Redox Conductivity in Covalent Organic Frameworks

Electroactive covalent organic frameworks (COFs) are attracting intense research interest due to their diverse applications in energy and environmental technologies. However, the underlying electron transport mechanism is not fully understood yet. Herein, we present experimental evidence that electron transport through electroactive COFs can operate through a redox-conductivity mechanism, i.e., by electron hopping between neighboring redox-active units that differ in oxidation states. Electroactive naphthalene diimide (NDI)-based COF thin films are prepared as they display two distinct and reversible one-electron redox waves, correlating to the [NDI]^0/•- and [NDI]^•-/2- redox pairs, respectively. Utilizing an operando UV-vis spectroelectrochemistry technique, the potential-dependent absorption of the COF film can be dynamically followed and unambiguously assigned. Therefore, the redox composition of the film can be precisely controlled by just varying the electrode potential. This enables a clear experimental demonstration of the potential- or redox composition-dependent bell-shaped conductivity distribution in the COF film, corresponding to the electron-hopping transport nature that has been observed in redox-active polymers and metal-organic frameworks (MOFs). Importantly, the COF film displays reversible and stable insulator-to-semiconductor transitions upon electrochemical modulation for 100 cycles, setting the foundation for many practical applications. Further, we demonstrate that both the apparent electron diffusion coefficient, D_e^app, and redox conductivities are cation-dependent, highlighting the cation-coupled electron transport nature. Finally, this hopping transport mechanism is found to be operative in two more NDI-based electroactive COFs, demonstrating its generality.