Mercaptoethanol and dimercaptopropanol form a complex with a cationic dye. Two dyes interacting with two SH groups in the complex with dimercaptopropanol exhibit two different absorptions and two different phosphorescences. Each one of them, present at a shorter wave length, corresponds to an absorption and a phosphorescence of the complex with mercaptoethanol. The dye-dimercaptopropanol complex is more photosensitive than the dye-mercaptoethanol complex. A phototropism observed in the former complex is based on an electron transfer.