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Solvent- and tatrz-induced single-crystal-to-single-crystal transformations within cobalt(II)-triazole MOFs: a luminescent probe for frankincense†

Jialin Feng‡ ^a, Jianshuai Mu‡*^a, Zhengyu Liu‡^a, Lin Cheng*^a, Siyu Li^a, Xin Cheng^a, Shaowei Zhang*^b, Xin Meng^a and Ying Wang*^a
aCollege of Chemistry, Tianjin Normal University, Tianjin 300387, P.R. China. E-mail: hxxymujianshuai@tjnu.edu.cn; hxxycl@tjnu.edu.cn; hxxywy@tjnu.edu.cn
^bSchool of Chemistry and Chemical Engineering, Key Laboratory of Theoretical Organic Chemistry and Functional Molecule of the Ministry of Education, Hunan University of Science and Technology, Xiangtan, Hunan 411201, China. E-mail: swzhang@hnust.edu.cn

First published on 10th August 2024

In supramolecular host–guest complexes, guest molecules can be encapsulated either partially or entirely into the cavities of host molecules. Therefore, it is extremely vital for the host cavity and guest molecules to possess the compatibility of shapes and sizes in the formation of the host–guest complex. In such systems, the mutual orientations of the host and guest units are determined by the specific interactions that participate between the recognition sites of the macrocyclic hosts and the functional groups of guest molecules.^16–19 The definite interactions between functional groups involved in a macrocyclic host's recognition sites and guest molecules has the final decision in terms of the reciprocity of host and guest molecules. Yet, SC–SC transformation possesses a relatively extraordinary particularity among the countless numbers of existing MOFs, especially for the ones composed of triazole-based cobalt networks. This is partially because of the borderline Lewis acidity of Co²⁺ cations; correspondingly, robust metal–ligand coordination interactions not only make bond cleavage and formation rather challenging but also make the crystallinity hard to maintain. A variety of non-covalent interactions, including electrostatic, hydrophobic, hydrogen bonding, π⋯π stacking interaction, and van der Waals interactions enhance the stability of host–guest complexes.

Inspired by our previous efforts involved in framework development during the SC–SC transformations based on transition-metal-triazole-based MOFs,²⁰ we explored the coordination chemistry between Co²⁺ ions and mixed ligands of 1-(9-(1H-1,2,4-triazol-1-yl)anthracen-10-yl)-1H-1,2,4-triazole (tatrz) and isophthalic acid (H₂ipa) in this contribution. Initially, the hydrothermal reaction of Co(NO₃)₂·6H₂O with tatrz and H₂ipa ligands at 90 °C for 3 d gave reddish-brown block crystals of {[Co(tatrz)(ipa]·H₂O}_n (1). When soaking crystal 1 in DMF for 6 h, another isomorphous MOF was gained by the solvent exchange of the lattice water molecule, {[Co(tatrz)(ipa)]·DMF}_n (2). In contrast, after immersing crystal 1 in the ethanol solution of tatrz for 3 days, {[Co(tatrz)(ipa)(H₂O)]·(tatrz)_0.5}_n (3) can be isolated (Scheme 1). The SC–SC transformations observed in MOFs 1 to 3 could be ascribed to the variation of the coordination environments of Co²⁺ ions. In addition, fluorescent results indicated that MOF 3 represents the first luminescent sensor for the detection of frankincense.

	Scheme 1 The schematic diagram of SC–SC transformations among MOFs 1–3.

X-ray crystallographic analyses demonstrate that MOFs 1–3 crystallize in the triclinic space group P and have similar skeletons composed of one crystallographically independent Co²⁺ ion, one tatrz and one ipa²⁻ except for different guest molecules (Fig. S1a–c†). The crystallographic data and selected bond lengths and angles for MOFs 1–3 are listed in Tables S1 and S2,† respectively.

Herein, MOF 1 is taken as an example. The Co1 ion exhibits a six-coordinate octahedral geometry with the O_h symmetry (Fig. S1d and Table S3†), which consists of two N atoms from two different tatrz ligands [Co1–N: 2.141(2) and 2.146(2) Å] and four carboxylic O atoms of three ipa²⁻ ligands in both bridging and chelating modes [Co1–O: 2.025(2)–2.208(2) Å]. In the coordination configuration of Co1 ion (Fig. S1b†), the four carboxylic O atoms (O1, O2, O3 and O4) construct the middle plane and N1 and N6 atoms are separately located in the two vertexes of the octahedron. The standard deviation from the least-square of the middle plane is 0.0022 Å, the distance between Co1 and the middle plane is 0.0069 Å, the distances between N1 and N6 atoms and the middle plane are severally 2.1320 and 2.1469 Å. The N1, O1 and O4 atoms, the N1, O1 and O2 atoms, the N1, O2 and O3 atoms, the N1, O3 and O4 atoms, the N6, O1 and O4 atoms, the N6, O1 and O2 atoms, the N6, O2 and O3 atoms, the N6, O3 and O4 atoms, are the eight sides of the octahedron, respectively. The dihedral angles between the eight surfaces and the middle plane are 60.2°, 47.7°, 53.0°, 61.5°, 56.4°, 49.4°, 57.4°, and 60.1°. The distances between Co1 and the eight surfaces are 1.1991, 1.3973, 1.1469, 1.0580, 1.1445, 1.4241, 1.2016, and 1.0364 Å, respectively. The above distances and dihedral angles indicate that the octahedron is conspicuously distorted. Adjacent Co²⁺ ions are bridged by tatrz ligands to generate a 1D double chain, which is further connected by ipa²⁻ ligands through carboxylic O atoms to present a 2D network with 1D channels, in which free lattice water molecules are located (Fig. S1e†). Moreover, the adjacent interlayers are occupied by free lattice water molecules (Fig. S2a†). In comparison, the guest molecules DMF in 2 and free tatrz in 3 are only located in adjacent interlayers due to their larger sizes (Fig. S2b and c†). To understand the structure of 1, it could be simplified by the application of topology analysis via the freely available computer program TOPOS.²⁰ If every {Co₂(CO₂)₂} unit is served as a four-connected node, the 2D network of 1 can be described as a four-connected ‘sql’-type topology (Fig. S1f†). Note that MOF 1 possesses lattice water molecules, as well as numbers of C, N and O atoms, which could serve as hydrogen bonding donors and acceptors to assist in the formation of hydrogen bonding. Moreover, the offset arrangements of anthracene rings of tatrz ligands and the shorter distances of layers favour the generation of π⋯π stacking interactions. Therefore, adjacent 2D layers could form a 3D supramolecular architecture through hydrogen bonding and π⋯π stacking interactions (Fig. S1g†).

