Multifunctionalities of an Azine-Linked Covalent Organic Framework

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2018, 122, 3245−3255

Multifunctionalities of an Azine-Linked Covalent Organic Framework: From Nanoelectronics to Nitroexplosive Detection and Conductance Switching Chandrima Chakravarty,† Bikash Mandal,‡ and Pranab Sarkar*,† †

Department of Chemistry, Visva-Bharati University, Santiniketan 731235, India School of Chemical Engineering, University of Ulsan, 93 Daehakro, Nam-Gu, Ulsan 44610, South Korea

‡

S Supporting Information *

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 (ACOF-1). 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 multifunctionalities of azine-linked COF (ACOF-1).

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INTRODUCTION Covalent organic frameworks (COFs) are well-defined, twodimensional (2D)1,2 or three-dimensional (3D)3−5 periodic crystalline materials. COFs are constituted by lightweight elements, such as C, H, N, B, O, and Si, which are combined via strong covalent bonds. Hence, they possesse low mass density and have permanent porosity with large surface areas, by means of which COFs find their applications in storage for gases, such as hydrogen, methane, carbon dioxide, etc.6 In addition, these materials are 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 multifunctional 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 is insignificant, indicating the robustness of COF’s intrinsic properties. Zhou et al.22 predicted that COF5 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 regard, Yang et al.23 explored how the alteration of elemental combination in X4Y can tune the band © 2018 American Chemical Society

gap of (X4Y)(O2B-C6H4-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 10 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 that the flat band characteristics in 2D boroxine-linked covalent organic frameworks are due to the delocalized π-conjugated electrons around phenyl ring and can be better understood by aromaticity theory. Pakhira et al.27 predicted that Fe-intercalation between two organic layers of COFs is a new strategy to obtain semiconducting property. Similarly, Meng and co-workers28 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 co-workers30 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 are key to applications of graphene. Thus, we may expect that 1D COF, a Received: November 25, 2017 Revised: January 27, 2018 Published: January 30, 2018 3245

DOI: 10.1021/acs.jpcc.7b11609 J. Phys. Chem. C 2018, 122, 3245−3255

Article

The Journal of Physical Chemistry C

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 gray, 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.

Li et al.43 very recently synthesized a stable azine-linked ACOF-1 by condensation of hydrazine hydrate and 1,3,5triformylbenzene 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 is 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 nitroaliphatic 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.

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 the 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 these electronic properties, COFs are also an exciting material for photocurrent 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 photocurrent switching with a large switching ratio of 8.0 × 10 4. Furthermore, Guo et al.37 have synthesized electronically conjugated CS-COF and found that it has high on−off photocurrent switching ratio. In this context, other interesting properties like magnetic and electronic switching behavior of manganese porphyrin-based model system were also investigated by Zeng et al.38 Despite this intensive research on photocurrent switching and magnetic and electronic switching, conductance switching39,40 behavior of COF has been 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 co-workers19 synthesized two new stable imide-based COFs, namely, TpBDH and TfPBDH. They showed that TfPBDH selectively detects nitroaromatic analytes over TpBDH, even for low analyte concentration. Thus, these are efficient fluorescence chemosensors. The use of COF is not limited to these applications only; it spread its wings in so many fields.41,42

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COMPUTATIONAL METHODS

Our density-functional theory (DFT) calculations were performed using SIESTA package.44 To represent the core and valence electrons, respectively, we have considered normconservative Troullier−Martins pseudopotentials45 and doubleζ plus polarization (DZP) basis set. To account for the electron−electron interactions, the generalized gradient approximation (GGA) in the Perdew−Burke−Ernzerhof46 (PBE) form is applied. A large vacuum space is considered along the 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 300 K. The tolerance for energy convergence is 0.001 eV. The k-point sampling for 3246


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The Journal of Physical Chemistry C 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 nonequilibrium Green’s function (NEGF).48 Similar sets of parameters such as basis functions, the exchange−correlation functional, and convergence criteria are used as our first-principle DFT calculations. In the NEGF self-consistent 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 sandwiching 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 been described elsewhere in detail.49−53 The total energy calculations are done by using self-consistent charge density functional tight-binding method (SCCDFTB)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 COFTNT composite system using density-functional theory (DFT) as implemented in SIESTA package.44 We have considered norm-conservative Troullier−Martins pseudopotentials45 and double-ζ plus polarization (DZP) basis set for representing core and valence electrons, respectively. The generalized gradient approximation (GGA) in the Perdew−Burke− Ernzerhof46 (PBE) form is applied to account for electron− electron interactions. A real space mesh cutoff of 200 Ry is used throughout the calculation, and the electronic temperature is set to 300 K. The tolerance for energy convergence is 0.001 eV. The optical mesh was set to 40 × 40 × 1.

