Dicationic imidazolium-based ionic liquids and ionic liquid crystals with variously positioned fluoro substituents

Article abstract of DOI:10.1039/b912873e

A series of new dicationic imidazolium-based organic salts bearing fluoro substituents on the bent-core unit and/or linker benzene-ring positions has been synthesized and characterized. The properties of these salts, including transition and decomposition temperatures, and density were determined, indicating that their performance is strongly dependent on the positions of the fluoro substituents, the number of carbon atoms in the alkyl group, and the anion present. Examination by polarized optical microscopy showed that the four salts (compounds 4c, 7a, 7b and 8b, where the alkyl group was C ₁₂H₂₅-) were mesomorphic and these data were supported by DSC measurements.

Full text of DOI:10.1039/b912873e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Dicationic imidazolium-based ionic liquids and ionic liquid crystals with
variously positioned fluoro substituents
a
Xiaoju Li, Duncan W. Bruce and Jean’ne M. Shreeve
b
a
*
Received 1st July 2009, Accepted 26th August 2009
First published as an Advance Article on the web 25th September 2009
DOI: 10.1039/b912873e
A series of new dicationic imidazolium-based organic salts bearing fluoro substituents on the bent-core
unit and/or linker benzene-ring positions has been synthesized and characterized. The properties of
these salts, including transition and decomposition temperatures, and density were determined,
indicating that their performance is strongly dependent on the positions of the fluoro substituents, the
number of carbon atoms in the alkyl group, and the anion present. Examination by polarized optical
microscopy showed that the four salts (compounds 4c, 7a, 7b and 8b, where the alkyl group was
C12H25–) were mesomorphic and these data were supported by DSC measurements.
for ion-conductive materials, organic reaction media or self-
Introduction
assembled nanostructured materials. Imidazolium salts, that
have been well-investigated as ionic liquids, have also been
introduced into ILCs systems.16a,17 Some imidazolium-based
ILCs including symmetrical and unsymmetrical molecular
frameworks were generated and most of them show smectic
phases. For example, a series of unsymmetrical monocationic
imidazolium-based ILCs which contain rigid cores are known18
as well as some self-organized imidazolium-based materials that
form columnar phases.19 Their mesomorphic properties and
potential applications as solvents, electrolytes, high ion transport
materials and templates have also been extensively developed.
Considering the potential applications, the introduction of fluoro
substituents at diverse positions in compounds as in more
conventional, neutral LCs (vide supra) is a feasible route.
However, although many fluorinated LCs have been synthesized
and shown to display excellent modified properties due to the
existence of fluoro substituents, investigations involving this kind
of ILC-bearing fluoro substituents have been reported only
rarely.20 In this work, we describe the design and syntheses
of twelve dicationic imidazolium-based organic salts with fluoro
substituents and study the impact of fluoro groups in varying
positions, of alkyl chain lengths, and of anions on their
properties.
Liquid crystals (LCs) as soft materials have received considerable
attention owing to their fascinating properties1 and their success
in commercial applications.2 Generally, LCs are fluids showing
different mesophases that depend strongly on the combination of
ordered and disordered constituent molecules and on the
molecular shape. Therefore, the constituent molecular architec-
tures are crucial factors both for mesomorphism and for the
physical/chemical properties that characterize LC mesophases.3
In order to investigate structure–property relationships and
generate targeted results, many materials have been prepared,4–6
but as is ever the case, only a very small percentage of them is of
suitable quality for commercial applications.
