Axial-bundle phases - New modes of 2D, 3D, and helical columnar self-assembly in liquid crystalline phases of bolaamphiphiles with swallow tail lateral chains

Article abstract of DOI:10.1021/ja110065r

Two series of polyphilic molecules composed of a rigid and linear p-terphenyl core, terminated at both ends with polar glycerol groups capable of hydrogen bonding, and two branched swallow tail-type lateral chains, composed of a fluorinated and a nonfluor

Full text of DOI:10.1021/ja110065r

ARTICLE
pubs.acs.org/JACS
Axial-Bundle Phases - New Modes of 2D, 3D, and Helical Columnar
Self-Assembly in Liquid Crystalline Phases of Bolaamphiphiles with
Swallow Tail Lateral Chains
Marko Prehm,† Feng Liu,‡ Xiangbing Zeng,‡ Goran Ungar,*,‡,§ and Carsten Tschierske*,†
†Organic Chemistry, Institute of Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt-Mothes-Strasse 2, D-06120 Halle,
Germany
‡Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield S13JD, United Kingdom
§WCU Program Chemical Convergence for Energy & Environment, School of Chemical and Biological Engineering,
Seoul National University, Seoul, Korea
S Supporting Information
b
ABSTRACT: Two series of polyphilic molecules composed of a
rigid and linear p-terphenyl core, terminated at both ends with
polar glycerol groups capable of hydrogen bonding, and two
branched swallow tail-type lateral chains, composed of a fluo-
rinated and a nonfluorinated branch or two fluorinated
branches, were synthesized and investigated by differential
scanning calorimetry, polarizing microscopy, and X-ray diffrac-
tion (XRD) with respect to their self-assembly in thermotropic liquid crystalline (LC) phases. Hexagonal columnar phases were
formed by all molecules, at least at the highest temperature. In these phases the columns are composed of a core of aromatic rods and
an aliphatic shell. The aromatic rods form bundles which are rotationally averaged and lie parallel to the column long axis. This
unique organization is proven by different optical and XRD methods. The aromatic and glycerol groups inside the rod bundles are
segregated into alternating segments. Depending on temperature and molecular structure, long-range intercolumnar correlation of
this periodicity could take place, leading to a 3D-ordered LC phase with rhombohedral R3m symmetry. The bundles are embedded
in the matrix of the lateral chains, which is divided into fluoroalkyl- and aliphatic-rich regions. In the 2D columnar phase the
fluorinated regions take the form of either straight columns running along the edges of the hexagonal Voronoi cells or, for
compounds with a higher degree of fluorination, fuse to a hexagonal honeycomb enclosing the aromatic cores. In the R3m phase the
fluorine-rich chains are preferentially found along right- and left-handed helices wound around the 31 screw axes between the main
aromatic columns.
’ INTRODUCTION
strongly coiled chains (e.g., oliogo(ethylene oxides), oligoo-
(propylene oxides),18 oligosiloxanes)19 and fluorinated chains,20
were attached in terminal position to calamitic or bent aromatic
cores21 to form columnar mesophases.4,22-27
The combination of mobility and order in soft self-assembled
superstructures1 is an indispensable feature of numerous self-
assembled structures that form the basis of both life and
numerous technical devices.2 Liquid crystalline (LC) phases
are prototypical self-assembled systems combining long-range
order and mobility.3-5 The discovery that disk-like molecules
can organize into LC phases in which the molecules stack up in
columns (Figure 1)6 initiated intense activity in the field of
columnar LC phases.7 Especially the overlapping π-systems of
aromatic disk-like molecules within the columns lead to inter-
esting materials for molecular electronics and photovoltaics.8,9
About 10 years after the discovery of discotic LC, it was shown by
Malthete, Destrade, Levelut, and Tinh that rod-like (calamitic)
molecules can also form columnar LC phases if two or three alkyl
chains were attached at the ends of rod-like molecules.10,11 Since
then the concept of polycatenar mesogens12 was extended
to metallomesogens,13 polymers,14 hydrogen-bonded,15 and
ionic16 LC systems. Besides linear alkyl chains, also branched
chains (swallow tail chains)17 and other bulky groups, such as
Columnar LC phases formed by the rod-like molecules
reported so far can be viewed as ribbon phases, i.e., layers sliced
up into infinite ribbons in which the rod-like cores are perpendi-
cular or tilted at a moderate angle with respect to the column long
axis (Figure 1c).10,11,28 Another, relatively new approach to
columnar phases combines rod-like segments with terminal
and lateral chains which are incompatible with each other. This
concept of polyphilic T-shaped tectons provided a series of novel
columnar LC phases described as honeycombs, where rod-like
units form polygonal cylinder frames, and the flexible lateral
chains fill the interior of the cylinder cells (Figure 1e).29-31 With
respect to the positions of rigid and flexible units, these meso-
phases are the inverse of those formed by disk-like molecules and
Received: November 9, 2010
Published: March 10, 2011
r
2011 American Chemical Society
4906
dx.doi.org/10.1021/ja110065r J. Am. Chem. Soc. 2011, 133, 4906–4916
|

Journal of the American Chemical Society
ARTICLE
assume the shape of either straight columns or helices. For
compounds with a higher degree of fluorination, the fluorous
columns fuse to a hexagonal honeycomb enclosing the aromatic
columns. These new phase structures were obtained with speci-
fically designed molecules, having branched (swallow tail) semi-
perfluorinated chains attached laterally to glycerol-terminated
rigid rod-like p-terphenyl units. The role of the glycerol groups,
attached at both ends, is to provide end-to-end linking of the rod-
like cores by hydrogen bonding. Two swallow tail substituents
were attached, with only one of the branches being fluorinated in
compounds 1 and both branches fluorinated in compounds 2.
’ RESULTS AND DISCUSSION
1. Synthesis. The synthesis (Scheme 1) starts with benzene
1,4-diboronic acid which is coupled in a Suzuki type reaction34
with two equivalents of the acetonide protected glycerol func-
tionalized arylbromide A.35 In the obtained terphenyl derivative
B, the benzyl groups were removed by catalytic hydrogenation,
and the resulting p-terphenyldiol C was used for etherification
reactions with semiperfluorinated and branched alkyl bromides
Figure 1. Major groups of columnar LC phases formed by different
types of molecular building blocks.
Scheme 1. Synthesis of compounds 1 and 2a
previously reported rod-like molecules where the rigid segments
occupy the center of the cells.
Here we report two modes of organization in columnar LCs
where rod-like units form the interior rather than the periphery of
the cells, but where, in contrast to previously reported columnar
phases of polycatenars, the rod-like molecules lie parallel to the
column axis in bundles. In one of them the columns are longi-
tudinally uncorrelated, giving a true 2D hexagonal columnar
phase, which was reported for only two examples in previous
communications.30f,32 The other one is a correlated type, giving a
new rhombohedral R3m phase with 3D order.
The molecular alignment of the π-conjugated rods parallel to
the columns is also in contrast to the columnar structures formed
by discotic LC widely used for charge carrier materials.8 In this
way the structures reported herein provide the benefits of LC
self-assembly (no grain boundaries, self-healing, directed
alignment) also to rod-like materials with their intramolecular
π-conjugation path parallel to the columns. Hence, the results could
contribute to understanding and improving the self-assembly and
alignment of π-conjugated oligomers and polymers representing
another important class of charge carrier materials.33
a Reagents and conditions: (i) Pd[PPh3]4, NaHCO3, glyme, H2O,
reflux; (ii) H2, Pd/C, EtOAc, 45 °C; (iii) K2CO3, DMF, 50-60 °C;
(iv) HCl, MeOH, reflux.
