Matthew J. Rosseinsky

A Guest-Responsive Fluorescent 3D Microporous Metal−Organic Framework Derived from a Long-Lifetime Pyrene Core

The carboxylate ligand 1,3,6,8-tetrakis(p-benzoic acid)pyrene (TBAPy)-based on the strongly fluorescent long-lifetime pyrene core-affords a permanently microporous fluorescent metal-organic framework, [In(2)(OH)(2)(TBAPy)].(guests) (1), displaying 54% total accessible volume and excellent thermal stability. Fluorescence studies reveal that both 1 and TBAPy display strong emission bands at 471 and 529 nm, respectively, upon excitation at 390 nm, with framework coordination of the TBAPy ligands significantly increasing the emission lifetime from 0.089 to 0.110 ms. Upon desolvation, the emission band for the framework is shifted to lower energy: however, upon re-exposure to DMF the as-made material is regenerated with reversible fluorescence behavior. Together with the lifetime, the emission intensity is strongly enhanced by spatial separation of the optically active ligand molecules within the MOF structure and is found to be dependent on the amount and chemical nature of the guest species in the pores. The quantum yield of the material is found to be 6.7% and, coupled with the fluorescence lifetime on the millisecond time scale, begins to approach the values observed for Eu(III)-cryptate-derived commercial sensors.

A Water‐Stable Porphyrin‐Based Metal–Organic Framework Active for Visible‐Light Photocatalysis

Metal–organic frameworks (MOFs) permit the combination of high internal surface area with chemical and physical functionality conferred by the molecular linker. Porphyrins are versatile functional molecules in catalysis, light harvesting, and molecular sensing. Porphyrins have been used as building blocks for MOFs, affording catalysts, light harvesting and selective sorption in liquid and gas phases. MOFs based on Alcarboxylate coordination chemistry are amongst the most thermally and chemically stable of such systems reported to date. Here we report a waterstable porous porphyrin MOF with a BET surface area of 1400 m g 1 which performs visible-lightdriven hydrogen generation from water. The freebase porphyrin can be metalated within the rigid host structure. The reaction of AlCl3·6 H2O with the free-base meso-tetra(4-carboxyl-phenyl) porphyrin H2TCPP (Figure 1b) in water under hydrothermal conditions at 180 8C followed by washing with dimethyl formamide (DMF) to remove unreacted ligand leads to the formation of the microcrystalline porous red compound H2TCPP[AlOH]2(DMF3(H2O)2) 1 (referred to as Al-PMOF, experimental details are given in section 1.1 in the Supporting Information). The linker consists of four benzoate groups around the central porphyrin core. The analyzed composition reveals that no aluminium is coordinated within the porphyrin ring, consistent with the need to use reactive trialkylaluminium reagents for metalation of the porphyrin in solution. The reaction temperature is required to solubilize the porphyrin linker. The crystal structure of 1 was solved and refined from synchrotron powder Xray diffraction collected at 100 K. Indexing and Pawley refinement revealed an orthorhombic cell (a = 31.978(3) , b = 6.5812(4) , c = 16.862(2) , V= 3548.7(6) ) consistent with the C222, Cmm2, and Cmmm space groups. Each of these candidate space groups was evaluated by simulated annealing using a semi-rigid body to describe the TCPP unit (Figure S1 in the Supporting Information) with eight refined parameters describing distances and angles within the porphyrin. The best results were obtained for the benzoic acid group perpendicular to the central porphyrin ring, which can be best described in Cmmm symmetry, and zero occupancy for Al at the center of the porphyrin. This model was used in the final Rietveld analysis (Figure 1a). Fourier mapping revealed a single guest atom in the channels attributed to oxygen from water, which was included in the final refinement (Figure S2 in the Supporting Information). Each porphyrin linker in 1 is coordinated to eight aluminium centers (Figure 1c–e) through the four carboxylate groups which each bridge two aluminium units. There is Figure 1. a) Final Rietveld refinement of 1 (100 K) showing observed (gray crosses), calculated (line a), and difference (line b) plots (Q = 2p/d). Bragg peak positions are indicated. b) TCPP porphyrinic linker in 1. c–e) Crystal structure of 1 viewed down [001], [100], and [010] directions, respectively.

An Adaptable Peptide-Based Porous Material

Swelling Pores Porosity is a key parameter when selecting materials for catalysts, chemical separations, gas storage, host-guest interactions, and related chemical processes. In most cases the porosity of a material is fixed. Rabone et al. (p. 1053; see the Perspective by Wright) have described a molecular material in which the size of the pores changed during the sorption process. The porosity increased because a dipeptide linker between metal centers reoriented during uptake of some gases, thus improving the capacity of the material to adsorb. Conformational changes in a porous material during the sorption of small molecules lead to a dynamic increase in porosity. Porous materials find widespread application in storage, separation, and catalytic technologies. We report a crystalline porous solid with adaptable porosity, in which a simple dipeptide linker is arranged in a regular array by coordination to metal centers. Experiments reinforced by molecular dynamics simulations showed that low-energy torsions and displacements of the peptides enabled the available pore volume to evolve smoothly from zero as the guest loading increased. The observed cooperative feedback in sorption isotherms resembled the response of proteins undergoing conformational selection, suggesting an energy landscape similar to that required for protein folding. The flexible peptide linker was shown to play the pivotal role in changing the pore conformation.

