Phase behaviour and Janus hierarchical supramolecular structures based on asymmetric tapered bisamide

Article abstract of DOI:10.1039/c2sm07332c

A precisely defined molecular Janus compound based on asymmetric tapered 1,4-bis[3,4,5-tris(alkan-1-yloxy)benzamido] benzene bisamide (abbreviated as C₂₂PhBAEO₃) was designed and synthesized, and its phase behavior was fully investigated. The C₂₂PhBAEO₃ compound possesses a rigid core with three aromatic rings connected with amide bonds which possess the ability to form hydrogen (H) bonds. Three hydrophobic alkyl flexible tails and three hydrophilic flexible methyl terminated triethylene glycol tails are located at the other end. Major phase transitions and their origins in C₂₂PhBAEO₃ were studied via DSC and 1D WAXD techniques. Its hierarchical supramolecular crystal structure was further identified through combined techniques of 2D WAXD and SAXS as well as SAED. Results based on computer simulations confirmed the structure determination. It was found that the C₂₂PhBAEO₃ possesses three phases through various thermal treatments including a micro-phase separated columnar liquid crystal (col.) phase, a metastable crystal I phase and a stable crystal II phase. Among them, the crystal II phase showed that the columnar structure possesses 3D inter-column order and highly crystalline alkyl tails with a long-range overall orientational order. Four C₂₂PhBAEO₃ molecules self-assembled into a phase-separated disc with an ellipsoidal shape having a C₂ symmetry along the disc normal. These discs then stacked on top of each other to generate a 1D asymmetric column through H-bonding, and further packed into a 3D long-range ordered monoclinic lattice. The unit cell parameters of this lattice were determined to be a = 5.08 nm, b = 2.41 nm, c = 0.98 nm, α = 90°, β = 90°, and γ = 70.5°. The alkyl chain tails crystallize within the hydrophobic layers and possess a relatively fixed orientation with respect to the column packing due to the selective interactions based on the hydrophobic/hydrophilic microphase separation. Both phase behaviour and unit cell structure showed significant difference compared with the symmetrically tapered counterparts. The results provided a new approach of fine-tuning not only in the Janus supramolecular structures but also in the formation pathway of the self-assembling process in order to meet the specific requirements for optical and biological applications.

Full text of DOI:10.1039/c2sm07332c

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PAPER
Phase behaviour and Janus hierarchical supramolecular structures based on
asymmetric tapered bisamide†
Hao-Jan Sun,a Chien-Lung Wang,ab I-Fan Hsieh,a Chih-Hao Hsu,a Ryan M. Van Horn,a Chi-Chun Tsai,a
c
Kwang-Un Jeong, Bernard Lotz and Stephen Z. D. Cheng
d
a
*
Received 9th December 2011, Accepted 14th February 2012
DOI: 10.1039/c2sm07332c
A precisely defined molecular Janus compound based on asymmetric tapered 1,4-bis[3,4,5-tris(alkan-1-
yloxy)benzamido] benzene bisamide (abbreviated as C22PhBAEO3) was designed and synthesized, and
its phase behavior was fully investigated. The C22PhBAEO3 compound possesses a rigid core with three
aromatic rings connected with amide bonds which possess the ability to form hydrogen (H) bonds.
Three hydrophobic alkyl flexible tails and three hydrophilic flexible methyl terminated triethylene
glycol tails are located at the other end. Major phase transitions and their origins in C22PhBAEO3 were
studied via DSC and 1D WAXD techniques. Its hierarchical supramolecular crystal structure was
further identified through combined techniques of 2D WAXD and SAXS as well as SAED. Results
based on computer simulations confirmed the structure determination. It was found that the
C22PhBAEO3 possesses three phases through various thermal treatments including a micro-phase
separated columnar liquid crystal (col.) phase, a metastable crystal I phase and a stable crystal II phase.
Among them, the crystal II phase showed that the columnar structure possesses 3D inter-column order
and highly crystalline alkyl tails with a long-range overall orientational order. Four C22PhBAEO3
molecules self-assembled into a phase-separated disc with an ellipsoidal shape having a C2 symmetry
along the disc normal. These discs then stacked on top of each other to generate a 1D asymmetric
column through H-bonding, and further packed into a 3D long-range ordered monoclinic lattice. The
unit cell parameters of this lattice were determined to be a ¼ 5.08 nm, b ¼ 2.41 nm, c ¼ 0.98 nm, a ¼
90ꢀ, b ¼ 90ꢀ, and g ¼ 70.5ꢀ. The alkyl chain tails crystallize within the hydrophobic layers and possess
a relatively fixed orientation with respect to the column packing due to the selective interactions based
on the hydrophobic/hydrophilic microphase separation. Both phase behaviour and unit cell structure
showed significant difference compared with the symmetrically tapered counterparts. The results
provided a new approach of fine-tuning not only in the Janus supramolecular structures but also in the
formation pathway of the self-assembling process in order to meet the specific requirements for optical
and biological applications.
Introduction
Since the concept of ‘‘Janus grains’’ was introduced in de Gennes’
Nobel lecture two decades ago,1 Janus particles have received
extensive attention over the last few decades. They are named
after the double-faced Roman god Janus, a representative of
dichotomy. In material science, particles that have special
asymmetric architecture of two distinct sides or interfaces with
different chemical compositions or polarities are named Janus
particles. They were found to be both surface active and
amphiphilic.2,3 The non-centrosymmetric (an inversion center
operation is absent at the geometric center) character makes
them capable of forming complex hierarchical structures
showing desired properties for applications such as bio-chemical
sensors,4–10 self-motile particles,11,12 and interface stabilizers.3,13,14
Several review articles have summarized the synthetic strategies
aDepartment of Polymer Science, College of Polymer Science and Polymer
Engineering, The University of Akron, Akron, OH, 44325, USA. E-mail:
scheng@uakron.edu
bDepartment of Applied Chemistry, National Chiao Tung University, 1001
Ta Hsueh Road, Hsinchu, Taiwan 300, ROC
cPolymer Bin Fusion Research Center, Department of Polymer Nano-
Science and Technology, Chonbuk National University, Jeonju, Jeonbuk
561-756, Korea
dInstitut Charles Sadron, 23, Rue du Loess, Strasbourg 67034, France
† Electronic supplementary information (ESI) available: Fig. S1–S12
include synthetic route, 1H NMR spectrum, 13C NMR spectrum,
MALDI-TOF mass spectrum, FT-IR spectrum, TGA diagram,
supporting DSC and ED data, ED fiber pattern, disc formation
mechanism, and POM morphology of two crystal phases. See DOI:
10.1039/c2sm07332c
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and organization pathways as well as potential applications of
the Janus entities.15–19 In general, Janus entities can be derived
from inorganic materials, colloidal particles, polymers, liquid
crystalline compounds, and supramolecular assemblies. They
can be divided into several common classes such as spheres (0D),
cylinders (1D), and discs (2D), according to their architectures
and dimensionalities. Other architectures including snowman or
dumbbell-like, hamburger, and raspberry-like were also repor-
ted.20–22 The lack of centrosymmetry is a common feature to all of
these particles and has become the major challenge in their
preparation. Syntheses toward molecules with Janus features
often require that symmetry breaking of the underlining building
blocks takes place first. In order to ensure the formation of 3D
long-range ordered structures, molecular Janus entities with high
uniformity and molecular precision have been required as
reported, including amphiphilic dendrimers,23,24 colloidal
photonic crystals,25 unimolecular micelles of block copolymers,10
and polymer brushes.26,27 However, the demands of sophisticated
synthesis techniques and non-trivial purifications have made
commercially driven applications less practical. In addition,
molecular Janus entities often lack volume- or shape-persistence
due to their flexible conformations. On the other hand, facile
synthetic routes or self-assembly procedures such as microfluidic
co-flow synthesis,10,28 Pickering emulsion,29 electrospinning with
a bi-phasic nozzle,30 homopolymer blends,31 and block copoly-
mer self-assembly in bulk state32–34 can produce larger quantities
of asymmetrical particles; yet, size distribution and composition
uniformity are often not guaranteed. Furthermore, engineering
approaches such as microfluidic devices10,35 and pre-patterned
templates36,37 are usually required for those particles to access
larger scale Janus structures. Therefore, precisely defined
molecular Janus entities obtained through simple preparation
procedures that can self-assemble into 3D long-range ordered
hierarchical structures with volume- and shape-persistent
features are of great interest to develop. In our view, molecular
Janus compounds, which readily scale-up and self-assemble into
highly ordered supramolecular structures, are closer to this goal.