Table 1. Lattice Parameter and 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

Å).56,57 The N−N bond length (1.3872 Å) is much smaller than the previously reported value (1.47 Å).58 These bond shortening and lengthening can be explained in the following manner. A closer look at Figure 1a reveals that diazabutadiene (−CN−NC−) unit is attached to the cross-conjugated59 position. As can be seen from Figure 1b, 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 Figure 1c. 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 ∠C5−N−N. ∠C5−N−N bond angle is 111.195°, much smaller than 120°. From Figure 1b it is clear that 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 lonepair−bond-pair repulsion, which squeezes the ∠C5−N−N. We now turn our attention toward the electronic properties of ACOF-1. Notably, the ground state of the studied system is nonmagnetic (NM). The band structure is plotted in Figure 2a. 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 Figure 2a, valence band maximum (VBM) and conduction band minimum (CBM) exhibit flat band characteristics. Moreover, these 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 Figure 2b and Figure 2c, respectively. Clearly, the charge densities of VBM state arise mainly from the azine (−N−N−) linkage of the framework, as shown in Figure 2b. 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 metaconnected 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. Hatanaka60 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

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RESULTS AND DISCUSSION The optimized structure of 2D covalent organic framework, ACOF-1, is shown in Figure 1a. The framework has hexagonal planar structure and displays idealized P6/m symmetry.55 The optimized lattice constant is 14.9873 Å, 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 1,3,5-triformylbenzene (B). Stegbauers and his coworkers55 named this COF as azine-benzene COF, in short, AB-COF. As shown in Figure 1a, 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 (1.4635 Å) bond length is much less than the C− C single bond distance (1.54 Å). The bond length of C5−N (1.2993 Å) is larger than the previously reported value (1.279 3247


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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 correspond 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.

considered to be unstable and we have calculated the electronic properties of 1D COF considering NM state. Before discussing the 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 Figure S1a in Supporting Information. Our results indicate that the magnetic moment mainly appears in −CN−NC− unit and −CN− moiety of edge. As can be seen from Figure S1a, each −CN− unit is a combination of up and down spin density; hence this is a singlet pair with total magnetic moment of zero. As a result, such singlet pairs result in the antiferromagnetic ground state. We further investigated the electronic properties of 1D derivative of COF in AFM ground state. The spin-resolved electronic band structure is plotted in Figure S1b. 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 exhibits almost flat band characteristics around the Fermi level. In order to explain these characteristics, we have plotted charge density, as shown in Figure S1c,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 Figure 2d, with band gap value of 2.1787 eV. The flat band feature around Fermi level can again be understood via charge density plot, as shown in Figure 2e,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 to flat band in CBM state. Quite

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-hexahydroxytriphenylene (HHTP) and 1,4-phenylenebis(boronic acid) (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-triformylbenzene unit. According to such result of COF-5, it presents type II band alignment and becomes a promising candidate for performance in photovoltaic cell. Furthermore, Zhou et al.22 showed 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 lacks 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 Figure 2d shows the unit cell structure of 1D derivative along the periodic X-direction. The unit cell is same as the sheet; the only difference is that 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 KBT (KB = Boltzmann constant; T = 300 K). Hence, AFM state is 3248


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

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 represents 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 represents Fermi level. (f) VBM and (g) CBM state of COF-RDX composite.

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, which means it remains almost the 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 increasing the band gap; (ii) short-range direct conjugation that decreases the band gap value. For both 2D and 1D, the wave functions corresponding to VBM and CBM states are the same. Hence, cross-conjugation and shortrange direct conjugation have almost similar impact on the band gap. Consequently, the band gap remains almost the same. Thus, we conclude that 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 that reveal that COFs have been widely used in chemical sensing of nitroexplosives. Structurally, our studied system consists of azine-linkage, which acts 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 also be able to capture guest molecules through H-bonding interactions, as shown in Figure 3a. 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, for example, TNT, TNP, DNT, and TNR; (ii) nitroaliphatics, such as RDX, DMNB, HMX, PETN. First, the isolated explosive molecules are optimized. The optimized structures are addressed in Figure 3b and Figure 3c, respectively. In order to investigate the effect of these 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 3249


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Figure 5. Schematic representation of two-probe systems (both top and side view) for transport calculation along z-axis of enol and keto forms. The vertical dotted line indicates 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.

