On the Photochemical Behavior of Some Xanthenes and Xanthones and the Photochemical Formation of Dixanthenes, Mono-, and Dihydroxydixanthenes

Abstract By ultraviolet spectrometric measurements we studied the photochemical behavior of xanthene, xanthone, 9, 9′-dihydroxydixanthene and dixanthene in aerated and deaerated ethanolic solutions in the range of concentration 5·10−3 m to 10−4 m by Osram-HBO 200- and Hanau TQ 81- lamps. We found: Xanthenes are photostable similar to monoacridans in the absence of oxygen; in the presence of oxygen photooxidation to the corresponding xanthones takes place. The xanthones are photochemically unstable; in oxygen free solvents containing H-atoms, like methanol, ethanol, benzene, cyclohexane, dioxane, they are reduced to xanthenes. In cyclohexane we found a soluble xanthene, 9-cyclohexylxanthydrol. In hexane there is formed a precipitate with a yield above 80%; this is the dimeric xanthene 9, 9′-dihydroxydixanthene. The photo-reaction presumably is going over a diradical, H-atom transfer from the solvent and formation of a xanthydrol monoradical in the primary step and dimerization to the 9, 9′-dihydroxydixanthene. Irradiation of solutions of xanthone and xanthene in benzene furnish 9-hydroxydixanthene. Solutions of equal molarity of xanthone and acridan yield under irradiation also a solid photoproduct; this is 9, 9′-diacridan. The first step of reaction is photoreduction, where xanthene tranfers a H-atom to xanthone and the hydroxyxanthyl radical dimerizes with the xanthyl radical or the acridanyl radical dimerizes with themselves and forms the diacridan.