On the mesophase formation of 1,3-dialkylimidazolium ionic liquids

Article abstract of DOI:10.1021/cg4004593

A series of seven different 1,3-dialkylimidazolium-based ion-pair salts with the same molecular weight and size but different symmetries was synthesized. For all salts, bromide was chosen as the counterion, giving the series ([CnIMCm][Br]), where IM = imidazolium and Cn and Cm are varying N-alkyl substituents with n + m = 13. Thus, the effect of symmetry on the physicochemical properties, such as thermal transitions, densities and viscosities and particularly mesophase formation, is investigated herein. All salts are fully characterized by NMR spectroscopy and mass spectrometry, and their physicochemical properties such as thermal transitions, densities, and viscosities are reported. Single crystal X-ray structure analysis is reported for 1-tridecylimidazolium bromide ([C0IMC13][Br]) and 1-ethyl-3- undecylimidazolium bromide ([C2IMC11][Br]). Salts 1-tridecylimidazolium bromide ([C0IMC13][Br]) and 1-dodecyl-3-methylimidazolium bromide ([C1IMC12][Br]) exhibit thermotropic liquid crystal behavior, confirmed by differential scanning calorimetry, polarized optical microscopy, and small-angle X-ray diffraction to be the SmA mesophase. A structure with interdigitation of alkyl chains is observed for all of [C0IMC13][Br], [C1IMC12][Br], and [C2IMC11][Br], despite the absence of thermotropic liquid crystalline behavior for the latter (and all other isomers with an alkyl chain length less than 12 carbon atoms). This allows us to draw the conclusion that for the liquid crystal phase of an ionic liquid to exist, not only are the calamitic shape and integral length of a molecule important but a minimal alkyl chain length of n = 12 is also required. Therefore, a dodecyl group could be considered as the functional group responsible for liquid crystalline behavior.

Full text of DOI:10.1021/cg4004593

Article
pubs.acs.org/crystal
On the Mesophase Formation of 1,3-Dialkylimidazolium Ionic Liquids
Mei Yang, Bert Mallick, and Anja-Verena Mudring*
Anorganische Chemie III − Materials Engineering and Characterization, Fakultat
̈
̈ ̈
fur Chemie and Biochemie, Ruhr-Universitat
Bochum, 44780, Bochum, Germany
*
S Supporting Information
ABSTRACT: A series of seven different 1,3-dialkylimidazolium-
based ion-pair salts with the same molecular weight and size but
different symmetries was synthesized. For all salts, bromide was
chosen as the counterion, giving the series ([CnIMCm][Br]), where
IM = imidazolium and Cn and Cm are varying N-alkyl substituents
with n + m = 13. Thus, the effect of symmetry on the
physicochemical properties, such as thermal transitions, densities
and viscosities and particularly mesophase formation, is investigated
herein. All salts are fully characterized by NMR spectroscopy and
mass spectrometry, and their physicochemical properties such as
thermal transitions, densities, and viscosities are reported. Single
crystal X-ray structure analysis is reported for 1-tridecylimidazolium
bromide ([C0IMC13][Br]) and 1-ethyl-3-undecylimidazolium bro-
mide ([C2IMC11][Br]). Salts 1-tridecylimidazolium bromide
(
[C0IMC13][Br]) and 1-dodecyl-3-methylimidazolium bromide ([C1IMC12][Br]) exhibit thermotropic liquid crystal behavior,
confirmed by differential scanning calorimetry, polarized optical microscopy, and small-angle X-ray diffraction to be the SmA
mesophase. A structure with interdigitation of alkyl chains is observed for all of [C0IMC13][Br], [C1IMC12][Br], and
[
C2IMC11][Br], despite the absence of thermotropic liquid crystalline behavior for the latter (and all other isomers with an alkyl
chain length less than 12 carbon atoms). This allows us to draw the conclusion that for the liquid crystal phase of an ionic liquid
to exist, not only are the calamitic shape and integral length of a molecule important but a minimal alkyl chain length of n = 12 is
also required. Therefore, a dodecyl group could be considered as the functional group responsible for liquid crystalline behavior.
1
. INTRODUCTION
applications, such as organic solvent replacements, thermal
fluids, gas storage, and chemical synthesis and catalysis.
7
Ionic liquid crystals (ILCs) combine the self-organization
features of liquid crystals (LCs) and the solvent properties of
ionic liquids (ILs). “Liquid crystals” are substances which have
properties at an intermediate stage between the perfectly
ordered periodic structure of crystalline solids and the
completely disordered structure of isotropic liquids, gases and
amorphous solids. Such an intermediate state of matter is called
a “mesomorphic state”, the physical and chemical properties of
Imidazolium based ILs represent the most studied system so
far. Holbrey and Seddon reported the thermal behavior of 1-
alkyl-3-methylimidazolium based ILs. A mesophase formation
was observed when the substituted alkyl chain bore 12 or more
1
−
−
carbon atoms in combination with anions such as Cl , Br and
−
−
8
BF4 but not bis(trifluoromethylsulfonyl)imide (NTf ). 1-
2
Dodecyl-3-methylimidazolium is considered as one of the most
9
well-known example of rod shaped ILC cations, yet the choice
2
which, can be influenced by the orientations of LC molecules.
