Temperature-dependent solvatochromic probe behavior within ionic liquids and (ionic liquid + water) mixtures

Article abstract of DOI:10.1021/jp102217u

Spectroscopic responses of absorbance probes, betaine dye 33, N,N-diethyl-4-nitroaniline, and 4-nitroaniline, and fluorescence dipolarity probes, pyrene (Py) and pyrene-1-carboxaldehyde (PyCHO) within ionic liquids (ILs) 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]), and aqueous mixtures of [bmim][BF₄] are used to assess the changes in important physicochemical properties with temperature in the range 10-90 ?°C. ET^N obtained from betaine dye 33, indicating dipolarity/ polarizability and/or hydrogen bond donating (HBD) acidity, decreases linearly with increasing temperature within the two ILs. Changes in Kamlet-Taft parameters dipolarity/polarizability (??*), HBD acidity (?±), and HB accepting (HBA) basicity (?2) with temperature show interesting trends. While ??* and ?± decrease linearly with increasing temperature within the two ILs, ?2 appears to be independent of the temperature. Similar to ET_Nand ??*, the first-to-third band intensity ratio of probe Py also decreases linearly with increasing temperature within the ILs. The lowest energy fluorescence maxima of PyCHO, though it decreases significantly within water as the temperature is increased from 10 to 90 ?°C, it does not change within the two ILs investigated. The temperature dependence of the dipolarity/polarizability as manifested via betaine dye 33 behavior is found to be more within the aqueous mixtures of [bmim][BF₄] as compared to that within neat [bmim][BF₄] or neat water. The sensitivity of ??* toward temperature increases as IL is added to water and that of ?± decreases. The temperature dependent Py behavior shows no clear-cut trend within aqueous mixtures of [bmim][BF₄]; insensitivity of the PyCHO response toward temperature change is reasserted within aqueous IL mixtures. All-in-all, the temperature-dependent behavior of solvatochromic probes within [bmim][PF₆], [bmim][BF₄], and aqueous mixtures of [bmim][BF₄] is found to depend on the identity of the probe. ? 2010 American Chemical Society.

Full text of DOI:10.1021/jp102217u

8
118
J. Phys. Chem. B 2010, 114, 8118–8125
Temperature-Dependent Solvatochromic Probe Behavior within Ionic Liquids and (Ionic
Liquid + Water) Mixtures
†
‡
†
,†
Shruti Trivedi, Naved I. Malek, Kamalakanta Behera, and Siddharth Pandey*
Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India, and
Applied Chemistry Department, Sardar Vallabhbhai National Institute of Technology (SVNIT),
Surat 395007, India
ReceiVed: March 11, 2010; ReVised Manuscript ReceiVed: April 16, 2010
Spectroscopic responses of absorbance probes, betaine dye 33, N,N-diethyl-4-nitroaniline, and 4-nitroaniline,
and fluorescence dipolarity probes, pyrene (Py) and pyrene-1-carboxaldehyde (PyCHO) within ionic liquids
(
ILs) 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF
tetrafluoroborate ([bmim][BF ]), and aqueous mixtures of [bmim][BF
important physicochemical properties with temperature in the range 10-90 °C. E
3, indicating dipolarity/polarizability and/or hydrogen bond donating (HBD) acidity, decreases linearly with
6
]) and 1-butyl-3-methylimidazolium
4
4
] are used to assess the changes in
N
T
obtained from betaine dye
3
increasing temperature within the two ILs. Changes in Kamlet-Taft parameters dipolarity/polarizability (π*),
HBD acidity (R), and HB accepting (HBA) basicity (ꢀ) with temperature show interesting trends. While π*
and R decrease linearly with increasing temperature within the two ILs, ꢀ appears to be independent of the
N
temperature. Similar to E
T
and π*, the first-to-third band intensity ratio of probe Py also decreases linearly
with increasing temperature within the ILs. The lowest energy fluorescence maxima of PyCHO, though it
decreases significantly within water as the temperature is increased from 10 to 90 °C, it does not change
within the two ILs investigated. The temperature dependence of the dipolarity/polarizability as manifested
via betaine dye 33 behavior is found to be more within the aqueous mixtures of [bmim][BF
to that within neat [bmim][BF ] or neat water. The sensitivity of π* toward temperature increases as IL is
added to water and that of R decreases. The temperature dependent Py behavior shows no clear-cut trend
within aqueous mixtures of [bmim][BF ]; insensitivity of the PyCHO response toward temperature change is
reasserted within aqueous IL mixtures. All-in-all, the temperature-dependent behavior of solvatochromic probes
4
] as compared
4
4
within [bmim][PF
of the probe.
6 4 4
], [bmim][BF ], and aqueous mixtures of [bmim][BF ] is found to depend on the identity
Introduction
play an important role in altering the behavior of dye probes
and porphyrin aggregation within solution.9
The newly rediscovered room-temperature ionic liquids (ILs),
consisting entirely of ions that are molten at room temperature
with negligible vapor pressure, are showing enormous potential
as media with unusual properties. Consequently, interest in
investigating the properties of ILs has been intensified in many
fields of science and technology. They appear to be promising
future replacements for volatile organic compounds (VOCs)
though this proposition is yet to be established. The fact that
an IL is composed of ions alone and is still liquid at ambient
conditions is reason enough to evoke curiosity among research-
ers as far as properties and applications are concerned. Further,
the option of fine-tuning the physicochemical properties by an
appropriate choice of cations and anions has stimulated much
of the current excitement with respect to these compounds and
has led to the term “designer solvents”. Almost every named
synthesis and many more organic/inorganic/organometallic
reactions have been reported in ILs.
As mentioned earlier, properties of ILs could be specific to
the cation-anion combination; however, mixtures of ILs with
other solvents have been proposed to possess altered (and in
1
1
0
some cases, even improved) physicochemical properties.
2
Mixtures of ILs with water, in this respect, have garnered
6
increased attention. The major reason for this could be traced
3
to the fact that aqueous mixtures of ILs may form a class of
10
“
hybrid green” system. The fact that many ILs are hygroscopic
in nature further contributes to the growing interest in under-
standing aqueous IL mixtures. Due to the possibility of strong
intermolecular H-bonding interactions, among others, between
water and the constituent ions of ILs,6 addition of water may
potentially alter the physicochemical properties of ILs in
significant fashion. In this regard, though the investigation of
,11
4
12
structural features of the solution as well as measurement of
bulk physicochemical properties13 of aqueous mixtures of ILs
are of certain importance, understanding the behavior of solutes
dissolved in such media may directly furnish crucial information
on solute-solvent interactions(s). Key insights on physico-
chemical properties of aqueous mixtures of ILs along with the
information on solute solvation within such systems would be
obtained in the process.
2
a-c
Novel analytical
5
applications of ILs are emerging day by day. Due to the
possibility of widespread applications in various fields, ILs are
employed with other environmentally benign systems such as
6
7
8
aqueous, polymer, and surfactant-based systems, etc. ILs also
*
To whom correspondence should be addressed. E-mail: sipandey@
chemistry.iitd.ac.in. Phone: +91-11-26596503. Fax: +91-11-26581102.
†
Responses of solvatochromic absorbance and fluorescence
probes usually change as a result of changes in the cybotactic
Indian Institute of Technology.
Sardar Vallabhbhai National Institute of Technology (SVNIT).
