Multi-Functionalities of an Azine-Linked Covalent-Organic Framework

Subscriber access provided by University of Florida | Smathers Libraries

Article

Multi-Functionalities of an Azine-Linked Covalent-Organic Framework: From Nanoelectronics to Nitro-Explosive Detection and Conductance Switching Chandrima Chakravarty, Bikash Mandal, and Pranab Sarkar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11609 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 31, 2018

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 free 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 accessible to all readers and 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.

The Journal of Physical Chemistry C 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 35 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

The Journal of Physical Chemistry

Multi-functionalities of an Azine-linked Covalent-Organic Framework: From Nanoelectronics to Nitro-Explosive Detection and Conductance Switching Chandrima Chakravarty,† Bikash Mandal,‡ and Pranab Sarkar∗,† Department of Chemistry, Visva-Bharati University, Santiniketan- 731235, India, and School of Chemical Engineering, University of Ulsan, 93 Daehakro, Nam-Gu, Ulsan-44610, South Korea E-mail: [email protected]

Abstract By using the state-of-the-art theoretical method, we have investigated the electronic structures of recently synthesized two-dimensional azine-linked covalent-organic framework (ACOF1). Our result indicates that ACOF-1 is a direct band gap semiconductor, suggesting useful application in nanoelectronics. Its one-dimensional (1D) structure also exhibits semiconducting properties. Furthermore, this azine-linked COF is found to be practically useful for selective sensing of nitroaromatics over nitroaliphatics. Lastly, our calculations reveal a more realizable way for using two tautomers of ATFG-COF, a derivative of ACOF-1, in conductance switching device by means of transport property calculation. Therefore, our present study may provide a guideline for multi-functionalities of azine-linked COF (ACOF-1). ∗ To

whom correspondence should be addressed University ‡ University of Ulsan † Visva-Bharati

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

Page 2 of 35

Introduction Covalent-Organic Frameworks (COFs) are well-defined, two-dimensional (2D) 1,2 or three dimensional (3D) 3–5 periodic crystalline materials. COFs are constituted by light-weight elements, such as C, H, N, B, O and Si, which are combined via strong covalent bonds. Hence, it possesses low mass density and have permanent porosity with large surface areas. By means of which, COFs find its applications in storage for gases, such as hydrogen, methane, carbon dioxide etc. 6 In addition, these materials grow an active and exciting research field due to their large-scale production, mechanical properties, application in gas adsorption, catalysis, photoelectricity, chemical sensing, semiconductors, optoelectronics etc. 7–15 Over the past few years, experimental studies mainly focused on the rational design and controlled synthesis of COFs to explore its multi-functional potential applications in gas storage/separation, 16 gas adsorption, 17 energy conversion, 18 chemical sensing, 19 catalyst for fuel cell 20 etc. Nowadays, several computational and theoretical studies also reveal many exciting electronic properties of COFs. In 2012, Zhu et al. 21 investigated that increasing the organic-chain links leads to systematic reduction of band gap. However, the effect of substrate on its electronic properties are insignificant, indicating the robustness of COF’s intrinsic properties. Zhou et al. 22 predicted that COF-5 exhibits type-II band alignment and shows effective spatial carrier separation of electron and holes with band offset. They also studied the optical properties of TP-COF and NiPc-PBBA COF and found their suitability for application in optoelectronic devices. In this regards, Yang et al. 23 explored how the alteration of elemental combination in X4 Y can tune the band gap of (X4 Y)(O2 BC6 H4 -BO2 )3 COF. Thus, the optical responses of these COFs can also be tuned from UV to visible range of spectrum. Moreover, Yang et al. 24 predicted a series of ten new COFs, having the band gaps ranging from 1.5 to 2.1 eV, which correspond to visible to near infrared. Lukose et al. 25 have also studied the electronic properties of some new and existing COFs and found that their band gaps are ranging from 1.7 to 4.0 eV. Wang et al. 26 have analyzed the flat band characteristics in 2D boroxine-linked covalent organic frameworks is due to the delocalized π-conjugated electrons around phenyl ring and can be better understood by aromaticity theory. Pakhira et al. 27 predicted 2


Page 3 of 35 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


that Fe-intercalation between two organic layers of COFs is a new strategy to obtain semiconducting property. Similarly, Meng and coworkers 28 proposed a calcium intercalated COF, which is useful in H2 storage. More recently, Er et al. 29 have explored how the number of layers and the stacking order in COF, affect the carrier mobility and photoconductivity along the vertical direction. Interestingly, for the first time, Liu and coworkers 30 investigated the thermal conductivity of 3D boron-based COFs and gave a mechanistic insight of heat transfer. In the recent past, e-beam lithography technique is widely explored to create one dimensional (1D) graphene nanoribbon from 2D graphene sheet 31–34 and the 1D analogue of 2D graphene shows many exciting properties that shows up large key of applications of graphene. Thus, we may expect that, 1D COF, a derivative of 2D COF can show exciting properties and therefore 1D COF is highly desirable for basic research as well as for their immense technological impacts. Wen et al. 35 and Gutzler et al. 36 have computationally studied the electronic structure of single-layer covalent organic framework in 2D (2D conjugated polymer) and 1D (1D polymer). These two polymers are built from same parent molecular repeating unit cell. The 2D polymer is actually a covalently linked network of the repeating unit cell with bonding in two orthogonal directions. 35 Apart from this electronic properties, COFs are also an exciting material for photo current switching. In this context, it is to be noted that Wan et al. 15 demonstrated that TP-COF is capable of on-off switching of electric current, which is described from I-V curve. The same group also prepared a polypyrene-based COF (PPy-COF), which shows its promising performance for repetitive on-off photo current switching with a large switching ratio of 8.0 ×104 . Furthermore, Guo et al. 37 have synthesized electronically conjugated CS-COF and found that it has high on-off photo current switching ratio. In this context, other interesting properties like magnetic and electronic switching behavior of manganese porphyrin-based model system was also investigated by Zeng et al. 38 Despite this intensive research on photo current switching, magnetic and electronic switching, still conductance switching 39,40 behavior of COF has overlooked. Very recently, Dalapati et al. 14 have shown that a highly chemically stable azine-linked COF can selectively sense the presence of 2,4,6-trinitrophenol explosive. Furthermore, Das and coworkers 19 synthesized two new stable

3



imide-based COFs, namely TpBDH and TfPBDH. They shown that TfPBDH selectively detect nitroaromatic analytes over TpBDH, even for low analyte concentration. Thus, these are efficient fluorescence chemo sensor. The use of COF is not limited to these applications only, it spreaded its wings in so many fields. 41,42 Li et al. 43 very recently synthesized a stable azine-linked ACOF-1 by condensation of hydrazine hydrate and 1,3,5-triformyl benzene under solvothermal condition. To explore the applicability of this COF in different fields, we should have clear understanding of the electronic structure of this COF. In view of this, in this article, we have made an attempt to investigate the electronic properties of a stable azine-linked ACOF-1. Although there are extensive experimental and theoretical studies on different COFs, the 1D derivative of these COFs are still missing. So herein, we would like to perform the electronic property calculations of its 1D derivative to predict its use in electronic devices. We further investigated the nitroaromatic and nitro-aliphatic explosive induced change in electronic properties of ACOF-1 to detect its selective sensing properties. Finally, based on two tautomeric structures of one of its derivative, we studied the transport properties to understand its conductance switching behavior.

