Enhancing the singlet oxygen capture and release rate of metal−organic frameworks through interpenetration tuning
Jing Hao , Feifan Lang , Liqin Hao , Yi Yang , Lulu Zhang , Hao Zhang , Quan-Wen Li , Jiandong Pang , Xian-He Bu
Singlet oxygen (1O2) is an excited state of molecular oxygen, which can be obtained via chemical reactions, cellular metabolisms, or photoexcitation of ground-state oxygen [1,2]. The controlled capture and release of 1O2 is of great significance owing to its potential applications in industrial wastewater treatment, photochemical organic synthesis, photodynamic therapy, and so on [3–6]. It has been confirmed that some specific aromatic derivatives able to capture 1O2 through endo-peroxidation reaction (endo-peroxidation reaction is the [4 + 2] cycloaddition reaction of forming endoperoxides from singlet oxygen and possibly canthracene, pyridone, or naphthalene.) with 1O2 to form endoperoxides (EPOs) and release it under external thermal stimulation or ultraviolet irradiation [7]. In which, most of the compounds capable in such process were functionalized with anthracene, 2-pyridinone, or naphthalene [8–10]. However, the reported aromatic compounds usually show low 1O2 capture and release efficiency due to their poor solubility, amorphous aggregation, and non-porous structures in general, which lead to the inaccessibility of the active sites [10]. In addition, many EPOs based on polycyclic aromatic hydrocarbons are unstable under ambient conditions and exhibit poor recyclability, which seriously affects their applications [3]. Besides, the precise regulation of 1O2 capture and release rates is of great importance for achieving practical applications due to its short lifetime and high reactivity, which so far has not been perfectly solved [11]. Therefore, it is urgently needed to construct new porous materials with regular functional channels, high stability, and good recyclability, in order to further achieve accurate regulation of the trapping and releasing rate of 1O2.
Due to their highly tunable pore structures and definite structural-property relationships, metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) have attracted widely attentions, and show potential applications in gas adsorption and separation [12–16], chemical sensing, drug delivery, and catalysis [17–24]. With the development of crystal engineering, the structures of MOFs can be predesigned and predicted according to several useful strategies such as reticular chemistry and soft/hard acid/base principle [25–29]. In addition, the pore environment of the as-synthesized MOFs can be precisely tuned by post-synthetic modification, including metal/linker exchange or installation [30–32]. Therefore, MOFs with various as-designed functions can be obtained.
Many pioneering works have confirmed that MOFs constructed from anthracene-based linkers can capture 1O2 through cycloaddition reactions to form EPOs, which exhibit excellent 1O2 release properties under heating or ultraviolet (UV) irradiation [33–35]. For example, a two-fold interpenetrating MOF based on the anthracene-containing DPA linker was reported by Matsuda and co-workers, which reacts with 1O2 and produces anthracene endoperoxide proved by crystallographic data. Quantitatively reverse reaction proceeds under heat and the material can be used for the removal of gaseous oxygen in water under ppm level [36]. However, the regulation of 1O2 capture and release rates by structural switching in MOFs has been rarely reported. We believe that the varied post-synthetic methods can bring new opportunities to the 1O2 capture and release property tuning of MOFs [22,37,38]. Based on the previous research, we demonstrated that single-crystal to single-crystal structural transformation of an anthracene-based MOF (NKM-181, "NKM" stands for School of Materials Science and Engineering, Nankai University) from seven-fold to six-fold interpenetration (NKM-182) can be achieved by post-synthetic modification under the assisted of solvent, showing an increased porosity and more accessible active anthracene sites, which led to a dramatically enhanced 1O2 capture and release efficiency.