Moreover, the continuum between the columns is divided into
fluorous and aliphatic regions, whereby the fluorinated domains
4907
dx.doi.org/10.1021/ja110065r |J. Am. Chem. Soc. 2011, 133, 4906–4916

Journal of the American Chemical Society
Table 1. Mesophases, Phase Transitions, and Other Parameters of Compounds 1, Depending on the Size of the Lateral Chainsa
ARTICLE
T / °C
Lattice parameter/nm
comp.
n
RF
ΔH / kJ mol-1
ahex
ahex, cb
fR
fRF
ncell
3
1a
7
C4F9
Cr 103
7.8
R3m
150 Colhex
159 Iso
11.2
2.95
3.11
3.32
5.19, 2.27
0.64
0.22
10.4
-
1bc
1c
9
C6F13
C8F17
Cr 81
21.3
R3m
R3m
130 Colhex
154 Iso
7.8
5.45, 2.25
5.87, 2.24
0.69
0.73
0.28
0.32
9.8
9.7
-
11
Cr 76
125 Colhex
156 Iso
31.3
-
8.0
a Transition temperatures and enthalpy values (lower lines) were taken from first DSC heating curves (peak temperatures, 10 K min-1); for DSC traces
of compound 1a and 1c see Figures S1, Supporting Information and 8a, respectively; R3m/Colhex transitions were determined by temperature-
dependent XRD; abbreviations: Cr = crystalline solid; Colhex = hexagonal columnar LC phase; R3m; correlated Colhex phase with rhombohedral 3D-
lattice; Iso = isotropic liquid; fR = total volume fraction of the nonpolar lateral chains, including the fluorinated and aliphatic segments (calculated using
crystal volume increments);37 fRF = volume fraction of the fluorinated part of the lateral chains only; ncell number of molecules in a hypothetical “unit cell”
of the Colhex phase, calculated according to ncell = Vcell/Vmol, where the volume of the “cell” (Vcell) is the product of the cell area and the molecular length
a2 ꢀ sin(60°) ꢀ 2.2 nm, and the molecular volume (Vmol) is obtained from crystal volume increments, considering the reduced packing density in the
LC state (for more details of calculations, see Table S10, Supporting Information). b Refers to the R3m phase. c Preliminary report, see ref 32.
Table 2. Mesophases, Phase Transitions, and Other
Parameters of Compounds 2 Depending on the Length
of the Fluorinated Segmentsa
Perkin-Elmer), polarizing microscopy (Optiphot 2, Nikon po-
larizing microscope in conjunction with a FP 82 HT heating
stage, Mettler) and different X-ray diffraction (XRD) techniques,
using powder samples and surface aligned samples. Investiga-
tions of surface aligned samples were performed using a 2D
detector (HI-Star, Siemens). Uniform orientation was achieved
by slowly cooling a drop of the compound on a glass surface. The
X-ray beam was applied parallel to the substrate (exposure time:
between 30 min and 3 h). For selected compounds, a synchro-
tron X-ray source was used for the investigation of the powder
samples (beamline I22 at Diamond Light Source) and the surface
aligned samples on silicon substrates by grazing incidence
(GISAXS) (beamlines BM28-XMaS at ESRF, and I07 at
diamond). Based on the powder data, electron density recon-
struction was carried out. Details of the reconstruction process
are described in Supporting Information. Transition tempera-
tures, transition enthalpies and observed mesophases of com-
pounds 1 and 2 are collated in Tables 1 and 2, respectively. All
compounds show enantiotropic (thermodynamically stable) LC
phases over broad temperature ranges.
2.1. Hexagonal Columnar Phases of Compounds 1 and 2.
The mesophases formed by compounds 1 and 2 are character-
ized by mosaic-like textures (Figure 2a) or by spherulitic textures
as typical for columnar phases (Figure 2b). Homeotropically
aligned samples (columns perpendicular to the surfaces) appear
completely dark between crossed polarizers, indicating optical
uniaxiality of the mesophases. The XRD patterns have a diffuse
wide angle halo confirming the absence of crystalline order on
the atomic scale (see Figures 3a and S6a, Supporting In-
formation). The maximum of this diffuse scattering shifts with
increasing degree of fluorination from d ≈ 2π/q = 0.50 nm
(compound 1a) to 0.55 nm (compound 2c). The d-values of the
T / °C
comp. RF
2a
ΔH / kJ mol-1
a / nm fR fRF ncell
3
C4F9 Cr 83 M 97 Colhex 161 Iso 3.68 0.78 0.27 9.9
12.3 4.8 9.9
2b C6F13 Cr 83 Colhex 175 Iso
22.9 11.7
2c C10F21 Cr 122 Colhex 191 Iso
108.9 10.8
3.89 0.80 0.35 9.8
4.49 0.84 0.47 10.7
a Transition temperatures and enthalpy values (lower lines) were taken
from first DSC heating curves (peak temperatures, 10 K min-1); for
DSC traces of compound 2a as representative example, see Figure S3,
Supporting Information; abbreviations: M = unknown mesophase, for
other abbreviations, see explanation in Table 1.
(D, E) in a Williamson ether synthesis to introduce the lateral
chains. After purification of the acetonides the glycerol groups
were deprotected, yielding compounds 1a-c and 2a-c.36
2. Mesomorphic Self-Assembly. The compounds were in-
vestigated by differential scanning calorimetry (DSC, DSC-7,
4908
dx.doi.org/10.1021/ja110065r |J. Am. Chem. Soc. 2011, 133, 4906–4916

Journal of the American Chemical Society
ARTICLE
Figure 2. Typical textures of the Colhex phases of compounds 1 and 2 as
observed between crossed polarizers: (a) mosaic-like texture with
pseudoisotropic areas of compound 1a at T = 150 °C and (b) spherulitic
texture of compound 1c at T = 150 °C; directions of polarizer and
analyzer are indicated by arrows.
√
small angle reflections have the ratio 1: 1/ 3: 1/2, characteristic
of hexagonal symmetry, which is confirmed by diffraction
patterns of surface aligned samples. Diffraction patterns of
compound 2a, as representative examples, are shown in
Figure 3a and b (for more details see also Figures S2, S4, and
S6 and Tables S1-S9, Supporting Information).
Enlarging the lateral chains (i.e., increasing fR, see Tables 1 and
2) continuously increases the hexagonal lattice parameter from
ahex = 2.95 nm for compound 1a (fRF = 0.22) to ahex = 4.49 nm
for compound 2c (fRF = 0.47).38 This continuous swelling is
important for the interpretation of the structure of these Colhex
phases, as it excludes the possibility of a hexagonal honeycomb
structure. In polygonal honeycombs, which are the most com-
mon structures in rod-like bolaamphiphiles with lateral chains
(see Figure 1e),30a,b the lattice parameter is fixed within narrow
limits by the length and the number of rod-like cores making up
the circumference of a given polygon; significant expansion
of the lattice would thus require a change of symmetry.39,40
This is clearly not the case for the Colhex phases of compounds
1 and 2.
The electron density map reconstructed from the small-angle
powder diffraction pattern of the Colhex phase of compound 2a is
shown as an example in Figure 3c. Each column is characterized
by a circular high electron density center (purple/blue), sur-
rounded by a shell of lowest density (red). As the lowest electron
density is provided by the aliphatic segments of the lateral chains,
it is reasonable to assume that it is these segments that constitute
the low-density ring surrounding the aromatic/glycerol central
maximum. Furthermore, six electron density maxima are located
at the vertices of the hexagonal Voronoi cell, and these are
attributed to regions rich in fluorinated (C4F9) end groups.
