Excited-State Intramolecular Proton Transfer Reaction and Ground-State Hole Dynamics of 4′-N,N-Dialkylamino-3-hydroxyflavone in Ionic Liquids Studied by Transient Absorption Spectroscopy

The excited-state intramolecular proton transfer (ESIPT) of 4'-N,N-dialkylamino-3-hydroxyflavone (CnHF) having different alkyl chain lengths (ethyl, butyl, and octyl chains) was investigated in ionic liquids (ILs) by steady-state fluorescence and transient absorption spectroscopy. Upon photoexcitation, CnHF underwent ESIPT from the normal form to the tautomer form, and dual emissions from both states were detected. For C4HF and C8HF, the tautomerization yields determined from the fluorescence intensity ratios increased with the increasing number of alkyl chain carbon atoms in the cation and on reducing the excitation wavelength as reported for C2HF [K. Suda et al., J. Phys. Chem. B. 117, 12567 (2013)]. The transient absorption spectra of CnHF were measured at excitation wavelengths of 360, 400, and 450 nm. The ESIPT rate determined from the induced emission of the tautomer was correlated with the tautomerization yield for C2HF and C4HF. In addition, the recovery of the ground-state bleach was found to be strongly dependent on the excitation wavelength. This result indicates that the solvated state of the molecule before photoexcitation is dependent on the excitation wavelengths. The time constant for the groundstate relaxation was slower than that for the excited state.

Automated summary

In this study, the excited-state intramolecular proton transfer (ESIPT) of CnHF with different alkyl chain lengths in ionic liquids was investigated using fluorescence and transient absorption spectroscopy. ESIPT was observed from the normal form to the tautomer form upon photoexcitation, with dual emissions detected. The tautomerization yields and ESIPT rates were found to be influenced by the alkyl chain lengths and excitation wavelengths, indicating a strong dependence on solvation states and transition processes in the molecules.