Series Of Metal-organic Frameworks Research Articles

Ab Initio Flexible Force Field for Metal-Organic Frameworks Using Dummy Model Coordination Bonds.

We present force fields developed from periodic density functional theory (DFT) calculations that can be used in classical molecular simulations to model M-MOF-74 (M = Co, Fe, Mg, Mn, Ni, Zn) and its extended linker analogs. Our force fields are based on cationic dummy models (CDMs). These dummy models simplify the methodology required to tune the parameters and improve the accuracy of the force fields. We used our force fields to compare mechanical properties across the M-MOF-74 series and determine that increasing the size of the linker decreases the framework rigidity. In addition, we applied our force fields to an extended linker analog of Mg-MOF-74 and characterized the free energy of a previously reported deformation pattern in which the one-dimensional hexagonal channels of the framework become irregular. The free energy profiles confirm that the deformation is adsorbate induced and impossible to access solely by a pressure stimulus. On the basis of our results, we conclude that the force fields presented here and others that may be developed using our methodology are transferable across metal-organic framework series that share a metal center topology. Finally, we believe that these force fields have the potential to be adapted for the study of complex problems in MOF chemistry, including defects and crystal growth, that have thus far been beyond the scope of classical molecular simulations.

Water sorption properties of functionalized MIL-101(Cr)-X (X=-NH2, –SO3H, H, –CH3, –F) based composites as thermochemical heat storage materials

Abstract Thermochemical heat storage based on inorganic salt hydrates is a promising approach to relieve the heat energy supply and demand conflicts, but there exist lots of defects of pure salts when applied to the thermochemical. This paper attempted to use functionalized Metal-Organic Frameworks (MOFs) as the porous matrices to improve the heat storage performance of inorganic salt hydrates. A series of MOFs/CaCl2 composites based on MIL-101(Cr)-X (X = -NH2, –SO3H, H, –CH3, –F) were successfully synthesized and fully characterized by combining multiple techniques (i.e. powder X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis, N2 adsorption-desorption, Fourier transform infrared spectroscopy, thermogravimetric and differential scanning calorimetry). The water sorption properties of the series of MOFs/CaCl2 composites were studied during the hydration process, and a continuous adsorption/desorption experiment was also performed to investigate the cycle stability of the MOFs/CaCl2 composites. The results show that the hydrophilic MOFs based composites have higher water sorption capacity (0.6 g(H2O)/g(sample) for MIL-101(Cr)–SO3H/CaCl2) accompanied with higher heat storage density (1274 J/g for MIL-101(Cr)–SO3H/CaCl2). Moreover, the series of MOFs all presented excellent cycle performance with little loss after continuous 17 adsorption/desorption cycles.

Spectroscopic characterization of metal ligation in trinuclear iron-μ3-oxo-based complexes and metal-organic frameworks.

The adsorption-based functionalities of porous metal-organic framework (MOF) materials that lead to applications such as catalysis and gas separation rely on specific host-guest interactions often involving the framework metal sites. These interactions are difficult to probe on the molecular level and consequently poorly understood. Conventional X-ray absorption spectroscopy (XAS) methods can provide molecular-level insights but, as the sole method of characterization, often lack the ligand sensitivity required to probe the relevant local metal coordination changes associated with MOF adsorption processes. Here, we investigate a series of trinuclear iron μ3-oxo-based MOFs under different metal-coordinating guest environments (water, pyridine, propylene, and guest-free) using a multipronged spectroscopy approach, including valence-to-core X-ray emission spectroscopy (vtc-XES) along with conventional XAS and vibrational spectroscopy, in an effort to characterize their local metal site coordination environments, including ligand identity. Closely related iron μ3-oxo reference complexes with known coordination are characterized as well for comparison to evaluate the ligand diagnostic nature of the combined spectroscopy approach. Density functional theory calculations aid the vtc-XES band assignments and provide insights into the molecular orbital parentage of the vtc transitions. This series of MOFs and complexes illustrates the advantages and limitations of using this combination of complementary techniques for distinguishing subtle differences in framework metal node coordination environments.

