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Unraveling the Optoelectronic and Photochemical Behavior of Zn4OBased Metal Organic Frameworks Hossein Khajavi,*,† Jorge Gascon,*,† Juleon M. Schins,‡ Laurens D. A. Siebbeles,‡ and Freek Kapteijn† †
Catalysis Engineering, Chemical Engineering Department, and ‡Optoelectronic Materials, Delft University of Technology, Julianalaan 136, 2628 BL, Delft, The Netherlands
bS Supporting Information ABSTRACT:
Zn4O clusters corresponding with those present in MOF-5, IRMOF-8, and IRMOF-9 have been synthesized as monounits with the respective monodentate organic linkers attached. Ultraviolet (UV) absorption analysis of synthesized clusters and metal organic frameworks (MOFs) reveals that the organic moieties in the MOFs lose their individual absorption and emission characteristics, in contrast with their subunits which retain these characteristics. Observed photoluminescence (PL) quantum yields of the MOFs are higher than those of their corresponding subunits. This is explained by the isolation of clusters in monounits, where linkers are only attached to one cluster, resulting in greater charge localization compared to MOFs. The photocatalytic activity of the clusters in the nonporous monounits crystals turned out to be higher than that of their MOF counterparts and that of zinc oxide in the oxidation of propene at room temperature under UV illumination. Clearly porosity does not offer in this case any significant advantage in contrast to charge localization at Zn4O clusters. This observed difference in photocatalytic activity was found to be inversely proportional to the difference in PL quantum yields between monounits and MOFs, highlighting competition between decay of excited states by photon emission and electron transfer. Detection of the same products of partial oxidation with similar selectivities for the crystals of the synthesized monounits as well as for the MOFs led to the conclusion that Zn4O clusters in the monounits are in fact photochemical equivalents of those in their corresponding MOFs.
1. INTRODUCTION Ordered porous solids are an essential class of materials from an economic viewpoint as they are crucial for many industrial applications: from catalysis and separation to the fine chemicals industry.1 Metal organic frameworks (MOFs) are a relatively new class of materials that fall under this category15 but have not yet been utilized on such a scale. MOFs are composed of inorganic clusters connected to organic linkers possessing complexing and chelating groups such as carboxylates, phosphonates, and nitrogen-containing compounds. Many MOFs exhibit absorption in the ultraviolet region and emissions in the visible region qualitatively similar to semiconductors, while with additional chemical modification of linkers strong and highly tunable luminescent activities are possible. MOFs can therefore be adapted for a vast variety of optical applications, including luminescent pH sensors,6 tunable light emitters,7 and photocatalysis.8,9 In spite of their poor stability, among the different MOF candidates for photo-based applications, the so-called isoreticular MOFs have so far stood out. MOF-5, also known as IRMOF-1, is the best known and understood structure,10 offering an excellent playground for fundament studies into the different factors that r 2011 American Chemical Society
dominate charge transfers in these hybrid materials.11 The scope of this work involves the preparation and optoelectronic characterization of several MOFs based on the well-known Zn4O cluster (MOF-5, IRMOF-8, and IRMOF-9) along with that of their respective inorganic subunit complexes and the isolated Zn4O cluster. To date, a large number of Zn4O core containing inorganic complexes have been reported.1222 Basic zinc acetate (BZA), also known as zinc acetate oxide and zinc oxyacetate, with the chemical formula Zn4O (CH3COO)6, was already synthesized in 1924 by heating zinc acetate under vacuum, giving off acetic anhydride while its structure was established in 1954.1214 Since then, it has been found that yields can be improved by reacting zinc oxide and acetic acid, filtering the produced slurry followed by heating under vacuum.15 This important complex has been thoroughly investigated, as it is the most isolated form of Zn4O. Similarly tetranuclear zinc Received: February 22, 2011 Revised: May 17, 2011 Published: May 24, 2011 12487
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The Journal of Physical Chemistry C benzoate (TNZB), chemical formula Zn4O (C6H5CO2)6, has been synthesized by thermal decomposition of zinc benzoate.20 The Zn4O cluster is also familiar in metal organic frameworks, present as the node or “connector” in a wide variety of MOFs. Coordination of Zn4O nodes in MOFs through oxygen bridges to six organic linkers in three dimensions can result in a continuous lattice.23 In MOF-5 terephthalates link Zn4O clusters in the lattice. Similarly, naphthalene 2,6-dicarboxylate and (1,10 -biphenyl) 4,40 -dicarboxylate are the linkers joining Zn4O clusters in IRMOF-8 and IRMOF-9,24 respectively.
