Calamitic bolaamphiphiles with (semi)perfluorinated lateral chains: Polyphilic block molecules with new liquid crystalline phase structures

Article abstract of DOI:10.1021/ja036213g

Novel bolaamphiphiles consisting of a rigid biphenyl unit, two terminal polar 1,2-diol units and laterally attached (semi)perfluorinated chains have been synthesized via palladium-catalyzed cross coupling reactions as the key step. The thermotropic liquid

Full text of DOI:10.1021/ja036213g

Published on Web 08/14/2003
Calamitic Bolaamphiphiles with (Semi)Perfluorinated Lateral
Chains: Polyphilic Block Molecules with New Liquid
Crystalline Phase Structures
Xiaohong Cheng,† Marko Prehm,† Malay Kumar Das,‡ Jens Kain,‡ Ute Baumeister,‡
Siegmar Diele,‡ Dag Leine,‡ Alfred Blume,‡ and Carsten Tschierske*,†
Contribution from the Institute of Organic Chemistry, UniVersity Halle, Kurt-Mothes-Strasse 2,
D-06120 Halle, Germany, and Institute of Physical Chemistry, UniVersity Halle, Mu¨hlpforte 1,
D-06108 Halle, Germany
Received May 19, 2003; E-mail: Tschierske@chemie.uni-halle.de
Abstract: Novel bolaamphiphiles consisting of a rigid biphenyl unit, two terminal polar 1,2-diol units and
laterally attached (semi)perfluorinated chains have been synthesized via palladium-catalyzed cross coupling
reactions as the key step. The thermotropic liquid crystalline behavior of these compounds was investigated
by polarized light optical microscopy, DSC, and X-ray scattering, and the influences of the length, number,
structure, and position of the lateral chain on the mesomorphic properties were studied. A wide variety of
unique liquid crystalline phases were found upon elongation of the lateral semiperfluorinated chains. For
short- and medium-chain length a series of columnar phases were observed, and upon further elongation
of the lateral chain a series of novel mesophases with layer structures were found. In the columnar phases,
the nonpolar lateral chains segregate into columns, which are embedded in honeycomb-like networks of
cylinders consisting of the biphenyl units. Strings of hydrogen-bonding networks of the diol groups provide
cohesive forces, which maintain the overall structure. Changing the length of the lateral chains influences
the diameter of the columns and thus determines the number of biphenyl units which are required to surround
these columns. The number of these units [four (c2mm, p4mm), five (p2gg), six (p6mm), eight (c2mm) or
10 (p2gg)] defines the shape of the cylinders as well as the lattice type of the columnar phase. It is proposed
that the columnar phases with a p2gg lattice result from the regular organization of pairs of cylinders which
have a pentagonal cross sectional shape. In the mesophases with layer structure the aromatic rodlike
cores are arranged parallel to the layer planes, and the onset of orientational and positional ordering of the
biphenyl segments leads to a sequence of subtypes for these lamellar phases (LamIso-LamN-LamX).
Introduction
or complimentary units, and (ii) minimization of the free volume,
which in the case of rigid molecules or supermolecular ag-
In the past 20-30 years liquid crystalline materials have
received significant attention. The combination of order and
mobility in these systems leads to novel functional materials
which have had a great impact on recent development of mobile
information technologies.1 Although more than 100 000 liquid
crystalline compounds have been synthesized thus far, their basic
structures are quite similar. They represent either anisometric
molecules with a specific rodlike or disklike shape, or am-
phiphilic molecules, as well as oligomers, polymers, and
dendrimers derived from these fundamental structures.2 There
are two main driving forces for self-organization in such
systems: (i) segregation of incompatible molecular parts into
separate subspaces,3 accompanied by aggregation of compatible
gregates with a rodlike or disklike shape leads to a parallel
alignment of these anisometric units.2 The self-organized
structures of such conventional LC materials are quite simple:
flexible binary amphiphiles give rise to a sequence of lamellar,
bicontinuous cubic, columnar, and spheroidic cubic mesophases
with respect to the volume fraction of the incompatible parts
(e.g., surfactant solvent systems,4 thermotropic mesophases of
amphiphiles,5 flexible block copolymers,6 and low-molecular
mass block molecules7). These mesophases are characterized
(3) (a) Tschierske, C. J. Mater. Chem. 2001, 11, 2647-2671. (b) Tschierske,
C. J. Mater. Chem. 1998, 8, 1485-1508. (c) Chen, W.; Wunderlich, B.
Macromol. Chem. Phys. 1999, 200, 283-311. (d) Skoulios, A.; Guillon,
D. Mol. Cryst. Liq. Cryst. 1988, 165, 317-332. (e) Hendrikx, Y.; Levelut,
A. M. Mol. Cryst. Liq. Cryst. 1988, 165, 233-263.
† Institute of Organic Chemistry.
(4) Seddon, J. M.; Templer, R. H. In Handbook of Biological Physics;
Lipowsky, R., Sackmann, E., Eds.; Elsevier: Amsterdam, 1995; Vol 1, pp
97-160.
‡ Institute of Physical Chemistry.
(1) Petty, M. C.; Bryce, M. R.; Bloor, D. Introduction to Molecular Electronics;
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1997, 22, 427-443. (b) Borisch, K.; Diele, S.; Go¨ring, P.; Kresse, H.;
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9
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J. AM. CHEM. SOC. 2003, 125, 10977-10996
10977

A R T I C L E S
Cheng et al.
by positional order in one, two, or three dimensions. Rodlike
and disklike molecules on the other hand align parallel and give
rise to orientational order, which leads to the formation of
nematic phases (orientational ordering only), and smectic or
columnar LC phases if there is additional positional order.2,8
New and more complex mesophase morphologies can arise if
these driving forces are combined in a competitive way, or if
there are other forces which compete with these fundamental
organizations. Chirality, for example, favors a helical organiza-
tion of molecules, which is in competition with a layered
organization, leading to quite complex superstructures (TGB
phases, blue phases, and other complex 3D ordered structures).9
In bent-core mesogens, as another example, a polar order results
from the directed organization of the molecules in layers, which
is in competition with a layerlike organization, leading to
frustrated phase structures and supramolecular chirality.10
From the field of multiblock copolymers, it is known that
increasing the number of incompatible units fixed to one another
is an alternative way to amazingly complex morphologies.11,12
However, this concept is less explored in low-molecular mass
LC materials. There are some reports about rodlike,13,14 taper-
shaped, and dendritic polyphilic molecules,15,16 where different
incompatible units were connected in a linear fashion. However,
this molecular topology inherently favors the organization
provided by the molecular shape, leading predominately to the
formation of triple-layer structures for calamitic molecules and
formation of onion-like columns or spheroids for taper-shaped
molecules. Another approach aims at the competitiVe combina-
tion of the organization provided by rigid segments with a
polyphilic molecular structure.3a,b,17-20
with complex morphologies. These compounds can be regarded
as block molecules consisting of three incompatible units: a
rigid rodlike aromatic unit, two hydrophilic terminal groups,
and a liphophilic lateral alkyl chain. All three structural units
are incompatible with respect to one another, and therefore, they
are forced to organize within different subspaces. Furthermore,
a strong competition arises between the parallel alignment of
the rodlike biphenyl units and the steric interaction caused by
the lateral alkyl chains. The space requirement of the lateral
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Betterton, K. Liq. Cryst. 1991, 10, 389-396. (b) Doi, T.; Sakurai, Y.;
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G. J. Am. Chem. Soc. 2000, 122, 10273-10281. (b) Percec, V.; Cho, W.-
D.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 2001, 123, 1302-
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G. J.; Percec, V.; Ungar, G.; Zhau, J. P. Macromolecules 1996, 29, 646-
660. (b) Percec, V.; Johansson, G.; Ungar, G.; Zhou, J. J. Am. Chem. Soc.
1996, 118, 9855-9866. (c) Percec, V.; Glodde, M.; Bera, T. K.; Miura,
Y.; Shiyanovskaya, I.; Singer, K. D.; Balagurusamy, V. S. K.; Heiney, P.
A. Schnell, I.; Rapp, A.; Spiess, H.-W.; Hudson, S. D.; Duan, H. Nature
2002, 419, 384-387. (d) Cheng, X.; Das, M. K.; Diele, S.; Tschierske, C.
Langmuir 2002, 18, 6521-6529.
(17) Different columnar phases were reported for thermotropic and lyotropic
mesophases of calamitic p-terphenyl derivatives with lateral polar groups:
(a) Hildebrandt, F.; Schro¨ter, J. A.; Tschierske, C.; Festag, R.; Kleppinger,
R.; Wendorff, J. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 1631-1633.
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Eur. J. 2001, 7, 2758-2764. (b) Barbera´, J.; Gime´nez, R.; Marcos, M.;
Serrano, J. L. Liq. Cryst. 2002, 29, 309-314.
Rigid bolaamphiphiles with flexible lateral alkyl chains 1/n21
represent one class of compounds in which this approach was
very successfully used to obtain novel liquid crystalline phases
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copolymers: (a) Mogi, Y.; Nomura, M.; Kotsuji, H.; Ohnishi, K.;
Matsushita Y.; Noda, I. Macromolecules 1994, 27, 6755-6760. (b)
Matsushita, Y.; Tamura, M.; Noda, I. Macromolecules 1994, 27, 3680-
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semiperfluorinated chains and lateral aliphatic substituents: (a) Arehart,
S. V.; Pugh, C. J. Am. Chem. Soc. 1997, 119, 3027-3037. (b) Small, A.
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Cryst. 2001, 28, 991-1001.
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123, 6809-6818.
(12) Examples of complex morphologies in ABC heteroarm star terpolymers:
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Phys. 2000, 201, 296-307.
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10978 J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003

Calamitic Bolaamphiphiles with Lateral Chains
A R T I C L E S
elongation of the lateral chain of the compounds 1/n gives rise
to a transition from a smectic monolayer structure (n ) 0, 1:
SmA1) via a mesophase with a strongly distorted structure (n
) 3-7: SmA+), a centered rectangular columnar phase (n )
6-9: Colr/c2mm), and a noncentered rectangular columnar
phase (n ) 10-12: Colr/p2gg) to a hexagonal columnar phase
(n g 12: Colh/p6mm).21b This phase sequence results from the
microsegregation of the lateral alkyl chains from the rigid
aromatic units which occurs above a certain minimum length
for these chains (g3). The segregated alkyl chains organize into
distinct regions. For short-chain compounds (1/3, 1/5) and for
medium-chain compounds at higher temperatures (1/6, 1/7),
there is no long-range correlation between these lipophilic
regions. This leads to a special type of mesophase which is
characterized by a typical SmA-like optical texture and by an
X-ray diffraction pattern in which a diffuse scattering in the
small-angle region appears adjacent to or instead of the sharp
layer reflection (SmA+).21b Longer chains organize in infinite
columns which adopt a positional long-range order in two
dimensions, leading to columnar phases. In these mesophases
the bolaamphiphilic units form networks of cylinder shells
around the lipophilic columns (see Figure 1).25 The terminal
diol groups are organized into hydrogen-bonding networks
which stabilize this arrangement by providing attractive forces
at the ends of the calamitic units. The space required by the
alkyl chains with respect to the length of the bolaamphiphilic
units determines the number of biphenyl units which have to
be organized in the shells around the lipophilic cylinder cores.
