Construction of Coordination Nanosheets Based on Tris(2,2

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Construction of Coordination Nanosheets Based on Tris(2,2'Bipyridine)-Iron(Fe2+) Complexes: As Potential Electrochromic Materials Manas Kumar Bera, Taizo Mori, Takefumi Yoshida, Katsuhiko Ariga, and Masayoshi Higuchi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22568 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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ACS Applied Materials & Interfaces

Construction of Coordination Nanosheets Based on Tris(2,2'-Bipyridine)-Iron(Fe2+) Complexes: As Potential Electrochromic Materials Manas Kumar Bera,† Taizo Mori,‡,§ Takefumi Yoshida,† Katsuhiko Ariga,‡,§ and Masayoshi Higuchi*,† † Electronic

Functional Macromolecules Group, Research Center for Functional Materials, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ World Premier International (WPI) Center for Materials, Nanoarchitectonics (NAMA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan ABSTRACT: The coordination nanosheets (CONASHs) are emerging as a new class of functional two-dimensional materials, which is one of the most active research areas of chemistry and physics in this decade. Despite the success of various structural and functional CONASHs, development of a new molecular structure to discover alluring functional CONASHs remains challenging. Herein, we report successful preparation of two novel CONASHs (NBP1 and NBP2) through coordination between one of the unexplored molecular frameworks of bis(2,2'-bipyridine) based ligands (BP1 and BP2) and Fe2+ ion. Using liquid-liquid interface as a platform, large-scale thin films of multi-layer CONASHs have been prepared without any support, which can be deposited onto any desired substrate. Detailed characterization of the CONASHs using various microscopic and spectroscopic techniques reveal homogeneous and flat morphology of nanometer thickness with quantitative formation of tris(2,2'-bipyridine)-Fe2+ complex motif in the nanosheet frameworks. The color of the films has been tuned from blue to magenta by suitable molecular design of the ligands. Owing to insolubility of the CONASH films in any solvent and presence of redox active Fe2+, we explore the functionality of these nanostructured thin films deposited on indium tin oxide (ITO) as electrochromic materials. The CONASHs exhibit color-to-colorless and color-to-color electrochromic transitions with attractive response times, switching stabilities, and colorations efficiencies. Finally, we demonstrate solid-state electrochromic devices of the CONASHs operated at a potential range from +2.5 V to -2.5 V, which are electrochemically stable up to several switching cycles, suggesting these CONASHs are potential electrochromic materials for next generation display applications. KEYWORDS: bipyridine-based ligand, nanosheets, interfacial Synthesis, iron, electrochromism, solid state electrochromic device

INTRODUCTION The nanosheets represent two-dimensional polymeric structural materials, which have received remarkable research interest in this decade because of their structural diversity and attractive functionalities.1-6 The iconic nanosheet is graphene with various unique, and useful properties, such as high carrier mobilities, large thermal conductivity, and high mechanical strength, which ensouled the emergence of two-dimensional materials as a new paragon in science and technology.7-8 The breakthrough of graphene research has boosted interest in the study of graphene alternative nanosheets, such as metal oxides,9-10 metal hydroxides,11-12 metal sulfides,13-14 boron nitrides,15-16 and silicon nanosheets,17-18 some of which possess additional interesting functionalities like semiconductivity,19 photoluminescence,14 and magnetism.18 These kind of nanosheets have been prepared by various synthetic protocols which are mainly based on two synthetic strategies,

top-down and bottom-up. The top-down approaches involve mechanical or chemical exfoliation of nanosheets from bulk-layered crystalline mother materials. Therefore, the structures and properties of top-down nanosheets are sealed by their constituent elements and difficult to modify at will. In contrast, bottom-up approaches involve direct preparation of nanosheets from molecular, atomic, and ionic components.20-29 The utmost advantage of bottom-up over top-down nanosheets is that their structures can be tailored by proper design and choice of the components, which may offer some attractive physicochemical properties. Recently, a new class of intriguing non-conjugated nanosheet materials have been developed, which is known as coordination nanosheets (CONASHs) that contain coordination complexes of organic ligand and metal ion.21,2325,30-32 Presence of a wide range of functional organic ligands and metal ions provide much room to regulate the

