Transforming MOFs for Energy Applications Using the Guest@MOF

Forum Article pubs.acs.org/IC

Transforming MOFs for Energy Applications Using the Guest@MOF Concept Andrew M. Ullman, Jonathan W. Brown, Michael E. Foster, François Léonard, Kirsty Leong, Vitalie Stavila, and Mark D. Allendorf* Chemistry, Combustion, and Materials Center, Sandia National Laboratories, Livermore, California 94551, United States ABSTRACT: As the world transitions from fossil fuels to clean energy sources in the coming decades, many technological challenges will require chemists and material scientists to develop new materials for applications related to energy conversion, storage, and efficiency. Because of their unprecedented adaptability, metal−organic frameworks (MOFs) will factor strongly in this portfolio. By utilizing the broad synthetic toolkit provided by the fields of organic and inorganic chemistry, MOF pores can be customized to suit a particular application. Of particular importance is the ability to tune the strength of the interaction between the MOF pores and guest molecules. By cleverly controlling these MOF−guest interactions, the chemist may impart new function into the Guest@MOF materials otherwise lacking in vacant MOF. Herein, we highlight the concept of the Guest@MOF as it relates to our efforts to develop these materials for energy-related applicatons. Our work in the areas of H2 and noble gas storage, hydrogenolysis of biomass, light-harvesting, and conductive materials will be discussed. Of relevance to light-harvesting applications, we report for the first time a postsynthetic modification strategy for increasing the loading of a light-sensitive electron-donor molecule in the pores of a functionalized MIL-101 structure. Through the demonstrated versatility of these approaches, we show that, by treating guest molecules as integral design elements for new MOF constructs, MOF science can have a significant impact on the advancement of clean energy technologies. catalytic devices.14 This versatility, unparalleled in other classes of microporous materials, stems from the ability to rationally design them utilizing well-understood metal coordination modes and linker topologies. Moreover, MOFs possess unprecedented synthetic versatility by virtue of having both constituent metal ions and organic “linker” groups. This allows customization of pore sizes, shapes, and chemical environments to suit particular applications. The pores of as-synthesized MOFs are filled with solvent molecules and residual reactants, which are removed (a process referred to as “activation”) to allow for gas sorption. These socalled “guest” molecules were initially viewed as inconsequential to the material properties. However, with the visibility of MOFs greatly heightened as a result of the gas storage research, some were lead to inquire “what molecules besides gases can be put into a MOF pore?” The answer to this question is “almost anything”. MOFs hosting dye molecules,15 fullerenes,16,17 polymers,18 and even proteins19 now populate the literature. Initially, these achievements were primarily curiosities, as their utility was unclear. However, their use as templates and as a means to stimulate structural switching in MOFs was recognized early on by S. Kitagawa and co-workers.18 It was soon realized that MOFs could do more than merely confine molecules; they could also create new functions in these or

1. INTRODUCTION Energy conversion and storage is perhaps the defining scientific challenge of our time. Of the many critical problem areas affecting individuals and society in life-changing ways, none has such far-reaching implications as anthropogenic climate change.1 This necessitates the continued adoption of carbonneutral energy sources and development of efficient technologies for their conversion and storage. With recent shocking spikes in global surface temperatures,2 there is a mounting sense of urgency, making the consideration of novel high-risk/ high-payoff solutions more justified than might otherwise be the case.3 New material chemistries will play a critical role in developing solutions to mitigate the impact of climate change over the remainder of the century. During the 1990s, the search for new sources of clean energy led to expanded efforts to develop hydrogen-powered fuel cells. The lack of high-capacity, low-cost hydrogen storage materials that could meet targets set by the U.S. Dept. of Energy was recognized as a barrier to progress. Enter metal−organic frameworks (MOFs), a class of microporous materials that, by virtue of their unprecedented high surface areas, have the potential to solve the vehicular hydrogen storage problem. This sparked a boom in MOF research related to gas storage.4−7 It is now apparent that MOFs possess a unique combination of properties that make them attractive materials for many other energy-related problems. For example, they have been used as fuel cell membranes,8 components of batteries, and as active materials in photovoltaic,9−12 thermoelectric,13 and photo© XXXX American Chemical Society

Special Issue: Metal-Organic Frameworks for Energy Applications Received: April 14, 2016

A

DOI: 10.1021/acs.inorgchem.6b00909 Inorg. Chem. XXXX, XXX, XXX−XXX

Forum Article

Inorganic Chemistry enable use of the guest in new ways. As such, the guest becomes more than an innocent bystander. An early example is the stabilization of catalytic nanoparticles without protective groups while retaining access to reactant species.20,21 More recently, work by us and others demonstrated altered reaction pathways and kinetics,22 enhanced light absorption and emission,17 sizeselective biocatalysis,23 and new electronic properties13,24 as a result of guest infiltration. These results raise the intriguing possibility that guests within MOF pores should be considered a further design element that can be used to create materials with emergent properties.25 Motivated by energy applications, our group has sought to control and tailor guest−MOF interactions. In this Forum Article, we illustrate the “Guest@MOF” theme in this context, beginning with weak interactions of light gases to the much stronger interactions of large molecules coordinated to open metal sites (OMS). First, we highlight our work in the area of noble gas adsorption in which subtle differences in heats of adsorption can be used to separate mixtures of noble gases from one another, results that have implications for energyefficient gas separations as well as storage. From there, we consider MOFs as nanoreactors, showing that MOFs infiltrated with metal atoms/clusters can catalyze the regeneration of complex metal hydrides used in hydrogen storage or facilitate hydrogenolysis of intractable bonds in byproducts of biofuel production. Light-absorbing MOFs for photovoltaic devices are discussed next followed by the synthesis of MOFs with electrical conductivity and thermoelectric properties using “non-innocent” guest molecules. One way to characterize the nature of MOF−guest interactions is by the magnitude of the isosteric heats of adsorption Qst. These range from very weak, as in noble gases and hydrogen (Qst = 5−20 kJ mol−1), to very strong when the guest chemically binds to the MOF (Figure 1). Physisorption

We are unaware of any reports of binding energies for large molecules (larger than small molecule gases such as H2, N2, CO2, and CH4) coordinated to MOF OMS, but there are many examples of strong small-molecule binding. Chemisorption of O2 is a case in point. The highest heats of adsorption for physisorbed O2 (11−24 kJ mol−1; Figure 1) are still low enough that adsorbed gas molecules should be removable by a combination of mild heating and dynamic vacuum.30 In contrast, the Qst of O2 binding to the Fe(II) sites in MOF74(Fe) is 41 kJ mol−1 at low temperatures ( Kr > Ar ≈ N2) and with the increasing polarizability of the linker (-I > -Br > -Cl > -F). We also observed that GCMC simulations always overestimated the adsorption profile found experimentally, although the values were closer for Kr than for the other two noble gases. One possible explanation for this overprediction is the presence of small amounts of interpenetrated frameworks that adsorb these gases more strongly. Alternatively, the GCMC method may insert adsorbate molecules into

problematic, whereas the long-range order of MOFs provides a well-defined platform for probing these intrapore energetics using, for example, periodic DFT codes. Such interactions surely go beyond simple physisorption, but the precise nature of the effect is not understood. Together, these examples from our work and related research from other groups illustrate the continuum and variety of MOF−guest interactions. For the most part, guest molecules have not been treated as an integral part of a MOF-based material, but our research over the past ten years produced a paradigm shift in our thinking. We are now treating guest molecules as integral design elements in MOF constructs and believe that the small number of examples discussed below represent simply a first glimpse of this burgeoning subfield of MOF science.

2. NOBLE GAS ABSORPTION AND SEPARATION MOFs are excellent sorbents for gases due to their large surface areas38 compared to those of other porous materials, such as mesoporous silica, carbons, and zeolites, as well as their aforementioned synthetic flexibility.39 The ability to tailor the organic linkers within the framework allows for control over the chemical environment inside the pores40−42 as well as the pore size.43−45 Modulation of these properties has engendered certain MOFs with extraordinary high affinities for guest molecules such as CO2, CH4, and H2.46−48 Consequently, understanding and controlling these absorption properties has been and remains a major focus of MOF research. As research efforts aimed at H2 storage in MOFs has burgeoned, new approaches to MOF modification, such as interpenetration of frameworks and doping MOFs with alkali earth metals,49,50 have been developed to tune the H2− framework interaction. These approaches are general enough that they can be used to tailor frameworks for sorption of other molecules of interest, such as noble gases. The noble gases Ar, Kr, and Xe have many industrial uses, including in lasers, lighting,51,52 as carrier gases,53 and in numerous medical applications.54 Similar to H2, noble gases interact weakly with the MOF framework through physisorption within the MOF pores. The MOF−gas interaction energy can be tuned systematically by controlling the size of the pores. Indeed, molecular dynamics simulations show that the uptake of specific noble gases within nanopores is exquisitely sensitive to the pore limiting diameter (PLD) and could potentially be used to separate these from other similarly sized diatomic gases by C


Forum Article

Inorganic Chemistry

and no vibrational modes to complicate simulations, both experiment and theory are inherently limited. Measured isotherms assume a pure material (e.g., with no strongly adsorbing minority phases and no pore collapse), whereas the GCMC method does not account for the pore limiting diameter when placing gas particles within the pore. Although an improved GCMC method is conceivable (but might not be computationally efficient), it is difficult at this point to see how MOF synthesis methods could be improved to yield highly pure materials and minimize, if not eliminate, pore collapse. Clearly, there is much that remains to be learned with regard to this heavily investigated subdiscipline of MOF science.

