Ab initio study of the excited-state proton transfer mechanisms for 3-hydroxy-2-(thiophen-2-yl)chromen-4-one

Abstract

The optical absorption properties and fluorescence properties of 3-hydroxy-2-(thiophen-2-yl)chromen-4-one (3-HTC) were simulated by the time-dependent density functional theory (TDDFT) method. The experimental absorption spectrum and fluorescence spectrum were reproduced well by the calculated vertical excitation energies of 3-HTC-a, which corresponds to the intramolecular hydrogen bond O2⋯H1–O1. The excited state intramolecular proton transfer (ESIPT) mechanism of 3-HTC has been systematically investigated. The constructed potential energy curves (PECs) of 3-HTC-a in the S₀ and S₁ states indicate that, after photo-excitation, the intramolecular proton transfer reaction with the H1 atom moving from O1 to O2 occurs more readily in both the dynamics and thermodynamics aspects. In sharp contrast, the weak S⋯H1–O1 bond in 3-HTC-b was weakened in the S₁ state compared with that in the S₀ state, suggesting that the ESIPT process is forbidden. For the first time, it is found that the strong hydrogen bond is further strengthened and the weak hydrogen bond is further weakened in the excited state of 3-HTC. Moreover, the vibrational frequencies of the CO and O–H groups involved in the hydrogen bonds were also studied, and a more clear-cut signature of the hydrogen-bonding dynamics of 3-HTC is provided in this work.