of anion remains important The driving forces for the
formation of mesophases in thermotropic ILCs are, for
example, hydrophobic interactions between large alkyl groups
including van der Waals interactions; Coulombic forces
between ions; dipole−dipole interactions; hydrogen bonding
and cation−π interactions as well as π−π stacking of the
Thermotropic LCs, which are able to form mesophases when
the temperature is varied, have been very intensively
investigated. A typical thermotropic LC is composed of rod-
3
or disc-shaped molecules. Nowadays, liquid crystalline
materials can be found in a wide range of applications, such
as in the manufacturing of displays, molecular sensors and
detectors, and as highly ordered solvents for many chemical
1
,10
commonly used aromatic core groups etc. Only a balance of
all these interactions could promise the formation of a
mesophase. All these intermolecular forces are also displayed
in conventional ILs and can be influenced by cation/anion
combinations and geometrical factors of both ions. For
4
reactions. ILs are salts with a melting point at or below 100
5
°C. ILs have become the subject of a large amount of research
6
over last decades. The ability to change chemical and physical
properties by specific cation−anion combinations has led to
their promotion as “designer solvents”. Because of their unique
properties such as nonvolatility, high thermal stability and
nonflammability, ILs have been used in a wide variety of
Received: March 27, 2013
Revised: May 27, 2013
©
XXXX American Chemical Society
A
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example, formation of mesophase was observed for the salt 1-
methyl-3-octylimidazolium octylsulfonate [C1IMC8][C8SO3],
in which an amphiphilic character is imparted into both the
Scheme 1. Different Types of Ionic Liquid Cations under
Investigation, Bromide Was Used as Counter Ion
1
1,12
cation and anion.
Therein, the synergistic effect of cation
and anion are seen, as separately these are rather poor
amphiphiles. Furthermore, changing cation geometries can also
provide the opportunity to diversify the physiochemical
properties of ILs, such as varying the polar and nonpolar
domains. Indeed, the segregation of nonpolar and polar regions
in ILs, previously demonstrated by molecular dynamics
1
3
14
simulations and coarse-grain modeling, dictates their ability
to interact with given solutes. For this reason ILs are regarded
as highly structured solvents and have already been used in size
13−15
controlled nanoparticle synthesis.
By studying these interactions in ILs it is hoped to gain
fundamental insights into the structures of ILCs, which is
clearly required for rational design of ILCs for further
applications. A fundamental question needs to be answered
here: how does one design a cation topology that accompanied
by a suitable anion results in an IL capable of forming a
mesophase? What are the necessary factors for an ionic
mesophase formation?
Symmetrical 1,3-dialkylimidaolium based ILs with dialkyl
moieties ranging from dimethyl to bisdecyl have been reported
16
as hexafluorophosphate salts, and the bisdodecyl analogue as
17
the perchlorate and tetrafluoroborate salt. Only for the
bisdodecyl imidazolium IL a mesophase formation is reported,
not for the lighter cations. Interestingly, the dihexyl IL forms a
crystalline phase, while dipentyl forms a RTIL (room
temperature ionic liquid) with a low temperature glass
transition. Between diheptyl and dioctyl, a drastic drop in
melting point is observed alongside a poorer tendency to
crystallize for the odd-numbered chain lengths. It could be that
asymmetric cations with similar molecular size and weight
possess properties intermediate to the reported symmetrical
species and perhaps form a mesophase. Therefore, herein we
report a series of ILs with asymmetric cations, all of which are
structural isomers of 1-dodecyl-3-methylimidazolium. The rod-
shaped molecule 1-dodecyl-3-methylimidazolium, has an
integral length equal to 13 methyl/methylene carbons and an
imidazolium headgroup. In our present study, we synthesize
and characterize a series of 1,3-dialyklimidazolium ILs with an
identical integral length but different symmetries (Scheme 1).
As counterion, bromide was chosen. Hence, the physicochem-
ical properties such as thermal transitions, densities and
viscosities can be compared for ILs with same size and shape,
but different symmetries.
(
(
Darmstadt, Germany). 1-Hexylimidazole (98%) and 1-octylimidaole
98%) were purchased from IoLiTec (Heilbronn, Germany). 1-
Ethylimidazole (95%), 1-butylimidazole (98%), imidazole (99.5%),
sodium hydride (95%) and the all 1-bromoalkanes were purchased
from Sigma-Aldrich (Steinheim, Germany). All the chemicals were
used as received without further purification.
Synthesis of 1-alkylimidazole is outlined in Scheme 2. The
1
8
procedures were modified from a reported method. The synthesis
of 1-propylimidazole is as an example reported below.
Scheme 2. Synthesis of 1-Alkylimidazole
NaH (1.8 g, 70.2 mmol) suspended in anhydrous THF (∼50 mL)
was added to a 100 mL flask. Upon addition of imidazole (3.7 g, 54
mmol) hydrogen gas evolution set in. The reaction mixture was stirred
at 40 °C until the gas evolution ceased. Then 1-bromopropane (4.4
mL, 5.9 g, 48 mmol) was added and the reaction mixture was heated
under reflux (70 °C) overnight. After cooling to room temperature the
solid precipitate was filtered off. The filtrate was concentrated under
reduced pressure. A light brown oil was yielded and redissolved in
CH Cl . Then the CH Cl phase was washed three times with water.
The organic phase was dried over sodium sulfate. After filtration, the
solvent was removed via rotary evaporation. The product was dried
under vacuum overnight to give a yellow-brownish oil. Yield = 4.6 g,
4
For convenience, the salts are named by the number of
carbon atoms in the two alkyl chains at the first and third
+
positions of the imidazolium ring. For example, [C4IMC9] , 1-
2
2
2
2
+
butyl-3-nonylimidazolium, as C4C9 and [C6IMC7] , 1-hexyl-3-
heptylimidazolium, as C6C7.
1
1.6 mmol, 77%. H NMR (298K, 200 MHz, DMSO-d ): δ [ppm] =
6
2. EXPERIMENTAL SECTION
0
.80 (3H, t), 1.71 (2H, sx), 3.89 (2H, t), 6.87 (1H, s), 7.12 (1H, s),
2
.1. Sample Preparation. The synthesis and sample handling of
7.58 (1H, s).
anhydrous salts were carried out using standard Schlenk and argon-
glovebox techniques. All the bromide salts were hygroscopic. The
samples were dried under 10 mbar at 60 °C for 3 days. The water
content was determined by Karl Fischer titration. In all cases the water
content was below 50 μg/g.