‡
1
0.1021/jp102217u  2010 American Chemical Society
Published on Web 06/02/2010

Solvatochromic Probe Behavior within Ionic Liquids
J. Phys. Chem. B, Vol. 114, No. 24, 2010 8119
region of the probe. The changes in the cybotactic region could
be due to the changes in the specific solute-solvent interactions
or they could be due to the changes in the properties of the
milieu as a result of changing conditions, e.g., changing
temperature and/or pressure. Depending on the solute-solvent
interaction(s), a solubilizing medium may exert a profound effect
on the electronic transition of a solute. In solvatochromic
absorbance and fluorescence probes, such changes in electronic
transitions are systematic with respect to some property of the
medium. Many of such properties, e.g., dipolarity/polarizability,
dipolarity, static dielectric constant, hydrogen-bond donating
ratories, respectively. Doubly distilled deionized water with
>18.0 MΩ cm resistivity was obtained from a Millipore Milli-Q
Academic water purification system. Pyrene and pyrene-1-
carboxaldehyde were purchased in the highest purities available
from Sigma-Aldrich. Ethanol was obtained from Merck.
4 6
ILs [bmim][BF ] and [bmim][PF ] were obtained from Merck
(
ultrapure, halide content <10 ppm, water content <10 ppm)
and stored under dry argon. Alternatively, these ILs were
synthesized and purified in our laboratories using the procedures
17
provided in the literature. Specifically, [bmim][BF
4
] was
synthesized from [bmim][Cl]. To obtain [bmim][Cl], a mixture
of 1-methylimidazole (24.64 g, 0.3 mol), 1-chlorobutane (37.03
g, 0.4 mol), and acetonitrile (50 mL) was refluxed for 48 h,
after which the solvent and an excess of 1-chlorobutane were
removed. The remaining pale yellow oil was redissolved in 50
mL of acetonitrile and added dropwise to 200 mL of ethyl
acetate under nitrogen with vigorous agitation. The mixture was
cooled to -30 °C and kept at this temperature for 2 h; a white
(
HBD) acidity, hydrogen-bond accepting (HBA) basicity, etc.,
are readily manifested through molecular absorbance or fluo-
rescence spectra of a variety of solvatochromic probes.13
To assess the temperature dependence of important physi-
cochemical properties of ILs and aqueous mixture of ILs, we
have chosen to observe the behavior of different solvatochromic
absorbance and fluorescence probes as a function of temperature.
Specifically, we have used three common electronic absorbance
probes, betaine dye 33, N,N-diethyl-4-nitroaniline, and 4-ni-
troaniline, and two popular fluorescence probes, pyrene and
pyrene-1-carboxaldehyde, for this purpose (structures of the
probes are provided in Scheme S1, Supporting Information).
While the behaviors of absorbance probes furnish information
on empirical parameters of importance of a medium, the two
fluorescence probes are common empirical polarity probes (vide
infra). To understand the effect of temperature on solvatochro-
mic probe behavior that may manifest in changes in important
physicochemical properties of ILs as a function of ions
constituting ILs, we have selected two ILs having the same
cation but different anions: 1-butyl-3-methylimidazolium hexaflu-
solid was formed. The supernatant was removed and the washing
17a
step was repeated to obtain [bmim][Cl]. A solution of NaBF
(
4
3
0.060 mol) in water (50 cm ) was added slowly to a cooled,
rapidly stirring solution of [bmim][Cl] (0.060 mol) in water (200
3
17b
cm ). The product precipitated as a waxy solid and was
3
collected by filtration, dissolved in dichloromethane (300 cm ),
3
and washed with water (2 × 100 cm ). The organic layer was
4
collected, dried over MgSO , and filtered and the solvent was
removed in vacuo to yield the tetrafluoroborate salt, which was
recrystallized from methanol as a colorless product. The minimum
volume of water was used to dissolve the chloride salts. The
tetrafluoroborate salt did not initially separate from the aqueous
phase but was preferentially extracted by dichloromethane.
orophosphate ([bmim][PF
tetrafluoroborate ([bmim][BF
mation). While [bmim][PF ] is considered a “hydrophobic” IL
with minimal aqueous solubility, “hydrophilic” IL [bmim][BF
6
]) and 1-butyl-3-methylimidazolium
6
To prepare [bmim][PF ], hexafluorophosphoric acid (1.3 mol)
4
]) (Scheme S1, Supporting Infor-
was added (slowly to prevent the temperature from rising
significantly) to a mixture of [bmim][Cl] (1 mol) in 500 mL of
water. After stirring for 12 h, the upper acidic aqueous layer
was decanted and the lower IL portion was washed with water
6
4
]
14
is completely miscible with water. Subsequently, temperature
dependent probe behavior is investigated within aqueous
(
10 × 500) mL until the washings were no longer acidic. The
4
mixtures of [bmim][BF ] in the complete composition regime
IL was heated under vacuum at 70 °C to remove any excess
water.1 We observed our results to be similar irrespective of
whether the ILs were purchased or synthesized.
in the temperature range 10-90 °C.
7c
It is well-established that temperature has a profound effect
1
5
on the physicochemical properties of solutions. Recently, El
Seoud groups investigated thermosolvatochromism in binary
mixtures of water and ILs (1-allyl-3-alkylimidazolium chlorides)
Methods. The required amount of probes was weighed using
a Mettler Toledo AB104-S balance with a precision of (0.1
mg. All absorbance and fluorescence probe stock solutions were
prepared in absolute ethanol and stored in sealed ambered glass
to assess the relative importance of solute-solvent solvophobic
interactions.16a Previously, the same group had reported ther-
4
vials at 4 ( 1 °C. Aqueous [bmim][BF ] solutions were prepared
mosolvatochromic behavior of certain dyes in aqueous
by mass using an Ohaus AR2130 balance with a precision of
(1 mg. An appropriate amount of the probe solution from the
stock was transferred to precleaned quartz cuvette. Ethanol was
evaporated using a gentle stream of high-purity dry, filtered
[
4
bmim][BF ] and compared it with behaviors of aqueous
11d
alcohols in the temperature range 10-60 °C. An earlier report
on thermosolvatochromic behavior of chloronickel complexes
in 1-hydroxyalkyl-3-methylimidazolium cation based ILs sug-
nitrogen gas. [bmim][PF
6 4
], [bmim][BF ], water, or aqueous
gested changes in the structure of the complex as the temperature
was changed.16b Interesting results were found by Kumar et al.
[bmim][BF ] mixture was added to the cuvette to achieve the
4
desired probe concentration.
by contrasting thermosolvatochromic trends in three series of
1
5m
A Varian Cary 100-bio double-beam spectrophotometer with
variable bandwidth was used for the acquisition of the UV-vis
molecular absorbance spectra. Steady-state fluorescence spectra
were acquired on a Jobin-Yvon Fluorolog-3 (model FL-3-11)
modular spectrofluorometer equipped with single grating
Czerny-Turner monochromators as wavelength selection de-
vices, a 450 W Xe arc lamp as the excitation source, and a
photomultiplier tube as the detector. All absorbance and
ILs in the temperature range 298-353 K. The temperature
dependent polarity of [bmim][PF ] was studied by Baker et al.
6
emphasizing that HBD strength of imidazolium cation was
strongly temperature dependent but HBA abilities were weak
1
k
functions of temperature and added water.
Experimental Section
2
Reagents and Supplies. 2,6-Dichloro-4-(2,4,6-triphenyl-N-
pyridino)phenolate (betaine dye 33), 4-nitroaniline, and N,N-
diethyl-4-nitroaniline were purchased in the highest available
purity from Fluka, Spectrochem. Co. Ltd., and Frinton Labo-
fluorescence data were acquired using 1 cm quartz cuvettes.
All spectroscopic measurements were performed in triplicate
with samples prepared individually, and the results were
averaged. All spectra were duly corrected by measuring the

8
120 J. Phys. Chem. B, Vol. 114, No. 24, 2010
Trivedi et al.
Figure 1. Variation of ET
of the linear regression analysis.