Supramolecular architectures are considered to be chemistry
beyond molecules. Instead of connecting building blocks by
covalent bonds, the long-range ordered supramolecular struc-
tures are constructed by intermolecular physical interactions
such as hydrogen (H)-bonding, electrostatic (ion–ion, ion–
dipole, and dipole–dipole) interactions, p–p stacking interac-
tions, van der Waals interactions, hydrophobic–hydrophilic
phase separations, and others. Recently, we reported our effort
toward self-assembled, hierarchical supramolecular structures in
the bulk through the design and synthesis of polyhedral oligo-
meric silsesquioxane (POSS), [60]fullerene (C60), and porphyrin
based molecular Janus particles.38–40 These dumbbell shaped
Janus particles were effectively synthesized by thiol–ene click
chemistry and their hierarchical architectures consisting of
bilayered structures further organized into 3D orthorhombic
lattice within layers were reported.
formed by a series of symmetric tapered bisamides (abbreviated
as CnPhBA, where n is the number of carbon atoms in each tail,
n ¼ 6, 8, 10, 12, 14, and 16) having three alkyl chain tails at each
end of the three aromatic ring cores.44–46 A similar molecular
architecture has also been reported by Ungar et al.47 From our
previous results, four CnPhBA were found to generate a disc-like
building block through the intermolecular H-bonding, and the
supramolecular discs then further assembled into two oblique
columnar phases in response to the environment temperature.
With these experiences, we would like to investigate which
structures and phase behaviours are altered if the symmetry of
the bisamide building molecules is lost and the Janus feature is
introduced.
In this respect, a series of asymmetrically tapered bisamides
(CMPhBAEON) were designed and synthesized by Steglich
esterification. The asymmetric bisamide has a rigid core of 1,2-bis
[3,4,5-tris(alkan-1-yloxy) benzamido] benzene in the middle,
carbon hydrophobic alkyl chain tails with M carbon numbers at
one end, and hydrophilic methyl terminated ethylene glycol (EO)
tails with N repeat units at the other end. In this article, we chose
to report one compound in this series of which M ¼ 22 and N ¼ 3
(C22PhBAEO3). The core possesses a rigid plane that consists of
three phenylenes connected with amide bonds. In addition to
intermolecular H-bonding interactions and core–tail micro-
phase separation force, hydrophilic/hydrophobic microphase
separation of the tails has been added to this system (Scheme 1).
Thermodynamic properties of the phase transitions in
C22PhBAEO3 were studied by differential scanning calorimetry
(DSC) experiments. The phase structures were identified by wide
angle X-ray diffraction (WAXD), small angle X-ray scattering
(SAXS), and selected area electron diffraction (SAED) experi-
ments. Molecular conformations and packing schemes were
obtained by structure analysis and supported by computer
simulations. The phase morphologies were also investigated by
transmission electron microscopy (TEM). It has been found that
Holding on to the same principles, our attention has been
extended to H-bonding induced columnar supramolecular
structures. H-bonding assisted structures have thrived in
a dynamic research field due to the moderate bonding energy,
reversibility, selectivity and directionality of H-bonding.41–43
Previously, we have also reported a discotic liquid crystal phase
Scheme 1 A brief synthetic route of C22PhBAEO3 (1) is shown. Reagents
and conditions: (i) 1,4-phenylenediamine, N,N0-diisopropylcarbodiimide
(DIPC), 4-(dimethylamino) pyridinium toluene-p-sulfonate (DPTS),
THF/CH2Cl2 ¼ 1/4, 25 ꢀC; (ii) N-(3-dimethylaminopropyl)-N0-ethyl-
carbodiimide hydrochloride (EDC$HCl), 1-hydroxy-benzotriazole
(HOBT), CH2Cl2, 25 ꢀC.
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four C22PhBAEO3 Janus molecules self-assemble into a 2D
phase-separated ellipsoid disc that is formed by the intermolec-
ular H-bonding between the cores. These discs are then further
stacked into a supramolecular asymmetric column and further
self-organized into a 3D monoclinic lattice with a bilayer char-
acter that is constructed by phase separated hydrophobic and
hydrophilic tails. Therefore, instead of oblique LC columnar
phases as found in symmetric bisamides,44–46 the asymmetric
bisamides formed a 3D long-range ordered hierarchical crystal-
line structure which can be obtained through moderate temper-
ature annealing.
cycles were also subjected to SAXS measurements conducted at
room temperature.
The 2D WAXD measurements were performed on mechan-
ically sheared sampꢀles. A 2 mg sample was placed on a glass slide
and heated to 120 C followed by cooling from the I melt. The
sample was mechanically sheared with a sharp razor blade at
temperature around 75 ꢀC. Then, the film was removed from the
glass surface as a free standing film at 30 ꢀC before recording
diffraction patterns. The typical sample thickness was ꢁ0.2 mm.
After the first set of experiments, the same piece of sample was
ꢀ
annealed at 87 C for 2 hours and then, brought back to room
temperature for another set of 2D WAXD experiments. The
diffraction patterns were recorded with the beam aligned along
three directions: the through, front, and shear directions,
respectively (as indicated in Fig. 3a).
Experimental
Materials
To prepare the TEM samples for SAED experiments and
morphological investigation, a dilute solution of C22PhBAEO3
was prepared in CH2Cl2 (0.02% w/v). The solution was sprayed
onto carbon coated copper grids. After solvent evaporation, the
copper grids were subject to the identical thermal treatments as
the samples prepared for DSC and 1D WAXD experiments.
To study the surface topology of the self-assembled structure
in the thin films, the polyethylene surface lamellar decoration
(PE-SLD) technique49,50 was applied to study the surface texture
of the supramolecular structures. This technique was widely used
in studying fold surface of polymer single crystals.49–52 In a short
description of this method, 0.2 mg of linear HDPE (Mn ¼ 17.3 kg
molꢃ1 with PDI ¼ 1.11, Phillips Petroleum Co.) was placed in
a tungsten basket in a vacuum evaporator with sample grids
inside. The PE molecules degraded into small fragments during
heating in the vacuum chamber. The fragments evaporated and
deposited onto the sample surface to reveal the surface texture of
the self-assembled structure. Each fragment possessed a length of
ꢁ20 nm and, thus, the width of the PE rod crystals.
