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Photoactive Metal−Organic Framework and Its Film for Light-Driven Hydrogen Production and Carbon Dioxide Reduction Pengyan Wu, Xiangyang Guo, Linjuan Cheng, Cheng He,* Jian Wang, and Chunying Duan State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, 116024, China S Supporting Information *

ABSTRACT: The design of a new photocatalytic system and integrating the essential components in a structurally controlled manner to create artificially photosynthetic systems is high desirable. By incorporating a photoactive triphenylamine moiety to assemble a Gd-based metal−organic framework as a heterogeneous photosensitizer, new artificial systems were constructed for the proton and carbon dioxide reduction under irradiation. The assembled MOFs exhibited a one-dimensional metal-oxygen pillar that was connected together by the depronated TCA3− ligands to form a three-dimensional noninterpenetrating porous framework. The combining of proton reduction and/or the carbon dioxide reduction catalysts, i.e., the Fe-Fe hydrogenase active site models and the Ni(Cyclam) complexes, initiated a photoinduced single electron transfer from its excited state to the substrate. The system exhibited an initial TOF of 320 h−1 of hydrogen per catalyst and an overall quantum yield of about 0.21% and is able to reduce carbon dioxide under irradiation. The deposit of the photoactive Gd-TCA into the film of an α-Al2O3 plate provided a platform for the practical applications through prolonging the lifetime of the artifical system and allowed the easily operated devices being recyclable as a promising photocatalytic system.

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INTRODUCTION A renewable and clean energy source has been paid more and more attention in recent years due to the exhausting fossil fuel and the world’s growing demand for green energy. Hydrogen was regarded as a clean fuel of the next generation to reduce consumption of fossil fuels and emission of greenhouse gases.1,2 The promising strategy is hydrogen production from water utilizing solar energy, since the harnessing of solar energy would contribute significantly to our electrical and chemical needs, reducing the carbon dioxide emission.3,4 To date, several types of hydrogen evolution photocatalysts that operated in heterogeneous systems, including the heteroatom-doped, nanoxide type, dye-sensitized, and Z-scheme types, have been developed.5,6 Inspirited by the structures and mechanism of the natural photosynthetic systems, light-driven hydrogen evolution has been also realized by the employing of a multicomponent system consisting of a sacrificial electron donor, a photosensitizer, and a hydrogen evolution catalyst in a homogeneous systems.7,8 These systems have shown beneficial features in each of their fields; the design of new photoactive systems and integrating the components in a structurally controlled manner to create more efficient functional devices is still high desirable.9 Metal−organic frameworks (MOFs) are a new family of hybrid solids with infinite structures built from organic bridging ligands and inorganic connecting points.10,11 The intrinsic crystalline property provided precise knowledge about the pore structure and the nature and distribution of catalytically active © XXXX American Chemical Society

sites. In comparison to these heterogeneous catalytic systems that were examined earlier, the design flexibility and framework tenability resulting from the huge variations of metal nodes and organic linkers make MOFs interesting photocatalysts in the light-driven hydrogen production from water.12 By incorporating a triphenylamine moiety as the backbone of the organic linker, we report herein the preparation of a new gadoliniumbased lanthanide−organic framework and its film for the application as a heterogeneous photosensitizer in the lightdriven hydrogen production and carbon dioxide reduction. We envisioned that the redox potential of the excited MOF-based material is negative enough to reduce these proton reduction catalysts and/or the carbon dioxide reduction catalysts, i.e., the Fe-Fe hydrogenase active site models13 and the Ni(Cyclam) complexes,14 as the triphenylamine moiety is a stable blue emitter and an efficient electron donor to initiate a photoinduced single electron transfer from its excited state to the substrate.15,16 (Scheme 1) The choice of gadolinium as the nodes of the framework partly is due to that the Gd3+ has no energy level below 32 000 cm−1 to accept any energy from the TCA3− moiety, which potentially prevents the unnecessary energy loss in the case of other lanthanide ions.17−19 The pores of the photoactive MOFs provided the possibility to adsorb redox catalysts with suitable Received: May 25, 2016

