Self-assembly of imidazolium-based rodlike ionic liquid crystals: transition from lamellar to micellar organization

Article abstract of DOI:10.1002/chem.200903210

By using aryl-amination chemistry, a series of rodlike 1-phenyl1H- imidazole-based liquid crystals (LCs) and related imidazolium-based ionic liquid crystals (ILCs) has been prepared. The number and length of the C-terminal chains (at the noncharged end of the rodlike core) and the length of the N-terminal chain (on the imidazolium unit in the ILCs) were modified and the influence of these structural parameters on the mode of self-assembly in LC phases was investigated by polarizing microscopy, differential scanning calorimetry, and X-ray diffraction. For the single-chain imidazole derivatives nematic phases (N) and bilayer SmA₂ phases were found, but upon increasing the number of alkyl chains the LC phases were lost. For the related imidazolium salts LC phases were preserved upon increasing the number and length of the C-terminal chains and in this series it leads to the phase sequence SmA-columnar (Col)-micellar cubic (Cuby1/Pm3n). Elongation of the N-terminal chain gives the reversed sequence. Short Nterminal chains prefer an end-to-end packing of the mesogens in which these chains are separated from the C-terminal chains. Elongation of the N-terminal chain leads to a mixing of N- and C-terminal chains, which is accompanied by complete intercalation of the aromatic cores. In the smectic phases this gives rise to a transition from bilayer (SmA₂) to monolayer smectic (SmA) phases. For the columnar and cubic phases the segregated end-to-end packing leads to core-shell aggregates. In this case, elongation of the N-terminal chains distorts core-shell formation and removes Cub₁ and Col phases in favor of single-layer SmA phases. Hence, by tailoring the length of the N-terminal chain, a crossover from taper-shaped to polycatenar LC tectons was achieved, which provides a powerful tool for control of self-assembly in ILCs.

Full text of DOI:10.1002/chem.200903210

DOI: 10.1002/chem.200903210
Self-Assembly of Imidazolium-Based Rodlike Ionic Liquid Crystals:
Transition from Lamellar to Micellar ACTHUNRTGNEUNGOrganization
Xiaohong Cheng,*[a] Xueqing Bai,[a] Shan Jing,[a] Helgard Ebert,[b] Marko Prehm,[b] and
Carsten Tschierske*[b]
Abstract: By using aryl-amination
chemistry, a series of rodlike 1-phenyl-
1H-imidazole-based liquid crystals
(LCs) and related imidazolium-based
ionic liquid crystals (ILCs) has been
prepared. The number and length of
the C-terminal chains (at the non-
charged end of the rodlike core) and
the length of the N-terminal chain (on
the imidazolium unit in the ILCs) were
modified and the influence of these
structural parameters on the mode of
self-assembly in LC phases was investi-
gated by polarizing microscopy, differ-
ential scanning calorimetry, and X-ray
diffraction. For the single-chain imida-
zole derivatives nematic phases (N)
and bilayer SmA2 phases were found,
but upon increasing the number of
alkyl chains the LC phases were lost.
For the related imidazolium salts LC
phases were preserved upon increasing
the number and length of the C-termi-
nal chains and in this series it leads to
the phase sequence SmA–columnar
(Col)–micellar cubic (CubI/Pm3n).
Elongation of the N-terminal chain
gives the reversed sequence. Short N-
terminal chains prefer an end-to-end
packing of the mesogens in which these
chains are separated from the C-termi-
nal chains. Elongation of the N-termi-
nal chain leads to a mixing of N- and
C-terminal chains, which is accompa-
nied by complete intercalation of the
aromatic cores. In the smectic phases
this gives rise to a transition from bi-
layer (SmA2) to monolayer smectic
(SmA) phases. For the columnar and
cubic phases the segregated end-to-end
packing leads to core–shell aggregates.
In this case, elongation of the N-termi-
nal chains distorts core–shell formation
and removes CubI and Col phases in
favor of single-layer SmA phases.
Hence, by tailoring the length of the
N-terminal chain,
a crossover from
taper-shaped to polycatenar LC tectons
was achieved, which provides a power-
ful tool for control of self-assembly in
ILCs.
Keywords: cubic phases · imidazoli-
um salts · ionic liquids · liquid crys-
tals · self-assembly
Introduction
ticles or nanorods,[4] and as ion-conducting materials.[2,5]
Liquid crystals (LCs) combining order at the nanometer
scale with mobility have found widespread application, for
example, in display technology.[6,7] Combining the self-or-
ganization forces of liquid crystals with the unique solvent
properties and ion-conducting properties of imidazolium
ionic liquids yielded imidazolium-based ionic liquid crystals
(ILCs).[6,8,9] These LC materials lead to novel applications
as, for example, in direction-dependent ion-conducting ma-
terials[6] and have improved the light-to-electric energy con-
version efficiency of dye-sensitized solar cells.[10,11] Very re-
cently, mesomorphic imidazolium salts were also used as
new vectors for small interfering RNA (siRNA) transfec-
tion.[12] Hence, for the exploration of further applications
the study of the self-assembly behavior and the structure–
property relationships of imidazolium-based ILCs is essen-
tial.
Imidazolium-based ionic liquids (IL) have attracted consid-
erable attention as environmentally benign solvents for nu-
merous applications, as in various chemical reactions,[1] for
preparation of dye-sensitized solar cells,[2] and light-emitting
electrochemical cells,[3] for the stabilization of gold nanopar-
[a] Prof. Dr. X. Cheng, X. Bai, S. Jing
Key Laboratory of Medicinal Chemistry for Natural Resources
Yunnan University, Kunming, Yunnan 650091 (P.R. China)
Fax : (+86)871-5032905
E-mail: xhcheng@ynu.edu.cn
[b] Dipl.-Chem. H. Ebert, Dr. M. Prehm, Prof. Dr. C. Tschierske
Institute of Chemistry, Martin-Luther University Halle-Wittenberg
Kurt-Mothes Strasse 2, 06120 Halle/Saale (Germany)
Fax : (+49)345-55-27346
E-mail: carsten.tschierske@chemie.uni-halle.de
There are several distinct modes of self-assembly in
liquid-crystalline soft matter (see Figure 1). These are the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903210.
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FULL PAPER
tinct types of cubic phases (see Figure 1). The bicontinuous
cubic phases (CubV), appearing as intermediate phases at
the transition from lamellar to columnar organization, form
different types of continuous interwoven 3D networks of
channels. The other type, the so-called micellar cubic phases
(CubI) is based on the complex self-assembly of spheroidic
supramolecular aggregates. Hence, inducing a transition be-
tween anisotropic mesophases with distinct directionality of
conducting properties (Sm=2-d; Col=1-d) and isotropic
cubic phases that can be either insulators (CubI) or isotropic
conducting materials (CubV; provided that the ion-conduct-
ing path is inside the aggregates in all cases) is a useful tool
for tailoring the application properties of ionic liquid-crys-
talline materials (Figure 1).[14,15,18c] Though cubic phases are
well-known modes of self-assembly in lyotropic systems, in
thermotropic LCs these superstructures are relatively rare.