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Figure 1. (a) Chemical structure of the ligands (BP1 and BP2) with chemdraw structure of the CONASHs (NBP1 and NBP2). (b) Optimized structure by density functional theory calculation (DFT) of tris(2,2'-bipyridine)-Fe2+ complex unit present in the CONASHs indicating the octahedral coordination mode of Fe2+. (c) Schematic representation of the interfacial synthesis of CONASHs thin film. (d) Side (i & iii) and top view (ii & iv) of NBP1 and NBP2 thin film in a glass beaker synthesized by interfacial process.

molecular composition and/or the physicochemical properties of CONASHs. Researcher started the synthesis and structural analysis of CONASHs few years before, but in recent years, CONASHs have fascinated researchers due to their attractive potential functionalities. Literally, the exploration of the functionalities of CONASHs is just starting to blossom. For example, Schlü ter and co-workers addressed synthetic and analytical study of monolayer bottom-up CONASHs containing bis(terpyridine)metal(II) complexes at an air/water interface, which are able to exchange metal ions by transmetalation.20,27 Nishihara and co-workers first studied redox-modulated electrical conductivity of bottomup CONASHs and very recently, some other interesting functionalities of CONASHs containing the bis(dithiolene)metal(II),21,33 bis(dipyrrinato)metal(II),24,31 and bis(terpyridine)metal(II) complex motifs were demonstrated.23,30 Moreover, CONASHs can also exhibit electrocatalytic activities.32,34-35 Indeed, bottom-up CONASHs provide a very simple and straightforward way to fabricate thin film of the CONASHs with unlimited lateral dimension and desired thickness (single, few, and/or multilayers of

nanometer to micrometer), which could accelerate their comprehensive implementation in near future. Motivated by the above pioneer works, in the same pursuit, we are interested to develop new functional bottom-up CONASHs. Taking the advantage of the varieties in coordination modes and ligand design offered by bottom-up CONASHs, herein, we introduce one of the unexplored molecular building blocks based on bis(2,2'-bipyridine) derivative as coordinating ligand to prepare CONASHs. The 2,2'bipyridine is an attractive metal coordinating ligand due to its robust redox stability and ease of functionalization.36-38 Therefore, it has been extensively used as a versatile building block in supramolecular and macromolecular chemistry.39-46 Furthermore, 2,2'-bipyridine is a neutral ligand. For this, it forms charged complexes with metal cations which may contribute to exhibit some special functionality. Thus, we envision that such characteristic of 2,2'-bipyridine may be applied for the construction of functional bottom-up CONASHs. We have synthesized two bis(2,2'-bipyridine) derivatives (BP1 and BP2) to prepare CONASHs namely,


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NBP1 and NBP2, with Fe2+ as coordinating metal ion. A liquid-liquid (dichloromethane-water) interfacial polymerization strategy was adopted to prepare the CONASHs films, which can be deposited on any substrate like indium tin oxide (ITO), silicon wafer, and mica. The color of the films was varied from blue (NBP1) to magenta (NBP2) by changing the spacer between two 2,2'-bipyridine units in the ligands. The CONASHs were characterized by various spectroscopic and microscopic techniques, which reveal homogeneous and flat morphology with quantitative formation of tris(2,2'-bipyridine)-Fe2+ complex motif in the nanosheet frameworks. In a second part, utilizing the redox property of Fe2+ ion in CONASHs, we explore the potential functionality of these CONASHs as electrochromic (EC) materials. Electrochromism is a special functionality of materials, optical properties of which can be altered reversibly and persistently by the application of a small current or voltage.43-44,47-48 The EC materials have shown a widespread of promising applications, including information displays, smart windows for energy efficient buildings, self-dimming mirrors, and electrochromic e-skins.49-51 Polypyridyl-metal and related complexes are thought to be ideal chromophores to make EC materials because of their oxidation state dependent light absorption and remarkable stability.41,47,52-53 To date, a variety of materials (metal oxides, transition metal complexes, organic molecules, conducting polymers, and metallo-supramolecular polymers) have been developed as EC materials which involves various coating process including chemical vapor deposition,54 spin or spray coating,55-56 solgel,54 and LBL assembly to make electrochromic film on a conducting substrate.57 But, the bottom-up CONASHs are new addition in the group of EC materials which offer a simple and direct preparation of electrochromic film that can be transferred on any conducting substrate. Although, a few studies on synthesis and functionality of CONASHs have been done during last few years, there has been only one study introduced electrochromism phenomena of nanosheets based on terpyridine ligands (terpyridine is a tridentate ligand compared to bipyridine which is bidentate ligand), but without much insight of electrochromic properties like optical contrast (ΔT), coloration time (tc), bleaching time (tb), coloration efficiency (η), and durability of the solid state electrochromic device, which incompletely explore electrochromic properties of CONASHs.23 In this context, an electrochromic material should exhibit the properties of large color contrast, low switching times, long-term redox stability, and high coloration efficiency. Thus, a detailed investigation of electrochromic properties of bottom-up CONASHs is yet to be reported to establish these as a promising class of electrochromic materials. The EC properties of the CONASHs were investigated by transferring the films on ITOs, which exhibit reversible color-to-colorless (blue to colorless for NBP1) and color-to-color (magenta to yellow for NBP2) electrochromic change with low switching times, large color contrast, high coloration efficiencies, and long term reversible redox switching (for at least 1500 cycles). Furthermore, we demonstrate solid-state electrochromic devices with these newly developed bottom-up CONASHs as electrochromic layer. This work represents the first example of the preparation and characterization of bis(2,2'bipyridine) ligand-based bottom-up CONASHs and also,

