Selective Photooxidation of Amines and Sulfides Triggered by a

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 3016−3023

Selective Photooxidation of Amines and Sulfides Triggered by a Superoxide Radical Using a Novel Visible-Light-Responsive Metal− Organic Framework Hongxia Wei, Zhifen Guo, Xiao Liang, Peiqi Chen, Hui Liu, and Hongzhu Xing* Provincial Key Laboratory of Advanced Energy Materials, College of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China

ACS Appl. Mater. Interfaces 2019.11:3016-3023. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/25/19. For personal use only.

S Supporting Information *

ABSTRACT: Photocatalysis is an efficient and sustainable approach to convert solar energy into chemical energy, simultaneously supplying valuable chemicals. In this study, a novel metal−organic framework (MOF) compound is constructed from anthracene-based organic linkers, which shows visible-light absorption and efficient photoinduced charge generation property. It was applied for triggering photooxidation of benzylamines and sulfides in the presence of environmental benign oxidants of molecular oxygen or hydrogen peroxide. Results show that it is a highly selective photocatalyst for oxidation reactions to produce valuable imines or sulfoxides. We further investigate the underlying mechanism for these photocatalytic reactions by recognizing reactive oxygen species in the reactions. It has been demonstrated that the superoxide radical (O2•−), generated by electron transfer from a photoexcited MOF to oxidants, serves as the main active species for the oxidations. The work demonstrates the great potential of photoactive MOFs for the transformation of organic chemicals into valuable complexes. KEYWORDS: oxidation, photocatalysis, sulfide, benzylamine, metal−organic framework

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INTRODUCTION Great progress in photocatalysis has been made in recent years to convert sustainable solar energy into chemical energy.1−3 Very recently, photocatalytic organic transformations have attracted tremendous research interest owing to the high selectivity, the moderate reaction conditions, as well as the green and sustainable concerns.4,5 Certain kinds of important reactions, such as oxidation of alcohols, amines, and sulfides and atom-transfer radical polymerization, have been achieved in different photocatalytic systems.6−9 In these reactions, various visible-light-harvesting materials including inorganic semiconductors and molecular coordination complexes have been employed as photocatalysts to realize organics transformation.10−12 However, these photocatalytic materials exhibit potential restrictions such as over-oxidation and limited conversion rate. Thus, it is indispensable to develop novel photocatalysts for efficient, green, and selective transformation of organic chemicals. Metal−organic frameworks (MOFs) constructed from organic linkers and metal clusters are promising materials for photocatalytic application, owing to their tunable character in structure and property.13−16 More importantly, there might be various photoinduced charge/energy transfer processes in MOFs, such as metal-to-ligand charge transfer, ligand-to-ligand energy transfer, ligand-to-metal charge transfer, and so forth.17−19 These processes are important and essential for © 2019 American Chemical Society

MOF-mediated photocatalytic reactions. The rapid development of MOFs has generated intense photocatalysis studies in which MOFs have been generally applied as heterogeneous photocatalysts to drive desirable photochemical reactions.20−23 Recently, the use of MOFs to carry out photoinduced organics transformation has attracted increasing attention.24−26 Emerging studies have demonstrated that MOFs are intriguing photocatalysts for oxidizing reactions.27−32 Zeng et al. reported the aerobic oxidation of benzylamines and benzylalcohols over a perylene-diimide-based coordination polymer.33 Porphyrinbased PCN-222 has been applied as a visible-light photocatalyst for selective aerobic oxidation of amines.34 Another photoactive porphyrin-based MOF UNLPF-10 has been demonstrated to be an effective photocatalyst for the oxidation of thioanisole to sulfoxide.35 Despite these success studies, MOF-mediated photocatalytic transformation of organic chemicals is still at the beginning, and very limited photoactive MOFs have been used for the oxidation of amines and sulfides under visible light. We report here a new three-dimensional indium-based MOF (1) constructed from anthracene-based linkers under solvothermal condition. The prepared 1 shows a wide-range Received: October 18, 2018 Accepted: December 27, 2018 Published: January 10, 2019 3016