Figure 1. Schematic structure of BZA (protons not shown).
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So far, reported Zn4O complexes have been viewed as furthering molecular models of zinc oxide25 and as a tool for understanding the electronic structure and properties of zinc oxide defect centers and the nonstoichiometry that occurs in the bulk.26 This has been done experimentally through coupling of UV techniques with X-ray photoelectron spectroscopy (XPS) and theoretically by the first local density function principle.25,26 Experimentally, it has been shown that the tetrahedral arrangement of basic zinc acetate provides a good electronic model of the oxygen chemical environment in bulk zinc oxide. Theoretical results indicate that the lowest energy electronic absorption transition mimics that of zinc oxide in the bulk, resulting in similar, if not identical, photophysical properties. While these simple Zn4O-based clusters have been viewed as molecular models of zinc oxide, zinc oxide itself has been considered as a semiconductor equivalent of Zn4O-based MOFs, specifically MOF-5 in view of its zinc oxide like quantum dot behavior.27 This suggests that Zn4O-based MOFs are wide band gap semiconductors or possibly insulators. Photoexcitation studies of MOF-5 have revealed charge separation as well as a band gap ranging from 3.5 to 4.0 eV (dependent on crystal size, preparation method, and sample treatment), the lowest value being similar to that of zinc oxide,2830 widely accepted to be 3.4 eV. In this respect the overall electronic properties of Zn4O-based MOFs can be characterized by the inorganic clusters that bring the lattice system to the wide band gap semiconductor character of zinc oxide. It must be realized that the filling of the highest valence bands (VB) is mostly due to contributions from the π-orbitals of the aromatic rings in the close vicinity, with minor contributions of distant conjugated systems and negligible
Figure 2. Schematic structure of MOF-5, IRMOF-8, and IRMOF-9 and their corresponding inorganic subunit complexes (protons not shown). 12488
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The Journal of Physical Chemistry C contributions from carboxylate functional groups forming the oxygen bridges. It can be speculated whether only the band gaps of these materials are altered by changing the ligands involved. Understanding the linker to metal charge transfer (LMCT) between the Zn4O clusters and linkers, the potential arises for tuning the effective band gaps of MOFs by using linkers with higher degrees of aromaticity.28,30 This is the objective of the present study. In recent literature it has been suggested that comparisons between MOFs and zinc oxide can be associated with the instability of MOFs and their susceptibility to hydrolysis, which can form zinc oxide nanoparticles within the structure.31 All semiconductor characteristics observed for Zn4O MOFs are then attributed to the presence of zinc oxide. Though it can be expected that zinc oxide and zinc acetate are formed when less stable MOFs such as MOF-5 are hydrolyzed, the free linkers produced in the process have not yet been identified. That in combination with cutoffs in the UV range of solvents used for luminescence studies gives insufficient grounds to draw such solid conclusions. In the context of this work, great care has been taken to ensure the purity of produced compounds as well as the absence of zinc oxide. In the following the synthesized MOFs and subunits will be identified by X-ray diffraction (XRD), Brunauer Emmet and Teller (BET) surface area from nitrogen adsorption, proton and carbon liquid nuclear magnetic resonance (1H, 13C NMR), liquid chromatography mass spectroscopy (LCMS), and inductive coupled plasma (ICP) elemental analysis. Optical characterization is done by UVvis absorption and photoluminescence (PL) spectroscopy. The effective quantum yield of the identified materials has been determined. Finally, the photocatalytic activity of these materials is demonstrated for the room temperature partial oxidation of propene under UV illumination.30
