Excited-state Intramolecular Proton Transfer Mechanisms of Thiazole-based Chemosensor: a TD-DFT Study

The excited-state intramolecular proton transfer (ESIPT) mechanisms of 2-(2-hydroxyphenyl)-4-phenylthiazole (HPT) and 2-(5-bromo-2-hydroxyphenyl)-4-phenylthiazole (BrHPT) have been systematically investigated by density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, approaching along with the polarizable continuum model (PCM). The calculated primary bond lengths and bond angles demonstrate that HPT and BrHPT can form intramolecular hydrogen bonds in the ground state (S-0), which can be significantly strengthened in the first excited state (S-1). Our calculated results well reproduce the experimental absorption and emission spectra. Upon addition of F-, the proton can move close to F- and the hydroxy moieties are deprotonated, which cause a red-shift in absorption and a new emission peak in fluorescence emission with the disappearance of the dual fluorescence emission. The calculated Mulliken's charge distribution and frontier molecular orbitals further demonstrate that the ESIPT processes are more likely to occur in the S-1 state. The constructed potential energy curves of the S-0 and S-1 states confirm that the proton transfer processes are hard to occur in the S-0 state due to the high energy barriers. Moreover, much lower energy barriers are found in the S-1 state, which proves that the ESIPT processes are more likely to take place in the Si state. In addition, compound with electron withdrawing (-Br) group might result in much stronger intramolecular hydrogen bond and owns lower energy barrier, which can facilitate the ESIPT processes.