English

• 0.   Introduction

  Coordination polymers, as a class of highly attractive functional materials, have garnered widespread interest over the past few years owing to their versatile structural tunability and multifunctional characteristics. Among various functionalities, photochromic behavior refers to reversible color changes under light irradiation, which has attracted particular attention due to its potential applications in optical storage^[1-8], sensors^[9-13], and smart materials^[14-19]. Meanwhile, photoluminescent properties provide additional functionality, enabling these materials to be used in optoelectronic devices^[20-23] and bioimaging^[24-26]. The integration of both photochromic and photoluminescent properties into a single coordination polymer offers unique opportunities for advanced technological applications, as the interplay between these two phenomena can lead to novel responsive behaviors.

  Viologen-based ligands have been widely explored in the construction of photochromic materials owing to their ability to undergo reversible redox reactions upon light exposure, accompanied by distinct color changes. Moreover, viologen derivatives exhibit diverse coordination modes with metal ions, allowing them to act as versatile structural building blocks for synthesizing functional coordination polymers. As an important category of transition metal ions, zinc ions not only possess diverse coordination geometries and relatively low toxicity but also a specific d¹⁰ electronic configuration. This unique electronic structure enables zinc ions to endow the resulting complexes with luminescent properties, making them a preferred choice as central metal ions in such systems. Their use further facilitates the formation of coordination polymers with both structural stability and structural diversity, laying a foundation for integrating multiple functionalities.

  Based on the above considerations, a novel coordination polymer [Zn(CV)_0.5(BDC)(H₂O)]·2H₂O has been successfully synthesized via the coordination of the 1, 1′-bis(2-carboxyethyl)-4, 4′-bipyridinium ((H₂CV)²⁺) ligand with zinc ions, along with the incorporation of terephthalic acid (H₂BDC). This complex features a 1D chain structure. Notably, complex 1 exhibits photochromic behavior: it undergoes a distinct color change from colorless to blue upon light irradiation, and the blue-colored sample can gradually revert to its original color when placed in air, demonstrating excellent reversibility. This reversible photochromic property endows 1 with great potential in ink-free printing. In addition, complex 1 also shows photoluminescent behavior, emitting blue light in the dark. Interestingly, this photoluminescent property is inversely correlated with the photochromic phenomenon (luminescence intensity decrease when photochromism occurs). The coexistence of these two distinct optical properties in 1 endows the complex with photo-controllable switching performance. This dual characteristic holds significant value, as unraveling the fundamental mechanisms behind these properties and investigating their synergistic interactions may pave the way for the creation of more sophisticated and versatile materials.

  1.   Experimental

  1.1   Materials and physical measurement

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

  1.2   Synthesis of complex 1

  ZnBr₂ (22.5 mg, 0.1 mmol), (H₂CV)Cl₂ (30 mg, 0.1 mmol), and H₂BDC (16.6 mg, 0.1 mmol) were dissolved in a mixed solvent consisting of EtOH (2 mL), DMF (2 mL), and H₂O (2 mL), followed by stirring at room temperature for 10 min. The resulting mixture was then transferred into a 25 mL Teflon-lined autoclave. The autoclave was placed in an oven, where the temperature was programmed to rise to 70 ℃ over 12 h, and the reaction was maintained at this temperature for 72 h. Then, the products were cooled to room temperature, and filtration yielded colorless and transparent crystals. The yield was 34% (based on ligand (H₂CV)Cl₂). Elemental analysis Calcd. for C₁₆H₁₈NO₉Zn(%): C, 44.27; H, 4.15; N, 3.23. Found(%): C, 44.31; H, 4.18; N, 3.25.

  1.3   X-ray crystallography

  Single crystals of complex 1 were grown via solvothermal synthesis of a reaction in a mixed solution of EtOH (2 mL), DMF (2 mL), and H₂O (2 mL) at 70 ℃. A suitable crystal with dimensions of 0.6 mm×0.4 mm×0.3 mm was selected and mounted on a Bruker SMART APEX Ⅱ CCD diffractometer. X-ray diffraction data were collected using graphite-monochromated Mo Kα radiation (λ=0.071 073 nm) at 297.16 K. The crystal structure was solved and refined by full-matrix least-squares techniques on F ² using Olex2. Absorption corrections were performed via the multi-scan method.

  The crystallographic data of 1 are summarized in Table 1, and the selected bond lengths, bond angles, and hydrogen bonds are tabulated in Table S1-S3.

