Understanding Charge-Transport in Metal−Organic Frameworks: Impact of the Framework-Topology and Electronic Structure on the Redox Hopping

Abstract

Metal−organic frameworks (MOFs) are an emerging class of high surface area molecular compositions for electro- and photoelectrochemical energy conversion and storage. For these, work producing charges need to be delivered to catalytic centers and/or externally appended electrodes. Understanding and improving the charge-transport processes within the redox-active MOFs are important. Here, we discuss some key parameters that controls the redox-hopping process in a series of hydrolytically robust topologically different Zr(IV)-MOFs made of tetrakis(4-carboxyphenyl)porphyrinato iron(III), TCPP(FeIII). Topological variation offers fixed but different center-to-center (Fe-Fe) distances to define the hopping rate. Furthermore, we also discuss how axial coordination at the Fe-center can alter its spin-state to impact the TCPP(FeIII/II) -centered reorganization energy of this self-exchange process. Interestingly, the ability to attain a complete hexa-coordinated TCPP(Fe) through two-axial coordination is also determined by the pore diameter –i.e. the space available to accommodate two ligand-molecules. The improved hopping rate possibly be enough to drive electrochemical transformations, but the real question to address would be –how slow it is for photoelectrochemical processes?