Theoretical Investigation of Substituent Effects on Excited-State Intramolecular Proton Transfer in Amino-Functionalized Benzene-Derived Fluorophores.

Molecules that exhibit excited-state intramolecular proton transfer (ESIPT) have demonstrated great promise in fluorescent probes. The electronic effect of substituents has an important influence on the ESIPT process. In this study, we investigated the effects of substituents on the ESIPT mechanism and the photophysical behavior of single-benzene fluorophore (SBF) derivatives with computational chemistry methods. The ω-B97XD/TZVP level of theory was used to calculate bond lengths, interaction energies, infrared spectra, electron-hole analysis, and the energy barriers of ESIPT in SBFs. Our results confirmed the presence of intramolecular hydrogen bonds in both the ground and excited states in SBFs. Transition state calculation demonstrated that increasing the number of electron-withdrawing groups enhances hydrogen bonding interaction and facilitates ESIPT. However, excessive substitution saturates the electron-withdrawing effect, suppresses C═O basicity, and potentially induces conformational constraints, leading to a higher barrier. Our findings provide some important theoretical basis for the design of ESIPT fluorescent dyes.