Metal-organic frameworks MOF-808-X as highly efficient catalysts for direct synthesis of dimethyl carbonate from CO2 and methanol

A series of metal-organic frameworks MOF-808-X (6-connected) were synthesized by regulating the ZrOCl2·8H2O/1,3,5-benzenetricarboxylic acid (BTC) molar ratio (X) and tested for the direct synthesis of dimethyl carbonate (DMC) from CO2 and CH3OH with 1,1,1-trimethoxymethane (TMM) as a dehydrating agent. The effect of the ZrOCl2·8H2O/BTC molar ratio on the physicochemical properties and catalytic performance of MOF-808-X was investigated. Results showed that a proper ZrOCl2·8H2O/BTC molar ratio during MOF-808-X synthesis was fairly important to reduce the redundant BTC or zirconium clusters trapped in the micropores of MOF-808-X. MOF-808-4, with almost no redundant BTC or zirconium clusters trapped in the micropores, exhibited the largest surface area, micropore size, and the number of acidic-basic sites, and consequently showed the best activity among all MOF-808-X, with the highest DMC yield of 21.5% under the optimal reaction conditions. Moreover, benefiting from the larger micropore size, MOF-808-4 outperformed our previously reported UiO-66-24 (12-connected), which had even more acidic-basic sites and larger surface area than MOF-808-4, mainly because the larger micropore size of MOF-808-4 provided higher accessibility for the reactant to the active sites located in the micropores. Furthermore, a possible reaction mechanism over MOF-808-4 was proposed based on the in situ FT-IR results. The effects of different reaction parameters on DMC formation and the reusability of MOF-808-X were also studied.

Porosity Dependence of Compression and Lattice Rigidity in Metal-Organic Framework Series.

Porous materials, including metal-organic frameworks (MOFs), are known to undergo structural changes when subjected to applied hydrostatic pressures that are both fundamentally interesting and practically relevant. With the rich structural diversity of MOFs, the development of design rules to better understand and enhance the mechanical stability of MOFs is of paramount importance. In this work, the compressibilities of seven MOFs belonging to two topological families (representing the most comprehensive study of this type to date) were evaluated using in situ synchrotron X-ray powder diffraction of samples within a diamond anvil cell. The judicious selection of these materials, representing widely studied classes of MOFs, provides broadly applicable insight into the rigidity and compression of hybrid materials. An analysis of these data reveals that the bulk modulus depends on several structural parameters (e.g., void fraction and linker length). Furthermore, we find that lattice distortions play a major role in the compression of MOFs. This study is an important step toward developing a predictive model of the structural variables that dictate the compressibility of porous materials.

Six Isomorphous Window-Beam MOFs: Explore the Effects of Metal Ions on MOF-Derived Carbon for Supercapacitors.

Six isomorphous metal-organic frameworks (MOFs) with a 3D window-beam architecture have been synthesized from solvothermal reactions, and are named Zn, Cd, Ni, Co, Mn and Cu-MOF, respectively. The series of MOFs was utilized as precursors to synthesize MOF-derived carbon with different morphologies. Zn and Cd-MOFs lead to the derivation of porous carbons (PCs), which exhibit remarkable BET specific surface areas. For derivates of Ni, Co and Mn-MOFs, graphitized carbons (GCs) show some carbon graphitization, but their BET specific surface areas are relatively small. C-Cu has the smallest BET specific surface area, and there is no carbon graphitization. Therefore, the metal ion of the parent MOF exerts a crucial effect on the preparation of MOF-derived carbon, such as the pore-forming effect of Zn and Cd species, and catalytic graphitization of Ni, Co, and Mn species. The capacitances of MOF-derived carbon follow the sequence of PCs>GCs>C-Cu, which reveals that the specific surface area plays a dominant role in the capacitive performance of electrical double layer capacitors (EDLCs), and that the graphitization could improve the capacitance. Significantly, PC-Zn exhibits the best specific capacitance (138 F g-1 at 0.5 Ag-1 ), and excellent life cycle, which can be applied as an electrode material in supercapacitors.