2. EXPERIMENTAL METHODS Reagents and solvents were purchased from Sigma-Aldrich and were used without further purification. All XRD patterns were recorded under a nitrogen atmosphere using a Bruker-AXS D5005 theta/theta diffractometer equipped with incident beam Cu KR1 monochromator. An AutoSorb 6B was used for N2 adsorption measurements. Scanning electron microscopy (SEM) was done using a Philips XL20. NMR spectra were made using Varian Unity Inova 300 MHz with deuterated benzene (D6C6) as solvent unless stated otherwise. LCMS was done through direct injection using electrospray ionization/atmospheric pressure chemical ionization (ESI/APCI) Shimadzu LCMS-2010A in methanol. A Perkin-Elmer Optica 3000dv was used for ICP elemental analysis. UV/vis and PL spectra have been measured in solid state on 25 mg of dry powder samples dispersed over 1 cm2 quartz glass (Ted Pella) under a pure nitrogen atmosphere. UV/ vis spectra were measured using Perkin-Elmer UV/vis/NIR Lambda 900 in diffusive reflectance mode, and PL spectra were measured using a Photon Technology International (PTI) QuantaMaster 20 with excitation at 200 nm. Estimation of quantum yields was done with a stable suspension of the solids in chloroform (UV cutoff at 200 nm). The photocatalytic oxidation of propene was carried out in a Harrick three window cell (TWC) using a Newport Oriel Apex fiber illuminator 100 W Hg lamp equipped with a 650 lm filter for illumination. 2.1. MOF Preparation. MOF-5 was synthesized by the room temperature method reported elsewhere;32 IRMOF-8 and
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IRMOF-9 were also made according to the literature.24 Further cleaning of all MOFs made was done using continuous extraction with chloroform overnight, followed by drying under high vacuum at ambient temperature and storing under dry nitrogen. 2.2. BZA Preparation. BZA was produced by dissolving 10 g of zinc acetate dihydrate in 25 mL of ultrapure water at 85 °C. To this clear solution, 4.25 g of zinc oxide was added, and this slurry was kept at 85 °C for 30 min under reflux. Then the solution was transferred to an oven where the water was slowly evaporated after which the remaining white powder was heated to 175 °C for up to 18 h. BZA was isolated by continuous extraction with benzene and then recrystallized in benzene for a minimum of 30 days. The crystals were then filtered under a nitrogen atmosphere, washed with the mother liquor, dried under high vacuum at room temperature, and stored under dry nitrogen. Yield: 18.6%. Elemental analysis: 38.56 wt % Zn. 1H NMR in chloroform δ: 1.45 (d, methyl), 5.15 (m, methine) ppm. 13C NMR in chloroform δ: 25.9, 79.0 ppm. LCMS m/z: 659.55 (BZA þ Na), 637.75 (BZA), 571.61 (BZA C2H4O2), 454.69 (BZA 3C2H4O2). 2.3. TNZB Preparation. TNZB was obtained by a modified procedure found in the literature20 similar to the preparation BZA: by preparing a slurry of 14 g of zinc benzoate and 4.25 g of zinc oxide in ultrapure water. After refluxing for 1.5 h and cooling, the water was slowly evaporated, after which the remaining white powder was heated to 275 °C for a minimum of 12 h under a dry nitrogen atmosphere. After continuous extraction and recrystallization in benzene for a minimum of 30 days the crystals were filtered under a nitrogen atmosphere, dried under high vacuum at room temperature, and stored under dry nitrogen. It was found that TNZB could also be prepared by a similar synthesis procedure as for MOF-5 under reflux at 153 °C using benzoic acid instead of the terephthalic acid linker, followed by cold crystallization and drying under a nitrogen flow at 100 °C. Procedure from literature: yield 10.3%. Elemental analysis: 26.99 wt % Zn. 1 H NMR in chloroform δ: 6.28 (s, methine), 7.38 (s, benzene), 7.40 (m, benzene) ppm. 13C NMR in chloroform δ: 90.3, 126.5, 128.3, 129.5, 143.2 ppm. LCMS: m/z 950.77 (TNZB C6H6), 888.88 (TNZB C7H6O2), 766.53 (TNZB 2C7H6O2), 656.00 (TNZB 3C7H6O2), 508.23 (1/2 TNZB þ 3H). Optimized proposed procedure: yield 35.1%. Elemental analysis: 27.87 wt % Zn. 1H NMR in chloroform δ: 6.25 (s, methine), 7.35 (s, benzene), 7.37 (m, benzene) ppm. 13C NMR in chloroform δ: 89.3, 125.8, 127.6, 128.9, 142.0 ppm. LCMS m/z: 950.77 (TNZB C6H6), 888.88 (TNZB C7H6O2), 766.53 (TNZB 2C7H6O2), 656.00 (TNZB 3C7H6O2), 508.23 (1/2 TNZB þ 3H). 2.4. Mono IRMOF-8 and Mono IRMOF-9 Preparation. Mono IRMOF-8 and mono IRMOF-9 units were prepared separately according to the synthesis procedure of their respective MOFs but using 2-naphthoic acid and 4-phenylbenzoic acid instead of the linkers. Using a 12.5 molar ratio of zinc acetate dihydrate to acid in N,N-dimethylformamide. After refluxing overnight the product was cold crystallized and then recrystallized in benzene for over 60 days. The yellowish crystals were dried under a nitrogen flow at 100 °C and stored under dry nitrogen. Mono IRMOF-8: yield 17.1%. Elemental analysis: 21.32 wt % Zn. 1H NMR δ: 6.21 (s, methine), 7.18 (d, naphthalene), 7.50 (s, naphthalene), 7.55 (m, naphthalene), 7.97 (m, naphthalene), 8.01 (m, naphthalene) ppm. 13C NMR δ: 90.0, 126.0, 127.3, 127.5, 128.0, 131.8, 133.7, 135.2 ppm. LCMS m/z: 811.45 (mono IRMOF 8 3C11H8O2). Mono IRMOF-9: yield 13.8%. Elemental analysis: 18.31 wt % Zn. 1H NMR δ: 6.24 (s, methine), 7.40 (m, benzene), 7.50 (m, benzene) ppm. 12489
