Liquid-crystalline triangle honeycomb formed by a dithiophene-based X-shaped bolaamphiphile

Article abstract of DOI:10.1002/anie.200903247

Soft supramolecular triangles: The molecule shown displays a new liquid-crystalline phase formed by a periodic array of triangular cylinders. The cylinders are fused to form a honeycomb by hydrogen-bonding networks running along the vertices, and the cells are filled by molten alkyl chains. The thickness of the walls separating the compartments is equal to the width of the π-conjugated rods.

Full text of DOI:10.1002/anie.200903247

Communications
DOI: 10.1002/anie.200903247
Liquid Crystals
Liquid-Crystalline Triangle Honeycomb Formed by a Dithiophene-
Based X-Shaped Bolaamphiphile**
Xiaohong Cheng,* Xing Dong, Guanghui Wei, Marko Prehm, and Carsten Tschierske*
The formation of well-defined two-dimensional (2D) arrays is
of current interest because of potential nanotechnological
applications. Solid-state 2D arrays have been obtained with
metallorganic frameworks and as grids on solid surfaces.[1]
Series of liquid-crystalline (LC) arrays of polygonal honey-
comb-like cylinder networks have been reported recently as
self-assembled equilibrium structures formed by T-shaped
polyphilic molecules composed of a rigid aromatic core, two
hydrogen-bonding polar end groups, and a hydrophobic
lateral chain.[2] Though these LC phases represent highly
Figure 1. Tiling patterns and their 2D lattices; the views result from
cuts through the self-assembled LC polygonal honeycomb phases
perpendicular to the cylinder long axis (white dots: columns incorpo-
dynamic fluid structures, they can form well-defined 2D
lattices over quite large areas owing to their self-healing
ability. In these LC arrays the honeycomb frames are formed
by walls of parallel arranged rodlike aromatics units aligned
perpendicular to the resulting channels. The hydrogen-bond-
ing networks between the end groups fuse these walls to form
polygonal honeycombs, and the resulting prismatic cells
inside these honeycombs (the channels) are filled with the
lateral chains.[2] The cross sections of the known honeycombs
range from rhombuses and squares to pentagons and hex-
agons (see Figure 1b–d) to a series of giant cylinders with a
circumference of up to ten molecules.[2–4]
Herein we report a new LC phase which represents the
smallest possible LC honeycomb, composed of triangular cells
(Figure 1a).[5,6] In this array the hydrogen-bonding groups are
organized to form vertices with a valence of six. This is the
first example of a self-assembled honeycomb array (including
grids and solid-state structures) where hydrogen bonding
forms vertices with such a high valence. It is also shown that
this new LC phase structure requires a change of the
molecular topology from T-shaped to X-shaped.
rating the H-bonding networks, bold lines: aromatic cores). a) A
triangle honeycomb formed by the X-shaped bolaamphiphile 2/6 and
b–d) tilings with rhombuses, squares, and pentagons, as reported
previously for T-shaped bolaamphiphiles.[2, 3]
Two series of new bolaamphiphiles incorporating a rigid
rodlike 5,5’-diphenyl-2,2’-dithiophene core[7] and with gly-
cerol groups at both ends were synthesized (see the Support-
ing Information). The T-shaped compounds 1 have an alkyl
chain at only one of the thiophene rings, whereas in
compounds 2 both thiophenes are alkylated. In the preferred
molecular conformation, in which the thiophene rings are
oriented in an alternating fashion,[8] the two lateral chains are
located at opposite sides of the aromatic core, and hence
molecules of type 2 adopt an X-shaped conformation (see
Figure S5a in the Supporting Information).[9,10] Compounds 1
and 2 form LC phases; the phase types and transition
temperatures are collated in Table 1. The LC phases of
compounds 1(12,0), 1(18,0), and 2 represent enantiotropic
phases (thermodynamic equilibrium structures), whereas
compound 1(10,0) has only a monotropic (metastable) LC
phase which rapidly crystallizes. Crystallization also takes
place upon cooling compounds 1(12,0) and 1(18,0) below the
melting point, whereas no crystallization was observed for the
double-chain compounds 2 even after prolonged storage
(> 3 months) at room temperature. Between crossed polar-
izers, all compounds show optical textures typical for LC
phases with 2D periodicity (columnar phases; see Figure 3a
and Figure S1 in the Supporting Information). In all cases
some areas of the textures appear dark between crossed
polarizers, which indicates that all columnar phases are
optically uniaxial. This restricts the possible phase symmetries
to square and hexagonal. All columnar phases are optically
negative; that is, the high refractive index axis, which is known
to be parallel to the long axis of the p-conjugated cores, is
perpendicular to the column axis as indicated by the textures
recorded with a l retarder plate where the direction of the
blue fans is parallel to the high index axis (see Figure 3b).
