Observation of a single proton transfer fluorescence in a biaxially symmetric dihydroxy diflavonol

The proton transfer (PT) emission of the diflavonol molecule 2,8-diphenyl-3,7-dihydroxy-4H,6H-pyrano[3,2-g]-chromene-4,6-dione (D3HF) is reported for the first time. The D3HF molecule possesses a chemical structure almost equivalent to two 3-hydroxyflavone molecules symmetrically condensed on one-benzene ring. Thus, D3HF contains two five-membered intramolecular hydrogen bonds (IMHB), and it may undergo double-PT. The discovery via experiments of the D3HF PT emission conforms to the theoretical finding, by using time-dependent (TD) density functional theory (DFT) (with the B3LYP hybrid functional), of a stable single PT tautomer either in the first excited electronic state (S1) or in the ground electronic state. In the S1 state, once the first single PT process occurs, the second one is hindered.