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Abstract：
Organic photocatalysts frequently possess dual singlet and triplet photoreactivity and a thorough photochemical characterization is essential for efficient light-driven applications. In this article, the mode of action of a polyazahelicene catalyst (Aza-H) was investigated using laser flash photolysis (LFP). The study revealed that the chromophore can function as a singlet-state photoredox catalyst in the sulfonylation/arylation of styrenes and as a triplet sensitizer in energy transfer catalysis. The singlet lifetime is sufficiently long to exploit the exceptional excited state reduction potential for the activation of 4-cyanopyridine. Photoinduced electron transfer generating the radical cation was directly observed confirming the previously proposed mechanism of a three-component reaction. Several steps of the photoredox cycle were investigated separately, providing deep insights into the complex mechanism. The triplet-excited Aza-H, which was studied with quantitative LFP, is formed with a quantum yield of 0.34. The pronounced triplet formation was exploited for the isomerization reaction of (E)-stilbene to the Z-isomer and the cyclization of cinnamyl chloride. Catalyst degradation mainly occurs through the long-lived Aza-H triplet (28 µs), but the photostability is greatly increased when the triplet efficiently reacts in a catalytic cycle such that turnover numbers exceeding 4400 are achievable with this organocatalyst.
摘要：
有机光催化剂通常具有双重单线态和三重态光反应性，彻底的光化学表征对于有效的光驱动应用至关重要。在这篇文章中,使用激光闪光光光解（LFP）研究了聚氮杂苯催化剂（Aza-H）的作用方式。研究表明，发色团可以在苯乙烯的磺酰化/芳基化中充当单线态光氧化还原催化剂，并在能量转移催化中充当三重态敏化剂。单线态寿命足够长，可以利用特殊的激发态还原电位来激活4-氰基吡啶。直接观察到产生自由基阳离子的光诱导电子转移，证实了先前提出的三组分反应机理。分别研究了光氧化还原循环的几个步骤，提供对复杂机制的深刻见解。三重态激发的Aza-H,用定量LFP研究，以0.34的量子产率形成。明显的三重态形成用于（E）-二苯乙烯到Z-异构体的异构化反应和肉桂酰氯的环化。催化剂降解主要通过长寿命Aza-H三重态（28µs）发生，但是当三重态在催化循环中有效反应时,光稳定性大大增加,使得使用该有机催化剂可实现超过4400的转换数。
