A photochromic metal–organic framework with a rare 3D self-interpenetrated architecture and an ultrahigh MnO₄⁻ sensing ability†

Jinfang Zhang *^a, Yinlong Yue ^a, Xingyu Tao ^a, Jiarun Zhang ^a, Dejing Yin ^b and Chi Zhang *^ac
aInternational Joint Research Center for Photoresponsive Molecules and Materials, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, P. R. China. E-mail: zjf260@jiangnan.edu.cn; chizhang@jiangnan.edu.cn
^bSchool of Biotechnology, Jiangnan University, Wuxi 214122, P. R. China
^cSchool of Chemical Science and Engineering, Tongji University, Shanghai 200092, P. R. China

First published on 18th June 2024

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Introduction

Metal–organic frameworks (MOFs) have unique structures^1–4 and extensive applications, such as sensing,^5–9 catalysis,^10–12 adsorption,¹³ separation,^14–17 biomedical imaging,^18–20etc. Photochromic MOFs are an important type of MOFs and have very promising applications, such as metal oxidation state modulation,²¹ erasable printing without ink,²² conductivity,²³ anticounterfeiting,²⁴ sensing,^25,26etc. Photochromic functional units play a decisive role in the construction of photochromic MOFs. Photochromic molecules show significantly different absorption spectra before and after light irradiation, so the incorporation of photochromic ligands into MOFs typically results in different absorptions and/or emissions, resulting in multi-response and/or enhanced properties.

In modern industrial society, many pollutants cause harm to human health and the ecological environment. The MnO₄⁻ anion is a common strong oxidant in laboratories and factories, which is harmful to organisms and the environment.^27,28 The U. S. Environmental Protection Agency (EPA) has listed MnO₄⁻ as an important pollutant.^29,30 MnO₄⁻ is widely used in the fishing and aquaculture industries.^31,32 But excess MnO₄⁻ is carcinogenic to cells and leads to all sorts of cancer disease.³³ Thus, it is very necessary to develop convenient and efficient methods to detect MnO₄⁻. Luminescent metal–organic frameworks (LMOFs) have exhibited obvious advantages in the sensing field.^34–37 Organic ligands will significantly affect the functions of MOFs. Therefore, it is important for the development of special functional organic ligands to construct LMOF-based MnO₄⁻ sensing materials, but faces huge challenges.

Recently, our group developed new diarylethene-type photochromic organic ligands to construct LMOF-based sensing materials.^38–40 For example, {[Cd_1.5(L)(HOBA)(EtOH)(SO₄)(H₂O)]·3H₂O}_n (a)³⁸ and [Cd₂L(HBTC)₂·2H₂O·2i-PrOH]_n (b)³⁹ are fabricated using a new dihydroanthracene-based photochromic ligand L. a has a 1D triple-chain architecture and can sense MnO₄⁻ with extremely high sensitivity and good selectivity after photochromism. b shows a 2D (4,6)-connected structure and efficient MnO₄⁻ sensing ability after photochromism. Herein, a new photochromic luminescent Ni(II)–organic framework [Ni₂L(OBA)₂(H₂O)_1.5·3.5i-PrOH]_n (1) was constructed using L. This MOF shows a more complicated three-dimensional (3D) self-interpenetrated structure and can detect MnO₄⁻ in H₂O before and after photochromism. Especially, 1′ (1 after photochromism) has the second highest K_sv and nearly lowest LOD values, and can identity MnO₄⁻ in the presence of Cr₂O₇²⁻.

Experimental section

Materials and methods

The photochromic organic ligand L was synthesized according to a literature method.³⁸ The chemicals used in this work were purchased directly (Table S1, ESI†). Thermogravimetric analysis (TGA) curves were collected on a TGA/1100SF analyzer. Powder X-ray diffraction (PXRD) patterns were obtained on a Bruker D8 Advance X-ray diffractometer. Simulated PXRD patterns were achieved using Mercury. UV-vis absorption spectra were acquired on a TU-1901 double-beam spectrophotometer. FT-IR spectra were obtained using a Nicolet Impact 470 FT-IR spectrometer with KBr pellets. Fluorescence spectra were recorded using a PTI QM-TM fluorescence spectrometer.

