Spontaneous Mirror-Symmetry Breaking in Isotropic Liquid Phases of Photoisomerizable Achiral Molecules

Article abstract of DOI:10.1002/anie.201508097

Spontaneous mirror-symmetry breaking is of fundamental importance in science as it contributes to the development of chiral superstructures and new materials and has a major impact on the discussion around the emergence of uniform chirality in biological

Full text of DOI:10.1002/anie.201508097

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International Edition: DOI: 10.1002/anie.201508097
German Edition: DOI: 10.1002/ange.201508097
Chirality
Spontaneous Mirror-Symmetry Breaking in Isotropic Liquid Phases of
Photoisomerizable Achiral Molecules
Mohamed Alaasar,* Marko Prehm, Yu Cao, Feng Liu,* and Carsten Tschierske*
[
Abstract: Spontaneous mirror-symmetry breaking is of fun-
damental importance in science as it contributes to the
development of chiral superstructures and new materials and
has a major impact on the discussion around the emergence of
uniform chirality in biological systems. Herein we report
chirality synchronization, leading to spontaneous chiral con-
glomerate formation in isotropic liquids of achiral and photo-
isomerizable azobenzene-based rod-like molecules. The posi-
tion of fluorine substituents at the aromatic core is found to
have a significant effect on the stability and the temperature
range of these chiral liquids. Moreover, these liquid conglom-
erates occur in a new phase sequence adjacent to a 3D
tetragonal mesophase.
ness (labelled here as Iso1 *] phases).[11,12] Formation of these
liquid conglomerates obviously requires a locally twisted
cluster structure of the liquids, providing cooperativity and
acting as a template for chirality synchronization of the
conformers.[11] However, the few known Iso1 *] phases are
[
metastable, with only few exceptions, meaning that they could
only be detected on cooling if the formation of the competing
crystalline phases is suppressed. Moreover, they were found
at high temperatures around 2108C, where decomposition
becomes an issue, thus making their investigation and
application difficult.
Herein we report the first chiral liquids formed by
photoisomerizable achiral azobenzene-derived compounds
1–4 (Scheme 1). These chiral liquids have Iso1 *] phases in
[
T
he observation by Pasteur of chiral conglomerate forma-
tion during the crystallization of tartrates marked the birth of
stereochemistry.[1] Since then, symmetry breaking in racemic
mixtures of chiral molecules and the synchronization of chiral
conformers of achiral molecules during crystallization in
chiral space groups have been found in numerous cases.[2]
Additionally, enantiophobic chirality discrimination between
permanently chiral molecules and chirality synchronization of
transiently chiral conformers[3] on surfaces[4] and in fibrous
aggregates have been well documented.[5] The spontaneous
development of chiral conglomerates has also been observed
in soft-matter systems, especially in liquid-crystalline (LC)
phases of achiral bent-core molecules,[6–9] and this has
considerable impact on the problem of the development of
uniform chirality in biotic systems.[10] Surprisingly, sponta-
neous mirror-symmetry breaking was recently found even in
isotropic liquids of achiral molecules, which form conglom-
erates of two immiscible chiral liquids with opposite handed-
Scheme 1. Synthetic route to polycatenar molecules 1–4. Reagents and
conditions: i) SOCl2, DMF, reflux; ii) Et3N, pyridine, CH2Cl2, reflux.
DMF=dimethylformamide.
convenient temperature ranges. These azo-functionalized
materials are of special interest because of their photo-
sensitive nature, leading to photoisomerizable chiral liquids
which could be addressed by linearly or circularly polarized
light and thus used for optical, optoelectronic, and sensing
devices.[13–15]
[*] Dr. M. Alaasar, Dr. M. Prehm, Prof. Dr. C. Tschierske
Institute of Chemistry
Martin Luther University Halle-Wittenberg
Kurt-Mothes Str. 2, 06120 Halle/Saale (Germany)
E-mail: carsten.tschierske@chemie.uni-halle.de
The synthetic pathway to compounds 1–4 is shown in
Scheme 1. Detailed synthetic procedures and analytical data
are reported in the Supporting Information and the transition
temperatures are summarized in Table 1.
Dr. M. Alaasar
Department of Chemistry, Faculty of Science
Cairo University
P.O. 12613 Giza (Egypt)
E-mail: malaasar@sci.cu.edu.eg
Figure 1, taken as a representative example, shows the
differential scanning calorimetry (DSC) curves obtained on
heating and cooling of compound 1 (X = Y= H). This
compound transforms into a low birefringent tetragonal
mesophase (Tet) at T= 1218C, which melts into an isotropic
liquid at T= 1758C. There are two isotropic liquid phases
Y. Cao, Prof. Dr. F. Liu
State Key Laboratory for Mechanical Behavior of Materials
Xi’an Jiaotong University
Xi’an 710049 (P. R. China)
E-mail: feng.liu@mail.xjtu.edu.cn
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201508097.