Among the tools available to synthetic chemists to tune LC
properties, selective fluorination is among the most productive,7
having been extensively developed and explored since the intro-
duction of diversely located fluoro substituents including
terminal chain,8 linking arm,9 or core position10 provide an
excellent opportunity for investigating the relationship between
structures and properties and for modifying and optimizing the
physical/chemical properties of compounds. In 20 years every
semiconductor and LCD plant will have its own on-site fluorine
generator.7b Therefore, targeted materials can be generated
which may lead to active matrix applications11 as well as other
display applications enabling ferroelectric–antiferroelectric
switchable devices,12 electroluminescence,13 and semi-
conductors.14
Experimental section
General methods
Ionic liquid crystals (ILCs), that are considered to be the
combination of LCs and ionic liquids (ILs), are also a class of
liquid-crystalline compounds.15 Owing to the presence of anions
and cations, some properties of ILCs, such as ion conductivity,
non-volatility, low viscosity, low melting points and tunable
polarity, are significantly different from conventional, neutral
LCs, but these materials still display mesophases16 that can allow
All the reagents were available commercially and were used as
1
purchased without further purification. H, 19F and 13C NMR
spectra were recorded in CDCl3 on a 300 MHz spectrometer
(Bruker AMX 300) operating at 300, 282 and 75 MHz, respec-
tively, by using CDCl3 or CD3CN as locking solvents unless
otherwise indicated. Chemical shifts are reported in ppm relative
1
to CFCl3 for 19F NMR spectra and TMS for H and 13C NMR
spectra. Differential scanning calorimetry (DSC) measurements
were performed using a calorimeter equipped with an autocool
accessory and calibrated using indium. The following procedure
was used in experiments for each sample: cooling from 40 ꢀC to
aDepartment of Chemistry, University of Idaho, Moscow, ID, 83844-2343,
USA. E-mail: jshreeve@uidaho.edu; Fax: +1 208 885 9146
bDepartment of Chemistry, University of York, Heslington, York, UK
Y010 5DD
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0 ꢀC, heating to 400 ꢀC, cooling from 400 ꢀC to ꢁ30 ꢀC at 10 ꢀC
minꢁ1, and then heating and cooling again. The transition
temperature, Tm, was taken as peak maximum. Density was
measured at room temperature using a Micromeritics Accupyc
1330 gas pycnometer. Thermogravimetric analysis (TGA)
measurements were carriꢀed out by heating samples at 10 ꢀC
was purified using column chromatography (SiO2) to give pure
2a–c and 6.
Compound 2a–c. 1H NMR (CDCl3) analyses were in good
agreement with the literature.18,21
minꢁ1 from 25 C to 600 C in a dynamic nitrogen atmosphere.
Compound 6. White solid (1.9 g), yield 56%; 1H NMR (CDCl3):
d 7.73 (s, 1H), 7.24 (s, 1H), 7.16 (s, 1H), 7.14 (m, J ¼ 3.0 Hz, 1H),
7.10 (m, J ¼ 6.0 Hz, 1H), 7.06–6.98 (m, J ¼ 24.0 Hz, 2H), 4.04 (t,
J ¼ 12.0 Hz, 2H), 1.82 (m, J ¼ 27.0 Hz, 2H), 1.25 (m, 18H), 0.86
(t, J ¼ 12.0 Hz, 3H).
ꢀ
Elemental analyses were determined using an Exeter CE-440
Elemental Analyzer. Analysis by polarized optical microscopy
was carried out using an Olympus BX50 polarizing microscope
in conjunction with a Linkam LTS350 hot stage and a Linkam
TMS92 control unit.
General procedures for the preparation of compounds 3a–c and
7a–c
General procedures for the preparation of precursors of 1a–c and
5 (Williamson alkylation method)
Imidazole precursors 2a–c (1.2 mmol) with 1,4-bis(chlor-
omethyl)tetrafluorobenzene (0.5 mmol) for 3a–c; imidazole
precursors 2c and 6 (1.2 mmol) with a,a0-dibromo-p-xylene
(0.5 mmol) for 7a and 7b, respectively; 6 (1.2 mmol) and 1,4-
bis(chloromethyl)tetrafluorobenzene (0.5 mmol) for 7c; and
CH3CN (5 mL) were placed in a Pyrex glass tube. After the
mixture was cooled to ꢁ78 ꢀC, the tube was evacuated and
sealed, then warmed to room temperature. The reaction mixture
was heated to 110 ꢀC and stirred for 12 h. After cooling and
carefully opening the tube, the reaction mixture was removed
and dried under reduced pressure. The residue was washed three
times with hexane and pure compounds 3a–c and 7a–c were
obtained.
A mixture of 4-iodophenol (4.40 g, 20 mmol) for 1a–c and 4-
bromo-2-fluorophenol (3.8 g, 20 mmol) for 5, K2CO3 (5.5 g,
40 mmol) and KI (0.664 g, 4 mmol) in DMF (30 mL) was stirred
for 30 min at 25 ꢀC. Then 1-bromooctane (4.6 g, 24 mmol) for 1a,
1-bromodecane (5.3 g, 24 mmol) for 1b, and 1-bromododecane
(6.0 g, 24 mmol) for 1c and 5 were added slowly. The reaction
mixture was heated at 150 ꢀC for 12 h; then cooled to room
temperature, filtered and concentrated to 20 mL. 1 M HCl
(15 mL) was added to the concentrated solution, and the mixed
solution was extracted with CH2Cl2 (3 ꢂ 20 mL). The combined
organic layers were washed with 1 M HCl (2 ꢂ 20 mL), H2O (3 ꢂ
20 mL), and dried over Na2SO4. After the solvents were
removed, the residue was purified using recrystallization from
methanol to give 1a–c and 5.
