Three water-stable isostructural metal–organic frameworks (MOFs) of the general formula [M2(TCS)(BPY)] (M = Co(1), Ni(2) and Cu(3); H4TCS = tetrakis(4-carboxyphenyl) silane, BPY = 4,4′-bipyridine) were synthesized and fully characterized. MOFs 1–3 are stable in pH = 5–11, 2–11, 3–11 aqueous solution respectively for at least 24 h at room temperature. Although H4TCS absorbs only UV light, MOFs 1–3 absorb both UV and visible light in broad ranges (250–800 nm) and absorb more visible light than the ligand BPY. The rapid anodic photocurrent responses of MOFs 1–3 under UV and visible light illumination were observed. The photocurrent densities increase in the order of MOF 3 < 1 < 2 under visible light illumination (430 nm). The band gaps of MOFs 1–3 determined based on UV-Vis diffuse reflectance spectra and electrochemical (EC) analysis are 1.28, 1.35 and 0.67 eV, respectively. MOF 1 is able to photocatalyze the reduction of CO2 to CH4 under visible light, producing CH4 (1.44 μmol g−1 in 8 h), which is unprecedented in MOFs. The catalytic activity of MOF 1 (0.75 μmol g−1 after 4 h) under the irradiation of a 300 W xenon lamp is significantly better than those of MOFs 2 and 3 (0.14 μmol g−1 after 4 h). The band structures, density of states and band gaps of MOFs 1–3 were calculated by the GGA-PBE and GGA-PBE+U method implemented in VASP code. The calculations show that all the three compounds can be viewed as bulk intermediate band (IB) materials. The density of states of the IB in MOF 1 is high, which could suppress the non-radiative recombination. The density of states of the IB levels in MOFs 2 and 3 are low, making these levels very effective recombination centres, thus jeopardizing the photocatalytic activities of MOFs 2 and 3. The calculated results are in good agreement with experimental results and explain the photocatalytic activity differences. This study is the first to successfully address the question of how the types of unpaired electron containing electron-rich metal ions (i.e. Cu(II), Co(II), Ni(II)) affect the band gaps and band structures of MOFs and thus their photoelectronic properties.

Photovoltaics (PV), which directly convert solar energy into electricity generally using semiconductors, offer a practical and sustainable solution to the current energy shortage and environmental pollution crisis. Photovoltaic applications of metal-organic frameworks (MOFs) belong to a relatively new area of research. Given that UV light accounts for only 4% while visible light contributes 43% of solar energy, it is rather imperative to develop semiconductors with narrow band gaps so that they could absorb visible light. In this work, three water-stable, narrow band semiconducting MOFs of [Cu(H2TCS)(H2O)] (1), [Co(H2TCS)(BPB)] (2) and [Ni(H2TCS)(BPB)] (3) were synthesized using tetrakis(4-carboxyphenyl)silane (H4TCS) and 1,4-bis (pyridyl)benzene (BPB) in water, and structurally characterized by single-crystal X-ray diffractions. MOF 1 has a 2D structure. MOF 2 and 3 are isostructrual and have 3D frameworks formed by interwoven 2D layers. All three MOFs are stable in acidic water solutions and can be stable in water for 7 days. MOFs 1–3 absorb UV and visible light and have band gaps of 0.50, 1.77 and 1.49 eV, respectively. Rapid and stable photocurrent responses of MOFs 1–3 under UV and visible light illuminations are observed. This work demonstrates that using electron rich Cu2+, Co2+, or Ni2+ as metal nodes can effectively decrease the band gaps of MOFs to make them absorbing visible light. To increase the conjugation in the linker is generally considered to be the method to decrease the band gap of MOFs. The conjugation in H4TCS is not significant and this ligand basically only absorbs UV light. However, by using electron rich Cu2+ ions as metal nodes, the prepared [Cu(H2TCS)(H2O)]·H2O (1) absorbs broadly in the visible light region. Thus, this work suggests that by using electron rich Cu2+, many narrow-band semiconductor MOFs can be prepared even by using ligands which only absorbs UV light.Three water-stable, narrow-band semiconducting MOFs of [Cu(H2TCS)(H2O)] (1), [Co(H2TCS)(BPB)] (2) and [Ni(H2TCS)(BPB)] (3) (H4TCS = tetrakis(4-carboxyphenyl)silane, BPB = 1,4-bis (pyridyl)benzene) were synthesized. Rapid and stable photocurrent responses of compounds 1-3 under UV and visible light were observed. This work demonstrates that using electron rich Cu2+, Co2+, or Ni2+ as metal nodes can effectively decrease the band gaps of MOFs to make them absorbing visible light, and using Cu2+ and a UV absorbing ligand (e.g. H4TCS) can make a visible light absorbing semiconducting MOF (e.g. 1).Download high-res image (378KB)Download full-size image

