Hunting for smectic C in calamitic azobenzene ionic liquid crystals with different cationic head groups

Article abstract of DOI:10.1002/poc.3779

The tilted smectic C phase is a rather uncommon phase in ionic liquid crystals (ILCs), whereas the orthogonal smectic A phase is the most common phase in ILCs. We now present 2 new groups of mesogens with an azobenzene core that exhibit smectic C as well

Full text of DOI:10.1002/poc.3779

Received: 20 July 2017
DOI: 10.1002/poc.3779
Revised: 12 September 2017
Accepted: 14 September 2017
R E S E A R C H A R T I C L E
Hunting for smectic C in calamitic azobenzene ionic liquid
crystals with different cationic head groups
2
1
1
2
2
Nadia Kapernaum | Eugen Wuckert | Wolfgang Frey | Sebastian Marino | Manuel Wahl |
2
1
Frank Giesselmann
| Sabine Laschat
1
Institut für Organische Chemie,
Abstract
Universität Stuttgart, Stuttgart, Germany
2
The tilted smectic C phase is a rather uncommon phase in ionic liquid crystals
(ILCs), whereas the orthogonal smectic A phase is the most common phase in
ILCs. We now present 2 new groups of mesogens with an azobenzene core that
exhibit smectic C as well as smectic A phases. Their phase sequences and tilt
angles were studied by polarizing microscopy, and their temperature‐depen-
dent layer spacings and orientational order parameters were investigated by
X‐ray diffraction. We present 1 new amidinium azobenzene mesogen that
forms enantiotropic smectic C and A phases and another amidinium as well
as 2 new guanidinium azobenzene mesogens that exhibit monotropic smectic
C and enantiotropic smectic A phases. With this study, we show that
azobenzene is indeed an SmC‐promoting group in ILCs. Comparing these
results with our earlier results on azobenzenes with an N‐methylimidazolium
head group (N Kapernaum et al, ChemPhysChem 2016, 17, 4116‐4123), we
show that the aromaticity of the imidazolium head group plays an important
role in the formation of smectic C phases.
Institut für Physikalische Chemie,
Universität Stuttgart, Stuttgart, Germany
Correspondence
Nadia Kapernaum, Institut für
Physikalische Chemie.
Email: n.kapernaum@ipc.uni‐stuttgart.de
Sabine Laschat, Institut für Organische
Chemie, Universität Stuttgart,
Pfaffenwaldring 55, D‐70569 Stuttgart,
Germany.
Email: sabine.laschat@oc.uni-stuttgart.de
Funding information
Deutsche Forschungsgemeinschaft, Grant/
Award Number: La 907/17‐1 and Gi 243/8‐
1
; Bundesministerium für Bildung und
Forschung, Grant/Award Number:
1RI05177; Carl‐Schneider Stiftung Aalen;
0
Ministerium für Wissenschaft, Forschung
und Kunst des Landes Baden
Württemberg; Fonds der Chemischen
Industrie
KEYWORDS
amidinium, de Vries material, guanidinium, imidazolium, ionic liquid crystals, smectic C
1
| INTRODUCTION
much rarer as compared with the corresponding SmA/
SmC ratio in nonionic thermotropic liquid crystals.
[1–8]
Among the huge variety of different low molecular weight
compounds forming thermotropic mesophases, the class
of ionic liquid crystals (ILCs) has received considerable
interest because they combine the unique features of ionic
liquids, such as low volatility, adjustable polarity and sol-
ubility, and high thermal and electrochemical stability
with those of thermotropic liquid crystals and can thus
There are relatively few examples of ILCs with
SmC phases described in the literature, most notably
[
9–15]
[16]
calamitic biphenyls,
phenylpyrimidines,
and
[
17–19]
azobenzenes
with various cationic head groups.
However, general design principles for SmC promoting
mesogens are unknown for ILCs in contrast to the guide-
lines successfully developed by Lemieux for noncharged
[
1–8]
[20–22]
be considered as ordered anisotropic fluid salts.
liquid crystals.
From our previous findings that
Another remarkable feature of ILCs is that the relative
occurrence of various mesophases is not the same as in
nonionic thermotropic liquid crystals. For example, the
lamellar nontilted SmA phase is the most common
mesophase found in ILCs, while tilted SmC phases are
alkoxybiphenyls with tethered guanidinium moiety
displayed indeed SmC phases in addition to the SmA
[23]
phase, when the spacer was sufficiently long,
we
concluded that the connection of the cationic head
group with a rigid calamitic unit via a flexible tether
J Phys Org Chem. 2017;e3779.
wileyonlinelibrary.com/journal/poc
Copyright © 2017 John Wiley & Sons, Ltd.
