Room-temperature thermotropic ionic liquid crystals: Viologen bis(triflimide) salts

Article abstract of DOI:10.1080/15421400490478272

Several dicationic salts with bis(triflimide) as counterions - otherwise known as viologens - were prepared by metathesis reaction of the corresponding viologen dibromides (diiodides) with lithium triflimide in a polar solvent. They were characterized for

Full text of DOI:10.1080/15421400490478272

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Room-Temperature
Thermotropic Ionic
Liquid Crystals: Viologen
Bis(Triflimide) Salts
Pradip Bhowmik a , Haesook Han a , Ivan Nedeltchev
a & James Cebe a
a Department of Chemistry, University of Nevada
Las Vegas, 4505 Maryland Parkway, Box 454003, Las
Vegas, Nevada, 89154-4003, USA
Published online: 18 Oct 2010.
To cite this article: Pradip Bhowmik , Haesook Han , Ivan Nedeltchev & James
Cebe (2004) Room-Temperature Thermotropic Ionic Liquid Crystals: Viologen
Bis(Triflimide) Salts, Molecular Crystals and Liquid Crystals, 419:1, 27-46, DOI:
10.1080/15421400490478272
To link to this article: http://dx.doi.org/10.1080/15421400490478272
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Mol. Cryst. Liq. Cryst., Vol. 419, pp. 27–46, 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490478272
ROOM-TEMPERATURE THERMOTROPIC IONIC
LIQUID CRYSTALS: VIOLOGEN BIS(TRIFLIMIDE) SALTS
Pradip K. Bhowmikꢀ, Haesook Han, Ivan K. Nedeltchev,
and James J. Cebe
Department of Chemistry, University of Nevada Las Vegas,
4505 Maryland Parkway, Box 454003, Las Vegas,
Nevada 89154-4003, USA
Several dicationic salts with bis(triflimide) as counterions—otherwise known
as viologens—were prepared by metathesis reaction of the corresponding violo-
gen dibromides (diiodides) with lithium triflimide in a polar solvent. They
were characterized for their thermotropic liquid-crystalline properties with
various experimental techniques. Two of them had low crystal-to-smectic phase
transitions (Tm) at –4 and 28ꢁC; the other two also had low Tm values, at 51 and
58ꢁC. Each had a wide range of LC phase. Two had relatively low crystal-to-
isotropic (Tm) transitions at 117 and 131ꢁC. Each had excellent thermal stab-
ility in the temperature range of 327–396ꢁC. They exhibited photoluminescence
property in both nonpolar and polar solvents as well as in the solid state. Their
emission spectra in the solid state exhibited hypsochromic shift when
compared with those in solution of dimethoxyethane.
Keywords: hypsochromic; ionic liquid; photoluminescence; smectic phase; thermotropic;
viologen bis(triflimide salt)s
INTRODUCTION
The 1,10-dialkyl-4,40-bipyridinium salts are usually known as viologens. They
are an important class of compounds that exhibit a number of interesting
P. K. Bhowmik acknowledges the University of Nevada Las Vegas (UNLV) for the startup;
Stimulation, Implementation, Transition and Enhancement (SITE); New Investigator Awards
(NIA) and Planning Initiative Awards (PIA) grants; the Donors of the Petroleum Research
Fund (PRF), administered by the American Chemical Society; and an award from Research
Corporation for the support of this research. J. J. Cebe acknowledges the Graduate College
at UNLV for providing a Summer Graduate Research Education and Training (GREAT) Assis-
tantship for the support of this research. The authors also thank D. L. Walsh for her generous
donation for the support of this research.
ꢀCorresponding author. E-mail: bhowmikp@unlv.nevada.edu
27

