Y., Rolling Up a Monolayer MoS2 Sheet, Small 2016, 12, 3770. (37) Hwang, D. Y.; Choi, K. H.; Park, J. E.; Suh, D. H., Highly Efficient Hydrogen Evolution Reaction by Strain and Phase Engineering in Composites of Pt and MoS2 Nano-Scrolls, Phys. Chem. Chem. Phys. 2017, 19, 18356. (38) Hwang, D. Y.; Choi, K. H.; Park, J. E.; Suh, D. H., Evolution of Magnetism by Rolling up Hexagonal Boron Nitride Nanosheets Tailored with Superparamagnetic Nanoparticles, Phys. Chem. Chem. Phys. 2017, 19, 4048. (39) Hwang, D. Y.; Choi, K. H.; Park, J. E.; Suh, D. H., Highly Thermal-Stable Paramagnetism by Rolling up MoS2 Nanosheets, Nanoscale 2017, 9, 503. (40) Schwab, M. G.; Hamburger, M.; Feng, X.; Shu, J.; Spiess, H. W.; Wang, X.; Antonietti, M.; Müllen, K., Photocatalytic Hydrogen Evolution Through Fully Conjugated Poly(Azomethine) Networks, Chem. Commun. 2010, 46, 8932. (41) Smith, B. J.; Overholts, A. C.; Hwang, N.; Dichtel, W. R., Insight into the Crystallization of Amorphous Imine-Linked Polymer Networks to 2D Covalent Organic Frameworks, Chem. Commun. 2016, 52, 3690. (42) 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, 4999.


28

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43) Jorgensen, W. L.; Maxwell, D. S.; TiradoRives, J., Development and Testing of the OPLS All-Atom Force Field on Conformational Energetics and Properties of Organic Liquids, J. Am. Chem. Soc. 1996, 118, 11225. (44) Pronk, S.; Pall, S.; Schulz, R.; Larsson, P.; Bjelkmar, P.; Apostolov, R.; Shirts, M. R.; Smith, J. C.; Kasson, P. M.; van der Spoel, D.; Hess, B.; Lindahl, E., GROMACS 4.5: A HighThroughput and Highly Parallel Open Source Molecular Simulation Toolkit, Bioinformatics 2013, 29, 845. (45) Li, H. Y.; Chavez, A. D.; Li, H. F.; Li, H.; Dichtel, W. R.; Brédas, J.-L., Nucleation and Growth of Covalent Organic Frameworks from Solution: The Example of COF-5, J. Am. Chem. Soc. 2017, 139, 16310. (46) Ramachandran, K. I.; Deepa, G.; Namboori, K. Computational chemistry and molecular modeling: principles and applications; Springer, Berlin, 2008. (47) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen, L. G., A Smooth Particle Mesh Ewald Method, J. Chem. Phys. 1995, 103, 8577. (48) Martinez, L.; Andrade, R.; Birgin, E. G.; Martinez, J. M., PACKMOL: A Package for Building Initial Configurations for Molecular Dynamics Simulations, J. Comput. Chem. 2009, 30, 2157. (49) Nose, S., A Molecular-Dynamics Method for Simulations in the Canonical Ensemble, Mol. Phys. 1984, 52, 255. (50) Nose, S., A Unified Formulation of the Constant Temperature Molecular-Dynamics Methods, J. Chem. Phys. 1984, 81, 511. (51) Hoover, W. G., Canonical Dynamics - Equilibrium Phase-Space Distributions, Phys. Rev. A 1985, 31, 1695. (52) Parrinello, M.; Rahman, A., Polymorphic Transitions in Single-Crystals - a New Molecular-Dynamics Method, J. Appl. Phys. 1981, 52, 7182. (53) Kumar, S.; Bouzida, D.; Swendsen, R. H.; Kollman, P. A.; Rosenberg, J. M., The Weighted Histogram Analysis Method for Free-Energy Calculations on Biomolecules. I. The Method, J. Comput. Chem. 1992, 13, 1011. (54) Hub, J. S.; de Groot, B. L.; van der Spoel, D., g_wham-A Free Weighted Histogram Analysis Implementation Including Robust Error and Autocorrelation Estimates, J. Chem. Theory Comput. 2010, 6, 3713. (55) Chai, J.-D.; Head-Gordon, M., Long-Range Corrected Hybrid Density Functionals with Damped Atom-Atom Dispersion Corrections, Phys. Chem. Chem. Phys. 2008, 10, 6615. (56) Hehre, W. J.; Ditchfield, R.; Pople, J. A., Self—Consistent Molecular Orbital Methods. XII. Further Extensions of Gaussian—Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules, J. Chem. Phys. 1972, 56, 2257. (57) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.;


