Distinct influence of the anion and ether group on the polarity of ammonium and imidazolium ionic liquids

Article abstract of DOI:10.1039/c2nj20965a

The polarity of ionic liquids (ILs), usually denoted as E_T(30) by the solvatochromic probe Reichardt's dye, is one of the most fundamental properties that remarkably affect the solvation and chemical reaction in ILs. It was generally accepted that the E_T(30) of ILs was dominated by the nature of the cation. However, in this work, it was found that the common ammonium-based ILs showed strongly anion-dependent E_T(30). For example, the E_T(30) value for [N₁₁₂₄][DCA] and [N ₁₁₂₄][NTf₂] is 49.0 and 59.0 kcal mol^-1, respectively, while the corresponding imidazolium ILs bearing the same anions possess nearly identical E_T(30), the E_T(30) value for [BMIm][DCA] and [BMIm][NTf₂] is 51.4 and 51.6 kcal mol^-1, respectively. Moreover, introduction of an ether group was found to increase the E_T(30) of imidazolium ILs while having no obvious effect on that of ammonium-based ILs. The Kamlet-Taft parameters and density functional theory (DFT) calculations indicated that the distinct result is related to different stabilization of the ground state of Reichardt's dye 30. In imidazolium ILs, the main interactions between ILs and zwitterionic dye involve both coulombic interaction (between the cation and the phenolate oxygen atom) and H-bonding interaction (between the acidic hydrogen on imidazolium ring and the phenolate oxygen atom). However, with the ammonium ILs lack of active hydrogen, the dye is only stabilized by the coulombic interaction between the cation and the phenolate oxygen atom. Interestingly, in both imidazolium and ammonium-based ILs, the spiropyran-merocyanine equilibrium exhibit obvious anion-dependent photochromism, solvatochromism, and thermal relaxation. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012.

Full text of DOI:10.1039/c2nj20965a

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
NJC
Cite this: New J. Chem., 2012, 36, 1043–1050
www.rsc.org/njc
PAPER
Distinct influence of the anion and ether group on the polarity of
ammonium and imidazolium ionic liquidsw
Shiguo Zhang, Zhengjian Chen, Xiujuan Qi and Youquan Deng*
Received (in Montpellier, France) 18th November 2011, Accepted 25th January 2012
DOI: 10.1039/c2nj20965a
The polarity of ionic liquids (ILs), usually denoted as ET(30) by the solvatochromic probe Reichardt’s
dye, is one of the most fundamental properties that remarkably affect the solvation and chemical
reaction in ILs. It was generally accepted that the ET(30) of ILs was dominated by the nature of the
cation. However, in this work, it was found that the common ammonium-based ILs showed strongly
anion-dependent ET(30). For example, the ET(30) value for [N1124][DCA] and [N1124][NTf2] is 49.0 and
59.0 kcal molÀ1, respectively, while the corresponding imidazolium ILs bearing the same anions
possess nearly identical ET(30), the ET(30) value for [BMIm][DCA] and [BMIm][NTf2] is 51.4 and
51.6 kcal molÀ1, respectively. Moreover, introduction of an ether group was found to increase the ET(30)
of imidazolium ILs while having no obvious effect on that of ammonium-based ILs. The Kamlet–Taft
parameters and density functional theory (DFT) calculations indicated that the distinct result is related to
different stabilization of the ground state of Reichardt’s dye 30. In imidazolium ILs, the main
interactions between ILs and zwitterionic dye involve both coulombic interaction (between the cation and
the phenolate oxygen atom) and H-bonding interaction (between the acidic hydrogen on imidazolium
ring and the phenolate oxygen atom). However, with the ammonium ILs lack of active hydrogen, the
dye is only stabilized by the coulombic interaction between the cation and the phenolate oxygen atom.
Interestingly, in both imidazolium and ammonium-based ILs, the spiropyran–merocyanine equilibrium
exhibit obvious anion-dependent photochromism, solvatochromism, and thermal relaxation.
presence of H2O2.19 Diels–Alder reactions processed in highly
1. Introduction
polar ILs produced a much higher endo/exo ratio as compared to
others.18 Solvatochromism, photochromism and thermal rever-
sion of spiropyran indeed showed obviously polarity-dependence
in ILs.20,21 Polarity and hydrogen-bonding ability also play
crucial roles in designing ILs as solvents for cellulose.22
Ionic liquids (ILs) are salts melting below 373 K, which have
received increasing interest as suitable candidates in various
areas, such as organic synthesis,1–5 catalysis,6–8 electrochemical
devices,9–12 and solvent extraction,13–15 because of their attractive
properties not available in molecular solvents. ILs are frequently
referred to as ‘tuneable’, ‘tailored’, ‘task-specific’ or ‘designer’
solvents due to the ability to vary the ions, thereby modifying
and optimizing a salt’s physicochemical properties for specific
applications, such as increasing reactivity, selectivity, catalyst
recyclability, and so on.2
Polarity is the sum of all possible interactions between the
solvent and any potential solute, except for those that lead to a
permenant chemical transformation, according to the IUPAC
definition. Analogous to molecular solvents, the polarity of ILs
has been intensively investigated using a series of solvatochromic
dyes. Amongst all probes used, Reichardt’s betaine dye 30
(2,6-diphenyl-4-(2,4,6-triphenyl-N-pyridino) phenolate) betaine is
the most common solvatochromic dye used in these experiments,16
based on its large negative solvatochromic shift of the long-
wavelength intramolecular p–p* charge transfer (CT) absorption
band, and it has one of the most popular empirical solvent
polarity scales (ET(30) or ETN), which can be easily studied and
compared with other solvents. Systematic and fundamental
studies demonstrated that except protic ILs, common ILs exhibit
lower polarity comparable to short chain alcohols, and the
