Synthesis and proton-conductive behaviour of two MOFs with covalently bonded imidazoles in the channels.

Immobilization of imidazole molecules as proton carriers into MOFs to facilitate proton conduction is a general strategy for developing high proton conductive materials. Herein, we designed two imidazole substituted phthalic acid ligands and constructed two novel MOFs, {[Zr6(OH)16(H3L1)4]Cl8·20H2O}n [Zr-MOF; H3L1 = 2-(1H-imidazol-4-yl) methylaminoterephthalic acid] and {Gd(HCOO)(H2L2)2}n [Gd-MOF; H3L2 = 5-(1H-imidazol-4-yl)methylaminoisophthalic acid] and fully studied their porous nature, stability and water-assisted proton conduction. The resulting Zr-MOF exhibits a high proton conductivity of 1.82 × 10-2 S cm-1 at 98% RH and 80 °C, while Gd-MOF has a proton conductivity of 3.01 × 10-3 S cm-1 at 98% RH and 60 °C.

Anchoring of Fe-MIL-101-NH2 to the Polymer Membrane Matrix through the Hinsberg Reaction to Promote Conductivity of SPEEK Membranes.

SCPEEK@MOF proton exchange membranes, where SCPEEK is sulfinyl chloride polyether ether ketone and MOF is a metal-organic framework, were prepared by doping Fe-MIL-101-NH2 into polymers. The amino group in the MOF and the -SOCl2 group in thionyl chloride polyether ether ketone cross-link to form a covalent bond through the Hinsberg reaction, and the prepared composite membrane has stronger stability than other electrostatic interactions and simple physical doping composite membranes. The formation of covalent bonds improves the water absorption of the composite membrane, which makes it easy for water molecules to form hydrogen bonds. Moreover, SPEEK as a proton conductive polymer and the synergy of MOFs improve the proton conductivity of composite membranes. The composite membranes were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The swelling rate, water absorption, mechanical stability, ion exchange capacity, and proton conductivity of the pure sulfonated polyether ether ketone (SPEEK) membrane were compared with those of the mechanically doped SPEEK/MOF membrane and the composite membrane SCPEEK@MOF doped with different ratios of Fe-MIL-101-NH2, and all of the SCPEEK@MOF showed superior performance. When the Fe-MIL-101-NH2 loading rate of the composite membrane is 2%, the proton conductivity of the composite membrane can reach 0.202 S cm-1 at 363 K and a 98% relative humidity, which is much higher than that of the SPEEK/MOF membrane obtained by simple physical doping under the same conditions.

High Proton Conductivity of the UiO-66-NH2-SPES Composite Membrane Prepared by Covalent Cross-Linking.

A sulfonated poly(ethersulfone) (SPES)-metal-organic framework (MOF) film with excellent proton conductivity was synthesized by anchoring UiO-66-NH2 to the main chain of the aromatic polymer through the Hinsberg reaction. The chemical bond was formed between the amino group in MOFs and the -SO2Cl group in chlorosulfonated poly(ethersulfones) to conduct protons in the proton channel of the membrane, making the membrane have excellent proton conductivity. UiO-66-NH2 is successfully prepared as a result of the consistency of the experimental and simulated powder X-ray diffraction (PXRD) patterns of MOFs. The existence of absorption peaks of characteristic functional groups in Fourier transform infrared (FTIR) spectra proved the successful preparation of SPES, PES-SO2Cl, and a composite film. The results of the AC impedance test indicate that the composite film with a 3% mass fraction has the best proton conductivity of 0.215 S·cm-1, which is 6.2 times higher than that of the blended film without a chemical bond at 98% RH and 353 K. To our knowledge, there are rarely any reports on the preparation of a composite membrane by directly linking MOFs and the membrane matrix with chemical bonds. This work provides a good way to synthesize the highly conductive proton exchange film.

Pyrazine-Functionalized Donor-Acceptor Covalent Organic Frameworks for Enhanced Photocatalytic H2 Evolution with High Proton Transport.