1 of 12
https://doi.org/10.1002/poc.3779

2
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KAPERNAUM ET AL.
seems to be a promising design motive for that purpose.
2 | RESULTS AND DISCUSSION
2.1 | Synthesis of the ionic liquid crystals
[24]
[25]
In addition, Westphal
and we
independently
disclosed that azobenzenes tethered to imidazolium head
groups formed tilted mesophases. Moreover, we discov-
ered that these imidazolium salts were the first ILCs with
The known azobenzene ILCs MIM(C )Br (n = 4, 6, 8)
n
[25]
de Vries‐like SmA to SmC phase transitions.
The
with N‐methylimidazolium head group were prepared in
2 steps from 4‐dodecyloxy‐4′‐hydroxyazobenzene 1 accord-
so‐called de Vries materials exhibit a very small layer
[25]
contraction in the tilted SmC phase after the phase transi-
ing to the previously described method (Scheme 1). Key
intermediate 1 was also used as a precursor for the corre-
sponding guanidinium and amidinium ILCs (Scheme 1).
Williamson etherification of the known ω‐bromo‐
[26–29]
tion from the nontilted SmA phase.
In conventional,
non‐de Vries materials, the SmA‐to‐SmC phase transition
is accompanied by a strong contraction of the smectic
layer thickness by a factor of cosθ depending on the tilt
angle θ between the director n and the layer normal
k of the SmC phase. As ILCs with SmC phases are quite
rare and show interesting properties like de Vries‐type
behavior, novel ILCs with SmC phases are highly
desirable. Thus, we anticipated that azobenzenes with
a flexible spacer connecting the cationic unit might
be a promising SmC promoting group. However, the
specific role of the head group remained unclear. In the
current manuscript, we prepared a dedicated library of
tethered azobenzenes with tetramethylguanidinium
and dimethylamidinium head group and varying
spacer lengths and explored their mesomorphic and phys-
ical properties with a special focus on SmA versus
SmC phases. These tetramethylguanidinium and
dimethylamidinium ILCs were compared with the corre-
sponding known N‐methylimidazolium ILCs. The results
are reported below.
[
23]
alkyl‐N‐phthalimides 2 provided the azobenzene deriv-
atives 3(C ) with terminal N‐phthalimido‐protecting
group. Subsequent hydrazinolysis followed by reaction
with tetramethylformamidinium chloride and
triethylamine according to the method by Kantlehner
and treatment with HCl or HBr (method A) yielded the
guanidinium chlorides Gua(C )Cl (n = 4, 6, 8) and bro-
mides Gua(C )Br (n = 4, 8), respectively, in 42 to 56%.
The bromide with C spacer Gua(C )Br was obtained
via salt metathesis (method B) from the corresponding
chloride Gua(C )Cl in 99%. For the synthesis of the
amidinium salts, N‐phthalimides 3(C ) were submitted
n
[30]
n
n
6
6
6
n
to hydrazinolysis and the resulting free amines were
treated with N,N‐dimethylformamide dimethylacetal in
DMF at 80°C under microwave conditions followed by
reaction with HCl and HBr, respectively, to give the
amidinium chloride Am(C
Am(C )Br (n = 4, 6, 8) in 66 to 82%.
)Cl in 78% and bromides
4
n
SCHEME 1 Synthesis of guanidinium
(
Gua(C
Am(C
n
)X) and amidinium salts
)X) starting from azobenzene 1
(
n

KAPERNAUM ET AL.
3 of 12
2
.2 | Solid state structures of the ionic
The guanidinium salt displays a tilted bilayer packing
along the c‐axis with strong interdigitation (Figure 1B).
There are 2 types of interdigitating interactions evident.
First, there is an interdigitated orientation between 2 mol-
ecules with coplanar phenyl moieties. The C33‐C11 dis-
tance is 3.94 Å, and the lengthening factor is 1.29.
