Carbohydrate rod conjugates: Ternary rod-coil molecules forming complex liquid crystal structures

Article abstract of DOI:10.1021/ja0535357

T-shaped polyphilic triblock molecules, consisting of a rodlike p-terphenyl unit, a hydrophilic and flexible laterally attached oligo(oxyethylene) chain terminated by an 1-acylamino-1-deoxy-D-sorbitol unit, and two end-attached lipophilic alkyl chains, have been synthesized by palladium-catalyzed cross-coupling reactions as the key steps. The thermotropic liquid crystalline behavior of these compounds was investigated by polarized light microscopy, differential scanning calorimetry (DSC), and X-ray scattering. We investigated the mode of self-organization as a function of the length and position of the lateral polar chain and the length of the terminal alkyl chains. Depending on the size of the polar and lipophilic segments, a series of unusual liquid crystalline phases was detected. In three of these phases, the space is divided into three distinct periodic subspaces. In addition to a hexagonal channeled layer phase (ChL_hex) consisting of layers that are penetrated by polar columns, there are also two honeycomb-like network structures formed by square (Col_squ/p4mm) or pentagonal cylinders (Col_squ/p4gm) . The cylinder walls consist of the terphenyl units fused by columns of alkyl chains, and the interior contains the polar side chains. In addition, a hexagonal columnar phase was observed in which the polar columns are organized in a continuum of terphenyls and alkyl chains with an organization of the terphenyl cores tangentially around the columns with the long axis perpendicular to the columns. For one compound, a reversal of birefringence was observed, which is explained by a reorientation of the terphenyl cores. The addition of protic solvents induces lamellar phases.

Full text of DOI:10.1021/ja0535357

Published on Web 11/02/2005
Carbohydrate Rod Conjugates: Ternary Rod-Coil Molecules
Forming Complex Liquid Crystal Structures
Bin Chen,† Ute Baumeister,‡ Gerhard Pelzl,‡ Malay Kumar Das,‡ Xiangbing Zeng,§
Goran Ungar,§ and Carsten Tschierske*,†
Contribution from the Institute of Organic Chemistry, UniVersity Halle, Kurt-Mothes-Strasse 2,
D-06120 Halle, Germany, Institute of Physical Chemistry, UniVersity Halle, Mu¨hlpforte 1,
D-06108 Halle, Germany, and Centre for Molecular Materials, UniVersity of Sheffield,
Robert Hadfield Building Mappin Street, Sheffield S1 3JD, Great Britain
Received May 31, 2005; Revised Manuscript Received September 14, 2005; E-mail: carsten.tschierske@chemie.uni-halle.de
Abstract: T-shaped polyphilic triblock molecules, consisting of a rodlike p-terphenyl unit, a hydrophilic
and flexible laterally attached oligo(oxyethylene) chain terminated by an 1-acylamino-1-deoxy-D-sorbitol
unit, and two end-attached lipophilic alkyl chains, have been synthesized by palladium-catalyzed cross-
coupling reactions as the key steps. The thermotropic liquid crystalline behavior of these compounds was
investigated by polarized light microscopy, differential scanning calorimetry (DSC), and X-ray scattering.
We investigated the mode of self-organization as a function of the length and position of the lateral polar
chain and the length of the terminal alkyl chains. Depending on the size of the polar and lipophilic segments,
a series of unusual liquid crystalline phases was detected. In three of these phases, the space is divided
into three distinct periodic subspaces. In addition to a hexagonal channeled layer phase (ChLhex) consisting
of layers that are penetrated by polar columns, there are also two honeycomb-like network structures formed
by square (Colsqu/p4mm) or pentagonal cylinders (Colsqu/p4gm). The cylinder walls consist of the terphenyl
units fused by columns of alkyl chains, and the interior contains the polar side chains. In addition, a hexagonal
columnar phase was observed in which the polar columns are organized in a continuum of terphenyls and
alkyl chains with an organization of the terphenyl cores tangentially around the columns with the long axis
perpendicular to the columns. For one compound, a reversal of birefringence was observed, which is
explained by a reorientation of the terphenyl cores. The addition of protic solvents induces lamellar phases.
Introduction
to give rise to positional and orientational long-range order.
Increasing the complexity of liquid crystals6-8 should open new
perspectives for their use as sophisticated functional materials.9
Furthermore, it is hoped that such systems might contribute to
the fundamental understanding of self-assembly in other com-
plex systems, such as solid crystals and living systems.
On the basis of the concept of competitive polyphilicity, the
bolaamphiphiles shown in Figure 1, incorporating a rigid
biphenyl core, a pair of 2,3-dihydroxypropoxy terminal groups,
The optimization of materials properties requires control of
the organization of molecules.1 The design of solid crystals with
a predetermined structure, for example, is still more a hit-or-
miss activity rather than a rational design.2,3 The difficulty lies
in the large number of intermolecular interactions with similar
energies that are hard to predict and evaluate accurately. In
contrast, in fluid systems some of the interactions are averaged
out by molecular motion, leaving only the strongest determining
the overall structure and simplifying the problem considerably.
Liquid crystals4 are ordered fluids in which the segregation of
incompatible units5 and parallel alignment of rigid units combine
(5) (a) Tschierske, C. J. Mater. Chem. 1998, 8, 1485-1508. (b) Chen, W.;
Wunderlich, B. Macromol. Chem. Phys. 1999, 200, 283-311. (c) Tsch-
ierske, C. J. Mater. Chem. 2001, 11, 2647-2671.
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Auze´ly-Velty, R.; Benvegnu, T.; Plusquellec, D. Chem. Commun. 1998,
2057-2070. (b) Tschierske, C. Ann. Rep. Prog. Chem., Ser. C 2001, 97,
191-267. (c) Tschierske, C. Curr. Opin. Colloid Interface Sci. 2002, 7,
69-80.
† Institute of Organic Chemistry.
‡ Institute of Physical Chemistry.
§ University of Sheffield.
(1) (a) Petty, M. C.; Bryce, M. R.; Bloor, D. Introduction to Molecular
Electronics; Edward Arnold: London, 1995. (b) Simon, J.; Bassoul, P.
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Wiley: Chichester, U.K., 2000.
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A.; Serrano, J. L. J. Mater. Chem. 2001, 11, 2808-2813. (b) Ungar, G.;
Liu, Y.-S.; Zeng, X.-B.; Percec, V.; Cho, W.-D. Science 2003, 299, 1208-
1211. (c) Zeng, X.-B.; Ungar, G.; Liu, Y.-S.; Percec, V.; Dulcey, A. E.;
Hobs, J. Nature 2004, 428, 157-160. (d) Percec, V.; Mitchell, C. M.; Cho,
W.-D.; Uchida, S.; Glodde, M.; Ungar, G.; Zeng, X.; Liu, Y.; Balagurusamy,
V. S. K.; Heiney, P. A. J. Am. Chem. Soc. 2004, 126, 6078-6094. (e)
Gehringer, L.; Bourgogne, C.; Guillon, D.; Donnio, B. J. Am. Chem. Soc.
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(c) Zaworotko, M. J. Chem. Commun. 2001, 1-9. (d) Sarma, J. A. R. P.;
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K. Cryst. Growth Des. 2002, 2, 565-474. (f) Robson, R. J. Chem. Soc.,
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(4) Demus, D.; Goodby, J.; Gray, G. W.; Spiess, H. W.; Vill, V. Handbook of
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9
16578
J. AM. CHEM. SOC. 2005, 127, 16578-16591
10.1021/ja0535357 CCC: $30.25 © 2005 American Chemical Society

Rod−Coil Molecules Forming Liquid Crystal Structures
A R T I C L E S
Figure 1. Mesophases reported for calamitic bolaamphiphiles with nonpolar lateral chains,10a,b abbreviations of the mesophase types: SmA ) smectic A
phase (molecules are organized in layers without in-plane order and in average perpendicular to the layers). Colrec ) rectangular columnar phase. Colsqu
)
square columnar phase. Colhex ) hexagonal columnar phase. Lam ) laminated phase; the Lamiso phase is built up by a sequence of two sets of isotropic
sublayers; in the LamN phase the rodlike units have an orientational order, and these units are arranged in average parallel to the layers; in the LamSm phase
the molecules are thought to adopt an additional positional order within the layers.
and a lipophilic lateral chain, have been developed recently.10,11
For these compounds, a series of new and complex types of
organizations in liquid crystal (LC) systems has been discovered.
