Perylene Diimide-Based Multicomponent Metallacages as Photosensitizers for Visible Light-Driven Photocatalytic Oxidation Reaction

Abstract

Open AccessCCS ChemistryCOMMUNICATION5 Aug 2022Perylene Diimide-Based Multicomponent Metallacages as Photosensitizers for Visible Light-Driven Photocatalytic Oxidation Reaction Yali Hou, Zeyuan Zhang, Lingzhi Ma, Ruping Shi, Sanliang Ling, Xiaopeng Li, Gang He and Mingming Zhang Yali Hou State Key Laboratory for Mechanical Behavior of Materials, Shaanxi International Research Center for Soft Matter, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049 Google Scholar More articles by this author , Zeyuan Zhang State Key Laboratory for Mechanical Behavior of Materials, Shaanxi International Research Center for Soft Matter, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049 Google Scholar More articles by this author , Lingzhi Ma State Key Laboratory for Mechanical Behavior of Materials, Shaanxi International Research Center for Soft Matter, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049 Google Scholar More articles by this author , Ruping Shi State Key Laboratory for Mechanical Behavior of Materials, Shaanxi International Research Center for Soft Matter, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049 Google Scholar More articles by this author , Sanliang Ling Advanced Materials Research Group, Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD Google Scholar More articles by this author , Xiaopeng Li College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518055 Google Scholar More articles by this author , Gang He Frontier Institute of Science and Technology (FIST), Xi’an Jiaotong University, Xi’an 710049 Google Scholar More articles by this author and Mingming Zhang *Corresponding author: E-mail Address: [email protected] State Key Laboratory for Mechanical Behavior of Materials, Shaanxi International Research Center for Soft Matter, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049 State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101382 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Self-assembled supramolecular structures with efficient singlet oxygen (1O2)-generation ability are expected to enable photocatalytic oxidation reactions. Herein, we use two photosensitizers, perylene diimide and benzothiadiazole derivative, as the boards to prepare two barrel-shaped metallacages via metal-coordination-driven self-assembly. The integration of two photosensitizers takes advantage of Pt(II)-based coordination bonds to promote the intersystem crossing, offering the metallacages with high 1O2 generation efficiency (quantum yield up to 56%). The metallacages are further used to oxidize a series of alkenes. The yields of the photooxidation reaction exceed 90% when even 1 mol % metallacage is used. This study uses a type of metallacages for visible light-driven photocatalytic oxidation reactions, which will guide the future design and applications of metallacages toward photocatalysis. Download figure Download PowerPoint Introduction Singlet oxygen (1O2) plays a vital role in many natural organic and inorganic processes owing to its strong oxidizing properties.1 It has been extensively used for photocatalytic oxidation2–4 and photodynamic therapy.5–7 A photosensitizer with efficient 1O2 production generally calls for intense light absorption, good photostability, and effective intersystem crossing from the singlet to the triplet state.8,9 However, it is challenging to develop a single photosensitizer to meet all these requirements simultaneously. Therefore, considerable effort has been devoted to the integration of multiple photosensitizers into a single molecule or polymer (Schemes 1a and 1b) to avoid their degradation and self-aggregation, to increase the stability and efficiency of 1O2 generation.10,11 However, covalent functionalization often entails tedious synthesis and suffers from poor yields, which greatly limit their practical application. Therefore, the development of a new strategy to prepare photosensitizers with good stability and 1O2 generation ability is urgently needed. Scheme 1 | Cartoon representations of the integration of multiple photosensitizers into a single entity by the formation of (a) small molecule (b) polymer and (c) multicomponent metallacage. Download figure Download PowerPoint Metal-coordination-driven self-assembly has proven to be a powerful way for the construction of supramolecular coordination complexes (SCCs).12–19 This strategy uses the highly directional metal-coordination bonds to prepare SCCs spontaneously and endows the SCCs with high structural and functional diversity.20–28 Especially, the preparation of photosensitizers using this method not only avoids their self-aggregation through the formation of geometric structures, but also facilitates intersystem crossing owing to the introduction of heavy metal atoms, giving high 1O2 generation ability and photostability.29–34 Moreover, different building blocks capable of generating 1O2 