DOI: 10.1021/acsami.8b18206 ACS Appl. Mater. Interfaces 2019, 11, 3016−3023

Research Article

ACS Applied Materials & Interfaces

benzylamine were placed in a Pyrex vial containing 1 mL of dimethyl sulfoxide (DMSO) in the dark. Diphenyl was added as internal standard. The sealed Pyrex vial (3 mL) was then stirred magnetically at O2 atmosphere. The photocatalytic oxidation at room temperature was then irradiated under visible light using a 300 W xenon lamp with a cut-off filter at 420 nm to remove UV light. The distance between the lamp and the reaction vial placed in a cycling water system was ca. 15 cm, and the optical power at the vial was measured to be ca. 18 mW cm−2. A small amount of reaction suspension was taken out at fixed time intervals and the suspension was injected into GC-MS after it was filtered through a porous membrane (22 μm in diameter). The chemical structures of products were confirmed by comparison with standard chemicals. However, for the oxygenation of sulfides, 4.0 mg of 1 was placed into a dried Pyrex vial equipped with a stir bar. Then, 0.4 mmol sulfide in 2 mL of mixed solvent (CHCl3/MeOH = 4/1) was added. The reaction mixture in the sealed vial was stirred under air atmosphere and 1 mmol H2O2 (30% aqueous solution) was added. Decane was added as internal standard for the GC measurement. Photocatalytic reactions were monitored using GC-MS. EPR Trapping Studies. For the reaction of amines, the solvent is changed from DMSO to methanol in order to avoid the disturbance of the carbonyl group from DMSO. The photocatalyst 1 (4 mg) in methanol (1 mL) was bubbled with O2 under dark condition. The trapping agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) for the detection of O2•− was added into the mixture in the dark. The mixture containing 1 and DMPO was placed into an EPR tube. EPR measurements were then carried out under irradiation with a 300 W xenon lamp (λ > 420 nm) at room temperature. Furthermore, benzylamine was added in the system to study the interaction between reactive species with the substrate. The EPR measurement for the detection of 1O2 was carried out following the same procedure but using 2,2,6,6-tetramethylpiperidine (TEMP) as trapping agent. As for the reaction of sulfide, a very similar procedure was adopted.

absorption in the visible light region and exhibits efficient photoinduced charge generation. It was applied for photooxidation reactions in the presence of environmental benign oxidants of molecular oxygen or hydrogen peroxide, in which benzylamines and sulfides could be efficiently and selectively transformed into valuable imines and sulfides, respectively. More importantly, it has been demonstrated that the photoinduced peroxide radical (O2•−), resulting from electron transfer of excited In-MOF to oxidants, serves as the main reactive oxygen species (ROS) in the reactions.