These small fluorinated segments tend to partially mix with
aliphatic segments, hence the large areas of continuum at column
boundaries with medium electron density (green/light blue).
Figure 3. Colhex phase of compound 2a: (a) XRD pattern of a surface
aligned sample at T = 143 °C (X-ray beam parallel to the surface); (b)
small-angle X-ray scattering (SAXS) powder pattern at T = 140 °C; (c)
surface-and-contour plot of electron density distribution in the plane
normal to the column axis, reconstructed from diffraction pattern in (b).
Only at the vertices, which are difficult to reach by the aliphatic
segments, a considerably higher concentration of electron-rich
fluorinated chains is found.
In contrast to compounds 1c and 2a, in compound 2c, which
has the largest RF segments, the electron-rich fluorinated columns
are fused to form an uninterrupted hexagonal honeycomb around
the core-shell columns (Figures 4b and 5d). The difference
between the discrete fluorine-rich columns in compounds 1c and
2a, onthe onehand, andthe continuous fluorinated honeycombin
2c is illustrated schematically in Figure 11a and b (Section 2.2).
4909
dx.doi.org/10.1021/ja110065r |J. Am. Chem. Soc. 2011, 133, 4906–4916

Journal of the American Chemical Society
ARTICLE
Figure 5. Comparison of electron density maps of the Colhex phases of
compounds (a) 1b32 and (b) 1c, having only one fluorinated chain in
each swallow tail (T = 140 °C), and (c) 2a (T = 140 °C) and (d) 2c (T =
160 °C), both latter compounds having two fluorinated branches in each
swallow tail. The unit cell sizes are normalized to the same length scale.
Note however that the electron densities are scaled individually, from
red (lowest) to purple (highest) each. This is why, e.g., the column
centers have different colors, although their electron densities are
expected to be the same in all compounds; fR = volume fraction of the
lateral chains.
Figure 4. (a) Small angle XRD pattern of Colhex phase of compound 2c
recorded at T = 160 °C (the inset shows the wide-angle region); (b)
surface-and-contour plot of the electron density reconstructed from (a).
Note that the electron densities are not directly comparable with those in
Figure 3 as each map is scaled individually between red (lowest) and
purple (highest).
The maps of the Colhex phase of compounds 1b and 1c, having
only one fluorinated branch in each swallow tail, are shown in
Figure 5a and b. In compound 1c the high electron density
regions of C8F17 chains form separate columns at the vertices of
the Voronoi cells. However, the relatively short and sparse C6F13
groups in compound 1b seem to mix with the alkyl groups
resulting in a continuum at column periphery that has higher
averageelectron density than the columncore(Figure5a). Indeed,
calculation gives average electron densities of the terphenyl þ
glycerol region as 560 el/nm3, as compared to 610 el/nm3 for the
mixed region of lateral chains. Figure 5c and d show 2D maps for
compounds 2a and 2c, for comparison.
As shown in Figure 6, the aromatic cores of compounds 1 and
2 may lie either parallel (Figure 6b) or perpendicular (Figure 6a)
to the column axis. The orientation of the aromatic core with
respect to the columns was investigated by two methods.
Compound 2a was doped with Disperse-red-1, (0.5 weight-%).
Disperse-red-1 (Figure 7) is an anisometric (rod-like) and
dichroic azo dye with the transition moment for the absorption
at λ = 504 nm parallel to the molecular long axis.41 Due to the
polar groups at both ends of the azobenzene moiety, this dye has a
bolaamphiphilic and rod-like structure, and hence, it should mix
with and be parallel to the terphenyls. Between crossed polarizers
the mixture shows a texture (see Figure 7a,) with homeotropically
aligned regions (dark areas, columns perpendicular to the sub-
strate surfaces) and regions with spherulitic texture where the
columns form concentric rings which are predominantly parallel
to the substrate surfaces. In Figure 7b the same region is shown
but with parallel polarizers. Red color, indicating an alignment of
the dye molecules with their long axis parallel to the substrate (i.e.,
with the conjugated π-system perpendicular to the direction of
the light beam allowing maximal light absorption) is observed in
the regions with spherulitic texture. The observation of less
absorbing (yellow) areas in the homeotropically aligned regions
indicates that chromophore and column axis coincide and that
both are perpendicular to the surface. This excludes honeycomb-
like or any other organization of the terphenyls perpendicular to
the column axis (e.g., Figure 6a) and indicates that the aromatic
cores of the host molecules 2a are parallel to the columns, as
shown in Figures 6b and 7c.
That the orientation of the aromatics is coaxial with the
columns for all compounds 1 and 2 was additionally confirmed
by investigation of the textures with a λ-plate retarder between
crossed polarizers. In the micrographs taken with a λ-plate (e.g.,
Figures 8d and S7, Supporting Information), the yellow and blue
colors define the orientation of the high-index axis as tangential
rather than radial within fans or “spherulites”. Since the columns
are known to be tangential, and the high-index axis parallel to the
4910
dx.doi.org/10.1021/ja110065r |J. Am. Chem. Soc. 2011, 133, 4906–4916

Journal of the American Chemical Society
ARTICLE
Figure 6. Alternative models of the arrangement of compounds 1 and 2
in the Colhex phase: (a) terphenyl segments are perpendicular to the
column and (b) terphenyls are parallel to the column axis; in this case the
terphenyl cores of approximately 10 molecules should form the cross
section of the column (for calculations see Table S10, Supporting
Information), and a periodicity corresponding to the molecular length
could arise if the glycerol groups register into distinct hydrogen-bonding
networks; a circular column cross section is thought unlikely, as the
lateral chains on the occluded molecules would not easily reach
the surface of the bundle, hence flat bundles (ribbons) are envisaged,
that are orientationally averaged; however, for simplicity, in the models
the columns are shown as space- and time-averaged circular cylinders.32
terphenyl long axis, it follows that the terphenyls are coaxial with
the columns.
The coaxial organization of the molecules could favor segrega-
tion of the aromatic units from the polar glycerol groups, leading
to segmentation of the columns (see Figure 6b). To check for
this possibility, IR spectra of compound 2a were recorded
between 50 and 170 °C. The spectra (Figure S5, Supporting
Information) show a broad absorption between 3200 and
3600 cm-1 at all temperatures, confirming the presence of re-
latively large hydrogen-bonding clusters42 in the LC state as well
as in the crystalline and the isotropic liquid states. The presence
of hydrogen-bonding networks in a structure with the terphenyls
aligned parallel to the columns suggests that, within the columns,
bundles of aromatic cores are separated by domains of glycerol
units. This should lead to periodicity along the column axis
corresponding to the length of bolaamphiphilic core (Figure 6b).
Indeed, a periodicity of 2.23 nm, corresponding to the molecular
length (L = 2.0-2.4 nm depending on the conformation of the
glycerol groups) was found in the low-temperature phase (R3m)
occurring below the Colhex phase of all compounds 1 (see next
section). However, a 2.23 nm reflection was not observed in
X-ray patterns of the high-temperature Colhex phase, meaning
that the segment positions are not correlated between the
columns.
Figure 7. Optical microscopy of a mixture of compound 2a and
Disperse-red 1 (0.5 wt %, T = 140 °C): (a) spherulitic texture as seen
between crossed polarizers; (b) same region as observed with parallel
polarizers indicating the alignment of the dye with respect to the light
beam as shown in (c).
(with other conformations) due to the rotation around the two
aryl-aryl bonds of the terphenyl core. Nevertheless, it is likely
that in the rod-bundles formed by on average 10 molecules in the
cross section, the Π-shaped conformation is dominant as this
allows a more efficient segregation of the flexible lateral chains
from the rigid aromatics and a dense packing of the terphenyl
moieties.