Tetrahydrofuran (THF) and methanol were purchased from
Aldrich as reagent grade and used as received. Anhydrous grade
CH2Cl2 was purchased from Acros. All other reagents were
purchased as reagent grade and used without further purification
unless otherwise noted. Compounds 3,4,5-tris-docosoxybenzoic
acid
and
methyl-3,4,5-tris(2-(2-(2-hydroxyethoxy)ethoxy)-
ethoxy)benzoic acid were synthesized according to the litera-
ture.46,48 Scheme 1 shows the synthetic route of C22EO3Bisamide
(C22PhBAEO3, 1). Detailed preparation procedure and
compound characterization data can be found in the ESI†.
Computed molecular dimensions by Cerius2 software in the
extended conformation can also be found at the bottom of
Scheme 1. The samples were stored in a vacuum desiccator before
conducting characterization.
Sample preparation
For DSC experiments, material with a weight of ꢁ3.0 mg was
sealed in an aluminium pan. The sample and reference pan
weights were kept constant with a precision of ꢂ0.001 mg. The
temperature scanning rate was kept constant at 10 ꢀC minꢃ1. The
thin film sample for Fourier transform infrared spectroscopy
(FT-IR) was prepared by dilute solution casting from a CH2Cl2
solution (0.1% w/v) onto a KBr plate. The plate was heated to
120 ꢀC after solvent evaporation to remove previous thermal
history.
To measure the crystal density, a small amount of crystals
(ꢁ 0.5 mg) were collected and placed in a vial filled with water
followed by ultrasonication to remove the air bubbles embedded
within the sample. The sample then sank to the bottom of the vial
due to its higher density compared with water. Potassium iodide
(KI) was then added into the solution at ꢁ0.1 g per aliquot to
gradually increase the solution density. KI was added at an
interval of at least 20 min to ensure that equilibrium within the
solution was reached. When the sample was suspended in the
middle of the mixed solution, the density of the sample was
identical to that of the solution. The solution density was
determined by the composition when the sample was suspended.
For 1D WAXD powder measurements, around 30 mg of
material was melted on an aluminium plate to form a film with
thickness of ꢁ1 mm. The phase transitions and the correspond-
ing structural transformations were monitored by 1D WAXD
experiments in the following three thermal processes according
to the transition temperatures observed in DSC thermal
diagrams. First, the sample was heated to the isotropic (I) melt at
110 ꢀC to remove previous thermal histories, and then, the
subsequent cooling and heating thermal diagrams were recorded
at a scan rate of 10 ꢀC minꢃ1. Second, the sample was heated to
the I melt and cooled to room temperature. It was reheated to
87 ꢀC and annealed there for 2 hours. The sample was then
brought back to room temperature where the 1D WAXD scan
was recorded. Finallyꢀ, the sample was heated to the I melt and
directly cooled to 90 C and annealed there for 1 day. The 1D
WAXD scan was recorded after the sample returned to room
temperature. The samples prepared by the first two thermal
Equipment and experiments
Various characterization techniques were utilized to identify the
chemical structure of the synthesized sample. 1H NMR spectrum
analysis was performed on a Varian Gemini 300 NMR at
300 MHz; the spectrum was referenced to the residual proton
impurities in CDCl3 at d 7.27 ppm. 13C NMR spectrum analysis
was performed on a Varian Gemini 300 NMR at 75 MHz; the
spectrum was referenced to 13CDCl3 at d 77.00 ppm.
Bruker Ultraflex III TOF equipped with a Nd:YAG laser emit-
ting at 355 nm (Bruker Daltonics, Inc., Billerica, MA) was used to
carry out the matrix-assisted laser desorption/ionization-time of
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flight (MALDI-TOF) measurement. The positive reflector mode
was adopted when recording all spectra. Instrument calibration
was performed with external standards, poly(methyl methacrylate)
and polystyrene, prior to every measurement. Data analysis was
performed on the flexAnalysis software package. FT-IR (Digilab
Win-IR Pro FTS 3000) equipped with a Bruker heating stage was
used to study the hydrogen bonding and molecular chain confor-
mations (results are reported in ESI†).
The d-spacings were calibrated by using evaporated thallous
chloride standard, which has the largest first-order spacing
diffraction of 0.384 nm.
Morphological optical textures of ordered phases at the
micrometre scale were observed by a polarized optical micro-
scope (Olympus BH-2) coupled with a Mettler hot stage (FP-90).
Polarizer, analyzer, and tint plate were inserted in the light path
to study the molecular orientation within the supramolecular
structures (results are reported in ESI†).
Differential scanning calorimetry (DSC) experiments were
carried out on a Perkin-Elmer PYRIS Diamond differential
scanning calorimeter with an Intracooler 2P apparatus. Cali-
bration was performed by using standard materials on both
temperature and heat flow scales at various heating and cooling
rates (2.5–40 ꢀC minꢃ1). Transition temperatures were deter-
mined according to the onset temperatures which were defined as
the cross-point of the peak slope and the baseline in the DSC
thermal diagrams. The onset temperature was determined on the
high-temperature side during cooling and on the low-tempera-
ture side during heating. Thermogravimetric measurements were
conducted prior to the DSC experiments on a TA TGA Q500
Molecular packing modeling and crystallographic simulation
was performed using the Accelrys Cerius2 simulation package
(version 4.6) with COMPASS force field. The global equilibrium
conformation with lowest energy at 0 K was chosen as the initial
molecular conformation. Unit cell parameters determined via 1D
WAXD, 2D WAXD, SAXS, and SAED patterns were adopted
as initial values for constructing the simulated crystal unit cell.
Results and discussion
Synthesis of C22PhBAEO3
instrument at
atmosphere.
a
10 ꢀC minꢃ1 scan rate under nitrogen
Compound C22PhBAEO3 (1) was obtained by first reacting
3,4,5-tris-docosoxybenzoic acid with excess 1,4-phenylenedi-
amine using Steglich esterification to give a single substituted
primary amine derivative (C22NH2, 2). The amine derivative
further underwent esterification reaction with 3,4,5-tris(2-(2-(2-
hydroxyethoxy)ethoxy)-ethoxy)benzoic acid to give the desired
product. The molecular purity and integrity were confirmed by
1H NMR, 13C NMR, MALDI-TOF mass spectrometry, and IR
spectroscopy (see Fig. S2–S4 in ESI†). The compound possessed
a light yellow color with good thermal stability. It revealed 1%
weight loss at 277 ꢀC during thermogravimetric measurement
under N2 atmosphere (Fig. S5 in ESI†).
1D WAXD measurements were performed on a Rigaku
Multiflex 2 kW Automated Diffractometer equipped with CuKa
radiation (l ¼ 0.1542 nm). A hot stage calibrated with a devia-
tion within ꢂ0.5 ꢀC is coupled with the diffractometer for
structure phase transition studies as a function of temperature.