[*] Prof. Dr. X. Cheng, X. Dong, G. Wei
Key Laboratory of Medicinal Chemistry for Natural Resources,
Yunnan University
Kunming, Yunnan 650091 (P.R. China)
Fax: (+86)871-503-2905
E-mail: xhcheng@ynu.edu.cn
Dr. M. Prehm, Prof. Dr. C. Tschierske
Institut fꢀr Chemie
Martin-Luther-Universitꢁt Halle-Wittenberg
Kurt-Mothes-Strasse 2, 06120 Halle/Saale (Germany)
Fax: (+49)345-552-5664
E-mail: carsten.tschierske@chemie.uni-halle.de
[**] This work was supported by funds from the National Natural
Science Foundation of China (no. 20472070), the PhD Programs
Foundation of the Ministry of Education of China (no. 2007067300),
the European Science Foundation, the DFG, the EC Sixth Framework
Programme under contract ERAS-CT-2003-989409 (EUROCORES
programme, SONS project SCALES), and the Fonds der Chem-
ischen Industrie (Germany).
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Table 1: Data on bolaamphiphiles 1 and 2.[a]
Cmpd.(m,n)
Phase transitions T/8C
[DH/kJmolꢀ1
a [nm]
ncell (nwall)
]
1(10,0)
1(12,0)
1(18,0)
2(6,6)
Cr 154 Colsqu/p4mm 150 Iso
—-[31.7]—————-[4.6]
Cr 155 Colsqu/p4mm 166 Iso
—-[19.2]—————-[3.9]
Cr 95 Colsqu/p4gm 170 Iso
—-[23.8]————-[4.6]
3.0
3.0
7.5
2.8
2.8
4.3 (2.1)
4.2 (2.1)
21.4 (2.1)
3.1 (1.0)
2.6 (1.3)
Figure 3. Triangular cylinder phase (Colhex/p6mm) of compound 2(6,6).
a) Texture between crossed polarizers at T=708C; the dark areas
represent homeotropically aligned regions. b) Texture with l retarder
plate and indicatrix; in the fans the high-index axis is radial; since the
columns are tangential in the fans the high-index axis is perpendicular
to the columns.[13] c,d) Model showing the organization of the mole-
cules; the hexagonal lattice (green) coincides with the cylinder walls
formed by the p-conjugated rodlike cores. e,f) Alternative hexagonal
lattices (red) formed by hexagonal or rhombic cylinders which would
give much larger lattice parameters (L=molecular length).
Cr <20[b] Colhex/p6mm 85 Iso
—————————-[1.0]
Cr <20[b] Colsqu/p4mm 62 Iso
—————————-[1.0]
2(12,12)
[a] Determined by differential scanning calorimetry (10 Kminꢀ1) and
confirmed by polarizing spectroscopy, peak temperatures from the first
heating scan are given; abbreviations: Cr=crystalline solid, Iso=iso-
tropic liquid, Colhex/p6mm hexagonal columnar phase with plane group
p6mm (triangular cylinders), Colsqu/p4mm=square columnar phase
with plane group p4mm (square cylinders), Colsqu/p4gm=square
these square phases can be assigned to a simple lattice with
pffiffi
plane group p4mm (ratio of reciprocal spacing is 1: 2(: 2),
see Figure 2a and Table S1 and Figures S2a and S3 in the
Supporting Information). The lattice parameter is close to the
length of the molecule (L = 2.9 nm between the ends of the
glycerol groups in the most stretched conformation, see
Figure S5a in the Supporting Information). This is in line with
a square honeycomb structure of these mesophases.
columnar phase with plane group p4gm (pentagonal cylinders); ncell
number of molecules per unit cell with an assumed height of 0.45 nm,
wall =number of molecules in the cross-section of the cylinder walls (see
=
n
Table S2 in the Supporting Information); the structures of the LC phases
are shown in Figure 1; for 1(10,0) values in parenthesis refer to
monotropic phase transitions (in this case the values for clearing
transition were taken from the second heating scan). [b] No crystalliza-
tion was observed even after storage for three months.
Compound 1(18,0) has a square phase with a completely
different diffraction pattern where the 10 reflex is missing
(Figure 2b). Based on the observed diffraction pattern (hk no
conditions, h0: h = 2n and 0k: k = 2n), it can be indexed to a
p4gm plane group with asqu = 7.5 nm. The intensity distribu-
tion of the reflections in the p4gm phase of compound 1(18,0)
is identical to that observed previously for related compounds
with p4gm lattice forming pentagonal honeycomb structures
(see Figure S4 in the Supporting Information).[2b,7a,11] Also the
lattice parameter is in agreement with a pentagonal honey-
comb, where pairs of pentagonal cylinders are arranged in a
herringbone-like fashion (Figure 1d). Moreover, this phase
assignment is completely in line with the development of the
phase types in the series 1(10,0) to 1(18,0), where increasing
the chain length changes the shape of the cylinders from
square to pentagonal.[12]
Figure 2. SAXS powder pattern of a) 1(12,0) at 1438C (Colsqu/p4mm);
b) 1(18,0) at 1508C (Colsqu/p4gm); c) 2(6,6) at 708C (Colhex/p6mm)
with indexation.