As shown in Figure 1, different honeycomb-like cylinder
structures10 and lamellar phases11 were detected.
crystalline phases in total are found. The evolution of mesophase
structures is monitored in a systematic way as a function of
length of the terminal alkyl chains and as a function of the
number of oxyethylene units in the polar chains. In addition,
the effects of the position of the polar group, of branching of
the alkyl chains, and of the nonsymmetric distribution of these
chains at both ends are reported. The results provide a lesson
in principles of molecular self-assembly in soft matter and offer
clues for designing molecules and tailoring complex superstruc-
tures in a predictable way. Although there are some similarities
with the phase sequence observed for the bolaamphiphiles, there
are also significant differences due to the exchange of positions
of the amphiphilic groups. In particular, a new hexagonal
columnar mesophase is reported here for the first time. Its polar
columns penetrate an orientationally ordered continuum, and it
shows a temperature-induced reversal of mesogen alignment
with respect to the column axis. Furthermore, it is shown that
lamellar phases are induced by protic solvents, indicating
amphotropic17 behavior of these compounds.
In this report, another class of ternary block molecules is
described in which the position of amphiphilic groups is
reversed; that is, the polar group is attached to a lateral position
on the rigid core (1,1′-4′1′′-terphenyl units in all cases), and
the lipophilic chains are tethered at the termini. Previously, we
have sporadically reported selected examples of such facial
amphiphiles with the lateral chains ending with different polar
groups, mainly carboxylate,12 COOH groups,13 or diol14 groups.
Herein, the development of soft-matter nanostructures is reported
for several complete series of facial amphiphiles with a bulky
carbohydrate unit as the polar group (1-acylamino-1-deoxy-D-
sorbitol derivatives).15,16 A series of seven different liquid
(10) (a) Ko¨lbel, M.; Beyersdorff, T.; Cheng, X.-H.; Tschierske, C. J. Am. Chem.
Soc. 2001, 123, 6809-6818. (b) Cheng, X.-H.; Prehm, M.; Das, M. K.;
Kain, J.; Baumeister, U.; Diele, S.; Leine, D.; Blume, A.; Tschierske, C. J.
Am. Chem. Soc. 2003, 125, 10977-10996. (c) Cheng, X.-H.; Das, M. K.;
Baumeister, U.; Diele, S.; Tschierske, C. J. Am. Chem. Soc. 2004, 126,
12930-12940.
The general structure of the compounds investigated is shown
below. The polar hydrogen-bonding units are attached via an
amide group and flexible oligo(oxyethylene) chains of variable
length (n ) 0-6) to the rigid and linear p-terphenyl core either
(11) (a) Prehm, M.; Cheng, X.-H.; Diele, S.; Das, M. K.; Tschierske, C. J. Am.
Chem. Soc. 2002, 124, 12072-12073. (b) Cheng, X.-H.; Das, M. K.; Diele,
S.; Tschierske, C. Angew. Chem., Int. Ed. 2002, 41, 4031-4035. (c) Prehm,
M.; Diele, S.; Das, M. K.; Tschierske, C. J. Am Chem. Soc. 2003, 125,
614-615. (d) Patel, N. M.; Dodge, M. R.; Zhu, M.-H.; Petschek, R. G.;
Rosenblatt, C.; Prehm, M.; Tschierske, C. Phy. ReV. Lett. 2004, 92, 015501.
(e) Patel, N. M.; Syed, I. M.; Rosenblatt, C.; Prehm, M.; Tschierske, C.
Liq. Cryst. 2005, 32, 55-61.
(15) Preliminary communication: Chen, B.; Baumeister, U.; Diele, S.; Das, M.
K.; Zeng, X.-B.; Ungar, G.; Tschierske, C. J. Am. Chem. Soc. 2004, 126,
8608-8609.
(16) Carbohydrate LC incorporating terminally attached rodlike segments: (a)
Mu¨ller, H.; Tschierske, C. Chem. Commun. 1995, 645-646. (b) Ewing,
D. F.; Glew, M., Goodby, J. W.; Haley, J. A.; Kelly, S. M.; Komanschek,
B. U.; Letellier, P.; Mackenzie, G.; Mehl, G. J. Mater. Chem. 1998, 8,
871-880. (c) Lafont, N. L. D.; Dumoulin, F.; Boullanger, P.; Mackenzie,
G.; Kouwer, P. H. J.; Goodby J. W. J. Am. Chem. Soc. 2003, 125, 15499-
15506.
(12) Chen, B.; Zeng, X.-B.; Baumeister, U.; Diele, S.; Ungar, G.; Tschierske,
C. Angew. Chem., Int. Ed. 2004, 43, 4621-4625.
(13) Chen, B.; Zeng, X.-B.; Baumeister, U.; Ungar, G.; Tschierske, C. Science
2005, 307, 96-99.
(14) (a) Schro¨ter, J. A.; Plehnert, R.; Tschierske, C.; Katholy, S.; Janietz, D.;
Penacorada, F.; Brehmer, L. Langmuir 1997, 13, 796-800. (b) Plehnert,
R.; Schro¨ter, J. A.; Tschierske, C. Langmuir 1999, 15, 3773-3781.
(17) Reviews about amphotropic LC: (a) Tschierske, C. Prog. Polym. Sci. 1996,
21, 775-852. (b) Tschierske, C. Curr. Opin. Colloid Interface Sci. 2002,
7, 355-370.
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A R T I C L E S
Chen et al.
Scheme 1. Synthetic Route to the Facial Amphiphiles Am/na
in a central position (compounds Am/n, where m is the length
of the terminal alkyl chains and n is the number of oxyethylene
units in the lateral chain) or in a peripheral position (compounds
Bm/n).18 The flexible polyether chain has two functions: it
increases the size of the lateral group and decouples the
carbohydrate unit from the rigid core, thus reducing the melting
point and broadening the LC range. The terminal chains are
linear alkyls with identical length at both ends (m ) 4-16) in
most cases. In selected examples, the terminal chains are
(racemic) branched 3,7-dimethyloctyl groups (A10*/3 and
A10*/4), whereas in others, the chains at each end have very
different lengths (hexyl and hexadecyl).
Results and Discussion
1. Synthesis. Three different strategies were used to synthe-
size compounds Am/n containing two identical alkyl chains and
the lateral polar chain in the central 2′-position. The routes to
the carboxylic acids are outlined in Scheme 1 (see also Schemes
S1 and S2 in the Supporting Information). In all synthetic routes,
the key step is a Suzuki-type coupling19 of a 2-substituted 1,4-
dichlorobenzene derivative with two equiv of a 4-substituted
benzeneboronic acid using Pd(OAc)2, 2-(di-tert-butylphosphi-
no)biphenyl as a catalyst and KF as the base (Buchwald
procedure).20 The amides Am/n were prepared from these acids
by esterification with pentafluorophenol using DCC as a
condensing agent followed by aminolysis of the resulting
pentafluorophenyl esters21 with 1-amino-1-deoxy-D-sorbitol. The
2-substituted compounds Bm/n and the compounds Am.m′/n
with two different alkyl chains at the terphenyl core were built
up in a stepwise manner as shown in Schemes S3-S5 of the
Supporting Information. All final products were purified by
column chromatography on silica gel, then by repeated crystal-
a Reagents and conditions: i) 1. TsCl, Py, 0 °C; 2. 2,5-dichlorophenol,
K2CO3, CH3CN, (n-Bu)4NI, reflux, 8 h; ii) Pd(OAc)2, 2-(di-tert-butylphos-
phino)biphenyl, KF, THF, rt, 24 h; iii) NaOH, H2O, reflux 16 h, then HCl
(10% aq), Et2O, rt, 1 h; 16 h; iV) Pd(OH)2, MeOH, cyclohexene, reflux; V)
CmH2m+1Br, K2CO3, CH3CN, (n-Bu)4NI, reflux, 8 h; Vi) K2CO3, CH3CN,
(n-Bu)4NI, reflux, 8 h.; Vii) pentafluorophenol, DCC, THF, 20 °C, 24 h;
Viii) 1-amino-1-deoxy-D-sorbitol, THF, 24 h.
lization from appropriate solvents and characterized by 1H, 13
C
compounds were investigated using synchrotron radiation, and
the precise diffraction data obtained in this way were used to
calculate electron density maps23 that additionally confirmed
the predicted mesophase structures. The transition temperatures
and corresponding enthalpy values are collected in Tables 1-3.