28
P. K. Bhowmik et al.
properties, including electrical conductivity, photochromism, electrochro-
mism, and thermochromism [1]. With the suitably modified chemical archi-
tectures, they also exhibit thermotropic liquid crystalline (LC) properties
[2–6]. For example, the ranges of thermotropic LC phase for a series of
these compounds with variation of alkyl chain lengths (n ¼ 5–10 and 18)
and anions (X– ¼ Cl–, Br–, I–, ꢂ BF4, ꢂOOCCF3, ꢂO3SCH3, and ꢂOTs)
are sensitive to both the alkyl chain lengths and the anions. For a fixed
alkyl chain length, considerable variability in thermal transitions is
observed even when the anions are restricted to the halide ions. Among
these compounds, the reversibility of various thermal transitions is poor;
all three compounds decompose prior to exhibiting a stable isotropic liquid
phase. Other anions show thermal properties ranging from the absence of
an LC phase formation with ꢂBF4 to reversible polymesomorphism (more
than one LC phase) with ꢂO3SCH3. Generally, all of them form very viscous
melts when compared with conventional, nonionic thermotropic LC com-
pounds [2]. The 1,10-di(3,6,9-trioxatridecyl)-4,40-bipyridinium dibromide
and 1,1,0-di(3,6,9,12-tetraoxatetradecyl)-4,40-bipyridinium diiodide have LC
properties from room temperature up to about 200 and 154ꢁC, respectively.
The other compound, 1,10-di(3,6,9-trioxatridecyl)-4,40-bipyridinium diio-
dide, has a crystal-to-LC phase transition (Tm) at 63ꢁC and an LC-to-isotropic
transition (Ti) at 216ꢁC [3,4]. In contrast, the 1,10-di(3,6,9-trioxadecyl)-4,40-
bipyridinium bis(tetrafluoroborate) and 1,10-di(3,6,9-trioxadecyl)-4,40-
bipyridinium ditosylate are room-temperature (RT) ionic liquids [5]. The
1,10-diphenyl-4,40-bipyridinium dialkylbenzene sulfonates that contain alkyl
chain lengths of 10-, 13-, and 15-carbon atoms have Tm values (205, 190,
and 168ꢁC, respectively) above which each exhibits a SA phase, and the SA
phase for each persists up to their decomposition temperatures (280–
290ꢁC) [6]. Thus, it is evident that the viologen compounds, having suitably
designed chemical architectures (dicationic salts), like 1-alkyl-3-methylimi-
dazolium, N-alkylpyridinium, quaternary ammonium, and quaternary phos-
phonium salts, each of which is a monocationic salt [7–10], have great
potential for the preparation of both RT ionic liquids (green solvents) and
RTLC ionic liquids (ordered reaction media). Additionally, there has been
recent development in the structural features of RTLC ionic salts—for
example, as ordered solvents that can impart selectivity in reactions by order-
ing reactants [11], as templates for the synthesis of mesoporous and zeolitic
materials [12], and in the formation of ordered thin films [13].
In this article, we describe the facile preparation of a new series of
viologen bis(triflimide) salts (1–6) by the metathesis reaction of either
the corresponding 1,10-dialkyl-4,40-bipyridinium dibromides or 1,10-diben-
zyl -4,40-bipyridinium dibromide with lithium triflimide in a polar organic
solvent (Scheme 1) and the characterization of their thermotropic
LC properties by differential scanning calorimetry (DSC), polarizing light

Room-Temperature Thermotropic Ionic Liquids
29
SCHEME 1
microscopy (PLM), and thermogravimetric analysis (TGA). To our knowl-
edge, they are the first examples of viologen bis(triflimide) salts that exhi-
bit thermotropic LC properties at as low as room temperature, as well as at
relatively low temperatures (51 and 58ꢁC). Their photoluminescent pro-
perty in both nonpolar and polar organic solvents as well as in the solid
state is also included in this study. Their emission spectra in the solid state
displayed hypsochromic shift when compared with those in solution of non-
polar solvent such as dimethyoxyethane. The triflimide organic counterion
is selected because of its unique ability to depress the melting points of
ionic salts [14–16]. It also has great thermal stability when compared with
other inorganic or organic counterions. For example, the thermal stability
of 1-ethyl-3-methylimidazolium triflimide—a RT ionic liquid—in both air
and nitrogen is up to 400ꢁC [14]. Triflimide is undoubtedly an interesting
organic counterion, because it plays a pivotal role for the synthesis of
several major classes of RT ionic liquids [14,15].
EXPERIMENTAL
The 1,10-dialkyl-4,40-bipyridinium dibromides (diiodides) were prepared by
adding the corresponding slight excess of two equivalents of alkyl bromides
(diiodides) to a solution of one equivalent of 4,40-bipyridyl in acetonitrile.
After heating to reflux for 24 h the yellow crystalline dialkylated product
was filtered from the reaction mixture (0ꢁC), washed with acetone, and
recrystallized twice from H2O=acetone (15=85) [2]. Their purity was
checked by 1H NMR spectroscopy (the absence of peaks at d ¼ 7.83,
8.32, 8.64, and 8.83 ppm of monoalkylated product in D2O) and elemental
analysis. The viologens 1–6 were prepared by the metathesis reaction of
either the corresponding 1,10-dialkyl -4,40-bipyridinium dibromides (diio-
dides for 1 and 2) or 1,10-dibenzyl -4,40-bipyridinium dibromide with lith-
ium triflimide in a common organic solvent. A typical procedure was as
follows: 2.0 g (4.5 mmol) of 1,10-dimethyl-4,40-dipyridinium diiodide was

