Molecular Diode-Based Covalent Organic Frameworks: Imine Orientation-Driven Acid-Base Switching Photocatalytic H₂...

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.