The anthracene derivative 9,10-di(pyridin-4-yl)anthracene (DPA) and the elongated ditopic carboxylic linker 2′,5′-bis(trifluoromethyl)-[1,1′:4′,1″-terphenyl]-4,4″-dicarboxylic acid (H2TPDC-2CF3) were selected to prepare the 1O2 trapping MOFs. Due to the poor solubility of the elongated ditopic carboxylic linker, we introduced -CF3 groups to improve both the solubility of ligand and the stability of the MOFs. NKM-181 can be synthesized by reaction of DPA, H2TPDC-2CF3 with Cd(NO3)2·4H2O in DMF, EtOH and H2O for 72 h at 100 ℃, and NKM-182 can be obtained after soaking NKM-181 in methanol for 4 h under ambient temperature (Fig. 1). Single-crystal X-ray analysis revealed that NKM-181 crystallizes in the monoclinic space group C2/c and the asymmetric unit consists of one Cd2+ ion, one TPDC-2CF32− fragment, one DPA ligand, and one coordinated H2O molecule. The Cd2+ ion is coordinated in a trigonal-bipyramidal geometry defined by two pyridyl nitrogen atoms of two DPA ligands, two carboxylate oxygen atoms from two TPDC-2CF32− fragments and one oxygen atom from the coordinated H2O molecule. Each TPDC-2CF32− fragment links to two independent Cd2+ ions via two oxygen atoms from two carboxyl groups, and each DPA ligand coordinates to two Cd2+ ions via two pyridyl donors. On the whole, a three-dimensional framework with quadrilateral pores was formed, which can be descripted as a dia topology framework with the Cd2+ ions simplified as 4-connected nodes. Such dia framework consists with large quadrilateral pores but are filled via mutual interpenetration to another six of its equivalents into an overall seven-fold 3D network with no potentially accessible voids determined. Different from that of NKM-181, NKM-182 crystallizes in the monoclinic space group P2/c and the asymmetric unit consists of one Cd2+ ion, one TPDC-2CF32− fragment and one DPA ligand. The Cd2+ ion is coordinated in a twisted octahedral geometry defined by two pyridyl nitrogen atoms from two DPA ligands and four carboxyl oxygen atoms from two TPDC-2CF32− fragments. The carboxyl oxygen atom on each TPDC-2CF32− fragment binds to two independent Cd2+ ions in a bidentate chelated coordination fashion, and each DPA ligand coordinates to two Cd2+ ions via two pyridyl groups (Figs. S1 and S2 in Supporting information). Interestingly, the terminal coordinated water molecules observed in NKM-181 were removed through methanol soaking (no coordinated methanol observed). Similar dia topological networks can be obtained if the Cd2+ ion was simplified as 4-connected nodes. The size of the quadrilateral channel in a single framework of NKM-182 is slightly smaller than that of in NKM-181 due to the increased steric hindrance from the anthracene groups that are now oriented more orthogonal to the channels. A six-fold interpenetrated network, rather than seven-fold, was formed in NKM-182 with preserved rectangular open channels (dimensions of approximately 9.17 Å × 14.29 Å). The active anthracene sites are therefore more exposed than those of in NKM-181, which possibly dominating the significant increase in 1O2 capture and release rates.
The robustness of two MOFs was tested before 1O2 trapping measurements, and both of NKM-181 and NKM-182 show good thermal and chemical stabilities. Combination of thermogravimetric analysis curves and temperature-variable PXRD patterns confirmed that NKM-181 and NKM-182 are very stable until reached ~320 ℃ and ~360 ℃, respectively (Figs. S3 and S4 in Supporting information). The chemical stability of the MOFs was investigated by immersing crystalline samples in aqueous solutions with pH ranging between 2 and 12 or various solvents. The PXRD patterns proved that both two frameworks remained intact after treated in weak acidic or basic solution (pH 4-12, Fig. S5 in Supporting information), or in common types of organic solvents apart from the combination of NKM-181/methanol (Fig. S6 in Supporting information). A drastic single-crystal-to-single-crystal phase transition occurred from NKM-181 (seven-fold interpenetration) to NKM-182 (six-fold interpenetration) when immersing the former into methanol. Therefore, the PXRD patterns remarkably changed for the methanol treated NKM-181, which is now consistent with the simulated ones for NKM-182.
In order to verify whether NKM-181 and NKM-182 can effectively capture and release of 1O2, we prepared EPO-DPA by [4 + 2]-cycloaddition of DPA and 1O2 under 660 nm light-emitting diode (LED) irradiation using 5,10,15,20-tetraphenylporphyrin (TPP) as the photosensitizing agent. As shown in Fig. 2a, a new peak centered at 84 ppm appeared on the 13C NMR spectrum of EPO-DPA, which can be attributed to the formation of peroxidation bonds in EPO-DPA. Time resolved 1H NMR spectrum was used to trace the trapping process of 1O2. As shown in Fig. 2b, a new peak centered at 7.1 ppm appeared on the 1H NMR spectrum after 20 min, indicating the generation of EPO-DPA. The electrospray ionization mass spectrometry (ESI-MS) data of EPO-DPA showed the quasi-molecular ion peak at mass-to-charge ratio (m/z) of 365.1, indicating the successfully capture of 1O2 by the DPA linkers (Fig. S7 in Supporting information).