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EXPERIMENTAL SECTION

Synthesis of 1. The photoactive dicarboxylate ligand of 4,4′(diethynylanthracene-9,10-diyl) dibenzoic acid (ADBEB) was synthesized by coupling reaction according to reported procedure.36,37 Other chemicals without special descriptions were commercially available and used without further purification. Orange crystals of 1 (also named NNU-45) were synthesized by a solvothermal method. Typically, a suspension of In(NO3)3·4H2O (10 mg, 0.025 mmol), ADBEB (5 mg, 0.01 mmol), and 2 M HCl (100 μL, aq.) dissolved in a mixed solvent of dimethylformamide (DMF)/H2O (3.5 mL, 6:1, v/ v) was transferred into an autoclave and placed at 85 °C for 3 days. Then, it was cooled down at ambient conditions. The crystals were recollected and washed with DMF and ethanol several times. Yield: ca. 64% based on the ADBEB linker. Physical Characterizations. Single-crystal X-ray diffraction data of a selected crystal was collected on a Bruker SMART APEXII CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. The crystal structure was solved by a direct method and refined using full-matrix least-squares on F2 with anisotropic displacement using SHELXTL.38 The X-ray crystallographic coordinates of 1 reported in this article have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition number CCDC 1860843. The details of the crystal data collection and refinement are supported, see Table S1. Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku SmartLab X-ray diffractometer operated at 40 kV/30 mA with Cu Kα radiation (λ = 0.15417 nm) with 2θ ranging from 3° to 40° at room temperature. UV−vis spectra of solid-state samples were recorded on a HITACHI U-4100 spectrophotometer. The UV−vis spectrum for benzylamines and sulfides in liquid was collected on a SHIMADZU UV-2550 spectrophotometer. The Brunauer−Emmett− Teller (BET) surface area and N2 sorption isotherm are measured with an ASAP 2020 instrument. Electron paramagnetic resonance (EPR) spectra were obtained on a JES-FA 200 EPR under visible-light irradiation. Scanning frequency: 9.42 GHz; central field: 336 mT; scanning power: 0.998 mW; scanning temperature: 25 °C. Scanning electron microscopy (SEM) measurements were taken on a HITACHI SU8010 with an acceleration voltage of 3 kV. The Fourier transform infrared (FTIR) spectra were measured using KBr pellets in the range of 400−4000 cm−1 on a Mattson Alpha-Centauri spectrometer. Thermogravimetry (TG) measurement was carried on a PerkinElmer TGA-7 thermogravimetric analyzer from 40 to 800 °C with a heating rate of 10 °C min−1 under air atmosphere. The photoluminescence (PL) spectra were measured on FLSP920 fluorescence spectrometers. Gas chromatography-mass spectrometry (GC-MS) was recorded on AGILENT-6890/5973 under the following conditions: oven temperature 300 °C, injector temperature 250 °C, column temperature program 10 °C/min, from 150 to 250 °C holding for 20 min. A 300 W xenon arc lamp was used as the light source for the catalytic experiments where a cut-off filter at 420 nm has been used to remove UV light. Photocurrent measurements were performed in a standard three-electrode system with the MOF-coated indium tin oxide as the working electrode, Pt and Ag/AgCl electrodes as the counter electrode and reference electrode, respectively. Aqueous solution of Na2SO4 (0.2 M) was used as electrolyte. Photocatalytic Activities’ Evaluation. In a typical reaction of benzylamines oxidation, 4 mg of photocatalyst 1 and 0.2 mmol

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RESULTS AND DISCUSSION Crystal Structure and Characterizations of 1. Singlecrystal X-ray diffraction analysis shows that 1 possesses a threedimensional network and crystallizes in the monoclinic space group C2/m with a formula of [In(OH)(ADBEB)]·DMF. Each In(III) ion in the structure is coordinated by four oxygen atoms from four ADBEB linkers and two hydroxyl groups, affording the [InO4(OH)2] octahedron (Figures 1a and S1a).

Figure 1. (a) View showing inorganic chains and stacking interactions between linkers in the structure. (b) Perspective view of 1 showing twofold interpenetration. 3017


