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A Naphthalenediimide-Based Metal−Organic Framework and Thin Film Exhibiting Photochromic and Electrochromic Properties Yi-Xin Xie,† Wen-Na Zhao,*,‡ Guo-Chang Li,† Peng-Fei Liu,† and Lei Han*,†,§ †

Institute of Inorganic Materials, School of Materials Science & Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, China ‡ Key Laboratory for Molecular Design and Nutrition Engineering of Ningbo, Ningbo Institute of Technology, Zhejiang University, Ningbo, Zhejiang 315100, China § Key Laboratory of Photoelectric Materials and Devices of Zhejiang Province, Ningbo University, Ningbo, Zhejiang 315211, China S Supporting Information *

groups,23,28−34 the tetrazole-based NDI derivatives have not been investigated as potential linkers to construct MOFs to date. Here, a novel zinc-based MOF single-crystal derived from an NDI-related tetrazole ligand (Scheme 1), [Zn(NDI-ATZ)

ABSTRACT: A multifunctional metal−organic framework, NBU-3, has been explored as a 2D three-connected network based on a naphthalenediimide-based ligand. The NBU-3 crystals display photochromic properties, and NBU-3 thin films on FTO substrates exhibit electrochromic properties. NBU-3 is the first example of MOF materials containing both photochromic and electrochromic properties, which can be desirable for thin film devices.

Scheme 1. Ligand NDI-ATZ

M

(DMF)2]n (NBU-3), is described, which displays a 2D planar structure, excellent thermal stability, and reversible photochromic properties. NBU-3 thin films were prepared on fluorine-doped tin oxide (FTO)-coated glass substrates, and their electrochromic properties were determined. The organic ligand NDI-ATZ was synthesized from 1,4,5,8naphthalenetetracarboxylic dianhydride and 5-aminotetrazole monohydrate in N,N-dimethylformamide (DMF) solution under a refluxing procedure at 140 °C. NBU-3 was successfully obtained as yellowish block crystals via the solvothermal reaction of NDI-ATZ and Zn(NO3)2·6H2O in DMF at 80 °C for 72 h (SI). Single-crystal X-ray crystallography analyses revealed that NBU-3 crystallized in the monoclinic P21/c space group, and the asymmetric unit contained one Zn center, one deprotonated NDI-ATZ, and two coordinated DMF molecules (Figure S1). Each Zn center was 5-coordinated to nitrogen and oxygen atoms, with 3 nitrogen atoms from tetrazoles and 2 oxygen atoms from DMF molecules. The Zn−O bond distances range from 2.106(5) to 2.151(4) Å, while the Zn−N bond distances are between 2.003(6) and 2.008(9) Å. Each NDI-ATZ linker has three nitrogen atoms involved in coordination. NBU-3 possesses a 2D planar structure (Figure 1a) and displays a 3-connected net topology containing both 4-member rings and 8-member rings (Figure S2). Due to the planar aromatic nature of NDI moiety, two neighboring NDI-ATZ molecules have strong face-to-face π−π stacking interactions between the conjugated π-deficient NDI rings. The centriod-to-centriod distance of the rings is 3.744(5) Å. This π-deficient functionality of NDI groups can be utilized to implement strong, directional π−π stacking

etal−organic frameworks (MOFs) have attracted much attention due to their routinely high porosity and surface areas.1−8 The tunability through either predesigned linkers or postsynthetic modifications makes MOFs great candidates for a variety of applications in the fields of gas storage,9 separation,10 catalysis,11 drug delivery,12 chemical sensing,13 and electronic devices.14 Therefore, a more desirable form has been developed to process MOFs as thin films, generally more desirable for use in smart devices.15,16 Recently, multifunctional MOFs crystalline or film materials have engendered enormous scientific interest in order to spread their applications in memory switch. Several combined nonlinear optical and ferroelectric,17 ferromagnetic and ferroelectric,18 resistive switching and ferroelectric,19 photochromic and photomagnetic20 MOFs have been extensively explored. However, the combinations of photochromic and electrochromic features have been little investigated.21−23 The studies on the photochromic and electrochromic MOFs showed that their photophysical and electrochemical properties could be tuned by changing the metal centers and organic linkers in such a way that light and redox reactions may be employed to affect reversible conversions between an array of states. Therefore, the use of color-switching, redox-active organic linkers might be considered for the design of desired photochromic and electrochromic MOFs materials. W describe a novel photochromic and electrochromic MOF and thin film (NBU-3) containing a naphthalenediimide (NDI) linker. NDI can undergo a one-electron reduction to form stable radical anions as redox-active units for studying photoinduced electron transfer, leading to design and construction of optoelectronically active MOFs.24,25 Although some MOFs have been described based on NDI organic ligands with carboxylic acid or pyridine moieties by our26,27 and other © XXXX American Chemical Society

Received: October 27, 2015

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DOI: 10.1021/acs.inorgchem.5b02480 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 2. (a, b) Photochromic effect of NBU-3 single crystals from photographic images. (c) ESR spectra of NBU-3 and NBU-3b. (d) Diffuse-reflectance UV/vis spectra of NBU-3 and NBU-3b. (e) Emission spectra of NBU-3 and NBU-3b.

