Dynamic Covalent Spiropyran Exchange for Rapid Structural Diversification

Here we disclose that spiropyrans are able to undergo dynamic covalent exchange via their corresponding merocyanine isomers. In the latter, the indolinium moieties can be exchanged by a Michael-type addition-elimination sequence, in which a methylene indoline attacks a merocyanine and subsequently the initial indoline fragment is cleaved. The rate and position of the exchange equilibrium strongly depend on the reaction conditions as well as the substitution pattern on the methylene indoline fragments. Importantly, spiropyran cross exchange is catalyzed by indolinium salts and provides the opportunity for rapid in situ structural diversification of spiropyrans. This was used as synthetic tool to access various, hitherto inaccessible spiropyrans as potential dual-color photoinitiators for xolography. Moreover, we foresee application to screen dynamic covalent libraries of spiropyrans for photochromic properties and to exploit their light-sensitivity to bias the thermal equilibrium.