Computational Methods Our density-functional theory (DFT) calculations were performed using SIESTA package. 44 To represent the core and valance electrons, respectively, we have considered norm-conservative Troullier-Martins pseudo-potentials 45 and double-ζ plus polarization (DZP) basis set. To account the electron-electron interactions, the generalized gradient approximation (GGA) in the PerdewBurke-Ernzerhof 46 (PBE) form is applied. A large vacuum space is considered along z direction to neglect the interaction between neighboring cells. Conjugate gradient method is employed to fully relax all the atomic positions without any geometrical constraints so that the maximum force becomes less than 0.01 eV Å−1 . The entire calculations are done using a real space mesh cutoff of 200 Ry and the electronic temperature is set to 300K. The tolerance for energy convergence is

4


Page 4 of 35

Page 5 of 35 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


0.001 eV. The k-point sampling for unit cell was performed with a 6 × 6 × 1 Monkhorst-Pack grid, 47 respectively. We performed spin transport calculation using TranSIESTA module within the SIESTA package. This is the combination of density functional theory and non-equilibrium Green’s function (NEGF). 48 Similar set of parameters such as basis functions, the exchange-correlation functional and convergence criteria are used as our first-principle DFT calculations. In the NEGF selfconsistent loop, the charge density was integrated over 400 energy points along the semicircle in the complex plane. We have used Au(111) surface as the contact lead. Left (LE) and right electrodes (RE) are constructed by three layered unit cell of Au(111) surface. The central scattering region consists of the molecule associated with two layered Au(111) surfaces. Thereby, sand witching the central scattering region (SR) by three layered unit cell of Au(111) surface. To demonstrate the electronic structure of all COF-explosive composite systems, herein, we have employed self-consistent charge density-functional tight-binding (SCC-DFTB) method. This method has described elsewhere in details. 49–53 The total energy calculations are done by using self-consistent charge density-functional tight-binding method (SCC-DFTB) 49–54 as implemented in the dftb+ code. A large vacuum distance is maintained in the z-direction to avoid the interaction between the neighboring cell. The interaction between COF and explosives is inevitable thus, we have included the dispersion interaction through Slater-Kirkwood model, using a previously derived set of parameters. 50,53 We also used the conjugate gradient algorithm for geometry optimization until the forces on each atoms are below 0.0001 eV/Å. A (4 × 4 × 1) Monkhorst-Pack grid is found to be appropriate by convergence test on the k-point sampling along the periodic direction. We have calculated the optical properties of COF and COF-TNT composite system using density-functional theory (DFT) as implemented in SIESTA package. 44 We have considered normconservative Troullier-Martins pseudo-potentials 45 and double-ζ plus polarization (DZP) basis set for representing core and valance electrons, respectively. The generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof 46 (PBE) form is applied to account for electron-electron

5



interactions. A real space mesh cutoff of 200 Ry is used throughout the calculation and the electronic temperature is set to 300K. The tolerance for energy convergence is 0.001 eV. The optical mesh was set to 40×40 × 1.

Results and Discussion The optimized structure of 2D covalent-organic framework, ACOF-1 is shown in Fig. 1(a). The framework has hexagonal planar structure and displays idealized P6/m symmetry. 55 The optimized lattice constant is 14.9873Å, which is slightly larger than the experimental reported value (14.724 Å). 43 The unit cell of ACOF-1 consists of two building blocks, namely, azine (A) and 1,3,5triformyl benzene (B). Stegbauers and his coworkers 55 named this COF as azine-benzene COF, in shortly, AB-COF. As shown in Fig. 1(a), C1 , C2 , C3 are three different carbon atom sites from benzene unit and C4 , C5 are two carbon sites from formyl unit connected with benzene ring. The bond lengths are listed in Table 1. C1 -C2 and C2 -C3 bond lengths are close to the C-C bond lengths in benzene. On the other hand, C1 -C5 (1.4635Å) bond length is much less than the CC single bond distance (1.54Å). The bond length of C5 -N (1.2993Å) is larger than the previously reported value (1.279Å). 56,57 The N-N bond length (1.3872Å) is much smaller than the previously reported value (1.47Å). 58 These bond shortening and lengthening can be explained in the following manner. A closer look in Fig. 1(a) reveals that diazabutadiene (-C=N-N=C-) unit is attached to the cross-conjugated 59 position. As can be seen from Fig. 1(b), the isolated diazabutadiene has π conjugated pathway in this planar configuration. 57 Herein, the cross-conjugated meta-connected ACOF-1 displays a short-range direct conjugation pathway starting from one benzene ring to other via diazabutadiene (-C=N-N=C-) unit, as shown in Fig. 1(c). However, cross-conjugation inhibits the long-range conjugation pathway through antiresonance. This short-range resonance allows the bond length shortening and lengthening. In addition, we may point out that N atom changes its hybridization and this is reflected in N-N bond shortening and large bond angle deviation of 6 C5 N-N. 6 C5 -N-N bond angle is 111.195◦ , much smaller than 120◦ . From Fig. 1(b) it is clear that,

6


Page 6 of 35

Page 7 of 35 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


the lone pair electron is localized on N atom. The electronegativity of N atom is larger than C atom, hence N=C bonded electron-pair will be more centered around N atom. As a result, there is lone-pair - bond-pair repulsion, which push the 6 C5 -N-N to squeeze. We now turn our attention towards the detail electronic properties of ACOF-1. Notably, the ground state of the studied system is nonmagnetic (NM). The band structure is plotted in Fig. 2(a). The result indicates that, ACOF-1 is semiconductor with direct band gap of 2.1767 eV at Γ point. Thus, this may be a promising candidate for semiconducting application. It should be noted that PBE/DFT calculation underestimates the band gap of a semiconductor. It is important to notice that, in Fig. 2(a), valance band maximum (VBM) and conduction band minimum (CBM) exhibit flat band characteristics. Moreover, this flat band characteristics of VBM and CBM states exist over the whole Γ-M-K-Γ range. This feature is also demonstrated in the previous calculation by Er et al. 29 and Wang et al. 26 Our result indicates that the wave function corresponding to VBM and CBM states must be highly localized. In order to understand the reason behind flat band, in detail, we have analyzed the charge densities of VBM and CBM state of ACOF-1. The charge densities are plotted in Fig. 2(b) and (c), respectively. Clearly, the charge densities of VBM state arises mainly from the azine (-N-N-) linkage of the framework, as shown in Fig. 2(b). Thus, due to this localized state, flat band appears in VBM state. In contrast, the wave function corresponding to CBM is contributed by diazabutadiene unit and some carbon atoms of two meta-connected benzene ring connecting the former unit. We also note that, as an effect of cross-conjugation, the nodes are centered on diazabutadiene unit including the carbon atom of benzene ring. Consequently, this node will give rise to dispersionless band. Hatanaka 60 reported that this zero dispersion phenomenon is originating from zero HOMO-HOMO and LUMO-LUMO interactions between nodal points. Naturally, as a consequence of this nondispersive band, flat band appears. In this situation, we may point out that ACOF-1 is semiconductor because of the absence of conjugation throughout the structure, although CBM consists of short-range resonance. For comparison, Liu et al. 61 and Zhou et al. 22 have shown that both VBM and CBM states are spatially separated in COF-5 and located in 2,3,6,7,10,11-hexahrdroxytriphenylene (HHTP) and 1,4-phenylenebis(boronic acid)

