Achieving the cooperative and stepwise regulation of ESIDPT process in AFBD by introducing different electronic groups

In this paper, the effect of different electronic group substituents on the excited state intramolecular double proton transfer (ESIDPT) in AFBD is comprehensively researched by using quantum chemistry methods. Two electronic groups with comparable capabilities, including the electron-withdrawing group (-Br) and the electron-donating group (-OCH3), are chose to accomplish this study. The obtained geometric parameters, reduced density gradient and infrared spectra calculations have demonstrated that AFBD and its two derivatives experience the ESIDPT process. Subsequently, the conclusion of the potential energy surface and Bonn-Oppenheimer molecular dynamics further unveil that AFBD undergoes a cooperative ESIDPT process at ∼15 fs. Nevertheless, it should be noted that the introduction of -Br and -OCH3 transforms the proton transfer path. That is: "the cooperative ESIDPT is converted into the stepwise ESIDPT". Besides, we also confirm that the substitution of the electronic groups OCH3→H→Br can accelerate the ESIDPT process. Our work not only regulates the path of ESIDPT process, but also provides a time scale for this process, which is conducive to developing new intelligent fluorophores based on ESIDPT behavior.