Fluorescence spectroscopic measurements were used to demonstrate the successfully capture and release of 1O2 by the MOFs. 5 mg NKM-181 or NKM-182, 0.5 mg meso‑tetraphenylporphyrin (TPP) and 20 mL DMF were added to a round-bottom flask and treated with ultrasound in an ice bath for 20 min to give a purple suspension. The mixture was then illuminated by light of a wavelength 660 nm under pure oxygen flow, before fluorescence spectra were collected at different reaction times. As expected, NKM-181 and NKM-182 behave similar during 1O2 trapping, thus NKM-182 will be discussed in detail as a representative for brevity. As shown in Fig. 3a, the fluorescence peak intensity of NKM-182 gradually decreased with the increase of irradiation time and hardly show further changes after about 60 min of reaction, indicating the continuous formation of the endoperoxides EPO-NKM-182 before the saturation of the reaction between anthracene group and 1O2.
Then the photodegradation reaction of EPO-NKM-182, i.e., the release of the 1O2 from the endoperoxides, was also carefully investigated. According to the literature, the photocycloreversion reaction of endoperoxide derivatives occurred in the upper excited singlet state π-π*, while the decomposition reaction was taken place in the lowest excited state S1 (π*σ*), corresponding to the partly excited peroxide chromophore [7]. There are two absorption bands in the UV–vis spectrum of EPO-NKM-182, containing a high-energy absorption at ~265 nm and low-energy absorptions that exceed 290 nm (Fig. S8 in Supporting information). Therefore, the 254 nm light was chosen to trigger the decomposition reaction of EPO-NKM-182, and time resolved fluorescence spectrum was recorded to track the reaction process. The fluorescence intensity increased with the irradiation time extended, and the reaction basically completed in about 30 min (Fig. S9 in Supporting information). In addition to light irradiation, heating is another commonly used method to initiate the endoperoxide decomposition reaction. Upon heating at 100 ℃, the weakened fluorescence intensity of EPO-NKM-182 gradually raise back, and almost restored as of before capture within only 11 min (Fig. 3b). The 1O2 capture and release processes of NKM-182 are reversible and can tolerate at least six cycles without significant decrease of the loading amount (Fig. 3c). To verify the oxygen released from the system was under excited state, 1,3-diphenylisobenzofuran and EPO-NKM-182 were dispersed into 20 mL of DMF and heated at 100 ℃, and the characteristic absorption of 1,3-diphenylisobenzofuran at 410 nm decreased in the UV–vis absorption spectra, revealing the release of 1O2 from EPO-NKM-182 (Fig. 3d). Additionally, the X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), elemental analysis (EA), and scanning electron microscope (SEM) results displayed that the content of oxygen in EPO-NKM-182 increased after binding 1O2, which proves that 1O2 has been successfully captured (Figs. 3e-g and Figs. S10-S13 in Supporting information).
The ratio of the fluorescence intensity to the original fluorescence intensity (I/I0) of NKM-181 and NKM-182 were used to further evaluate the 1O2 trapping and releasing efficiency. As shown in Fig. 4a and Table S3 (Supporting information), the rate of 1O2 trapping and releasing for NKM-182 is much faster than that of NKM-181. The PXRD studies indicated that the structural integrity of NKM-181 and NKM-182 were fully retained after the completion of six reaction cycles (Fig. 4b). The fluorescence emission peak under solid state remained at 445 nm when the excitation wavelength increased from 320 nm to 420 nm in the three-dimensional fluorescence spectrogram of NKM-181 (Fig. 4c). The excitation wavelength of NKM-182 ranges from 290 nm to 370 nm, but the position of the fluorescence emission peak is consistent with that of NKM-181 (Fig. 4d). Therefore, the delayed fluorescence spectra at 445 nm before and after 1O2 capture of both MOFs were tested. As shown in Figs. 4e and f, the fluorescence lifetime for both compounds were all weakened after binding 1O2, as a consequence of destroyed conjugated structure after trapping 1O2. Compared with NKM-181, the efficiency of trapping and releasing 1O2 of NKM-182 is significantly improved under the same test conditions (Figs. S14-S19 in Supporting information).