MOF-derived binary mixed carbon/metal oxide porous materials for constructing simultaneous determination of hydroquinone and catechol sensor

It is a top priority to simultaneously and accurately detect hydroquinone (HQ) and catechol (CC). Here, a new strategy for constructing simultaneous determination of HQ and CC sensor was proposed by one-step pyrolysis of MIL series metal-organic frameworks materials (MIL-125 (Ti), MIL-101 (Cr), and MIL-101 (Fe)) to obtain uniform-mixed carbon/metal oxide porous materials (TiO2/C900, Cr2O3/C900, and Fe2O3/C900, respectively). And, cyclic voltammetry (CV) was utilized to investigate the electrochemical behavior of the composite materials. It was found that the simultaneous detection of catechol (CC) and hydroquinone (HQ) could be achieved by the sensor consisted of TiO2/C900 with the superior BET-specific surface area and micro-mesoporous characteristics. And, the linear range and detection limit of HQ and CC for the TiO2/C900 sensor were further studied. In addition, it was also found that the pyrolysis temperature and metal centers would affect the internal structures and component of the materials, thus affecting the properties of materials. The experiment provides a new idea for optimizing the simultaneous detection of the dihydroxybenzene isomers.

SuFEx in Metal-Organic Frameworks: Versatile Postsynthetic Modification Tool.

A new type of click reaction, sulfur(VI) fluoride exchange (SuFEx), has been utilized to prepare five postsynthetically modified UiO-67 series metal-organic frameworks (MOFs). The postsynthetic modification (PSM) via SuFEx can be achieved selectively for the sulfonyl fluoride (R-SO2F) without degrading the MOF structure as confirmed by X-ray crystallographic analysis. The present SuFEx method provides a straightforward tool for introducing new functionality inside MOFs. Introduction of an imidazolium group into the MOF afforded a heterogeneous catalyst for the benzoin condensation reaction.

Modification of Cu2+ into Zr-based metal–organic framework (MOF) with carboxylic units as an efficient heterogeneous catalyst for aerobic epoxidation of olefins

A series of metal-organic framework (MOF) based catalysts for aerobic epoxidation were reported. A post-synthetic modification strategy using a solvothermal method enabled the incorporation of three different copper salts on a Zr-based MOF material UiO-66-(COOH)2; catalysts Cu@UiO-1, Cu@UiO-2 and Cu@UiO-3 were obtained. When analyzed by SEM and EDX, no impact of the post-synthetic modification on the core structure of the UiO-66-(COOH)2 framework was noticed. Studies showed Cu@UiO-1 to be the optimal catalyst for aerobic epoxidation thanks to its broad substrate scope, thermal stability, excellent recyclability. Observed catalytic properties are also better than those of previously reported copper catalysts.

A regulatable oxidative valorization of furfural with aliphatic alcohols catalyzed by functionalized metal-organic frameworks-supported Au nanoparticles