A

DOI: 10.1021/acs.inorgchem.6b01267 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Construction of Gd-TCA Consolidated by the Dinuclear Units Exhibits the Opening of the Pores along an Axis and Its Application on the Light-Driven Hydrogen Production with Fe-Fe Hydrogenase Model Compound and Carbon Dioxide Reduction with Ni(Cyclam) Complex, Respectively

Figure 1. (a) The coordination configuration of metal centers in GdTCA. Symmetry codes: A. 1 − x, −y, −z; B. 1 − x, −y, 1 − z; C. x, 1 + y, z; D. 2 − x, 1 − y, 1 − z. (b) View of the crystal packing of Gd-TCA along the b direction showing the cavities channels with metal ions having labile solvent molecules and photoactive moieties exposed.

redox potential to form artificial systems, from which the close proximity between the adsorbed redox catalyst and the photosensitizer around the confined pores is expected to facilitate the efficient photoinduced electron transfer, where the unwanted energy-transfer and electron-transfer process potentially was avoided.16 In the meantime, the deposition of MOFbased material onto the α-Al2O3 plate to create a membrane was expected to integrate the components in a structurally controlled manner to create more efficient functional devices. It will combine the large surface areas with pore characteristics of MOFs to prolong the lifetime of the systems for light-driven hydrogen evolution and carbon dioxide reduction, leading the MOFs to exhibit potentially practical applications.20

for 24 h. The experimental results demonstrate a dye uptake equivalent to as much as 15% of the framework weight.22,23 Confocal laser scanning microscopy of the guest-adsorbed crystals gave a strong green fluorescence response22 that can be assigned to 2′,7′-dichorofluorescein. The uniform distribution of the dye molecules throughout the crystals suggests that the dyes penetrated deeply into the channels, rather than remaining on the external surface.23The results demonstrate the ability of Gd-TCA to adsorb organic substrates within its open channels. The solid-state UV−vis absorption spectrum of compound Gd-TCA exhibited an absorption band centered at 350 nm typically assignable to the π−π* transition of the triphenylamine group.24 Upon excitation at this band (350 nm), GdTCA showed an intense luminescence band at about 435 nm. Solid-state electrochemistry exhibited a redox potential at 0.82 V (vs SCE), assignable to the redox potential of the Gd-TCA+/ Gd-TCA couple (Figure 2). The redox potential of the excited-

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RESULTS AND DISCUSSION Synthesis and Characterization of Gd-TCA. Solvothermal reaction of 4,4′,4″-tricarboxyltriphenylamine (H3TCA) with Gd(NO3)3·6H2O in a DMF/ethanol (1:1 in v:v) mixed solvent gave a new compound Gd-TCA in a yield of 60%. Elemental analyses along with powder X-ray analysis indicated the pure phase of its bulky sample. Single-crystal structure analysis reveals the formation of a three-dimensional noninterpenetrating porous framework.21 Each gadolinium ion is coordinated by a bidentate carboxyl group, two neutral water molecules, and four oxygen atoms from four different dimonodentate carboxyl groups (Figure 1a). The di-monodentate carboxyl groups were divided into two parts, each part contains a pair of 2-fold bridging carboxyl groups to connect the neighboring Gd3+ ions into a one-dimensional metal-oxygen pillar along the a axis. The pillars were further connected together by the depronated TCA3− ligands to form a threedimensional framework. Such a metal-oxygen pillar-like network is stable in aqueous solution, allowing the MOF to be used in photocatalysts that operated in aqueous media. The largest running openings along the a axis are a tetragon consolidated by two Gd3+ ions and two TCA3− ligands with the interatomic Gd···Gd separation, and the separations between the two N atoms were ca. 13.72 and 9.19 Å, respectively (Figure 1b). This opening enables the ingress and ingress of the proton reducing catalysts, i.e., the Fe-Fe hydrogenase active site model compound 1 to interact with the photoactive catalytic sites within the pores. A dye-uptake study was displayed by soaking crystals of GdTCA in a methanol solution containing 2′,7′-dichorofluorescein