30
P. K. Bhowmik et al.
dissolved in 30 ml methanol. To the methanol solution of diiodide salt, 30 ml
of lithium triflimide (2.9 g, 10.1 mmol, 11% excess) in methanol was slowly
added on stirring. The resulting solution was stirred overnight. After
removing methanol by a rotary evaporator, deionized water was added to
the solid products to dissolve LiI and excess lithium triflimide, thus result-
ing the desired compound 1. It was further purified by simply washing with
deionized water several times and dried in vacuum at RT for several days.
The yield of metathesis reaction for the preparation of this compound 1
was essentially quantitative. Its purity was checked by both 1H and
13C NMR spectroscopy, as well as elemental analysis. No special equip-
ments, such as a glove box, and precautions were adopted for the
synthesis of these viologen salts.
Selected data for 1: IR (KBr)=mmax cmꢂ1: 3136, 3069, 1644, 1572, 1507,
1439, 1354, 1193, 1136, 1048, 833, 795, 741, 706, 618, 570, 515; dH
(CD3OD, 400 MHz, ppm): 9.12–9.14 (4H, d, J ¼ 6.56 Hz), 8.58–8.59 (4H,
d, J ¼ 6.23 Hz), 4.51–4.53 (6H, d, J ¼ 7.29 Hz); dC (CD3OD, 100 MHz,
ppm): 151.21, 148.02, 127.94, 125.96, 122.77, 119.59, 116.40, 49.06; Anal.
Calcd for C16H14N4O8F12S4: C, 25.74; H, 1.89; N, 7.50; S, 17.18. Found: C,
25.83; H, 2.12; N, 7.57; S, 17.01.
Selected data for 2: IR (KBr)=mmax cm ꢂ1: 3068, 2974, 2882, 1639, 1558,
1509, 1453, 1352, 1231, 1194, 1137, 1049, 841, 788, 739, 620, 570, 515; dH
(CD3OD, 400 MHz, ppm): 9.20–9.21 (4H, d, J ¼ 6.79 Hz), 8.59–8.61 (4H, d,
J ¼ 6.32 Hz), 4.71–4.75 (4H, t, J ¼ 7.70 Hz), 2.05–2.20 (4H, m), 1.44–1.55
(4H, m), 1.01–1.05 (6H, t, J ¼ 7.35 Hz); dC (CD3OD, 100 MHz, ppm):
151.46, 147.97, 147.02, 146.11, 128.36, 125.93, 122.74, 119.56, 116.37,
63.22, 34.40, 20.42, 13.72; Anal. Calcd for C22H26N4O8F12S4: C, 31.81; H,
3.15; N, 6.74; S, 15.44. Found: C, 31.84; H, 3.44; N, 6.85; S, 15.63.
Selected data for 3: IR (KBr)=mmax cm ꢂ 1: 3135, 3074, 2965, 2877, 1640,
1564, 1509, 1449, 1352, 1192, 1136, 1056, 838, 787, 739, 652, 620, 598, 570,
512; dH (CD3OD, 400 MHz, ppm): 9.20–9.21 (4H, d, J ¼ 6.79 Hz), 8.59–8.61
(4H, d, J ¼ 6.32 Hz), 4.71–4.75 (4H, t, J ¼ 7.70 Hz), 2.05–2.20 (4H, m),
1.44–1.55 (4H, m), 1.01–1.05 (6H, t, J ¼ 7.35 Hz); dC (CD3OD, 100 MHz,
ppm): 151.39, 146.95, 128.29, 125.86, 122.67, 119.49, 116.30, 63.30,
32.13, 29.19, 23.07, 14.04; Anal. Calcd for C24H30N4O8F12S4: C, 33.57; H,
3.52; N, 6.52; S, 14.93. Found: C, 33.60; H, 3.41; N, 6.78; S, 14.74.
Selected data for 4: IR (KBr)=mmax cm ꢂ1: 3130, 3072, 2941, 2869, 1640,
1564, 1509, 1449, 1350, 1329, 1193, 1133, 1060, 842, 787, 738, 648, 597,
572, 511; dH (CD3OD, 400 MHz, ppm): 9.21–9.22 (4H, d, J ¼ 5.30 Hz),
8.60–8.61 (4H, d, J ¼ 5.56 Hz), 4.71–4.74 (4H, t, J ¼ 8.47 Hz), 2.07–2.20
(4H, m), 1.38–1.45 (12H, m), 0.91–0.94 (6H, m); dC (CD3OD, 100 MHz,
ppm): 151.47, 147.02, 128.36, 125.96, 122.77, 119.58, 116.40, 63.42,
32.49, 32.27, 26.86, 23.43, 14.25; Anal. Calcd for C26H34N4O8F12S4: C,
35.21; H, 3.86; N, 6.32; S, 14.47. Found: C, 35.11; H, 3.78; N, 6.11; S, 14.54.

