Isothermal Chirality Switching in Liquid-Crystalline Azobenzene Compounds with Non-Polarized Light

Article abstract of DOI:10.1002/anie.201705559

Spontaneous mirror-symmetry breaking is a fundamental process for development of chirality in natural and in artificial self-assembled systems. A series of triple chain azobenzene based rod-like compounds is investigated that show mirror-symmetry breaking

Full text of DOI:10.1002/anie.201705559

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Accepted Article
Title: Isothermal Chirality Switching with Non-Polarized Light
Authors: Mohamed Alaasar, Silvio Poppe, Qingshu Dong, Feng Liu,
and Carsten Tschierske
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201705559
Angew. Chem. 10.1002/ange.201705559
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Isothermal Chirality Switching with Non-Polarized Light
Mohamed Alaasar,*[a,b] Silvio Poppe,[a] Qingshu Dong,[c] Feng Liu,*[c] and Carsten Tschierske*[a]
Abstract: Spontaneous mirror symmetry breaking is a fundamental
process for development of chirality in natural and in artificial self-
assembled systems. Here we report a new series of triple chain
azobenzene based rod-like compounds, which for the first time show
mirror symmetry breaking in an isotropic liquid occurring adjacent to
a lamellar LC phase. The transition between the lamellar phase and
the symmetry broken liquid is affected by trans-cis
photoisomerization which allows a fast and reversible photoinduced
switching between chiral and achiral states with non-polarized light.
between the molecules. Only in the photo-induced nematic
phase without these layers the molecular chirality can couple
with the nematic director field and a helical superstructure
evolves (N* phase).[10] Recently, it was shown that for achiral
bent mesogenic dimers with azobenzene units
a helical
superstructure is spontaneously formed in the twist-bend
nematic phase (NTB, see Scheme S2). This spontaneously chiral
superstructure is removed by trans-cis photonisomerization with
non-polarized light, leading to a transition to a nematic phase
which, due to the absence of molecular chirality, is achiral (N).[11]
Chirality of molecular systems has developed into a major topic
in chemistry since its discovery by L. Pasteur.[1] It is considered
as a prerequisite for emergence of life[2] and has numerous
technological implications in pharmaceutical and agricultural
chemistry[3] as well as for applications in nano-technology and
advanced materials.[4] In condensed matter phases chirality can
be based on permanent (configurational) or transient
A4
A5
(conformational)
molecular
chirality
and
on
chiral
superstructures, arising from the (in most cases helical)
organization of molecules (superstructural chirality).[5] Circular
polarized light represents a chiral environment[6] and therefore
has often been used, either to induce chirality, or to modify
chirality by diastereomeric interaction with existing molecular
and superstructural chirality.[7,8] In contrast, non-polarized light is
achiral (racemic) and therefore cannot induce chirality in the
absence of a properly oriented magnetic field or another source
of chirality.[2b,6] Accordingly, chirality switching by non-polarized
light requires chiral molecules, preferably organized in liquid
crystalline (LC) phases combining the molecular order required
for helix formation with the mobility required for switching.
Photoisomerization changes the molecular shape and this
modifies the self-assembly. Examples are light induced SmC*-
SmA and SmA-N* transitions (see Scheme S2 in the SI) of
azobenzene derived rod-like LCs. In the first case,
photoisomerization removes the molecular tilt in the SmC*
structure which is the basis of helix formation perpendicular to
the layers.[9] In the latter case photoisomerization removes the
layer structure (SmA), being incompatible with a helical twist
[ ]
Iso1 *
A6
Cub
SmA
SmCa
A7
Cr
A8
100
120
140
160
180
200
T / °C
Figure 1. Phase transitions of compounds An as determined from the 2nd
heating (upper columns) and cooling DSC scans (lower columns) with rate 10
K min-1; abbreviations: SmA = non-tilted lamellar phase; SmCa = anticlinic
tilted lamellar phase; Cub = bicontinuous cubic phase with Ia3¯d lattice; Cr =
solid crystal; Iso1 *] = chiral isotropic conglomerate liquid mesophase;[17] the
[
superscript [*] indicates the presence of a chiral superstructure though the
involved molecules are achiral; for numerical data and transition enthalpies
see Table S1.
