Electronic and Ionic Conductivity of Metal-Organic Frameworks

This article provides a critical review on two topical conductive properties, namely, electronic and ionic conductivities, related to the burgeoning porous materials field of metal–organic frameworks (MOFs). The intense current interest stems from fundamental importance as well as technological relevance for energy conversion and storage technologies (e.g., batteries, photovoltaics, fuel cells, and sensing devices), and recent progresses have been extensively reviewed. The raison d'être of the review, consequently, lies in the personal account and reflections weaved into the fabric of current developments of the two areas. For the part of electronic conductivity, large, fused aromatics (e.g., triphenylene or even larger π-electron building blocks) figure prominently because of the intrinsically rich and diverse electronic properties that can be accessed for the synthesis of potentially highly conductive networks in the solid state. For the part of ionic conductivity, we characterize the unifying design scheme as one of the establishing strong anionicity on the host frameworks so as to facilitate the incorporation of potentially mobile protons or other cations in the porous domain. Interestingly, the pursuit of the two apparently disparate conductive properties can both be greatly facilitated by the common and abundant element of sulfur, because of its versatile chemical functionalities. For example, whereas thiol-equipped porous MOF grid can readily lock in soft and polarizable metal ions to install metal–thiolate bridges across the organic π-electron systems, and thus to potentially boost electronic transport throughout the solid-state framework, similar thiol-equipped net can be treated with oxidants (e.g., H₂O₂ or O₃) to generate postsynthetically the highly ionic sulfonate function to impart strong ionic conductivity.