Effect of external electric fields on the ESIPT process and photophysical properties of benzothiadiazole derivative

Focusing theoretically on the Excited State Intramolecular Proton Transfer (ESIPT) process, this study evaluates how external electric fields (EEFs) modulate both ESIPT dynamics and the photophysical behavior of a benzothiadiazole derivative, 2-(Benzo[c] Weller (1955), Huang et al. (2024), Lu and He (2021) [1, 2, 5] thiadiazol-4-yl)-N, N-diethylpyridin-4-amine (BZ-4, Chem. Commun., 2024, 60, 9105) using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. Analyses of dihedral angle variations in molecular structures, including infrared (IR) vibrational spectra related to bond lengths, bond angles, and intramolecular hydrogen bond (IHB) parameters, demonstrate that hydrogen bond strength varies under different EEFs. Furthermore, applying EEFs in different directions differentially impacts the molecular potential energy curves (PECs). Notably, while the direction of the EEFs determines the high or low barrier regime of the PECs, the variation of field intensity causes only minimal fluctuations in the energy barrier height. Moreover, the enhancement of IHB induced by the application of a negative electric field will inhibit the proton transfer. Specifically, distortion of the dihedral angle θ hinders the completion of ESIPT. Moreover, the applied electric field suppresses the twisted intramolecular charge transfer (TICT) process, thus enhancing the fluorescence intensity. This theoretical investigation offers valuable guidance on modulating molecular photophysical behaviors through external electric field regulation.