In particular, the thermotropic micellar cubic phase was
only recently discovered, at first for carbohydrate-based am-
phiphiles.[16] Although nowadays these mesophases have
been found in the phase sequences of several different types
of taper-shaped and dendritic amphiphilic molecules[17–24]
based on nanosegregation,[25] including the first examples of
taper-shaped imidazolium salts (see Scheme 1),[14,26] the
Figure 1. Mesophase morphologies observed in nanosegregated thermo-
tropic LC systems occurring with increasing interface curvature in the
self-assembled system and their inherent properties; 1D, 2D, and 3D indi-
cate the dimensionality of the crystallographic lattice; 0-d,1-d, 2-d, and 3-
d indicate the dimensions of connectivity of the aggregates, as for exam-
ple expressed by the directionality of conducting properties (3-d=con-
duction in all three directions;1-d, 2-d, in one and two directions, respec-
tively; 0-d=insulator, if the conducting path, shown in black, is inside
the aggregates[14,18c]); abbreviations Sm=smectic (lamellar) phase,
CubV =bicontinuous cubic phase, Colhex =hexagonal columnar phase, and
CubI =micellar (discontinuous, spheroidic) cubic phase.[16,18,25]
nematic phases, widely used nowadays in LC displays,
having only orientational order (not shown in Figure 1), the
smectic phases representing lamellar organizations (Sm), the
columnar phases formed by cylinders packed on an, in most
cases, hexagonal 2D lattice (Colhex), and two distinct types
of cubic phases (CubV and CubI). In contrast to the nematic,
lamellar, and columnar LC phases with direction-dependent
properties, such as electron-, hole-, or ion-conducting,[6,13] as
well as optical properties, the cubic phases have isotropic,
that is, direction-independent properties. There are two dis-
Abstract in Chinese:
Scheme 1. Previously reported examples of imidazolium salts forming mi-
cellar cubic phases.[14, 26]
design of materials forming these micellar cubic phases is
still a challenge. In particular, for the exploration of further
applications of imidazole-based ILCs, the study of the LC
self-assembly behavior dependent on the molecular struc-
ture of these ILCs is essential.
Presently, there are four classes of imidazole-based ILCs:
1) Imidazolium-based amphiphilic LCs with one or two long
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X. Cheng, C. Tschierske et al.
alkyl chains,[27–30] in some cases incorporating an additional
benzene ring in the lipophilic chain(s),[31] giving predomi-
nately thermotropic (in the absence of solvents) and lyo-
tropic (in the presence of solvents) smectic phases. 2) Taper-
shaped imidazolium salts with multiple alkyl chains provide
predominately columnar phases,[14,15b,c,32–34] but more recent-
ly, the first examples of bicontinuous cubic,[12] micellar
cubic,[14,26] and 3D tetragonal phases[26] were observed for
dendritic imidazolium compounds, such as I and II (see
Scheme 1). The LC properties of these imidazolium salts
originate mainly from their strongly amphiphilic character
and the space filling of the lipophilic chains. 3) The third
type is provided by rodlike[3,15a,35–40] and disclike meso-
gens,[41–45] which are connected with imidazolium moieties
by relatively long flexible spacers. In this case, mesogenity is
mainly based on the self-assembly of the anisometric meso-
genic unit (smectic for rodlike and columnar for disclike
units), which is then modified by the segregation of the imi-
dazolium unit.[15a,42,46] Because of the strong intermolecular
electrostatic interactions, smectic and in some cases also col-
umnar phases are stabilized and nematic mesophases are re-
moved for such systems.[9,28,47] 4) Rodlike or slightly bent
molecules in which the imidazolium group is an integral
part of the rigid core unit either in the center[48–50] or at the
periphery[51] represent the fourth and least investigated class
of imidazole-based ILCs. Whereas for noncharged 3-substi-
tuted 1H-imidazole amphiphiles the number of alkyl chains
attached to the imidazole-terminated rodlike core was modi-
fied and in this way columnar and micellar cubic mesophas-
es based on hydrogen-bonded aggregates were ob-
tained,[52,53] to our knowledge no such investigation has been
carried out for charged imidazolium-based rodlike meso-
gens. Only rodlike molecules (mostly 1-(4’-substituted 1,1’-
biphenyl-4-yl)-1H-imidazolium salts) with single chains at-
tached to both ends have been recently reported by Kouwer
and Swager with respect to the effects of chain length, chain
branching, and the nature of the counterion on the meso-
phase properties.[51] Due to this calamitic (rodlike) molecu-
lar structure, smectic phases (mostly SmA phases) were ex-
clusively observed that have either a bilayer or monolayer
structure.[51]
Scheme 2. Imidazole-based nonionic (1n/0–3n/0) and ionic (1n/m–3n/m)
rodlike, taper-shaped, and polycatenar mesogens under investigation and
the notations used for these compounds; the compound numbers 1–3 in-
dicate the number of C-terminal chains, m indicates the length of the N-
terminal chain (0 means the absence of a chain, that is, noncharged imi-
dazole-based mesogens) and the superscript number (n) gives the
number of C atoms in each of the C-terminal chains. The assignment of
the compounds as single-, double-, and triple chain is based on the
number of C-terminal chains, irrespective of whether there is an addition-
al N-terminal chain or not.
based on the number of C-terminal chains, irrespective of
whether there is an additional N-terminal chain or not.
Only nematic (N) and bilayer smectic phases (SmA2)
were observed for the single-chain nonionic molecules 1n/0,
and LC phases were lost by increasing the number of C-ter-
minal chains, but in the series of related imidazolium salts a
transition from SmA phases via hexagonal columnar phases
(Colhex) to micellar cubic phases (CubI) with a Pm3n lattice
was observed by increasing the number and the length of
the C-terminal chains. These are the first micellar cubic
phases formed by imidazolium-based ILCs with extended
rodlike aromatic cores. It was also found that the mode of
self-assembly in this class of ILC is mainly determined by
the length of the N-terminal chain. Very short chains prefer
an end-to-end packing of the mesogens in which the N-ter-
minal chains are segregated from the C-terminal chains.
Elongation of the N-terminal chain leads to a mixing of N-
and C-terminal chains that is accompanied by complete in-
tercalation of the aromatic cores. In the smectic phases this
gives rise to a transition from bilayer (SmA2) to monolayer
smectic (SmA) phases. For the columnar and cubic phases
the segregated end-to-end packing leads to core–shell aggre-
gates, composed of lipophilic cores that contain the N-termi-
nal chains, which are enclosed by shells of the ionic moieties
and the aromatic parts. These core–shell aggregates adopt a
long-range order in the lipophilic continuum of the fluid C-
terminal chains. Elongation of the N-terminal chains inhibits
core–shell formation and removes CubI and Col phases in
favor of single-layer SmA phases. Effectively, by tailoring
the length of the N-terminal chain, a crossover from taper-
Herein we present the first systematic study of the effects
of length and number of alkyl chains on the LC properties
of phenyl benzoate based rodlike imidazolium salts 1n/m–
3n/m[54] and their nonionic precursors, the 1-phenyl-1H-imi-
dazoles 1n/0–3n/0.[55] In all the LC imidazolium salts reported
herein bromide anions were used as counterions. In the no-
tation of the compounds (see Scheme 2) the compound
number indicates the number of alkyloxy chains at the ben-
zoate moiety (C-terminal chains); the superscript n indicates
the length of these C-terminal chains (even-numbered
chains with n=6–18); the number m (after the slash) identi-
fies the length of the alkyl chain attached to the imidazole
moiety (N-terminal chain, m=0 for noncharged imidazoles
and m=2–6, 8, 10, 12, 14, and 18 for the imidazolium salts).
In this report the assignment of the compounds to the series
of single-, double-, or triple-chain compounds is exclusively
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Imidazolium-Based Ionic Liquid Crystals
FULL PAPER
shaped to polycatenar LC tectons was achieved that allows
the control of ILC self-assembly.
microscopy (POM, Optiphot 2, Nikon, in conjunction with a
FP 82 HT heating stage, Mettler), differential scanning calo-
rimetry (DSC, DSC-7, Perkin–Elmer), and for selected com-
pounds, by X-ray diffraction (XRD) using surface-aligned
samples on a glass plate by means of a 2D detector (HI-
STAR, Siemens, beam direction parallel to the surface).
