Non-interpenetrated metal-organic frameworks based on copper(II) paddlewheel and oligoparaxylene-isophthalate linkers: synthesis, structure and gas adsorption

Two metal–organic framework materials, MFM-130 and MFM-131 (MFM = Manchester Framework Material), have been synthesized using two oligoparaxylene (OPX) tetracarboxylate linkers containing four and five aromatic rings, respectively. Both fof-type non-interpenetrated networks contain Kagomé lattice layers comprising [Cu2(COO)4] paddlewheel units and isophthalates, which are pillared by the OPX linkers. Desolvated MFM-130, MFM-130a, shows permanent porosity (BET surface area of 2173 m2/g, pore volume of 1.0 cm3/g), high H2 storage capacity at 77 K (5.3 wt% at 20 bar and 2.2 wt% at 1 bar), and a higher CH4 adsorption uptake (163 cm3(STP)/cm3 (35 bar and 298 K)) compared with its structural analogue, NOTT-103. MFM-130a also shows impressive selective adsorption of C2H2, C2H4, and C2H6 over CH4 at room temperature, indicating its potential for separation of C2 hydrocarbons from CH4. The single-crystal structure of MFM-131 confirms that the methyl substituents of the paraxylene units block the windows in the Kagomé lattice layer of the framework, effectively inhibiting network interpenetration in MFM-131. This situation is to be contrasted with that of the doubly interpenetrated oligophenylene analogue, NOTT-104. Calculation of the mechanical properties of these two MOFs confirms and explains the instability of MFM-131 upon desolvation in contrast to the behavior of MFM-130. The incorporation of paraxylene units, therefore, provides an efficient method for preventing network interpenetration as well as accessing new functional materials with modified and selective sorption properties for gas substrates

Photophysical pore control in an azobenzene- containing metal-organic framework †

The synthesis and structure of an azobenzene functionalized isoreticular metal-organic framework (azo-IRMOF-74-III) [Mg 2 (C 26 H 16 O 6 N 2 )] are described and the ability to controllably release a guest from its pores in response to an external stimulus has been demonstrated. Azo-IRMOF-74-III is an isoreticular expansion of MOF-74 with an etb topology and a 1-D hexagonal pore structure. The structure of azo-IRMOF-74-III is analogous to that of MOF-74, as demonstrated by powder X-ray diffraction, with a surface area of 2410 m 2 g À1 BET. Each organic unit within azo-IRMOF-74-III is decorated with a photoswitchable azobenzene unit, which can be toggled between its cis and trans conformation by excitation at 408 nm. When propidium iodide dye was loaded into the MOF, spectroscopic studies showed that no release of the luminescent dye was observed under ambient conditions. Upon irradiation of the MOF at 408 nm, however, the rapid wagging motion inherent to the repetitive isomerization of the azobenzene functionality triggered the release of the dye from the pores. This light-induced release of cargo can be modulated between an on and an off state by controlling the conformation of the azobenzene with the appropriate wavelength of light. This report highlights the ability to capture and release small molecules and demonstrates the utility of self-contained photoactive switches located inside highly porous MOFs

