Tolane-based bent bolaamphiphiles forming liquid crystalline hexagonal honeycombs with trigonal symmetry

Article abstract of DOI:10.1039/c4nj01855a

Bolaamphiphiles consisting of a bent 1,3-bis(phenylethynyl)benzene core with two terminal glycerol units, a lateral n-alkyl chain in the bay-position and a methyl group at the apex of the central 1,3-substituted benzene ring have been synthesized via Sono

Full text of DOI:10.1039/c4nj01855a

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Tolane-based bent bolaamphiphiles forming
liquid crystalline hexagonal honeycombs with
trigonal symmetry†
Cite this: New J. Chem., 2015,
39, 2060
Hongfei Gao,‡a Huifang Cheng,‡a Qijing Liu,a Yulong Xiao,a Marko Prehm,b
Xiaohong Cheng*a and Carsten Tschierske*b
Bolaamphiphiles consisting of a bent 1,3-bis(phenylethynyl)benzene core with two terminal glycerol units,
a lateral n-alkyl chain in the bay-position and a methyl group at the apex of the central 1,3-substituted
benzene ring have been synthesized via Sonogashira coupling reactions as key steps. The thermotropic and
solvent-modified or solvent-induced liquid crystalline (LC) phases of these compounds were investigated
by POM, DSC and X-ray scattering. Compounds with medium alkyl chain length form a hexagonal honeycomb
LC phase with non-centrosymmetric trigonal p3m1 symmetry as a monotropic LC phase. The addition
of water increases the LC phase range, leading to enantiotropic p3m1 phases and induces LC phases for
the non-mesomorphic compounds with shorter and longer lateral alkyl chains.
Received (in Montpellier, France)
21st October 2014,
Accepted 6th January 2015
DOI: 10.1039/c4nj01855a
www.rsc.org/njc
significant interest in the recent years, because such achiral
molecules can organize into chiral superstructures and into polar
1. Introduction
The design of new functional molecules which can self- lamellar and columnar LC phases with ferroelectric and antiferro-
organize into well defined two-dimensional (2D) arrays with electric properties (Scheme 1a).10 Adding a lateral alkyl chain into
potential nanotechnological applications is of current interest. the bay position of these bent-core mesogens removes polar order
Especially, liquid crystals (LCs)1 represent soft self-assembled in the LC phases.11 On the other hand, introducing a chain which
materials with significant technological importance1,2 and is incompatible with the terminal chains or groups into the bay
provide routes to increased complexity in soft self-assembly.3 position of the bent core, as for example by combination of polar
Among them, a series of polygonal honeycomb LCs were end groups with a lipophilic lateral alkyl chain, leads to the
created by the self-assembly of T-shaped and X-shaped bolaam- formation of hexagonal columnar phases representing hexagonal
phiphiles composed of a rod-like core, two polar end groups honeycombs (Scheme 1b and c).12 Two different types of hexago-
and one (T-shaped)4,5 or two (X-shaped) laterally attached nal honeycombs were obtained, depending on the ratio of the
lipophilic or fluorophilic chains.6,7 In these molecules strongly bent core size and lateral chain length, those with six molecules in
incompatible building blocks are combined in a competitive the circumference of the honeycomb cells, having a relatively large
manner which leads to new LC phases with enhanced complexity hexagonal lattice parameter and hexagonal p6mm symmetry
compared to the usually observed nematic, smectic and columnar (Colhex-6/p6mm, Scheme 1c) and those with only three molecules
LC phases.1 By combination of different lipophilic and fluoro- in the periphery, having much smaller lattice parameters and a
philic lateral chains new modes of molecular self-assembly could non-centrosymmetric trigonal p3m1 symmetry (Colhex-3/p3m1,
be achieved, providing multicompartment LC phases.8,9 Bent- Scheme 1b).10 Non-centrosymmetric soft matter structures are
shaped molecules composed of a non-linear, approximately 1201 of significant interest, as for example, for use in NLO applica-
bent rigid core with terminal alkyl chains have also attracted tions.13,14 In order to obtain significant NLO activity a strong
molecular dipole moment is required, which can be achieved
by introduction of appropriate substituents at the apex of the
p-conjugated bent core. Therefore, the investigation of the general
effects of such additional substituents on mesomorphic self-
assembly, and specifically on the ability of these bent-core
bolaamphiphiles to form hexagonal honeycombs with trigonal
symmetry, is of interest.