NMR, and elemental analysis. The purity was additionally
checked by TLC. The experimental details and analytical data
of all compounds are collated in the SI.
2. Mesomorphic Properties. The synthesized compounds
were investigated by polarized light microscopy, differential
scanning calorimetry (DSC), and X-ray scattering.22 Selected
(22) Most of the compounds are hygroscopic. To remove traces of water, the
open samples were heated to 170 °C for ca. 5 s; then, they were immediately
covered or sealed and investigated. The weight loss corresponds to the
amount of water absorbed by the material, which can be determined
according to: Dunkel, R.; Hahn, M.; Borisch, K.; Neumann, B.; Ru¨ttinger,
H.-H.; Tschierske, C. Liq. Cryst. 1998, 24, 211-213. Filling of the
capillaries for X-ray investigations was also done at that temperature.
(23) (a) Balagurusamy, V. S. K.; Ungar, G.; Percec, V.; Johansson, G. J. Am.
Chem. Soc. 1997 119, 1539-1555. (b) Dukeson, D. R.; Ungar, G.;
Balagurusamy, V. S. K.; Percec, V.; Johansson, G.; Glodde, M. J. Am.
Chem. Soc. 2003, 125, 15974-15980.
(18) Rod-coil molecules with a terminally attached carbohydrate unit: Kim,
B.-S.; Yang, W.-Y.; Ryu, J.-H.; Yoo, Y.-S.; Lee, M. Chem. Commun. 2005,
2035-2037.
(19) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483.
(20) Wolfe, J. P.; Singer, R. A.; Yang, B. H.; Buchwald, S. L. J. Am. Chem.
Soc. 1999, 121, 9550-9561.
(21) Baars, M. W. P. L.; So¨ntjens, S. H. M.; Fischer, H. M.; Peerlings H. W. I.;
Meijer, E. W. Chem.-Eur. J., 1998, 4, 2456-2466.
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Rod−Coil Molecules Forming Liquid Crystal Structures
A R T I C L E S
Table 1. Mesophases, Phase Transition Temperatures, Phase Transition Enthalpy Values, and Other Parameters of the Facial Amphiphiles
Am/na
a Enthalpy values are shown in the lower lines in italics; the transition temperatures were determined by DSC (second heating scan, 10 K min-1).
b Abbreviations: Cr ) crystalline phase; G ) glassy state of the LC phase; Colsqu ) square columnar phase, the lattice type is additionally given; Colhex
)
d
hexagonal columnar phase; ChLhex ) hexagonal channeled layer phase; I ) isotropic liquid. c a, c, ) lattice parameters; d ) layer spacing. Vcell ) volume
e
of a unit cell (for the columnar phases, a stratum height of h ) 0.45 nm was assumed).24 Vmol ) molecular volume calculated using the crystal volume
f
increments reported by Immirzi.25 ncell ) number of molecules per unit cell, calculated as ncell ) Vcell/Vmol; ncyl ) average number of molecules arranged
in a stratum around each polar column (only given for the p4gm phases, where four columns are organized in a unit cell, for the p4mm phases; this value
corresponds to ncell). g Two coexisting mesophaes, asqu1 refers to the p4gm phase, asqu2 refers to the p4mm phase. h Structure of this compound is slightly
different from the general formula as there is a direct coupling of the CONH group to the aromatic core. i Values for the ChLhex phase. j Racemic 3,7-
dimethyloctyloxy chains.
All compounds show enantiotropic (thermodynamically stable)
liquid crystalline phases over a broad temperature range. Some
compounds do not crystallize even after prolonged storage (>1
year) and form glassy LC materials at reduced temperature. The
mesophase stability and the mesophase types are strongly
dependent upon the size of the polar lateral chain and the length
of the terminal alkyl chains.
2.1. Effect of the Size of the Lateral Polar Group. Figure
2 shows the dependence of the properties of compounds
A10/n, having two terminal decyloxy chains, on the length of
the oligo(oxyethylene) chain connecting the 1-acylamino-1-
deoxy-D-sorbitol unit to the rigid terphenyl core. With increasing
length of this chain, the transition temperatures decrease, and
three different LC phases are observed: SmA, ChLhex, and
Colsqu/p4mm.
2.1.1. The SmA Layer Structure. Compound A10/0,26 in
which the carbohydrate unit is directly coupled to the rigid
terphenyl core, shows a simple smectic (SmA) phase, as
indicated by the typical texture (fanlike texture and optically
isotropic homeotropic alignment after shearing can be seen
between crossed polarizers) and X-ray diffraction pattern (layer
reflection and diffuse scattering in the wide-angle region).26 In
this liquid crystalline phase, the molecules are organized in
layers, where the terphenyl units are arranged, on average,
perpendicular to the layer planes but without in-plane order.
These sublayers are separated by layers of alkyl chains. The
(24) The value h ) 0.45 nm corresponds to the maximum of the wide-angle
scattering in the diffraction pattern.
(25) Immirzi, A.; Perini, B. Acta Crystallogr., Sect. A 1977, 33, 216-218.
(26) Plehnert, R.; Schro¨ter, J. A.; Tschierske C. J. Mater. Chem. 1998, 8, 2611-
2626.
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A R T I C L E S
Chen et al.
Figure 2. Mesophase types and transition temperatures (T/°C) of com-
pounds A10/n, depending on the number of oxyethylene units n. Compound
A10/026 is slightly different from the other compounds as the CONH group
is directly attached to the aromatic core.
Figure 3. ChLhex phase of A10/1: (a) texture of birefringent mosaics as
seen between crossed polarizers at 130 °C; (b) X-ray diffraction pattern of
an aligned sample of this mesophase at 100 °C; (c) model of the molecular
organization in the ChLhex phase.
terphenyl layers must also incorporate the lateral carbohydrate
units. This is surprising because it can be expected that these
bulky groups should strongly disturb such an arrangement. The
layer spacing (d ) 3.3 nm) is significantly smaller than the
molecular length. (L ) 4.1 nm between the ends of the alkyl
chains, assuming the most stretched all-trans conformation as
measured with CPK models, see Figure S2.) This indicates either
a high degree of disorder within the layers or a partial
intercalation of the alkyl chains. Although it can be expected
that the lateral groups, which are normally incompatible with
the rigid and lipophilic aromatic cores, would segregate into
their own distinct domains, no indication of such domains can
be found in the X-ray diffraction pattern. Probably, hydrogen
bonding between the polyhydroxy groups and also hydrogen
bonding to the π-systems of the aromatic cores and the O atoms
of the alkoxy chains have a stabilizing effect upon this layer
arrangement, which compensates for the destabilizing steric
effects.27
2.1.2. The Hexagonal Channeled Layer Phase. Introduction
of the 1,4-dioxapentylene spacer unit between the terphenyl core
and the polar group (compound A10/1) does not change the
isotropization temperature, but it changes the mesophase
structure and eliminates the crystalline phase. The mesophase
of this compound appears almost completely black between
crossed polarizers. Additionally, this mesophase shows a very
high viscosity and a viscoelastic response to mechanical stress,
indicating a structure with 3D order. In some regions, the texture
is characterized by strongly birefringent mosaics (see Figure
3a). All of these observations indicate an optically uniaxial
mesophase with a 3D-lattice. The X-ray diffraction pattern of
compound A10/1 shows only diffuse wide-angle scattering,
which indicates that this is a liquid crystalline phase. In the
small-angle region (see diffraction pattern of an aligned sample
in Figure 3b), the positions of the Bragg reflections (001, 100,
101, 110, and 002) indicate a 3D hexagonal lattice (P6/mmm)
with parameters a ) 4.0 nm and c ) 3.76 nm. All of these
optical, rheological, and crystallographic data are in line with a
hexagonal channeled layer phase (ChLhex). This mesophase was
recently investigated in detail, and the structure (Figure 3c) was
confirmed by electron density calculations using high-resolution
scattering data.12 Accordingly, this mesophase consists of
alternating layers of aromatic units and aliphatic chains,
penetrated at right angles by columns with undulating profiles
containing the polar lateral groups (Figure 3c). The columns
are arranged on a 2D hexagonal lattice. Thus, the structure
consists of layers perforated by an array of polar channels. The
proposed structure is in complete agreement with the molecular
dimensions, as for example the length of an extended molecule
of 10/1 (L ) 4.1 nm) fits very well with the parameter c )
3.76 nm.28 The likely reason for the appearance of this unusual
(27) Hildebrandt, F.; Schro¨ter, J. A.; Tschierske, C.; Festag, R.; Kleppinger,
(28) This small negative deviation (d ) 0.92L) is typical for LC phases with a
R.; Wendorff, J. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 1631-1633.
layer structure and is due to conformational disorder of the alkyl chains.