3. MOFS FOR H2 STORAGE The use of hydrogen as a chemical fuel has garnered intense interest due to its multiple advantages over other sources. It is nontoxic, produces no greenhouse gas emissions at the point of use, and can be converted in high efficiency to electricity in proton exchange membrane (PEM) fuel cells. However, highdensity storage of hydrogen gas remains problematic, especially for transportation. In 2012, the DOE issued targets for onboard hydrogen storage for light-duty fuel cell vehicles. Specifically, the H2 storage gravimetric capacity goals are 55 g of H2/kg by 2020 and ultimately 75 of g H2/kg of system weight.60 The system weight includes all stored hydrogen, sorbent, additives, and system components; consequently, the gravimetric capacity of the sorbent alone must significantly exceed these values. Hydrogen fuel-cell cars are now being marketed in California,61 but these store hydrogen as a compressed gas at either 350 or 700 bar.62 Even at these pressures, it is physically impossible to meet the DOE volumetric capacity target, and the cost of compressors needed for refueling stations is also prohibitive. Fuel tanks filled with solid-state materials, in the form of either porous sorbents6,63 or metal-hydrides,64 can exceed the maximum capacity achievable using compressed gas. Unfortunately, as seen in Figure 4, although some storage materials possess gravimetric densities in the required range (>10 wt %), none have the adsorption enthalpy necessary for ambienttemperature storage and delivery using the residual heat of a

Figure 3. IRMOF-2 and its -F, -Cl, and -I variants.

experimentally inaccessible locations (GCMC does not take into account the trajectory of the gas particles), which may exist as a result of the halogen on the linker pointing toward the Zn4O cluster. These results allowed us to estimate gas selectivities by comparing Henry’s constants from both GCMC simulations and experimental data. The Ar/N2 selectivity is approximately one for all MOFs studied, and there is no significant preference for Ar over N2 even as the polarizability of the MOF increases. However, selective adsorption is observed in both experiment and simulation for Kr, Xe, and Rn relative to N2 and for Xe relative to Kr, and this selectivity increases with increasing polarizability of the MOF. Rn adsorption was not measured experimentally due to the radioactive nature of this gas, but simulations showed that at low pressures ( IRMOF-2-F > -Cl > -Br > -I. In this pressure range, Rn appears to saturate the pores, leading to deviations from Henry’s law behavior as the loading begins to depend more strongly on surface area and total pore volume rather than on polarizability. In summary, we find that the combined application of experiment and modeling is an effective way of determining qualitative structure−function relationships concerning the uptake of noble gases and, by implication, of other weakly interacting gases such as H2. In the case of these halogenated MOFs, it was possible to obtain a clear picture of the influence of linker polarizability on the uptake of noble gases, one that is likely applicable to other gases that weakly interact with MOFs. However, the discrepancy between GCMC predictions and the measured isotherms for all gases but Xe highlights the limitations of this combined approach for determining quantitative relationships between MOF structural features and gas sorption. In particular, it is likely that, even for noble gases for which there are no dipole or quadrupole moments

Figure 4. Comparison of current hydrogen storage materials, relating enthalpy of adsorption to gravimetric H2 storage density. Data are from ref 60. Square indicates the region encompassing the U.S. DOE targets. D


Forum Article

Inorganic Chemistry

Figure 5. (a) Depiction of the 12 Å pores of MOF-74(Mg) infiltrated with NaAlH4. (b) Temperature-programmed desorption data showing improved kinetics of H2 desorption from nanoconfined NaAlH4 versus bulk NaAlH4. Reprinted with permission from ref 22. Copyright 2012, American Chemical Society.

uptake and release of hydrogen, thus enhancing the reaction kinetics.70 In addition, the thermodynamics of the process can also be altered, and reversibility is often improved compared to that for bulk hydrides as a result of shorter diffusion pathways and more efficient mixing of the reagents. There are several reports regarding the encapsulation of metal hydrides in MOF “nanopockets”, including hydrides such as ammonia borane NH3BH371−73 and sodium alanate NaAlH4.22,36,37 Our group demonstrated that the MOF HKUST-1 can be infiltrated with a THF solution of NaAlH4 without framework degradation,36 forming eight formula-unit NaAlH4 clusters (on average) inside the pores.37 The dehydrogenation reaction to form NaH, Al, and H2 occurs with an activation energy reduced by more than 60 kJ (mol H2)−1 relative to the bulk. Importantly, in contrast to bulk material, H2 release from nanoscale NaAlH4 occurs in a single step, avoiding the formation of intermediate Na3AlH6, decomposition of which is rate limiting. Unfortunately, HKUST-1 is unstable under H2 pressures over 10 bar; thus, the regeneration of NaAlH4 by H2 uptake could not be tested. To understand the regeneration step, we undertook experiments to nanoconfine NaAlH4 in MOF-74(Mg), which displays remarkable stability to hydrogen up to 195 °C (Figure 5). Moreover, doping this MOF with a titanium catalyst such as TiCl4 renders H2 uptake material reversible with only a modest reduction in capacity from 4.1 to 3.6 wt % over four cycles, which compares very favorably to an analogous Ti-doped porous carbon host.74 In this context, the discovery of the isoreticular IRMOF-74(X)75 series opens new possibilities to systematically probe the effects of hydride particle size and morphology on the thermodynamics and kinetics of hydrogen release.

fuel cell and regeneration at pressures of 100 bar or less (Figure 4).60 A viable solid-state hydrogen storage technology would allow H2 to be endothermically released upon mild heating and exothermically absorbed when pressurized with H2 gas. In addition, it should display long cycle life and release and absorb H2 under mild conditions of temperature and pressure. The ultimate material would have a high gravimetric capacity of 10 wt % hydrogen or greater to meet DOE targets,60 have high reversibility (survive 1500 cycles), and release hydrogen at 80− 85 °C, which would utilize the waste heat from a PEM fuel cell for desorption. Finally, it must be “kinetically fast”, meaning that the material is capable of releasing hydrogen at the demanded rate (up to 2 g of H2/second) and absorbing hydrogen fast enough so the tank can be refueled within the desired time (∼15 g of H2/second). MOFs are proving to be attractive H2 sorbents65−67 due to their record high surface areas and potential to strongly interact with H2 molecules at OMS or along pore surfaces.6 Some MOFs come close to meeting the US DOE gravimetric adsorption standards, albeit at 77 K, and significant research has focused on understanding the factors that affect H2 adsorption in MOFs. Several studies68,69 report that smaller pores increase H2 uptake, as this allows the H2 molecules to interact with multiple portions of the framework instead of a single linker or metal site. This discovery led to renewed efforts to synthesize interpenetrated structures to reduce pore dimensions while maintaining the overall framework structure.68 In addition, MOFs can serve as porous hosts for metal hydride nanoparticle guests.70 The hypothesis is that an appropriately designed host can prevent sintering and enable faster hydrogen release and absorption; with their exceptional synthetic tailorability, MOFs seem to offer the ultimate in design flexibility. Nanoscale hydrides display dramatically different chemical and physical properties compared to their bulk counterparts due to increased surface area, reduced diffusion distances, increased density of atoms at the grain boundaries, and close contact between reacting species. In metal hydrides, it is now well-established that nanostructuring and nanoconfinement lead to significant improvements in the

4. HYDROGENOLYSIS Encouraged by the discoveries revealed by infiltrating MOFs with metal hydrides, our group extended the concept of nanoconfinement to heterogeneous catalysis. In particular, it appears that H2 is activated by molecular-scale titanium in the pore, which is itself stabilized in some way that improves cycling of the material. These observations suggest that Tidoped IRMOF-74(Mg) could be an effective catalyst for E


Forum Article

Inorganic Chemistry hydrogenolysis. An energy-related application we identified is the need for robust, low-cost catalysts for the production of value-added chemicals from lignin.76 Lignin is a byproduct of cellulose extraction from biomass and is the most abundant source of renewable aromatics with 200−300 Mtons/yr projected production by a US biofuels industry that would process ∼1 B tons of biomass. However, there are currently no efficient processes for extracting these aromatics and converting them to value-added chemicals and drop-in fuels. The technical and economic challenges are staggering due to the quantities of material involved and lignin’s recalcitrance to depolymerization. Conventional lignin degradation processes use aggressive reagents, are energy intensive (400−800 °C), and yield complex product mixtures. Milder reaction conditions and narrower product distributions could be achieved using lignindegrading enzymes, but these are too fragile to be practical for large-scale biorefining. Our objective was to develop lignin valorization methods in which a feedstock of lignin-derived oligomers would be selectively digested using robust catalysts based on MOFs under industrially relevant conditions. MOF-74 was selected as a starting point for catalyst development because the topology presents several advantages. First, the recently reported isoreticular IRMOF-74(n) series (where n = the number of linear aryl groups in linkers based on 2,5-dihydroxyterephthalic acid parentage) provides hexagonal 1-D channels with diameters between 1.2 and 9.8 nm that can accommodate a range of substrate sizes. Second, the density of open metal sites (OMS) in these MOFs, which can behave as Lewis acids to activate C−O bonds, is the highest known for this class of materials. Finally, IRMOF-74(n) can be synthesized with a wide range of metals, (Mg, Mn, Fe, Co, Ni, Cu, and Zn) as well as mixed-metal compounds of up to 10 different metals,77 allowing the reactivity of the OMS to be readily tuned. IRMOF-74-I(Mg) and IRMOF-74-II(Mg) were initially tested without additives and following infiltration with TiClx and Ni nanoparticles, both of which have been shown to catalyze aryl ether hydrogenolysis.78−80 The model compounds selected phenylethylphenyl ether (PPE), benzylphenyl ether (BPE), and diphenyl ether (DPE) (Figure 6), which are representative substrates that incorporate the common β-O-4, α-O-4, and 4-O-5 linkages (highlighted in bold) found in lignin. Reactions involving neat IRMOF-74-(I,II) catalysts showed that all three ethers react with H2 to generate small amounts of phenol and the corresponding aromatic hydrocarbon. In all instances, higher conversions were obtained using the IRMOF74-II catalyst despite optimized substrate geometries computed using density functional theory (DFT) showing that all three substrates will fit within the pores of either MOF. When TiClx or Ni-infiltrated MOF catalysts were used, significantly increased conversion efficiencies were observed. The conversions obtained using both doped and undoped MOFs follow the trend PPE > BPE > DPE, and the activity of the catalysts indicates Ni@IRMOF-74 > Ti@MOF-74 > IRMOF-74 regardless of substrate. Repeated cycling of the catalysts did not affect their performance or yield any loss in MOF crystallinity, indicating that the MOF structure is robust under hydrogenolysis conditions. Analysis of the supernatant following the reactions yielded no evidence of Mg, Ti, or Ni leaching, confirming that the MOF itself is the active catalyst and not a solubilized metal component. Substrate confinement within the MOF pore plays a key role in the efficiency and selectivity of these catalysts. DFT