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 Figure 4e. 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 remains 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 COFRDX is addressed in Figure 4f and Figure 4g, which confirms that VBM is localized on azine-linkage and CBM is dispersed over some space of COF. Interestingly, comparison of Figure 2b,c and Figure 4g,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 explored 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 also has 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, exhibits an reversible tautomeric structural equilibrium in two forms: one is −OH and other one is −NH form. For sake of convenience, we referred −OH and −NH forms as enol and keto forms, 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

in Figure 4a and Figure 4e, 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 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 appears 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. Figure 4b and Figure 4c 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 maybe leading 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 resulting in a red shift. We have calculated the optical properties of isolated COF and COF-TNT composite, as shown in Figure 4d. 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 plays a crucial role. The hydroxyl (−OH) group participates in H-bonding interactions with the azinelinkage (−N−N−) of COF.67−70 Nitro group has 2-fold functionality; it controls the electron deficiency of benzene ring, and thus structurally, TNR is the most electron deficient in this series. On the other hand, the strength of acidity depends on the presence of NO2 group, which in turn affects the Hbonding interactions. Thus, most electron deficient TNR facilitates strong interactions with azine linkage, which 3250


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exhibit remarkably distinct I−V curves. Especially, the keto form has significantly large current value as compared to enol form. Not only that, keto form shows small negative differential resistance74 (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 Figure 6b. 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 Figure 6a. 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 suitable 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, 0.1, and 0.2 V, respectively, in Figure 7a. 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 Figure 7a. 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 density of states (PDOS) of the scattering region (SR), the left electrode (LE), and right electrode (RE) at bias voltages 0.0, 0.1, and 0.2 V, as shown in Figure 7b. At bias

ATFG-COF, we have considered a molecular building block of ATFG-COF as a representative model system. This moiety can exist in two tautomeric forms, which are referred to as enol and keto, respectively, which are being interconverted through proton transfer. Previous work of Weckbecker et al.73 motivated us to choose these molecular building blocks. To test whether the keto and enol moiety can fulfill the abovementioned criteria, we have calculated the transport properties of such model systems, as schematically represented in Figure 5. Their current−voltage (I−V) characteristics are shown in Figure 6a. The most striking result is that enol and keto forms

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

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 V. The zero energy refers to the Fermi energy. The region within blue dotted line indicates the bias window. 3251


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Figure 8. (a) Transmission coefficient at zero bias for keto and enol structures. Red and black colors 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) Spatial distribution of HOMO−3, HOMO−2, HOMO−1, HOMO, and LUMO at zero bias for enol and keto, respectively.

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 Figure 8b further strengthens 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 not only is localized on S end group but extends over the molecular backbone to some extent. This leads to strong coupling, and as a result larger current and broad peak are observed in Figures 6a and 8a, respectively. Furthermore, to gain insight into the most prominent and stronger transmission peak for both, as shown in Figure 8a, 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 as 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 has 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, the larger is the orbital density, the larger will be molecule− electrode coupling and hence high conductance. So the

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 (Figure 7a) 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 reduce 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 Figure 8a. The results indicate that for enol form, the transmission spectra exhibit no significant peaks near the Fermi level. The peaks started to appear far from the Fermi level, which is 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 to −0.653 eV. We notice that a strong peak corresponding to −0.549 eV energy value is expected to be the main conducting channel. Similarly, a prominent transmission peak for enol form is 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 spatial distributions of the frontier molecular orbitals for both ON and OFF states, as shown in Figure 8b. 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 Figure 8b. 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 electrode−molecule contact structures are same; the only 3252



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difference in current is due to orbital density distribution in keto and enol forms. In this regard, Weckbecker et al.73 have found that keto and keto−enol forms exhibit OFF and ON states, respectively, and this result is completely different from us.

CONCLUSION We have investigated the electronic properties of a newly synthesized covalent organic framework, ACOF-1. Our results show that ACOF-1 exhibits direct band gap semiconducting property with flat band characteristics near the Fermi level. This is because of localization of VBM on azine-linkage and shortrange 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 directband gap semiconducting features. Dimensionality decrease and cross-conjugation both explain the flat band features. The same spatial localization of charge density explores the constant band gap. The 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 that 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 calculations 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 work may inspire the experimentalist to design the optimized situation in experiments to explain the multidirectional uses of ACOF-1 as semiconducting materials, molecular switchers, and lastly explosive sensors. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b11609. Magnetic charge density (Figure S1a), spin-resolved band structure (Figure S1b), and charge density plot of antiferromagnetic 2 × 1 × 1 supercell of 1D COF (Figure S1c,d) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pranab Sarkar: 0000-0003-0109-6748 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from DST NanoMission, Government of India, New Delhi, through Research Grant SR/NM/NS-1005/ 2016 is gratefully acknowledged. C.C. is grateful to CSIR, New Delhi, for the Award of Senior Research Fellowship (SRF) [CSIR Award 09/202(0051)/2015-EMR-I]. 3253


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Multifunctionalities of an Azine-Linked Covalent Organic Framework

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