a Key Laboratory of Medicinal Chemistry for Natural Resources, Chemistry
Department, Yunnan University, Kunming, Yunnan 650091, P. R. China.
E-mail: xhcheng@ynu.edu.cn; Fax: +86 871 5032905
b Institute 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/
c4nj01855a
In the first step, to evaluate the influence of steric effects,
we report herein the effect of an additional methyl group.
‡ Both authors contributed equally to this work.
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Scheme 1 Principal modes of LC self-assembly of bent-core mesogens with
or without a chain in the bay position; SmCP = tilted polar smectic phase,
Colhex = hexagonal columnar phase; the Colhex phases have a honeycomb
structure and are shown as cross sections through these honeycombs
perpendicular to the column long axis; Colhex-3 phases have 3 molecules
in the circumference of each honeycomb cell, providing p3m1 symmetry
and Colhex-6 phases have 6 molecules in the circumference of each
honeycomb cell, providing p6mm symmetry.
Scheme 3 Synthesis of bent shaped bolaamphiphilies 1/n; Reagents and
conditions: (i) I2, KI, ammonia, MeOH, 25 1C, 2 h, 52%; (ii) CnH2n+1Br, K2CO3,
DMF, 95 1C, 20 h, 75–86%; (iii) 2-methylbut-3-yn-2-ol, THF, Et3N, Pd(PPh3)4,
CuI, 25 1C 89%; (iv) KOH, toluene, reflux, 15 h, 88%; (v) THF, Et3N, Pd(PPh3)4,
CuI, 25 1C 64–88%; (vi) PPTS, CH3OH, 25 1C, 15 h, 77–87%.
This group was introduced at the apex of the bent core (in position
5 of the central 1,3-disubstituted benzene ring) in molecules 1/n
consisting of a p-conjugated 1,3-bis(phenylethynyl)benzene
bent-core, two terminal glycerol units and having a long lateral
alkyl chain of variable length n in the 2-position (bay position) at
the central benzene ring (see Scheme 2). Specifically, the influence
of the length of the lateral chain and the effect of water as a polar
solvent on LC self-assembly were investigated.
phenylacetylene 617 in a Sonogashira coupling reaction,18 leading to
the 1,2-O-isopropylidene glycerol substituted 1,3-bis(phenylethynyl)-
benzenes 7/n. In the last step the 1,2-O-isopropylidene groups were
cleaved with pyridinium p-toluenesulfonate (PPTS) in CH3OH.12,19
The final compounds 1/n were purified by repeated crystallization
from petroleum ether/ethyl acetate mixtures (1 : 2). The detailed
procedures and corresponding analysis data are collected and given
in the ESI.†
2.2. Mesomorphic properties
2. Results and discussion
The LC self-assembly of compounds 1/n was investigated by
polarized optical microscopy (POM), differential scanning calori-
metry (DSC), and X-ray diffraction (XRD). The phase transition
temperatures and the corresponding enthalpy values of the
newly synthesized compounds 1/n together with the previously
reported compound 2/12 without the 5-methyl group and a
lateral C12H25 alkyl chain12 are collated in Table 1. Monotropic
(metastable) LC phases were observed only for compounds 1/14
and 1/16 with a medium length of the alkyl chain in the bay
position and the LC phases were lost by decreasing or increasing
the alkyl chain length n (n = 6, 12, 18, and 22).