Single crystals of compounds 110/0 and 112/0 were obtained
by slow crystallization from CHCl3 and these were used for
crystal-structure determination on a diffractometer with a
CCD area detector (Smart Apex II, Bruker, MoKa, l=
0.71073 ꢁ). The complete conditions of the data collection
and structure refinement are given in Figures S23–S27 in the
Supporting Information.
Results and Discussion
Synthesis: The synthesis of the nonionic imidazoles 1n/0–3n/0
and the imidazolium salts 1n/m–3n/m is shown in Scheme 3
(for experimental details, see the Supporting Information).
Liquid-crystalline properties of the nonionic imidazole de-
rivatives: The transition temperatures of the nonionic imida-
zoles 1n/0–3n/0 are collected in Table 1. Single-chain com-
Table 1. Mesophases, transition temperatures (T in 8C), and transition
enthalpy values (DH in kJmolꢀ1) of the 4-(1H-imidazol-1-yl-phenyl) ben-
zoates 1n/0–3n/0.[a]
R1
R2
R3
Phase and T[DH]
16/0
17/0
18/0
110/0
112/0
118/0
H
H
H
H
H
H
OC6H13
OC7H15
OC8H17
OC10H21
OC12H25
OC18H37
H
H
H
H
H
H
H
H
Cr 120ACTHNGUTERNNU[G 33.6] (N 100ACHTUNGTRNE[NUNG 0.4]) Iso
Cr 107 SmA2 110 N 125 Iso[b]
Cr 105 SmA2 126 Iso[b]
Cr 100
N
ACHTUNGTRENNUNG[2.9] Iso
Cr 97
Cr 99
C
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
212/0 OC12H25 OC12H25
216/0 OC16H33 OC16H33
Cr1 89ACHTNUTRGENNUG[37.2] Cr2104ACHTUNGTERN[NUGN 29.6] Iso
312/0 OC12H25 OC12H25 OC12H25 Cr 63 Iso[b]
314/0 OC14H29 OC14H29 OC14H29 Cr 87 Iso[b]
Scheme 3. Synthesis of compounds 1n/m–3n/m. Reagents and conditions:
i) SOCl2, reflux, 5 h; ii) K2CO3, CuI, N-methyl-2-pyrrolidone, 24 h,
1508C; iii) 48% HBr, 18 h, 1308C; iv) NEt3, THF, RT, 48 h; v) 1-bromoal-
kane, toluene, reflux, 18 h.
316/0 OC16H33 OC16H33 OC16H33 Cr 79
ACTHNUTRGNEU[GN 107] Iso
[a] Transition temperatures and enthalpy changes (in square brackets)
were determined by DSC (peak temperature, first heating scan, 10 K
minꢀ1), values in parentheses indicate monotropic (metastable) phases.
Abbreviations: Cr=crystalline solid, N=nematic phase, SmA2 =smectic
A phase with intercalated bilayer structure,[60] Iso=isotropic liquid state;
for more details see Figures S1–S5 in the Supporting Information.
[b] Transition temperatures were determined by POM. [c] See ref. [51].
The key step in the synthesis was an Ullman-type cou-
pling[56] of 4-bromoanisole with imidazole, which gave 1-(4-
methoxyphenyl)-1H-imidazole (6).[51] Demethylation of 6
using 48% HBr[57] yielded 4-(1H-imidazol-1-yl)phenol (7),
which was acylated with the appropriate benzoyl chloride
(5), leading to the 1-(4-benzoyloxyphenyl)-1H-imidazoles
1n/0–3n/0. These compounds were quaternized with appropri-
ate n-alkyl bromides to yield the imidazolium-based ILCs
1n/m–3n/m. Purification of the products was done by crystal-
lization from methanol/chloroform (10:1) or by column
chromatography; the structure and purity of all compounds
were confirmed by 1H NMR spectroscopy and elemental
analysis (see Tables S3–S6 in the Supporting Information).
pounds 1n/0 exhibit thermotropic nematic phases for short-
chain compounds and smectic A phases for compounds with
longer chains, while double- and triple-chain derivatives 2n/0
and 3n/0 are only crystalline materials without LC phases.
Under POM, the nematic phases are characterized by their
typical marble or schlieren textures (Figure S1a in the Sup-
porting Information). The SmA phases were identified by
textures consisting of focal conics, fanlike textures, and
pseudoisotropic regions with oily streaks, depending on the
alignment conditions (Figure S1b in the Supporting Informa-
tion). The pseudoisotropic regions indicate an on average
orthogonal organization of the molecules with respect to the
layer planes.[58] The SmA phases of compounds 110/0 and
Investigations: The liquid-crystalline properties of the non-
ionic imidazoles 1n/0–3n/0 and the corresponding imidazoli-
um compounds 1n/m–3n/m were studied by polarizing optical
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X. Cheng, C. Tschierske et al.
112/0 were additionally confirmed by X-ray investigation of
the aligned samples (Figure 2 and Figures S2–S5 in the Sup-
porting Information). The diffraction pattern of 110/0, for ex-
ample, shows strong sharp fundamental layer reflections on
Single-crystal structures of the crystalline phases were ob-
tained for compounds 110/0 and 112/0. For both compounds, a
¯
triclinic symmetry was observed with space group P1 with
two molecules per unit cell.[61] Typical motifs of the organi-
Figure 2. a) X-ray diffraction pattern of an aligned sample of the SmA
phase of compound 110/0 at T=1108C; b) oily streaks texture of com-
pound 112/0 (crossed polarizers) at 1058C (dark areas represent homeo-
tropically aligned regions).
Figure 3. Models of two possible structures of SmA phases of compounds
1n/0: a) bilayer structure with parallel organization of the aromatic cores
inside the layers and antiparallel organization of the layers with interca-
lated C-terminal chains (SmA2)[60] as observed for compounds 1n/0;
b) monolayer SmA structure with antiparallel organization of the rodlike
cores inside the layers; this arrangement is only observed if sufficiently
long alkyl chains are attached to the N-terminal ends of the molecules.
the meridian along with diffuse outer scatterings with
maxima of the intensity on the equator. This confirms a
layer structure in which the long axes of the molecules are
on average perpendicular to the layers. The maxima of the
outer diffuse scatterings at d=0.45 nm correspond to the
average lateral distances between the molecules in the fluid
LC state. The layer spacings in the SmA phases of com-
pounds 110/0 and 112/0 are d=3.4 and 3.7 nm, respectively.