The oxidative upgrading of furfural (FUR) and aliphatic alcohols is an important way to produce desirable precursor of jet fuel or value-added furanic compound. Therein, developing a highly active catalytic system with switchable product selectivity still remains a challenge. In this work, we report a novel strategy on regulating the oxidative condensation and oxidative esterification of FUR with aliphatic alcohol in the presence of molecular oxygen. Firstly, Au@UiO-66 is prepared using different methods and employed as the catalyst for the oxidative valorization of FUR with methanol. It is found that the impregnation-reduction-H2 (I-H) method is the best where a 100% selectivity of methyl-2-furoate with a complete conversion was obtained using Au@UiO-66 as catalyst. Then, a series of metal-organic frameworks (MOFs) supported Au nanoparticles (Au@UiO-66-X) such as Au@UiO-66, Au@UiO-66-NH2, Au@UiO-66-NO2, Au@UiO-66-COOH and Au@UiO-66-NH3Cl have been prepared with I-H method and employed for oxidative valorization of furfural with ethanol. Experimental results showed that, in “FUR-ethanol-O2” system, the Au@UiO-66-X can efficiently regulate the oxidative condensation and oxidative esterification as two competitive reaction pathways. With Au@UiO-66-COOH as the catalyst, the oxidative condensation process is dominant in which 84.1% selectivity of furan-2-acrolein is attained; Meanwhile, the Au@UiO-66 is beneficial to the occurrence of oxidative esterification and generation of ethyl-2-furoate. At last, based on the catalyst characterization and the numerous control experiments, a possible catalytic reaction mechanism for conversion of FUR is proposed.

Molecular Vise Approach to Create Metal-Binding Sites in MOFs and Detection of Biomarkers.

We report a new approach to create metal-binding site in a series of metal-organic frameworks (MOFs), where tetratopic carboxylate linker, 4',4'',4''',4''''-methanetetrayltetrabiphenyl-4-carboxylic acid, is partially replaced by a tritopic carboxylate linker, tris(4-carboxybiphenyl)amine, in combination with monotopic linkers, formic acid, trifluoroacetic acid, benzoic acid, isonicotinic acid, 4-chlorobenzoic acid, and 4-nitrobenzoic acid, respectively. The distance between these paired-up linkers can be precisely controlled, ranging from 5.4 to 10.8 Å, where a variety of metals, Mg2+ , Al3+ , Cr3+ , Mn2+ , Fe3+ , Co2+ , Ni2+ , Cu2+ , Zn2+ , Ag+ , Cd2+ and Pb2+ , can be placed in. The distribution of these metal-binding sites across a single crystal is visualized by 3D tomography of laser scanning confocal microscopy with a resolution of 10 nm. The binding affinity between the metal and its binding-site in MOF can be varied in a large range (observed binding constants, Kobs from 1.56×102 to 1.70×104 L mol-1 ), in aqueous solution. The fluorescence of these crystals can be used to detect biomarkers, such as cysteine, homocysteine and glutathione, with ultrahigh sensitivity and without the interference of urine, through the dissociation of metal ions from their binding sites.

Influence of lactic acid on the catalytic performance of MDABCO for ring-opening polymerization of l-lactide

A series of metal organic frameworks (MOFs) MDABCO (M=Co, Ni, Cu, Zn; DABCO=1,4-diazabicyclo[2.2.2]octane) are utilized as catalysts for the bulk ring-opening polymerization of l-lactide. The catalytic activity varies in the order CuDABCO, NiDABCO<ZnDABCO, CoDABCO. By adding lactic acid, we unexpectedly observed enhanced catalytic activity. The combined use of lactic acid and ZnDABCO during polymerization results in a highly active polymerization system, along with the gradual change of MOFs structure caused by an increased acid concentration. Compared to related homogeneous fragments, such as zinc nitrate and 1,4-diazabicyclo[2.2.2]octane, ZnDABCO is found to afford highly isotactic polylactide, which would be ascribed to the unique structural properties of MOFs.

CO2 Cycloaddition to Epoxides by using M-DABCO Metal-Organic Frameworks and the Influence of the Synthetic Method on Catalytic Reactivity.

