Developing design tools for introducing and tuning structural order in ionic liquids

Article abstract of DOI:10.1039/d0ce01672a

Ionic liquids (ILs) are receiving growing interest as highly tunable, multifunctional materials. Remarkably for liquids, they tend to display a high level of structural order. This structural order may even lead to the formation of mesophases such as liquid crystals (LCs). Imidazolium compounds are by far the most popular ILs, because they offer a widely versatile platform for property tuning. To investigate what is driving structural order in imidazolium-based ILs a series of asymmetrical 1-dodecyl-2-methyl-3-alkylimidazolium bromides, [C₁₂C₁C_nim][Br] withn= 0-12 have been synthesized, fully characterized and their structures and properties compared with the analogous 1-dodecyl-3-alkylimidazolium as well as the 1,2,3-triazolium bromides. The aim is to examine the influence of the replacement of the most acidic 2-H proton on the imidazolium head group by methylation on the properties and structure of ILs. For all compounds, except for compounds with butyl- and hexyl-chains as well as the protonated species, mesophase formation can be observed. Obviously, the simple presence of long alkyl chains such as dodecyl (a design concept frequently put forward in the literature) is not sufficient to support mesophase formation alone. Rather, for the formation of a liquid crystalline phase, a balance between attractive van der Waals forces, hydrogen bonds, and electrostatic interactions is required. Data from temperature-dependent small-angle X-ray scattering (SAXS) and polarizing optical microscopy (POM) suggest three different cation conformations for the studied [C₁₂C₁C_nim][Br]: cations with 0 ≤n≤ 4 exhibit a near-linear conformation; for 5 ≤n≤ 10 a V-shape is adopted, and forn= 11 or 12 a U-shape is found. We demonstrated that the structural possibility for an interdigitation of the long chains is an influential factor for the formation of a mesophase.

Full text of DOI:10.1039/d0ce01672a

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Developing design tools for introducing and
tuning structural order in ionic liquids†
Cite this: CrystEngComm, 2021, 23,
1785
a
Olivier Renier, a Guillaume Bousrez,
Mei Yang,b Milena Hölter,b Bert Mallick,b
Volodymyr Smetana a and Anja-Verena Mudring
ab
*
Ionic liquids (ILs) are receiving growing interest as highly tunable, multifunctional materials. Remarkably for
liquids, they tend to display a high level of structural order. This structural order may even lead to the for-
mation of mesophases such as liquid crystals (LCs). Imidazolium compounds are by far the most popular
ILs, because they offer a widely versatile platform for property tuning. To investigate what is driving struc-
tural order in imidazolium-based ILs a series of asymmetrical 1-dodecyl-2-methyl-3-alkylimidazolium bro-
mides, [C12C1Cnim][Br] with n = 0–12 have been synthesized, fully characterized and their structures and
properties compared with the analogous 1-dodecyl-3-alkylimidazolium as well as the 1,2,3-triazolium bro-
mides. The aim is to examine the influence of the replacement of the most acidic 2-H proton on the
imidazolium head group by methylation on the properties and structure of ILs. For all compounds, except
for compounds with butyl- and hexyl-chains as well as the protonated species, mesophase formation can
be observed. Obviously, the simple presence of long alkyl chains such as dodecyl (a design concept fre-
quently put forward in the literature) is not sufficient to support mesophase formation alone. Rather, for
the formation of a liquid crystalline phase, a balance between attractive van der Waals forces, hydrogen
bonds, and electrostatic interactions is required. Data from temperature-dependent small-angle X-ray scat-
tering (SAXS) and polarizing optical microscopy (POM) suggest three different cation conformations for the
studied [C12C1Cnim][Br]: cations with 0 ≤ n ≤ 4 exhibit a near-linear conformation; for 5 ≤ n ≤ 10 a
V-shape is adopted, and for n = 11 or 12 a U-shape is found. We demonstrated that the structural possibil-
ity for an interdigitation of the long chains is an influential factor for the formation of a mesophase.
Received 18th November 2020,
Accepted 8th January 2021
DOI: 10.1039/d0ce01672a
rsc.li/crystengcomm
which can be beneficially used in the designed synthesis of
new materials. For example, they can be used for templating
Introduction
Imidazolium-based ionic liquids (ILs) have become an ex-
tremely popular class of compounds over the past decade due
to their unique properties and large number of
applications.1–8 ILs are generally defined as salts exhibiting a
melting point below 100 °C.9,10 Hydrogen-bonding interac-
tions in these ILs mainly involving the hydrogen atom located
on the C2 position of the imidazolium head group, have been
found to play an important role in the order of the IL.11–15
They lead to the formation of supramolecular structures,
open-framework materials,16 to allow the morphological con-
trol of nanoparticles17 and even for phase control of polymor-
phic materials.18 These effects rely both on the interaction of
the IL ions with each other, leading to the formation of a
highly structured solvent where clear segregation of domains
occurs,19,20 but also on the interaction of the IL ions with the
solutes and nuclei of materials formed in them during syn-
thesis. It is believed that hydrogen-bonding plays an impor-
tant role in both interactions.21,22 Indeed, it has been ob-
served that altering or blocking possible hydrogen-bonding
sites significantly affects the physicochemical properties of
the corresponding ILs by inducing drastic changes in the in-
teractions and structures.14,23 In imidazolium ILs, the proton
at the C2 position is known to interact strongly with the IL
