Tuning the Band Structure of Covalent Triazine Frameworks for Sequential Reductive-Oxidative Photoredox Catalysis

Interrupted atom transfer radical addition (ATRA) processes through radical-polar crossover (RPC) manifolds constitute a powerful strategy for the rapid assembly of complex molecular architectures, yet their implementation using heterogeneous photocatalysts remains largely unexplored due to the challenge of simultaneously sustaining reductive and oxidative elementary steps within the same catalytic manifold. Herein, we demonstrate that systematic band-structure engineering of covalent triazine frameworks (CTFs) through aromatic spacer modulation enables dual photoredox activity under visible light. Three CTF materials featuring progressively elongated aromatic spacers were synthesized using superacid-mediated nitrile trimerization. Structural and photophysical studies revealed that spacer elongation modulates donor-acceptor interactions, charge-carrier dynamics, and energetic band positions. Among them, CTF-3 displayed the unique energetic balance required to efficiently promote both photocatalytic oxidation and reduction processes. More importantly, this material enables an unexplored interrupted ATRA RPC cyclization toward dihydro-oxazolines under heterogeneous photocatalytic conditions. Mechanistic studies support a sequential catalytic manifold involving SET halide activation, radical addition to olefins, oxidation-induced RPC, and intramolecular cyclization. These results demonstrate how molecular engineering of CTF semiconductors affords control over complex dual photoredox reactivity.