Proton‐Conducting Magnetic Coordination Polymers

Three isostructural lanthanide‐based two‐ dimensional coordination polymers (CPs) {[Ln₂(L)₃(H₂O)₂]_n ? 2n CH₃OH) ? 2n H₂O} (Ln=Gd³⁺ ( 1 ), Tb³⁺ ( 2 ), Dy³⁺ ( 3 ); H₂L=cyclobutane‐1,1‐dicarboxylic acid) were synthesized by using a low molecular weight dicarboxylate ligand and characterized. Single‐crystal structure analysis showed that in complexes 1 – 3 lanthanide centers are connected by μ₃‐bridging cyclobutanedicarboxylate ligands along the c axis to form a rod‐shaped infinite 1D coordination chain, which is further linked with nearby chains by μ₄‐connected cyclobutanedicarboxylate ligands to form 2D CPs in the bc plane. Viewing the packing of the complexes down the b axis reveals that the lattice methanol molecules are located in the interlayer space between the adjacent 2D layers and form H‐bonds with lattice and coordinated water molecules to form 1D chains. Magnetic properties of complexes 1 – 3 were thoroughly investigated. Complex 1 exhibits dominant ferromagnetic interaction between two nearby gadolinium centers and also acts as a cryogenic magnetic refrigerant having a significant magnetic entropy change of ?ΔS_m=32.8 J kg^?1 K^?1 for ΔH=7 T at 4 K (calculated from isothermal magnetization data). Complex 3 shows slow relaxation of magnetization below 10 K. Impedance analysis revealed that the complexes show humidity‐dependent proton conductivity (σ=1.5×10^?5 S cm^?1 for 1 , σ=2.07×10^?4 S cm^?1 for 2 , and σ=1.1×10^?3 S cm^?1 for 3 ) at elevated temperature (>75 °C). They retain the conductivity for up to 10 h at high temperature and high humidity. Furthermore, the proton conductivity results were correlated with the number of water molecules from the water‐vapor adsorption measurements. Water‐vapor adsorption studies showed hysteretic and two‐step water vapor adsorption (182000 μL g^?1 for 1 , 184000 μL g^?1 for 2 , and 1874000 μL g^?1 for 3 ) in the experimental pressure range. Simulation of water‐vapor adsorption by the Monte Carlo method (for 1 ) confirmed the high density of adsorbed water molecules, preferentially in the interlayer space between the 2D layers.     

Ion‐selective Electrodes with 3D Nanostructured Conducting Polymer Solid Contact

Until now both ion‐to‐electron transducers as well as large surface area nanostructured conducting materials were successfully used as solid contacts for polymer‐based ion‐selective electrodes. We were interested to explore the combination of these two approaches by fabricating ordered electrically conducting polymer (ECP) nanostructures using 3D nanosphere lithography and electrosynthesis to provide a high surface area and capacitive interface for solid contact ion‐selective electrodes (SC‐ISEs). For these studies we used poly(3,4‐ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT(PSS)) films with 750 nm diameter interconnected pores as the intermediate layer between a glassy carbon electrode and a Ag⁺ ‐selective polymeric membrane. We also investigated the feasibility of loading the voids created in the polymer film with a lipophilic redox mediator (1,1’‐dimethylferrocene) to provide the respective ISEs with well‐defined/controllable E⁰ values. These expectations were fulfilled as the standard deviation of E⁰ values were reduced with almost an order of magnitude for 3D nanostructured SC‐ISEs filled with the redox mediator as compared to their redox mediator‐free analogs. The detrimental effect of the redox mediator extraction into the plasticized PVC‐based ion‐selective membrane (ISM) was efficiently suppressed by replacing the PVC‐based ISMs with a low diffusivity silicone rubber matrix.     

Excellent Stability of a Lithium‐Ion‐Conducting Solid Electrolyte upon Reversible Li+/H+ Exchange in Aqueous Solutions

Batteries with an aqueous catholyte and a Li metal anode have attracted interest owing to their exceptional energy density and high charge/discharge rate. The long‐term operation of such batteries requires that the solid electrolyte separator between the anode and aqueous solutions must be compatible with Li and stable over a wide pH range. Unfortunately, no such compound has yet been reported. In this study, an excellent stability in neutral and strongly basic solutions was observed when using the cubic Li₇La₃Zr₂O₁₂ garnet as a Li‐stable solid electrolyte. The material underwent a Li⁺/H⁺ exchange in aqueous solutions. Nevertheless, its structure remained unchanged even under a high exchange rate of 63.6 %. When treated with a 2 M LiOH solution, the Li⁺/H⁺ exchange was reversed without any structural change. These observations suggest that cubic Li₇La₃Zr₂O₁₂ is a promising candidate for the separator in aqueous lithium batteries.     