Sample scanning rate was fixed at 1ꢀ minꢃ1 within the 2q range of
1.8–40ꢀ. The sample chamber was evacuated before recording the
spectrum. 2D-WAXD fiber experiments were carried out on
a Rigaku 18 kW rotating anode X-ray generator attached to an
R-AXIS-IV image plate system. The mechanically sheared
samples were exposed for 40 to 60 minutes to ensure the high
quality diffraction patterns. The X-ray beams were mono-
chromatized by graphite crystals. The observed 2q diffraction
positions above 15ꢀ were calibrated by silicon crystals with
known periodic 2q diffractions, and silver behenate was used for
calibrating diffraction positions below 15ꢀ. The deviation of the
measured diffraction angle was ꢂ0.05ꢀ. The crystal unit cell
determination was achieved by construction of the reciprocal
lattice. Solutions with the least error between calculated values
and experimental results were calculated and compared with
computer refinements.53,54
At the bottom of Scheme 1, the geometric shape and dimen-
sion, which were constructed by Cerius2 4.6 software, of each
part of the asymmetric tapered C22PhBAEO3 are shown in the
extended conformation. The molecule contains three parts:
a core with three phenylenes connected by two amide bonds
which can form hydrogen bonding with neighboring molecules
during the phase formation, three hydrophobic alkyl chain tails
each having 22 carbon atoms located on one side of the molecule,
and three hydrophilic methyl terminated triethylene glycol tails
(EO chain with three repeat units) located on the other. The
hydrophobic and hydrophilic tails on both sides of C22PhBAEO3
are immiscible yet covalently connected with the core, therefore
only micro-phase separation at a nano-scale is allowed during
supramolecular structure formation. One should note that not
only does the Janus asymmetric character manifest in the
polarity of these two tails but also the geometrical length of the
hydrophobic part is about twice that of the hydrophilic part.
SAXS experiments were performed at beamline 23A1 at the
National Synchrotron Radiation Research Center (NSRRC)
located at Hsinchu, Taiwan.55 The source energy and sample-to-
detector distance were 10 keV and 2444 mm, respectively. An
exposure time of about 20 seconds was required to ensure high
quality patterns. The SAXS intensity profiles were plotted
against q ¼ 4psin q/l, where l is the wavelength of X-ray
(l ¼ 0.124 nm) and 2q is the scattering angle. The SAXS spectra
were recorded by a 2D Mar CCD detector with 512 ꢄ 512 pixel
resolution. Silver behenate with primary reflection peak at
1.067 nmꢃ1 was used to calibrate the q value.
Phase behavior and the corresponding structure evolution
Fig. 1a shows a set of DSC thermal diagrams of C22PhBAEO3
during heating and cooling. The top two scans are the cooling
scan from the I melt to 40 ꢀC and the subsequent heating of
C22PhBAEO3. During the cooling, two thermal transition
TEM bright field images were obtained with a JEOL (1200 EX
II) TEM using an accelerating voltage of 120 kV. SAED exper-
iments were carried out by a TEM rotating–tilting stage to
provide the ED diffraction patterns and the unit cell parameters.
The camera length was set in a range between 0.8 m and 1.2 m.
ꢀ
processes are found at the onset temperatures of 85 C (with a
heat of transition of ꢃ19.2 kJ molꢃ1) and 71 ꢀC (ꢃ83.3 kJ molꢃ1).
Upon the subsequent heating, two endothermic thermal
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can be explained by the Gibbs free energy–temperature (G–T)
diagram of the phases as shown in this figure (Fig. 1b). The
diagram reveals the relative stability of each phase (the free
energy) as a function of temperature. At temperatures higher
than 104 ꢀC, the I phase is the most stable phase since it possesses
the lowest free energy. The compound should thermodynami-
cally transfer into the most stable crystal II phase when the
temperature decreases and passes the cross-point between the I
and the crystal II phases. However, the energy barrier in forming
this crystal II phase is kinetically too high, and this transition is
thus practically forbidden at temperatures above the col. phase
formation at a cooling rate of 10 ꢀC minꢃ1. The compound has to
be directly transferred to the col. phase at 85 ꢀC since the energy
formation barrier of this LC phase is low. When the temperature
ꢀ
passes through 73 C, the compound enters the crystal I phase
since it becomes more stable than the col. phase below the cross-
point between those two phases in the G–T diagram. Annealing
within the crystal I phase was not able to bring the compound
into the crystal II phase. Upon heating at a rate of 10 ꢀC minꢃ1
,
the crystal I and col. phases melt at 73 ꢀC and 92 ꢀC, respectively.
However, if the compound was annealed within the col. phase at
ꢀ
around 87 C, the most stable crystal II phase forms. Further-
more, after the compound was quenched from the I phase
directly to 87 ꢀC without the formation of the col. phase, the
crystal II phase did not form even within a prolonged annealing
time. In other words, the col. phase has to be the precursor in
forming the stable crystal II phase.
DSC experiments in Fig. 1a have provided the heat transfer
information during the phase transitions. To gain insights about
the phase structures and their evolution associated with the
thermal transition processes, a series of 1D WAXD patterns have
been acquired as shown in Fig. 2. Fig. 2a shows a set of WAXD
patterns obtained during cooling starting at 110 ꢀC. Fig. 2c shows
the subsequent heating patterns. In Fig. 2a, the I phase is
confirmed at 110 ꢀC by the observation of two amorphous halos
located at 2q of ꢁ6.5ꢀ and 19.5ꢀ, respectively. The high-angle halo
represents the short-range order of the average distance among
the amorphous chains; while, the low-angle halo may indicate the
average periodicity of electron density fluctuations between the
micro-phase-separated aromatic core and tails and the two types
of tails. In the WAXD pattern recorded at 80 ꢀC, the high-angle
halo remained and a small peak with d-spacing of 0.49 nm (2q ¼
18.1ꢀ) appeared that corresponds to the repeat distance between
H-bonded cores. This confirms the formation of the LC columnar
structure. Also, two diffraction peaks appeared in the low-angle
region with d-spacings of 3.22 nm and 2.19 nm, respectively. The
q-value ratio of 2 : 3 was identified, indicating that a layered
structure has formed (see also the SAXS result in Fig. 4). The
formation of H-bonded core columns induces the phase separa-
tion of the amphiphilic tails. The phase separation of hydrophilic
and hydrophobic tails forces the columns to have parallel align-
ment in a smectic-like fashion within the tail interface. The layer
normal direction is perpendicular to the column direction (see also
Fig. 7 and 9 for simulated molecular packing results). Therefore,
the lamellar diffractions are attributed to the periodic arrange-
ment of the core columns. When the temperature decreases to
30 ꢀC, as shown in the bottom 1D WAXD pattern of Fig. 2a (the
crystal I phase, the magnified pattern is shown in Fig. 2b, and see
also, 2D WAXD in Fig. 3b and c), two new reflection peaks
Fig. 1 DSC thermal diagrams and a Gibbs free energy–temperature
(G–T) diagram of C22PhBAEO3 at a scan rate of 10 ꢀC minꢃ1. (a) The top
two thermal scans: a cooling diagram from the I phase and subsequent
heating. The bottom thermal diagram: a heating diagram after annealing
at 87 ꢀC for 2 hours. (b) Schematic illustration of the free energies with
respect to temperature at atmospheric pressure of the isotropic and three
ordered phases (the assignments of col., crystal I and crystal II phases are
by the structural analysis based on WAXD and ED experiments).
transitions can also be observed at a scan rate of 10 ꢀC minꢃ1 at
onset temperatures of 73 C (79.9 kJ molꢃ1) and 92 C (20.5 kJ
molꢃ1). Based on the heats of transitions obtained during both
cooling and heating, it can be recognized that two endothermic
transitions observed during heating correspond to the
exothermic transitions observed in the cooling, and the high-
temperature transition exhibits a slight supercooling.