Compared with the molecular lengths in the most stretched
conformations (L=2.9 nm and 3.2 nm, respectively[59]), the
layer thickness is larger than the molecular length (d/L=
1.2, see Table 2), but significantly smaller than twice the
length, indicating bilayer structures with complete intercala-
tion of the alkyl chains (SmA2) as shown in Figure 3a.[60]
zation of the molecules in these crystal structures (see
Figure 4 and Figures S23–S27 in the Supporting Informa-
tion) are staggered end-to-end pairs of the aromatic cores,
which are arranged side-by-side in an antiparallel fashion in
Table 2. Comparison of X-ray data and molecular dimensions of the
SmA phases of imidazoles 1n/0 and the single-, double-, and triple-chain
imidazolium bromides 1n/m–3n/m.[a]
R1
R2
R3
m
d [nm]
L
d/L
(T [8C])
[nm]
110/0
112/0
112/3
112/4
112/12
112/18
212/4
212/12
312/8
H
H
H
H
H
H
OC10H21
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
OC12H25
H
H
H
H
H
H
H
H
–
–
3
4
12
18
4
12
8
3.4 (110)
3.7 (120)
5.4 (150)
4.1 (170)
3.7 (175)
4.1 (170)
4.4 (100)
3.6 (160)
3.8 (70)
2.9
3.2
3.6
3.7
4.7
5.5
3.7
4.7
4.2
1.2
1.2
1.5
1.1
0.8
0.8
1.2
0.8
0.9
OC12H25
OC12H25
OC12H25
OC12H25
[a] Abbreviations: d=layer spacing; L=molecular length measured be-
tween the ends of the terminal chains and assuming a most stretched con-
formation with all-trans conformation of the alkyl chains.[59] For more de-
tails see Figures S2–S12, S16 and Table S1 in the Supporting Information.
Figure 4. a) Unit cell of the single-crystal structure of 112/0; b) crystal
packing on the (bc) plane; for other details and the crystal structure of
110/0, see Figure S23–S27 in Supporting Information.
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Imidazolium-Based Ionic Liquid Crystals
FULL PAPER
Table 3. Mesophases, transition temperatures (T in 8C), and transition
enthalpy values (DH in kJmolꢀ1) of the single-chain imidazolium bro-
mides 1n/m.[a]
ribbons (Figure 4b) leading to an overall structure that can
be regarded as a broken lamellar structure with strongly
tilted molecules (Figure 4b). The end-to-end organization is
also observed for the liquid-crystalline SmA2 phases of com-
pounds 110/0 and 112/0, but in contrast to the crystal struc-
tures, in the LC phases there is a nontilted and parallel side-
by-side packing of the molecules inside the layers, leading to
the bilayer structure with intercallated alkyl chains (see Fig-
ure 3a). This difference between crystal and LC structure
might be due to the different properties of the alkyl chains
in these two states of matter. In the crystalline state the
alkyl chains are rigid and pack parallel to the aromatic rods,
leading to ribbons of antiparallel aromatics separated by rib-
bons of antiparallel alkyl chains to optimize space filling
and maximize attractive intermolecular interactions. In the
LC state the alkyl chains are molten and these flexible
chains become incompatible with the still rigid aromatics
(rigid–flexible incompatibility), which induces the segrega-
tion of these chains into infinite layers to minimize the aro-
matic–aliphatic interfaces. The antiparallel end-to-end pack-
ing is retained, but the side-to-side packing becomes parallel
to allow complete segregation of the flexible alkyl chains
and the rigid aromatics (see Figure 3a). As will be shown
later, if an N-terminal alkyl chain of sufficient length is in-
troduced, the antiparallel side-by-side packing can be re-
stored, leading to monolayer smectic phases as sketched in
Figure 3b. Hence, the crystal structures contribute to the un-
derstanding of the observed structural transitions in the LC
phases.
n
m
Phases and T[DH]
16/12
16/18
18/4
6
6
8
8
10
12
12
12
12
18
18
12
18
4
12
4
3
4
12
18
6
Cr 166
ACHTUNGTNER[NUNG 17.6] SmA >200 Iso (decomp)
Cr 172ACTHNUGRTNEUNG[23.1] SmA >200 Iso (decomp)
Cr 199 Iso[b]
18/12
110/4
112/3
112/4
112/12
112/18
118/6
118/12
Cr 190 SmA >200 Iso (decomp)[b]
Cr 162
Cr1 103
Cr1 89[5.4] Cr2 151
[23.5] SmA >200 Iso (decomp)
[12.6] Cr 149[12.6] SmA >200 Iso (decomp)
[22.4] SmA >200 Iso (decomp)
A
[0.3][c]) Iso
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
R
ACHTUNGTRENNUNG
Cr 169
Cr 112
Cr 165
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
12
Cr 167 SmA >200 Iso (decomp)[b]
[a] Transition temperature and enthalpy changes (in square brackets)
were determined by DSC (peak temperature, first heating scan, 10 K
minꢀ1; no transition enthalpies are given for clearing temperatures associ-
ated with decomposition). Abbreviations: SmA=SmA phase with mono-
layer structure, for the other abbreviations, see Table 1. [b] Transition
temperatures were determined by POM. [c] Transitions not resolved.
[d] At 858C, formation of a birefringent Schlieren texture is observed in
the homeotropically aligned regions of the SmA phase, most likely indi-
cating a transition to a SmC phase.
Imidazolium bromides with one C-terminal chain 1n/m: The
transition temperatures of the single-chain imidazolium bro-
mides 1n/m are collated in Table 3. These compounds have
two flexible chains, one at each end of the rodlike aromatic
core, and hence, these compounds can be regarded as rod-
like ILCs. Compound 18/4, which has a total of 12 carbon
atoms in the hydrocarbon chain, is a high-melting crystalline
solid. All the other single-chain compounds with a total
number of more than 12 carbon atoms in their chains form
SmA phases with focal conic textures and homeotropic
areas separated by oily streaks (see Figure 2b). Only com-
pound 110/4, the shortest homologue with LC properties, has
a very small range of a nematic phase above the SmA
phase. With the exception of this compound all the other
compounds have exclusively SmA phases with high clearing
temperatures, in most cases above 2008C. The clearing tem-
peratures for most of these compounds cannot be deter-
mined precisely, due to significant decomposition at these
high temperatures. The SmA-to-Iso transition temperatures
seems at first to decrease with increasing length of the N-
terminal chain from 112/3 to 112/4 and then, after going
through an apparent minimum, increase again to >2008C
for compounds 112/12–112/18 with long N-terminal chains
(see Table 3).
In summary, for the single-chain compounds 1n/0, upon
elongation of the terminal alkoxy chain, the mesophase type
changes from nematic to bilayer SmA2, accompanied by
almost constant melting points and a slight increase of the
clearing points, giving rise to slightly increasing mesophase
ranges due to the increase of van der Waals interactions and
nanosegregation upon increasing the alkyl chain length.
Mesophases were lost if the number of chains was increased;
all synthesized compounds 2n/0 and 3n/0 with two or three
alkyl chains show only crystalline phases (see Table 1).
Liquid-crystalline properties of the ionic imidazolium salts:
In contrast to the series of noncharged imidazoles 1n/0–3n/0,
in the series of the imidazolium salts 1n/m–3n/m most com-
pounds show LC phases and the variety of different phase
types is much broader, including monolayer and bilayer
smectic phases as well as columnar and cubic phases. Due to
the additional attractive electrostatic interactions provided
by the charged polar groups and also due to the stronger
segregation of these groups from the lipophilic molecular
parts (polar–apolar incompatibility) the transition tempera-
tures of these compounds, especially those with only one C-
terminal chain, are very high. Due to this mesophase stabi-
lizing effect most of the compounds with two or more C-ter-
minal chains also form stable LC phases. The transition tem-
peratures of the synthesized imidazolium bromides are col-
lated in Tables 3, 4, and 5.