polarity of ILs varied according to the structure of ILs. It was
previously suggested that ET(30) of general ILs appears to be
dominated by the nature of the cation.23,24 For instance, ET(30)
Polarity, as one of the most fundamental parameters for ILs,
is of great importance for solvation and chemical reaction
conducted in ILs,16,17 especially for polarity-sensitive reaction
and catalysis.18,19 For example, the high polarity of a Brønsted
acidic pyrrolidinium ILs was considered to be an important
factor for the oxidative desulfurization of diesel fuel in the
Center for Green Chemistry and Catalysis, Lanzhou Institute of
Chemical Physics, Chinese Academy of Sciences, Lanzhou,
730000, China. E-mail: ydeng@licp.cas.cn; Fax: +86-931-4968116;
Tel: +86-931-4968116
w Electronic supplementary information (ESI) available: structure para-
meters and atom coordinates calculated at the B3LYP/6-31+G(d,p)
level of theory. see DOI: 10.1039/c2nj20965a
for alkylimidazolium salts has mean values about 52 kcal molÀ1
,
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 1043–1050 1043

View Article Online
comparable with ethanol, replacing the proton at the 2-position
of the ring with a methyl group resulted in lower values
(48 kcal molÀ1). This is entirely consistent with the expected
hydrogen-bond donor properties of these cations.23,24 In 2005, Kaar
and co-workers reported a series of 1-methyl-1-(2-methoxyethyl)-
pyrrolidinium ([MOEMPy])-based ILs with anions including
acetate, nitrate, trifluoroacetate, trifluoromethylsulfonate, and
methanesulfonate,25 which exhibit unexpectedly anion-dependent
polarity with ENT (ETN = (ET(30) À 30.7)/32.4)16 ranging from
0.37–0.91. Unfortunately, this unique anion-dependent polarity was
not further investigated in detail. More recently, we have found that
hydroxyl groups on imidazolium exhibit a significant effect on the
ET(30) of ILs.21 Although most of the nonhydroxyl 1-ethyl-3-
methylimidazolium ILs exhibit anion-independent polarity with
similar ET(30) in the narrow range of 49.7–52.6 kcal molÀ1, the
ET(30) of the 1-(2-hydroxyethyl)-3-methylimidazolium ILs covers a
rather wide range (51.2–61.7 kcal molÀ1) and is strongly anion-
dependent.21 Kamlet–Taft parameters and density functional
theory calculations indicated that the greatly expanded range
of ET(30) of hydroxyl ILs is correlated to an intramolecular
synergistic solvent effect of the ionic hydrogen-bonded complexes
between the anions and the hydroxyl group on cations, wherein the
hydroxyl group exhibits a significant differentiating effect on the
strength of H-bonding and thus the polarity. Lee et al. also
investigated the effect of functional groups on the polarity of
pyrrolidinium-based ILs, it was revealed that the presence of a
cyano group on the pyrrolidinium cation slightly decreased the
ET(30),26 while a vinyl or disulfide group has a slight influence.
Introducing an ether functional group into the alkyl chain on the
cation was also found to increase the polarity of imidazolium ILs.18
Although polarity of ILs has been widely investigated, the
conclusion to date was generally stemmed from the commonly
used imidazolium ILs. In contrast, the case of ammonium ILs
remains poorly understood, mostly because of their relatively
high melting points. Based on our previous work on the synthesis
and characterization of dialkoxy functionalized quaternary
ammonium ILs,27 we further investigated the polarity behavior
of ether functionalized ILs by ET(33), ET(30) and Kamlet–Taft
parameters in-depth in this work, together with the results of
imidazolium ILs for comparison. The solvation behavior, including
polarity and hydrogen bonding ability of ammonium-based ILs
with or without an ether group were investigated to understand
the effect of anion and ether group on the ET(33), ET(30),
hydrogen-bond acidity a, hydrogen-bond basicity b, and
dipolarity/polarizability p*. In contrast to the imidazolium
ILs, it was interestingly found that the ammonium-based ILs
exhibited strongly anion-dependent polarity, while the ether
function has no obvious effect. Moreover, the [NTf2]-based
ammonium ILs possess higher polarity (close to glycol) that
can fill the polarity gap between water and general molecular
solvents, and can be used as a single aprotic ionic solvent for
polarity-specific reaction and synthesis.
1-(2-methoxyethyl)-3-methylimidazolium ([1O2MIm]), N-butyl-
N-ethyl-N,N-diethylammonium ([N1124]), N-(2-methoxyethyl)-N-
butyl-N,N-diethylammonium ([N114,1O2]); anions: nitrate ([NO3]),
dicyanamide ([DCA]), tetrafluoroborate ([BF4]), hexafluoro-
phosphate ([PF6]), bis(trifluoromethanesulfonyl)amide ([NTf2]),
were synthesized according to a typical two-step protocol27,28
and the purity was confirmed by 1H-NMR, elemental analysis,
water content, and AgNO3 test (for details, see supporting
informationw).
2.2. Determination of polarity and Kamlet–Taft Parameters
The polarity and Kamlet–Taft parameters were determined by
using the following solvatochromic probes, i.e., Reichardt’s
dye, N,N-dimethyl-4-nitroaniline, 4-nitroaniline, and spiropyran
(Fig. 2), according to our previous work.21
2.3. Computational analysis (DFT calculations)
All the calculations were performed with the Gaussian 03
programs using the B3LYP/6-31+G(d,p) method.29 No
restrictions on symmetries were imposed on the initial structures.
Therefore the geometry optimization for the saddle points
occurred with all degrees of freedom. The presence of a minimum
amount of energy was ensured by the lack of imaginary
vibrational frequencies. A scaling factor was not applied to
the calculated frequency in the calculations. The gas-phase
energy of the ion-pair formation (DE) has been estimated
using eqn (1), according to Turner et al.30
DE(kJ molÀ1) = 2625.5[EAX(au) À (EA+(au) + EXÀ(au))]
(1)
where DE is the energy of the ion-pair formation, and EAX
,
EA+, EXÀ are the energy of ion pair, the isolated cation and
anion, respectively.
