X-ray and UV Dual Photochromism, Thermochromism

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X-ray and UV Dual Photochromism, Thermochromism, Electronchromism and Amine-Selective Chemochromism in an Anderson-like Zn7 Cluster Based 7-Fold Interpenetrated Framework Shi-Li Li, Min Han, Yan Zhang, Guo-Ping Li, Mei Li, Gang He, and Xian-Ming Zhang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b04930 • Publication Date (Web): 21 Jul 2019 Downloaded from pubs.acs.org on July 21, 2019

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Journal of the American Chemical Society

X-ray and UV Dual Photochromism, Thermochromism, Electronchromism and Amine-Selective Chemochromism in an Anderson-like Zn7 Cluster Based 7-Fold Interpenetrated Framework Shi-Li Li,† Min Han†, Yan Zhang†, Guo-Ping Li‖, Mei Li†, Gang He‖* and Xian-Ming Zhang†,‡* †

Key Laboratory of Magnetic Molecules & Magnetic Information Materials (Ministry

of Education), Institute of Chemistry and Culture, School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, P. R. China ‡Institute

of Crystalline Materials, Shanxi University, Taiyuan 030006 P. R. China

‖Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi Province, 710054 P. R. China ABSTRACT: Smart materials are highly desirable over the recent decade due to the growing demand on complicated nature. Stable stimuli-responsive smart materials exhibit widespread potential for applications in smart windows, sensors, separators, chemical valves and release platforms but are rare. In spite of being good candidates, viologen-based multifunctional smart materials are still a challenging task for chemists. To obtain such materials, the judicious strategy is to introduce polynuclear metal-carboxylate clusters as electron donors into stable framework to increase chromic sensitivity. Towards this endeavor, we have synthesized a novel viologen-based polymer with a unique Anderson-like metal-carboxylate cluster, namely [Zn7(bpybc)3(o-BDC)6]·2NO3·6H2O (bpybc = 1,1 ’ -bis(4-carboxyphenyl)-

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4,4’-bipyridinium, o-BDC = o-benzenedicarboylic acid) (1), which is a particular 7-fold interpenetrated framework with 3D pcu network in which bpybc ligand as linker and Zn7O30C12 second building unit (Zn7 SBU) as 6-connected nodes. More importantly, it shows excellent chromic behavior in response to multiple external stimuli especially soft X-ray and UV dual light, temperature, electricity and organic amines, which stand out in the viologen-based polymers. Interestingly, the coloration process of 1 from “core” to “edge” is observed upon heating at the appropriate temperature, which has not yet been found in other reported thermochromic materials. Of particular interest for 1 is the couple of quaternary stimuli sensitive abilities because it simultaneously meets the following condition: (i) the capability of withstanding high light, higher temperature, extreme of pH, and other harsh conditions; (ii) the high sensitivity to external stimuli keeping away from photodegradation, thermal relaxation, side reactions and so on. To be noted, 1 has the high thermal stability and chemical stability, which are the excellent advantages as smart materials. To further develop possible practical utilization, 1 has been doped into the polymer matrices to construct a hybrid film, which not only keep the response to external stimuli but also significantly improve the repeatability of the photochromic process, indicating that a new smart device with multistimuli-responsive functions will emerge successively in future. KEYWORDS: 1. INTRODUCTION Driven by the need to develop intelligent technology, stimuli-sensitive materials


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have received tremendous scientific attention in various applications such as mimicking of photosynthetic solar energy conversion, molecular shuttles, recyclable catalysis, sensing, coating, separation, nanotechnology, biology, tissue engineering, energy harvesting, secret writing, smart window, rewritable copy paper, controlled drug delivery vehicle and so forth.1-7 Of various stimuli-sensitive materials, crystalline chromic materials such as diarylethenes,8-9 benzo[b]phosphole,10 spiropyran,11 spirooxazine,12 viologens,13-14 and azobenzene compounds15-16 with reversible color changes have attracted a great deal of interest,17-18 because of their highly designable and controllable geometric structures, the convenient and efficient chromic behavior and the readable corresponding signals with regenerated and reused characters in response to external chemical or physical stimuli.19 Among various crystalline chromic materials, viologens with electron-deficient nature, whose color can be adjusted via chemical, electrical or optical stimuli, are well-known for their great potentials in smart technologic applications.20-21 With the assistance of electron-rich species, they can undergo two step redox process and exhibit three different oxidation states (neutral V0, radical cation V•+, dication V2+) via the electron transfer from electron-donating inorganic ions or organic ligands to electron-deficient viologen components of frameworks.22-23 These three different oxidation states own disparate electronic dissociation, which lead to different electronic absorption bands, accompanied with the visible color change.24 On the basis of the character, viologen materials can directly display a revisable and visible color change in response to the external stimuli such as pH, heat, light irradiation, electric potential, solvent and