1-Tridecylimidazole was produced from imidazole (0.6 g, 9 mmol)
and 1-bromotridecane (2.4 mL, 2.5 g, 9.5 mmol) with the general
procedure. Yield = 1.9 g, 7.6 mmol, 85%. H NMR (298 K, 200 MHz,
−6
1
DMSO-d ): δ [ppm] = 0.84 (3H, t), 1.22 (22H, s), 1.70 (2H, qn),
6
3.91 (2H, t), 6.85 (1H, s), 7.11 (1H, s) 7.57 (1H, s).
2
.2. Synthesis. Acetonitrile (99.5%), ethyl acetate (95%),
methanol (99%), dichloromethane (99.9%), and tetrahydrofuran
extra dry, >99%) were used as received from J.T. Backer (Deventer,
Holland). Hydrobromic acid (47%) was obtained from Merck
The bromide salts were made via direct alkylation of the respective
1-alkylimidazole with alkyl bromide in a 1:1.1 molar ratio in
1
b
(
acetonitrile (Scheme 3). The reaction mixture was heated for two
to three days at 90 °C. Afterward, the products were purified by
B
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2.4. Crystal Structure Analysis. Crystals of [C0C13]Br were
Scheme 3. Synthesis of 1,3-Dialkylimidazolium Bromides
obtained from recrystallization of the salt in dichloromethane and
diethyl ether. Crystals of [C2C11]Br were obtained from cooling the
molten salt to room temperature. Suitable single crystals were sealed in
Lindeman glass capillaries. All data were collected on a Stoe IPDS-I
single-crystal X-ray diffractometer with graphite monochromated Mo
Kα radiation (λ = 0.71073 Å at 100 K). Crystal structure solution by
direct methods using SIR 92 yielded the heavy atom positions.
Refinement with SHELXL-97 allowed for the localization of the
remaining atom positions. Hydrogen atoms were added and treated
with the riding atom mode. Data reduction was carried out with the
program package X-Red and numerical absorption correction was
carried out with the program X-Shape. To illustrate the crystal
structures, the program Diamond 3 was used.
crystallization. For this, the crude products in solution were added
dropwise to cold ethyl acetate (T ≈ −30 °C). The crystalline products
were then dried at room temperature for several days under dynamic
vacuum. The preparation of [C3C10]Br is given in detail as
representative procedure for synthesis of 1,3-dialkylimidazolium
bromides.
1-Propylimidazole (2.0 g, 18.1 mmol) was dissolved in 30 mL of
acetonitrile. 1-Bromodecane (3.8 mL, 4.0 g, 20 mmol) was added
dropwise and the reaction mixture was heated under reflux for 72 h. By
the end of the reaction, the solution changed color from pale yellow to
brown. The hot reaction mixture was added to cold (−30 °C) ethyl
acetate (300 mL). The product precipitated as a white powder.
Afterward, the product was filtered under inert environment, washed
2.5. Temperature-dependent SAXS Experiments. SAXS
(small-angle X-ray scattering) measurements were carried out at the
A2 Beamline of DORIS III, Hasylab, DESY, Hamburg, Germany, at a
fixed wavelength of 1.5 Å. The data were collected with a MarCCD
detector. The detector was calibrated with silver behenate. The
sample−detector position was fixed at 635.5 mm. For measurements,
the samples were placed in a copper sample holder between aluminum
foil. The sample temperature was controlled by a JUMO IMAGO 500
multichannel process and program controller. Data reduction and
analysis, correction or background scattering and transmission were
carried out by using the program a2tool (Hasylab).
2.6. Differential Scanning Calorimetry. The DSC measure-
ments were performed with a computer-controlled Phoenix DSC 204
F1 thermal analyzer (Netzsch, Selb, D) with argon as protection gas.
The samples were placed in aluminum pans which were cold-sealed
under argon. Experimental data are displayed in such a way that
exothermic peaks occur at negative heat flow and endothermic peaks at
positive heat flow. DSC runs included heating and subsequent cooling
at 5 °C/min. Given temperatures correspond to the onset of the
respective thermal process. For the thermotropic liquid crystalline
samples, much lower heating and cooling rate were used (1 °C/min)
in order to confirm reproducability between cycles and eliminate the
possibility of fractional decomposition. The DSC scans with lower rate
are collected in Supporting Information.
repeatedly with ethyl acetate and dried in vacuum for at least 3 days at
1
6
0 °C. Yield = 4.9 g, 14.7 mmol, 81%. H NMR (298 K, 200 MHz,
DMSO-d ): δ [ppm] = 0.81 (6H, m), 1.19 (15H, m), 1.76 (4H, m),
6
4
.11 (4H, m), 7.74 (2H, s), 9.17 (1H, s). ESI-MS (positive, ([%])) m/
+
z = 251.1 [M − Br] (100).
[
C1C12]Br was synthesized from 1-methylimidazole (5 g, 60
mmol) and 1-bromododecane (8.3 g, 6.7 mmol). Yield = 18.0 g, 54
1
mmol, 91%. H NMR (298 K, 200 MHz, DMSO-d ) δ [ppm] = 0.82
6
(
(
3H, t), 1.20 (17H, m), 1.74(2H, t), 3.81 (s, 3H), 4.17 (2H, t), 7.61
2H, s), 9.27 (1H, s). ESI-MS (positive, ([%])) m/z = 251.1 [M −
+
Br] (100).
[
C2C11]Br was synthesized from 1-ethylimidazole (3 g, 29.6
mmol) and 1-bromoundecane (7.1 g, 30.0 mmol). Yield = 8.5 g, 25.6
1
mmol, 86%. H NMR (298 K, 200 MHz, DMSO-d ) δ [ppm] = 0.84
6
(
(
3H, t), 1.23 (17H, m), 1.41 (3H, t), 1.78(2H, t), 4.17 (4H, qu), 7.82
2H, s), 9.27 (1H, s). ESI-MS (positive, ([%])) m/z = 251.1 [M −
+
Br] (100).
