Effect of central linkages on mesophase behavior of imidazolium-based rod-like ionic liquid crystals

Article abstract of DOI:10.1039/c2sm06854k

Two series of phenylbenzylether and benzanilide based rod-like imidazolium bromides and their nonionic precursors, the 1-phenyl-1H-imidazoles have been synthesized and the influence of the number and length of the alkyl chain(s) and the structure of the linking group in the aromatic core (-CH₂O-, -COO-, -CONH-) on their mesophase self-assembly in ionic liquid crystalline phases were studied by POM, DSC and XRD. Upon decreasing the length of the N-terminal chain or by enlarging the number and length of the C-terminal chains, the sequence smectic (SmA)-hexagonal columnar (Col_hex)-micellar cubic (Cub_I/Pm3n) was found for the ether based imidazolium salts; while only SmA and Col_hex phases were observed for the related amides. The influence of the polarity of the central linkages, namely -CH ₂O- and -CONH-, on the mesophase structure and stability is discussed and compared with related -COO- connected ILC.

Full text of DOI:10.1039/c2sm06854k

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PAPER
Effect of central linkages on mesophase behavior of imidazolium-based
rod-like ionic liquid crystals†
a
a
a
a
b
Xiaohong Cheng, Fawu Su, Rong Huang, Hongfei Gao, Marko Prehm and Carsten Tschierske
b
*
*
Received 29th September 2011, Accepted 28th November 2011
DOI: 10.1039/c2sm06854k
Two series of phenylbenzylether and benzanilide based rod-like imidazolium bromides and their
nonionic precursors, the 1-phenyl-1H-imidazoles have been synthesized and the influence of the
number and length of the alkyl chain(s) and the structure of the linking group in the aromatic core
(–CH2O–, –COO–, –CONH–) on their mesophase self-assembly in ionic liquid crystalline phases were
studied by POM, DSC and XRD. Upon decreasing the length of the N-terminal chain or by enlarging
the number and length of the C-terminal chains, the sequence smectic (SmA)–hexagonal columnar
(Colhex)–micellar cubic (CubI/Pm3n) was found for the ether based imidazolium salts; while only SmA
and Colhex phases were observed for the related amides. The influence of the polarity of the central
linkages, namely –CH2O– and –CONH–, on the mesophase structure and stability is discussed and
compared with related –COO– connected ILC.
unit is connected with an alkoxylphenyl group through
a carboxylate linking group (Scheme 1). A mesophase sequence
SmA–Colhex–CubI/Pm3n was observed upon decreasing the
length of the N-terminal chain or by increasing the number and
length of the C-terminal chains (Scheme 2).33 The strong self-
assembly tendency of such rod-like imidazolium salts prompted
us to study the influence of further variations of the molecular
structure.
1. Introduction
Ionic self-assembly of soft matter provides interesting possibili-
ties for producing well defined nanometre sized structures.1 Such
soft ionic materials can be used as adaptive ion conducting
materials for batteries, fuel cells and capacitors.2–4 Also in living
systems self-assembly of ionic soft matter is of significant
importance, for example, in DNA self assembly,5 for the inter-
actions of DNA with peptides6 and with anionic surfactants for
gene delivery.7,8 Imidazolium based ionic liquid crystals
(ILCs)9,10 combine the self-organization forces of liquid crystals
with the unique solvent properties and ion-conducting properties
of imidazolium based ionic liquids.11 These LC materials lead to
new applications, as for example in direction dependent ion-
conducting materials,12 and have improved the light-to-electric
energy conversion efficiency of dye sensitized solar cells.13,14
Recently, mesomorphic imidazolium salts were also used as new
vectors for si-RNA transfection.15 Hence, for the further explo-
ration of new applications and optimization of application
relevant properties, the study of the self-assembly behavior, as
well as the fundamental structure–property relations of ILCs16–32
is required.
In this contribution, the –COO– linkage in the phenyl-
benzoates C1n/m–C3n/m was replaced by a less polar –CH2O–
linkage and a more polar amide linkage (–CONH–), respectively,
leading to two new series of rod-like and taper-shaped imida-
zolium salts, the ethers E1n/m–E3n/m and the amides A1n/m–A3n/m.
In this notation the letters A, C and E specify the type of
Recently we have systematically studied the structure–prop-
erty relation of a series of phenylbenzoate based rod-like imi-
dazolium salts C1n/m–C3n/m in which the 1-phenyl-1H-imidazol
aChemistry Department, Yunnan University, Kunming, Yunnan, 650091,
P. R. China. E-mail: xhcheng@ynu.edu.cn; Fax: (+86) 871 5032905
bInstitute of Chemistry, Organic Chemistry, Martin-Luther University
Halle-Wittenberg, Kurt-Mothes Str. 2, 06120 Halle/Saale, Germany.
E-mail: carsten.tschierske@chemie.uni-halle.de; Fax: (+49) 345 55 27346
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2sm06854k
Scheme 1 Structures of reported rod-like imidazolium salts with phe-
nylbenzoate units (‘‘carboxylates’’) C1n/m–C3n/m,33 benzylphenylether
derivatives E1n/m–E3n/m (‘‘ethers’’) and benzanilide derivatives
A1n/m–A3n/m (‘‘amides’’) under investigation.
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Scheme 2 Development of the mesophase structure depending on the
length and number of the C-terminal chains and the length of the N-
terminal chains attached to the imidazolium based rod-like core as
observed for the phenylbenzoate based imidazolium salts C1n/m–C3n/m.33
Scheme
3
Synthesis of the imidazolium bromides; Reagents and
conditions: (i) CnH2n+1Br, K2CO3, DMF, 90 ꢀC, overnight; (ii) LiAlH4,
THF, 50 ꢀC, 4 h; (iii) SOCl2, DMF, THF, 5 h; (iv) KOH, EtOH, reflux; (v)
SOCl2, DMF, THF, reflux, 5 h; (vi) K2CO3, CuI, DMF, N2, reflux, 24 h;
(vii) K2CO3, DMF, 90 ꢀC, overnight; (viii) NaH, DMF, 90 ꢀC, overnight;
(ix) CmH2m+1Br, toluene, reflux, 16–48 h.
linking group (A ¼ amide ¼ –CONH–, C ¼ carboxylate ¼
–COO– and E ¼ ether ¼ –CH2O–), the next number indicates
the number of alkyloxy chains attached to the nonionic end of
the molecule (‘‘C-terminal chains’’), the following superscript n
defines the length of these chains, and the last number m
identifies the number of carbon atoms in the N-terminal
hydrocarbon chain attached to the imidazolium salt structure.