Figure 6. Model compounds used as reactants in the hydrogenolysis reaction catalyzed by IRMOF-74-1(Mg) with the observed reaction products. Reprinted (adapted) with permission from ref 76. Copyright 2016, American Chemical Society.

calculations indicate that the OMS in the IRMOF-74 pores bind and orient the substrate. A cluster model consisting of four five-coordinate Mg2+ ions, each connected to five oxygen donor atoms approximating one wall of the hexagonal pore, was used to compute binding energies for all substrates. These calculations indicate that all three substrates are chemisorbed to the cluster through an interaction of electron density of an aromatic ring with the Lewis acidic Mg2+ ion. It is also interesting to note that the MOF OMS may play a role in activating or at least orienting H2 in the pores because both neutron scattering studies81 and DFT calculations82 indicate that the Mg2+ OMS in Mg-IRMOF-74(I) are the strongest H2 binding sites. Cleaving the aryl-ether bond is a new reaction category for MOFs. The results clearly indicate that the MOF itself actively participates in the reaction, most likely through interaction of the OMS with the substrate. Overall conversion is increased by trapping transition metal dopants within the MOF pores, and although the catalytic activity is slightly lower compared to the best-known C−O hydrogenolysis catalysts, these MOF-based catalysts do not require the addition of a base like NaOtBu. Together, these results demonstrate that MOFs possess all of the elements of a “nanoreactor”: substrate confinement and orientation, reactant activation, and dopant stabilization, all of which are highly encouraging for their use in this problematic application. The substrates in the studies mentioned above are relatively simple small molecules compared to the large lignin-derived oligomers that are the true targets. A remaining challenge for the MOF experimentalist will be to find new MOFs and catalytic conditions that enable the decomposition of these larger substrates while still maintaining high activity. As seen in Figure 7, the IRMOF-74-(I-V) series offers a possible strategy for catalyzing reactions involving larger lignin oligomers than the model compounds used in our initial investigation. By choosing an IRMOF-74-(n) that has a pore aperture matching the dimensions of the lignin-derived oligomer, it may be possible to transfer the optimum condition found for small model complexes and the IRMOF-74-I nanoreactor to larger, more valuable substrates. This work is ongoing. F


Forum Article

Inorganic Chemistry

Figure 7. Comparison of lignin-derived oligomers and monomer dimensions with pore apertures of the IRMOF-74-(I−V) series. Reprinted with permission from ref 75. Copyright 2012, American Association for the Advancement of Science.

5. MOFS FOR LIGHT HARVESTING AND PHOTOVOLTAIC APPLICATIONS The above sections highlight our work concerning small molecule MOF−guest interactions, wherein the size and polarizability of the MOF pores are the primary determinants of function. These tunable properties make MOFs attractive not only for gas storage/separation and catalysis, as discussed above, but also for photovoltaic (PV) applications. There are numerous ways that MOFs may be modified to impart photovoltaic function. For example, in our early work, we showed that incorporating luminescent molecules, such as stilbene, into a MOF structure increases the rigidity of the linker, which in turn enhances fluorescence lifetime compared to that of the linker in dilute solution.83 Dincă and co-workers subsequently applied these ideas to the design of MOF-based turn-on fluorescence sensors.84−86 Although still a nascent aspect of MOF science, these early results are stimulating new research into the use of light-absorbing organic molecules within MOF topologies, either as linkers or as guests, to impart tailored light-harvesting properties. In this section, we highlight two strategies for creating lightharvesting MOFs for PV devices. In accordance with the theme of this Article, we first discuss the use of MOFs as hosts for photoactive guest molecules. In this approach, the MOF linkers are not designed for light absorption but are instead chosen to impart a particular 3-dimensional topology that enables high guest loading and can potentially be used to place the photoactive guest molecules in a desired orientation. Two synthetic methods for introducing donor and acceptor molecules are described: first, solution-based infiltration, and second, a new route that we report for the first time in which Schiff base chemistry is used to covalently bind guests to the framework. We next consider the use of MOFs with photoactive linkers, highlighting computational and synthetic work to design new photoactive conjugated organic molecules for use as electron-donating linkers in PV devices. 5.1. Solution-Based Infiltration of Light-Harvesting Guests. When considering the use of MOFs in PV applications, it is important to emphasize that most known MOFs are electronically insulating. This is due to the poor overlap between orbitals of the metal atoms of the secondary building unit (SBU) and the π orbitals of the aromatic linkers, which impedes the formation of dispersed, conductive bands (for a discussion of electronic overlap between molecular

orbitals of linkers and metallic SBUs, see refs 12 and 87). Truly semiconducting MOFs are extremely rare,88 although in section 6 we discuss strategies for enhancing charge mobility within MOF architectures. Therefore, even if the linkers themselves may possess desirable light-harvesting properties, such as a high molar absorptivity and broad visible light absorption, separating excitons into mobile charges (holes and electrons) is challenging when the only transport path is through the framework. A strategy that avoids the need for semiconducting MOFs is to use MOFs as a platform for organizing guest molecules that are known to be both good light absorbers as well as charge transport materials, such as those found in conjugated polymer-based solar cells.89,90 To this end, we are exploring the use of MOFs for organizing light-harvesting donor−acceptor pairs, specifically, α,ω-dihexylsexithiophene (DH6T) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). These materials were chosen as faithful representatives of active materials found in some of the most efficient bulk heterojunction organic photovoltaic (OPV) devices.91 DH6T is a truncated molecular version of the polymer poly(3-hexylthiophene), whereas PCBM is a commonly used fullerene electron acceptor. It is well-recognized that the performances of solar cells based on thin-film polythiophene:fullerene composites depends strongly on the material properties and processing conditions (deposition solvent, annealing time and temperature, etc.), which control the extent to which the donor/acceptors aggregate or phase separate.92 We reasoned that by judiciously selecting a MOF with the appropriate channel and cavity size, it would be possible to use the 3-dimensional structure of the MOF to organize these donor−acceptor molecules in close proximity, such that light absorption and charge transport could be controlled and possibly enhanced. We also were intrigued by the possibility of the MOF linker to act as a photon antenna for these guest species through the relay of absorbed energy via fluorescent resonance energy transfer (FRET). DH6T, PCBM, and 1:1 mixtures of the two molecules were solution-infiltrated into MOF-177, a zinc-carboxylate-based MOF with a benzene-tribenzoate (BTB) linker,93 which was chosen because of its large pore size and demonstrated ability to be infiltrated by fullerene molecules.16 High loadings of PCBM (2−3 molecules per unit cell) could be obtained, whereas loadings of DH6T that we could achieve were significantly lower (only ∼0.1 molecules per unit cell). This G


Forum Article

Inorganic Chemistry

DH6T, which absorbs moderately at the 345 nm excitation wavelength, or from FRET from the exited state of the BTB linker to a nearby DH6T. The latter explanation is consistent with the observation that, in solution phase, increasing concentrations of DH6T quench the emission of H3BTB with an associated increase in the emission intensity of the DH6T. The quenching of the linker emission is even more apparent in PCBM@MOF-177; however, no emission from the infiltrated PCBM was observed, so the nature of this quenching mechanism could not be elucidated. When both DH6T and PCBM are guests within the MOF, the quenching of the linker emission was found to be even more effective with a quantum yield of 88% compared to 65% with PCBM alone. The possibility remains that some of the quenching of the linker excited state is due to nonradiative charge transfer processes, and importantly, it is still to be experimentally determined if excitation of (DH6T+PCBM)@MOF-177 samples leads to charge transport from the DH6T donor to the PCBM acceptor within the pore. Therefore, time-dependent DTF calculations were used to investigate whether excitations could result in direct charge transfer from linker to guests. Indeed, it was found that many excitations in the UV region resulted in the movement of electron density from BDC to PCBM; however, in the case of donor−acceptor pair BTB−DH6T, no such charge transfer was manifested in the calculation. Overall, these results highlight the importance of the MOF linker in the lightharvesting process, and in subsequent sections we will elaborate on our approach to using MOFs as visible and near-IR absorbers. However, we will first address our recent efforts to improve the loading levels of thiophene electron donors. 5.2. Schiff Base Attachment of Fullerenes. The large size and rigidity of DH6T precludes its facile incorporation into MOF-177 using solution infiltration methods, resulting in poor loading levels as mentioned above. As an alternative, we developed a new postsynthetic modification (PSM) approach that dramatically increases the loading of the thiophene, enabling equally high loading levels of both donor and acceptor molecules within a MOF. The results of this approach are presented here for the first time. PSM is an incredibly versatile strategy for developing new functionality in MOFs94−97 with applications in sorption, catalysis, and sensors.98−100 We reasoned that the driving force supplied by the formation of a Schiff base between an aldehyde-functionalized thiophene molecule and an amino-functionalized MOF would substantially increase the loading levels of the electron donor. The MOF MIL-101-NH2 (Al) was chosen for testing this PSM

result is somewhat surprising based on the similarities in calculated binding energies for these molecules within the MOF pore (55 kcal mol−1 for DH6T and 57 kcal mol−1 for PCBM). However, these calculations neglected the solvent effects, which may have a significant influence, especially since the bonding interactions between the guests and linkers are based on weak π−π stacking. Fluorescence spectroscopy was used to characterize each of the Guest@MOF samples. The emission spectra for DH6T@ MOF-177, PCBM@MOF-177, and (DH6T+PCBM)@MOF177 are shown in Figure 8 and exhibit significant quenching of

Figure 8. Comparisons of the photoluminescence spectra of solid state samples of (a) MOF-177, DH6T@MOF-177, and DH6T (inset) and (b) MOF-177, PCBM@MOF-177, and (DH6T+PCBM)@MOF-177. Reprinted with permission from ref 93. Copyright 2014, Royal Society of Chemistry.

the linker-associated emission peak as compared to that MOF-177 alone. For DH6T@MOF-177, a weak band observed at lower energy that corresponds to the emission DH6T. This emission feature could result from excitation

of is of of

Scheme 1. Postsynthetic Modification to Attach a Thiopene Moiety To MIL-101

H


Forum Article

Inorganic Chemistry

Figure 9. (a) PXRD data of MIL-101-NH2 (Al) (black line, post DMF wash; blue line, post MeOH wash) and MTTC@MIL-101 (Al). SEM images of (b) MIL-101-NH2 (Al) and (c) MTTC@MIL-101 (Al).

