English

• 0.   Introduction

  In the field of materials science, thermochromic materials have attracted much attention due to their ability to change color in response to changes in ambient temperature. Such materials have broad application prospects in many fields, such as smart windows^[1-5], temperature indicators^[6-11], and anti‐counterfeiting^[12-16]. As a typical redox‐active organic molecule, viologen plays an important role in the study of thermochromic materials due to its unique electron transfer properties and reversible color change behaviors. However, the thermal stability of organic viologen ligands is not good, so to solve this intractable problem, the construction of viologen‐based inorganic‐organic coordination polymers has aroused great concern.

  With the deepening of the research on thermochromic materials, viologen‐based coordination polymers have gradually become a research hotspot in this field because of their excellent thermochromic properties and controllability^[17-26]. The combination of electron transfer capability and structural tunability of the viologen‐based coordination polymers provides a new way for the development of thermochromic materials. Thermochromic properties, such as temperature range and reversibility of color change, can be further optimized by adjusting the composition, structure, and coordination mode of the coordination polymers.

  In this work, a novel viologen‐based coordination polymer [Cd(bcbpy)I₂]·2H₂O (1) was successfully constructed. The complex displays a 1D chain structure and has thermochromic properties in response to temperature stimulation. In addition, the thermochromic films and electrochemical properties of complex 1 were also investigated.

  1.   Experimental

  1.1   Materials and physical measurements

  The materials and physical measurements can be found in the Supporting information.

  1.2   Synthesis of complex 1

  Complex 1 was obtained by solvothermal synthesis method, by the reaction of Cd(NO₃)₂·4H₂O (59.4 mg, 0.2 mmol) with H₂bcbpy·2Cl (39.2 mg, 0.1 mmol), isophthalic acid (16.6 mg, 0.1 mmol) and KI (49.8 mg, 0.3 mmol) in a mixed solvent of DMF (2 mL) and H₂O (4 mL). The mixture was stirred at room temperature for 10 min, then sealed in a 25 mL Teflon‐lined stainless steel container and heated to 90 ℃in 12 h, and maintained constant temperature for 96 h. The mixture was then slowly cooled to room temperature and filtered to produce dark green crystals. The yield of complex 1 was 65% (based on the ligand H₂bcbpy·2Cl). Elemental analysis Calcd. for C₅₂H₄₆N₄O₁₁Cd₂I₄(%): C 38.16, H 2.81, N 3.42; Found(%): C 38.21, H 2.83, N 3.45.

  1.3   X‐ray crystallography

  X‐ray diffraction data for complex 1 were obtained using a Bruker SMART APEX Ⅱ CCD, and the graphite Mo Kα was a monochromatic X‐ray (λ=0.071 073 nm) at 293 K. Absorption correction was performed by a multi‐scan technique. The structure of complex 1 was analyzed by the Olex2 software package. Anisotropic refinement of the thermal parameters of all non‑ hydrogen atoms in complex 1 was refined anisotropically. The crystal data of complex 1 are listed in Table 1, and the important bond lengths and bond angles of complex 1 are listed in Table S1 and S2, respectively.

  Table 1

  

  Table 1.  Crystallographic data of complex 1

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  Parameter	1
  Empirical formula	C52H46N4O11Cd2I4
  Formula weight	1 635.35
  Crystal system	Monoclinic
  Space group	C2/c
  a / nm	2.122 1(4)
  b / nm	1.068 0(3)
  c / nm	2.4653(6)
  β / (°)	103.551(8)
  Volume / nm3	5.432(2)
  Z	4
  F(000)	3 128.0
  Goodness‐of‐fit on F 2	1.089
  Final R indexes [I≥2σ(I)]*	R1=0.050 5, wR2=0.119 4
  Final R indexes (all data)*	R1=0.064 8, wR2=0.127 6
  $ * R_1=\sum\left\|F_{\mathrm{o}}\left|-\left|F_{\mathrm{c}} \| / \sum\right| F_{\mathrm{o}}\right|, w R_2=\left[\sum w\left(F_{\mathrm{o}}^2-F_{\mathrm{c}}^2\right)^2 / \sum w\left(F_{\mathrm{o}}^2\right)^2\right]^{1 / 2} .\right.$

