Effects of solvents on the excited state intramolecular proton transfer and hydrogen bond mechanisms of alizarin and its isomers

In this paper, we use time-dependent density functional theory (TD-DFT) to investigate the effects of protic and aprotic solvents on the excited state intramolecular proton transfer (ESIPT) and hydrogen bond (ESHB) mechanisms of the alizarin. The calculation results show that there are mainly two forms of alizarin: alizarin-A and alizarin-B (both are isomers), in which alizarin-A is more stable than alizarin-B. In the aprotic (dimethylsulfoxide and n-heptane) solvents, alizarin-A is more prone to proton transfer reactions than alizarin-B in the excited state. In protic (methanol) solvent, the ESIPT process of alizarin-A is carried out without energy potential barrier, while the energy potential barrier of alizarin-B is only 0.04 kcal/mol, which is easier to occur than in aprotic solvent conditions. The scatter plot and isosurface of the reduced density gradient function indicate that the ESHB of alizarin in the protic solvent is stronger than the aprotic solvent. In other words, the hydroxyl group at the proton transfer site undergoes more hydrogen bonding interactions in the protic solvent, which will facilitate the occurrence of the ESIPT process. In general, our work will contribute to regulate the ESIPT process of alizarin and its derivatives through solvent effects in the future, as well as application to more fields.