Theoretical study on the trihydroxy-anthraquinone tautomerism in the ground and excited states

Abstract

Proton tautomerism of 1,5,8-trihydroxy-3-methoxyl-6-methyl-9,10-anthraquinone was studied using HF and configuration interaction all single-excitations methods with 6-31g(d, p) basis set for the ground and singlet excited states. The calculations indicate that the compound exhibits three strong intramolecular hydrogen bonds (IHB), and shows similar characters in their proton transfer processes considering the geometries and Mulliken charge population. It exhibits normal intramolecular proton transfer in the region III where only one pair of adjacent hydroxyl group and carbonyl group exists in a peri region. However, intramolecular proton transfer (IPT) is not favored in view of the energy trend along IPT coordinate in the region I and II, where two hydrogens of hydroxyl groups bond with a common oxygen of carbonyl group and exists two IHBs in the peri region. This energy trend is verified by the potential energy surface. Hereby, it can be conjectured that IPT is absence in the similar structure such as hypericin's peri region and exists in hypomycin B from a theoretical point of view. Calculation results on the photophysical process show that the isomerization process is competitive with the intersystem crossing process, which facilitates the increase of triplet state quantum efficiency and photosensitive activity.