Metal-organic frameworks (MOFs) are composed of metal-oxide units stitched together covalently by organic linkers to make architecturally stable extended structures supporting permanent porosity. Both inorganic and organic constituents of MOFs can be varied in their shape, size, composition, geometry and branching modality to produce a versatile class of porous crystalline solids. Thus, the pore shape and size are designed nearly at will to produce MOFs with ultrahigh porosity (greater than 10,000 m2/g surface area) and pore sizes up to 98 Angströms. The rigidity and strong bonding within and to the metal oxide units (referred to as secondary building units, SBUs) has led to a large number of MOFs having high architectural, thermal and chemical stability. These characteristics have allowed their covalent functionalization whereby their interior is modified by carrying out reactions on the organic linkers and open metal sites. These modifications span the gamut of organic reactions, coordination of ligands to open metal sites, and metalation of the organic linkers. The precision with which MOFs can be made and modified, coupled with the preservation of their high crystallinity after modification, has motivated their study in many applications such as gas adsorption, selective separations, catalysis, and imaging, to mention a few examples.
Schematic of the variability of metal-organic frameworks. Secondary Building Units and organic linkers can be varied in their shape, size, composition, geometry and branching modality. The empty pore space (yellow sphere) can be further modified through covalent functionalization. Hydrogen atoms are omitted for clarity. Color code: grey, C; red, O; green, N; purple, Cl; blue, metal.
Reducing our society’s reliance on fossil fuels presents the most pressing energy and environmental challenges facing our country. Hydrogen, methane, and carbon dioxide, which are some of the smallest and simplest molecules known, may lie at the center of solving this problem through realization of a carbon-neutral energy cycle. Potentially, this could be achieved through the deployment of hydrogen as the fuel of the long-term, however the abundance of methane might open new possibilities for energy independence in the foreseeable future. In addition, carbon dioxide capture and clean coal technologies might serve as immediate solutions. The figure above details production pathways of key gases in the provision of energy. In this context, MOFs can provide a basis for addressing today’s energy problem through materials design at the molecular level.
Rod MOFs are metal-organic frameworks in which the metal-containing secondary building units consist of infinite rods of linked metal-centered polyhedra. For such materials the points of extension are often atoms at the interface between the organic and inorganic constituents of the structure. The pattern of points of extension defines the shape of the building such as a helix, ladder, helical ribbon or cylinder. The linkage of these shapes into a 3-dimensional framework in turn defines the net characteristic of the original structure.
Two properties of rod MOFs are intimately related to their structures: The first is “forbidden catenation”. It means that for rod MOFs, no matter how large the pores, there can be no multiple intergrowths of the same structure. This simply results from the fact that the periodicity along the rods is at most a few ångströms – generally insufficient to allow intercalation of a second copy of the structure. This property has recently been used to make a large suite of isoreticular rod MOFs with pore diameters ranging from 14 to 98 Å. The second property that has been the subject of considerable interest is that rod MOFs have flexible frameworks and can “breathe” (change volume) with uptake or desorption of guests. It should be remarked, however, that not all rod MOF structures breathe, and some breathing structures are not rod MOFs. However, there is considerable scientific interest in breathing rod MOFs pertaining to their applications as selective sorbents or sensors.
MOFs have demonstrated great potential as chemical sensors because of the precision with which they can be designed to create favorable interactions between the pore interior and diverse analytes. MOF sensors usually operate through luminescence, solvatochromic/vapochromic, interferometry, localized surface plasmon resonance, with colloidal crystals or conductivity and electromechanical detection. Most studies were carried out on luminescence-based MOF sensors for the detection of hazardous materials and high explosives. The characteristics of a good sensor are generally summarized as the “4S”: sensitivity, selectivity, stability, and speed of response and recovery times.In most luminescent MOF sensors, the response is based on quenching, or enhancement of the emission intensity upon guest adsorption.
MOFs can be either rigid or flexible, depending on the linker component, the free space within the structure, and on different host–guest interactions. The extreme sensitivity and selectivity of flexible MOFs towards guest molecules is a promising feature regarding possible applications such as chemical sensors. For chemical sensing, membrane separations, optical devices, and for the assembly of complex nanoscale structures, it would be highly desirable to obtain thin oriented MOF films for this purpose.
Huge progress has been made over the past years to introduce several well-established reaction mechanisms and covalent bond formations to the field of extended nets. Although several elegant examples have been reported there is still an ongoing demand on utilizing hitherto unexplored chemical bonding and building blocks. An increase in structural and chemical diversity will impart new material properties and potential applications. Such developments have already been witnessed in the MOF area during the past two decades, where nets with predictable topology and fine-tunable structural properties can now be made nearly at will. However, purely organic nets, such as covalent organic frameworks, are limited in such capacity mainly through the synthetic efforts as well as the feasibility to prepare diverse building units. We are therefore interested in new pathways of design and synthesis that may overcome such limitations and opens up a broad, new spectrum of potential applications.