Excited-State Engineering of Viologen Photocatalysts via Synergistic Through-Space and Through-Bond Electronic Coupling

Precise manipulation of excited-state charge dynamics represents a fundamental challenge in the molecular design of organic photocatalysts. Herein, we introduce a synergistic electronic coupling strategy for engineering long-lived charge separation states by integrating through-space and through-bond electronic coupling in viologen-based photocatalysts. A family of through-space-conjugated viologens (TSCVs) featuring a dumbbell-shaped donor-acceptor architecture was constructed by integrating extended π-conjugated scaffolds with TSCV units. This unique architecture establishes an electronically cooperative framework that accelerates photoinduced charge separation, promotes charge delocalization, and suppresses charge recombination. Spectroscopic, electrochemical, and theoretical investigations reveal that the synergistic coupling pathway enables persistent charge-separated excited states with enhanced charge-carrier utilization. Consequently, TSCV-T-c functions as a highly efficient metal-free photocatalyst for tandem C(sp3)-H functionalization/annulation of N,N-dimethylanilines with maleimides, affording tetrahydroquinoline products in yields up to 98%. Beyond demonstrating an effective photocatalytic system, this work establishes electronic coupling engineering as a molecular strategy for controlling excited-state dynamics and unlocking enhanced photocatalytic reactivity in organic semiconducting molecules.