[
C4C9]Br was synthesized from 1-butylimidazole (3 g, 23.7 mmol)
and 1-bromononane (5.18 g, 25 mmol). Yield = 6.3 g, 19.0 mmol,
2.7. Polarizing Optical Microscopy. The POM pictures of
mesophases were acquired with an Axio Imager A1 microscope (Carl
1
8
0%. H NMR (298 K, 200 MHz, DMSO-d ): δ [ppm] = 0.89 (6H,
6
m), 1.23 (15H, m), 1.78 (4H, qn), 4.18 (4H, dt), 7.84 (2H, s), 9.35
̈
Zeiss MicroImaging GmbH, Gottingen, Germany) equipped with a
+
(
1H, s). ESI-MS (positive, ([%])) m/z = 251.1 [M − Br] (100).
hot stage, THMS600 (Linkam Scientific Instruments Ltd., Surrey,
UK), and a Linkam TMS 94 (Linkam Scientific Instruments Ltd.,
Surrey, UK) temperature controller and crossed polarizers. Images
were recorded at a magnification of 100× as video with a digital
camera after initial heating during the cooling stage. Heating and
cooling rates were 5 K/min. For the measurement the samples were
placed under argon between two coverslips which were sealed with
two-component adhesive (UHU plus 300, UHU GmbH & Co. KG,
[
C5C8]Br was synthesized from 1-octylimidazole (2 g, 16.4 mmol)
and 1-bromopentane (2.7 g, 18 mmol). Yield = 3.9 g, 11.6 mmol, 70%.
1
H NMR (298 K, 200 MHz, DMSO-d ): δ [ppm] = 0.82 (6H, m),
6
1
.20 (15H, m), 1.76 (4H, qn), 4.13 (4H, t), 7.73 (2H, s), 9.14 (1H, s).
+
ESI-MS (positive, ([%])) m/z = 251.1 [M − Br] (100).
[
C6C7]Br was synthesized from 1-hexylimidazole (3 g, 19 mmol)
and 1-bromoheptane (3.8 g, 21 mmol). Yield = 3.3 g, 9.8 mmol, 52%.
1
H NMR (298 K, 200 MHz, DMSO-d ): δ [ppm] = 0.85 (6H, t), 1.25
Bu
̈
hl, Germany).
6
(
15H, m), 1.79 (4H, qu), 4.17 (4H, t), 7.82 (2H, s), 9.27 (1H, s). ESI-
2.8. Density Measurements. Densities of the ILs, which were
+
MS (positive, ([%])) m/z = 251.1 [M − Br] (100).
liquid at room temperature, were determined from ratio of their mass
m to their volume V. The ILs were filled into a pycnometer with 0.956
mL volume (Carl Roth, Karlsruhe, Germany). Mass of the ILs for this
volume were obtained by weighting from laboratory scales. The
experiments carried out in an argon purged glovebox at 25 °C.
[
C0C13]Br was prepared by adding dropwise a slight excess of
concentrated hydrobromic acid (1 mL, 48%, 11 mmol) to 1-
tridecylimidazole (1.5 g, 5.9 mmol) at 0 °C in diethyl ether. The
bromide salt precipitated as white solid immediately. This solid was
purified by successive washing with cold ether, followed by vacuum
1
drying. Yield = 1.6 g, 4.6 mmol, 78%. H NMR (200 MHz, DMSO-
3
. RESULTS AND DISCUSSION
d6): δ [ppm] = 0.84 (3H, t), 1.23 (20H, m), 1.78 (2H, t), 4.17 (2H, t),
7
=
.68 (1H, s), 7.78 (1H, s), 9.13 (1H, s). ESI-MS (postive, ([%])) m/z
The salts [C0C13]Br, [C1C12]Br, [C2C11]Br exist as white
crystalline solids. [C3C10]Br, [C4C9]Br, [C5C8]Br, [C6C7]
Br were colorless highly viscous liquids.
+
251.3 [M − Br] (100).
2.3. Rheological Measurements. Rheological measurements
were performed on a stress-controlled rheometer (RheoStress75,
Thermo Fisher Scientfic Inc., St. Louis, MO) with CR (controlled
rate)-mode under argon atmosphere. Oscillatory shear measurements
were performed using a parallel plate geometry with a 20-mm-
diameter PP20-sensor on rotor and 21-mm-diameter PP20-sensor on
stator. The gap between the plates is 1 mm. After each heating process
the sample was kept for 10 min on the plate to reach equilibrium. The
temperature was regulated within ±0.1 °C.
3
.1. Structural Aspects. Crystals of suitable quality for
single crystal X-ray structure analysis could be obtained for the
salts [C0C13]Br and [C2C11]Br. Crystallographic data, data
collection, and structure refinement details are given in Table 1.
The salt [C0C13]Br crystallizes in the triclinic space group P1
̅
+
(No. 2). The asymmetric unit contains one [C0C13] cation
−
and one Br as the counterion. The amphiphilic cations are
C
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Crystal Growth & Design
Article
Table 1. Crystallographic and Refinement Details for
graphic ac and bc planes (Figure 1, bottom). Imidazolium core
planes are parallel but shifted with respect to each other, so that
no directly face-to-face formation exists. The core planes tilt
with respect to the ab plane and form an angle of 30.8° and the
alkyl chains form an angle with the ab plane of 63.7°.
[C0C13]Br and [C2C11]Br
[
C0C13]Br
[C2C11]Br
Empirical formula
Formula weight
Temperature
C16 H30 Br N2
330.33
170 K
−
Each imidazolium is further linked with a Br anion through
nonclassical hydrogen bonds. The hydrogen bonds are
observed between the bromide ion and all hydrogen atoms of
neighboring C−H and N−H. They show distances ranging
from 2.64 to 3.01 Å. The most significant Br···H−N and
Br···H−C2 interactions are highlighted in red in Figure 2. The
hydrogen bonding interactions are summarized for clarity in
Table 2.