Compound 3a. White solid (0.35 g), yield 89%; 1H NMR
(CDCl3): d 11.00 (s, 2H), 8.73 (s, 2H), 7.61 (d, J ¼ 9.0 Hz, 4H),
7.49 (s, 2H), 6.95 (d, J ¼ 9.0 Hz, 4H), 6.08 (s, 4H), 3.92 (t, J ¼
15.0 Hz, 4H), 1.75 (m, J ¼ 30.0 Hz, 4H), 1.28 (m, 20H), 0.85 (t,
J ¼ 15.0 Hz, 6H); 19F NMR: d ꢁ139.8 (s, 4F); 13C NMR:
d 160.42, 135.81, 127.25, 123.45, 120.65, 115.89, 68.58, 41.49,
31.73, 29.28, 29.26, 29.11, 29.02, 25.91, 22.59, 14.03. Anal. Calcd
for C42H52N4O2F4Cl2 (791.79): C, 63.71; H, 6.62; N, 7.13; found:
C, 63.70; H, 6.73; N, 7.16%.
Compounds 1a and 1c. 1a: yellow liquid (4.8 g), yield 72%; 1c:
white solid (6.3 g), yield 81%; 1H NMR (CDCl3) analysis was in
good agreement with the literature.21
Compound 1b. Yellow liquid (6.8 g), yield 94%; 1H NMR
(CDCl3): d 7.52 (d, J ¼ 9.0 Hz, 2H), 6.64 (d, J ¼ 9.0 Hz, 2H), 3.89
(t, J ¼ 15.0 Hz, 2H), 1.74 (m, J ¼ 30.0 Hz, 2H), 1.25 (m, 14H),
0.86 (t, J ¼ 12.0 Hz, 3H).
Compound 3b. White solid (0.39 g), yield 92%; 1H NMR
(CDCl3): d 11.05 (s, 2H), 8.77 (s, 2H), 7.62 (d, J ¼ 9.0 Hz, 4H),
7.44 (s, 2H), 6.97 (d, J ¼ 9.0 Hz, 4H), 6.09 (s, 4H), 3.94 (t, J ¼
12.0 Hz, 4H), 1.74 (m, J ¼ 30.0 Hz, 4H), 1.26 (m, 28H), 0.85 (t,
J ¼ 12.0 Hz, 6H); 19F NMR: d ꢁ139.8 (s, 4F); 13C NMR:
d 160.43, 135.84, 127.22, 125.05, 123.45, 120.63, 115.93, 68.58,
41.48, 31.88, 29.54, 29.34, 29.30, 29.04, 25.94, 22.67, 14.11. Anal.
Calcd for C46H60N4O2F4Cl2 (847.89): C, 65.16; H, 7.13; N, 6.61;
found: C, 65.10; H, 7.38; N, 6.71%.
Compound 5. White solid (5.0 g), yield 70%; 1H NMR (CDCl3):
d 7.22–7.18 (m, J ¼ 12.0 Hz, 1H), 7.16–7.12 (m, J ¼ 12.0 Hz, 1H),
6.81 (t, J ¼ 21.0 Hz, 1H), 3.97 (t, J ¼ 12.0 Hz, 2H), 1.78 (m, J ¼
30.0 Hz, 2H), 1.28 (m, 18H), 0.86 (t, J ¼ 15.0 Hz, 3H); 19F NMR:
d ꢁ131.2 (t, J ¼ ꢁ19.7 Hz, 1F).
General procedures for the preparation of precursors of 2a–c and
6 (Ullmann-type coupling reaction)
A mixture of 1a–c (10 mmol) for 2a–c, 5 (10 mmol) for 6, imid-
azole (1.0 g, 15 mmol), K2CO3 (4.1 g, 30 mmol), CuI (0.19 g,
1 mmol) and L-proline (0.23 g, 2 mmol) was stirred in dry DMSO
(30 mL) in nitrogen atmosphere at 110 ꢀC for 12 h; then cooled to
room temperature and filtered. 1 M NH3$H2O (20 mL) was
added to the solution, and then the mixed solution was extracted
with CH2Cl2 (3 ꢂ 20 mL). The combined organic layers were
washed with 1 M NH3$H2O (2 ꢂ 20 mL), H2O (3 ꢂ 20 mL), and
dried over Na2SO4. After the solvent was removed, the residue
Compound 3c. White solid (0.39 g), yield 86%; 1H NMR
(CDCl3): d 11.04 (s, 2H), 8.77 (s, 2H), 7.60 (d, J ¼ 9.0 Hz, 4H),
7.45 (s, 2H), 6.97 (d, J ¼ 9.0 Hz, 4H), 6.09 (s, 4H), 3.94 (t, J ¼
15.0 Hz, 4H), 1.76 (m, J ¼ 27.0 Hz, 4H), 1.25 (m, 36H), 0.85 (t,
J ¼ 12.0 Hz, 6H); 19F NMR: d ꢁ139.7 (s, 4F); 13C NMR:
d 160.45, 135.88, 127.25, 125.07, 123.40, 120.58, 115.94, 68.60,
41.50, 31.87, 29.63, 29.59, 29.57, 29.56, 29.52, 29.32, 29.04, 25.93,
22.65, 14.05. Anal. Calcd for C50H68N4O2F4Cl2 (904.00): C,
66.43; H, 7.58; N, 6.20; found: C, 65.75; H, 7.61; N, 6.31%.