This influences the geometry of the cylinders and gives rise to
the above-mentioned sequence of two-dimensional(2D) lattices.21b
To further extend this designing principle, we have synthe-
sized rigid bolaamphiphiles with perfluorinated (compounds 4/n)
and semiperfluorinated lateral chains (compounds 2/n, 3/n, 5/n-
7/n, see Scheme 1). There are several distinct effects of (semi)-
perfluorinated chains which can influence the self-organization
of such molecules.7b,14,16,19,28,29 First, the perfluorinated segments
are incompatible with hydrogenated molecular parts and also
with polar segments. This fluorophobic effect usually increases
segregation and stabilizes the mesophases. Second, perfluori-
nated chains have a significantly larger volume than alkyl chains.
Finally, such chains have preferred conformations (helical)
which are different from those adopted by alkyl chains (all-
trans).30 For these reasons, it was hoped that the introduction
of perfluorinated segments would lead to further novel meso-
phases being observed. The structure of all compounds is based
on the bolaamphiphilic31,32 4,4′-bis(2,3-dihydroxypropoxy)-
biphenyl unit.33 Systematically changing the length, the degree
Figure 1. Dependence of the liquid crystalline phases of compounds 1/n
with respect to the length of the lateral alkyl chains and models for the
organization of the molecules 1/n in their mesophases.21b Cr ) crystal, SmA
) smectic A phase (liquid crystalline phase with layer structure, without
long-range positional order in the layers and with an arrangement of the
biphenyl units parallel to the layer normal), SmA+ ) disordered mesophase
(typical SmA texture, but a diffuse reflection is found adjacent or instead
of the sharp layer reflection in the small-angle region of the X-ray diffraction
pattern),21b Colr ) centered (c2mm) or noncentered (p2gg) rectangular
columnar phases;26 Colh ) hexagonal columnar phase (p6mm).27 Models:
blue ) hydrogen-bonding networks of the terminal diol groups; white )
microsegregated regions of the lateral alkyl chains; gray ) rigid biphenyl
units.
chains disturbs the original layer structure.22 However, the
cooperative and dynamic networks of intermolecular hydrogen
bonding23 between the diol groups provide sufficient cohesive
energy to inhibit a complete collapse of the molecular order
and support the segregation of the incompatible parts,24 leading
to a series of novel mesophases. As shown in Figure 1,
(22) Weissflog, W. In Handbook of Liquid Crystals; Demus, D., Goodby, J.
W., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim,
1998; Vol 2B, pp 835-863.
(23) (a) Frank, H. S.; Wen, W.-Y. Discuss. Faraday Soc. 1957, 24, 133-140.
(b) Bellamy, L. J.; Pace, R. L. Spectrochim. Acta 1966, 525-534. (c)
Kleeberg, H. In Intermolecular Forces, An Introduction to Modern Methods
and Results; Huyskens, P. L., Luck, W. A. P., Zeegers, T., Eds.; Springer:
Berlin, 1999; pp 251-280.
(24) The strong incompatibility of the polar 2,3-dihydroxypropyloxy groups and
other diol groups with alkyl chains is well documented and widely used
for the design of amphotropic liquid crystals: (a) Tschierske, C. Prog.
Polym. Sci. 1996, 21, 775-852. (b) Blunk, D.; Praefcke, K.; Vill, V. In
Handbook of Liquid Crystals; Demus, D., Goodby, J. W., Gray, G. W.,
Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol 3, pp
305-340. (c) Tschierske, C.; Brezesinski, G.; Kuschel, F.; Zaschke, H.
Mol. Cryst. Liq. Cryst., Lett. 1989, 6, 139-144. (d) van Doren, H. A.; van
der Geest, R.; Kellog, R. M.; Wynberg, H. Recl. TraV. Chim. Pays-Bas
1990, 109, 197-203. (e) Lattermann, G.; Staufer, G. Liq. Cryst. 1989, 4,
347-355. (f) Praefcke, K.; Marquardt, P.; Kohne, B.; Stephan, W. J.
Carbohydr. Chem. 1991, 10, 539-548. (g) Vill, V.; Hashim, R. Curr. Opin.
Colloid Interface Sci. 2002, 7, 395-409.
(25) To avoid confusion the term (cylinder) shell is used for the shell of
bolaamphiphilic units arranged around a lipophilic column [(cylinder) core].
In the resulting 2D lattice shells of adjacent cylinders form the (cylinder)
walls, i.e. these cylinder walls separate the cylinder cores.
(26) The model of the p2gg phase has been revised (see section 2.2) and is
different from the earlier proposed structures.21
(27) In ref 21b two different models were discussed for the Colh phases of
compounds 1/12-1/18, but on the basis of the new model for the p2gg
phases and the other results reported herein the alternative star-like model
can be excluded.
(28) Disclike LC with perfluorinated chains: (a) Dahn, U.; Erdelen, C.;
Ringsdorf, H.; Festag, R.; Wendorff, J. H.; Heiney, P. A.; Maliszewskyj,
N. C. Liq. Cryst. 1995, 19, 759-764. (b) Terasawa, N.; Monobe, H.;
Kiyohara, K.; Shimizu, Y. Chem. Lett. 2003, 32, 214-215.
(29) Pegenau, A.; Cheng, X. H.; Tschierske, C.; Go¨ring, P.; Diele, S. New. J.
Chem. 1999, 23, 465-467.
(30) Smart, B. E. In Organofluorine Chemistry Principles and Commercial
Applications; Banks, R. E., Smart, B. E., Tatlow, J. C., Eds.; Plenum
Press: New York, 1994; pp 57-88.
9
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A R T I C L E S
Cheng et al.
Scheme 1. Structural Formulas of the Molecules under
Investigation
Scheme 2. Synthesis of Compounds 2/n, 5/n (R ) H), 6/n (R )
a
CnH2n+1) and 7/n (R ) (CH2)3CnF2n+1)
of fluorination (compounds 2/n-4/n), and the position (com-
pounds 5/n) of the lateral chain gives rise to a series of different
liquid crystalline phases with unique structures.
Results and Discussion
1. Synthesis. Scheme 2 describes the synthesis of most of
the compounds. Compounds 2/n, 5/n, 6/n, and 7/n containing
perfluorinated lateral chains attached via a propylene spacer were
synthesized by the Pd0-catalyzed addition of 1-iodoperfluoro-
alkanes16a,34 to 2-allylphenol or 3-allylanisole, followed by para-
selective bromination using HBr/AcOH/DMSO35 (phenols) or
NBS/CH3CN36 (anisoles). Then a Suzuki-coupling37 with ap-
propriate benzene boronic acids gave the biphenyl cores, and
finally the introduction of the terminal propane-2,3-diol groups
led to the desired compounds.38,39 The first step in the synthesis
of compounds 4/n with perfluorinated chains directly attached
to the aromatic core was a coupling reaction between 2-
iodoanisole and 1-iodoperfluoroalkanes in the presence of
reactive Cu-powder.40 This was followed by bromination with
NBS in trifluoroacetic acid41 and continued according to Scheme
a Reagents and conditions: i) Mg, Et2O, reflux; ii) 1. CH2dCHCH2Br,
Et2O, 0 °C, 2. HCl, H2O, 0 °C; iii) Pd(PPh3)4, RFI, 25 °C, 36 h; iV) 1.
LiAlH4, Et2O, reflux, 2 h, 2. H2O, H2SO4, 0 °C; V) NBS, CH3CN, 25 °C,
24 h; Vi) DMSO, HBr/AcOH, 5 °C; Vii) CH3I, K2CO3, CH3CN, reflux, 2 h;
Viii) Pd(PPh3)4, NaHCO3, glyme, H2O, reflux, 6 h; ix) BBr3, CH2Cl2; x)
CH2dCH-CH2Br, K2CO3, CH3CN, reflux, 6 h; xi) OsO4, N-methylmor-
pholine-N-oxide, H2O, acetone, 25 °C 2 h.
2. To further elongate the lateral chain, compounds 3/n, in which
the perfluorinated segments are attached via a hexamethyleneoxy
spacer, were synthesized by etherification of bis-4,4′-(2,2-
dimethyl-1,3-dioxolan-4-ylmethoxy)-3-biphenylol42 with ap-
propriate 1H,1H,2H,2H,3H,3H,4H,4H,5H,5H,6H,6H-perfluo-
roalkylhalides.42 All final products were purified by preparative
centrifugal thin-layer chromatography with a Chromatotron
(Harrison Research) and, if possible, by repeated crystallization
(31) Fuhrhop, J.-H.; Fritsch, D. Acc. Chem. Res. 1986, 19, 130-137.
(32) (a) Festag, R.; Hessel, V.; Lehmann, P.; Ringsdorf, H.; Wendorff, J. H.
Recl. TraV. Chim. Pays-Bas 1994, 113, 222-230. (b) Hessel, V.; Ringsdorf,
H. Festag, R.; Wendorff, J. H. Makromol. Chem. Rapid Commun. 1993,
14, 707-718. (c) Dahlhoff, W. V. Z. Naturforsch. 1988, 43b, 1367-1369.
(d) Hentrich, F.; Tschierske, C.; Zaschke, H. Angew. Chem., Int. Ed. Engl.
1991, 30, 440-441. (e) Hentrich, F.; Diele, S.; Tschierske, C. Liq. Cryst.
1994, 17, 827-839.
(33) Hentrich, F.; Tschierske, C.; Diele, S.; Sauer, C. J. Mater. Chem. 1994, 4,
1547-1558.
(34) Chen, Q.-Y.; Yang, Z. Y.; Zhao, C.-X.; Qiu, Z. M. J. Chem. Soc., Perkin
Trans. 1 1988, 563-567.
from appropriate solvents and characterized by 1H, 13C, and 19
F
(35) Majetch, G.; Hicks, R.; Reister, S. J. Org. Chem. 1997, 62, 4321-4326.
(36) Carreno, M. C.; Ruano, J. G.; Sanz, G.; Toledo, M. A.; Urbano, A. J. Org.
Chem. 1995, 60, 5328-5331.
NMR and elemental analysis. The purity was additionally
checked by TLC. Experimental details and analytical data are
reported in the Supporting Information.
(37) (a) Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981, 11, 513-
519. (b) Hird, M.; Gray, G. W.; Toyne, K. J. Mol. Cryst. Liq. Cryst. 1991,
206, 187-204 (c) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-
2483.
(40) (a) McLoughlin, V. C. R.; Thrower, J. Tetrahedron 1969, 25, 5921-5946.
(b) Pozzi, G.; Cavazzini, M.; Cinato, F.; Montanari, F.; Quici, S. Eur. J.
Org. Chem. 1999, 1947-1955.
(41) Duan, J.; Zhang, L. H.; Dolbier, W. R., Jr. Synlett 1999, 8, 1245-1246.
