Skillfully tuning 1-hydroxy-9H-fluoren-9-one forward-backward ESIPT processes by introducing electron-withdrawing groups: A theoretical exploration

Abstract

Judiciously tuning the forward-backward excited state intramolecular proton transfer (ESIPT) barriers can effectively influence the generation of white light. Harnessing the 1-hydroxy-9H-fluoren-9-one (HHF) ESIPT characteristics as the prototype, two electron-withdrawing substituents (nitro group and formyl group) were introduced. The influence of electron-withdrawing group on the energy barrier of the forward-backward ESIPT processes has been studied by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The frontier molecular orbitals, infrared spectra, non-covalent interactions, electrostatic potential, electronic spectra, molecular structure have cross-validated our results. These results indicate that the electron-withdrawing groups dramatically promote the occurrence of the forward ESIPT process and hinder the backward ESIPT. The potential curves revealed that the forward ESIPT energy barrier was obviously reduced to 5.73 kcal/mol (HHF-CHO) and 4.55 kcal/mol (HHF-NO2) compared with HHF (7.64 kcal/mol). Meanwhile, the backward ESIPT energy barrier is availably improved to 3.57 kcal/mol (HHF-CHO) and 3.84 kcal/mol (HHF-NO2) compared with HHF (2.51 kcal/mol). Through the introduction of the electron-withdrawing groups, the energy barrier of the forward-backward ESIPT is efficaciously modulated. Furthermore, the more stable and better white fluorescence has been achieved. Our work will be conducive to improve molecular ESIPT properties and exploit white luminescent molecules.