In spite of the lack of 3D long-range order, we can talk about
the volume occupied by a column segment (the “unit cell”) Vcell
,
2.2. Correlated Rod-Bundles: Rhombohedral R3m Phase of
Compounds 1.
calculated from ahex and L = 2.2 nm. Dividing Vcell by the
molecular volume gives about 10 molecules per cell for the Colhex
phases of all compounds 1 and 2 (see Table S10 of the
Supporting Information). Thus, about 10 terphenyl units are
arranged side-by-side in each bundle forming the aromatic
column core. In order to enable the side chains on all molecules
to have access to the column periphery, the local cross section of
the aromatic column core is likely to be elliptical (ribbon-like),
rather than circular (see Figure 6b, right); orientational averaging
over space and time then gives rise to hexagonal symmetry and
circular cores in the electron density maps Figures 3-5. The
molecules of compounds 1 and 2 can adopt different conforma-
tions with respect to the orientation of the two swallow tail
substituents to each other, an X- (chains at opposite sides) and a
Π-shaped (chains at the same side), which are in equilibrium
XRD pattern of compounds 1a-c changes in the meso-
morphic temperature range (at T = 150 °C for 1a, at T =
130 °C for 1b, and at 125 °C for 1c, see Table 1), though there is
no indication of any additional peak in the DSC curves (see
Figure 8a) or of any change in texture in the whole mesomorphic
temperature range (see Figure 8b and c). Only a continuous
increase in birefringence takes place with decreasing tempera-
ture, signifying an increase of the orientational order parameter
of the aromatics. In the powder XRD pattern of the Colhex phase
of compound 1c, for example, an additional very weak diffraction
peak, corresponding to d = 2.05 nm, appears at ca. 135 °C, which
continuously increases in intensity upon further cooling (see
Figure 9a). A number of additional hkl reflections can be
observed in the GISAXS pattern of a highly oriented LC film
4911
dx.doi.org/10.1021/ja110065r |J. Am. Chem. Soc. 2011, 133, 4906–4916

Journal of the American Chemical Society
ARTICLE
Figure 9. R3m phase of compound 1c: (a) SAXS spectra (synchrotron
source) recorded at decreasing temperatures showing the Colhex-R3m
transition indicated by the appearance of the (101) rhombohedral reflec-
tion; (hk) indices refer to Colhex and (hkl) to R3m phase using hexagonal
coordinates. (b) GISAXS pattern of the R3m phase recorded at 110 °Cwith
indices (mostly multiple) above the observed reflections; note that reflec-
tions (2-21) and (-22-1) are also superimposed on (3-30).
the Colhex-rhombohedral transition (Figure 8a) and the lack
of change in the OH-stretching absorption in the IR spectrum, i.
e., the lack of change in hydrogen-bonding (Figure S5, Support-
ing Information), suggest that segmented columns persist in the
entire range studied, from the Colhex phase right into the
isotropic liquid, albeit only with short-range order (see
Figure 6b). In the 3D electron density map of the R3m phase
of compound 1c in Figure 10a and b, the red undulated
cylinders, enclosing the low electron density areas, contain
the column cores formed by the bolaamphiphilic units
(terphenyls þ glycerols). The alternation of aromatic and
glycerol groups along these columns is in line with the undula-
tion of the electron density along the columns. The projections
of the undulating columns are arranged on a 2D hexagonal
lattice, and there are three columns in a rhombic unit cell. In
one unit cell each column contains one segment, i.e., a
10-molecule bundle, shifted along the z-direction by either 0,
c/3 or 2c/3, as represented in red, green, and yellow, respec-
tively, in Figure 10c. Based on the lattice parameter ahex of the
R3m phase, the distance l between neighboring columns is 3.39
nm (l = ahex/31/2), which is almost equal to the lattice
parameter of the Colhex phase observed at higher temperatures
(ahex = 3.32 nm, see Table 1). This suggests that upon cooling
through the phase transition the columns simply lock into
longitudinal register without any prominent transverse displa-
cement. The high electron density domains (blue, Figure 10a
and b) enclose the regions with highest concentration of the
fluorinated chains. In spite of having a lower fluorine content,
the continuum surrounding these regions still has a higher
electron density than the bolaamphiphilic column cores, mean-
ing that the continuum still contains a mixture of RH and RF
Figure 8. Compound 1c: (a) DSC heating (solid line) and cooling
curves (dashed line, rates: 10 K min-1) and textures (crossed polarizers)
as observed for: (b) the Colhex phase at T = 150 °C; (c) the R3m phase at
T = 100 °C; (d) the Colhex phase at T = 150 °C with λ-plate retarder
(different sample). The indicatrix orientation in the compensator and in
the two orientations of the fans is shown in the inset; position of
polarizer and analyzer are shown with white arrows.
aligned on Si surface (Figure 9b). These were indexed on a
rhombohedral lattice, space group R3m, with lattice parameters
ahex = 5.87 nm, and c = 2.24 nm. The orientation is planar, as in
the Colhex phase, with z-axis in the film plane.
The rhombohedral lattice results from the establishment of
3D register of the column segments. The c = 2.24 nm
periodicity is well within the range of the bolaamphiphilic core
length (L = 2.0-2.4). The absence of noticeable latent heat of
4912
dx.doi.org/10.1021/ja110065r |J. Am. Chem. Soc. 2011, 133, 4906–4916

Journal of the American Chemical Society
ARTICLE
Figure 11. Different modes of organization of the RF chains in the
mesophases of compounds 1 and 2: (a) in the Colhex phase of 1c and 2a,
(b) in the Colhex phase of 2c, and (c) in the rhombohedral R3m phase of
1c; green: RF-rich regions; gray: columns of the rod-bundles. The two
helices in the front in (c) are decorated with purple blobs which
represent the regions of highest RF concentration.
with decreasing temperature (Figure 9a) is the result of a
progressive increase in longitudinal register (positional order
parameter) of the column segments, seen as increasing undula-
tion amplitude of column cores in the electron density maps,
see the effect of temperature on column undulation in Figure
S8, Supporting Information.
It should be noted that the RF domains are arranged on
alternative right- and left-handed helices as indicated in
Figure 10a and b. The helical structure results from the 31 screw
axes inherent to R3m symmetry and is linked to the 1/3 shift of
the individual columns as shown in Figure 10c, where the arrows
trace the path between the glycerol zones of adjacent columns.
These arrows follow a right- (black) and a left-handed (blue) 31
screw axis. The helical RF regions appear on cooling through the
Colhex-R3m phase transition by deformation and condensation
into blobs of the continuous straight RF columns of the Colhex
/
p6mm phase (see Figures 5b, blue column projections, and
Figure 11a, green columns). These blobs appear as high electron
density regions (blue/purple) in the electron density maps in
Figure 10a and b and in the schematic in Figure 11c.