ꢀ
ꢀ
ꢀ
If C22PhBAEO3 is annealed at a temperature of 87 C for 2
hours, which is in between the two endothermic transitions
observed during heating, the subsequent heating reveals the
existence of another phase with an endothermic transition
occurring at 104 ꢀC with a heat of transition of 120.8 kJ molꢃ1, as
shown in the thermal diagram at the bottom of Fig. 1a. There-
fore, there are three ordered phases that exist in this compound,
and the compound apparently exhibits a monotropic phase
behavior.56–59
Denoting these three phases as the high-temperature
(the liquid crystal (LC) columnar phase, col.), low-temperature
(the crystal I) phase as observed during cooling and heating at
10 ꢀC minꢃ1 and the annealed (the crystal II) phase found during
the isothermal experiment, these phase structural identifications
were determined via results of WAXD and ED experiments
(see below). The phase behavior observed in DSC experiments
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Fig. 2 Set of 1D WAXD patterns of C22PhBAEO3. (a) The cooling process from 110 ꢀC to RT. (b) The enlarged crystal I phase pattern obtained by
cooling from the I phase to 30 ꢀC at cooling rate 10 ꢀC minꢃ1. (c) The subsequent heating process from RT to 110 ꢀC. (d) The enlarged crystal II phase
pattern obtained by annealing the crystal I phase at 87 ꢀC for 2 hours. The top patterns in (b) and (d) are the magnified results within the area marked by
red rectangular boxes. A scan rate of 1ꢀ minꢃ1 was used.
develop in the high-angle halo with a d-spacing of 0.42 nm and
0.39 nm, respectively. These two high 2q-angle diffractions cor-
responded to the (110) and (200) inter-chain packing distance of
the alkyl tails, respectively, which is similar to that found in
ordinary polyolefin crystals.60 The diffractions in the small angle
region remain unchanged as well as the diffraction of d-spacing
0.49 nm corresponding to the repeatdistance along the H-bonding
direction (or the half-disc thickness). This indicates that the layer
structure and dimension do not change during the low-tempera-
ture transition even when the alkyl tails crystallized in the
hydrophobic layers with relatively random orientation and low
crystallinity (simulated molecular packing results can be found in
Fig. 3 2D WAXD patterns of the crystal I (b and c) and the crystal II (d, e and f) phases after shearing with incident X-ray beam along (b) and (d) the
through direction (TD), (c and e) the front direction (FD), and (f) the shear direction (SD). (a) Relative orientation of sample and X-ray beam. Solid
arrow indicates the direction of mechanical shearing force applied on the sample. The hollow arrows indicate the direction of the incident X-ray beam.
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Fig. 9). Note that very few diffractions can be identified between
2q ¼ 5ꢀ and 20ꢀ (Fig. 2b), indicating that the core supramolecular
columns in the crystal I phase possess a rather poor 3D ordered
packing.
Supramolecular structure determinations of C22PhBAEO3
A combination of the DSC and 1D WAXD results provides the
structure evolution of C22PhBAEO3. Experimental techniques of
SAXS and 2D WAXD can provide more insight about the
detailed supramolecular structures and packing symmetry. Fig. 3
shows the 2D WAXD patterns of the crystal I and crystal II
phases after mechanical shearing. For the crystal I phase, the
sample was sheared within the col. phase (at 75 ꢀC). The
geometric orientation of the incident X-ray beam with respect to
the sheared sample is shown in Fig. 3a. Fig. 3b and c are 2D
patterns of the crystal I phase obtained from the through and
front directions (TD and FD), respectively. These two patterns
are essentially identical. In the low-angle region along the
equator (<5ꢀ), two arcs of d-spacing at 3.22 nm (2q ¼ 2.74ꢀ) and
2.18 nm (2q ¼ 4.04ꢀ), with a q-value ratio of 2 : 3, can be
observed, revealing again a layered structure parallel to the
column axis that shows a periodic change of the electron density
along the lateral direction of the aligned column axis. In the high-
angle region (>5ꢀ) along the meridian direction, an arc with
d-spacing 0.49 nm appeared that corresponds to the bisamide
core-to-core distance along the hydrogen bonding direction.44–46
The (200) arc at 2q ¼ 21.2ꢀ along the equator and the (110) arc at
2q ¼ 22.8ꢀ in the quadrant are attributed to the packing of the tail
crystals and their orientation.60 This is thus a ‘‘fiber pattern’’ of
the alkyl chain crystals with the b-axis along the fiber (column)
direction. In other words, the b-axis of the tail crystals is parallel
to the shear direction and thus, the column axis. Comparison
between experimental and simulated X-ray fiber patterns of alkyl
chain crystals is located in Fig. S10 in ESI†.
Fig. 2c shows a set of 1D WAXD patterns recorded in
a subsequent heating. At 82 ꢀC, the alkyl tail crystals melt and the
system undergoes a transition from a crystal structure to
a ‘‘rotator phase’’. Note that this melting temperature is much
higher than that of pure C22H46 (42 ꢀC). This observation is
similar to that made with the symmetric tapered bisamides
reported before.44–46 In C22PhBAEO3, the peaks of the (200) and
(110) diffractionsꢀ gradually merged together and completely
overlapped at 82 C, revealing a transition from the crystal to
a pseudo-hexagonal rotator phase. This rotator phase has been
described such that the alkyl tails rotate along the chain axis
locally in the unit cell, in which the all trans-conformation is still
kept yet the bond orientation order is lost.61–63 This rotator phase
generally appears in chains where the number of carbons is
between 11 and 40 in n-alkenes.61–63 It is observed that in Fig. 2c,
the tails’ re-organization process towards the rotator phase takes
place immediately after the melting of the alkyl tail crystals (a re-
construction exothermic peak was observed immediately after
the melting peak of the crystal I phase, as shown in Fig. S6 of
ESI†). During the process of temperature equilibrium and
angular X-ray scanning (1ꢀ minꢃ1), the more stable crystal II
phase starts to develop as evidenced by the observation of the
growing crystalline diffractions in the vicinity of 2q ꢁ22ꢀ that
start out the top of the amorphous halo in the patterns obtained
above 75 ꢀC. In the WAXD pattern collected at 87 ꢀC, one strong
low-angle diffraction peak appears at d-spacing of 2.38 nm (2q ¼
3.71ꢀ) and a set of weaker diffractions corresponding to the
ordered inter-column packing spacings develops between 2q ¼
5ꢀand 20ꢀ (a 5ꢄ magnified WAXD pattern is included in Fig. 2d).
It is evident that the packing scheme of the columns has been
changed as compared with the crystal I, and the long-range order
of this crystal II structure has been improved within the
isothermal annealing time period of the WAXD data collection
at 87 ꢀC. A strong diffraction observed at 2q ¼ 20.8ꢀ results from
the crystalline rotator phase of the alkyl tails.61–63 It is actually
a merge of the (110) and (200) diffractions of the alkyl chain that
gradually form a pseudo-hexagonal packing. The thermal
expansion coefficient of the (200) plane is larger than that of the
(110) plane, and as the (200) spacing increases, the two diffrac-
tions gradually merge to become one during heating (the
evidence can be found in a series of WAXD patterns obtained
during heating in Fig. S9 of ESI†).
Fig. 3d–f are 2D WAXD patterns of the crystal II phase
obtained along the TD, FD, and SD, respectively, for the
sample obtained by heating the crystal I phase to 87 ꢀC and
annealed there for 2 hours. Two patterns shown in Fig. 3d and e
are identical with an arc of a d-spacing of 0.49 nm along the
meridian direction. This again suggests that the columnar
structure remained unchanged along the column axis within the
crystal II phase and that this column axis is parallel to the shear
direction. However, the lateral packing of the columns becomes
different. Compared with the 2D WAXD patterns of crystal I,
the two strong arcs appearing in the small 2q angle region now
are at d-spacings of 4.79 nm (2q ¼ 1.84ꢀ) and 2.38 nm (2q ¼
3.71ꢀ), respectively. In addition, many relatively weak diffrac-
tion arcs can be observed between 2q ¼ 5ꢀ and 20ꢀ on the
equator and quadrants, revealing a better 3D packing of the
bisamide core columns in the crystal II phase. The arcs corre-
sponding to the alkyl tail crystals remained at the same posi-
tions as in the case of crystal I since this 2D WAXD pattern was
taken at room temperature. These diffractions are stronger and
sharper due to a higher crystallinity and better ordered crystals.
