AbstractElectron‐deficient acridones and in situ generated acridinium salts are reported as potent, closed‐shell photooxidants that undergo surprising mechanisms. When bridging acyclic triarylamine catalysts with a carbonyl group (acridones), this completely diverts their behavior away from open‐shell, radical cationic, ‘beyond diffusion’ photocatalysis to closed‐shell, neutral, diffusion‐controlled photocatalysis. Brønsted acid activation of acridones dramatically increases excited state oxidation power (by +0.8 V). Upon reduction of protonated acridones, they transform to electron‐deficient acridinium salts as even more potent photooxidants (*E1/2=+2.56–3.05 V vs SCE). These oxidize even electron‐deficient arenes where conventional acridinium salt photooxidants have thusfar been limited to electron‐rich arenes. Surprisingly, upon photoexcitation these electron‐deficient acridinium salts appear to undergo two electron reductive quenching to form acridinide anions, spectroscopically‐detected as their protonated forms. This new behaviour is partly enabled by a catalyst preassembly with the arene, and contrasts to conventional SET reductive quenching of acridinium salts. Critically, this study illustrates how redox active chromophoric molecules initially considered photocatalysts can transform during the reaction to catalytically active species with completely different redox and spectroscopic properties.
摘要据报道,缺电子的吖啶酮和原位生成的吖啶鎓盐是强效的闭壳光氧化剂,其作用机制令人惊讶。当用羰基(吖啶酮)桥接无环三芳基胺催化剂时,它们的行为完全从开壳、自由基阳离子、"超越扩散 "的光催化转变为闭壳、中性、扩散控制的光催化。吖啶酮的布氏酸活化可显著提高激发态氧化能力(+0.8 V)。质子化吖啶酮还原后,会转化为缺电子的吖啶鎓盐,成为更强的光氧化剂(*E1/2=+2.56-3.05 V vs SCE)。它们甚至能氧化缺电子的烯烃,而传统的吖啶鎓盐光氧化剂迄今为止仅限于氧化富电子的烯烃。令人惊讶的是,在光激发时,这些缺电子的吖啶鎓盐似乎会经历两个电子的还原淬火,形成吖啶阴离子,并通过光谱检测到其质子化形式。这种新行为在一定程度上得益于催化剂与炔的预组装,并与吖啶鎓盐的传统 SET 还原淬火形成鲜明对比。重要的是,这项研究说明了最初被认为是光催化剂的氧化还原活性发色分子是如何在反应过程中转变为具有完全不同的氧化还原和光谱特性的催化活性物种的。