Figure 1. (a) 2D structure of NBU-3. (b) TG curve of NBU-3. (c) NBU-3 thin films on FTO substrates. (d) PXRD patterns of NBU-3, NBU-3b, and NBU-3 thin films on FTO substrates.

interactions for crystal engineering. Thermogravimetric analysis (TGA) in flowing air indicated a weight loss of 23.93 wt % during heating from room temperature to 310 °C due to desorption of the 2-coordinated DMF molecules (calc. 23.89 wt %). The weight loss from 310 to 510 °C corresponded to decomposition of the coordinated NDI-ATZ ligand (Figure 1b). In light of electrochemical applications, fabrication of MOF thin films on conducting substrates is necessary.35−38 NBU-3 thin films were successfully prepared on FTO-coated glass using relatively simple solvothermal conditions (SI). In Figure 1c, scanning electron micrographs (SEM) of FTO substrates revealed a uniform coverage of the 640 nm thick NBU-3 thin films. Indeed, no distinct particles were observed by SEM, suggesting the presence of densely packed crystallites. Furthermore, the phase purity of NBU-3 and thin films were confirmed by powder X-ray diffraction (PXRD) (Figure 1d). NBU-3 was sensitive to sunlight and underwent a photochromic transformation from yellowish to dark green (NBU-3b) upon irradiation by sunlight for 15 min. NBU-3b was stable in air and returned to a yellowish color after being held in the dark for 1 h at room temperature (Figure 2a,b). Their diffuse-reflectance UV/vis spectra were different (Figure 2d), but PXRD revealed that the crystal structure of NBU-3b was identical to that of NBU-3 (Figure 1d). These indicated that their photoresponsive behaviors may have resulted from a charge-transfer chemical process and not from a structural transformation. The diffusereflectance UV/vis spectrum of NBU-3 displayed strong absorption bands at 220 and 333 nm, corresponding to n−π* and π−π* transitions of aromatic organic ligands (Figure S3). The diffuse-reflectance UV/vis spectrum of NBU-3b displayed a different broad absorption band in the 455−615 nm region, which may have been caused by a photoinduced charge-transfer transition.39 NDI is redox-active and can generate radicals upon irradiation by light. Aside from by charge-transfer, the photochromic process can be caused by photoinduced radical generation of organic ligands. Radical generation has been confirmed by electron spin resonance (ESR) spectra. NBU-3 exhibited no ESR signal, while NBU-3b had a single-peak radical signal with a g value of 2.0626 (Figure 2c).26 Photoluminescent properties of NBU-3 and NBU-3b were also investigated in the solid state at room temperature and exhibited similar emission peaks at 468 and 470 nm under excitation 322 nm (Figure 2e).

NBU-3 displayed a slight blue shift compared with the NDI-ATZ ligand emission (λem = 470 nm, λex = 328 nm) (Figure S4). These emissions could be attributed to ligand-to-ligand charge transfer (LLCT) related to π−π stacking interactions and/or ligand-tometal charge transfer (LMCT).26 Electrochromic performance of NBU-3 was also studied using NBU-3 thin films. Cyclic voltammetry (CV) measurements were carried out in DMF solutions containing 0.1 M [(nBu)4N]PF6 using a three-electrode electrochemical cell with an FTO thin film working electrode, platinum mesh as the counter electrode, and Ag/Ag+ the reference electrode. As shown in Figure 3, the film exhibited well-behaved redox events. The NBU-3 thin film exhibited reversible color change from colorless to yellow at E1/2 = −1.18 V, perhaps in response to the [NDI].−/2− redox couple. After 6 cycles, color change was no longer observed, possibly because of a loss of a small amount of the thin film and capacitor

Figure 3. (a−f) Images of observed color changes at NBU-3 thin film electrodes at reducting potentials. (g) Cyclic voltammograms of NBU-3 thin films measured at varying scan rate. (h) Cyclic voltammogram of NBU-3 thin film on FTO substrate after 6 cycles. B

DOI: 10.1021/acs.inorgchem.5b02480 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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attenuation (confirmed by PXRD after 6 cycles, Figure S5). CV measurements of the free NDI-ATZ ligand in solution using a glassy carbon button electrode showed fully reversible [NDIATZ].−/2− redox behavior at E1/2 = −1.13 V (Figure S6). The slight cathodic shift in the reduction potential NDI in the MOF film (approximately 50 mV) relative to the free ligand was consistent with greater negative charge density of the deprotonated ligand in the MOF material. In conclusion, a novel multifunctional 2D MOF material has been prepared based on a naphthalenediimide ligand. The NBU3 MOF crystals exhibit reversible photochromic properties varing in color from yellowish to dark green. Furthermore, NBU3 thin films with electrochromic properties were also successfully prepared. To our best knowledge, NBU-3 is the first example of a MOF material containing both photochromic and electrochromic features. These results provide a useful contribution to the design and construction of photochromic MOF materials based on photoinduced electron-transfer chemical processes. These materials can be used as potential electro-optical switches and memory devices in molecular electronics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02480.

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Experimental preparation, crystal structure, UV/vis and emission spectra, and cyclic voltammograms. (PDF) X-ray crystallographic data in CIF format. (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.-N. Zhao). *E-mail: [email protected] (L. Han). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (91122012, 21471086), Social Development Foundation of Ningbo (2014C50013), and K.C. Wong MagnaFund in Ningbo University.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02480 Inorg. Chem. XXXX, XXX, XXX−XXX