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Compound 7a. White solid (0.40 g), yield 86%; 1H NMR
(d6-DMSO): d 9.86 (s, 2H), 8.20 (s, 2H), 7.95 (s, 2H), 7.65 (d, J ¼
12.0 Hz, 4H), 7.55 (s, 4H), 7.14 (d, J ¼ 9.0 Hz, 4H), 5.47 (s, 4H),
4.01 (t, J ¼ 15.0 Hz, 4H), 1.70 (m, J ¼ 15.0 Hz, 4H), 1.21 (m,
36H), 0.82 (t, J ¼ 15.0 Hz, 6H); 13C NMR: d 159.93, 135.71,
129.53, 128.02, 123.92, 123.41, 122.35, 115.99, 68.52, 52.29,
31.68, 29.41, 29.38, 29.36, 29.12, 29.08, 28.91, 25.82, 22.45, 14.33.
Anal. Calcd for C50H72N4O2Br2 (920.94): C, 65.21; H, 7.88; N,
6.08; found: C, 65.40; H, 7.95; N, 6.19%.
Calcd for C50H60N6O10F16S4 (1337.28): C, 44.91; H, 4.52; N,
6.28; found: C, 45.45; H, 4.65; N, 6.50%.
Compound 4c. White solid (0.63 g), yield 90%; 1H NMR
(CDCl3): d 8.89 (s, 2H), 7.56 (s, 2H), 7.42 (d, J ¼ 3.0 Hz, 4H),
7.40 (s, 2H), 6.95 (d, J ¼ 9.0 Hz, 4H), 5.63 (s, 4H), 3.95 (t, J ¼
12.0 Hz, 4H), 1.77 (m, J ¼ 27.0 Hz, 4H), 1.25 (m, 36H), 0.86 (t,
J ¼ 12.0 Hz, 6H); 19F NMR: d ꢁ78.9 (s, 12F), ꢁ140.5 (s, 4F); 13
C
NMR: d 160.90, 134.56, 126.93, 126.02, 124.10, 121.76, 115.92,
68.69, 41.34, 31.91, 29.65, 29.58, 29.55, 29.36, 29.33, 29.15, 29.06,
25.94, 22.67, 14.08. Anal. Calcd for C54H68N6O10F16S4
(1393.39): C, 46.55; H, 4.92; N, 6.03; found: C, 46.59; H, 4.94; N,
6.04%.
Compound 7b. White solid (0.40 g), yield 82%; 1H NMR
(d6-DMSO): d 9.88 (s, 2H), 8.23 (s, 2H), 7.96 (s, 2H), 7.81 (d, J ¼
3.0 Hz, 2H), 7.77 (d, J ¼ 3.0 Hz, 2H), 7.54 (s, 4H), 7.40 (t, J ¼ 18.0
Hz, 2H), 5.46 (s, 4H), 4.01 (t, J ¼ 15.0 Hz, 4H), 1.72 (m, J ¼ 15.0
Hz, 4H), 1.21 (m, 36H), 0.81 (t, J ¼ 12.0 Hz, 6H); 19F NMR:
d ꢁ139.7 (t, J ¼ 28.2 Hz, 2F); 13C NMR: d 153.10, 150.51, 147.99,
136.04, 135.47, 129.58, 123.48, 122.31, 118.96, 115.96, 111.46,
111.15, 69.64, 52.38, 31.70, 29.41, 29.36, 29.07, 28.82, 25.73, 22.49,
14.35. Anal. Calcd for C50H70N4O2F2Br2 (956.92): C, 62.76; H,
7.37; N, 5.85; found: C, 61.78; H, 7.42; N, 5.76%.
