Solvatochromic Probe Response
within Ionic Liquids
and Their Equimolar Mixtures with Tetraethylene Glycol
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Abstract
Synergism
in a probe response within a mixture hints at the presence
of strong interactions involving the solvent constituents of the mixture
and possibly the probe. Unusual and rare “hyperpolarity”
resulting from the synergism in probe response exhibited by ionic
liquid (IL) mixtures with glycol family solvents is investigated in
detail for equimolar mixtures of tetraethylene glycol (TEG) with many
structurally different ILs using several UV–vis absorbance
and fluorescence solvatochromic probes. Thirteen different ILs, of
the same cation 1-butyl-3-methylimidazolium and different anions,
of the same anion bis(trifluoromethylsulfonyl)imide and different
cations, and of C2 methyl-substituted imidazolium cations, are used
to assess the structural dependence of the IL on synergism exhibited
by (IL + TEG) mixture. Responses from UV–vis absorbance probes
are used to obtain <i>E</i><sub>T</sub> [dipolarity/polarizability
and/or H-bond donating (HBD) acidity] and Kamlet–Taft parameters
[π* (dipolarity/polarizability), α (HBD acidity), and
β (HB accepting basicity)] within (IL + TEG) mixtures. The band
I-to-band III fluorescence intensity ratio of dipolarity probe pyrene
along with the lowest energy fluorescence band maxima of pyrene-1-carboxaldehyde
(PyCHO, a probe for the permittivity of the medium), coumarin-153
and <i>N</i>,<i>N</i>-dimethyl-6-propionyl-2-naphthylamine
PRODAN (neutral photoinduced charge-transfer fluorescence probes),
and 6-<i>p</i>-toluidine-2-naphthalenesulfonic acid (TNS)
and l-anilinonaphthalene-8-sulfonate (ANS) (ionic photoinduced charge-transfer
fluorescence probes) are used to assess whether synergism is exhibited
by (IL + TEG) equimolar mixtures. Probe responses within TEG equimolar
mixtures with ILs are compared to those with common organic solvents.
An attempt is made to establish a correlation between the synergism
observed in the probe response within an (IL + TEG) mixture and the
structural features of the cation and anion of the IL, such as acidity
of the protons of the cation, aromaticity of the cation, and size,
shape, and coordinating ability of the anion. It is established that
the solvatochromism exhibited by the probes within (IL + TEG) mixtures
is due to complex coupling of several different interactions and dynamical
processes involving the probe as well as IL and TEG within the mixture