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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
Published on: July 17, 2020
Prolonging the Triplet Excited-State Lifetime via Tandem Through-Space Conjugation in Tetracationic Cyclophanes for
Yawen Li1,2,3, Qi Sun1, Zengrong Wang1
1Frontier Institute of Science and Technology, Interdisciplinary Research Center of Frontier Science and Technology, State Key Laboratory for Strength and Vibration of Mechanical Structures, Engineering Research Center of Key Materials for Efficient Utilization of Clean Energy of Shaanxi Province, Xi'an Key Laboratory of Electronic Devices and Material Chemistry, Xi'an Jiaotong University, Xi'an, Shaanxi Province, P. R. China.
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 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.
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