Preparation of 1

Ni(NO₃)₂·6H₂O (0.10 mmol, 0.0290 g), L (0.05 mmol, 0.0256 g), H₂OBA (0.10 mmol, 0.0258 g), H₂O (3 mL) and i-PrOH (3 mL) were taken in a Teflon-lined stainless-steel vessel, stirred and then heated at 140 °C for 72 h. Then the mixture was cooled to room temperature at a rate of 5 °C h⁻¹. The green crystals were collected and dried in air (yield: 51% based on L). IR (cm⁻¹): 3378(s); 3059(s); 2957(s); 1590(vs); 1520(s); 1417(vs); 1387(vs); 1239(vs); 1153(vs); 1090(m); 950(s); 880(s); 782(s); 660(m).

Single crystal structure

A single crystal of 1 was selected to measure its structure using a D8 VENTURE CMOS X-ray diffractometer with Cu Kα radiation. The cell parameter was refined on all observed reflections by using the program APEX-3. Then the structure of 1 was determined by the direct method and refined using the SHELXTL program.⁴¹ All non-hydrogen atoms were anisotropically refined. Single-crystal diffraction experiment and refinement data of 1 are listed in Table 1 (CCDC number: 2351838†).

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Luminescence detection experiments

Before performing luminescence sensing experiments, the synthesized crystals were gradually ground in a mortar to reduce their size, which could directly enhance the dispersibility of 1 in water. 1 mg of 1 was dispersed in 40 mL of water, sonicated for 45 min, and left in the dark for 72 h to form a stable suspension (0.025 mg mL⁻¹). After irradiating the H₂O suspension of 1 under 365 nm ultraviolet light for 45 min and standing for 3 days, a stable suspension of 1′ was obtained (0.025 mg mL⁻¹).

Luminescent signals were collected with or without anions in the H₂O suspension of 1 or 1′ for 2 min at 2 mL, including F⁻, Cl⁻, Br⁻, I⁻, CO₃²⁻, HCO₃⁻, SO₄²⁻, NO₃⁻, PO₄³⁻, HPO₄²⁻, H₂PO₄⁻, Cr₂O₇²⁻ and MnO₄⁻. The dependence of spectral intensity on concentration was analyzed by gradually adding anions into the H₂O suspension of 1 and 1′. Detailed titration experiments were carried out using a stepwise addition method. 1 mM MnO₄⁻ aqueous solution was gradually added into 2 mL of H₂O suspension of 1 and 0.1 mM MnO₄⁻ aqueous solution was gradually added into 2 mL of H₂O suspension of 1′, respectively. When excited at 350 nm, the emission spectrum of 1 was recorded from 370 nm to 600 nm. Under excitation of 400 nm, 1' is emitted in the range of 420–620 nm.

Results and discussion

Crystal structure of 1

Single-crystal structural analysis demonstrates that 1 belongs to a monoclinic crystal system with a P2₁/c space group (Table 1) and exhibits a self-interpenetrated 3D (4,4)-connected framework, which is composed of 4-connected Ni²⁺, 4-connected L units and OBA²⁻ bridges (Fig. 1 and 2). As shown in Fig. S1 (ESI†), the asymmetric unit of 1 contains two Ni²⁺, one L, two OBA²⁻, one and a half H₂O and three and a half i-PrOH units. The two kinds of crystals are independent Ni²⁺ with the same coordination environment. Each Ni²⁺ coordinates with four O atoms from two OBA²⁻ and two N atoms from different L ligands and one water molecule to form a six-coordinated octahedral pattern (Fig. 2). Each L coordinates with two Ni1 and two Ni2 by N atoms from four pyridyl groups (Fig. S2a, ESI†). The dihydroanthracene group is heavily twisted. The angles between two pyridyl groups in one side of dihydroanthracene are 113.833° and 112.056°, which are similar to those from a (116.117° and 111.644°) and b (113.793° and 108.01°).

Two OBA²⁻ units show the same coordination and connection manners. Each OBA²⁻ bonds with one Ni²⁺ by one carbonyl O atom and chelates with one Ni²⁺ by two O atoms from another side carbonyl (Fig. S2b, ESI†). Therefore, each OBA²⁻ unit is in a μ₂-η¹η¹η¹ manner. The lengths of the Ni–N and Ni–O bonds range from 2.047(3) Å to 2.111(3) Å and 2.006(2) Å to 2.207(3) Å, respectively.