(42) The synthesis of this compound will be reported in a separate paper.
(38) Grieco, P. A.; Nishizawa, M.; Oguri, T.; Burke, S. D.; Marinovic N. J.
Am. Chem. Soc. 1977, 99, 5773-5780.
(39) Van Rheenen, V.; Cha, D. Y.; Hartley, W. M. Org. Synth. 1978, 58, 43-
51.
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Calamitic Bolaamphiphiles with Lateral Chains
A R T I C L E S
Figure 3. Optical photomicrographs (left, crossed polarizers) and X-ray
diffraction pattern (right) of the columnar mesophases of group I com-
pounds: (a, b) Colr phase (c2mm) of 2/3 at 115 °C; (c) Colr phase (p2gg)
of 2/4 growing into the isotropic liquid at 135 °C (black areas are residual
regions of the isotropic liquid); (d) Colr phase (p2gg) of 2/4 at 127 °C; (e)
Colh phase of 2/i7 at 170 °C (here, the black areas are homeotropic domains
in which the columns are perfectly aligned perpendicular to the glass
substrates); (f) Colh phase of 2/i7 at 175 °C.
Figure 2. Dependence of the liquid crystalline phases of compounds 2/n
and 3/n with respect to the length of the semiperfluorinated lateral chain
(2/n: R ) (CH2)3CnF2n+1; 3/n: R ) O(CH2)6CnF2n+1).
2. Mesomorphic Properties
2.1. General Trends. The obtained compounds were inves-
tigated by polarized light optical microscopy, differential
scanning calorimetry, and X-ray diffraction. The transition
temperatures and corresponding enthalpy values for all com-
pounds are collated in Tables 1-3. All materials with one
(semi)perfluorinated lateral chain show thermotropic liquid
crystalline phases. The mesophase stability and the mesophase
type are strongly dependent upon the length of the semiperflu-
orinated chain. However, the type of attachment of the perflu-
orinated chains to the aromatic units (direct connection in
compounds 4/n or via spacer units in all other compounds) and
the position of the chains (position 2 in compounds 5/n or
position 3 in all other compounds) are less important. Figure 2
graphically shows the dependence of the phase transitions on
the chain length for the 3-substituted compounds 2/n and 3/n.
Three distinct groups of compounds can be distinguished with
regard to the phase behavior. Compounds 2/3-2/i9 (group I)
have either rectangular or hexagonal columnar mesophases,
similar to those found for the alkyl-substituted compounds 1/n.
Compounds 2/10-2/12 with a medium length of the semiper-
fluorinated chain (group II) have mesophases with unusually
large lattice parameters, and two additional lamellar mesophases
(Lam) were found for compound 2/12. Compounds 3/10 and
3/12 (group III) which have the longest chains show exclusively
lamellar phases.
comparison with the corresponding hydrocarbon analogues 1/n
(see Figure 1) with the same number of C-atoms in the lateral
chain shows that most fluorinated compounds 2/n have reduced
melting points, but all have significantly enhanced mesophase
stabilities. This gives rise to significantly enlarged mesophase
regions for all fluorinated compounds with the exception of
compound 2/3, which exhibits only monotropic (metastable)
behavior. It is also interesting to note that branching of the
terminal chains (compound 2/i7 and 2/i9) appears to have no
significant influence upon the mesophase behavior. Compounds
2/i7, 2/6, and 2/8 all exhibit the same mesophase type (Colh),
and the clearing temperature of 2/i7 lies between those of
compounds 2/6 and 2/8 which contain unbranched chains.
Compounds of group I exhibit four different types of
columnar mesophases. Typical textures and small-angle X-ray
diffraction pattern of the three main phase types are shown in
Figure 3. Diffuse scattering in the wide-angle region of all X-ray
diffraction patterns confirm the liquid crystalline nature of these
mesophases. Mosaic-like textures and spherulitic textures were
detected for compound 2/3 (Figure 3a). The powder-like X-ray
diffraction pattern of this mesophase (Figure 3b) is characterized
by three non-equidistant sharp reflections in the small-angle
region, which were indexed on the basis of a centered
rectangular 2D lattice. In accordance with results obtained during
previous investigations of bolaamphiphiles with lateral alkyl
chains21b the c2mm 2D space group has been assumed. The
lattice parameters were then calculated to be a ) 3.3 nm and b
) 3.4 nm. These lattice parameters are between the single
2.2. Influence of the Length of the Semiperfluorinated
Chain on the Mesophase Morphology: Columnar Meso-
phases of Group I Compounds. In this section the mesomor-
phic properties of compounds 2/3-2/i9 will be discussed. A
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A R T I C L E S
Cheng et al.
Table 1. Mesophases, Phase Transition Temperatures, Phase Transition Enthalpies (lower lines in italics), and Other Parameters of the
Bolaamphiphiles 2/n-4/na
a The transition temperatures were determined by DSC (first heating scan, 10 K min-1); a,b,d ) lattice parameters, Vcell ) volume of a unit cell with a
height of h ) 0.45 nm;43 Vmol ) volume for a single molecule as calculated using the crystal volume increments reported by Immirzi;44 ncell ) number of
molecules in a unit cell, calculated according to n ) Vcell/Vmol; ncell,calc ) number of molecules arranged in the cross section of a unit cell as theoretically
required by the model of the mesophase, this number is calculated from the number of biphenyl units required for the formation of the polygon and the
number of columns per unit cell; nwall ) average thickness of the cylinder walls given by the average number of biphenyl units arranged side by side in the
walls separating two adjacent cylinder cores;25 fR ) volume fraction of the lateral chains; abbreviations: LamIso ) lamellar phase with an arrangement of
rodlike units parallel to the layer planes and without additional order; LamN ) lamellar phase with an arrangement of biphenyl units parallel to the layer
planes and with an in-plane orientational order; LamX ) lamellar phase for which an arrangement of biphenyl units parallel to the layer planes with an
in-plane orientational and possibly also an positional order is proposed (see Figure 15); Colsq ) square columnar mesophase; Colr ) rectangular columnar
phases, the lattice type is additionally given; M1 ) mesophase with unknown structure; Iso ) isotropic liquid; the perfluorinated segments of compounds
2/i7, 2/i9 and 2/i11 are branched at their terminal ends (iso-perfluoroalkyl segments) whereas all other chains are linear. b This phase transition was only
observed in the first heating scan. c Values for the Colr/p2gg phase. d This phase transition was only observed by X-ray diffraction. e The phase transitions
Colr/c2mm-LamN-LamIso are not resolved. f Values for the Colr/c2mm phase. g Value for the LamIso phase. h On cooling additional transitions to crystalline
mesophases can be observed at 101 °C and 77 °C. i The phase transitions LamX-LamN-LamIso are not resolved. k On cooling, an additional transition to
a crystalline mesophases can be observed at 122 °C. l Value for the Colh phase.
molecular length (L ) 2.1 nm)45 and twice that length. The
number of molecules located within a hypothetical unit cell with
a height of 0.45 nm43 has been calculated according to the
equation n ) Vcell/Vmol, yielding a value of approximately eight
(44) Immirzi, A.; Perini, B. Acta Crystallogr., Sect. A 1977, 33, 216-218.
(45) The molecular lengths are calculated as the distance between the ends of
the terminal propane-2,3-diol units and have a value between 1.7 and 2.1
nm, depending on the conformation assumed for the propane-2,3-diol units.
The molecular length of 2.1 nm is found in all cylinder structures of
compounds 1/n, 2/n, and 5/n. It seems that the organization into cylinders
leads to a maximum stretching of the bolaamphiphilic cores due to the
confined geometry in which the flexible chains are enclosed. To get minimal
interfaces the cross section of the cylinders should be as large as possible,
which leads to a stretching of the cylinder walls. In contrast, a value of
only 1.7 nm was found for a correlated layer structure.61 Within layer
structures, expansion of the lipophilic regions is easily achieved by changing
the thickness of the nonpolar sublayers, so that the conformation of the
hydrogen bonding is more independent from the organization of the
nonpolar chains. Here the molecules organize in such a way that a maximum
number of hydrogen-bonding sites are utilized most efficiently. This is
obviously achieved if the propane-2,3-diol groups adopt a compact
conformation with strong interdigitation.
(43) Herein, the value h ) 0.45 nm was used in all calculations, independent of
the actual position of the maximum of the wide-angle scattering in the
diffraction pattern. The reason is that the bolaamphiphilic units (their
average diameter is 0.45 nm) are responsible for the lattice of the cylinder
shells, whereas the semiperfluoralkyl chains only fill the channels within
this structure. Because the cross section of the perfluorinated segments is
larger than the rest of the molecules, there is a drift of the maximum of the
diffuse scattering in the X-ray diffraction pattern to larger D-values with
increasing degree of flourination. However, this shift is due to a change of
a molecular parameter rather than due to a change of the structrure. Because
the diameter of the flourinated segments is larger than that of the
nonfluorinated bolaamphiphilic moieties, which form the cylinder walls,
there is a mixing of the chains of adjacent (hypothetical) unit cells along
the cylinder, i.e. the perfluorinated chains in one unit cell also contribute
to the space filling in the adjacent unit cell. This is in line with the models
of the mesophases (see Figures 4f, 6c, and 10b), where some vacant space
is available for parts of the chains from the neighboring cells.
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Calamitic Bolaamphiphiles with Lateral Chains
A R T I C L E S
molecules per unit cell (see Table 1). A cylinder model, in which
approximately four molecules are arranged in the cross-section
of each cylinder and two cylinders are arranged in the unit cell,
provides the most accurate fit to these data. This organization
of cylinders with an elliptical cross-sectional area in a c2mm
lattice is similar to the model proposed for the Colr/c2mm phases
of 1/6-1/9 shown in Figure 1.
with the previously discussed lattice shown in Figure 4a),
composed of only two cylinders per unit cell is more likely.
These small lattices, shown in Figure 4c, are more likely as
they only contain one type of hydrogen-bonding network (three-
wall connections), whereas the p2gg lattice described in Figure
4b contains three distinct types of hydrogen-bonding networks,
i.e. two-, three-, and four-wall connections.
A similar p2gg lattice containing four cylinders arranged in
pairs is possible when five biphenyl units form the cylinder shell.