Figure 10. Top left: FTIR spectra of MIL-101-NH2 (Al) (blue line) and MTTC@MIL-101 (Al) (black line). Top right: XPS MIL-101-NH2 (Al) (black) and MTTC@MIL-101 (Al) (blue). Bottom: C 1s, N 1s, and S 2p XPS spectra of MTTC@MIL-101 (Al).

amino group needed to form the Schiff base. Furthermore, MIL-101-NH2, like many of the MIL family of MOFs pioneered by Férey and co-workers,102 has greater stability

protocol because it possesses both a large pore size (1.2 nm pentagonal window and 1.6 nm hexagonal window101) capable of accommodating both of the fullerene molecules and the I


Forum Article

Inorganic Chemistry

roughly a factor of 30. This concentration is similar to that of PCBM-infiltrated MOF-177 described above, suggesting that it should be feasible to have an essentially 1:1 donor:acceptor ratio in such frameworks. We also characterized the photophysical properties of MTTC@MIL-101 (Al) both experimentally and computationally. The diffuse reflectance spectra of MIL-101-NH2 (Al) and MTTC@ MIL-101 (Al) are presented in Figure 11. An absorption feature around 500 nm appears after PSM, which we attribute to an imine-based transition (Figure 11).

than that of Zn-based MOFs, such as MOF-177. This is important because water is a byproduct in the dehydration step of the Schiff base reaction. MIL-101-NH2 (Al) was synthesized using a reported method by Serra-Crespo et al.103 and characterized using powder X-ray diffraction (PXRD). The crystallite size was determined to be roughly 26 nm from refinement of the diffraction data. MIL101-NH2 (Al) powders were then soaked in a saturated solution of 5-(5-methylthiophen-2-yl)thiophene-2-carbaldehyde (MTTC) in methanol at room temperature (Scheme 1). After a few hours, the color of the powder changes from yellow to orange, suggesting the formation of an imine bond. When the MIL-101-NH2 (Al) powders were soaked in toluene solutions of PCBM only, and MTTC with PCBM, the colors of the powders changed to dark red and reddish-orange, respectively. PXRD measurements of the postsynthetically modified MTTC@MIL-101 are in good agreement with simulated data indicating that the crystalline structure maintains its integrity after PSM (Figure 9a). Furthermore, a comparison of the SEM images of MTTC@MIL-101 and as-synthesized MIL-101 demonstrate that the crystal morphologies are the same (Figure 9b vs c). The MTTC@MIL-101 sample was characterized further using FT-IR, XPS, and elemental analysis to determine chemical bonding and composition; the results are consistent with reaction shown in Scheme 1. The FTIR spectrum of MTTC@ MIL-101 (Al) shows the presence of a strong characteristic absorption band at 1690 cm−1 corresponding to the formation of the imine bond (R-CN-R) (Figure 10, top). X-ray photoelectron sprectroscopy (XPS) survey scans of MIL101-NH2(Al) and MTTC@MIL-101-NH2(Al) show a change in the coordination environment of the MOF following PSM (Figure 8, middle). The XPS spectrum of MIL-101-NH2(Al) indicates the presence of the following elements: aluminum, carbon, nitrogen, and oxygen. The XPS survey scan of MTTC@MIL-101-NH2(Al) shows an additional peak at binding energy ∼163.6 eV corresponding to the sulfur atom in MTTC. High resolution elemental scans of C 1s from MTTC@MIL-101-NH2(Al) show the following bonding motifs: CO, CN, C−S, and C−C. Binding energies of CN and C−S validate the covalent attachment of MTTC onto the MOF’s framework. A high resolution scan of the N 1s region of MTTC@MIL-101-NH2(Al) shows binding energies at 400.33 and 402 eV corresponding to NC and -NH2 groups, respectively. The high-resolution scan of the S 2p region shows two different coordination environments for sulfur, which we attribute to the difference between the sulfur atom closer to the terminal methyl group on MTTC and the sulfur atom on the thiophene moiety adjacent to the imine bond. Elemental analysis was used to quantify the loading of MTTC in samples of MTTC@MIL-101 (Al). Upon infiltration, the carbon content increased from 41.62 wt % for MIL101-NH2 (Al) to 46.90 wt % for MTTC@MIL-101 (Al), and sulfur was detected at 1.44 wt % for MTTC@MIL-101 (Al). Using these values, we calculated a loading level of 54 MTTC molecules per unit cell. To compare this loading value with that of DT6H in MOF-177, we normalized for the disparate unit cell volumes of the two MOFs (MOF-177, unit cell volume = 35746 Å3; MIL-101, unit cell volume = 704969 Å3). Using these volumes and the previously determined value for the number of DH6T found per unit cell (0.09),17 we find that the PSM strategy improved the electron donor guest loading by

Figure 11. Diffuse reflectance spectra of MIL-101-NH2 (Al) and MTTC@MIL-101 (Al).

The steady-state luminescence of the PSM MTTC@ MIL101-NH2 (Al) was measured and compared with those of the unmodified MIL-101-NH 2 (Al) and PCBM-infiltrated MTTC@MIL-101-NH2 (Al) (Figure 12). As in the case of DH6T@MOF-177 discussed above, the inclusion of the thiophene electron donor in MTTC@MIL-101 results in a new emission band at a lower energy than that of the MOF (Figure 12). The increased intensity of this feature compared to DH6T@MOF-177 attests to the higher loading of MTTC achieved using the PSM method. Introduction of the electron-

Figure 12. Photoluminescence spectra of solid state samples of MIL101 (black curve), MTTC@MIL-101 (orange curve), and (see inset) MTTC@MIL-101(Al) + PCBM (blue curve). Dashed lines: excitation spectra (detection at 445 nm). Solid lines: emission spectra (excitation wavelength: 375 nm). J


Forum Article

Inorganic Chemistry

inspiration, we turned to the field of conjugated polymers110 and, in particular, those based on electron-deficient benzothiadiazole (BT) and electron-rich thienothiophene (TT) units. As a design constraint, the linkers were terminally functionalized with hydroxybenzoate rings for incorporation into the IRMOF-74 topology. As mentioned above, it has been demonstrated that the pore diameter of IRMOF-74 can be systematically altered using this strategy,75 which is important for incorporating guests of varying sizes. Importantly, by increasing conjugation of the alternating BT-TT units by lengthening the linker, gas phase calculations predict a periodic decrease in absorbance maxima from 450 to 650 nm, corresponding to a band gap shift from 2.8 to 1.9 eV (Figure 12, top).109 The experimentally determined absorption spectrum of the BT-TT-BT linker matched well with that predicted from theory, validating the computational approach. Further calculations were then performed on the bulk IRMOF74-BT-TT-BT material to understand what effect stacking of the linkers within the MOF would have on the band gap and band positioning. As shown in the lower portion of Figure 14,

accepting PCBM results in a marked quenching of the emission from the MTTC linker, which we attribute to charge injection, although at this time we cannot rule out some contribution from energy transfer. The HOMO and LUMO energy levels of MIL-101 linker (2aminoterepthalic acid) and the PSM linker were calculated using DFT for comparison with PCBM. As seen in Figure 13,

Figure 13. MIL-101-NH2 (Al) and MTTC@MIL-101 (Al) linkers and PCBM band alignment predicted by B3LYP/6-311G(d,p).

the incorporation of MTTC results in a substantial decrease in the optical bandgap relative to that of the unmodified MOF linker. Additionally, alignment of the modified linker with the HOMO and LUMO levels of PCBM is now favorable for charge transfer. For creating bulk heterojunction (BHJ) PV cells, the offset between the donor thiophene and PCBM is too high; a value closer to 0.3 eV is thought to be ideal for P3HT− PCBM BHJ solar cells.90 The bandgap is also too large, but this is to be expected from this rather small thiophene. Nevertheless, the results suggest that our combined approach for functionalizing host MOFs via a combination of PSM and solution-phase guest infiltration can lead to high loadings of both donors and acceptors, a prerequisite for achieving efficient light harvesting. 5.3. Computational Linker Design for Visible Light Absorption. Our investigations thus far concerning BHJ-like MOF architectures relies on the light absorption properties of thiophene-based guests within the MOF pores. Importantly, we have shown that the MOF linker, which was initially viewed as a passive scaffold, can act as a light harvester through an energy transfer cascade mechanism.17 However, most common MOF linkers absorb little light, if any, in the visible portion of the solar spectrum.104 This result prompted us to explore novel MOF linkers for broad-band visible light absorption. Most light-harvesting MOFs rely on porphyrin or M(bpy)32+ (M = Ru(II) or Os(II))-based linkers,105,106 which without significant synthetic manipulation107 suffer from less than ideal molar absorptivities over the range 375−900 nm, where 66% of the total solar flux falls.108 To address this problem, we set out to computationally design a new series of light-absorbing linkers that exhibit high oscillator strengths over a broad spectral range and have the proper band alignment to inject charge into guest acceptor molecules (like PCBM).109 For