The XRD patterns of the smectic phases are similar to
those observed for the noncharged imidazoles, characterized
by the fundamental layer reflection, in some cases associat-
ed with the second-order layer reflection, and a diffuse
wide-angle scattering (see Figures S6–S9 in the Supporting
Information). Unexpectedly, the layer spacing d strongly de-
creases with elongation of the N-terminal chain from d=
Chem. Eur. J. 2010, 16, 4588 – 4601
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4593

X. Cheng, C. Tschierske et al.
5.4 nm for m=3 to d=3.7 nm for m=12 and then slightly
increases again to d=4.1 nm for m=18 (see Table 2). The
layer spacing of compounds 112/3 and 112/4 with short N-ter-
minal chains is larger than the molecular length (d/L=1.5
and 1.1, respectively) but significantly smaller than twice the
length, which indicates a bilayer structure with significant in-
tercalation of the alkyl chains (SmA2, see Figure 5a), similar
resulting monolayer structure the parallel side-by-side pack-
ing of the aromatic units inside the layers is removed, which
seems to be favorable as it maximizes entropy and allows
direct dipole compensation between adjacent molecules
inside the layers (see Figure 5b). Moreover, in the monolay-
er structure the same antiparallel side-by-side packing of the
aromatics, as observed in the crystal structures of com-
pounds 110/0 and 112/0 (see Figure 4) is achieved. Hence, it
seems that monolayer formation is favored as soon as com-
patibility of the lengths of the N-terminal and the C-termi-
nal chains is achieved and this gives rise to extremely stable
smectic phases with high transition temperatures. The differ-
ence between L and d in the single-layer structure might be
a consequence of a relatively low order parameter of the ar-
omatics, which is reduced due to the presence of the distor-
tion caused by the bulky bromide counterions, by chain fold-
ing and by intercalation of the alkyl chains to achieve opti-
mal space filling (see Figure 5b).
Imidazolium bromides with two C-terminal chains 2n/m: In
the series of compounds 2n/m with two C-terminal alkyloxy
chains (see Table 4) SmA phases were exclusively found as
Table 4. Mesophases, transition temperatures (T in 8C), and transition
enthalpy values (DH in kJmolꢀ1) of the double-chain imidazolium bro-
mides 2n/m.[a]
n
m
Phases and T[DH]
Cr 51[24.8] SmA2 >225 Iso (decomp)
Cr 62 SmA 223 Iso[b]
Cr1 95[23.8] Cr2 106[14.4] SmA 186
Cr 74[40.4] SmA2 170[0.1] Iso
212/4
212/8
212/12
216/4
12
12
12
16
4
8
12
4
ACHTUNGTRENNUNG
Figure 5. CPK models[59] and schematic sketches showing a) the organiza-
tion of 112/3 in a double-layer SmA2 phase and b) the organization of
compound 112/12 in a monolayer SmA phase.
G
E
ACHTUNGTRENNUNG[1.5] Iso
A
ACHTUNGTRENNUNG
[a] Transition temperature and enthalpy changes (in square brackets)
were determined by DSC (peak temperature, first heating scan, 10 K
minꢀ1). For abbreviations see Table 1. [b] Transition temperatures were
determined by POM.
to the SmA2 phases of the imidazoles 1n/0, shown in Fig-
ACHTUNGTRENNUNG
ure 3a.[60] The d/L ratio decreases with increasing length of
the N-terminal chain from 112/3 to 112/4 and for compound
112/12 with a long N-terminal chain the layer spacing is
smaller than the molecular length[59] (d/L=0.8), which indi-
cates a monolayer structure as shown in Figure 5b. Further
increase of the length of the N-terminal chain (compound
112/18) does not change the d/L ratio significantly. It is pro-
posed that in the bilayer SmA2 phases of the imidazolium
salts with short N-terminal chains the charged imidazolium
groups are separated by a thin lipophilic layer formed by
the N-terminal chains (see Figure 5a). Elongation of the
chains increases the thickness of these lipophilic layers,
which reduces the stability of the bilayer smectic phase,
probably due to the creation of high-energy interfaces be-
tween the highly polar surfaces of the imidazolium sides of
the aromatic sublayers and the nonpolar layers formed by
the N-terminal chains. The bilayer SmA2 phase is replaced
by a monolayer structure if the N-terminal chain becomes
sufficiently long to accommodate the C-terminal chains,
giving rise to a mixing of N- and C-terminal chains. In the
confirmed by the typical textural features as well as by X-
ray diffraction (see Figure 6 and Figures S10–S12 in the Sup-
porting Information). The SmA-to-Iso transition tempera-
tures are lower than those of related single-chain com-
pounds 1n/m and these temperatures decrease upon elonga-
tion of either the C-terminal or the N-terminal chain(s),
thus indicating a distortion of layer formation by chain elon-
gation, which is opposite to the trends seen for the single-
chain compounds 1n/m. This indicates that the increasing
space required by the additional chains distorts the packing
in the layers. Nevertheless, a similar dependence of the d
spacing on the length of the N-terminal chain as reported
for the single-chain compounds 1n/m was also observed in
the series of double chain compounds 2n/m, that is, com-
pound 212/4 forms an intercalated double layer SmA2 phase,
whereas the SmA phase of compound 212/12 with a longer
N-terminal chain is a single-layer SmA phase (see Table 2).
4594
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Chem. Eur. J. 2010, 16, 4588 – 4601

Imidazolium-Based Ionic Liquid Crystals
FULL PAPER
with a length of the N-terminal chain of m>4. The stability
of these SmA phases is much lower than those of the related
single-chain and double-chain imidazolium salts and this is
in line with the layer distortion that grows with increasing
number of chains. This is, for example, seen in the develop-
ment of the SmA–Iso transition temperatures as observed
with growing chain number in the series 112/12 (>2008C)–
212/12 (1868C)–312/14 (no SmA phase).
The stability of the SmA phase also decreases with in-
creasing length of the N-terminal chain (compare, for exam-
ple, compounds 316/5–316/8 in Table 5) and it slightly increas-
es with increasing length of the C-terminal chains (compare
312/8 and 316/8). The XRD pattern of 312/8 (Figure S16 in the
Supporting Information) provides a layer distance of d=
3.8 nm that corresponds to a monolayer organization with
only weak intercalation of the alkyl chains (d/L=0.9).
Beside the SmA phases in the series of compounds 3n/m
there are also columnar and cubic phases that can only be
found for molecules with shorter N-terminal chains.
Figure 6. X-ray diffraction pattern of an aligned sample of the SmA
phase of compound 212/4 at T=1008C (after subtraction of the diffraction
pattern of the isotropic phase; the layer reflections are perpendicular to
the equator as indicated by the dashed line; shading gives rise to a lower
intensity of the diffuse wide-angle scattering in the lower part).
The reasons for this structural change upon elongation of
the N-terminal chain should be essentially the same as dis-
cussed for the single-chain compounds in the previous sec-
tion.
Columnar phases: Columnar phases were found for com-
pounds 312/3, 312/4, 314/4, and 316/4, that is, for all compounds
with short N-terminal alkyl chains with m=4 and in the
series 312/m, also for m=3. Elongation of the N-terminal
chain leads to a dramatic reduction of the stability of the
columnar phases as seen by comparison of compounds 312/3
and 312/4, in which the Col-to-Iso transition is reduced by
more than 76 K (Table 5). The textures of these mesophases,
as observed by POM, are characterized by typical spherulitic
domains as shown in Figure 7a–c for the columnar phase of
compounds 312/4, 314/4, and 316/4 as examples.[62] The colum-
nar phases of these three compounds were also investigated
by XRD (see Figure 7d,e, and Figures S14, S15, S18, S21,
and S22 and Table S1 in the Supporting Information). Be-
Imidazolium bromides with three C-terminal chains 3n/m
Smectic phases: A series of different LC phases was ob-
served for compounds 3n/m with three C-terminal chains
(Table 5). SmA phases (confirmed by typical textures using
POM) are formed by compounds 312/8, 314/14, and 316/5–316/8
Table 5. Mesophases, transition temperatures (T in 8C), and transition
enthalpy values (DH in kJmolꢀ1) of the triple-chain imidazolium bro-
mides 3n/m.[a]
sides the diffuse wide-angle scattering three small-angle re-
1
=
2
flections with a ratio of their reciprocal spacings 1:3 :2 were
typically observed,[63] which can be indexed to the 10, 11,
and 20 reflections of a hexagonal lattice (see, for example,
Figure 7d). The 2D X-ray diffraction pattern of a surface-
aligned sample of compound 312/4 is shown as an example in
Figure 7e, confirming the hexagonal lattice (Colhex/p6mm).