anion through hydrogen bonds.24 The introduction of a
methyl group on the C2 position is therefore thought to cause
significant property changes of these ILs,23,25 such as an in-
crease in the melting points,26–28 viscosities29 and thermal- or
chemical stabilities,26,29,30 along with a decrease in densities31
a Physical Materials Chemistry, Department of Materials and Environmental
Chemistry, Stockholm University, Svante Arrhenius väg 16c, 10691 Stockholm,
Sweden. E-mail: anja-verena.mudring@mmk.su.se
b Anorganische Chemie III – Materials Engineering and Characterization, Fakultät
für Chemie and Biochemie, Ruhr-Universität Bochum, 44780, Bochum, Germany
† Electronic supplementary information (ESI) available: Experimental details, in-
strumentation, SCXRD table, NMR spectra, IR spectra, TGA spectra, DSC traces,
SAXS and POM. CCDC 1912603–1912605. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/d0ce01672a
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and polarities.32 Even the chemical reactivity can be affected
after C2 methylation.33
Mesophase formation was observed for 1-alkyl-3-methyl-
imidazolium bromides when the substituted alkyl chains
contained twelve carbon atoms or more.34 A mesophase, like a
liquid crystalline phase, is an intermediate between state the
perfectly ordered crystalline and the disordered liquid state.35
ILs which are able to form a liquid crystalline phase are also
addressed as ionic liquid crystals (ILCs). ILCs combine the self-
organization features of liquid crystals and the solvent proper-
ties of ILs.36 The importance of ILCs has been realized in di-
verse applications such as ion-conductive materials, organized
media or self-assembled nanostructured materials.36–43 Thus, it
is of strong interest to understand how structural order can be
introduced into ILs and learn which kind of secondary bonding
forces evoke it. It was proposed that van-der-Waals interactions
become so strong in 1-alkyl-3-methylimidazolium salts when the
alkyl chain reaches twelve carbon atoms that LC formation in-
duced.32 However, aside from van der Waals interactions,
hydrogen-bonds have been found to be involved in the forma-
tion of liquid crystalline phases.7,44 There is experimental evi-
dence that methylation of the C2 position of an imidazolium
cation can decrease or even eliminate the liquid crystallinity.45
However, for 1-hexadecyl-2,3-dimethylimidazolium bromide, a
mesophase formation has been reported.46 Thus, increasing the
lengths of the alkyl-tails and enhancing van der Waals interac-
tions between the IL cations can outbalance the effect of C2
methylation in imidazolium bromides. So far, no understanding
between mesophase formation, i.e. liquid crystallinity,
hydrogen-bond ability as well as the influence of van der Waals
forces has been reached for ILs,47 and the effect of a change in
the H-donor site on the imidazolium ring on the liquid crystal-
line behaviour is yet to be determined. For this reason, we have
prepared a series of 1-dodecyl-2-methyl-3-alkylimidazolium bro-
mides ([C12C1Cnim][Br], n = 0–12) (Scheme 1). The thermal and
structural properties are investigated via differential scanning
calorimetry (DSC), polarizing optical microscopy (POM),
thermogravimetric analysis (TGA), and small angle X-ray scatter-
ing (SAXS). The influence of C2 methylation on the physico-
chemical properties is studied by comparison with their proton-
ated C2 (ref. 44) and triazolium counterparts.48
Scheme 1 Different types of ionic liquid cations (and acronyms used
for identification) under investigation.
imidazole (Scheme 2). Because of the lack of commercial
availability of methyl bromide, the 1,2-dimethylimidazole has
been alkylated at the N-3 position with a small excess of
dodecylbromide to obtain 1-dodecyl-2,3-dimethylimidazolium
(C12C1C1). Compounds C12C1C2–C12C1C12 were synthesized
by alkylation at the N-3 position with the corresponding alkyl
bromide. For the synthesis of compound C12C1C0, the proton-
ation of the nitrogen has been realized by addition of hydro-
bromic acid (HBr) at low temperature.
For convenience, the same compound naming conven-
tions as in previous reports has been used.7,44 The com-
pounds are named by the number of carbon atoms in the
three alkyl chains at the first, second and third positions of
the imidazolium ring. For example, [C12C1C5im][Br],
1-dodecyl-2-methyl-3-pentylimidazolium bromide, as C12C1C5
and [C12C1C6im][Br], 1-dodecyl-2-methyl-3-hexylimidazolium
bromide, as C12C1C6.
All compounds could be obtained as white crystalline
solids apart from C12C1C5 and C12C1C7 which were isolated
as extremely viscous oils.
Results and discussion
Synthesis
Thermal investigations
To obtain the different 1-dodecyl-2-methyl-3-alkylimidazolium
salts, the respective 2-methylimidazole was reacted with a
small excess of dodecylbromide to yield 1-dodecyl-2-methyl-
To study their thermal behaviour in TGA, DSC, and POM
were undertaken. Table 1 lists the decomposition tempera-
tures as recorded by TGA. The decomposition temperatures
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difficult to determine the phase behaviour of the studied
compounds from one method alone. However, it is the com-
bination of different methods supports the phase assign-
ments made. The transition temperatures (in °C), enthalpies
(in J g−1) and phase transition assignments as obtained from
DSC measurements are listed in Table S3.† The samples were
first cooled at a thermal ramp of 5 °C min−1 to −60 °C with-
out recording thermal data. During sample preparation con-
tact with the atmosphere and some water uptake could not
be completely avoided. Both the first full thermal cycle and
the second, showing the anhydrous compounds, are
reported. All further thermal cycles were identical with the
second unless otherwise stated.