(A), π* (B), R (C), and ꢀ (D) with temperature in [bmim][PF
] (1), [bmim][BF
] (9), and water (b). Lines show results
N
6
4
spectral responses from suitable blanks prior to data analysis
and statistical treatment.
28591.5/λ am b as x (nm). However, in the present work betaine dye
33 [corresponding molar transition energy is subsequently
T
termed as E (33)] is used due to certain advantages over betaine
-
6
Results and Discussion
dye 30. Specifically, the low solubility (<10 M) of betaine
dye 30 in water renders it somewhat unsuitable to investigate
Temperature-Dependent Probe Behavior within Neat
Ionic Liquids. We have utilized an effective UV-vis molecular
absorbance probe 2,6-dichloro-4-(2,4,6-triphenyl-N-pyridino)phe-
nolate (betaine dye 33) (see structure in Scheme S1, Supporting
Information) to assess its temperature dependence, which is
subsequently manifested into the temperature dependence of
dipolarity/polarizability and/or hydrogen bond donating (HBD)
acidity of neat ILs as well as IL-water mixtures. The more
popular 2,6-diphenyl-4-(2,4,6-triphenyl-N-pyridino)phenolate
aqueous-based solutions.1 Betaine dye 33 has a pK
k,20
value of
a
4
.78 ( 0.05, and it remains unprotonated at physiological pH.
An established empirical relationship was used to convert E (33)
(30), which was
T
10a
values to the more commonly employed E
T
N
subsequently used to arrive at the corresponding E
T
, as suggested
in the literature.21
N
Figure 1A presents measured E
bmim][BF
T
within [bmim][PF
], and water, respectively, in the temperature range
0-90 °C. A cursory examination reveals that an increase in
temperature results in decrease in E within each of the solvents
6
],
[
4
1
(
betaine dye 30) exhibits an unusually high solvatochromic band
N
18
shift. The lowest energy intramolecular charge-transfer absorp-
tion band of betaine dye 30 is hypsochromically shifted by ca.
T
N
investigated. A decrease in E
T
implies a decrease in the
dipolarity/polarizability and/or the HBD acidity of the medium
with increasing temperature. In general, “polarity” is suggested
to usually decrease with increasing temperature due in major
part to the increased average thermal reorientation of the
3
57 nm on going from relatively nonpolar diphenyl ether (λmax
810 nm) to water (λmax ∼ 453 nm). There is a considerable
∼
charge transfer from the phenolate to the pyridinium part of
the zwitterionic molecule. Because of its zwitterionic nature,
the solvatochromic probe behavior of betaine dye 30 is strongly
affected by the HBD acidity of the solvent; HBD solvents
22
dipoles. This results in usually a decrease in dielectric constant
with increasing temperature of polar liquids due partly to the
1
9
22
stabilize the ground state more than the excited state. This is
one of the most widely used probes of its kind; the empirical
scale of solvent “polarity”, E
transition energy of the dye in kcal mol- at room temperature
and normal pressure according to the expression E (30) )
destruction of the cooperative effect. For example, the static
dielectric constant of water is observed to decrease as the
23
T
(30), is defined as the molar
temperature is increased to 100 °C. It is important to mention
N
1
that the observed decrease in E
similar to that observed by Baker et al. for [bmim][PF
T
with increasing temperature is
] using
T
6

Solvatochromic Probe Behavior within Ionic Liquids
TABLE 1: Slopes Recovered from the Linear Regression Analysis of EN
J. Phys. Chem. B, Vol. 114, No. 24, 2010 8121
T
, π*, r, ꢀ, Pyrene I
1
/I
Pyrene-1-carboxaldehyde Lowest Energy Fluorescence Maxima (PyCHO λmax), Respectively, versus Temperature in
bmim][PF ], [bmim][BF ], and Water
3 1 3
(Py I /I ) and
fluo
[
6
4
slope
probe response
[bmim][PF
6
]
[bmim][BF
4
]
water
N
-4 -1
-4 -1
-4 -1
E
T
-8.50 ((0.36) × 10
-8.37 ((0.78) × 10
-9.29 ((0.90) × 10
-5.15 ((1.43) × 10
-4.24 ((0.70) × 10
K
-7.55 ((0.79) × 10
-9.26 ((0.67) × 10
-6.64 ((1.35) × 10
-5.26 ((1.32) × 10
-4.21 ((0.29) × 10
K
-9.10 ((0.55) × 10
-0.57 ((0.02) × 10
K
-
-
-
-
4
4
4
3
-1
-1
-1
-1
-4 -1
-4 -1
π*
R
K
K
K
K
K
K
-4 -1
-4 -1
K
-17.2 ((1.7) × 10
5.63 ((2.88) × 10
-2.99 ((0.33) × 10
K
-4 -1
-4 -1
ꢀ
K
K
-3 -1
-3 -1
Py I
PyCHO λmax
1
/I
3
K
K
fluo
-2
-1
-2
-1
-2
-1
-1.50 ((0.83) × 10 nm K
3.33 ((1.91) × 10 nm K
-9.50 ((1.79) × 10 nm K
betaine dye 301k and by the El Seoud group for [bmim][BF
and for 1-allyl-3-alkylimidazolium chlorides (alkyl being methyl,
]11d
4
6 4
the two ILs [bmim][PF ] and [bmim][BF ], respectively, are
observed to be statistically similar, as evidenced from the two
slope values, indicating two ILs to have similar temperature-
dependent dipolarities/polarizabilities. However, the reduction
in π* as the temperature is increased within each of the ILs is
very different from that observed within water where the
decrease in π* is almost insignificant. While the static dielectric
constant of water is known to decrease with increasing tem-
perature,23 it is not manifested in the behavior of absorbance
probe DENA. This may be attributed to the compensatory
contributions from polarizability, although it is proposed that
1
6a
1
-butyl, and 1-hexyl, respectively) using different UV-vis
N
absorbance-based probes. Linear regression analysis of E
temperature for [bmim][PF ], [bmim][BF ], and water, respec-
tively, reveals a decrease in E with increasing temperature to
T
versus
6
4
N
T
be linear in nature (Figure 1A and Table S1, Supporting
Information). The temperature dependence of dipolarity/polar-
izability and/or HBD acidity as revealed by betaine dye 33 is
N
represented by the slope of the E
T
versus temperature best fit
straight line (Table 1). Though the dipolarity/polarizability and/
or HBD acidity decreases the most within water followed by
2
3
polarizabilities are only very slightly temperature dependent.
For IL [bmim][PF ], Bright’s group also observed a linear
decrease in π* with increasing temperature. It is clear that,
unlike water, the dipolarity/polarizability of ILs [bmim][PF
and [bmim][BF ] decreases as the temperature is increased.
Variations in HBD acidity (R) with temperature within
[bmim][PF ], [bmim][BF ], and water, respectively, however,
that within ILs [bmim][PF
6
] and [bmim][BF
4
], the differences
6
1k
in slopes are statistically not very significant. It may be inferred
that the decrease in dipolarity/polarizability and/or HBD acidity
6
]
with increasing temperature within ILs [bmim][PF
bmim][BF ], respectively, is not too different from that
observed within water. Further, ILs constituted of the same
6
] and
4
[
4
6
4
+
bmim cation but two different anions PF
contrasting aqueous solubilities to the two ILs ([bmim][PF
hydrophobic”, while [bmim][BF ] is “hydrophilic”), show
6
and BF
4
, furnishing
are found to be different. The parameter R is observed to
decrease with increasing temperature within each of
[bmim][PF ], [bmim][BF ], and water. The extent of decrease
6 4
6
] is
“
4
similar decreases in dipolarity/polarizability and/or HBD acidity
as the temperature is increased based on the behavior of betaine
dye 33. It is concluded that the increased average thermal
reorientations of the dipoles leading to the destruction of the
cooperative effect resulting in decreased dielectric constant with
in R with increasing temperature (represented by the slope of
R versus temperature best fit straight lines, Table 1) is observed
to be a maximum within water followed by that within
[bmim][PF
IL [bmim][BF
in R with increasing temperature within [bmim][PF
important to mention that while π* (and to a lesser extent E
of [bmim][BF ] is slightly higher than that of [bmim][PF ] in
6
]; the extent is observed to be a minimum within
4
]. Bright’s group has reported a similar decrease
1
k
increasing temperature for both the ILs [bmim][PF
6
] and
6
]. It is
N
[
bmim][BF ] are very similar. The HBD acidity, which is
4
T
)
+
decided in major part by the C2-H of bmim of these ILs,
may be assumed to change in a similar fashion with temperature
as both the ILs possess the same cation (vide infra).