Figure 2. (a) Normalized absorption (red line) and emission spectrum (black line) of Gd-TCA, excitation at 350 nm. (b) Solid-state CV of Gd-TCA in the scan range 0.5−1.1 V (scan rate: 50 mV s−1) in phosphate-buffered saline with a platinum-wire counter electrode and saturated calomel reference electrode.

state Gd-TCA+/Gd-TCA* couple was calculated as −2.30 V on the basis of a free energy change (E0−0) between the ground state and the vibrationally related excited state of 3.12 eV.24 This potential was more negative than that of the [FeFe]H2ases model compound 1 (E0−0 = −1.71 V), suggesting the thermodynamically feasibility for the electron reduction processes of compound 1 by the photogenerated of Gd-TCA. Progressive addition of compound 1 into the Gd-TCA suspension in CH3CN/H2O (7:3 in v/v) quenched the emission dramatically (Figure 3a) with the coefficient KSV calculated as 1.80 ± 0.06 × 104 M−1.25 The lifetime for the B

DOI: 10.1021/acs.inorgchem.6b01267 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

photosystem I, use iron, nickel, and manganese ions, cobalt has also emerged in the past decade as the most versatile non-noble metal for the development of synthetic hydrogen and oxygen evolving catalysts.25,27,28 Fluorescence titration upon addition of compound 2, [Co(bpy)3]Cl2, revealed the significant quenching of Gd-TCA emission with the Stern−Volmer constant as KSV = 7.13 ± 0.4 × 103 M−1 (Figure 4a). LightFigure 3. (a) Family of emission spectra of the Gd-TCA suspension (0.02%) upon addition of compound 1 in 7:3 CH3CN/H2O up to 11 μM. (b) H2 evolution of Gd-TCA (1 mg), in 5 mL of solution containing NiPr2EtH·OAc (0.8 M) and 1 with various concentrations.

emission band of 2.45 ns for Gd-TCA is reduced to 1.73 ns in the presence of compound 1. The result indicated the occurrence of a direct photoinduced electron-transfer process from Gd-TCA* to the redox catalyst 1,26 which suggests the possibility of the excited state of Gd-TCA to reduce the model compound 1. Light-Driven Hydrogen Production Using [FeFe]H2ases Model Compound 1. Photolysis of a suspension of Gd-TCA (1.0 ppm) and compound 1 (8.0 mM) in a solvent mixture of 7:3 CH3CN/H2O at 25 °C in the presence of NiPr2EtH·OAc (0.80 M) as sacrificial electron donors results in a H2 generation, which was quantified at the end of the photolysis by GC analysis of the headspace gases, which shows the effect of varying the catalyst concentration on the overall yield of the hydrogen evolution. Increasing the catalyst concentration increases the overall rate of hydrogen evolution and the total amount of hydrogen evolved (Figure 3b). The initial rates for H2 evolution are obtained from the linear portion of each curve during the first hour of illumination and indicate a first-order dependence on the catalyst concentration. At higher catalyst concentration (>10 mM), while more hydrogen is evolved, the rate of hydrogen production does not increase linearly with the catalyst concentration, due to the poor solubility of compound 1 in the solution. After 6 h of irradiation, the rate of hydrogen evolution decreased dramatically, indicating the decomposition of at least one system component. The further addition of Gd-TCA (1 mg) to a reaction flask that contained Gd-TCA (1 mg), compound 1 (8.0 mM), and NiPr2EtH·OAc (0.8 M) after cessation of the 6 h irradiation for hydrogen evolution did not cause any additional hydrogen evolution, whereas the addition of NiPr2EtH·OAc (0.80 M) led to ca. 15% enhancing of hydrogen evolution, and the further addition of compound 1 (8 mM) led to continued hydrogen production up to 85% of the above-mentioned system added. The addition of both compound 1 and NiPr2EtH·OAc at the same amount was able to resume the production of hydrogen directly. This result demonstrated the fast decomposition of compound 1 during the light-driven H2 evolution, providing the possibility of the Gd-TCA complex being reused. In fact, the time course of photocatalytic H2 evolution through the addition of a fresh solution containing compound 1 (8.0 mM) and NiPr2EtH·OAc (0.80 M) to the filtration of the used photosensitizer Gd-TCA revealed continuous H2 production from the beginning of the irradiation period, and the total amount of produced hydrogen reached to 15 mL after 40 h (4 rounds repeating) irradiation. Light-Driven Hydrogen Production Using [Co(bpy)3]Cl2. While the active sites of enzymes involved in the overall water-splitting in natural systems, namely, hydrogenase and