The single-crystal structures of NKM-181 and NKM-182 were carefully analysed in order to figure out the reason for the improvement of the 1O2 trapping and release properties. As shown in Fig. 5a, NKM-181 is a densely packed structure without solvent accessible voids, while open channels with dimensions of approximately 9.17 Å × 14.29 Å can be observed in NKM-182. Therefore, we infer that the regular pores in NKM-182 can be beneficial to the transfer of 1O2 in the MOFs. Furthermore, the EPO-NKM-181 and EPO-NKM-182 can be used as smart materials for anti-fake applications. The EPO-NKM-181 (Fig. S21 in Supporting information) or EPO-NKM-182 (Fig. 5b) powder was sprinkled on a glass substrate with the shape of "181" or "182" characters and irradiated with 365 nm light to emit a weak fluorescence. The intensity of the blue fluorescence for "181" and "182" remarkably increased after heated the glass at 100 ℃, which was due to the release of the captured 1O2.
In conclusion, interpenetration tuning of the anthracene-based MOFs from seven-fold (NKM-181) to six-fold (NKM-182) have been successfully achieved through solvent assisted single-crystal to single-crystal transformation. Both NKM-181 and NKM-182 present good thermal and chemical stability, with 1O2 capture and release properties proved and investigated from multiple perspectives. Compared to NKM-181, NKM-182 shows increased structural porosity, better accessible active anthracene sites, and enhanced 1O2 capture and release efficiency. In addition, the prepared endoperoxides EPD-NKM-181 and EPD-NKM-182 exhibit attractive potential for anti-fake purpose. This work provides a new platform for 1O2 capture, storage, and related promising applications.
The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.
The authors acknowledge the financial support of the National Natural Science Foundation of China (Nos. 22035003 and 22201137), Nature Science Fund of Tianjin, China (No. 19JCZDJC37200), Fundamental Research Funds for the Central Universities (No. 63223020) and the Haihe Laboratory of Sustainable Chemical Transformations (No. YYJC202101).
Supplementary material associated with this article can be found, in the online version, at doi:
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Figure 1 Synthesis and structural description of NKM-181 and NKM-182: (a) The synthesis of NKM-181; (b, c) Coordination modes of the Cd2+ ions in NKM-181 and NKM-182; (d, f) The dia topological networks in NKM-181 and NKM-182; (e, g) The seven-fold and six-fold interpenetration frameworks in NKM-181 and NKM-182.
Figure 2 (a) 13C NMR spectra of DPA before (blue) and after (pink) photo-oxygenation (star peak represents peroxidation bonds). (b) Time resolved 1H NMR spectra of the 1O2 capturing by DPA.
Figure 3 (a) Time resolved fluorescence spectra of NKM-182 in the process of 1O2 capturing and (b) releasing under heat. (c) Fluorescence intensities of NKM-182 in multiple cycles of 1O2 capture and thermal triggered release. (d) Time resolved UV–vis absorption spectra of 1,3-diphenylisobenzofuran in DMF at 373 K in the presence of EPO-NKM-182. (e) XPS spectra of NKM-182 and EPO-NKM-182 (O 1 s) (7.41% and 11.31% represented for the O element atom percentage of NKM-182 and EPO-NKM-182). (f) Oxygen elemental contents of NKM-182 and EPO-NKM-182. (g) TEM image and EDX elemental mapping of NKM-182.
Figure 4 (a) Comparison of NKM-181 and NKM-182 for 1O2 trapping rates and thermal triggered releasing rates (blue axes: capture; black axes: release). (b) PXRD patterns of NKM-181 and NKM-182, pre- and after-reaction and simulated. Solid-state fluorescence excitation-emission matrix spectra of (c) NKM-181 and (d) NKM-182. Fluorescence decay profiles of (e) NKM-181 and (f) NKM-182.
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