In relation to the helical arrangement of fluorine-rich domains
we note that all compounds 1 comprise four sterogenic centers in
the molecules, and they actually represent mixtures of diastere-
omers in racemic form. While overall the compounds and the
structure are optically inactive, it is possible that a degree of
enantiomeric separation develops on the local scale, as different
isomers may fit better to helices of one or the other sense. While
this is difficult to establish here, a related case of enantiomer
separation into right- and left-handed helical columns has
recently been reported in racemic liquid crystal helicenes.43
In contrast to compounds 1 no transition to a 3D ordered
phase was observed for any of the compounds 2 with longer and
more bulky lateral chains having two fluorinated branches. The
reason might be that the shorter chains of compounds 1 allow a
stronger coupling between the aromatic columns, which induces
positional correlation between them. The intercolumnar inter-
actions are weaker for compounds 2 due to the larger distances
Figure 10. R3m phase of compound 1c: (a and b) Oblique and top
views of the reconstructed 3D electron density map obtained from the
powder pattern at 120 °C; red = low density, in this map corresponding
to the terphenyl and glycerol groups, including a portion of the alkyl
segments of the side-chains); blue = high density (pure or nearly pure RF
groups); transparent continuum = mixed alkyl and RF groups; schematic
molecules are added in (a). In (b) symbols indicate location of three-fold
rotation axes and 31 screw axes, while curved arrows show the helical
relationship between RF domains (each arrow connects three RF
domains shifted along z by c/3, 2/3c, and c). (c) Bundles of molecular
cores (terphenyl and glycerol groups) in the R3m phase (schematic); the
green and yellow bundles are shifted along z by 1/3c and 2/3c,
respectively; the arrows indicate the action of two adjacent 31 axes on
glycerol zones in adjacent columns. Black arrows follow a right-handed
helix starting with z = 0 on the red column at rear left (indicated by
dotted line, arrow hidden by green column) passing via the green (z = c/
3) and yellow (z = 2c/3) columns and ending again on the starting red
column at z = c. Similarly, the left-handed helix starts at the red column in
the front, right and follows the blue arrows via the green and yellow back
to the red column.
between the columns (ahex = 3.7-4.5 nm compared to ahex
=
3.0-3.3 nm for compounds 1).44 Moreover, the side-by-side
fixation of a fluorinated and a hydrocarbon chain in each of the
swallow tail branches of compounds 1 may cause an increased
tendency for segregation of RF and RH along z-axis and not just in
the xy plane as in compounds 2. The arrangements of RF-rich
domains in different phases of compounds 1 and 2 are schema-
tically represented in Figure 11.
segments. In fact there is a slightly higher electron density spine
in the center of the red undulated columns in Figure 10a and b,
albeit less than in the high-temperature Colhex phase, see
Figure 5b. The intensification of the (101) diffraction peak
4913
dx.doi.org/10.1021/ja110065r |J. Am. Chem. Soc. 2011, 133, 4906–4916

Journal of the American Chemical Society
ARTICLE
hexagonal cross section (Colrec/c2mm, Figure 12b).29,30 Further
increase in the lateral chain volume, either through adding extra
atoms or by increasing the temperature, leads to a transition to
lamellar phases where the bolaamphiphilic cores lie in the layer
planes (LamN and LamSm, Figure 12c and d).29,35,50 Further
increasing the size of the lateral chains causes the Lam phases
to transform to a new family of columnar LC phases with
rod-bundle structure and 2D or 3D periodicity.30f,32 With respect
to the positions of rigid and flexible units, the mesophases at the
extreme right and left of Figure 12 are the inverse of each other.
The reversal of the position of flexible and rigid parts in these
rigid/flexible amphiphiles is analogous to swapping the polar and
nonpolar moieties in polar/apolar amphiphiles. In addition to
this, a complete reorganization of the rod-like moieties with
respect to the column long axis takes place in two steps. In the
first step the polygonal cylinders of the honeycomb phases burst
parallel to the hydrogen-bonding networks (i.e., perpendicular to
the direction of the aromatic cores) resulting in the formation of
layers (Lam phases).30e In the second step the layers of these
Lam phases split parallel to the rod-like cores into infinite
ribbons, giving rise to the axial-bundle phases.
The mesophases reported here are in some respect related to
columnar mesophases of hairy rod main-chain polymers,51 as the
systems can be viewed as hydrogen-bonded supramolecular
main-chain polymers52 with the lateral chains as hairy side
groups. However, in contrast to these polymers the molecules
reported here are organized in bundles. Even some flexible and
semiflexible polymers without lateral chains exhibit a main-chain
columnar LC phase, but there again each chain forms a column of
its own.53 The hairy rod polymers51,54,55 represent an important
class of fluorescent and semiconducting organic materials for
potential use in molecular electronics,56 organic light-emitting
diodes, organic transistors, and photovoltaic devices.33,57 Hence,
investigation of the low molecular weight molecules reported
herein can provide clues concerning the controlled organization
of these important functional materials by directed design of
molecular tectons. More generally, these investigations have
contributed to the general understanding of soft matter self-
assembly and the molecular design principles required for
generation of distinct types of complex phase morphologies.
Figure 12. Sequence of different phase types formed by LC self-
assembly of rod-like bolaamphiphiles with incompatible lateral and
terminal chains, as seen upon increasing the size of the lateral chain(s):
(a and b) polygonal cylinder phases; (c and d) Lam phases (rod-like
cores parallel to the layer planes); and (e) axial-bundle phases.
’ SUMMARY AND CONCLUSIONS
In summary, new types of rod-like bolaamphiphiles with
lateral swallow-tailed chains were synthesized, and new modes
of LC self-assembly are reported with the aromatic rods grouped
in bundles parallel to the column axis (axial-bundle columnar
phases). Besides the polygonal cylinder phases (see for example
Figure 12a and b) and the laminated phases (Figure 12c and d),
these axial-bundle phases (Figure 12e) represent the third
fundamental mode of self-assembly of ternary amphiphiles with
(branched) lateral chains. Moreover, the axial-bundle phases
represent a new type of columnar organization in general, where
rod-like units are aligned parallel to the column axis and not
across it as in columnar phases of polycatenar compounds10,11
or rod-like mesogens with bulky end chains.17-20 The bundles
stack up longitudinally forming segmented ribbons of elliptical
cross-section. The segmentation is due to the segregation of the
aromatics from the terminal glycerol groups.45 The ribbons are
surrounded by the disordered swallow-tailed lateral chains.
There are at least two distinct subtypes, one with 2D and the
other with 3D periodicity, i.e., a hexagonal columnar phase in
which the ribbons are rotationally averaged giving rise to
hexagonal symmetry with no indication of long-range axial
30f,32
’ ASSOCIATED CONTENT
positional correlation between column segments (Colhex
)
and a 3D mesophase where the periodicity along the columns
locks into register giving a 3D phase with rhombohedral R3m
symmetry,46 in which the bundles are longitudinally shifted by
(1/3 molecular length between adjacent columns. Regarding
the nature of the latter phase, although there is 3D long-range
order in density fluctuations, we still regard it as LC, since there is
no preferred position for individual molecules in the x,y-plane,
hence the diffuse wide-angle X-ray scattering.47
S
Supporting Information. Synthesis procedures and ana-
b
lytical data, additional DSCs, textures, and XRD data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
g.ungar@sheffield.ac.uk, carsten.tschierske@chemie.uni-halle.de
The formation of a Colhex phase of axially aligned hard
spherocylinders has been observed in early MC/MD simulations
by Frenkel et al.48 The 2D-to-3D transition observed for com-
pounds 1, and the fact that it appears to be second order, is also
important from the theoretical point of view, and parallels can be
’ ACKNOWLEDGMENT
This work, as part of the ESF EUROCORES Programme
SONS, was supported by funds from the DFG, EPSRC, and the
EC 6th Framework Programme, under contract ERAS-CT-2003-
989409 and by the Fonds der Chemischen Industrie. G.U. also
acknowledges the support from the WCU program through the
National Research Foundation of Korea funded by the Ministry
of Education, Science and Technology (R31-10013). M.P.
drawn with the nematic-smectic A transition.49
With respect to the molecular structure, the compounds
discussed here are related to the previously reported T-shaped
amphiphiles which organize into honeycomb-type polygonal
cylinder phases in which the cylinders adopt a rhombic, square,
pentagonal, hexagonal (Colhex, Figure 12a), or stretched
4914
dx.doi.org/10.1021/ja110065r |J. Am. Chem. Soc. 2011, 133, 4906–4916

Journal of the American Chemical Society
ARTICLE
acknowledges the support by the Cluster of Excellence “Nano-
structured Materials” and DFG (FG 1145). For help with the
synchrotron experiments we thank Drs. Nick Terrill, Jen Hiller
(beamline I22), Chris Nicklin, and Tom Arnold (I07) at
Diamond and Drs. Oier Bikondoa, Simon Brown, and Paul
Thompson (BM28-XMaS) at ESRF.