Second, there is an interdigitated orientation between 2
molecules with a perpendicular orientation of the phenyl
moieties. The C33‐C18 distance is 3.97, and the lengthen-
ing factor is 1.51 (Figure 1C).
liquid crystals
Fortunately, single crystals were obtained for
guanidinium chloride Gua(C )Cl, amidinium chloride
Am(C )Cl, and imidazolium bromide MIM(C )Br,
respectively, which were suitable for X‐ray crystal struc-
ture determination.
crystallizes with 1 molecule in the asymmetric unit, 1
chlorine as counterion, and 2 chloroform solvent mole-
4
4
4
[
31]
Guanidinium chloride Gua(C )Cl
4
cules in the centrosymmetric space group P2 /n. We
Derivative Am(C )Cl crystallizes with 1 ion pair in
1
4
found a hydrogen bond with the N–H function of the
guanidinium head as donor and the chlorine as acceptor.
The hydrogen bond is nearly linear with a N1···Cl1 dis-
tance of 3.279(2) Å and a H1···Cl1 distance of 2.45(3) Å
and an angle N1‐H1···Cl1 of 167(3)°. In addition, a consid-
erable shortening of the O1‐C1 distance of 6.1 Å due to
folding of the C4 spacer was observed. For comparison,
the distance of the linear folded tail atoms C27···C33 is
the asymmetric unit of the centrosymmetric monoclinic
space group P2 /n (Figure 2A). An intermolecular H bond
1
is evident between the N2‐H2 function of the amidinium
head group as donor and the Cl counterion as acceptor.
The N2···Cl1 distance is 3.11(1) Å and that of H2···Cl1 is
2.27 Å. The N2‐H2···Cl1 angle is 159°. The folding of the
C4 spacer yields also to a pronounced decrease of the
O1···C3 distance (5.34 Å) bringing the amidinium head
group closer to the azobenzene. For comparison, the
7
.6 Å (Figure 1A).
FIGURE 1 X‐ray structure of Gua(C
Cl in the solid state (A) and packing
4
)
diagrams (B and C) (H atoms omitted for
clarity)

4
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KAPERNAUM ET AL.
FIGURE 2 X‐ray structure of Am(C
Cl in the solid state (A) and packing
4
)
diagram (B) (H atoms omitted for clarity).
The crystal quality of Am(C )Cl was not
well indicated by a relatively high R(int) of
.23 and a high R1 value of 17.4%.
4
0
Nevertheless, data are sufficient for a
qualitative discussion
C25···C31 distance of the tail atoms in an extended all‐
extended all‐trans conformation, which yields in a short-
ened O1‐C1 distance to the imidazolium head between
5.71 and 5.83 Å. For comparison, the tail C26···C32 dis-
trans conformation is 7.63 Å. In contrast to Gua(C )Cl,
4
an antiparallel tilted bilayer packing along the b‐axis
without any interdigitation was observed (Figure 2B).
tance is 7.65 Å. In contrast to both Gua(C )Cl and
4
The MIM(C )Br system crystallizes with 4 indepen-
Am(C )Cl, the bend spacer of the imidazolium salts
4
4
dent ion pairs in the asymmetric unit of the centrosym-
metric triclinic space group P1. Additional 4 water
molecules co‐crystallize in the structure (Figure 3). The
structure is stabilized by a complex network of intermo-
lecular hydrogen bond interactions. The H atoms of the
water molecules act as donor, where the Br anions work
as acceptor. On the imidazolium head group, the C–H
groups are also donors of hydrogen bonds, of which the
water oxygen atoms and Br anions work alternatively as
acceptors. The C4 spacer shows in all conformers no
resulted in S‐shaped dimers forming a tilted bilayer pack-
ing along the c‐axis. As in amidinium salt Am(C )Cl, an
4
interdigitation of the antiparallel oriented tails of the mol-
ecules is not evident (Figure S1).
2
.3 | Mesomorphic and physical
properties of the ionic liquid crystals
The 3 homologous guanidinium bromides Gua(C )Br,
4
Gua(C )Br, and Gua(C )Br as well as the 3 amidinium
6
8
4
FIGURE 3 X‐ray structure of MIM(C )Br in the solid state (packing diagram see Figure S1)

KAPERNAUM ET AL.
5 of 12
bromides Am(C )Br, Am(C )Br, and Am(C )Br were
Br exhibited a fan‐shaped texture in the SmA phase and
a broken fan‐shaped texture in the SmC phase (Figure 4).
