Metal-Organic Frameworks (MOFs) are an emerging class of porous materials for electrochemical applications. An understanding of the underlying charge transport process will facilitate in better design of these materials for desirable results. A series of four chemically identical but structurally different ZrIV-MOFs constructed from tetrakis(4-carboxyphenyl) porphyrinato iron (III), TCPP(FeIII) are examined to understand how their topological construction affects redox hopping conductivity. A structural variation fixes centre-to-centre distances and defines the hopping rate. On the other hand, altering the central metal spin-state can further tune the TCPP(FeIII/II) reorganization energy of the self-exchange process. The hopping rate was further improved upon axial coordination of 1-methyl imidazole which converts the weakly halide bound high-spin TCPP(FeIII/II) to the six-coordinated low-spin complex. Electrochemical and resonance Raman data reveal that pore geometry that defines the centre-to-centre distance in these frameworks dictate the steric demand to accommodate two 1-methyl imidazole molecules, and thus, the population of low-spin vs high-spin species is a function of topology in the presence of an excess ligand.

Improving charge transport in metal-organic frameworks: Alteration of iron-porphyrin centered redox hopping rate

ACS Fall 2021

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