A series of high‐quality M2(BDC)2(DABCO) metal–organic frameworks (abbreviated as M‐DABCO; M=Zn, Co, Ni, Cu; BDC=1,4‐benzene dicarboxylate; DABCO=1,4‐diazabicyclo[2.2.2]octane), were synthesized by using a solvothermal (SV) method, and their catalytic activity for the cycloaddition of CO2 to epoxides in the absence of a co‐catalyst or solvent was demonstrated. Of these metal–organic frameworks (MOFs), Zn‐DABCO exhibited very high activity and nearly complete selectivity under moderate reaction conditions. The other members of this MOF series (Co‐DABCO, Ni‐DABCO, and Cu‐DABCO) displayed lower activity in the given sequence. Samples of Zn‐DABCO, Co‐DABCO, and Ni‐DABCO were recycled at least three times without a noticeable loss in catalytic activity. The reaction mechanism can be attributed to structural defects along with the acid–base bifunctional characteristics of these MOFs. Moreover, we illustrate that the synthetic method of M‐DABCO influences the yield of the reaction. In addition to the SV method, Zn‐DABCO was synthesized by using spray drying due to its industrial attractiveness. It was found that the synthesis procedure clearly influenced the crystal growth and thus the physicochemical properties, such as surface area, pore volume, and gas adsorption, which in turn affected the catalytic performance. The results clarified that although different synthetic methods can produce isostructural MOFs, the application of MOFs, especially as catalysts, strongly depends on the crystal morphology and textural properties and, therefore, on the synthesis method.

Impacts of Gas Impurities from Pipeline Natural Gas on Methane Storage in Metal–Organic Frameworks during Long-Term Cycling

The development of adsorbed natural gas (ANG) technology creates opportunities for use of pipeline natural gas as clean fuel in vehicles. Metal–organic frameworks (MOFs) are one class of materials that have received considerable attention as possible adsorbents in ANG applications. We examine how accumulation of trace components from pipeline natural gas will impact the performance of MOFs in ANG during long-term cycling. Our approach combines information from grand canonical Monte Carlo (GCMC) simulations of single-component adsorption, ideal adsorbed solution theory (IAST) of multicomponent adsorption, and an isothermal model of tank cycling to assess accumulation of heavy hydrocarbons and tert-butyl mercaptan (TBM). In a series of MOFs, a reduction in deliverable energy up to 50% is observed after 200 cycles. These results highlight the importance of considering multicomponent effects during consideration of adsorbents for ANG applications.

Cu3(BTC)2 catalyzed oxidation of silane to silanol using TBHP or water as oxidants

In the present work, a series of metal organic frameworks are examined for the conversion of Si-H to Si-OH using either t-butylhydroperoxide (TBHP) or water as oxidants. The reaction is optimized using dimethylphenylsilane (1) as a model substrate. It is observed that Cu3(BTC)2 (BTC: 1,3,5-benzenetricarboxylate) exhibits a comparable activity with Zr(BDC) (BDC: 1,4-benzenedicarboxylate) while the activity of Fe(BTC) is lower than Cu3(BTC)2 using TBHP as oxidant. On the other hand, the reaction of 1 with water in the presence of Cu3(BTC)2 as a catalyst showed complete conversion of 1 with 99% selectivity to the corresponding silanol, but other MOFs like Fe(BTC) and Zr(BDC) are inactive under identical reaction conditions. A series of control experiments indicate that Cu2+ is essential to convert 1 to 2 under the present experimental conditions. Further, Cu2+ in Cu3(BTC)2 acts as redox centre with TBHP whereas it behaves as a Lewis acid using water as oxidant. High conversion and selectivity is observed for all the silanes studied under the present experimental conditions. The catalyst stability is assessed by powder XRD, FT-IR and SEM images and observing no structural deterioration of Cu3(BTC)2 either in TBHP or water as oxidants. Furthermore, hot filtration test indicated the absence of copper under the present reaction conditions, thus confirming the stability of Cu3(BTC)2.

Chemical Warfare Agents Detoxification Properties of Zirconium Metal-Organic Frameworks by Synergistic Incorporation of Nucleophilic and Basic Sites.