  Table 1

  

  Table 1.  Crystallographic data of complex 1

  下载: 导出CSV

  Parameter	1
  Empirical formula	C16H18NO9Zn
  Formula weight	433.68
  Crystal system	Monoclinic
  Space group	P21/n
  a / nm	1.029 16(14)
  b / nm	1.630 0(3)
  c / nm	1.041 04(18)
  β / (°)	97.407(5)
  V / nm3	1.731 8(5)
  Z	4
  F(000)	892.0
  GOF (F 2)	1.031
  Final R indexes* [I≥2σ(I)]	R1=0.040 6, wR2=0.077 4
  Final R indexes (all data)	R1=0.068 4, wR2=0.088 5
  *R1=∑||Fo|-|Fc||/∑|Fo|, wR2={∑w[(Fo)2-(Fc)2]2/∑w[(Fo)2]2}1/2.

  2.   Results and discussion

  2.1   Crystal structures of complex 1

  Through single-crystal X-ray diffraction analysis, it was found that complex 1 crystallizes in the monoclinic system and belongs to the space group P2₁/n. The asymmetric unit of 1 is composed of one Zn²⁺ ion, half of one CV ligand, one BDC^2- ligand, one coordinated water molecule, and two free water molecules. As illustrated in Fig.1a, each Zn²⁺ ion is four-coordinated, bonding with two oxygen atoms from two BDC^2- ligands, one oxygen atom from a CV ligand, and one oxygen atom from a coordinated water molecule, thus adopting a distorted tetrahedral geometry. The Zn²⁺ ions, CV ligands, and BDC^2- ligands are interconnected to form a 1D chain structure (Fig.1b). The Zn—O bond lengths range from 0.191 6 to 0.201 5 nm. The shortest hydrogen bond, O2—H2B…O4 (H2B…O4 0.184 nm), is a strong O—H…O interaction and contributes significantly to the structural stability of the coordination polymer framework. Thermogravimetric analysis (TGA) of 1 showed a stable plateau from 30 to 105 ℃, indicating that 1 exhibits good thermal stability within this temperature range. The TGA also revealed a weight loss of 13% from 105 to 196 ℃, which corresponds to the removal of two free water molecules and one coordinated water molecule (Calcd. 12.46%). Above 196 ℃, 1 underwent a significant weight loss, suggesting that the framework of 1 began to collapse (Fig.S1).

  Figure 1

  

  Figure 1.  (a) Coordination environment of complex 1; (b) 1D chain structure of 1 along the a-axis

  Symmetric codes: ^ⅰ 1+x, y, z; ^ⅱ-1+x, y, z; ^ⅲ 1-x, 1-y, 1-z.

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  2.2   Photochromic properties of complex 1

  Complex 1 changed color from colorless to blue upon irradiation with sunlight or a 300 W xenon lamp. Notably, a visible color change could be observed as early as 1 s after irradiation, and the color gradually deepens with increasing irradiation time (Fig.2a). The blue sample after irradiation was designated as 1P. When 1P was stored in the dark, it would gradually revert to its original color. To investigate the photochromic mechanism of 1, solid-state UV-Vis diffuse reflectance spectroscopy was performed. As shown in Fig.2b, before light irradiation, 1 showed no obvious absorption peaks. In contrast, 1P exhibited two strong absorption peaks at 410 and 606 nm in the solid diffuse reflectance absorption spectrum, and the intensity of these absorption peaks increased with prolonged irradiation time. These peaks were similar to the absorption peaks of viologen-based complexes reported previously^[27-33], indicating that they originate from the formation of viologen radicals. To confirm the generation of viologen radicals during the photochromic process, electron paramagnetic resonance (EPR) spectroscopy was conducted on samples of 1 before and after irradiation. As depicted in Fig.2c, no signal peak was observed for 1 before irradiation, while a strong signal peak appeared at g=2.035 1 after irradiation. This result demonstrates that the photochromic property of 1 stems from the generation of viologen radicals^[34-41]. Powder X-ray diffraction (PXRD) and IR analyses revealed that the framework of 1 remains unchanged before and after irradiation (Fig.3), indicating that no photoisomerization or photodecomposition occurs during the coloring and fading processes of 1^[42].

  Figure 2

  

  Figure 2.  (a) Color of complex 1 as a function of different light irradiation durations; (b) Solid UV-Vis diffuse reflectance spectra of 1 with irradiation time variations; (c) EPR spectrum of 1 before and after light irradiation

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  Figure 3

  

  Figure 3.  (a) Simulated and experimental PXRD patterns of complex 1 in the photochromic processes; (b) IR spectra of 1 in the photochromic process

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  2.3   Ink-free erasable printing properties of complex 1

  Given that complex 1 exhibits excellent photochromic properties, it can serve as a medium for ink-free erasable printing. A certain amount of ground sample of 1 was dispersed in a specific volume of ethanol, and the mixture was ultrasonicated for 30 min. The suspension was dropped onto filter paper. The paper was stored in the dark to dry, and this operation was repeated five times. Subsequently, by placing a template with patterns on the printed paper, the photo-printed content could be obtained after irradiation. As shown in Fig.4, after irradiating the printed paper containing 1 for 30 s and removing the template, a blue flower pattern appeared on the white paper, which showed a distinct color contrast with the white background, facilitating the visual reading of relevant content. The printed content could be erased after approximately 7 d, and upon re-irradiation, the blue pattern reappeared, demonstrating the reusability of the composite paper.

