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• The demand for advanced disposable paper has increased dramatically accompanied by growing environmental problems, such as deforestation and chemical pollution [1]. Inkless erasable printing paper based on reversible photochromic materials holds great promise to solve the above problems due to its advantages of zero printing ink, low energy consumption, and reusability [2–7]. Photochromic materials with reversible color changes triggered by light have been regarded as excellent candidates for erasable inkless printing [8]. To date, some well-known photochromic small molecules, including diarylethene, spiropyran, naphthalenediimide (NDI), and viologens, have been extensively studied [9–11]. Their photochromic mechanisms are mainly divided into photoinduced changes in the electronic state and molecular structure. Compared to photoinduced structural transformation, photoinduced electron transfer (PET) isomers generally lead to a lesser degree of structure change that renders better cycling performance, which is more favorable for the applications of photochromic materials [12,13]. Currently, research on PET molecules is focused on the viologen and their derivatives [14–17]. Viologen-based materials have limitations of either high biotoxicity or poor stability during the photo-response process. In contrast, NDI derivatives as classical PET compounds are prized for their easy availability, low price, low toxicity, and high stability. However, the low color contrast and short photo-generated color lifetime hamper their applications due to the difficulty of content recognition and information storage [18].

  Metal-organic frameworks (MOFs), typically constructed from metal nodes or metal clusters and organic ligands, are characterized by designability, highly ordered structures, and good crystallinity, providing numerous opportunities to create promising stimuli-responsive materials with special functions [19–22]. In this regard, the desirable photo-responsive MOFs can be precisely designed by adjusting the position and types of the photo-responsive functional moieties. Moreover, the highly ordered structure with a uniform arrangement of the response functional groups allows for faster and more efficient energy transfer between the external stimulus signal and the response sites. High crystallinity affords precise structures crucial for probing the structure-property relationships of targeted photo-responsive materials. Based on these advantages, many photochromic MOFs have been discovered and the outstanding photochromic behavior allows them for various applications, such as information storage, optoelectronic devices, inkless printing [23–27]. To date, some progress has been made in inkless erasable printing using photochromic MOFs [20,22]. However, the resolution of inkless erasable paper based on photochromic MOFs is still not comparable to commercial printers and the cycling performance has not yet been explored in depth. Therefore, inkless and erasable printing papers based on photochromic materials remain significant challenges in achieving high resolution and long recycling performance.

  Herein, a N, N'-bis(5-isophthalic acid)naphthalenediimide (H₄BINDI) ligand, composed of electron-deficient NDI cores [18,28–30] and electron-rich carboxylate that facilitates the electron transfer, was embedded into porous coordination frameworks to obtain a new photochromic MOF [La(H₂O)(BINDI)_0.5(DMF)₃][NO₃] (1, H₄BINDI = N, N'-bis(5-isophthalic acid)naphthalenediimide, DMF = N, N'-dimethylformamide). It was found that 1 exhibits distinct color contrast from light yellow to dark brown after UV light irradiation, delivering a rapid response within 10 s and long lifetime over one week isolated from air, which is suitable as an inkless and erasable printing material. The resulting inkless printing paper based on 1 shows high printing resolution reaching up to 0.2 mm and excellent cycling performance (197 cycles), surpassing most reported inkless printing materials.

  Single-crystal X-ray diffraction (SCXRD) analysis reveals that 1 crystallizes in a monoclinic space group C2/c. The refinement details and selected bond lengths and angles are given in Tables S1 and S2 (Supporting information), respectively. The asymmetric unit of 1 contains one La³⁺ cation, one water molecule, one-half BINDI⁴⁻ ligand, three coordinated DMF molecules, and one free NO₃⁻ ion (Fig. S1a in Supporting information). Each La³⁺ cation is eight coordinated with three oxygen atoms from DMF molecules, one oxygen atom from H₂O molecule, and four carboxylate oxygen atoms from BINDI⁴⁻ ligands, forming a distorted square antiprism [LaO₈] coordination geometry (Fig. S1b in Supporting information). The pair of symmetry-related La³⁺ cations are chelated by four carboxylate groups to form a classic paddle-wheel secondary building unit (La₂-SBU). The adjacent La₂-SBUs are connected by BINDI⁴⁻ ligands to form a 3D framework with a rhombic channel (19.9 Å × 11.4 Å) along c axis (Fig. S1c in Supporting information). This 3D framework features a 4, 4-connected lvt topology in which La₂-SBU serves as a 4-connected node (Fig. 1). The phase purity of the as-synthesized 1 was confirmed by the powder X-ray diffraction (PXRD) patterns (Fig. S2 in Supporting information), which is in good agreement with the simulated patterns.

  Figure 1

  

  Figure 1.  (a) The 3D framework of 1 with the lvt topology. (b) The 3D structure of 1. The cyan, red, grey, and blue balls represent La, O, C, and N atoms, respectively. H atoms and free solvent molecules are omitted for clarity.

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  Complex 1 can change its color from yellow to dark brown (Fig. S3 in Supporting information) within 10 s upon UV light irradiation and return to its original yellow after 2 min at ambient conditions, indicating the reversible photochromic behavior of 1. To further explore the photochromic behavior of 1, the time-dependent diffuse-reflectance UV–vis spectra were carried out. As shown in Fig. 2a, new bands around 481, 607, 686, and 765 nm were observed after irradiation for 10 s, and the intensity increased with the duration of irradiation. The enhancement of absorption peaks tends to saturation after 20 min.

  Figure 2

  

  Figure 2.  (a) Time-dependent diffuse-reflectance UV–vis spectra of 1. (b) EPR spectra of 1 and after 2 and 10 min irradiation.