Here ‘‘0’’ means that there is no alkyl chain, i.e. these
compounds represent the nonionic precursors (1-phenyl-1H-
imidazoles) from which the imidazolium salts were prepared
by quaternization (Scheme 1). Upon decreasing the length of
the N-terminal chain or increasing the number and length of
the C-terminal chains, the sequence SmA–Colhex–CubI/Pm3n
was found for the ether based imidazolium salts, similar to
that found for the previously reported esters,33 while only
SmA and Colhex phases were detected for the related amides
(Scheme 4). Compared with the corresponding carboxylates C,
the ethers E generally show lower phase transition tempera-
tures and reduced mesophase stability and in most cases also
reduced melting points. Both effects can be explained by
the enhanced flexibility of the ether linkage. Surprisingly, the
effect of the amide group is very distinct in the series of the
non-charged 1H-Imidazoles and the corresponding imidazo-
lium salts. Whereas the mesophases of the 1H-imidazoles are
stabilized due to the enhanced polarity of the amide group,
the mesophases of the imidazolium salts are destabilized
and the interface curvature is reduced, i.e. micellar cubic and
hexagonal columnar phases are replaced by columnar and
smectic phases, respectively. It is suggested that the interaction
between the amide groups and the imidazolium moieties
modifies the molecular packing.
phenol 6 was etherified with appropriate alkoxysubstituted
benzyl chlorides 4, leading to the 1-(4-benzyloxyphenyl)-1H-
imidazoles E1n/0–E3n/0. The amine
7 was acylated with
appropriate alkoxysubstituted benzoyl chlorides 5, leading to
the 1-(4-benzamidophenyl)-1H-imidazoles A1n/0–A3n/0. These
compounds were quaternized with appropriate n-alkyl bromides
yielding the imidazolium based ILCs. Purification of the
products was done by crystallization from methanol/chloroform
(10/1) or by column chromatography; the structure and purity of
2. Results and discussion
2.1. Synthesis
The synthesis of the compounds under discussion is shown in
Scheme 3 (for experimental details, see the ESI†). The key step in
the synthesis was an Ullman-type coupling34 of 4-bromophenol
or 4-bromoaniline with imidazole which gave 4-(1H-imidazol-1-
yl)phenol 6 or 4-(1H-Imidazol-1-yl)aniline 7, respectively. The
Scheme 4 Development of the mesophase structure depending on the
length and number of the C-terminal chains and the length of the N-
terminal chains attached to the imidazolium based rod-like aromatic core
and the structure of the linking group X.
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all compounds were confirmed by 1H NMR and elemental
analysis (see Table S3–S6, ESI†).
mesophase (M, see Table 2 and Fig. 1b) which is most likely
a columnar phase. The amide group increases the polarity of the
rod-like core and introduces the possibility of intermolecular
hydrogen bonding, which both should be responsible for the
formation of the stable SmA phases of the single chain nonionic
amide derivatives and the occurrence of LC behavior (M) in one
of the three-chain amides. However, due to crystallization this
mesophase M could not be investigated in more detail.
2.2. Investigations
The liquid crystalline properties of the synthesized compounds
were studied by polarizing optical microscopy (POM, Optiphot
2, Nikon, in conjunction with a FP 82 HT heating stage, Met-
tler), differential scanning calorimetry (DSC, DSC-7, Perkin
Elmer). The X-ray diffraction patterns of aligned or partially
aligned samples were recorded with a 2D detector (HI-STAR,
Siemens). Ni filtered and pin hole collimated Cu Ka radiation
was used. The exposure time was normally 60 min. The sample to
detector distance was 8.8 cm and 26.9 cm for the wide angle and
small angle measurements, respectively. Alignment was achieved
upon slow cooling (rate: 1 K minꢁ1–0.1 K minꢁ1) of a small
droplet of the sample on a glass plate and takes place at the
sample–glass or at the sample–air interface, with domains fiber-
like disordered around an axis perpendicular to the interface.
The aligned samples were held on a temperature-controlled
heating stage. The X-ray beam was parallel to the substrate.
The SmA phases were identified by polarizing microscopy by
their typical appearance consisting of focal-conics, fan-like
textures and pseudoisotropic regions with oily streaks, depend-
ing on the alignment conditions (see Fig. 1a, 2b). The pseudoi-
sotropic regions indicate an on average orthogonal organization
of the molecules with respect to the layer planes.35
The SmA phases of the shortest and longest LC amides A112/0
and A118/0 were additionally investigated by X-ray scattering
of aligned samples (Table 3, Table S1†). The diffraction
patterns show strong fundamental layer reflections on the
meridian along with diffuse outer scatterings with intensity
maxima on the equator (Fig. 2a, Fig. S3†). This confirms
layer structures in which the long axes of the molecules are on
average parallel to the layer normal. The maxima of the outer
diffuse scattering at d ¼ 0.46–0.47 nm correspond to the
average lateral distances between the molecules in the fluid LC
state. Compared with the molecular length in the most
stretched conformations (L ¼ 2.9 nm for A112/0 and L ¼ 3.9
nm for A118/036), the layer spacings (d ¼ 3.9 and 4.7 nm,
respectively) are larger than the molecular lengths (d/L ¼ 1.2–
1.3, see Table 3), but significantly smaller than twice the
lengths, indicating bilayer structures with complete interdigi-
tation of the alkyl chains (SmA2) as shown in Fig. 3.37
This is the same phase structure as previously reported for
related 1-phenyl-1H-imidazoles incorporating ester linkages
(C1n/0)33 and therefore it is likely that formation of this type of
interdigitated bilayer structure is a general feature of single chain
1-phenyl-1H-imidazoles, independent from the linking group.
2.3. Liquid crystalline properties of the nonionic imidazole
derivatives
The phase transition temperatures and enthalpies of the nonionic
1H-imidazoles with ether linking units (E1n/0–E3n/0) and amide
units (A1n/0–A3n/0) are collected in Tables 1 and 2, respectively.
