Photoredox catalysis has driven a revolution in the field of organic chemistry, but direct mechanistic insights into reactions of genuine synthetic utility remain relatively scarce. Herein we report ultrafast time-resolved spectroscopic observation of a bimolecular organocatalyzed photoredox reaction, from catalyst photoexcitation through to photoinduced electron transfer (PET) and intermediate formation, using transient vibrational and electronic absorption spectroscopy with sub-picosecond time resolution. Specifically, the photochemical dynamics of initiation in organocatalyzed atom-transfer radical polymerization (O-ATRP) are elucidated for two complementary photoredox organocatalysts (N,N-diaryl-5,10-dihydrophenazines). Following photoexcitation, a dissociative bimolecular electron transfer is observed from the first excited singlet state of both photocatalysts to methyl 2-bromopropionate in dichloromethane, toluene, and dimethylformamide. The photocatalyst excited donor state, ground state, and radical cation are tracked in real time alongside the debrominated radical fragment. Our work challenges previously proposed mechanisms of initiation in O-ATRP and indicates that PET from short-lived excited singlet states can exert control of polymer molecular weight and dispersity by suppressing the steady-state concentration of the reactive debrominated radical. More broadly, we aim to demonstrate the potential of ultrafast absorption spectroscopy to observe directly transient, open-shell intermediates in mechanistic studies of photoredox catalysis.

中文翻译：

---

光氧化还原反应机理的超快观察：有机催化原子转移自由基聚合中的光引发

光氧化还原催化推动了有机化学领域的一场革命，但对真正合成效用的反应的直接机制洞察仍然相对稀缺。在这里，我们报告了双分子有机催化光氧化还原反应的超快时间分辨光谱观察，从催化剂光激发到光诱导电子转移 (PET) 和中间体形成，使用具有亚皮秒时间分辨率的瞬态振动和电子吸收光谱。具体而言，阐明了两种互补的光氧化还原有机催化剂（N,N-二芳基-5,10-二氢吩嗪）在有机催化原子转移自由基聚合（O-ATRP）中引发的光化学动力学。光激发后，在二氯甲烷、甲苯和二甲基甲酰胺中，观察到从两种光催化剂的第一激发单线态到 2-溴丙酸甲酯的解离双分子电子转移。光催化剂激发的供体态、基态和自由基阳离子与脱溴自由基片段一起实时跟踪。我们的工作挑战了先前提出的 O-ATRP 引发机制，并表明来自短寿命激发单线态的 PET 可以通过抑制反应性脱溴自由基的稳态浓度来控制聚合物的分子量和分散性。更广泛地说，我们旨在证明超快吸收光谱在光氧化还原催化机理研究中直接观察瞬态、开壳中间体的潜力。