7



(PBBA) building blocks, respectively. While, in our system VBM is localized on azine linkage and CBM has mixed contribution from both azine linkage and 1,3,5-triformyl benzene unit. According to such result of COF-5, it presents type-II band alignment and become a promising candidate for performance in photovoltaic cell. Furthermore, Zhou et al 22 shown that NiPc-PBBA COF has type-I band alignment. Additionally, TP-COF has localized CBM state and delocalized VBM state. Our results are different from this previously studied framework. Up to this end, we can see that 2D ACOF-1 exhibits direct-band gap semiconducting property, but still lacking of magnetism. Liu et al. 22 have made an attempt to include spin-polarization in COF-5 framework via B and N atom doping. Besides, they have also shown that COF-5 become spin-polarized metallic or semiconducting material. Hence, these systems are suitable for application in spintronic devices. Previous studies reveal that the 2D pristine graphene sheet is NM, while theoretical and experimental studies confirmed that its one dimensional derivative (1D), zigzag-edged graphene nanoribbon (ZGNR) has antiferromagnetic property. 62–65 Inspired by this concept, we have studied 1D derivative of 2D ACOF-1 sheet. The inset of Fig. 2(d) shows the unit cell structure of 1D derivative along the periodic X-direction. The unit cell is same as the sheet, only difference, the edge is passivated by H atoms. We found that antiferromagnetic (AFM) is energetically more stable magnetic state. However, the energy difference between AFM and NM state is very small compared to the KB T (KB = Boltzmann constant; T = 300K). Hence, AFM state is considered to be unstable and we have calculated the electronic properties of 1D COF considering NM state. Before discussing about NM 1D derivative of COF, we tried to understand the origin of magnetism and magnetic charge density distribution on 2× 1 × 1 supercell. We have calculated the magnetic charge density in AFM state, as shown in Fig. S1(a). Our result indicate that the magnetic moment mainly appears in -C=N-N=C- unit and -C=N- moiety of edge. As can be seen from Fig. S1(a), each -C=N- unit is a combination of up and down spin density, hence this is a singlet pair with total magnetic moment zero. As a result, such singlet pairs resulting the antiferromagnetic ground state. We further investigated the electronic properties of 1D derivative of COF in AFM ground

8


Page 8 of 35

Page 9 of 35 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


state. The spin-resolved electronic band structure is plotted in Fig. S1(b). In this case, the figure clearly demonstrates that it is a direct-band gap semiconductor at Γ point. Both spin states are degenerate, which is consistent with its antiferromagnetic property. Note that similar to 2D COF, 1D system also exhibit almost flat band characteristics around the Fermi level. In order to explain this characteristics, we have plotted charge density, as shown in Fig. S1(c)-(d). As an effect of localized state, both for VBM and CBM state, flat band appears. Similar to AFM 1D COF, nonmagnetic 1D derivative of COF also shows direct band gap features at Γ point, as shown in Fig. 2(d), with band gap value 2.1787 eV. The flat band feature around Fermi level can again be understood via charge density plot, as shown in Fig. 2(e)-(f). Similar to 2D structure, VBM is mainly contributed by the azine (-N-N-) linkage, hence VBM is localized state and generates flat bands. On the other hand, CBM is still a combination of short-range resonance and node. Due to same unit cell structure, here also cross-conjugation plays an important role for charge localization in the same region as ACOF-1. This leads flat band in CBM state. Quite interestingly, most of the bands are flat because dimensionality reduction leads to the multiple short-range conjugation path reduction. Most interestingly, the band gap difference between 2D and 1D is 0.0020 eV only, that means it remain almost same. In order to understand the constant band gap, here, we should mention two competiting factors that governs the band gap - (i) cross-conjugation, which reduces conjugation and it left nodes, thus, increases the band gap. (ii) short-range direct conjugation that decreases the band gap value. For both 2D and 1D, the wave function corresponding to VBM and CBM states are same. Hence, cross-conjugation and short-range direct conjugation has almost similar impact on the band gap. Consequently, the band gap remains almost same. Thus, we conclude, 1D derivative of a COF can serve as a useful material for semiconducting application. Till now, we have discussed the electronic and magnetic properties of 2D and 1D ACOF-1. Apart from these, there are experimental studies which reveals that COFs have widely used in chemical sensing of nitro explosives. Structurally, our studied system consists of azine-linkage, which act as open docking sites for hydrogen-bonding. This fact is confirmed by very recent work of Dalapati et al. 14 Thus, we expect that ACOF-1 will be also able to capture guest molecules

9



through H-bonding interactions, as shown in Fig. 3(a). At last, to be of practical interest for sensing, we wish to emphasize on the sensing property of ACOF-1. To study the sensing of ACOF-1, we have chosen two kinds of explosives- (i) nitroaromatics, as example TNT, TNP, DNT and TNR (ii) nitroaliphatics, such as RDX, DMNB, HMX, PETN. Firstly, the isolated explosive molecules are optimized. The optimized structures are addressed in Fig. 3(b) and (c), respectively. In order to investigate the effect of this explosives in the electronic structure of ACOF-1, we have plotted partial density of states (PDOS) for two kinds of COF-explosive composites, as shown in Fig. 4(a) and (e), respectively. Left panel indicates the PDOS of individual component of nitroaromatic composites along with the total density of state (TDOS) of isolated COF. Very interestingly, we could found that nitroaromatics are highly efficient to shift the position of VBM and CBM with respect to Fermi level. This surely indicates a strong interaction with COF. Dramatically, the LUMO state of nitroaromatics appear in the band gap region of ACOF-1. Thus, band gap of ACOF-1 decreases for all studied nitroaromatics. As a result one should expect a red shift in the electronic absorption spectrum and this issue has been discussed later. Most interestingly, TNR reduces band gap over TNP and TNT over DNT. So, the order of sensing is TNR > TNP > TNT > DNT. Fig. 4(b) and (c) represent the charge density of COF-TNT composite system, as a representative example. We find that the VBM is localized on that azine-linkage (-N-N-), which is close to TNT. On the other hand, CBM is contributed by TNT solely. The electrons are transferred from CBM of COF to LUMO of TNT on excitation, thus, may leads to a quenching effect. 66 In order to explain the sequence of band gap reduction, we will take the help of structural merit of explosives. Here, all structures exhibit planar conformation of phenyl ring, which would increase π conjugation, thus results red shift. We have calculated the optical properties of isolated COF and COF-TNT composite, as shown in Fig. 4(d). We found that COF-TNT composite shows red shift as compared to the isolated COF. To differentiate the sensitivity of different composites, the presence of hydroxyl (-OH) and nitro (-NO2 ) groups play a crucial role. The hydroxyl (-OH) group participates in H-bonding interactions with the azine-linkage (-N-N-) of COF. 67–70 Nitro group has two-fold functionality - it controls the electron deficiency of benzene ring, thus structurally, TNR is the most