Herein we report the first example for the opposite
process of induction of superstructural chirality by
photoisomerization of achiral compounds with non-polarized
light. In this case mirror-symmetry breaking takes place at the
transition from a higher to a lower order phase which is contrary
to common knowledge, where symmetry breaking occurs at the
transition from lower to higher order (e.g. crystallization from
solutions or melts). This discovery is based on our previous
work on spontaneous mirror symmetry breaking in isotropic
[a] Prof. Dr. C. Tschierske, Dr. M. Alaasar, S. Poppe.
Institute of Chemistry, Martin-Luther University Halle-Wittenberg, Kurt-
Mothes Str.2, D-06120 Halle/Saale, Germany.
Fax: +49(0)3455527346; Tel: +49(0)34555256664.
E-mail: carsten.tschierske@chemie.uni-halle.
[b] Dr. M. Alaasar.
Department of Chemistry, Faculty of Science, Cairo University,
P.O. 12613 Giza, Egypt. Fax: +20235727556; Tel: +20235676595.
E-mail: malaasar@sci.cu.edu.eg
liquids (Iso1 *] phases) formed by achiral multi-chained rod-like
[
[c] Prof. Dr. F. Liu, Q. Dong.
(polycatenar[12]
)
molecules.[13-16] The spontaneous mirror
State Key Laboratory for Mechanical Behavior of Materials,
Xi’an Jiaotong University, Xi’an 710049, P. R. China.
Fax: +862982663453; Tel: +862982665768.
E-mail: feng.liu@xjtu.edu.cn
symmetry breaking in these liquids was proposed to result from
the combination of a locally twisted cybotactic cluster structure
with the chirality synchronization of the involved transiently
chiral molecules.[15] The local twist is thought to be due to the
local aggregation and parallel organization of the rods and the
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201705559
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density map of the Cub/ Ia3¯d phase obtained from g), see Section 3.4.1 in the
SI for more details.
simultaneous clashing of the bulky terminal alkyl chains. These
Iso1 *] phases typically occur besides cubic or non-cubic
[
mesophases with long range 3D lattice.[16,17] It was also shown
that polycatenar compounds involving azobenzene units photo-
isomerize in solution, though it was not possible to observe a
measurable effect on the bulk state.[18] We attributed this to a
kinetic hindrance of photoisomerization by the long range 3D
lattice. In order to replace these 3D phases by fluid lamellar LC
The transition temperatures of compounds An are shown
graphically in Figure 1 (for numerical data, see Table S1). All An
compounds with n = 5-8 show spontaneous chiral conglomerate
formation in a certain temperature range of their isotropic liquid
phases (Iso1 *]). The formation of these Iso1 *] phases from the
achiral isotropic liquid state (Iso) is indicated under the polarizing
microscope by the occurrence of chiral domains, identified upon
slight rotation of the analyzer by a small angle out of the 90°
position, leading to dark and bright domains which invert their
brightness after rotating the analyzer by the same angle into the
opposite direction (Figures 2c,d and S12). The domains have
circular shapes and flow under gravity as typical for isotropic
[
[
phases, we have designed
a new class of polycatenar
compounds involving photoisomerizable azobenzene
a
group[19] and having only three instead of the previously used
four terminal alkyl chains (tricatenar compounds An, see Figure
1). These compounds have been synthesized (Scheme S1),
purified and analyzed as described in the Supporting
Information.
[
liquids.[20] The Iso1 *] phases are accompanied by different types
a)
Cr
Iso
]
[
b)
SmA
Iso1
*
of LC phases, depending on the chain length n. As shown in
Figure 1, a cubic phase with Ia¯3d lattice is found for compounds
with chain length n = 7 and 8 (A7: acub = 10.0 nm, see Figure 2g,
h and Table S2; A8: 10.2 nm, see Table S3; for more details of
structure elucidation, see Section 3.4.1 of the SI). The cubic
phases are achiral, in line with the racemic Ia3¯d structure being
composed of two enantiomorphic networks of branched helical
columns (Figures 2h and S16).[16] With decreasing chain length
the optical isotropic cubic phase is replaced by less viscous and
birefringent LC phases. Polarizing optical microscopy of the LC
phases of compounds A4-A6 indicates uniaxial SmA phases
characterized by typical fan textures in planar alignment (layers
perpendicular to the substrate) and dark isotropic textures in
homeotropic alignment (layers parallel to the substrate, see
Figure 2b). Small angle X-ray scattering (SAXS) confirms the
lamellar organization and provides a layer thickness of dSmA= 5.2
nm for the SmA phase of A6, as an example (Figure 2e, see
also Tables S4-S6). This value is slightly larger than the
molecular length (Lmol = 4.4 nm), indicating an antiparallel
organization of the molecules with intercalation of the single
alkyl chains between the biphenyl units of adjacent molecules
(Figure 4a,c). In all cases on cooling the SmA phase an
anticlinic tilted lamellar phase (SmCa phase) is observed (Figure
1). This is for some compounds followed by additional tilted and
non-tilted smectic, hexatic and crystalline low temperature
mesophases which will be discussed in a forthcoming report.