Single chain ether derivatives exhibit monotropic smectic A
phases if the alkyl chain length is at least n ¼ 16 (E116/0 and E118/0),
whereas all two- and three-chain ether derivatives are not liquid
crystalline. For the amide derivative the required alkyl chain
length is reduced to n ¼ 12 and enantiotropic SmA mesophases
are observed (A112/0, A116/0 and A118/0). Although most of the
two- and three-chain amides are crystals, one of these amides
with three dodecyloxy chains (A312/0) exhibits a monotropic
Table 1 Mesophases, transition temperatures and transition enthalpy values of the 1-phenyl-1H-imidazoles E1n/0–E3n/0 with an ether linking groupa
Comp
R1
R2
R3
T/ꢀC [DH/kJ molꢁ1
]
fRC
E16/0
H
H
H
H
OC6H13
OC12H25
OC16H33
OC18H37
OC16H33
OC12H25
OC14H29
OC16H33
OC18H37
H
H
H
H
Cr 112 [32.2] Iso
Cr 126 Isob
0.34
0.49
0.56
0.59
0.72
0.75
0.78
0.80
0.82
E112/0
E116/0
E118/0
E216/0
E312/0
E314/0
E316/0
E318/0
Cr 111 [68.7] (SmA2 91 [8.5]c) Iso
Cr 109 [61.7] (SmA2 89 [3.1]c) Iso
Cr 92 [89.9] Iso
OC16H33
OC12H25
OC14H29
OC16H33
OC18H37
H
OC12H25
OC14H29
OC16H33
OC18H37
Cr 43 [36.1] Iso
Cr 59 [91.4] Iso
Cr 71 [109.8] Iso
Cr 80 [115.1] Iso
a
Transition temperatures and enthalpy changes (in square brackets) were determined by DSC (peak temperature, first heating scan, 10 ꢀC minꢁ1), values
in round parentheses indicate monotropic (metastable) phases, abbreviations: Cr ¼ crystalline solid, N ¼ nematic phase, SmA2 ¼ smectic A phase with
interdigitated double layer structure,37 Iso ¼ isotropic liquid state; fRC ¼ volume fraction of the C-terminal alkyl chains R1–R3 including the ether
oxygens. b Transition temperature was determined by POM. c Phase transitions observed on cooling.
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Table 2 Mesophases, transition temperatures, and transition enthalpy values of the 1-phenyl-1H-imidazoles A1n/0–A3n/0 with amide linking groupsa
Comp
R1
R2
R3
T/ꢀC [DH/kJ molꢁ1
]
fRC
A16/0
H
H
H
H
OC6H13
OC12H25
OC16H33
OC18H37
OC16H33
OC6H13
OC10H21
OC12H25
OC14H29
OC18H37
H
H
H
H
Cr 148 [33.4] Iso
0.34
0.49
0.56
0.59
0.72
0.61
0.72
0.75
0.78
0.82
A112/0
A116/0
A118/0
A216/0
A36/0
Cr 132 [42.1] SmA2 169 [3.6] Iso
Cr 129 [40.3] SmA2 183 [3.8] Iso
Cr 127 [49.5] SmA2 184 [4.4] Iso
Cr 136 [51.3] Iso
OC16H33
OC6H13
OC10H21
OC12H25
OC14H29
OC18H37
H
OC6H13
OC10H21
OC12H25
OC14H29
OC18H37
Cr 62 [19.2] Iso
Cr 72 [55.6] Iso
A310/0
A312/0
A314/0
A318/0
Cr 58 [62.9] (M 24 [0.5]b) Iso
Cr 67 [79.2] Iso
Cr 86 [100.5] Iso
a
Transition temperatures and enthalpy changes (in square brackets) were determined by DSC (peak temperature, first heating scan, 10 ꢀC minꢁ1), values
in round parentheses indicate monotropic (metastable) phases, abbreviations: M ¼ unknown mesophase most probably a columnar phase according to
b
textural observations (Fig. 1b); for other abbreviations, see Table 1. Phase transition observed on cooling.
Table 3 Comparison of the XRD data of the SmA phases and the
molecular dimensions of the single chain 1-phenyl-lH-imidazoles
A1n/0 with those of the imidazolium bromides E112/ma
Comp
R1
R2
R1
m
d/nm (T/ꢀC)
L/nm
d/L
A112/0
A118/0
E112/4
E112/12
H
H
H
H
OC12H25
OC18H37
OC12H25
OC12H25
H
H
H
H
—
—
4
3.9 (165)
4.7 (140)
5.9 (80)
2.9
3.9
3.4
4.6
1.3
1.2
1.7
0.9
12
4.0 (100)
a
Abbreviations: d ¼ layer spacing; L ¼ molecular length measured
between the ends of the terminal chains and assuming a most stretched
conformation with all-trans conformation of the alkyl chains;36 for
details see Fig. 5, S3,S4 and Table S1.
Fig. 1 Representative textures as seen between crossed polarizers: (a)
SmA2 phase of compound A116/0 at T ¼ 182 ꢀC and (b) M phase of
compound A312/0 at T ¼ 24 ꢀC.
Fig. 2 Compound A112/0: (a) X-ray diffraction pattern of an aligned
sample of the SmA2 phase at T ¼ 165 ꢀC; (b) oily streaks texture (between
crossed polarizers) at the same temperature (dark areas represent
homeotropically aligned regions).
For the mesophase stability the order is ether < ester < amide,
which is in line with the increasing polarity of the linking groups.
2.4. Liquid crystalline properties of the ionic imidazolium salts
In contrast to the series of non-charged imidazoles E1n/0–E3n/0,
in the series of the ether based imidazolium bromides E1n/m–E3n/m
Fig. 3 Model of the structure of the SmA phases38 of compounds
A1n/0 (SmA2).33,37
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a variety of different LC phases including smectic phases,
columnar and cubic phases is found. The amide based imidazo-
lium bromides A1n/m–A3n/m have much higher melting points
and only smectic and columnar mesophases could be observed.
2.4.1 Imidazolium bromides with a single C-terminal chain.
The transition temperatures of the synthesized single chain imi-
dazolium bromides incorporating amide and ether linking units
are collected in Table 4, together with related compounds
incorporating ester linkages.33 All these compounds can be
regarded as typical rod-like ionic liquid crystals (ILCs) because
they all have two flexible chains, one at each end of the rod-like
aromatic core. The melting temperatures strongly increase in the
order ether < ester < amide where the difference in melting
temperature between each class of compounds is about nearly
100 K. The amide derivatives A1n/m are all crystals with very high
melting points and no LC phases can be observed for these
compounds.
