Linkage-Locking Cyclization Enables Hydrolysis-Resistant Imine COFs for Photocatalytic Water Splitting.

Area of Science:

• Materials Science
• Catalysis
• Computational Chemistry

Background:

• Imine-linked covalent organic frameworks (COFs) are promising metal-free photocatalysts for overall water splitting (OWS).
• Their practical application is hindered by hydrolysis of the imine linkage under acidic conditions.
• Developing robust COFs is crucial for efficient solar fuel production.

Purpose of the Study:

• To investigate the impact of post-synthetic imine-linkage cyclization on the hydrolytic stability and photocatalytic performance of COFs.
• To explore cyclized derivatives of triazine-imine-triazine (TIT) COFs with varying heteroatoms (X = NH, O, S, Se).
• To computationally assess the potential of cyclization as a strategy for enhancing COF durability.

Main Methods:

• First-principles density functional theory (DFT) calculations were employed.
• Proton adsorption affinities and reaction pathways were analyzed.
• Electronic structures, exciton properties, and charge carrier dynamics were simulated.
• Predicted solar-to-hydrogen (STH) efficiencies were calculated for the parent and cyclized COFs.

Main Results:

• Cyclization significantly reduces proton adsorption affinity at the imine linkage by approximately 0.6 eV.
• The preferred site for hydrogen evolution reaction (HER) shifts from the imine nitrogen to the triazine unit in cyclized frameworks.
• Cyclization leads to enhanced interfragment polarization, reduced exciton binding energy, and more delocalized excited-state carriers.
• The TIT-NH derivative exhibited the highest predicted STH efficiency of 2.4%, surpassing the parent TIT COF (2.2%).

Conclusions:

• Imine-linkage cyclization is a viable strategy to improve the hydrolytic robustness of imine-based COFs.
• This modification preserves, and in some cases slightly enhances, photocatalytic water splitting performance.
• The TIT-NH derivative shows particular promise as a stable and efficient metal-free photocatalyst for OWS.