Compound 2(6,6) behaves distinctly from all other com-
pounds, as it forms a hexagonal columnar phase (ratio of
pffiffi
This confirms that in the LC phases of 1 and 2 the arenes are
perpendicular to the columns, as required for polygonal
honeycombs.
The X-ray diffraction patterns show diffuse wide-angle
scatterings with maxima around d = 0.45–0.46 nm, confirming
the LC nature of all mesophases (see Figure S2 in the
Supporting Information). Indexing of the small-angle X-ray
scattering (SAXS) patterns (Figure 2) leads to the phase
assignments with plane groups shown in Table 1 (see also
Tables S1 and S2 in the Supporting Information).
reciprocal spacing is 1: 3 : 2, see Table S1 in the Supporting
Information and Figures 2c and 3a–d) from below room
temperature up to 858C. The lattice parameter ahex = 2.8 nm is
in the range of the molecular length; this is expected for
triangular cylinder phases, as in this case the position and
direction of the rodlike cores coincides with this lattice
(Figure 3c,d).[5] In any alternative honeycomb structure with
hexagonal symmetry, formed by either hexagons or rhom-
buses, ahex would be much larger and in this case would
correspond to 1.73L (see Figure 3e,f).[2b] For the proposed
triangular cylinder structure the number of molecules in a
hypothetical 3D unit cell is calculated to be ncell = 3.1 on
Square columnar phases were identified for compounds 1
and compound 2(12,12). In all cases, with exception of 1(18,0),
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Communications
average (see Table S2 in the Supporting Information). Since
structures. This fundamental concept should not be restricted
to liquid-crystalline soft-matter structures, but should also be
applicable to the design of grids on surfaces and solid-state
materials.
there are three walls per unit cell, the average thickness of the
walls (nwall = ncell/3) is only one rodlike core (nwall = 1.0). This
should result from the steric disturbance of a side-by-side
packing caused by the two lateral groups at opposite sides of
the aromatic core in the preferred X-shaped conformation.
Figure 3d shows that complete space filling and correct lattice
parameters are achieved in this arrangement.[14,15]
Received: June 16, 2009
Published online: September 15, 2009
In contrast to compounds 2 with two lateral chains[16] all
compounds 1 with only one chain form cylinder walls where
the average thickness of the walls (nwall, see Table 1) is about
two aromatic units placed side-by-side (Figure 4d,e). This
Keywords: hydrogen bonding · liquid crystals · nanostructures ·
.
self-assembly · soft matter
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Figure 4. Dependence of wall thickness on the number of lateral
chains, shown for square honeycomb phases; a,e) cuts through one
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a,b) Single-wall structure of X-shaped bolaamphiphiles; c) staggered
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dependence of the cylinder wall thickness on the number of
chains at the aromatic unit leads to the surprising observation
that compound 2(6,6), with two hexyl chains, forms triangular
cylinders, whereas compound 1(12,0), with its single lateral
chain occupying approximately the same volume, and even
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larger square cylinders. A major reason should be that in the
double-wall honeycombs of the T-shaped molecules the
aromatic cores and the polar glycerol groups are located
inside the unit cells, and these groups fill a significant part of
the space inside the cells (see Figure 4e). In contrast, in the
case of single-wall cylinders formed by X-shaped molecules
the bolaamphiphilic units are positioned on the lattice and
hence, they contribute to the space filling in two adjacent
cells, effectively leaving more space available for the lateral
chains (Figure 4a).[17] Moreover, the relatively short hexyl
chains (length 0.9 nm) would not be able to reach the centers
of square cylinders built up by the relatively long diphenyldi-
thiophene cores (side length ca. 3 nm), thus disfavoring any
honeycomb with square or larger cells.
In summary, the triangular cylinder phase is a new LC
phase of ternary bolaamphiphiles, and it is shown that the
change from a T-shaped to an X-shaped structure provides an
additional tool for the directed design of polygonal honey-
combs. In addition, the dynamic hydrogen-bonding networks
provided by the flexible glycerol groups[2] allow access to
networks having vertices with high valences which cannot be
achieved with the more restricted classical hydrogen-bonding
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Chemie
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[14] An alternative structure of the Colhex phase with the same lattice
parameter and negative birefringence could be possible if the
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