Experimental Manifestation of Redox-Conductivity in Metal-Organic Frameworks and its Implication for Semiconductor/Insulator Switching

Abstract Electrical conductivity through metal-organic frameworks (MOFs) is almost exclusively interpreted in terms of band theory. This common practice neglects the fact that MOFs with electronically isolated redox active units will in fact show redox conductivity. Herein, we demonstrate unambiguous experimental evidence for redox conductivity in a MOF for the first time. The study was conducted on FTO-grown thin films of Zn(pyrazol-NDI) (pyrazol-NDI = 1,4-bis[(3,5-dimethyl)-pyrazol-4-yl]naphthalenediimide). Following Nernstian behavior, the proportion of reduced and oxidized NDI linkers can be adjusted by the applied potential. Through a series of conductivity measurements, it is demonstrated that the MOF exhibits minimal electric resistance at the mid-point potentials of the NDI linker, and conductivity is enhanced by more than 10000-fold compared to that of either the neutral or completely reduced films. The generality of redox conductivity is demonstrated in different MOFs, and its implication for applications that require switching between insulating and semiconducting regimes is discussed.