n = 6
[
]
Iso1
*
Iso
170 175 180 185
T/°C
[
*
]
Iso
SmA
1
Iso
SmC
Heating
a
SmA
[
]
*
Cooling SmC
a
Iso
Iso
1
SmA
100
120
140
160
180
T / °C
[
Iso1 *]
[
Iso1 *]
d)
c)
(-)
(-)
(+)
(+)
100 µm
5.5
60
d = 5.2 nm
40
f)
e)
600
SmA
dSmA
5.0
4.5
Iso[
*
]
I/a.u.
20
400
I/a.u.
200
d = 0.46 nm
0
d/nm
dnet,cub
14 16 18 20 22 24 26
2
4.0
d
Iso1[ ]
*
d = 4.0 nm
0
3.5
5
6
7
8
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
n
2
g)
h)
Though the Iso-Iso1 *] transition is easily detected by POM,
[
Iq2/
a.u.
the associated enthalpies are very small and the corresponding
DSC peaks are broad, more typical for a Cp-anomaly (see inset
in Figure 2a).[14,17,21] Also the XRD patterns in the Iso1 *] and Iso
[
ranges are very similar. As shown in Figure 2e for the Iso1 *]
[
phase of A6, there are only two diffuse scatterings, one in the
small and the other in the wide angle region of the XRD pattern
(at d = 0.46 nm), in line with the presence of only short range
order (black line, Figure 2e with inset). The intensity of the small
angle scattering is significantly larger than the wide angle
scattering, but it is also significantly broader compared to the
smectic phases (red line, Fig 2e), as typical for liquids with a
locally ordered structure (cybotaxis).[14,17,22] As previously shown,
the change of the position and line width of the XRD small angle
q /nm-1
Figure 2. Characterization of the mesophases of compounds An: a) heating
and cooling DSC curves (10 K min-1) recorded for A6. b) Texture of the SmA
phase (T
= 170 °C) of A5 between crossed polarizers with coexisting
homeotropic (dark area) and planar alignment (birefringent area). c, d) Chiral
domains observed for the Iso1 *] phase of compound A6 in a thin film between
[
two glass plates (~20 m) at T = 181 °C, c) after rotating one polarizer by 12
from the crossed position in anticlockwise and d) in clockwise direction (see
also Figure S12). e) SAXS pattern of the SmA and Iso1 *] phase of A6 at T =
[
180 and 160 °C, respectively; the inset shows the corresponding WAXS
patterns. f) Development of the d-spacings dIso1[*] (2K below Iso-Iso1 *]
[
scattering at the Iso-Iso1 *] transitions is continuous without any
[
transition) and dSmA depending on chain length (n); dnet,cub = (3½acub)/4 is the
distance between the two networks in the Cub/Ia3¯d phases. g) SAXS pattern
of the Cub/Ia3¯d phase of A7 (T = 160 °C) and h) reconstructed electron
distinct jump.[14] This indicates that the Iso and Iso1 *] phases are
[
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formed by small clusters which grow continuously on cooling
and reach the critical size for chirality synchronization at the Iso-
SmA and Iso phases, as well as the involved molecules, are
achiral. It is therefore likely that the chiral information is stored at
the surfaces of the glass substrates via surface stabilization of
residual helical clusters or chiral surface defects, which then
Iso1 *] transition temperature.
[
Compound A6 was selected for photoisomerization
experiments in solution and in the distinct LC phases.