2.1. Synthesis
The bent-core bolaamphiphiles 1/n have been synthesized via
Sonogashira coupling reactions as key steps, as shown in Scheme 3.
Accordingly, 2,6-diiodo-4-methylphenol 2, obtained by iodination
of p-cresol,15 was alkylated with appropriate n-alkyl bromides16
and then coupled with 1,2-O-isopropylidene glycerol substituted
The optical textures of the monotropic LC phases of com-
pounds 1/14 and 1/16, as obtained by cooling from the isotropic
liquid and observed between crossed polarizers, are character-
ized by highly birefringent spherulitic domains coexisting
with large homotropically aligned regions appearing completely
dark and interrupted by filament-like defects (see Fig. 1a and c).
Scheme 2 Structure of compounds 1/n under investigation.
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Table 1 LC phase transitions and other parameters of compounds 1/na compared with the previously reported compound 2/12 12 and the effect of
water on mesophase stability
Compound
X
R
Phase transitions T/1C [DH/kJ molÀ1
]
a/nm
ncell
nwall
Tcl b/1C
1/6
CH3
CH3
CH3
CH3
CH3
CH3
H
C6H13
Cr 92 Isoc
B96d
B92d
B93d
74
66
39
1/12
1/14
1/16
1/18
1/22
2/12
C12H25
C14H29
C16H33
C18H37
C22H45
C12H25
Cr 94 Isoc
Cr 48[12.1] (Colhex-3/p3m1 45[1.2]) Iso
Cr 44[12.3] (Colhex-3/p3m1 41[1.7]) Iso
Cr 74[11.1] Iso
2.74
2.77
2.7
2.65
1.8
1.8
Cr 82 Isoc
Cr 94[24.5] (Colhex-3/p3m1 85[6.3]) Iso
2.82
3.1
2.1
a
Transition temperatures were determined by DSC (peak temperatures, second heating scan, 10 K minÀ1); abbreviations: Cr = crystalline solid;
Colhex-3/p3m1 = hexagonal honeycomb LC phase with a trigonal p3m1 lattice; Iso = isotropic liquid; values in parentheses represent monotropic
b
phase transitions only observed upon cooling. Maxima of the LC–Iso transition temperatures observed by POM in the contact region between
c
d
compounds 1/n and water. Transition temperatures were determined by POM. Values 490 1C cannot be determined with certainty, due to rapid
water evaporation at these temperatures.
These textures indicate columnar LC phases which are optically compound 2/12 (ahex = 2.82 nm, see Table 1) for which the
uniaxial. As confirmed by investigations using a l retarder plate trigonal symmetry was additionally confirmed by reconstruction
(see Fig. 1b and d) the birefringence of the LC phases is of electron density maps.12 If there would be larger 6-hexagon
negative, i.e. the high index axis, which is known to be parallel cylinders, leading to a hexagonal p6mm symmetry (Colhex-6/p6mm,
to the long axis of the p-conjugated cores, is perpendicular to Scheme 1), a lattice parameter ahex around 4–5 nm would be
the column long axis. This means that the bent aromatic cores expected.12 This means that in the Colhex-3/p3m1 phases of
of compounds 1/n are aligned perpendicular to the column compounds 1/14 and 1/16 each side of the hexagon is made up
long axis in the plane of the 2D lattice and this is in line with a of one arm of the bent molecules, as shown in Scheme 1b and
polygonal honeycomb structure of these LC phases with a Fig. 1g. In the honeycomb frames three molecules are organized
uniaxial square or hexagonal lattice.
