We report a strategy to expand the pore aperture of metal-organic frameworks (MOFs) into a previously unattained size regime (>32 angstroms). Specifically, the systematic expansion of a well-known MOF structure, MOF-74, from its original link of one phenylene ring (I) to two, three, four, five, six, seven, nine, and eleven (II to XI, respectively), afforded an isoreticular series of MOF-74 structures (termed IRMOF-74-I to XI) with pore apertures ranging from 14 to 98 angstroms. All members of this series have noninterpenetrating structures and exhibit robust architectures, as evidenced by their permanent porosity and high thermal stability (up to 300°C). The pore apertures of an oligoethylene glycol-functionalized IRMOF-74-VII and IRMOF-74-IX are large enough for natural proteins to enter the pores.

If individual molecules are to be used as building blocks for electronic devices, it will be essential to understand charge transport at the level of single molecules. Most existing experiments rely on the synthesis of functional rod-like molecules with chemical linker groups at both ends to provide strong, covalent anchoring to the source and drain contacts. This approach has proved very successful, providing quantitative measures of single-molecule conductance, and demonstrating rectification and switching at the single-molecule level. However, the influence of intermolecular interactions on the formation and operation of molecular junctions has been overlooked. Here we report the use of oligo-phenylene ethynylene molecules as a model system, and establish that molecular junctions can still form when one of the chemical linker groups is displaced or even fully removed. Our results demonstrate that aromatic pi-pi coupling between adjacent molecules is efficient enough to allow for the controlled formation of molecular bridges between nearby electrodes.

We determine and compare, at the single molecule level and under identical environmental conditions, the electrical conductance of four conjugated phenylene oligomers comprising terminal sulfur anchor groups with simple structural and conjugation variations. The comparison shows that the conductance of oligo(phenylene vinylene) (OPV) is slightly higher than that of oligo(phenylene ethynylene) (OPE). We find that solubilizing side groups do neither prevent the molecules from being anchored within a break junction nor noticeably influence the conductance value.

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.

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.