4
6
8
investigated to characterize their mesomorphic and phys-
ical properties. Polarizing optical microscopy (POM) and
differential scanning calorimetry were used to identify
the mesophases and to determine the phase transition
temperatures and transition enthalpies (Table 1). For the
POM investigations, a red band‐pass filter with 630 nm
was used to prevent the light‐induced cis‐trans‐isomeriza-
Am(C )Br showed in the liquid crystal test cell a well
8
aligned planar texture in the SmA phase and a domain tex-
ture in the SmC phase (Figure 6). Under homeotropic con-
ditions in the SmA phase, Gua(C8)Br (Figure 5, top)
showed a black homeotropic texture, while for Am(C )Br
8
(Figure 7, top), an oily streak texture is observed. In the
SmC phase, both materials exhibit a Schlieren texture
under homeotropic conditions (Figures 5 and 7, bottom).
The temperature‐dependent smectic layer spacing
d(T) of all 6 materials was measured with small‐angle X‐
ray scattering. The layer spacing for the guanidinium
mesogens is shown in Figure 8A and for the amidinium
mesogens in Figure 8B. Gua(C )Br and Am(C )Br
[32,33]
tion of the azo‐group.
For better comparison of the
quanidinium and amidinium mesogens with the
[25]
mesogens with imidazolium head group,
only the bro-
mides were studied because the imidazolium chlorides
MIM(C )Cl neither are available via alkylation of the
n
chloroalkyl precursor nor by subsequent salt metathesis
of MIM(C )Br. Furthermore, the chloride salts are usu-
n
4
4
ally more hygroscopic than the bromides.
exhibit only an SmA phase and therefore show a continu-
ously rising layer spacing with decreasing temperature
due to increasing orientational order at lower tempera-
tures (small insets in Figure 8A and B).
All 6 mesogens exhibit SmA phases. Furthermore, an
SmC phase was observed in both series for the homologs
with a spacer length of 6 and 8 carbon atoms, while the
2
homologs with a spacer length of 4 carbon atoms only
The layer spacings of Gua(C )Br, Gua(C )Br,
6
8
showed an SmA phase. The mesogen Am(C )Br exhibits
Am(C )Br, and Am(C )Br increase likewise in the
6 8
6
an enantiotropic SmC phase, while Gua(C )Br, Gua(C )
SmA phase until the typical layer shrinkage of the SmC
phase takes place. For the enantiotropic SmC phase of
6
8
Br, and Am(C )Br show monotropic SmC phases that
8
only appear on cooling. The phase transition from the
SmA to the SmC phase showed small peaks in the differ-
ential scanning calorimetry for all 4 mesogens. This indi-
cates weakly first‐order phase transitions with transition
Am(C )Br as well as for the monotropic SmC phases of
6
Gua(C )Br and Gua(C )Br, it was possible to measure
6
8
the layer spacing at least until 10 K below the tilting tran-
sition. The monotropic SmC phase of Am(C )Br on the
8
−
1
enthalpies ΔHAC in the range of 0.8 to 1.8 kJ mol . These
are typical values for ΔHAC as the phase transition
enthalpies of first‐order SmA‐to‐SmC phase transition of
contrary crystallized during the X‐ray measurements
already 3 K below the phase transition from SmA to
SmC. The layer shrinkage 10 K below the SmA to SmC
[34,35]
nonionic mesogens are in the same range.
phase transition is 12% for Gua(C )Br, 12.7% for
6
Textures of the SmA and SmC phases under the polar-
Gua(C )Br, and 7.2% for Am(C6)Br. This indicates that
8
izing microscope are shown in Figures 4 and 5 for Gua(C8)
these mesogens are conventional non‐de Vries‐type mate-
rials, which show a regular layer shrinkage below their
SmA‐to‐SmC phase transition.
Br and in Figures 6 and 7 for Am(C )Br. In liquid crystal
8
test cells coated with single side rubbed Nylon Gua(C8)
TABLE 1 Phase sequences on heating (top row) and cooling (bottom row) determined by polarizing optical microscopy (POM) and tran-
sition enthalpies ΔHAC of the SmA‐to‐SmC phase transition determined by differential scanning calorimetry (DSC) of guanidinium and
amidinium bromides
Mesogen
Phase Sequence
ΔHAC
Gua(C
Gua(C
Gua(C
4
6
8
)Br
)Br
)Br
Cr 141°C SmA 217°C Iso
Iso 213°C SmA 121°C Cr
Cr 124°C SmA 166°C Iso
Iso 163°C SmA 102°C (SmC) 63°C Cr
−
−
1
1
−0.8 kJ mol
−1.4 kJ mol
Cr 124°C SmA 167°C Iso
Iso 166°C SmA 118°C (SmC) 83°C Cr
Am(C
Am(C
Am(C
4
)Br
)Br
)Br
Cr 155°C SmA 226°C Iso
Iso 225°C SmA 150°C Cr
−
−
1
1
6
8
Cr 135°C SmC 146°C SmA 214°C Iso
Iso 213°C SmA 145°C SmC 110°C Cr
+1.2 kJ mol
−1.6 kJ mol
Cr 153°C SmA 178°C Iso
Iso 178°C SmA 148°C (SmC) 136°C Cr
−
1
−1.8 kJ mol

6
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KAPERNAUM ET AL.