Inspired by SC–SC transformations induced by solvent-, anion- or host–guest exchanges in the triazole ligands-based systems,¹⁹ we exploited crystal 1 as a precursor in DMF for 6 h; solvent-exchange experiments suggested that dissociative water molecules in 1 could be completely replaced by DMF as evidenced by the product of {[Co(tatrz)(ipa)]·DMF}_n (2). A careful investigation shows that the intercalated DMF and water molecules play the key role in differentiating the two structures. The structure of 2 is essentially isostructural to 1, with similar cell dimensions and the same gross structural features (detailed bond parameters can be obtained for the archived CIF files). When reddish-brown block crystals of 1 were exposed to the ethanol solution of tatrz for 3 days, light red block crystals of {[Co(tatrz)(ipa)(H₂O)]·(tatrz)_0.5}_n (3) can be isolated, showing sponge-like dynamic behaviour with retention of the crystalline integrity. Single crystal analyses indicate that the 2D network of 3 encapsulates free tatrz as the template, which represents the first example of reactive organic group functionalized large aromatic guest (tatrz) loaded host–guest complex.

Considering the coexistence of coordinated tatrz and H₂ipa with π-conjugated structures, as well as the free tatrz ligand in MOF 3, which may favour the development of luminescent materials, the emission spectra of the free ligands tatrz and ipa as well as MOF 3 have been measured in DMF solution at room temperature (Fig. S9†). Upon the excitation wavelength of 343 nm, the free tatrz ligand displays an obvious emission band at ca. 441 nm, and the free H₂ipa ligand exhibits an apparent emission band at ca. 428 nm. However, MOF 3 shows two emission bands with slight blue shifts at ca. 425 and 405 nm, respectively, which indicate that the emission signs of MOF 3 may be mainly derived from the intra-ligand emission states for the π* → n and/or π* → π transitions rather than ligand-to-metal charge transfer (LMCT) or metal-to-ligand charge transfer (MLCT).

Frankincense essential oil and its main component α-pinene have also been shown to have anti-proliferation and pro-apoptotic effects on various types of cancer cells.²¹ The exploration of a rapid and simple method for the sensitive detection of frankincense is of great significance. Therefore, essential oil derivatives containing frankincense, lavender, ginger, fennel, basil and lemongrass were selected to evaluate the luminescent sensitivity of MOF 3 (Fig. S10†). The results indicated that MOF 3 may be considered as luminescent probes of frankincense (Fig. 1). The main component of frankincense is α-pinene, while the key ingredient of lavender, ginger, fennel, basil and lemongrass is linalool, α-zingiberene, anethene, ocimene and citral, respectively. Their constitutional formula is shown in Fig. S14.† As is depicted in Fig. S14,† the size of α-pinene in frankincense is comparatively smaller than that of other essential oils used for aromatherapy purposes. Because the channel diameter of MOF 3 is not large enough to accommodate the guest recognition complex such as lavender, ginger, fennel, basil and lemongrass, only frankincense was expected to be located at the inner surface. The main components of lavender, ginger, fennel, basil and lemongrass opt b3lyp 6-31g(d) were optimized using Gauss and the sizes of the six molecules were calculated using multiwfn.²² The corresponding calculation results are shown in Fig. S23.† The size of the main component of frankincense is the smallest; it can be recognized among the channel of MOF 3. To evaluate the detection limit and sensitivity of 3 towards frankincense, the concentration-dependent intensity experiments were performed. Upon gradually adding the aqueous solution of frankincense, the luminescent intensities gradually decreased (Fig. S10a†). When the concentration of frankincense reached 0.1 mM, the luminescent intensity weakened by 31.9%; when the amount reached 1.0 mM, the luminescent intensity was nearly quenched and the intensity weakened by 90.3%. The limit of detection (LOD) was 3.39 × 10⁻⁶ mol L⁻¹ obtained using the expression 3σ/k, where σ is the standard deviation of luminescent intensities of 10 field blanks and k is the slope (Fig. S10b†). A reasonable explanation for the sensing mechanism is that α-pinene interacts with MOF 3 and activates non-emissive deexcitation channels, thus quenching the emission. In addition, other components of frankincense may be responsible for the observed fluorescence quenching. However, note that a deep understanding of such quenching effects is still lacking. A series of studies need to be done to explore the possible detection mechanism and will be discussed in the following work.

	Fig. 1 Photoluminescent intensity of MOF 3 (1 × 10−3 mol L−1) in DMF solution at room temperature with the addition of different essential oils (4000 ppm). All the compounds are excited at 343 nm. The emission peaks are located at 410–440 nm.

Conclusions

Data availability

Conflicts of interest

Acknowledgements

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