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Rod−Coil Molecules Forming Liquid Crystal Structures
A R T I C L E S
Figure 4. Representative textures of compound A10/2 as seen between
crossed polarizers: (a) Colsqu/p4mm phase at 142 °C (homogeneous
alignment); (b) Colsqu/p4mm phase at 142 °C (predominately homeotropic
alignment); (c) optically isotropic texture of the homeotropically aligned
ChLhex phase as obtained by cooling the homogeneously aligned Colsqu
/
p4mm phase to 53 °C (same region as a); (d) birefringent texture of the
ChLhex phase as obtained by cooling the homeotropically aligned Colsqu
/
p4mm phase to 53 °C (same region as b).
mesophase is the enlargement of the incompatible polar lateral
group brought about by the introduction of the spacer between
the terphenyl core and the carbohydrate unit. This leads to
segregation of the lateral groups from the aromatic cores and
enables fusion of the polar domains from adjacent layers into
infinite polar columns.
Figure 5. Colsqu/p4mm phase of compound A10/4: (a) X-ray diffraction
pattern at 137 °C; (b) model of the Colsqu/p4mm phase; (c) electron density
map of the Colsqu/p4mm phase15 (combined surface and contour plot); the
blue regions are the high electron density domains (polar columns), and
the red regions are the low electron density areas (alkyl chains).
2.1.3. The Colsqu/p4mm Square Cylinder Phase. The
mesophase of compounds A10/3-A10/6 and the high-temper-
ature phase of compound A10/2 show spherulitic textures or
filament textures with optically isotropic regions, as shown in
Figure 4a,b. This mesophase exhibits a much lower viscosity
than the ChLhex phase. All these observations indicate a fluid
and optically uniaxial columnar phase. The X-ray diffraction
pattern of the aligned mesophase of compound A10/4 is shown
in Figure 5a. In the wide-angle region, a diffuse outer scattering
indicates the liquid crystal nature of the phase. In the small-
angle region, the sharp reflections (10, 11, and 01) can be
indexed on the basis of a Colsqu/p4mm lattice with the lattice
parameter asqu ) 4.0 nm.
This diffraction pattern can be explained by the model shown
in Figure 5b. Accordingly, the aromatic cores form cylinder
shells with a square-shaped cross section enclosing columns
containing the segregated lateral polar groups. The nonpolar
alkyl chains form columns located at the corners of the squares,
running along the edges of the cylinders. These alkyl columns
interconnect the cylinder walls.29 The reconstructed electron
density map of the Colsqu/p4mm phase of compound A10/4
(Figure 5c)15 confirms this model. It clearly shows the square
arrangement of the alkyl columns (low electron density, red
regions) and the polar columns (high electron density, blue
regions). The lattice parameter asqu increases from 3.7 nm for
A10/2 to 4.2 nm for A10/6, which indicates that the columns
expand to a certain degree and that this expansion is limited by
the length of the molecule in the most stretched conformation
(L ) 4.1 nm).
For compound A10/2, which has a relatively short lateral
chain, a second mesophase has been observed below the Colsqu
/
p4mm phase. The typical spherulitic texture of the Colsqu/p4mm
phase (see Figure 4a) breaks up upon cooling at 115 °C, and
the whole sample becomes optically isotropic at 102 °C (Figure
4c).30 Under other experimental conditions, the Colsqu/p4mm
phase can be grown with predominately homeotropic alignment
(columns predominately perpendicular to the glass surface),
characterized by large optically isotropic domains and birefrin-
gent filaments (see Figure 4b). If this sample is cooled in the
same temperature range between 115 °C and 102 °C, a
birefringent mosaic texture develops; i.e., most of the optically
isotropic areas become birefringent (Figure 4d). The textural
features of this low-temperature phase are very similar to those
of the ChLhex phase of compound A10/1. In addition, the
viscosity of the low-temperature phase is significantly higher
than in the Colsqu phase. All of these observations suggest that
the low-temperature phase should represent a ChLhex phase. (For
X-ray data, see Table S1 in the Supporting Information.)31
(29) The mesophases reported herein represent ordered fluids, which means that
there is no long-range order in atomic or molecular positions. Hence, there
are no well-defined crystal-like structures, as might be suggested by the
schematic models shown. Instead, there are distinct regions with enhanced
concentration of aromatic cores, polar groups, and aliphatic chains. Due to
this disorder, only diffuse scattering can be found in the wide-angle region
of the diffraction pattern.
(30) The transition takes place between 102 °C and 115 °C on heating and
cooling, with the DSC peak maximum at T ) 107 °C; the broadness of the
transition might be due to the fact that a 3D ordered mesophase is involved
in the transition and that a complete reorganization of the molecules takes
place.
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A R T I C L E S
Chen et al.
Figure 6. Mesophase types and transition temperatures (T/°C) of com-
pounds Am/3 as a function of the length m of terminal alkyl chains (for
A6/3 the p4gm and the p4mm phase coexist over the whole temperature
range as indicated by X-ray scattering (see Table S1 of the Supporting
Information).
Figure 7. Mesophase types and transition temperatures (T/°C) of com-
pounds Am/4 as a function of the length of terminal alkyl chains. The Colhex
phases have either negative (bright yellow) or positive (dark yellow)
birefringence; in compound A4/4, a change of the sign of birefringence is
seen within the temperature range of the Colhex phase.
Hence, as shown in Figure 2, elongation of the polar lateral
chain leads to a transition from simple layers via hexagonal
channeled layers to a square cylinder structure.
2.2. Influence of the Length of the Terminal Alkyl Chains.
Figure 6 graphically summarizes the dependence of the phase
transitions on the length of the terminal alkyl chains for the
homologous series of compounds Am/3, which have the polar
group connected by a tris(oxyethylene) spacer unit to the
p-terphenyl core. As the terminal alkyl chains are reduced from
hexadecyl (A16/3) to decyl (A10/3), the mesophase types change
from ChLhex to Colsqu/p4mm. Compound A6/3 has two coexist-
ing mesophases, the Colsqu/p4mm (asqu ) 3.6 nm) and another
square columnar phase with a p4gm lattice and a much larger
lattice parameter (asqu ) 7.9 nm). Compound A4/3, with the
shortest alkyl chains, forms a hexagonal columnar phase. Hence,
(T ) 98 °C); this value is about twice as large as that observed
for the Colsqu/p4mm phases (see Table 1). This parameter is
also more than twice as large as the molecular length (L ) 3.1
nm), and this excludes a simple square cylinder structure. About
25 to 26 molecules are arranged in a unit cell with a height of
0.45 nm.24 This Colsqu/p4gm phase is also made up of cylinders,
but in this case with a pentagonal cross-sectional shape. The
polar lateral chains fill the interior of these cylinders, and the
columns of the alkyl chains interconnect the cylinder walls.
These pentagonal cylinders can only organize in a regular and
periodic way if they form cylinder pairs (indicated by dotted
green lines in Figure 8b). These pairs have symmetry higher
than that of the individual pentagonal cylinder and adopt a 90°
turn herringbone-like packing, leading to the p4gm-lattice.
with decreasing alkyl chain length, the sequence ChLhex
-
Colsqu/p4mm - Colsqu/p4gm - Colhex is observed.