FIGURE 4 Planar textures of Gua(C
cells coated with single‐side rubbed nylon with a cell gap of
.8 μm under the polarizing microscope. Top: fan‐shaped texture of
8
)Br in liquid crystal test
FIGURE 5 Homeotropic textures of Gua(C
8
)Br between 2 plain
glass plates under the polarizing microscope. Top: black
homeotropic texture of the SmA phase with few defects at 150°C.
Bottom: Schlieren texture of the SmC phase at 84°C
1
the SmA phase at 130°C. Bottom: broken fan‐shaped texture of the
SmC phase at 105°C
The optical tilt angles θ were measured for all 4 SmC
mesogens with the polarizing microscope (Figure 9). The
R ¼ θXray=θ:
(2)
2
guanidinium mesogens showed higher tilt angles than
In the case of conventional behavior, R is 1, while it is
the amidinium compounds. The highest tilt angles of
0 for perfect de Vries behavior. The best de Vries materials
[38]
about 38° were found for Gua(C )Br, while the tilt angles
known show R values in the range of 0.17.
The
8
of the shorter homolog Gua(C )Br reached only 27°. The
mesogens Gua(C )Br and Am(C )Br have R values of 1
6
6
6
2
amidinium compounds Am(C )Br and Am(C )Br
or even higher, which means that they do not show de
Vries behavior and behave completely conventional.
6
8
exhibit similar values of their tilt angles with a maximum
tilt of approximately 23°.
Gua(C )Br has an R value of 0.78. This shows that its
8
The optical tilt angles θ were compared with the tilt
angles θXray calculated from the layer shrinkage in the
small‐angle X‐ray scattering measurements for Gua(C6)
X‐ray layer shrinkage is a little bit smaller than what is
expected from the optical tilt angles but still exhibits a
huge layer shrinkage and is thus also a conventional
non‐de Vries‐type material.
Br, Gua(C8)Br, and Am(C )Br 10 K below the SmA‐to‐
6
SmC phase transition to check whether these compounds
exhibit de Vries‐type behavior (Table 2). The X‐ray tilt
angles θXray were calculated according to Equation 1:
For the 2 mesogens Gua(C )Br and Am(C )Br, the
6
8
orientational order parameters S of the SmA phases were
2
determined by X‐ray measurements on cooling from the
isotropic phase (Figure 10). The orientational order
parameter measures the quality of the parallel orientation
of the long axes of the rod‐like mesogens with respect to
−1
θXray ¼ cos ðd =d Þ:
(1)
C
A
where d denotes the maximum layer spacing in the SmA
the director. Experimental values of S are obtained from
A
2
phase and d the layer spacing in the SmC phase 10 K
the directional intensity profile of the diffuse wide‐angle
C
below the SmA‐to‐SmC phase transition. These X‐ray
tilt angles θXray were compared with the optical tilt angles
scattering arcs of aligned monodomain samples according
to the procedure by Davidson and Levelut.
[
39]
θ at T − T = −10 K to receive the figure of merit R
The orientational order parameters of the SmA phases
were in the range of 0.5 for both mesogens and nearly
AC
[36,37]
(Equation 2)

KAPERNAUM ET AL.
7 of 12
FIGURE 6 Planar textures of Am(C
8
)Br in liquid crystal test
FIGURE 7 Homeotropic textures of Am(C )Br between 2 plain
8
cells coated with single‐side rubbed nylon with a cell gap of
.8 μm under the polarizing microscope. Top: aligned planar texture
of the SmA phase at 155°C. Bottom: planar domain texture of the
SmC phase at 140°C
glass plates under the polarizing microscope. Top: oily streak
1
texture of the SmA phase at 155°C. Bottom: Schlieren texture of the
SmC phase at 138°C
layers like most smectic A phases of ILCs. The ratio of
independent of temperature. In the SmA phases of non-
1.8 seems to indicate that the SmA phase is built of par-
2
ionic thermotropic liquid crystals, S usually exhibits
tially interdigitated double layers. Nevertheless, for this
simple calculation, the orientational order was not taken
2
[40]
values of about 0.7.