pH‐Dependent Proton Conducting Behavior in a Metal–Organic Framework Material

A porous metal–organic framework (MOF), [Ni₂(dobdc)(H₂O)₂]?6 H₂O (Ni₂(dobdc) or Ni‐MOF‐74; dobdc^4?=2,5‐dioxido‐1,4‐benzenedicarboxylate) with hexagonal channels was synthesized using a microwave‐assisted solvothermal reaction. Soaking Ni₂(dobdc) in sulfuric acid solutions at different pH values afforded new proton‐conducting frameworks, H⁺@Ni₂(dobdc). At pH 1.8, the acidified MOF shows proton conductivity of 2.2×10^?2 S cm^?1 at 80 °C and 95 % relative humidity (RH), approaching the highest values reported for MOFs. Proton conduction occurs via the Grotthuss mechanism with a significantly low activation energy as compared to other proton‐conducting MOFs. Protonated water clusters within the pores of H⁺@Ni₂(dobdc) play an important role in the conduction process.     

The Diversity of Electron‐Transport Behaviors of Molecular Junctions: Correlation with the Electron‐Transport Pathway

We report the electron‐transport behaviors of a number of molecular junctions composed of π‐conjugated molecular wires. From calculations performed by using density functional theory (DFT) combined with the non‐equilibrium Green’s function (NEGF) method, we found that the length–conductivity relations are diverse, depending on the particular molecular structures. The results reveal that the conductance–length dependence follows an exponential law for many conjugated molecules with a single channel, such as oligothiophene, oligopyrrole and oligophenylene. Therefore, a quantitative relation between the energy gap (E_g)_∞ of the molecular wire and the attenuation factor β can be defined. However, when the molecular wires have multichannels, the decay of conductance does not follow the exponential relation. For example, the conductance of porphyrin‐based oligomers and fused thiophene decays almost linearly. The diversity of electron‐transport behaviors of molecular junctions is directly dominated by the electron‐transport pathway.     

Sulfophenylation of Polysulfones for Proton‐Conducting Fuel Cell Membranes

Polysulfone has been sulfophenylated by lithiation and anionic reaction with 2‐sulfobenzoic acid cyclic anhydride. This provides a new convenient method to modify polysulfones by attaching pendant sulfonated phenyl groups via ketone links. Membranes of the sulfophenylated polysulfones show promise for use in proton‐exchange‐membrane fuel cells. For example, a membrane with 0.9 sulfophenyl units per repeating polysulfone unit and 30 wt.‐% water was found to have a proton conductivity of 32 mS/cm at 60°C.     

Conducting Polymer‐Based Solid‐State Ion‐Selective Electrodes

Conducting polymers, i.e., electroactive conjugated polymers, are useful both as ion‐to‐electron transducers and as sensing membranes in solid‐state ion‐selective electrodes. Recent achievements over the last few years have resulted in significant improvements of the analytical performance of solid‐contact ion‐selective electrodes (solid‐contact ISEs) based on conducting polymers as ion‐to‐electron transducer combined with polymeric ion‐selective membranes. A significant amount of research has also been devoted to solid‐state ISEs based on conducting polymers as the sensing membrane. This review gives a brief summary of the progress in the area in recent years.     

Two‐Dimensional Mesoscale‐Ordered Conducting Polymers

Despite the availability of numerous two‐dimensional (2D) materials with structural ordering at the atomic or molecular level, direct construction of mesoscale‐ordered superstructures within a 2D monolayer remains an enormous challenge. Here, we report the synergic manipulation of two types of assemblies in different dimensions to achieve 2D conducting polymer nanosheets with structural ordering at the mesoscale. The supramolecular assemblies of amphipathic perfluorinated carboxylic acids and block co‐polymers serve as 2D interfaces and meso‐inducing moieties, respectively, which guide the polymerization of aniline into 2D, free‐standing mesoporous conducting polymer nanosheets. Grazing‐incidence small‐angle X‐ray scattering combined with various microscopy demonstrates that the resulting mesoscale‐ordered nanosheets have hexagonal lattice with d‐spacing of about 30 nm, customizable pore sizes of 7–18 nm and thicknesses of 13–45 nm, and high surface area. Such template‐directed assembly produces polyaniline nanosheets with enhanced π–π stacking interactions, thereby resulting in anisotropic and record‐high electrical conductivity of approximately 41 S cm^?1 for the pristine polyaniline nanosheet based film and approximately 188 S cm^?1 for the hydrochloric acid‐doped counterpart. Our moldable approach creates a new family of mesoscale‐ordered structures as well as opens avenues to the programmed assembly of multifunctional materials.     

