Hydrogen bond capability tunable different “relay-race” mechanisms of the excited-state proton transfer process for 4′-methoxy-3-hydroxyflavone

Abstract

Abstract The different “relay-race” mechanisms of proton transfer process are investigated for 4′-methoxy-3-hydroxyflavone (MHF) in methanol (MeOH) and dimethylformamide (DMF) characterized by different hydrogen bond (HB) capabilities. Based on the density functional theory and time-dependent density functional theory, we demonstrate that the asynchronous excited-state intermolecular double proton transfer via two intermolecular HBs happens for MHF in MeOH. Differently, in DMF solvent, it breaks the intramolecular HB and forms a single intermolecular HB. And the intermolecular single proton transfer occurs in DMF upon photoexcitation. The excited-state intermolecular HBs both in MHF-MeOH and MHF-DMF complex are proved to be significantly strengthened, which facilitate the proton transfer process. Importantly, it reveals that the higher electrostatic potential and the stronger proton capture ability of DMF than MeOH induce a faster proton transfer for MHF-DMF in experiment. The results highlight the significance of the solvent HB capability in effecting the proton transfer mechanism, and hence have potential to design and develop excellent fluorescent probes.