Exploring and elaborating the excited state mechanism of a novel AIE material 2-(5-(4-carboxyphenyl)-2-hydroxyphenyl)benzothiazole

A novel aggregation-induced emission material 2-(5-(4-carboxyphenyl)-2-hydroxyphenyl)benzothiazole (2-CHBT) has been investigated based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Via reduced density gradient (RDG) versus sign(λ₂)ρ analyses, we firstly verify the formation of intramolecular hydrogen bond in 2-CHBT system. Analyzing the primary geometrical parameters in 2-CHBT and comparing the changes about infrared (IR) vibrational spectra, we confirm that the hydrogen bond O-H···N is strengthened in the S₁ state upon excitation. Exploring photo-excitation process, the frontier molecular orbitals (MOs) and charge density difference (CDD) analyses have been performed using TDDFT/B3LYP/TZVP theoretical level, based on which we verify the charge transfer phenomenon referring to the S₀?→?S₁ transition. And the CDD around the hydrogen bond moiety provides the tendency of ESIPT for 2-CHBT molecule. Comparing the energy gap between HOMO and LUMO orbitals in cyclohexane, toluene, chloroform, and DMSO solvents, we predict the ESIPT reaction might be more active in non-polar solvents. In addition, constructing potential energy curves and searching transition state (TS) structures, we further clarify the ESIPT mechanism and verify that non-polar solvents might facilitate the ESIPT process. Our simulated results reappear experimental spectral results and successfully explain experimental phenomenon.