Room-Temperature Thermotropic Ionic Liquids
31
Selected data for 5: IR (KBr)=mmax cm ꢂ 1: 2922, 2853, 1684, 1643, 1560,
1509, 1470, 1351, 1195, 1136, 1060, 835, 789, 740, 654, 621, 600, 514; dH
(CD3OD, 400 MHz, ppm): 9.21–9.22 (4H, d, J ¼ 6.89 Hz), 8.60–8.62 (4H,
d, J ¼ 6.63 Hz), 4.70–4.74 (4H, t, J ¼ 5.97 Hz), 2.07–2.20 (4H, m), 1.29–
1.43 (36H, m), 0.87–0.91 (6H, t, J ¼ 6.68 Hz); dC (CD3OD, 100 MHz,
ppm): 151.44, 147.03, 128.35, 125.94, 122.76, 119.57, 116.38, 63.42,
33.04, 32.53, 30.69, 30.61, 30.44, 30.11, 27.20, 23.70, 14.42; Anal. Calcd
for C38H58N4O8F12S4: C, 43.26; H, 5.54; N, 5.31; S, 12.15. Found: C,
43.30; H, 5.62; N, 5.48; S, 12.31.
Selected data for 6: IR (KBr)=mmax cm ꢂ1: 3067, 1684, 1634, 1558, 1508,
1497, 1450, 1364, 1331, 1201, 1139, 1052, 853, 813, 793, 748, 725, 701, 616,
570, 552, 515; dH (CD3OD, 400 MHz, ppm): 9.23–9.25 (4H, d, J ¼ 6.52 Hz),
8.58–8.60 (4H, d, J ¼ 6.44 Hz), 7.47–7.54 (10H, m), 5.93 4H, s); dC
(CD3OD, 100 MHz, ppm): 151.80, 146.99, 134.06, 131.29, 130.86. 130.40,
128.63, 125.94, 122.75, 119.57, 116.38, 66.04; Anal. Calcd for
C28H22N4O8F12S4: C, 37.42; H, 2.47; N, 6.23; S, 14.27. Found: C, 37.56; H,
2.43; N, 6.26; S, 14.53;
The FTIR spectra of compounds 1–6 were recorded with a Nicolet FTIR
1
analyzer with their neat films on KBr pellets. Their H and 13C NMR spec-
tra were recorded with three RF channels operating at 400 and 100 MHz,
respectively, in CD3OD using TMS as an internal standard. The phase tran-
sition temperatures were measured by the differential scanning calor-
imetry (TA 2100 DSC) in nitrogen at both heating and cooling rates of
10ꢁC=min. The TGA measurements were performed with a TA 2100 instru-
ment at a heating rate of 20ꢁC=min in nitrogen. The optical textures of LC
phases were observed by using a Nikon polarizing light microscope
equipped with a Mettler FP80 hot-state. UV-vis absorption spectra of com-
pounds 1–6 in spectrograde dimethoxyethane (DME) and methanol were
recorded with a Varian Cary 3 Bio UV-Vis spectrophotometer at room tem-
perature. Their photoluminescence spectra in solutions of both DME and
methanol were recorded with a Perkin-Elmer LS 55 luminescence spec-
trometer with a xenon lamp source. The photoluminescence spectra of 3
and 5 in thin films cast from DME were also recorded with this instrument
by using a front surface accessory.
RESULTS AND DISCUSSION
The 1,10-dimethyl-4, 40-bipyridinium diiodide had a weak crystal-to-crystal
transition at 264ꢁC (DH ¼ 1.1 kJ=mol), and a crystal-to-liquid transition at
365ꢁC (mp 325ꢁC [17]), accompanied by immediate decomposition as
determined by DSC at a heating rate of 10ꢁC=min in nitrogen. In contrast,
the viologen 1 showed a single endotherm in the first and second heating

32
P. K. Bhowmik et al.
cycles of its DSC thermograms under identical conditions. In each of the
corresponding cooling cycles, there were several exotherms. Among them,
two exotherms at 103 and 108ꢁC were more prominent than others. These
multiple exotherms were presumably related to the polymorphism of an
ionic compound on cooling from the isotropic liquid phase [18]. In conjunc-
tion with the PLM studies, it was determined that the endotherm corre-
sponded to the crystal-to-liquid transition (Tm ¼ 132ꢁC with DHm ¼ 39.0
kJ=mol), which was far below its decomposition temperature at 396ꢁC
(Table 1). Thus, 1 had no thermotropic LC property, but its much lower
melting transition was related to the presence of bis(triflimide) counter-
ions in this salt when compared with the 1,10-dimethyl-4,40-bipyridinium
diiodide.
The 1,10-dibutyl-4,40-bipyridinium diiodide exhibited no crystal-to-crys-
tal transition but did exhibit a melting endotherm at 270ꢁC [17] with the
concomitant decomposition as determined by DSC at a heating rate of
10ꢁC=min in nitrogen. The TGA measurement also indicated its decompo-
sition temperature at 269ꢁC at a heating rate of 10ꢁC=min in nitrogen.
Figure 1 shows the DSC thermograms of 2 obtained at both heating and
cooling rates of 10ꢁC=min. In each of the heating cycles there were dis-
tinctly two endotherms. Correspondingly, there were two major exotherms
TABLE 1 Transition Temperatures and Enthapies from DSC Thermograms and
Decomposition Temperatures from TGA Measurements for Viologen Salts
First heading
T(ꢁC)
Second heading
Compound Transition
DH (kJ=mol)
T(ꢁC)
DH (kJ=mol)
Td a(ꢁC)
1
2
cryst-iso
cryst-SA
SA-iso
132.3
51.1
87.8
39.0
14.7
27.6
3.9
132.3
50.4
87.8
39.0
5.7
395.5
366.2
27.2
b
3
4
5
cryst-SA
SA-iso
ꢂ 4.3
42.2
58.4
–
364.4
361.7
355.5
19.0
31.0
5.0
42.2
58.1
19.3
30.3
5.0
cryst-SA
SA-iso
cryst-SX
78.2
78.3
18.3
28.3c
120.3
198.8
68.0
27.2
8.0
18.5
1.4
0
SX-SX
S
119.3
201.9
68.0
0
X -SX
00
9.9
3.6
6
glass transition
cryst-iso
327.3
117.5
34.9
117.7
34.9
a Thermal stability (Td) was determined in nitrogen at a heating rate of 20ꢁC=min at which
5 wt% loss of the compound occurred, with the exceptions of 2 and 5, which were determined
at a heating rate of 10ꢁC=min.
b This transition was not detected in the second heating cycle for this salt.
c There was a shoulder peak at 24.3ꢁC with DH ¼ 0.8 kJ=mol in the first heating cycle.