A similar sequence of phases was observed for the series of
compounds Am/4, having a longer tetra(oxyethylene) spacer unit
(see Figure 7). Due to the larger size of the lateral group in
compounds Am/4, the lattice parameters are slightly enlarged,
and the phase types are shifted to compounds with longer alkyl
The proposed pentagonal cylinder structure of the Colsqu/p4gm
phase was confirmed by electron density calculations based on
high-resolution powder X-ray diffraction pattern (synchrotron
X-ray source). In the electron density map (see Figure 8c), the
high electron density regions (polar chains, shown in blue and
purple) form pentagons that are separated by squares and
triangles of low electron density (yellow to red color), containing
the alkyl chains.13 The network of terphenyl cores can be
regarded as a pentagonal net interconnected by threefold and
fourfold nodes (columns of alkyl chains). The interior of the
cylinder frame is filled by the polar columns.29
chain length. For example, compound A6/4 has a uniform Colsqu
/
p4gm phase (asqu ) 8.0 nm) and a Colhex phase at higher
temperature. In addition, compound A4/4 shows a reversible
inversion of the birefringence within the temperature range of
the Colhex phase at 91 °C, which is not found for the related
compound A4/3.
2.2.1. The Colsqu/p4gm Pentagonal Cylinder Phase. The
X-ray diffraction pattern of the square columnar phase of
compound A6/4 (see Figure 8a) is clearly distinct from those
of the Colsqu/p4mm phases. There are numerous small-angle
reflections, which can be indexed to a square lattice of the plane
group p4gm. The calculated lattice parameter is asqu ) 8.0 nm
2.2.2. Hexagonal Columnar Phases: Columns In An
Ordered Continuum. Extending the polar chain (see Table 1,
compounds A6/n), raising the temperature (compounds A4/2,
A6/4, and A6/5), or reducing the alkyl chain length in both Am/3
and Am/4 series leads to a transition from square to hexagonal
columnar phases. The change in birefringence pattern at the
Colsqu/p4gm to Colhex phase transition of compound A6/4 is
shown in Figure 9a. The texture itself does not change, but the
phase transition can be detected by a slight decrease in
(31) In addition, these changes in texture indicate that the direction of the polar
columns with respect to the surfaces changes at the phase transition Colsqu
/
p4gm to ChLhex, either from parallel to perpendicular (Figure 5 a to c) or
from perpendicular to parallel (Figure 5 b to d).
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Rod−Coil Molecules Forming Liquid Crystal Structures
A R T I C L E S
Figure 8. Colsqu/p4gm phase of compound A6/4: (a) X-ray diffraction
pattern of an aligned sample of the Colsqu/p4gm phase13 at 98 °C; (b) model
of the Colsqu/p4gm phase: blue columns incorporate the polar lateral groups,
white columns incorporate the alkyl chains interconnecting the terphenyl
units at the nodes; (c) electron density map of the Colsqu/p4gm phase; the
high electron density regions (polar chains) are shown in blue and purple
and low electron density regions (alkyl chains) in yellow to red.13
Figure 9. Hexagonal columnar phase of compound A6/4: (a) transition
from the Colsqu/p4gm phase (upper left) to the Colhex phase (lower right)
observed at 104 °C (the black regions are air bubbles; arrows indicate the
borderline between the two phases); (b) small-angle range of the X-ray
diffraction pattern of an aligned sample at 106 °C; (c) electron density map
calculated from synchrotron X-ray data at T ) 108 °C; high electron density
regions (polar chains) are shown in blue and purple, and low electron density
regions (alkyl chains plus aromatic cores) are shown in yellow to red.
birefringence. X-ray diffraction confirms a 2D hexagonal lattice
(see Figure 9b), but the lattice parameter (ahex ) 4.3 nm) is
significantly smaller than expected for the hexagonal cylinder
structure shown in Figure 10a. The expected value of the lattice
parameter for this structure should correspond to the diameter
of the cylinder inscribed in the hexagonal frame and can be
calculated according to 2rhexagon ) ahex,calc ) 31/2L ) 5.36 nm,
using the molecular length L ) 3.1 nm (most stretched
conformation). Furthermore, it is remarkable that the phase
transition enthalpy of the transition Colsqu/p4gm to Colhex (∆H
) 1.6 kJ mol-1) is higher than that of the Colhex to isotropic
transition (∆H ) 0.7 kJ mol-1, see Table 1). This indicates
that a significant change takes place at the Colsqu/p4gm to Colhex
transition. Because this transition is induced either by raising
the temperature or by reducing the alkyl chain length, it is
reasonable to assume that the segregation of the aromatic cores
from the alkyl chains is lost or strongly reduced at the transition
into this Colhex phase. Indeed, in the electron density map shown
in Figure 9c, a nearly constant electron density within the low
electron density region (yellow to red) around the high electron
density columns (blue to pink, polar groups) can be seen, which
confirms the above assumption. Another notable observation
is that, at the Colsqu/p4gm to Colhex phase transition, the defect
structure (i.e., the optical textures) remains unchanged. This
indicates that during this phase transition, the polar columns
retain their orientation. Furthermore, there is only a slight
decrease in birefringence, indicating that the orientation of the
terphenyl units does not change fundamentally (see Figure 9a).
Hence, in the Colhex phase, the terphenyl units should, on
average, align nearly perpendicular to the column axis, as in
the Colsqu/p4gm phase. Had the orientation of the terphenyl units
changed completely with respect to the column axis, aligning
parallel to it, the sign of the birefringence would have changed
at the phase transition, and consequently, the birefringence
would have passed through zero. Evidently, this was not the
case. Nevertheless, the reduction in birefringence at the Colsqu
/
p4gm to Colhex phase transition indicates a reduction in
orientational order of the terphenyl units or an increasing tilt
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J. AM. CHEM. SOC. VOL. 127, NO. 47, 2005 16585

A R T I C L E S
Chen et al.
Figure 10. Possible models of molecular organization in the Colhex phases
of compounds Am/n arranged from (a) to (d) in the order of decreasing
segregation between aromatic cores and aliphatic chains; white columns
contain alkyl chains, blue columns, the polar lateral groups, and the rigid
cores are shown in gray. (a) Hexagonal honeycomb cylinder net (only one
cylinder is shown, aliphatic chains and aromatic cores are segregated); (b)
rotationally disordered pentagonal cylinders (each polar cylinder is framed
by five aromatic blocks in a stratumswithin the cylinder shells, the aromatic
and aliphatic moieties are segregated, but there is no interconnection of the
aliphatic columns belonging to adjacent cylinders; a related structure is also
possible with hexagonal cylinders; (c) hexagonal organization of columns
in a continuum of terphenyl and alkyl chains, where the terphenyls are
arranged tangentially around the columns with the long axis perpendicular
to the columns; this arrangement requires the presence of some residual
segregation in the form of cybotactic clusters of cylinder shells as shown
in (b); (d) hexagonal organization of columns in a nematic continuum of
terphenyl and alkyl chains, where the terphenyls are arranged parallel to
the columns; this organization is favored by the minimization of the excluded
volume.
Figure 11. Textures of the Colhex phase of compound A4/4 at different
temperatures: (a) birefringent texture at 95 °C; (b) optically isotropic state
at 91 °C; (c) thick sample as observed at 91 °C with polychromatic light
between crossed polarizers and with prolonged exposure time (ca. fivefold
magnification with respect to a, b, and d); (d) birefringent texture at 87 °C.
temperature columnar phase n| is smaller than n (n| < n ,
where n| and n are the refractive indices parallel and
perpendicular to the polar columns, respectively). In other words,
below 91 °C, the birefringence of the Colhex phase is negative.
Above 91 °C, the domain becomes yellow, indicating that the
birefringence is positive. The birefringence inversion is com-
pletely reversible, but no transition enthalpy is found at 91 °C,
and also, no significant change can be detected in the X-ray
diffraction pattern. The observed inversion in birefringence can
be explained in the following way: In the continuum between
the polar cylinders, there is preferred parallel (“nematic-like”)
alignment of the terphenyl cores. On the other hand, at lower
temperature, the terphenyl cores are aligned predominately
perpendicular to the polar columns, as shown in Figure 10c.
