Elaborating H-bonding effect and excited state intramolecular proton transfer of 2-(2-hydroxyphenyl)benzothiazole based D–π–A fluorescent dye

Abstract

2-(2-Hydroxyphenyl)benzothiazole (HBT) derivatives with donor–π–acceptor (D–π–A) structure have received extensive attention as a class of excited state intramolecular proton transfer (ESIPT) compounds in the fields of biochemistry and photochemistry. The effects of electron-donors (triphenylamine and anthracenyl), the position of substituents and solvent polarity on the fluorescence properties and ESIPT mechanisms of HBT derivatives were investigated through time-dependent density functional theory (TDDFT) calculations. Potential energy curves (PECs) and frontier molecular orbitals (FMOs) reveal that the introduction of the triphenylamine group on the benzene ring enhances intramolecular HB, thereby benefiting the ESIPT process. Analysis of their spectra reveals that P-TPA (para position for TPA) and M-TPA (meta position for TPA) are both excellent candidates for fluorescent dyes because of their large Stokes shifts. The PECs of four derivatives indicate that the ESIPT process of P-TPA in dimethyl sulfoxide (DMSO) solvent is the most likely to occur. The research revealed that both P-TPA and P-En (para positions for both TPA and En) can undergo a spontaneous transformation from the enol to the keto form in the S₁ state. Furthermore, the ESIPT process was found to be enhanced with an increase in polarity. The energy barrier of P-TPA(N*) → P-TPA(K*) is 3.06 kcal mol⁻¹ in the S₁ state and its reversed energy barrier is 4.47 kcal mol⁻¹. The para triphenylamine group could accelerate the ESIPT reactions, as it has a greater impact on the excited state intramolecular hydrogen bond (ESIHB) compared to meta-substitution of the triphenylamine group.