Photochromism of o‑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.

邻硝基苯基取代的氧杂双环化合物1通过一锅法串联多组分反应制备得到。将其置于二氯甲烷（CH₂Cl₂）中暴露于紫外光（UV）下时，会变为紫色，加热后可恢复至初始颜色。通过三甲基硅基氰对光生产物进行原位捕获，得到加合物4，从而证实了开环两性离子物种2的生成。针对模型化合物7开展的电子顺磁共振（EPR）光谱数据、N-叔丁基-α-苯基硝酮（PBN）自旋捕获实验以及光谱电化学研究，为化合物1的光致变色过程涉及两性离子双自由基中间体5提供了实验依据；该中间体5是由胺基团向邻近的邻硝基苯基基团发生光诱导分子内电子转移而生成的。此外，研究人员分离并表征了两种导致化合物1发生光致疲劳的光生副产物，并提出了其生成的可能反应机理。