For C12C1C1 the first heating from −60 °C reveals four irre-
versible phase transitions. First, two solid–solid transitions at
53.1 °C (Cr → Cr′) and 67.2 °C (Cr′ → Cr″) were observed,
followed by a crystalline solid–liquid crystalline (Cr″ → SmA)
transition at 77.4 °C. The liquid crystalline phase was
assigned to a smectic A phase (ESI† Fig. S89d). The clearing
point (SmA → Liso) lies at 148.2 °C. Upon cooling, the transi-
tion from isotropic liquid to liquid crystalline (Liso → SmA) is
revealed in the cooling scan at 120.5 °C and a liquid crystal-
line to crystalline phase (SmA → Cr′) at 36.2 °C. It appears
that under the given cooling rate only a fraction of the sam-
ple crystallized. Upon subsequent heating, cold crystallization
can be observed at 27.7 °C (Cr′ → Cr). A comparison of the
associated enthalpies shows that indeed cold crystallization
at 27.7 °C lead to a full crystallization of the sample before it
melts to a LC state at 81.9 °C (Cr → SmA). The clearing point
(SmA → Liso) is observed at 120.0 °C. The reverse transition
(Liso → SmA) occurs at 120.0 °C upon cooling. At 36.2 °C par-
tial crystallization is observed (SmA → Cr). With more ther-
mal cycles the fraction of material that recrystallizes is in-
creased as becomes evident from the phase transition
enthalpies, which are −49.5 J g−1 for the first cooling vs. −54.7
J g−1 for the fifth cooling. At the same time, the crystallization
peaks, on cooling become more pronounced and sharp re-
vealing a solid–solid transition close to the point of crystalli-
zation. The associated cold crystallization peak observed on
heating also shows a split into two. All thermal transitions
were fully reproducible from the second cooling cycle on-
wards. A reason why the first heating deviates from all the
subsequent cycles is that there is water uptake during sample
preparation in ambient air. Similar thermal behaviour has
been observed for [Cnmim]Br·xH2O, (n = 10, 12; x = 0, 1).2
There it was observed that water enlarges the mesophase win-
dow by pushing the clearing point to higher temperatures.
Evaluation of TG data (ESI†) reveals the presence of ∼1 mole-
cule of H2O per formula unit.
Scheme
(C12C1C1–C12C1C12
compounds. i) r.t., K2CO3, THF; ii) 80 °C, 48 h, acetonitrile; iii) 0 °C, 1
h, diethyl ether.
2
Syntheses of 1-dodecyl-2-methyl-3-alkylimidazolium
and 1-dodecyl-2-methylimidazolium (C12C1C0)
)
Table 1 Decomposition temperatures of compounds C12C1C0–C12C1C12
Compound
T decomposition (°C)
C12C1C0
C12C1C1
C12C1C2
C12C1C3
C12C1C4
C12C1C5
C12C1C6
C12C1C7
C12C1C8
C12C1C9
C12C1C10
C12C1C11
C12C1C12
262
273
266
268
271
269
269
270
266
259
266
264
263
of all compounds are similar within the error limits of mea-
surement and occur around 265 10 °C. It is remarkable that
this series of compounds appears to have a low tendency to
take up water, despite having the most hydrogen-bond accep-
tor and donor sites (due to experimental limitations, it was
not possible to transfer the compounds under strictly inert
conditions into the thermal analyser. While all samples were
handled the same, for some water uptake was observed
whilst for others none. See ESI† on the thermal traces). This
is an interesting observation as one would expect the hydro-
phobic character to increase with increasing length of the al-
kyl chain. Generally speaking, 1,3-dialkylimidazolium ILs ex-
hibit a decomposition temperature higher than that of their
analogous triazolium ILs.49
Phase transitions were studied in greater detail by
employing DSC, POM, SAXS and SXRD (for the latter two, vide
infra). Initial phase assignments were made based upon char-
acteristic textures observed in the POM micrographs and con-
firmed by SAXS and SXRD. The complexity and the different
sensitivity to water (which changes the phase behaviour by al-
tering the transition temperatures and observed phases in-
cluding the formation of lyotropic LC phases) does make it
The DSC trace of C12C1C2 reveals a sharp endothermic phase
transition at 63.1 °C during the first heating scan. This can be
identified as a melting transition (Cr′ → Liso). It is preceded by a
solid–solid transition (Cr → Cr′) at −13.6 °C. Upon cooling crys-
tallization can be observed, but via the transition to a metasta-
ble LC phase at −3.4 °C. Vitrification (transition from liquid to
glass) and devitrification (transition from glass to liquid) occur
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around −28 °C. The subsequent cycles all exhibit the same tran-
sitions. Upon heating, a solid–liquid crystalline transition is ob-
served at 10.5 °C (Cr → SmA) followed by a liquid crystalline–
isotropic liquid transition at 65.0 °C (Sm → Liso). It is important
to note that several liquid crystalline phases can be seen to com-
pete under the polarizing optical microscope, rendering the
identification impossible (ESI,† Fig. S88).
The first heating trace of C12C1C3 reveals that, aside from a
glass transition at −35.5 °C, there are two non-reversible transi-
tions around −40 °C and 10 °C. The second transition was in
fact identified to be two overlapping transitions of which the
first is the transition to a smectic A phase and the second was
the clearing point (28.9 °C). The existence of the mesophase
was proven by POM measurements at 25 °C, where smectic A
phase characteristic texture could be seen. Upon cooling from
the isotropic liquid, only a glass transition around −35 °C is ob-
served, which is reversed upon heating at 30.7 °C.
The first heating DSC trace of C12C1C4 exhibits only one
endothermic transition. It is observed at 34.6 °C and can be
attributed to a solid–liquid transition (Cr → Liso). The cooling
trace shows a glass transition at −33.7 °C. Upon subsequent
heating devitrification (Tg → Liso) and cold crystallization
(Liso → Cr) occur at −27.2 °C and 8.2 °C respectively. The
clearing point (Cr → Liso) can then be observed at 35.3 °C. All
subsequent thermal cycles are fully reproducible.