Room-Temperature Thermotropic Ionic Liquids
33
FIGURE 1 DSC thermograms of viologen 2 obtained at both heating and cooling
rates of 10ꢁC=min.
in each of the cooling cycles. In conjunction with PLM studies, it was
determined that the low-temperature endotherm was related to the crys-
tal-to-smectic LC phase transition (Tm ¼ 51ꢁC) and the high-temperature
endotherm was related to the smectic-to-isotropic transition (Ti ¼ 88ꢁC).
Both of these transitions were conveniently well below its decomposition
temperature at 366ꢁC (Table 1). The high-temperature exotherm
(ꢂDH ¼ 26.3 kJ=mol) in the second cooling cycle, which underwent a
low degree of hysteresis (i.e., a supercooling of 16ꢁC), corresponded to
the transition from the isotropic liquid phase to the smectic LC phase.
The low-temperature exotherm ( ꢂ DH ¼ 2.7 kJ=mol), which underwent
a high degree of supercooling of 56ꢁC, was related to the transition from the
smectic LC phase to crystalline phase. Additionally, the enthalpy change for
the Ti remained essentially identical in both the heating cycles, but the
enthalpy change for the Tm in the second heating cycle was much lower
than that in the first heating cycle (Table 1). This result was presumably
related to the imperfect crystallization under the experimental conditions
used. Thus 2, unlike the 1,10-dibutyl-4-40-bipyridinium diiodide, exhibited
an enantiotropic LC phase and had a wide range of LC phase of 37ꢁC. These
results suggested that a simple exchange of diiodide counterion of viologen
salt by bis(triflimide) as counterions was conducive to the formation of LC
ionic salt.

34
P. K. Bhowmik et al.
The 1,10-dipentyl-4,40-bipyridinium dibromide exhibited a crystal-to-
crystal transition at 192ꢁC (DH ¼ 29.3 kJ=mol) in the first heating cycle
of DSC thermogram, but it showed three endotherms (164, 187, and
208ꢁC) in the second heating cycle, none of which was related to crystal-
to-liquid transition. Its melting transition was also very close to its
decomposition temperature at 261ꢁC, as determined by DSC at a heating
rate of 10ꢁC=min. The TGA measurement also confirmed its decomposition
temperature at 272ꢁC [17] at a heating rate of 20ꢁC=min in nitrogen.
Figure 2 displays the DSC thermograms of 3 obtained at both heating
and cooling rates of 10ꢁC=min. In the first heating cycle, this salt showed
distinctly two endotherms. The low-temperature endotherm was its Tm
( ꢂ 4ꢁC), and the high-temperature one was its Ti (42ꢁC), as verified with
PLM studies. In the first cooling cycle, there was a rather broad exotherm
that corresponded to the transition from isotropic liquid phase to the smec-
tic LC phase. The absence of exotherm for smectic LC phase-to-crystalline
phase in the first cooling cycle and the absence of Tm in the second heating
cycle suggested that this salt retained the smectic LC phase without the
interference of crystallization even at low temperature (ꢂ30ꢁC) on cooling
at a rate of 10ꢁC=min. Thus, it was found that 3 is a RT ionic salt that has a
FIGURE 2 DSC thermograms of viologen 3 obtained at both heating and cooling
rates of 10ꢁC=min.

Room-Temperature Thermotropic Ionic Liquids
35
range of LC phase of 46ꢁC. It also has excellent thermal stability as high as
364ꢁC (Table 1).
The 1,10-dihexyl-4,40-bipyridinium dibromide showed clearly an
endotherm in each of the heating cycles. Correspondingly, there was an
exotherm in each of the cooing cycles of its DSC thermograms. In conjunc-
tion with PLM studies it was found that the endotherm was the crystal-to-
LC phase transition (Tm ¼ 144ꢁC, and DH ¼ 14.2 kJ=mol), which persisted
up to its decomposition temperature at 286ꢁC by DSC. The TGA measure-
ment showed its Td ¼ 269ꢁC obtained at a heating rate of 10ꢁC=min in
nitrogen. The exotherm in the cooling cycle at 124ꢁC (DH ¼ 12.0 kJ=mol)
mol) corresponded to the LC-to-crystal phase transition that underwent a
relatively low degree of supercooling of 20ꢁC. Figure 3 displays the DSC
thermograms of 4 obtained at both heating and cooling rate of 10ꢁC=min.
In the first heating cycle, there were two endotherms. In the first cooling
cycle, there were two exotherms at low-temperature region and an
exotherm at high-temperature region. The DSC thermograms in the
second heating and cooling cycles were essentially identical to those in
the first heating and cooling cycles. Again in corroboration with PLM
studies, it was found that the low-temperature endotherm corresponded
to the crystal-to-smectic LC phase transition (Tm ¼ 58ꢁC) and the high-
temperature one corresponded to the smectic LC phase-to-isotropic
FIGURE 3 DSC thermograms of viologen 4 obtained at both heating and cooling
rates of 10ꢁC=min.