Fig. 4 Representative textures of SmA phases as seen between crossed
polarizers: (a) for E112/4 at T ¼ 170 ꢀC (SmA2) with predominately
homeotropic alignment (dark regions) and oily streaks, and (b) for E118/6
at T ¼ 185 ꢀC (SmA) with predominately homogeneous alignment (fan-
like texture).
In contrast, broad regions of SmA phases (for representative
textures see Fig. 4 and Fig. S1a, ESI†) were found for the ethers
E1n/m. Their stability increases with growing total alkyl chain
length (m + n). Though the stability of the SmA phases is reduced
by replacing the –COO– group by the –CH2O– group, the
reduction of the melting point is much larger. This makes the
ethers E1n/m, having melting points between 43 and 87 C and
ꢀ
Fig. 5 (a) XRD pattern of a partly aligned sample of the SmA phase of
compound E112/12 at T ¼ 100 ꢀC; (b) 2q-scan over this diffraction
pattern, and (c) enlarged wide angle region (the lower part of the XRD
pattern is shaded by the heating stage).
mesophase ranges of up to 160 K, the most useful smectic ILC in
this series.
The layer spacing of compound E112/4 with a short N-terminal
chain (d ¼ 5.9 nm, as determined by XRD) is larger than the
molecular length (L ¼ 3.4 nm, d/L ¼ 1.7) but smaller than twice
the length (Table 3), indicating a bilayer structure with
interdigitation of the short N-alkyl chains and the long C-alkyl
chains in distinct, separate layers (SmA2 phase, Fig. 6a).37 In this
SmA2 phase there is a parallel-side-by-side packing of the
aromatic cores, allowing the segregation of the polar imidazo-
lium units from the less polar alkoxy-substituted benzene rings at
the opposite end.
The layer spacing (d ¼ 4.0 nm) of compound E112/12 with two
long terminal chains is a bit smaller than the molecular length36
(L ¼ 4.6 nm, d/L ¼ 0.9, see Table 3), indicating a monolayer
structure as shown in Fig. 6b, where the N-terminal and C-
terminal chain are mixed in common layers. In the resulting
monolayer structure there is an on average antiparallel side-by-
side packing of the aromatic units which is favored for entropic
reasons. The difference between L and d in the single layer
structure might be a consequence of a relatively low order
parameter of the aromatics and alkyl chains due to the presence
of a distortion caused by the bulky bromide counter ions.
Table 4 Mesophases, transition temperatures, and transition enthalpy
values of the single chain imidazolium bromides E1n/m and A1n/m
incorporating ether and amide linkages in comparison with the related
imidazolium bromides C1n/m33 with ester linking unitsa
Compd
X
n
m
T/ꢀC [DH/kJ molꢁ1
]
E112/3
E112/4
E112/12
E114/12
E116/3
E118/6
C112/4c
C112/12c
C118/6c
A112/4
A116/4
CH2O
CH2O
CH2O
CH2O
CH2O
CH2O
COO
COO
COO
CONH
CONH
12
12
12
14
16
18
12
12
18
12
16
3
Cr 43 [15.2] SmA2 139 [1.9] Iso
Cr 81 [13.0] SmA2 174 Isob
4
12
12
3
6
4
12
6
4
Cr 87 [28.7] SmA1 212 Iso (dec.)
Cr 60 [32.7] SmA1 224 Iso (dec.)
Cr 52 [21.2] SmA2 213 [0.7] Iso
Cr 68 [35.2] SmA 216 Iso (dec.)
Cr 151 [15.1] SmA2 196 Iso (dec.)
Cr 169 [23.5] SmA1 > 200 Iso (dec.)
Cr 156 [22.4] SmA > 200 Iso (dec.)
Cr 246 [32.6] Iso (dec.)
2.4.2 Imidazolium bromides with two and three C-terminal
chains. Table 5 shows that for compounds with a short N-
terminal chain (m ¼ 3) by increasing the number of C-terminal
chains (C16) from one to three a transition from SmA via
a hexagonal columnar phase (Colhex) to a cubic phase (Cub)
takes place. As the volume fraction of the C-terminal hydro-
carbon chains (fRC) increases on going from E116/3 to E316/3 the
curvature of the aromatic–aliphatic interface increases and
therefore the cubic phase of E316/3 should represent a discontin-
uous (micellar) cubic phase (CubI).33,39 This is in line with the
XRD diffraction pattern obtained for the cubic phase of
4
Cr 242 [42.1] Iso (dec.)
a
Transition temperatures and enthalpy changes (in square brackets)
were determined by DSC (peak temperature, first heating scan, 10 K
minꢁ1; no transition enthalpies are given for clearing temperatures
associated with decomposition), abbreviations: SmA1 ¼ SmA phase
with monolayer structure; SmA ¼ SmA phase with unknown layer
b
periodicity; for the other abbreviations, see Table 1. Transition
temperature were determined by POM (Fig. 4a). c See ref. 33.
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from m ¼ 2 to m ¼ 5, corresponding to an increase of the volume
fraction of the N-terminal chains fRN from 0.04 to about 0.10 (see
Fig. 8), leads to a phase sequence CubI–Colhex–SmA, i.e. in this
way at first the cubic and then also the Colhex phase is removed.
In the same order also the mesophase stability is significantly
reduced, so that for some compounds with m ¼ 5, 6 the SmA
phases become monotropic or cannot be investigated due to
rapid crystallization. All ether based imidazolium salts have
lower transition temperatures than the ester based imidazolium
salts due to the increased molecular flexibility and reduced
polarity of the –CH2O– linkage.44 The effect on mesophase
stability is larger than on the melting points, leading to smaller
mesophase ranges for the ether based imidazolium salts. Never-
theless, the lower transition temperatures make them advanta-
geous over the previously reported esters C3n/m as the
mesomorphic temperature range is shifted to lower temperatures
and hence, thermal decomposition in the mesophase region is
avoided.