Compound C12C1C8 exhibits two processes of which only
one was found to be reversible. The first, non-reversible pro-
cess, is found at 22.3 °C and is attributed to the melting
point (Cr → SmC). The reversible transition from liquid crys-
tal to liquid (Sm ⇋ Liso) can be found at 106.4 °C and the re-
verse process at 109.0 °C. POM observations confirmed the
formation of a liquid crystalline phase which was identified
as smectic C phase. This transition is followed by a glass
transition at −39.8 °C. The following cycles all exhibit the
same behaviour; a glass transition upon heating at −36.0 °C
followed by the clearing point (SmC → Liso) at 98.6 °C. The
cooling traces then reverse this transition (Liso → Sm) at
100.9 °C and the vitrification occurs at −39.8 °C.
The DSC trace for compound C12C1C9 is very similar to
the previous compound's trace. The first process can be
found at 33.6 °C and comprises two overlapping events, of
which the latter was identified by POM as a solid to liquid
crystalline transition (Cr → SmC) into a smectic C phase.
The clearing point was observed at 122.9 °C. Its reverse
transition is observed at 127.9 °C. Vitrification of the smec-
tic C phase can then be observed at −33.3 °C. As for the
previous compound, the next traces only exhibit a clearing
point at 129.1 °C which is reversed at 127.7 °C, as well as
glass transitions around −33 °C. These processes are highly
reproducible.
Compound C12C1C5 exhibits a cold crystallization from a
smectic A phase at −30.1 °C during the first heating, followed
by a solid–liquid crystalline transition (Cr → SmA) occurs at
4.8 °C. POM allows identification of the LC phase as a smec-
tic A phase. The clearing point can be found at 51.7 °C. Upon
cooling, only a glass transition can be observed at −37.9 °C.
All further cycle only exhibited devitrification and vitrification
at −27.1 °C and −37.9 °C respectively. From this, it can be
concluded that crystallization is kinetically hindered.
Compound C12C1C6 exhibits only one solid–solid transi-
tion at −13.1 °C before melting into an isotropic liquid at
38.9 °C. The following thermal cycles reveal, similarly to the
previous sample, only a glass transition around −33.0 °C
upon cooling and −30.5 °C upon heating.
For compound C12C1C7, a cold recrystallization can be ob-
served upon the first heating at −12.6 °C, followed by melting
to a LC phase at −1.0 °C. This transition was then later iden-
tified by POM to be a smectic C phase. The final transition is
then attributed to a liquid crystalline–isotropic liquid (clear-
ing point) transition. Upon cooling a transition from the iso-
tropic liquid to a smectic C phase is observed at 53.5 °C. A
vitrification can then be observed at −33.7 °C which is then
reversed upon heating at −28.6 °C. This is then followed by a
liquid crystalline–isotropic liquid transition at 52.6 °C, which
is reversed upon cooling at 52.2 °C. These processes are then
reproducible in all subsequent cycles. Like for C12C1C1 TG
analysis shows the presence of about one molecule of water
per formula unit. Thus, it is expected that the first heating cy-
cle will look different from the following. Like for C12C1C1,
the presence of water moves the clearing point to higher tem-
peratures and enlarges the mesophase window.
Compound C12C1C10 exhibits a particular thermal behav-
iour due to being hydrated, as confirmed by TGA and IR-
spectroscopy. In fact, during the first heating cycle, four tran-
sitions can be observed. The first event can be observed at
−41.2 °C and is attributed to a solid–solid transition (Cr →
Cr′). A second transition of this type (Cr′ → Cr″) can then be
observed at −16.4 °C. A solid–liquid crystalline phase transi-
tion can then be observed at 22.4 °C. The phase was con-
firmed to be a smectic C phase by POM. The clearing point is
observed at 132.9 °C. Upon cooling from the isotropic liquid
a smectic C phase forms at 147.7 °C. A glass transition can
also be seen at −32.9 °C. The ensuing traces exhibit two tran-
sitions that are both reversible. Devitrification and vitrifica-
tion occur at −31.0 °C and −33.9 °C, respectively. The clearing
point (SmC → Liso) is observed at 140.7 °C upon heating and
is reversed at 145.7 °C upon cooling.
The first heating cycle for C12C1C11 and C12C1C12 display
similar behaviour. In fact, for both compounds, solid–solid
transitions (Cr → Cr′) which involve at least for C12C1C11 two
solid forms, can be observed preceding a solid–liquid crystal-
line transition (Cr′ → SmC) involving two LC states which
could not be resolved (as indicated by the peak shape) (Fig.
S65†) and a clearing point. The first transition is observed at
−19.4 °C (C12C1C11) and −24.0 °C (C12C1C12). The next events
can be found at 33.2 °C (C12C1C11) and 37.9 °C (C12C1C12).
Both compounds exhibit clearing points at 127.6 °C
(C12C1C11) and 140.3 °C (C12C1C12). In fact, C12C1C11 exhibits
a reversal of its clearing point at 158.4 °C followed by a par-
tial recrystallization at −16.4 °C. C12C1C12 displays a similar
behaviour as it reverses its clearing point at 166.6 °C and re-
crystallizes at −11.8 °C.
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The second heating traces of the two compounds differ
slightly. C12C1C11 does not display its solid–solid transition
any longer and only shows two reversible processes identified
as a melting transition at −27.4 °C and a clearing point at
152.8 °C. The transitions are then reversed upon cooling at
158.4 °C and −18.4 °C respectively. These process are then
highly reproducible.
C12C1C12 on the other hand displays a very similar behav-
iour as in its first trace, albeit with a lower enthalpy content,
pointing to kinetically hindered effects. A solid–solid transi-
tion (Cr → Cr′) is found at −18.5 °C followed by a solid–liquid
crystalline transition (Cr′ → SmC) at 39.2 °C and a clearing
point at 161.5 °C. All transitions are reversible and appear at
165.9 °C (Liso → SmC) and −11.8 °C (SmC → Cr). These pro-
cesses are then highly reproducible.