Because the birefringence of the phase is determined primarily
by the orientation of the terphenyl units with respect to the
columns, i.e., the optic axis, this mesophase has negative
birefringence.
The vacillating alignment of terphenyl mesogens in the Colhex
phase is an indication of the frustration caused by two opposing
tendencies: one for the alkyl chains to phase separate in discrete
columns (Figure 10b), and the other to forego such separation
and maximize the interaction between mesogens by allowing
them to form a nematic-like continuum (Figure 10d). The latter
tendency prevails; i.e., microphase separation is abandoned and
nematic-like alignment achieved in A4/4, the compound with
the shortest alkyl chains, as would be expected. The “transition”
between mesogen alignments perpendicular and parallel to the
polar columns suggests an analogy with the upper critical
dissolution point in macroscopically immiscible liquids. It thus
seems that segregation of aromatic and aliphatic parts may not
toward the column axis. Further information concerning the
phase structure was obtained from the investigation of the Colhex
phase in compound A4/4.
Compound A4/4 (see Figure 7) shows an inversion of
birefringence within the temperature range of the Colhex phase.
Upon heating, at 91 °C, the birefringent texture (Figure 11d)
becomes completely dark between crossed polarizers (Figure
11b). In a thick sample, distinct domains with blue and red
interference color can be distinguished (see Figure 11c), which
result from the wavelength dependence of the refractive indices,
a very typical feature of birefringence inversion.32 Upon further
heating, the birefringence increases again until the clearing
temperature is reached (Figure 11a). The sign of the birefrin-
gence of the Colhex phase was experimentally determined by
examining the texture between crossed polarizers. At first,
monodomains of the sample were oriented with the columnar
axis at 45° to the crossed polarizers. A first-order λ-plate was
placed with the direction of nR (smaller refractive index) parallel
to the columnar axis. In the optically isotropic state at 91 °C,
the sample adopts the red interference color of the λ-plate. Below
91 °C, the domain becomes blue. This indicates that in the low-
(32) There are only a few examples of inversion of birefringence in LC
systems: (a) Pelzl, G.; Sackmann, H. Mol. Cryst. Liq. Cryst. 1971, 15,
75-87. (b) Zimmermann, H.; Poupko, R.; Luz, Z.; Billard, J. Z. Natur-
forsch. 1986, 41a, 1137-1140. (c) Ungar, G.; Abramic, D.; Percec, V.;
Heck, J. Liq. Cryst. 1996, 21, 73-86. (d) Reiffenrath, V.; Bremer, M. Abstr.
Pap. Am. Chem. Soc. 1999, 218, 70-ORGN. (e) Olssen, N.; Dahl, I.; Helgee,
B.; Komitov, L. Liq. Cryst. 2004, 1555-1568.
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Rod−Coil Molecules Forming Liquid Crystal Structures
A R T I C L E S
Table 2. Mesophases, Phase Transition Temperatures, Phase
be completely lost at the Colsqu to Colhex transition, and locally,
residues of the cylinder shells remain. If present, such residual
pentagonal or hexagonal clusters33 would be rotationally
disordered around the polar columns. (Hence, the real structure
is probably somewhere between the models b and c shown in
Figure 10.) Upon heating, these clusters decrease in size, and
the excluded volume effect becomes more dominating, giving
rise to an average alignment of the terphenyls closer to parallel
to the column axis, leading to positive birefringence. Because
the inversion of birefringence is not associated with a distinct
first-order phase transition, it seems that the reorganization of
the terphenyl units is a phenomenon similar to the formation
of cybotactic clusters found in some nematic phases close to
the Sm-to-N transition.34 In contrast to most cybotactic N phases,
which exist only over short temperature ranges, in the Colhex
phases reported herein, the cybotactic behavior is predominant;
i.e., no real parallel organization of terphenyls and polar columns
(as shown in Figure 10d) is reached before the transition to the
isotropic state.
The hexagonal columnar phases of compounds A4/2, A4/3,
A6/4, A6/5, and A6/6 should all have the same structure with
a perpendicular organization of columns and terphenyls, as
additionally proven for that of compound A6/6 by X-ray
experiments with aligned samples. (The maxima of the diffuse
wide-angle scatterings are located on the equator; see Figure
S1 in the Supporting Information,) However, a change in the
sign of birefringence, as found for A4/4, was not observed for
any of these Colhex phases. Only for compound A4/3, which
has one oxyethylene unit less than A4/4, is a continuous decrease
in birefringence with increasing temperature observed, but
apparently in this case no reversal in birefringence occurs before
isotropization.
Transition Enthalpies, and Lattice Parameters of the Facial
Amphiphiles A6.16/n and A16.6/na
T/
°
‚
C
asqu/nm
(T/ C)
1
cmpd
m
m′
n
∆
H/kJ
mol-
°
A6.16/3
6
16
16
16
6
3
Cr 51 Colsqu/p4mm 137 I
49.6 2.3
Cr 52 Colsqu/p4mm 135 I
42.3 2.5
Cr 56 Colsqu/p4mm 120 I
50.3 2.4
Cr 49 Colsqu/p4mm 138 I
50.3 2.6
Cr 43 Colsqu/p4mm 134 I
38.7 2.9
Cr 32 Colsqu/p4mm 119 I
51.3 3.0
4.1 (130)
4.2 (90)
4.2 (130)
4.4 (90)
4.5 (80)
A6.16/4
A6.16/6
A16.6/3
A16.6/4
A16.6/6
6
6
4
6
3
4
6
16
16
16
4.0 (130)
4.2 (80)
4.2 (95)
6
6
4.4 (95)
a Enthalpy values are given in the lower lines in italics.
Table 3. Mesophases, Phase Transition Temperatures, Phase
Transition Enthalpy Values, and Lattice Parameters of the Facial
Amphiphiles B10/na
2.3. Other Variations of the Molecular Structure.
T/
°
‚
C
a
squ/nm
2.3.1. Branched Alkyl Chains. Compounds A10*/3 and
A10*/4 (see Table 1) with branched decyl chains (3,7-
dimethyloctyloxy chains) show Colsqu/p4mm phases the same
as those of the corresponding linear derivatives A10/3 and A10/4
with the same number of C-atoms. It seems that this structural
variation slightly reduces the clearing temperatures, whereas the
mesophase type is not changed.
1
cmpd
n
∆
H/kJ
mol-
(T/ C)
°
B10/3
3
Cr 117 Colsqu/p4mm 155 I
48.9 4.1
Cr 114 Colsqu/p4mm 154 I
66.9 5.6
4.0 (130)
4.1 (90)
4.0 (150)
4.2 (100)
B10/4
4
a Enthalpy values are given in the lower lines in italics.
2.3.2. Nonequal Terminal Chains. Compounds A6.16/n and
A16.6/n comprise two different terminal alkyl chains at the ends
of the p-terphenyl units, see Table 2. For all six compounds,
only the Colsqu/p4mm phase was observed, irrespective of the
position of the longer chain relative to the lateral group. Also,
the mesophase stability of the two sets of isomeric compounds
A6.16/n and A16.6/n is nearly the same as that of similar
compounds with identical end chains of comparable total length
(e.g., compounds A10/n).
2.3.3. Position of the Lateral Chain (Y-Shaped Am-
phiphiles). The two compounds B10/n with a polar chain at
the peripheral 3-position show only the Colsqu/p4mm phase, see
Table 3. Comparison with the corresponding centrally substi-
tuted (2′-substituted) compounds A10/3 and A10/4 shows that
changing the position of the lateral chain does not change the
mesophase type. The Colsqu/p4mm phase is slightly more stable
in the former compounds; their isotropization temperatures are
10 °C higher, but because the melting points are also increased,
the overall mesophase range is actually reduced. This slight
increase in clearing temperature might be due to a less serious
steric destabilizing effect of the peripheral substituent in
compounds B10/n or to a change in conformation of the
p-terphenyl unit (reduction of the twist of the terphenyl core).35
2.4. Solvent-Induced Mesophases: New Types of Lami-
nated Mesophases. Actually, all the compounds discussed
above are amphotropic liquid crystals17 because their meso-
morphic properties can be influenced by specific interaction of
appropriate solvents with one of the incompatible units: lipo-
(33) The measured parameter ahex ) 4.3 nm (compound A6/4) is only slightly
larger than the calculated diameter of a cylinder inscribed in a regular
pentagon formed by these molecules (2rpentagon ) 4.2 nm). Also the number
of molecules arranged around the columns ncyl (see Table 1) does not change
at the phase transition, assuming an unchanged 0.45-nm stratum thickness
of the column; moreover, ncyl becomes even slightly smaller in some cases
(compound A4/2). This suggests that the pentagonal shape remains, but
the pentagonal cylinders become rotationally disordered.