can be integrated into a single metallacage. Such a metallacage possesses a three-dimensional (3D) cavity, which is capable of encapsulating guest molecules (Scheme 1c). The oxidation of substrates in the inner cavities of metallacages enhances the efficiency of 1O2 transfer from the metallacage to the encapsulated guests, which could further increases the catalytic efficiency.16 Therefore, such metallacages need to be developed and explored for photocatalysis. Perylene diimide (PDI)35 and benzothiadiazole36,37 derivatives are well-known photosensitizers which can generate 1O2 effectively, benefiting their wide application in photocatalysis and photodynamic therapy. Herein, we combine the two photosensitizers and prepare two metallacages via the multicomponent self-assembly of PDI-based tetrapyridyl ligand ( 1), benzothiadiazole-derived tetracarboxylic ligands ( 2a and 2b) ( Supporting Information Figures S1–S8) and cis-Pt(PEt3)2(OTf)2 ( 3) (Figure 1a and Supporting Information Figures S9–S14). The 1O2 quantum yields of metallacages 4a and 4b are 0.35 and 0.56 (λex = 520 nm), respectively, much higher than those of their building blocks, owing to the synergistic effect of the ligands. These metallacages are further used for the oxidation of a series of alkenes, showing good catalytic efficiency upon visible light irradiation. This study represents the successful usage of metallacages as efficient 1O2 photosensitizers, which will promote the development of metallacages for photocatalytic oxidation. Figure 1 | (a) Cartoon illustrations of the self-assembly of metallacages 4a and 4b and metallacycle 4c. Partial 31P{1H} NMR (b–d) and 1H NMR (e–g) spectra (162 or 400 MHz, CD3CN, 295 K) of 1 (e), 3 (b), 4a (c and f) and 4b (d and g). ESI-TOF-MS spectra of 4a (h) and 4b (i). Download figure Download PowerPoint Results and Discussion The formation of metallacages 4a and 4b was confirmed by 31P{1H}, 1H NMR, and electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS). The 31P{1H} NMR spectra of 4a and 4b split into two doublet peaks at 5.37 and −0.26 ppm for 4a, and 5.17 and −0.32 ppm for 4b, respectively (Figures 1b–1d). These two doublet peaks shared the same intensity with concomitant 195Pt satellites, indicating the formation of discrete, charge-separated metallacages. In the 1H NMR spectra, the α-pyridyl protons Ha and the β-pyridyl protons Hb split into two sets of signals with obvious chemical shifts (Figures 1e–1g), corresponding to the protons inside and outside the metallacages.27 To further confirm the successful formation of the metallacages, coordination stoichiometries of 4a and 4b were determined by ESI-TOF-MS (Figures 1h and 1i). Isotopically well-resolved peaks with charge states ranging from 3+ to 8+ were found for the two metallacages due to the loss of counterions (OTf−), suggesting the composition of the metallacages. Orange triclinic single crystals of 4a suitable for X-ray diffraction analysis were obtained by slow diffusion of dioxane into its dimethylformamide (DMF) solution ( Supporting Information Table S1). The single-crystal structure (Figures 2a and 2b) unequivocally confirmed the 3D barrel-shaped structure of 4a. Each metallacage was composed of two tetrapyridyl PDI units and two tetracarboxylic ligands joined by eight Pt atoms, forming a highly symmetrical, barrel-shaped metallacage. The two PDI faces were parallel with each other. The distance between the two faces was 1.29 nm, offering good π–π stacking interactions with conjugate aromatic molecules. The dimensions of the metallacage were 1.62 × 1.47 × 1.08 nm, based on the distance between Pt atoms. The large portals made it accessible for the guest molecules to enter into the cavity of the metallacage to form host–guest complexes. In the packing model (Figure 2c and Supporting Information Figure S18), the metallacages are aligned to form nanochannels, beneficial for the transportation of guest molecules. Figure 2 | (a and b) Crystal structure of 4a in different views. (c) Crystal packing model of 4a. Hydrogen atoms, counterions, solvent molecules, and triethylphosphine units are omitted for clarity.b Download figure Download PowerPoint UV–vis absorption and emission spectra in acetonitrile were collected to investigate the photophysical properties of ligand 1, metallacages 4a and 4b, and metallacycle 4c (Figure 3a and Supporting Information Figures S15–S17). Three typical absorption bands centered at 456, 487, and 522 nm were observed for ligand 1. Besides these three absorption bands, metallacages 4a and 4b exhibited an extra absorption band centered at 379 and 363 nm, respectively, derived from the absorption of tetracarboxylic ligands 2a and 2b ( Supporting Information Figure S25). Two emission bands were found for ligand 1 (λmax = 539 and 576 nm), metallacages 4a and 4b (λmax = 545 and 578 nm), and metallacycle 4c (λmax = 539 and 576 nm). In the case of 4a, it also showed a weak emission peak at 455 nm, which agreed well with the characteristic emission of benzothiadiazole