A novel pathway to photoinduced colour formation of colourless flourane dye using iodonium salts as activators

Abstract

A photoinduced colour formation system consisting of flourane dye (FR) and iodonium salt was investigated. The thermodynamic data indicate that the photoreaction of colour formation mainly accurs via an electron transfer pathway. The processes of electron transfer colour formation were proposed by separating and analysing the photoproducts. On irradiation, the excited flourane dye and iodonium salt may undergo electron transfer to give a flourane radical (FR ·+) and diphenyliodonium radical ( ·IPH 2). The latter rapidly decomposes to a phenyl radical (Ph ·) and iodobenzene, and the ring-opened coloured forms may be subsequently created via two pathways: abstraction of hydrogen by the FR ·+ radical intermediate or coupling between FR ·+ and Ph ·. The acidic ring-opened coloured form from the abstraction of hydrogen is predominant, and easily undergoes a colour disappearance reaction in non-polar solvents or basic medium, but the structure of FR-Ph from radical coupling possesses good stability. In order to improve the solubility of the iodonium salt, several diaryliodonium salts containing different flexible phenolic chains or counterions were used as activators. The influences of these two factors on colour formation were studied; the results demonstrate that the former has no distinct effect on the rate of colour formation or maximal optical density at 580 nm, whereas the latter has a significant effect. The influence of concentration was also studied.