Tunable mesogens based on shape-persistent aromatic oligoamides: From lamellar, columnar, to nematic liquid crystalline phase

Article abstract of DOI:10.1021/ol301057g

Crescent aromatic oligoamides are shown to form thermotropic lamellar columnar, rectangular columnar, and discotic nematic mesophases according to structural variation, demonstrating their capability to serve as a new class of diverse mesogens of liquid c

Full text of DOI:10.1021/ol301057g

ORGANIC
LETTERS
2012
Vol. 14, No. 14
3584–3587
Tunable Mesogens Based on
Shape-Persistent Aromatic Oligoamides:
From Lamellar, Columnar, to Nematic
Liquid Crystalline Phase
Shuliang Zou,† Lutao He,† Jing Zhang,‡ Youzhou He,† Lihua Yuan,*,† Lixin Wu,*,‡
Jian Luo,† Yinghan Wang,† and Wen Feng†
College of Chemistry, Key Laboratory for Radiation Physics and Technology of
Ministry of Education, Institute of Nuclear Science and Technology, State Key
Laboratory of Polymer Materials Engineering, College of Polymer Science and
Engineering, Sichuan University, Chengdu 610064, China and College of Chemistry,
State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Changchun 130012, China
lhyuan@scu.edu.cn; wulx@jlu.edu.cn
Received April 22, 2012
ABSTRACT
Crescent aromatic oligoamides are shown to form thermotropic lamellar columnar, rectangular columnar, and discotic nematic mesophases
according to structural variation, demonstrating their capability to serve as a new class of diverse mesogens of liquid crystals.
Liquid crystals (LC) based on organic molecules with
different shapes and large or extended cores, particularly
discotic LC, have arrested increasing attention in the past
decade.1 Apart from the conventional rod-like and disk-
like shape of mesogenic molecules, a variety of novel
molecular shapes such as cone,2 bent-core,3 and ring4 have
been reported to display LC behaviors. Shape-persistent
molecules as defined by their noncollapsible framework
are intriguing in terms of their well-defined structures and
† Sichuan University.
‡ Jilin University.
(1) For selected examples, see: (a) Laschat, S.; Baro, A.; Steinke, N.;
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10.1021/ol301057g
Published on Web 06/29/2012
2012 American Chemical Society

unique capability of forming mesophase.5 To date a
number of liquid crystalline shape-persistent compounds
havebeengeneratedsuchasphenylacetylene macrocycles,6
dendrimers,7 and V-shaped3 molecules containing five-
membered heterocycles.
strong aggregation and self-assembly.9f,10a It is reasoned
that with flat stiff cores defined by the constitutional
backbones and flexible peripheral side chains appropri-
ately positioned, it would be possible for these oligoamide
molecules to form well-organized mesophases. However,
the mesomorphism for these interesting shape-persistent
compounds has remained unknown since they were first
described 10 years ago.9a
Herein we report on our findings of diverse mesognic
phases of crescent oligoamides 1À3 (Scheme 1) and tuning
of their LC mesophases via structural variation by extend-
ing the crescent molecular backbones and adjusting per-
ipheral side chains. These compounds were shown to form
thermotropic lamellar columnar (LCol for 1a), rectangular
columnar (Colr for 2c, 2d and 3a), and discotic nematic
(ND for 3b) mesophases.
Aromatic oligoamides with backbones rigidified by in-
tramolecular hydrogen bonds are attracting intense re-
search interests in recent years.8 Among them, crescent
aromatic oligoamides containing three-center hydrogen
bonds reported by Gong et al.9 represent an interesting
class of shape-persistent molecules due to their character-
istic features in folded conformations9 and internal cavities
for ion-complexation.9g In particular, short oligomers
consisting of less than six benzene residues take a flat,
crescent conformation.9a,c,g These molecules, along with
their cyclic analogues as reported recently by us and the
collaborators,10 have large aromatic surfaces favorable for
πÀπ stacking and thus demonstrated a high propensity for
Scheme 1. Molecular Structures of Aromatic Oligoamides 1À3
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The key compounds for our investigation are oligoa-
mides 1À3 with 4À6 long alkyl or triethylene glycol (TEG)
chains attached to the backbones. They were synthesized
according to the similar reported procedures.9 All compounds
were characterized with NMR and mass spectroscopy.11
Thermogravimetric analysis (TGA) of compounds 1À3
revealed no mass loss up to 270 °C, well above the
isotropization temperature,11 indicating thermal stability
of the oligoamides. However, whether the intramolecular
three-center hydrogen bonds are strong enough to endure
the high temperature condition at which they were exam-
ined for DSC or high temperature XRD experiments is still
not clear. Thus, 2b was chosen as a representative example
for variable temperature FTIR (VT FTIR) examination in
solid state. The NH stretching frequency at 3347 cmÀ1 was
found to experience almost no change upon cooling from
190 to 40 °C, and no new band pertinent to the NH amide
was observed.11 Since the change of hydrogen bonding is
directly reflected in the change of stretching frequencies of
amide NH groups,9b these data showed that intramolecular
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Cheng, S.; Furukawa, M.; Parra, R. D.; Kovalevsky, A. Y.; Mills, J. L.;
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(11) See the Supporting Information for details.
Org. Lett., Vol. 14, No. 14, 2012
3585