Figure 4. (a) Family of emission spectra of the Gd-TCA suspension (0.02%) upon addition of complex 2 [Co(bpy)3]Cl2 up to 1.1 × 10−4 M in 1:1 CH3CN/H2O. (b) H2 evolution of Gd-TCA (1 mg), in 5 mL of a solution containing Et3N (2.5%) and 2 ([Co(bpy)3]Cl2) with various concentrations (right), respectively.

driven hydrogen evolution was also observed after irradiating the system containing Gd-TCA (1 mg), and compound 2 (50 μM) in 5 mL of a 1:1 CH3CN/H2O solution. The maximum of hydrogen evolution efficiency was achieved at pH = 10.0 with the concentration of the sacrificial electron donor triethylamine (Et3N) of 2.5% (v/v). The system exhibited an initial TOF of 320 h−1 of hydrogen per cobalt compound for the first hour. This result is comparable to the best results for [Co(bpy)3]Cl2 as catalyst with the ruthenium compound as photosensitizer.29 The overall quantum yield for the light-driven H2 evolution is about 0.21%. The activity increases with the concentration increasing of 2 until 50 μM. Further adding 2 caused a little increase of hydrogen evolved, but the volume of hydrogen evoluted does not scale linearly with the catalyst concentration (Figure 4b). These results indicate the possible decomposition of 2 during the irradiation. The addition of a fresh solution of 2 (50 μM) and Et3N (2.5% in v/v) resumed the hydrogen production directly. The reaction is able to be repeated, and the lifetime of the system was prolonged to more than 20 h (5 rounds repeating) with hydrogen evolution reaches to 22 mL. Since the α-Al2O3 pattern with high affinity for carboxylic groups is a commonly used support for the MOF-based thin films to be combined, our MOF-based film was prepared according to the classic method of MOFs film formation.30 The XRD pattern of Gd-TCA film matches very well with the simulated pattern, indicating that the formed film is a pure phase of Gd-TCA (Figure 5c). The SEM images in Figure 5a,b revealed that the surface of the deposited support was completely covered with a continuous and dense Gd-TCA layer. The amounts of the loading Gd-TCA material were quantified by ultraviolet−visible spectroscopy; an uptake equivalent was demonstrated as much as 1 mg for a 1.5 × 0.5 cm2 film. The Gd-TCA deposited film exhibits high stability for photocatalyzing hydrogen evolution with the initial rate of about 1.71 mL/h and about 3.8 mL for the first 5 h with 1.5 × 0.5 cm2 film in a fresh solution containing 2 (50 μM) and Et3N (2.5% v/v). While the rate of hydrogen decreased dramatically after 5 h irradiation, the light-driven hydrogen evolution could be resumed by simply pulling out the film and reusing through putting down in the above-mentioned solution. The preliminary experiments suggested that a system lifetime was C

