Research

Our uniform multifunctionalization strategy depends on our ability to construct mixed-linker frameworks that have large enough channels to accommodate additional functionality. In this context, we have provided evidence to support the theory that the use of organic linkers that have hydrogen-bonding substituents can prevent the interpenetration of frameworks. We synthesized a non-catenated, mixed-ligand MOF that, to our knowledge, has the largest pore volume and lowest density of its topology (Figure 1, Link). We have subsequently synthesized several other non-catenated mixed-linker frameworks where at least one of the organic linkers bears a functional group capable of H-bonding interactions.

Figure 1. The self-assembly of modifiable organic linkers that have H-bonding substituents to generate a large-pore, low-density, independently functionalizable MOF material, KSU-100.

Our ability to synthesize non-catenated and well-defined mixed-linker MOFs has allowed us to develop strategies to uniformly multifunctionalize the frameworks post-synthesis. By constructing pillared MOFs using linkers that bear differently (Link), we are able to uniformly functionalize the fameworks via independent and quantitative reaction steps. Extending this work with the use of orthogonally reactive functional groups has allowed us to generate uniformly multifunctional MOFs simultaneously in one pot (Figure 2, Link).

Figure 2. A: The self-assembly of modifiable organic linkers with zinc cations to generate a bifunctionalizable metal-organic framework (MOF) material, KSU-1. B: Schematic representation of the sequential, independent functionalization of KSU-1 to yield a uniformly multifunctional MOF material.

With these multifunctional MOFs we have observed confinement effects that result in unexpected behavior. We found that, when they are immobilized in MOFs individually, amine-functionalized and glycol-functionalized linkers react as expected with isocyanates: the amines react faster than the hydroxyls despite being immobilized in a MOF with smaller pores. However, when the two linkers are immobilized together in a MOF, the reactivity reverses for less electrophilic isocyanates, with the hydroxls reacting to a larger extent than the amines (Figure 3, Link). This so-called emergent behavior is exciting as it is reminiscent to that observed for some amino acid residues in enzyme active sites.

Figure 3. The reversl of the relative reactivities of amine-functionalized and glycol-functionalized organic linkers when they are immobilized together in MOF pores.