Tuning Redox Hopping Charge-Transport in Metal−Organic Frameworks

Metal−organic frameworks (MOFs) define high surface area molecular compositions for electrochemical and photoelectrochemical energy conversion and storage. For these, the electrode-injected or photoinduced charges need to be delivered to catalytic centers (or be collected at the external electrode). Understanding and improving the charge-transport processes within the redox-active MOFs are important. Here, we discuss some key tools to control 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 find that axial coordination at the Fe-center can alter its spin-state to impact the TCPP(FeIII/II) -centered reorganization energy involved in this self-exchange process. Interestingly, the ability to attain a complete hexa-coordinated TCPP(Fe) is spatially demanding and determined by the pore size. The improved hopping rate may have a significant impact on the catalytic performance.