DOI: 10.1021/acs.inorgchem.6b01267 Inorg. Chem. XXXX, XXX, XXX−XXX

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the excited state of Gd-TCA to the model compound 3. For the system containing Gd-TCA (1 mg), and compound 3 (50 μM) in 5 mL of CH3CN/H2O (1:1) solution, the maximum of hydrogen evolution efficiency was achieved at pH = 11.0 with the concentration of the sacrificial electron donor triethylamine of 10.0% (in v/v). After irradiation of 12 h, about 0.11 mL of hydrogen was produced. Further addition of compound 3 did not enhance the volume of the product hydrogen of the system. To optimize the reaction conditions of the carbon dioxide reduction under irradiation, we used a cyclic carbonate, 4methyl-1,3-dioxolan-2-one, 4 as the model compound. As shown in Figure 4, after irradiation of the system containing a Gd-TCA suspension (1.0 ppm), redox catalyst 3 (50 μM), Et3N (10.0% v/v) as sacrificial electron donor, and the model compound 4 (5 μL, 11 mM), in a solvent mixture of 7:3 CH3CN/H2O (5 mL) at 25 °C for 12 h, a total of 17.1% of the cyclic carbonate was reduced, by measuring the quantum of the glycol using GC. Of course, at these reaction conditions, with the absence of light, only negligible (lower than 1.8%) hydrolysis of the model compound 4 was found. The reduction procedure of carbon dioxide displayed by bubbling the carbon dioxide into the solution up to solution saturation and carefully adjusting the pH value of the system to 11.0 of a CH3CN/H2O (7:3 in volume 5.0 mL) mixture solvent at 25 °C. Upon irradiation of the aforementioned solution containing a Gd-TCA suspension (1.0 ppm), redox catalyst 3 (50 μM), and Et3N (10.0% v/v) for 12 h, the system gives the concentration of the HCOO− of about 22.7 μM. Furthermore, the isotopic 13CO2 was employed in the photocatalytic reaction, and the product was identified by 13C NMR spectroscopy to confirm the origin of HCOO−. The 13C NMR spectrum clearly gave a peak at 164.6 ppm, corresponding to HCOO−, when 13 CO2 was introduced, while that signal was not detected with 12 CO2 only or with 13CO2 in dark. The results unambiguously demonstrate that the produced HCOO− anion indeed comes from CO2.32

Figure 5. (a) SEM image of Gd-TCA film on α-Al2O3 support (top view). (b) SEM image of Gd-TCA film on α-Al2O3 support (side view). (c) XRD patterns for the Gd-TCA film, α-Al2O3, and the simulation pattern of the single crystal of Gd-TCA. (d) Time course of photocatalytic hydrogen production over a 1.5 × 0.5 cm2 film (containing 1 mg of Gd-TCA) for a total of 40 h with a fresh solution containing catalyst 2 (50 μM) and Et3N (2.5%) was added every 5 h.

prolonged to more than 40 h with a total of 33.5 mL (about 1000 TON per TCA moiety, Figure 5d) H2 evolved. The index of XRD patterns of the Gd-TCA film pulled out from the catalytic reaction evidences the maintenance of the framework and the stability of the materials (Figure S6). To our best knowledge, this is the first example of MOF film for photochemical reduction of water into H2 molecules. Light-Driven Carbon Dioxide Reduction by Using Ni(Cyclam)2+. Most importantly, the excited state potential of Gd-TCA is negative enough to reduce the well-known carbon dioxide electrochemical reduction catalyst Ni(Cyclam)Cl2 3;14,31 we try to create the artificial system for the light-driven reduction of carbon dioxide. As shown in Figure 6, progressive

