Photochromism of <i>o</i>‑Nitrophenyl-Substituted Oxazabicycles

The o-nitrophenyl-substituted oxazabicycle 1 was prepared via a one-pot tandem multicomponent reaction. Upon exposure to UV light in CH2Cl2, it turned purple and reverted back to its original color while being heated. The in situ trapping of the photogenerated product with trimethylsilyl cyanide yielded the adduct 4, which confirmed the formation of the ring-opened zwitterionic species 2. The EPR spectroscopic data, PBN spin-trapping experiments, and spectroelectrochemical studies of the model compound 7 provided evidence to support that the photochromism of 1 involved the zwitterionic biradical intermediate 5, generated by photoinduced intramolecular electron transfer from the amine moiety to the nearby o-nitrophenyl group. Further, two photogenerated side products responsible for the photofatigue of 1 were isolated and characterized, and a possible mechanism for their formation was proposed.