explores their potential functionality as promising electrochromic materials.

RESULT AND DISCUSSION Synthesis and characterization of the CONASHs (NBP1 and NBP2). The 2,2'-bipyridine is a bidentate chelating ligand that has a strong coordination ability with various metal ions under mild reaction conditions.38 It can coordinate with d6 metal ions [e.g. Fe2+] in an octahedral fashion.45 Therefore, we have designed and synthesized bis(2,2'-bipyridine) derivatives, (trans)-1,2-bis(4'-methyl-[2,2'-bipyridin]-4-yl)ethene (BP1)58 and 1,4-bis((trans)-2-(4'-methyl[2,2'-bipyridin]-4-yl)vinyl)benzene (BP2), where two bipyridine units are separated by a vinyl and bis(vinyl)benzene spacer, respectively (Figure 1a). So, both sides of the ligands can make octahedral coordination complex with metal ion to build a 2D network structure as proposed in Figure 1a,b. The spacer between two bipyridine units of the ligands has been introduced to modulate optical properties of ligands and hence, of the corresponding metal complexs/polymers. The ligands were synthesized via the Wittig reaction59 and Horner-Wordsworth-Emmons (HWE) reaction60 and the details of step-wise synthetic procedure and characterization are given in experimental section (see section S-2,S-3, and S-7 in Supporting Information (SI)). For the synthesis of CONASHs, a room temperature liquid/liquid interfacial coordination reaction was followed between the ligands and Fe2+ ion, which has been schematically depicted in Figure 1c (see section S-2 in for the details of synthesis). Typically, three solvents layers were employed for the synthesis of CONASHs. The ligands (1 × 10-4 M) were dissolved in dichloromethane (DCM) as bottom layer, then pure water as middle layer, and the topmost layer was Fe2+ salt (as Fe(BF4)2·6H2O, 1 × 10-2 M) in water. The DCM and water are two immiscible solvents; thus, a liquid-liquid interface is created at the contacting surface of these two solvents. The system was kept in room temperature and reaction proceeds under static condition. After 24 h, thin blue and magenta color films of NBP1 and NBP2 were formed at the interface (Figure 1d). The CONASH films are insoluble in either water or organic solvent, which indicates the formation of network polymer structure as proposed in Figure 1a,b. The formation of the CONASHs was confirmed by different spectroscopic and microscopic techniques after transferring these on various substrate like silicon wafer, mica, and ITO (vide infra). The optical microscopy (OM) and scanning electron microscopy (SEM) images of the CONASHs (NBP1 and NBP2) were investigated on the ITO substrate, and silicon wafer, respectively. The OM and SEM images revealed flat, and homogeneous film-like morphology with cracks and wrinkles, indicating the formation of sheet like structure (Figure 2a,b, Figure 3a,b, Figure S2a,b). The transmission electron microscopy (TEM) images studied on carbon coated copper grid showed layered structure of the



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Figure 2. Microscopic and spectroscopic characterization of NBP1. (a) Optical microscopic image on ITO substrate, (b) SEM image, (c) TEM image, (d) AFM image and its height obtained from cross section analysis, (e) Elemental mapping for C, N, Fe, F, and B from TEM/EDX, (f) XP spectra focusing on the N 1s, Fe 2p, F 1s, and B 1s core levels, (g) UV-vis spectra (λmax, MLCT), (h) Scan rate dependent (5-50 mV) CV spectra of thin film deposited on ITO investigated in three electrode systems, and (i) Linear correlations between the peak current and the scan rate during oxidation (top) and reduction (bottom) process (R2 > 0.99 for fitting).