10


Page 10 of 35

Page 11 of 35 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


electron deficient in this series. On the other hand, the strength of acidity depends on the presence of NO2 group, which in turn affect the H-bonding interactions. Thus, most electron deficient TNR facilitate strong interactions with azine linkage , which promotes the most red shift. Following these two factors, TNP shows its sensing ability and so on. In contrast to nitroaromatics, nitroaliphatics are unable to affect the electronic structure of COF very significantly, as shown in Fig. 4(e). The inclusion of nitroexplosive into the pore of COF will only affect the position of VBM and CBM state of COF and the band gap remain almost the same. The states of nitroaliphatics are unable to introduce new states in the band gap region of COF. The charge density plot of COF-RDX is addressed in Fig. 4(f) and (g), which confirms that VBM is localized on azine-linkage and CBM is dispersed over some space of COF. Interestingly, comparison of Fig. 2(b)-(c) and Fig. 4(g)-(f) confirms that nitroaliphatics changes the electron density distribution over COF, after composite formation, but could not change the spatial distribution as nitroaromatics. This result indicates that ACOF-1 can detect the nitroaromatic and nitroaliphatic explosives very selectively. In this context, Odbadrakh et al. 71 explore the effect of RDX in the electronic structure modification of IRMOF-8. Thereafter they concluded that appearance of density of states of RDX in the band gap region of IRMOF-8 will cause the band gap decrease, which in turn can be exploited for spectroscopic explosive detecting technologies. The recent research trend shows that this particular class of materials have also its applications in designing molecular switch. We found that conductance switching is also an active and exciting field for application in molecular switch. The properties of a molecular switching device can be tailored by small modification in the building unit. 72 Very recently, Stegbauer et al. 55 showed that 1,3,5-triformylphloroglucinol (ATFG-COF), which is a derivative of ACOF-1, exhibit an reversible tautomeric structural equilibrium in two form - one is -OH and other one is -NH form. For sake of convenience, we referred -OH and -NH form as enol and keto form, respectively. We are now trying to understand whether two tautomeric forms may represent ON and OFF state of a switch, respectively. To realize this conductance switching behavior of ATFG-COF, we have considered a molecular building block of ATFG-COF, as a representative model system. This moiety can

11



exists in two tautomeric forms, which are referred to as enol and keto, respectively, which are being inter-converted through proton transfer. Previous work of Weckbecker et al. 73 motivated us to choose this molecular building blocks. To test whether the keto and enol moiety can fulfill the above mentioned criteria, we have calculated the transport properties of such model systems, as schematically represented in Fig. 5. Their current-voltage (I-V) characteristics are shown in Fig. 6(a). The most striking result is that enol and keto form exhibit remarkably distinct I-V curves. Especially, the keto form have significantly large current value as compared to enol form. Not only that, keto form shows small negative differential resistance 74 (NDR) feature, while enol has no such effect. This leads keto form as ON state of the switch and enol form as OFF state of the same. As bias increases, the current carried by the keto form increases significantly, while there is small increase in current for enol. To deeply understand how their switching performance works, we have calculated the switching ratio (SR), defined as I(Vbias )ON /I(Vbias )OFF and plotted in the inset Fig. 6(b). Notably, the SR starts with a large value at bias voltage 0.1 V, then steeply decreases at bias voltage 0.2 V. This is consistent with the NDR effect at bias voltage 0.2 V, as shown in Fig. 6(a). Following the variation in I-V curve, SR also shows NDR effect for 0.6 V and so on. Thus, the SR curve actually has an oscillatory variation with applied bias voltages. As the SR is maximum for low-bias voltage at 0.1 V, this model system can be suited for low-bias molecular switch. In order to understand the NDR mechanism, 75–77 we have plotted the transmission function at bias voltage 0.0 V, 0.1 V and 0.2 V, respectively in Fig. 7(a). We have calculated the maximum peak value ratios (PVR) (defined as the ratio of Imax and Imin within a range of bias). These are 1.649, 1.3742, 1.187; as the studied system shows multiple NDR characteristics. There is a large transmission peak in the bias window at 0.1 V, as shown in Fig. 7(a). However, at bias voltage 0.2 V, there is a drop in transmission peak value. As a result the current drops. Hence, NDR feature is observed at 0.2 V. Other two NDR features follow the same reason. In order to have better understanding of the underlying mechanism of NDR behavior, we have followed the analysis made by Kuang et al. 78 and Zeng et al. 79 We have thus plotted the projected

12


Page 12 of 35

Page 13 of 35 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


density of states (PDOS) of the scattering region (SR), the left electrode (LE) and right electrode (RE) at bias voltages 0.0 V, 0.1 V and 0.2 V, as shown in Fig. 7(b). At bias voltage 0.1 V, we found that the PDOS of left electrode is very strong within the bias window. The figure suggests that the states of left electrode contribute to the DOS mainly as compared to the states of scattering region and right electrode. It is evident that the states of scattering region and the left electrode coupled strongly within the bias window, hence the transmission peak is very sharp (Fig. 7(a)) in the bias window. Thus, the current value is large. In contrast, the contributions of the states of left electrode and the scattering region within the bias window reduces sharply at bias voltage 0.2 V. The coupling between these two states weakened and we found reduced transmission function within the bias window, at bias voltage 0.2 V. This results in current drop and hence NDR effect. To explain the difference in conductance between ON and OFF state, we have calculated the transmission function at zero bias, as shown in Fig. 8(a). The results indicate that for enol form, the transmission spectra exhibits no significant peaks near the Fermi level. The peaks started to appear far from the Fermi level, that’s why the current carried by the enol is very small. In contrast, the keto form exhibits large number of spectra around the Fermi level. More precisely, keto isomer exhibits an appreciable transmission spectra in the wide energy range of -0.187 - -0.653 eV. We notice that, a strong peak corresponds to -0.549 eV energy value is expecting to be the main conducting channel. Similarly, a prominent transmission peak for enol form also seen at -0.597 eV. This certainly implies an important conducting channel for its conductivity. In order to provide more fundamental molecular origin of this characteristics, we have calculated the the spatial distributions of the frontier molecular orbitals for both ON and OFF states, as shown in Fig. 8(b). Notably, our aim is to find the main conductance channel responsible for conductivity. Thus, we have plotted spatial distributions of HOMO-3, HOMO-2, HOMO-1, HOMO and LUMO in Fig. 8(b). The HOMO levels are closer to the Fermi level than LUMO. These results are also found for phenyl, BiPh and TriPh dithiol molecule. 80–83 Hence, occupied molecular orbitals mainly contribute to the conductivity. In this context, we should mention that OFF state is an aromatic enol-hydrazone and ON state is a nonaromatic keto-enhydrazine. For both states, gold

