Kuang-Lieh Lu

Self‐Assembled Arrays of Single‐Walled Metal–Organic Nanotubes

Many recent advances in the field of metal–organic framework (MOF) materials have been reported, not only from the standpoint of the potential applications, ranging from gas storage to catalysis and drug delivery, but also because of their intriguing architectures and framework topologies. 2] Conceptually, endless structures can be produced by assembling judiciously selected molecular building blocks. Just as the notable saying in crystal engineering goes “the limits are mainly in our imagination”, any conceivable MOF might be obtained in the future, although it is all currently imagination. Since the first discovery of carbon nanotubes (CNTs) by Iijima in 1991, discrete hollow tubular structures such as various CNTs and other synthetic nanotubes (SNTs) prepared from inorganic, organic, or biological precursors have been successfully developed, because they possess useful functionalities and can serve as molecular capillaries, sieves, and biological models. In theory, the curling-up or rollingup mechanism of topology transformations from 2D flat sheets to 1D hollow tubes is achievable. Thanks to effective design and synthesis strategies, many porous MOFs with various interesting network topologies have been reported over the past decade. Compared with the focus on CNTs and SNTs, it is surprising that significantly less effort has been directed to the preparation of metal–organic nanotubes (MONTs). In particular, discrete MONT structures are extremely rare to date. As part of our ongoing efforts in the design and synthesis of functional crystalline materials, 8d, 10] we wish to report herein on a unique type of MOF of [{[Cd(apab)2(H2O)]3(MOH)·G}n] (MAS-21–23, M I = Cs, K, Na, respectively; for MAS-22, G = 18 H2O·6C2H5OH·3C4H8O; apab = 4-amino-3[(pyridin-4-ylmethylene)amino]benzoate; MAS = materials of Academia Sinica), all of which consist of a large singlewalled metal–organic nanotube of [{Cd(apab)2(H2O)}3n] (MONT-A1) with an exterior wall diameter of up to 3.2 nm and an interior channel diameter of 1.4 nm. These MONTs are held together by alkaline cations to form 3D nanotubular supramolecular arrays (Figure 1). To the best of our knowledge, a single-walled MONT with such a large diameter is unprecedented. Compounds MAS-21–23 were synthesized by reaction of cadmium perchlorate, 4-amino-3-[(pyridin-4-ylmethylene)amino]benzoic acid (Hapab), and MOH (M = Cs, K, and Na, respectively) in an EtOH/THF/H2O solvent diffusion system at 4 8C through a single-step, self-organization process (Scheme 1). The appropriate choice of an organic ligand with specific functional groups and geometry is a major factor in achieving these large nanotube-based structures. The multifunctional Schiff base ligand of Hapab was designed deliberately and possesses a bending angle of 1208 between the pyridyl and carboxylate groups. Unlike similar bananashaped organic linkers, the use of the apab scaffold favors the formation of a tubular structure, rather than a spherical network.

One-Step Orthogonal-Bonding Approach to the Self-Assembly of Neutral Rhenium-Based Metallacycles: Synthesis, Structures, Photophysics, and Sensing Applications