CONASHs (Figure 2c, Figure 3c, Figure S2c,d). The atomic force microscopy (AFM) imaging on silicon wafer also showed flat morphology and the height analysis from the silicon wafer to film surface revealed the thickness of the films of approximately 165 nm for NBP1 and 200 nm for NBP2 (Figure 2d, Figure 3d). The main constituent elements of the CONASHs framework are composed C, N, and Fe, whereas counteranion part made of F and B. The energydispersive X-ray spectroscopy (EDX) elemental mapping by TEM confirmed the presence of these elements in the CONASHs (Figure S2e,f). The Figure 2e, and Figure 3e showing the elemental mapping of C, N, Fe, F, and B, indicating that all the elements are homogeneously distributed in the nanosheets. The formation of the CONASHs was also examined by Xray photoelectron spectroscopy (XPS). To confirm the quantitative formation of nanosheets, we also prepared the mononuclear complex (MC) of Fe2+ with 4,4'-dimethyl-2,2'-bipyridine as reference (see section S-5 for details of synthesis). We measured XP spectra of the CONASHs, MC, and the ligands. The survey spectra of all compounds are given in Figure S4. The XPS analysis of the CONASHs focused on Fe 2p, N 1s, B 1s, and F 1s core levels are shown in Figure 2f, and Figure 3f. Upon coordination with Fe2+, the binding energy of N 1s peak of BP1 at 396.9 eV shifted to 398.9 eV in NBP1.23 Similarly, for NBP2, N 1s also shifted to 398.8 eV

compared with BP2 at 397.1 eV. The N 1s binding energy of the reference complex MC (398.7 eV) is almost same to that of CONASHs. The binding energies of Fe 2p core level of the CONASHs were also equal to that of MC (Figure S4). The XPS analysis revealed the calculated atomic ration of N/Fe was ~6.6:1 in NBP1, and ~6.3:1 in NBP2, which is much closure to the atomic ration of the reference complex MC (N/Fe ~ 6:1), suggesting quantitative formation of tris(2,2'-bipyridine)-Fe2+ complexes in the CONASHs. The XP spectra of NBP1, NBP2, and MC also indicated B 1s and F 1s peak of BF- which is present as counteranion. The calculated atomic ration of F/B in the CONASHs were ~3.8:1, ~4.6:1, which also close to the ratio of MC (F/B ~4.1:1). In order to get further confirmation about the formation of CONASHs, we used FT-IR and Raman analysis. The FTIR spectra indicated characteristic C=C stretching frequencies of BP1 and BP2 at 1590 and 1588 cm-1, which were shifted to higher wavelength of 1612 cm-1 for NBP1 and 1606 cm-1 for NBP2 (Figure S5a). These observations are consistent with other reported polypyridyl metal complexes.23, 61 Additionally, broad peak at around 1056 cm-1 which can be assigned for BF- anion, was observed in both the CONASHs. The IR spectra of the reference MC revealed the presence of 1606 cm-1 and 1056 cm-1 for C=C and BF- anion, confirming the complete formation of coordination complex in the CONASHs.


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Figure 3. Microscopic and spectroscopic characterization of NBP2. (a) Optical microscopic image on ITO substrate, (b) SEM image, (c) TEM image, (d) AFM image and its height obtained from cross section analysis, (e) Elemental mapping for C, N, Fe, F, and B from TEM/EDX, (f) XP spectra focusing on the N 1s, Fe 2p, F 1s, and B 1s core levels, (g) UV-vis spectra (λmax, MLCT), (h) Scan rate dependent (5-50 mV) CV spectra of thin film deposited on ITO investigated in three electrode systems, and (i) Linear correlations between the peak current and the scan rate during oxidation (top) and reduction (bottom) process (R2 > 0.99 for fitting).

Figure 4. (a) Schematic representation of monolayer NBP1 formation at air-water interface using Langmuir-Blodgett system. (b) AFM image with height profile from cross section analysis and (c) UV-vis spectra of monolayer NBP1.