4
6
the temperatures range 10-90 °C, the R within the two ILs are
very similar at lower temperatures and only start to become
different at very high temperatures. It is easily conceivable as
To assess separately the temperature dependence of dipolarity/
polarizability, HBD acidity, and HBA basicity of ILs, we used
a well-documented empirical procedure for the estimation of
Kamlet-Taft parameters using UV-vis molecular absorbance
probes N,N-diethyl-4-nitroaniline (DENA) and 4-nitroaniline
+
the C2-H of the bmim is the major contributor to the HBD
acidity of ILs constituted of bmim cation.1
9,25
It appears that
the anion of an IL has an insignificant effect on HBD acidity
of ILs having the bmim cation. Only at higher temperatures
24
+
(
NA) (Scheme S1, Supporting Information). We measured the
wavelength of electronic absorbance maxima of DENA and NA,
respectively, within each [bmim][PF ], [bmim][BF ], and water
in the temperature range 10-90 °C, and by combining it with
(30) (vide supra), we obtained empirical parameters π*
may the other factors such as the interaction of bmim with
the anion start to play a role in HBD acidity of the bmim-type
6
4
ILs. The temperature dependence of R is a little more pro-
nounced within IL [bmim][PF
[bmim][BF ].
HBA basicity (ꢀ) does not change in a distinct manner in
any of [bmim][PF ], [bmim][BF ], or water as the temperature
6
] as compared to that within IL
E
T
4
(
dipolarity/polarizability), R (HBD acidity), and ꢀ (HBA basic-
24
ity). It is important to mention at this point that π*, R, and ꢀ
for the two ILs assessed by us at ambient temperature are in
6
4
is changed (Figure 1D and Table 1). This is clearly represented
by the high values of the standard deviations of the slopes; errors
associated with each of the ꢀ measurements is also fairly
significant. The linear regression analyses of ꢀ versus temper-
ature data also result in not-so-good fits (Table S1, Supporting
Information). Bright and co-workers have also reported ꢀ to be
good agreement with those reported by other groups in the recent
past.1
d,k,11e,19
Measured π* (panel B), R (panel C), and ꢀ (panel
D) within [bmim][PF
6
], [bmim][BF ], and water, respectively,
4
are presented in Figure 1, while the slopes recovered from the
linear regression analysis of each of the parameter versus
temperature are reported in Table 1. Some interesting inferences
can clearly be drawn from our experimental outcomes. The
extents of decrease in π* with an increase in temperature within
1
k
not a strong function of temperature between 10 and 70 °C. It
appears that ꢀ within each of the IL decreases ever-so-slightly
to a similar extent with increasing temperature, it appears to

8
122 J. Phys. Chem. B, Vol. 114, No. 24, 2010
Trivedi et al.
N
Similar to the observations for E
with increasing temperature within each of the two ILs and
water. Further, the slopes of Py I /I versus temperature best fit
T
and π*, the Py I
1 3
/I decreases
1
3
straight lines (Table 1) representing the extent of temperature
dependence is observed to be almost identical for the two ILs.
Interestingly, this slope for water is observed to be lower.
1 3
Dependence of Py I /I on temperature, in this regard, is more
similar to that observed for the parameter π*. It is easy to
comprehend as these two empirical parameters are supposed to
represent dipolarity/polarizability of the medium in general. Two
important inferences, that the dipolarities of the two ILs
investigated here decrease more rapidly as compared to that of
water and that these decreases are similar for both the ILs
irrespective of their different anions, are reemphasized nonetheless.
The temperature-dependent behavior of another fluorescence
probe pyrene-1-carboxaldehyde (PyCHO), a pyrene probe
analog containing an aldehyde functionality (Scheme S1,
Supporting Information), within the two ILs is found to be
different from that observed for pyrene. PyCHO has two types
of closely lying excited singlet states (n-π* and π-π*), both
of which show emission in fluid solution. In nonpolar solvents,
the emission from PyCHO is highly structured and weak (φ <
F
-
3
1
0
in hexane), arising exclusively from the n-π*state. On
increasing the polarity of the medium, however, the π-π* state
is brought below the n-π* state via solvent relaxation to
become the emitting state, manifested by broad, moderately
intense emission (φ
increasing solvent dielectric. Figure 2B presents
experimental PyCHO λmax within [bmim][PF ], [bmim][BF ],
F
≈ 0.15 in MeOH) that red shifts with
1e,7f,26a,27c,e
fluo
6
4
and water, respectively, in the temperature range 10-90 °C. A
Figure 2. Variation of pyrene (1 µM) I
carboxaldehyde (10 µM) lowest energy fluorescence maxima λmax (B)
with temperature in [bmim][PF ] (1), [bmim][BF ] (9), and water (b)
excitation ) 337 and 365 nm, and excitation and emission slits are at
/1 and 2/2 nms for pyrene and pyrene-1-carboxaldehyde, respectively).
1 3
/I (A) and pyrene-1-
fluo
careful examination of the data reveals an interesting outcome:
fluo
within each of the two ILs investigated, PyCHO λmax appears
6
4
fluo
(
2
λ
to be insensitive to the temperature variations (the PyCHO λmax
between 10 and 90 °C are within the experimental error for
Lines show results of the linear regression analysis.
both the ILs). Further, the resulting linear fits of the data between
fluo
PyCHO λmax and temperature are not satisfactory (Table S1,
Supporting Information). It is noteworthy that in water, PyCHO
increase slightly with increasing temperature within water. It is
reemphasized from other reports in the literature that IL
fluo
λ
4
max does decrease significantly from 474((1) nm at 10 °C to
66((1) nm at 90 °C. This is in agreement with the decrease
[
bmim][BF
4
] possesses a significantly higher HBA basicity as
2
5c
in static dielectric constant of water from 83.96 at 10 °C to
5
static dielectric constant of the medium,
compared to the case for IL [bmim][PF
6
]. This is obviously
29
8.12 at 90 °C. As PyCHO is known to be a probe of the
25b,26a,b,27c
attributed to the different anions of the ILs, as the anion of an
IL usually plays the major role in deciding the HBA basicity.25c
it may be
inferred that either the static dielectric constants of ILs
bmim][PF ] and [bmim][BF ] do not change significantly with
Molecular fluorescence from an appropriate fluorophore is
well-suited to furnish information regarding complex systems
owing to the high sensitivity and orthogonality of information
[
6
4
a change in temperature in the range 10-90 °C or the
fluo
temperature independence of PyCHO λmax is specific to this
1
8b,26
inherent to fluorescence techniques.
