A detailed theoretical investigation on the excited-state intramolecular proton-transfer mechanism of 3-BTHPB chemosensor

In the present work, the density functional theory and time-dependent density functional theory (TDDFT) methods have been adopted to investigate the detailed excited-state intramolecular proton-transfer (ESIPT) dynamics of a new chromophore N-(3-(benzo[d]thiazol-2-yl)-4-(hydroxyphenyl)benzamide (Hu et al. in Angew Chem Int Ed 49:4915-4918, 2010). The method we adopted reproduces the absorption and fluorescence spectra reported in the experiment, which demonstrates that the TDDFT theory is reasonable and effective for this chromophore. The phenomenon of hydrogen-bond strengthening in the S-1 state based on the primary bond length, bond angles, infrared spectra and the calculated hydrogen-bonding energy indicated the tendency of excited-state proton transfer. The corresponding frontier molecular orbitals and Mulliken's charge distribution as well as natural bond orbital have been analyzed, manifesting that the ESIPT process could happen due to the intramolecular charge transfer. The constructed potential energy curves of the S-0 state and S-1 state demonstrate that the proton transfer is more likely to occur in the S-1 state due to the relative lower barrier. And the ESIPT process can be facilitated on account of the photoexcitation.