Rapid photo-oxidation reactions of imidazole derivatives accelerated by planar quinoid oxidation-state structures

The photo-oxidation reaction of 2,4,5-triphenylimidazole (lophine) and its derivatives has been studied in-depth since the discovery of its chemiluminescence phenomenon. It has been well agreed that the photostability of lophine could be maintained if hydrogen in imidazole is substituted by benzyl or alkyl groups. However, recently, it has been discovered that a lophine derivative (DPA-PIM) substituted with benzene at the N position of the imidazole ring undergoes a rapid photo-oxidation reaction after the introduction of diphenylamine as the donor group into lophine. Based on DPA-PIM, a series of lophine derivatives with different donor groups were designed and synthesized. In situ absorption spectra indicated that lophine derivatives linked with the p-C position of the arylamine group exhibit photo-oxidation activity under UV irradiation. In comparison, when the N position of the corresponding arylamine group is linked to benzene-substituted lophine, the photostability of derivatives can be maintained. ESR and electrochemical measurements indicated that the arylamine group linked in the p-C position would help lophine derivatives to form a rearranged stable planar quinoid oxidation state structure under UV irradiation, which tends to be easily attacked by self-sensitized singlet oxygen. The planar quinoid structure greatly contributes to the rapid reaction rate of this photo-oxidation reaction. Therefore, we tentatively put forward the mechanism for this kind of photo-oxidation reaction with two main intermediate states: the quinoid oxidation-state structure and the 1,2-dioxetane-like intermediate. It is believed that this finding can deepen the knowledge of the photostability of lophine derivatives or imidazole-based materials. Owing to their rapid reaction rate, some of the imidazole derivatives can also serve as high-sensitivity oxygen sensor materials.