Crystal system
Space group
triclinic
monoclinic
P1
̅
P2 /c
1
Unit cell dimensions
a = 5.5208(5) Å
b = 7.8840(7) Å
c = 20.727(2) Å
α = 95.61(1)°
β = 95.53(1)°
γ = 97.86(1)°
a = 17.125(2) Å
b = 8.2532(7) Å
c = 13.007(1) Å
α = 90°
β = 103.10(1)°
γ = 90°
1790.5(3) Å3
3
Volume
883.9(1) Å
Z
2
4
3
1.228 g/cm3
2.288 mm −1
2.75−24.99°
12051/3037
Calculated density
Absorption coefficient
Θ-range for data collection
Reflections collected/unique
Refinement method
Data/parameters
1.241 g/cm
2.317 mm−
2.62−24.99°
7098/2624
1
Full-matrix least-squares on F2
2624/189
3037/174
0.855
2
Goodness-of-fit on F
1.022
Final R indices [I > 2sigma(I)]
R1 = 0.0379
R1 = 0.0515
wR = 0.0906
2
wR = 0.1000
2
R indices (all data)
R1 = 0.0422
R1 = 0.0919
wR = 0.0919
2
wR = 0.1100
2
characterized by a rod-like shape with one alkyl chain being
perpendicular to imidazolium core plane. The alkyl chain and
imidazolium core plane form an angle (defined as the angle
between the imidazolium ring centroid and the terminal C
atom of alkyl chain) of 92°.
The interdigitated alkyl chains build a bilayer lamellar
structure with a repeating layer distance of 20.5 Å (Figure 1,
top). The interdigitation is observed in both the crystallo-
Figure 2. Hydrogen bonding interactions in detail for [C0C13]Br.
Only interacting hydrogen atoms are shown for clarity. The most
significant H-bonds are highlighted in red.
Table 2. Hydrogen Bonds in [C0C13]Br and [C2C11]Br
D−A/Å
D−H/Å
H···A/Å
D−H···A/deg
[
C0C13]Br
0.867
N1−H1··Br·
N1′−H1′···Br
C4−H4B···Br
C1−H1···Br
C4′−H4A···Br
C2−H···Br
3.307
3.469
3.659
3.659
3.840
3.511
2.655
2.926
3.009
2.829
2.976
3.000
132.580
122.190
127.993
146.938
154.207
115.42
0.867
0.934
0.946
0.936
0.945
[
C2C11]Br
0.970
C4−H···Br
C4′−H′···Br
C6−H···Br
C1−H1···Br
3.902
3.732
3.775
3.407
3.010
2.805
2.877
2.904
153.392
160.072
127.993
115.28
0.970
0.970
0.930
The interdigitation of alkyl chains is observed in both the
crystallographic ab and bc planes (Figure 3). The cations stack
in such a way that their undecyl chains alternate in orientation
in neighboring layers, hence forming a bilayer structure. The
bilayer spacings are 18.5 Å. Imidazolium core planes lie directly
face-to-face, but with opposite orientations; while for one the
ethyl chain points up, for the other the ethyl group is directed
down. Nonclassical hydrogen bonds between cations and
anions can be discussed. The C−H···Br interactions are
observed with both the C2 position on imidazolium core and
Figure 1. Packing diagram of [C0C13]Br. View along crystallographic
b axis (top) and a axis (bottom), hydrogen atoms are omitted for
clarity.
D
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Figure 4. Hydrogen bonding interactions in detail for [C2C11]Br.
Only interacting hydrogens are shown for clarity. The most significant
H-bonds are highlighted in red.
Figure 3. Packing diagrams of [C2C11]Br; view along crystallographic
b axis (top) and c axis (bottom), hydrogen atoms are omitted for
clarity.
starting C atom on ethyl and undecyl chains. The most
significant interactions, which force two imidazolium-rings to
align face-to-face are highlighted in red. (Figure 4).
3
.2. Thermal Investigations. The thermal properties of all
the salts were examined by polarizing optical microscopy
POM) and differential scanning calorimetry (DSC). Thermal
transitions are summarized in Table 2.4. [C0C13]Br and
C1C12]Br both show liquid crystalline behavior. Each
Figure 5. DSC scan for C1C12 (heating rate: 5 °C/min).
(
Upon the first heating DSC scan, [C1C12]Br shows two
reversible phase transitions. These thermal effects are similar to
the scans for [C0C13]Br: a crystalline solid ↔ liquid crystalline
(S↔LC) transition at 42.5 °C with an enthalpy ΔH = 29.7 kJ/
mol; and a liquid crystalline ↔ isotropic liquid (LC↔L)
transition at 100.5 °C, enthalpy value of this transition is 0.6 kJ/
mol. The reverse transitions of these two thermal effects are
revealed in the cooling DSC traces: the phase transition from
isotropic liquid to liquid crystalline (L↔LC) occurs at 100 °C
(ΔH = −0.6 kJ/mol) and a liquid crystalline to crystalline
phase (LC↔S) at −14.2 °C (ΔH = −6.0 kJ/mol). An extensive
supercooling was observed for the crystallization process.
[C1C12]Br exists as liquid crystal at room temperature. The
enthalpy value of LC↔S transition is significantly smaller than
the value from the heating scan, while the enthalpy value for
LC↔L phase during the heating is the same as for the cooling.
In contrast, the second heating scan of [C1C12]Br shows
complex thermal behavior (Figure 6, top). At −1.3 °C, a raised
baseline is observed. This small endothermic peak is directly
followed by two exothermic effects (1.2 and 5.7 °C). These
[
mesophase was identified with POM and SAXS measurements.
The DSC scans of [C0C13]Br and [C1C12]Br at 5 K/min are
shown in Figure 5 and 6 (top). The DSC scans obtained from
the second heating cycle for these two ILCs are strikingly
different from the first run.
