Modulation of color changing paths for flavylium salts by solvent and concentration

The modulations of the color changing paths for flavylium salt (2-phenylbenzopyrilium, abbreviated as FV) by solvent and their concentrations have been studied by observing changes in the UV-Vis absorption spectra. The feature of the spectral changes varies with the solvent and solution FV concentration. The color of the solutions in ethanol at high concentration changed from yellow to red via green. It has been proposed that the yellow, green, and red species of FV are a monomer, a monomer with charge-transfer character, and a dimer and/or aggregate of FV, respectively. The spectral changes showed different behaviors in different solvents. In ethanol solution with a low concentration, a chalcone was generated by nucleophilic addition. The PM3 calculation revealed a new reaction path where the green FV is converted to a chalcone in propylene glycol. The color changing paths for FVs were able to be modulated in different solvents and by their concentration change. The previous proposed scheme well explained the experimental results in various solvents.