3. Result and discussion
3.1. Effect of anion on the polarity of ILs
Polarity scales (ET(33) and ET(30), kcal molÀ1) of all ILs
investigated were listed in Table 1, where comparative values
for some conventional organic solvents are also included. For
all ammonium and imidazolium ILs, there is indeed a linear
relationship in polarity scales between ET(33) and ET(30)
(Fig. S1w), as described in the following equation:
ET(30) = 0.99382(Æ0.1109)ÁET(33)
À 8.74797(Æ7.04428), n = 10, r = 0.95363 (2)
where n is the number of solvents employed and r is the
correlation coefficient. The result verified the feasibility of both
2. Experimental
2.1. Synthesis and characterization of ILs
All ILs with different cation and anion structures (Fig. 1),
namely, cations: 1-butyl-3-methylimidazolium ([BMIm]),
Fig. 1 Structures and abbreviations of the cations and anions.
c
1044 New J. Chem., 2012, 36, 1043–1050
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

View Article Online
Fig. 2 Dyes and probe reaction used for probing polarity and Kamlet–Taft parameters of ILs: Reichardt’s dye 30 (A), Reichardt’s dye 33 (B),
N,N-dimethyl-4-nitroaniline (C), 4-nitroaniline (D), and spiropyran (SP) 2merocyanine (MC) (E).
Table 1 ET(30) and ET(33) scales,a and Kamlet–Taft parameters
of ILs
We first focused on the nonether ILs with a [DCA] or [NTf2]
anion. Clearly for the [BMIm]-based ILs, comparable polarity
that similar to ethanol (ENT = 0.654) was observed between the
two anions, [DCA] and [NTf2], with ENT 0.640 and 0.645,
respectively, which is also comparable to the previous result
([BMIm][DCA]: 0.639,34 [BMIm][NTf2]: 0.642,23 0.64535). In
fact, it was reported that alteration of the anion has very little
effect on the polarity of imidazolium ILs,21,23,36 [BMIm]-based
ILs for other anions typically have ENT E 0.65 at room
temperature (trifluoroacetate: 0.630,25 tetrafluoroborate:
0.680,37 hexafluorophosphate: 0.66738). The result is consistent
with the previous conclusion that the polarity of general ILs
appears to be largely cation controlled.23,24 However, it did
not hold true for ammonium-based ILs with the same [DCA]
or [NTf2] anion. Both the ET(33) and ET(30) values are
indicative of the significant influence of anion on the polarity
of nonether ammonium ILs. ET(30) value of [N1124][NTf2] is
58.9 kcal molÀ1, comparable with that of glycerol, and much
higher than those of most imidazolium ILs, while [N1124][DCA]
still possess ET(30) of 49.0 kcal molÀ1, slightly lower than
general imidazolium ILs and comparable to 1-propanol.39
Taking into account of the lack of suitable conventional
ILs
ET(33) ET(30) ENT
p*
a
b
[BMIm][DCA]
[BMIm][NTf2]
[1O2MIm][DCA]
[1O2MIm][NTf2]
[N1124][DCA]
61.4
60.7
60.6
66.2
58.0
66.2
58.7
66.5
51.4
51.6
52.4
54.5
49.0
59.0
49.0
58.8
56.1
60.8
49.2
51.9
55.4
57.0
63.1
0.640 1.06 0.53 0.57
0.645 0.97 0.60 0.24
0.669 1.11 0.51 0.56
0.733 0.96 0.78 0.29
0.563 0.97 0.43 0.58
0.868 0.89 1.16 0.27
0.563 0.99 0.42 0.61
0.867 0.87 1.16 0.38
[N1124][NTf2]
[N114,1O2][DCA]
[N114,1O2][NTf2]
[HOEMIm][DCA]b 65.1
0.784
0.929
—
—
—
—
—
—
[HOEMIm][NTf2]b
1-propanolc
Ethanol
70.6
—
—
—
—
—
0.571 0.48 0.76 0.84
0.654 0.54 0.86 0.75
0.762 0.60 0.98 0.66
0.812 0.62 1.21 0.51
1.000 1.09 1.17 0.47
Methanol
Glycerol
Water
a
b
c
Data from ref. 21. Data of molecular solvents from
kcal molÀ1
ref. 39.
.
Reichardt’s dye probes in studying the polarity of ILs. The
coefficients in eqn (2) were further compared with those for
conventional molecular solvents and other series of ILs, as
listed in the following equations:
solvents with polarity range of ET(30) = 55–63 kcal molÀ1 39
,
the [NTf2]-based ammonium ILs, possessing relatively high
polarity, will act as a single aprotic ionic solvent filling the
polarity gap between water and general molecular solvents.
The longest wavelength intramolecular charge-transfer p–p*
absorption bands of Reichardt’s betaine dye 30 in the four ILs
are shown in Fig. 3. Obviously, Reichardt’s dye 30 in
[BMIm][NTf2] and [BMIm][DCA] showed UV-vis absorption
both around 555 nm, while that of quaternary ammonium-
based ILs exhibit distinct absorption for different anions, 584 nm
for [N1124][DCA] and 485 nm for [N1124][NTf2], respectively.
Since the solvatochromic absorption band of this betaine dye
lies within the visible region of the spectrum, the drastic anion-
dependent polarity for ammonium-based ILs can be easily seen
colorimetrically and a visual estimate of solvent polarity can
often be made: the solution colour is violet in [BMIm][NTf2] and
[BMIm][DCA], while [N1124][DCA] and [N1124][NTf2] are cyan
and orange, respectively.
