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

## TL;DR

This study explores how electrons and ions move in a special material called a covalent organic framework, showing how it can switch between being an insulator and a semiconductor.

## Contribution

The first observation of a potential-dependent, bell-shaped redox conductivity profile in covalent organic frameworks.

## Key findings

- Electron hopping in a redox-active COF is influenced by ion size, ion pairing, and solvent polarity.
- Redox conductivity in the COF increases by up to 4 orders of magnitude at intermediate redox states.
- The material can reversibly switch between insulating and semiconducting regimes.

## Abstract

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 (NDI0/•–, NDI•–/2– by applied potential and NDI0/•– 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.