The Raman spectra of the CONASHs showed several types of peak (including C=C stretching, C=N stretching, and ringbreathing) which are almost similar with the reported bipyridine-iron complexes.57 The major peaks for C=C stretching, C=N stretching, and ring-breathing for the ligands, MC, and CONASHs have been assigned in Figure S5b. A shifting of the ring-breathing, C=N, and C=C stretching peaks of non-coordinated ligand was observed upon complexation (in CONASHs).62 For example, the ring-breathing (995 cm-1), C=N stretching (1434 cm-1), and C=C stretching (1605, 1642 cm-1) peaks of BP1 shifted to 1019, 1481, 1542, and 1611 cm-1, respectively in NBP1. The Raman spectrum of MC was almost identical to that of NBP1 and NBP2, which also clearly indicates the formation of tris(2,2'-bipyridine)-Fe2+ complexes in CONASHs. The powder X-ray diffraction (PXRD) patterns of the CONASHs showed two peaks, one at 2θ = ~ 9.5° and ~ 9.2°, another broad peak at 2θ = ~ 20.5°, and ~ 20.1° for NBP1 and NBP2, respectively (Figure S6a). The broad peak at 2θ = ~ 20.5°, and ~ 20.1° could be indexed to the π-π stacking between successive layers of the CONASHs films. The π-π stacking distance between the successive layers of the CONASHs was calculated to be ~4.32 Å and ~4.41 Å for NBP1 and NBP2, respectively. Thermal stability of the CONASHs was monitored by thermogravimetric analysis (TGA) under N2 atmosphere. The TGA analysis revealed thermal stabilities up to 350 °C, indicating the high thermal stability of the CONASHs (Figure S6b).



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Figure 5. Electrochromic properties of NBP1 film in three electrode systems. (a) Transmittance spectra (at 590 nm), (b) Photographs of the film (1 cm × 1 cm) before (color state) and after oxidation (bleached state), (c) Transmittance change (ΔT, %) at different pulse width, (d) SEC switching stability (ΔT vs. time) up to 1500 cycles with a pulse width of 5 s; each segment corresponds to the first few switching cycles of every interval measurement, (e) Flow of current as a function of time during SEC switching stability measurement, and (f) Response time for coloration (tc), bleaching (tb). In each case, the applied voltage was switched between 1 and 0 V.

Optical and electrochemical properties of NBP1 and NBP2. The optical properties of the CONASHs (deposited film on ITO substrate) were monitored by UV-vis spectroscopy. The UV-vis spectra of NBP1 showed three peaks; 317 nm due to π-π* transition, 412, and 590 nm (λmax) which can be assigned to metal-to-ligand-charge-transfer (MLCT; dπ*) transition (Figure 2g, Figure S6d in SI). Similarly, NBP2 showed π-π* transition at 298 nm and d-π* MLCT transition at 378, and 568 nm (λmax) (Figure 3g, Figure S6d). The combination of two MLCT bands of the CONASHs are responsible for observed color (blue and magenta for NBP1 and NBP2, respectively). This kind of MLCT bands are quite similar to the other reported polypyridine-Fe2+-complexes.40, 57 The electrochemical properties of the CONASHs were investigated in a three electrode electrochemical cell where CONASH deposited ITO substrate as the working electrode, platinum flag as the counter electrode, and Ag/AgCl as the reference electrode in 0.1 M LiClO4 / CH3CN as electrolyte. The cyclic voltammetry (CV) study revealed a reversible redox characteristic of Fe2+/Fe3+ couple, with a half-wave potential (E1/2) of 0.68 and 0.61 V for NBP1 and NBP2 at a scan rate 50 mV/s (Figure S6e in SI). Scan rate dependent (5-50 mV/s) CV study of the CONASHs indicated that the peak current was linearly proportional to the scan rate (Figure 2h,i, Figure 3h,i,), which suggest a surface-confined electrochemical redox phenomenon that is not limited by slow diffusion.63

Synthesis of monolayer NBP1 and its thickness measurement by AFM. To get idea about the thickness of individual layer (monolayer) of the CONASHs, we synthesized monolayer NBP1 at an air-water interface using LangmuirBlodgett system.24 The detailed experimental procedure of monolayer synthesis is given in supporting information (section S-8) and a schematic presentation for stepwise synthesis is given in Figure 4a. In brief, a very small amount DCM solution (50 μL) of BP1 (2.7 × 10-3 M) was gently spread at ambient temperature on to an aqueous surface containing Fe(BF4)2·6H2O (1× 10-3 M). After prompt evaporation of DCM, coordination complexation happened between BP1 and Fe2+ ion at air-water interface, which produced monolayer NBP1. The resultant transparent monolayer could be horizontally transferred on various flat substrate like silicon wafer. To measure the thickness of monolayer, AFM analysis was performed which revealed flat and sheet like morphology. A topography and phase images of the monolayer have been shown in Figure S7a,b which showed uniform nature of film with some folded parts. The height profile measurement of monolayer gives average height of 2.4 nm, which agree reasonably well with the size of tris(2,2'-bipyridine)-Fe2+ complexes (Figure 4b, Figure S8 in SI). Considering the height of the monolayer, the no of layers in multilayer CONASHs described above could be estimated to be 66 layers in NBP1 and 80 layers in NBP2