These biphenyl units would form a pentagonal shape around
the cylinder cores as shown in Figure 4, d-f. When a pentagonal
cross-sectional shape is adopted, the formation of pairs of
cylinders is the only possible structure by which a regular 2D
organization of this type of column can be realized, i.e. smaller
lattices are not possible. Furthermore, in this structure only two
types of hydrogen bonding are combined, three-wall connections
(as found in the p6mm lattice of the higher homologues) and
four-wall connections (as found in the c2mm lattice of the shorter
homologue). In this respect a p2gg lattice formed by the
organization of pentagons is intermediate between the c2mm
and the p6mm lattice. As shown in Figure 4f, the model con-
sisting of slightly distorted pentagonal cylinders is in very good
agreement with measured lattice parameters.45 This model also
predicts the large value of the number of molecules contained
within the unit cell (ncell ) 22). Assuming that the wall thickness
in the network structure is approximately twice the width of
the biphenyl units, a theoretical value ncell,calc ) 20 can be
calculated (five molecules in each cylinder shell and four
cylinders forming the unit cell) which is very close to the experi-
mental value ncell. Therefore, it is suggested, that the p2gg
lattices of compounds 1/10-1/12 and compound 2/4 are
organized in this type of arrangement. Moreover, it is proposed
that in this class of compounds the p2gg lattice containing four
cylinders in each unit cell is an indication of a regular 2D
organization of pairs of cylinders with pentagonal cross-
sectional shape.46,47 In these p2gg mesophases, the specific pen-
tagonal shape is provided by the self-organization of the rigid
biphenyl cores, and it is not averaged out by rotation around
the long axis of the cylinders, because this rotation is inhibited
by the segregation of the intermolecular hydrogen-bonding
networks of the terminal diol groups from the rigid aromatic
cores. This segregation fixes the positions of the biphenyl cores
(and H-bonding networks) within the cylinder walls.
The high-temperature mesophase of 2/4 shows a quite
different optical texture. Upon cooling from the isotropic, a fern-
like texture is observed (see Figure 3c) which coalesces to form
mosaic-like and spherulitic regions. To determine the structure
of this and other mesophases X-ray scattering investigations
were performed on aligned samples. The alignment was
achieved by slow cooling (0.01 K min-1) of a droplet of the
sample (diameter ca. 1-1.5 mm) on a glass plate. Surface
interactions at the sample-glass interface or at the sample-air
interface cause a special alignment of the cylinders within the
columnar phases: their long axes are packed in planes parallel
to the surfaces of the sample. Within these planes, the cylinder
axes are orientationally disordered around the layer normal with
a more or less equal distribution of domains with different
orientations. The incident X-ray beam (wavelength of the Cu
KR line, 0.154 nm) is applied parallel to the substrate glass
plate, and the scattering beam intensity was recorded in
transmission using a 2D detector (HI-Star, Siemens). The X-ray
diffraction pattern of an aligned sample of compound 2/4 at
127 °C clearly excludes a centered cell due to the appearance
of the 21 and 12 reflections (see Figure 3d). These reflections
can be explained if a p2gg 2D noncentered rectangular lattice
is assumed. The lattice parameters were then calculated to be a
) 5.4 nm and b ) 5.9 nm. These values are more than twice
the molecular length (L ) 2.1 nm), and approximately 22
molecules are arranged in the cross section of the unit cell.43
The model shown in Figure 4a was first proposed for the
p2gg lattice of compounds 1/10-1/12.21 Generally, a p2gg
lattice requires a herringbone-like arrangement within the lattice,
which was realized in the model in Figure 4a by including two
elliptical cylinders per unit cell, separated by bilayer-like
aggregates of the bolaamphiphilic units.21 However, such a
model does not fit with the cylinder-shell models of the other
columnar mesophases discussed in this article. Alternatively,
the formation of a p2gg lattice is also possible by the regular
organization of four cylinders per unit cell. This is achieved if
the large cylinders shown in Figure 4a are divided into pairs of
cylinders at the same geometrical positions. Such a model with
four cylinders per unit cell can easily explain the large lattice
parameter found.
On further cooling, an additional phase transition was
observed in the X-ray diffraction pattern of compound 2/4 at
approximately 67 °C. At this temperature the spotlike reflections
in the small-angle region are replaced by a single sharp ring at
θ ) 2.09°, corresponding to d ) 2.14 nm (the diffuse wide-
angle scattering remains). It is interesting to note that this
reflection shows clear maxima positioned at the equator and at
the meridian. A possible explanation for this observation (square
columnar phase Colsq) will be given in section 2.4.
The p2gg lattice is located between the Colr/c2mm phase and
the Colh (p6mm) phase. The c2mm lattice is formed by the
arrangement of four molecules around a cylinder, whereas the
p6mm lattice has six molecules arranged around each cylinder.
Therefore, it can be assumed that in the p2gg lattice each
cylinder is surrounded by the bolaamphiphilic units of either
five or six molecules. If a six-molecule arrangement for the p2gg
lattice is assumed, then the cylinder shell must adopt an
elongated quadrilateral shape. This shape is formed by the
organization of an end-to-end dimer on two sides and a single
molecule on the others, as shown in Figure 4b. However, a much
simpler lattice, such as the c2mm lattice or an alternative p2gg
lattice (however, this two-cylinder-lattice should not be confused
Mesophase textures of compounds 2/6-2/i9 are identical and
can be optically characterized by the observation of large
(46) In the p2gg phase of the hydrocarbon compounds 1/10-1/12 a number of
19-23 molecules per unit cell was calculated,21 which is also in perfect
agreement with the pentagonal cylinder model.
(47) This model corresponds to an elemental tilling of 2D space with the
topological class [32‚4‚3‚], for details see: (a) Gru¨nbaum, B.; Shepard, G.
C. Tillings and Patterns; W. H. Freeman: New York, 1986. (b) Simon, J.;
Bassoul, P. Design of Molecular Materials: Supramolecular Engineering;
John Wiley: Chinchester, 2000.
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A R T I C L E S
Cheng et al.
Figure 4. Models for the molecular arrangement of 1/10-1/12 and 2/4 in rectangular columnar mesophases. (a) Model with only two elliptical columns per
unit cell as initially proposed for the p2gg phases of compounds 1/10-1/12;21 (b) p2gg lattice containing four columns per unit cell as formed by the
organization of pairs of cylinders with a quadrilateral cross sectional shape (only the biphenyl units are shown); (c) other 2D lattices of columnar phases
arising from the regular organization of cylinders with a quadrilateral cross-sectional shape (additional oblique lattices are possible); (d) p2gg lattice containing
four columns per unit cell as formed by the organization of pairs of cylinders with a pentagonal cross-sectional shape; (e) revised molecular model for the
arrangement of 1/10-1/12 and 2/4 in the Colr/p2gg phases; the dotted lines indicate the pairs of columns; (f) CPK models showing 10 molecules of 2/4
forming a pair of pentagons (central pair of the model shown in d and e).
homeotropic aligned regions containing birefringent filaments,
typical for hexagonal columnar phases (see Figure 3e). X-ray
diffraction also proof a hexagonal columnar structure for these
mesophases and the diffraction pattern of compound 2/i7 is
shown in Figure 3f as an example. The lattice parameters are
calculated to be between ahex ) 3.45 nm and ahex ) 3.50 nm
for all investigated Colh-phases. In all cases approximately six
molecules are arranged (on average) in the cross section of each
cylinder (see Tables 1 and 2). In this mesophase six biphenyl
units form the cylinder shells around the nonpolar cores as
shown in Figure 1 (for the Colh phases of compounds 1/12-
1/18).27
Therefore, with respect to the changes in chain length, the
phase sequence Colr/c2mm, Colr/p2gg, Colh/p6mm was found,
which is the same as that found for the alkyl-substituted
compounds 1/n.21b Also similar molecular arrangements for the
mesophases of the fluorinated group I compounds 2/3-2/i9 and
the hydrocarbon compounds 1/n can be proposed. These
mesophases represent cylinder structures in which the semiper-
fluorinated lateral chains form infinite columns around which
the rigid bolaamphiphilic units form a shell. These bola-
amphiphilic units are connected end-to-end and side-by-side by
elongated hydrogen-bonding networks between the diol groups,
and these are arranged parallel to the nonpolar columns. The
relative volume required by the semifluorinated chains with
respect to the length of the rigid segments determines the number
of biphenyl units organized in the shells around the cylinder
cores, and this number increases from four in the Colr/c2mm
and Colsq/p4mm phases (see section 2.4) via five in the Colr/
p2gg phases to six in the Colh/p6mm phases. Hence, the number
of biphenyl units arranged around the nonpolar cylinder cores
is well defined, and this leads to distinct polygonal shapes of
the cylinder shells. These shapes must enable a regular and
uniform packing in 2D space as well as a segregation of the
biphenyl units from the diol groups, which allows interconnec-
tion of the biphenyl units to occur exclusively at their H-bonding
sites (commensurate packing).
Although the phase sequence observed is the same as that
found for the hydrocarbon analogues 1/n, the overall chain
length necessary to achieve these columnar phases is signifi-
cantly reduced. The Colh phase, for example, occurs for the
fluorinated compound 2/6 (with a total of nine C-atoms in the
lateral chain), whereas the same phase is not observed in the
1/n series until the lateral hydrocarbon chain contains at least
12 CH2 groups (compound 1/12). This observation can be
explained due to the much larger volume occupied by semi-
fluorinated chains when compared to that of the corresponding
alkyl chains. Indeed, the volume fraction of the lateral chains
(fR, see Table 1), required for the formation of the different
types of columnar phases is identical for the bolaamphiphiles
with fluorinated and nonfluorinated lateral chains: Colr/c2mm:
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Calamitic Bolaamphiphiles with Lateral Chains
A R T I C L E S
Table 2. Mesophases, Phase Transition Temperatures, Phase Transition Enthalpies (lower lines in italics) and Other Parameters of the
Bolaamphiphiles 5/na
a Abbreviations: Colsq ) columnar mesophase with square lattice; M2 ) unknown mesophase. b Refers to the Colh phase.
fR ) 0.28-0.36; Colr/p2gg: fR ) 0.36-0.43; Colh/p6mm: fR
) 0.43-0.53.
semifluorinated chains with the rigid biphenyl units (compounds
4/n) and position of the lateral chain (compounds 5/n) will be
made.
The organization into hexagonal columnar phases can be
found over the broadest range of volume fraction of the lateral
chains, which indicates that it is especially favorable. Further-
more, it is remarkable that the diameter of the columns of the
Colh phases does not change upon elongation of the lateral chain
from compound 2/6 to compound 2/i9. This shows that the
diameter of the columns is wholly defined by the length of the
bolaamphiphilic units forming the cylinder shells. Therefore, it
can be deduced that the additional space required by the larger
chains is provided by the stretching of the cylinders along the
cylinder long axis, leading to a reduction in the average number
of molecules per cylinder segment with h ) 0.45 nm43 from
ncell ) 6.6 (compound 2/6) to ncell ) 5.6 (compound 2/i9) for
the series 2/n (and from ncell ) 6.6 to ncell ) 5.5 for compounds
5/n, see Table 2, section 2.4). This stretching of the cylinders
is accompanied by a slight reduction in the thickness of the
cylinder walls. This means, that the number of biphenyl units
located side by side within the walls separating the columns
(nwall, see Table 1) can vary to optimize the overall space filling
within the nonpolar regions of the cylinders. In most cases, an
average of two biphenyl units are arranged side by side. For
the Colh phases the value is between 2.2 and 1.8, but in some
cases this value can be as low as 1.6 (Colsq phase of compound
4/4, see section 2.4, Table 2) or as high as 2.3 (c2mm
mesophases of the group II compounds, see section 2.5). It is
also important to note here, that all these structures are highly
dynamic and the positions of the molecules within the columns
are not fixed. This means that there is no formation of disklike
entities or any long-range correlation of the positions of the
biphenyl units along the cylinder shells, i.e. there is no perfect
order as the models shown might suggest. Therefore, the number
of biphenyl units arranged side by side in the cross section of
the cylinder walls is an average value and noninteger numbers
are allowed.