derivatives.36 The absolute fluorescence quantum yields of ligand 1 and metallacages 4a and 4b in acetonitrile were determined to be 95.21%, 38.46%, and 20.03% (λex = 365 nm), respectively ( Supporting Information Figures S19–S24). The quantum yields of metallacages 4a and 4b were lower than that of ligand 1, suggesting that a nonradiative decay process may have taken place, and the metallacages generated 1O2 upon irradiation. Figure 3 | (a) UV–vis absorption (dash line) and fluorescence (solid line) spectra of ligand 1, metallacages 4a and 4b, and metallacycle 4c in acetonitrile (λex = 365 nm, 10 μM). (b) Plots of the absorption decays of DPBF at 410 nm versus irradiation time using 1, 4a, 4b, or 4c as the photosensitizer in acetonitrile (λex = 520 nm). Download figure Download PowerPoint The 1O2 generation capacity by photosensitization with 1, 2a, 2b, 4a, 4b, and 4c in acetonitrile was first studied by measuring the phosphorescence spectra of 1O2 upon visible light irradiation (λex = 520 nm). Ligand 1 and complexes 4a–4c showed a clear peak centered at 1270 nm ( Supporting Information Figure S26), which is a typical characteristic of 1O2 generation.3 The quantum yields of 1O2 generation (ΦΔ) of all the species were measured using a reactive 1O2 scavenger, 1,3-diphenylisobenzofuran (DPBF). Solutions of a mixture of DPBF and different compounds were irradiated by a light source at 520 nm, and their time-dependent UV–vis absorption spectra were recorded ( Supporting Information Figures S27a–S27g). A gradual decrease of the typical absorption band centered at 410 nm for DPBF was observed as a function of exposure time, suggesting the accumulation of 1O2. The absorption bands corresponding to 4a, 4b, and 4c remained invariant ( Supporting Information Figures S27e–S27g), suggesting their good photostability. ΦΔ of all the compounds was calculated using a visible light photosensitizer Rose Bengal (RB) with known efficiency (ΦΔRB = 0.54)38 as the reference. The ΦΔ values were measured to be 15%, 0,a 23%, 35%, 56%, and 29% for 1, 2a, 2b, 4a, 4b, and 4c, respectively (Figure 3b and Supporting Information Figure S27). The 1O2 generation of 4a mainly came from the photosensitization of its PDI ligand 1 because ligand 2a showed negligible absorption in the visible region ( Supporting Information Figure S29). However, there was a good spectral overlap between the emission of 1 and the absorption of 2b ( Supporting Information Figure S28), so fluorescence resonance energy transfer took place from 1 to 2b, which will further increased the efficiency of singlet oxygen generation of the metallacage. It is worth noting that the metallacages exhibited higher ΦΔ than their building blocks, owing to the integration of different photosensitizers via metal-coordination bonds, which further enhanced the intersystem crossing and thus increased the efficiency of 1O2 generation. The 1O2 generation property of metallacages 4a and 4b inspired us to explore their applications in photooxidation reactions. A series of alkenes including 1,5-dihydroxynaphthalene (DHN, G5), anthracene (G6), 10-methylanthracene (G7), 9,10-dimethylanthracene (G8), 1,4-diphenylbutadiene (G9), and hexamethylbenzene (G10) were used as the substrates (Table 1) to examine the scope of the reaction ( Supporting Information Figures S35 and S36). The metallacages showed good photocatalytic efficiency toward all the test alkenes upon visible light irradiation, offering the oxidized products with conversions and yields over 90% for G5, G6, G7, and G8. For alkenes G9 and G10, the metallacages also showed moderate oxidation properties. To study the difference on the catalytic properties between the metallacages and ligand 1, time-dependent oxidation experiments of DHN by 1, 4a, and 4b were performed ( Supporting Information Figures S39–S42). Upon excitation at 520 nm in the presence of 1 mol % metallacages 4a or 4b, the peaks of the protons on DHN gradually decreased, while a set of new peaks related to its oxidized product, juglone, gradually emerged. After 40 min, almost all the DHN was consumed ( Supporting Information Figures S41 and S42). Only a very small amount of DHN was oxidized even though 2 mol % ligand 1 was used as photosensitizer after 40 min ( Supporting Information Figure S39). The reaction yields versus irradiation time in acetonitrile were plotted (Figure 4a). The yields catalyzed by 1 mol % metallacages 4a, 4b, and 2 mol % ligand 1 after 40 min were 88.0%, 92.4%, and 24.7%, respectively. Table 1 | Photooxidation of Organic Substrates Using Metallacage 4a as Photosensitizer Substrate Conversion (%) Yield (%)a TOF (min−1) Time (min) G5 94.0 88.0 2.22 40 G6 99.0 96.0 2.40 40 G7 98.9 96.5 3.24 30 G8 99.0 95.0 2.40 40 G9 96.0 56.4 0.240 210 G10 83.0 67.1 0.447 150 Reaction conditions: substrate (20 μmol), 4a (1 mol %), acetonitrile (2 mL), room temperature, O2 atmosphere. Samples were irradiated with a light-emitting diode (LED) lamp (λex = 520 nm, 4.36 W). aCrude yields determined from 1H NMR based on the