Fig. 6 Schematic sketches showing (a) the organization of single chain
imidazolium salts in an interdigitated double layer SmA2 phase (d > L)
and (b) in a monolayer SmA1 phase (d < L).33
Table 5 Effect of increasing the number of C-terminal alkyl chains on
the properties of ether based imidazolium saltsa
The effect of increasing the length of the C-terminal chains on
the mesophase morphology and mesophase stability is much
smaller; nevertheless Colhex phases can be replaced by CubI
phases upon elongation of the C-terminal chain (compare
compounds E314/3–Colh and E316/3–CubI in Fig. 7).
In a series of related amides A3n/m only the phase sequence
SmA–Colhex is observed upon elongation of the N-terminal
chain, but the CubI phases are missing for these compounds
(Table 6, bottom). Besides the more limited variety of different
mesophases, the melting points of the amide based imidazolium
salts are significantly enhanced compared to the related esters
and ethers and the stability of the mesophases is reduced
(compare E316/2 and A316/2 as examples).
Compd
R1
R2
R3
T/ꢀC [DH/kJ molꢁ1
]
fRC
E116/3
E216/3
E316/3
R
R
R
H
R
R
H
H
R
Cr 52 [21.2] SmA 213 [0.7] Iso
Cr 72 [41.0] Colhex 242 [5.2] Iso
Cr 89 [34.7] CubI 184 [0.7] Iso
0.48
0.65
0.74
a
R ¼ OC16H33
.
Hexagonal columnar phases. The textures of the columnar
mesophases, as observed by POM, are characterized by typical
spherulitic domains as shown in Fig. 9a,c. These columnar
phases are optically uniaxial as indicated by the completely dark
appearance of regions with homeotropic alignment of the
columns (columns perpendicular to the glass substrates) and they
are optically negative as indicated by investigation with a l-
retarder plate (Fig. 9b,d). This means that in the columns the
preferred direction of the intramolecular p-conjugation pathway
(i.e. the long axis of the rod-like aromatic cores) is on average
perpendicular to the column long axis.
compound E316/3, which can be indexed on a Pm3n lattice (see
below) and the Pm3n lattice is known to be the most favoured for
thermotropic micellar cubic phases.39–43 A similar phase sequence
SmA–Colhex–CubI is observed if fRC is related to the observed
phase structures for a broader range of molecules as shown in
Fig. 7 for compounds with m ¼ 3. In this series SmA phases were
found for fRC < 0.5, Colhex phases occur between fRC ¼ 0.6 and
0.72 and for fRC > 0.72 CubI phases were found.
In the series of ether based imidazolium salts E314/m with three
C-terminal chains only slight elongation of the N-terminal chain
Fig. 7 Plot of phase transition temperatures and phase types (indicated
by color) against the volume fraction of the C-terminal chains (fRC) for
compounds (;) ¼ E112/3, (A) ¼ E116/3, (stars) ¼ E216/3 (C) ¼ A312/3,
(:) ¼ E314/3, (-) ¼ E316/3, (hexagons) ¼ E318/3.
Fig. 8 Plot of phase transition temperatures and phase types (indicated
by color) against the volume fraction of the N-terminal chains (fRN) for
the series of compounds E114/m.
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Table 6 Mesophases, transition temperatures and transition enthalpy values of the three-chain imidazolium bromides E3n/m and A3n/ma
Comp
X
n
m
T/ꢀC [DH/kJ molꢁ1
]
fRC
E312/4
E312/5
E314/2
E314/3
E314/4
E314/5
E314/6
E316/2
E316/3
E316/4
E316/5
E318/3
E318/5
A312/2
A312/3
A316/2
A316/4
CH2O
CH2O
CH2O
CH2O
CH2O
CH2O
CH2O
CH2O
CH2O
CH2O
CH2O
CH2O
CH2O
CONH
CONH
CONH
CONH
12
12
14
14
14
14
14
16
16
16
16
18
18
12
12
16
16
4
5
2
3
4
5
6
2
3
4
5
3
5
2
3
2
4
Cr 91 [51.7] Colh 109 [0.56] Iso
Cr 98 [37.2] (SmA 84 [0.38]) Iso
0.68
0.66
0.73
0.72
0.71
0.70
0.68
0.75
0.74
0.73
0.72
0.76
0.74
0.70
0.68
0.75
0.73
Cr 43 [61.8] CubI 177b c Iso
,
Cr 68 [101] Colh 180 [0.2] Iso
Cr 94 [65.6] Colhex 133 [0.48] Iso
Cr 98 [33.2] (SmAb 57 [0.31]) Iso
Cr 96 [32.6] (SmAb 68 [0.27) Iso
Cr 96 [61.6] CubI 217b c Iso
,
Cr 89 [35.0] CubI 184 [0.66] Iso
Cr 100 [78.7] CubI 100d Colh 148 [0.33] Iso
Cr 97 [32.7] Iso
Cr 94 [67.2] CubI 189 [1.9] Iso
Cr 92 [20.5] Iso
Cr 77 [16.6] Colh 183 [0.18] Iso
Cr 93 [31.9] Colhex 122 [0.37] Iso
Cr 115 [62.3] Colh 189 [0.44] Iso
Cr 127 [28.0] (SmAb 92 [0.34]) Iso
a
Transition temperatures and enthalpy changes (in square brackets) were determined by DSC (peak temperature, first heating scan, 10 ꢀC minꢁ1),
abbreviations: Colhex ¼ hexagonal columnar phase as confirmed by XRD; Colh ¼ hexagonal columnar phase (assignment based only on optical
investigations, no XRD performed or only 01-reflection in non-aligned sample observed); CubI ¼ cubic mesophase, most probably CubI/Pm3n ¼
b
c
micellar cubic mesophase with space group Pm3n; for other abbreviations, see Table 1. Determined by POM. No peak could be found for this
d
transition in the DSC traces, probably due to slow transition and partial decomposition at the transition temperature. Phase transitions observed
on heating, on cooling Colhex 95 CubI.