The well-defined 2D or 3D structure of covalent organic frameworks (COFs) makes it have great potential in photoelectric conversion and ions conduction fields. Herein, a new donor-accepter (D-A) COF material, named PyPz-COF, constructed from electron donor 4,4',4″,4'″-(pyrene-1,3,6,8-tetrayl)tetraaniline and electron accepter 4,4'-(pyrazine-2,5-diyl)dibenzaldehyde with an ordered and stable π-conjugated structure is reported. Interestingly, the introduction of pyrazine ring endows the PyPz-COF a distinct optical, electrochemical, charge-transfer properties, and also brings plentiful CN groups that enrich the proton by hydrogen bonds to enhance the photocatalysis performance. Thus, PyPz-COF exhibits a significantly improved photocatalytic hydrogen generation performance up to 7542 µmol g-1 h-1 with Pt as cocatalyst, also in clear contrast to that of PyTp-COF without pyrazine introduction (1714 µmol g-1 h-1 ). Moreover, the abundant nitrogen sites of the pyrazine ring and the well-defined 1D nanochannels enable the as-prepared COFs to immobilize H3 PO4 proton carriers in COFs through hydrogen bond confinement. The resulting material has an impressive proton conduction up to 8.10 × 10-2 S cm-1 at 353 K, 98% RH. This work will inspire the design and synthesis of COF-based materials with both efficient photocatalysis and proton conduction performance in the future.

Bifunctional Metal-Organic Framework Functionalized by Dimethylamine Cations: Proton Conduction and Iodine Vapor Adsorption.

A metal-organic framework, {Zn3(BTB)2(µ3-OH)[(CH3)2NH2](H2O)}n (1), was synthesized based on H3BTB (1,3,5-tri(4-carboxyphenyl)benzene). An AC impedance test proves that 1 has a relatively high conductivity performance of 1.52 × 10-3 S·cm-1 at 338 K and 98% RH. The proton conductivity of the composite film 1@CS-9 (CS = chitosan) reaches 1.84 × 10-1 S·cm-1 at 328 K and 98% RH. In addition, 1 is discovered to have a good adsorption effect on iodine vapor, and the adsorption capacity reaches 726 mg·g-1. The multifunctionality caused by dimethylamine cations was investigated for the first time, which has implications for multifunctionality generated by host-guest molecules.

Slow magnetic relaxation in a Dy3 triangle and a bistriangular Dy6 cluster.

Two lanthanide single-molecule magnets (SMMs) [Dy3(µ3-OH)(HL-1)3(H2O)3](NO3)2·3H3O (1, H3L-1 = (E)-3-(((8-hydroxyquinolin-2-yl)methylene)amino)propane-1,2-diol) and [Dy6(µ3-OH)4(H2L-2)4(HL-2)2(L-2)2] (2, H3L-2 = (E)-2-hydroxy-N'-(2-hydroxy-3-methoxybenzylidene)benzohydrazide) were synthesized and characterized structurally and magnetically. Complex 1 contains a triangular Dy3 core in which the three Dy3+ ions share a µ3-OH- anion and the deprotonated ligands of (HL-1)2- serve both capping and bridging functions, while 2 displays a centrosymmetric hexanuclear DyIII structure with two similar Dy3 triangular cores ligated by two fully deprotonated (L-2)3- ligands, each of which shares two µ3-OH- anions. All the DyIII ions are eight-coordinated with quasi D2d or C2v symmetry. Magnetic studies reveal that 1 exhibited two-step magnetic relaxation under an applied dc field of 800 Oe, with effective energy barriers of 40.1 and 31.0 K for the slow relaxation (SR) and fast relaxation regimes (FR), respectively. Meanwhile, 2 only showed a tail of slow magnetic relaxation at above 2 K. Ab initio calculations have been carried out to show the nature of their different magnetic properties.

Enantiomorphic Single Crystals of Linear Lead(II) Bromide Perovskitoids with White Circularly Polarized Emission.