ET(30) = 0.979 Â ET(33) À 7.461
(3)
(4)
(5)
n = 32 molecular solvents, r = 0.990531
ET(30) = 0.9986 Â ET(33) À 8.6878
n = 10 molecular solvents32
ET(30) = 0.9953 Â ET(33) À 8.1132
n = 20 (19 molecular solvents + 1 IL), r = 0.992633
ET(30) = 1.09396 Â ET(33) À 15.15814
n = 10 ILs, r = 0.992621
(6)
Obviously, excellent linear correlations between ET(30) and
ET(33) scales were obtained for both molecular solvents and
ILs. The coefficients in eqn (2) are comparable to the case of
molecular solvents (eqn (3)–(5)) while exhibiting some devia-
tion over the results from other ILs. This indicated that ILs
maybe exhibit different solvation behavior towards the probe
compared to molecular solvents, and thus a substantial corre-
lation derived from the specific ILs is necessary.
3.2. Effect of the ether group on the polarity of ILs
After ether functionalization, the polarity of [NTf2]-based
imidazolium ILs, [1O2MIm][NTf2], was enhanced compared
to nonether ILs and are comparable with that of methanol,
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 1043–1050 1045

View Article Online
3.3. Photochromism, solvatochromism and thermal reversion
of spiropyran in ammonium and imidazolium ILs
Spiropyran was proposed as a multi-parameter probe for its
immediate molecular environment, based on photochromic
behavior, solvatochromism of the MC, and rate of thermal
relaxation of MC to SP form (Fig. 2). Recently, photo-
chromism of spiropyran in ILs were found to be sensitive to
the solvation environments. Some distinctive features, including
polarity and hydrogen bonding basicity-dependent photo-
chromism, negative activation entropies, and good linear shape
between ET(30) scales and the ESP scales were obtained.20,44
Here the photochromism of spiropyran was investigated in
these ammonium and imidazolium ILs.
As can be seen from Table 3, positive photochromism was
observed for all ILs investigated, which agree with the conclusion
drawn by Wu et al.20 that spiropyran showed positive photo-
chromism with the Esp (28591/lmax) o 53.9 kcal molÀ1, while
negative ones with the Esp 4 53.9 kcal molÀ1 (when the
thermodynamically less stable state of spiropyran is the more
deeply coloured form, the system is called a ‘‘positive photo-
chromism’’, otherwise it is called a ‘‘negative (or inverse)
photochromism’’46). Unlike the case in hydroxyl imidazolium-
based ILs,21 no significant polarity-dependent Esp but an
unexpected anion-dependence was found in these ILs, irrespective
of the nature of the cation. For example, Esp of [NTf2]-based ILs
are nearly constant (52.1–52.3 kcal molÀ1) although their ET(30)
scales changed from 51.4 to 58.8 kcal molÀ1 with cations of
[BMIm], [1O2MIm], [N114,1O2] and [N11,1O2,1O2]. This anion-
dependent Esp still hold true for imidazolium ILs such as
[EDMIm][NTf2] and [EMIm][DCA] (Table 3). Similar to the
case of solvatochromism, the rate constants (k) and half-
lifetimes (t1/2) for MC thermal isomerization tabulated in
Table 3 exhibit no polarity dependence, but slight anion-
dependence. t1/2 for ILs containing [NTf2] is obviously larger
than that of [DCA] for each common cation, indicating that
the MC was more stablized in [DCA]-based ILs. The obvious
anion-dependent solvatochromism and thermal relaxation
of MC is rather similar to the case of [Cu(acac)(tmen)][X]
(X = BPh4 or ClO4), as reported by Gordon et al.,23 where its
Fig. 3 Longest wavelength intramolecular charge-transfer p–p* absorp-
tion bands of Reichardt’s betaine dye 30 in [N1124][DCA] (a), [BMIm][NTf2]
(b), [BMIm][DCA] (c) and [N1124][NTf2] (d). Inset is a photograph
illustrating the appearance of IL solutions of Reichardt’s dye 30.
which is also consistent with the previous result.18 However,
polarity of [1O2MIm][DCA] is nearly constant, as indicated by
both ET(33) and ET(30). This is somewhat similar to the effect
of hydroxyl on imidazolium ILs but is of a smaller magnitude,21
the anion-dependent polarity of hydroxyl ILs relative to
[BMIm]-based ILs increased by 4.7 and 9.2 kcal molÀ1 for
[HOEMIm][DCA] and [HOEMIm][NTf2], respectively, which
was ascribed to the fact that OH in the imidazolium cation
is a better hydrogen bond donor than C2–H to differentiate the
(anionÁ Á ÁHO) H-bonding strength.21 In contrast, for quaternary
ammonium-based ILs, introduction of an ether group has no
obvious influence on their polarity, although it was reported that
the ether group lead to significant ion crystal packing of ILs and
notable differences in their thermal properties and viscosity,40,41
and the location of ether oxygen showed significant influence on
their reductive stability of ammonium ILs.42
To further investigate the distinct influence of anions and ether
groups on the polarity, ET(33) scales of a series of ammonium and
imidazolium ILs, including anions with different hydrogen bond
accepting (HBA) ability were measured. The results are given in
Table 2 and plotted in Fig. 4. Obviously, the above rule still hold
true for all ILs. In detail, for the imidazolium ILs with anions
[NO3], [DCA], [BF4], [PF6] and [NTf2], comparable ET(33) scales
(60.7–61.6 kcal molÀ1) were obtained, which is in good agreement
with the previous result,21,23,36 and suggested little effect of the
anion on the polarity. Introduction of an ether group on the cation,
lmax values are significant anion-dependent, irrespective of the
nature of cation. The anion-dependent response of spiropyran
in both imidazolium and tetraalkylammonium ILs may be
caused by the zwitterionic form in these ILs with short alkyl
chain or non-function groups preferentially positioning itself
near the anion, thus not being significantly influenced by the
cation. Although both probes have similar zwitterionic structures,
the above result indicated that spiropyran in ILs undergo different
however, greatly expanded the ET(33) scales (59.2–66.2 kcal molÀ1
)
with the following trend: [1O2MIm][NO3] o [1O2MIm][DCA] o
[1O2MIm][BF4] o [1O2MIm][PF6] o [1O2MIm][NTf2]. As for
ammonium ILs, a comparable study of ammonium ILs with and
without ether groups strongly suggested the significant effect of the
anion rather than the ether group on the ET(33) scales. The
changed trend in ET(33) scales is roughly similar to the case of
[1O2MIm]-based imidazolium ILs, wherein an IL with an anion of
lower HBA ability gives a higher ET(33) value. Note that ET(33)
scales of [N1124][NO3], [N1124][BF4] and [N1124][PF6] were not
determined because of their solid state at room temperature. For
convenience, we will only discuss the comparable cases of
[DCA] and [NTf2]-based ILs in the next context.