in the circumference of each hexagonal cell with the bent
The XRD patterns of both compounds are characterized by a aromatic cores arranged in the plane of the 2D-lattice and
diffuse wide angle scattering at d = 0.44–0.45 nm, confirming aligned perpendicular to the resulting channels, in line with
the presence of liquid like disorder and a series of sharper the experimentally observed negative birefringence of these LC
small angle scatterings, indicating the presence of a long range phases. In the resulting honeycomb walls two individual legs
periodicity, as typically observed for LCs (Fig. 1e and f). There is of adjacent bent shaped aromatic units are arranged parallel
one very strong and three weak small angle diffraction peaks side-by-side. The dynamic hydrogen bonding networks between
with a ratio of their reciprocal spacings of 1 : 2 : 71/2 : 121/2 which the glycerol end-groups fuse these walls to hexagonal honey-
can be indexed to the 10, 20, 21 and 22 reflections of a hexagonal combs and the prismatic cells resulting inside these honey-
lattice. This indicates hexagonal columnar mesophases for com- combs are filled with the long alkyl chains in the 2-position.
pounds 1/14 and 1/16 (Fig. 1e and f and Table S1, ESI†). The The nodes of the honeycomb are alternating, only three of them
hexagonal lattice parameters ahex are 2.74 and 2.77 nm for are formed by the hydrogen bonding networks of the glycerol
compounds 1/14 and 1/16, respectively. This relatively small groups and those between them are made up by the apexes of
value of ahex, compared to the molecular dimensions, confirms the bent-core units, thus leading to the reduced trigonal
that both hexagonal columnar mesophases should be formed by symmetry (plane group p3m1). According to the calculation
3-hexagons, leading to a trigonal columnar phase with p3m1 in Table S2 (ESI†), the number of molecules in each unit cell
symmetry (Colhex-3/p3m1 phases).12 In this case the side length of the with an assumed height of 0.45 nm (the maximum of diffuse
hexagons (lhex) is related to the hexagonal lattice parameter (ahex
)
wide angle scattering) is about three (ncell = 2.7–2.8), which is
through lhex = ahex/31/2 B 1.6 nm and this value is in agreement with in line with the proposed mode of molecular organization
the length of each leg (L = 1.5 nm) of the 1,3-bis(phenylethynyl)- (see Table 1 and Table S2, ESI†). The slight deviation from
benzene unit of the molecules 1/n as measured with molecular being exactly three could be due to a deviation of the core–
models between the middle of the 5-methylresorcinol core and the core distance along the columns from the assumed value of
end of the primary OH group of the glycerol unit, assuming a most 0.45 nm and to a slight staggering of the aromatic cores along
extended conformation. The hexagonal lattice parameters of 1/14 the honeycomb walls, as known for previously reported LC
and 1/16 are also very similar to that of the previously reported honeycombs.4–8
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Fig. 1 Colhex-3/p3m1 phases of compounds 1/14 and 1/16: (a) texture of 1/14 as seen between crossed polarizers upon cooling from the isotropic state
at T = 42 1C; (b) with additional l-plate, the indicatrix orientation in the compensator is shown in the inset; (c) texture of 1/16 at T = 42 1C; (d) with l-plate;
(e) XRD pattern of 1/14 at T = 35 1C; and (f) of 1/16 at T = 35 1C; (g) models of molecular organization in the hexagonal honeycombs with p3m1 lattices;
the bent aromatic cores are shown in gray; green circles/columns represent the hydrogen bonding networks of the glycerol groups, the yellow region
represents the honeycomb cells filled by the alkyl chains.
Compound 1/12 has a higher melting point at T = 94 1C, chain length is required to obtain LC phases for the methyl
which can be supercooled to T = 41 1C, when the compound substituted compounds 1/n, but upon further chain elongation
slowly crystallizes without formation of any LC phase. Even at the mesophase stability is reduced (1/14 4 1/16), and for
T = 30 1C no LC phase could be detected between the developing compounds 1/18 and 1/22 the LC phases are removed again. So
crystallites (see Fig. S1, ESI†). Thus, comparison of compound the major conclusion is that, though the non-centrosymmetric
2/12 without the 5-substituent and compound 1/12 with the trigonal columnar organization can be retained, the 5-methyl
5-methyl group (Table 1) indicates that the melting point is group has a significant mesophase destabilizing effect. This
increased and the LC phase is strongly depressed or even disturbance can partly be compensated by optimizing the
completely removed by introduction of the 5-substituent. 2-alkyl chain length and, as described further below, by adding
This clearly indicates a mesophase destabilizing effect of this appropriate solvents, stabilizing the columns comprising the
substituent, which is easily explained by steric distortion of the hydrogen bonding networks.