(14) Achard, M. F.; Tinh, N. H.; Richard, H.; Mauzac, M.; Hardouin,
F. Liq. Cryst. 1990, 84, 533–544. Percec, V.; Heck, J.; Ungar, G.
Macromolecules 1991, 24, 4957–4962.
(15) (a) Ungar, G.; Abramic, D.; Percec, V.; Heck, J. A. Liq. Cryst.
1996, 21, 73–86. (b) Sauer, C.; Diele, S.; Lindner, N.; Tschierske, C. Liq.
Cryst. 1998, 25, 109–116.
(16) For bent polycatenar dithiolium salts formation of columnar
phases was reported, where the organization of the molecules inside the
columns is more driven by the amphiphatic feature of these salts than by
the shape of the molecules: Artzner, F.; Veber, M.; Clerc, M.; Levelut,
A. M. Liq. Cryst. 1997, 23, 27–33.
’ REFERENCES
(1) Steed, J. W.; Atwood, J. L. Encyclopedia of Supramolecular
Chemistry; Marcel Dekker: New York, 2004.
(2) (a) Stewart, G. T. Liq. Cryst. 2003, 30, 541–557. (b) Stewart,
G. T. Liq. Cryst. 2004, 31, 443–471. (c) Lydon, J. Liq. Cryst. Today 2006,
15, 1–10. (d) Lydon, J. Liq. Cryst. Today 2004, 13, 1–13. (e) Nakata, M.;
Zanchetta, G.; Chapman, B. D.; Jones, C. D.; J. O. Cross, J. O.; Pindak,
R.; Bellini, T.; Clark, N. A. Science 2007, 318, 1276–1279.
(3) Collings, P. J.; Hird, M. Introduction to Liquid Crystals; Taylor &
Francis: London, 1997. Demus, D.; Goodby, J.; Gray, G. W.; Spiess,
H.-W.; Vill, V. Handbook of Liquid Crystals, Wiley-VCH: Weinheim,
Germany, 1998; Vol. 1-3. Pauluth, D.; Tarumi, K. J. Mater. Chem.
2004, 14, 1219–1227.
(17) Weissflog, W. in Demus, D.; Goodby, J.; Gray, G. W.; Spiess,
H.-W.; Vill, V. Handbook of Liquid Crystals; Wiley-VCH: Weinheim,
Germany, 1998; Vol. 2B, pp 835-863.
(18) Lee, M.; Cho, B.-K.; Zin, W.-C. Chem. Rev. 2001, 101, 3869–3892.
Lee, M.; Yoo, Y.-S. J. Mater. Chem. 2002, 12, 2161–2168. Ryu, J.-H.; Cho,
B.-K.; Lee, M. Bull. Korean Chem. Soc. 2006, 27, 1270–1282. Ryu, J.-H.; Lee,
M. Struct. Bonding (Berlin) 2008, 128, 63–98.
(19) Nishikawa, E.; Samulski, E. T. Liq. Cryst. 2000, 27, 1463–1471.
(20) Guillevic, M.-A.; Bruce, D. W. Liq. Cryst. 2000, 27, 153–156.
Lose, D.; Diele, S; Pelzl, G.; Dietzmann, E.; Weissflog, W. Liq. Cryst.
1998, 24, 707–717.
(21) Reddy, R. A.; Tschierske, C. J. Mater. Chem. 2006, 16, 907–961.
Takezoe, H.; Takanishi, Y. Jpn. J. Appl. Phys. 2006, 45, 597–625.
(22) (a) Pegenau, A.; Hegmann, T.; Tschierske, C.; Diele, S. Chem.—
Eur. J. 1999, 5, 1643–1660. (b) Pegenau, A.; Cheng, X. H.; Tschierske, C.;
G€oring, P.; Diele, S. New J. Chem. 1999, 23, 465–467. (c) Cheng, X. H.;
Diele, S.; Tschierske, C. Angew. Chem., Int. Ed. 2000, 39, 592–595.
(23) Rosen, B. M.; Wilson, C. J.; Wilson, D. A.; Peterca, M.; Imam,
M. R.; Percec, V. Chem. Rev. 2009, 109, 6275–6540.
(4) Tschierske, C. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem. 2001,
97, 191–267.
(5) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed.
2006, 45, 38–68. Ungar, G.; Zeng, X. Soft Matter 2005, 1, 95–106.
Donnio, B.; Guillon, D. Adv. Polym. Sci. 2006, 201, 45–155. Saez, I. M.;
Goodby, J. W. Struct. Bonding (Berlin) 2008, 128, 1–62.
(6) Chandrasekhar, S.; Sadashiva, B. K.; Suresh, K. A. Pramana 1977,
9, 471–480.
(7) Guillon, D. In Structure and Bonding, Liquid Crystals II; Mingos,
D. M. P, Ed.; Springer: Berlin, Germany, 1999; Vol. 95, pp 41-82.
Bushby, R. J.; Lozman, O. R. Curr. Opin. Colloid Interface Sci. 2002,
7, 343–354. Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.;
(24) Borisch, K.; Diele, S.; G€oring, P.; Kresse, H.; Tschierske, C. J.
Mater. Chem. 1998, 8, 529–543. Fuchs, P.; Tschierske, C.; Raith, K.; Das,
K.; Diele, S. Angew. Chem., Int. Ed. 2002, 41, 628–631. Cheng, X. H.; Das,
K.; Diele, S.; Tschierske, C. Langmuir 2002, 18, 6521–6529. Kohlmeier,
A.; Janietz, D. Chem. Mater. 2006, 18, 1483–1489.
€
Hagele, C.; Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.;
Schreivogel, A.; Tosoni, M. Angew. Chem., Int. Ed. 2007, 46, 4832–4887.
(8) (a) Pisula, W.; Zorn, M.; Chang, J. Y.; M€ullen, K.; Zentel, R.
Macromol. Rapid Commun. 2009, 30, 1179–1202. (b) Wu, J.; Pisula, W.;
M€ullen, K. Chem. Rev. 2007, 107, 718–747. (c) Sergeyev, S.; Pisula, W.;
Geerts, Y. H. Chem. Soc. Rev. 2007, 36, 1902–1929. (d) Pisula, W.; Feng,
X.; M€ullen, K. Adv. Mater. 2010, 22, 3634–3649.
(9) Chen, Z.; Stepanenko, V.; Dehm, V.; Prins, P.; Siebbeles,
L. D. A.; Seibt, J.; Marquetand, P.; Engel, V.; W€urthner, F. Chem.—
Eur. J. 2007, 13, 436–449. Chen, Z.; Baumeister, U.; Tschierske, C.;
W€urthner, F. Chem.—Eur. J. 2007, 13, 450–465.
(25) Seddon, J. M.; Templer, R. H. In Handbook of Biological Physics;
Lipowsky, R., Sackmann, E., Eds.; Elsevier: Amsterdam, The Nether-
lands, 1995; Vol. 1, pp 97-160. Hassan, S.; Rowe, W.; Tiddy, G. J. T.