In some previous
probe, which may be a consequence of solute-solvent interac-
tions between the aldehyde functionality of PyCHO and the
constituents of ILs, rendering this probe unsuitable as a
thermosolvatochromic probe.
investigations, we have demonstrated the effectiveness of using
a variety of fluorescence probes to obtain key insight into
solute-solvent and solvent-solvent interactions within IL-based
solvent mixtures.1
e,h,k,27
To further assess the temperature
Temperature-Dependent Probe Behavior within Aqueous
dependence of the properties via the probe behavior within ILs,
we have utilized two valuable fluorescence polarity probes,
pyrene and pyrene-1-carboxaldehyde. Pyrene (Scheme S1,
Supporting Information) is one of the most widely used neutral
Mixtures of [bmim][BF
properties of water mixtures of IL[bmim][BF
4
]. The temperature dependence of the
] is acquired
4
through the behavior of the probes used in the previous section
at 0.1 mol fraction intervals in the temperature range 10-90
8
,26,28
fluorescence probes for polarity studies.
polarity scale (Py I /I ) is defined by its I
ratio, where I
arising from the S
sponds to the solvent-insensitive S
The I /I ratio increases with increasing solvent dipolarity and
is a function of both the solvent dielectric (ε) and the refractive
The pyrene solvent
°
C. IL [bmim][BF
completely miscible with water whereas IL [bmim][PF
4
] is selected for this purpose, as this IL is
] shows
])
1
3
1 3
/I emission intensity
1
is the intensity of the solvent-sensitive band
(V)0) f S (V)0) transition and I corre-
(V)0) f S (V)1) transition.
6
limited solubility in water [only ∼2 wt % water (or [bmim][PF
6
1
1
0
3
4
26,28
is miscible in [bmim][PF ] (or water) at ambient conditions].
6
1
0
N
1 3
, π*, R, ꢀ, Py I /I
, and PyCHO λ mfl ua ox were
Parameters E
T
1
3
obtained at 10 deg intervals in the temperature range 10-90
°C at nine different compositions of aqueous [bmim][BF
mixtures in [bmim][BF
0.1-0.9 (Figures S1-S6, Supporting Information). Results of
2
index (n) via the dielectric cross term, f(ε,n ). Measured Py I
within [bmim][PF ], [bmim][BF ], and water, respectively,
in the temperature range 10-90 °C are plotted in Figure 2A.
1
/
4
]
I
3
6
4
4
] mole fraction range X[bmim][BF4] )

Solvatochromic Probe Behavior within Ionic Liquids
J. Phys. Chem. B, Vol. 114, No. 24, 2010 8123
Figure 3. Slopes recovered from the linear regression analysis of ET
4
of [bmim][BF ] mole fraction (X[bmim][BF4]) within aqueous [bmim][BF ].
(A), π* (B), R (C), and ꢀ (D), respectively, versus temperature as function
N
4
-
N
the linear regression analysis of the parameters versus temper-
ature at each composition of aqueous [bmim][BF ] mixtures are
and BF
4
T
of IL and -H of water render E of the mixture extra
4
sensitivity toward temperature change. The same may not be
presented in Table S2 (Supporting Information) and are shown
as dark lines in Figures S7-S12 (Supporting Information). It
is important to mention that at ambient temperature the values
of these parameters are similar to those reported earlier.6d
Though from a careful examination of the data plotted in Figures
S7-S12 (Supporting Information) and parameters reported in
Table S2 (Supporting Information) it appears that the fit of the
parameters versus the temperature data to a linear relationship
may not be entirely satisfactory at all compositions for all
implied about the changes in π* with temperature within
aqueous [bmim][BF ] mixtures (Figure 3B). The extent of
4
decrease in π* with temperature appears to be similar in the
composition range X[bmim][BF4] ) ∼0.3-1, but it decreases
dramatically in the water-rich region, i.e., X[bmim][BF ] e 0.3. This
4
is conceivable as π* is almost insensitive to temperature changes
in water but it decreases with increasing temperature in IL
[bmim][BF
temperature becomes more prominent as water is added to IL
[bmim][BF ] (Figure 3C), as R in water is more sensitive to
4
]. Similarly, the decrease in R with increasing
parameters, we chose to demonstrate the slope as a function of
4
N
X[bmim][BF4] for all parameters. Figure 3 presents slopes of E
T
temperature changes. The slope values appear to reach a plateau
(
panel A), π* (panel B), R (panel C), and ꢀ (panel D),
respectively, versus temperature at each composition within
aqueous [bmim][BF ] mixtures. It is clear from Figure 3A that,
as suggested by betaine dye 33, dipolarity/polarizability and/or
HBD acidity of aqueous mixtures of IL [bmim][BF ] are more
sensitive to the changes in temperature than that of IL
bmim][BF ] alone-the decrease in dipolarity/polarizability and/
or HBD acidity with temperature is more in [bmim][BF ] +
at X[bmim][BF4] ∼ 0.7 and increase again when X[bmim][BF4] e 0.3.
4
We infer that as water is added to [bmim][BF ], an increase in
4
temperature dependence of R combined with the temperature
independence of π* results in an overall increase in temperature
4
dependence of the dipolarity/polarizability and/or HBD acidity
N
manifested via the increase in temperature dependence of E
T
.
[
4
However, a subsequent decrease in temperature dependence of
π* as we move toward the water-rich region results in a decrease
in temperature dependence of the dipolarity/polarizability and/
or HBD acidity after reaching a maximum value. As indicated
4
water mixtures. It is suggested by our data that the decrease in
dipolarity/polarizability and/or HBD acidity with temperature
is the most at X[bmim][BF4] ∼ 0.7. It is interesting to note that the
4
earlier, parameter ꢀ even in aqueous mixtures of [bmim][BF ]
N
temperature dependence of E
T
in the range 0.1 e X[bmim][BF ]
e
is not affected significantly by the change in temperature; it is
not possible to observe any trends in the extent of change in ꢀ
with temperature (Figure 3D). Goodness of the fits to the linear
expression between ꢀ and temperature at most compositions is
also not acceptable (Table S2 and Figures S4 and S10,
Supporting Information).
4
0
4
.8 is more than that observed in either the neat [bmim][BF ]
or the neat water. It is suggested that dipolarity/polarizability
and/or HBD acidity of the IL-water mixture is very sensitive
to the temperature change. It appears that the presence of HB
+
interactions between C2-H of bmim of IL and -O- of water

8
124 J. Phys. Chem. B, Vol. 114, No. 24, 2010
Trivedi et al.
temperature dependence of the dipolarity/polarizability and/or
HBD acidity within ILs [bmim][PF
tively, is observed to be less than that within water; in aqueous
mixtures of [bmim][BF ], this temperature dependence increased
6 4
] and [bmim][BF ], respec-
4
and was observed to be the most at X[bmim][BF4] ∼ 0.7. Dipolarity/
polarizability (π*) obtained from the molecular absorbance of
DENA show a trend similar to that by betaine dye 33 for ILs,
but within water the behavior of this probe is nearly temperature
4
independent. As [bmim][BF ] is added to water, the temperature
dependence of π* increases. HBD acidity (R), obtained from
the behavior of betaine dye 33 and DENA, of each of the ILs
and water decreases with increasing temperature; the temperature
dependence is less pronounced in ILs as compared to that in
4
water. Subsequently, addition of [bmim][BF ] to water decreases
the temperature dependence of R. HBA basicity (ꢀ) obtained
in concert with another absorbance probe NA appears to be
independent of temperature within ILs as well as aqueous
mixtures of IL [bmim][BF ]. The fluorescence dipolarity probe
4
pyrene behaves similarly to betaine dye 33 within two ILs and
water as the temperature is changed; the behavior is different
within the [bmim][BF
fluorescence wavelength maxima of pyrene-1-carboxaldehyde
a probe of static dielectric constant of the medium), though
4
] + water mixture. The lowest energy
(
decreasing significantly with increasing temperature within
water, is found to be insensitive to the temperature changes
within the two ILs and the aqueous [bmim][BF ] mixtures.