Table 2 ET(33)a scales of ammonium and imidazolium ILs with
[NO3], [BF4] and [PF6] anion
[NO3]
[BF4]
[PF6]
[BMIm]
[1O2MIm]
61.2
59.2
—
63.2
61.6c
65
—
63.3
61.4
66.0
—
65.4
b
b
b
[N1124
[N114,1O2
]
]
a
b
kcal molÀ1
.
Data of [N1124][NO3], [N1124][BF4] and [N1124][PF6]
c
were not determined since they are solid at room temperature. Data
from ref. 43.
c
1046 New J. Chem., 2012, 36, 1043–1050
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

View Article Online
ln k = À7.10(7.09 Â 10À5) + 0.22(0.6988)*a À 0.95(0.3963)*b
ln k = À4.49(0.0102) À 0.54(0.4834)*b À 2.66(0.0947)*p*
ln k = À3.01(0.2073) À 0.37(0.5025)*a À 4.14(0.0683)*p*
Since not all coefficients proved significant, we next test all
three variables individually {a}, {b}, {p*}
ln k = À7.71(4.08 Â 10À8) + 0.52(0.2572)*a
ln k = À6.84(2.17 Â 10À7) À 1.21(0.1618)*b
ln k = À4.27(0.0073) À 3.12(0.0305)*p*
It is clear that only the third relationship contains significant
parameter. This indicates that the rate constants depend only
upon {p*}.
Fig. 4 Comparison of ET(33) scales for ammonium and imidazolium
ILs with different anions.
3.4. Kamlet–Taft parameters and DFT calculation
Table 3 Photochromism and thermal reversion of spiropyran in ILs
(10À4 M) at 298 K.a
Empirical Kamlet–Taft parameters (dipolarity/polarizability, p*;
HBD acidity, a; HBA basicity, b) were determined and the result
is shown in Table 1 and plotted in Fig. 5. The p* values of all ILs
are high in comparison with molecular solvents (except water)
because the effect of Coulombic interactions from the ions is
incorporated as well as dipole and polarizability effects.24,48 For
all ILs, no matter ammonium or imidazolium-based, and no
matter ether group functionalized, the [DCA]-based ILs possess
higher p* and b than [NTf2] but lower a. This was due to the
higher electronegativity and nucleophilicity of [DCA] than that
of [NTf2]. Introduction of an ether group on both cations was
found to somewhat expand the p* difference between the two
anions, while narrowing the b difference.
Plot of the dipolarity/polarizability, HBD acidity, and HBA
basicity vs. the ET(30) scale for all ILs investigated, respectively,
is also shown in Fig. 5. Obviously, no significant correlation
between p* or b and the ET(30) scale was obtained according to
least squares analysis. Although a value increased with ET(30)
value, with good linearity (r = 0.98), it is not unexpected
because hydrogen bonding acidity is known to contribute
mainly to polarity (ca. two thirds as reported by Taft and
Kamlet49), and for protic solvents the ET(30) scale is largely a
measure of the hydrogen bond donor strength. Moreover, the
a parameter was determined indirectly, from the measure-
ments of ET(30) and polarizability of the solvent.21 In light
of this, it is clear that since a significant change is not observed
in the p* values as the anion of the ammonium ILs is varied
from [DCA] to [NTf2], thus a of these ILs will reasonably
change as the ET(30) values and the unusual polarity behavior
of ammonium ILs cannot be simply explained by the hydrogen
bonding acidity.50
t1/2
/
c
b
ILs
ET(30)b lmax Esp 103 k/sÀ1 ln k min
Photod
[BMIm][DCA]
[BMIm][NTf2]
[1O2MIm][DCA] 52.4
[1O2MIM][NTf2] 54.5
[N1124][DCA]
[N1124][NTf2]
[N114,1O2][DCA] 49.0
[N114,1O2][NTf2] 58.8
[EMIm][DCA]f 51.6
[EDMIm][NTf2]g 51.8
51.4
51.6
550 52.0 0.367
549 52.1 1.010
552 51.8 0.548
547 52.3 0.911
552 51.8 0.694
547 52.3 0.711
552 51.8 0.611
547 52.3 0.828
551 51.9 0.382
548 52.2 —
À7.91 31.4
À6.90 11.4
À7.51 21.1
À7.00 12.7
À7.27 16.6
À7.25 16.3
À7.40 18.9
À7.10 13.9
À7.87 30.2
Pe
p
p
p
p
p
p
p
p
p
49.0
59.0
—
—
Detailed measurement see our previous work.44 b kcal molÀ1
.
The
a
c
wavelength corresponding to maximum absorption, nm. d Photochromism.
e
f
Positive photochromism. [EMIm] = 1-ethyl-3-methylimidazolium,
g
data from ref. 21. [EDMIm] = 1-ethyl-2,3-dimethylimidazolium,
data from ref. 45.
specific and non-specific interactions between ionic solute
within ILs compared to Reichardt’s dye, probably due to its
more flexible conformation.
