Anthraquinone-Based Donor-Acceptor Covalent Organic Frameworks with Thiazole Linkages to Enhance Direct Hydrogen Atom Transfer and Charge Separation in Photocatalytic Oxidation of C(sp3)─H Bonds.

Abstract

The design of efficient covalent organic frameworks (COFs) as photocatalysts for C(sp3)─H bond oxidation under green and mild conditions is highly desirable. Herein, TpAQ-TZ COF and TpAR-TZ COF, featuring thiazole linkages, were synthesized by combining 1,3,5-tricarboxylcarboxaldehyde (Tp), sulfur (S8), and direct hydrogen atom transfer (d-HAT) components (2,6-diaminoanthraquinone, AQ; 2,6-diaminoanthrone, AR). The TpAQ-TZ COF demonstrates impressive photocatalytic activity, achieving a 93% yield for phthalan oxidation under heterogeneous conditions-a rare example of photocatalytic C(sp3)─H bond oxidation in water under ambient conditions. Theoretical calculations reveal enhanced hydrogen atom abstraction capability coming from the increased number of d-HAT catalytic sites. Moreover, BdAQ-TZ COF and HbAQ-TZ COF were prepared by replacing Tp with 2,4-dihydroxy-1,3,5-triformylbenzene (Bd) or 2-hydroxy-1,3,5-triformylbenzene (Hb), respectively. Dipole moment calculations and femtosecond transient absorption spectroscopy show that the increased number of hydroxyl groups on the benzene-1,3,5-tricarbaldehyde monomer improves the charge separation efficiency within the three COFs, thereby accounting for the enhanced photocatalytic activity of TpAQ-TZ COF. This work opens up new opportunities for designing highly active photocatalysts by using the synergistic effects of d-HAT, O2 •-, and 1O2 within anthraquinone-based donor-acceptor COF platforms, offering a sustainable route for the oxidation of C(sp3)─H bonds under environmentally friendly conditions.