Concerning the color change of pyrromethene 650 dye in electron-donor solvents

This paper studies the change in the color from violet to green of pyrromethene 650 (PM650) dye observed in certain N,N′-dialkyl-amides characterized by their high electron-donor ability and low proton-donor capacity. For this purpose, the photophysical properties of PM650 are registered in diluted solutions of several amides, including dimethylformamide/dioxane mixtures. The color change of PM650 is discussed on the basis of the formation of new hypsochromic absorption and fluorescence bands, which are assigned to the formation of new chromophoric species as consequence of a specific reaction between PM650 and the amide solvents. The influence of the ageing time of the sample and the temperature on the color change is also discussed. In some cases the ageing leads to the bleaching of the dye.