Electron Injection from Photoexcited Metal–Organic Framework

Mar 26, 2018 - We report the design of two new metal–organic frameworks (MOFs), Ru-TBP and Ru-TBP-Zn, based on Ru2 secondary building units (SBUs) a...
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Electron Injection from Photoexcited MOF Ligands to Ru Secondary Building Units for Visible-Light-Driven Hydrogen Evolution Guangxu Lan, Yuan-Yuan Zhu, Samuel S. Veroneau, Ziwan Xu, Daniel Micheroni, and Wenbin Lin J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b01601 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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Electron Injection from Photoexcited MOF Ligands to Ru2 Secondary Building Units for Visible-Light-Driven Hydrogen Evolution Guangxu Lan,1,† Yuan-Yuan Zhu,1,2,† Samuel S. Veroneau,1 Ziwan Xu,1,3 Daniel Micheroni,1 and Wenbin Lin1* 1

Department of Chemistry, The University of Chicago, 929 E 57th Street, Chicago, Illinois 60637, United States School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China 3 College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China 2

Supporting Information Placeholder ABSTRACT: We report the design of two new metal-organic frameworks (MOFs), Ru-TBP and Ru-TBP-Zn, based on Ru2 secondary building units (SBUs) and porphyrin-derived tetracarboxylate ligands. The proximity of Ru2 SBUs to porphyrin ligands (~1.1 nm) facilitates multi-electron transfer from excited porphyrins to Ru2 SBUs to enable efficient visible-light-driven hydrogen evolution reaction (HER) in neutral water. Photophysical and electrochemical studies revealed oxidative quenching of excited porphyrin by Ru2 SBUs as the initial step of the HER process and the energetics of key intermediates in the catalytic cycle. Our work provides a new strategy to building multifunctional MOFs with synergistic ligands and SBUs for efficient photocatalysis.

the design of two novel MOFs, Ru-TBP and Ru-TBP-Zn, built from Ru2 paddlewheel SBUs and porphyrin-derived tetracarboxylate ligands for efficient visible-light-driven HER (Figure 1). Upon visible-light (λ> 400 nm) irradiation, the excited porphyrin ligands inject electrons into adjacent Ru2 SBUs to produce hydrogen from water (Figure 1c).

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Metal-organic frameworks (MOFs) have provided a versatile material platform for studying key steps in artificial photosynthesis in the past few years.1-2 By taking advantage of their synthetic tunability and structure regularity, MOFs can hierarchically integrate photosensitizing and catalytic components into a single porous material to enable artificial photosynthesis and to establish structure-function relationships.1 For example, MOFs built from luminescent ligands have afforded novel model systems to study energy transfer,3-5 revealing facile triplet excited state hopping in phosphorescent MOFs6-8 and efficient singlet excited state migration in fluorescent MOFs due to contributions from jumping beyond nearest neighbors.9-10 MOFs have also provided porous materials to study water oxidation,11-13 proton reduction,14-16 and carbon dioxide reduction.17-20 Because photo-driven water splitting is recognized as a pathway to convert solar energy into chemical energy,21-24 MOFs have been examined for photocatalytic hydrogen evolution reaction (HER) via hierarchical assembly of photosensitizers (PSs) and catalytic centers.25-29 Photo-excited or photo-reduced PSs in MOFs can efficiently inject multi-electrons to adjacent catalytic centers to enable HER. For instance, by loading Pt nanoparticles (NPs) or polyoxometalates (POMs) as HER catalysts into the cavities of photosensitizing MOFs, Lin and coworkers observed facile electron injection from photosensitizing frameworks to Pt NPs or POMs for photocatalytic HER.25-27 Direct incorporation of HER catalysts in the secondary building units (SBUs) of photosensitizing MOFs should provide an even more efficient system, but, due to the kinetic inertness of catalytically relevant precious metals such as Ru, Pd, Ph, Pt, Ir, their incorporation as SBUs into photosensitizing MOFs has not been achieved. Herein we report

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Figure 1. (a) Perspective view of Ru-TBP crystal structure down the (100) direction. The distance between adjacent porphyrin centers and Ru2 SBUs is ~1.1 nm, whereas the distance between adjacent Ru2 SBUs is ~1.6 nm. (b) Coordination environment of Ru2 paddlewheel SBUs. (c) Schematic showing visible-light-driven HER catalyzed by Ru-TBP or Ru-TBP-Zn. Photoexicted porphyrin ligands inject electrons to adjacent Ru2 SBUs to reduce protons to hydrogen. The oxidized porphyrin ligands are then reduced by sacrificial TEOA to regenerate the photocatalytic system.

