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.