Substituent control of dynamical process for excited state intramolecular proton transfer of benzothiazole derivatives

Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, in this work, the excited state intramolecular proton transfer (ESIPT) mechanisms of 2-(2′-hydroxyphenyl)benzothiazole (HBT) derivatives obtained by switching the ortho-position functional group of the hydroxyl group have been investigated. Analysis of bond parameters, bond energy, and infrared (IR) vibrational spectra show that hydrogen bonds are enhanced in the first excited state. The core-valence bifurcation (CVB) index and bond critical point (BCP) parameters are further applied to evaluate hydrogen bonds. The maximum absorption and emission peaks of molecules mainly originate from the transition of electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Exploring potential energy curves (PECs) and transition state (TS) energy profiles, we clarify the ultrafast ESIPT behavior of HBT and its derivatives are easier in the excited state. It is clarified that the HBT derivatives in the excited state with electron withdrawing group at the ortho-position weaken the intramolecular hydrogen bond OH···N, while are not conducive to the ESIPT process. And the electronic effect of the substituents can be employed to adjust the emission of the fluorophore from blue to near-infrared. The calculated results also show that the enol form is stable in the ground state, while the keto form is the main conformation in the excited state.