2.3. Compounds with Directly Attached Perfluorinated
Chains. Two compounds 4/4 and 4/8 have been synthesized,
in which the perfluorinated chain is directly attached to the
biphenyl units (see Table 1). Compound 4/4, which has the
shortest chain of all synthesized molecules shows a fanlike
texture, typical for SmA phases. This is the only bolaamphiphilic
compound, reported herein, which shows a smectic A phase
where the biphenyl units are thought to be arranged in average
perpendicular to the layer plane,48 but this phase is only
monotropic, so further investigations were not possible.
Compound 4/8 shows a Colh phase and an additional low-
temperature phase (M1), which was not investigated in detail.
This material shows an unusually low-melting transition for
compounds which contain a perfluorinated alkyl chain attached
directly to the mesogenic core. When the transition temperatures
and mesophase type for compound 4/8 (C8-chain) is compared
with compounds of the 2/n series it was found to fit between
2/4 (C7-chain, Colr/p2gg) and 2/6 (C9-chain, Colh). This means
that the volume fraction of the lateral chain with respect to the
length of the bolaamphiphilic unit is the dominating factor which
determines the mesophase structure. There is no significant
influence of the type of connection between the fluorinated
segment and the biphenyl unit on mesophase type and meso-
phase stability.
2.4. Influence of the Position of the Lateral Chain: A
Square Columnar Phase. For bolaamphiphiles with lateral
alkyl chains, it was found that the position of substitution at
the biphenyl unit only has an influence upon the mesophase
stability, and not the mesophase type.21b To check if this also
applies for the perfluorinated compounds a series of compounds
5/4, 5/6, 5/8, and 5/10 have been synthesized, in which the
semiperfluorinated chain is attached at the 2-position on the
aromatic core. Compounds 5/6 and 5/8 which are related to the
isomeric compounds 2/6 and 2/8 show Colh phases; however,
Before discussing compounds 2/n and 3/n with longer chains
(group II and III) a discussion of the connection of the
(48) Also a SmA+ structure21b could be possible for this mesophase.
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A R T I C L E S
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these materials have significantly reduced clearing temperatures
(see Table 2). This reduction in clearing temperature was also
observed in other amphiphilic49 and non-amphiphilic liquid
crystals50 when the lateral chain is moved to a more central
position. Two possible explanations for this phenomenon have
been proposed: (i) that changing the position of the lateral
substitution would cause a change of the conformation of the
biphenyl unit, i.e. a change in the dihedral angle between the
planes of the adjacent benzene rings and (ii) that a change of
substitution would disturb the parallel alignment of the rigid
units.
However, compound 5/4 is different from 5/6 and 5/8. This
compound shows a texture (Figure 5a) which is quite different
from that of the isomeric compound 2/4. The mesophase is
optically uniaxial, and the texture is similar to those found for
the hexagonal columnar phases in series 2/n and 4/n. However,
the 2D X-ray diffraction pattern of an aligned sample (Figure
5b) indicates a liquid crystalline phase with a square lattice
(p4mm), which is in full accordance with the optical uniaxiality
of this mesophase.51 The lattice parameter can be calculated to
asq ) 2.14 nm, which is close to the molecular length in a
stretched conformation (L ) 2.1 nm).45 The model shown in
Figure 5c is in good agreement with the experimentally
determined lattice parameter. Accordingly, the arrangement of
the molecules in this mesophase is similar to those found in
the centered rectangular columnar c2mm phase of compound
2/3. In both mesophases four bolaamphiphilic units form the
cylinder shells around the nonpolar columns of the semifluori-
nated chains. The main difference is that the columns in the
Colsq phase have a circular to square-like52 instead of an elliptical
cross-sectional shape. This enhances the symmetry and gives
rise to the change in lattice type. Hence, the Colsq-phase of 5/4
can be regarded as an intermediate stage in the transition from
the c2mm lattice to the p2gg lattice. In this square-columnar
phase the space between the four bolaamphiphilic units is filled
to the maximum possible extend, and therefore the columns
adopt a square cross section. However, it was found that in this
mesophase approximately 3.2 molecules are contained within
the unit cell. It is thought that this unusually low number of
molecules contained within the unit cell arises due to an
overcrowding of the interior of the columns in this square phase.
This leads to an expansion of the columns along their long axes
(as discussed in section 2.2. for the Colh phases), and therefore
a reduction in the number of molecules organized in the cross
section of the cylinder walls is found. The same results were
obtained from the CPK model shown in Figure 5d. The length
of each side of this square is ca. 2.3 nm, which means that the
(49) Ko¨lbel, M.; Beyersdorff, T.; Tschierske, C.; Diele, S.; Kain, J. Chem. Eur.
J. 2000, 6, 3821-3837.
(50) (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.;
Tschierscke, C.; Diele, S.; Lose, D. J. Mater. Chem. 1996, 6, 1297-1307.
(51) Earlier reports about Colsq phases: (a) Ohta, K.; Watanabe, T.; Hasebe,
H.; Morizumi, Y.; Fujimoto, T.; Lelievre, D.; Simon, J. Mol. Cryst. Liq.
Cryst. 1991, 196, 13-26. (b) Praefcke, K.; Marquardt, P.; Kohne, B.;
Stephan, W. Mol. Cryst. Liq. Cryst. 1991, 203, 149-158. (c) Komatsu, T.;
Ohta, K.; Watanabe, T.; Ikemoto, H.; Fujimoto, T.; Yamamoto, I. J. Mater.
Chem. 1994, 4, 537-540. (d) Hatsusaka, K.; Ohta, K.; Yamamoto, I.; Shirai,
H. J. Mater. Chem. 2000, 11, 423-433.
(52) In Figure 5c, the cross-sectional shape of the cylinder cores is represented
by a circle; however, the shape of the cylinder shells is a square. Therefore,
the cylinder cores must, in actual fact, have a cross-sectional shape which
is between a circle (minimized interfacial areas) and a square (maximal
space filling), i.e. it should be a “square with rounded corners”. Related
arguments also concern the models of all other columnar phases discussed
herein.
Figure 5. (a) Texture (crossed polarizers) of the Colsq phase of compound
5/4 at 95 °C; (b) X-ray diffraction pattern of an aligned sample of the Colsq
phase at 99 °C (small-angle region); (c) model of the organization of the
molecules in the Colsq phase (two biphenyl units are shown in each cylinder
wall; however, in reality this number is only about 1.6). (d) CPK models
showing four molecules of 5/4 forming a square.
lattice parameter of 2.14 nm can only be realized if the effective
thickness of the cylinder walls is less than the usually observed
value of two molecules.
A Colsq phase could also be one possible explanation for the
low-temperature phase observed in the isomeric compound 2/4.
Although no well-aligned samples have been obtained for this
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mesophase, the observation of a sharp ring in the small-angle
region could result from the 10 and 01 reflections of a Colsq
phase (multidomain sample). Assuming such an indexation
would lead to a ) 2.14 nm, which exactly corresponds to the
lattice parameter found for the Colsq phase of 5/4. This means
that changing the position of the (CH2)3C4F9 chain has a
stabilizing effect upon the Colsq/p4mm phase with respect to
the Colr/p2gg phase. This would seem to suggest that a chain
in the 2-position requires less space than that in the 3-position
and hence enables a slightly more efficient packing. Probably,
the more pronounced T-like shape of 5/4 is also favorable for
the organization in a square lattice.
A similar influence of the position of the chain on the
mesophase type is evident if the isomeric compounds 2/10 and
5/10 are compared. Compound 5/10 exhibits a broad region of
a Colh phase, whereas the isomeric compound 2/10 exhibits a
mesophase with a novel c2mm lattice (see section 2.5). However,
at low temperature a mesophase with unknown structure (M2)
was found for 5/10, which could possibly be related to the
mesophase type found for 2/10.
2.5. Mesophases of Group II Compounds: Columnar
Phases with Large Lattice Parameters. Compounds 2/10,
2/i11, and 2/12 show two new columnar phases. The texture of
the mesophase of 2/10 (Figure 6a) is different from any other
known liquid crystalline phase. In the X-ray pattern only
reflections with h + k ) 2n have been observed in the small-
angle region (Figure 6b). These reflections can be assigned to
a centered rectangular lattice (c2mm) with the lattice parameters
a ) 3.7 nm and b ) 9.6 nm. An additional weak reflection on
the meridian corresponding to a periodicity d ) 3.4 nm could
be explained by a contribution of a differently aligned
monodomain. The lattice parameter b in this c2mm phase is
much larger than that found for the c2mm phase of compound
2/3. One possible explanation for the organization of molecules
within this columnar mesophase is shown in Figure 6, c and d.
In this model the bolaamphiphilic units again form shells around
the cylinder cores of microsegregated semiperfluorinated chains.
In this phase the shells contain eight bolaamphiphilic units in
the cross section instead of six as found for the Colh phases.
As regular octagons cannot efficiently pack into a 2D lattice,
the shell adopts a deformed hexagonal shape. Within these
hexagons two opposite sides are stretched, i.e. two sides of these
hexagons are formed by an end-to-end dimer. This causes a
reduction of the symmetry, leading to a change of the lattice
type from p6mm to c2mm. Within this c2mm lattice the lattice
parameter a has the same order of magnitude as ahex for the
Colh phases of compounds 2/n, (about 31/2 L), whereas the
parameter b is significantly enlarged and has a value equivalent
to 4-5 times the molecular length. This can be seen in Figure
6c, and these lattice parameters predicted are in excellent
agreement with the experimental data.
Figure 6. (a) Texture of the mesophase of compound 2/10 as obtained on
cooling from the isotropic liquid at T ) 170 °C; (b) X-ray diffraction pattern
of an aligned sample of 2/10 at 150 °C; (c) molecular arrangement within
the rectangular columnar mesophase Colr with a c2mm 2D space group;
(d) CPK model of eight molecules 2/10 arranged so that the semiperflu-
orinated chains form a central region around which eight bolaamphiphilic
units are arranged.
The optical texture of compound 2/i11 (see Figure 7a, b) is
quite different from 2/10. However, both diffraction patterns
can be indexed on the basis of a centered rectangular c2mm
lattice, and the lattice parameters obtained for these two
compounds are almost identical. Therefore, the model for the
organization of the molecules in this mesophase is similar to
that shown in Figure 6c.53 The apparent differences in the
diffraction patterns are thought to arise due to differing
alignments within the rotationally disordered sample. In com-
pound 2/i11 the crystallographic (110) plane of the lattice is
parallel to the surfaces; thus, the sample is rotationally
disordered about the normal to this plane. Figure 7c shows two
(53) In contact preparations of compounds 2/10 and 2/i11 a continuous change
of the isotropization temperature was found without any minimum. This
may be another indication that both materials show a mesophase structure
of the same type. Also in the contact region between 2/i7 (Colh) and 3/12
(Lam) only one single new columnar mesophase is induced, as expected.
