Excited-State
Proton Transfer of Fluorescein Anion
as an Ionic Liquid Component
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Abstract
Fluorescent
ionic liquids (FILs) incorporating the fluorescein
anion have been prepared by anion exchange of the parent quaternary
ammonium chloride (Quat<sup>+</sup>Cl<sup>–</sup>) ionic liquid.
By controlling the molar ratio of fluorescein to Quat<sup>+</sup>Cl<sup>–</sup>, ionic liquids incorporating different prototropic
forms of fluorescein were prepared. The 1:1 molar ratio ionic liquid
(FIL1) is essentially composed of monoanionic fluorescein, while dianionic
fluorecein is predominant in the FIL with a 1:2 molar ratio (FIL2).
The fluorescence excitation spectrum of FIL2 is markedly different
from its absorption spectrum. Absorption features the fluorescein
dianion, while the excitation spectrum is exclusively due to the monoanion.
In FIL1, the absorption and excitation spectra are both characteristic
of the monoanion. In both FILs, emission of the dianion is observed
upon excitation of the monoanion. This unusual behavior is interpreted
in the context of a fast deprotonation of the monoanion in the excited
state. The presence of residual water in the ionic liquid is important
for the proton transfer process. By lowering the pH of FIL1, the transient
proton transfer is inhibited, and the emission of the monoanion could
be observed. The FILs have completely different spectroscopic properties
from solvated fluorescein in Quat<sup>+</sup>Cl<sup>–</sup>, where the prototropic equilibrium is shifted toward the neutral
forms