Metal organic frameworks (MOFs) are porous structures consisting of metal centers and organic linkers known for their ability to adsorb molecules from gas and solution phases. In general, MOFs are electrical insulators, but conductive MOFs would have diverse applications as electrocatalysts, chemiresistive sensors, and battery electrodes. Stoichiometric loading of the pores with covalently-bound dye molecules can anisotropically increase the conductivity of certain MOFs such as HKUST-1. To understand the tunable conductivity of HKUST-1 and inform the future design of conductive MOFs, it is critical to understand the mechanism of electron transfer and how it is impacted by structure and conditions. These structure-function relationships are studied through solid-state high harmonic generation (sHHG), a process in which electrons undergo photo-induced tunnel ionization into the conduction band followed by interactions with the periodic lattice potential and eventual recombination and photoemission. With the sensitivity to band structure, symmetry, and anisotropic conductivity that sHHG provides, it has been underutilized in studying structure-function relationships in widely applicable materials like conductive MOFs. In this study, it is shown that MOFs such as HKUST-1 undergo efficient sHHG which is simultaneously detected with photoluminescence (PL). The direction of maximum PL emission informs on the orientation of the lattice while individual harmonics exhibit anisotropic characteristics that correspond to specific symmetries within the structure. Loading HKUST-1 with different amounts of the dye TCNQ and using angle-resolved pump-probe sHHG can reveal the structural and charge transfer dynamics that lead to tunable conductivity while maintaining high surface area. Understanding the mechanism of conductivity in a highly porous MOF will inform the future design of diverse conductive MOFs for a wide variety of electrochemical applications.

Solid-state high harmonic generation as a probe of electronic structure and dynamics in a dye-loaded metal organic framework

ACS Spring 2022

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