13



electrode-molecule contact structures are same, only difference is in the molecular backbone due to tautomerism. Consequently, the presence of aromaticity in enol form may imply the π delocalization and we expect the increase in current value. Surprisingly, we found that the real situation is remarkably different. The conductance depends on two factors: (a) energetic alignment of frontier molecular orbitals with the Fermi energy of the electrode (b) effective molecule-electrode orbital overlap. 83 Ratner et al. 81 predicted that transport property of a molecular junction is determined by the molecular states whose energy lies closet to the metal Fermi level. In our studies, for both keto and enol, the HOMO levels or the occupied levels are closer to the metal Fermi level. Thus, we may say that this HOMO or occupied molecular energy levels are mainly responsible for molecular conductance of these two systems. The charge density plot in Fig. 8(b) further strengthen the explanation, as occupied energy levels are found to be the main conducting channels. Here, the orbital density is the key point to determine the current drop. We found that both leads have high orbital densities for both enol and keto form. At the same time for enol, the orbital density is highly localized on the S end group in the HOMO orbital. A weak coupling to the electrode is found and hence, transmission peaks are narrow. On the other hand, for keto the HOMO density is not only localized on S end group but it extends over the molecular backbone to some extent. This leads to strong coupling and as a result larger current and broad peak is observed in Fig. 6(a) and 8(a), respectively. Furthermore, to gain insight into the most prominent and stronger transmission peak for both, as shown in Fig. 8(a), we now analyze the lower frontier molecular orbitals. We found that HOMO-2 and HOMO-3 are the main conducting channels because of the large delocalization of orbital density over the whole molecular backbone as well in the electrode. This stronger coupling results in larger conductance. While, the large difference in transmission peaks in two forms is due to lack of orbital density distribution. The LUMO has less contribution to the conductance and have small transmission peaks because of high orbital density on gold electrode not on molecular backbone. Finally, we may conclude that presence of delocalization does not always signify larger current. However, larger the orbital density, larger will be molecule-electrode coupling and hence high conductance. So, difference in current is due to orbital density distribution in keto and enol

14


Page 14 of 35

Page 15 of 35 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


form. In this regards, Weckbecker et al. 73 have found keto and keto-enol forms exhibits OFF and ON states, respectively and this result is completely different from us.

Conclusion Conclusively, we have investigated the electronic properties of a newly synthesized covalentorganic framework, ACOF-1. Our results show that ACOF-1 exhibits direct-band gap semiconducting property with flat band characteristics near Fermi level. This is because of localization of VBM on azine-linkage and short-range resonance in CBM state, which is separated by nodes due to cross-conjugation effect. For the first time we reported the 1D structure of ACOF-1, which shows almost flat band direct-band gap semiconducting features. Dimensionality decrease and crossconjugation both explain the flat band features. Same spatial localization of charge density explore the constant band gap. These 1D COF may find its application for further advancement in the field of semiconductors. In addition, ACOF-1 has azine linkages as docking site to capture guest molecule within the pore through H-bonding interactions. We find, ACOF-1 can selectively detect the nitroaromatic explosives over nitroaliphatics through the modification of electronic structure after COF-explosive composite formation. Furthermore, ATFG-COF derivative of ACOF-1 exhibits keto-enol tatutomeric structure. Transport property calculation of the building block of keto-enol tatutomers of ATFG-COF present ON and OFF state of a switch, respectively, thus enabling conductance switching behavior. This conductance switching behavior can be utilized in molecular switching devices. Therefore, we hope that our works may inspire experimentalist to design the optimized situation in experiments to explain the multi-directional uses of ACOF-1 as semiconducting materials, molecular switchers and lastly explosive sensors.

Supporting Information Available Magnetic charge density (Fig. S1a), spin-resolved band structure (Fig. S1b) and charge density plot of antiferromagnetic 2×1×1 supercell of 1D COF (Fig. S1c-d). This material is available

15



free of charge via the Internet at http://pubs.acs.org/. Acknowledgments This paper is dedicated to Prof. S. P. Bhattacharyya, the Ph.D. supervisor of Prof. Pranab Sarkar on the happy occasion of his reaching 70. The financial support from DST NanoMission, Govt. of India, New Delhi, through research grant [Ref. No. SR/NM/NS-1005/2016] is gratefully acknowledged. Chandrima Chakravarty is grateful to CSIR, New Delhi for the award of Senior Research Fellowship (SRF)[CSIR Award No. 09/202(0051)/2015-EMR-I].

16


Page 16 of 35

Page 17 of 35 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


References (1) Côté, A. P.; Benin, A. I.; Ockwig, N. W.; O’keeffe, M.; Matzger, A. J.; Yaghi, O. M. Porous, crystalline, covalent organic frameworks. Science 2005, 310, 1166–1170. (2) Feldblyum, J. I.; McCreery, C. H.; Andrews, S. C.; Kurosawa, T.; Santos, E. J.; Duong, V.; Fang, L.; Ayzner, A. L.; Bao, Z. Few-layer, large-area, 2D covalent organic framework semiconductor thin films. Chem. Commun. 2015, 51, 13894–13897. (3) El-Kaderi, H. M.; Hunt, J. R.; Mendoza-Cortés, J. L.; Côté, A. P.; Taylor, R. E.; O’keeffe, M.; Yaghi, O. M. Designed synthesis of 3D covalent organic frameworks. Science 2007, 316, 268–272. (4) Klontzas, E.; Tylianakis, E.; Froudakis, G. E. Designing 3D COFs with enhanced hydrogen storage capacity. Nano Lett. 2010, 10, 452–454. (5) Uribe-Romo, F. J.; Hunt, J. R.; Furukawa, H.; Klöck, C.; O’Keeffe, M.; Yaghi, O. M. A crystalline imine-linked 3-D porous covalent organic framework. J. Am. Chem. Soc. 2009, 131, 4570–4571. (6) 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, 8875–8883. (7) Hunt, J. R.; Doonan, C. J.; LeVangie, J. D.; Côté, A. P.; Yaghi, O. M. Reticular synthesis of covalent organic borosilicate frameworks. J. Am. Chem. Soc. 2008, 130, 11872–11873. (8) Jackson, K. T.; Reich, T. E.; El-Kaderi, H. M. Targeted synthesis of a porous borazine-linked covalent organic framework. Chem. Commun. 2012, 48, 8823–8825. (9) Fang, Q.; Gu, S.; Zheng, J.; Zhuang, Z.; Qiu, S.; Yan, Y. 3D microporous base-functionalized covalent organic frameworks for size-selective catalysis. Angew. Chem. Int. Ed. 2014, 53, 2878–2882. 17