In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K.,
Ed.; Wiley: Chichester, U.K., 2002; Vol. 1, pp 465-508. Hyde, S. T.
In Handbook of Applied Surface and Colloid Science; Holmberg, K., Ed.;
Wiley: Chichester, U.K., 2002; Vol. 2, pp 299-332.
(26) (a) Tschierske, C. J. Mater. Chem. 1998, 8, 1485–1508. (b)
Tschierske, C. J. Mater. Chem. 2001, 11, 2647–2671.
(10) Malthete, J.; Levelut, A. M.; Tinh, N. H. J. Phys., Lett. 1985,
46, L875–880. Tinh, N. H.; Malthete, J.; Destrade, C. Mol. Cryst. Liq. Cryst.
Lett. 1985, 2, 133–138. Tinh, N. H.; Destrade, C.; Levelut, A. M.; Malthete,
J. J. Phys. (Paris) 1986, 47, 553–557. Guillon, D.; Skoulios, A.; Malthete, J.
Europhys. Lett. 1987, 3, 67–72. Destrade, C.; Tinh, N. H.; Roubineau, A.;
Levelut, A. M. Mol. Cryst. Liq. Cryst. 1988, 159, 163–171. Levelut, A. M.;
Malthete, J.;Destrade, C.;Tinh, N. H.Liq. Cryst. 1987, 2, 877–888. Nguyen,
N. T.; Destrade, C.; Malthete, J. Liq. Cryst. 1990, 8, 797–811.
(11) Reviews:(a) Malthete, J.; Nguyen, H. T.; Destrade, C. Liq.
Cryst. 1993, 13, 171–187. (b) Nguyen, H. T.; Destrade, C.; Malthete, J.
Adv. Mater. 1997, 9, 375–388.(c) Nguyen, H. T.; Destrade, C.; Malthete,
J. In Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W.,
Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, Germany, 1998;
Vol. 2B, pp 865-900. (d) Gharbia, M.; Gharbi, A.; Nguyen, H. T.;
Malthete, J. Curr. Opin. Colloid Interface Sci. 2002, 7, 312–325. (e) Bruce,
D. W. Acc. Chem. Res. 2000, 33, 831–840.
(27) Ostrovskii, B. I. In . Liquid Crystals I; Mingos, D. M. P., Ed.;
Springer: Berlin, Germany, 1999; Vol. 94, pp 199-240.
(28) Donnio, B.; Heinrich, B.; Allouchi, H.; Kain, J.; Diele, S.;
Guillon, D.; Bruce, D. W. J. Am. Chem. Soc. 2004, 126, 15258–15268.
(29) Tschierske, C. Chem. Soc. Rev. 2007, 36, 1930–1970.
(30) (a) K€olbel, M.; Beyersdorff, T.; Cheng, X. H.; Tschierske, C.;
Kain, J.; Diele, S. J. Am. Chem. Soc. 2001, 123, 6809–6818. (b) Cheng,
X. H.; Prehm, M.; Das, M. K.; Kain, J.; Baumeister, U.; Diele, S.; Leine,
D.; Blume, A.; Tschierske, C. J. Am. Chem. Soc. 2003, 125, 10977–10996.
(c) Cheng, X.-H.; Das, M. K.; Baumeister, U.; Diele, S.; Tschierske, C. J.
Am. Chem. Soc. 2004, 126, 12930–12940. (d) Prehm, M.; G€otz, G.;
B€auerle, P.; Liu, F.; Zeng, X.; Ungar, G.; Tschierske, C. Angew. Chem., Int.
Ed. 2007, 46, 7856–7859. (e) Prehm, M.; Liu, F.; Baumeister, U.; Zeng,
X.; Ungar, G.; Tschierske, C. Angew. Chem., Int. Ed. 2007,
46, 7972–7975. (f) Prehm, M.; Enders, C.; Anzahaee, M. Y.; Glettner,
B.; Baumeister, U.; Tschierske, C. Chem.—Eur. J. 2008, 14, 6352–6368.
(g) Cheng, X.; Dong, X.; Huang, R.; Zeng, X. B.; Ungar, G.; Prehm, M.;
Tschierske, C. Chem. Mater. 2008, 20, 4729–4738. (h) Glettner, B.; Liu,
F.; Zeng, X. B.; Prehm, M.; Baumeister, U.; Walker, M.; Bates, M. A.;
Boesecke, P.; Ungar, G.; Tschierske, C. Angew. Chem., Int. Ed. 2008,
47, 9063–9066.
(12) Originally the term “phasmidic” referred to rod-like molecules
with three alkyl chains at both ends; “biforked” mesogens have two alkyl
chains at both ends; now these molecules are generally named “poly-
catenar”, where six-, five-, four-, and three-chain compounds are labeled
hexa-, penta-, tetra-, and tricatenar, respectively; see ref 11c.
(13) Donnio, B.; Heinrich, B.; Gulik-Krzywicki, T.; Delacroix, H.;
Guillon, D.; Bruce, D. W. Chem. Mater. 1997, 9, 2951–2965. Bruce,
D. W. Acc. Chem. Res. 2000, 33, 831–840.
(31) (a) Chen, B.; Zeng, X.-B.; Baumeister, U.; Diele, S.; Ungar, G.;
Tschierske, C. Angew. Chem., Int. Ed. 2004, 43, 4621–4625. (b) Chen, B.;
Zeng, X.-B.; Baumeister, U.; Ungar, G.; Tschierske, C. Science 2005,
4915
dx.doi.org/10.1021/ja110065r |J. Am. Chem. Soc. 2011, 133, 4906–4916

Journal of the American Chemical Society
ARTICLE
307, 96–99. (c) Chen, B.; Baumeister, U.; Pelzl, G.; Das, M. K.; Zeng
X.-B.; Diele, S.; Ungar, G.; Tschierske, C. J. Am. Chem. Soc. 2005,
127, 16578–16591. (d) Cook, A. G.; Baumeister, U.; Tschierske, C. J.
Mater. Chem. 2005, 15, 1708–1721. (e) Liu, F.; Chen, B.; Baumeister,
U.; Zeng, X.; Ungar, G.; Tschierske, C. J. Am. Chem. Soc. 2007,
129, 9578–9579. (f) Liu, F.; Chen, B.; Glettner, B.; Prehm, M.; Das,
M. K.; Baumeister, U.; Zeng, X. B.; Ungar, G.; Tschierske, C. J. Am.
Chem. Soc. 2008, 130, 9666–9667.
(ΔH = 4.8 kJ mol-1, see Figure S3, Supporting Information). The
precise structure of this mesophase M, though it is clearly a LC phase
(maintenance of the diffuse wide angle scattering), has not yet been
resolved.
(45) T-shaped rod-coil molecules with lateral coils but without
terminal groups were also reported recently, however these molecules
show a different self assembly behavior (stepped stripes) as there is no
attractive end-to-end interaction, which is essential for the formation of
the polygonal cylinder phases and the rod-bundle phases, see Hong,
D.-J.; Lee, E.; Lee, J.-K.; Zin, W.-C.; Han, M.; Sim, E.; Lee, M. J. Am.
Chem. Soc. 2008, 130, 14448–14449.
(46) A R3m phase was recently also observed for the organization of
functionalized gold nanoparticles in a calamitc nematic matrix. In this
case the 3D packing is provided by the optimization of the packing of the
nanoparticles in the nematic director field: Zeng, X. B.; Liu, F.; Fowler,
A. G.; Ungar, G.; Cseh, L.; Mehl, G. H. J.; Macdonald, E. Adv. Mater.
2009, 21, 1746–1750. An orthorhombic LC phase with columnar
elements has been reported in dendrimers, space group Pmna: Rosen,
B. M.; Peterca, M.; Huang, C.; Zeng, X. B.; Ungar, G.; Percec, V. Angew.
Chem., Int. Ed. 2010, 49, 7002–7005.