Further LSER (Linear Solvation Energy Relationships)
analysis of ln k was conducted according to the established
procedures,47 using the Kamlet–Taft data in Table 1 (Kamlet
–Taft data of [EMIm][DCA] is p* = 1.08, a = 0.53, b = 0.35
from ref. 44). In the simplest Kamlet–Taft LSER’s (XYZ =
XYZ0 + a*a + b*b + s*p*), there are four parameters:
a, b, s, and the intercept (XYZ0). The standard procedure for
LSER analysis is as follows:
If we perform a linear regression using all variables {a, b, p*},
we obtain:
The reason for the outstanding negative solvatochromism of
Reichardt’s dye 30 in ILs stems from the unequal, differential
solvation of the dipolar electronic ground and less dipolar
excited state of the probe with increasing solvent (ILs) polarity,
according to its inherent molecular structure.16 Since the pheno-
late oxygen atom exhibits a highly basic EPD (electron pair
donor) centre, suitable for the interaction with H-bonding
donors and Lewis acids (EPA = electron pair acceptor),16 the
zwitterionic probe in ILs is thus suitable for the registration of
ln k = À2.33(0.3198) À 0.69(0.2708)*a À 0.99(0.2642)*b
À 4.18(0.0656)*p*
The numbers in italics represent the p-values for each coeffi-
cient. Since nearly all coefficients proved significant (p o 0.05
is typical for smaller data sets to indicate significance47), we
must then perform a regression using each pair of variables
{a, b}, {b, p*}ss,29
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 1043–1050 1047

View Article Online
above or below the plane. In the case of the tetraalkylammonium
cation, the optimized strutures are that the anion is located
on top of the two methyl groups, which are about 4.0 (for
[N1124][NTf2]) and 3.41 kJ molÀ1 (for [N1124][DCA]) stabilized
as compared to the cases that the anion located on top of the
two methylene groups, due to the extra stabilization by the
other two methylene groups. The optimized geometries for
these ILs are similar to the previous theoretical reports at
different levels.55
According to Welton’s proposition,47 the ability of the
cation in ILs to interaction with a probe appears to come
from an antagonistic relationship between its anions, as
described below:
[Cation]+ + [Anion]À " [Cation]+- - - [Anion]À
[Cation]+ + solute " [Cation]+- - - solute
Fig. 5 (Left) Comparative study of the Kamlet–Taft parameters
between [DCA]À ) and [NTf2]À (m)-based ILs, (Right) Plot of p*,
a, b vs. ET(30) scale, respectively. The result of correlation analysis is
the following: ET(30) = À3.07885 + 0.07068a, r = 0.98.
(
The nature of the anion will lead to different degrees of
interaction with a cation and thus different degrees of inter-
action of a cation with the solute. With changing the anion
from [DCA]À to [NTf2]À, the negative charge of the anion is
delocalized and the distance between the cation and anion is
increased, thus the interaction energy between cation and
anion decreased. As can be seen from Table 4, ion pairs
[N1124][DCA] are more strongly bound than [N1124][NTf2] by
25.33 kJ molÀ1, indicating a tendency for very strong ion
pairing in the former ILs, thus the residue effective cation
charge to interact with the phenolate oxygen atom increased
and the dipolar electronic ground of Reichardt’s dye 30 in
[N1124][NTf2] is much more stabilized than that in
[N1124][DCA]. Taking into account the above conclusion that
the polarity of ammonium ILs was controlled by coulombic
interaction between cation and the phenolate oxygen
(Scheme 1), the resulting strong coulombic interaction in
[N1124][NTf2] gives higher ET(30). This change of polarity of
tetraalkylammonium ILs is rather similar to that observed in
acetonitrile solution with addition of metal salts, where the
long-wavelength CT absorption band of Reichardt’s dye 30
hypsochromic shifted with increasing effective cation charge
(i.e. ion charge/ion radius).51
(i) the coulombic interaction between the cation and the pheno-
late oxygen atom (EPA/EPD interaction); (ii) the H-bonding
interaction between acidic hydrogen on cation and the phenolate
oxygen atom. In contrast, as confirmed by the experimental
results,51 coulombic interaction between the pyridinium moiety
and anion was small and practically not registered because the
positive charge is delocalized and sterically somewhat shielded,
thus the betainic dye is neither a hydrogen-bond donor (HBD)
nor an electron-pair acceptor (EPA).52
In imidazolium ILs, the strong delocalization of positive
charge on the imidazolium ring caused weak coulombic
interaction with the negative phenoxide oxygen, while the
hydrogen bonding donor acidity of the imidazolium cation
(for example, pKa = 21–23 for the C2-H53) suggested that the
ground state of Reichardt’s dye 30 was dominantly stabilized
by the H-bonding interaction between hydrogen (mainly
C2-H) and the phenolate oxygen. It has long been recognized
by experiment and molecular dynamics study that the ENT scale
for ILs is directly related to the specific interactions involving
the phenoxide oxygen atom.54 However, for ammonium ILs,
the stabilization of the dipolar electronic ground state mainly
originated from the coulombic interaction between the cation
and the phenolate oxygen atom (Scheme 1), since the tetra-
alkylammonium is a poor H-bonding donor by the lack of
acidic hydrogens, and moreover, its positive charge is less
delocalized than the imidazolium ring.