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Figure 8. Optical textures (crossed polarizers) of compound 2/12 as
observed on cooling a homeotropically aligned sample (layers are parallel
to the surfaces of the glass substrates, left) and a sample showing a fan-
shaped texture (layers are perpendicular to the glass substrates, right): (a)
LamIso phase at 181.5 °C (in the upper part the schlieren texture of the
LamN phase develops; (b) LamIso phase at 183 °C; (c, d) LamN phase at
181 °C; (e, f) Colr/c2mm phase at 160 °C; (g) transition from the Colr/
c2mm phase (lower right corner) to the Colr/p2gg phase (upper left corner)
at 155 °C; (h) Colr/p2gg phase at 145 °C.
fall together. This different sample alignment could also be a
possible explanation for the different optical textures observed
between crossed polarizers.
Compound 2/12 shows four different mesophases (see Figures
8 and 9). The two high-temperature mesophases are lamellar
phases (LamIso and LamN phases) which are described in the
next section. Figure 8 shows changes in the textures of these
lamellar mesophases upon cooling (between crossed polarizers),
and the corresponding X-ray diffraction patterns of aligned
samples are shown in Figure 9.
The mesophase which is observed below the lamellar phases
appears to have a 2D lattice. The diffraction pattern of this
mesophase (Figure 9b) can also be assigned to a c2mm lattice,
where the lattice parameters are a ) 9.7 nm, b ) 4.0 nm. The
value of b is slightly larger than that observed for the c2mm
phase of 2/i11 (b ) 3.7 nm). At first glance the diffraction
pattern looks quite distinct from those obtained for the c2mm
lattices of 2/10 and 2/i11. This is due to the almost perfect
cylindrical averaging in the sample, which is again aligned
parallel to the (110) plane of the lattice as in the case of 2/i11.
A complete change of the diffraction pattern (see Figure 9c)
Figure 7. (a) Texture of the mesophase of compounds 2/i11 as obtained
on cooling from the isotropic liquid at 178 °C; (b) X-ray diffraction pattern
of an aligned sample of 2/i11 at 177 °C; (c) the same pattern showing the
two orientations of the 2D reciprocal lattice.
orientations of the reciprocal lattice that give rise to the
diffraction pattern. The different reflection intensities observed
for both orientations may be due to a “nonperfect” disorder in
the sense that a nonequally distributed orientation of the cylinder
axes exists around the normal of the (110) plane. In the sample
of 2/10 the (010) plane is parallel to the surface, and the rotation
axis is perpendicular to this plane. Since this axis lies within
one mirror plane of the lattice, the reflections of both orientations
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A R T I C L E S
section 2.2 for the p2gg phase of 2/4. When a comparison of
the area of the high-temperature c2mm lattice (S ) 38.8 nm2,
two columns per cell) and the lower-temperature p2gg lattice
(S ) 88.8 nm2, four columns per cell) of 2/12 is made, it is
apparent that the area of this p2gg lattice is more than twice
the area of the c2mm lattice. This is thought to arise due to an
increase in the number of molecules within the cross section of
each column. Calculations based on these areas give a value of
9.3 molecules per column in the p2gg phase. If it is additionally
assumed that a more efficient packing of molecules occurs at
lower temperature, then it is reasonable to assume that this
number is in fact 10 molecules per column. As each unit cell
contains four columns, then a number of ncell,calc ) 40 molecules
is expected to be located in each unit cell with a height of h )
0.45 nm. A value of ncell ) 42.7 was calculated from the
experimental data, and this is in good agreement with the ideal
value proposed. As suggested in section 2.2 the occurrence of
a p2gg lattice in this class of compounds is a strong indication
for a columnar mesophase formed by the organization of
cylinders with a pentagonal cross-sectional shape. Furthermore,
a pentagon can be easily built up by a number of 10 molecules
if each side of the pentagon is formed by two molecules.
Therefore, the most probable arrangement for this p2gg lattice
is a regular and commensurate packing of four cylinders, each
cylinder having a pentagonal cross-sectional shape made up of
10 molecules in the cross section in which each side of the
pentagon is formed by end-to-end dimers as shown in Figure
10a. Figure 10b shows the CPK model of 10 molecules of 2/12
arranged in a pentagon, and the lattice parameters measured
for the model are in good agreement with the experimental
values obtained. The formation of such an unusual structure is
thought to arise due to an increase in the stiffness of the
perfluorinated chains upon reducing the temperature.
These results show that further elongation of the lateral chains
leads to a further expansion of the cylinders which gives rise
to a transition from the Colh phases to new columnar phases
with a cylinder-shell morphology and very large unit cell
parameters. In these mesophases at least two sides of the cylinder
shells are elongated, and these sides are formed by two end-
to-end connected bolaamphiphilic units. These columnar phases
are thought to represent intermediary phases at the transition
from hexagonal cylinder structures to a novel type of lamellar
organization, and these lamellar phases will be discussed in the
next section.
2.6. Lamellar Mesophases of Group III Compounds. As
mentioned in the previous section compound 2/12 forms not
only two distinct columnar mesophases, but also two lamellar
high-temperature mesophases. Much broader regions of these
lamellar mesophases were found for compounds 3/1054 and 3/12
which have longer lateral chains. In this section attention will
be focused on the lamellar phases of these two compounds. Both
compounds contain three different mesophases with layered
structure.
The DSC heating and cooling scans of 3/12 are shown in
Figure 11, and the textures of the different mesophases are
collated in Figure 12. Upon cooling the first mesophase behaves
like a conventional SmA phase. This is characterized by a typical
fanlike texture (Figure 12b) which can be easily aligned
Figure 9. X-ray diffraction pattern of an aligned sample of 2/12 (a) LamIso
phase at 184 °C; (b) Colr/c2mm phase at 167 °C and (c) Colr/p2gg phase
at 151 °C.
and of the texture (see Figure 8, g and h) was observed on
further cooling at 155 °C. The diffuse outer scattering remains,
but more than 100 sharp spots appear in the small-angle region
of the diffraction pattern. These reflections can be assigned to
a noncentered rectangular lattice with the space group p2gg and
lattice parameters a ) 11.5 nm and b ) 7.7 nm. These lattice
parameters are much larger than those found for all other
mesophases of compounds 1/n-5/n. Such large lattice param-
eters would suggest that this p2gg lattice results from the
organization of four columns in the unit cell, as proposed in
(54) Preliminary communication: Prehm, M.; Cheng, X. H.; Diele, S.; Das, M.
K.; Tschierske, C. J. Am. Chem. Soc. 2002, 124, 12072-12073.
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Figure 10. (a) Possible molecular arrangement of the molecules in the
rectangular columnar mesophase Colr of compound 2/12 with a p2gg 2D
space group, the dotted lines indicate pairs of columns; (b) CPK models
showing an arrangement of 10 molecules 2/12 in a pentagon, in which each
side is formed by end-to-end connected dimers (the upper right cylinder
within the frame of the p2gg lattice in (a) is shown, the hydrogen-bonding
site A is located in the center of the unit cell, the distances A-B and A-C
correspond to 0.5 a and 0.5 b, respectively).
Figure 12. Optical textures (crossed polarizers) of compound 3/12 observed
on cooling a homeotropically aligned sample (left) and a sample with fan-
shaped texture (right): (a, b) LamIso phase at 180 °C; (c) LamN phase at
168 °C; (d) LamIso-LamN transition at 169 °C; (e, f) LamN phase at 163
°C; (g, h) LamX phase at 139 °C.
feature of this schlieren textures is the absence of any four-
brush-disclinations. This excludes a SmC-like organization in
which the molecules are tilted with respect to the layer planes,
as often found below the SmA phases in conventional LC
materials. Upon cooling, in the regions with a fan-shaped texture
a strong change in the birefringence can be observed at the phase
transition; however, the fans themselves are unaffected (see
Figure 12, d and f). These textural features are typical for
transitions from uniaxial SmA phases to biaxial SmAb phases55,56
in which the layer structure remains unchanged; however, there
is a change of the ordering within the layers.
Upon transition to the third mesophase the fan texture remains
unaffected (see Figure 12h), whereas in the schlieren texture a
distinct featherlike pattern appears, as well as an increase in
birefringence (Figure 12g). There is also an associated increase
in the viscosity of the sample at the transition to this low-
temperature mesophase.
Figure 11. DSC heating and cooling scans of compound 3/12 (10 K min-1).
homeotropically (in this alignment the layers are parallel to the
glass surfaces of the measuring cell) and which appears almost
completely black between crossed polarizers, but contains some
oily streaks and some other defects due to a nonperfect
alignment when viewed through crossed polarizers (Figure 12a).
Upon further cooling of this homeotropically aligned sample a
fluid schlieren texture appears (Figure 12c), indicating the
transition to a fluid, optically biaxial mesophase. A characteristic
However, the X-ray diffraction patterns of all three phases
are almost identical, and are characterized by a diffuse scattering
(55) Brand, H. R.; Cladis, P. E.; Pleiner, H. Macromolecules 1992, 25, 7223-
7226.
(56) (a) Pratibha, R.; Madhusudana, N. V.; Sadashiva, B. K. Science 2000, 288,
2184-2187. (b) Hegmann, T.; Kain, J.; Pelzl, G.; Diele, S.; Tschierske, C.
Angew. Chem., Int. Ed. 2001, 40, 887-890. (c) Sadashiva, B. K.; Reddy,
A.; Pratibha, R.; Madhusudana, N. V. Chem. Commun. 2001, 2140-2141.
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Figure 14. Temperature dependence of the νO-H stretching frequency in
the IR spectra of compound 3/10 in the region measured between 150 and
180 °C at 3 K intervals.
However, the layer thickness does not change at the transitions
between the different phases. Only the number of higher-order
reflections increases with decreasing temperature. Another
important feature of the X-ray diffraction pattern of aligned
samples is the diffuse wide-angle scattering which forms a
noncircular ring with maxima corresponding to D ) 0.53 nm
at the equator and to D ) 0.48 nm at the meridian. The
maximum at the equator is thought to be due to the perfluori-
nated chains, whereas the maxima at the meridian result from
the rest of the molecule. Therefore, it is assumed that the
perfluorinated chains are aligned in average parallel to the
meridian (layer normal), whereas the aromatic parts are aligned
perpendicular to it.
Compound 3/10 was investigated by IR spectroscopy between
150 and 180 °C to include all three mesophases. This investiga-
tion focused on the O-H stretching frequency region of the IR
spectra; however, no significant changes in the spectra were
observed (see Figure 14). The OH stretching region for all
phases was characteristic of extended intermolecular hydrogen-
bonding networks23,58 and would seem to indicate that structural
changes are due to the reorganization of the biphenyl units rather
than to significant changes in the hydrogen-bonding networks.
This is supported by the observation of low enthalpies (Figure
11) at the transitions between the different lamellar phases and
into the isotropic liquid.
Figure 13. X-ray diffraction pattern of an aligned sample of compound
3/10 (a) at 147 °C (LamX), (b) at 167 °C (LamN), (c) at 185 °C (LamIso),
and (d) scans of the pattern obtained at 185 °C along the meridian (1) and
along the equator (2, 3).