(32) Prehm, M.; Liu, F.; Zeng, X. B.; Ungar, G.; Tschierske, C. J. Am.
Chem. Soc. 2008, 130, 14922–14923.
(33) Anthony, J. E. Chem. Rev. 2006, 106, 5028–5048. Kroon, R.;
Lenes, M.; Hummelen, J. C.; Blom, P. W. M.; De Boer, B. Polym. Rev.
2008, 48, 531–582. Cheng, Y. J.; Yang, S.-H.; Hsu, C.-S. Chem. Rev.
2009, 109, 5868–5923. Kaeser, A.; Schenning, A. P. H. J. Adv. Mater.
2010, 22, 2985–2997.
(34) (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.
(35) Prehm, M.; Diele, S.; Das, M. K.; Tschierske, C. J. Am. Chem.
Soc. 2003, 125, 614–615.
(47) Pershan, P. S. Structure of Liquid Crystal Phases; World Scien-
(36) Due to the presence of the stereogenic centers in the two
glycerol groups, and for compounds 1 also at the branching points in the
lateral chains, the investigated systems actually represent racemic
mixtures of several stereoisomers.
(37) Immirzi, A.; Perini, B. Acta Crystallogr., Sect. A 1977, 33,
216–218.
tific: Singapore, 1988.
(48) Veerman, J. A. C.; Frenkel, D. Phys. Rev. A 1991, 43,
4334–4343.
(49) de Genns, P. J.; Prost, J. Physics of Liquid Crystals; Oxford
Science Publishers: Oxford, England, 1975.
(50) (a) Cheng, X. H.; Das, M. K.; Diele, S.; Tschierske, C. Angew.
Chem., Int. Ed. 2002, 41, 4031–4035. (b) Prehm, M.; Cheng, X. H.;
Diele, S.; Das, M. K.; C. Tschierske, C. J. Am. Chem. Soc. 2002,
124, 12072–1273. (c) Patel, N. M.; Dodge, M. R.; Zhu, M. H.; Petschek,
R. G.; Rosenblatt, C.; Prehm, M.; Tschierske, C. Phys. Rev. Lett. 2004,
92, 015501. (d) Patel, N. M.; Syed, I. M.; Rosenblatt, C.; Prehm, M.;
Tschierske, C. Liq. Cryst. 2005, 32, 55.
(51) Fu, K.; Sekine, N.; Sone, M.; Tokita, M.; Watanabe, J. Polymer J.
2002, 34, 291–297. Carbonnier, B.; Andreopoulou, A. K.; Pakula, T. J.;
Kallitis, K. Macromol. Chem. Phys. 2005, 206, 66–76. Kim, L.; Arnt, T.;
Atkins, E.; Tew, G. N. Chem.—Eur. J. 2006, 12, 2423–2427. Riala, P.;
Andreopoulou, A.; Kallitsis, J.; Gitsas, A.; Floudas, G. Polymer 2006,
47, 7241–7250.
(38) Enlarging the fluorinated segments also leads to mesophase
stabilization in the series of compounds 2 (Table 2) due to the
fluorophobic effect (see for example,Johannsson, G.; Percec, V.; Ungar,
G.; Zhu, J. P. Macromolecules 1996, 29, 646–660); whereas there is no
significant effect of elongation of the RF segments on the mesophase
stability of compounds 1 (Table 1) with only one fluorinated branch.
(39) For hexagonal cylinder phases of related p-terphenyl derivatives
with only one lateral chain or with chains at opposite sides of the
terphenyl unit parameters ahex = 3.8-4.1 nm were found:Kieffer, R.;
Prehm, M.; Glettner, B.; Pelz, K.; Baumeister, U.; Liu, F.; Zeng, X. B.;
Ungar, G.; Tschierske, C. Chem. Commun. 2008, 3861–3863.
(40) Calculation of the number of molecules in the unit cells (ncell),
defined by the hexagonal lattice parameter ahex and assuming a height of
the cell of h = 0.45 nm (corresponding to the lateral mean distance
between the aromatic cores in such a hexagonal cylinder array) would
amount only ncell = 2.2 for all compounds (see Table S10 of the
Supporting Information), which is too small for a hexagonal cylinder
structure. Such a cylinder structure would require a number of at least
(52) Rieth, S.; Baddeley, C.; Badjic, J. D. Soft Matter 2007,
3, 137–154.Supramolecular Polymers, 2nd ed.; Ciferri, A., Ed.; Marcel
Dekker, Inc.: New York, 2006; Hoogenboom, R.; Schubert, U. S. Chem.
Soc. Rev. 2006, 35, 622–629. Rowan, S. J.; Mather, P. T. Struct. Bonding
(Berlin) 2008, 128, 119–149.
(53) Ungar, G. Polymer 1993, 34, 2050–2059.
n
cell = 3, which is the lowest limit for stable cylinder phases. In fact, values
(54) Zheng, R.-Q.; Chen, E.-Q.; Cheng, S. Z. D.; Xie, F.; Yan, D.; He,
T.; Percec, V.; Chu, P.; Ungar, G. Macromolecules 1999, 32, 3574–3582.
(55) Watanabe, J.; Sekine, N.; Nematsu, T.; Sone, M.; Kricheldorf,
H. R. Macromolecules 1996, 29, 4816–4818.
(56) Petty, M. C. Molecular Electronics. From Principles to Practice;
Wiley: New York, 2008. Funahashi, M.; Shimura, H.; Yoshio, M.; Kato,
T. Struct. Bonding (Berlin) 2008, 128, 151–179.
(57) Pankaj, S.; Beiner, M. Soft Matter 2010, 6, 3506–3516. Kreyes,
A.; Ellinger, S.; Landfester, K.; Defaux, M.; Ivanov, D. A.; Elschner, A.;
Meyer-Friedrichsen, T.; Ziener, U. Chem. Mater. 2010, 22, 2079–2092.
of ncell around or above 6 were found for all previously reported Colhex
phases of T-shaped ternary amphiphiles (see refs 29 and 30a, b, f, and g),
i.e., there are about two aromatic cores in the cross section of the cylinder
walls; only X-shaped amphiphiles with lateral chains at opposite sides of
an aromatic core form predominately single wall cylinders with slightly
more than one (1.1-1.3) aromatic core in the cross section of the
cylinder walls, i.e., ncell is about four for the hexagonal honeycombs
formed by these compounds.39
(41) Wang, Y.-J.; Carlisle, G. O. J. Mater. Sci.: Mater. Electron. 2002,
13, 173–178.
(42) (a) Frank, H. S.; Wen, W.-Y. Disc. Farad. 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-
Huyskens, T., Eds.; Springer: Berlin, Germany, 1992; pp 251-280.
(43) Shcherbina, M. A.; Zeng, X. B.; Tadjiev, T.; Ungar, G.; Eich-
horn, S. H.; Phillips, K. E. S.; Katz, T. J. Angew. Chem., Int. Ed. 2009,
48, 7837–7840.
(44) Only compound 2a with the shortest swallow-tailed chains in
this series shows an additional phase transition at T = 97 °C, but this
transition is distinct from the 2D-to-3D transitions observed for
compounds 1, as it is associated with a significant transition enthalpy
4916
dx.doi.org/10.1021/ja110065r |J. Am. Chem. Soc. 2011, 133, 4906–4916