(10) Lukose, B.; Kuc, A.; Heine, T. Stability and electronic properties of 3D covalent organic frameworks. J. Mol. Model. 2013, 19, 2143–2148. (11) Doonan, C. J.; Tranchemontagne, D. J.; Glover, T. G.; Hunt, J. R.; Yaghi, O. M. Exceptional ammonia uptake by a covalent organic framework. Nature Chem. 2010, 2, 235–238. (12) Ding, S.-Y.; Gao, J.; Wang, Q.; Zhang, Y.; Song, W.-G.; Su, C.-Y.; Wang, W. Construction of covalent organic framework for catalysis: Pd/COF-LZU1 in Suzuki–Miyaura coupling reaction. J. Am. Chem. Soc. 2011, 133, 19816–19822. (13) Dogru, M.; Handloser, M.; Auras, F.; Kunz, T.; Medina, D.; Hartschuh, A.; Knochel, P.; Bein, T. A photoconductive thienothiophene-based covalent organic framework showing charge transfer towards included fullerene. Angew. Chem. 2013, 125, 2992–2996. (14) Dalapati, S.; Jin, S.; Gao, J.; Xu, Y.; Nagai, A.; Jiang, D. An azine-linked covalent organic framework. J. Am. Chem. Soc. 2013, 135, 17310–17313. (15) Wan, S.; Guo, J.; Kim, J.; Ihee, H.; Jiang, D. A belt-shaped, blue luminescent, and semiconducting covalent organic framework. Angew. Chem. 2008, 120, 8958–8962. (16) Pramudya, Y.; Mendoza-Cortés, J. L. Design principles for high H2 storage using chelation of abundant transition metals in covalent organic frameworks for 0–700 bar at 298 K. J. Am. Chem. Soc. 2016, 138, 15204–15213. (17) Mendoza-Cortés, J. L.; Han, S. S.; Furukawa, H.; Yaghi, O. M.; Goddard III, W. A. Adsorption mechanism and uptake of methane in covalent organic frameworks: theory and experiment. J. Phys. Chem. A 2010, 114, 10824–10833. (18) DeBlase, C. R.; Silberstein, K. E.; Truong, T.-T.; Abruña, H. D.; Dichtel, W. R. β Ketoenamine-linked covalent organic frameworks capable of pseudocapacitive energy storage. J. Am. Chem. Soc. 2013, 135, 16821–16824.

18


Page 18 of 35

Page 19 of 35 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


(19) Das, G.; Biswal, B. P.; Kandambeth, S.; Venkatesh, V.; Kaur, G.; Addicoat, M.; Heine, T.; Verma, S.; Banerjee, R. Chemical sensing in two dimensional porous covalent organic nanosheets. Chem. Sci. 2015, 6, 3931–3939. (20) Xiang, Z.; Xue, Y.; Cao, D.; Huang, L.; Chen, J.-F.; Dai, L. Highly efficient electrocatalysts for oxygen reduction based on 2D covalent organic polymers complexed with non-precious metals. Angew. Chem. Int. Ed. 2014, 53, 2433–2437. (21) Zhu, P.; Meunier, V. Electronic properties of two-dimensional covalent organic frameworks. J. Chem. Phys. 2012, 137, 244703. (22) Zhou, Y.; Wang, Z.; Yang, P.; Zu, X.; Gao, F. Electronic and optical properties of twodimensional covalent organic frameworks. J. Mater. Chem. 2012, 22, 16964–16970. (23) Yang, L.-M.; Pushpa, R. Tuning electronic and optical properties of a new class of covalent organic frameworks. J. Mater. Chem. C 2014, 2, 2404–2416. (24) Yang, L.-M.; Dornfeld, M.; Hui, P.-M.; Frauenheim, T.; Ganz, E. Ten new predicted covalent organic frameworks with strong optical response in the visible and near infrared. J. Chem. Phys. 2015, 142, 244706. (25) Lukose, B.; Kuc, A.; Frenzel, J.; Heine, T. On the reticular construction concept of covalent organic frameworks. Beilstein J. Nanotechnol. 2010, 1, 60. (26) Wang, R.-N.; Zhang, X.-R.; Wang, S.-F.; Fu, G.-S.; Wang, J.-L. Flatbands in 2D boroxinelinked covalent organic frameworks. Phys. Chem. Chem. Phys. 2016, 18, 1258–1264. (27) Pakhira, S.; Lucht, K. P.; Mendoza-Cortes, J. L. Iron intercalation in covalent–organic frameworks: A promising approach for semiconductors. J. Phys. Chem. C 2017, 121, 21160– 21170. (28) Gao, F.; Ding, Z.; Meng, S. Three-dimensional metal-intercalated covalent organic frameworks for near-ambient energy storage. Sci. Rep. 2013, 3, 1882. 19



(29) Er, D.; Dong, L.; Shenoy, V. B. Mechanisms for engineering highly anisotropic conductivity in a layered covalent-organic framework. J. Phys. Chem. C 2015, 120, 174–178. (30) Liu, Y.; Feng, Y.; Huang, Z.; Zhang, X. Thermal conductivity of 3D boron-based covalent organic frameworks from molecular dynamics simulations. J. Phys. Chem. C 2016, 120, 17060– 17068. (31) Han, M. Y.; Özyilmaz, B.; Zhang, Y.; Kim, P. Energy band-gap engineering of graphene nanoribbons. Phys. Rev. Lett. 2007, 98, 206805. (32) Han, M. Y.; Brant, J. C.; Kim, P. Electron transport in disordered graphene nanoribbons. Phys. Rev. Lett. 2010, 104, 056801. (33) Hwang, W. S.; Tahy, K.; Li, X.; Xing, H.; Seabaugh, A. C.; Sung, C. Y.; Jena, D. Transport properties of graphene nanoribbon transistors on chemical-vapor-deposition grown waferscale graphene. Appl. Phys. Lett. 2012, 100, 203107. (34) Chen, Z.; Lin, Y.-M.; Rooks, M. J.; Avouris, P. Graphene nano-ribbon electronics. Physica E: Low Dimens. Syst. Nanostruct. 2007, 40, 228–232. (35) Wen, J.; Luo, D.; Cheng, L.; Zhao, K.; Ma, H. Electronic structure properties of twodimensional π-conjugated polymers. Macromolecules 2016, 49, 1305–1312. (36) Gutzler, R.; Perepichka, D. F. π-Electron conjugation in two dimensions. J. Am. Chem. Soc. 2013, 135, 16585–16594. (37) Guo, J.; Xu, Y.; Jin, S.; Chen, L.; Kaji, T.; Honsho, Y.; Addicoat, M. A.; Kim, J.; Saeki, A.; Ihee, H. Conjugated organic framework with three-dimensionally ordered stable structure and delocalized π clouds. Nat. Commun. 2013, 4, 2736. (38) Zeng, J.; Chen, K.-Q. Spin filtering, magnetic and electronic switching behaviors in manganese porphyrin-based spintronic devices. J. Mater. Chem. C 2013, 1, 4014–4019.

