The Relation between the Absorption Spectra and the Chemical Constitution of Dyes. XXVII. cis-trans Isomerism and Hydrogen Bonding in Indigo Dyes¹

Abstract

The ultraviolet and visible absorption spectra of indigo and of six halogen-substituted indigo dyes were determined in chloroform solution. In contrast with the behavior of thioindigo dyes, no evidence was found for photochemical isomerization. This was believed to be attributable to the preferential stabilization of the trans configuration by hydrogen-bonding.The discovery of phototropism in , '-diacetylmdigo and a comparison of the infrared absorption spectra of indigo and thioindigo provided evidence for this hypothesis. In addition, the effect of halogen substitution on the ultraviolet and visible spectra was considered.

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

“…The photostability of indigo was attributed originally to the lack of trans – cis photoisomerization 7. In contrast, indigo derivatives like thioindigo ( 2 ) readily isomerize about the central double bond8 and, unlike indigo, show strong fluorescence 9.…”

“…In contrast, indigo derivatives like thioindigo ( 2 ) readily isomerize about the central double bond8 and, unlike indigo, show strong fluorescence 9. It was proposed that the trans isomer of indigo is stabilized by hydrogen bonds formed between the NH and CO groups connecting the two halves of the molecule 7. 10 It was also discussed that after photoexcitation an intramolecular proton transfer followed by a nonradiative relaxation to the ground state occurs 11.…”

High performance liquid chromatography of slightly soluble brominated indigoids from Tyrian purple

“…The solubility of indigoids decreases with the number of bromine atoms attached to their structure in positions 6 and 6 [25]. Also, indigotins are less soluble than indirubins because of the presence of intramolecular hydrogen bonds between the hydrogen from amino groups and oxygen from ketone groups [26,27]. In indigotins, these bonds are formed on both sides of the molecule of the most thermodynamically stable trans isomer while in indirubins only one hydrogen bond is sterically possible, leaving other polar groups accessible for solvents.…”

Photostabilität von 4,4′‐Dihydroxythioindigo: ein Mimetikum des Indigo

“…Die Photostabilität von Indigo wurde ursprünglich auf fehlende trans ‐ cis ‐Photoisomerisierung zurückgeführt 7. Indigoderivate wie Thioindigo ( 2 ) hingegen isomerisieren ungehindert um die zentrale Doppelbindung8 und weisen im Unterschied zu Indigo starke Fluoreszenz auf 9.…”

“…Indigoderivate wie Thioindigo ( 2 ) hingegen isomerisieren ungehindert um die zentrale Doppelbindung8 und weisen im Unterschied zu Indigo starke Fluoreszenz auf 9. Vermutet wurde eine Stabilisierung des trans ‐Isomers von Indigo durch Wasserstoffbrücken, die zwischen der NH‐ und der CO‐Gruppe die beiden Hälften des Indigomoleküls verbinden 7. 10 Weiterhin wurde vorgeschlagen, dass nach Photoanregung ein intramolekularer Protonentransfer auftritt, dem ein nichtstrahlender Relaxationsprozess zurück in den Grundzustand folgt 11.…”