hexagon, confirming a hexagonal columnar mesophase (Fig. 10
and S6). The hexagonal lattice parameter is ahex ¼ 6.1 nm for
E216/3 with only two C-terminal chains. For the compounds with
three chains it is around 5.0 nm for the ethers and between 4.3–
4.6 nm for the amides (see Table 7). Though not for all columnar
phases XRD has been carried out, based on textural similarities,
it is very likely that also the other columnar phases (indicated as
Colh in Table 6) represent Colhex phases. The number of mole-
cules organized in a slice of the columns with a height of h ¼ 0.45
nm (maximum of the diffuse wide angle scattering) were
Table 7 Comparison of XRD data and molecular dimensions of the
columnar and cubic phases of imidazolium bromides with three C-
terminal chainsa
Comp
n
m
a/nm (T/ꢀC)
n
E216/3
E312/4
E314/4
E316/3
E316/4
16
12
14
16
16
3
4
4
3
4
Colhex/p6mm
Colhex/p6mm
Colhex/p6mm
CubI/Pm3n
Colhex/p6mm
CubI/Pm3n
6.1 (100)
4.9 (100)
4.9 (120)
10.6 (120)
5.1 (120)
10.8 (100)
4.3 (140)
4.6 (140)
10.3
6.0
5.5
80
5.5
83
Fig. 9 Typical textures (crossed polarizers) of the Colhex phases of the
imidazolium bromides: (a) compound A316/2 at 180 ꢀC (dark area at the
rigt top is a homeotropically aligned region; (b) same region with l-
retarder plate and (c) E316/4 at 120 ꢀC; (d) same region with l-retarder
plate, the inset shows the indicatrix orientation in the l–compensator.
A312/2
A316/2
12
16
2
2
Colhex/p6mm
Colhex/p6mm
4.8
4.5
XRD investigations (see Table 7, S1 and S2) show two or three
small angle reflections with a ratio of their reciprocal spacing
1 : 31/2 or 1 : 31/2 : 2 which can be indexed to the 10, 11 (and 20)
reflections of a hexagonal lattice (Fig. 10, S5, S6). In some cases
only the 10 reflection can be observed which in the 2D diffraction
pattern of oriented samples is spitted into 6 spots on a regular
a
Abbreviations: a ¼ lattice parameter determined by XRD (ahex and
acub, respectively); n ¼ number of molecules 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).
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Fig. 10 X-Ray diffraction pattern of the Colhex phase of compound
E316/4 at T ¼ 120 ꢀC, (a) wide angle pattern and (b) small angle pattern
(the lower part is shaded by the heating stage).
estimated using eqn (1) and assuming a density of r ¼ 1 g cmꢁ3
(NA ¼ Avogadro constant; M ¼ molecular mass, see also Table 7
and S2). This number decreases from n ¼ 10 for the two-chain
ether E216/3 to n ¼ 5.5–6.0 for the investigated three-chain ethers
E3n/m and to n ¼ 4.5–4.8 for the corresponding three chain
amides A3n/m.
.
pffiffi
n ¼ ða2 2Þ 3hðNA=MÞr
(1)
Based on the results of optical investigations, XRD, the
development of phase type and lattice parameters depending on
the chain lengths and also considering the results obtained for the
related carboxylates C3n/m,33 the following model could be
deduced for the organization of the molecules in the columnar
phases. Accordingly, in the Colhex phases the short N-terminal
alkyl chains form a lipophilic core, which is surrounded by
a polar stratum of the ionic parts and aromatic units (Fig. 11).
These core–shell cylinders are assembled in the lipophilic
continuum of the C-terminal alkyl chains on a hexagonal lattice.
This kind of organization allows a complete segregation of the
incompatible units, each in its own domain, and it requires an
interdigitation of the C-terminal alkyl chains of molecules in
adjacent columns to completely fill the space in the lipophilic
periphery. The effective diameters of the columns (considering
this interdigitation) correspond to the experimentally observed
hexagonal lattice parameters ahex. The large decrease of the
number of molecules organized in the cross section of the
columns, observed with increasing number of C-terminal chains
is due to the increase of the taper angle with rising space
requirement of the C-terminal chains.
Fig. 11 Models showing schematically the molecular organization in the
Colhex phases: (a) cross section through a single column of the Colhex
phase of the two-chain compound E216/3, further improved space filling
in the middle would be obtained by more disorder, as shown in (b); (b)
cross section through a single column of the Colhex phase of the three-
chain compound E316/4; in both cases a dense packing in the periphery is
achieved by interdigitation with the chains of adjacent columns (as shown
in (a), but not in (b)); the yellow regions show the dimensions provided by
the experimentally observed hexagonal lattice parameters; brown spheres
represent the bromide counter ions; (c) shows the arrangement of the
columns on a hexagonal lattice.
optically isotropic mesophases were observed, which are not
fluid, but represent soft solids with viscoelastic response on shear
forces, as typical for cubic mesophases.45 The compounds E3n/2
and E3n/3 (with n ¼ 14, 16, 18) form the cubic phase in the whole
mesomorphic temperature range, whereas compound E316/4
represents a special case which forms the cubic phase below
a Colhex phase (Table 7, S1, S2 and Fig. 12).
The isotropic mesophases of compounds E316/3 and E316/4
were investigated by XRD. The diffraction patterns are charac-
terized by a diffuse scattering in the wide-angle region, indicating
the fluid LC state. The positions of the sharp spot-like reflections
in the small-angle region are in line with a cubic lattice with Pm3n
space group (see Table 7–9, S1, S2 and Fig. S7, S8†). The
calculated lattice parameter is acub ¼ 10.6 nm for E316/3 and
acub ¼ 10.8 nm for E316/4. The Pm3n lattice is the most
commonly observed lattice for micellar cubic phases (CubI)
occurring in systems formed by spheroidic aggregates with soft
corona.46,47 In this type of phase the molecules are organized in
closed spheroidic aggregates and there are 8 of these aggregates
In the proposed arrangements the space available for the
packing of the N-terminal alkyl chain at the apex of the taper
shaped molecules in the middle of the column cores is strongly
limited and only slight enlargement of these chains causes the
columns to burst. This is thought to be the reason for the strong
decrease of the stability of the Col phases upon slight elongation
of the N-terminal chains by only one CH2 group (compare, for
example, E314/3 and E314/4 in Table 6) and the transition to SmA
phases, or even the loss of LC phases upon further elongation of
this chain.