The optical observations in conjugation with the X-ray
patterns give rise to the proposed molecular arrangements,
shown in Figure 15, a-d. The layer structure arises due to
segregation of the nonpolar lateral chains from the bola-
amphiphilic units (biphenyl units and terminal diol groups) into
distinct sublayers, however, unlike conventional smectic phases,
the calamitic biphenyl units are organized parallel to the layer
planes. The aromatic layers are separated by the nonpolar layers
of the lateral chains. These chains are strongly disordered
in the wide-angle region, and in the small-angle region by
several sharp equidistant reflections on the meridian, indicating
the presence of well-defined fluid layer structures. The X-ray
diffraction patterns for the mesophases of 3/10 are shown in
Figure 13. The layer thickness is 3.94 nm for 3/10 and strongly
increases by elongation of the lateral chain (3/12: d ) 4.5 nm).57
(57) This strong increase of d with the chain length is unexpectedly large
compared to observations made with smectic phases of conventional LC
materials. A detailed discussion of this effect is however difficult, because
not only the degree of intercalation of the chains, but also the thickness of
the aromatic sublayers can change, depending on the chain length, and
both have a strong influence upon the layer distances.
(58) Siegel, G. G.; Huyskens, P. L. In Intermolecular Forces, An Introduction
to Modern Methods and Results; Huyskens, P. L., Luck, W. A. P., Zeegers,
T., Eds.; Springer: Berlin, 1999; pp 387-395.
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(liquidlike) but with a certain preferred direction of the
fluorinated segments perpendicular to the layer planes (the
maximum of the diffuse scattering at the equator is very close
to the value for the mean distance between perfluorinated chains
corresponding to D ) 0.54 nm). The experimentally determined
layer distance requires that the chains are intercalated and that
the thickness of the aromatic sublayers should correspond to,
on average, approximately 3 times the width of the biphenyl
unit. The diffraction pattern and the layer distances do not
change at the phase transitions, and therefore, the biphenyl units
remain parallel to the layer planes in all three lamellar phases.
It is suggested that the symbol Lam should be used for each of
these special types of lamellar mesophases, to distinguish them
from the conventional smectic phases of rodlike mesogens and
the smectic phases of the bolaamphiphiles 1/0-1/9, and 4/4, in
which the rodlike cores are aligned perpendicular to the layer
planes (SmA) or tilted (SmC). As indicated by the IR investiga-
tions, the phase transitions seems to be accompanied by a change
of the shape and distribution of the hydrogen-bonding networks
rather than by a change in the size of the networks as all phases
have quite large dynamic hydrogen-bonding networks. In the
isotropic liquid phase these networks are thought to be randomly
distributed in space, whereas in the Lam phases they are
concentrated within the sublayers of the bolaamphiphilic
moieties. Hence, there are clusters of molecules in all meso-
phases.
Three subtypes of Lam phases were observed, which show
different optical properties as a function of temperature. The
high-temperature phase is optically uniaxial as observed for
SmA and lyotropic LR phases, and upon cooling a transition to
an optically biaxial Lam phase is observed. These two meso-
phases and the transition between them will be discussed first.
There are two different organizations which could explain the
uniaxiality of the high-temperature phase. One possible structure
for the uniaxial phase is a lamellar organization in which there
is no other ordering within the plane.59 The bolaamphiphilic
moieties are organized in clusters which are irregularly distrib-
uted within the layers, as shown in Figure 15b. Because there
is an isotropic distribution of the calamitic units within the layer
planes this phase is assigned as LamIso (D∞h symmetry). At the
transition to the optically biaxial mesophase below the LamIso
phase, the biphenyl units adopt a long-range orientational
ordering within the aromatic sublayers, and the individual layers
become orientationally correlated to one another. Therefore,
biaxiality is observed throughout the bulk (see Figure 15c). This
phase can be regarded as a lamellar phase built up of quasi-2D
layers with nematic-like ordering within the plane, separated
by layers of the fluid lateral chains, and therefore this mesophase
is designated laminated nematic (LamN) and exhibits a D2h
symmetry. Another model for the optically uniaxial phase is
also possible, in which a nematic order within the aromatic
sublayers is already present, but in which an orientational
correlation between adjacent layers does not exist (noncorrelated
LamN phase). This arrangement would also be optically uniaxial,
and the transition to the optical biaxial mesophase in this case
would arise due to the onset of the orientational correlation
between the layers, which may be caused by a reduction in the
conformational (internal) mobility of the molecules at reduced
Figure 15. (a) CPK model of six molecules of 3/10 arranged into layers.
On average the aromatic sublayers have a thickness 3 times the width of
the aromatic unit. The marked d-spacing corresponds to the experimentally
determined value. (b-d) Models of the organization of the molecules in
the different lamellar mesophases. The rigid bolaamphiphilic units (gray)
are arranged parallel to the layer planes, and the space between the layers
is filled by the fluid lateral semiperfluoroalkyl chains. The LamSm structure
shown in (d) represents a possible structure of the LamX low-temperature
phase of compounds 3/n.
temperature. This would lead to a preferred direction within
the fluorinated sublayers, and as a consequence correlation
between the layers would arise. However, there is no direct
experimental evidence for either of these two possibilities, but
replacement of the biphenyl unit by an elongated p-terphenyl
unit60 leads to a complete loss of the optically uniaxial phase
and a strong stabilization of the biaxial mesophase, which is
predicted by using the LamIso model of the uniaxial mesophase.
Therefore, it seems likely that the optically uniaxial high-
temperature phase is a LamIso phase and that the formation of
a long-range orientational order within the individual layers is
coupled with the formation of long-range orientational correla-
tion between adjacent layers.
The structure of the low-temperature mesophases LamX for
compounds 3/10 and 3/12 is even more difficult to understand
and still remains somewhat unclear. A crystallization in one of
the sublayers can be excluded due to the presence of diffuse
scattering in the wide-angle region of the X-ray diffraction
pattern. If an analogy is drawn with the phase sequence Iso-
N-Sm, seen for calamitic molecules upon decreasing temper-
ature, it can be assumed that the same sequence occurs in the
2D sublayers of the bolaamphiphilic units. Hence, it is thought
that in the low-temperature phase an additional periodicity
occurs parallel to the aromatic sublayers arising from the
organization of the hydrogen-bonding networks into stripes.
These stripes are separated by ribbons of biphenyl units
organized in a parallel fashion. Such a phase should exhibit a
reflection at the equator of the diffraction pattern; however, this
has not been observed. A possible explanation for the absence
of this reflection may be that the number of scattering units
(59) A related mesophase was reported for anthracene derivatives: Norvez, S.;
Tournilhac, F. G.; Bassoul, P.; Hersonn, P. Chem Mater 2001, 13, 2552-
2561.
(60) Cheng, X. H.; Tschierske, C. Unpublished results.
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Calamitic Bolaamphiphiles with Lateral Chains
A R T I C L E S
Table 3. Transition Temperatures (T/°C) of the Bolaamphiphiles
6/n
contributing to the overall scattering volume is small and that
the electron density modulation parallel to the layer planes is
much lower than perpendicular to the layers. Both effects would
cause this reflection to have very low intensity. There are two
possible organizations of layers with such an additional in-plane
periodicity; the layers could be either correlated or noncorrelated,
positionally. A correlated layer structure was recently reported
for a molecule, related to 3/n but with a branched semiperflu-
orinated lateral chain, and as expected, a 2D lattice as typical
for a columnar phase (Colr/p2mm) was indicated by several
reflections out of the meridian of the diffraction pattern.61 This
shows that such a 2D lattice should be detectable by X-ray
diffraction. Therefore, the other possible structure, i.e. a
mesophase without positional correlation between adjacent
layers (but with orientational correlation), designated herein as
LamSm (see Figure 15d), seems more likely for this mesophase.62
Theoretically, a streaky vertical reflection should be observed
on the equator of the diffraction pattern of such a mesophase,
but according to the arguments given above such a reflection
might not be detected due to its low intensity. Therefore, it is
proposed that a LamSm structure is the most likely possibility
for the LamX phase, but further investigations are necessary to
completely understand this special type of mesophase.63
Apart from a temperature dependence, the chain length (i.e.
the space required by the lateral chains) also has a significant
influence on the subtype of Lam phases observed. The broadest
regions of the LamIso and LamN phases were found for 3/12,
whereas a stabilization in the LamX phase was observed on
reducing the chain length. This leads to a rather small region
of the LamN phase for compound 3/10.54 It seems that the space
required by the lateral chains in the nonpolar sublayers also
has an influence upon the organization of the rigid calamitic
units within the aromatic sublayers. It is thought that upon
increasing the chain length there is not only an increase of the
layer distance d but also an expansion of the area required by
the chains within the layer planes. This leads to an increased
separation of the rigid units, which disfavors the occurrence of
orientational and positional in-plane ordering. For this reason
compound 2/12, which has a shorter chain than compounds 3/n,
has only very short LamIso- and LamN-phase regions. However,
this compound does not exhibit a LamX phase, but instead,
different columnar mesophases; this has already been discussed
in section 2.5.
cmpd
n
T/°C
6/1
6/6
1
6
Cr 97 Colh 134 Iso
Cr 137 Iso
6/12
12
Cr 134 Iso
crystalline phase. This shows that enlargement of the nonpolar
regions is not possible by attaching two lateral chains to a
biphenyl unit.
3. Summary and Conclusions
In this investigation novel complex mesophase morphologies
were obtaineded by careful molecular design. This was achieved
by attaching a nonpolar chain laterally to a rigid rodlike
bolaamphiphilic core. Rodlike bolaamphiphiles are known to
form extremely stable smectic monolayer phases due to the
parallel organization of the rodlike moieties, the segregation of
the polar terminal groups from the aromatic units, and the
cohesive forces provided by the hydrogen-bonding networks
between terminal diol groups.32d,e,33 Introduction and successive
enlargement of lateral alkyl-, semiperfluoroalkyl-, or perfluo-
roalkyl chains disturb the parallel organization of the biphenyl
cores within these layer structures. However, the cohesive forces
(H-bonding) inhibit the complete collapse of positional ordering
to yield nematic or isotropic liquid phases as usually observed.22
Instead, a variety of new columnar phases with core-shell
morphologies and novel types of smectic phases, in which
rodlike aromatic units are organized parallel to the layer planes
(Lam), were observed. As shown in Figure 16, all the columnar
phases occur as intermediary phases at the transition between
conventional SmA phases and the novel laminated phases (Lam).
2.7. Compounds with Two Lateral Chains. Three different
types of bolaamphiphiles with two lateral chains have also been
synthesized. Compounds 6/n (see Table 3) have a semiperflu-
orinated chain and an alkyl chain, whereas compounds 7/n have
two semiperfluorinated chains. Compound 6/1 is similar to 2/6,
but has an additional methyl group attached to the aromatic core.