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CONCLUSION In a summary, a new strategy to create MOFs as photosensitizer for light-driven H2 evolution and carbon dioxide reduction was achieved through incorporating a triphenylamine moiety as the photoactive organic linkers. Gd-TCA exhibited excellent photocatalytic efficiency comparable to the commonly used photosensitizer Ru(bipy)32+ in aqueous media with redox catalysts including Fe-Fe hydrogenase models and the tris(bipyridine)cobalt complexes for the H2 evolution. The prolonged lifetime of MOFs through simply repeating the catalytic reactions and the easy preparation of the film ensured them as promising candidates for the photocatalytic H2 evolution and carbon dioxide reduction. The deposition of MOF-based material into the α-Al2O3 plate to create a membrane combined with the large surface areas with pore characteristics of a MOF allows the MOF-based materials exhibiting potentially practical application.

Figure 6. Left: Family of emission spectra of the Gd-TCA suspension (0.02%) upon addition of complex 3 up to 1.1 × 10−4 M in 1:1 CH3CN/H2O. Right: 13C NMR spectra for the product obtained from reaction with (a) 13CO2 under irradiation; (b) 13CO2 in the dark, and (c) 12CO2 under irradiation.

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EXPERIMENTAL SECTION

Material and Methods. Unless otherwise stated, all chemical materials were purchased from commercial sources and used without further purification. 4,4′,4″-Tricarboxytriphenylamine (H3TCA) was synthesized according to the published procedure.331H NMR spectra were measured on a Varian INOVA 400 M spectrometer. Elemental analyses were obtained on an Elemental Analyzer Vario EL III. Powder XRD diffractograms were obtained on a Riguku D/Max-2400 X-ray

addition of compound 3 into the Gd-TCA suspension in CH3CN/H2O (1:1) quenched the emission with the coefficient KSV calculated as 2.65 ± 0.2 × 103 M−1. The lifetime for the emission band of 2.38 ns for Gd-TCA is reduced to 1.58 ns in the presence of compound 3. These results confirmed the occurrence of a photoinduced electron-transfer process from D

DOI: 10.1021/acs.inorgchem.6b01267 Inorg. Chem. XXXX, XXX, XXX−XXX

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APEX CCD diffractometer with graphite monochromated Mo−Kα (λ = 0.71073 Å) using the SMART and SAINT programs.34 The structure was solved by direct methods and refined on F2 by full-matrix least-squares methods with SHELXTL version 5.1.35 Of two benzoic acid moieties in the ligand, a total of 10 carbon atoms on the two benzene rings and 2 carboxylic acid groups were disordered into two parts and their site occupied factor (s. o. f.) fixed as 0.75 and 0.25, respectively. The oxygen atoms in other benzoic acid moieties were disordered into two parts and their (s. o. f.) were fixed as 0.75 and 0.25, respectively. Except for these disordered carbon and oxygen atoms with minor site occupied factors (0.25), non-hydrogen atoms of the ligand backbones were refined anisotropically. Except for the solvent methanol and coordinated water molecules, hydrogen atoms were fixed geometrically at calculated positions and allowed to ride on the parent non-hydrogen atoms. Bond distances constraints corresponding to the disordered phenyl rings and the isolated solvent molecules were used to help the refinement on the disordered parts. The A alert in the checkCIF file is due to the close distance between the atoms in different parts of the disordered phenyl rings. The first B alert in the checkCIF file is due to that the hydrogen atoms were not fixed on the solvent molecules, and the second B alert in the checkCIF file is due to the possible presence of high disordered solvent molecules in the lattice. Typical Procedure for Hydrogen Production and Carbon Dioxide Reduction. For [FeFe]-H2ase model compound 1 as catalysts, each sample was made in a 23 mL flask with a volume of 5 mL in MeCN/water (7:3 v/v). Typically, the sample contained 0.02% Gd-TCA, 8 mM model compound 1, and 0.8 M NiPr2EtH·OAc as the sacrificial electron donor. The flask was sealed with a septum and protected from light, then degassed by bubbling nitrogen for 15 min under atmospheric pressure at room temperature. After that, the samples were irradiated by a 500 W xenon lamp, and the reaction temperature was 293 K by using a water filter to absorb heat. The generated photoproduct of H2 was characterized by GC 7890T instrument analysis using a 5 Å molecular sieve column (0.6 m × 3 mm) and a thermal conductivity detector, and nitrogen was used as carrier gas. The amount of hydrogen generated was determined by the external standard method. Hydrogen in the resulting solution was not measured, and the slight effect of the hydrogen gas generated on the pressure of the flask was neglected for calculation of the volume of hydrogen gas.27a,36 Quantum yield for light-driven H2 evolution was determined according to the literature method.29 The difference between the power of light passing through the blank and that through the sample was considered to be absorbed by Gd-TCA. Each value of quantum yield was tested three times to reduce errors. The powers of the light passing through the blank and the sample were measured with an L30A-BB-13 thermal sensor and a Nova II power meter using a 500 W Xe lamp equipped with a 365 nm band-pass filter. For [Co(bpy)3]2+ as catalysts, each sample was made in a 23 mL flask with a volume of 5 mL in MeCN/water (1:1 v/v) at pH = 10. Typically, the sample contained 0.02% Gd-TCA, 50 μM [Co(bpy)3]2+, and 2.5% v/v of Et3N as the sacrificial electron donor. The flask was sealed with a septum and protected from light, then degassed by bubbling nitrogen for 15 min under atmospheric pressure at room temperature. After that, the samples were irradiated by a 500 W xenon lamp, and the reaction temperature was 293 K by using a water filter to absorb heat. The photon flux was determined by eq 1