Cubic phases. For the triple chain ether-based imidazolium
bromides E3n/m with very short N-terminal chains (m ¼ 2, 3, 4)
and three relatively long C-terminal chains (n ¼ 14, 16, 18)
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Fig. 12 Texture of the CubI phase of compound E316/4 (dark areas) (a) growing from the hexagonal columnar mesophase (bright areas) on cooling at
T ¼ 95 ꢀC and (b) CubI–Colhex transition on heating at T ¼ 100 ꢀC; (c) shows a model of the arrangement of the spheroidic aggregates in the CubI phase
with Pm3n lattice (the interior of the spheroids is filled by the N-terminal alkyl chains, the continuum is formed by the C-terminal alkyl chains).
Table 9 Crystallographic data of the Pm3n cubic phase (a ¼ 10.8 nm) of
in each unit cell which have, depending on their crystallographic
position, a slightly different shape.39,41,47
E316/4 at 100 ꢀCa
For the investigated compounds each micelle is built up of
dobs
dcalc
ꢁ
ꢀ
approximately 80–83 molecules (see Table 7 and S2). Consid-
ering the molecular structure, these cubic phases can be regarded
as micellar cubic phases (CubI phases), where spheroids (short
column segments39c) incorporating the short N-terminal chains,
the imidazolium units, bromide ions and aromatic cores adopt
fixed positions on a long range cubic Pm3n lattice in a continuum
of the fluid and partly interdigitated C-terminal alkyl chains
(Fig. 12c). As already proposed for the ester based imidazolium
salts and confirmed by the experimental observations made here,
the spheroids should have a core–shell structure with aliphatic
cores formed by the short N-terminal chains in closed shells
formed by the charged units and the rod-like cores (Fig. 12c).
This packing is only possible if the N-terminal chains are very
short, so that they can be accommodated in the cores of the
spheroids and do not significantly disturb the packing of the
polar imidazolium salt units. Only slight elongation of the N-
terminal chains distorts this arrangement and the closed spher-
oidic micelles burst and are replaced at first by infinite columns
and finally by layers. Beside this steric effect, elongation of the
N-terminal chains also allows these chains to mix with the longer
C-terminal chains more easily. This favors an antiparallel side-
by-side packing of the molecules (as in the SmA1 phases of the
related compounds with only one C-terminal chain, see section
2.4.1) and in this way additionally increases the steric disturbance
caused by these chains. For this reason the phase sequence CubI–
Colhex–SmA is observed by elongation of the N-terminal chain
by only 2–3 CH2 groups (see Fig. 8), whereas the effect of the
length of the C-terminal chain on mesophase type is only
moderate (see Fig. 7).
qobs
/
dobs/nm
hkl
dcalc/nm
0.83
0.92
1.00
1.51
1.83
5.33
4.81
4.40
2.92
2.41
200 (O4)
210 (O5)
211(O6)
320 (O13)
420 (O20)
5.38
4.81
4.39
2.98
2.41
ꢁ0.05
0.00
0.01
ꢁ0.06
0.00
Compound E316/4 has a cubic phase which forms slowly on
cooling the Colhex phase to T ¼ 95 ꢀC (Fig. 12a) whereas the
phase transition of the Cub phase to the Colhex phase on heating
takes place at T ¼ 100 ꢀC (Fig. 12b). This hysteresis is typical for
cubic phases45b and the Pm3n space group was confirmed by
XRD (Table S1, Fig. S7, ESI†). Therefore, it is reasonable to
assume that this cubic phase is a micellar cubic phase, too,48 the
same as observed for the homologues with slightly shorter N-
terminal chains. However, formation of a micellar cubic phase
(CubI) at temperatures below a Colhex phase is a bit surprising, as
a bicontinuous cubic phase would be expected,39,45 but the
observed phase sequence could also be explained considering the
special core–shell structure of the spheroids. As described above,
only a slight increase of the N-terminal chain leads to the
bursting of the spheroids with formation of columns. Therefore,
thermal expansion of the N-terminal chain has a much stronger
effect on mesophase morphology than thermal expansion of the
C-terminal chains and this leads to a transition CubI–Colhex
upon rising temperature. In this way the usually observed phase
sequence Colhex–CubI on rising temperature,39 which results
from a stronger thermal expansion of the aliphatic periphery, is
reversed in this case. This means that reducing the temperature
has a similar effect as reducing the length of the N-terminal
chain.
Table 8 Crystallographic data of the Pm3n cubic phase (a ¼ 10.6 nm) of
E316/3 at 120 ꢀC.a
dobs
dcalc
ꢁ
Columnar phases of the amides A3n/m. The behavior of the
amides A3n/m is different from the previously discussed
compounds, as for the amide derivatives A3n/m with n ¼ 12, 16
and a short N-terminal chain (m ¼ 2, 3) only columnar phase and
no CubI phases were observed. This indicates that it is more
difficult to curve the polar/apolar interfaces of the amide
aggregates. In this context it is interesting to note, that for the
three-chain amides the number of molecules organized in a slice
of the columns with a height of h ¼ 0.45 nm is a bit smaller
ꢀ
qobs
/
dobs/nm
hkl
dcalc/nm
0.83
0.94
1.03
1.53
1.89
5.36
4.72
4.29
2.88
2.34
200 (O4)
210 (O5)
211 (O6)
320 (O13)
420 (O20)
5.28
4.72
4.31
2.92
2.36
0.08
0.00
ꢁ0.02
ꢁ0.04
0.02
a
qobs ¼ experimental scattering angles; dobs ¼ experimental and dclac
¼
calculated d spacing; hkl ¼ assigned indices.
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Fig. 13 CPK-models showing the antiparallel organization of four molecules A316/2 in a slice of a column in the Colhex phase: (a) with radial distri-
bution of the alkyl chains, requiring interdigitation of adjacent columns and (b) ribbon-like organization with nearly parallel alkyl chains, in this case
time and space averaging of the orientation of the ribbons leads to an on average hexagonal lattice; (c) shows the complete interdigitation of the polar
aromatic cores allowing polar interactions between the amide groups and the imidazolium units; (d) shows the arrangement of the column cores on
a hexagonal lattice.