Framework functionalisation triggers metal complex binding

Post-synthetic derivatisation of a porous material produces a functionalized material that binds the metal complex V(O)acac2, in contrast to the unfunctionalized precursor, which is inactive for complex binding.

Synthesis and characterization of alkali metal fullerides: AxC60

Abstract Alkali metal fullerides (A x C 60 ) are a subject of considerable current interest because of the occurrence of superconductivity for A x C 60 at temperatures surpassed only by the high T c copper oxides. The preparation and characterization of A x C 60 (A = alkali metal, x = 2,3,4,6) by powder X-ray diffraction, NMR ( 13 C, 23 Na and 87 Rb), and d.c. magnetization are reported. The structures are described as intercalation compounds of the FCC structure of pristine c 60 or of hypothetical BCC or BCT structures. The structures and phase diagrams can be rationalized on the basis of ion size and electrostatic considerations. Only the A 3 C 60 compounds are metallic (and superconducting). The superconducting T c increases nearly linearly with unit cell size. EHT (Extended Huckel Theory) calculations and 13 C NMR relaxation measurements indicate higher densities of states for the higher T c compositions.

Bulk superconductivity at 38 K in a molecular system

C(60)-based solids are archetypal molecular superconductors with transition temperatures (Tc) as high as 33 K (refs 2-4). Tc of face-centred-cubic (f.c.c.) A(3)C(60) (A=alkali metal) increases monotonically with inter C(60) separation, which is controlled by the A(+) cation size. As Cs(+) is the largest such ion, Cs(3)C(60) is a key material in this family. Previous studies revealing trace superconductivity in Cs(x)C(60) materials have not identified the structure or composition of the superconducting phase owing to extremely small shielding fractions and low crystallinity. Here, we show that superconducting Cs(3)C(60) can be reproducibly isolated by solvent-controlled synthesis and has the highest Tc of any molecular material at 38 K. In contrast to other A(3)C(60) materials, two distinct cubic Cs(3)C(60) structures are accessible. Although f.c.c. Cs(3)C(60) can be synthesized, the superconducting phase has the A15 structure based uniquely among fullerides on body-centred-cubic packing. Application of hydrostatic pressure controllably tunes A15 Cs(3)C(60) from insulating at ambient pressure to superconducting without crystal structure change and reveals a broad maximum in Tc at approximately 7 kbar. We attribute the observed Tc maximum as a function of inter C(60)separation--unprecedented in fullerides but reminiscent of the atom-based cuprate superconductors--to the role of strong electronic correlations near the metal-insulator transition onset.

Generation of a solid Brønsted acid site in a chiral framework

Protonation of chiral porous materials introduces a Brønsted acid centre, the structure of which is unique to the heterogeneous phase requiring pore wall confinement for stable isolation.

Interstitial oxide ion conductivity in the layered tetrahedral network melilite structure

High-conductivity oxide ion electrolytes are needed to reduce the operating temperature of solid-oxide fuel cells. Oxide mobility in solids is associated with defects. Although anion vacancies are the charge carriers in most cases, excess (interstitial) oxide anions give high conductivities in isolated polyhedral anion structures such as the apatites. The development of new families of interstitial oxide conductors with less restrictive structural constraints requires an understanding of the mechanisms enabling both incorporation and mobility of the excess oxide. Here, we show how the two-dimensionally connected tetrahedral gallium oxide network in the melilite structure La(1.54)Sr(0.46)Ga(3)O(7.27) stabilizes oxygen interstitials by local relaxation around them, affording an oxide ion conductivity of 0.02-0.1 S cm(-1) over the 600-900 degrees C temperature range. Polyhedral frameworks with central elements exhibiting variable coordination number can have the flexibility needed to accommodate mobile interstitial oxide ions if non-bridging oxides are present to favour cooperative network distortions.

13C NMR Spectroscopy of KxC60: Phase Separation, Molecular Dynamics, and Metallic Properties

The results of 13C nuclear magnetic resonance (NMR) measurements on alkali fullerides KxC60 are reported. The NMR spectra demonstrate that material with 0 < x < 3 is in fact a two-phase system at equilibrium, with x = 0 and x = 3. NMR lineshapes indicate that C3–60 ions rotate rapidly in the K3C60 phase at 300 K, while C6–60 ions in the insulating K6C60 phase are static on the time scale of the lineshape measurement. The temperature dependence of the 13C spin-lattice relaxation rate in the normal state of K3C60 is found to be characteristic of a metal, indicating the important role of the C3–60 ions in the conductivity. From the relaxation measurements, an estimate of the density of electronic states at the Fermi level is derived.

The disorder-free non-BCS superconductor Cs3C60 emerges from an antiferromagnetic insulator parent state.

The body-centered cubic A15-structured cesium fulleride Cs3C60 is not superconducting at ambient pressure and is free from disorder, unlike the well-studied face-centered cubic A3C60 alkali metal fulleride superconductors. We found that in Cs3C60, where the molecular valences are precisely assigned, the superconducting state at 38 kelvin emerges directly from a localized electron antiferromagnetic insulating state with the application of pressure. This transition maintains the threefold degeneracy of the active orbitals in both competing electronic states; it is thus a purely electronic transition to a superconducting state, with a dependence of the transition temperature on pressure-induced changes of anion packing density that is not explicable by Bardeen-Cooper-Schrieffer (BCS) theory.