In contrast, the difference in binding energy with changing
anion from [DCA] to [NTf2] (DEdisp) for imidazolium ILs is
about half of the ammonium ILs (Table 4). Moreover, due to
the presence of C2-H, polarity of imidazolium ILs experienced
by Reichardt’s dye 30 resulted mainly from H-bonding inter-
action with phenolate oxygen but less affected by the Coulombic
interaction.23 However, for general imidazolium ILs the
difference in intensity of hydrogen bonds for various anions
is not enough to differentiate their polarities, since the C2-H is
less acidic and free to interact with the phenolate oxygen of
Based on the suggested interaction between ILs and Reichardt’s
dye probe, DFT computation of four typical imidazolium and
tetraalkylammonium ILs with the B3LYP method and basis set
6–31+g(d,p) was conducted to get optimized structure and ionic
energy and to deeper understand the different polarity behavior at
the molecular level. Shown in Fig. 6 are the optimized structures
for the calculated ion pairs. In the case of the imidazolium cation,
the anion was located in front of the imidazolium ring, where its
atoms of more negative charge are close to the methyl, ethyl, in
particular C2-H (Fig. 6). In detail, [DCA]À nearly located in the
plane of the imidazolium ring and [NTf2]À located its S–N–S core
in the plane of the imidazolium ring with each CF3 group
Scheme 1
A cartoon illustrating plausible interaction between
Reichardt’s dye 30 and ILs.
c
1048 New J. Chem., 2012, 36, 1043–1050
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

View Article Online
Fig. 6 Optimized geometries of ion-pair conformations of ILs in this work: (a) [BMIm][DCA], (b) [BMIm][NTf2], (c) [N1124][DCA] and
(d) [N1124][NTf2].
support. The authors thank the supercomputing center,
CNIC, CAS for Gaussian calculations.
Table 4 Bonding energy (DE) and displacement energies (DEdisp
)
calculated for four imidazolium and ammonium ILs.a
b
ILs
DE (kJ molÀ1
)
DEdisp (kJ molÀ1
)
Notes and references
[BMIm][DCA]
[BMIm][NTf2]
[N1124][DCA]
[N1124][NTf2]
À327.08
À313.96
À314.86
À289.53
À13.12
0.0
1 J. P. Hallett and T. Welton, Chem. Rev., 2011, 111, 3508–3576.
2 P. Wasserscheid and T. Welton, Ionic liquids in synthesis,
2nd Edition, Wiley-VCH, 2007, vol. 2.
3 M. J. Earle and K. R. Seddon, Pure Appl. Chem., 2000, 72, 1391–1398.
4 H. Weingaertner, Angew. Chem., Int. Ed., 2008, 47, 654–670.
5 J. Dupont and P. A. Z. Suarez, Phys. Chem. Chem. Phys., 2006, 8,
2441–2452.
À25.33
0.0
a
b
Atom coordinates see supporting information.w For displacement
energy [BMIm][NTf2] and [N1124][NTf2] was denoted as reference,
respectively.
6 V. I. Parvulescu and C. Hardacre, Chem. Rev., 2007, 107, 2615–2665.
7 T. Fischer, A. Sethi, T. Welton and J. Woolf, Tetrahedron Lett.,
1999, 40, 793–796.
8 J. S. Wilkes, J. Mol. Catal. A: Chem., 2004, 214, 11–17.
9 H. Ohno, Electrochemical Aspects of Ionic Liquids, Wiley Inter-
science, New York, 2005.
10 S. A. Forsyth, J. M. Pringle and D. R. MacFarlane, Aust. J. Chem.,
2004, 57, 113–119.
11 T. Sato, G. Masuda and K. Takagi, Electrochim. Acta, 2004, 49,
3603–3611.
12 P. Hapiot and C. Lagrost, Chem. Rev., 2008, 108, 2238–2264.
13 L. C. Branco, J. G. Crespo and C. A. M. Afonso, Angew. Chem.,
Int. Ed., 2002, 41, 2771–2773.
14 A. E. Visser, R. P. Swatloski, W. M. Reichert, R. Mayton, S. Sheff,
A. Wierzbicki, J. H. Davis and R. D. Rogers, Environ. Sci. Technol.,
2002, 36, 2523–2529.
Reichardt’s dye, as indicated by their comparable hydrogen
bond donating ability (a values in Table 1).21 Thus imidazolium
ILs with the two anions give comparable polarity scales.
Introduction of an ether group resulted in different effects
on the polairty of imidazolium and ammonium ILs. The
increase in polarity of [1O2MIm][NTf2], although less significant
than 1-(2-methoxyethyl)-1-methylpyrrolidinium-based ILs,25 was
ascribed by Crowhurst et al. to the electron withdrawing effect of
the oxygen atom, which actually increases the overall acidity of
the ion.48 In contrast, the ether group affected the coulombic
interaction between ammonium cation and anions less.
15 J. Esser, P. Wasserscheid and A. Jess, Green Chem., 2004, 6,
316–322.
16 C. Reichardt, Green Chem., 2005, 7, 339–351.
17 L. Crowhurst, N. L. Lancaster, J. M. P. Arlandis and T. Welton,
J. Am. Chem. Soc., 2004, 126, 11549–11555.
18 S. V. Dzyuba and R. A. Bartsch, Tetrahedron Lett., 2002, 43,
4657–4659.
19 D. S. Zhao, J. L. Wang and E. P. Zhou, Green Chem., 2007, 9,
1219–1222.
20 Y. S. Wu, T. Sasaki, K. Kazushi, T. Seo and K. Sakurai, J. Phys.
Chem. B, 2008, 112, 7530–7536.
21 S. G. Zhang, X. J. Qi, X. Y. Ma, L. J. Lu and Y. Q. Deng, J. Phys.
Chem. B, 2010, 114, 3912–3920.
22 H. Ohno and Y. Fukaya, Chem. Lett., 2009, 38, 2–7.
23 M. J. Muldoon, C. M. Gordon and I. R. Dunkin, J. Chem. Soc.,
Perkin Trans. 2, 2001, 433–435.
4. Conclusion
In conclusion, a distinct influence of anion and ether group on
the polarity of ammonium and imidazolium ILs was found.
The polarity of the ammonium ILs appears to be strongly
anion dependent, but less affected by the ether group, unlike
the case of imidazolium ILs. The polarity of imidazolium ILs
was much more affected by the ether group than the anions.