The tails preserved the original orientation after the tail rear-
rangement in crystal II. It is evident that the layer arrangement
of smectic-like columnar structure of the bisamide cores in the
crystal I and col. phases has changed into a long-range ordered
supramolecular crystalline structure according to the spacing
shift in the small-angle region and the appearance of the inter-
column order diffractions on the equator. Moreover, the layer
structure from the phase separation of tails must remain in the
crystal II phase.
Fig. 2d shows the 1D WAXD pattern of the crystal II phase
collected at room temperature. The two separated diffractions
between 2q ¼ 21ꢀ and 24ꢀ are attributed to the alkyl tail ordered
packing that returns to crystals from the rotator phase during
cooling. The (hkl) plane index assignments of the stable crystal II
phase in Fig. 2d are obtained based on the structure determi-
nation via combined 2D WAXD, SAXS, and SAED experiments
(see below). Furthermore, this crystal II phase cannot be ach-
ieved by directly quenching the sample from the I phase to 87 ꢀC
even if annealed there for one day as shown in supporting DSC
and XRD results located at Fig. S7 and S8 in ESI†, respectively.
The result again indicated that the col. phase has to be the
precursor of crystal II phase.
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The ring pattern obtained along the shear direction in Fig. 3f
provides additional evidence to prove that Fig. 3d and 3e are
fiber patterns of the crystal II phase. The tail packing orientation
is again aligned along the b-axis of the alkyl chain unit cell and
the SD at room temperature. In the core packing, the column
axis (the c-axis) is parallel to the SD (along the meridian), while
the a- and b-axes are rotating within the equator plane. Based on
these patterns, the a and b angles of the unit cell can thus be
determined to be 90.0ꢀ, and the dimension of c-axis can be
measured as 0.98 nm. According to the information collected
thus far, the crystal structure lattice of the supramolecular core
packing in the crystal II phase could be either orthorhombic or
monoclinic.
actually an overlap of the (200)/(110) diffractions with d-spacings
of 2.39 nm and 2.38 nm. With this piece of information in hand
and combined with all other diffractions observed in the WAXD
patterns, a 2D a*b* lattice can be constructed to fit with all the
diffractions on the equator of the 2D WAXD pattern. Careful
structural analysis via the refinement of the reciprocal lattice
reveals a monoclinic unit cell with dimensions of a ¼ 5.08 nm,
b ¼ 2.41 nm, c ¼ 0.98 nm, a ¼ b ¼ 90.0ꢀ, and g ¼ 70.5ꢀ, which fits
the best to the experimental diffraction results.
Crystallographic density calculations based on the molecular
weight of C22PhBAEO3 (1776.66 g molꢃ1) and the unit cell
volume (11.31 nm3) give rise to a theoretical density of 1.04 g
cmꢃ3 with four molecules per unit cell. The theoretical value
SAXS experiments were conducted to confirm the actual phase
structures, and the results are shown in Fig. 4. In this figure, the
bottom pattern is obtained from the crystal I phase. The layered
structure is confirmed due to the observation of diffraction peaks
with d-spacings of 6.41, 3.22, and 2.18 nm which possess q value
ratios of 1 : 2 : 3. This layer structure, with the layer normal
perpendicular to the columnar direction, is formed due to both
the phase separation of the hydrophilic and hydrophobic tails
and the formation of core columns at the interface of the two
kinds of tails. The phase separation behaviour of the tails gives
rise to the individual supramolecular column to exhibit a Janus
character. Furthermore, it also explains why the col. phase plays
an important role as a precursor in the formation of the stable
crystal II phase since the molecular lateral packing and the
crystallinity evolution have to be based on the phase-separated
layers which form after the formation of supramolecular
columns.
matches well with the experimentally measured one, 1.03 g cmꢃ3
.
Fig. 5a shows the 2D WAXD pattern of the crystal II phase with
the mechanical shearing direction marked as SD. The diffrac-
tions on the meridian are indexed according to the monoclinic
unit cell determined above as shown in Fig. 5b (see also in
Fig. 2d). Those on the equator are also indexed as shown in
Fig. 5c. Table 1 is the list of experimentally observed and
calculated d-spacings and 2q values according to the unit cell
parameters.
A bright field TEM image of the crystal I phase is shown in
Fig. 6a. Concentric circular bands were observed parallel to the
tangential circular boundary. The PE-SLD result shown in
Fig. 6b shows the existence of columns that are constructed by
the H-bonded aromatic cores wrapped with the amphiphilic
flexible tail shell. The PE fragments are deposited between the
The top pattern in Fig. 4 is the SAXS data obtained from the
crystal II phase at room temperature. Four diffraction peaks
with 2q ¼ 1.84ꢀ, 3.71ꢀ, 3.89ꢀ, and 4.38ꢀcan be identified from the
pattern. As discussed above, the diffractions in this pattern are
dominated by the packing of the cores. Based on the triangula-
tion 2D lattice building method,53,54 the four diffractions were
assigned as the (100), (200)/(110), (010), and (210) diffractions,
respectively. Note that the second diffraction is an overlap of
the (200) and (110) diffractions with d-spacings of 2.39 nm and
2.38 nm. The high resolution SAXS results reveal that the strong
peak observed at 2q ¼ 3.71ꢀ in both 1D and 2D WAXD is
Fig. 4 SAXS patterns of the crystal I and crystal II phases. Top pattern:
the crystal II phase obtained by annealing the crystal I phase at 87 ꢀC for
2 hours. Bottom pattern: the crystal I phase obtained by cooling from the
Fig. 5 2D WAXD pattern of the mechanically sheared crystal II phase
(a). The assignment of meridian diffractions (b) and equatorial diffrac-
tions (c) between 2q ¼ 4.7ꢀ and 21.5ꢀ based on the 2D a*b* lattice.
I phase to RT at a cooling rate of 10 ꢀC minꢃ1
.
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Table 1 Crystallographic parameters of C22 PhBAEO3 bisamide crystal
II phase monoclinic crystals
that the morphological difference is attributed to the crystallinity
difference of the alkyl tails in these two phases. In the crystal I
phase, the alkyl chain tails possess low crystallinity and less
molecular orientation and thus, have a lower modulus. This may
lead to the bent, circular morphology. On other hand, the alkyl
tails in crystal II phase possess higher positional and molecular
orientational order even when they are in the rotator phase (see
XRD and ED results in Fig. 2, 3 and 8). A higher modulus and
the straight lath-like morphology are thus observed in the crystal
II phase. The alkyl tails in the crystal II phase crystallized toward
extended chain and interdigitated conformations (simulation
results locate in Fig. 8 and 9); therefore, the boundaries between
columns, which will direct the PE rods orientation, become
blurred and lead to a different and less orientationally ordered
PE-SLD pattern compared with crystal I (Fig. 6).
2q/ꢀ
d-spacing/nm
No.