organic amines.25-26 Unfortunately the most known viologen based chromic materials only respond to mono-stimulus. Actually, the natural stimuli are multiple and generally include light, electricity, heat and chemical.27 Thus, the known viologen based materials have limited application. Another important issue in the field is how to obtain a viologen material with a fast responsibility and high stability since reported viologen materials usually show a slow responsive rate (photochromism time usually more than 10 min under light irradiation and the fading of color from several hours to months, thermochromism time generally more than one hour and the fading time taking much longer) and poor stability (the coloration usually unstable).28-29 Development of a fast and stable multi-stimuli responsive viologen-based hybrid remains a tough task to meet with practical applications. To develop stable and fast multi-stimuli responsive materials, the judicious strategy is to incorporate the viologen derivatives into the excellent electron donors such as metal-carboxylate and polyoxometalate clusters and assemble the interpenetrating structural motif for stabilizing the reduced radicals and creating multi-bridges for electron transfers.30-31 Both polyoxometalate and metal-carboxylate clusters have inherent rich-electron groups, good redox activity, photo-activity, and excellent stability, which is beneficial to transfer electron to viologen to reach chromic properties.32-33 The metal-viologen materials constructed by polyoxometalate clusters such as Keggin or Anderson type clusters as electron donors generally have the dark color, thus their color change cannot be easily detected by naked eyes.21 While metal-viologen materials constructed by metal-carboxylate clusters usually possess


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the obvious color change, the reasons are as follows: i) almost no electronic absorption bands are observed in the visible region before exposure to the external stimuli;34 ii) many electron-transfer pathways are good at the colored free radicals generated under different stimuli.28,

35

When viologens are dispersed in aprotic

metal-organic framework, generated viologen radical cations can be stabilized by the surrounding solid matrix due to restriction of thermal back electron transfer and air oxidation.36 In order to pursue a stable and fast multi-stimuli responsive chromic material, d10 Zn ion and o-benzenedicarboylic acid (o-H2BDC) are chosen to construct the electron-rich zinc-oxide clusters (ZnO clusters) which are favorable for increasing the

sensitivity

to

external

stimuli,37

and

the

flexible

viologen

ligand

(1,1’-bis(4-carboxyphenyl)-4,4′-bipyridinium, bpybc) is chosen as an electron acceptor to interact with electron-rich ZnO clusters. As expected, a seven-fold interpenetrated

Anderson-like

cluster

based

Zn(II)

MOF,

namely

[Zn7(bpybc)3(o-BDC)6]·2NO3·6H2O (1), can exhibit a stable and fast response to various external stimuli, especially soft X-ray, UV-light, heat, electricity and some organic amines, mainly due to that the abundant O atoms in the Anderson-like clusters are favorable for forming much more electron transfer pathways and further improving the possibility for the effective electron transfer under the external stimuli. To be noted, sample 1 has the high thermal stability and chemical stability, which are the excellent advantages as smart materials for far-reaching application. Thus, to further develop the utilization value, sample 1 has been doped into the polymer matrices to construct a new poly(methyl methacrylate)-supported (PMMA) hybrid



film, which not only keep the response to external stimuli but also significantly improve the repeatability of the photochromic process. 2. EXPERIMENTAL SECTION 2.1. Synthesis and Characterization. Compound 1 was prepared by a solvothermal reaction of Zn(NO3)2.6H2O, o-benzenedicarboylic acid (o-H2BDC), 1,1’-bis(4-carboxyphenyl)-4,4′-bipyridinium dichloride (H2bpybcCl2) in a mixture solvents of dimethylformamide (DMF), ethanol (EtOH), and water. The ligand H2bpybcCl2 was synthesized according to previously reported literature.38 Synthesis of 1: Zn(NO3)2·6H2O (0.149 g, 0.5 mmol), o-H2BDC (0.035 g, 0.2 mmol), H2bpybcCl2 (0.099 g, 0.2 mmol) were dissolved in a mixture of DMF (2 mL), EtOH (2 mL) and H2O (2 mL). The pH value was adjusted to 7 with NaOH solution. The solution was placed in a 15 ml Teflon-lined stainless container, which was then heated at 90 ºC for 7 days and then slowly cooled to room temperature at 5 C/min. After being filtered off and dried at room temperature, 1 was obtained as a yellow block crystal in 40% yield based on H2BpybcCl2. Anal. Calcd (%) for 1 C126H118N7O56Zn7: C, 49.07; H, 3.86; N, 3.18. Found: C, 48.96; H, 3.83; N, 3.19. IR (KBr, cm−1): 3442(s), 3117(w), 2346(w), 2031(w), 1600(s), 1381(s), 1121(s), 812(w), 759(m), 668(w), 615(m), 551(w), 471(w). The solid-state UV−vis diffuse-reflectance spectrum was determined on a TU-1901 spectrophotometer. Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku MiniFlex II X-ray diffractometer. Thermogravimetric (TG) analysis data were carried out on a SETARAM LABSYS equipment. Elemental analyses were recorded


