Facile and Versatile Mechanochemical Synthesis of Indigoid Photoswitches.

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.