4
Acknowledgment. This work is generously supported by the
Department of Science and Technology (DST), Government of
India, through a grant to S.P. [grant number SR/S1/PC-16/2008].
S.T. and K.B. thank UGC and CSIR for fellowships.
Figure 4. Slopes recovered from the linear regression analysis of
pyrene I /I (A) and pyrene-1-carboxaldehyde lowest energy fluores-
cence maxima λmax (B), respectively, versus temperature as function
of [bmim][BF ] mole fraction (X[bmim][BF4]) within aqueous [bmim][BF ].
1
3
fluo
4
4
Supporting Information Available: Scheme S1 (structures
of the ionic liquids and probes), Tables S1 (slopes) and S2
(
linear regression analysis results), and Figures S1-S12 (varia-
1 3
/I
(panel A) and PyCHO λ mfl ua ox (panel B),
Slopes of Py I
respectively, versus temperature at each composition for aqueous
bmim][BF ] mixtures are presented in Figure 4. It appears from
the data presented in Figure 4A that an aqueous mixture of IL
bmim][BF ] having a small amount of IL (i.e., X[bmim][BF4] only
0.1) shows significant enhancement in the temperature de-
pendence of Py I /I . This is attributed to our earlier observations
tions of results with mole fraction and temperature). This
material is available free of charge via the Internet at http://
pubs.acs.org.
[
4
[
∼
4
References and Notes
(
1) (a) Rogers, R. D.; Seddon, K. R. Ionic Liquids: Industrial
1
3
Applications for Green Chemistry, Eds.; ACS Symposium Series 818;
American Chemical Society, Washington, DC, 2002. (b) Seddon, K. R.
Green Chem. 2002, 4, G25–G26. (c) Huddleston, J. G.; Visser, A. E.;
Reichert, W. M.; Willauer, H. D.; Broken, G. A.; Rogers, R. D. Green
Chem. 2001, 3, 156. (d) Cammarata, L.; Kazarian, S. G.; Salter, P. A.;
Welton, T. Phys. Chem. Chem. Phys. 2001, 3, 5192. (e) Fletcher, K. A.;
Pandey, S. Appl. Spectrosc. 2002, 56, 266. (f) Singh, T.; Kumar, A. J. Phys.
Chem. B 2008, 112, 4079. (g) Scurto, A. M.; Aki, S. N. V. K.; Brennecke,
J. F. J. Am. Chem. Soc. 2002, 124, 10276. (h) Pandey, S.; Fletcher, K. A.;
Baker, S. N.; Baker, G. A. Analyst 2004, 129, 569. (i) Solinas, M.; Pfaltz,
A.; Cozzi, P. G.; Leitner, W. J. Am. Chem. Soc. 2004, 126, 16142. (j) Pal,
A.; Samanta, A. J. Phys. Chem. B 2007, 111, 4724. (k) Baker, S. N.; Baker,
G. A.; Bright, F. V. Green Chem. 2002, 4, 165.
that dilute aqueous solutions of ILs show significantly altered
physicochemical properties as compared to those of neat water.6c
For X[bmim][BF4] g 0.1, the extent of decrease in Py I
increasing temperature, in general, appears to become smaller
before increasing again to attain its value in IL [bmim][BF ].
1 3
/I with
4
N
The latter behavior is in contrast with what is observed for E
T
(
Figure 3A). Perhaps the effect of HBD acidity manifesting itself
N
T
through theE could be the reason for this observation (vide
supra).1k,19,24
fluo
Slopes of PyCHO λmax versus temperature (Figure
4
B) at different mixture compositions show a trend similar to
(2) (a) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-
that observed for probe pyrene (Figure 4A). However, too much
emphasis may not be given to this trend as the slope values
appear to be rather insignificant (e.g., a slope of 0.05 nm K
VCH: New York, 2003. (b) Welton, T. Chem. ReV. 1999, 99, 2071. (c)
Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667.
(
d) Poole, C. F. J. Chromatogr. A 2004, 1037, 49. (e) Blanchard, L. A.;
-
1
Hancu, D.; Beckman, E. J.; Brennecke, J. F. Nature 1999, 399, 6731. (f)
Baker, S. N.; Baker, G. A.; Kane, M. A.; Bright, F. V. J. Phys. Chem. B
2001, 105, 9663. (g) Baker, S. N.; Baker, G. A.; Munson, C. A.; Chen, F.;
Bukowski, E. J.; Cartwright, A. N.; Bright, F. V. Ind. Eng. Chem. Res.
suggests that on changing temperature by 100 K, a mere change
fluo
of 5 nm in PyCHO λmax is observed). All-in-all, the insensitivity
fluo
of PyCHO λmax toward temperature change within our IL and
2
1
003, 42, 6457. (h) Baker, G. A.; Baker, S. N. Aust. J. Chem. 2005, 58,
aqueous IL systems is reasserted.
74. (i) Lantz, A. W.; Pino, V.; Anderson, J. L.; Armstrong, D. W.
J. Chromatogr. A 2006, 1115, 217. (j) Velasco, S. B.; Turmine, M.; Di
Caprio, D.; Letellier, P. Colloids Surf. A 2006, 275, 50. (k) Gao, Y.; Li,
N.; Zheng, L. Q.; Zhao, X. Y.; Zhang, S. H.; Han, B. X.; Hou, W. G.; Li,
G. Z. Green Chem. 2006, 8, 43. (l) Chakrabarty, D.; Seth, D.; Chakraborty,
A.; Sarkar, N. J. Phys. Chem. B 2005, 109, 5753.
Conclusions
Temperature-dependent behavior of solvatochromic probes
within [bmim][PF
bmim][BF ] is found to depend on the identity of the probe.
Manifested through the behavior of betaine dye 33, the
6 4
], [bmim][BF ], and aqueous mixtures of
(
3) (a) Pham, T. P. T.; Cho, C. W.; Yun, Y. S. Water Res. 2010, 44,
52. (b) Stolte, S.; Matzke, M.; Arning, J.; Boeschen, A.; Pitner, W. R.;
Welz-Biermann, U.; Jastorff, B.; Ranke, J. Green Chem. 2007, 9, 1170.
[
4
3

Solvatochromic Probe Behavior within Ionic Liquids
J. Phys. Chem. B, Vol. 114, No. 24, 2010 8125
(
4) (a) Freemantle, M. Chem. Eng. News 1998, 30, 32. (b) Newington,
J. Org. Chem. 2006, 71, 9068. (f) Webb, M. A.; Morris, B. C.; Edwards,
W. D.; Blumenfeld, A.; Zhao, X.; McHale, J. L. J. Phys. Chem. A 2004,
108, 1515. (g) Bastos, E. L.; Silva, P. L.; El Seoud, O. A. J. Phys. Chem.
A 2006, 110, 10287. (h) Nicolet, P.; Laurence, C. J. Chem. Soc., Perkin
Trans. 1986, 2, 1071. (i) Laurence, C.; Nicolet, P.; Helbert, M. J. Chem.
Soc., Perkin Trans. 1986, 2, 1081. (j) Varadaraj, R.; Bock, J.; Valint, P.,
Jr.; Brons, N. Langmuir 1990, 6, 1377. (k) Nishida, S.; Morita, Y.; Fukui,
K.; Sato, K.; Shiomi, D.; Takui, T.; Nakasuji, K. Angew. Chem., Int. Ed.
2005, 44, 7277. (l) Zhao, X.; Burt, J. A.; Knorr, F. J.; McHale, J. L. J.
Phys. Chem. A 2001, 105, 11110. (m) Khupse, N. D.; Kumar, A. J. Phys.
Chem. B 2010, 114, 376.