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to vibrations of carboxylate groups in the linker. The peak at 1656 cm−1 (vCO) and peaks near 3000 cm−1 (vC−H) should be due to the presence of the guest DMF molecule in the structure.37 The broad peak at ca. 3300 cm−1 should result from the hydroxyl group (−OH) in the structure.42 As shown by the UV−vis spectrum in Figure 2b, 1 exhibits optical absorption in the visible light region from 400 to 700 nm with a band gap energy of ca. 2.19 eV estimated from the Tauc plot (Figure S7). The first absorption band at ca. 400− 500 nm should result from the ground-state transition of the ADBEB linker, whereas the second band at ca. 550−700 nm might be due to excitonic absorption in view of the dense stacking interaction between ADBEB linkers in the structure. It is well known that photoinduced excitons could be easily ionized to generate photocurrent at the interface. Figure 2c records the photocurrent responses of 1, where the photocurrent density was produced instantly and increased sharply once illuminated by visible light. This indicates that 1 can achieve an effective separation of photoexcited electrons and hole pairs. Besides, the fluorescence of 1 at 500−750 nm is very weak, indicating that the nonradiation process dominates the depopulation of photoexcited 1 (Figure 2d). These results suggest that 1 is inherently beneficial to photocatalytic applications. Photocatalytic Oxidation of Benzylamines. Imines are vital intermediates in the synthesis of dyes, pharmaceuticals, and agrochemicals.43,44 The photocatalytic performance of 1 to synthesize imines through oxidative coupling reaction of amines was then studied with benzylamine as model substrate. Molecular oxygen, one of the environmental-friendly and economic oxidants, has been adopted as oxidant according to reported studies.32−35 As shown in Table 1 (entry 1) and Figure 3, the experiment suggests that 1 is capable of oxidizing benzylamine under visible light irradiation. Control experiments suggest that DMSO is the most optimal reaction medium among various solvents examined (Table S2). Thus, the following oxidative coupling reactions of amines were carried out in DMSO. Series of experiments were performed to illustrate the nature of the coupling reaction over 1 (Table S3). There is no reaction of substrate without either MOF or visible light, indicating the photocatalytic nature of the reaction. The use of organic ligand ADBEB as photocatalyst gave a comparable catalytic performance with that shown by 1 (Table S3, entry 9), indicating the photocatalytic ability of 1 results from aromatic organic linkers. However, it should be noted that the use of 1 to carry out the reaction is more advanced than the use of organic linkers as 1 is insoluble in DMSO, serving as heterogeneous catalyst. The result suggested that oxygen is important for the reaction (Table S3, entry 3). If the reaction was carried out under air atmosphere, the conversion of benzylamine is as low as 26% at 160 min (Table S3, entry 4). However, the complete transformation of benzylamine at 160 min could be achieved when the reaction was carried out under pure oxygen atmosphere (Table S3, entry 5). The turnover number (TON) of the reaction is estimated to be about 33.3 and the turnover frequency (TOF) is equal to 13.4 h−1. As shown in Table S4, such a conversion rate is much higher than that shown by the typical MOF photocatalyst of NH2-MIL125(Ti),32 among the best performances shown by MOF photocatalysts of Zn-PDI and PCN-222.33,34 Benzylamine derivatives with various substitutional groups were examined for the visible-light-induced photocatalytic

The In−O bond lengths are in the range of 2.078(4)− 2.165(3) Å, which are among typical values reported for InMOFs.39,40 The In(III) ions are connected by bringing hydroxyl groups through In−OH−In bonds to from inorganic chains along the a-axis (Figures 1a and S1b). Then, the inorganic chains are further connected by ADBEB linkers to construct a three-dimensional framework (Figure 1b). It is notable that the ADBEB linker deviates from a planar configuration, where the anthracene moiety is approximately perpendicular to in-plane parts of ethynyl-substituted benzoic acids. Long-range stacking interaction between anthracene moieties is observed along the a-axis and the face-to-face distance is measured to be ca. 3.5 Å (Figures 1a and S1c). As shown in Figure 1b, 1 possesses twofold interpenetrating networks because of the large size of ADBEB molecules. The interpenetration makes ADBEB linkers in different networks intersect with each other, enclosing hydrophobic channels. The potential solvent-accessible spaces occupied by disordered guest DMF molecules is estimated to be 46.8% using the PLATON program.41 After guest exchange in acetone for 2 days, the N2 isotherm of activated 1 shows a BET surface area of 411.2 m2 g−1 with a void space of 0.13 cm3 g−1 (Figure S2). The SEM image displays that 1 is a strip-shaped crystal (Figure S3). The purity of the bulk sample was studied by PXRD, where the experimental PXRD pattern fits well with the one simulated from the crystal structure, indicating the asprepared sample is in good purity (Figure 2a). PXRD patterns

Figure 2. (a) Experimental and simulated PXRD patterns of 1. (b) UV−vis spectrum. (c) Photocurrent response of 1 under visible light (λ > 420 nm) irradiation. (d) PL spectrum excited at 470 nm.