The development of protective self-detoxifying materials is an important societal challenge to counteract risk of attacks employing highly toxic chemical warfare agents (CWAs). In this work, we have developed bifunctional zirconium metal-organic frameworks (MOFs) incorporating variable amounts of nucleophilic amino residues by means of formation of the mixed ligand [Zr6O4(OH)4(bdc)6(1-x)(bdc-NH2)6x] (UiO-66-xNH2) and [Zr6O4(OH)4(bpdc)6(1-x)(bpdc-(NH2)2)6x] (UiO-67-x(NH2)2) systems where bdc = benzene-1,4-dicarboxylate; bdc-NH2= benzene-2-amino-1,4-dicarboxylate; bpdc = 4,4'-biphenyldicarboxylate; bpdc-(NH2)2 = 2,2'-diamino-4,4'-biphenyldicarboxylate and x = 0, 0.25, 0.5, 0.75, 1. In a second step, the UiO-66-xNH2 and UiO-67-x(NH2)2 systems have been postsynthetically modified by introduction of highly basic lithium tert-butoxide (LiOtBu) on the oxohydroxometallic clusters of the mixed ligand MOFs to yield UiO-66-xNH2@LiOtBu and UiO-67-x(NH2)2@LiOtBu materials. The results show that the combination of pre and postsynthetic modifications on these MOF series gives rise to fine-tuning of the catalytic activity toward the hydrolytic degradation of both simulants and real CWAs in unbuffered aqueous solutions. Indeed, UiO-66-0.25NH2@LiOtBu is able to hydrolyze both CWAs simulants (diisopropylfluorophosphate (DIFP), 2-chloroethylethylsulfide (CEES), and real CWAs (soman (GD), sulfur mustard (HD)) quickly in aqueous solution. These results are related to a suitable combination of robustness, nucleophilicity, basicity, and accessibility to the porous framework.

Using Predefined M3(μ3-O) Clusters as Building Blocks for an Isostructural Series of Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) have attracted much attention in the past decade owing to their unprecedented internal surface areas, tunable topologies, designable surfaces, and various potential applications. One bottleneck in the field regarding MOF synthesis is controlling the metal-containing secondary building unit (SBU) incorporated into the structure. In this work we report the synthesis and characterization of five trimeric [M3(μ3-O)(CH3CO2)6]x clusters (where M = Fe3+, Cr3+, Fe3+/Cr3+, Fe3+/Co2+, or Fe3+/Ni2+ and x = +1 or 0). The monocarboxylate capping ligand, acetate in this case, readily undergoes exchange with several difunctional counterparts, including 1,4-benzenedicarboxylic acid (H2-BDC) and biphenyl-4,4'-dicarboxylic acid (H2-BPDC), for the formation of an isostructural series of MOFs, several of which are newly reported (for M = Fe3+/Cr3+, Fe3+/Co2+, and Fe3+/Ni2+) and show excellent CO2 adsorption properties. In this report, a host of techniques including NMR, ICP, and ESI-MS are used to probe the ligand exchange process and composition of the SBUs, and XAS is used to monitor the Fe3+ and Cr3+ environment throughout the reactions, giving strong evidence that the clusters stay intact throughout the MOF synthesis. This work reveals that predefined SBUs is an effective means to create metal-substituted analogues of known frameworks. Further, CO adsorption and in situ IR are used to probe accessibility of the metals after solvent removal. We show for the first time that the incorporation of the neutral clusters, containing weaker Lewis acids like Ni2+ and Co2+, can promote the formation of open metal sites in the MOF frameworks, structural features known to enhance the binding energy of small guest molecules like CO2.