  2.   Results and discussion

  2.1   Crystal structure of complex 1

  X‑ray single‑crystal diffraction analysis shows that complex 1 belongs to the monoclinic system, and the space group is C2/c. The asymmetric unit of complex 1 involves one Cd²⁺ ion, one bcbpy ligand, two coordinated I^- ions, and two free water molecules. As shown in Fig. 1a, each Cd²⁺ ion shows a quadrigonal geometry, coordinated by two iodine ions and three oxygen atoms from two bcbpy molecules. The bond length of Cd—O is in a range of 0.227 4‐0.255 0 nm. The bond lengths of Cd—I vary from 0.274 9 to 0.280 0 nm, and the metal center combines with bcbpy to form a spiral 1D chain structure (Fig. 1b). Thermogravimetric analysis of complex 1 showed slow weight loss at 30 ℃, and a stable plateau appeared at 137 ℃ (Fig.S1). When heated to 271 ℃, the framework of complex 1 gradually began to collapse.

  Figure 1

  

  Figure 1.  (a) Asymmetric unit of complex 1 with thermal ellipsoids set at a 50% probability level; (b) 1D chain structure of complex 1

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  2.2   Thermochromic properties of complex 1

  Interestingly, complex 1 was able to undergo a distinct color change after heating. However, the unground samples showed different color changes in response to temperature stimuli than the ground samples, which may be due to the different crystal forms that lead to different effects on light absorption. As shown in Fig. 2a, the unground samples were dark green before heating, and gradually changed to brownish yellow as the temperature increased. When the temperature increased to 130 ℃, all the samples turned orange. After being ground, the sample was green before heating, turned almost golden when the temperature reached 90 ℃, and completely turned golden when the temperature reached 100 ℃. To investigate the thermochromic mechanism of complex 1, UV‐Vis solid diffuse reflection, electron spin resonance (ESR), powder X‐ray diffraction (PXRD), and IR tests were performed. It can be seen from Fig. 2b that the heated samples had absorption peaks at 636 nm, which were similar to the characteristic absorption peak of viologen radicals^[27-29], and the intensity of the absorption peak gradually increased with the increase of temperature. However, as the temperature rose to 120 ℃, the absorption peak at 636 nm began to disappear. These results indicate that complex 1 can produce viologen free radicals under thermal stimulation, but the high temperature will quench the free radicals. When the temperature was higher than 120 ℃, the absorption peaks of the samples disappeared. This may be caused by the rate of free radicals quenching being greater than the rate of formation. ESR tests were performed on complex 1. As shown in Fig. 3, complex 1 had no signal peak before heating, but when it was heated to 80 ℃, an obvious absorption peak appeared at g=1.988 3 in the ESR spectrum. This indicates that the thermochromic process involves the production of viologen radical^[30-37]. At 140 ℃, no free radicals were detected (Fig.S2), indicating that excessive temperature would quench free radicals. However, the PXRD pattern and IR spectrum of complex 1 after heating did not change significantly (Fig.S3), which further indicated that the thermochromic properties of complex 1 were caused by the generation of viologen radical rather than the collapse of the framework during the thermochromic process^[38-39].

  Figure 2

  

  Figure 2.  (a) Color of complex 1 varying with temperature (First row: unground samples, Second row: ground samples); (b) UV‐Vis solid diffuse reflectance spectra of complex 1 with temperature variation

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  Figure 3

  

  Figure 3.  ESR spectra of complex 1 before and after heating

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  2.3   Film applications of complex 1

  Complex 1 was made into a thermochromic film. It was mixed with polyvinylidene fluoride at a ratio of 8∶1, and an appropriate amount of ethanol was added for grinding. After being ground for 1 h, the color of complex 1 changed from green to yellow, and the film was coated on the conductive glass to obtain a golden yellow film. When the film was heated to 120 ℃, the color of the film turned to dark yellow. The color of the film turned orange when heated to 160 ℃ (Fig. 4). Thus, the complex has potential for temperature detection applications.

