Photo-substitution reactions of perylene red dyes

Irradiation of Lumogen Red 305 (3) in ethyl acetate with intense blue LED light led to very slow bleaching, showing a high photochemical stability. However, irradiation in ethanol led to the formation of a low yield product in which one of the phenoxy groups is substituted by an acetoxy group, the oxidized form of an ethoxy group. This reaction product was only formed in the presence of blue light and oxygen. Adding a small amount of acetic acid to the solutions resulted in a much higher yield of the product, indicating a possible photo oxidation of ethanol to acetic acid during the process in this solvent. A similar dye (5) containing less phenoxy side groups than Lumogen Red 305 showed the same reaction although the product formed showed lower chemical stability. The photo-substitution of phenoxy groups by acetoxy groups is a new reaction. The results are used to discuss the structural effects on the stability of these type of fluorescent dyes under irradiation conditions, potential reaction mechanisms and the possibility of using the photochemical transformations for preparative purposes.