| Summary: | Excited state intramolecular proton transfer (ESIPT) within molecules in solvents plays important roles in photo-chemistry and photo-biology. Herein, the influence of 1-ethyl-3-methyl-imidazolium bis (trifluoromethylsulfonyl) imide ([EMIm][NTf<sub>2</sub>]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF<sub>6</sub>]) on the ESIPT of 4′-N,N-diethylamino-3-hydroxyflavone (DEAHF) was explored. The density functional theory and time-dependent density functional theory methodologies were used. The calculated fluorescence spectrum reveals that the fluorescence peaks of DEAHF in [EMIm][NTf<sub>2</sub>] and [BMIm][PF<sub>6</sub>] originate from the emission of N* and T* forms. The structure’s optimization, infrared spectra, non-covalent interactions and the scanning of potential energy curves collectively demonstrate that the ESIPT of DEAHF likely happen more in [EMIm][NTf<sub>2</sub>] than in [BMIm][PF<sub>6</sub>]. The solvation effects in [BMIm][PF<sub>6</sub>] exhibit greater prominence compared to those in [EMIm][NTf<sub>2</sub>], as evidenced by the free energy curve. The alterations in dipole moment indicate a substantial solvation relaxation during the ESIPT processes. Our aforementioned research offers backing for the advancement of novel fluorescent probes.
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