Compound 8a. White solid (0.67 g), yield 92%; 1H NMR
(d6-DMSO): d 9.80 (s, 2H), 8.20 (s, 2H), 7.94 (s, 2H), 7.64 (d, J ¼
9.0 Hz, 4H), 7.54 (s, 4H), 7.14 (d, J ¼ 9.0 Hz, 4H), 5.46 (s, 4H), 4.02
(t, J ¼ 15.0 Hz, 4H), 1.70 (m, J ¼ 12.0 Hz, 4H), 1.22 (m, 36H), 0.81
(t, J ¼ 6.0 Hz, 6H);19FNMR: d ꢁ78.6 (s, 12F); 13C NMR: d 160.99,
136.74, 130.54, 129.05, 124.96, 124.44, 123.42, 117.02, 69.55,
53.38, 32.71, 30.44, 30.42, 30.39, 30.15, 30.11, 29.94, 26.85, 23.50,
15.35. Anal. Calcd for C54H72N6O10F12S4 (1321.42): C, 49.08; H,
5.49; N, 6.36; found: C, 49.28; H, 5.54; N, 6.36%.
Compound 7c. White solid (0.36 g), yield 76%; 1H NMR
(CDCl3): d 11.10 (s, 2H), 8.82 (s, 2H), 7.59 (d, J ¼ 6.0 Hz, 2H),
7.50 (m, J ¼ 30.0 Hz, 2H), 7.45 (s, 2H), 7.06 (t, J ¼ 18.0 Hz, 2H),
6.10 (s, 4H), 4.05 (t, J ¼ 12.0 Hz, 4H), 1.82 (m, J ¼ 21.0 Hz, 4H),
1.24 (m, 36H), 0.86 (t, J ¼ 15.0 Hz, 6H); 19F NMR: d ꢁ128.9 (t,
J ¼ 19.9 Hz, 2F), ꢁ139.7 (s, 4F); 13C NMR: d 154.08, 150.75,
148.95, 136.16, 126.77, 125.27, 120.29, 118.55, 115.34, 110.87,
69.74, 41.56, 31.83, 29.56, 29.44, 29.25, 28.91, 25.75, 22.59, 14.00.
Anal. Calcd for C50H66N4O2F6Cl2 (939.98): C, 63.89; H, 7.08; N,
5.96; found: C, 62.82; H, 7.31; N, 5.96%.
Compound 8b. White solid (0.64 g), yield 94%; 1H NMR
(d6-DMSO): d 9.83 (s, 2H), 8.22 (s, 2H), 7.94 (s, 2H), 7.79 (m, J ¼
15.0 Hz, 2H), 7.53 (s, 4H), 7.40 (t, J ¼ 18.0 Hz, 4H), 5.45 (s, 4H),
4.10 (t, J ¼ 12.0 Hz, 4H), 1.69 (m, J ¼ 21.0 Hz, 4H), 1.21 (m,
36H), 0.81 (t, J ¼ 15.0 Hz, 6H); 19F NMR: d ꢁ78.6 (s, 12F),
ꢁ131.5 (s, 2F); 13C NMR: d 153.34, 150.71, 148.07, 147.93,
136.00, 135.42, 129.53, 127.66, 123.47, 122.33, 118.95, 115.92,
111.44, 69.61, 52.42, 31.66, 29.38, 29.33, 29.07, 29.04, 28.79,
25.70, 22.48, 14.31. Anal. Calcd for C54H70N6O10F14S4
(1357.41): C, 47.78; H, 5.20; N, 6.19; found: C, 47.43; H, 5.09; N,
6.03%.
General procedures for the preparation of compounds 4a–c and
8a–c
Compounds 3a–c or 7a–c (0.5 mmol) and LiNTf2 (0.43 g,
1.5 mmol) were added into CH3OH (20 mL). The reaction
mixture was stirred at 25 ꢀC for 24 h, then the solvent was
evaporated under reduced pressure, and the residue was washed
five times with water, and then dried under vacuum. Pure
compounds 4a–c and 8a–c were generated.
Compound 8c. White solid (0.65 g), yield 91%; 1H NMR
(CDCl3): d 8.90 (s, 2H), 7.57 (s, 2H), 7.42 (s, 2H), 7.26 (m, J ¼
12.0 Hz, 4H), 7.03 (t, J ¼ 21.0 Hz, 2H), 5.63 (s, 4H), 4.03 (t, J ¼
15.0 Hz, 4H), 1.82 (m, J ¼ 27.0 Hz, 4H), 1.25 (m, 36H), 0.86 (t,
J ¼ 15.0 Hz, 6H); 19F NMR: d ꢁ78.9 (s, 12F), ꢁ129.1 (s, 2F),
ꢁ140.5 (s, 4F); 13C NMR: d 154.11, 135.04, 123.84, 122.49,
119.33, 115.22, 111.68, 111.37, 69.87, 41.42, 31.91, 29.64, 29.57,
29.52, 29.33, 28.97, 25.82, 22.67, 14.08. Anal. Calcd for
C54H66N6O10F18S4 (1429.37): C, 45.38; H, 4.65; N, 5.88; found:
C, 45.67; H, 4.67; N, 5.88%.
