DFT/TDDFT based study to decipher the excited state intramolecular proton transfer process for 3-benzothiazol-2-yl-2-hydroxy-5-methyl-benzaldehyde-oxime

Abstract

In this study, we theoretically investigate the interactions of hydrogen bonding in excited states and the mechanism of intramolecular proton transfer (ESIPT) for the novel compound 3-benzothiazol-2-yl-2-hydroxy-5-methyl-benzaldehyde-oxime (HBT-phos) theoretically. Herein, the effects of atomic electronegativity of chalcogen (O, S and Se) are also focused. Firstly, employing quantum theory of atoms-in-molecules (QTAIM) method, we confirm intrinsic quality of O-H···N within HBT-phos derivatives (HBT-phos-O, HBT-phos-S and HBT-phos-Se). By astutely examining changes in geometries and infrared (IR) spectra, we elegantly demonstrate O-H···N is remarkably strengthened in S₁ state, indicating a pronounced tendency towards ESIPT. Computational hydrogen bonding energies further magnificently support this conclusion. Probing into photo-induced excitation, we discover heightened electronic densities surrounding N atom that play pivotal roles in attracting protons, thereby facilitating ESIPT. Ultimately, upon comparing potential energy curves in excited states, we deduce ESIPT process is expected to be exceptionally rapid with low atomic electronegativity (O → S → Se), which elucidates why detecting fluorescence peak of HBT-phos-S itself proves challenging in experiment. This study not only fills the void in understanding ESIPT mechanism within HBT-phos derivatives and authentically reveals its unambiguous mechanism, but also presents atomic-electronegativity-dependent ESIPT behaviour and the explanation for fluorescence quenching observed in prior experiment.

GRAPHICAL ABSTRACT

KEYWORDS:

• Electronic density difference
• charge reorganisation
• frontier molecular orbital
• excited state intramolecular proton transfer
• atomic electronegativity

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data are available on request from the corresponding author.

Additional information

Funding

This work was supported by the National Natural Science Foundation (No. 11904082), and the Fundamental Research Funds for the Central Universities (No. 2022MS120).