Chiroptical hybrid organic-inorganic perovskites are emerging as a new class of promising materials with mirror optical signal responses for optoelectronic applications. However, chiroptical white-emission materials have been scarcely unearthed. Herein, four pairs of hybrid lead(II) bromide perovskitoids were obtained, namely, (R)- and (S)-(H2 MPz)PbBr4 (R/S-MPz=(R)-(-)/(S)-(+)-2-methylpiperazine) (1 and 2), (R)- and (S)-(H2 MPz)3 Pb2 Br10 ⋅2 DMAc (3 and 4), (R)- and (S)-(H2 MPz)PbBr4 ⋅0.5 MeCN (5 and 6) and (R)- and (S)-(H2 MPz)2 Pb2 Br8 ⋅DCM (7 and 8). Notably, they all exhibit ultrabroadband emission and chiroptical signals. Perovskitoids 3-6 even achieve white circularly polarized emission with a high dissymmetric factor (glum ) (±3×10-3 for 3 and 4; ±8×10-3 for 5 and 6). This new type of hybrid perovskitoids will attract attention and find applications in chiroptical fields because of the extensively and easily tunable photophysical properties.

A dual-function Cd-MOF with high proton conduction and excellent fluorescence detection of pyridine.

Metal-organic frameworks have great potential in the field of proton conducting materials and fluorescent probes due to their structural tunability and designability. A novel water-stable metal organic framework material [Cd2(Hdpb)(H2O)3] (Cd-MOF) was synthesized based on H5dpb (H5dpb = 3,5-diphosphonobenzoic acid) and Cd2+ ions. Cd2+ ions are connected with phosphonates and carboxyl groups of H5dpb to form an infinitely extended 1D chain, which is further connected by the Hdpb4- ligand and coordinated water to form a three-dimensional network structure. There are hydrogen bond networks in the 3D structure of the Cd-MOF, which are favorable for proton transfer, achieving its maximum proton conductivity of 2.97 × 10-3 S cm-1 at 338 K and 98% relative humidity (RH). To realize its application in fuel cells, the Cd-MOF was introduced into the chitosan (CS) matrix, and a series of composite membranes (Cd-MOF@CS-X) with high proton conductivity were obtained. The results of AC impedance show that the proton conductivity of Cd-MOF@CS-5 reaches 3.55 × 10-1 S cm-1 at 358 K and 98% RH, which is comparable to the highest values reported for MOF-polymer complexes. Moreover, the Cd-MOF can be used as a selective fluorescent probe for pyridine detection, and its detection limit can reach 1.0 × 10-6 M. A bifunctional MOF with proton conduction and pyridine recognition is reported for the first time, and has important reference value for the practical application of functional MOFs in both electrochemical and luminescence sensing.

Designable Guest-Molecule Encapsulation in Metal-Organic Frameworks for Proton Conductivity.

Metal-organic frameworks (MOFs), as a porous frame material, exhibit considerable electrical conductivity. In recent decades, research on the proton conductivity of MOFs has made gratifying progress. In this review, the designable guest molecules encapsulated into MOFs are summarized and generalized into four types in terms of promoting proton conductive performance, and then recent progress in the promotion of proton conductivity by MOFs encapsulating guest molecules is discussed. The existing challenges and prospects for the development of this strategy for promoting MOFs' proton conductivity are also listed.

Dual-Functional Coordination Polymer with High Proton Conductivity and a Low-Detection-Limit Fluorescent Probe.

A coordination polymer with dual functions of high proton conductivity and highly sensitive fluorescent sensors demonstrates a great application potential. In this work, a cadmium-based coordination polymer (denoted as CP 1) with hydrothermal stability was synthesized. The abundant coordination water, lattice water, and amino groups make an extended hydrogen-bonding pathway for efficient proton migration, which endows CP 1 with the highest proton conductivity of 2.41 × 10-3 S·cm-1 at 353 K and 98% RH. Especially, the proton conductivity of the chitosan (CS) hybrid membrane containing CP 1 reaches a maximum value of 2.62 × 10-2 S·cm-1 under 343 K and 98% RH, which increases almost 7 times higher than that of the pure CS membrane due to the host-guest collaboration. Furthermore, luminescence studies revealed that CP 1 is a high-sensitivity and good-selectivity fluorescent probe for the detection of trace amounts of l-histidine with a lowest detection limit of 1.0 × 10-8 M.