29


Page 30 of 31

Farkas; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision D.01; Gaussian Inc.: Wallingford, CT, 2009. (58) Kronik, L.; Stein, T.; Refaely-Abramson, S.; Baer, R., Excitation Gaps of Finite-Sized Systems from Optimally Tuned Range-Separated Hybrid Functionals, J. Chem. Theory Comput. 2012, 8, 1515. (59) Smith, B. J.; Parent, L. R.; Overholts, A. C.; Beaucage, P. A.; Bisbey, R. P.; Chavez, A. D.; Hwang, N.; Park, C.; Evans, A. M.; Gianneschi, N. C.; Dichtel, W. R., Colloidal Covalent Organic Frameworks, ACS Central Science 2017, 3, 58. (60) Gole, B.; Stepanenko, V.; Rager, S.; Grüne, M.; Medina, D. D.; Bein, T.; Würthner, F.; Beuerle, F., Microtubular Self‐Assembly of Covalent Organic Frameworks, Angew. Chem., Int. Ed. 2018, 57, 846. (61) Li, H. F.; Li, H. Y.; Dai, Q. Q.; Li, H.; Brédas, J. L., Hydrolytic Stability of Boronate Ester ‐Linked Covalent Organic Frameworks, Adv. Theory Simul. 2018, 1, 1700015. (62) Wan, S.; Guo, J.; Kim, J.; Ihee, H.; Jiang, D., A Belt-Shaped, Blue Luminescent, and Semiconducting Covalent Organic Framework, Angew. Chem., Int. Ed. 2008, 47, 8826. (63) Wan, S.; Gándara, F.; Asano, A.; Furukawa, H.; Saeki, A.; Dey, S. K.; Liao, L.; Ambrogio, M. W.; Botros, Y. Y.; Duan, X.; Seki, S.; Stoddart, J. F.; Yaghi, O. M., Covalent Organic Frameworks with High Charge Carrier Mobility, Chem. Mater. 2011, 23, 4094. (64) Duhović, S.; Dincă, M., Synthesis and Electrical Properties of Covalent Organic Frameworks with Heavy Chalcogens, Chem. Mater. 2015, 27, 5487. (65) Crowe, J. W.; Baldwin, L. A.; McGrier, P. L., Luminescent Covalent Organic Frameworks Containing a Homogeneous and Heterogeneous Distribution of Dehydrobenzoannulene Vertex Units, J. Am. Chem. Soc. 2016, 138, 10120. (66) Medina, D. D.; Sick, T.; Bein, T., Photoactive and Conducting Covalent Organic Frameworks, Adv. Energy. Mater. 2017, 7, 1700387. (67) Jin, E.; Asada, M.; Xu, Q.; Dalapati, S.; Addicoat, M. A.; Brady, M. A.; Xu, H.; Nakamura, T.; Heine, T.; Chen, Q.; Jiang, D., Two-Dimensional sp2 Carbon–Conjugated Covalent Organic Frameworks, Science 2017, 357, 673. (68) Sun, B.; Zhu, C. H.; Liu, Y.; Wang, C.; Wan, L. J.; Wang, D., Oriented Covalent Organic Framework Film on Graphene for Robust Ambipolar Vertical Organic Field-Effect Transistor, Chem. Mater. 2017, 29, 4367. (69) Haldar, S.; Chakraborty, D.; Roy, B.; Banappanavar, G.; Rinku, K.; Mullangi, D.; Hazra, P.; Kabra, D.; Vaidhyanathan, R., Anthracene-Resorcinol Derived Covalent Organic Framework as Flexible White Light Emitter, J. Am. Chem. Soc. 2018, 140, 13367. (70) Albacete, P.; Martínez, J. I.; Li, X.; López-Moreno, A.; Mena-Hernando, S. a.; PlateroPrats, A. E.; Montoro, C.; Loh, K. P.; Pérez, E. M.; Zamora, F., Layer-Stacking-Driven Fluorescence in a Two-Dimensional Imine-Linked Covalent Organic Framework, J. Am. Chem. Soc. 2018, 140, 12922. (71) Xu, Z.; Zheng, B. N.; Chen, J. W.; Gao, C., Highly Efficient Synthesis of Neat Graphene Nanoscrolls from Graphene Oxide by Well-Controlled Lyophilization, Chem. Mater. 2014, 26, 6811.


30

Page 31 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC graphic


31

Nanoscrolls Formed from Two-Dimensional Covalent Organic

Recommend Documents