A computational study of the ground and excited state acidities of synthetic analogs of red wine pyranoanthocyanins

The color of mature red wines is due in large part to the chemical transformation of grape anthocyanins into pyranoanthocyanins. Given the difficulties of isolation and purification of pyranoanthocyanins from wines, experimental investigations have focused on pyranoflavylium cations, synthetic analogs that contain the basic chromophoric moiety of pyranoanthocyanins. Quantum chemical methodologies have been extensively employed to predict the physical, spectroscopic and photophysical properties of anthocyanins and, more recently, pyranoflavylium cations. In the present work, we employ TD-DFT with the B3-LYP functional and the def2-TZVP basis set, combined with estimation of solvation free energies via COSMO, to estimate a priori the pK_a values of the ground state and the lowest excited singlet and triplet states of a series of seven substituted hydroxypyranoflavylium cations. For the ground state and the first excited singlet state, the quantum chemical results compare favorably to experimental values for most of these compounds. Although there are currently no experimental data for the lowest excited triplet state, the acidity is predicted to be more similar to that of the ground state than to the excited singlet state, as is generally found experimentally for photoacids.