I.; Perez-Arlandis, J. M.; Welton, T. Org. Lett. 2007, 9, 5247. (c) Plechkova,
N. V.; Seddon, K. R. Methods and Reagents for Green Chemistry 2007,
05. (d) Rogers, R. D.; Visser, A. E.; Swatloski, R. P.; Reichert, W. M.
Abstracts of Papers, 220th ACS National Meeting; American Chemical
Society: Washington, DC, 2000.
1
(
5) (a) Pandey, S. Anal. Chim. Acta 2006, 556, 38. (b) Electrochemical
Aspects of Ionic Liquids; Ohno, H., Ed.; Wiley-Interscience: New York,
2
2
005. (c) Baker, G. A.; Baker, S. N.; Pandey, S.; Bright, F. V. Analyst
005, 130, 800.
(
6) (a) Malham, I. B.; Letellier, P.; Turmine, M. J. Phys. Chem. B
2
2
2
006, 110, 14212. (b) Sarkar, A.; Pandey, S. J. Chem. Eng. Data 2006, 51,
051. (c) Ali, M.; Sarkar, A.; Tariq, M.; Ali, A.; Pandey, S. Green Chem.
007, 9, 1252. (d) Sarkar, A.; Ali, M.; Baker, G. A.; Tetin, S. Y.; Ruan,
(16) (a) Sato, B. M.; de Oliveira, C. G.; Martins, C. T.; El Seoud, O. A.
Phys. Chem. Chem. Phys. 2010, 12, 1764. (b) Wei, X.; Yu, L.; Wang, D.;
Chen, G. Z. Green Chem. 2008, 10, 296.
Q.; Pandey, S. J. Phys. Chem. B 2009, 113, 3088. (e) Dominguez-Vidal,
A.; Kaun, N.; Ayora-canada, M. J.; Lendl, B. J. Phys. Chem. B 2007, 111,
(17) (a) Dupont, J.; Consorti, C. S.; Suarez, P. A. Z.; Souza, R. F. Org.
Synth. Collect. 2004, 10, 184. (b) Holbrey, J. D.; Seddon, R. J. Chem. Soc.,
Dalton Trans. 1999, 2133. (c) Huddleston, J. G.; Willauer, H. D.; Swatloski,
R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 1765.
(18) (a) Reichardt, C. Chem. ReV. 1994, 94, 2319. (b) Reichardt, C.
Pure Appl. Chem. 2004, 76, 1903. (c) Reichardt, C.; Asharin-Fard, S.; Blum,
A.; Eschner, M.; Mehranpour, A. M.; Milart, P.; Niem, T.; Schafer, G.;
Wilk, M. Pure Appl. Chem. 1993, 65, 2593.
4
446. (f) Jeon, Y.; Sung, J.; Kim, D.; Seo, C.; Cheong, H.; Ouchi, Y.;
Ozawa, R.; Hamaguchi, H. O. J. Phys. Chem. B 2008, 112, 923. (g) Mele,
A.; Tran, C. D.; Lacerda, S. H. D. P. Angew. Chem., Int. Ed. 2003, 42,
4364. (h) K o¨ ddermann, T.; Wertz, C.; Heintz, A.; Ludwig, R. Angew. Chem.,
Int. Ed. 2006, 45, 3697. (i) Moreno, M.; Castiglione, F.; Mele, A.; Pasqui,
C.; Raos, G. J. Phys. Chem. B 2008, 112, 7826. (j) Ali, M.; Kumar, V.;
Pandey, S. Chem. Commun. 2010, DOI: 10.1039/c0cc00620c.
(19) Muldoon, M. J.; Gordon, C. M.; Dunkin, I. R. J. Chem. Soc., Perkin
Trans. 2001, 2, 433.
(7) (a) Triolo, A.; Russina, O.; Keiderling, U.; Kohlbrecher, J. J. Phys.
Chem. B 2006, 110, 1513. (b) Wang, I.; Chen, X.; Cahi, Y.; Hao, J.; Sui,
Z.; Zhuang, W.; Sun, Z. Chem. Commun. 2004, 2840. (c) Araos, M. U.;
Warr, G. G. J. Phys. Chem. B 2005, 109, 14275. (d) Meli, L.; Lodge, T. P.
Macromolecules 2009, 42, 580. (e) Patrascu, C.; Gauffre, F.; Nallet, F.;
Border, R.; Oberdisse, J.; de Lauth-Viguerie, N.; Mingotaud, C. Chem. Phys.
Chem. 2006, 7, 99. (f) Sarkar, A.; Trivedi, S.; Baker, G. A.; Pandey, S. J.
Phys. Chem. B 2008, 112, 14927.
(20) Kessler, M. A.; Wolfbeis, O. S. Phys. Chem. Lipids 1989, 50, 51.
(21) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry,
3rd ed.; Wiley-VCH: Weinheim, 2003.
(22) (a) Marcus, Y. Introduction to Liquid State Chemistry; Wiley-
Interscience: New York, 1977. (b) Eyring, H.; John, S. H. Significant Liquid
Structures; John Wiley & Sons, Inc.: New York, 1969.
(23) Lide, D. R. (ed.) CRC Handbook of Chemistry and Physics, 87th
ed.; CRC Press: Boca Raton, FL, 2006.
(24) (a) Kamlet, M. J.; Abboud, J. L.; Taft, R. W. J. Am. Chem. Soc.
1977, 99, 6027. (b) Taft, R. W.; Kamlet, M. J. J. Am. Chem. Soc. 1976, 98,
2886. (c) Kamlet, M. J.; Taft, R. W. J. Am. Chem. Soc. 1976, 98, 377. (d)
Kamlet, M. J.; Abboud, J. L.; Abraham, M. H.; Taft, R. W. J. Org. Chem.
1983, 48, 2877.
(8) (a) Behera, K.; Dahiya, P.; Pandey, S. J. Colloid Interface Sci. 2007,
3
07, 235. (b) Behera, K.; Pandey, S. J. Colloid Interface Sci. 2007, 316,
03. (c) Behera, K.; Pandey, S. J. Colloid Interface Sci. 2009, 331, 196.
8
(
d) Behera, K.; Om, H.; Pandey, S. J. Phys. Chem. B 2009, 113, 786. (e)
Behera, K.; Pandey, M. D.; Porel, M.; Pandey, S. J. Chem. Phys. 2007,
27, 184501. (f) Behera, K.; Pandey, S. J. Phys. Chem. B 2007, 111, 13307.
g) Behera, K.; Pandey, S. Langmuir 2008, 24, 6462. (h) Behera, K.; Kumar,
1
(
(25) (a) Carmichael, A. J.; Seddon, K. R. J. Phys. Org. Chem. 2000,
13, 591. (b) Fletcher, K. A.; Storey, I. K.; Hendricks, A. E.; Pandey, S.;
Pandey, S. Green Chem. 2001, 3, 210. (c) Crowhurst, L.; Mawdsley, P. R.;
Perez-Arlandis, J. M.; Salter, P. A.; Welton, T. Phys. Chem. Chem. Phys.
2003, 5, 2790.
V.; Pandey, S. Chem. Phys. Chem. 2010, 11, 1044. (i) Behera, K.; Malek,
N. I.; Pandey, S. Chem. Phys. Chem. 2009, 10, 3204.
(
9) (a) Ali, M.; Pandey, S. J. Photochem. Photobiol. A: Chem. 2009,
07, 288. (b) Ali, M.; Baker, G. A.; Pandey, S. Chem. Lett. 2008, 37, 260.
c) Ali, M.; Sarkar, A.; Pandey, M. D.; Pandey, S. Anal. Sci. 2006, 22,
051. (d) Ali, M.; Kumar, V.; Baker, S. N.; Baker, G. A.; Pandey, S. Phys.
