Electronic Engineering of Donor-Acceptor Covalent Organic Frameworks via Fluorine Substitution for Efficient Solar...

Covalent organic frameworks (COFs) are gaining prominence in photocatalysis due to their adjustable topologies, expanded π-conjugation, large surface areas, and superior chemical stability. Their modular architecture enables accurate control over electronic properties by rational design using appropriate building parts. Herein, we design and synthesize four pyrene-based COFs by systematically varying the number of fluorine substituents in the framework, using organic linkers containing no fluorine (Py-3Ph), mono-fluoro (Py-1Fl), difluoro (Py-2Fl), and tetrafluoro (Py-4Fl) groups. All four COFs exhibit high crystallinity and permanent porosity with distinct optoelectronic properties. These changes in the number of fluorine substituents in the structural unit of COFs have led to pronounced differences in their light absorption, charge transport, and redox potentials, ultimately resulting in different photocatalytic hydrogen evolution activities. Specifically, Py-2Fl COF showed the best H₂ evolution rate (HER) of 7150 µmol g^-1 h^-1 over visible light in the presence of a cocatalyst (1 wt.% Pt). Notably, the COF exhibits appreciable HER activity (614 µmol g^-1 h^-1) even without a platinum co-catalyst and retains its structural stability and catalytic activity even after multiple reuse cycles. This work offers important insights into rational engineering of the structure-property-performance of COFs for improved photocatalytic HER by tuning the number of fluorine substituents in the framework.