Hydrophobic
deep eutectic solvents (DESs) exhibit immense potential
as viable environmentally benign inexpensive alternatives to both
nonpolar organic solvents as well as hydrophobic ionic liquids. Pyrene
fluorescence and its quenching by five different nitro compounds are
used as a tool to examine structural features and solute dynamics
within a prototypical hydrophobic DES formed by mixing salt tetra-n-butylammonium chloride (TBAC) as H-bond acceptor with n-decanoic acid (DA) as H-bond donor in 1:2 mol ratio, named
TBAC–DA, in the temperature range 298.15–358.15 K. Changes
in fluorescence emission intensity, empirical polarity scale, and
excited-state intensity decay of pyrene with change in temperature
within TBAC–DA are compared and contrasted with those reported
within common and popular hydrophilic DESs and water miscible and
immiscible ionic liquids. All five nitro compoundsnitromethane,
nitrobenzene, 4-nitrobenzaldehyde, 1-chloro-4-nitrobenzene, and 4-nitroanisolequench
the fluorescence from pyrene in TBAC–DA; the quenching follows
a simplistic Stern–Volmer relation and is purely dynamic in
nature. Quenching of pyrene fluorescence by nitromethane is more within
TBAC–DA as compared to the hydrophilic DES reline and ionic
liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.
It is attributed to the possible stabilization of partial positive
charge that develops on the excited pyrene during electron/charge
transfer to the quencher by partially/completely dissociated acid
groups of the H-bond donor n-decanoic acid. The dynamic
quenching constant (K
D) and bimolecular
quenching rate constant (k
q) within TBAC–DA
are significantly higher for nitrobenzene in comparison to the other
four quenchers. Conformity to the empirical Arrhenius expression is
exhibited by the linear behavior of ln k
q versus 1/T for all five nitro compounds.
While overall pyrene–quencher data does not comply with the
Stokes–Einstein relation, each of the pyrene–quencher
pair data does. This suggests the dependence of diffusion behavior
on the structure of the quencher within TBAC–DA. Pyrene fluorescence
is established as an effective tool to characterize such DESs; the
DESs can be used as solubilizing media to detect and assess the important
class of nitro compounds.