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The Journal of Physical Chemistry C C NMR δ: 67.8, 127.6, 128.9, 139.8, 141.1 ppm. LCMS m/z: 872.12 (mono IRMOF 10 3C12H10O2). 2.5. PL Quantum Yield Estimation Procedure. Using the method reported by Williams et al.,33 comparative PL quantum yields have been calculated using a relative basis with a comparable reference molecule whose exact PL is known and increases linearly with concentration. This concept finds its origin in the ratio of photons absorbed against those emitted which is the defined quantum yield here. In this context quinine sulfate, which in 0.1 M H2SO4 solution has a reported quantum yield of 0.54 and an effective emission range of 400600 nm, was used.34 The procedure is as follows: the absorption and PL spectra of a sample in solution with six different concentrations were measured, and the recorded background is subtracted. The integrated fluorescence intensity and the area of the fluorescence spectrum are calculated from spectra corrected for background and solvent emissions. A graph is plotted of the integrated fluorescence intensity against absorbance. A straight line with gradient (m) and an intercept of zero is fitted. The same procedure is applied for the reference solution. The quantum yield (φ) is then defined as the gradient of the sample divided by that of the reference multiplied by the quantum yield of the reference. See Supporting Information for further details. Chloroform was an appropriate solvent here due to absence of π-bond stacking which can give rise to fluorescence phenomena.35 Despite this, an overall blue shifting of spectra was observed, associated with weak coupling of conduction bands of samples with those of the solvent. The shifting of spectra does not affect the calculated values of quantum yields as it is constant for all samples and does not shift emission peaks outside of the range of accuracy of the used reference. 2.6. Photoinduced Oxidation of Propene. The oxidation of propene was carried out in a Harrick three-window cell (TWC). One window was used for illumination while the other two windows were used for Operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to follow the reaction on the sample bed surface with time.35 Before recording the infrared (IR) spectra, samples were pretreated under a steady flow of helium at 100 °C for up to 45 min. After pretreatment the sample was exposed to a gas flow rate of 10 mL/min at room temperature consisting of 28.5 mol % propene and 14.3 mol % O2 from dry air. After 45 min stabilization under flow conditions a background was taken before the sample was illuminated. On this basis new IR bands observed after illumination correspond with products of photoinduced oxidation on the surface.30 The obtained spectra have been normalized to the number of Zn4O clusters present in the sample, with BZA having the highest and mono IRMOF-9 the lowest number of clusters on a weight basis. 13
3. EXPERIMENTAL RESULTS From the XRD patterns (Supporting Information; S1) MOF5, IRMOF-8, and IRMOF-9 can be identified.24,32,36 In the latter sample reflections for both the interpenetrated and noninterpenetrated forms are visible. Density separation indicated the presence of 8.6 wt % of the interpenetrated form of IRMOF-9. XRD patterns of BZA and MOF subunits in combination with SEM pictures (Supporting Information; S2) show that with increasing size of the organic moieties the crystal size increases and therefore the crystallinity; the diffraction intensity of mono IRMOF-8 and mono IRMOF-9 increased in comparison with BZA and TNZA, while the reflection widths are not significantly