20


Page 20 of 35

Page 21 of 35 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


(39) Pramanik, A.; Sarkar, P. Understanding the conductance switching of permethyloligosilanes: A theoretical approach. J. Chem. Phys. 2015, 143, 114314. (40) Pramanik, A.; Sarkar, P. Theoretical studies on the carrier tunability of oxidized oligothiophenes. Phys. Chem. Chem. Phys. 2015, 17, 26703–26709. (41) Cai, S.-L.; Zhang, Y.-B.; Pun, A. B.; He, B.; Yang, J.; Toma, F. M.; Sharp, I. D.; Yaghi, O. M.; Fan, J.; Zheng, S.-R. Tunable electrical conductivity in oriented thin films of tetrathiafulvalene-based covalent organic framework. Chem. Sci. 2014, 5, 4693–4700. (42) Stegbauer, L.; Schwinghammer, K.; Lotsch, B. V. A hydrazone-based covalent organic framework for photocatalytic hydrogen production. Chem. Sci. 2014, 5, 2789–2793. (43) Li, Z.; Feng, X.; Zou, Y.; Zhang, Y.; Xia, H.; Liu, X.; Mu, Y. A 2D azine-linked covalent organic framework for gas storage applications. Chem. Commun. 2014, 50, 13825–13828. (44) Soler, J. M.; Artacho, E.; Gale, J. D.; García, A.; Junquera, J.; Ordejón, P.; Sánchez-Portal, D. The SIESTA method for ab initio order-N materials simulation. J. Phys.: Condens. Matter 2002, 14, 2745. (45) Troullier, N.; Martins, J. L. Efficient pseudopotentials for plane-wave calculations. Phys. Rev. B 1991, 43, 1993. (46) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. (47) Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188. (48) Datta, S. Electronic transport in mesoscopic systems; Cambridge university press, 1997. (49) Porezag, D.; Frauenheim, T.; Köhler, T.; Seifert, G.; Kaschner, R. Construction of tightbinding-like potentials on the basis of density-functional theory: Application to carbon. Phys. Rev. B 1995, 51, 12947. 21



(50) Elstner, M.; Porezag, D.; Jungnickel, G.; Elsner, J.; Haugk, M.; Frauenheim, T.; Suhai, S.; Seifert, G. Self-consistent-charge density-functional tight-binding method for simulations of complex materials properties. Phys. Rev. B 1998, 58, 7260. (51) Niehaus, T. A.; Suhai, S.; Della Sala, F.; Lugli, P.; Elstner, M.; Seifert, G.; Frauenheim, T. Tight-binding approach to time-dependent density-functional response theory. Phys. Rev. B 2001, 63, 085108. (52) Aradi, B.; Hourahine, B.; Frauenheim, T. DFTB+, a sparse matrix-based implementation of the DFTB method. J. Phys. Chem. A 2007, 111, 5678–5684. (53) Elstner, M.; Hobza, P.; Frauenheim, T.; Suhai, S.; Kaxiras, E. Hydrogen bonding and stacking interactions of nucleic acid base pairs: a density-functional-theory based treatment. J. Chem. Phys. 2001, 114, 5149–5155. (54) Singh, A. K.; Penev, E. S.; Yakobson, B. I. Vacancy clusters in graphane as quantum dots. ACS Nano 2010, 4, 3510–3514. (55) Stegbauer, L.; Hahn, M. W.; Jentys, A.; Savasci, G.; Ochsenfeld, C.; Lercher, J. A.; Lotsch, B. V. Tunable water and CO2 sorption properties in isostructural azine-based covalent organic frameworks through polarity engineering. Chem. Mater. 2015, 27, 7874–7881. (56) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. Tables of bond lengths determined by X-ray and neutron diffraction. Part 1. Bond lengths in organic compounds. J. Chem. Soc., Perkin Trans. 2 1987, S1–S19. (57) McLoughlin, C.; Clyburne, J. A.; Weinberg, N. Azines: conjugation stoppers or conjugation switches. J. Mater. Chem. 2007, 17, 4304–4308. (58) Sutton, L. The interatomic bond distances and bond angles in molecules and ions. London Chem. Soc. London 1958, (59) Phelan, N. F.; Orchin, M. Cross conjugation. J. Chem. Educ. 1968, 45, 633. 22


Page 22 of 35

Page 23 of 35 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


(60) Hatanaka, M. Band structures of porous graphenes. Chem. Phys. Lett. 2010, 488, 187–192. (61) Liu, X.; Tan, J.; Wang, A.; Zhang, X.; Zhao, M. Electron spin-polarization and spin lattices in the boron-and nitrogen-doped organic framework COF-5. Phys. Chem. Chem. Phys. 2014, 16, 23286–23291. (62) Tapasztó, L.; Dobrik, G.; Lambin, P.; Biró, L. P. Tailoring the atomic structure of graphene nanoribbons by scanning tunnelling microscope lithography. Nat. Nanotechnol. 2008, 3, 397– 401. (63) Datta, S. S.; Strachan, D. R.; Khamis, S. M.; Johnson, A. C. Crystallographic etching of few-layer graphene. Nano Lett. 2008, 8, 1912–1915. (64) Berger, C.; Song, Z.; Li, X.; Wu, X.; Brown, N.; Naud, C.; Mayou, D.; Li, T.; Hass, J.; Marchenkov, A. N. Electronic confinement and coherence in patterned epitaxial graphene. Science 2006, 312, 1191–1196. (65) Son, Y.-W.; Cohen, M. L.; Louie, S. G. Energy gaps in graphene nanoribbons. Phys. Rev. Lett. 2006, 97, 216803. (66) Wang, C.; Tian, L.; Zhu, W.; Wang, S.; Wang, P.; Liang, Y.; Zhang, W.; Zhao, H.; Li, G. Dye@ bio-MOF-1 composite as a dual-emitting platform for enhanced detection of a wide range of explosive molecules. ACS Appl. Mater. Interfaces 2017, 9, 20076–20085. (67) Ray, D.; Dalapati, S.; Guchhait, N. Spectral properties of a simple azine Schiff base and its sensing ability towards protic environment through hydrogen bonding interaction. Spectrochim. Acta, Part A 2013, 115, 219–226. (68) Gole, B.; Shanmugaraju, S.; Bar, A. K.; Mukherjee, P. S. Supramolecular polymer for explosives sensing: role of H-bonding in enhancement of sensitivity in the solid state. Chem. Commun. 2011, 47, 10046–10048.

23



(69) Kim, T. K.; Lee, J. H.; Moon, D.; Moon, H. R. Luminescent Li-based metal–organic framework tailored for the selective detection of explosive nitroaromatic compounds: direct observation of interaction sites. Inorg. Chem. 2012, 52, 589–595. (70) Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. Detection of nitroaromatic explosives based on photoluminescent polymers containing metalloles. J. Am. Chem. Soc. 2003, 125, 3821–3830. (71) Odbadrakh, K.; Lewis, J. P.; Nicholson, D. M. Interaction of the explosive molecules RDX and TATP with IRMOF-8. J. Phys. Chem. C 2010, 114, 7535–7540. (72) Michoff, M. E. Z.; Castillo, M. E.; Leiva, E. P. A reversible molecular switch based on the biphenyl structure. J. Phys. Chem. C 2013, 117, 25724–25732. (73) Weckbecker, D.; Coto, P. B.; Thoss, M. Controlling the conductance of a graphene–molecule nanojunction by proton transfer. Nano Lett. 2017, 17, 3341–3346. (74) Zeng, J.; Chen, K.-Q.; Sun, C. Q. Electronic structures and transport properties of fluorinated boron nitride nanoribbons. Phys. Chem. Chem. Phys. 2012, 14, 8032–8037. (75) Ren, H.; Li, Q.-X.; Luo, Y.; Yang, J. Graphene nanoribbon as a negative differential resistance device. Appl. Phys. Lett. 2009, 94, 173110. (76) Mandal, B.; Sarkar, S.; Pramanik, A.; Sarkar, P. Theoretical prediction of a new twodimensional carbon allotrope and NDR behaviour of its one-dimensional derivatives. Phys. Chem. Chem. Phys. 2013, 15, 21001–21006. (77) Ma, J.; Yang, C.-L.; Wang, M.-S.; Ma, X.-G. Controlling the electronic transport properties of the tetrapyrimidinyl molecule with atom modified sulfur bridge. RSC Adv. 2015, 5, 10675– 10679. (78) Kuang, G.; Chen, S.-Z.; Yan, L.; Chen, K.; Shang, X.; Liu, P.-N.; Lin, N. Negative differential