Compound C12C1C0 exhibits four events in its first heating
cycle. The first at −10.1 °C is attributed to a solid–solid transi-
tion (Cr → Cr′), and two additional solid–solid transitions (Cr
→ Cr″) at 22.3 °C and (Cr″ → Cr‴) at 28.8 °C can be distin-
guished. The final event at 84.0 °C was identified as a solid–
liquid transition (Cr‴ → Liso). Upon cooling, at 67.0 °C, the
clearing point is reversed (Liso → Cr‴) followed by two solid–
solid transitions at 59.7 °C (Cr‴ → Cr″) and 41.7 °C (Cr″ →
Cr′). Upon subsequent heating cold crystallization occurs at
24.7 °C before melting at 85.3 °C. Upon cooling the trace fol-
lows the first trace.
Compounds C12C1C4, C12C1C6 and C12C1C0 do not form
liquid crystalline phases. A similar observation for the
1-dodecyl-3-alkylimidazolium based bromide salts with ethyl-
and hexyl-unit was reported.44
For C12C1C0 the melting point is almost the same as for
its non-methylated counterpart. However, for a chain length
of Cn (n ≥ 8), the opposite is observed. The reason for this in-
crease has previously been attributed to a reduction in en-
tropy.11,23,50 The triazolium analogues seem to exhibit a lower
melting point when the chain length is Cn (n ≤ 7) with the ex-
ception of C1. Longer chain lengths usually exhibit the
highest melting point of all the compounds.
bromides. In contrast, the enthalpy values of Cr → Sm transi-
tions for the liquid crystalline compounds are significantly
higher than for the liquid crystalline 1-dodecyl-3-
alkylimidazolium compounds. For example, [C12C12im][Br]
undergoes a Cr → Sm transition with an enthalpy ΔH = 26.3 J
g−1, whereas C12C1C12 undergoes a Cr → Sm transition with
an enthalpy ΔH = 72.31 J g−1. Therein, according to the re-
sults obtained from quantum chemical calculations by
Hunt,11 the methylation on C2 position inhibits the rotation
of the alkyl side chains, which leads to a decreased entropy.
In the case of ILs with bromide anions and short alkyl side
chains (n = 2–4), no decrease in the enthalpy was observed in
the previous report. As already mentioned, for the long-chain
containing ILs in the present study, a significant increase in
transition enthalpies after the C2 methylation was found.
The phase transition temperature is described by the quo-
tient of transition enthalpy and entropy. Therefore, the reduc-
tion in the entropy values and the increasing enthalpy values
lead to the increase of melting point.10,51
The specific changes in the melting points displayed in
Fig. 1 show an inverse relation with the non-methylated com-
pounds. In fact, the melting points are alternating from high
to low depending on the parity of the number of carbons
within the chain. For the non-methylated compounds, the
melting point follows the trend discovered for alkanedi-
carboxylic acids in which an even number of carbon atoms
would always result in a slightly higher melting point.52 The
opposite is observed for the methylated compounds. The
characteristic changes in the clearing point follow the same
trend for our observations of C2 protonated 1-dodecyl-3-
alkylimidazolium bromides44 and triazolium bromides.48 The
clearing points decrease with increasing n for n = 1–3. For n =
4–6, a slight increase of clearing points can be observed. For
n ≥ 7, the clearing points increase rapidly with rising n.
Interestingly, 1-dodecyl-2-methyl-3-alkylimidazolium salts
have much higher clearing points and therefore wider meso-
phase temperature ranges than their 1-dodecyl-3-
In general, the Sm ⇄ Liso transitions are reversible and
can be observed with similar enthalpy values in the cooling
scans of each liquid crystalline sample except C12C1C5. In
contrast, the Cr → Sm transitions could almost only be ob-
served in the first heating scans and could not be considered
reversible. Some materials show a cold crystallization process
in the second heating scan preceding the melting transitions
(e.g. C12C1C4), whereas in others, no crystallization is ob-
served (e.g. C12C1C3 and C12C1C5). In addition, the enthalpy
values of the Cr → Sm transitions in the further heating cy-
cles are significantly lower than the enthalpy values from the
first heating cycles, while the enthalpy values for Sm → Liso
phase transition do not vary. The previously mentioned ob-
servations indicate that the crystallization kinetics are rela-
tively slow and inhibited by the rapid cooling rate. Further-
more, the enthalpy values of Sm → Liso transitions for the
liquid crystalline compounds in this series are very similar to
those of the liquid crystalline 1-dodecyl-3-alkylimidazolium
Fig.
1 Comparison of the melting point (Cr → Sm/Liso) of the
[C12C1CnIm][Br] (methylated), [C12CnIm][Br] (not methylated)42 and
[C12CnTr][Br] (1,2,3-triazolium).46
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alkylimidazolium bromides and triazolium counterparts
(Fig. 2). In fact, for compound C12C1C12, the triazolium bro-
mide and 1,3-didodecylimidazolium exhibit an almost identi-
cal clearing point. In general, it is noted that the removal of
the most acidic proton on the imidazole (proton on the C2
position) dramatically increases both the melting and clear-
ing point of the salt.
The falling trend indicates structural disorganization by
insertion of short alkyl chains into well-ordered dodecyl
chains. The short alkyl chain (n ≤ 3) hinders the close pack-
ing of molecules, whereas parallel chain packing maximizes
the van der Waals forces between the dodecyl chains for
C12C1C12. It is well-known that the melting point can also be
affected by the degree of interdigitation in the solid-state. A
smaller d value in the crystalline state indicates a high extent
of alkyl chain interdigitation, which leads to a high melting
point. The trend in the melting transitions for 4 ≤ n ≤ 12
confirms this statement. In addition, it has been reported
that odd length chains are more likely to adopt an all-trans
configuration,53 resulting in a higher melting point for com-
pound C12C1C9 than its immediate neighbours with even
lengths of n (Fig. 3).