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Figure 6. Electrochromic properties of NBP2 film in three electrode systems. (a) Transmittance spectra (at 568 nm), (b) Photographs of the film (1 cm × 1 cm) before (color state) and after oxidation (bleached state), (c) Transmittance change (ΔT, %) at different pulse width, (d) SEC switching stability (ΔT vs. time) up to 1500 cycles with a pulse width of 5 s; each segment corresponds to the first few switching cycles of every interval measurement, (e) Flow of current as a function of time during SEC switching stability measurement, and (f) Response time for coloration (tc), bleaching (tb). In each case, the applied voltage was switched between 1 and 0 V.

films. The monolayer formation was further confirmed by investigating in situ surface UV-vis spectroscopy. The UVvis spectra of monolayer indicated similar type of absorption as like multilayer NBP1. The monolayer showed three peaks, 371 nm due to π-π* transition, 464, and 600 nm (λmax) for metal-to-ligand-charge-transfer (MLCT; d-π*) transition (Figure 4c). The small differences in the peak positions of monolayer compared with multilayer NBP1, may be due to the different measurement conditions (air-water interface for monolayer vs. ITO substrate for multilayer of NBP1). Electrochromic properties of NBP1 and NBP2. The interfacial CONASHs film is insoluble in any solvent, which is more superior to use it as EC layer for fabrication of EC device compared with 1D polymers. The EC film preparation of 1D polymers involves synthesis of soluble polymer (in any solvent) followed by spin or spray coating process, which may sometime result redissolution in the media and thus, reduce the contrast of the device. But, no such kind of coating process is required for the CONASHs film preparation. It can be directly used as EC layer by transferring on any conducting substrate like ITO. One of the advantages of this interfacial synthesis is the control of film thickness by varying the reaction conditions (metal ion concentration or reaction time).23 For example, when dilute solution of Fe(BF4)2·6H2O (5 mM) was used for the interfacial synthesis,

the thickness of NBP1 was obtained as 78 nm with a reaction time of 24 h (Figure S9 in SI). The AFM image of this thin film also showed smooth and flat morphology. However, we opted the CONASHs films which were prepared using 10 mM Fe2+ salt with 24 h reaction time for electrochromic application. The EC properties of the CONASHs film (1 cm × 1 cm) were investigated by spectroelectrochemical (SEC) measurements in three electrode systems (CONASH containing ITO glass substrate as the working electrode, platinum flag as the counter electrode, and Ag/AgCl as the reference electrode in 0.1 M LiClO4 / CH3CN as electrolyte) by applying double potential steps (1 and 0 V) as a function of time (chronoamperometry) and in situ monitoring the percentage of transmittance change (%T) over time. The SEC measurement of NBP1 revealed a reversible change of %T upon oxidation-reduction by applying a potential of 1 and 0 V. It showed 62% optical contrast (ΔT) (Figure 5a) recorded at λ = 590 nm with reversible change in the color of the film between blue and colorless (Figure 5b). The ΔT can be altered as a function of the pulse width (0.25-20 s) as shown in Figure 5c, where retention of ΔT (>98%) was observed with a pulse width of 0.5-1 s. To check durability of the film, SEC switching stability was investigated by monitoring the optical contrast (ΔT) with a pulse width of 5 s for at least 1500 redox cycles. The first



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Table 1. Optical and electrochromic properties of NBP1 and NBP2 Maximum wavelength

Operation

(λmax, nm)

voltage (V)

Transmittance change (ΔT, %)

Coloration time (tc, s)

Bleaching time (tb, s)

Charge/discharge amount (Q, mC/cm2)

Coloration efficiency (η, cm2/C)

NBP1

590

0-1

62

0.48

0.57

1.16 / 1.51

431

NBP2

568

0-1

57

0.50

0.54

1.12 / 1.40

382

CONASHsa

a All values corresponding to the electrochromic properties (in three electrode systems) were averaged from three measured data

points with less than ±5% error.