New functional molecules in molecular junctions

The trend of decreasing the size of top-down fabricated silicon based electronic devices led to an increasing interest in alternative concepts which allow overcoming physical but also economical limits. The concept of integrating functional molecules into electronic devices, referred to as molecular electronics, might be a concept to fabricate smaller and cheaper devices in the future. The goal of this thesis was the design and synthesis of new functional molecules for molecular junctions. To achieve molecular electronic devices, molecules need to perform functions. Among them the most appealing ones are rectification and switching. However, to enable the design of electronic functions on a molecular level, it is equally important to develop a thorough comprehension of the relationship between the molecule’s structure and its transport properties. The contents of this thesis thus contribute to various aspects of molecular electronics. A row of model compounds consisting of a non-varying oligo(phenylene ethynylene) (OPE) backbone was synthesized. As a reference molecule, an OPE with two terminal thiol anchor groups was inspected in a mechanical controllable break junction (MCBJ) on a single molecule level. Certain components were systematically varied to examine the role of contacts and intermolecular interactions in molecular junctions. After the basic investigations the molecule’s function was further enriched and molecular switches and molecule based rectifiers moved into the focus of interest. Two redox switches based on ferrocene were proposed. In both switches, the ferrocene was functionalized in the 1,1’-position with two linkers comprising thiol anchor groups. Whereas in the first model compound the linkers consist of rather poor conducting alkyl groups, the linkers of the second switch are conjugated vinyl groups to provide an efficient electronic coupling to the electrodes. The latter switch was electrochemically characterized and a fully reversible redox reaction was observed. It was possible to immobilize single molecules of the switch in an MCBJ setup. Combining the MCBJ setup with an electrochemical cell, the conductance of the immobilized molecule was investigated in relation to an applied potential with respect to a reference electrode. Indeed, a change in the conductance correlating with the oxidation state was observed, thus the molecular junction can be considered as a reversible single molecule redox switch. Furthermore a new switching concept based on the interplay of electrode and molecule was proposed. Therefore, different pathways for electrons in a cruciform type structure consisting of two crossed rods were considered. One of the rods bears terminal sulphur groups, whereas the second transversal rod is functionalized terminally with pyridine subunits. In an electrochemically controllable MCBJ the coordination of the pyridine nitrogen is expected to bind to the gold surface in dependence of the surface potential of the electrodes. The synthesis and characterization of several cruciform model compounds were synthesized to investigate the proposed coordination induced switch. The model compounds were investigated on a single molecule level in a MCBJ setup. No switching behaviour was obtained yet, nevertheless, the obtained results revealed that the molecules are functional and that electron transport is possible through both of the crossed rods. Further a molecular rectifier based on two meta-substituted benzene units, one functionalized with an electron donating group, the other with an electron withdrawing group was proposed. The synthesis was performed, but no integration experiments have yet been performed. An additional signal from a molecule other than the electrical response would be very appealing to obtain information from integrated molecules. We propose to profit from electroluminescence (EL) of a molecule connected to two electrodes. EL as signal from molecules is very appealing as it indicates that molecules are bridging the electrodes and the emitted light is characteristic for the fluorophore. Three target structures possessing fluorophores were designed to bridge a 5 nm gap of two single-walled carbon nanotube electrodes. The synthesis and characterization of three target structures with a length of around 7.5 nm was described. The photophysical investigations in solution confirmed the possibility of tuning the absorption and emission bands. The integration into a single-walled carbon nanotube device was successfully performed and the nanotube-molecule-nanotube junction was characterized by its I/V-characteristics. Furthermore, the characteristic emission signal of integrated molecules was detected upon electrical excitation. The main work of this thesis was from synthetic nature, but in collaboration with physicists, it was contributed to several aspects of molecular electronics

Novel Cruciform Structures as Model Compounds for Coordination Induced Single Molecule Switches

We have synthesized various molecular cruciforms consisting of two different crossing π-systems and comprising crosswise arranged thiol- and pyridine-anchor groups. With these model compounds we strive towards the investigation of a new switching concept based on the potential dependent coordination of pyridines to gold electrodes in an electrochemical set-up. Integration of these cruciform molecules between both electrodes of a mechanically controlled break junction in a liquid environment gave insight into their single molecule transport properties. These studies allowed individual transport characteristics to be assigned to the bar subunits of the cruciforms but also revealed the remaining experimental challenges to realize the suggested switching concept

Controllability of the Coulomb charging energy in close-packed nanoparticle arrays

We studied the electronic transport properties of metal nanoparticle arrays, particularly focused on the Coulomb charging energy. By comparison, we confirmed that it is more reasonable to estimate the Coulomb charging energy using the activation energy from the temperature-dependent zero-voltage conductance. Based on this, we systematically and comprehensively investigated the parameters that could be used to tune the Coulomb charging energy in nanoparticle arrays. We found that four parameters, including the particle core size, the inter-particle distance, the nearest neighboring number, and the dielectric constant of ligand molecules, could significantly tune the Coulomb charging energy

Electroluminescence from a single nanotube-molecule-nanotube junction

The positioning of single molecules between nanoscale electrodes has allowed their use as functional units in electronic devices. Although the electrical transport in such devices has been widely explored, optical measurements have been restricted to the observation of electroluminescence from nanocrystals and nanoclusters and from molecules in a scanning tunnelling microscope setup. In this Letter, we report the observation of electroluminescence from the core of a rod-like molecule between two metallic single-walled carbon nanotube electrodes forming a rigid solid-state device. We also develop a simple model to explain the onset voltage for electroluminescence. These results suggest new characterization and functional possibilities, and demonstrate the potential of carbon nanotubes for use in molecular electronics

CCDC 1469866: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

CCDC 1469865: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

Efficient Long-Range Stereochemical Communication and Cooperative Effects in Self-Assembled Fe₄L₆ Cages

A series of large, optically active Fe₄L₆ cages was prepared from linear 5,5′-bis­(2-formylpyridines) incorporating varying numbers (<i>n</i> = 0–3) of oligo-<i>p</i>-xylene spacers, chiral amines, and Fe^II. When a cage was constructed from the ligand bridged by one <i>p</i>-xylene spacer (<i>n</i> = 1) and a bulky chiral amine, both a homochiral Fe₂L₃ helicate and Fe₄L₆ cage were observed to coexist in solution due to a delicate balance between steric factors. In contrast, when a less bulky chiral amine was used, only the Fe₄L₆ cage was observed. In the case of larger cages (<i>n</i> = 2, 3), long-range (>2 nm) stereochemical coupling between metal centers was observed, which was minimally diminished as the ligands were lengthened. This communication was mediated by the ligands’ geometries and rigidity, as opposed to gearing effects between xylene methyl groups: the metal-centered stereochemistry was not observed to affect the axial stereochemistry of the ligands