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Figure 2. TEM image (a) and HRTEM image and FFT pattern (inset) (b) of Ru-TBP. TEM image (c) and HRTEM image and FFT pattern (inset) (d) of Ru-TBP-Zn. PXRD patterns (e), nitrogen sorption isotherms (f), Ru 3p XPS spectra (g), and UV-visible absorption spectra (h) of Ru-TBP and Ru-TBP-Zn. Plate-shaped single crystals of Ru-TBP were synthesized through a solvethermal reaction between RuCl3·xH2O and 5,10,15,20-tetra(p-benzoic acid)porphyrin (H4TBP) in N,N- dimethylformamide (DMF) with acetic acid as the modulator at 120 °C. X-ray single crystal diffraction studies revealed the coordination of RuIII ions to the nitrogen atoms of TBP during the MOF synthesis, with a DMF molecule occupying the axial positions. The metalated TBP ligands are linked by Ru2 paddlewheel SBUs to form a 3D framework of sql topology (Figure 1a). The two RuIII centers in each Ru2 SBUs are bridged by three benzoate groups whereas adjacent Ru2 SBUs are linked by one benzoate group to form a 1-D chain (Figure 1b). In each Ru2 SBU, one RuIII is tetrahedrally coordinated to four carboxylate oxygen atoms and the other RuIII is octahedrally coordinated to four carboxylate oxygen atoms and two water molecules. Ru-TBP thus has a framework formula of [Ru2(TBP-Ru-DMF)(H2O)2]Z+ based on the crystal structure.

By decreasing the concentration of acetic acid modulator, we optimized the synthesis of Ru-TBP to produce nanoplates of ~500 nm in diameter as shown by transmission electron microscopy (TEM, Figure 2a and S2, SI). High-resolution transmission electron microscopy (HRTEM) image of Ru-TBP nanoplates showed lattice points corresponding to Ru2 SBUs with the fast Fourier transform (FFT) revealing a four-fold symmetry (Figure 2b), which is consistent with the projection of Ru-TBP crystal structure in the (100) direction. The distance between two adjacent lattice points in the HRTEM image was measured to be 1.6 nm, matching the distance between two adjacent Ru2 SBUs. The analogous MOF with ZnII-metalated TBP ligands, RuTBP-Zn, was similarly synthesized from RuCl3·xH2O and 5,10,15,20-tetra(p-benzoato)porphyrinatozinc (H4TBP-Zn) and exhibited similar size and morphology to Ru-TBP (Figure 2c and S3, SI). Ru-TBP-Zn is isostructural to Ru-TBP as demonstrated by HRTEM imaging (Figure 2d) and power X-ray diffraction (PXRD) studies (Figure 2e). The porous structures of Ru-TBP and Ru-TBP-Zn were confirmed by type I nitrogen adsorption isotherms at 77 K (Figure 2f) with Brunauer−Emmett−Teller (BET) surface areas of 441 m2/g and 422 m2/g, respectively. These BET surface areas are similar to those reported for MOFs constructed from Cu2 or Zn2 paddlewheels and TBP ligands.30-31 The Ru oxidation states in Ru-TBP and Ru-TBP-Zn were studied by X-ray photoelectron spectroscopy (XPS). The Ru 3p3/2 peak of Ru-TBP and Ru-TBP-Zn exhibited binding energies of 462.7 eV and 462.6 eV (Figure 2g), respectively, indicating the RuIII oxidation state.32 Additionally, X-ray absorption near edge structure (XANES) spectra of Ru-TBP and Ru-TBP-Zn showed a similar energy edge to RuCl3, confirming the RuIII centers in the MOFs (Figure S6, SI). The RuIII and ZnII occupancies in the metalated porphyrin ligands in the Ru-TBP and Ru-TBP-Zn nanoplates were investigated by a combination of UV-visible spectroscopy, thermogravimetric analysis (TGA), and inductively coupled plasma mass spectrometry (ICP-MS). The UV-visible absorption spectra of Ru-TBP showed a small peak at 648 nm (Figure 2h), which corresponds to the last Q-band of free TBP ligand, indicating incomplete metalation of TBP ligand in Ru-TBP nanoplates. The ratio of TBP-Ru ligand to TBP ligand in Ru-TBP nnaoplates was calculated to be 0.71:0.29 by TGA (Figure S4a, SI), affording a formula of Ru2(TBP-Ru-DMF)0.71(TBP)0.29)(H2O)2]Cl2.71. For Ru-TBP-Zn nanoplates, the UV-visible absorption spectra showed the same Q bands as H4TBP-Zn (Figure 2h), indicating complete metalatiom of TBP ligands by ZnII. TGA analysis revealed a formula of [Ru2(TBP-Zn)(H2O)2]Cl2 (Figure S4b, SI). ICP-MS analysis gave a Ru to Zn ratio of 1.92 ± 0.09, supporting the formulation of Ru-TBP-Zn. These Ru-TBP and Ru-TBP-Zn nanoplates were used for subsequent HER and mechanistic studies. We hypothesized that the integration of photosensitizing porphyrin ligands and Ru2 SBUs in the MOFs could facilitate multielectron injection from the excited ligands to catalytic SBUs to drive HER. The discrete Ru2 paddlewheel (Ru2-PD) with a formula of RuIIRuIII(CH3COO)4(DMF)Cl was synthesized and used as a homogeneous control (Figure S1, SI). The visible-light driven (λ > 400 nm) HER catalytic activities of Ru-TBP, Ru-TBP-Zn, and the homogenous control (Ru2-PD + H4TBP-Zn) were studied in an oxygen-free CH3CN solution with H2O as proton source and triethanolamine (TEOA) as sacrificial electron donor (CH3CN:H2O:TEOA = 20:1:5, V:V:V). The amount of generated H2 was quantified by gas chromatography (GC) analysis of the headspace gas in the reactor. H2 production increased linearly with time at a rate of 0.13 mmol·h-1·g-1 for Ru-TBP and 0.24 mmol·h-1·g-1 for Ru-TBP-Zn, with respect to the Ru2 moieties (or the porphyrin derivatives). The higher HER catalytic activity of Ru-TBP-Zn than that of Ru-TBP is likely due to the better photosensitizing ability of the TBP-Zn ligand than the TBP-Ru or TBP