Figure 7. (a) Structure of solid-state electrochromic device based on CONASHs. (b,e) Photographs of the electrochromic device (2 cm × 1.5 cm) before (color state) and after oxidation (bleached state), (c,f) SEC switching stability (ΔT vs. time) with a pulse width of 5 s, and (d,g) corresponding current flow through the device as a function of time for NBP1 (at λ = 590 nm) and NBP2 (at λ = 568 nm), respectively.

few switching cycles (ΔT vs. time) of every interval measurement are given in Figure 5d and the corresponding current changes with time are shown in Figure 5e. The results indicated that the change of ΔT was almost unaltered after 1500 cycles with steady flow of current. The coloration (tc) and bleaching times (tb) (defined as the time taken for 95% change of ΔT) were calculated to be 0.48, and 0.57 s (Figure 5f). The coloration efficiency (η) is a parameter for determining the quality of EC materials, which is related to the amount of optical density change (ΔOD) and injected /ejected electronic charge (Qd) by the following equation:

𝜂=

ΔOD 𝑇𝑏 = log /𝑄d 𝑄d 𝑇c

where Tb and Tc are the bleached and colored transmittance values, respectively.64 The Qd was calculated from charge/discharge curve (obtained by plotting change in current with time upon redox switching) (Figure S10a). The

amount of charge (during transformation from color to bleached state of the film by applying 1V) was calculated to be 1.16 mC/cm2 and the amount of discharge (during transformation from bleached to color state by applying 0 V) was calculated to be 1.51 mC/cm2. Using the above equation and the date summarized in Table 1, the η value for NBP1 was calculated to be 431 cm2/C. Such a high optical contrast (62 %) and coloration efficiency (431 cm2/C) are observed for the first time in 2D coordination polymer, which has rarely been observed in 1D or 3D type coordination polymers.65 Similarly, NBP2 showed 57% optical contrast (ΔT) recorded at λ = 568 nm with reversible color to color electrochromic change (Figure 6a). The initial color of the film was magenta (Fe2+) which becomes yellow upon oxidation (Fe3+) due to presence of strong transmission band at 412 nm (Figure 6b). The change of ΔT as a function of the pulse width (0.25-20 s) are shown in Figure 6c, where ΔT was unaltered (>98%) with a pulse width of 0.5-1 s. The durability


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of the film was very high, tested for at least 1500 switching cycles with a pulse width of 5 s. The first few switching cycles of ΔT and corresponding current changes indicated a steady change after several cycles (Figure 6d,e). The response times for coloration (tc) and bleaching (tb) were calculated to be 0.50 and 0.54 s (Figure 6f). From the charge/discharge curve, the amount of charge and discharge were calculated to be 1.12 mC/cm2 and 1.40 mC/cm2 (Figure S10b). Finally, the coloration efficiency (η) was calculated to be 382 cm2/C (Table 1). We further investigated the electrochromic color of NBP1 and NBP2 before and after oxidation by reflection color measurement. The colors are shown in CIE chromaticity diagram with chromaticity coordinates in Figure S11, which covers blue, colorless (before and after oxidation of NBP1), magenta and yellow region (before and after oxidation of NBP2). So, the CONASHs (NBP1 and NBP2) display very good EC properties with high color contrast and low response time for coloration and bleaching. The coloration efficiencies of the 2D CONASHs are also very high. The EC properties of these CONASHs are comparable to some of the best-performing EC materials, such as, inorganic oxides, organic polymers, metal-organic molecular assemblies etc. (see section S-11 in SI for comparison of EC properties of these CONASHs with different types of EC material).66-69 Solid state electrochromic devices. The solid-state electrochromic devices were fabricated using the as prepared NBP1 and NBP2 films. The details of device fabrication process have been given in experimental section (see section S1) and a schematic presentation is given in Figure S12. The devices were fabricated by sandwiching two ITO substrates; one ITO contained CONASH film (2 cm ×1.5 cm) and other semi-gel electrolyte (Figure 7a). An operating voltage of +2.5 V to -2.5 V was applied at two ITO electrodes using a DC voltage regulator. The NBP1-based device exhibited reversible color to colorless transition (blue to colorless) with 60% optical contrast (ΔT) (Figure 7b,c). The SEC switching stability measurement revealed that the device is stable for at least 300 redox cycles with constant current flow through the device over time (Figure 7c,d, Figure S13a,c). Similarly, NBP2-based device showed reversible color to color transition (magenta to yellow) with 61% optical contrast (ΔT) (Figure 7e,f) and it was also stable for at least 300 redox cycles with constant current flow through the device (Figure 7f,g, Figure S13b,d). The response times for coloration (tc) and bleaching (tb) were calculated to be 0.73 s, 1.44 s for NBP1, and 1.71 s, 2.5 s for NBP2 (Figure S13e,f). Both the devices showed electrochromic color change in an applied potential range from +2.5 V to -2.5 V, which indicates CONASHs are promising materials to fabricate low-voltage operated solid-state electrochromic devices. The durability of the CONASHs based devices is also high (for at least 300 redox cycles) which has been rarely observed for other types of electrochromic material (see section S-11 for comparison). This observation indicates that the CONASHs could be a potential electrochromic material for development of high durable solid-state EC devices.