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on a vario EL-II analyzer. Fourier transform infrared (FT-IR) spectra were performed on a Nicolet 5DX spectrometer. Photochromic properties were determined upon ultraviolet light irradiation (22.4 mw/cm2). Cyclic voltammetry (CV) was performed on a CHI 660E electrochemical analyzer (Shanghai). Electron paramagnetic resonance (EPR) spectra were performed using a Bruker EMX spectrometer. 2.2. X-ray Crystallography. X-ray single-crystal diffraction data were performed on an Agilent Technologies Gemini EOS diffractometer at 293(2) K with Cu−Kα radiation (λ = 1.5418 Å). Pertinent crystallographic data and structural results for 1 and 1P are summarized in Table 1. Table 1. Crystallographic Data and Structural Refinement for 1 Compound

1

1P

Empirical

C126H118N7O56Zn7

C126H118N7O56Zn7

Fw

3084.03

3084.03

Crystal system

trigonal

trigonal

Space group

R-3

R-3

a (Å)

19.8583(4)

19.8621(4)

b (Å)

19.8583(4)

19.8621(4)

c (Å)

30.3819(8)

30.3966(7)

α (deg)

90

90

β (deg)

90

90

γ (deg)

120

120



aR = 1

V (Å3)

10376.0(5)

10385.0(5)

Z

3

3

calc,(g cm-3)

1.356

1.355

, (mm-1)

2.002

2.000

F(000)

4320.0

4320.0

Reflections

11214/4553

10538/4553

Crystal size(mm)

0.23×0.21×0.16

0.23×0.21×0.16

Tmax/Tmin

1.00000/0.88822

1.00000/0.82574

Data/parameters

4553/261

4553/281

S

1.068

1.068

R1a

0.0491

0.0455

wR2b

0.1686

0.1557

max/min(eÅ-3)

0.655/-1.428

0.512/-1.572

Fo-Fc/Fo. bwR2 = [[w(Fo2-Fc2)2]/[w(Fo2)2]]1/2.

3. RESULTS AND DISCUSSION 3.1. Description of Crystal Structures. Single-crystal X-Ray diffraction studies show that compound 1 has an Anderson-like Zn7 cluster based 7-fold interpenetrated 3D structure. It crystallizes in trigonal space group R-3, and there are two crystallographically independent Zn ions, half bpybc ligand, one o-BDC2– ligand and one lattice water molecule in the asymmetric unit (Figure 1a). Zn ions display two types of coordination environments.


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Zn(1) shows a slightly distorted tetrahedron coordination sphere, coordinated by four monodentate carboxyl oxygen atom from one bpybc ligand and three different o-BDC2– ligands (Figure 1a). Zn(2) ion in a special position lies on a 3-fold axis with a site occupancy of 1/6, coordinated by six monodentate carboxylate oxygens from o-BDC2– ligands, in which the axial position are occupied by two carboxyl oxygen atoms with O-Zn-O angle of 180° and the equatorial plane is occupied by four oxygen atoms with O-Zn-O angle in the range of 88.95(8)–91.05(8)°, indicating a slightly distorted octahedral coordination geometry. As shown in Figure 1b, Zn(2) as a center is surrounded by six adjacent Zn(1) ions, which is connected through six o-BDC2– ligands with μ2-η1:η1 bidentate mode, resulting inorganic core of Zn7O30C12 second building unit (Zn7 SBU) (Figure S1). The Zn7 SBU can be described as Anderson-like cluster, which is constructed by six ZnO4 tetrahedra moieties arranged in a cyclic manner (Zn6(CO2)6 ring) through Zn–(μ2-η1:η1 OCO)–Zn bridges connected to the central ZnO6 octahedron (Figure 1b). Each Anderson-like Zn7 SBU is bridged by six bpybc ligands, and each bpybc ligand is linked by two Zn7 SBUs (Figure 1c), thus forming a pcu topological framework with quadrilateral rings (Figure S2). The quadrilateral rings are so large that seven identical but independent sets of networks interpenetrate to give 7-fold interpenetrated 3D framework (Figure 1d and 1e). Despite of 7-fold interpenetration, it is still a porous framework with channels containing guest molecules, whose potential void spaces have been estimated to 20.6% per unit volume by SQUEEZE routine of PLATON. To be noted, the porous framework is positively charged, which is balanced by nitrates. The presence of NO3—



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anion can be confirmed by FT-IR spectra and ion chromatography (Figure S3, S4 and S5). After exposure to UV light or X-rays, there are obvious bond-length changes in the crystal structure. In detail, Zn–O distances and most other band distances only increased with a small variation of

X-ray and UV Dual Photochromism, Thermochromism

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