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  To gain insights into the photochromic mechanism of 1, PXRD patterns and Fourier transform infrared (FT-IR) spectra before and after irradiation were recorded. No obvious variations in the crystal structure and organic components were observed (Figs. S2 and S4 in Supporting information). From structural analysis, 1 is easily amenable to electron transfer because the NDI core has a strong ability to accept electrons and the electron-rich carboxylate groups can act as electron donors. In particular, the new absorption peak at 607 nm can be ascribed to typical absorption bands of NDI radicals [20], so the photochromic mechanism of 1 might arise from the generation of NDI radicals by photoinduced electron transfer. To further confirm the existence of photogenerated radicals, the electron paramagnetic resonance (EPR) measurement of 1 was further performed. As shown in Fig. 2b, a weak signal was found around g = 2.004 in unirradiated samples. This may be due to the high photosensitivity of 1 to sunlight resulting in the production of few radicals. After UV light irradiation, the peak intensity increases dramatically with the irradiation time, indicating the generation of photoinduced NDI radicals. [20,31,32]. These results reveal that the photochromic behavior of 1 is triggered by photoinduced electron transfer.

  To further explore the potential of 1 as an inkless printing material, irradiated samples were sealed in quartz tubes isolated from air and left in the darks. Surprisingly, the color of irradiated samples remains brown after one week. Furthermore, the EPR spectrum of irradiated samples shows that the radical signal is still obvious (Fig. S5 in Supporting information), indicating that 1 would be an excellent inkless erasable material. To obtain a better printing effect, the inkless printing paper based on 1 was prepared, involving three key components, namely the supporting layer (filter paper), coloring layer (powder of 1), and protecting layer (HEC/glycol gel, HEC = hydroxyethyl cellulose) (Fig. 3a). The protecting layer not only helps coloring layer to attach to the support layer but also insulates it from air, thereby getting a longer coloring lifetime. The inkless printing paper without the protecting layer shows a poor printing effect and mechanical properties (Fig. S6 in Supporting information). The printed picture was a bit blurry after irradiation for 1 min and faded in one day. After applying a protective layer over the coloring layer, the inkless printing paper could remain brown for 3 days and return to its original yellow when put at 80 ℃ for 2 h (Fig. 3b). The thermal gravimetric analysis (TGA) reveals that 1 can retain the framework stability up to 420 ℃ (Fig. S7 in Supporting information), indicating samples are not damaged during erasuring process at 80 ℃. Additionally, the resolution of this paper can reach at least 0.2 mm, comparable to the commercial printer (HP LaserJet P1606dn, 1200 dpi) (Figs. 4a and b). The cycling performance of the inkless printing paper was explored by monitoring the chromatism between printed dots and blank (Fig. S8 in Supporting information). The results show that the inkless printing paper based on 1 can be recycled at least 197 times and the printed picture is still clear and readable (Fig. 3c and Fig. S9 in Supporting information), which is much superior to some other reported materials. The quick response (QR) codes of the homepage of Nankai University (NKU) were also printed (Fig. 4c) and can be successfully recognized by mobile phones.

  Figure 3

  

  Figure 3.  (a) Schematic representation for the preparation process of the inkless printing paper coated with 1. (b) The printing and erasing process of inkless printing paper based on 1. (c) Printed pictures of 10, 20, 30, 50, 125, and 197 cycles.

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  Figure 4

  

  Figure 4.  (a) Resolution of inkless erasable printing paper. (b) Comparison of resolution of the printed picture between commercial printer (top) and printing paper (bottom); line width increases equally from 0.1 mm to 1.1 mm and the distance between two lines was set at 1 mm. (c) Printed QR codes of the NKU homepage.

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  In conclusion, we have successfully designed and synthesized a new photochromic MOF by integrating rare-earth metals and photochromic groups with both electron-deficient and electron-rich components. Benefiting from precise selection and ingenious combination of structure moieties, 1 shows the rapid color response and good reversibility upon UV light irradiation. The inkless printing paper based on 1 delivers high resolution up to 0.2 mm and excellent cycling performance of 197 cycles. This work paves the way for exploiting MOF materials available for inkless printing with excellent properties.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Le-Tian Zhang: Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation. Bin Xia: Writing – original draft, Methodology, Investigation, Formal analysis. Nan Lu: Visualization, Formal analysis. Quan-Wen Li: Visualization, Investigation. Xia Zhang: Methodology, Investigation, Data curation. Na Li: Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding acquisition, Conceptualization. Xian-He Bu: Supervision, Project administration, Funding acquisition.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 21905142 and 22035003) and the Programme of Introducing Talents of Discipline to Universities (No. B18030).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110534 .

  
  
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• 

  Figure 1  (a) The 3D framework of 1 with the lvt topology. (b) The 3D structure of 1. The cyan, red, grey, and blue balls represent La, O, C, and N atoms, respectively. H atoms and free solvent molecules are omitted for clarity.

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  Figure 2  (a) Time-dependent diffuse-reflectance UV–vis spectra of 1. (b) EPR spectra of 1 and after 2 and 10 min irradiation.

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  Figure 3  (a) Schematic representation for the preparation process of the inkless printing paper coated with 1. (b) The printing and erasing process of inkless printing paper based on 1. (c) Printed pictures of 10, 20, 30, 50, 125, and 197 cycles.

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  Figure 4  (a) Resolution of inkless erasable printing paper. (b) Comparison of resolution of the printed picture between commercial printer (top) and printing paper (bottom); line width increases equally from 0.1 mm to 1.1 mm and the distance between two lines was set at 1 mm. (c) Printed QR codes of the NKU homepage.

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