The possible reason is that the polarity of imidazolium ILs
experienced by Reichardt’s dye 30 resulted mainly from
H-bonding interaction with the phenolate oxygen, while the
polarity of ammonium ILs is controlled by coulombic inter-
action between cation and the phenolate oxygen. Note that the
[NTf2]-based ammonium ILs possess higher polarity (close to
glycol) that can fill the polarity gap between water and general
molecular solvents, and can be used as a single aprotic ionic
solvent for polarity-specfic reaction and synthesis.
24 Ionic Liquids in Synthesis, ed. P. Wasserscheid and T. Welton,
Wiley-VCH, 2007.
25 J. L. Kaar, A. M. Jesionowski, J. A. Berberich, R. Moulton and
A. J. Russell, J. Am. Chem. Soc., 2003, 125, 4125–4131.
26 J. M. Lee and J. M. Prausnitz, Chem. Phys. Lett., 2010, 492, 55–59.
27 Z. J. Chen, S. M. Liu, Z. P. Li, Q. H. Zhang and Y. Q. Deng,
New J. Chem., 2011, 35, 1596–1606.
28 P. Bonhote, A. P. Dias, N. Papageorgiou, K. Kalyanasundaram
and M. Gratzel, Inorg. Chem., 1996, 35, 1168–1178.
29 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, J. A. Montgomery Jr., T. Vreven, K. Kudin, J. C. N.
Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,
B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson,
H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21103208 and 21173240) for financial
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 1043–1050 1049

View Article Online
H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross,
C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala,
K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain,
O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford,
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,
B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople,
Gaussian 03, revision E.01, Gaussian, Inc.:Pittsburgh, PA, 2003.
30 E. A. Turner, C. C. Pye and R. D. Singer, J. Phys. Chem. A, 2003,
107, 2277–2288.
31 E. B. Tada, L. P. Novaki and O. A. E. Seoud, J. Phys. Org. Chem.,
2000, 13, 679–687.
32 C. P. Fredlake, M. J. Muldoon, S. N. V. K. Aki, T. Welton and
J. F. Brennecke, Phys. Chem. Chem. Phys., 2004, 6, 3280–3285.
33 A. Sarkar, S. Trivedi, G. A. Baker and S. Pandey, J. Phys.
Chem. B, 2008, 112, 14927–14936.
41 H. Shirota, H. Fukazawa, T. Fujisawa and J. F. Wishart, J. Phys.
Chem. B, 2010, 114, 9400–9412.
42 C. M. Lang and P. A. Kohl, J. Electrochem. Soc., 2007, 154,
F106–F110.
43 T. Masaki, K. Nishikawa and H. Shirota, J. Phys. Chem. B, 2010,
114, 6323–6331.
44 S. G. Zhang, Q. H. Zhang, B. X. Ye, X. L. Li, X. P. Zhang and
Y. Q. Deng, J. Phys. Chem. B, 2009, 113, 6012–6019.
45 R. Byrne, K. J. Fraser, E. Izgorodina, D. R. MacFarlane,
M. Forsyth and D. Diamond, Phys. Chem. Chem. Phys., 2008,
10, 5919–5924.
46 J. Zhou, Y. Li, Y. Tang, F. Zhao, X. Song and E. Li, J. Photochem.
Photobiol., A, 1995, 90.
47 M. A. Ab Rani, A. Brant, L. Crowhurst, A. Dolan, M. Lui,
N. H. Hassan, J. P. Hallett, P. A. Hunt, H. Niedermeyer,
J. M. Perez-Arlandis, M. Schrems, T. Welton and R. Wilding,
Phys. Chem. Chem. Phys., 2011, 13, 16831–16840.
48 L. Crowhurst, P. R. Mawdsley, J. M. Perez-Arlandis, P. A. Salter
and T. Welton, Phys. Chem. Chem. Phys., 2003, 5, 2790–2794.
49 R. W. Taft and M. J. Kamlet, J. Am. Chem. Soc., 1976, 98, 2886–2894.
50 N. D. Khupse and A. Kumar, J. Phys. Chem. B, 2011, 115,
711–718.
34 Y. Yoshida, O. Baba and G. Saito, J. Phys. Chem. B, 2007, 111,
4742–4749.
35 H. Tokuda, S. Tsuzuki, M. A. B. H. Susan, K. Hayamizu and
M. Watanabe, J. Phys. Chem. B, 2006, 110, 19593–19600.
36 O. Zech, J. Hunger, J. R. Sangoro, C. Iacob, F. Kremer, W. Kunz
and R. Buchner, Phys. Chem. Chem. Phys., 2010, 12, 14341–14350.
37 S. Park and R. J. Kazlauskas, J. Org. Chem., 2001, 66, 8395–8401.
38 P. Wasserscheid, C. M. Gordon, C. Hilgers, M. J. Muldoon and
I. R. Dunkin, Chem. Commun., 2001, 1186–1187.
39 Y. Marcus, Chem. Soc. Rev., 1993, 22, 409–416.
40 W. A. Henderson, V. G. Young, D. M. Fox, H. C. De Long and
P. C. Trulove, Chem. Commun., 2006, 3708–3710.
51 C. Reichardt, Chem. Soc. Rev., 1992, 21, 147–153.
52 V. Sans, N. Karbass, M. I. Burguete, V. Compan, E. Garcıa-
Verdugo, P. S. V. Luis and M. Pawlak, Chem.–Eur. J., 2011, 17,
1894–1906.
53 T. L. Amyes, S. T. Diver, J. P. Richard, F. M. Rivas and K. Toth,
J. Am. Chem. Soc., 2004, 126, 4366–4374.
54 V. Znamenskiy and M. N. Kobrak, J. Phys. Chem. B, 2004, 108,
1072–1079.
55 K. J. Fraser, E. I. Izgorodina, M. Forsyth, J. L. Scott and
D. R. MacFarlane, Chem. Commun., 2007, 3817–3819.
c
1050 New J. Chem., 2012, 36, 1043–1050
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012