Effects of Twisted Intramolecular Charge Transfer Behavior on Excited-State Intramolecular Proton Transfer Reactions of Methyl Benzoate Derivatives in Water Solution

Many methyl benzoate derivatives were found to show intramolecular charge transfer (ICT), intramolecular proton transfer, and other properties, which have extensive applications in lasing media, metal ion sensors, active materials, and fluorescence probe fields. However, the intrinsic relationship and reaction mechanism between the excited-state intramolecular proton transfer (ESIPT) and ICT between methyl benzoate derivatives with different substituents have not been explained. In this paper, the density functional theory and time-dependent density functional theory methods were used to study the ESIPT and ICT behaviors of p-aminosalicylic acid methyl ester and p-dimethylaminosalicylic acid methyl ester in water and obtain the intrinsic interaction between the two behaviors. The bond parameters, infrared spectra, reduced density gradient scatter plots, and topological analyses of these two molecules in the ground state and excited state were analyzed to confirm the enhancement of the excited-state intramolecular hydrogen bonds (IHBs). The simulated absorption and fluorescence spectra of these molecules agreed well with the experimental values. Based on the optimized structure, we also plotted the natural transition orbitals, electron density difference maps, and frontier molecular orbitals (FMOs), which showed the changes of the charge distribution of these molecules intuitively upon photoexcitation. In addition, we also found that the degree of IHB enhancement with −N(CH3)2 substituents was less than that with −NH2, reflecting an inhibition effect of twisted intramolecular charge transfer (TICT) on ESIPT reaction. This conclusion was confirmed by our calculated potential energy curves. This work may better deepen the comprehension of the intrinsic relationship between ESIPT and TICT behavior and sequentially provide better theoretical guidance for the synthesis of fluorescent molecules related to these two behaviors.