Room-temperature phosphorescence of manganese-based metal halides.

Room-temperature phosphorescent (RTP) materials can be used in anti-counterfeiting, organic light-emitting diodes and displays. However, designing RTP materials with a long luminescence lifetime and high solid-state emission efficiency is still a challenge. Due to the strong quantum confinement effect and the hydrogen bond network structure formed by polyamino sites, 0D RTP materials usually have a higher fluorescence quantum yield and longer phosphorescence lifetime. Here, we synthesized four manganese-based metal halides of different dimensions with a long lifetime and high luminous efficiency by changing organic cations: {[H2DAP]MnCl4}n (1, DAP = 1,3-propanediamine, 2D), {[(H2MELA)2MnCl5]Cl}n (2, MELA = melamine, 1D), [H2TAP]2MnCl6 (3, TAP = 2,4,6-triaminopyrimidine, 0D) and [H2MXD]2MnCl6 (4, MXD = m-xylylenediamine, 0D). [H2MXD]2MnCl6 (4) has a long lifetime (11 ms) and the maximum photoluminescence quantum yield (31.05%). Our work provides a new procedure for the development of RTP materials with high quantum yields.

High Proton Conductivity of a Cadmium Metal-Organic Framework Constructed from Pyrazolecarboxylate and Its Hybrid Membrane.

A new type of metal-organic framework, [Cd2(pdc)(H2O)(DMA)2]n (pdc = 3,5-pyrazoledicarboxylic acid; DMA = dimethylamine), named Cd-MOF, was synthesized and characterized. There are regular rectangular pore channels containing a large number of dimethylamine cations in the crystal structure. AC impedance test results show the proton conductivity of Cd-MOF reaches 1.15 × 10-3 S cm-1 at 363 K and 98% RH. In order for its application in fuel cells, the Cd-MOF was introduced into a sulfonated polyphenylene oxide matrix to prepare a hybrid membrane, and the proton conductivity of the hybrid membrane has a high value of 2.64 × 10-1 S cm-1 at 343 K and 98% RH, which is higher than those of most MOF polymer hybrid membranes. The proton conductivity of the hybrid membrane of the SPPO polymer still maintains a certain degree of stability in a wide temperature range. To the best of our knowledge, it is the first proton exchange membrane that combines pyrazolecarboxylate cadmium MOFs and an SPPO polymer with high proton conductivity and good stability. This research may help to further develop the application of MOFs in the field of proton exchange membrane fuel cells.

Promotion of Proton Conductivity by Encapsulation of Metal-Organic Polyhedra in Metal-Organic Frameworks.

A Zr-based metal-organic polyhedron (MOP) was self-assembled in a porous MOF host, DUT-68, successfully to synthesize MOP-1@DUT-68. The MOP guest (MOP-1) has a diameter of about 20 Å, larger than that of the square windows (pore sizes of ∼14 Å) of DUT-68 but smaller than that of the rhombicuboctahedral cage (27.7 Å), which means that the migration and leaching of MOP-1 could be effectively prohibited if MOP-1 is encapsulated in the MOF's cavities. The proton conductivity of MOP-1@DUT-68 is 1.14×10-3  S cm-1 (at 80 °C under 98 % relative humidity), which is three orders of magnitude higher than that of DUT-68. Compared with MOP-1⊂DUT-68, which was synthesized by impregnation, MOP-1@DUT-68 is more prone to form faster proton-conduction pathways and thus provides higher proton conductivity.

Dual-functional coordination polymers with high proton conduction behaviour and good luminescence properties.