In a temperature range from −80 to 160 °C, salt [C0C13]Br
reveals two reversible phase transitions in the heating cycle.
The first heating scan is identical to the second one. A broad
endothermic peak is found at 71.3 °C corresponding to a
“
melting transition”, from a crystalline solid into a liquid
crystalline state (S↔LC), with an enthalpy ΔH = 34.0 kJ/mol;
a small endothermic peak appears at 141.5 °C with an enthalpy
ΔH = 0.8 kJ/mol, belonging to a liquid crystalline ↔ isotropic
liquid (LC↔L) transition. Between those transitions is a
mesophasic region (79−142 °C). Compared to the heating
scan, the cooling scan shows an extension of this mesophasic
region (140−30 °C), due to inhibited solidification and
supercooling.
E
dx.doi.org/10.1021/cg4004593 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

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In contrast to [C0C13]Br, [C1C12]Br and [C2C11]Br,
which are solid at room temperature, [C3C10]Br, [C4C9]Br
and [C5C8]Br, [C6C7]Br are viscous liquids. Since their
melting points are below room temperature, [C3C10]Br,
[C4C9]Br, [C5C8]Br and [C6C7]Br are also room temper-
ature ionic liquids (RTILs). The thermal behavior of these
RTILs is very similar. They exhibit only glass transitions at very
low temperatures. It should be mentioned that for all these
salts, the first heating cycles are identical to further heating
cycles. Only thermograms of the second heating scans are
summarized in Figure 7.
Figure 6. DSC scan for [C1C12]Br (top) and [C2C12]Br (bottom)
with heating rate at 5 K/min. The DSC scan for first heating in this
plot is upset by +0.5 mW/mg for clarity.
effects could be determined as solid−solid transitions (S↔S)
by further analytics, such as POM and SAXS. This can be
associated to an order−disorder transition of the long alkyl
chains. By reducing the heating and cooling rate from 5 to 1
°
C/min, these solid−solid transitions split up into two distinct
exothermic thermal events at 3.1 and 23.1 °C, respectively
Supporting Information). After the recrystallization process,
(
the S↔LC transition occurs at 38.5 °C, followed by LC↔L
Figure 7. DSC thermograms for [C3C10]Br, [C4C9]Br, [C5C8]Br
and [C6C7]Br (heating rate: 5 K/min), second heating scans (left)
and second cooling scans (right).
transition at 100 °C. The enthalpy value of both effects (ΔH =
2
6.9 kJ/mol for S↔LC and ΔH = 0.5 kJ/mol for LC↔L) are
similar to the value from the first heating cycle.
C2C11]Br is obtained as a white crystalline powder. In
[
Salts [C3C10]Br, [C4C9]Br and [C5C8]Br show similar
contrast to [C0C13]Br and [C1C12]Br, only one sharp
endothermic peak at 37.6 °C is observed upon first heating.
This is identified as a melting transition (S↔L) with an
enthalpy ΔH = 33.2 kJ/mol. Upon cooling the liquid sample of
glass transition temperatures (T ) around −60 °C, with a
g
remarkable tendency: T drops with reducing chain length of
g
the longest alkyl chain. This may be due to the fact that
decreasing the alkyl chain length would reduce the attractive
van der Waals interactions between the long hydrocarbon
chains. In contrast, [C6C7]Br, the salt with the shortest alkyl
[
C2C11]Br from the isotropic melt to −50 °C, no transition
was observed. Upon heating, the sample undergoes a broad
exothermic transition corresponding to a recrystallization at 0
chain length shows a T at −57 °C. It is well-known that
g
°C, followed by two narrow endothermic peaks at 23.3 and 28.0
°C. The DSC scans at 5 K/min are shown in Figure 6
(bottom). All the further heating cycles are reproducible and
melting point (T ) and glass transition temperature (T )
m
g
19
increase with increasing symmetry. The relatively high T of
g
[C6C7]Br can be associated with the pseudosymmetry of this
are similar to the first cooling and second heating scans.
salt, which enables more efficient ion−ion packing.
F
dx.doi.org/10.1021/cg4004593 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Article
Glass transition occurs in the case of a hampered
structure. As shown in Figure 10, both salts exhibit layered
structures in the crystalline and liquid crystalline phase. Two
diffraction peak for [C0C13]Br emerge in the small-angle
region. Both peaks are consistent with a d-spacing of 20.5 Å at
25 °C corresponding to the (001), the most intense peak in the
low angle region, and (002) reflections, respectively. This is in
agreement with the data from single-crystal XRD. In the
mesophase at 90 °C, the layer distance extends to 29 Å. This is
possibly due to a dispersing alkyl chain arising from the
formation of a mesophase, where the rod-like molecules move
with respect to one another in the direction of the
crystallization of ILs. Vitrification takes place when a liquid is
2
0
cooled sufficiently fast to elude crystallization. For a
crystallization process with a well-defined phase transition
temperature, an extremely low cooling rate is required. The
melting point, T , of a substance can be related to its glass
m
transition temperature, T , via many approaches. Since several
g
salts only show glass transitions, while the others show well-
defined melting points, for further comparisons, we estimate
the melting points T of those RTILs from their glass transition
m
6
,21
temperatures T with a simple equation: T ≈ 2/3T (in K).
The melting points and the evaluated melting points are plotted
g
g
m
2
2
interdigitated alkyl chains during heating processes.
versus n, the length of longest alkyl chains, as shown in Figure
. A tendency is immediately noticeable; the melting points
[C1C12]Br exhibits similar lamellar structures. The calculated
layer spacing from the SAXS data are 24 Å for crystalline state
and 30 Å in the mesophase. For both salts, an extending layer
distance from crystalline state to liquid crystalline state was
observed. SAXS data of several N-alkyl-imidazolium ILCs and
their LC behavior have been previously reported by Bradley et
8
22
al. They suggested two different cation conformations
according SAXS data, since single-crystal X-ray data were
missing. According our crystal data, the layer spacing in the
crystalline state and in the mesophase can be explained by a
model shown in Figure 11. However, the thermal transitions
and SAXS data from our investigations deviate from the data
reported in the literature. The reason for the deviations is the
hygroscopic nature of these salts meaning that the previous
investigations were carried out using partially hydrated
2
2,23
samples.
