# Molecular Diode‐Based Covalent Organic Frameworks: Imine Orientation‐Driven Acid‐Base Switching Photocatalytic H2 Production

## TL;DR

Researchers designed COFs that switch photocatalytic hydrogen production based on pH by controlling electron transfer through imine bond orientation.

## Contribution

The study introduces COFs with pH-responsive acid-base switching for selective photocatalytic H2 production.

## Key findings

- PyAm-PhAl-COF shows 172 times higher H2 evolution under alkaline conditions.
- PyAl-PhAm-COF shows 25 times higher activity under acidic conditions.
- Imine bond orientation drives pH-responsive electron transfer and photocatalytic switching.

## Abstract

Donor–acceptor type covalent organic frameworks (D–A COFs) have emerged as a promising class of photocatalytic materials due to their highly porous structures and excellent photo charge separation. However, the role of linkage between donor and acceptor in regulating charge transport and reaction selectivity remains not fully elucidated. Inspired by molecular diodes with similar D–A structure and specific rectification character, we designed and synthesized two molecular diode‐based COF isomers (PyAm‐PhAl‐COF and PyAl‐PhAm‐COF), and systematically investigated their switchable electron transfer and photocatalytic hydrogen evolution. These isomers exhibit pronounced pH‐responsive current rectification during photo‐induced electron transfer, with the behavior directly driven by imine bond orientation. Specifically, PyAm‐PhAl‐COF facilitates efficient electron transfer under alkaline conditions, achieving a hydrogen evolution rate 172 times higher than that under acidic conditions. In contrast, PyAl‐PhAm‐COF displays an opposite trend, with a 25‐fold activity enhancement under acidic vs. alkaline environments. This “acid‐base switching effect” originates from protonation/deprotonation‐induced reversal of the imine bond dipole: the dipole change dynamically regulates the intramolecular electron transport pathway, thereby governing the selective oxidation of different electron donors. These findings not only deepen the understanding of structure‐performance relationships in D–A COFs but also provide a new design strategy for developing adaptive smart photocatalytic systems.

Molecular diode‐based covalent organic frameworks (COFs) enable pH‐responsive current rectification, dynamically reversing electron flow to toggle photocatalytic hydrogen evolution activity between acidic and alkaline environments.