Photostability of 4,4′-Dihydroxythioindigo, a Mimetic of Indigo

• Published in: Angewandte Chemie International Edition Vol. 53; no. 2; pp. 591 - 594
• Main Authors: Dittmann, Marc, Graupner, Franziska F., Maerz, Benjamin, Oesterling, Sven, de Vivie-Riedle, Regina, Zinth, Wolfgang, Engelhard, Martin, Lüttke, Wolfgang
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 07.01.2014; WILEY‐VCH Verlag; Wiley; Wiley Subscription Services, Inc
• Edition: International ed. in English
• Subjects: Chemistry; Chemistry, Multidisciplinary; Derivatives; Drug Stability; Dyes; dyes/pigments; Hydrogen bonds; Indigo; Indigo Carmine - analogs & derivatives; Indigo Carmine - chemistry; Indigo Carmine - radiation effects; Isomerism; Models, Chemical; Models, Molecular; Photochemical; Photochemicals; Photochemistry; Photoexcitation; Physical Sciences; proton transfer; Protons; Science & Technology; Strategy; thioindigo; ultrafast spectroscopy; Ultraviolet Rays; UV stabilizers; thioindigo; pigments; SPECTROSCOPY; CHROMOPHORE; photochemistry; ultrafast spectroscopy; EXPEDIENT SYNTHESIS; proton transfer; dyes; DIRECTED METALATION; dyes/pigments
• ISSN: 1433-7851; 1521-3773; 1521-3773
• DOI: 10.1002/anie.201307016

The photochemical properties of indigo, a widely used industrial dye, has attracted both experimentalists and theoreticians from the beginning. Especially the high photostability of indigo has been the subject of intensive research. Recently, it was proposed that after photoexcitation an intramolecular proton transfer followed by a nonradiative relaxation to the ground state promote photostability. In indigo the hydrogen bond and the proton transfer occur between the opposing hemiindigo parts. Here, we provide experimental and theoretical evidence that a hydrogen transfer within one hemiindigo or hemithioindigo part is sufficient to attain photostability. This concept can serve as an interesting strategy towards new photostable dyes for the visible part of the spectrum. Beyond the blue: The vastly different photochemical properties of 4,4′‐dihydroxythioindigo and its dimethoxy derivative demonstrate that excited‐state proton transfer within the rigid ring system of only one hemithioindigo moiety improves photostability. This promoting mechanism is the same as that in indigo and in many UV stabilizers. This concept may serve as an interesting strategy towards new photostable dyes for the visible part of the spectrum.