The values for S for Gua(C )Br
2 6
and Am(C )Br are thus substantially smaller, but these
into account. For these ILCs, S is in the range of 0.5 (see
8
2
[25,33]
low values are typical for ILCs.
In the case of ILCs,
Figure 10). The orientational order parameter measures
the quality of the orientational order of the long axes of
the mesogens with respect to the director. The value of
0.5 indicates that most mesogens are randomly tilted with
respect to the director in all possible directions, and this
involves a contraction of the smectic layers. To take this
impact of the orientational order on the layer spacing d
into account the effective molecular length, Leff is needed.
The effective molecular length can be interpreted as the
layer spacing at a certain temperature if the rod‐like
mesogens had perfect orientational order. It is calculated
the driving force for the formation of liquid crystal phases
is the nanosegregation between the ionic and the non-
ionic parts of the molecule, which leads to a strong
[41,42]
layering.
The orientational order is thus only a sec-
ondary order parameter and often remains quite low
and shows only a weak temperature dependence.
With all the data collected so far, we will now present
a structure model for these smectic ILCs by using the
example of Gua(C )Br for the guanidinium series and
6
Am(C )Br for the amidinium series; as for these 2
8
[
43,44]
mesogens, the values of their orientational order parame-
ters are known. The molecular lengths L of both mole-
cules are calculated by molecular modeling. It is 36.9 Å
for Gua(C )Br and 41.3 Å for Am(C )Br. These values
after (Equation 3):
3⋅dðTÞ :
Leff ¼
(3)
6
8
S ðTÞ þ 2
2
for the molecular length are compared with the layer
spacing d of the smectic A phase of the 2 mesogens, and
we receive for both mesogens a ratio of approximately d/
L ≈ 1.8. This means that the smectic A phases of the
guanidinium and the amidinium mesogens investigated
where Leff is determined for all temperatures where the
orientational order parameter S is known (Figure 10).
2
The averaged values for the effective molecular length
are Leff = 80.6 Å for Am(C )Br and L = 77.5 Å for
8
eff
in this study are SmA phases, which are built of double
Gua(C )Br. By comparing these values with the
6
2

8
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KAPERNAUM ET AL.
FIGURE 8 Layer spacing d versus temperature T relative to the
SmA‐SmC transition temperature TAC for (A) the 3 guanidinium
FIGURE 9 Tilt angle versus temperature difference to the SmA‐
6 8
SmC phase transition for (A) Gua(C )Br and Gua(C )Br and (B)
mesogens Gua(C
4
)Br, Gua(C
6
)Br, and Gua(C
8
)Br and (B) the 3
)Br
Am(C )Br and Am(C )Br
6
8
amidinium mesogens Am(C
4
)Br, Am(C
6
)Br, and Am(C
8
TABLE 2 Optical tilt angle θ, layer shrinkage, X‐ray tilt angle
Xray, and for Gua(C )Br, Gua(C )Br, and Am(C )Br 10 K below
the SmA to SmC phase transition
calculated molecular length L, we get a ratio L /L of 1.95
eff
θ
6
8
6
for Am(C )Br and of 2.1 for Gua(C )Br. This indicates
8
6
that the smectic A phases of these ILCs are within exper-
2
θ/deg Layer Shrinkage/%
θXray/° R Value
imental error built of full double layers without any con-
siderable degree of interdigitation.
Gua(C
Gua(C
6
8
)Br 27.3
12.0
12.7
7.2
28.3
29.1
21.9
1
)Br 37.3
0.78
>1
6
Am(C )Br 16.6
2.4 | Comparison of the mesomorphic
properties with our earlier results on
[
25]
imidazolium azobenzenes
groups, only the homologs with spacer lengths of 6 and 8
formed an SmC phase. Furthermore, these SmC phases
The results on our azobenzene bromides with
guanidinium or amidinium head groups are compared
with our previous results on the same azobenzene bro-
were only monotropic in the case of Gua(C
Gua(C )Br, and Am(C )Br. Only Am(C )Br showed
)Br,
6
8
8
6
mides but with imidazolium head groups MIM(C )Br
an enantiotropic SmC phase over a temperature range of
approximately 10 K.
n
[25]
(
n = 4, 6, 8) (Scheme 1 and Kapernaum et al ). The
phase transition temperatures for these 9 mesogens are
summarized in Figure 11. The imidazolium compounds
exhibited more stable SmC phases than the amidinium
and guanidinium mesogens. All 3 imidazolium mesogens
showed an enantiotropic SmC phase over at least 25 K,
while in the case of the guanidinium and amidinium head
The smectic layer shrinkage in the SmC phase 10 K
below the SmA‐to‐SmC phase transition of the
imidazolium compounds measured by X‐ray scattering
was much smaller than those of amidinium and
guanidinium bromides. In the case of imidazolium
mesogens, it was in the range of 4% for MIM(C )Br to
4

KAPERNAUM ET AL.