A new UiO-67-type zirconium metal organic framework (MOF) material UiO-67-bpy-Me (bpy = 2,2-bipyridine-4,4′-dicarboxylic acid, Me = methyl) was prepared by N-quaternization of the pyridine sites in UiO-67-bpy. After N-quaternization, the pristine neutral framework turned cationic while its high thermal and chemical stabilities were primarily preserved. Fast and enhanced anionic dye adsorption was observed in UiO-67-bpy-Me. In addition, despite the decrease in surface area and pore volume, UiO-67-bpy-Me exhibited an evident increase in CO2 uptake compared to UiO-67-bpy. The enhancement was ascribed to the strong interactions between CO2 and the N-quaternized framework. More importantly, as the N-quaternization has changed the electronic structure of the organic linker. UiO-67-bpy-Me showed optical absorption up to ca. 800 nm with a large red shift of 450 nm compared to the pristine UiO-67-bpy (ca. 350 nm). The extended optical absorption may lead to more efficiency in light utilization. A proof-of-concept demonstration showed that UiO-67-bpy-Me could more efficiently catalyze methyl-orange degradation under UV-Vis light irradiation than the pristine UiO-67-bpy. These findings demonstrate that N-quaternization could serve as a facile post-synthetic modification method to tune the chemical/physical properties of free pyridyl-containing MOFs.

A new (4,8)-connected Zr-MOF porous zirconium metal–organic framework (Zr-MOF) with flu topology, Zr6(μ3-O)4(μ3-OH)4(TCPS)2(H2O)4(OH)4 (1, TCPS = tetrakis(4-carboxyphenyl) silane) with a BET specific area of 1402 m2 g−1 has been constructed and fully characterized. 1 is stable in air and acid media but unstable in water and basic media, and thermally stable up to 200 °C. The new MOF is a wide band gap semiconductor with Eg = 3.95 eV. The excitation of 1 at 260 nm gives a ligand-based emission peak at 435 nm. After solvent exchange processes and activation at 200 °C, this MOF exhibits high storage capacities for H2, CH4 and CO2. We summarized the hydrothermal stability data of Zr-MOFs, calculated the NBO (natural bond orbital) charges of the coordinating oxygen atoms of the corresponding carboxylate ligands and analyzed the influencing factors. Besides the known reasons of hydrothermal stabilities of Zr-MOFs, we demonstrated that NBO charges of coordinating atoms of the ligands can be used to explain the hydrothermal stabilities of Zr-MOFs.

Based on the ligands tetrakis(4-carboxyphenyl)silane (TCS) and 4,4′-bipy, a novel 3D doubly interpenetrated Zn(II)-paddlewheel metal–organic framework Zn2TCS(4,4′-bipy) (1) was synthesized via facile conversion of its non-interpenetrated isomer (2) upon solvent removal at ambient temperature. It exhibits high CO2 and CH4 adsorption capacities and exceptional water stability.

Highlights•An uncommon 3D structure and a W-shape-like 2D layer structure have been reported.•Using both flexible and rigid ligands is a strategy for construction of these novel structures.•Changing the positions of imidazole group results in different structures.•The optical properties are related to the local linker environment.Two new Cd(II) coordination polymers, [Cd(BDC)(m-bitmb)(H2O)] (1) and [Cd2(BDC)2(p-bitmb)1.5(H2O)2] (2), have been solvothermally synthesized by reacting cadmium sulfate and 1,4-benzenedicarboxylic acid (H2BDC) with flexible ligands 1,3-bis(1-imidazol-1-ylmethyl)-2,4,6-trimethylbenzene (m-bitmb) or 1,4-bis(1-imidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (p-bitmb), respectively. Compound 1 shows an uncommon 3D 4-connected 66 topology, a distorted diamond structure, while compound 2 is a novel 2D (3,4)-connected W-shape-like layer framework with a novel (63·66) topology. 1 and 2 have different fluorescence and thermal properties. Compound 1 shows a ligand based fluorescence, while compound 2 shows a red-shifted emission compared to the ligands and is assigned to the ligand-to-ligand charge transfer (LLCT) emission. These results demonstrated the structure–property relationship of MOF material.Graphical abstractChanging the positions of imidazole group of the flexible ligands resulted in different structures and different luminescent behaviors.