suggest also that 1 is stable in organic solvents of DMSO, chloroform, and methanol, as well as in aqueous solution with a pH range of 3−8 (Figure S4). The TG measurement of 1 shows a two-step weight loss (Figure S5). The first weight loss of 10.9 wt % before 250 °C corresponds to the removal of guest DMF molecules (ca. 10.9 wt %). The following sharp weight loss above 400 °C (ca. 58.1 wt %) is attributed to the decomposition of the organic linker. The bulk sample of 1 was also characterized by FTIR spectroscopy (Figure S6). The characteristic peak of ethynyl groups (−CC−) in the ADBEB linker is observed at ca. 2189 cm−1 on the spectrum. The intense absorptions at 1417, 1582, and 1599 cm−1 belong 3018


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ACS Applied Materials & Interfaces Table 1. Photocatalytic Oxidative Coupling of Various Amines To Produce Iminesa

a

Reaction conditions: 0.2 mmol amine, 4 mg of 1, O2 atmosphere, 1 mL of DMSO. bDetermined by GC-MS.

coupling reaction using 1 as photocatalyst. As shown in Table 1, it is notable that all substrates exhibit high selectivity (>99%) to produce imines, avoiding any by-product. The reaction times for complete conversion differ from each other (120−210 min). Results manifest that the conversion of benzylamines modified by electron-donating groups (−CH3/− OCH3) was much faster than those with electron-withdrawing groups (−F/−Cl). Meanwhile, it is found that the position of the substituent on the phenyl group of benzylamine is important for the reaction rate. Taking the methyl (−CH3) substituent as an example, the reaction rates are in the order of ortho < meta < para substitution, indicating an influence of a steric effect (Table 1, entry 2−4). The substitution of methoxyl (−OCH3) with stronger electron-donating effect further

Figure 3. (a) Time evolution reactions for the oxidation of benzylamine. (b) Conversion and selectivity in different runs.

Table 2. Photocatalytic Oxidation of Sulfides To Produce Sulfoxidesa

a

Reaction conditions: 0.4 mmol sulfide, 4 mg of 1, 1 mmol H2O2, 1.6 mL of CHCl3, 0.4 mL of CH3OH. bDetermined by GC-MS. 3019


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moderate conversion rate and selectivity (Table S7).50−52 Different from these conventional reactions, we demonstrate here the use of H2O2 as oxidant in photocatalytic reaction for sulfide oxidation. It is notable that both the conversion rate and selectivity of the reaction are improved significantly (Table S7). Recycling experiments suggest that thioanisole could be converted into sulfoxide with high yield and selectivity in three consecutive runs using recollected 1 (Figure 4). Studies of FTIR, PXRD, and SEM suggest that the structure of 1 was maintained during the recycling experiments (Figures S6, S8 and S9).