diffractometer with a Cu sealed tube (λ = 1.54178 Å). Thermogravimetric analysis (TGA) was carried out at a ramp rate of 5 °C/min in a nitrogen flow with a Mettler-Toledo TGA/SDTA851 instrument. FTIR spectra were recorded as KBr pellets on a JASCO FT/IR-430. Solid UV−vis spectra were recorded on an HP 8453 spectrometer. The solid fluorescent spectra were measured on a JASCO FP-6500. Both excitation and emission slits were 3 nm wide. The HCOO− in the liquid phase was analyzed using an IC (DIONEX ICS-5000, (Thermo Fisher Scientific Inc.) with an IonPac AS11-HC column and an IonPac AG11-HC column. The solution of Gd-TCA was prepared in a CH3CN/H2O mixture solution with the concentration of 0.02%. Stock solutions of the [FeFe]-H2ases model compound 1 (1.0 × 10−2 M) and [Co(bpy)3]Cl2 (4.0 × 10−2 M) were prepared in 7:3 CH3CN/H2O and 1:1 CH3CN/ H2O solvents, respectively. Solid-state voltammograms were measured by using a carbon-paste working electrode, and a well-ground mixture of each bulk sample and carbon paste (graphite and mineral oil) was set in the channel of a glass tube and connected to a copper wire. A platinum wire with a 0.5 mm diameter counter electrode and saturated calomel reference electrode were used. Measurements were performed by using a threeelectrode system in phosphate-buffered saline [PBS] at a scan rate of 50 mV s−1, in the range of 0.5−1.1 V. Dye-uptaking experiments were displayed by soaking Gd-TCA (1.2 mg, 2 μmol) in a methanol solution of 2′,7′-dichorofluorescein dye (24 mM, 2 mL) overnight. The resulting crystalline powders were washed thoroughly with methanol until the solution became colorless. The washed samples were digested by Na2EDTA (0.01 M, 2 mL) and NaOH (1.5 M, 0.1 mL), and the resultant clear solution with light olivine color was diluted to 25 mL and adjusted to a pH of 1.5. Absorption experiments were performed on a TU-1900 UV−vis spectrophotometer. The concentration of 2′,7′-dichorofluorescein was determined by comparing the UV−vis absorption with a standard curve. Syntheses and Characterizations. Synthesis of Gd-TCA. A mixture of 4,4′,4″-tricarboxytriphenylamine (H3TCA) (94 mg, 0.25 mmol) and Gd(NO3)3·6H2O (455 mg, 1 mmol) was dissolved in 15 mL of mixed solvents of DMF and ethanol in a screw-capped vial. The resulting mixture was kept in an oven at 100 °C for 3 days. Yellow block-shaped crystals were obtained after filtration and dried in vacuum. Yield: 60%. Anal. Calcd For C21H17NO6Gd·2H2O (%): C, 44.44; H, 2.47; N, 2.84; Found: C, 44.40; H 2.43; N, 2.87. FTIR (KBr pellet) (cm−1): 3428 (w), 1659 (m), 1591 (m), 1532 (m), 1510 (m), 1311 (m), 1174 (m), 1105 (s), 1015 (s) cm−1. Synthesis of Gd-TCA Film.20b α-Al2O3 particles (1.5 cm × 0.5 cm) with a large average pore size of 100 nm and porosity of 30−40% were treated with 4 mL of APTES in 150 mL of toluene at 120 °C for 24 h under N2 conditions, leading to an APTES monolayer deposited on the α-Al2O3 particle surface. Then, the crystal seedings were deposited onto the surface of the APTES functionalized α-Al2O3 patterns by dispersion in mother solution with a microwave method. The solvothermal reaction of H3TCA and Gd(NO3)3·6H2O in the presence of the functionalized α-Al2O3 pattern in a mixed solvent of DMF and ethanol gave Gd-TCA film in a yield of 10%, and the films were washed with pure ethanol and then dried in a vacuum oven (Scheme 2).