9 of 12
FIGURE 10 Orientational order parameter S
2
versus temperature
T in the SmA phase of Gua(C )Br and Am(C )Br measured on
6 8
FIGURE 11 Phase transition temperatures T on heating (top)
and cooling (bottom) for the mesogens with guanidinium
cooling
(Gua(C
n
n
)Br), amidinium (Am(C )Br), and imidazolium
(
MIM(C
n
)Br) head groups measured by polarizing optical
[
25]
2
.3% for MIM(C )Br (Figure 12 and Kapernaum et al ).
microscopy (POM)
8
The layer shrinkage of the 2 guanidinium mesogens was
about 12%, while it was 7.2% for Am(C )Br.
6
The maximum tilt angles of the imidazolium com-
[25]
pounds were in the range of 30°
(Figure 12 inset for
MIM(C )Br). This is quite similar to the values for
8
Gua(C )Br and the 2 amidinium mesogens of about 25°.
6
The maximum tilt angles of Gua(C )Br were with 38°
8
slightly higher than those of imidazolium bromides. The
azobenzene mesogens from the 3 series therefore exhibit
usual tilt angles in their SmC phases.
Due to the small layer shrinkage of the imidazolium
compounds, their figures of merit for the de Vries charac-
ter, namely, their R values, were much smaller than the
ones of the amidinium and guanidinium compounds.
The R value for Gua(C )Br and Am(C )Br was approxi-
6
6
FIGURE 12 Layer spacing d and tilt angle θ versus temperature
mately 1, and it was 0.78 for Gua(C )Br, which indicates
8
difference to the SmA‐SmC phase transition T‐TAC for the
conventional behavior. For the imidazolium compounds,
[25]
8
imidazolium mesogen MIM(C )Br
R values of 0.61 for MIM(C )Br, 0.54 for MIM(C )Br,
4
6
[25]
and 0.48 for MIM(C )Br were calculated.
This means
shrinkage that is smaller than what is expected from the
optical tilt angles.
To summarize the results on the 3 azobenzene series,
we found the most stable and therefore broadest smectic
8
that the amidinium and guanidinium mesogens show
conventional behavior, while the imidazolium series
exhibits aspects of de Vries‐like behavior with a layer

10 of 12
KAPERNAUM ET AL.
phase range for the mesogens with imidazolium head
group, while the smectic phase ranges of the mesogens
with amidinium and guanidinium head groups were sim-
ilar to each other and significantly less broad than for the
imidazolium head group (see Figure 11). Furthermore, we
observed that the imidazolium head group was the stron-
gest SmC promoting head group of all 3 and led to the
most stable SmC phases. The amidinium as well as the
guanidium head group led to less stable SmC phases. Of
these 2 head groups, the amidinium head group seemed
to be more SmC promoting, as Am(C )Br was the only
6
mesogen of these 2 series that formed an enantiotropic
SmC phase. Although we observed de Vries‐like behavior
with a reduced layer shrinkage with respect to the optical
tilt angles in the imidazolium series, the guanidinium as
well as the amidinium compounds exhibited conventional
behavior with the layer shrinkage in the SmC phase being
in accordance with their optical tilt angles.
To rationalize these different behaviors of the head
groups, their geometries and elongations were considered.
All 3 head groups were modeled with only a methyl group
as substituent, and their energies were minimized.
Figure 13 shows the 3 head groups as well as their dimen-
sions, which are in agreement with the obtained X‐ray
crystal structure data discussed above.
Whereas
N,N‐dimethylamino
moieties
in
guanidinium and amidinium head groups possess rota-
−1
tional freedom with an energy barrier of 47 to 53 kJ mol
as determined previously by variable temperature NMR
FIGURE 13 Molecular models of the
methylimidazolium, dimethylamidinium, and
3
head groups N‐
[45]
for related guanidinium salts,
resulting in a 3D oval
tetramethylguanidinium with an additional methyl group. The
shape for guanidinium and a 3D spherical shape for
amidinium units, the heterocyclic N‐methylimidazolium
head group consists of a flat shape (see Figure 13). These
structures were energy‐minimized by molecular modeling
[
48]
3
D shapes together with the elongations of the head groups
rod‐shaped thermotropic mesogens.