Compound 4a. White solid (0.60 g), yield 93%; 1H NMR
(CDCl3): d 8.86 (s, 2H), 7.54 (s, 2H), 7.44 (d, J ¼ 6.0 Hz, 4H),
7.39 (s, 2H), 6.93 (d, J ¼ 9.0 Hz, 4H), 5.60 (s, 4H), 3.93 (t, J ¼
15.0 Hz, 4H), 1.75 (m, J ¼ 27.0 Hz, 4H), 1.28 (m, 20H), 0.86 (t,
J ¼ 15.0 Hz, 6H); 19F NMR: d ꢁ79.0 (s, 12F), ꢁ140.7 (s, 4F); 13
C
NMR: d 161.64, 135.29, 127.86, 124.86, 122.61, 116.84, 69.50,
42.06, 32.61, 30.15, 30.10, 30.03, 29.99, 29.90, 23.47, 14.87. Anal.
Calcd for C46H52N6O10F16S4 (1281.17): C, 43.12; H, 4.09; N,
6.56; found: C, 43.70; H, 4.15; N, 6.60%.
Results and discussion
Initially, we synthesized six dicationic imidazolium-based
compounds (3a–c and 4a–c) with various alkyl groups (–C8H17,
–C10H21 and –C12H25), and discovered that the length of the
alkyl group plays an important role in the performance proper-
ties of the salts, especially on the appearance of mesophases.
When the alkyl group is C-12 (compound 4c), the mesophase
may be formed. Then using –C12H25 as a substituent group,
another six dicationic imidazolium-based compounds (7a–c and
8a–c) were synthesized, in which 7b–c and 8b–c were fluorinated
with the introduction of fluoro substituents at different positions.
Compound 4b. White solid (0.61 g), yield 91%; 1H NMR
(CDCl3): d 8.92 (s, 2H), 7.57 (s, 2H), 7.42 (d, J ¼ 24.0 Hz, 6H),
6.96 (d, J ¼ 9.0 Hz, 4H), 5.63 (s, 4H), 3.94 (t, J ¼ 12.0 Hz, 4H),
1.78 (m, J ¼ 21.0 Hz, 4H), 1.26 (m, 28H), 0.86 (t, J ¼ 12.0 Hz,
6H); 19F NMR: d ꢁ78.9 (s, 12F), ꢁ140.6 (s, 4F); 13C NMR:
d 160.88, 134.62, 126.94, 126.03, 124.08, 123.35, 115.92, 68.68,
41.33, 31.88, 29.54, 29.35, 29.30, 29.06, 26.00, 22.64, 14.07. Anal.
8234 | J. Mater. Chem., 2009, 19, 8232–8238
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Compounds 7a, 7b and 8b show LC mesophases. All twelve
compounds, 3a–c, 4a–c, 7a–c and 8a–c, were characterized by
nuclear magnetic resonance (NMR) spectroscopy, differential
scanning calorimetry (DSC), thermogravimetric analysis (TGA)
and elemental analysis (EA). Compounds 4c, 7a, 7b and 8b were
also analyzed by polarized optical microscopy (POM). The
synthetic routes using standard organic methodology are shown
in Schemes 1 and 2.
Compounds 3a–c and 4a–c were prepared by the convergent
syntheses of 1,4-bis(chloromethyl)tetrafluorobenzene and rigid
bent-core imidazole derivatives with different alkyl groups.
Compounds 4a–c were generated from the anion exchange
of compounds 3a–c with LiNTf2 (lithium bis(tri-
fluoromethanesulfonyl)amide) (Scheme 1). The chemical/phys-
ical properties of compounds 3a–c and 4a–c including transition
and thermal decomposition temperatures, and densities are given
in Table 1. For the six compounds, changing of the anion from
ꢁ
Clꢁ to NTf2 results in an increase in density and thermal
decomposition temperature which demonstrates the positive
ꢁ
effect of the stabilizing NTf2 anion over that of chloride.
Scheme 2 Syntheses of compounds 7a–c and 8a–c. Reagents and
conditions: (i), (ii), (iii) and (iv) are the same as in Scheme 1.
