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, Deapp, 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 (NDI0/•−, NDI•−/2− by applied potential and NDI0/•− by light). The conductivity at intermediate redox states is enhanced by up to four 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.