The excited-state intramolecular proton transfer (ESIPT) of 4′-N,Ndialkylamino-3-hydroxyflavone (C n HF) 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, C n HF underwent ESIPT from the normal form to the tautomer form, and dual emissions from both states were detected. For C 4 HF and C 8 HF, 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 C 2 HF [K. Suda et al., J. Phys. Chem. B. 117, 12567 (2013)]. The transient absorption spectra of C n HF 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 C 2 HF and C 4 HF. 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.

No abstract