The hexagonal lattice parameters (see Table 6) are around
5 nm for all of the investigated compounds. Although XRD
was not carried out on all of the columnar phases, based on
textural similarities, it is very likely that also the other co-
n
m
Phases and T[DH]
Cr1 92[49.9] Col >200 Iso (decomp)
Cr 111[72.2] Colhex 124[0.5] Iso
Cr 53[61.1] SmA 76[0.3] Iso
Cr1 47[26.0] Cr2 65[57.8] Iso
Cr 102[62.5] CubI/Pm3n 225 Iso (decomp)
[65.8] Col 178
[1.9] Iso/Iso 160 CubI <20 Cr[b,c,d]
[58.5] Colhex 120 Iso[b,c]
[58.8] (SmA 52[0.6]) Iso
[38.3] CubI/Pm3n 225 Iso (decomp)
312/3
312/4
312/8
312/14
314/2
314/3
314/4
314/14
316/2
316/3
316/4
316/5
316/6
316/8
12
12
12
12
14
14
14
14
16
16
16
16
16
16
3
4
8
14
2
3
4
14
2
3
ACHTUNGTRENNUNG
U
ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Cr 95
Cr 93
Cr 78
Cr 98
R
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Cr 106
Cr 109
C
ACHTUNGTRENN[UNG 0.3] Iso
AHCTUNGTRENNUNG
4
5
6
8
ACHTUNGTRENNUNG
ized in a slice of the columns with a height of h=0.45 nm
(maximum of the diffuse wide-angle scattering), estimated
using Equation (1) and assuming a density of 1=1 gcmꢀ3, is
between n=5 and n=6 for the investigated compounds
(NA =Avogadro constant, M=molecular mass; see Tables 6
and Table S2 in the Supporting Information). There is a
slight decrease of this number with increasing chain length,
which is usually observed and that is due to the increase of
the taper angle with the increasing space requirement of the
C-terminal chains.[18,21]
Cr 82
N
ACHTUNGTRENN[UNG 0.3] Iso
Cr1 51
N
N
ACHTUNGTRENN[UNG 0.3] Iso
[a] Transition temperature and enthalpy changes (in square brackets)
were determined by DSC (peak temperature, first heating scan, 10 K
minꢀ1). Abbreviations: Col=columnar phase, most probably Colhex
;
Colhex =hexagonal columnar phase; CubI =cubic mesophase, most proba-
bly CubI/Pm3n; CubI/Pm3n=micellar cubic mesophase with space group
Pm3n; for other abbreviations, see Table 1. [b] Transition temperatures
were determined by POM. [c] No peak could be found for this transition
in the DSC traces. [d] Phase transitions observed on cooling.
Chem. Eur. J. 2010, 16, 4588 – 4601
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4595

X. Cheng, C. Tschierske et al.
Table 6. Comparison of X-ray data and molecular dimensions of the col-
umnar and cubic phases of imidazolium bromides with three C-terminal
chains.[a]
n
m
a [nm] (T [8C])
L [nm]
n
312/4
314/4
316/4
314/2
316/2
316/3
12
14
16
14
16
16
4
4
4
2
2
3
Colhex/p6mm
Colhex/p6mm
Colhex/p6mm
CubI/Pm3n
CubI/Pm3n
CubI/Pm3n
4.9 (110)
5.1 (100)
5.0 (150)
10.4 (150)
10.8 (150)
10.8 (130)
3.6
4.0
4.2
3.7
4.0
4.1
6.0
5.9
5.1
85
87
84
[a] Abbreviations: a=lattice parameter determined by XRD (ahex and
acub, respectively); L=molecular length of a molecule measured between
the ends of the terminal chains and assuming a most stretched conforma-
tion with all-trans conformation of the alkyl chains;[59] n=number of mol-
ecules in the cross section of a column in the Colhex phases (with assumed
height of 0.45 nm)/respective number of molecules in each micelle of the
cubic Pm3n lattice (ncell/8); for more details see Table 7, and Figures S14,
S15, S17–S22 and Tables S1, S2 in the Supporting Information.
pffiffiffi
ꢀ
a2
ð1Þ
n ¼ ð
Þ
3hðNA=MÞ1
2
There are two distinct modes of self-assembly of taper-
shaped molecules; a rosettelike organization and an antipar-
allel arrangement in ribbons (Figure 8a, b). A rosettelike or-
ganization of the molecules (Figure 8a) leads to cylinders
with a circular cross section, which gives rise to columnar
LC phases with a hexagonal lattice (Colhex).[17,18b] In this ar-
rangement the N-terminal alkyl chains of compounds 3n/m
form a lipophilic core in the middle of the columns, which is
surrounded by a polar stratum of the ionic and aromatic
parts. These core–shell cylinders are assembled on a hexago-
nal lattice in the lipophilic continuum of the C-terminal
alkyl chains (see Figure 8d). This kind of organization
allows a complete segregation of the incompatible units
(polar/nonpolar groups as well as short/long alkyl chains),
each in its own domain and it requires an interdigitation of
the lipophilic C-terminal chains of adjacent columns to fill
the space remaining at the periphery (Figure 8e). The effec-
tive diameters of the columns (considering this interdigita-
tion) correspond to the experimentally observed hexagonal
lattice parameters ahex. This end-to-end organization of the
molecules with strong interdigitation of the C-terminal alkyl
chains is related to the SmA2 double-layer structure ob-
served for the smectic phases of the single- and double-
chain compounds with short N-terminal chains (see Fig-
AHCTUNGTREGuNNNU res 3a and 5a). However, in this columnar arrangement the
space available for the packing of the N-terminal chain
inside the column cores is strongly limited and enlarging
these chains strongly disfavors this type of organization.
This is thought to be the reason for the strong decrease of
the stability of the Col phases upon slight elongation of the
Figure 7. Colhex phases of the imidazolium bromides with three C-termi-
nal chains: Selected texture (crossed polarizers) of compound: a) 312/4 at
1008C, b) 314/4 at 1008C, c) 316/4 at 1368C; d) XRD pattern of 314/4
(small-angle region, inset shows wide-angle region) at T=1108C; e) X-
ray diffraction pattern of an aligned sample of compound 312/4 at T=
1108C (multidomain sample); for a color version of the textures, see Fig-
ure S13 in the Supporting Information.
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Imidazolium-Based Ionic Liquid Crystals
FULL PAPER
Figure 8b, more favorable for formation of SmA phases
with flat interfaces. The layer distance measured for the
SmA phase of compound 312/8 (d=3.8 nm) is in full agree-
ment with this model (Figure 8c). This organization in the
SmA phases is fundamentally the same as in the single-layer
SmA phases observed for the single- and double-chain com-
pounds with long N-terminal alkyl chains. These SmA
phases are destabilized upon elongation of the N-terminal
chains (compare compounds 316/5–316/8 in Table 5), as the
lipophilic regions expand, which is unfavorable for the
dense packing of the aromatic cores. In contrast, the meso-
phase stability of the SmA phases of single-chain com-
pounds increases upon chain elongation (see Tables 1 and
3), probably because in this case the packing density of the
alkyl chains is improved. In some respects, the change of the
molecular structure achieved by elongation of the N-termi-
nal chain provides an example of a change of the mode of
self-assembly upon transition from a taper-shaped to a poly-
catenar molecular structure of the mesogens.
