Pyrylenes: A New Class of Tunable, Redox-Switchable, Photoexcitable Pyrylium–Carbene Hybrids with Three Stable Redox-States

A new synthetic and modular access to a large family of redox-switchable molecules based upon the combination of pyrylium salts and carbenes is presented. The redox-properties of this new molecule class correlate very well with the π-accepting properties of the corresponding carbenes. While the pyrylium moiety acts as a chromophore, the carbene moiety can tune the redox-properties and stabilize the corresponding radicals. This leads to the isolation of the first monomeric pyranyl-radical in the solid-state. The three stable oxidation states could be cleanly accessed by chemical oxidation, characterized by NMR, EPR, UV–vis, and X-ray diffraction and supported by (TD)-DFT-calculations. The new hybrid class can be utilized as an electrochemically triggered switch and as a powerful photoexcited reductant. Importantly, the pyrylenes can be used as novel photocatalysts for the reductive activation of aryl halides and sulfonamides by consecutive visible light induced electron transfer processes.本研究报道了一种全新的模块化合成策略，可用于构建基于吡喃鎓盐与卡宾（carbenes）结合的大型氧化还原可切换分子家族。该新型分子类别的氧化还原特性与对应卡宾的π接受能力呈现极强的相关性。当吡喃鎓结构单元作为生色团（chromophore）时，卡宾结构单元可调控其氧化还原特性并稳定对应的自由基。这一特性使得研究人员成功分离得到首例固态单吡喃基自由基。该分子家族可通过化学氧化精准获得三种稳定氧化态，并通过核磁共振（NMR）、电子顺磁共振（EPR）、紫外-可见（UV–vis）光谱、X射线衍射（X-ray diffraction）进行表征，同时辅以含时密度泛函理论[(TD)-DFT]计算验证。这种新型杂化分子类别可作为电化学触发开关，同时亦是一种高效的光激发还原剂。尤为重要的是，吡喃烯类（pyrylenes）化合物可作为新型光催化剂，通过连续可见光诱导电子转移过程，实现芳基卤化物与磺酰胺的还原活化。