In this work, we report the first CASPT2//CASSCF study of the mechanism of the photodecarboxylation of N-phthaloylglycine. The charge transfer excited state S CT （ 1∏∏＊） is initially populated upon irradiation at 266 nm. As a result of a fast internal conversion to the lowest excited singlet state S CT-N （ 1∏∏＊）, this state becomes a favorable precursor state for proton transfer, which triggers decarboxylation. Actually, the excited state intramolecular proton transfer （ESIPT） and decarboxylation processes proceed in an asynchronous concerted way. The ESIPT process is accomplished in the S CT-N （ 1∏∏＊） state, but the CO 2 molecule is finally formed in the ground state via the S CT /S 0 conical intersection. Azomethine ylide is formed in the ground state as a complex with CO 2 . A barrier of ~15 kcal/mol indicates that azomethine ylide is stable in the ground state, which is consistent with the experimental findings. This work provides mechanistic details about the formation of azomethine ylide by photoreaction of N-phthaloylglycine.