24


Page 24 of 35

Page 25 of 35 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


conductance in polyporphyrin oligomers with non-linear backbones. J. Am. Chem. Soc. 2018, 140, 570–573. (79) Zeng, J.; Chen, K.-Q.; Tong, Y.-X. Covalent coupling of porphines to graphene edges: Quantum transport properties and their applications in electronics. Carbon 2018, 127, 611–617. (80) Xue, Y.; Ratner, M. A. Theoretical principles of single-molecule electronics: a chemical and mesoscopic view. Int. J. Quantum Chem. 2005, 102, 911–924. (81) Xue, Y.; Ratner, M. A. Microscopic study of electrical transport through individual molecules with metallic contacts. I. Band lineup, voltage drop, and high-field transport. Phys. Rev. B 2003, 68, 115406. (82) Xue, Y.; Ratner, M. A. Local field effects in current transport through molecular electronic devices: Current density profiles and local nonequilibrium electron distributions. Phys. Rev. B 2004, 70, 081404. (83) Cohen, R.; Stokbro, K.; Martin, J. M.; Ratner, M. A. Charge transport in conjugated aromatic molecular junctions: Molecular conjugation and molecule- electrode Coupling. J. Phys. Chem. C 2007, 111, 14893–14902.

25



Table 1: Lattice parameters, calculated bond lengths of optimized 2D COF Lattice constant (Å) dC5 −N (Å) dC1 −C5 (Å) dC1 −C2 (Å) dC2 −C3 (Å) dC3 −C4 (Å) dN−N (Å) 14.9873 1.2993 1.4635 1.4198 1.4061 1.4637 1.3872

26


Page 26 of 35

Page 27 of 35 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


pz

(a)

(b)

1.3872 Å

1.4198 Å 1.4061 Å

1.2993 Å

C5 C2C1 C4C3 1.4637 Å

Y

1.4635 Å Quasi-double bond

(c) sp2 hybridised double bonded C atom

C X

Two Single bonds Short range resonance

Figure 1: (a) Optimized structure of 2D covalent-organic framework (ACOF-1), unit cell is represented within the box. C, H and N atoms are denoted by grey, white and blue color, respectively. Different carbon sites are denoted as C1 , C2 , C3 , C4 , and C5 , respectively. Red circle area represents the cross-conjugated position. Different bond lengths are marked by blue arrows. (b) Schematic representation of planar diazabutadiene (C=N-N=C-) unit, sp2 hybridized orbitals are shown by red arrows. Red and blue arrow represents the lone pair-bond pair repulsion and quasi-double bond, respectively. (b) Schematic representation of short range resonance through -C=N-N=C- unit. Conjugation stopper single bonds are indicated by double line.

27



(a)

Page 28 of 35

(d)

1D

(e)

(c)

(b)

VBM

(f)

VBM

CBM

CBM

Figure 2: (a) Band structure plot of ACOF-1, indicating direct band gap at Γ point. The Fermi level is marked by black dashed line. Red and blue isosurfaces corresponds to (b) VBM and (c) CBM charge densities, respectively. (d) band structure plot of 1D derivative of ACOF-1, inset figure represents unit cell. (e) VBM and (f) CBM charge densities of 1D derivative, respectively. The isosurface value is 0.02 e/Å−3 .

28


Page 29 of 35 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


(a)

(b)

Cavity

TNT

DNT

TNR

TNP

Nitro -Aromatic (c) Docking unit, Lock guest molecule by H-bonding interaction

RDX

DMNB

HMX

PETN

Nitro -Aliphatic

Figure 3: (a) Schematic representation of cavity and docking unit for guest molecule capture of ACOF-1. (b)-(c) Different kinds of nitro-explosives.

29



(a)

Page 30 of 35

(e)

(b) (f) CBM@TNT

(c) CBM@RDX

VBM@TNT

(g)

(d)

VBM@RDX

Figure 4: (a) Total density of states (green) and partial density of states (PDOS) contributions from COF (blue) and nitroaromatic explosive (red). The black vertical dotted line represent Fermi level. (b) VBM and (c) CBM state of COF-TNT composite. (d) Optical absorption spectra of COF (black) and COF-TNT (red) composite. (e) Total density of states (green) and partial density of states (PDOS) contributions from COF (blue) and nitroaliphatic explosive (red). The black vertical dotted line represent Fermi level. (f) VBM and (g) CBM state of COF-RDX composite.

30


Page 31 of 35 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


Enol

Keto Top view

Top view

Side view

Side view

Tilt molecule

Left Electrode (LE)

Scattering Region (SR)

Right Electrode (RE)

Left Electrode (LE)

Scattering Region (SR)

Right Electrode (RE)

Figure 5: Schematic representation of two-probe systems (both top and side view) for transport calculation along z-axis of enol and keto form. The vertical dotted line indicate the central scattering region and three layers of Au atoms in each lead. Golden, yellow, gray, white, red and blue balls represent Au, S, C, H, O and N atom, respectively.

31



(a)

(b)

Oscillatory Variation Consistent with NDR

Figure 6: (a) I-V characteristics of ON (keto) and OFF (enol) state. Inset figure (b) ON/OFF switching ratio as a function of bias voltage. Red circle area represents corresponding current drop points as in I-V curve.

32


Page 32 of 35

Page 33 of 35 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


(a)

(b)

Figure 7: (a) Transmission functions at 0.0, 0.1 and 0.2 V, respectively. (b) PDOS of the scattering region (SR) and the electrodes (LE and RE) at bias 0.0, 0.1 and 0.2 volts. The zero energy refers to the Fermi energy. The region within blue dotted line indicates the bias window.

33



(b)

Page 34 of 35

Keto

Enol LUMO

(a) HOMO

HOMO-1

HOMO-2 Main conducting channels

HOMO-3

Figure 8: (a) Transmission coefficient at zero bias for keto and enol structures. Red and black color represent transmission coefficient of enol and keto, respectively. Fermi level is marked by blue dotted line. The positions of molecular orbitals of keto(enol) are marked by black(red) color. (b) The spatial distribution of HOMO-3, HOMO-2, HOMO-1, HOMO and LUMO at zero bias for enol and keto, respectively.

34


Page 35 of 35 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


Semiconductor

Conductance switching

Explosive sensing

OFF

2D-COF

Figure 9: TOC Graphic

35


Multi-Functionalities of an Azine-Linked Covalent-Organic Framework

Recommend Documents