enhances the reaction, showing faster conversion rate (Table 1, entry 5). Contrarily, the chloro-substituted benzylamine with a more electron-withdrawing effect than that of the fluorosubstituted substrate shows a decreased conversion rate (Table 1, entry 6−7). The reusability of 1 has been verified by recycling experiments during which 1 was reused by centrifugation. 1 retained its photocatalytic activity in consecutive runs (Figure 3). FTIR, PXRD, and SEM measurements suggest the conservation of the photocatalyst (Figures S6, S8 and S9). Photocatalytic Oxygenation of Sulfides. Encouraged by the remarkable photocatalytic performance shown in oxidative coupling reactions, we further explored the photooxidation of sulfides to produce sulfoxides that have broad applications in pharmaceutical agents and pesticide because of their biological activities.45,46 Thioanisole was selected as model substrate. The photocatalytic experiment was first carried out under O2 atmosphere, where 50% thioanisole was transformed into sulfoxide at 4 h (Table S5, entry 1). The conversion was even much lower (ca. 6%) if the reaction was carried out under air atmosphere (Table S5, entry 2). Beyond molecular oxygen, hydrogen peroxide (H2O2) has been demonstrated to be an effective oxidant for the catalytic oxygenation of sulfides and it is a benign chemical to the environment.47−49 However, to the best of our knowledge, the use of it in the photocatalytic system of MOFs for the oxidation of sulfides has not been investigated so far. After removal of molecular oxygen by three freeze−pump−thaw cycles, the oxidation of sulfide by H2O2 was first investigated under N2 atmosphere. Remarkably, 80% of thioanisole was transformed into sulfoxide within 4 h (Table S5, entry 3). If the H2O2 was added directly under air atmosphere, the complete conversion (>99%) of thioanisole could be achieved at 4 h (Table S5, entry 4). These results suggest that both H2O2 and molecular O2 are valid for sulfide oxygenation. The following photocatalytic reactions were then performed under air atmosphere in the presence of H2O2. Blank runs showed that both 1 and visible light are vital to the reaction, indicating a photocatalytic nature (Table S5, entry 5−6). When the organic ligand of ADBEB was used as a control group, it showed 60% conversion of substrate, which is much smaller than that of 1 (Table S5, entry 10). We also studied the influence of reaction medium including methanol, ethanol, acetonitrile, chloroform, and some mixed solvents (Table S6). The mixed solvent of CHCl3/CH3OH (v/v = 4/1) has been demonstrated to be the most optimal solvent. As shown in Table 2, several sulfides with different R1 and R2 groups were examined for the photocatalytic oxidation reaction. It is notable that the oxidation reaction is highly selective to produce sulfoxide. The reaction of sulfides modified by electron-donating groups retained both conversion rate and selectivity as compared to thioanisole (Table 2, entry 2−3), whereas the substitution of thioanisole by electron-withdrawing groups decreased the conversion rate and selectivity (Table 2, entry 4−5). These results suggest the electronic property of substituents on sulfides is important for the photocatalytic reaction, where the use of the electrondonating group is more beneficial to get higher selectivity and conversion rate. The TON number for the reaction of thioanisole is calculated to be ca. 66.7 with a TOF equal to 16.7 h−1. Prior to this study, several reports have demonstrated that H2O2 is effective for the oxidation of sulfides in MOFmediated catalytic reactions at different temperatures, showing

Figure 4. (a) Time evolution conversions for consequent oxidation of thioanisole. (b) Conversion and selectivity to produce sulfoxide in different runs.

Photocatalytic Mechanism. As mentioned before, the activated 1 is porous and thus the substrates of benzylamine and thioanisole could possibly diffuse into it. It was found that thioanisole could not diffuse into the pore of 1 (Figure S10b), indicating the reaction of sulfide oxidation should primarily happen on the surface of the MOF. In contrast, the diffusion of benzylamine into 1 has been observed. To ensure the influence of porosity on the oxidizing of benzylamine, both the as-made and the activated 1 were used for the reaction. Results suggested that they showed almost identical photocatalytic performances, indicating the porosity of the material merely influences the reactions, possibly because the reaction mainly occurs on the surface and the diffusion of benzylamine into/ out of porous 1 is fast (Figure S10a). We further evaluate the mechanism of the oxidation reactions by recognizing ROS. It has been widely reported that these oxidation reactions may experience two mechanisms mediated either by the superoxide radical (O2•−) or by singlet oxygen (1O2) as ROS.34,53,54 The former specie depends on electron transfer, whereas the latter one relies on the energy transfer process between photosensitizers and oxidants. This was first studied by quenching experiments. According to reported studies, BQ (p-benzoquinone) and DABCO (1,4diazabicyclo [2.2.2] octane) are sensitive scavengers for O2•− and 1O2, respectively.33,55 Thus, BQ and DABCO were added into these reactions as quenchers. The conversion of benzylamine decreased sharply to 23% upon the addition of BQ (Table S3, entry 6). In contrast, the addition of DABCO did not significantly quench the conversion of benzylamine (Table S3, entry 7). This phenomenon suggests that the main reactive specie in photocatalytic coupling reactions for amines should be O2•−. This inference is consistent with the observed phenomenon that DMSO is the most profitable solvent for the reaction, because it is well known that DMSO is detrimental to the existence of singlet oxygen. As for the oxygenation of sulfides, quenching experiments suggest also the existence of O2•− in the reaction, where BQ instead of DABCO quenched 3020


