Scutellarein in organic solvents: changes in spectroscopic properties caused by solute-solvent interactions

In this work, the spectroscopic properties of scutellarein (6-hydroxyapigenin) were studied in three organic solvents (methanol, acetonitrile and N,N-dimethylformamide) taking into account possible ionization and isomerization (tautomerization) processes. Significant visible colour changes were reported in the case of scutellarein in N,N-dimethylformamide. It was shown that isomerization processes can be one of the reasons for the observed changes in absorption spectrum, because some scutellarein isomers have an absorption band at about 623 nm while other forms of scutellarein show no absorption in this region. Moreover, spectroscopic properties were studied for cases of scutellarein in acetonitrile and methanol. The molar extinction coefficient has been found in the case of methanol solution which could be used to determine scutellarein concentration in this solvent using spectroscopic methods in future studies. The quantum-chemical calculations were performed for neutral and anionic forms and for two types of possible isomers of scutellarein in each solvent. The results help explain the experimentally observed rising absorption in the 500-750 nm wavelength range. Another important result of the quantum-chemical calculations is a prediction of excited state intramolecular proton transfer (ESIPT) in scutellarein. This result has been obtained for free molecule in vacuum and in the cases of methanol, acetonitrile and N,N-dimethylformamide solution. It was found that the excited state energy of the normal molecular form is higher than the excited state energy of the tautomer form of scutellarein.