Quantum chemical investigation of the ground- and excited-state acidities of a dihydroxyfuranoflavylium cation

The two hydroxyl groups of the 4 ',7-dihydroxyfurano-3,2 '-flavylium cation (1), a synthetic analog of the aurone pigments of plants, have been shown to have different relative acidities in the ground state (S-0) and the lowest excited singlet state (S-1). In the ground state, the 4 '-OH group is slightly more acidic, while in the excited state, the molecule is strongly photoacidic and deprotonation occurs preferentially from the 7-OH group. In order to compare the relative acidities of these two OH groups via quantum chemical methodology, a common reference state was employed in which an explicit water molecule was hydrogen-bonded to each of the OH groups of 1. The relative acidities of the two OH groups were then inferred from the differential change in energy along the coordinate for proton transfer to the explicit water molecule via time-dependent density functional calculations (B3-LYP with Grimme's D3 dispersion correction; TZVP basis set; and PCM to simulate an aqueous environment). The calculated acidity changes confirm the experimentally observed inversion in the relative acidities between S-0 and S-1. The enhanced photoacidity of S-1 was also mirrored in the natural transition orbitals and the decrease in the negative change on the oxygen atoms of the OH groups. Employing a common reference state with an explicit water as the proton acceptor should thus serve as a convenient strategy for exploring the relative ground- and excited-state acidities of the OH groups of natural or synthetic dyes, especially when the values are not readily accessible through experiment.