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ACS Applied Materials & Interfaces the reaction obviously (Table S5, entry 7−8) In addition, diphenyl sulfide that is unreactive toward singlet oxygen can be oxidized into sulfoxide (Table 2, entry 6), indicating the reaction should be mediated by a superoxide radical.56 We further performed EPR studies to confirm the existence of O2•−. 5,5-dimethyl-pyrroline-N-oxide (DMPO) was applied as a sensitive trapping agent to probe O2•− in both reactions.57 The EPR spectra were monitored in situ when reaction suspensions were exposed to visible light using a Xe lamp. As shown in Figure 5a, there was no EPR signal detected before

Scheme 1. Photocatalytic Oxidation of Benzylamines and Sulfides over 1 with Superoxide Radicals as Reactive Active Species

amines in the presence of molecular oxygen. Photocatalytic experiments indicate it is an efficient visible light photocatalyst for the reaction, showing highly selective transformation of substrates to produce valuable imines. Importantly, another popular environmentally benign oxidant of hydrogen peroxide for the oxygenation of sulfides had been studied for the first time in MOF-mediated photocatalytic reactions. Significantly, the photocatalytic reaction in the presence of hydrogen peroxide is more efficient than those reactions achieved under heating condition. We further carried out different quenching experiments and EPR trapping measurements to recognize the ROS in these reactions. Results suggest that the peroxide radicals O2•−, resulting from photoinduced electron transfer of excited MOF, should be the main active species in the reactions. This study demonstrates the great potential of photoactive MOFs for the organics transformation to produce valuable chemicals. It is believed that more and more novel photoactive MOFs could be rationally synthesized and utilized for desirable photocatalytic reactions.

Figure 5. (a) EPR detection of O2•− formation over 1 trapped by DMPO in the oxidative coupling of benzylamine; (b) EPR spectra of the O2•− trapped by DMPO in the oxidation of thioanisole.

light illumination in the presence of DMPO. Interestingly, characteristic EPR signals for DMPO captured with the superoxide radical (complex of DMPO−O2•−) emerged under visible light illumination, suggesting clearly the existence of O2•−. When the reaction substrate of benzylamine was added into the suspension, the EPR signals decreased significantly, indicating the strong interaction between benzylamine and O2•−. As shown in Figure 5b, a very similar EPR response has been detected for the oxidation of sulfide. We also tested the probable existence of 1O2 in the reactions by EPR trapping experiments. EPR measurements showed both reactions are silent to TEMP that is a sensitive trapping agent to detect 1O2 (Figure S11).57 These EPR studies clearly show that O2•−, produced by electron transfer from excited 1 to oxidants, serves as the main ROS in both reactions. In fact, the capability of 1 to generate photoinduced electrons has been confirmed above by its photocurrent response (Figure 2c). On the basis of these coherent findings, the photooxidation of benzylamine and sulfide catalyzed by 1 relies on the reactive species of the peroxide radical. In view of this mechanism, the role of H2O2 in the reaction is believed to provide superoxide radicals either through directly oxidizing by photoinduced holes on MOF or in an indirect way of decomposition, producing oxygen followed by reception of photoinduced electrons.54,58 Furthermore, hole-trapping experiments have also been carried out to study the impact of holes on both reactions. When the reducing agent of KI was added into both reactions (Table S3, entry 8 and Table S5, entry 9),59 the photocatalytic transformation of substrates was severely blocked, indicating the holes would oxidize the substrateforming cationic species to drive the reactions (Scheme 1).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b18206. SEM, UV−vis spectrum, PXRD patterns, IR spectra, TGA curves, crystal data, and structure refinement (PDF) Crystallographic information of 1 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hongzhu Xing: 0000-0001-7179-0394 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21473024) and Natural Science Foundation of Jilin Province (20180101290JC).

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CONCLUSIONS In summary, we prepared here a novel three-dimensional InMOF with broad-range absorption in a visible light region owing to the incorporation of photoactive organic linkers. The photoactive MOF was utilized for the oxidation of benzyl-

REFERENCES

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