Theoretical investigation of the ESPT mechanism in Alloxazine: effects of substituents and DMSO concentration

Flavin-type molecules have good biocompatibility and spectral properties, and they are widely studied in photophysics and electrochemistry, most of them are derivatives of Alloxazine (All). Some scholars have studied the spectral properties of All in water-organic mixtures at the molecular level. Nevertheless, the photophysical properties and reaction mechanism of simple flavin-water in an aprotic solvent remain poorly explored. Our research delves into the proton transfer mechanism of All, as well as investigates the effects of substituents and DMSO concentration on its photophysical properties. It is found that the intermolecular hydrogen bond between the solvent and solute is crucial for excited-state proton transfer (ESPT). ESPT occurs spontaneously when at least two water molecules and All to form three intermolecular hydrogen bonds (All-2H₂O). The time-resolved excited-state non-adiabatic dynamics illustrate that the long-range ESPT of All-2H₂O is an ultrafast kinetic reaction, the bond cleavage of proton donors N-H first occurs on a time scale of less than 100 fs. It indicates the hydrogen-supplying capacity of the proton donor is dominant in the ESPT process. Moreover, F substitution (All-F) and 50 % DMSO concentration transform the first excited state of All into a bright state with allowed transitions. This work will offer valuable theoretical insights for developing and applying flavin-type based on ESPT.