Cubic phases: For the triple-chain imidazolium bromides 3n/
m with very short N-terminal chains (m=2, 3) and three rel-
atively long C-terminal chains (n=14, 16) optically isotropic
mesophases were observed, which are not fluid, but soft and
viscoelastic. By increasing the temperature, sharp transitions
to highly fluid isotropic liquids occur at defined tempera-
tures, which can also be detected as distinct peaks in the
DSC traces (Figure 9). The formation of the isotropic meso-
phases can be supercooled as typical for mesophases with
3D periodicity (see inset in Figure 9). These POM and DSC
observations are indicative of cubic mesophases.[64]
Figure 8. Models showing the organizations of the triple-chain molecules
in the LC phases: a) schematic sketches of a rosette, forming cylinders
with a circular cross section; b) an antiparallel organization of the mole-
cules leading to ribbonlike aggregates; CPK molecular models showing
the organization of the molecules c) in the SmA phase of compound 312/8
with complete intercalation of the aromatic cores; d,e) in the Colhex
phase of compound 312/3. (d) shows the cross section through a single
column, composed of a lipophilic core containing the N-terminal chains
in the middle, the polar shell mainly formed by the bromide ions and the
imidazolium units, and the lipophilic shell of the C-terminal alkyl chains
around it; it is clear that a dense packing in the periphery cannot be ach-
ieved in this arrangement and this requires the intercalation with the
chains of adjacent columns. (e) shows the same column with the alkyl
chains of adjacent columns filling the space in the lipophilic periphery by
intercalation; the regions inside the dotted circles/lines show the dimen-
sions provided by the experimentally observed d values (d=3.8 nm) and
hexagonal lattice parameters (ahex =4.9 nm), respectively; the organiza-
tion in the cross section of the spheroidic aggregates forming the CubI
phases is related to those shown in (d) and (e).
The isotropic mesophases of compounds 314/2, 316/2, and
316/3 were investigated by XRD. The diffraction patterns are
characterized by a diffuse scattering in the wide-angle
region, indicating the fluid LC state, and several reflections
in the small-angle region that could be indexed to a cubic
lattice with Pm3n space group (see Table 7, and Table S1
and Figures S17, S19, and S20 in the Supporting Informa-
tion). The Pm3n lattice is the most commonly observed lat-
N-terminal chains by only one CH2 group (312/3 vs. 312/4, see
Table 5).
Elongation of the N-terminal chains also enhances the in-
fluence of the rodlike shape of the aromatic core on the
self-assembly of compounds 3n/m, as it favors an antiparallel
packing of the molecules. This antiparallel packing is favora-
ble for the compensation of dipole moments and it reduces
the excluded volume between the rods. The complete inter-
digitation of the aromatic parts reduces the aromatic–lipo-
philic interface curvature and integrates the N-terminal
alkyl chains into the lipophilic regions formed by the C-ter-
minal chains (see Figure 8c). These two effects lead to a
transition from a rosettelike organization in the Colhex
phases to a ribbonlike antiparallel organization as shown in
Figure 9. DSC heating and cooling curves of compound 316/3
(10 Kminꢀ1); the inset shows an enlargement of the DSC peaks corre-
sponding to the Iso–Cub and Cub–Iso transition a and b, respectively, in-
dicating a significant supercooling of this phase transition.
Chem. Eur. J. 2010, 16, 4588 – 4601
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4597

X. Cheng, C. Tschierske et al.
Table 7. Crystallographic data of the Pm3n cubic phase of compound 316/
3 at T=1708C.[a]
similar to the models shown for the Colhex phases (Fig-
AHCTUNGTREGuNNNU re 8d,e and Figure 10). These core–shell spheroids develop
2qobsd [8]
dobsd [nm]
hkl
dcalcd [nm]
dobsdꢀdcalcd
0.09
0.00
ꢀ0.01
0.04
from the Colhex phases by increasing the apolar–polar inter-
face curvature of the core–shell cylinders that is achieved by
increasing the length of the C-terminal chains from n=12
(Colhex) to n=16 (CubI; compare compounds 312/3, 314/3,
and 316/3 in Table 5).[67–69]
p
1.615
1.837
2.015
3.041
3.270
3.720
5.47
4.81
4.38
2.91
2.70
2.38
200 ( 4)
p
5.38
4.81
4.39
2.87
2.69
2.41
2.35
210 ( 5)
p
220 ( 6)
p
321 ( 14)
p
400 ( 16)
p
0.01
It is interesting to note that in the series of compounds
316/m the columnar phase is limited to a single compound
(316/4). Reduction of the length of the N-terminal chain by
only one CH2 group leads to the micellar cubic phase (CubI/
Pm3n), whereas adding only one additional CH2 gives an
SmA phase. This is very surprising and unusual, as the tran-
sition between these three completely different types of
mesophases with very different interface curvatures usually
requires very distinct molecular structures.[18–22,24,25] This is
the first example of a series of three directly neighboring
homologues that shows a Sm–Col–CubI transition. This ob-
servation can be explained with the change of the molecular
shape from taper to a polycatenar structure (specifically a
tetracatenar structure with an unequal distribution of the
terminal chains).[70] If the N-terminal chains are short (m=
2–4), then mixing of these chains with the C-terminal chains,
though entropically favorable, seems to be energetically un-
favorable and hence these chain form their own domains.
The segregation of these short alkyl chains from the polar
groups leads to spheroidic, columnar, or lamellar aggregates,
depending on the curvature imposed on the polar regions by
the C-terminal chains at the periphery. However, for longer
N-terminal chains (m>4) there is not sufficient space in the
cores of the spheroidic and cylindrical aggregates, and the
mixing of N-terminal and C-terminal chains becomes the en-
ergetically and entropically favorable packing mode. It
favors the antiparallel side-by side packing of the rods dra-
matically, which reduces the interfacial curvature between
the aromatic/ionic regions and the lipophilic regions, driving
the transition from strongly curved spheroidic via columnar
to noncurved (lamellar) aggregates.
420 ( 20)
ꢀ0.03
p
421 ( 21)
0.03
[a] qobsd =experimental scattering angles; dobsd =experimental and dcalcd
calculated d spacing; hkl=assigned indices.
=
tice of micellar cubic phases (CubI) occurring in thermotrop-
ic systems formed by spheroidic aggregates with soft coro-
na.[20,65c] On the other hand, a Pm3n lattice has never been
reported for a bicontinuous cubic phase,[17,18,66] hence, it is
most likely that the Pm3n cubic phases of compounds 3n/m
represent micellar cubic phases composed of closed spheroi-
dic aggregates.[65] This assumption is in line with the position
of these cubic phases in the phase sequence SmA–Col–CubI,
as observed upon increasing the number and length of the
C-terminal chains. The calculated lattice parameter increas-
es from acub =10.4 nm for compound 314/2 to acub =10.8 nm
for compounds 316/2 and 316/3 with longer C-terminal chains.