Figure 14. Top: Calculated absorption spectra and oscillator strengths of the three linkers depicted on the right. Bottom: Energy diagram for the proposed charge injection of photoexcited BT-TT-BT linker to a fullerene guest acceptor with a structural depiction of this process within an IRMOF-74 topology. Reprinted with permission from ref 109. Copyright 2014, Royal Society of Chemistry.

the band energies of the BT-TT-BT linker in the MOF align well with the electron acceptor PC71BM, suggesting that visible light excitation of the linker will result in a rapid charge-transfer event. Incorporation of a fullerene-infiltrated IRMOF-74-BT-TTBT material into a working photovoltaic device requires that the electron−hole pair produced following photoexcitation has sufficient mobility to diffuse apart before relaxing to the ground state by reverse electron transfer. Electron mobilities in bulk fullerenes are usually high enough to be nonlimiting in conjugated polymer BHJ cells,111 and if ordered in MOF pores, the mobility may be expected to increase. On the other hand, hole mobility from one BT-TT-BT linker to an adjacent one is not expected to be high because of the electronic isolation of the linkers. However, the IRMOF-74 topology is an K


Forum Article

Inorganic Chemistry

manifested in, for example, conventional silicon photovoltaic solar cells in which a p−n junction made of only one material is the central unit that absorbs light, generates the electron−hole pairs, separates the electrons and holes, and transports the electrons and holes. A key question, therefore, is can MOFs be elevated to the status of a new class of solid-state semiconductors that possess not only useful electronic transport properties but also entirely new properties (in particular, porosity) that can be harnessed for novel applications. Realizing this vision is a grand challenge because virtually all MOFs are insulators. This is not surprising because typical MOF organic linkers have HOMO−LUMO gaps of several electron-volts, making electron transport through the framework extremely difficult. In addition, the electronic structure of MOFs can often be approximated as a lattice of noninteracting metal centers separated by organic linkers. The resulting poor coupling leads to low band dispersion and therefore low carrier transport velocities. Fortunately, the large chemistry toolbox and the uniquely versatile MOF structure point to several potential approaches to impart electrical conductivity, a few of which have been recently realized experimentally.112 The first approach is to create intrinsically conducting MOFs (Figure 16a) wherein the electronic transport proceeds through

excellent test case for designing hole mobility into MOFs because the linkers are forced into a π−π stacked arrangement due to the coordination environment of the SBU. Therefore, it may be possible to enhance the electronic communication by increasing the overlap between π-orbitals of adjacent linkers. To test this idea, we calculated the band structure (DFT/B3LYP) along the π stacking direction for a series of BT-TT-BT linkers with varying stacking distances. As shown in Figure 15, as the distance decreases from 5 Å (found experimentally in IRMOF-74 structures) to 3 Å,

Figure 15. Calculated band structures for the π stacked BT-TT-BT linkers at intermolecular distance of 5, 4, and 3 Å. Blue lines: filled bands. Red lines: unfilled bands.

significant band dispersion is observed. At the shortest distance (3 Å), the gap between the valence and conduction bands vanishes, rendering the material metallic. However, intermolecular repulsive forces make this geometry energetically unfavorable. Realizing these band dispersion phenomena in practice may be possible by enforcing the necessary linker arrangement through clever SBU design. For example, it may be possible to synthesize a MOF using a divalent metal such as Ti2+ or V2+ that can be oxidized aerobically, thereby contracting the surrounding metal−ligand bond lengths and forcing the linkers into closer proximity. An alternative strategy would be to introduce π stacking guest molecules that could be inserted between the linkers. Experimental results aimed at proving these concepts are needed.

Figure 16. Illustration of potential charge transport schemes in MOFs: (a) electronic transport through band transport or hopping between metal centers, (b) electronic transport across proximal linker planes without going through the metal centers, and (c) electronic transport enabled by guests.

6. CONDUCTIVE MOFS The examples described above of designing MOFs for energyrelated applications clearly show that their unique properties can be beneficial in a number of energy areas. This success raises the question of whether an even broader set of energy applications could benefit from the new modalities that MOFs provide. For example, solid-state semiconductor materials find broad use in a wide range of technologies such as photovoltaics, thermoelectrics, energy storage, and lighting. In addition, solidstate semiconductors play a key role in enabling low-power computing, and much research is underway to identify new approaches to further reduce energy use during computation. The critical enablers for the broad success of solid-state semiconductors are their tunable electrical conductivity and the ability to create both p- and n-type materials. These are

the complete framework (i.e., metal centers + linkers). This in principle can be achieved by controlling the alignment of energy levels between the metal centers and the linker, although few examples exist so far.112 An alternative is to circumvent the issue with the bonding between the metal centers and the linkers and design frameworks where linkers are in close proximity, enabling electronic transport perpendicular to the linkers (Figure 16b). This approach has recently been demonstrated for a class of MOFs based on tetrathiafulvene (TTF) linkers.113 In solid-state semiconductors, the approach to impart tunable conductivity is to use atomic dopants that create free electrons or holes in the semiconductor. In a sense, the dopant atoms serve as guests in the host semiconductor. Thus, a L


Forum Article

Inorganic Chemistry

dissolved TCNQ with the immersion time used to control the conductivity. Initial ab initio calculations24 suggested that TCNQ can serve as a bridge between Cu dimer SBUs, enabling charge transport by creating a donor−acceptor−donor path. Subsequent ab initio calculations13 suggested a different mechanism where TCNQ serves as a passive dopant by transferring holes to the MOF framework. In practice, the complex environment of the pore, which may include water molecules coordinated to the axial positions of the copper dimers, makes this distinction difficult to probe, and it is possible that both mechanisms are present. The combination of tunable electrical conductivity and the ability to make thin film devices opens a number of avenues for energy applications. One such application we recently demonstrated is a thin film thermoelectric MOF material. Thermoelectric devices convert thermal gradients to electricity (and vice versa) by using the Seebeck effect. They can be used to harvest waste heat or for cooling. In the power generation mode, a temperature gradient ΔT across the material generates a voltage difference ΔV between the hot and cold sides given by

second approach to impart conductivity in MOFs is to generalize this approach and insert guests in the pores. The guests in this case can be either individual molecules or other species (e.g., quantum dots) chosen to create the necessary properties for electronic transport. This could be achieved in different ways. For example, in systems where the energy alignment between the metal centers and the linkers is already favorable, the guests can be used to transfer charge to the framework (blue triangle and left-most green line, Figure 16c) and increase the carrier concentration, similar to the role of dopants in semiconductors. Another possibility is for the guest to serve as a bridge between metal centers (Figure 16c, rightmost green line) and create the necessary transport path. We recently demonstrated the use of guests in MOFs to impart tunable electrical conductivity,24 as we now discuss. The molecule 7,7,8,8-tetracyanoquinododimethane (TCNQ) has been extensively explored for its charge transport properties.114,115 As a strong electron acceptor, it can be paired with electron donors (e.g., TTF) to form charge transfer complexes with tunable electronic conductivity,116 and both nonporous and porous117 coordination polymers involving TCNQ as a ligand are known. We recently reported the use of TCNQ as the guest in the MOF Cu3(BTC)2 (also known as HKUST-1118) and found that this induced a significant increase in electrical conductivity, achieving conductivities as high as 0.07 S cm−1 in polycrystalline thin films (Figure 17).24

ΔV = −SΔT

(1)

in which S is the Seebeck coefficient. The energy conversion efficiency of the thermoelectric material is governed by the figure of merit ZT =

S2σT κ

(2)

where σ is the electrical conductivity, and κ is the thermal conductivity. The best thermoelectric materials reach ZT values of ∼1, and much effort has been devoted to finding new ways to improve this value. However, because electronic and thermal transport are interrelated, increasing ZT is challenging because increases in σ also increase κ. In the past decade, the thermoelectric research community has explored nanostructuring as a new approach to decouple electrons and phonons.119 In this context, MOFs present a unique set of characteristics because they inherently contain nanoscale pores organized in a 3D network. This type of porosity is of interest for thermoelectrics because the length scales for electron and phonon scattering can be different; therefore, MOFs may provide low thermal conductivity, and if their electrical conductivity can be increased, this could lead to an interesting new class of thermoelectric materials. However, until recently there had been no measurements of the Seebeck coefficient of MOFs.

Figure 17. Infiltration of the Cu3(BTC)2 MOF with TCNQ leads to substantial electrical conductivity over a wide temperature range. Reprinted with permission from ref 24. Copyright 2014, American Association for the Advancement of Science.