A unique anionic 6-fold interpenetrated (10,3)-b Cd(II) framework was obtained using a novel nanosized tripodal aromatic acid ligand, in which both the Cd(II) atoms and tripodal aromatic acid ligands act as three-connected nodes. This compound displays intense and blue-shifted photoluminescence compared to that of the free H3L ligand.

A nonporous neutral framework [CuCl2(m-bttmb)2]n (1) was changed into a porous ionic {[Cu(m-bttmb)2(H2O)Cl]Cl(CH3CN)0.5(H2O)2.75}n (2) by simply increasing the amount of CH3CN in the mixed solvent (CH3CN and H2O) or temperature in the reactions of CuCl2·2H2O with 1,3-bis(triazol-1-ylmethyl)-2,4,6-trimethylbenzene (m-bttmb). 1 undergoes transformation into 2 when treated with CH3CN. Both 1 and 2 have 2D 4-connected (4,4) network architectures but in different packing arrangements. These compounds have been characterized by single-crystal X-ray diffraction analysis, elemental analysis, IR spectra and thermogravimetric analysis. This work may provide a way to control the formation of neutral or ionic frameworks, as well as porosities by adjusting the polarity and components of the solvents.

By using a rigid octadentenate carboxylate linker 5,5′,5″,5″′-silanetetrayltetraisophthalic acid (H8L), three new microporous coordination polymers, namely, [Cu4L(H2O)4]·2DMF·10H2O (1), [Zn5L2(H2O)4][NH2(CH3)2]6·2DMF·4H2O (2), and [Cd3(H2L)2][NH2(CH3)2]6·4DMF·14H2O (3), were solvothermally synthesized and structurally characterized. Compound 1 is a 3D framework built from a square paddlewheel [Cu2(O2CR)4] and a 8-connected cube L ligand to give a (4,8)-connected scu structure. Compound 2 is a 3D 4-nodal (4,4,4,8)-connected network with small 1D channels built on three tetrahedral Zn units and a 8-connecting ligand L. Compound 3 possesses a 3D structure with a (4,4,6)-connected net built on two tetrahedral Cd units and a 6-connecting ligand H2L6–. All the three compounds consist of large solvent accessible voids, but only compound 1 possesses permanent porosity as confirmed by N2, H2, O2, CO2 and CH4 gas adsorption measurements. Compounds 2 and 3 exhibited strong blue fluorescent emissions with a peak at 414 and 412 nm, respectively, at RT upon excitation owing to a ligand-centered excited state.

MOF-5 is an important metal–organic framework and has been intensely studied, especially in its hydrogen storage properties. In this study, we obtained the interpenetrated MOF-5 materials (MOF-5-int) using N,N′-dimethylformamide (DMF) or N,N′-diethylformamide (DEF) as solvents. The Langmuir surface area of MOF-5-int determined by N2 adsorption is 950–1100 m2 g−1, much lower than the non-penetrated MOF-5 (3000 m2 g−1). However, it can store 1.54–1.82 wt% by volumetric method hydrogen at 77 K and 1 atm, which is higher than the amount stored by the non-penetrated MOF-5. The MOF-5-int was also characterized by XRD-powder diffraction, thermogravimetric analysis (TGA), nitrogen adsorption/desorption analysis, scanning electron microscope (SEM) and X-ray single-crystal structure diffraction. In addition, we found grinding greatly facilitates the decomposition of the MOF-5-int material by H2O to a nonporous phase ZnBDC·xH2O (within 2–5 min, BDC = 1,4-benzenedicarboxylate), even under low humidity (30%), which calls for careful handling of the MOF-5 material. The effects of the water content, reaction time, reaction temperature, molar ratio of Zn(NO3)2 to H2BDC, addition of H2O2, rapid stirring and dilution on the synthesis of MOF-5-int were studied and the synthetic conditions were optimized. Moreover, Hafizovic et al. (J. Am. Chem. Soc., 2007, 129, 3612) found the intensity ratio of the powder XRD peak at 9.7° to that at 6.8° (referred to as the R1 value) of MOF-5 can be used to predict its porosity. The lower the intensity ratio, the more porous it is. In this study, we showed that MOF-5-int can have a very low R1 value but also a low porosity. The low specific surface area (SSA) is mainly due to its interpenetrated structure instead of the entrapped zinc species or the mesopores in the material, as previously proposed in the literature, and associated with the characteristic, very strong peak at 13.8° in its XRD-powder diffraction pattern. A high R2 value (the ratio of the intensity of the peak at 13.8° to that at 6.8°) suggests an interpenetrated structure, especially when the R1 value is low. In addition, we found that although entrapped ZnO or solvent molecules can increase the R1 value, and a low R1 value implies no zinc species or solvent molecules entrapped in the MOF-5 framework, a high R1 value does not necessarily suggest the presence of entrapped molecules.