Two coordination polymers, [M(5-hip)(H2O)3]n (M = Cd2+ (1), Zn2+ (2), 5-hip = 5-hydroxyisophthalic acid), have been synthesized under hydrothermal conditions. The crystal structure reveals that complexes 1 and 2 have 1D chain structures by the coordination of metal ions and 5-hip. 1D chains are connected by hydrogen bonds to form a 3D structure. AC impedance analysis shows that the proton conductivity of complexes 1 and 2 comes up to 1.58 × 10-3 S cm-1 (98%RH, 343 K) and 5.27 × 10-4 S cm-1 (98%RH, 353 K), respectively. To further improve the proton conductivity, a hybrid membrane was prepared by the solution casting method with complexes as fillers and sulfonated polyether ether ketone (SPEEK) as the organic matrix. The proton conductivity of hybrid membranes 1@SPEEK-5 and 2@SPEEK-5 is 1.97 and 1.58 times higher than that of pure SPEEK membranes, respectively. Furthermore, the two complexes are excellent fluorescent sensors, which could detect Cr2O72- in aqueous solution with high sensitivity and selectivity. Both of them have low detection limits for Cr2O72- in aqueous solution, where the detection limit of complex 1 is 0.8 µM and that of complex 2 is 1 µM. The above work demonstrates that the two complexes are dual-functional materials with high proton conduction and good fluorescence properties.

Concomitant Photoresponsive Chiroptics and Magnetism in Metal-Organic Frameworks at Room Temperature.

Stimulus-responsive metal-organic frameworks (MOFs) can be used for designing smart materials. Herein, we report a family of rationally designed MOFs which exhibit photoresponsive chiroptical and magnetic properties at room temperature. In this design, two specific nonphotochromic ligands are selected to construct enantiomeric MOFs, {Cu2(L-mal)2(bpy)2(H2O)·3H2O}n (1) and {Cu2(D-mal)2(bpy)2(H2O)·3H2O}n (2) (mal = malate, bpy = 4, 4' - bipyridine), which can alter their color, magnetism, and chiroptics concurrently in response to light. Upon UV or visible light irradiation, long-lived bpy- radicals are generated via photoinduced electron transfer (PET) from oxygen atoms of carboxylates and hydroxyl of malates to bpy ligands, giving rise to a 23.7% increase of magnetic susceptibility at room temperature. The participation of the chromophores (-OH and -COO-) bound with the chiral carbon during the electron transfer process results in a small dipolar transition; thus, the Cotton effects of the enantiomers are weakened along with a photoinduced color change. This work demonstrates that the simultaneous responses of chirality, optics, and magnetism can be achieved in a single compound at room temperature and may open up a new pathway for designing chiral stimuli-responsive materials.

A dual-functional MOF for high proton conduction and sensitive detection of ascorbic acid.

A new metal-organic framework (MOF), [Eu2(HBDPP)2(H2O)2(DMF)2](H2O)2 (H4BDPP = 3,5-bis(3,5-dicarboxylphenyl) pyridine; DMF = N,N-dimethylformamide) (Eu-MOF), has been successfully synthesized under solvothermal conditions. A 1D chain was formed by the adjacent Eu2(COO)24+ dinuclear cluster and HBDPP3-, and further connected by HBDPP3- to form an infinitely extended 3D structure. In order to further improve the proton conductivity of the Eu-MOF, imidazole was encapsulated in its pores to form a composite material named Im@Eu-MOF. AC impedance analysis shows that the highest proton conductivity of the Im@Eu-MOF reaches up to 4.53 × 10-4 S cm-1 at 348 K and 98% RH, which is about 10 times higher than that of the Eu-MOF. In addition, the Eu-MOF can be considered an excellent luminescence-based sensor with a high sensitivity and low detection limit (0.1 µM) for the detection of trace amounts of ascorbic acid.

High proton conductivity in a nickel(ii) complex and its hybrid membrane.