In contrast to [C0C13]Br and [C1C12]Br, the salt [C2C11]
Br exhibits no LC behavior. The single crystal data provides an
insight into the relationship between structure and the
likelihood of a mesophase formation before melting and allows
to reason why in that case no mesophase is adopted. As
previously mentioned, the positively charged imidazolium rings
are forced by H-bonds to align directly face-to-face
conformations (Figures 3 and 4). It might be argued that the
elimination of liquid crystallinity is a consequence of this face-
to-face conformation: The structure would expand during the
heating process due to additional thermal energy. This would
induce two positively charged imidazolium planes to move
closer to each other and lead to a repulsive force between the
layers. As a result, the attractive van der Waals interaction
between the alkyl chain stacks is not strong enough to stabilize
a liquid crystalline state when less than 12 carbon atoms are
involved in the chain, and the salt melts directly into an
isotropic liquid without exhibiting mesomorphic properties. A
representative illustration is shown in Figure 12.
Figure 8. Plot of melting points versus length of the longest alkyl chain
n. True melting points, measured from first heating scans, are shown as
●
, the calculated melting points as gray ■, for salts, which form LC
phase, the first melting point (crystalline solid ↔ liquid crystalline
S↔LC) transition) are as shown as gray ●.
(
increase with increasing chain length above n = 8, except for a
sudden fall between n = 10 and n = 11, although this could be
simply due to the inaccuracy of estimating T from measured
m
T data. The decrease of T between n = 7 and n = 8 can be
g
m
explained by the aforementioned symmetry effect, as a higher
symmetry leads to a higher melting point. [C6C7]Br is
considered as the salt with the highest symmetry in this study.
Interestingly, the characteristic changes in the melting point for
n ≥ 8 follow the same trend observed by Holbrey et al. for 1-
alkyl-3-methylimidazolium tetrafluoroborate with asymmetric
8
N-substitution. The scatter of the data show that the length of
3.4. Physical Properties: Densities and Viscosities.
Temperature-dependent viscosity measurements indicate that
the symmetry of the cation affects viscosity of these ILs (Figure
13). [C3C10]Br and [C4C9]Br with similarly asymmetric
cations are more viscous than [C5C8]Br and [C6C7]Br. Salt
[C5C8]Br reveals the same viscosity as [C3C10]Br and
[C4C9]Br below 35 °C. Above 35 °C, the viscosity of
[C5C8]Br decreases dramatically with increasing temperature.
For example, the viscosity decreases 40% by a temperature
change from 35 to 40 °C. [C6C7]Br, the pseudosymmetrical
IL, surprisingly shows the lowest viscosity. It is well-know that
the longest has the most dominant effect on the melting point;
the influence of cation symmetry is not discernible (see Table
3
).
3
.3. Mesophase Investigations with POM and SAXS.
Liquid crystalline mesophases were observed during the process
of heating and cooling only for salts [C0C13]Br and
[
C1C12]Br. Polarizing optical micrographs confirm the
temperature ranges and the existence of the aforementioned
mesophases. For [C0C13]Br, an oily streak texture was
observed (Figure 9, left) and for [C1C12]Br a focal conic fan
texture (Figure 9, right). Both textures are characteristic of a
lamellar smectic A phase.
24
highly asymmetric cations exhibit lower viscosities. The
results here could be explained as a consequence of an alkyl tail
25
Temperature dependent SAXS data support the assignment
segregation, which has been indicated by SWAXS (short and
wide-angle X-ray scattering) data and MD (molecular dynamic)
simulations. Alkyl tail segregation denotes the formation of
of lamellar phases. The primary d-spacing peak (d(100)
)
represents the average interlayer distance in the lamellar
G
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Article
Table 3. Thermal Behavior of Ionic Liquids and Ionic Liquid Crystalsa,b,c
compound
phase transition behavior
[
C0C13]Br
C1C12]Br
71.3(34.0)
141.5(0.8)
Cr HoooooooooooI SmA Hooooooooo oo I Iso
30.1(−36.7)
141.9(−0.7)
[
1st thermal cycle
42.5(29.7)
100.5(0.6)
Cr HoooooooooooI SmA Hooooooooo oo I Iso
−14.2(−6.0)
100.0(−0.6)
3
8.5
100.0
(0.5)
2nd thermal cycle
−1.3
3.6)
1.2
(−6.6)
5.7
(−6.3)
(
(26.9)
Cr ⎯⎯ ⎯⇀ Cr′ ⎯⎯⎯⎯ ⎯⇀ Cr″ ⎯⎯⎯⎯ ⎯⇀ Cr″ Hooo oo I SmA HooooI Iso
−
14.2
100.9
(
−6.0)
(−0.6)
0
.9
23.3
(5.1)
28.0
(15.2)
[C2C11]Br
37.6(33.2)
Cr ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ ⎯⇀ Iso
(−17.1)
st
1
thermal cycle
Iso ⎯⎯⎯⎯⎯ ⎯⇀ Cr″ ⎯⎯ ⎯⇀ Cr″ ⎯⎯⎯ ⎯⇀ Iso
nd
2 thermal cycle
d
[
[
[
[
C3C10]Br
−61.1(0.73)
Glass Hooooooooo oo I Iso
−63.6(0.28)
d
C4C9]Br
−62.7(0.67)
Glass Hooooooooo oo I Iso
−65.0(0.27)
d
C5C8]Br
−63.2(0.68)
Glass Hooooooooo oo I Iso
−68.2(0.26)
d
C6C7]Br
−57.4(0.19)
Glass Hooooooooo oo I Iso
−60.4(0.26)
a
b
Phase-transition temperatures (°C) and enthalpies (kJ/mol, in parentheses) are measured during heating and cooling at 5 K/min. Values were
c
determined from the second heating cycle if not mentioned, in this case, the thermal behavior was identical for all three heating cycles. Phase
transitions were identified by POM and SAXS measurements. These values are given as T : glass transition temperatures (°C) and heat capacity
changes at T (J/K g ) are given in parentheses. Cr = crystalline phase, LC = smectic-A phase, Iso = isotropic phase.
d
g
−1 −1
g
Figure 9. Representative POM images of thermotropic liquid
crystalline phases of [C0C13]Br and [C1C12]Br:[C0C13]Br at 105
°C (left) and [C1C12]Br at 67 °C (right).