Chem. Chem. Phys. 2010, 12, 1886.
2
(
2
(
26) (a) Acree, W. E., Jr. In Absorption and Luminescence Probes. In
Encyclopedia of Analytical Chemistry: Theory and Instrumentation; Meyer,
R. A., Ed.; John Wiley & Sons, Ltd.: Chichester, U.K., 2000 (see also
references cited therein). (b) Bredereck, K.; Forster, Th.; Oenstein, H. G.
In Luminescence of Inorganic and Organic Materials; Kallman, H. P.,
Spruch, G. M., Eds; Wiley: New York, 1960. (c) Lakowicz, J. R. Principles
of Fluorescence Spectroscopy, 3rd ed.; Kluwer Academics/Plenum Pub-
lisher: New York, 2006. (d) Turro, N. J. Modern Molecular Photochemistry;
University Science Books: Sausalito, CA, 1991. (e) Suppan, P.; Ghoneim,
N. SolVatochromism; Royal Society of Chemistry: Cambridge, U.K., 1997.
(
10) (a) Sarkar, A.; Trivedi, S.; Pandey, S. J. Phys. Chem. B 2008, 112,
9
7
042. (b) Sarkar, A.; Trivedi, S.; Pandey, S. J. Phys. Chem. B 2009, 113,
606.
(
11) (a) Saha, S.; Hamaguchi, H. J. Phys. Chem. B 2006, 110, 2777.
(
(
b) Almasy, L.; Turmine, M.; Perera, A. J. Phys. Chem. B 2008, 112, 2382.
c) Li, W.; Zhang, Z.; Han, B.; Hu, S.; Xie, Y.; Yang, G. J. Phys. Chem.
B 2007, 111, 6452. (d) Martins, C. T.; Sato, B. M.; El Seoud, O. A. J.
Phys. Chem. B 2008, 112, 8330. (e) Sun, B.; Jin, Q.; Tan, L.; Wu, P.; Yan,
F. J. Phys. Chem. B 2008, 112, 14251. (f) Zhang, L.; Xu, Z.; Wang, Y.; Li,
H. J. Phys. Chem. B 2008, 112, 6411. (g) Kato, H.; Nishikawa, K.; Murai,
H.; Morita, T.; Koga, Y. J. Phys. Chem. B 2008, 112, 13344. (h) Gardas,
R. L.; Dagade, D. H.; Coutinho, J. A. P.; Patil, K. J. J. Phys. Chem. B
(
f) Montalti, M., Credi, A., Prodi, L., Gandolfi, M. T., Eds. Handbook of
Photochemistry, 3rd ed.; CRC Press/Taylor and Francis Group: Boca Raton,
FL, 2006. (g) Pandey, S. ISRAPS Bull. 2006, 18, 9. (h) Pandey, S.; Redden,
R. A.; Fletcher, K. A.; Sasaki, D. Y.; Kaifer, A. E.; Baker, G. A. Chem.
Commun. 2004, 1318.
2
008, 112, 3380. (i) Chang, H. C.; Jiang, J. C.; Chang, C. Y.; Su, J. C.;
(
27) (a) Koel, M., Ed.; Ionic Liquids in Chemical Analysis; CRC Press:
Boca Raton, FL, 2009. (b) Fletcher, K. A.; Pandey, S. Langmuir 2004, 20,
3. (c) Fletcher, K. A.; Pandey, S. Appl. Spectrosc. 2002, 56, 1498. (d)
Hung, C. H.; Liou, Y. C.; Lin, S. H. J. Phys. Chem. B 2008, 112, 4351.
(
12) (a) Tokuda, H.; Baek, S. J.; Watanabe, M. Electrochemistry 2005,
3
7
4
3, 620. (b) Ozawa, R.; Hayahi, S.; Hamaguchi, H. Chem. Lett. 2003, 32,
Fletcher, K. A.; Pandey, S. J. Phys. Chem. B 2003, 107, 13532. (e) Fletcher,
K. A.; Baker, S. N.; Baker, G. A.; Pandey, S. New J. Chem. 2003, 27,
98.
(13) (a) Bowers, J.; Butts, C. P.; Martin, P. J.; Vergara-Gutierrez, M. C.;
1
706. (f) Trivedi, S.; Sarkar, A.; Pandey, S. Chem. Eng. J. 2009, 147, 36.
28) (a) Fletcher, K. A.; Pandey, S.; Storey, I. A.; Hendricks, A. E.;
Heenan, R. K. Langmuir 2004, 20, 2191. (b) Miki, K.; Westh, P.; Nishikawa,
K.; Koga, Y. J. Phys. Chem. B 2005, 109, 9014. (c) Katayanagi, H.;
Nishikawa, K.; Shimozaki, H.; Miki, K.; Westh, P.; Koga, Y. J. Phys. Chem.
B 2004, 108, 19451.
(
Pandey, S. Anal. Chim. Acta 2002, 453, 89. (b) Pandey, S. J. Dispersion
Sci. Technol. 2005, 26, 381. (c) Pandey, S.; Reddon, R. A.; Hendricks,
A. E.; Fletcher, K. A.; Palmer, C. P. J. Colloid Interface Sci. 2003, 262,
(
14) (a) Wong, D. S. H.; Chen, J. P.; Chang, J. M.; Chou, C. H. Fluid
5
79. (d) Baker, G. A.; Bright, F. V.; Pandey, S. Chem. Educ. 2001, 6, 223.
Phase Equilib. 2002, 194, 1089. (b) Anthony, J. L.; Maginn, E. J.;
Brennecke, J. F. J. Phys. Chem. B 2001, 105, 10942. (c) Liu, J. F.; Jiang,
G. B.; Chi, Y. G.; Cai, Y.; Zhou, Q. X.; Hu, J. T. Anal. Chem. 2003, 75,
(
e) Waris, R.; Acree, W. E., Jr.; Street, K. W., Jr. Analyst 1988, 113, 1465.
f) Dong, D. C.; Winnik, M. A. Can. J. Chem. 1984, 62, 2560. (g)
(
Karpovich, D. S.; Blanchard, G. J. J. Phys. Chem. 1995, 99, 3951. (h) Mazur,
M.; Blanchard, G. J. J. Phys. Chem. B 2005, 109, 4076. (i) Kalyana-
sundaram, K.; Thomas, J. K. J. Phys. Chem. 1977, 81, 2176. (j) Pandey,
S.; Redden, R. A.; Fletcher, K. A.; Palmer, C. P. Macromol. Chem. Phys.
5
870. (d) Alfassi, Z. G.; Huie, R. E.; Milman, B. L.; Neta, B. Anal. Bioanal.
Chem. 2003, 377, 159. (e) Kl a¨ hn, M.; St u¨ ber, C.; Senduraman, A.; Wu, P.
J. Phys. Chem. B 2010, 114, 2856.
(
15) (a) Reichardt, C. Chem. Soc. ReV. 1992, 21, 147. (b) Silva, P. L.;
2
003, 204, 425. (k) Aschi, M.; Fontana, A.; Di Meo, E. M.; Zazza, C.;
Amadei, A. J. Phys. Chem B 2010, 114, 1915.
29) Archer, D. G.; Wang, P. J. Phys. Chem. Ref. Data 1990, 19, 371.
Bastos, E. L.; El Seoud, O. A. J. Phys. Chem. B 2007, 111, 6173. (c) Tada,
E. B.; Silva, P. L.; El Seoud, O. A. Phys. Chem. Chem. Phys. 2003, 5,
(
5
378. (d) Martins, C. T.; Lima, M. S.; Bastos, E. L.; El Seoud, O. A. Eur.
J. Org. Chem. 2008, 1165. (e) Martins, C. T.; Lima, M. S.; El Seoud, O. A.
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