The Influence of Intramolecular C═C Number on the ESIPT Reactions of HBTM Derivatives: A Theoretical Study.

In the work, theoretical calculations were utilized to explore how the count of intramolecular double bonds influences the excited-state intramolecular proton transfer (ESIPT) reaction in HBTM derivatives. Three derivatives, namely, HBTM-1 (HBTM: 4-methyl-2-benzothiazolyl-6-pyrimidine vinylphenol), HBTM-2, and HBTM-3, were designed, and their geometric structures in the S₀ and S₁ states were optimized in DMSO solvent using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The results show that as the number of intramolecular C═C double bonds increases, the strength of intramolecular hydrogen bonds gradually weakens, accompanied by an increase in the energy barrier of the ESIPT reaction, making the process more difficult. Additionally, analyses of frontier molecular orbitals and Sr reveal a negative correlation between the count of double bonds and the intramolecular charge transfer effect. It was also found that a rise in the number of double bonds results in a redshift of the fluorescence emission wavelength and an increase in the Stokes shift. These findings not only provide a theoretical basis for the molecular design of fluorescent derivatives based on the HBTM scaffold but also offer new insights into the fine-tuning of the ESIPT process.