Infiltration of TCNQ in the Cu3(BTC)2 pores was realized by immersing a Cu3(BTC)2 thin film in a solution containing

Figure 18. Thermoelectric MOF: (a) SEM image of a Cu2(BTC)3 thin film used for the thermoelectric measurements, (b) measured Seebeck coefficient as a function of temperature, and (c) ZT factor as a function of temperature. Reprinted with permission from ref 13. Copyright 2015, John Wiley and Sons. M


Forum Article

Inorganic Chemistry To address this deficiency, our group made the first thermoelectric measurements13 on a MOF thin-film device using the TCNQ@Cu3(BTC)2 system. As shown in the SEM image of Figure 18a, the MOF film of thickness ∼150 nm has a rough appearance with grain sizes estimated to be on the order of 50−100 nm. The Seebeck coefficient was measured between two metal contacts located under the MOF thin film and separated by a few hundred micrometers. Two Peltier devices, one operated in heating mode and the other in cooling mode, were positioned under the device to generate a temperature gradient between the two contacts. The resulting ΔT was measured using an infrared camera, and the Seebeck coefficient was extracted from eq 1. Over the temperature range 280−315 K, the Seebeck coefficient is positive (Figure 18b) and attains quite large values. At room temperature, ZT is ∼375 μV K−1; in contrast, the ZT of bismuth telluride (Bi2Te3), a well-known thermoelectric material, is only 170 μV K−1.120 We also used time-domain thermoreflectance (TDTR) to measure the thermal conductivity of the thin film. As expected from the porous nature of the material, the thermal conductivity is quite low, around 0.2 W mK−1, at room temperature. When combined with the measured electrical conductivity, the figure of merit ZT is on the order of 10−4. Although this value is not particularly large for thermoelectric applications, our independent measurements of the quantities that enter ZT provide a path toward improvement. For example, the high Seebeck coefficient and low thermal conductivity imply that the low electrical conductivity is the factor limiting ZT. We anticipate that improving the quality of the MOF thin film may significantly improve the electrical conductivity while keeping the thermal conductivity low due to the nanoporous nature of the material. In addition to assessing the potential of MOFs for thermoelectric applications, the thermoelectric measurements also provide a means to identify the carrier type. Indeed, the positive value of the Seebeck coefficient implies that holes are the primary carriers. Our ab initio calculations13 show that the Fermi level in the TCNQ@Cu3(BTC)2 system is located within the MOF valence band due to the strong TCNQ electron acceptor character. These experimental and theoretical results highlight the importance of controlling carrier density to realize conducting MOFs. They also suggest that judicious choice of the guest could be used to control thermoelectric properties to, for example, realize an n-type Guest@MOF material.

First, accurate simulation of the energetics of Guest−MOF interactions is very challenging. When guest molecules interact strongly enough with the pore environment to change the overall properties of the material, higher levels of theory are demanded than are provided by classical force field methods that have been the tools of the trade for modeling gas sorption. Quantum chemistry methods capable of accurately predicting the energetics of guest coordination to an OMS, for example, are computationally expensive and thus not readily implemented in high-throughput screening approaches. Additionally, the repertoire of experimental probes must be expanded to provide the detailed spectroscopic information required to validate such calculations. Second, the large unit cells typical of MOFs generally require a cluster approach to model the MOF−guest interaction; often, only a single SBU is involved. This has the unfortunate consequence that long-range effects, such as charge mobility and energy transfer, are not taken into account. Thus far, MOF properties have been (largely) understood in an essentially molecular context: linker properties are assumed to be those of the molecule, and SBUs behave as if they were isolated coordination complexes. This paradigm cannot be used to successfully model long-range energy transfer and charge transport, for which band structure theory may be essential. Cooperative chemisorption phenomena, such as the recently reported “phase-change” sorbents,121 also require more rigorous theoretical treatments. Finally, other occupants within the pore or coordinated to OMS may exert a strong influence on Guest@MOF behavior. As discussed in section 6 above, coordinated water molecules appear to play a role in the mechanisms controlling charge transport in TCNQ-infiltrated HKUST-1. Extending electron transfer theory to such systems becomes problematic. For example, what is the meaning of the “reorganization parameter” in the context of a supramolecular structure that is itself somewhat flexible and is also permeated with solvent molecules that interact with both framework and guest? Despite these and other challenges not mentioned, it is nevertheless our view that guest molecules are valid design elements that open the door to new and unexpected properties. In addition, they provide a means of probing MOF geometric, electronic, and magnetic structure. Our group is pursuing both avenues, and we anticipate that forthcoming results will further illuminate the properties of this remarkable class of materials.

■

EXPERIMENTAL SECTION

MOF Synthesis and Infiltration. Aluminum chloride hexahydrate, 2-amino terephthalic acid, and 5-(5-methylthiophen-2-yl)thiophene-2-carbaldehyde (MTTC) were purchased from SigmaAldrich (St. Louis, MO) and used as received. Phenyl-C61-butyric acid methyl ester (PCBM) was purchased from American Dye Source (Quebec, Canada). Solvents used for MOF activation and exchange were stored over dried molecular sieves (4 Å). MIL-101-NH2 (Al) was synthesized using a reported method by Serra-Crespo et al.103 Briefly, amino-MIL-101 (Al) was synthesized by a solvothermal treatment involving N,N-dimethylformamide (DMF) as solvent. Starting reactants are aluminum chloride hexahydrate (AlCl3·6H2O, Sigma-Aldrich, 99%, 0.51 g), 2-amino terephthalic acid (HO2C−C6H3NH2−CO2H, Sigma-Aldrich, 99%, 0.56 g), and N,Ndimethylformamide ((CH3)2NCHO, Sigma-Aldrich, >99.9%, 30 mL). The reactants were placed in a Teflon-lined autoclave and heated for 72 h at 403 K in an oven under static conditions. The resulting yellow powder was filtered under vacuum and washed with acetone. For organic species trapped within the pores to be removed, the samples were activated in boiling methanol overnight and stored at 373 K.

7. PERSPECTIVE AND CHALLENGES Given the large number of possible MOF topologies and the virtually limitless possibilities with respect to linker design and choice of metal ion, it may seem unnecessary to add guest molecules as an additional element of synthetic design. However, the examples described above demonstrate that this is indeed a fruitful new aspect of MOF chemistry, one that is leading to unexpected emergent properties. This “out-of-thebox” dimension is a welcome one, as it provides another avenue by which to address some vexing problems in MOF design, particularly in the area of electronic materials. Although the discoveries described above are exciting and will, we hope, stimulate additional work in this area, design principles are only beginning to emerge, and much remains poorly understood. We see the following as key challenges to the expanded development of the Guest@MOF concept: N


Forum Article

Inorganic Chemistry

III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change; IPCC: Geneva, Switzerland, 2014; p 151. (2) NOAA National Centers for Environmental Information, State of the Climate: Global Analysis for February 2016. https://www.ncdc. noaa.gov/sotc/global/201602 (accessed March 29, 2016). (3) Wattles, J. Bill Gates launches multi-billion dollar clean energy fund. http://www.greentechmedia.com/articles/read/sorry-bill-gatesyou-are-wrong-on-clean-energy (accessed March 28, 2016). (4) D’Alessandro, D. M.; Smit, B.; Long, J. R. Angew. Chem., Int. Ed. 2010, 49, 6058−6082. (5) Makal, T. A.; Li, J. R.; Lu, W. G.; Zhou, H. C. Chem. Soc. Rev. 2012, 41, 7761−7779. (6) Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D. W. Chem. Rev. 2012, 112, 782−835. (7) Jacoby, M. Chem. Eng. News 2005, 2016, 42−47. (8) Wang, L.; Han, Y. Z.; Feng, X.; Zhou, J. W.; Qi, P. F.; Wang, B. Coord. Chem. Rev. 2016, 307, 361−381. (9) Lee, D. Y.; Shin, C. Y.; Yoon, S. J.; Lee, H. Y.; Lee, W.; Shrestha, N. K.; Lee, J. K.; Han, S. H. Scientific Reports 2014, 4. (10) Liu, J. X.; Zhou, W. C.; Liu, J. X.; Howard, I.; Kilibarda, G.; Schlabach, S.; Coupry, D.; Addicoat, M.; Yoneda, S.; Tsutsui, Y.; Sakurai, T.; Seki, S.; Wang, Z. B.; Lindemann, P.; Redel, E.; Heine, T.; Woll, C. Angew. Chem., Int. Ed. 2015, 54, 7441−7445. (11) Maza, W. A.; Haring, A. J.; Ahrenholtz, S. R.; Epley, C. C.; Lin, S. Y.; Morris, A. J. Chem. Sci. 2016, 7, 719−727. (12) Stavila, V.; Talin, A. A.; Allendorf, M. D. Chem. Soc. Rev. 2014, 43, 5994−6010. (13) Erickson, K. J.; Leonard, F.; Stavila, V.; Foster, M. E.; Spataru, C. D.; Jones, R. E.; Foley, B. M.; Hopkins, P. E.; Allendorf, M. D.; Talin, A. A. Adv. Mater. 2015, 27, 3453−3459. (14) Wang, S. B.; Wang, X. C. Small 2015, 11, 3097−3112. (15) Dong, M.-J.; Zhao, M.; Ou, S.; Zou, C.; Wu, C.-D. Angew. Chem., Int. Ed. 2014, 53, 1575−1579. (16) Chae, H. K.; Siberio-Perez, D. Y.; Kim, J.; Go, Y.; Eddaoudi, M.; Matzger, A. J.; O’Keeffe, M.; Yaghi, O. M. Nature 2004, 427, 523−527. (17) Leong, K.; Foster, M. E.; Wong, B. M.; Spoerke, E. D.; Van Gough, D.; Deaton, J. C.; Allendorf, M. D. J. Mater. Chem. A 2014, 2, 3389. (18) Tanaka, D.; Kitagawa, S. Chem. Mater. 2008, 20, 922−931. (19) Chen, Y.; Lykourinou, V.; Vetromile, C.; Hoang, T.; Ming, L. J.; Larsen, R. W.; Ma, S. Q. J. Am. Chem. Soc. 2012, 134, 13188−13191. (20) Hermes, S.; Schroter, M. K.; Schmid, R.; Khodeir, L.; Muhler, M.; Tissler, A.; Fischer, R. W.; Fischer, R. A. Angew. Chem., Int. Ed. 2005, 44, 6237−6241. (21) Schroder, F.; Fischer, R. A.; Schroder, M. Top. Curr. Chem. 2009, 293, 77−113. (22) Stavila, V.; Bhakta, R. K.; Alam, T. M.; Majzoub, E. H.; Allendorf, M. D. ACS Nano 2012, 6, 9807−9817. (23) Chen, Y.; Lykourinou, V.; Hoang, T.; Ming, L. J.; Ma, S. Q. Inorg. Chem. 2012, 51, 9156−9158. (24) Talin, A. A.; Centrone, A.; Ford, A. C.; Foster, M. E.; Stavila, V.; Haney, P.; Kinney, R. A.; Szalai, V.; El Gabaly, F.; Yoon, H. P.; Léonard, F.; Allendorf, M. D. Science 2014, 343, 66−69. (25) Allendorf, M. D.; Foster, M. E.; Leonard, F.; Stavila, V.; Feng, P. L.; Doty, F. P.; Leong, K.; Ma, E. Y.; Johnston, S. R.; Talin, A. A. J. Phys. Chem. Lett. 2015, 6, 1182−1195. (26) Greathouse, J. A.; Ockwig, N. W.; Criscenti, L. J.; Guilinger, T. R.; Pohl, P.; Allendorf, M. D. Phys. Chem. Chem. Phys. 2010, 12, 12621−12629. (27) Greathouse, J. A.; Kinnibrugh, T. L.; Allendorf, M. D. Ind. Eng. Chem. Res. 2009, 48, 3425−3431. (28) Parkes, M. V.; Demir, H.; Teich-McGoldrick, S. L.; Sholl, D. S.; Greathouse, J. A.; Allendorf, M. D. Microporous Mesoporous Mater. 2014, 194, 190−199. (29) Feng, P. L.; Leong, K.; Allendorf, M. D. Dalt. Trans. 2012, 41, 8869−8877. (30) Zeitler, T. R.; Van Heest, T.; Sholl, D. S.; Allendorf, M. D.; Greathouse, J. A. ChemPhysChem 2013, 14, 3740−3750.