MOF-5 is an important metal–organic framework and has been intensely studied, especially in its hydrogen storage properties. In this study, we obtained the interpenetrated MOF-5 materials (MOF-5-int) using N,N′-dimethylformamide (DMF) or N,N′-diethylformamide (DEF) as solvents. The Langmuir surface area of MOF-5-int determined by N2 adsorption is 950–1100 m2 g−1, much lower than the non-penetrated MOF-5 (3000 m2 g−1). However, it can store 1.54–1.82 wt% by volumetric method hydrogen at 77 K and 1 atm, which is higher than the amount stored by the non-penetrated MOF-5. The MOF-5-int was also characterized by XRD-powder diffraction, thermogravimetric analysis (TGA), nitrogen adsorption/desorption analysis, scanning electron microscope (SEM) and X-ray single-crystal structure diffraction. In addition, we found grinding greatly facilitates the decomposition of the MOF-5-int material by H2O to a nonporous phase ZnBDC·xH2O (within 2–5 min, BDC = 1,4-benzenedicarboxylate), even under low humidity (30%), which calls for careful handling of the MOF-5 material. The effects of the water content, reaction time, reaction temperature, molar ratio of Zn(NO3)2 to H2BDC, addition of H2O2, rapid stirring and dilution on the synthesis of MOF-5-int were studied and the synthetic conditions were optimized. Moreover, Hafizovic et al. (J. Am. Chem. Soc., 2007, 129, 3612) found the intensity ratio of the powder XRD peak at 9.7° to that at 6.8° (referred to as the R1 value) of MOF-5 can be used to predict its porosity. The lower the intensity ratio, the more porous it is. In this study, we showed that MOF-5-int can have a very low R1 value but also a low porosity. The low specific surface area (SSA) is mainly due to its interpenetrated structure instead of the entrapped zinc species or the mesopores in the material, as previously proposed in the literature, and associated with the characteristic, very strong peak at 13.8° in its XRD-powder diffraction pattern. A high R2 value (the ratio of the intensity of the peak at 13.8° to that at 6.8°) suggests an interpenetrated structure, especially when the R1 value is low. In addition, we found that although entrapped ZnO or solvent molecules can increase the R1 value, and a low R1 value implies no zinc species or solvent molecules entrapped in the MOF-5 framework, a high R1 value does not necessarily suggest the presence of entrapped molecules.

A nonporous neutral framework [CuCl2(m-bttmb)2]n (1) was changed into a porous ionic {[Cu(m-bttmb)2(H2O)Cl]Cl(CH3CN)0.5(H2O)2.75}n (2) by simply increasing the amount of CH3CN in the mixed solvent (CH3CN and H2O) or temperature in the reactions of CuCl2·2H2O with 1,3-bis(triazol-1-ylmethyl)-2,4,6-trimethylbenzene (m-bttmb). 1 undergoes transformation into 2 when treated with CH3CN. Both 1 and 2 have 2D 4-connected (4,4) network architectures but in different packing arrangements. These compounds have been characterized by single-crystal X-ray diffraction analysis, elemental analysis, IR spectra and thermogravimetric analysis. This work may provide a way to control the formation of neutral or ionic frameworks, as well as porosities by adjusting the polarity and components of the solvents.