(hkl)
Exptla
Calc.b
Exptla
Calc.b
1
2
3
4
5
6
7
8
(100)
1.84
3.71
3.89
4.36
5.52
6.23
7.35
7.74
1.84
4.79
2.38
2.27
2.03
1.60
1.42
1.20
1.14
1.03
0.98
0.95
0.80
0.75
0.72
0.59
0.56
0.51
0.49
0.48
0.47
0.45
0.45
0.43
0.42
0.38
4.79
(110)(200)
(010)
(210)
(300)
(210)
(400)(120)
(020)
(120)
(001)
(500)
(600)(130)
(030)
(130)
(140)
(040)
(240)
(002)
(112)
(150)
3.71, +3.69
3.89
4.38
5.54
6.19
7.38, +7.38
7.79
8.58
9.02
9.23
11.08, +11.21
11.69
12.43
15.09
15.61
17.21
18.10
18.49
19.00
19.55
19.59
20.25
21.31
23.51
2.38, +2.39
2.27
2.02
1.60
1.43
1.20, +1.20
1.13
1.03
0.98
0.96
0.80, +0.79
0.76
0.71
0.59
0.57
0.51
0.49
0.48
0.47
9
8.58
9.02
9.27
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
11.09
11.74
12.33
15.05
15.69
17.26
18.10
18.42
18.89
19.53
19.62
20.35
21.22
23.65
Molecular packing of the stable crystal II phase
According to the experimental results obtained, we were able to
construct a packing model of the crystal II phase through
(050)
(222)
(150)
(332)
(060)
0.45
0.45
0.44
0.42
0.38
a
Experimental values observed in both WAXD and TEM. b Calculated
based on the monoclinic unit cell of a ¼ 5.08 nm, b ¼ 2.41 nm, c ¼ 0.98
nm, and a ¼ b ¼ 90ꢀ, g ¼ 70.5ꢀ.
edges of two columns and then, crystallized to form rod-like
edge-on crystals.49,50 In other words, the PE chains are parallel to
the column axis, and the long axis of the PE rod-like crystals is
perpendicular to it (the c-axis of the PE crystals is parallel to the
column direction). According to the PE crystal orientation in the
image, the column axis is judged to be perpendicular to the radius
direction of the circular bands (parallel to the tangential direc-
tion of the circular bands). Fig. 6c shows an image of thꢀe crystal
II phase obtained after annealing the same grid at 87 C for 2
hours. The images show a straight lath-like morphology. The
texture difference between the two phases can be readily identi-
fied, indicating the characteristics of different structures between
these two phases. Single crystal SAED patterns were obtained
from a single crystal II domain (for example, the circled area in
Fig. 6c), and the relative orientation between columns and alkyl
tails was deduced based on the ED pattern (see discussion
below). Fig. 6d shows the PE-SLD result of the crystal II phase.
The ordered PE rods reveal that the column orientation in the
crystal II phase is along the long axis of the lath-like crystal. The
difference of this decoration pattern compared with that of the
crystal I is clearly seen, indicating different supramolecular
column arrangement near the crystal surface. Mixed crystal I and
crystal II phases during the transition can also be observed as
shown in Fig. 6e. The crystal II single crystals are extruded out
from the circular crystal I morphology. Fig. 6f is the PE-SLD
pattern on the mixture of the two phases. The regions of the
crystal I and crystal II can be distinguished by the difference in
decoration patterns as well as the phase textures. We speculate
Fig. 6 TEM bright field images of (a) the crystal I phase, (b) the crystal I
phase after PE-SLD, (c) the crystal II phase obtained after annealing at
87 ꢀC, (d) PE-SLD on crystal II phase, (e) mixture of the crystal I and
crystal II phases during phase transition, (f) PE-SLD on a mixture of two
phases.
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molecular packing simulation and energy minimization. This
model involves hierarchical structures at different length scales.
The first level of the structure is a 2D phase separated disc
constructed as shown in Fig. 7a (top view). Fig. 7b is a side view.
In these two figures, the hydrophobic tails are in blue and
hydrophilic tails are in red for clarity. It can be seen that the disc
is separated into four sectors. The alkyl tails and TEG tails both
occupy two sectors in diagonal locations. The step-by-step disc
formation mechanism is proposed in Fig. S11 in the ESI†. This
arrangement will mediate the unbalanced stress within the disc
between the two tails of different sizes. Each disc contains four
C22PhBAEO3 Janus molecules, separated into two pairs.
According to the global energy minimization result, two
aromatic cores in the top pair adopt a face-to-face conformation
at a distance of 0.7–1.0 nm apart from each other. The other pair
of two cores located at the bottom adopts the same conformation
but rotates by 90ꢀ. The center-to-center distance between the two
pairs (the half-height of one disc) is about 0.49 nm as observed in
WAXD experiments, and they are connected by four hydrogen
bonds located at the aromatic core cross-points shown in Fig. 7a.
This structure can be stabilized due to the fact that the amide
cores are self-assembled through intermolecular hydrogen
bonding between neighboring C]O and N–H groups with a
distance of 0.28 nm.46,47 A molecular disc packing model
containing both cores and tails is constructed by Cerius2 4.6
software using the COMPASS force field. The schematic illus-
tration is shown in Fig. 7c. Due to the size difference of the tails,
the disc possesses an ellipsoid shape with the hydrogen bonded
amide core represented as a solid green circle.
The second level of the structure is the asymmetric ellipsoid
cylinder, attributed to stacking of the discs. Instead of p–p
stacking between discs in traditional columnar LCs, all discs are
connected by the intermolecular hydrogen bonding to build up
the bisamide supramolecular structure. Illustrations of the 1D
ellipsoid columns and the perspective view of the supramolecular
structure of the crystal II phase can be found in Fig. 7c. The
amphiphilic ellipsoid discs stack together to form phase-sepa-
rated columns with the hydrogen bonding direction parallel to
the column axis.
The third level of structure in the crystal II structure is
constructed via the packing of these columns. Flexible hydro-
philic tails that remain amorphous in the crystal II phase are
represented as a red rectangle located at the top and bottom sides
of the core. Crystalline hydrophobic tails which possess a zigzag
extended chain conformation in the crystal II phase at room
temperature are represented as a blue semi-ellipsoid located on
the other two sides of the core. A monoclinic unit cell with
Fig. 8 (a) SAED pattern of the circled area of the bright-field TEM
image in Fig. 6c. (b) SAED pattern obtained after clockwise tilting 28ꢀ
along the c-axis of the pattern in (a). (c) Schematic illustration of the
molecular arrangement of the supramolecular structure packing in the
monoclinic unit cell. Relative orientation between columns and tails in
the hierarchical structure is illustrated. (d) Schematic representation
of the 21 helix packing of the molecular pairs. Computer simulated
molecular packing of C22PhBAEO3 in the crystal lattice on (e) ab-plane
when projected from the c-axis and (f) ac-plane when projected from the
b*-axis.
Fig. 7 Top (a) and side (b) views of the Janus disc packing model.
Hydrophilic tails are represented as red balls, and hydrophobic tails are
represented as blue balls. (c) Computer constructed molecular packing
model inside a disc and illustrations of structure migration from 2D
asymmetric disc to 3D supramolecular structure. (d) Schematic unit cell
model with four molecules (one disc) for the 3D crystal II phase of
C
22PhBAEO3.
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alternating hydrophobic and hydrophilic layers can thus be
constructed, and the layer normal is perpendicular to the b-axis.
