Photochemistry of the piperidinoanthraquinones

An examination of the absorption spectra of the 1- and 2- piperidinoanthraquinones in a range of solvents of differing polarity shows that, in their unprotonated forms, their longest wavelength absorption band is intra-molecular charge-transfer (c. t.) in type. On protonation of the 1-piperidino derivative the longest wavelength band becomes nπ* and this is accom­panied by a marked difference in photochemical behaviour. Photoreduction of the unprotonated 1-piperidino derivative in alkaline solution occurs by the conventional reactions of hydrogen abstraction, followed by combination of two semiquinone radical anions to form the corresponding dianion. With 2-piperidinoanthraquinone, the final reduction product is the semiquinone radical anion formed by an electron transfer process from hydroxide and alkoxide ions to the photo-excited c. t. state of the quinone. The relevance of these observa­tions to the mechanism of phototendering of cellulose materials by vat dyes is discussed.