[
(Iso1 *] and Iso) with a phase transition between them at T=
1898C on heating (see Table 1; Figure 1). The achiral liquid
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Table 1: Mesophase types, phase-transition temperatures (T/8C), and
transition enthalpies [DH/kJmolÀ1] of compounds 1–4.[a]
Compd.
Phase sequence
[
1
Heating: Cr 121 [35.1] Tet 175 [0.7] Iso1 *] 189 [0.2] Iso
Cooling: Iso 185 [0.1] Iso1 *] 163 [0.3] Tet 76 [33.7] Cr
[
[
2
3
4
Heating: Cr 137 [45.7] Tet 158 [0.5] Iso1 *] 169 [<0.05] Iso
Cooling: Iso 162 [<0.05] Iso1 *] 150 [0.4] Tet 112 [45.9] Cr
[
Heating: Cr 121 [36.9] SmA 155 [0.3] N 170 [0.2] Iso
Cooling: Iso 168 [0.4] N 154 [0.1] SmA 80 [31.3] Cr
Heating: Cr 128 [47.6] Iso1 *] 144 [<0.05] Iso
[
Cooling: Iso 139 [<0.05] Iso1 *] 114 [2.4] Tet 100 [36.4] Cr
[
[a] Peak temperatures as determined from first heating and first cooling
DSC scans with rate 10 KminÀ1. Abbreviations: Cr=crystalline solid;
Tet =tetragonal 3D mesophase; SmA=smectic A phase; N=nematic
[
phase; Iso1 *] =chiral isotropic conglomerate liquid; Iso=achiral iso-
tropic liquid.
Figure 2. Textures of compounds 1 and 4 between nontreated glass
[
substrates (thicknessꢀ25 mm). a) Compound 1 in the Iso1 *] phase at
T=1808C after rotating one polarizer by circa 58 from the crossed
position in the anticlockwise and b) in the clockwise direction. Dark
and bright domains are evident, indicating the presence of a conglom-
erate of domains with opposite chirality. The conglomerate textures
observed for these isotropic liquids were found to be independent of
the used substrates, also in homeotropic and planar cells, as long as
any contamination with traces of chirality is strictly excluded; in this
case unequal areas or only one sign of handedness would be formed.
c) Tet phase of compound 1 at T=1608C. d) Tet phase of compound 4
at T=1138C.[21] The direction of the polarizers is indicated by arrows.
See Figure S12 for a color version of this figure.
phase is formed at T= 1638C, indicated by the rapid growth of
a low birefringent mosaic-like texture (see Figure 2c). This
mesophase is highly viscous and does not flow even under
medium mechanical stress, which is indicative of a mesophase
with a 3D lattice. The formation of this phase from the
Figure 1. DSC heating and cooling curves (10 KminÀ1) measured for 1.
Inset: Expanded temperature ranges (150–1958C) on heating and
cooling.
[
adjacent Iso1 *] phase is associated with a small transition
enthalpy of only DH ꢀ 0.3 kJmolÀ1 (Table 1). Despite the
solid-like crystalline optical appearance, XRD investigations
show a completely diffuse wide-angle scattering with a max-
imum at d = 0.45 nm, not very distinct from that in the Iso
phases, thus confirming that the individual molecules have no
fixed positions (Figure 3a, inset).
[
(Iso) as well as the chiral conglomerate liquid (Iso1 *]) appears
dark (optically isotropic) between crossed polarizers and both
have low viscosity and easily flow under gravity. However, in
[
the temperature range of the Iso1 *] phase, occurring below
There are numerous sharp reflections in the small-angle
range (Figure 3b; Table S1 in the Supporting Information)
which can be indexed to a tetragonal 3D lattice (Tet). In
previous work tetragonal mesophases were observed as
birefringent LC phases, often accompanying bicontinuous
cubic phases of rod-like molecules.[17–19] A tetragonal phase
was also reported for bent-shaped molecules.[20] There are
different subtypes of tetragonal phases with distinct lattice
types, but the structures and the reasons for their formation
have not been well understood. Unfortunately, the exact
symmetry of the lattice could not be determined from powder
XRD patterns and the oriented diffractions could not be
achieved at the current stage. The highest symmetry that fits
the diffraction pattern is P4/mmm, though a P4222 lattice
could also be possible. The lattice parameters (a = 13.9 and
c = 18.9 nm) are much larger than the molecular length
(Lmol = 4.9 nm in the most stretched conformation with all-
trans alkyl chains) and indicate a complex structure of this
1898C, uncrossing the polarizers by a small angle (circa 2–58)
in the clockwise or anticlockwise direction leads to the
appearance of dark and bright domains. After rotating the
analyzer by the same angle in the opposite direction, the dark
and bright domains are reversed (Figure 2a,b). Rotating the
sample between crossed polarizers does not lead to any
change, indicating that the distinct regions represent optically
active domains rotating the plane of polarized light into
[
opposite directions. The Iso1 *]–Iso phase transition is asso-
ciated with a small enthalpy change of 0.2–0.3 kJmolÀ1. The
shape of the peak, being relatively sharp at the high-temper-
ature end and having a significant tailing towards the low-
[
temperature side is typically detected for this Iso–Iso1 *]
transition[11,12] (Figure 1, inset).[16]
On cooling, the liquid–liquid transition between the
[
achiral Iso phase and the chiral conglomerate liquid (Iso1 *])
takes place at T= 1858C, and upon further cooling a meso-
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Modification of the parent structure 1 by introducing one
lateral fluorine substituent (X = F, compound 2) on the
terminal benzene ring next to the alkoxy chain does not
change the phase sequence, but reduces the range of the
[
Tet phase and narrows the Iso1 *] range (see Table 1). A
further reduction of the transition temperatures was achieved
by introducing two fluorine atoms (X = Y= F, compound 4;
see Table 1 and Figure 2d). Changing the position of the
single fluorine atom from X to Y (X = H, Y= F, compound 3)
results in a completely different behavior, where the Tet and
[
Iso1 *] phases are removed and replaced by lamellar (SmA)
and nematic (N) LC phases (Figure S11). In compound 3 the
larger F atom (compared to H) in the 2-position (Y= F) is
pointing to the center of the molecule. This slightly reduces
the imbalance of interfacial area between rigid cores and
alkyl-chain periphery, thus leading to decreased interface
curvature and the formation of SmA and N phases.