Transition temperature is one of the most important properties.
Concomitantly with the change in alkyl group from –C8H17,
–C10H21 to –C12H25, the transition temperatures distinctly
change. Compounds 3a,b and 4a,b, which contain –C8H17 and
–,C10H21 groups, respectively, display low melting points with
the exception for compound 3a which exhibits not only a low
observed; however, when the anion is NTf2ꢁ, compound 4c is an
ionic liquid crystal and does show liquid crystal properties. Thus,
the compound melts at 82 ꢀC (Cr–SmA) and clears at 79 ꢀC
(SmA–Iso). The investigation of the six compounds shows that
when the alkyl group was –C12H25, the mesophase may be
present.
ꢀ
melting point at 56 C but also a transition arising from super-
ꢀ
cooling from crystal to crystal at 36 C. The melting points for
salts 3a,b and 4a,b are below 100 ꢀC; therefore, they fall into the
ionic liquid class. For 3c and 4c with –C12H25 alkyl substituents,
the different anion defines their properties. For example, with
chloride ion, compound 3c is an ionic liquid since only one
transition temperature, namely the melting point at 60 ꢀC is
Therefore, using –C12H25 as the long alkyl group, we synthe-
sized six additional dicationic compounds (7a–c and 8a–c) with
the concomitant introduction of fluoro substituents (Scheme 2),
and investigated the effect of this phenomenon on the property–
structure relationship. The transition and thermal decomposition
temperatures, and densities of 7a–c and 8a–c are shown in
Table 1. As reported for 3a–c and 4a–c, the densities and thermal
Table 1 Thermal behavior of the new compounds
Compound
Density/g cmꢁ3
1.22
Transition
T/ꢀC
Td/ꢀC
3a
Cr–Cr0
Cr0–Iso
Cr–Iso
Cr–SmA
SmA–Iso (dec)
Cr–Iso
36
56
64
106
299
85
50
74
194
4a
7a
1.38
1.25
317
250
8a
3b
4b
7b
1.36
1.28
1.39
1.31
325
198
318
250
Cr–Iso
Cr–Iso
Cr–Cr0
75
Cr0–M
133
145
289
81
(33)
60
63
82
(79)
77
M–SmA
SmA–Iso (dec)
Cr–Iso
8b
1.38
320
˜
(SmA–Iso)
Cr–Iso
3c
4c
1.21
1.36
215
334
Cr–Cr0
Cr0–Iso
(SmA–Iso)
Cr–Iso
Scheme 1 Syntheses of compounds 3a–c and 4a–c. Reagents and
conditions: (i) RBr, K2CO3, KI, DMF, 150 ꢀC, 12 h; (ii) imidazole,
K2CO3, CuI, L-proline, dry DMSO, 110 ꢀC, 12 h; (iii) CH3CN, 110 ꢀC,
12 h; (iv) LiNTf2, CH3OH, rt, 24 h.
7c
8c
1.21
1.40
195
324
Cr–Cr0
Cr0–Iso
43
84
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decomposition temperatures of compounds 8a–c which contain
NTf2ꢁ are much higher than the corresponding compounds 7a–c
with Clꢁ/Brꢁ anions. The thermal decomposition temperature
cooling through the transition were minimal, although it was not
possible to obtain a very well-developed SmA texture from which
to observe. On balance, it is considered most likely that this is an
SmB phase, but the label M is retained in the absence of
a definitive identification.
ꢁ
data show that the compounds containing NTf2 anions have
very high stability which is beneficial for possible applications.
The densities of compounds bearing fluoro substituents are
higher than those of their congeners without such substituents.
As we had shown earlier, it is not surprising that compounds
exhibiting higher densities are those with the larger amounts of
fluorine.22
For compound 4c, the phase behavior was different. Thus on
heating, a Cr–Cr0 transition was seen at 63 ꢀC and then at 82 ꢀC
the compound melted directly to the isotropic liquid. It was
necessary to take care when observing this behavior for just
below the melting point at 79 ꢀC, a monotropic SmA phase was
seen. Observed between normal glass cover slips, it was not easy
to pick out a texture, but using 5 mm cells treated with nylon to
give planar alignment, tꢀhe phase was readily identifiable. Thus,
the cell was filled at 90 C and then allowed to cool—the char-
acteristic texture is seen in Fig. 1.
