Halochromism of alizarin yellow R in aqueous solution: A comparative theoretical analysis of neutral, anionic, and dianionic forms

Abstract

Photochemical transformations of alizarin yellow R with alternate elimination of two protons from the carboxyl and OH groups are modeled in the present work using TD-DFT, and the accompanying shifts of electron densities are estimated from the analysis of NPA charges. It turned out that the carboxyl group, although polarized upon excitation, is almost not involved in the photoinduced charge transfer from the aromatic A ring to the diazo linker and does not affect it. Therefore, the neutral form and the anion have an absorption maximum at the same wavelength (λ_max = 374 nm). On the contrary, the OH group, although also not directly involved in this, significantly affects this process: when a proton is eliminated from it, the path of charge transfer from the A ring through the diazo linker is extended and reaches the second B ring. This leads to a significant shift of the absorption maximum of the dianion (λ_max = 496 nm) relative to the neutral form and the monoanion. Moreover, photoinduced changes in the conformations of all three forms of the dye are local: in contrast to the famous parent unsubstituted azobenzene, alizarin yellow R has a trans conformation in both the ground and excited states.