Design of a Conducting Metal–Organic Framework: Orbital-Level Matching in MIL-140A Derivatives

On the basis of the results of first-principles band calculations, we report a strategy for the development of a conducting metal–organic framework (MOF). The charge carrier in a zirconium-based MOF, MIL-140A, is expected to be localized because of a mismatch of the energy levels of bridging ligands’ π* and Zr 4d orbitals. On the basis of the findings, we propose a candidate structure for a conducting MOF

The Role of a Three Dimensionally Ordered Defect Sublattice on the Acidity of a Sulfonated Metal–Organic Framework

Understanding the role that crystal imperfections or defects play on the physical properties of a solid material is important for any application. In this report, the highly unique crystal structure of the metal–organic framework (MOF) zirconium 2-sulfoterephthalate is presented. This MOF contains a large number of partially occupied ligand and metal cluster sites which directly affect the physical properties of the material. The partially occupied ligand positions give rise to a continuum of pore sizes within this highly porous MOF, supported by N₂ gas sorption and micropore analysis. Furthermore, this MOF is lined with sulfonic acid groups, implying a high proton concentration in the pore, but defective zirconium clusters are found to be effective proton trapping sites, which was investigated by a combination of AC impedance analysis to measure the proton conductivity and DFT calculations to determine the solvation energies of the protons in the pore. Based on the calculations, methods to control the p<i>K</i>_a of the clusters and improve the conductivity by saturating the zirconium clusters with strong acids were utilized, and a 5-fold increase in proton conductivity was achieved using these methods. High proton conductivity of 5.62 × 10^–3 S cm^–1 at 95% relative humidity and 65 °C could be achieved, with little change down to 40% relative humidity at room temperature

The Role of a Three Dimensionally Ordered Defect Sublattice on the Acidity of a Sulfonated Metal–Organic Framework

Understanding the role that crystal imperfections or defects play on the physical properties of a solid material is important for any application. In this report, the highly unique crystal structure of the metal–organic framework (MOF) zirconium 2-sulfoterephthalate is presented. This MOF contains a large number of partially occupied ligand and metal cluster sites which directly affect the physical properties of the material. The partially occupied ligand positions give rise to a continuum of pore sizes within this highly porous MOF, supported by N₂ gas sorption and micropore analysis. Furthermore, this MOF is lined with sulfonic acid groups, implying a high proton concentration in the pore, but defective zirconium clusters are found to be effective proton trapping sites, which was investigated by a combination of AC impedance analysis to measure the proton conductivity and DFT calculations to determine the solvation energies of the protons in the pore. Based on the calculations, methods to control the p<i>K</i>_a of the clusters and improve the conductivity by saturating the zirconium clusters with strong acids were utilized, and a 5-fold increase in proton conductivity was achieved using these methods. High proton conductivity of 5.62 × 10^–3 S cm^–1 at 95% relative humidity and 65 °C could be achieved, with little change down to 40% relative humidity at room temperature

An Electrically Conductive Single-Component Donor–Acceptor–Donor Aggregate with Hydrogen-Bonding Lattice

An electrically conductive D–A–D aggregate composed of a single component was first constructed by use of a protonated bimetal dithiolate (complex <b>1H</b>_<b>2</b>). The crystal structure of complex <b>1H</b>_<b>2</b> has one-dimensional (1-D) π-stacking columns where the D and A moieties are placed in a segregated-stacking manner. In addition, these segregated-stacking 1-D columns are stabilized by hydrogen bonds. The result of a theoretical band calculation suggests that a conduction pathway forms along these 1-D columns. The transport property of complex <b>1H</b>_<b>2</b> is semiconducting (<i>E</i>_a = 0.29 eV, ρ_rt = 9.1 × 10⁴ Ω cm) at ambient pressure; however, the resistivity becomes much lower upon applying high pressure up to 8.8 GPa (<i>E</i>_a = 0.13 eV, ρ_rt = 6.2 × 10 Ω cm at 8.8 GPa). The pressure dependence of structural and optical changes indicates that the enhancement of conductivity is attributed to not only an increase of π–π overlapping but also a unique pressure-induced intramolecular charge transfer from D to A moieties in this D–A–D aggregate

Use of Halogen Bonding in a Molecular Solid Solution to Simultaneously Control Spin and Charge

