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

ABSTRACT

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 O ₂ ^•− 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.