Excitation
Wavelength Dependence of Excited State
Intramolecular Proton Transfer Reaction of 4′‑N,N‑Diethylamino-3-hydroxyflavone
in Room Temperature Ionic Liquids Studied by Optical Kerr Gate Fluorescence
Measurement
Excited
state intramolecular proton transfer reactions (ESIPT)
of 4′-N,N-diethylamino-3-hydroxyflavone
(DEAHF) in ionic liquids have been studied by steady-state and time-resolved
fluorescence measurements at different excitation wavelengths. Steady-state
measurements show the relative yield of the tautomeric form to the
normal form of DEAHF decreases as excitation wavelength is increased
from 380 to 450 nm. The decrease in yield is significant in ionic
liquids that have cations with long alkyl chains. The extent of the
decrease is correlated with the number of carbon atoms in the alkyl
chains. Time-resolved fluorescence measurements using optical Kerr
gate spectroscopy show that ESIPT rate has a strong excitation wavelength
dependence. There is a large difference between the spectra at a 200
ps delay from different excitation wavelengths in each ionic liquid.
The difference is pronounced in ionic liquids having a long alkyl
chain. The equilibrium constant in the electronic excited state obtained
at a 200 ps delay and the average reaction rate are also correlated
with the alkyl chain length. Considering the results of the steady-state
fluorescence and time-resolved measurements, the excitation wavelength
dependence of ESIPT is explained by state selective excitation due
to the difference of the solvation, and the number of alkyl chain
carbon atoms is found to be a good indicator of the effect of inhomogeneity
for this reaction.