Halogen-bonding interactions have attracted increasing attention in various fields of molecular science. Here we report the first comprehensive study of halogen-bonding-utilized solid solution for simultaneous control of multifunctional properties. A series of anion-mixed molecular conductors (DIETSe)₂MBr_4<i>x</i>Cl_{4(1–<i>x</i>)} [DIETSe = diiodo­(ethylenedithio)­tetraselenafulvalene; M = Fe, Ga; 0 < <i>x</i> < 1] were synthesized without changing crystal structure utilizing strong halogen bonds between DIETSe molecules and anions. Detailed physical property measurements (<i>T</i> > 0.3 K, <i>H</i> < 35 T) using the single crystals demonstrated simultaneous control of both spin and charge degrees of freedom. The increase in Br content <i>x</i> gradually suppresses a metal–insulator transition attributed to the nesting instability of the quasi-one-dimensional Fermi surfaces. It suggests the dimensionality of π electrons is extended by increasing the anion size, which is opposite of the typical effect of chemical pressure. We found that the “negative” chemical pressure is associated with the characteristic halogen-bonding network. Br substitution also enhances the antiferromagnetic (AF) ordering of d-electron spins in the Fe salts, as indicated by the Néel temperature, AF phase boundary field, and saturation field. Furthermore, we observed hysteresis in both magnetization and resistivity only in halogen-mixed salts at very low temperatures, indicating simultaneous spin and charge manipulation by alloying

An Electrically Conductive Single-Component Donor–Acceptor–Donor Aggregate with Hydrogen-Bonding Lattice

An electrically conductive D–A–D aggregate composed of a single component was first constructed by use of a protonated bimetal dithiolate (complex <b>1H</b>_<b>2</b>). The crystal structure of complex <b>1H</b>_<b>2</b> has one-dimensional (1-D) π-stacking columns where the D and A moieties are placed in a segregated-stacking manner. In addition, these segregated-stacking 1-D columns are stabilized by hydrogen bonds. The result of a theoretical band calculation suggests that a conduction pathway forms along these 1-D columns. The transport property of complex <b>1H</b>_<b>2</b> is semiconducting (<i>E</i>_a = 0.29 eV, ρ_rt = 9.1 × 10⁴ Ω cm) at ambient pressure; however, the resistivity becomes much lower upon applying high pressure up to 8.8 GPa (<i>E</i>_a = 0.13 eV, ρ_rt = 6.2 × 10 Ω cm at 8.8 GPa). The pressure dependence of structural and optical changes indicates that the enhancement of conductivity is attributed to not only an increase of π–π overlapping but also a unique pressure-induced intramolecular charge transfer from D to A moieties in this D–A–D aggregate

Use of Halogen Bonding in a Molecular Solid Solution to Simultaneously Control Spin and Charge

Halogen-bonding interactions have attracted increasing attention in various fields of molecular science. Here we report the first comprehensive study of halogen-bonding-utilized solid solution for simultaneous control of multifunctional properties. A series of anion-mixed molecular conductors (DIETSe)₂MBr_4<i>x</i>Cl_{4(1–<i>x</i>)} [DIETSe = diiodo­(ethylenedithio)­tetraselenafulvalene; M = Fe, Ga; 0 < <i>x</i> < 1] were synthesized without changing crystal structure utilizing strong halogen bonds between DIETSe molecules and anions. Detailed physical property measurements (<i>T</i> > 0.3 K, <i>H</i> < 35 T) using the single crystals demonstrated simultaneous control of both spin and charge degrees of freedom. The increase in Br content <i>x</i> gradually suppresses a metal–insulator transition attributed to the nesting instability of the quasi-one-dimensional Fermi surfaces. It suggests the dimensionality of π electrons is extended by increasing the anion size, which is opposite of the typical effect of chemical pressure. We found that the “negative” chemical pressure is associated with the characteristic halogen-bonding network. Br substitution also enhances the antiferromagnetic (AF) ordering of d-electron spins in the Fe salts, as indicated by the Néel temperature, AF phase boundary field, and saturation field. Furthermore, we observed hysteresis in both magnetization and resistivity only in halogen-mixed salts at very low temperatures, indicating simultaneous spin and charge manipulation by alloying