In contrast, the F substituent at the peripheral 3-position
(that is, X = F) contributes to an increase in the interface
[
curvature[22] and thus favors the Tet and Iso1 *] phases for
compound 2 and for the disubstituted compound 4, which has
F substituents in both the 2- and 3-positions (X = Y= F;
Table 1). Compound 4 behaves similar to the nonsubstituted
[
compound 1, but forms the Iso1 *] phase at much lower
temperature and in a wider temperature region. Thus,
fluorination provides a tool for tailoring mirror-symmetry
breaking in liquids.
XRD investigation of the Iso phases was performed for
compound 1. As shown in Figure 3a, in both Iso phases there
is a diffuse small-angle scattering. The position of the
maximum of the small-angle scattering is at d = 4.3–4.4 nm,
corresponding approximately to the molecular length
[
(d/Lmol = 0.87–0.90). This indicates that the Iso1 *] phase has
a cybotactic structure. The cybotactic clusters appear to have
a helical or a twisted lamellar local structure acting as
a template for helical molecular packing with synchronization
of chiral conformations. The full width at half maximum of the
small-angle scattering changes continuously without a visible
[
step at the Iso–Iso1 *] transition, indicating a continuous
growth of the size of the cybotactic clusters with decreasing
temperature, being in the range of 28 nm at the transition (see
Figure 3c). With growing cluster size on decreasing temper-
ature, chiral fluctuations in the achiral Iso phase increase and
become long-range at the phase transition. By chirality
synchronization of molecular conformations, a denser pack-
ing is achieved, giving rise to the detected enthalpic gain.
Helical conformations result from the twist of the COO
groups in the phenylbenzoate units[6c] and probably also from
the twist of the biphenyl core. As the liquid state is retained,
conformational disorder and molecular motions are partly
retained at the transition and thus the overall entropic penalty
of this process is decreased. Therefore, the enthalpic gain of
chirality synchronization can exceed the entropic penalty of
demixing of the enantiomorphic conformers of these rela-
Figure 3. XRD data of compound 1: a) small-angle and wide-angle
(inset) XRD diffractograms at different temperatures for the Iso phase
[
at T=1958C, Iso1 *] phase at T=1808C, and Tet phase at T=1708C;
b) SAXS diffractogram of the Tet phase at T=1608C with indexation;
and c) full width at half maximum (FWHM) of the small-angle
scatterings (D(2q); fitted using Lorentzian line shapes) depending on
temperature as observed in the isotropic liquid phases.
[
mesophase with about 2500 molecules per unit cell (see the
Supporting Information). There is no indication of chirality in
the Tet phases of compounds 1, 2, and 4 but this could
probably be hidden under the birefringence and thus might
not be detectable by optical methods.[21]
tively large molecules.[11,23,24] At the Iso1 *]–Tet transition the
local clusters fuse to a long-range 3D structure forming the
tetragonal lattice.
Figure 4 shows the UV/Vis spectra of compound 1 in
chloroform solution. For the freshly prepared sample an
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Acknowledgements
The work was supported by the DFG (Grant Ts 39/24-1) and
the National Natural Science Foundation of China
(21374086). We thank beamline BL16B1 at SSRF (Shanghai
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Keywords: azo compounds · chirality · liquids · self-assembly ·
symmetry breaking
[
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How to cite: Angew. Chem. Int. Ed. 2016, 55, 312–316
Angew. Chem. 2016, 128, 320–324
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Received: August 29, 2015
Published online: October 22, 2015
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