Terahertz spectroscopy of MOFs reveals dynamic structure and contact free ultrafast photoconductivity

Abstract

Metal-organic frameworks (MOFs) are porous crystalline materials. Their large pores make them particularly interesting for membranes, gas separation, and gas storage. Furthermore, MOFs are ultralight, making them suitable for a large realm of exciting applications ranging from wearable devices to space technology. Optimizing MOFs for these applications demands a detailed understanding of their low energy dynamics and photophysics, which can be provided by terahertz (THz) spectroscopy. MOFs exhibit structural modes, or phonons, with energies in the meV range, which corresponds to the THz spectral range (0.1–10 THz, 0.4–40 meV). Understanding these modes is crucial in determining how a MOF interacts with guest molecules in the process of gas capture and storage. In this perspective, we discuss how gas-MOF interactions alter the MOFs’ spectral fingerprints. We demonstrate that THz spectroscopy can be used for gas adsorption monitoring and explain how density functional theory, together with THz spectra, can illuminate the dynamic structure of MOFs, providing unique insight into their functionality. THz is also a contact free probe for conductivity and allows us to measure short range conductivity within an individual MOF crystal. We will discuss the advantages of THz as a conductivity probe for MOFs as compared to more established direct current techniques. We will then expand our view to incorporate ultrafast photoconductivity in MOFs measured via optical pump-THz probe spectroscopy, in comparison to more established ultrafast spectroscopic tools such as optical transient absorption and photoluminescence. We will supplement this section with a discussion of THz studies on perovskites, which unveiled electron–phonon interactions not yet explored in MOFs.