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different. Specific BET surface areas of the MOFs (Supporting Information; S3) are within the range of areas reported in the literature while the synthesized monounits are nonporous. Absorption and PL spectra are presented in Figure 3, while band gaps are presented in Figure 4 with estimated quantum yields. Values obtained here for MOFs are in excellent agreement with previous work.30 The nearly identical absorption and emission spectra of zinc oxide and BZA in Figure 3a reveal why these compounds have been considered as semiconductor equivalents in previous works,25 with band gaps of 3.4 and 3.7 eV, respectively, and maximum emissions at 488 and 495 nm. The absorption intensities of the two compounds are comparable up to 335 nm where both compounds show saturation for shorter wavelengths. One significant difference is the tailing in the absorption of BZA between 365 and 400 nm associated with the acetyl groups residing close to the Zn4O clusters. This tailing is mirrored in the emission spectra, which show no independent emission of acetyl groups. Tailing effects are not observable if excitation is done at longer wavelengths.37 Absorptions similar to benzoic acid are predominant in the spectrum of TNZB (Figure 3b). The absorption spectrum of MOF-5 with its characteristic band gap at 4.0 eV resembles greatly that of zinc oxide blue-shifted and the loss of all absorptions associated with organics in the range of 355 and 400 nm. All the emission spectra in Figure 3b share a common profile with different extents of tailing, present the strongest in TNZB. The maximum emission wavelengths at 423 nm for MOF-5 and at 418 nm for TNZB show a constant Stokes shift for this MOF and its subunit. The absorption spectrum of mono IRMOF-8 strongly resembles that of 2-naphthoic acid (Figure 3c), while IRMOF-8 is lacking the absorptions associated with organic moieties between 360 and 425 nm and has a band gap of 3.3 eV. With maximum emission wavelengths of IRMOF-8 and mono IRMOF-8 occurring at 500 and 488 nm, respectively, the Stokes shift for these MOFs and their corresponding subunits is again the same. In contrast, the Stokes shifts of the two MOFs are different, as inferred by a difference of more than 25 nm in the maximum emission of IRMOF-9 (447 nm) and mono IRMOF-9 (472 nm). IRMOF-9 has a band gap of 3.9 eV (Figure 3d), significantly larger than IRMOF-8, and as in previous cases there is a loss of absorption from 365 to 400 nm for the MOF compared to its subunits and linker. Emission intensity comparisons relative to corresponding excitations using the estimated quantum yields (Figure 4) show that BZA has a very low quantum yield of 0.09. This value is of the same order as zinc oxide which has an exact quantum yield of 0.03.38 On the other hand, MOFs show marginally higher quantum yields in comparison to their monounits with TNZB (0.21), MOF-5 (0.24), mono IRMOF-9 (0.24), IRMOF-9 (0.31), mono IRMOF-8 (0.35), and IRMOF-8 (0.49) in ascending order. While MOFs show higher quantum yields than monounits, the band gaps of monounits are visibly smaller than their corresponding MOFs with on average a difference of 0.3 eV. As is known from classical organic chemistry, the conversion of propene can take place through a reversible route with water involved and through an irreversible route. The reversible route proceeds through intermediates among which isopropyl alcohol, acetone, and acetic acid prior to the formation of carbon dioxide, while the irreversible route produces propanal and propanoic acid before further oxidation to carbon dioxide. In this context the photoinduced oxidation of propene was carried out over the investigated MOFs and subunits as well as blank reference runs 12490
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The Journal of Physical Chemistry C
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Figure 4. Band gaps and estimated quantum yields of MOFs and subunits using quinine sulfate as reference.
Figures 5b,c illustrate the photochemical activity of IRMOF-8 and its subunit over 60 min by three spectra taken every 20 min. As with other MOFs and monounits, IRMOF-8 and mono IRMOF-8 show no significant difference in their spectra over the 60 min interval. In the region of interest, absorptions at 1780 and 1764 cm1 correspond to the carbonyl stretching of propanoic acid. The carbonyl absorption of saturated aliphatic aldehydes appears from 1740 to 1720 cm1. Like typical ketones, acetone shows a carbonyl stretching absorption band at 1715 cm1, which also has contributions from acetic acid. As with ketones if the carbon adjacent to the aldehyde functional group is unsaturated vibrations are shifted to lower wavenumbers between 1710 and 1685 cm1, and here the presence of unsaturated aldehydes is apparent. The observed band at 1415 cm1 has contributions from the various CH bending in the forementioned products of the propene partial oxidation. Besides the products of partial oxidation, trace amounts of carbon monoxide are formed and are observed by bands at 2202 and 2228 cm1 corresponding to two different conformations of CO.39 Regrettably, carbon dioxide evolution could not be followed due to the background concentration in the experimental setup.