Self-assembled metallacycles offer structural diversity and interesting properties based on their unique frameworks and host-guest chemistry. As a result, the design and synthesis of these materials has attracted significant research interest. This Account describes our comprehensive investigations of an effective orthogonal-bonding approach for the self-assembly of neutral Re-based metallacycles. We discuss the various types of assemblies that can be created based on the nuclearity of the luminophore, including bimetallic materials, rectangles, cages, and calixarenes. This approach permits the preparation of a rectangular molecule, rather than two molecular squares, in excellent yields. We extended this strategy to the high yield synthesis of a series of Re-based metallacycles with different shapes. With the rich spectroscopic and luminescence properties, Re(I) metallacycles provide an excellent platform for studies of host-guest interactions. When possible, we also present potential applications of the luminescent Re-based metallosupramolecular assemblies. The orthogonal-bonding approach involves the simultaneous introduction of two ligands: a bis-chelating ligand to coordinate to two equatorial sites of two fac-(CO)(3)Re cores and a monotopic or ditopic nitrogen-donor ligand to the remaining orthogonal axial site. Furthermore, by the appropriate choice of the predesigned organic ligands with various backbones and connectivity information and fac-Re(CO)(3) metal centers, we could also design other novel functional metallacycles including rotors, gondolas, cages, triangles, and metallacalixarenes in high yields. The incorporation of flexible ligands into the Re(I) metallacycles allows us to introduce various conformation states and novel structures. As a result, these structures acquire new functions, such as adaptive recognition properties. For example, we assembled Re(I)-based metallacyclic rotors via a one-step process. These rotors, which contain a para-phenylene unit that rapidly rotates within the metallacycles, are prototypes of a neutral altitudinal rotor. Most of the metallacycles are luminescent. The ability to chemically modify the organic ligands offers opportunities to create structural diversity and to tune the photophysical properties of these Re(I) metallacycles efficiently. Several strategies for increasing emission quantum yields and excited-state lifetimes and tuning the colors in Re(I) metallacycles are available. The cyclometalated ligands in Re(I) metallacycles improve excited state lifetimes and quantum yields, and these C-H bond-activated metallacycles are considerably more emissive than their non-C-H bond-activated analogues. The introduction of crown-ether-like recognition sites into neutral gondola-shaped metallacycles that selectively recognize metal ions also enhanced emission. Rhenium-based rectangular boxes, synthesized via a simple one-step route, contain a large and tunable hydrophobic inner cavity, which selectively recognizes benzene molecules. Such structures were the best host for benzene reported to date. In addition, we designed and synthesized novel neutral metallacalixarenes with tunable size, cavity, color, and functionality. These structures are efficient hosts for the recognition of planar aromatic guests.

Highly dispersed silica-supported nanocopper as an efficient heterogeneous catalyst : application in the synthesis of 1,2,3-triazoles and thioethers

In this paper, we report the synthesis of amine modified SiNPs (silica nanoparticles) by a sol–gel method and the role of synthesized SiO2 as a solid support for the nanocatalyst CuNPs (copper nanoparticles). The nanocatalyst is characterized by XRD, HRTEM, BET, AFM, SEM, EDX, UV-vis, FT-IR and TGA techniques. The Cu/SiO2 (catalyst A) serves as an efficient heterogeneous nanocatalyst exhibiting high catalytic activity for the synthesis of a series of 1,4-disubstituted-1,2,3-triazoles and thioethers. The catalyst A can be recycled and reused several times without any significant loss of catalytic activity as proved by XRD and HRTEM techniques.

A journey in search of single-walled metal–organic nanotubes

Single-walled metal–organic nanotubes (SWMONTs) represent a family of new, structured porous materials. Metal–organic nanotubes (MONTs) offer an attractive alternative to carbon nanotubes because cationic metal ions are incorporated into the backbones of MONTs. However, efforts regarding the preparation of metal–organic nanotubes have been few in number, compared with the focus on carbon nanotubes (CNTs) and synthetic nanotubes (SNTs). In particular, the preparation of single-walled metal–organic nanotubes (SWMONTs) remains largely unexplored. The goal of this feature article is to highlight synthetic strategies, the structural characteristics of this unique class of SWMONTs materials and explore possible applications.

Aggregation-Induced Emission Enhancement in Alkoxy-Bridged Binuclear Rhenium(I) Complexes: Application as Sensor for Explosives and Interaction with Microheterogeneous Media

The aggregation-induced emission enhancement (AIEE) characteristics of the two alkoxy-bridged binuclear Re(I) complexes [{Re(CO)3(1,4-NVP)}2(μ2-OR)2] (1, R = C4H9; 2, C10H21) bearing a long alkyl chain with 4-(1-naphthylvinyl)pyridine (1,4-NVP) ligand are illustrated. These complexes in CH2Cl2 (good solvent) are weakly luminescent, but their intensity increased enormously by almost 500 times by the addition of poor solvent (CH3CN) due to aggregation. By tracking this process via UV-vis absorption and emission spectral and TEM techniques, the enhanced emission is attributed to the formation of nanoaggregates. The nanoaggregate of complex 2 is used as a sensor for nitroaromatic compounds. Furthermore, the study of the photophysical properties of these binuclear Re(I) complexes in cationic, cetyltrimethylammonium bromide (CTAB), anionic, sodium dodecyl sulfate (SDS), and nonionic, p-tert-octylphenoxypolyoxyethanol (TritonX-100, TX-100), micelles as well as in CTAB-hexane-water and AOT-isooctane-water reverse micelles using steady-state and time-resolved spectroscopy and TEM analysis reveals that the nanoaggregates became small and compact size.