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To confirm the catalytic activity of Ru2 SBUs in MOFs and rule out the involvement of Ru NPs, H2 generation was measured hourly for six consecutive reactions (Figure. S5, SI). Both RuTBP and Ru-TBP-Zn showed similar H2 generation in six reactions, arguing against the role of Ru NPs in HER, which would have afforded increasing HER rates over time. In addition, the XANES spectra of Ru-TBP and Ru-TBP-Zn also showed no change of Ru valence states or formation of Ru NPs after photocatalysis (Figure S8, SI). After photocatalysis, both Ru-TBP and Ru-TBP-Zn showed the same PXRD patterns as freshly prepared MOFs (Figure 3b) and showed only 0.34% and 0.30% leaching of Ru (determined by ICP-MS), respectively, indicating their structural stability during HER. The HER mechanism was investigated by photophysical and electrochemical studies on Ru-TBP-Zn and Ru2-PD + Me4TBPZn homogenous control. To establish whether the excited TBP-Zn was quenched reductively by TEOA as electron donor or oxidatively by Ru2 moieties as electron acceptor, the luminescence spectra of Me4TBP-Zn were measured with addition of TEOA or Ru2-PD. As shown in Figure 4a and 4b, the luminescence of Me4TBP-Zn was efficiently quenched by Ru2-PD but not TEOA, indicating that the photocatalytic HER in Ru-TBP-Zn occurred via electron transfer from the excited TBP-Zn to Ru2 SBUs. The luminescence quenching of Me4TBP-Zn was fitted to the SternVölmer equation to afford a KSV value of 6.03 mL·mg-1 (Figure 4c, Supplementary S5, SI). In addition, time-resolved photoluminescence measurements showed a shorter lifetime of TBP-Zn in Ru-TBP-Zn (τ = 1.98 ns) than that in Me4TBP-Zn (τ = 2.13 ns) in CH3CN solution (Figure S7, SI), consistent to the oxidative quenching mechanism. Cyclic voltammograms (CVs) of Ru2-PD and Ru-TBP-Zn were then studied to provide additional insight into the HER process. Ru2-PD showed a reversible reduction peak at 0.08 V vs. NHE in CH3CN, corresponding to the reduction of RuIIRuIII to RuIIRuII, and an irreversible catalytic peak with an onset potential of -0.47 V (Figure 4d). Similar CV pattern was observed for Ru-TBP-Zn, suggesting that the Ru2 SBUs in Ru-TBP-Zn were first reduced to RuIIRuII before catalyzing proton reduction. The onset potential of catalytic peak of Ru-TBP-Zn was -0.50 V vs. NHE in CH3CN solution with H2O and trifluoroacetic acid (TFA) as proton source (CH3CN:H2O:TFA = 20:0.5:0.5, V:V:V), where the proton concentration is 107.23 higher than the photocatalytic HER condition (CH3CN:H2O = 20:1, V:V). The energy to drive HER by Ru2