Figure 3. UV/vis absorption (left) and PL spectra (right) of (a) a zinc oxide and BZA; (b) MOF-5, TNZB, and benzoic acid; (c) IRMOF-8, mono IRMOF-8, and 2-naphthoic acid; and (d) IRMOF-9, mono IRMOF-9, and 4-phenylbenzoic acid.
without sample. Illumination of a mixture of propene and oxygen yielded no reaction as was also the case with BZA (Figure 5a). The catalytic performance of the selected MOFs was in accordance with the literature,30 with MOF-5 showing no appreciable activity, IRMOF-8 being the most active and IRMOF-9 performing in between, similar as the order in their PL quantum yield. The tested MOFs and subunits do not show a distinguishable selectivity in their products.
4. DISCUSSION Apart from subtle differences between the optoelectronic properties of synthesized MOFs and subunits, as discussed later, there are systematic trends, namely the blue-shifting of the MOF absorptions compared to their subunits and to the organic linkers. Loss of the independent organic absorptions in the MOFs, in contrast with their subunits that retain these characteristic profiles (also visible in the emission spectra), can be due to the absence of π-bond stacking in MOFs. Inherent to the structure, aromatic rings in MOFs cannot stack as in the synthesized monounits. Observing the same Stokes shifts for monounits and MOFs indicated that nuclear rearrangements occurring are identical in both materials and are associated with the same transitions when considering the similarity of electronic states and molecular surroundings. The large Stokes shifts observed indicate that the charge-separated states formed are very stable, and the observed emissions are those of chargeseparated states instead of PL from organic centers. It is also 12491
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The Journal of Physical Chemistry C
Figure 5. DRIFT spectra of (a) IRMOFs and subunits collected after 60 min of illumination under reaction conditions normalized to number of Zn4O clusters; (b) DRIFT spectra recorded after 20, 40, and 60 min of illuminating IRMOF-8; and (c) mono IRMOF-8.
interesting to note that the relative emission intensities are the highest for the organic linker and the lowest for the subunit with the MOF being in between, quantified by the estimated quantum yields. The very low quantum yield of BZA suggests that the Zn4O cluster does not contribute to the PL of MOFs. MOFs, however, have higher PL quantum yields than their subunit counterparts but exhibit lower catalytic activities per Zn4O cluster, while at the same time monounits exhibit smaller band gaps than their corresponding MOFs. This is due to significantly greater isolation of Zn4O clusters in the monounit, where linkers are connected to a single cluster, which causes greater charge localization and therefore a higher catalytic activity. At the same
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time charge buildup can hinder the excitation of electrons while reducing relaxation by photon emission, leading to lower PL quantum yields. The quantum yield of IRMOF-9 and mono IRMOF-9 is lower than that of IRMOF-8 and mono IRMOF-8. This is attributed to the perpendicular orientation of the benzene rings in the biphenyl linker, impeding effective electron transfer to the nearest Zn4O cluster from the outermost benzene rings. In Figure 3d, the absence of tailing for mono IRMOF-9 suggests the formation of forbidden transitions. In the same spectra the Stokes shift difference can be accounted for by some interpenetration of IRMOF-9.20,31 Network interpenetration can indeed result in a higher local density of Zn4O clusters, causing a shift in the maximum emission. LMCT for each organic antenna is spread over a larger number of nodes by electron transfers between the interpenetrated networks. Activation of molecular oxygen by photoinduced processes is strongly dependent on the number of Zn4O clusters, where charge can build up, preventing the complete oxidation of propene under the given conditions. This is evident as products observed in the photoinduced oxidation of propene are predominantly due to single electron oxidation steps. From Figure 5a follows that the monounits are notably more active than their MOF counterparts. For comparison, the relative number of turnovers per time and per Zn4O cluster seems a reasonable parameter to consider by comparing observed IR band intensities. With this in mind, using the most intense bands as a reference point it is observed that the effective turnover frequency (TOF) for TNZB is 12% larger than MOF-5, in the case of mono IRMOF-9 and mono IRMOF-8 20 and 25% greater than their respective MOFs. At the same time the quantum yield of MOF-5 is 13% larger than TNZB, while IRMOF-9 and IRMOF-8 have quantum yields 23 and 27% larger than their corresponding subunits. This is indicative of the competitive nature of relaxation of excited electrons by photon emission and electron transfer for oxygen activations. For nonporous monounits only the outside surface of the crystals are active while in the case of the MOFs the internal surface area is accessible and can also contribute to the observed bands. Hence, it is apparent that monounits are indeed very active and that porosity of MOFs does not play a significant role in comparison with effective LMCT. This is far greater when organic antennas are bridged to only one cluster. Larger numbers of defects present in the arbitrarily arranged monounits can also contribute to the difference in activity, given that MOF lattices primarily have defects at crystal edges.41,42 This would have similar effects as isolating the clusters, with higher charges being built up at defect points. Regardless of structural differences, the nonporous MOF subunits are representative for the photochemical activity of their corresponding MOFs, as bands observed between 1800 and 1400 cm1 associated with reported products are consistent in the DRIFT spectra, in clear contrast to the selectivity found for zinc oxide.30 The contrast between products and selectivities observed for Zn4O clusters (in MOFs and monounits) and zinc oxide together with the significant difference in band gaps of zinc oxide and MOFs is testament to the absence of zinc oxide.