Dihydrobenzoic acid modified nanoparticle as a MALDI-TOF MS matrix for soft ionization and structure determination of small molecules with diverse structures

Efficient structural characterization is important for quality control when developing novel materials. In this study, we demonstrated the soft ionization capability of the hybrid of immobilized silica and 2,5-dihydrobenzoic acid (DHB) on iron oxide magnetic nanoparticles in MALDI-TOF MS with a clean background. The ratio between SiO2 and DHB was examined and was found to affect the surface immobilization of DHB on the nanoparticle, critically controlling the ionization efficiency and interference background. Compared with commercial DHB, the functionalized nanoparticle-assisted MALDI-TOF MS provided superior soft ionization with production of strong molecular ions within 5 ppm mass accuracy on a variety of new types of synthetic materials used for solar cells, light emitting devices, dendrimers, and glycolipids, including analytes with either thermally labile structures or poor protonation tendencies. In addition, the enhancements of the molecular ion signal also provided high-quality product-ion spectra allowing structural characterization and unambiguous small molecule identification. Using this technique, the structural differences among the isomers were distinguished through their characteristic fragment ions and comprehensive fragmentation patterns. With the advantages of long-term stability and simple sample preparation by deposition on a regular sample plate, the use of DHB-functionalized nanoparticles combined with high-resolution MALDI-TOF MS provides a generic platform for rapid and unambiguous structure determination of small molecules.

Filling carbon nanotubes with small palladium metal crystallites: the effect of surface acid groups

The filling of carbon nanotubes with palladium metal crystallites is described and it is shown that when the carbon nanotube samples are opened using nitric acid, surface acid groups (CO2H and OH) are present on both the nanotubes and nanoparticles and these interact strongly with palladium ions: ultrasound pre-treatment of the carbon nanotube samples prior to the oxidation with nitric acid increases the number of these groups.

Semiconductor Metal–Organic Frameworks: Future Low-Bandgap Materials

Metal-organic frameworks (MOFs) with low density, high porosity, and easy tunability of functionality and structural properties, represent potential candidates for use as semiconductor materials. The rapid development of the semiconductor industry and the continuous miniaturization of feature sizes of integrated circuits toward the nanometer (nm) scale require novel semiconductor materials instead of traditional materials like silicon, germanium, and gallium arsenide etc. MOFs with advantageous properties of both the inorganic and the organic components promise to serve as the next generation of semiconductor materials for the microelectronics industry with the potential to be extremely stable, cheap, and mechanically flexible. Here, a perspective of recent research is provided, regarding the semiconducting properties of MOFs, bandgap studies, and their potential in microelectronic devices.

Giant metal–organic frameworks with bulky scaffolds: from microporous to mesoporous functional materials

New concepts on the design and synthesis of crystalline metal-organic frameworks (MOFs) have made them a subject of considerable interest in the growing field of materials science. By creating larger cavity sizes by a nearly infinite combination of metal nodes and organic linkers, many innovative characteristics of microporous MOFs have been revealed. The primary goal of this perspective article is to highlight the frontiers in the development of giant MOFs that are deliberately constructed from metallated or metal-free bulky scaffolds. Incorporating these types of distinct bulky ligands into giant MOFs may lead to MOFs with a large cavity size, intriguing properties and new framework topology. Emerging applications of these materials in catalysis, adsorption, and sensors are also discussed.

Highly emissive cyclometalated rhenium metallacycles: structure-luminescence relationship.

We report on a series of new self-assembled cyclometalated dirhenium(I) metallacyclic complexes via an unprecedented rhenium-mediated C-H bond activation and the relationship between their structures and luminescence properties.