In these cubic phases the molecules are organized in closed
spheroidic aggregates and there are 8 of these aggregates in
each unit cell that have, depending on their crystallographic
position,
a slightly different shape (see Figure 1 and
10).[18c,20,21,65] For the compounds under discussion each mi-
celle is built up of approximately 85 molecules (see Table 6,
and Table S2 in the Supporting Information). The molecular
structure of these cubic phases can be regarded as reverse
(type 2) micellar cubic phases, in which spheroids adopt
fixed positions on a long-range cubic Pm3n lattice in a con-
tinuum of the molten alkyl chains provided by the three C-
terminal chains (CubI2 phases). The spheroids should have a
core–shell structure with an aliphatic core formed by the N-
terminal chains in a stratum formed by the charged units,
Conclusion
Imidazolium-based ionic LCs constitute a relatively new
class of materials in which self-organization and nanoscale
segregation are imparted to ionic liquids. The anisometric
properties of these materials and the possibility to switch
between distinct modes of self-organization[14,30] offer appli-
cations of these materials in functional devices, such as in
photovoltaics, in fuel cells, batteries, as
a templating
medium, and as direction-dependent ion-conducting materi-
als.[15b]
Herein we report the first systematic structure–property
relationship study of rodlike imidazolium salts focusing on
the effects of the number and length of C- and N-terminal
chains on the self-assembly into lamellar, columnar, and
spheroidic supermolecular aggregates and the modes of the
transitions between them.
Figure 10. Development of the mesophase structure depending on the
length and number of the C-terminal chains and the length of the N-ter-
minal chains attached to the imidazolium-based rodlike aromatic core.
4598
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Chem. Eur. J. 2010, 16, 4588 – 4601

Imidazolium-Based Ionic Liquid Crystals
FULL PAPER
Using aryl-amination chemistry a series of new ILCs has
been prepared wherein the charged imidazolium moiety has
been incorporated in the rigid part of rodlike mesogenic
cores. By modification of the number and length of the C-
terminal chains and the length of the N-terminal chain the
molecular shape has been modified from rodlike to taper or
polycatenar. The strong electrostatic interactions between
the ionic parts lead to a significant mesophase stabilization.
For this reason formation of columnar and micellar cubic
phases by increasing the number and length of the C-termi-
nal chains was only achieved for the imidazolium salts,
whereas for related nonionic imidazoles these structural
changes lead to a complete loss of mesogenity. Surprisingly,
the length of the N-terminal chain has an unusual strong
effect on the self-assembly of the ionic imidazolium-based
LCs. Very short chains prefer an end-to-end packing of the
mesogens, leading to distinct regions in which the N-termi-
nal chains are organized separately from the C-terminal
chains. Elongation of the N-terminal chains allows the
mixing of these chains with the C-terminal chains, and this
requires a complete intercalation of the aromatic cores. For
the single- and double-chain compounds this intercalation
gives rise to a transition from bilayer (SmA2) to monolayer
smectic phases. For compounds with three C-terminal chains
the end-to-end packing of short N-terminal chains leads to
spheroidic and columnar aggregates. In these aggregates the
N-terminal alkyl chains form the cores that are enclosed by
shells of the polar ionic imidazolium groups and the bro-
mide counterions. These core–shell aggregates form periodic
arrays in the lipophilic continuum of the fluid C-terminal
chains, resulting in spheroidic cubic phases with Pm3n sym-
metry and hexagonal columnar phases with p6mm symme-
try. For these triple-chain compounds, upon elongation of
the N-terminal chain, chain mixing and intercalation causes
the loss of interfacial curvature and the micellar cubic and
columnar phases were replaced by single-layer smectic
phases. Due to the strong restriction of the space available
for the N-terminal chains in the core–shell aggregates only
slight elongation of these chains is sufficient to give dramat-
ic effects. In this way it was possible to achieve the phase se-
quence CubI2/Pm3n–Colhex2/p6mm–SmA for a series of three
directly neighboring homologues differing by only one addi-
tional single CH2 unit from each other. The change of the
segregation regime depending on the length of the N-termi-
nal chain is also responsible for the unexpected phase se-
quence CubI–Col–SmA observed upon elongation of the N-
terminal chain. This is opposite to the effect of enlarging the
number and length of the C-terminal chains, which gives the
(usually observed) sequence SmA–Col–CubI (Figure 10).
Overall, these investigations contribute to the understanding
and control of the self-assembly behavior of rodlike ionic
liquid crystals.
Acknowledgements
This work was supported by funds from the National Natural Science
Foundation of China (no. 20972133), the Ph.D. Programs Foundation of
the Education Ministry of China (no. 2007067300), and the Fonds der
Chemischen Industrie (Germany). We thank Prof. M. J. Xie and Dr. L.
Du for a helpful discussion.
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Chem. Eur. J. 2010, 16, 4588 – 4601

Imidazolium-Based Ionic Liquid Crystals
FULL PAPER
10
¯
1 /0: C26H32N2O3; triclinic; space group P1; Z=2; a=6.938(4), b=
12.758(8), c=13.766(9) ꢁ; a=85.627(9), b=81.205(9), g=
81.976(9)8; V=1190.5(13) ꢁ3; 1calcd =1.173 mgmꢀ3; absorption coef-
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2953–2961.
ficient=0.077 mmꢀ1
ApexII; q range for data collection: 1.50 to 28.408; 10354 reflections
and 5407 with (I>2s(I)); maximum residual electron density
0.15 eꢁꢀ3
C28H38N2O3; triclinic; space group P1; Z=2; a=6.988(5), b=
12.565(5), c=15.054(5) ꢁ; a=80.395(5)5, b=83.604(5), g=
81.783(5)8; V=1284.7(11) ꢁ3; 1calcd =1.160 mgmꢀ3; absorption coef-
ficient=0.075 mmꢀ1; T=293 K; data collection: Bruker Smart 1000;
q range for data collection: 1.66 to 28.488; 12341 reflections and
10493 with (I>2s(I)); maximum residual electron density
0.14 eꢁꢀ3; R1 =0.0822, Rw2 =0.19.
; T=298 K; data collection: Bruker Smart
;
R1 =0.078,
R
w2 =0.14; b) Crystal data for 112/0:
¯
[62] An additional texture is shown in Figure S13a in the Supporting In-
formation.
[63] For compound 316/4 only a single reflection was observed in the
powder XRD pattern (see Figure S22 in the Supporting Informa-
tion), but the 2D diffraction pattern of an aligned sample (see Fig-
ure S21 in the Supporting Information) confirms a hexagonal lattice.
[64] In the case of compound 314/3 the cubic mesophase is only formed
upon cooling from the isotropic liquid state, whereas upon heating
from the crystalline state a hexagonal columnar phase was observed.
Remarkably the Col–Iso transition is 18 K higher than the tempera-
ture at which the cubic phase appears on cooling.
[65] a) K. Fontell, K. K. Fox, E. Hansson, Mol. Cryst. Liq. Cryst. Lett.
Sect. 1985, 1, 9–17; b) K. Fontell, Colloid Polym. Sci. 1990, 268,
264–285; c) P. Ziherl, R. D. Kamien, J. Phys. Chem. B 2001, 105,
10147–10158; d) G. M. Grason, B. A. DiDonna, R. D. Kamien,
Phys. Rev. Lett. 2003, 91, 058304; e) J. Charvolin, J. F. Sadoc, J. Phys.
1988, 49, 521–526; f) R. Vargas, P. Mariani, A. Gulik, V. Luzzati, J.
Mol. Biol. 1992, 225, 137–145.
[66] S. T. Hyde in Handbook of Applied Surface and Colloid Chemistry
(Ed.: K. Holmberg), Wiley, New York, 2001, pp. 299.
Received: November 23, 2009
Published online: March 16, 2010
Chem. Eur. J. 2010, 16, 4588 – 4601
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4601