OBA²⁻ bridges connect with Ni1 or Ni2 in a μ₂-η¹η¹η¹ manner, which results in 1D chains along the a-axis or c-axis (Fig. 3 and Fig. S3a, b, ESI†). Each L links with two Ni1 and two Ni2 and acts as 4-connected units through its four pyridine groups to form a 1D double-chain architecture along the b-axis (Fig. 3 and Fig. S3c, ESI†). 1D chains fabricated by OBA²⁻ and Ni1/Ni2 connect with 1D double-chains constructed using L, Ni1 and Ni2 units to form a 3D framework (Fig. 3). Interestingly, 1D chains fabricated by OBA²⁻ and Ni1/Ni2 units interpenetrate each other in this 3D framework (Fig. 4).

L connects with two Ni1 and two Ni2 in a 4-connected manner. Each Ni1/Ni2 links with two L units and another two NI1/Ni2 and is still in a 4-connected manner. From the perspective of topology, L and Ni1/Ni2 units can be regarded as 4-connected nodes. The Schlafli vertex symbols are 4².6².8² and 4.6⁴.8 for L and Ni1/Ni2 nodes, respectively. Therefore, 1 has a binodal (4,4)-connected topology with a Schlafli symbol of (4².6².8²)(4.6⁴.8)₂ (Fig. 1c). A lot of self-interpenetrated MOFs have been reported;^42,43 however, this binodal (4,4)-connected topology with a Schlafli symbol of (4².6².8²)(4.6⁴.8)₂ is very scarce.^44,45

Previously, L was used by our group to construct two MOFs, {[Cd_1.5(L)(HOBA)(EtOH)(SO₄)(H₂O)]·3H₂O}_n (a)³⁸ and [Cd₂L(HBTC)₂·2H₂O·2i-PrOH]_n (b).³⁹ They show a 1D triple chain and a 2D (4,6)-connected network, respectively. However, 1 features a more complicated 3D self-interpenetrated (4,4)-connected structure. L acts as a 4-connected building unit in these MOFs. The involvement of large coordinated molecules (EtOH and SO₄²⁻) and rigid HBTC²⁻ in a and b should limit the extension of their structure. 1 has a small H₂O coordinated molecule and long and flexible OBA²⁻ bridges, which results in a 3D self-interpenetrated architecture.

PXRD and TGA

The PXRD pattern of 1 shows good agreement with the simulated pattern from its single crystal data, which proves that 1 has a highly pure phase (Fig. S4, ESI†). The PXRD pattern of 1′ can agree well with that of 1 (Fig. S4, ESI†), which reveals that 1′ and 1 are isomorphic. After soaking in water for 7 days, the PXRD pattern and the mass of 1 have almost no changes (Fig. S4 and S5, ESI†), indicating the excellent water stability of 1. 1 was immersed in solutions with different pH values for 24 h. The PXRD patterns of 1 agree well with the original pattern within the pH range of 3–11 (Fig. S6, ESI†). As shown in Fig. S7 (ESI†), in a N₂ atmosphere, 1 loses its solvent isopropanol and coordinated water before 159 °C, but the framework can remain unchanged up to 338 °C. Therefore, 1 exhibits excellent water, pH and thermal stabilities.

Photochromism and luminescence of 1

The light green crystals were collected and dried in air (Fig. S8a, ESI†). After being irradiated under UV light, the color of the crystals after soaking in H₂O changed from green to yellow (Fig. S8b and c, ESI†). This is an obvious photochromic phenomenon. The suspension of 1 shows excitation and emission at 350 nm and 469 nm, respectively (Fig. 5). The color of the H₂O suspension of 1 is colorless. After being irradiated under UV light, the colorless suspension turns its color to yellow. The suspension of 1′ shows a new excitation peak at 400 nm and the corresponding emission peak at 504 nm (Fig. 5). This indicates that the excitation and emission wavelengths are red-shifted. The photochromism from 1 to 1′ should be attributed to photocyclodehydrogenation of L units.³⁸

MnO₄⁻ sensing

A 50 μL solution with different anions (5 mM) was added into 2 mL H₂O suspensions of 1 and 1′. As shown in Fig. 6, MnO₄⁻ and Cr₂O₇²⁻ can quench the emission of the suspension of 1. Only MnO₄⁻ can quench the emission of the suspension of 1′. Therefore, 1 and 1′ can act as promising MnO₄⁻ sensing materials. Titration experiments of 1 and 1′ toward 1 mM and 0.1 mM solutions of MnO₄⁻ were carried out, respectively (Fig. 7a and 8a).

