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.