Substituent effects of amines on genipin dye formation – Insights into dye origin

Abstract

Genipin, a naturally occurring iridoid, represents an interesting class of reactive dyes. This colorless molecule produces brilliant blue dyes in the presence of primary aliphatic amines. The mechanism behind this unique reaction has been suggested to proceed via an oxidative polymerization reaction. By utilizing aromatic amines, we demonstrate that the dye absorption properties can be redshifted to form green dyes as opposed to the blue that is most associated with aliphatic amines. These results led us to explore the mechanism of dye formation using X-Ray crystallography, DFT calculations, electrochemical experiments, mass spectrometry, and EPR spectroscopy on slightly structurally perturbed genipin scaffolds. Herein we report the first findings of a crystallographically characterized pseudoazulenic genipin dye intermediate and the impacts of redox chemistry on the resulting dyes, which shows electrochromic reversibility.