Investigation in solution indicated almost complete trans-cis
isomerization under UV light and back relaxation to the trans-
isomer by thermal relaxation as shown in Figure 3a. Irradiation
of the bulk sample with non-polarized 405 nm laser light (5 mW)
trigger the chirality sense in the developing bulk Iso1 *] phase. In
[
contrast, the Cub-Iso1 *] transitions of compounds A7 and A8 are
[
not affected by irradiation, as previously also observed for the
tetracatenars.[18] Probably the long range 3D connected
networks of columns in the Cub phase act as templates which
kinetically hinder the isomerization.[24]
lowered the SmA-Iso1 *] transition temperature of A6 by 10 K. An
[
equilibrium state is reached after a couple of seconds (<2 s) and
relaxes back to the initial transition temperatures (cis-to-trans)
immediately after switching off the light source (<2 s). This
allows an isothermal induction of chirality and a relatively fast
and reversible on-off switching of chirality by non-polarized light,
as shown in Figure 3b-d (see also the supporting video 1). The
c)
a)
a)
SmA
dcalc = 5.4 nm
Decreasing
d)
e)
effect of irradiation on the Iso-Iso1 *] transition temperature is
[
n, T
intercallation
[
Iso1 *]
b)
much smaller, leading to a reduction by only 2 K, thus overall
expanding the enantiotropic Iso1 *] range of A6 from 11 to ~20
[
K.[23]
Lmol = dcalc = 4.4 nm
Twist
0.5
a)
Cub
Ia3¯d
A
= before irradiation
0.4
0.3
0.2
Figure 4. Molecular organization in the LC phases and in the cybotactic
[
clusters of the Iso/Iso1 *] phases: a, b) Molecular models of A6 showing the
C = after thermal
relaxation
molecular dimensions and the changing intercalation at the transition from a)
the intercalated packing in the lamellar to b) the segregated packing in the
[
Iso1 *] and Cub/Ia¯3d phases (twist not shown). c-e) Schematic models showing
A /a.u.
0.1
the cross sections of c) the layers (SmA); d) the local organization in the
[
twisted ribbons of the Iso1 *] phases (from front to back the molecules are
B = after irradiation
shown red, purple, blue) and e) the helical columns forming the networks of
0.0
the Cub/Ia¯3d phase.
300
350
400
nm
450
500
550
600
h
In order to understand the development of the Iso1 *] phase
[
b)
c)
d)
SmA
in relation to the molecular structure and the self-assembly in the
adjacent LC phases, the phase structures depending on chain
length and temperature have been investigated in more detail.
The maximum of the small angle X-ray scattering in the Iso1 *]
[
phases is dIso1[*] = 3.9-4.3 nm, being significantly smaller than the
layer spacing dSmA in the adjacent SmA phase (dSmA = 5.1-5.2
nm, Figure 2f) and only a bit smaller than the molecular length
(Lmol = 4.4 nm for A6, Figure 4a,b). This suggests a non-
intercalated antiparallel packing of the molecules with
completely segregated alkyl chains and aromatic cores in the
[ ]
[ ]
Iso1 *
SmA
Iso1 *
Relaxation
Iso1 *] phase (Figure 4b). This non-intercalated packing is distinct
[
from the intercalated organization in the adjacent SmA phase
(Figure 4a) and provides an unbalanced ratio of two aromatic
cores vs. three alkyl chains, leading to layer frustration which is
released by development of curvature and twist. With growing
alkyl chain length the twist of the aggregates increases, thus
favoring the chirality synchronization of helical conformers.
However, on cooling, the connectivity of the clusters increases,
Figure 3. a) UV-vis spectra of A6 dissolved in chloroform at 25 °C (c = 20
molL-1). A: freshly prepared solution before irradiation; B: cis-isomer as
obtained after 1 h irradiation with  = 365 nm; C: after keeping the sample in
[
dark overnight. b-d) Isothermal photo induced SmA-to-Iso1 *] transition
observed for compound A6 in a thin film between two glass plates under
slightly uncrossed polarizers (arrows) at T = 168 °C: b) SmA phase before
illumination with non-polarized 405 nm laser light (5 mW); c) during SmA-to-
[
[
Iso1 *] transition upon starting of illumination and d) Iso1 *] phase as observed
during illumination. The reverse sequence is obtained immediately after
switching off the light source (see also the supporting videos).
too, until a highly connected network is formed at the Iso1 *]-
[
Cub/Ia3¯d phase transition (n = 7, 8) leading to the achiral cubic
phase (Figure 2h). Accordingly, the distance between the nets
forming the Cub/Ia3¯d phase (Figure 2h), dnet,cub = 4.4-4.5 nm is
close to diso1[*] (Figure 2f), confirming that the segregated
organization (Figure 4b) is retained in the Cub/ Ia3¯d phase.