(n ¼ 4.5–4.6) compared to n ¼ 5.3–6.7 for the ethers. This could
indicate that the benzanilide cores can adopt a more dense
packing, probably due to polar interactions between the amide
groups and the imidazolium unit of adjacent molecules. This
would favor an antiparallel and interdigitated organization of
the aromatic cores, as shown in Fig. 13 which reduces the
interface curvature between the polar aromatics and the apolar
alkyl chains. In addition, the ribbon-like structure of the column
cores with a dense antiparallel packing of the rod-like cores
makes the aromatic regions more rigid and a further curvature of
the polar/apolar interfaces becomes more difficult. Both effects,
the reduced interface curvature and the increased aggregate
rigidity, remove the CubI and Colhex phases of the ether E316/4
and replaces them by a SmA phase in the related amide A316/4.
The interdigitated antiparallel organization of the rod-like
moieties in the column cores requires fewer molecules to achieve
a compact packing in these cores and this leads to the experi-
the related non-ionic 1H-imidazoles and it might also be a result
of the antiparallel core packing. In this organization the N-
terminal chains can distort the packing of the molecules due to
the unfavorable mixing of these short chains with the long C-
terminal chains. Also in this respect the effect of exchanging
–CH2O–/–COO– linking groups by –CONH– is similar as
increasing the length of the N-terminal chain.
3. Conclusions
In summary, two new series of phenylbenzylether (X ¼ CH2O)
and benzanilide based (X ¼ CONH) rod-like imidazolium salts
and their nonionic precursors, the 1-phenyl-1H-imidazoles have
been synthesized and the influence of the number and length of
the alkyl chain(s) and the structure of the linking group in the
aromatic core on their mesophase self-assembly were studied.
Upon enlarging the number and length of the C-terminal chains,
the sequence SmA–Colhex–CubI/Pm3n was found for the ether
based and the previously reported ester based imidazolium salts
(Scheme 4). In this way they follow the general designing rules for
amphiphilic LC molecules where increasing the volume fraction
of the aliphatic chains at a given core structure leads to an
increasingly negative curvature of the aliphatic–aromatic inter-
face which changes the aggregate morphology from lamellar to
speroidic.39
mentally observed reduction of ahex
.
Overall, the amide groups seem to favor intercalation of the
aromatic cores and in this way replacing –CH2O– or –COO– by
–CONH– has a similar effect as increasing the length of the N-
terminal chains in the ether and ester based imidazolium salts.
Comparison of the compounds shown in Table 10 reveals that,
surprisingly, the mesophase stability of the three chain amides
A3n/m is considerably reduced in comparison to the related esters
C3n/m and ethers E3n/m. This is opposite to the trends seen for
Table 10 Comparison of the mesophases and transition temperatures of the phenylbenzoate based imidazolium salts C316/m33 with related benzyl-
phenylether based imidazolium salts E316/m and benzanilides A316/m
Compd
X ¼ CH2O (T/ꢀC)
Compd.
X ¼ COO (T/ꢀC)
Compd.
A316/2
X ¼ CONH (T/ꢀC)
Cr 115 Colh 189 Iso
Cr 127 (SmA 92) Iso
E316/2
E316/3
E316/4
E316/5
Cr 96 CubI 217 Iso
Cr 89 CubI 184 Iso
Cr 102 Colh 145 Iso
Cr 97 Iso
C316/2
C316/3
C316/4
C316/5
Cr 98 CubI 225 Iso
Cr 106 CubI 202 Iso
Cr 109 Colh 171 Iso
Cr 102 SmA 118 Iso
A316/4
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For compounds with three C-terminal chains, upon only slight
enlargement of the N-terminal chain the phase sequence CubI–
Colhex–SmA is observed, which is opposite to the effect of
enlarging the number and length of the C-terminal chains. This is
due to the core–shell structure of the aggregates where the N-
alkyl chains fill the core region. In the cores the aromatic–
aliphatic interface curvature is positive and hence the effect of
increasing the volume of the N-terminal chains is opposite to that
of the C-terminal chains covering the outside of the aggregates
with negative curvature. Due to the strong restriction of the
space available in the cores of spheroidic and cylindrical aggre-
gates only slight enlargement of the N-terminal chains is suffi-
cient to remove these curved aggregates and to replace them by
a lamellar structure.
Yunnan Science Foundation (2010CD018). MP acknowledges
the support by the Cluster of Excellence ‘‘Nanostructured
Materials’’ and DFG (FG 1145).
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The benzanilides incorporating the more polar –CONH–
group generally show enhanced melting points and for the non-
ionic 1H-imidazoles also enhanced mesophase stability due to
the increased intramolecular polarity contrast between the
benzanilide core and alkyl chains. In contrast, the related imi-
dazolium salts with amide groups show lower mesophase
stability than the related ethers and esters. The unexpected
reduction of mesophase stability by replacing –CH2O–/–COO–
by –CONH– suggests that the packing of the amide-based imi-
dazolium salts might be different from the related ether- and
ester-based imidazolium salts. It is proposed that the polar
intermolecular interaction between the amide moieties and the
ionic units of adjacent molecules favor an antiparallel and
completely interdigitated packing of the polar rod-like cores,
leading to ribbon-like aggregates with reduced aromatic–
aliphatic interface curvature, disfavoring the formation of
strongly curved interfaces, even if the N-terminal chain is very
short (Scheme 4). Hence, in this series of compounds it seems to
be not possible to achieve the CubI phases with three linear alkyl
chains, but branching of the alkyl chain or replacing them by
bulkier semiperfluorinated chains39e could enhance the amount
of the aliphatic–aromatic interface curvature and in this way
could probably lead to spheroidic aggregates.
17 J. H. Olivier, F. Camerel, J. Barbera, P. Retailleau and R. Ziessel,
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Overall, these studies provided general clues for the proper
design of ionic liquid crystals, especially for tailoring the aggre-
gate structure and the LC temperature range. This contributes to
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21074105 and No. 20973133), the
2284 | Soft Matter, 2012, 8, 2274–2285
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37 This type of interdigitated bilayer structure is assigned as SmA2
though the d-parameter is significantly smaller than d ¼ 2L and
corresponds to L < d < 2L, which is usually assigned to double
layer smectic phases abbreviated as SmAd; in SmAd phases there is
48 No bicontinuous cubic phase with Pm3n lattice is known,45 therefore
a bicontinuous structure, as expected if the phase sequence is
considered, is unlikely. Also the lattice parameter of this cubic
phase is in the same range as observed for E316/3 (see Table 7); the
slight increase of acub by 0.4 mn with respect to E316/3 is in line with
the increase of m from 3 to 4.
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