A new (4,8)-connected Zr-MOF porous zirconium metal–organic framework (Zr-MOF) with flu topology, Zr6(μ3-O)4(μ3-OH)4(TCPS)2(H2O)4(OH)4 (1, TCPS = tetrakis(4-carboxyphenyl) silane) with a BET specific area of 1402 m2 g−1 has been constructed and fully characterized. 1 is stable in air and acid media but unstable in water and basic media, and thermally stable up to 200 °C. The new MOF is a wide band gap semiconductor with Eg = 3.95 eV. The excitation of 1 at 260 nm gives a ligand-based emission peak at 435 nm. After solvent exchange processes and activation at 200 °C, this MOF exhibits high storage capacities for H2, CH4 and CO2. We summarized the hydrothermal stability data of Zr-MOFs, calculated the NBO (natural bond orbital) charges of the coordinating oxygen atoms of the corresponding carboxylate ligands and analyzed the influencing factors. Besides the known reasons of hydrothermal stabilities of Zr-MOFs, we demonstrated that NBO charges of coordinating atoms of the ligands can be used to explain the hydrothermal stabilities of Zr-MOFs.

Three water-stable isostructural metal–organic frameworks (MOFs) of the general formula [M2(TCS)(BPY)] (M = Co(1), Ni(2) and Cu(3); H4TCS = tetrakis(4-carboxyphenyl) silane, BPY = 4,4′-bipyridine) were synthesized and fully characterized. MOFs 1–3 are stable in pH = 5–11, 2–11, 3–11 aqueous solution respectively for at least 24 h at room temperature. Although H4TCS absorbs only UV light, MOFs 1–3 absorb both UV and visible light in broad ranges (250–800 nm) and absorb more visible light than the ligand BPY. The rapid anodic photocurrent responses of MOFs 1–3 under UV and visible light illumination were observed. The photocurrent densities increase in the order of MOF 3 < 1 < 2 under visible light illumination (430 nm). The band gaps of MOFs 1–3 determined based on UV-Vis diffuse reflectance spectra and electrochemical (EC) analysis are 1.28, 1.35 and 0.67 eV, respectively. MOF 1 is able to photocatalyze the reduction of CO2 to CH4 under visible light, producing CH4 (1.44 μmol g−1 in 8 h), which is unprecedented in MOFs. The catalytic activity of MOF 1 (0.75 μmol g−1 after 4 h) under the irradiation of a 300 W xenon lamp is significantly better than those of MOFs 2 and 3 (0.14 μmol g−1 after 4 h). The band structures, density of states and band gaps of MOFs 1–3 were calculated by the GGA-PBE and GGA-PBE+U method implemented in VASP code. The calculations show that all the three compounds can be viewed as bulk intermediate band (IB) materials. The density of states of the IB in MOF 1 is high, which could suppress the non-radiative recombination. The density of states of the IB levels in MOFs 2 and 3 are low, making these levels very effective recombination centres, thus jeopardizing the photocatalytic activities of MOFs 2 and 3. The calculated results are in good agreement with experimental results and explain the photocatalytic activity differences. This study is the first to successfully address the question of how the types of unpaired electron containing electron-rich metal ions (i.e. Cu(II), Co(II), Ni(II)) affect the band gaps and band structures of MOFs and thus their photoelectronic properties.

A new UiO-67-type zirconium metal organic framework (MOF) material UiO-67-bpy-Me (bpy = 2,2-bipyridine-4,4′-dicarboxylic acid, Me = methyl) was prepared by N-quaternization of the pyridine sites in UiO-67-bpy. After N-quaternization, the pristine neutral framework turned cationic while its high thermal and chemical stabilities were primarily preserved. Fast and enhanced anionic dye adsorption was observed in UiO-67-bpy-Me. In addition, despite the decrease in surface area and pore volume, UiO-67-bpy-Me exhibited an evident increase in CO2 uptake compared to UiO-67-bpy. The enhancement was ascribed to the strong interactions between CO2 and the N-quaternized framework. More importantly, as the N-quaternization has changed the electronic structure of the organic linker. UiO-67-bpy-Me showed optical absorption up to ca. 800 nm with a large red shift of 450 nm compared to the pristine UiO-67-bpy (ca. 350 nm). The extended optical absorption may lead to more efficiency in light utilization. A proof-of-concept demonstration showed that UiO-67-bpy-Me could more efficiently catalyze methyl-orange degradation under UV-Vis light irradiation than the pristine UiO-67-bpy. These findings demonstrate that N-quaternization could serve as a facile post-synthetic modification method to tune the chemical/physical properties of free pyridyl-containing MOFs.