Thermal behavior
The most important property is the transition temperature
determined by optical microscopy and DSC. The thermal
behavior is given in Table 1 and shows that with the exception of
compounds 7a and 7b, all compounds melt below 100 ꢀC and so
can formally be considered as ionic liquids. Structurally, 7a and
7b differ by having the central ring as a 1,4-phenylene and by
having a halide counter-anion, whereas all the other compounds
have the much more sterically demanding bis(tri-
fluoromethanesulfonyl)amide (NTf2ꢁ) anion and/or and 2,3,5,6-
tetrafluorophenylene ring. With the exception of these two
compounds and 8b and 4c, which are liquid-crystalline, the
compounds melt between 50 and 85 ꢀC with little systematic
variation in melting point. Some of the compounds show solid-
state polymorphism.
On heating, compound 8b melted directly to the isotropic
liquid at 81 ꢀC and, on cooling between glass slides showed some
evidence of a monotropic transition close to room temperature.
However, the sample was very weakly birefringent and so it was
difficult to discern what the texture was. Therefore, the sample
was prepared in 5 mm cells in both planar and homeotropic
(lecithin) alignment by filling in the isotropic state and then
cooling. In the planar cells, a transition was observed at about
ꢀ
ꢀ
25 C; re-heating the sample showed a clearing point of 33 C.
The discrepancy between the temperature at which the texture
appears on cooling and the observed thermodynamic clearing
point is much larger than one would normally expect to see. In
this case, the hysteresis is attributed to the very high viscosity of
the sample at these temperatures and support for this assertion
comes from the observation that on standing at 29 ꢀC having
cooled from the isotropic state, a texture is observed (Fig. 2a).
The texture that is seen is not characteristic of an SmA phase and
indeed shows features more commonly associated with
a columnar phase. However, on viewing the sample as a prepa-
ration under homeotropic alignment, the texture shown in
Fig. 2b was observed, namely Maltese crosses. This is charac-
Closer examination of 4c, 7a, 7b and 8b by polarized optical
microscopy (POM) revealed that they were ionic liquid crystals.
Compounds 7a and 7b behaved in a similar fashion. Thus, 7a,
ꢀ
melted at 106 C to give a highly viscous fluid that was clearly
birefringent and could, with difficulty, be spread by the appli-
cation of mechanical strain. As the temperature was increased,
the viscosity of the sample gradually dropped and it was clear
that the material was in a liquid crystal mesophase. On
approaching 280 ꢀC, the viscosity was such that the preparation
was now quite free flowing and it was possible to see textural
features characteristic of an SmA phase, including focal-conic
and homeotropic regions. Cooling from these temperatures
allowed quite well-developed textures to form. However, on
heating further to 299 ꢀC, the compound began to clear and
showed distinct signs of decomposition, confirmed by the fact
that cooling the cleared material did not lead to any signs of the
SmA phase seen on heating. Decomposition at such a tempera-
ture is not surprising.
˜
teristic of the SmA phase, a modulated ribbon phase with c2mm
symmetry; the molecular organization in such a phase is shown
as Fig. 3. The same organization is characteristic of the Colr and
B1 phases and it is this that explains the similarity with the
texture of a columnar phase.
Compound 7b behaved in a broadly similar manner, except
that on heating a transition was seen at 133 ꢀC at which
temperature the sample visibly softened and could be persuaded
to flow when subjected to mechanical strain, although its
viscosity was very high. Then, at 145 ꢀC, a second transition was
seen which could be identified as an SmA phase on further
heating as described for 7a. Clearing at 289 ꢀC was again
accompanied by extensive decomposition so that no texture
could be observed on further cooling. The identity of the phase
observed between 133 and 145 ꢀC (represented as M in Table 1) is
not at this stage obvious. Rather few mesophase types are found
for truly ionic liquid crystals, being mainly SmA and SmB with,
occasionally, less common phases such as the tetragonal T phase
described by Skoulios and co-workers.23 Textural changes on
Fig. 1 Representative POM texture of 4c: a typical focal-conic LC
texture in smectic A phase at 76 ꢀC on cooling with some homeotropic
area (black).
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liquids and ionic liquid crystals) by employing fluorine-contain-
ing substituents in different positions, with long-chain alkyl
groups (–C8H17, –C10H21 and –C12H25), and varying anions. The
investigation reveals that (a) the introduction of fluoro substit-
uents into the linker or core benzene-ring positions of the
compounds such as 4c and 8b in comparison with 8a and 8c,
respectively, is a feasible route for the synthesis of ionic liquid
crystals, and for the investigation of property–structure
relationships; and (b) the long alkyl groups and different anions
are also crucial factors for modifying the chemical/physical
properties.
Acknowledgements
The authors gratefully acknowledge the support of HDTRA1-
07-1-0024, NSF (CHE-0315275), and ONR (N00014-06-1-1032).
Notes and references
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