(35) (a) Gray, G. W.; Hird, M.; Toyne, K. J. Mol. Cryst. Liq. Cryst. 1991, 195,
221-237. (b) Hird, M.; Toyne K. J.; Hindmarrsh, P.; Jones, J. C.; Minter,
V. Mol. Cryst. Liq. Cryst. 1995, 260, 227-240. (c) Andersch, J.; Tschierske,
C.; Diele, S.; Lose, D. J. Mater. Chem. 1996, 6, 1297-1307. (d) Ko¨lbel,
M.; Beyersdorff, T.; Tschierske, C.; Diele, S.; Kain, J. Chem.-Eur. J. 2000,
6, 3821-3837.
(34) De Vries, A. Mol. Cryst. Liq. Cryst. 1970, 10, 219-236.
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A R T I C L E S
Chen et al.
philic solvents can swell the nonpolar regions, and polar solvents
can swell the polar regions.
Formamide can form hydrogen bonds with the poly(oxyeth-
ylene) chains as well as with the OH groups and the amide
group of the lateral chain; i.e., it interacts specifically with the
polar lateral chains. The influence of formamide upon the Colhex
mesophases of compound A4/4 was investigated by means of
the solvent-penetration technique. The mesophases developing
in the contact region between the amphiphile and the solvent
were recorded qualitatively by polarizing microscopy. Panels a
and b of Figure 12 display the contact region of A4/4 with
formamide at different temperatures and panel e is a sketch of
the binary phase diagram, as recorded on cooling. With an
increasing concentration of formamide, the original Colhex phase
of pure A4/4 was destabilized, and new types of mesophases
were made to appear. In the contact region, on cooling from
the isotropic state, a texture with homeotropic alignment and
some defects (small crosses and oily streaks) is formed, which
is typical of SmA phases (Figure 12a). This solvent-induced
SmA phase is separated from the Colhex phase of the pure
compound A4/4 by an isotropic liquid ribbon. By further
cooling, a schlieren texture with strong birefringence grows in
the center of the homeotropic SmA domain (Figure 12b). This
indicates the formation of an optically biaxial mesophase.
Crystallization occurs at 53 °C, and reheating gives a melting
point of approximately 77 °C; i.e., the biaxial mesophase is only
monotropic, which has inhibited a more detailed investigation.
The schlieren texture of this mesophase comprises two brush
disclinations, which excludes a synclinic-tilted SmC phase.
However, the overall appearance of this mesophase is similar
to the textures of the SmAb phases36 and also to those of the
LamN phases, which were recently reported for bolaamphiphiles
with large lateral semiperfluorinated alkyl chains (see Figure
1).10b,c,11 The model proposed for the bolaamphiphiles might
also be applicable to the solvent-induced phases of compound
A4/4. The effect of formamide is to increase the effective size
of the polar parts of the molecule by coordinating its molecules
to the polar groups. This leads to fusion of the rows of polar
columns into infinite layers. In the resulting layer structure, the
alkyl chains and the p-terphenyl cores are segregated into
common layers, whereas the polar parts of the molecules,
together with the solvent, form the second set of layers. At high
temperature, there is no in-plane order in the apolar layers
(Lamiso phase, see Figure 12c) but on cooling, the p-terphenyl
cores adopt orientational order, which gives rise to the charac-
teristic birefringent schlieren texture. Accordingly, this meso-
phase can be regarded as a biaxial smectic mesophase (SmAb-
like) or as a laminated nematic phase (LamN, see Figure 12d).
Although these experiments are only preliminary, they show
that it is possible, with these facial amphiphiles, to obtain
laminated mesophases similar to those observed for the bola-
amphiphiles with lipophilic lateral chains.10b,c,11
Summary and Conclusions
Figure 12. Contact region of A4/4 (top left) and formamide (bottom right)
as observed on cooling between crossed polarizers: (a) at 99 °C; (b) at 61
°C; (c) model of Lamiso phase; (d) model of LamN phase; (e) sketch of the
nonequilibrium binary phase diagram A4/4 + formamide as determined
by polarizing microscopy of the contact region on cooling (two-phase
regions not shown).
In this investigation, five complex supramolecular LC phases
and two solvent-induced LC phases were found for a series of
rodlike facial amphiphiles with carbohydrate headgroups. As
summarized in Figure 13, a phase sequence SmA - ChLhex
-
Colsqu/p4mm - Colsqu/p4gm - Colhex - LamN - Lamiso has
been observed by increasing the volume fraction of the lateral
(36) Hegmann, T.; Kain, J.; Diele, S.; Pelzl, G.; Tschierske, C. Angew. Chem.,
Int. Ed. 2001, 40, 887-890.
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16588 J. AM. CHEM. SOC. VOL. 127, NO. 47, 2005

Rod−Coil Molecules Forming Liquid Crystal Structures
A R T I C L E S
Figure 13. Schematic view of the phase sequence of the facial amphiphiles with lateral polyhydroxy groups as a function of the size of polar lateral and
lipophilic terminal groups (LamN and Lamiso are proposed for the solvent-induced mesophases).
polar chain or by reducing the volume fraction of the terminal
lipophilic chains. Some of the phase transitions are temperature-
or solvent-induced.
positions of the amphiphilic groups. Therefore, the positions
of the polar and nonpolar columns in the LC structures are also
exchanged, as can be seen, for example, by comparing the
Colsqu/p4mm phase in Figure 1 (bolaamphiphiles) with that
shown in Figure 13 (facial amphiphiles). The two Colsqu/p4mm
phases are reversed with respect to the position of the polar
columns. The latter serve either as wall-interconnecting columns
or as framed columns. Hence, the two Colsqu/p4mm structures
are assigned as color isomers. Because the regions with the
strongest cohesive forces (hydrogen bonding) have a different
position with respect to the rigid cores, some mesophases are
missing in facial amphiphiles, whereas others, not found in
bolaamphiphiles 1/n,10 emerge (e.g., the ChLhex phase and the
noncylinder Colhex phase).
This sequence of mesophases bears a relation to that found
for the bolaamphiphiles 1/n with nonpolar lateral chains, shown
in Figure 1.10 Starting with a conventional SmA phase by
increasing the volume fraction of the lateral chain at first
cylinder structures are formed, which then change into new
lamellar phases. This similarity indicates the generality of the
self-organization principles involved. Both series of compounds
represent ternary amphiphiles with a T-like shape in which a
rigid core of defined length is combined with two terminal
chains and one lateral chain, incompatible with each other. For
the columnar mesophases, the rigid-rod segments tend to restrict
the side length of the polygons within relatively narrow limits,
giving rise to columns with a well-defined polygonal shape.
The lateral chains fill the interior of the polygons; the terminal
chains form the corners of these polygons and connect the rigid
rods. Thus, the number of sides of the polygons critically
depends on the volume of the lateral chain and the length of
the molecule. The polygon that provides the exact volume
required by the lateral chains is selected. The most remarkable
In bolaamphiphiles, the strongest cohesive forces are located
at the ends of the molecules, which leads to a dominance of
cylinder structures ranging from rhombuses via squares, hexa-
gons, to giant cylinder structures. In the series of facial
amphiphiles, these giant cylinder structures are missing. In the
case of facial amphiphiles, the phase-separation tendency of the
aliphatic groups is relatively weak, and separate aliphatic
columns from the aromatic cores only for alkyl length m g 6.
