Interplay between intra- and intermolecular excited-state single- and double-proton-transfer processes in the biaxially symmetric molecule 3,7-dihydroxy-4H,6H-pyrano[3,2-g]-chromene-4,6-dione

The molecule 2,8-diphenyl-3,7-dihydroxy-4H,6H-pyrano[3,2-g]-chromene-4,6-dione (D3HF, diflavonol) has been synthesized. This molecule contains two mutually symmetric and identical intramolecular hydrogen-bond systems. D3HF exhibits diverse excited-state proton-transfer processes, which transform it in a novel molecule in proton-transfer spectroscopy. Ultraviolet-visible absorption and fluorescence spectroscopy have been applied to D3HF in hydrocarbon, dioxane, and protic solvents from 295 to 77 K, together with fluorescence quenching experiments. The interplay between intramolecular single-proton-transfer and intermolecular double-proton-transfer processes has been demonstrated in protic solvents. Density functional calculations (DFT) in the ground state (DFT-B3LYP) and in the singlet excited states (time-dependent DFT) have been executed for D3HF and for 3-hydroxiflavone (3HF). The theoretical calculations confirm the experimental photophysical evidence. The role of the phenyl torsion on the proton-transfer spectroscopy of D3HF and 3HF is also discussed. Contrary to previous semiempirical calculations, for the isolated 3HF molecule the most stable conformation is the coplanar phenyl ring conformation.