Facile and Versatile Mechanochemical Synthesis of Indigoid Photoswitches

Abstract We demonstrate the application of mechanochemistry in the synthesis of indolone‐based photoswitches (hemiindigos, hemithioindigos, and oxindoles) via Knoevenagel condensation reactions. Utilizing ball‐milling and an organic base (piperidine) acting as catalyst and solvent for liquid assisted grinding (LAG) conditions, we achieve rapid, solvent‐free transformations, obtaining a set of known and previously unreported photoswitches, including highly functional amino acid–based photoswitches, multichromophoric derivatives and photoswitchable cavitands based on resorcin[4]arenes. The reaction under mechanochemical conditions gives moderate‐to‐high yields and is highly stereoselective leading to Z ‐isomers of hemiindigos and hemithioindigos and E ‐isomers of oxindoles. For selected examples, reversible visible‐light photoswiching properties have been demonstrated.