CONCLUSIONS

In summary, we have successfully created two new bottom-up CONASHs (NBP1 and NBP2) based on a liquid-liquid interfacial coordination polymerization technique using bis(2,2'-bipyridine) derivatives (BP1 and BP2) as coordinating ligand with Fe2+ ion. A monolayer CONASHs has been also realized at air-water interface. The CONASHs were characterized by various microscopic (OM, SEM, TEM, and AFM) and spectroscopic (XPS, XRD, IR, Raman, UV-vis, and CV) techniques which indicated flat, uniform, and multilayer structure with thickness in nanometer range. Molecular design of the ligands allowed to control the optical characteristic of the CONASHs, for which the color of NBP1 and NBP2 is blue and magenta, respectively. The films are insoluble in any solvent and can be transferred on various substrate like silicon wafer, mica, and ITO. The metal center (Fe2+) in these CONASHs is redox active, which exhibit reversible redox behavior. The insolubility and redox activity of the films were favorable to disclose the functionality of these CONASHs as promising electrochromic materials. The electrochromic properties of the CONASHs deposited on ITO reveal a reversible color-to-colorless (blue to colorless for NBP1) and color-to-color (magenta to yellow for NBP2) transition with high optical contrast (up to 62 %), low switching times, and high coloration efficiencies (up to 431 cm2/C). The reversibility of the electrochromic transition is stable up to several cycles (for at least 1500 cycles). We have also shown successful fabrication of solid-state electrochromic devices using these CONASH films, which are operated at a potential range from +2.5 to -2.5 V and exhibit reversible electrochromic color change that is stable for at least 300 cycles. Overall, this work introduces a new molecular framework in the family of CONASHs and could lead to an initiation for further investigation on this framework. In addition, this study recognizes CONASHs for the first time as remarkable class of electrochromic materials. Moreover, the high durability of the CONASH film in solid-state EC device gives a hint that the CONASHs can compete as potential electrochromic materials for next generation display applications. Furthermore, there are immense new opportunities either in the design at molecular level of ligand or choice of various transition metal to discover various structural and functional CONASHs, which are currently under investigation in our group.

ASSOCIATED CONTENT Supporting Information. Materials, characterization details and additional experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (Dr. M. Higuchi)

ORCID ID Manas Kumar Bera: 0000-0003-4784-9530 Taizo Mori: 0000-0002-6974-5137 Takefumi Yoshida: 0000-0003-3479-7890 Katsuhiko Ariga: 0000-0002-2445-2955 Masayoshi Higuchi: 0000-0001-9877-1134

Funding Sources



This work was financially supported by CREST project (grant number: JPMJCR1533) from the Japan Science and technology Agency.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We sincerely Acknowledge the Materials Analysis Station, NIMS, Tsukuba, for providing the XPS facility. We acknowledge Namiki Foundry Station, NIMS, Tsukuba, for providing various instrumental facility. We also acknowledge Dr. Satish Laxman Shinde and Dr. Susanta Bera for support and helpful discussions.

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Table of Contents - 2.5 V to +2.5 V Glass

ITO Gel electrolyte

V

CONASH

ITO Glass NBP1

-2.5 V

+2.5 V

NBP2

-2.5 V

+2.5 V

Solid-State Electrochromic Device (2 cm x 1.5 cm)


13

Construction of Coordination Nanosheets Based on Tris(2,2

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