Photochemical Reactions of Indigo Dyes: Dynamics of Photoisomerization and Proton Transfer

Abstract

Photochemistry of indigo dyes is introduced in this review. The indigo chromophore is constituted from two pairs of adjacent NH and C=O groups with two intramolecular hydrogen bonds between them. Indigo itself is a stable pigment and does not exhibit photoisomerization because of the short excited state lifetime which is considered to be due to a nonradiative decay that occurs via intramolecular proton transfer. In fact, solvent-assisted photo-induced proton transfer was observed for indigo carmine in aqueous solution but not in other solvents. Indigo derivatives without intramolecular hydrogen bonds exhibit trans-cis photoisomerization. We have measured femtosecond transient absorption spectra of indigo derivatives, and found that those with absorption bands at shorter wavelengths have slow isomerization rates, while those at longer wavelengths have faster ones. The excited state lifetime of cis isomer is shorter than that of the trans isomer, and the quantum yield of cis → trans is larger than that of trans → cis photoisomerization.