Proton‐conducting polymers based on benzimidazoles and sulfonated benzimidazoles

A sulfonated derivative of polybenzimidazole is reported, and its properties are analyzed in comparison with related polybenzimidazole proton‐conducting materials. Poly(2,5‐benzimidazole), poly(m‐phenylenebenzobisimidazole), and poly[m‐(5‐sulfo)‐phenylenebenzobisimidazole] were prepared by condensation of the corresponding monomers in polyphosphoric acid. Several adducts of these polymers with phosphoric acid were prepared. The resulting materials were characterized by chemical analysis, Fourier transform infrared spectroscopy, and thermogravimetric analysis; also, the dc conductivity of doped and undoped derivatives was measured. Similar to what has been observed for the commercial polybenzimidazole polymer (also examined here for comparison), the title polymers exhibit high thermal stability. Furthermore, their doping with phosphoric acid leads to a significant increase in conductivity from less than 10^?11 Scm^?1 for the undoped polymers to 10^?4 Scm^?1 (both at room temperature) for their acid‐loaded derivatives. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3703–3710, 2002     

Bioinspired Radical Stetter Reaction: Radical Umpolung Enabled by Ion‐Pair Photocatalysis

A bioinspired, intermolecular radical Stetter reaction of α‐keto acids and aldehydes is disclosed that is contingent on a formal “radical umpolung” concept. Enabled by secondary amine activation, electrostatic recognition ensures that the α‐ketocarboxylic acids, which function as latent acyl radicals, are proximal to the in situ generated iminium salts. This photoactive contact ion pair is an electron donor–acceptor (EDA) complex, and undergoes facile single electron transfer (SET) and rapid decarboxylation prior to radical–radical recombination. Importantly, decarbonylation is mitigated by this strategy. The initial computational validation on which the process is predicated matches closely with experiment. Synergising organo‐ and photocatalysis activation principles finally expands the mechanistic and synthetic scope of the classic Stetter reaction to include α,β‐unsaturated aldehydes as acceptors.     

Direct Synthesis of Fully Sulfonated Polyarylenethioether Sulfones as Proton‐Conducting Polymers for Fuel Cells

Summary: SPTES polymers have been successfully synthesized by direct polymerization using tetramethylene sulfone as the solvent. The chemical structures of the SPTES polymers are confirmed by FT‐IR and NMR spectroscopy. The thermal stability is characterized by TGA, and the results show that the sulfonated groups on the polymer backbone are stable up to 300 °C. The measured proton conductivity reaches values above 300 mS · cm⁻¹ at 65 °C and 85% relative humidity. Tough, ductile, free‐standing membranes have been fabricated by solution casting from N,N‐dimethylacetamide, which indicates that the SPTES polymers have excellent membrane‐forming capability and mechanical property. The mono‐functional monomers are introduced into the polymerization to end‐cap the SPTES polymers. The end‐capping groups are effective in improving water resistance, oxidative stability, and retaining the proton conductivity.

Fully sulfonated polyarylenethioether sulfone.     

Ion‐Selective Electrodes Based on Hydrophilic Ionophore‐Modified Nanopores

We report the synthesis and analytical application of the first Cu²⁺‐selective synthetic ion channel based on peptide‐modified gold nanopores. A Cu²⁺‐binding peptide motif (Gly‐Gly‐His) along with two additional functional thiol derivatives inferring cation‐permselectivity and hydrophobicity was self‐assembled on the surface of gold nanoporous membranes comprising of about 5 nm diameter pores. These membranes were used to construct ion‐selective electrodes (ISEs) with extraordinary Cu²⁺ selectivities, approaching six orders of magnitude over certain ions. Since all constituents are immobilized to a supporting nanoporous membrane, their leaching, that is a ubiquitous problem of conventional ionophore‐based ISEs was effectively suppressed.     