Fig. 3 Plot of melting point and clearing point temperature as a
function of compound number. The melting points are plotted in black
and clearing points in red. The data were taken from the onset
temperature from the second DSC heating scans.
imidazolium ring centroid and the terminal C atom of alkyl
chain) of 152.2°. The chain is not completely straight and
shows a slight bow originating from a dihedral angle of
172.0° along C6–C10–C16. The dodecyl tails form parallel hy-
drophobic stacks with end-to-end packing (i.e. no alkyl chain
interdigitation is observed) resulting in bilayer lamellar struc-
ture. The imidazolium head groups are parallel to each other,
adopting a direct face-to-face conformation (Fig. 4). They tilt
with respect to the ab plane, forming a 51.8° angle, while the
alkyl chains form a 67.4° angle with the ab plane. The amphi-
philic character of the cation results in a separation of polar
and non-polar regions in the crystalline packing. The non-
polar regions are formed by the alkyl tail groups, held
Crystal structures
Single crystals of C12C1C1 and C12C1C0 were obtained by re-
crystallization from ethyl acetate. The monohydrate of
C12C1C12, could be crystallized from a dichloromethane solu-
tion of the anhydrous compound by isothermal evaporation
in air at room temperature. All crystals, as a rule, appeared in
the form of thin plates.
¯
Compound C12C1C0 crystallizes triclinic (P1, No. 2) and
contains two asymmetric units in the unit cell (Fig. 4a). The
dodecylalkyl chain of the imidazolium cation adopts an all-
trans conformation and forms an angle with the methyl-
imidazolium core plane (defined as the angle between the
Fig. 2 Comparison of the clearing point taken from the 2nd heating
(Cr/Sm → Liso) of the [C12C1CnIm][Br] (methylated), [C12CnIm][Br] (non-
methylated)42 and [C12CnTr][Br] (Tr = triazolium)46 compounds C12C1C5
and C12C1C6 a problem as only glass transitions observed.
Fig. 4 Asymmetric units of C12C1C0 (a), C12C1C1 (b) and C12C1C12·H2O
(c).
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Table 2 Shortest (below 3 Å) hydrogen-bonds in C12C1C0, C12C1C1 and
C12C1C12·H2O
together by van der Waals interactions. The polar parts are
constituted of charged methylimidazolium head groups and
bromide anions exhibiting rather strong π–π (dCg–Cg = 3.79 Å,
offset 1.63 Å) and Br⋯H interactions (Fig. 5). Both of them
are responsible for strong intralayer bonding. Each methyl-
imidazolium cation is further linked to bromide anions
through non-classical hydrogen bonds. The latter are also ob-
served between the bromide anions and practically all hydro-
gen atoms on the alkyl chain, though involving different mol-
ecules leading to a 3D network. The hydrogen bonds show a
distance range of 2.4–3.1 Å (Table 2) where 2.4 Å stands solely
for the Br⋯H–N pairs and all others are significantly longer
(Fig. 6).
Compound C12C1C1 crystallizes in the monoclinic space
group P21/c (No. 14) with four formula units in the unit cell.
The asymmetric unit contains a [C12C1C1im]+ cation and a
bromide anion (Fig. 4b). Both the C atoms from the methyl
groups and the starting C atom in the dodecyl chain are lo-
cated in the imidazolium core plane. In contrast to C12C1C0,
the dodecyl chains are straight and form interdigitated
stacks, while the angle between the dodecyl chain and
methylimidazolium plane is slightly reduced – from 152.2° to
144.5°. Interdigitation of the alkyl chains is observed in the
(101) plane. The cations stack in such a way that their
dodecyl chains alternate orientation in neighbouring layers,
hence forming a bilayer structure. When viewed along the
crystallographic c-axis, the polar head groups line up in a
row, with adjacent alkyl tails tilted in different directions,
forming a herringbone-like structure (Fig. 7). The angle be-
tween the adjacent alkyl chain planes is ∼32°. Instead of clas-
sic π–π interactions between the imidazole cores observed in
D–A/Å
D–H/Å
H⋯A/Å
D–H⋯A/°
C12C1C0
N1–H⋯Br
C1–H⋯Br
C4–H⋯Br
C5–H⋯Br
C11–H⋯Br
C12C1C1
3.232
3.703
3.768
3.739
3.950
0.84
0.93
0.96
0.97
0.97
2.393
2.880
2.820
3.001
2.995
174.2
148.2
169.5
133.9
169.1
C17–H⋯Br
C16–H⋯Br
C14–H⋯Br
C12C1C12·H2O
Ow–H⋯Br
Ow–H⋯Br
C4–H⋯Br
C5–H⋯Br
C1–H⋯Br
C1–H⋯Br
C3–H⋯O
3.676
3.646
3.791
0.93
0.93
0.96
2.769
2.771
2.948
165.1
157.3
147.1
3.332
3.387
3.790
3.916
3.907
3.918
3.223
0.90
0.90
0.93
0.93
0.97
0.97
0.93
2.438
2.559
2.937
2.960
2.970
2.972
2.327
169.7
154.1
153.2
168.4
165.5
169.1
161.6
reason why C12C1C1 does not crystallize isostructurally with
the analogous 1-dodecyl-3-methyl-1,2,3-triazolium bromide.48
The most distinct difference is that the packing of the alkyl-
chains is missing the herringbone structure, but exhibit a
packing more similar to 1-dodecyl-3-methylimidazolium bro-
mide monohydrate.1,2 In addition, the steric hindrance
caused by the addition of the methyl group hinders the pack-
ing into the same arrangements as the triazolium analogue.
Both of these structural differences also help in explaining
the dissimilarities in their thermal properties. In fact, the
triazolium exhibits more interdigitation which promotes a
larger liquid crystalline window.