36
P. K. Bhowmik et al.
transition (Ti ¼ 78ꢁC), thus resulting in a range of LC phase of 20ꢁC for 4.
As expected, the high-temperature exotherm exhibited the lowest degree
of supercooling among the exotherms observed for 4. Additionally, the
enthalpy changes for the Tm and Ti were essentially identical in both
the heating cycles. Thus, similar to 2 and 3, 4 also exhibited an enan-
tiotropic LC phase. It also had excellent thermal stability up to 362ꢁC
(Table 1), which was exceedingly higher than its Tm and Ti.
Although the 1,10-didodecyl-4,40-bipyridinium dibromide showed two
reversible endotherms (at 119ꢁC with DH ¼ 43.6 and at 170ꢁC with
DH ¼ 3.6 kJ=mol) in each of the heating cycles of DSC thermograms, it
exhibited no melting endotherm up to its decomposition temperature
(Td ¼ 283ꢁC), which was determined by TGA measurement at a heating
rate of 10ꢁC=min in nitrogen. In contrast, the viologen salt 5 exhibited three
endotherms in each of the heating cycles of its DSC thermograms. Corre-
spondingly, there were three exotherms in each of the cooling cycles
(Figure 4). The enthalpy changes for all of these transitions in the first
heating cycle were much higher than those in the second heating cycle
(Table 1). The lowest temperature was related to its crystal-to-smectic
LC phase transition (Tm ¼ 28ꢁC). The other two endotherms at 120
and 199ꢁC were related to the high-order-smectic-phase–to–low-order-
FIGURE 4 DSC thermograms of viologen 5 obtained at both heating and cooling
rates of 10ꢁC=min.

Room-Temperature Thermotropic Ionic Liquids
37
smectic-phase transitions, i.e., to the polymesomorphic phases. These
results are in excellent agreement with those of analogous viologen salts
than contain ꢂ O3SCH3 as counterions in which the polymesomorphic nat-
ure also increases with the length of alkyl chains [2]. The smectic phase of 5
persisted up to its decomposition temperature at 356ꢁC (Table 1). Thus, it
exhibited the widest range of LC phase among all of the viologen salts
(2–5).
The 1,10-dibenzyl-4,40-bipyridinium dibromide exhibited neither crystal-
to-crystal transition nor melting transition up to the decomposition tem-
perature (248ꢁC) at which a loss of 5 wt% of this compound occurred at
a heating rate of 20ꢁC=min in nitrogen. In contrast, the viologen salt 6
exhibited a Tg at 68ꢁC and a sharp endotherm in each of the heating cycles
of its DSC thermograms. Correspondingly, there were a Tg and a sharp
exotherm ineach of the cooling cycles of DSC thermograms. The sharp
endotherm corresponded to the crystal-to-liquid transition (Tm ¼ 117ꢁC)
and the sharp exotherm to liquid-to-crystal transition with a relatively
low degree of supercooling of 30ꢁC. Its Tm was also far below its Td ¼ 327ꢁC
(Table 1). Thus, similar to 1, 6 exhibited no thermotropic LC property, but
its much lower melting transition was again related to the presence of tri-
flimide counterion in this salt when compared with the 1,10-dibenzyl-4,40-
bipyridinium dibromide.
All of these results (vide supra), on the one hand, suggested that the
viologens 2–5 exhibited an enantiotropic LC phase at lower temperatures
when compared with those of several 1-alkyl-3-methylimidazolium (alkyl
chain lengths of 14, 16, and 18) hexafluorophosphate and N-alkylpyridi-
nium (alkyl chain lengths 16 and 18) hexafluorophosphate. On the other
hand, the salts 3 and 5 exhibited an enantiotropic LC phase at much lower
temperatures when compared with those of several 1-alkyl-3-methylimida-
zolium (alkyl chain lengths of 13–18) tetrafluoroborates; the salts 2 and 4
formed an enantiotropic LC phase at temperatures that are comparable
with those of several 1-alkyl-3-methylimidazolium (alkyl chain lengths of
13–18) tetrafluoroborates. The ranges of LC phase for these salts were
much higher in some cases and in other cases were much lower than those
of 1-alkyl-3-methylimidazolium and N-alkylpyridinium salts [19,20]. Fur-
thermore, the observation of enentiotropic LC phase for each of the violo-
gen salts 2–5 is in contrast to several 1-alkyl-3-methylimidazolium (alkyl
chain lengths of 12, 14, 16, and 18) triflimide salts, all of which undergo
crystal-to-isotropic transitions at relatively low temperatures in the range
of 16.6–44.8ꢁC [16]. Thus, it appeared that triflimide is an interesting coun-
terion that has a unique capacity not only to decrease the crystal-to-
isotropic (Tm) in the cases of 1 and 6 but also to decrease crystal-to-LC
phase (Tm) and LC-to-isotropic (Ti) transitions in the cases of 2–5, thus
depending on the chemical structures of the ionic salts employed.

38
P. K. Bhowmik et al.
The thermal stability of each of the viologen salts 1–6 was determined in
nitrogen at a heating rate of either 10 or 20ꢁC=min by TGA measurement.
The thermal stability limit was the temperature at which a 5% weight loss
of 1–6 occurred. They were in the range of 327–396ꢁC, which was much
higher than those of the corresponding dibromide compounds (vide
supra). Figure 5 shows the illustrative thermogravimetric analysis plots
of dibromide salt and bis(triflimide) salt 5, revealing the much greater ther-
mal stability of 5 when compared with the corresponding dibromide salt.
These results are in excellent agreement with those of other ionic salts
containing triflimide anion [14].
The viologen salts 2–5 formed turbid melts above their Tm values as low
as ꢂ4ꢁC to as high as 58ꢁC, which exhibited strong stir opalescence. This
property was taken as a preliminary indication of their LC property. For
further characterization of their melt morphology, they were evaluated
by visual observations with a PLM. Their optical textures were essentially
identical in the cases 2–4. They typically formed, after immediate melting
transitions, Schlieren textures mixed with small mosaic textures that grew
into large mosaic textures before their Ti transitions. On cooling from the
isotropic phase, each of them exhibited various types of mosaic textures
along with spherical birefringent particles that have the appearance of
FIGURE 5 TGA plots of 5 and its corresponding dibromide salt in nitrogen at a
heating rate of 10ꢁC=min.