molecular packing into a honeycomb structure due to packing
frustration at the nodes formed by the methyl substituted mole-
2.3. Fluorescence properties
cular apexes. However, if the 2-alkyl chain is further elongated the UV investigation of dilute solution as well as thin films of compound
melting points are significantly reduced and Colhex-3/p3m1 phases 1/14 was performed (Fig. 2). In dilute solution (10À6 mol LÀ1 in THF)
can be observed for compounds 1/14 and 1/16, though the there is only one peak with a maximum at 302 nm. In the thin
mesophase stability, i.e. the LC–Iso phase transition temperature film obtained after spin-coating from THF solution (thickness
is significantly lower than that observed for compound 2/12 of about 1 mm) the absorption is blue shifted to 285 nm and
without the methyl group. This means that an increased alkyl simultaneously the peak is broadened (Fig. 2 left, dotted line) in
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Fig. 3 Representative textures of columnar phases of compounds 1/n in
water saturated samples as observed between crossed polarizers: (a) com-
pound 1/6 at T = 84 1C; (b) compound 1/12 at T = 33 1C, (c) compound
1/14 at T = 55 1C and (d) compound 1/18 at T = 51 1C; the insets at the top
right show sections of the textures with additional l-retarder plates,
indicating negative birefringence in all cases; the indicatrix orientation is
shown in the bottom inset in (a).
Fig. 2 UV-vis absorption (left) and PL spectra (right) of 1/14 in THF
solution (10À6 M) (solid lines) and solid thin film (dot lines).
comparison to the solutions (solid line). These spectral changes
can be ascribed to the formation of p-stacked aggregates with
H-type parallel stacking mode. These optical features also
support a self-organized p-stacked organization in the Colhex
phases (Fig. 1g). The photoluminescence (PL) spectrum of 1/14
in the thin film exhibits a broad and red-shifted emission peak
at 391 nm, while the solution displays structured emission with
maxima at 383 nm (Fig. 2 right). Based on these properties the
potential uses of this molecule could be as an active material in
OFETs and a blue emitter in OLEDs.20
The mesophase regions of compounds 1/14 and 1/16 are
significantly expanded and become enantiotropic by addition
of water, not only as a result of mesophase stabilization, but
also due to the reduction of the melting temperatures by the
added solvent. This leads to broad temperature ranges of
a trigonal columnar LC phase down to ambient temperature.
The LC phase of the water saturated compound 1/16 was
investigated by XRD at 25 1C (Fig. 4). The diffraction pattern
is characterized by a diffuse wide angle scattering at d =
0.45 nm confirming the presence of an LC phase. The ratio of
2.4. Influence of solvents on the phase behavior
Though only a few compounds show thermotropic mesomorphism the reciprocal spacing in the small angle region is 1 : 2. Based
as pure materials the mesophase stability of compounds 1/14 on the observed textures a lamellar phase can be excluded and
and 1/16 can be enhanced and LC phases can be induced for all indexing as 10 and 20 reflections of a hexagonal lattice leads to
other investigated homologues by addition of water, being ahex = 2.95 nm, which is slightly larger than that found for the
incorporated into the hydrogen bonding networks of the glycerol pure 1/16 sample (ahex = 2.77 nm). Nevertheless, the value is
groups, increasing the number of attractive intermolecular still in the range expected for a hexagonal honeycomb phase
hydrogen bonding and thus stabilizing these networks (see with the p3m1 lattice (see Fig. S4 and Table S1, ESI†). This is in
Table 1).21 Therefore, compounds 1/14 and 1/16 represent line with the incorporation of the solvent molecules into the
amphotropic LCs and the other compounds can be considered columns formed by the hydrogen bonding networks between
as mesogens forming solvent induced LC phases.22 All solvent- the glycerol units, thus leading to the swelling and a slight
containing mesophases have spherulitic (Fig. 3a–c) or filament increase of the lattice parameter.