C12C1C0, C12C1C1 exhibits solely CH–π interactions (dCH–Cg
=
3.49, dCg–Cg = 4.47 Å, offset 3.05 Å). Hydrogen bonds are ob-
served between the bromide ion and hydrogen atoms of four
neighbouring methylimidazolium head groups as well as a
C–H⋯Br interaction at the terminal CH3 group of a dodecyl
chain. In contrast to C12C1C0 and perhaps due to the absence
of N–H⋯Br interactions, the bonds length distribution is
more regular, starting from 2.77 Å, and the role of hydrogen-
bonding in this structure is less significant. Non-classical hy-
drogen-bonds with still reasonable distances can be observed
solely with the methyl/methylene groups of the carbon
chains. Presumably, hydrogen-bonding could be the main
Though the attempts to obtain sufficient quality crystals
of the anhydrous C12C1C12 have so far been unsuccessful,
both anhydrous and monohydrate [C12C1C12im][Br]·H2O
(C12C1C12·H2O) compounds show similar layer distances
according to SAXS-measurements, vide infra. Therefore, we
believe the anhydrous compound would present similar char-
acteristics in its structure. C12C1C12·H2O crystallizes as
colourless thin needle-shaped crystals and in the triclinic
¯
space group P1 (No. 2) with two formula units in the unit
Fig. 5 Packing diagram of C12C1C0. Projection on the bc plane. Cg–Cg
contacts are shown by red and Br⋯H bonds by green dashed lines.
Fig. 6 Packing diagram of C12C1C1. Projection on the ab plane.
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Fig. 7 Packing diagram of C12C1C1. Projection on the ac plane. Br⋯H
bonds are shown by green dashed lines.
Fig. 9 Packing diagrams of C12C1C12·H2O. Projection on the bc plane.
Cg–Cg contacts are shown by red and Br⋯H bonds by green and O⋯H
by blue dashed lines.
cell. The asymmetric unit contains a [C12C1C12im]+ cation, a
bromide anion, and a water molecule (Fig. 4c). An exclusively
U-shaped conformation of the cations is observed in the crys-
tal packing. Both N-alkyl chains are approximately orthogonal
to the methylimidazolium plane and point in the same direc-
tion. The methylimidazolium cores are almost parallel to the
ab-plane. The U-shape can be defined by the angles formed
around the first C atom of the dodecyl chain, between the N
atom from the methylimidazolium ring and the terminal C
atom of the dodecyl chain, and are measured as 93.6° and
97.6° for the two dodecyl chains, respectively. Both dodecyl
chains adopt an all-trans conformation being almost
overlapped in the projection approximately on the ac plane. A
slight shift is observed only for the first three carbon posi-
tions that gradually decreases and becomes invisible for the
fourth one. Such a U-shaped conformation has been previ-
ously observed for benzimidazolium halides18 and
1,3-didodecylimidazolium with [N(CN)2]− and [C(CN)3]− as
counter ions.54
a bilayer structure. The dodecyl chains interdigitate in rows
and are aligned approximately with the crystallographic c-axis
(tilt 10.1° from the ab-plane normal). The interdigitated cat-
ion stacks form a bilayer with a thickness of 20.5 Å (Fig. 8),
which in turn form a lamellar structure. Similarly to C12C1C1,
complete separation of hydrophobic and hydrophilic regions
can be observed in the crystal packing. While the hydropho-
bic regions are formed by alkyl chains, the hydrophilic part is
now more populated and contains the imidazolium head
groups, the bromide anions and water. Cohesion inside the
crystal structure is ensured by incorporation of water mole-
cules, as neighbouring cations are held together by CH⋯Br,
CH⋯O and additional OH⋯Br interactions (Table 3). Inter-
estingly, as with C12C1C1, C12C1C12·H2O shows distinct differ-
ences between analogous 1,3-didodecyl-1,2,3-triazolium bro-
mide.48 Though all carbon chains in both compounds align
identically, no interdigitation has been observed in the latter,
while the orientation of the triazolium rings allows efficient
π–π interactions between and CH–π interactions within the
The cation head groups are held in a row connected side
by side through strong π–π interactions (dCg–Cg = 3.70 Å, off-
set 1.44 Å) and CH⋯Br hydrogen-bonds, while all the dodecyl
chains point in the same direction (Fig. 9). Two rows of cat-
ions stack in such a way that their dodecyl chains alternate
orientational direction in neighbouring layers, hence forming
Table 3 Layer distances d [Å] in the crystalline states and the liquid crys-
talline phases
Compound
dCr [Å]
dLC [Å]
d
C12C1C0
C12C1C1
C12C1C2
C12C1C3
C12C1C4
C12C1C5
C12C1C6
C12C1C7
C12C1C8
C12C1C9
C12C1C10
C12C1C11
C12C1C12
16.7
22.2
30.4
27.9
26.2
21.2
—
27.8
a
—
28.5
b
—
23.8
c
d
—
—
24.5
23.0
24.0
25.4
19.6
20.4
21.9
22.7
22.7
23.0
23.7
24.8
a
b
Mesophases coexist and could not be measured. The liquid
crystalline phase exists only in a very small temperature range, no
c
layer spacing could be detected. The solid phase exists only at low
temperature and recrystallization did not occur for this sample.
d
These compounds exhibit no mesophase. Cr = crystalline state; LC
Fig. 8 Packing diagram of C12C1C12·H2O. Projection on the ac plane.
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= liquid crystalline state.
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layers that in large extent can be attributed to a less intense
hydrogen bond network.
rapid change in the d value indicates a principal geomet-
ric change in the molecular conformation. This could be
a result of decreasing the angle of the varying alkyl chain
tilt with respect to the dodecyl chains. We believe
a
Mesophase investigations with POM and SAXS
V-shaped molecule could be formed in this way for
C12C1C5–C12C1C10, where the angle between the two alkyl
chains at both sides of the 2-methylimidazolium ring re-
mains roughly constant with rising n since the d values
in the crystalline state are almost unchanging. The slight
increase in the d values could be explained by the inser-
tion of methylene groups. The observation of blue-violet
domains in the POM images is also in agreement with
our supposition: a presence of a V-shaped structure.