In contrast, in the case of bolaamphiphiles 1/n, even the
relatively short 2,3-dihydroxypropyl units provide sufficient
cylinder phase is that built up by pentagonal cylinders (Colsqu
/
p4gm). It is known that regular pentagons (with identical sides
and angles) cannot tile the plane,37 but the LC state, combining
order and mobility, enables a slight deviation of the angles and
side lengths, which is essential for the formation of the observed
honeycomb-like periodic network composed exclusively of
pentagonal cylinders.38-40
(38) There is only one report about a pentagonal net in solid crystal structure,
but in this case the net deviates from planarity (wavelike deformation) in
order to allow for periodic tiling by identical pentagons: Moulton, B.; Lu,
J.-J.; Zaworotko, M. J. J. Am. Chem. Soc. 2001, 123, 9224-9225.
(39) Examples of supramolecular pentagons: (a) Hasenknopf, B.; Lehn, J.-M.;
Boumediene, N.; Dupont-Gervais, A.; Dorsselaer, A. V.; Kneisel, B. O.;
Baum, G.; Frenske, D. J. Am. Chem. Soc. 1997, 119, 10956. (b) Campos-
Fernandez, C. S.; Clerac, R.; Koomen, J. M.; Russell, D. H.; Dunbar, K.
R. J. Am. Chem. Soc. 2001, 123, 773-776.
However, self-assembly of these two classes of compounds
also shows major differences, mainly due to the different
(37) Gru¨nbaum, B.; Shephard, G. C. Tilings and Patterns; W. H. Freeman: New
(40) Fivefold symmetry in quasi-crystal lattices: Caspar, D. L. D.; Fontano, E.
Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 14271-14278.
York, 1987.
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A R T I C L E S
Chen et al.
tendency for separation to maintain the cylinder structure for a
wide variety of different molecules.
form layers rather than columns, resulting in the ChLhex phase
being the “tubular smectic” counterpart of the “tubular nematic”
phase.
In the series of facial amphiphiles reported herein, the Colsqu
/
p4mm cylinder phase is the dominant mesophase, found in most
compounds. At the same time, in the series of bolaamphiphiles,
The columnar cylinder phases (p4mm and p4gm), the non-
cylinder Colhex phase, and the ChLhex phase reported herein
represent new channel structures that might be of practical
interest. The polar columns are formed by oligo(oxyethylene)
chains that have been widely used as a medium for ion
conduction.45 In the reported mesophases, the ion conductive
channels are one-dimensional and located in a well-structured
lipophilic surrounding. These ordered soft-matter systems can
easily be organized at interfaces or modulated by electric fields
and guest molecules, allowing the preparation of well-defined
structures incorporating monodomains of such mesophases.
These fluid systems formed in the self-assembly process can
then be fixed by cross-linking, polymerization (using slightly
modified molecules or additives), or vitrification to obtain stable,
functional nanodevices.
the Colhex/p6mm cylinder phase is dominant, and the Colsqu
/
p4mm phase was found for only one compound in a small
temperature range.10b This appears to be due to the different
length of the rigid cores in the two series of compounds
(biphenyl vs terphenyl) and the different size of the end-groups.
The size of the lateral polar group of the facial amphiphiles is
not sufficient to fill the space inside a hexagon of six terphenyl
units. Also, the structure of the pentagonal cylinder phase is
slightly different in the two series. In bolaamphiphiles, there is
an additional slight deformation of the pentagons (all angles
are different), leading to the reduced phase symmetry Colrec
/
p2gg10 as compared to that of Colsqu/p4gm in facial amphiphiles.
Possibly, the alkyl and perfluoroalkyl chains,41 which are more
rigid than the oligo(oxyethylene) chains,42 might give rise to
the additional deformation of the pentagons.
In summary, the concept of ternary amphiphilic T-shaped
molecules provides a powerful design principle for new and
unexpected complex liquid crystalline superstructures. There is
an analogy with the approaches used in polymer systems (star-
shaped multiblock copolymers46,47 and hairy rods48,49) with the
difference that the LC structures have lower viscosity, higher
perfection, and a smaller (3-30 nm) length scale than those of
polymers. Hence, they are much easier to produce in a fast
thermodynamically driven self-assembly process, resulting in
structures that are homogeneous over large lengths (several
square centimeters). There should also be a strong input into
the field of crystal engineering.3 Solid-state channel struc-
tures,50,51 related to the fluid columnar cylinder phases reported
herein,were found for coordination polymers52 and hydrogen-
bonding networks.53 Presently, also in the design of crystal
The most striking feature is that the pentagonal cylinder
structure is the largest of the stable cylinder structures, and the
hexagonal cylinder structure could not be obtained with the
compounds investigated herein. The likely reason is that the
approach used, which is based on the reduction of the terminal
alkyl chain length, also reduces the incompatibility of these
chains with the aromatic cores. Therefore, in the hexagonal
columnar phases, the terphenyl cores and the alkyl chains are
not segregated. However, a parallel organization of the rodlike
cores remains, and cybotactic groups (residues of the cylinder
structure) keep the terphenyl cores in a predominately perpen-
dicular orientation with respect to the polar columns. This phase
has a unique structure, where columns are hexagonally organized
in an anisotropic matrix with a local nematic-like order of the
terphenyls, and the nematic director changes tangentially around
the columns.43 It is a new mesophase, which in some respect
can be regarded as complementary to the so-called “tubular-
nematic-columnar phase” reported by Saez et al. for laterally
appended polypedes.44 The latter type of Colhex (and also Colrec)
phases is formed by oligomesogens composed of rodlike
phenylbenzoate cores with terminal alkyl chains at both ends
and lateral alkyl chains that are connected to an oligosiloxane
branching point. In the mesophases of these compounds, the
oligosiloxane units form columns and the rodlike phenylben-
zoate cores with the terminal alkyl chains form a nematic
continuum between them. In contrast to the Colhex phases
reported here, the nematic director is parallel or only slightly
tilted to the columns (similar to Figure 10d).
(44) (a) Saez, I. M.; Goodby, J. W.; Richardson, R. M. Chem. Eur. J. 2001, 7,
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In addition, there is a much larger variety of different phases
in the present facial amphiphiles. For example, as the alkyl
length of m ) 16 is reached, the chains are sufficiently long to
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16590 J. AM. CHEM. SOC. VOL. 127, NO. 47, 2005

Rod−Coil Molecules Forming Liquid Crystal Structures
A R T I C L E S
structures, there is an increased thinking in terms of interfaces54
and volume fractions of incompatible units.2,55 Taking into
account multiple-level segregation and including the restrictions
given by the rigidity, shape, and molecular topology of some
building blocks will contribute to a more sophisticated prediction
and design of solid crystal structures. This can lead to a
unification of the fields of soft matter engineering (block
copolymers and liquid crystals) and crystal engineering. More
generally, the systematic investigation of the self-assembly of
small but well-designed molecular tectons will undoubtedly
contribute to the decoding of the mechanism of molecular
information processing during the evolution of complex super-
structures, including biological systems.
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F.; Whitehead, M. A.; Lebuis, A.-M.; Sleiman, H. F. Chem.-Eur. J. 2003,
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Ed. 2004, 43, 5033-5036. (c) Wu¨rthner, F.; You, C.-C.; Saha-Mo¨ller, C.
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Chem. ReV., 2001, 101, 1629-1658. (b) James, S. L. Chem. Soc. ReV. 2003,
32, 276-288. (c) Papaefstathiou, G. S.; MacGillivray, L. R. Coord. Chem.
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2003, 3, 547-554.
Acknowledgment. This work was supported by the DFG
(GRK 894/1) and the Fonds der Chemischen Industrie. We thank
A. Gleeson for help with the synchrotron experiments at SRS
Daresbury and CCLRC for the beamtime.
Supporting Information Available: 2D-diffraction pattern of
compound A6/6, tables with crystallographic data of the
mesophases, figure showing the CPK models of compounds
A10/0-A10/3, detailed synthesis schemes, experimental pro-
cedures, and analytical data (NMR, MS, and elemental analysis).
This material is available free of charge via the Internet at
http://pubs.acs.org.
(54) Von Schnering, H. G.; Nesper, R. Angew. Chem., Int. Ed. Engl. 1987, 26,
1059-1080.
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2002, 124, 123-137. (b) Gordon-Wylie, S. W.; Clark, G. R. Cryst. Growth
Des. 2003, 3, 453-456.
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