textures (Fig. 3d), very similar to the textures of the solvent-free
If the series of compounds 1/n is compared (see Table 1), it
samples, indicating columnar LC phases. As no change in the appears that the highest stability of the solvent induced LC
texture is observed in the whole temperature range of the solvent phase is achieved for compounds with short chains and chain
stabilized LC phases and also no discontinuity is observed in the elongation decreases the mesophase stability. A possible reason
contact region between 1/14 or 1/16 and water it is assumed that could be restricted water uptake by the more hydrophobic long
the added solvent does not modify the fundamental phase chain compounds. Moreover, the long alkyl chains can by
structure, but increases the mesophase stability by strengthen- themselves efficiently fill the space in the hexagonal cylinder
ing the hydrogen bonding networks. In all cases the highest cells thus leaving less space available for additional water
clearing temperature of the mesophase is observed directly at the molecules. This reduces the total number of possible hydrogen
1/n–water interface and it decreases with decreasing water bondings and the honeycombs cannot be efficiently stabilized.
concentration (see Fig. 4b), indicating that the highest meso- The thermotropic as well as the solvent stabilized LC phases
phase stability is obtained for the water saturated samples.
of compounds 1/14 and 1/16 represent Colhex-3/p3m1 phases,
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Fig. 4 (a) XRD pattern of the Colhex/p3m1 phase of the water saturated sample of compound 1/16 at T = 25 1C; (b) contact region between water and
1/14 as observed by POM during cooling at T = 66 1C; arrows indicate the interface between water and 1/14.
based on textural and XRD evidence. This is most likely also the case 4. Experimental
for the other homologues, as there is continuous development of
The detailed synthetic procedures, purification of the com-
the LC–Iso transition temperatures depending on the chain length
pounds together with the analytical data are reported in the
ESI.† All cross-coupling reactions were conducted under argon
and, in addition, all solvent-induced LC phases are optically negative
as typical for honeycomb phases and indicated by investigation with
a l-retarder plate (see insets in Fig. 3). Nevertheless, there are some
textural differences (see Fig. 3), which are most likely due to changes
and the glassware was oven-dried (140 1C). Commercially
available chemicals were used as received. 1H NMR and 13C
NMR spectra were recorded in CDCl3 solution on a Bruker-DRX-
in the preferred alignment of the honeycombs depending on the
500 spectrometer with tetramethylsilane (TMS) as the internal
chain length. Overall, it is most likely that all solvent induced LC
standard. Elemental analysis was performed using an Elemen-
phases indeed represent Colhex-3/p3m1 phases, though they cannot
be stated with full certainty at present.
tal VARIO EL elemental analyzer. Column chromatography was
performed using silica gel 60 (230–400 mesh) from Merck.
A Mettler heating stage (FP 82 HT) was used for polarizing
optical microscopy (POM, Optiphot 2, Nikon) and DSCs were
recorded using a DSC-7 calorimeter (Perkin Elmer) at 10 K minÀ1
.