MIL-101-NH2 (Al) powders were infiltrated by soaking in saturated solutions of MTTC (1.0 × 10−4 M) in MeOH and/or PCBM (20 mg/ mL) in toluene for 1 week. After soaking for 1 week, the crystals were thoroughly washed and rinsed with toluene. Then, the material was activated at 125 °C for 6 h. The carbon weight % for MTTC@MIL101 (Al) was 46.90 wt %; sulfur was detected at 1.44 wt % for MIL101-NH2 (Al). This loading level corresponds to 54 bithiophene molecules per unit cell. Characterization Methods. Powder X-ray diffraction experiments were carried out using a PANalytical Empyrean diffractometer equipped with a PIXcel-3D detector operating in scanning line detector mode with Cu Kα radiation (λα = 1.54187 Å). The samples were activated and then ground to a fine powder in ambient air, applied to a low background sample holder, and mounted on a flat sample stage. Raw data were then evaluated using the X’Pert HighScore Plus software V 3.0.0 (PANalytical, The Netherlands). SEM images were collected on a Hitachi 4500 field emission SEM. FTIR spectra were collected using solid samples on a Digitlab FTS 7000 Series spectrometer with an IntegratIR mid-infrared integrating sphere (Pike Technologies). Steady-state photoluminescence measurements were collected using a Horiba Jobin-Yvon Fluorolog 3-21 fluorimeter, employing a 450 W Xe arc lamp using crystalline powder samples of MOF contained in a powder stage holder. XPS spectra were collected on a PHI 5400 instrument at the Molecular Foundry, Lawrence Berkeley National Laboratory. Diffuse reflectance UV−vis spectra were recorded on a Shimadzu UV-3600 spectrophotometer equipped with a Praying Mantis diffuse reflectance accessory (Harrick Scientific). Elemental analyses were performed at ALS Environment, Houston TX. Computational Methods. DFT calculations for the band alignment of linkers of MIL-101-NH2 (Al) and MTTC@MIL-101 (Al) relative to PCBM were carried out using the B3LYP functional and the 6-31G(d,p) basis set. The reported HOMO/LUMO energies are based on the optimized geometries. Single-point 1D periodic DFT calculations on the BT-TT-BT linker using a 100 k-points, along gamma to Z, were performed at the B3LYP/6-31G(d,p) level for the BT-TT-BT linker with several π-π stacked distances. The geometry of the linker was fixed to the optimized geometry obtained in ref 109, and only the lattice parameter was changed to adjust the spacing.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy SunShot Program under award number DE-FOA-0000387-1923 and the Sandia National Laboratories Laboratory Directed Research and Development (LDRD) Program. XPS measurements were performed at the Molecular Foundry, Lawrence Berkeley National Laboratory, supported by the Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

■

REFERENCES

(1) Team, C. W.; Pachauri, R. K., Eds. L. A. M. IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and O


Forum Article

Inorganic Chemistry (31) Bloch, E. D.; Murray, L. J.; Queen, W. L.; Chavan, S.; Maximoff, S. N.; Bigi, J. P.; Krishna, R.; Peterson, V. K.; Grandjean, F.; Long, G. J.; Smit, B.; Bordiga, S.; Brown, C. M.; Long, J. R. J. Am. Chem. Soc. 2011, 133, 14814−14822. (32) Nie, X. W.; Kulkarni, A.; Sholl, D. S. J. Phys. Chem. Lett. 2015, 6, 1586−1591. (33) Meilikhov, M.; Yusenko, K.; Esken, D.; Turner, S.; Van Tendeloo, G.; Fischer, R. A. Eur. J. Inorg. Chem. 2010, 2010, 3701− 3714. (34) Rosler, C.; Fischer, R. A. CrystEngComm 2015, 17, 199−217. (35) Kitao, T.; Bracco, S.; Comotti, A.; Sozzani, P.; Naito, M.; Seki, S.; Uemura, T.; Kitagawa, S. J. Am. Chem. Soc. 2015, 137, 5231−5238. (36) Bhakta, R. K.; Herberg, J. L.; Jacobs, B.; Highley, A.; Behrens, R., Jr.; Ockwig, N. W.; Greathouse, J. A.; Allendorf, M. D. J. Am. Chem. Soc. 2009, 131, 13198−13199. (37) Bhakta, R. K.; Maharrey, S.; Stavila, V.; Highley, A.; Alam, T.; Majzoub, E.; Allendorf, M. Phys. Chem. Chem. Phys. 2012, 14, 8160− 8169. (38) Farha, O. K.; Eryazici, I.; Jeong, N. C.; Hauser, B. G.; Wilmer, C. E.; Sarjeant, A. A.; Snurr, R. Q.; Nguyen, S. T.; Yazaydın, A. O. z. r.; Hupp, J. T. J. Am. Chem. Soc. 2012, 134, 15016−15021. (39) Farrusseng, D. Metal-organic frameworks: applications from catalysis to gas storage; John Wiley & Sons, 2011. (40) Deng, H.; Doonan, C. J.; Furukawa, H.; Ferreira, R. B.; Towne, J.; Knobler, C. B.; Wang, B.; Yaghi, O. M. Science 2010, 327, 846−850. (41) An, J.; Rosi, N. L. J. Am. Chem. Soc. 2010, 132, 5578−5579. (42) Jiang, H.-L.; Feng, D.; Liu, T.-F.; Li, J.-R.; Zhou, H.-C. J. Am. Chem. Soc. 2012, 134, 14690−14693. (43) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469−472. (44) Yushin, G.; Dash, R.; Jagiello, J.; Fischer, J. E.; Gogotsi, Y. Adv. Funct. Mater. 2006, 16, 2288−2293. (45) Lin, X.; Telepeni, I.; Blake, A. J.; Dailly, A.; Brown, C. M.; Simmons, J. M.; Zoppi, M.; Walker, G. S.; Thomas, K. M.; Mays, T. J. J. Am. Chem. Soc. 2009, 131, 2159−2171. (46) Saha, D.; Bao, Z.; Jia, F.; Deng, S. Environ. Sci. Technol. 2010, 44, 1820−1826. (47) Morris, R. E.; Wheatley, P. S. Angew. Chem., Int. Ed. 2008, 47, 4966−4981. (48) Ma, S.; Zhou, H.-C. Chem. Commun. 2010, 46, 44−53. (49) Han, S. S.; Goddard, W. A. J. Am. Chem. Soc. 2007, 129, 8422− 8423. (50) Xiang, Z.; Hu, Z.; Yang, W.; Cao, D. Int. J. Hydrogen Energy 2012, 37, 946−950. (51) Jüstel, T.; Krupa, J.-C.; Wiechert, D. U. J. Lumin. 2001, 93, 179− 189. (52) Lomaev, M.; Sosnin, E.; Tarasenko, V.; Shits, D.; Skakun, V.; Erofeev, M. V.; Lisenko, A. Instrum. Exp. Tech. 2006, 49, 595−616. (53) Kuo, D.-H.; Cheung, B.-Y.; Wu, R.-J. Thin Solid Films 2001, 398, 35−40. (54) Liu, L. T.; Xu, Y.; Tang, P. J. Phys. Chem. B 2010, 114, 9010− 9016. (55) Perry, J. J., IV; Teich-McGoldrick, S. L.; Meek, S. T.; Greathouse, J. A.; Haranczyk, M.; Allendorf, M. D. J. Phys. Chem. C 2014, 118, 11685−11698. (56) Chung, Y. G.; Camp, J.; Haranczyk, M.; Sikora, B. J.; Bury, W.; Krungleviciute, V.; Yildirim, T.; Farha, O. K.; Sholl, D. S.; Snurr, R. Q. Chem. Mater. 2014, 26, 6185−6192. (57) Meek, S. T.; Perry, J. J., IV; Teich-McGoldrick, S. L.; Greathouse, J. A.; Allendorf, M. D. Cryst. Growth Des. 2011, 11, 4309−4312. (58) Meek, S. T.; Teich-McGoldrick, S. L.; Perry, J. J.; Greathouse, J. A.; Allendorf, M. D. J. Phys. Chem. C 2012, 116, 19765−19772. (59) Rowsell, J. L.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 1304− 1315. (60) Durbin, D. J.; Malardier-Jugroot, C. Int. J. Hydrogen Energy 2013, 38, 14595−14617. (61) California Fuel Cell Partnership. http://cafcp.org/ (accessed April 10, 2016).