Scheme 2. Synthetic Process of Gd-TCA Film

FPhoton = 2

Pλ SBeamhc

(1)

where P (in W) is determined by the difference in the power of light passing through the blank and the sample, respectively; λ is the irradiation wavelength number (365 nm); SBeam is the beam area; h is Planck’s constant; and c is the speed of light. The quantum yields (φ) were determined by eq 2

Single-Crystal X-ray Crystallography. Crystal data of Gd-TCA: C22H20NO9Gd, Mr = 599.64, Triclinic, space group P1̅, a = 9.541 (2) Å, b = 12.842(2) Å, c = 14.455(3) Å, α = 116.32(1)°, β = 109.20(1)°, γ = 90.15(1)°, V = 1476.1(5) Å3, Z = 2, Dc = 1.349 g cm−3, μ(Mo− Kα) = 2.287 mm−1, 5606 unique reflections [Rint = 0.0402]. Final R1 [with I > 2σ(I)] = 0.0587, wR2 (all data) = 0.1650, GOF = 1.036. CCDC No. 933349. Intensities were collected on a Bruker SMART

n(H 2) × N n(H 2) × Nhc ⎛1 ⎞ φ⎜ H 2 ⎟ = =2 ⎝ 2 ⎠ FPhotonSBeamt Pλ t E

(2)

DOI: 10.1021/acs.inorgchem.6b01267 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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where the amount of hydrogen produced (n(H2)) was estimated form GC analysis; N is the Avogadro constant; and t is the irradiation period. The reduction procedure of carbon dioxide displayed by bubbling the carbon dioxide into the solution up to solution saturation and carefully adjusting the pH value of the system to 11.0 of a CH3CN/ H2O (7:3 in volume 5.0 mL) mixture solvent at 25 °C. The aforementioned solution containing a Gd-TCA suspension (1.0 ppm), redox catalyst 3 (50 μM), and Et3N (10.0% v/v) was irradiated for 12 h.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01267. Experimental details and related spectra (PDF) Crystal data of Gd-TCA (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Natural Science Foundation of China (21421005 and 21531001), the Natural Science Foundation of Jiangsu Province (BK20140234).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b01267 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b01267 Inorg. Chem. XXXX, XXX, XXX−XXX