The alkyl chain tails are partially ‘‘confined’’ and crystallized
within the hydrophobic layers and therefore, a hierarchical
structure is generated. The phase separation of hydrophilic–
hydrophobic tails is induced during the formation of H-bonded
aromatic columns. The supramolecular core columns are thus
embedded within the amphiphilic tail interface during the
formation of third level hierarchical structure.
crystals is also parallel to the c-axis of the supramolecular crys-
tals formed by the columns. The [100] zone pattern of the
supramolecular crystals can be obtained by rotating the crystal
along its c-axis. Fig. 8b shows a pattern obtained by 28ꢀ clock-
wise rotation along the column c-axis. The diffraction spots
belonging to the (010), (020), (030), (040), (050), and (060) on the
b*-axis of the supramolecular crystal were identified. This
confirms the unit cell determination and d-spacing calculations as
discussed in the above sections. The streaks appearing in the
diffraction patterns in Fig. 8b (circled by dashed ellipsoids) may
come from packing imperfections introduced by the unbalanced
stress.64,65 Another possibility may be reminiscent of a helical
pattern found in a 21 helix.66 This feature could result from the
alternate rotation character of the molecular pairs as shown in
Fig. 7 and 8d. An illustration of the ab-plane combined with
column packing and tail packing is shown in Fig. 8c. Note that
the alkyl tails crystallized within the hydrophobic layers with the
chain axis (c-axis) perpendicular to the column axis and parallel
to the (110) plane of the supramolecular crystals according to the
SAED patterns. The computer simulated model constructed by
Accelrys Cerius2 package using the COMPASS force field is
shown in Fig. 8e and f. The model represents the most probable
molecular packing of C22PhBAEO3 in the monoclinic unit cell
and agrees well with the hierarchical structure determined above.
The phase morphologies of the crystal I and crystal II phases
observed under POM with and without the tint plate, which
provided further evidence for the hierarchical structure deter-
mination discussed, can be found in Fig. S12 in the ESI†.
Based on their structural characteristics, we can summarize
these three ordered phase structures as shown in Fig. 9. In the
columnar phase (Fig. 9a), the aromatic columns have smectic-like
arrangement within an amorphous yet phase-separated matrix
consisting of flexible tails. The columns are periodically arranged
within the tail interface to give a layer structure, but the lateral
order of columns is absent. In the crystal I phase (Fig. 9b), the
spacing of the layers remains unchanged but a portion of the
alkyl tails crystallized within the hydrophobic layer (the circled
area) with less ordered chain orientation. In the crystal II phase
(Fig. 9c), the aromatic columns pack into a 3D long-range
ordered monoclinic unit cell in which the alkyl tails are inter-
digitated and possess high crystallinity with a generally fixed
chain orientation.
Through the crystal structure analysis above, the cell param-
eters of a ¼ 5.08 nm, b ¼ 2.41 nm, c ¼ 0.98 nm, a ¼ b ¼ 90.0ꢀ,
and g ¼ 70.5ꢀ were determined. A schematic of molecular
packing in the unit cell of the supramolecular structure was built
and shown in Fig. 7d. Four molecules are accommodated in one
unit cell based on the density measurement. In addition,
each ellipsoid disc also contains four molecules stabilized by
H-bonding as shown in Fig. 7a. Therefore, the crystal must
possess a primitive lattice that has eight discs at the eight corners.
This leads to one disc per unit cell. Based on the cell parameters
determined, the crystal system is primitive monoclinic with space
group assigned as P2/m (no. 10). The c-axis is the H-bonding
direction and the stacking of discs along c-axis produced the
supramolecular columns.
The relative orientation between the packing of alkyl tails and
the aromatic core columns was derived from the SAED experi-
ments on a single domain of the crystal II phase as shown in
Fig. 8a. The pattern was obtained from the circled area of the
bright field image in Fig. 6c. Two sets of overlapped SAED
patterns can be identified. The weaker diffractions are attributed
to the packing of aromatic columns and the six strong diffrac-
tions are attributed to the tail packing. In the column packing,
a pair of spots with the smallest q value closest to the main beam
has a d-spacing of 2.38 nm and agrees with the (110) diffraction
at 2q ¼ 3.7ꢀ on the equator of the SAXS and 2D WAXD results.
Another two pairs of weaker diffractions with d-spacings of 0.48
nm and 0.45 nm belong to the (112) and (222) diffractions,
respectively. This indicates that the c-axis direction is parallel to
the substrate as shown on the pattern, and the zone for this
ꢀ
pattern is determined to be [110] for the column packing. Six
bright spots in the pattern belong to the diffractions of the alkyl
tail packing. They can be assigned as the (110) and (200)
diffractions as labeled which indicates that this is the [001]
pattern of the tail packing.
It is interesting that the transformation and amplification of
Janus feature is similar to the concept of chirality transformation
and amplification.67–69 In this case, its Janus feature transfers and
amplifies from the molecular level to the disc motif, inducing the
formation of an ellipsoid disc. This transformation further takes
According to the analysis, the a-axis of the crystals formed by
the alkyl tails is perpendicular to the (110) plane of the column
packing, while its b- and c-axes are parallel to it. Consistent with
the 2D-WAXD results (Fig. 3d and e), the b-axis of the alkyl
Fig. 9 Simulated molecular packing models for (a) columnar phase, (b) crystal I phase, and (c) crystal II phase.
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Soft Matter, 2012, 8, 4767–4779 | 4777

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place to form the asymmetric column. After multiple columns
self-assemble into the supramolecular structure, although the
phase-separation character remains, the transformation and
amplification of this Janus feature stop as a typical principle of
transformation and amplification of the Janus effect via hierar-
chal structure.70
A new crystal II phase has been discovered in this Janus molecule
system which was not observed previously in symmetric tapered
bisamides even when the length of alkyl tails was changed
significantly. We speculate that the reason for structural differ-
ence is symmetry breaking of the supramolecular building blocks
(discs and columns). Although the aromatic cores possess similar
21 helix arrangement, the resulting supramolecular columns of
symmetric-tapered bisamides preserve a circular cylindrical
shape that is considered as CN symmetry. The phase separation
behavior and size difference of the tails in asymmetric-tapered
bisamides reduced the column’s symmetry from CN to C2 along
the column axis. The results provided a new pathway of fine-
tuning not only the Janus supramolecular structures but also the
design of forming the self-assembling process in order to meet the
specific requirements for optical and biological applications.
Conclusions
In summary, an asymmetric-tapered Janus bisamide,
C22PhBAEO3, has been designed and synthesized. The evolution
of its hierarchical supramolecular structure through levels of
different length scales and thermotropic phase behavior have
been fully characterized. The molecule consists of a rigid amide
core and three flexible tails with different lengths and chemical
properties on either side. It has been found that the molecules
formed three levels of hierarchical structures. The first level is
a phase-separated disc consisting of four Janus bisamide mole-
cules connected by H-bonding. The second level is an asymmetric
supramolecular column constructed by the stacking of discs. The
column has a ridged aromatic core and flexible amphiphilic
shells. The third level is a 3D ordered assembly of columns while
still keeping the phase-separated character of the tails within the
structure. In addition, three phases with different orders for the
highest hierarchical structure are identified before the tempera-
ture reaches the isotropic I phase. They are assigned as the crystal
II phase, the crystal I phase, and the col. phase. These three
phases possess phase-separated columnar structures. Mono-
tropic phase behavior has been identified by thermal analysis. It
has been found that the col. phase and crystal I phase form at
85 ꢀC and 75 ꢀC during cooling, respectively. The crystal II phase
forms during annealing under the precursor col. phase and melts
Acknowledgements
This work was supported by NSF (DMR-0906898).
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