The quenching efficiency can be quantitatively analyzed using the Stern–Volmer (S–V) equation: I₀/I = 1 + K_sv × [M]. The K_sv value of 1 for MnO₄⁻ is found to be 8.32 × 10⁴ M⁻¹ (Fig. 7b). In addition, 3σ/k (where σ represents the standard deviation and k represents the slope) is used to calculate the LOD. The LOD value of 1 for detecting MnO₄⁻ is calculated as 1.07 × 10⁻⁷ M (Fig. 7c). But the K_sv and LOD values of 1′ for detecting MnO₄⁻ are found to be 6.11 × 10⁵ M⁻¹ and 3.96 × 10⁻⁸ M, respectively (Fig. 8b and c). The K_sv value of 1′ is over 7 times higher than that of 1, and the LOD value of 1′ is about 3 times lower than that of 1. More importantly, the LOD value of 1′ reaches the nearly lowest level; the K_sv value of 1′ is the second highest in the reported MOF-based MnO₄⁻ sensors (Table 2). Therefore, 1′ has ultrahigh MnO₄⁻ sensing sensitivity. The emission intensities and PXRD patterns of 1 (Fig. S9, ESI†) and 1′ (Fig. S10, ESI†) remain basically unchanged after 4 cycles of MnO₄⁻ sensing, which reveals their good MnO₄⁻ sensing recyclability.

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100 μL of 5 mM H₂O solution of anions was added to 2 mL of a stable water suspension of 1. These anions, except for Cr₂O₇²⁻, only slightly quenched the emission of 1. Cr₂O₇²⁻ can quench the emission of 1. When the same volume of H₂O solution of MnO₄⁻ (1 mM) was added into the above suspensions, the emission of 1 was obviously quenched (Fig. 9a). And for 1′, these anions only slightly quenched the emission of 1′. While the same volume of 0.1 mM MnO₄⁻ solution was added into the above suspensions, the emission of 1′ was apparently quenched. So 1′ had excellent MnO₄⁻ sensing selectivity (Fig. 9b).

Sensing mechanism

The PXRD patterns of 1 and 1′ are consistent before and after detecting MnO₄⁻ (Fig. S4, ESI†), which shows that the frameworks of 1 and 1′ have not collapsed before and after detecting MnO₄⁻. The crystal shapes of 1 and 1′ after detecting MnO₄⁻ have no changes (Fig. S7d and e, ESI†). A clear overlap can be observed between the excitation of 1 (260–400 nm) and the absorption of Cr₂O₇²⁻ (320–400 nm) and MnO₄⁻ (320–410 nm) (Fig. 10), which leads to competitive energy absorption between Cr₂O₇²⁻/MnO₄⁻ and 1. As a result, both Cr₂O₇²⁻ and MnO₄⁻ cause the emission quenching of 1. Accordingly, 1 cannot recognize MnO₄⁻ in the presence of Cr₂O₇²⁻. The new excitation band (350–450 nm) of 1′ overlaps well with the absorption band of MnO₄⁻ (320–460 nm) and deviates from the absorption bands of Cr₂O₇²⁻ (320–400 nm) (Fig. 10). Thus, competitive energy absorption can occur only between 1′ and MnO₄⁻. An efficient FRET will occur between 1′ and MnO₄⁻ due to a good overlap between the emission of 1′ (420–600 nm) and another absorption band (500–600 nm) of MnO₄⁻ (Fig. 10). These cause ultrahigh MnO₄⁻ sensing sensitivity and efficient selectivity for 1′.

Conclusions

An efficient LMOF-based sensor 1 was successfully constructed using a dihydroanthracene-based photochromic organic ligand. 1 exhibits a rare 3D (4,4)-connected self-interpenetrated architecture and has excellent water, pH and thermal stabilities. 1 and 1′ can detect MnO₄⁻ in H₂O through luminescence quenching effects. Especially, 1′ has extremely high sensing sensitivity and selectivity toward MnO₄⁻. Competitive absorption and FRET should be responsible for the highly efficient MnO₄⁻ sensing capability of 1′. This work focuses on the fundamental study of photochromic MOFs including their synthesis, crystal structures, and photochromic, luminescence and sensing properties. This will provide support for the future practical applications of photochromic MOFs such as sensing, anticounterfeiting, etc.

Data availability

Crystallographic data for 1 have been deposited at the CCDC under [2351838] and can be obtained from https://www.ccdc.cam.ac.uk/data_request/cif.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (21671082 and 51432006) and the 111 Project (B13025).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2351838. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4nj02133a

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