5. CONCLUSIONS We have successfully synthesized monounits as model compounds for three MOFs in the form of individual Zn4O nodes attached to monodentate analogues of the linkers in the MOFs. An improved synthesis procedure has been developed for 12492
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The Journal of Physical Chemistry C tetranuclear zinc benzoate (TNZB), the subunit of MOF-5. This procedure could be applied to produce monounits of IRMOF-8 and IRMOF-9. By ultraviolet (UV) and photoluminescence (PL) techniques, it has been shown that the Zn4O clusters in the synthesized monounits are optoelectronic equivalents of those found in their respective MOFs, while similarity in photochemical behavior has been shown by the photoinduced oxidation of propene. The optoelectronic behavior and photocatalytic performance of MOFs are due to the cooperative effect of nodes and linkers. The semiconductor behavior of these MOFs comes from the inorganic clusters while the position and conjugation of the linkers tune the overall semiconductor behavior by acting as antennas for the linker to metal charge transfer (LMCT). Photoluminescence quantum yields of MOFs are higher than their corresponding subunits, attributed to the isolation of Zn4O clusters which causes a greater charge localization in monounits and reduces the relaxation by photon emission. On the other hand, the photocatalytic activity of the nonporous crystals of the monounits is higher than that of their porous MOF counterparts as well as that of zinc oxide in the oxidation of propene. The greater charge localization at Zn4O clusters boosts the activity of monounits.
’ ASSOCIATED CONTENT
bS
Supporting Information. X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) of monounits, nitrogen adsorptionBET analysis, 1H and 13C nuclear magnetic resonance (NMR), liquid chromatography mass spectroscopy (LCMS), and sample photoluminescence (PL) quantum yield calculation. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail
[email protected] (H.K.),
[email protected] (J.G.); Telþ31 (0)15 278 6733, Faxþ31 (0)15 278 5006.
’ ACKNOWLEDGMENT TU Delft is acknowledged for financial support. J.G. gratefully acknowledges The Netherlands Science Foundation for his personal VENI grant. ’ REFERENCES (1) Valtchev, V.; Mintova, S.; Tsapatsis, M. Ordered Porous Solids Recent Advances and Prospects; Elsevier: Oxford, UK, 2009. (2) Ferey, G. Chem. Soc. Rev. 2008, 37, 191–241. (3) James, S. L. Chem. Soc. Rev. 2003, 32, 276–288. (4) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem. 2004, 116, 2388–2430. (5) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Chem. Soc. Rev. 2009, 38, 1450–1459. (6) Harbuzaru, B. V.; Corma, A.; Rey, F.; Jorda, J. L.; Ananias, D.; Carlos, L. D.; Rocha, J. Angew. Chem., Int. Ed. 2009, 48, 6476–6479. (7) Wang, M.; Guo, S.; Li, Y.; Cai, L.; Zou, J.; Xu, G.; Zhou, W.; Zheng, F.; Guo, G. J. Am. Chem. Soc. 2009, 131, 13572–13573. (8) Silva, C. G.; Luz, I.; Xamena, F. X. L. i; Corma, A.; García, H. Chem.—Eur. J. 2010, 16, 11133–11138. (9) Choi, J. R.; Tachikawa, T.; Fujitsuka, M.; Majima, T. Langmuir 2010, 26, 10437–10443.
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