Room-Temperature Thermotropic Ionic Liquids
39
more or less regular cloverleaf in shape (Figure 6). All of them were
indicative of smectic LC phases, specifically SA, on the basis of PLM studies
[21–23]. Figure 7 shows the photomicrographs of 5 taken at various
temperatures, revealing its polymesomorphic phases.
All of the viologens 1–6 in DME (relatively nonpolar solvent) showed
essentially identical absorption, excitation and emission spectra. Figure
8(a) shows the illustrative emission spectra of 3 in DME at various exci-
tation wavelengths. One the one hand, at low excitation wavelengths
(210–240 nm), it exhibited two major kem values at 298 and 598 nm along
with a shoulder peak at 622 nm. On the other hand, it showed a single kem
at 531 nm at high excitation wavelengths of 410–475 nm. The peak and in-
tensity of emission spectra were dependent on the excitation wavelengths
used. The excitation spectra of 3 in DME showed several kex values at 251,
272, 286, and 476 nm, which were not equivalent to its absorption spec-
trum (kabs ¼ 218 and 265 nm). In contrast, viologens 1–6 in CH3OH (polar
solvent) showed essentially identical absorption spectra, but their exci-
tation and emission spectra were quite different depending on the length
of alkyl chain groups or benzyl group. For example, 1 in CH3OH showed
emission spectra at kem ¼ 305 nm at excitation wavelengths of 220–
230 nm, but it showed different emission spectra at kem ¼ 356 nm at exci-
tation wavelengths of 310 and 320 nm. 2 exhibited kem values at 310 and
622 nm at excitation wavelengths of 220–230 nm, but showed a single
kem ¼ 529 nm at excitation wavelengths of 390, 410, 430, 450, and
470 nm. Figure 8(b) shows the emission spectra of 3 in CH3OH at various
excitation wavelengths. Its emission spectra contained several kem values
depending on the excitation wavelengths used. 4 exhibited only a single
kem ¼ 530 nm irrespective of the excitation wavelengths of 380, 400, 420,
440, 460, and 480 nm used. The emission spectrum of 5 in CH3OH obtained
at excitation wavelength of 220 nm contained two kem values at 390 and
622 nm, but its emission spectra obtained at excitation wavelengths of
360, 400, 440, and 480 nm contained a major kem ¼ 529 nm. However, at
excitation wavelengths of 360 and 400 nm, there were also additional
shoulder peaks in the emission spectra. 6 exhibited a single kem ¼ 533 nm
nm at various excitation wavelengths of 370, 390, 410, 430, 450, and
470 nm. The intensity of emission spectrum was sensitive to the excitation
wavelength used.
Figure 9 shows the emission spectra of 3 and 5 in thin films cast from
DME, that is, in their RT LC phase. The viologen salt 3 had two kem values
at 293 and 375 nm at various excitation wavelengths; the viologen salt 5
had a single kem ¼ 346 nm at excitation wavelengths 265, 275, and
285 nm. Additionally, it exhibited a single kem ¼ 360 nm at low excitation
wavelength of 240 nm. Their excitation spectra contained a major
kex ¼ 241 nm along with a minor peak(s) during monitoring at 376 nm.

40
P. K. Bhowmik et al.
FIGURE 6 Photomicrographs of 3 and 4: (a) taken at room temperature, and (b)
obtained on immediate cooling at a rate of 10ꢁC=min from the isotropic phase under
crossed polarizers, respectively, both of which exhibited a smectic LC phase (mag-
nification 400ꢃ).

Room-Temperature Thermotropic Ionic Liquids
41
FIGURE 7 Photomicrogaphs of 5 taken at (a) 35, (b) 80, (c) 230, and (d) 270ꢁC
under crossed polarizers displaying its various LC phases (magnification 400ꢃ).

42
P. K. Bhowmik et al.
FIGURE 7 Continued.

Room-Temperature Thermotropic Ionic Liquids
43
FIGURE 8 Emission spectra of 3 at various excitation wavelengths, (a) in DME
and (b) in methanol, respectively.

44
P. K. Bhowmik et al.
FIGURE 9 (a) and (b) Emission spectra of 3 and 5 in thin films cast from DME at
various excitation wavelengths.