The sudden decrease in layer distances in C12C1C10 and
C12C1C11 of approximately 5 Å can be considered to be a mo-
lecular conformational change from V-shaped to U-shaped.
Single crystal data shows that C12C1C12 arranges itself as
interdigitated U-shaped molecules in the solid-state. We
could imagine that such U-shaped conformation is already
adopted above n = 11. A crude representation of the three
shape types supposed for the 1-dodecyl-2-methyl-3-
alkylimidazolium cations is shown in Fig. 11.
C12C1C11 and C12C1C12 show an increased d value in the
mesophase (23.7 Å (C12C1C11) and 24.8 Å (C12C1C12) com-
pared to the crystal phase (19.6 Å (C12C1C11) and 20.4 Å
(C12C1C12)). The U-shaped molecules are able to move with
respect to one another in the direction of the interdigitated
alkyl chains during heating and cooling processes. A smectic
A phase can, therefore, be formed by simply decreasing the
interdigitated region.
Mesophase formation was observed for all the compounds ex-
cept for C12C1C4, C12C1C6 and C12C1C0. The corresponding
polarizing optical micrographs for all the liquid crystalline
compounds are shown in Fig. S89–S101, ESI.†
Focal conic fan textures, Schlieren textures and oily streak
textures can be observed. All of these are textures are charac-
teristic of smectic mesophases. For compounds C12C1C9 and
C12C1C10, a bluish domain texture without the polarising fil-
ter could be observed, changing to violet before melting (Fig.
S95 and S96†), which is considered to be indicative of bow-
shaped molecules. The layer spacing distances d of all com-
pounds in the crystalline states and the liquid crystalline
phases are summarized in Table 3.
Based on the crystalline structure of C12C1C1 as evidenced
by SXRD, the large layer spacing in the crystalline phase
could be explained by a model shown schematically in
Fig. 9 (left). The molecules form a herringbone-like bilayer ar-
rangement; the polar head groups line up along a row, and
the long-chain tails tilt in alternate directions. Upon heating,
the long alkyl chains presumably stretch parallel to the nor-
mal of the layer plane, resulting in an increased d spacing in
agreement with a smectic A phase (Fig. 10, right). According
to the SAXS data, smectic A phases also form for compounds
C12C1C3–C12C1C5.
For C12C1C7–C12C1C10, the layer spacings in the meso-
phases are shorter than the layer spacings in the crystals.
This indicates that the structure undergoes a transition to
smectic C phase, where the alkyl chains of cations are
tilted with respect to the layer normal. An increase in
interdigitation could explain the falling trend for the layer
spacing d with rising n. A decrease of 5 Å in layer spac-
ing could be observed between C12C1C4 and C12C1C5. The
Generally speaking, these findings correspond to the pre-
viously reported result on the non-methylated imidazolium
analogues.44 The triazolium analogues, on the other hand,
present another behaviour as even for longer chain length,
the layer spacing in the mesophase appears to be shorter
than in its crystalline form.48 For the shorter alkyl chains (C1
and C2), however, all the analogues present the same behav-
iour; the layer spacing in the liquid crystalline phase is much
larger than in the crystal form.
Fig. 10 Illustration of the changes in molecular arrangement of
Fig. 11 Illustration of the three shape types observed for C12C1Cn
C12C1C1 from the crystalline state to the mesophase: herringbone-like
cations: (near-linear structures are shown for n = 0–4, (left); a
structure in the crystalline phase (left) and lamellar smectic liquid crys-
talline phase (right).
V-shaped configuration might be formed for 5 ≤ n ≤ 10, (middle); a
U-shaped molecule might be supposed for n = 11, 12 (right)).
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Conclusions
Energimydigheten. We would like to thank Dr. Sergio Funari,
for support during the SAXS measurements.
In conclusion, we have prepared a series of 1-dodecyl-2-
methyl-3-alkylimidazolium bromides of which the thermal
behaviour has been investigated by DSC, POM and SAXS ex-
periments. The main focus was directed at elucidating their
thermal behaviour and structure. Furthermore, crystal struc-
tures of C12C1C1 and C12C1C12·H2O and C12C1C0 could be
obtained. All thirteen compounds could be considered ILs
since they exhibit a melting point below 100 °C. For C12C1C4,
C12C1C6 and C12C1C0 no thermotropic liquid crystalline be-
haviour is observed. The absence of mesophase formation for
these three compounds was anticipated according to findings
for the analogous 1-dodecyl-3-alkylimidazolium bromides. In
the previous study, this absence of liquid crystallinity was at-
tributed to the formation of a bilayer structure with an in-
crease in the complexity of interactions between the anion
and cation.44 One can assume a similar effect for the three
aforementioned compounds. Elimination of liquid crystallin-
ity after methylation at the C2 position has been reported for
imidazolium dodecylsulfonate by Mukai et al.45 For LC for-
mation, a balance between attractive van der Waals forces
and electrostatic repulsion is required.
In comparison with the 1-dodecyl-3-alkylimidazolium salts
reported in our previous report, after insertion of the methyl
group on the C2 position, an increase of both the melting
points and clearing points of all the compounds can be ob-
served in this series. This increase is consistent with the pre-
vious report and originates from the reduced entropy.11,23 In
addition, C2 methylation induces a decrease in liquid crystal-
linity for n ≤ 6, but above n ≥ 7 an increase in the meso-
phase temperature ranges can be observed and with signifi-
cantly higher clearing points.
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