A computational study of substituted flavylium salts and their quinonoidal conjugate-bases:: S0→S1 electronic transition, absolute pKa and reduction potential calculations by DFT and semiempirical methods

The electronic transitions for flavylium cations and quinonoidal bases of 17 substituted flavylium salts have been studied at semiempirical and DFT (density functional theory) levels. Solvent effect on electronic spectra was included by Polarizable Continuum Model, PCM. We assigned longest-wavelength absorption maxima to HOMO -> LUMO transition. Both levels of theory gave good results for electronic transitions of flavylium cations whereas only TDDF-TPCM calculations could be used for electronic transitions of their quinonoidal bases. We also performed absolute pK(a) calculations of nine flavylium salts at DFT level. The pK a calculated values by our PCM parameterization gave excellent results with mean absolute deviation less than a half of one pK a unit. One-electron reduction potentials were carried out for 5 flavylium cations at DFT level. The theoretical results found were in good agreement with experimental values after adjustment for a systematic deviation.