As with compound 2/6 it forms a hexagonal columnar phase,
but the mesophase stability is significantly reduced. All bo-
laamphiphiles with two long lateral chains rapidly crystallize
at high temperatures without the observation of any liquid
As shown in Table 4, there is a remarkably strong correlation
between the mesophase type and the volume fraction of the
lateral chain. The elongation of the lateral chain causes
distinctive changes in the molecular shape, amphiphilic pattern,
and segregation regime. Molecules with very long chains (group
III) organize in Lam phases and behave similarly to single-
headed amphiphiles. In these molecules the whole bola-
amphiphilic unit (the biphenyl unit together with the two polar
groups) can be regarded as polar “headgroup”. In the meso-
(61) Prehm, M.; Diele, S.; Das, M. K.; Tschierske, C. J. Am. Chem. Soc. 2003,
125, 614-615.
(62) A related phase, designated as sliding columnar phase was recently proposed
as a possibility of the organization of DNA strands in LC phases of DNA-
lipid complexes: Lubensky, T. C.; O’Hern, C. S. In Slow Dynamics in
Complex Systems; Tokuyama, M., Oppenheim, I., Eds.; The American
Institute of Physics: Woodbury, NY, 1999; pp 105-116.
(63) A related phase sequence was also found for bolaamphiphilic triols: Cheng,
X. H.; Das, M. K.; Diele, S.; Tschierske, C. Angew. Chem., Int. Ed. 2002,
41, 4031-4035.
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A R T I C L E S
Cheng et al.
Figure 16. Liquid crystalline phases formed by the rigid bolaamphiphiles with respect to the size of the nonpolar lateral chain.
Table 4. Correlation between Volume Fraction of the Nonpolar Lateral Chain (fR) and Observed Mesophase Type for the Bolaamphiphiles
1/n-5/n
phases of these materials an equivalent space filling of the polar
units and the intercalated nonpolar chains allows these molecules
to adopt an organization in layer structures. At enhanced
temperature the rodlike polar groups are rotationally disordered,
i.e. their specific shape is not recognized (LamIso), but by
reducing the temperature, the rigid rodlike structure of these
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Calamitic Bolaamphiphiles with Lateral Chains
A R T I C L E S
“headgroups” becomes important, and this leads to a stepwise
onset of orientational (LamN) and possibly also positional order
within these sublayers.
and rod-coil molecules,67 where ribbonlike aggregates of
calamitic units form the interiors of the columns which are
surrounded by the fluid continuum of the attached alkyl chains.
The columnar phases reported herein represent channel
structures, built up by the bolaamphiphilic units and filled with
the semiperfluorinated chains. Channel structures have also been
reported for crystal structures of shape-persistent organic
macrocycles,68-71 the solid-state structures of some H-bonding
organic molecules72 and coordination polymers.73,74 Mesoporous
channel structures were also obtained by surfactant based sol-
gel synthesis.75 However, in contrast to these solid-state
structures, the mesophases reported here represent fluid, self-
organized systems (ordered fluids) as indicated by the rheologic
properties (fluidity) and the X-ray diffraction patterns (diffuse
wide-angle scattering).76 This means that there is a high mobility
in these systems, provided by rapid molecular motions and
rotations of the whole molecules, as well as conformational
mobility. Therefore, the interfaces between the segregated
regions are diffuse. This means that the different regions shown
in the models represent regions with an enhanced concentration
of one of the incompatible components with respect to that of
the other. Hence, these structures are highly ordered on a
macroscopic scale but are rather disordered on a microscopic
scale.
To the best of our knowledge, there are only two other LC
systems with morphologies related to those reported herein.
These are semirigid LC-main-chain polymers containing long
lateral alkyl chains. Some of these polymers form columnar
mesophases (Colh) with special cylinder morphologies, where
the semirigid polymer backbones form the cylinder walls, filled
by the lateral alkyl chains.77 Other polymers of this type form
smectic mesophases, where the semirigid polymer backbones
are segregated and organized parallel to the layer planes as in
the Lam phases.78-80 Another class of mesophases is formed
by the organization of rigid aromatic molecules with peripheral
Compounds with intermediate chain lengths form numerous
different columnar phases that arise from a mismatch of the
space required by the nonpolar chains (RH, RF) and the polar
molecular parts (see Table 4). The fluid nonpolar parts are
organized in the interior of the columns, whereas the rigid
bolaamphiphilic units form shells around these columns. In this
respect these phases can be regarded as type I (normal type)
columnar phases,64 as for example often found in lyotropic
systems.4 However, in contrast to conventional amphiphiles, the
“headgroups” have a distinct rigid and bolaamphiphilic sub-
structure, which is recognized during self-organization in
columns. The segregation of the biphenyl units from the attached
diol groups, as well as the rigid rodlike shape of the polar group
only allows organization into certain distinct 2D structures.
Furthermore, within the cylinder shells, only commensurate
structures are possible, which means, that the cylinder shells
must contain a well-defined number (4, 5, 6, 8, or 10) of the
bolaamphiphilic units organized around the nonpolar cylinder
cores. The resulting columns must be able to arrange in 2D
space regularly, and within the resulting 2D lattice the intercon-
nection between the cylinder walls occurs exclusively at the
H-bonding sites. Two of these columnar mesophases are
especially remarkable; these are the p2gg lattices (compounds
2/4 and 2/12), in which the columns adopt a pentagonal cross-
sectional shape, and the c2mm lattice of compounds 2/10-
2/12, in which eight molecules are organized into a deformed
hexagon. These unusual mesophases have never been observed
for any other liquid crystalline system. It is also remarkable
that in this class of compounds no cubic or any other 3D-ordered
mesophase has been detected at the transition between columnar
and lamellar organization, as is typical for most other LC
systems. In these systems it appears that there is a direct
transition from cylinders to layers. Cylinders with eight
molecules in the cross section seem to represent the largest of
the stable cylinder structures; however, under special conditions
(e.g. at reduced temperature) a larger cylinder can also be
observed (e.g., the p2gg phase of 2/12).
(67) Lee, M.; Cho, B.-K.; Zin, W.-C. Chem ReV. 2001, 101, 3869-3892.
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araman, D.; Lee, S.; Zhang, J.; Moore, J. S. Nature 1994, 371, 591-593.
(69) Ho¨ger, S.; Morrison, D. L.; Enkelmann, V. J. Am. Chem. Soc. 2002, 124,
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R. Org. Lett. 2002, 4, 339-342.
(71) Gleiter, R.; Werz, D. B.; Rausch, B. J. Chem. Eur. J. 2003, 9, 2676-
Another interesting feature to note about these systems is that
the rigid units are located at the interfaces between the individual
columns (i.e., at the Wigner-Seitz lattice), whereas the fluid
chains are located at the interior of the resulting networks of
cylinders.65 In columnar LC phases formed by conventional
disklike LC molecules the Wigner-Seitz lattice is located within
the fluid continuum between the columns, and the rigid parts
are located in the centers. In this respect the mesophases reported
here are the reverse of the mesophases of polycatenar mesogens
(rodlike molecules with multiple or branched terminal chains)66
2683.
(72) Holman, K. T.; Pivovar, A. M.; Ward, M. D. Science 2001, 294, 1907-
1911. (b) Holman, K. T.; Martin, S. M.; Parker, D. P.; Ward, M. D. J. Am.
Chem. Soc. 2001, 123, 4421-4431.
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T. AdV. Mater. 2001, 13, 637-641. (b) Xu, Z.; Kiang, Y.-H.; Lee, S.
Lobkovsky, E. B.; Emmott, N. J. Am. Chem. Soc. 2000, 122, 8376-8391.
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Moulton, B.; Kravtsov, V.; Zaworotko, M. J. J. Am. Chem. Soc. 2002,
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(b) Raimondi, M. E.; Seddon, J. M. Liq. Cryst. 1999, 26, 305-339. (c)
Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147-
1160.
(76) Columnar phases formed by taper-shaped or dendritic LC molecules with
polyether chains or crown ether units at the apex can also be regarded as
channel structures incorporating three distinct subspaces (central polar core,
aromatic inner shell, and aliphatic or perfluoroalkyl outer shell). However,
with these compounds segregation leads to onion-like structures. In such
structures the rotation of the tapered segments around the column long
axis is not restricted, and therefore Colh phases were observed.15c,16b
(77) (a) Watanabe, J.; Sekine, N.; Nematsu, T.; Sone, M.; Kricheldorf, H. R.
Macromolecules 1996, 29, 4816-4818. (b) Fu, K.; Sekine, N.; Sone, M.;
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(64) Borisch, K.; Tschierske, C.; Go¨ring, P.; Diele, S. Langmuir 2000, 16, 6701-
6708.
(65) A liquid crystalline macrocycle, which represents a disklike molecule built
up by a rigid frame and an interior of fluid alkyl chains, was recently
reported, but only a nematic phase is formed by this compound: Ho¨ger,
S.; Enkelmann, V.; Bonrad, K.; Tschierske, C. Angew. Chem., Int. Ed. 2000,
39, 2267-2270.
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A R T I C L E S
Cheng et al.
polar groups (some dyes and drugs) in water.81 In these
chromonic lyomesophases, the space required by the water
molecules coordinated around these rigid cores (instead of the
nonpolar chains in compounds 1/n-5/n) determines the meso-
phase type. Hollow-pipe structures (related to the columnar
phases of groups I and II materials) and optically biaxial
brickwork-layer structures (related to the Lam phases of group
III materials) have been proposed for the organization of such
molecules.81 However, in these LC materials only two distinct
sets of subspaces exist, whereas most of the mesophases reported
herein have three distinct sets of subspaces, i.e. columns of the
lateral chains, ribbons of the aromatic units, and strings of
H-bonding networks. In this respect these morphologies are
unique in LC systems but are related to those observed in some
ABC triblock copolymers.11,12
the unfavorable effect of the entropy of mixing, and this can be
achieved if the individual blocks have a high degree of
incompatibility with respect to one another. The competitive
combination of cohesive hydrogen-bonding, the fluorophobic
effect, and the rigid-flexible incompatibility have turned out
to be a successful approach in this direction. Remarkably, the
wide variety of different phases was obtained using one, single,
basic molecular structure, where the only structural change
involved the elongation of the laterally attached chain. This
shows that careful design of quite simple block molecules by
tiny changes of the molecular structure can lead to unexpectedly
complex superstructures.
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft, the Fonds der Chemischen Industrie
and the European Commission (RTN LCDD); we are grateful
to Dr. A. G. Cook for a critical review of the manuscript.
This report shows that it is possible to design complex
mesophase morphologies, related to those of multiblock co-
polymers also with low-molecular mass LC materials. A major
challenge in such low-molecular weight systems is to overcome
Supporting Information Available: Tables with crystal-
lographic data, experimental procedures, and analytical data
(NMR, MS, elemental analysis) (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
(80) These materials have potential use in organic light-emitting materials and
organic semiconductors: Mitschke, U.; Ba¨uerle, P. J. Mater. Chem. 2000,
10, 1471-1507.
(81) Harrison, W. J.; Mateer, D. L.; Tiddy, G. J. T. J. Phys. Chem. 1996, 100,
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