A Stable Polyoxometalate‐Pillared Metal–Organic Framework for Proton‐Conducting and Colorimetric Biosensing

A stable metal–organic framework pillared by Keggin‐type polyoxometalate, Cu₆(Trz)₁₀(H₂O)₄[H₂SiW₁₂O₄₀]?8 H₂O (Trz=1,2,4‐triazole) ( 1 ), has been prepared under hydrothermal condition. The 2D layer structure with a 22‐member ring was formed by Cu²⁺ ions, which are connected with each other via the Trz ligands on the ab plane. Thus, the 2D layers are further interconnected through Keggin polyoxoanions to generate a 3D porous network with a small 1D channel. Moreover, the presence of polyoxoanions make it exhibit selective adsorption of water and proton‐conducting properties. Additionally it showed efficient intrinsic peroxidase‐like activity, providing a simple and sensitive colorimetric assay to detect H₂O₂.     

Ion‐Pair Recognition of Tetramethylammonium Salts by Halogenated Resorcinarenes

The non‐covalent interactions of different upper‐rim‐substituted C₂‐resorcinarenes with tetramethylammonium salts were analyzed in the gas phase in an Electrospray Ionization Fourier‐transform ion‐cyclotron‐resonance (ESI‐FTICR) mass spectrometer and by ¹H NMR titrations. The order of binding strengths of the hosts towards the tetramethylammonium cation in the gas phase reflects the electronic nature of the substituents on the upper rim of the resorcinarene. In solution, however, a different trend with particularly high binding constants for halogenated resorcinarenes has been observed. This trend can be explained by a synergetic effect originating from the interaction of the halogenated resorcinarenes with the counter anions through hydrogen bonding. This study highlights the importance of weak interactions in recognition processes and points out the benefits of comparing the gas‐phase data with results obtained from solution experiments.     

Electronic Properties of p‐Xylylene and p‐Phenylene Chains Subjected to Finite Bias Voltages: A New Highly Conducting Oligophenyl Structure

Recently, experimental and theoretical determination of electric currents induced by finite bias voltages in p‐xylylene chains attached to gold contacts revealed higher conductance of these systems in comparison with p‐phenylene homologous chains. To gain more insight into the conducting properties of these oligophenyl structures, ab initio studies were carried out on the electronic properties of two different p‐xylylene‐like chains (pX1 and pX2) and the p‐phenylene (pP) chain attached to gold contacts, with molecular formulas AuCH₂(C₆H₄)_nCH₂Au (n=1–5), Au₂C(C₆H₄)_nCAu₂ (n=1–5), and Au(C₆H₄)_nAu (n=1–5), respectively. The molecules were subjected to finite bias voltages ranging from 0 to 5 V. Analysis of the intramolecular electron transfer and electron delocalization revealed a completely opposite response to electric perturbation of pX2 in comparison with pX1 and pP. Thus, in pX2 the applied voltage causes an increase in the electron delocalization within the rings together with a large electron transfer and energetic stabilization. On the contrary, the same voltages partially destroy the electron delocalization in pX1 and pP, produce a large local electron polarization in the benzene rings, and a smaller energetic stabilization. These differences can be rationalized in terms of the role played by polarized valence bond structures in the total wave function. Theoretical estimation of the I/V profiles indicates that pX2 chains are much better electronic conductors than pX1 and pP.     

Optoelectronic Properties of Dicyanofluorene‐Based n‐Type Polymers

Three new donor–acceptor‐type copolymers ( P1 , P2 , P3 ) consisting of dicyanofluorene as acceptor and various donor moieties were designed and synthesized. Optoelectronic properties were studied in detail by means of UV‐visible absorption and fluorescence spectroscopy, cyclic voltammetry, space‐charge‐limited current (SCLC), flash‐photolysis time‐resolved microwave conductivity (FP‐TRMC), and density functional theory (DFT). All polymers showed strong absorption in the UV‐visible region and the absorption maximum undergoes redshift with an increasing number of thiophene units in the polymer backbone. SCLC analysis showed that the electron mobilities of the polymers in the bulk state were 1 to 2 orders higher than that of the corresponding hole mobilities, which indicated the n‐type nature of the materials. By using FP‐TRMC, the intrapolymer charge‐carrier mobility was assessed and compared with the interpolymer mobility obtained by SCLC. The polymers exhibited good electron‐accepting properties sufficiently high enough to oxidize the excited states of regioregular poly(3‐hexylthiophene) (P3HT (donor)), as evident from the FP‐TRMC analysis. The P3 polymer exhibited the highest FP‐TRMC transients in the pristine form as well as when blended with P3HT. Use of these polymers as n‐type materials in all‐polymer organic solar cells was also explored in combination with P3HT. In accordance with the TRMC results, P3 exhibited superior electron‐transport and photovoltaic properties to the other two polymers, which is explained by the distribution of the energy levels of the polymers by using DFT calculations.