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ligand. The turnover number (TON) [defined as n(1/2H2)] reached 21.2 for Ru-TBP and 39.4 for Ru-TBP-Zn after 72 h irradiation (Figure. 3a). In comparison, the Ru2-PD + H4TBP-Zn homogenous control exhibited a modest TON of 1.4. The significantly enhanced catalytic activity of Ru-TBP-Zn over the homogenous control confirmed that the important role played by hierarchical organization of photosensitizing ligands and catalytic SBUs in facilitating multi-electron transfer processes to drive photocatalytic HER. a b Ru-TBP-Zn after Ru-TBP

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Figure 4. Emission spectra of Me4TBP-Zn (0.1 mg/mL) after the addition of different amounts of Ru2-PD (a) and TEOA (b) in 2 mL CH3CN with 410 nm excitation. (c) Plots of I0/I as a function of the concentration of Ru2-PD. (d) CVs of 5 mM Ru2-PD and 0.5 mg Ru-TBP-Zn coated on electrode surface in a 20 mL 0.1 M TBAH/CH3CN solution with 500 µL H2O and 500 µL TFA. TBAH = Tetrabutylammonium hydroxide. (e) Proposed catalytic cycle for visible-light-driven hydrogen evolution catalyzed by RuTBP-Zn. ∆E1 = 2.05 eV, ∆E2 = -1.12 eV, ∆E3 = -0.93 eV and ∆E’ = 0.93 eV. Based on the photophysical and electrochemical data, we propose the photocatalytic HER mechanism of Ru-TBP-Zn as shown in Figure 4e. Under visible-light irradiation, the ligand TBP-Zn is excited to the (TBP-Zn)* state, which can transfer one electron to the Ru2 SBU to generate (TBP-Zn)+. After each Ru2 SBU first accepting two electrons to form RuIIRuII, further electron injections to the Ru2 SBU drive the proton reduction to generate H2. The (TBP-Zn)+ is reduced back to the TBP-Zn by the TEOA sacrificial donor to complete the catalytic cycle. Differential pulse voltammetry (DPV) showed that Me4TBP-Zn displayed first oxidation peak at 0.93V (Figure S8, SI), indicating the energy change from (TBP-Zn)+ to TBP-Zn was -0.93 eV (∆E3). As shown in Figure 4a, Me4TBP-Zn exhibited luminescence emission peak at 605 nm, corresponding to the energy increase from TBP-Zn ground state to (TBP-Zn)* excited state and of 2.05 eV (∆E1). Based on the proposed catalytic cycle, the energy loss from (TBPZn)* to (TBP-Zn)+ is 2.05 – 0.93 = 1.12 eV (-∆E2), which is larger than the 0.93 eV needed for photocatalytic HER by Ru2 SBUs in Ru-TBP-Zn. In summary, we have designed two novel MOFs built from Ru2 SBUs and porphyrin-derived ligands and realized photocatalytic HER by directly incorporating HER catalysts as the SBUs into photosensitizing MOFs. The proximity of photosensitizing porphyrin ligands to the catalytic Ru2 SBUs in the MOFs facilitated multi-electron transfer, leading to 28 times higher HER activity than the homogeneous control. Photophysical and electrochemical

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studies established the oxidative quenching of the (TBP-Zn)* excited states by Ru2 SBUs as the initiating step of HER and revealed the energetics of key intermediates in the catalytic cycle. This work provides a blueprint for designing multifunctional MOFs with catalytic SBUs and photosensitizing ligands for photocatalytic and other applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions †These authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by National Science Foundation (DMR1308229). We thank Dr. Zekai Lin, Mr. Yang Song, and Mr. Zhe Li for experimental help. Y.-Y.Z. acknowledges the China Scholarship Council for a fellowship. XAS analysis was performed at Beamline 10-BM, Advanced Photon Source (APS), Argonne National Laboratory (ANL). Single crystal diffraction studies were performed at ChemMatCARS, APS, ANL. ChemMatCARS Sector 15 is principally supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), National Science Foundation, under grant number NSF/CHE-1346572. Use of the PILATUS3 X CdTe 1M detector is supported by the National Science Foundation under the grant number NSF/DMR-1531283. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.

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