(62) High Pressure Hydrogen Refueling Stations. http://tsrc. berkeley.edu/HighPressureHydrogenRefuelingStation (accessed April 10, 2016). (63) Eberle, U.; Felderhoff, M.; Schuth, F. Angew. Chem., Int. Ed. 2009, 48, 6608−6630. (64) Graetz, J. Chem. Soc. Rev. 2009, 38, 73−82. (65) Allendorf, M. D.; Stavila, V. CrystEngComm 2015, 17, 229−246. (66) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. Science 2013, 341. (67) Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D.-W. Chem. Rev. 2012, 112, 782−835. (68) Kesanli, B.; Cui, Y.; Smith, M. R.; Bittner, E. W.; Bockrath, B. C.; Lin, W. Angew. Chem., Int. Ed. 2005, 44, 72−75. (69) Chun, H.; Dybtsev, D. N.; Kim, H.; Kim, K. Chem. - Eur. J. 2005, 11, 3521−3529. (70) de Jongh, P. E.; Allendorf, M.; Vajo, J. J.; Zlotea, C. MRS Bull. 2013, 38, 488−494. (71) Li, Y. Q.; Song, P.; Zheng, J.; Li, X. G. Chem. - Eur. J. 2010, 16, 10887−10892. (72) Li, Z. Y.; Liu, W.; Yang, H. J.; Sun, T.; Liu, K.; Wang, Z. H.; Niu, C. RSC Adv. 2015, 5, 10746−10750. (73) Srinivas, G.; Travis, W.; Ford, J.; Wu, H.; Guo, Z. X.; Yildirim, T. J. Mater. Chem. A 2013, 1, 4167−4172. (74) Nielsen, T. K.; Polanski, M.; Zasada, D.; Javadian, P.; Besenbacher, F.; Bystrzycki, J.; Skibsted, J.; Jensen, T. R. ACS Nano 2011, 5, 4056−4064. (75) Deng, H.; Grunder, S.; Cordova, K. E.; Valente, C.; Furukawa, H.; Hmadeh, M.; Gándara, F.; Whalley, A. C.; Liu, Z.; Asahina, S.; Kazumori, H.; O’Keeffe, M.; Terasaki, O.; Stoddart, J. F.; Yaghi, O. M. Science 2012, 336, 1018−1023. (76) Stavila, V.; Parthasarathi, R.; Davis, R. W.; El Gabaly, F.; Sale, K. L.; Simmons, B. A.; Singh, S.; Allendorf, M. D. ACS Catal. 2016, 6, 55−59. (77) Wang, L. J.; Deng, H.; Furukawa, H.; Gándara, F.; Cordova, K. E.; Peri, D.; Yaghi, O. M. Inorg. Chem. 2014, 53, 5881−5883. (78) Molinari, V.; Giordano, C.; Antonietti, M.; Esposito, D. J. Am. Chem. Soc. 2014, 136, 1758−1761. (79) Sergeev, A. G.; Webb, J. D.; Hartwig, J. F. J. Am. Chem. Soc. 2012, 134, 20226−20229. (80) Wang, X.; Rinaldi, R. ChemSusChem 2012, 5, 1455−1466. (81) Sumida, K.; Brown, C. M.; Herm, Z. R.; Chavan, S.; Bordiga, S.; Long, J. R. Chem. Commun. 2011, 47, 1157−1159. (82) Lee, D. Y.; Lim, I.; Shin, C. Y.; Patil, S. A.; Lee, W.; Shrestha, N. K.; Lee, J. K.; Han, S.-H. J. Mater. Chem. A 2015, 3, 22669−22676. (83) Bauer, C. A.; Timofeeva, T. V.; Settersten, T. B.; Patterson, B. D.; Liu, V. H.; Simmons, B. A.; Allendorf, M. D. J. Am. Chem. Soc. 2007, 129, 7136−7144. (84) Shustova, N. B.; McCarthy, B. D.; Dincă, M. J. Am. Chem. Soc. 2011, 133, 20126−20129. (85) Shustova, N. B.; Cozzolino, A. F.; Dincă, M. J. Am. Chem. Soc. 2012, 134, 19596−19599. (86) Shustova, N. B.; Ong, T.-C.; Cozzolino, A. F.; Michaelis, V. K.; Griffin, R. G.; Dincă, M. J. Am. Chem. Soc. 2012, 134, 15061−15070. (87) Komatsu, T.; Taylor, J. M.; Kitagawa, H. Inorg. Chem. 2016, 55, 546−548. (88) Kobayashi, Y.; Jacobs, B.; Allendorf, M. D.; Long, J. R. Chem. Mater. 2010, 22, 4120−4122. (89) Hains, A. W.; Liang, Z. Q.; Woodhouse, M. A.; Gregg, B. A. Chem. Rev. 2010, 110, 6689−6735. (90) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58−77. (91) Brabec, C. J.; Gowrisanker, S.; Halls, J. J. M.; Laird, D.; Jia, S.; Williams, S. P. Adv. Mater. 2010, 22, 3839−3856. (92) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. Nat. Mater. 2006, 5, 197−203. (93) Leong, K.; Foster, M. E.; Wong, B. M.; Spoerke, E. D.; Van Gough, D.; Deaton, J. C.; Allendorf, M. D. J. Mater. Chem. A 2014, 2, 3389−3398. P


Forum Article

Inorganic Chemistry (94) Jiang, J.; Zhao, Y.; Yaghi, O. M. J. Am. Chem. Soc. 2016, 138, 3255−3265. (95) Tanabe, K. K.; Cohen, S. M. Chem. Soc. Rev. 2011, 40, 498−519. (96) Wang, Z.; Tanabe, K. K.; Cohen, S. M. Inorg. Chem. 2009, 48, 296−306. (97) Song, Y.-F.; Cronin, L. Angew. Chem., Int. Ed. 2008, 47, 4635− 4637. (98) Wang, Z.; Tanabe, K. K.; Cohen, S. M. Chem. - Eur. J. 2010, 16, 212−217. (99) Morris, W.; Doonan, C. J.; Furukawa, H.; Banerjee, R.; Yaghi, O. M. J. Am. Chem. Soc. 2008, 130, 12626−12627. (100) Lu, G.; Hupp, J. T. J. Am. Chem. Soc. 2010, 132, 7832−7833. (101) Lebedev, O. I.; Millange, F.; Serre, C.; Van Tendeloo, G.; Férey, G. Chem. Mater. 2005, 17, 6525−6527. (102) Férey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F. Acc. Chem. Res. 2005, 38, 217−225. (103) Serra-Crespo, P.; Ramos-Fernandez, E. V.; Gascon, J.; Kapteijn, F. Chem. Mater. 2011, 23, 2565−2572. (104) Perry, J. J.; Feng, P. L.; Meek, S. T.; Leong, K.; Doty, F. P.; Allendorf, M. D. J. Mater. Chem. 2012, 22, 10235−10248. (105) So, M. C.; Wiederrecht, G. P.; Mondloch, J. E.; Hupp, J. T.; Farha, O. K. Chem. Commun. 2015, 51, 3501−3510. (106) Zhang, T.; Lin, W. Chem. Soc. Rev. 2014, 43, 5982−5993. (107) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Nat. Chem. 2014, 6, 242−247. (108) Hardin, B. E.; Snaith, H. J.; McGehee, M. D. Nat. Photonics 2012, 6, 162−169. (109) Foster, M. E.; Azoulay, J. D.; Wong, B. M.; Allendorf, M. D. Chem. Sci. 2014, 5, 2081−2090. (110) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1323−1338. (111) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533−4542. (112) Sun, L.; Campbell, M. G.; Dincă, M. Angew. Chem., Int. Ed. 2016, 55, 3566−3579. (113) Narayan, T. C.; Miyakai, T.; Seki, S.; Dincă, M. J. Am. Chem. Soc. 2012, 134, 12932−12935. (114) Dunbar, K. R.; Heintz, R. A.; Karlin, K. D. Progress in Inorganic Chemistry, Vol 45 1997, 45, 283−391. (115) Kaim, W.; Moscherosch, M. Coord. Chem. Rev. 1994, 129, 157−193. (116) Hunig, S.; Herberth, E. Chem. Rev. 2004, 104, 5535−5563. (117) Shimomura, S.; Yanai, N.; Matsuda, R.; Kitagawa, S. Inorg. Chem. 2011, 50, 172−177. (118) Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 283, 1148. (119) Snyder, G. J.; Toberer, E. S. Nat. Mater. 2008, 7, 105−114. (120) Lowhorn, N. D.; Wong-Ng, W.; Lu, Z. Q.; Thomas, E.; Otani, M.; Green, M.; Dilley, N.; Sharp, J.; Tran, T. N. Appl. Phys. A: Mater. Sci. Process. 2009, 96, 511−514. (121) McDonald, T. M.; Mason, J. A.; Kong, X.; Bloch, E. D.; Gygi, D.; Dani, A.; Crocellà, V.; Giordanino, F.; Odoh, S. O.; Drisdell, W. S.; Vlaisavljevich, B.; Dzubak, A. L.; Poloni, R.; Schnell, S. K.; Planas, N.; Lee, K.; Pascal, T.; Wan, L. F.; Prendergast, D.; Neaton, J. B.; Smit, B.; Kortright, J. B.; Gagliardi, L.; Bordiga, S.; Reimer, J. A.; Long, J. R. Nature 2015, 519, 303−308.

Q


Transforming MOFs for Energy Applications Using the Guest@MOF

Recommend Documents