A novel nickel(ii) complex, [Ni(2,2'-bpy)3](H2SBTC)(H2O)5 (1) (2,2'-bpy = 2,2'-bipyridyl), was synthesized based on [Ni(2,2'-bpy)3]2+ discrete units and a partially deprotonated sulfonate-carboxylate ligand of H4SBTC (5-sulfonyl-1,2,4-benzentricarboxylic acid). The adjacent [Ni(2,2'-bpy)3]2+ cationic units were connected to H2SBTC2- anions by hydrogen bonds to form a one-dimensional (1-D) chain. The 1-D chains were further linked using hydrogen bonds from free water molecules and H2SBTC2- anions to generate a two-dimensional (2-D) plane structure. The proton conductivity of 1 reaches 1.3 × 10-4 S cm-1 at 318 K, 98% RH. Furthermore, hybrid membranes composed of polyvinyl alcohol (PVA) and 1 as fillers were prepared with different mass percentages. The proton conductivity of 1@PVA-5 was found to be 1.6 × 10-3 S cm-1 at 313 K and 98% RH. The large number of alcoholic hydroxyl groups in PVA promote the resulting conductivity.

Photoinduced electron transfer and remarkable enhancement of magnetic susceptibility in bridging pyrazine complexes.

Three isostructural coordination polymers of the formula [MCl2(DMSO)2(pz)]n (pz = pyrazine, DMSO = dimethyl sulfoxide, M = Mn, Fe and Co for 1-3) were synthesized through the solvothermal reaction of MCl2, DMSO and pz. By introducing DMSO and chloride as electron donor ligands and pz as an accepter, a donor-metal-accepter (D-M-A) system was constructed. Radicals generated by photoinduced electron transfer (PET) from a donor to an accepter result in a dramatic change of vibronic absorption spectra in UV regions and remarkable enhancement of magnetic susceptibility. The mechanism of PET was also determined by PXRD, IR, EPR and XPS measurements. This is the first example of bridging pz complexes which exhibit both photochromism and photomagnetism in a PET process.

2D europium coordination polymer as a regenerable fluorescence probe for efficiently detecting fipronil.

Recently, considerable attention has been paid to European eggs that were found to contain the poisonous insecticide fipronil. Excessive consumption of fipronil can harm the human body, in particular, the liver, thyroid, nervous system and kidneys. As a consequence, there is an urgent need to develop a method to detect fipronil. In this work, a lanthanide coordination polymer [Eu(SIP)(H2O)4]n (1) (NaH2SIP = 5-sulfoisophthalic acid monosodium salt) was synthesized and characterized by single-crystal X-ray crystallography, elemental analysis, infrared spectrum and powder X-ray diffractions. The X-ray studies show that 1 is a two-dimensional layered structure, which is constructed of fused {[Eu(H2O)4]3(SIP)3} pseudo-hexagonal grids. The luminescence properties of 1 exhibit effective recognition for fipronil in methanol solution. Furthermore, a wide linear range (10-6-10-4 M), a low detection limit of 0.8 µM, high selectivity and excellent recyclability, reveal that 1 can potentially act as a luminescence-based sensor for quantitative and highly sensitive detection of fipronil. To the best of our knowledge, it is the first time that a metal-organic coordination polymer, as a fluorescent-based probe, has been used for the detection of fipronil. Moreover, fluorescent-based test papers of 1 have also been prepared, which can be regarded as a simple and practical method for detecting fipronil.

Alkylamine-Templated Niccolite Frameworks of [GaIIIMII(HCOO)6]- (M = Fe, Ni): Structure, Magnetism, and Dielectricity.

The five heterometallic formate frameworks [EtA][GaIIIFeII(HCOO)6] (1; EtA = CH3CH2NH3+), [DMA][GaIIIFeII(HCOO)6] (2; DMA = (CH3)2NH2+), [DEtA][GaIIIFeII(HCOO)6] (3; DEtA = (CH3CH2)2NH2+), [MA][GaIIINiII(HCOO)6] (4; MA = CH3NH3+), and [DMA][GaIIINiII(HCOO)6] (5) were synthesized through solvothermal methods. Complexes 1-5 are isotructural, and all crystallize in the trigonal P3̅1 c space group. Each metal center is 6-connected, with each HCOO- bridging ligand in an anti-anti mode to build a three-dimensional niccolite-like architecture. All of the complexes exhibit weak ferromagnetism at low temperature. A variable-temperature (VT) dielectric study indicates that the dielectric anomaly is induced by the freezing of motions from the protonated amines during the freezing process.