By adding an
lead to an increase of the volume of the head group from the
small and flat imidazolium head group to the spherical
amidinium head group to the biggest egg‐shaped
guanidinium head group. In non‐ILCs, it was found that
flexible and bulky groups at the end of the molecules cause
an increase of the tilt of the aromatic cores due to the entro-
additional aromatic group at the end of the molecule,
attractive noncovalent π‐π interactions between these
aromatic groups can improve the nanosegregation. Stron-
ger nanosegregation enhances the stability of smectic
phases, and it also stabilizes the SmC phase as stronger
nanosegregation reduces the out‐of‐layer fluctuations,
and this minimizes the entropic cost of molecular tilt in
[46,47]
pic pressure caused by the bulky end groups.
In our
[
21,46]
case, the mesogens behave completely contradictory to
these earlier observations. The bulkiest head group, the
guanidinium head group, has the lowest tendency to form
SmC phases, while the smallest head group, imidazolium,
shows most SmC phases with the broadest temperature
range of all 3 mesogens.
a smectic structure.
groups can form π ‐π interactions between the posi-
tively charged aromatic rings.
The aromatic imidazolium head
+
+
[
49,50]
+
+
These π ‐π interac-
tions have been shown in simulations by Nagata and his
[51]
coworkers to enhance the layering in ionic liquids.
It
should be noted that in the solid state structure of the
We therefore suggest aromaticity as another structural
characteristic that leads to the huge differences in the
imidazolium salt MIM(C )Br, no face‐to‐face orientation
4
was observed, but a dimer, where the imidazolium moie-
ties are shifted approximately 1 moiety to the next stack
(Figure S1B). The tilt angle between the neighbor moie-
ties is 17.8(4)°. Nevertheless, there is a weak electrostatic
interaction characterized by the distances C2C‐N1D of
behavior of the
3
groups of ionic mesogens.
Nanosegregation between the rigid aromatic cores and
the flexible aliphatic side chains is usually the driving
force for the formation of smectic phases in nonionic

KAPERNAUM ET AL.
11 of 12
3
3
.508(7) Å, C2C‐C3D of 3.594(8) Å, and N2C‐C3D of
.702(7) Å. The experimentally observed dimers in the
in particular with respect to the counterion and the role
of hydrogen bonding interaction is necessary to fully elu-
cidate the formation of SmC phases in ILCs in general.
solid state resemble the diagonal conformers calculated
by Matthews,
[49]
where reduction in Coulomb repulsion
facilitates cation‐cation close contact. As the nonaromatic
ACKNOWLEDGEMENTS
head groups amidinium and guanidinium cannot form
+
+
these π ‐π interactions, it can be concluded that the
nanosegregation in the case of the aromatic imidazolium
head group might be stronger than for the nonaromatic
amidinium or guanidinium head groups. This stronger
layering then leads to broader smectic phases, and it also
stabilizes the SmC phase. The stronger layering and there-
fore higher translational order of the mesogens with
imidazolium head group can also explain that we found
de Vries‐like behavior for the imidazolium ILCs, while
we could not observe it for the ILCs with amidinium
and guanidinium head groups. It was stated by Lagerwall
Generous financial support by the Deutsche
Forschungsgemeinschaft (La 907/17‐1 and Gi 243/8‐1),
the Carl‐Schneider Stiftung Aalen (shared instrumenta-
tion grant), the Bundesministerium für Bildung und
Forschung (shared instrumentation grant 01RI05177),
the Ministerium für Wissenschaft, Forschung und Kunst
des Landes Baden Württemberg, and the Fonds der
Chemischen Industrie is gratefully acknowledged.
ORCID
[52]
et al
and later experimentally confirmed by
Frank Giesselmann http://orcid.org/0000-0002-9974-9470
Sabine Laschat http://orcid.org/0000-0002-1488-3903
[44]
Nonnenmacher et al
that de Vries behavior originates
from the unusual combination of exceptionally low orien-
tational order with strong 1D translational order. This
means that the increase in nanoseggregation caused by
the π ‐π interactions of the imidazolium head groups
can also lead to stronger de Vries‐like behavior.
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