Photoinduced generation of carbocations enabled by the promotion of aromaticity

Although one type of important species in chemistry, the instability of carbocations remains a longstanding issue in view of the electron-deficiency feature. Herein, we report on-demand light-triggered creation of carbocations with the promotion of aromaticity as a driving force, achieving controlled stability, switchability and catalytic reactivity. With cyclic hemiaminals as latent N-substituted carbocations on the ethene bridge of diarylethenes, photochemical open-ring and closed-ring forms allowed turning off and on the formation of carbocations by selecting different wavelengths of light. Mechanistic studies revealed that the change from antiaromaticity to aromaticity accounts for the stabilization. The ring-opening capability and, thereby, the switchability of carbocations, was further modulated by substituent effects, leading to the recovery of original hemiaminals. Their utility in Lewis acid catalysis was demonstrated with aromatic electrophilic substitutions and cationic ring-opening polymerization, showcasing the potential for remotely controlling diverse transformations. The results should offer opportunities for dynamic assemblies, molecular switches and organocatalysis.