Anion Effect on the Excited-State Intramolecular Proton Transfer of 4′-N,N-Diethylamino-3-hydroxyflavone in Ionic Liquids

Abstract

The excited-state intramolecular proton transfer (ESIPT) reaction of 4′-N,N,-diethylamino-3-hydroxyflavone (C₂HF) was studied using time-resolved fluorescence measurements in ionic liquids (ILs) of various anions with a fixed cation (1-ethyl-3-methylimidazolium [C₂mim]⁺). C₂HF showed an ESIPT reaction from the normal excited state (N*; keto form) to the tautomer excited state (T*; enol form) where both states are emissive. The ESIPT rate and yield were obtained by analyzing the time-resolved fluorescence spectra measured using the optical Kerr gate method. Both the ESIPT rate and yield decreased with increasing hydrogen-bond accepting ability of the anion. According to density functional theory calculations, the complex formation energy between C₂HF and the anion became significantly negative with increasing the hydrogen-bond accepting ability of anion. The pseudoequilibrium constant between N* and T* ([T*]/[N*]) in the electronic excited state decreased with increasing hydrogen-bond accepting ability of the anion, while it increased with increasing the alkyl-chain length of alkyl sulfonate. The excitation wavelength dependence of the ESIPT rate and yield was studied for C₂HF in [C₂mim][C₆H₁₃SO₃]. The ESIPT yield decreased by nearly a factor of 2 with increasing excitation wavelength from 360 to 425 nm, although the change in the ESIPT rate was small. The solvation heterogeneity due to the alkyl chain in the anion was considered to be the reason for the excitation wavelength dependence.