  Figure 4

  

  Figure 4.  Ink-free erasable printed images

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  2.4   Photoluminescence of complex 1

  When complex 1 is irradiated with a xenon lamp in the dark, it emits blue luminescence (Fig.5a). Therefore, the luminescent properties of 1 were investigated. When 320 nm was used as the excitation wavelength, 1 exhibited a characteristic emission peak at 441 nm, and its fluorescence emission intensity gradually decreased with increasing irradiation time (Fig.5b). This is exactly opposite to the photochromic behavior of complex 1, suggesting that there may be an energy competition relationship between the photochromic behavior and photo-controlled fluorescence behavior of 1. As depicted in Fig.6, the fluorescence emission spectrum of pristine complex 1 showed significant overlap with the solid-state UV-Vis diffuse reflectance absorption spectrum of photoirradiated 1P, which further indicates that the light-controlled fluorescence property of 1 is caused by intramolecular energy transfer.

  Figure 5

  

  Figure 5.  (a) Luminescent image of complex 1; (b) Fluorescence spectra of 1 in a prolonged irradiation duration

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  Figure 6

  

  Figure 6.  Spectra overlap between the absorption spectrum of 1P and fluorescence emission of complex 1

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  2.5   Multiple photo-switching properties of complex 1

  As shown in Fig.7a and 7b, the initially colorless crystals of complex 1 underwent a distinct color transition to blue upon exposure to light irradiation. When 1 was subjected to ultraviolet light in a dark environment, it exhibited prominent blue luminescence (Fig.7c). In contrast, the luminescent brightness of sample 1P became relatively weaker when illuminated under the same dark conditions (Fig.7d). These phenomena clearly demonstrate that there exists a competitive interplay between the photochromic behavior and light-modulated fluorescence properties of this complex. Furthermore, such dual-responsive characteristics further confirm that the complex possesses photo-switchable functionality. Notably, complex 1 exhibits versatile switching capabilities, encompassing multiple responsive behaviors. It can undergo reversible transitions in both color and luminescence upon specific light stimuli, thus functioning as a multi-channel optical switch.

  Figure 7

  

  Figure 7.  Multiple optical switching patterns of complex 1

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  3.   Conclusions

  In conclusion, a viologen-based zinc coordination polymer has been successfully synthesized, which features a 1D chain structure. Notably, this complex exhibits multiple distinctive functionalities, including photochromism, ink-free printing capability, photo-induced fluorescence, and multiple photo-switching performances. The synergistic integration of these integrated optoelectronic properties endows complex 1 with considerable potential as a promising candidate for advanced applications in optoelectronic devices, smart printing materials, and other related fields.

  
  Acknowledgments: The current 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 (Grant No.24TYKY216), and the Provincial College Students′ Innovation and Entrepreneurship Training Program (Grant No.TYX2025030). 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) Coordination environment of complex 1; (b) 1D chain structure of 1 along the a-axis

  Symmetric codes: ^ⅰ 1+x, y, z; ^ⅱ-1+x, y, z; ^ⅲ 1-x, 1-y, 1-z.

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) Color of complex 1 as a function of different light irradiation durations; (b) Solid UV-Vis diffuse reflectance spectra of 1 with irradiation time variations; (c) EPR spectrum of 1 before and after light irradiation

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  Figure 3  (a) Simulated and experimental PXRD patterns of complex 1 in the photochromic processes; (b) IR spectra of 1 in the photochromic process

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  Figure 4  Ink-free erasable printed images

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  Figure 5  (a) Luminescent image of complex 1; (b) Fluorescence spectra of 1 in a prolonged irradiation duration

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  Figure 6  Spectra overlap between the absorption spectrum of 1P and fluorescence emission of complex 1

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  Figure 7  Multiple optical switching patterns of complex 1

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  Table 1.  Crystallographic data of complex 1

  Parameter	1
  Empirical formula	C16H18NO9Zn
  Formula weight	433.68
  Crystal system	Monoclinic
  Space group	P21/n
  a / nm	1.029 16(14)
  b / nm	1.630 0(3)
  c / nm	1.041 04(18)
  β / (°)	97.407(5)
  V / nm3	1.731 8(5)
  Z	4
  F(000)	892.0
  GOF (F 2)	1.031
  Final R indexes* [I≥2σ(I)]	R1=0.040 6, wR2=0.077 4
  Final R indexes (all data)	R1=0.068 4, wR2=0.088 5
  *R1=∑||Fo|-|Fc||/∑|Fo|, wR2={∑w[(Fo)2-(Fc)2]2/∑w[(Fo)2]2}1/2.

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