Ion Diffusion and (Photo)redox Conductivity in a Covalent Organic Framework

Covalent organic frameworks (COFs) have emerged as promising materials for energy-related applications, where precise control over charge and mass transport is critical, such as in electrocatalysis and battery technologies. Despite ongoing debates on the mechanisms of charge transport in COFs─particularly band transport versus electron hopping─experimental evidence for redox conductivity via hopping remains limited. In this work, we investigate redox hopping-mediated charge transport in a naphthalene diimide (NDI)-based redox-active COF (TAPT-NDI COF), examining the influence of ion and solvent environment. We show that electron hopping through ion-coupled self-exchange between oxidized and reduced linkers is strongly affected by ion size, ion pairing, and solvent polarity, as evidenced by variations in the apparent electron diffusion coefficients, D_e^app, obtained through potential step chronoamperometry. Notably, we report the first observation of a potential-dependent, bell-shaped redox conductivity profile in COFs. Furthermore, the redox states of the NDI units can be systematically modulated by both electrical potential and light (NDI^0/•-, NDI^•-/2- by applied potential and NDI^0/•- by light). The conductivity at intermediate redox states is enhanced by up to 4 orders of magnitude, enabling a highly reversible switching from an insulating (∼10^-9 S cm^-1) to semiconducting (∼10^-6 S cm^-1) regime. These findings offer new insights into redox transport in COFs and lay the groundwork for advancing their use in (photo)memristive devices, sensors, and (photo)electrocatalysis.