Room-Temperature Thermotropic Ionic Liquids
45
Their kem values were hypsochromically shifted in the LC phase when com-
pared with those of solution spectra in DME.
Although the photoluminescence quenching processes of a p-conjugated
polymer by numerous viologens in aqueous solution open an opportunity
for the development of biological and chemical sensors for use in medical
diagnostics and toxicology [24–27], the fluorescence property of viologens
1–6 in both nonpolar and polar organic solvents as well as in the solid state
is undoubtedly an additional attribute to this class of ionic salts. This
property of these ionic salts can be exploited further for development
of chemical sensors.
CONCLUSIONS
Several dicationic bis(triflimide) salts, that is, viologens, displayed LC
properties either at RT (ꢂ4 and 28ꢁC) or at relatively low temperatures
(51 and 58ꢁC), and their LC phases persisted over a broad range of tem-
peratures as determined with a number of experimental techniques on
one hand. On the other hand, two viologens had relatively low crystal-to-
isotropic transitions (117 and 131ꢁC). All of them had much better thermal
stability in the temperature range of 327–396ꢁC when compared with that
of the corresponding viologen dihalides. Thus, triflimide anion is a useful
organic counterion for the generation of not only RT ionic liquids but also
RT LC ionic liquids. They have high potential as unique solvents, since their
LC properties were maintained over a wide range of temperatures. Rigidity
of molecular ordering and mobility make these ionic liquids suitable for
many stereochemically controlled organic reactions [11,28]. Their fluores-
cence property both in solution and in the solid state also imparted an
additional attribute to this class of ionic salts that could be further
exploited for the development of chemical sensors.
REFERENCES
[1] Monk, P. M. S. (1998). The Viologens Physicochemical Properties, Synthesis and
Applications of the Salts of 4,40-Bipyridine, New York: Wiley.
[2] Yu, L.-P. & Samulski, E. T. (1984). Oriented Fluids and Liquid Crystals, Griffin, A. C., &
Johnson, J. F. (Eds.), New York: Plenum, Vol. 4, p. 697.
[3] Tabushi, I., Yamamura, K., & Kominami, K. (1986). J. Am. Chem. Soc., 108, 6409–6410.
[4] Yamamura, K., Okada, Y., Ono, S., Kominami, K., & Tabushi, I. (1987). Tet. Lett., 28, 6475–
6478.
[5] Hatazawa, T., Terrill, R. H., & Murray, R. W. (1996). Anal. Chem., 68, 597–603.
[6] Haramoto, Y., Yin, M., Matukawa, Y., Ujiie, S., & Nanasawa, M. (1995). Liq. Cryst., 19,
319–320.
[7] Seddon, K. R. (1997). J. Chem. Technol. Biotechnol., 68, 351–356.

46
P. K. Bhowmik et al.
[8] Welton, T. (1999). Chem. Rev., 99, 2071–2083.
[9] Wasserscheid, P. & Keim, W. (2000). Angew. Chem. Int. Ed., 39, 3772–3789.
[10] Abdallah, D. J., Robertson, A., Hsu, H.-F., & Weiss, R. G. (2000). J. Am. Chem. Soc., 122,
3053–3062.
[11] Weiss, R. G. (1988). Tetrahedron, 44, 3413–3475.
[12] Jervis, H. B., Raimondi, M. E., Raja, R., Maschmeyer, T., Seddon, J. M., & Bruce, D. W.
(1999). Chem. Commun., 2031–2032.
[13] Tollmer, K., Popovitz-Biro, R., Lahav, M., & Milstein, D. (1997). Science, 278, 2100–2102.
ˆ
[14] Bonhote, P., Dias, A.-P., Papageorgiou, N., Kalyanasundaram, K., & Gratzel, M. (1996).
¨
Inorg. Chem., 35, 1168–1178.
[15] Sun, J., Forsyth, M., & MacFarlane, D. R. (1998). J. Phys. Chem. B, 102, 8858–8864.
[16] Bradley, A. E., Hardacre, C., Holbrey, J. D., Johnston, S., McMath, S. E. J., &
Nieuwenhuyzen, M. (2002). Chem. Mater., 14, 629–635.
[17] Ross, J. H. & Krieger, R. I. (1980). J. Agric. Food Chem., 20, 1026–1031.
[18] Alami, E., Levy, H., Zana, R., Weber, P., & Skoulios, A. (1993). Liq. Cryst., 13, 201–212.
[19] Gordon, C. M., Holbrey, J. D., Kennedy, A. R., & Seddon, K. R. (1998). J. Mater. Chem., 8,
2627–2636.
[20] Holbrey, J. D. & Seddon, K. R. (1999). J. Chem. Soc., Dalton Trans., 2133–2139.
[21] Demus, D. & Richter, L. (1978). Textures of Liquid Crystals. (Weinheim: Verlag
Chemie).
[22] Candau, F., Ballet, F., Debeauvais, F., & Whittmann, J.-C. (1982). J. Colloid Interface
Sci., 87, 356–374.
[23] Kumar, S. (Ed.). (2001). Liquid Crystals Experimental Study of Physical Properties
and Phase Transitions (Cambridge, UK: Cambridge University).
[24] Wang, D., Wang, J., Moses, D., Bazan, G. C., & Heeger, A. J. (2001). Langmuir, 17, 1262–
1266.
[25] Gaylord, B. S., Wang, S., Heeger, A. J., & Bazan, G. C. (2001). J. Am. Chem. Soc., 123,
6417–6418.
[26] Wang, J., Wang, D., Miller, E. K., Moses, D., Bazan, G. C., & Heeger, A. J. (2000). Macro-
molecules, 33, 5153–5158.
[27] Chen, L., Mc Branch, D. W., Wang, H.-L., Helgeson, R., Wudl, F., & Whitten, D. G. (1999).
Proc. Natl. Acad. Sci. USA, 96, 12287–12292.
[28] Kansui, H., Hiraoka, S., & Kunieda, T. (1996). J. Am. Chem. Soc., 118, 5346–5352.