3. Conclusions
X-ray diffraction patterns of partially aligned samples were
The bent-core bolaamphiphiles 1/n can self-assemble into mono- recorded with a 2D detector (HI-STAR, Siemens) using a small
tropic 3-hexagon honeycomb type columnar liquid crystalline phases droplet on a glass surface; Ni filtered and pin hole collimated
with non-centrosymmetric p3m1 plane group symmetry if the alkyl Cu-Ka radiation was used and the beam was parallel to the
chains in the bay position have a certain length (n = 14, 16). In surface. The exposure time was 60 min. The sample-to-detector
comparison with related compounds 2/n12 it is evident that the distance was 8.8 cm and 26.9 cm for the wide angle and small
additional lateral methyl group at the apex reduces the mesophase angle measurements, respectively. 2D frame integration was used
stability, though the fundamental mode of self-assembly is not to convert the patterns into one-dimensional intensity profiles
changed. This means that the non-centrosymmetric liquid crystalline and the evaluated peak positions were used for phase assign-
phase is retained even in the presence of this substituent. This paves ments. UV-vis absorption spectra were recorded on a UV-240
the way to new molecules with acceptor substituents at the apex for UV-visible spectrophotometer (Shimadzu, Japan). Fluorescence
achieving nonlinear optically active LC materials.13 Moreover, water, spectra were recorded using a Hitachi F-7000 fluorescence
as a typical protic solvent, was shown to stabilize, and widen or even spectrometer (Hitachi, Japan). The spectral measurements were
might induce the non-centrosymmetric trigonal columnar phase carried out at room temperature.
ranges. This kind of trigonal columnar phase is extremely rare9,12,23
For investigation of the solvent-free samples the materials have
and most often hexasubstituted benzenes were previously used in been heated in an open sample to 4120 1C before investigation to
attempts to design this kind of non-centrosymmetric LC phase.14 remove all water traces, and exposure of the dry samples to humid air
Especially lyotropic trigonal liquid crystalline phases were previously was avoided during investigation. Investigation of the effects of water
unknown.1 The approach presented here provides easy access to on the samples was carried out between ordinary (non-treated)
such LC phases with broad phase ranges including ambient tem- circular microscopy glass slides (Plano) by inspection of the contact
perature. Further structural modifications could potentially lead region between 1/n and water. In addition, compounds 1/n were
to liquid crystalline materials with significant second harmonic mixed with excess water and heated between glass slides to determine
generation activity.13 Besides the non-centrosymmetric structure of the LC–Iso transition temperatures of the water saturated samples. In
the columnar phases the fluorescence properties of the investigated order to avoid evaporation of water from these samples at elevated
compounds are also of interest for potential applications.
temperature, the sandwiched glass plates were placed in an optical
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Chem., 2006, 16, 907–961; (c) H. Takezoe and Y. Takanishi,
Jpn. J. Appl. Phys., 2006, 45, 597–625; (d) A. Eremin and
A. Jakli, Soft Matter, 2013, 9, 615–637.
transparent glass pan with a planar surface together with an excess of
water around the sample.
11 K. Geese, M. Prehm and C. Tschierske, Chem. Commun.,
2014, 50, 9903–9906.
Acknowledgements
12 B. Glettner, F. Liu, X. B. Zeng, M. Prehm, U. Baumeister,
G. Ungar and C. Tschierske, Angew. Chem., Int. Ed., 2008, 47,
6080–6083.
13 S. R. Marder, Chem. Commun., 2006, 131–134.
14 (a) G. Hennrich, A. Omenat, I. Asselberghs, S. Foerier, K. Clays,
T. Verbiest and J. L. Serrano, Angew. Chem., Int. Ed., 2006, 45,
4203–4206; (b) G. Hennrich, P. D. Ortiz, E. Cavero, R. E. Hanes and
J. L. Serrano, Eur. J. Org. Chem., 2008, 4575–4579; (c) L. de Vega,
This work was supported by the National Natural Science Founda-
tion of China [no. 21364017 and 21274119], the Yunnan Natural
Science Foundation (2013FA007) and Scholarship Award for Excel-
lent Doctoral Student of Yunnan Province [no. ynuy201418].
M.P. and C.T. acknowledge the support by DFG (FOR 1145).
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