Prolonging the Triplet Excited-State Lifetime via Tandem Through-Space Conjugation in Tetracationic Cyclophanes for Efficient Metal-Free Photoredox Catalysis

Prolonging the triplet excited-state lifetime is essential for developing efficient photocatalytic systems. Herein, a series of viologen-based cyclophane derivatives (2, 6, and 8) featuring TSC were designed and synthesized, with TSC-boxes 6 and 8 incorporating a unique combination of cyclophane structures and TSC units. Compared with 2 and 6, which possess only a single TSC unit, TSC-box 8 with a tandem TSC structure exhibits enhanced spin-orbit coupling and a narrower singlet-triplet energy gap. These attributes endow TSC-box 8 with favorable photoelectric properties, including photoluminescence and an apparent one-step four-electron redox behavior. Nanosecond transient absorption spectroscopy revealed efficient formation of the triplet excited state, prolonging the triplet excited-state lifetime, thereby enabling effective generation of O₂^•− and •OH. As a photocatalyst, TSC-box 8 efficiently mediates metal-free photoredox catalysis, affording benzimidazole derivatives in up to 99% yield and enabling the selective oxidation of toluene to benzaldehyde within 1 h (yield: 90%, conversion: 96%) and to benzoic acid within 5 h (yield: 97%, conversion: 98%). This work provides a strategic approach to designing long-lived triplet excited-state systems for metal-free photocatalysis.