Modulating the ESIPT dynamics of 3HF derivatives via substitution and solvent effect: A theoretical study

Abstract

The ground-state and excited-state electronic structure, electronic properties, vertical excitation and emission energies, as well as the potential energy curve of the excite-state intramolecular proton transfer (ESIPT) process for 3-hydroxyflavone (3HF) derivatives 2-(4-(diphenylamine)phenyl)-3-hydroxy-4H-chromen-4-one (PPC), 2-(4-(diethylamino)phenyl)-3-hydroxy-4H-chromen-4-one (EPC) and 2-(4-methoxyl)-3-hydroxy-4H-chromen-4-one (MNC) have been examined with DFT and TDDFT methods. We found that the ESIPT dynamics could be controlled via both the solvent effect and the substituents. Structurally and electronically, the strength of excited-state intramolecular hydrogen bond and the twisted intramolecular charge transfer degree could be tuned by changing the electron donating ability of the substituent, which in turn influences the ESIPT energy barrier. In particularly, the stronger the electron donating ability, the longer the excited-state hydrogen bond distance, accompanied by more twisted intramolecular charge transfer, and the higher energy barrier for ESIPT. Besides, we found that the emission energies for the derivatives are very sensitive to the excited-state intramolecular charge transfer degree, with PPC exhibiting the smallest emission energies, followed by EPC and MNC, which is consistent with experimental observations. Furthermore, the ESIPT energy barrier of 3HF derivatives in solvent increases when the solvent becomes more polar. This work provides significant insight into tuning the ESIPT dynamics in 3HF derivatives which is useful for the design of new organic fluorescent probes with excellent performance.