  Figure 4

  

  Figure 4.  Picture of a color‐changing membrane prepared with complex 1

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  2.4   Electrochemical properties of complex 1

  To evaluate the photogenerating charge ability of complex 1, a photocurrent test was performed. As shown in Fig. 5a, complex 1 exhibited an obvious cyclable photocurrent signal, indicating that electron‐hole separation can occur in complex 1 under the excitation of UV‐Vis light, thus forming redox pairs. At the same time, Mott‐Schottky curves were tested at different frequencies. It can be seen from Fig. 5b that 1/C² had a positive slope relationship with applied voltage, indicating that complex 1 has n‐type semiconductor characteristics. The intersection point with the horizontal coordinate was -1.27 V (relative to the Ag/AgCl electrode), which indicates that complex 1 has semiconductor properties and can be used as a potential photocatalyst material.

  Figure 5

  

  Figure 5.  (a) Photocurrent curves of complex 1; (b) Mott‐Schottky curves for complex 1

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  3.   Conclusions

  In conclusion, we have successfully prepared a new 1D viologen‑based coordination polymer (1), which exhibits reversible thermochromic behavior in the solid state. Thus, complex 1 was made into a thermochromic film. At the same time, the electrochemical behavior of complex 1 was also explored, and it was found that complex 1 has n‐type semiconductor properties.

  
  Acknowledgments: The work was supported by the Natural Science Foundation of Shanxi Province (Grant No.202403021222366), the Talent Introduction of Scientific Research Start‑Up Foundation of Taiyuan University (No.24TYKY216), the Doctoral Starting Research Foundation of Taiyuan University (No.23TYQN20), and Provincial College Students′ Innovation and Entrepreneurship Training Program (No.TYX2024061). Conflicts of interest: All authors claimed no competing interests.
  Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Figure 1  (a) Asymmetric unit of complex 1 with thermal ellipsoids set at a 50% probability level; (b) 1D chain structure of complex 1

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  Figure 2  (a) Color of complex 1 varying with temperature (First row: unground samples, Second row: ground samples); (b) UV‐Vis solid diffuse reflectance spectra of complex 1 with temperature variation

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  Figure 3  ESR spectra of complex 1 before and after heating

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  Figure 4  Picture of a color‐changing membrane prepared with complex 1

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  Figure 5  (a) Photocurrent curves of complex 1; (b) Mott‐Schottky curves for complex 1

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  Table 1.  Crystallographic data of complex 1

  Parameter	1
  Empirical formula	C52H46N4O11Cd2I4
  Formula weight	1 635.35
  Crystal system	Monoclinic
  Space group	C2/c
  a / nm	2.122 1(4)
  b / nm	1.068 0(3)
  c / nm	2.4653(6)
  β / (°)	103.551(8)
  Volume / nm3	5.432(2)
  Z	4
  F(000)	3 128.0
  Goodness‐of‐fit on F 2	1.089
  Final R indexes [I≥2σ(I)]*	R1=0.050 5, wR2=0.119 4
  Final R indexes (all data)*	R1=0.064 8, wR2=0.127 6
  $ * R_1=\sum\left\|F_{\mathrm{o}}\left|-\left|F_{\mathrm{c}} \| / \sum\right| F_{\mathrm{o}}\right|, w R_2=\left[\sum w\left(F_{\mathrm{o}}^2-F_{\mathrm{c}}^2\right)^2 / \sum w\left(F_{\mathrm{o}}^2\right)^2\right]^{1 / 2} .\right.$

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