Figure 11. Illustration of the change in molecular arrangement of
C0C13]Br and [C1C12]Br from the crystalline state to the
[
mesophase.
structurally heterogeneous domains: polar regions with cation
rings and anions and nonpolar regions formed by alkyl groups.
The densities of the ILs are given in Table 4. The densities of
all four ILs are approximately identical at ρ = 1.09 g mL− when
considering experimental error. This result is consistent with
the observation that the density of ILs increases by almost
1
26
proportionately with increasing molecular size.
4
. CONCLUSIONS
Figure 10. SAXS patterns for [C0C13]Br at 25 °C (top, left) and 90
A series of asymmetric 1,3-n-dialkylimidazolium bromides with
°
C (top, right) and (b) [C1C12]Br at 30 °C (bottom, left) and 70 °C
the same molecular weight and empirical formula, C H BrN ,
1
6
30
2
(
bottom, right), respectively.
were synthesized. Physicochemical properties such as thermal
transitions, density, and viscosities of all the salts were
H
dx.doi.org/10.1021/cg4004593 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
symmetry in this study. For n ≥ 11, where n is the length of the
longest alkyl chain, the T increases with increasing n. This can
m
be explained by considering the increasing van der Waals forces
between the long alkyl chains. The thermal behavior of the
asymmetric dialkyl-imidazolium ILs shows the same trend as
previously observed for 1-alkyl-3-methylimidazolium tetrafluor-
oborates. This can be rationalized in terms of the influence of
the length of the longest alkyl chain on the thermal behavior
dominating symmetry effects.
(
c). Crystal structure analyses show for both [C0C13]Br and
C2C11]Br a bilayer lamellar structure. Remarkably, a
separation of hydrophobic (alkyl chains stacks) and hydrophilic
imidazolium head groups) groups in the cations was observed
[
(
for both salts, although only [C0C13] shows thermotropic
mesophase formation. This indicates that for a LC phase to
exist, not only are the calamitic shape and integral length of a
molecule important but also a minimal chain length of n = 12 is
required. It has previously been reported that [C1C11]I
Figure 12. Schematic representation of [C2C11]Br: the directly face-
to-face conformation of the imidazolium cores eliminates a mesophase
formation, owning to repulsive forces between the layers caused by the
positively charged regions moving toward each other.
27
exhibits no LC phase. This dodecyl unit realizes the balance
between the van der Waals interactions and electrostatic
repulsion, opening the door to mesophase formation. There-
fore, a dodecyl group could be considered as a functional group
responsible for liquid crystalline behavior. Furthermore, it could
be considered that a face-to-face conformation of imidazolium
core planes hinders mesophase formation in the case of short
chain imidazolium salts. Further experiments on imidazolium-
based ILs bearing a dodecyl group with different sizes and
symmetries will be required to confirm this observation.
(
d). The fact that the viscosities of the RTILs drop with
increasing symmetry is a consequence of a alkyl tail segregation,
which was reported previously. The RTILs show almost the
−1
same density of ρ = 1.09 g mL , which is rationalized in terms
of the same size and molecular weight of all the salts.
ASSOCIATED CONTENT
Figure 13. Comparison of temperature dependent viscosity data for
■
[
C3C10]Br, [C4C9]Br, [C5C8]Br and [C6C7]Br.
S
*
Supporting Information
Supplemental figures and crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
a
Table 4. Densities of RTILs
[
C3C10]Br [C4C9]Br [C5C8]Br [C6C7]Br
1.089 1.085 1.085 1.086
AUTHOR INFORMATION
Density [g mL−1]
■
a
Corresponding Author
E-mail: anja.mudring@ruhr-uni-bochum.de. Tel: +49-234-32-
7408.
At 25 °C.
*
2
investigated. Crystals of sufficient quality for X-ray structure
analysis could be obtained from [C0C13]Br and [C2C11]Br.
Our study can be summarized in the following aspects.
Notes
The authors declare no competing financial interest.
(
a). All of these salts are ILs, since they have a melting point
below 100 °C. Salts [C0C13]Br, [C1C12]Br and [C2C11]Br
exist as (thermodynamically stable) crystalline solids at room
temperature, while [C3C10]Br, [C4C9]Br, [C5C8]Br and
ACKNOWLEDGMENTS
■
This work was supported by the German Science Foundation
DFG through the priority program 1191 “Ionic Liquids”, the
DFG Cluster of Excellence RESOLV and the DESY (Deutsches
Elektronensynchrotron proposal no. I-20100011). We thank
Dr. Sergio Funari, for support during the SAXS measurements
and thank Dr. P.Campbell for helpful comments.
[
C6C7]Br are room temperature ionic liquids with high
viscosities. Furthermore, thermotropic liquid crystalline behav-
ior for [C0C13]Br and [C1C12]Br can be observed. The liquid
crystalline properties of these two salts were examined by
differential scanning calorimetry and polarized optical micros-
copy. The mesophases that they form can be identified with
POM images and X-ray scattering experiments at small angle
range as the SmA phase for both salts.
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