Rare Earth pcu Metal–Organic Framework Platform Based on RE4(μ3-OH)4(COO)62+ Clusters: Rational Design, Directed Synthesis, and Deliberate Tuning of Excitation Wavelengths

The Td point group symmetry of rare earth (RE3+) metal clusters RE4(μ3-OH)4(COO)62+ makes them attractive building blocks for creating metal-organic frameworks (MOFs) with controllable topologies. Herein, we describe the design and synthesis of a series of isoreticular MOFs featuring pcu topology [MOF-1114(RE) and MOF-1115(RE)] with variable rare earth metal ions (RE3+ = Y3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+) and linear amino-functionalized dicarboxylate linkers of different lengths. In total, we report 22 MOFs that vary in both composition and structure yet share the same RE4(μ3-OH)4 cluster motif. We demonstrate that these pcu MOFs are cationic and that anion exchange can be used to affect the MOF properties. We also investigate the luminescence properties of a representative member of this MOF series [MOF-1114(Yb)] that exhibits near-infrared emission. We show that the excitation energy for Yb3+ sensitization can be carefully adjusted to lower energy via covalent postsynthetic modification at the amino group sites within the MOF.

Functional Versatility of a Series of Zr Metal-Organic Frameworks Probed by Solid-State Photoluminescence Spectroscopy.

Many of the desirable properties of metal-organic frameworks (MOFs) can be tuned by chemical functionalization of the organic ligands that connect their metal clusters into multidimensional network solids. When these linker molecules are intrinsically fluorescent, they can pass on this property to the resultant MOF, potentially generating solid-state sensors, as analytes can be bound within their porous interiors. Herein, we report the synthesis of a series of 14 interpenetrated Zr and Hf MOFs linked by functionalized 4,4'-[1,4-phenylene-bis(ethyne-2,1-diyl)]-dibenzoate (peb2-) ligands, and we analyze the effect of functional group incorporation on their structures and properties. Addition of methyl, fluoro, naphthyl, and benzothiadiazolyl units does not affect the underlying topology, but induces subtle structural changes, such as ligand rotation, and mediates host-guest interactions. Further, we demonstrate that solid-state photoluminescence spectroscopy can be used to probe these effects. For instance, introduction of naphthyl and benzothiadiazolyl units yields MOFs that can act as stable fluorescent water sensors, a dimethyl modified MOF exhibits a temperature dependent phase change controlled by steric clashes between interpenetrated nets, and a tetrafluorinated analogue is found to be superhydrophobic despite only partial fluorination of its organic backbone. These subtle changes in ligand structure coupled with the consistent framework topology give rise to a series of MOFs with a remarkable range of physical properties that are not observed with the ligands alone.

Ground-State versus Excited-State Interchromophoric Interaction: Topology Dependent Excimer Contribution in Metal-Organic Framework Photophysics.

Metal-organic frameworks (MOFs) define emerging materials with unique optoelectronic properties that stem from the highly organized chromophoric linkers within their frameworks. The extent of ground- and excited-state interchromophoric interaction among the π-conjugated macrocyclic linkers was studied within three tetraphenyl-pyrene (1,3,6,8-tetrakis(p-benzoic acid)pyrene; H4TBAPy)-based MOFs: ROD-7 (In2(OH)2TBAPy, frz), NU-901 (scu), and NU-1000 (csq) via steady-state and time-resolved spectroscopic techniques. These experimental data along with computational results indicate that the extent of the interchromophoric interaction, leading to a reduced optical band gap, varies across the series of MOFs and is a function of the relative orientation of the TBAPy linkers determined by their respective framework topology. The trend in the S1 → S0 emission lifetime is consistent with their relative optical bandgap. Analyses of the transient emission decay profiles and time-resolved emission spectroscopic data, recorded in low dielectric media, reveal that a long-lived emissive excimer state appears ∼1850 ± 150 cm-1 lower in energy relative to their corresponding S1 → S0 transitions. The emissive contribution from this excimer state, as well as its corresponding transition energy and time constants, are also found to be dependent on MOF identity. Such variation in properties are particularly influenced by the number density of the TBAPy linkers presented by the topology of a given MOF that are primed to form such an excited state complex. The present work shows how the specific arrangement of the linkers can play a key role in the photophysical properties of MOFs.