Figure 2f also shows that dIso decreases with growing n, from n =
The sign of chirality in the chiral domains of the Iso1 *]
[
phase is retained during repeated switching cycles Iso1 *]-SmA-
[
Iso1 *] or Iso1 *]-Iso-Iso1 *] (see supporting video 2), though the
[
[
[
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COMMUNICATION
R. A. van Delden, Angew. Chem. Int. Ed. 1999, 38, 3419; Angew.
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in the Iso1 *] phases becomes more intercalated, and hence, less
[
[7]
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sterically frustrated with decreasing chain length (Figure 4b→a),
in this way favoring the lamellar organization. Deeper chain-core
intercalation at decreased temperature (larger contribution of
linear trans-conformers makes the chains more compatible with
the rod-like cores) reduces the curvature further, and the local
clusters can fuse to infinite layers with intercalated structure
(SmA, n = 5, 6; Figure 4a,c). However, in the chiral isotropic
liquids of compounds A5 and A6 a twisted organization is still
retained in the aggregates and the local twist is obviously still
sufficient for chirality synchronization to take place, thus leading
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[
sufficient twist cannot develop and the SmA phase is directly
formed from the achiral Iso phase. This provides an
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[
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phases for compounds A5 and A6 occurring besides an SmA
phase, just before the cross-over from lamellar to bicontinuous
cubic organization in the adjacent LC phases.
In summary, isothermal switching from an achiral LC
phase to a mirror symmetry broken isotropic liquid with non-
polarized light was reported for achiral compounds, providing
significant potential for technological and nano-technological
applications.
[14] C. Dressel, T. Reppe, M. Prehm, M. Brautzsch, C. Tschierske, Nat.
Chem. 2014, 6, 971.
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[
[17] The Iso1 *] phases are distinct from the so-called dark conglomerate
phases (DC phases) formed by
a completely different class of
compounds, the bent-core molecules. DC phases represent sponge-
like deformed lamellar phases, where chirality and conglomerate
Experimental Section
[
formation is due to the combination of tilt and polar order.[25a] The Iso1 *]-
Iso transition between two isotropic liquids is (almost) continuous,[14,15]
whereas the DC-Iso transition, representing a SmCP-Iso transition, is
discontinuous and associated with relatively high transition enthalpies
(H ~10-25 kJ mol-1).[25b]
Compounds An were synthesized according to the procedures
given in the accompanying Supplementary Information, where
also the used experimental methods are described.
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Poppe, Q. Dong, F. Liu, C. Tschierske, Chem. Commun. 2016, 52,
13869.
Acknowledgements
The work was supported by the DFG (Grand Ts 39/24-1) and
the National Natural Science Foundation of China (No.
21374086). We thank Beamline BL16B1 at SSRF (Shanghai
Synchrotron Radiation Facility, China) for providing the
beamtimes.
[19] H. M. Dhammika, S. C Burdette, Chem. Soc. Rev. 2012, 41, 1809.
[20] see supporting video in ref. [14].
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Chem. Soc. 2014, 136, 14658.
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[23] The SmA-Iso transition of A4 is reduced by -4 K during irradiation, but
[
no Iso1 *] phase is induced; the SmCa-SmA transition of A5 is shifted by
Keywords: Chirality, Photoisomerization, Azobenzene, Mirror-
symmetry breaking, Liquid crystal
-3 K.
[24] Irradiation of A7 and A8 in the tilted SmCa range can induce the cubic
phase, i.e. isomerization to the bent cis isomer increases the interfacial
curvature and the SmCa-Cub transition temperature is shifted by 7 K to
lower temperature, expanding the cubic phase range by
photoisomerization, similar as reported for azobenzene doped SmCs
phases.[26]
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10.1002/anie.201705559
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Chirality switching in liquids
Mohamed Alaasar,*[a,b] Silvio Poppe,[a]
Qingshu Dong,[c] Feng Liu,*[c] and
Carsten Tschierske*[a]*
C6H13
O
O
Achiral three chained azobezene
compounds An with proper chain
length n show a fast isothermal
switching between an achiral lamellar
phase and a symmetry broken chiral
liquid conglomerate by
O
O
O
O
N
OCnH2n+1
N
C6H13
An (n = 4-8)
Page No. – Page No.
Isothermal Chirality Switching with
Non-Polarized Light
photoisomerization with non-polarized
light.
This article is protected by copyright. All rights reserved.