starting material with internal standard mesitylene. Figure 4 | (a) Plots of the reaction yields versus irradiation time without photosensitizer (None) and using 1 mol % of 1, 4a, 4b, 4c, 4a⊃coronene, 4b⊃coronene, or RB as the photosensitizer. (b) Reusability of 4b for visible light-driven photooxidation reaction. Download figure Download PowerPoint To understand why the metallacages showed better catalytic performance than ligand 1, 1H NMR experiments ( Supporting Information Figure S36) were first performed to study the interactions between metallacage 4a and the substrates. Obvious upfield chemical shifts (ca. 0.1 ppm) of the protons of G5, G6, G7, and G8 were observed, while negligible shifts were found for the protons of G9 and G10. The binding constants (Ka) of the complexes between metallacage 4a and substrates G5, G6, and G7 were determined to be 188.11 ± 2.55, 225.16 ± 3.74, and 119.99 ± 2.60 M−1 in CD3CN, respectively, using a 1H NMR titration method ( Supporting Information Figures S37). The Ka values of 4a⊃G9 and 4a⊃G10 were too small to be measured. This agrees well with the good catalytic performance of G5–G8 by the metallacages and indicates that encapsulation plays an important role in the oxidation of the substrates. To further prove this, the photooxidation of DHN using metallacycle 4c and RB as photosensitizers was investigated. Under the same reaction conditions, much lower conversions and yields were obtained for these two photosensitizers (Figure 4a and Supporting Information Figures S43 and S44 and Table S2). Moreover, coronene, which is a stronger guest for the metallacage (for 4a⊃coronene, Ka = 2.05 × 103 M−1; for 4b⊃coronene, Ka = 1.95 × 103 M−1),27 was used to block the cavity of the metallacage. The yields of DHN upon photooxidation decreased dramatically when 4a⊃coronene or 4b⊃coronene was used as the photosensitizer ( Supporting Information Figures S38 and S45–S47). These results suggest that the increased catalytic efficiency was not only due to the improved 1O2 generation capacity of the metallacages but also to the fact that the encapsulation enhanced 1O2 transfer from metallacages to encapsulated guests and offered significantly improved catalytic efficiency. Moreover, the color of the solution faded, and the product was not increased after 40 min when RB was used as the photosensitizer, indicating its photodegradation upon irradiation. On the contrary, the metallacage can be reused for at least five cycles without loss of its catalytic performance (Figure 4b and Supporting Information Figures S48 and S49), suggesting its good photostability. Conclusions Two PDI-based multicomponent metallacages were successfully constructed by coordination-driven self-assembly. Owing to the synergistic effect of 1O2 generation efficiency of PDI and benzothiadiazole derivatives as well as the improved intersystem crossing by Pt(II)-based coordination bonds, these metallacages showed improved 1O2 generation quantum yields than their building blocks. These metallacages were further used as photosensitizers for the visible light-driven photooxidation, showing excellent catalytic performance and stability toward a series of alkenes. This study not only offers a self-assembling strategy to prepare photosensitizers with good stability and 1O2 generation ability but also explores their applications for visible light-driven photocatalytic oxidation reaction, which will promote the future applications of metallacages toward photocatalysis. Footnotes a Ligand 2a cannot generate 1O2 upon the excitation at 520 nm, because it shows little absorption at 520 nm ( Supporting Information Figure S25). It can generate 1O2 upon UV irradiation. b Deposition Number 2086159 ( 4a) contains the supplementary crystallographic data for this paper. The data is provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures. Supporting Information Supporting Information is available and includes experimental details, characterization, spectra (1H, 13C, ESI-MS spectra, absolute fluorescence quantum yields, UV–vis absorption and fluorescence spectra), crystal diffraction data for metallacage 4a, 1O2 generation and quantum yields, and a photocatalytic oxidation study. Conflict of Interest The authors declare no competing financial interest. Acknowledgments This work was supported by the National Natural Science Foundation of China (nos. 21801203 and 22171219), the Key Research and Development Program of Shaanxi Province (no. 2019KW-019), the Basic Research Program of Xi’an Jiaotong University (no. XZY022020018), the Open Fund of the Key Laboratory of Luminescence from Molecular Aggregates of Guangdong Province (no. 2020-kllma-03), and the Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates (no. 2019B030301003) from South China University of Technology. The authors thank Dr. Gang Chang and Yu Wang at the Instrument Analysis Center and Dr. Aqun Zheng and Junjie Zhang at the Experimental Chemistry Center of Xi’an Jiaotong University for NMR and fluorescence measurements.