hydrogen bonding was maintained in the temperature range
examined, suggesting the persistence of crescent conforma-
tion at high temperature.
The thermotropic behavior of these compounds was
then studied by polarized optical microscopy (POM),
differential scanning calorimetry (DSC), and variable
temperature X-ray diffraction (VT XRD), and the results
are summarized in Table 1.
striking. This result prompted us to probe the possibility of
otheroligoamidesasmesogenicmolecules. Thus, 1b, which
shares exactly the same backbone but differs in the polarity
of side chains, was prepared and analyzed by DSC and
POM. Unexpectedly, 1b failed to indicate mesomorphic
property but formed rectangular columnar lattice,11 sug-
gesting the great impact of polarity of side chains. It is
known that polar TEG chains have more conformational
flexibility compared to alkyl chains,12 which could reduce
capacity of packing and cause loss of LC property or
forming a less ordered ND phase;13 however, other factors
may also determine the formation of mesophases.
Table 1. Phase Behaviors of Oligoamides 1, 2, and 3
Figure 1. Polarized optical micrograph of (a) LCol phase of 1a at
200 °C; (b) Colr phase of 2c at 125 °C; (c) Colr phase of 3a at
100 °C; and (d) ND phase of 3b at 115 °C.
a The transition temperatures (°C) and enthalpies (listed in parenth-
eses, kJ/mol) were determined by DSC at 10 °C/min. Cr = crystalline
phase; LCol = columnar lamellar mesophase; Colr = rectangular
columnar mesophase; g = glassy; ND = discotic nematic mesophase;
I = isotropic liquid.
Allowing for the aromatic surface that is crucial for
aggregation and intermolecular stacking,9f further exten-
sion of the backbone should provide chances for the
generation of LC properties. Thus, compound 2b, which
has one more added aromatic residue along the back-
bone than 1b, was prepared. Unexpectedly, 2b showed
nonmesomorphic property.11 The negative result made us
decide to prepare pentamer 2a, which resembles 1a to a
great extent in structures at one of the terminus and in the
number of long alkyl side chains. Surprisingly, contrary to
our expectation, 2a failed to exhibit mesogenic phases.11
We presume that increasing the surface area necessary for
stacking interactions via extending the backbone alone
does not help give rise to LC property, and it is likely that
the balance between the core dimension and the flexible
periphery is critical as observed in many other liquid
crystalline molecules.6b,14
The LC property was first noticed in tetramer 1a. It
melts at 41.6 °C and exhibits a broken fan-like texture at
200 °C upon cooling from isotropic phase (Figure 1a). The
XRD pattern at 140 °C for the oriented sample revealed
˚
five reflections at 36.9, 19.1, 12.6, 9.5, and 7.6 A in the
low angle regime with the reciprocal spacing ratio of
1:1/2:1/3:1/4:1/5 assigned as the (001), (002), (003), (004),
and (005) reflections, indicative of a layered phase with the
˚
periodicity of 36.9 A (Figure 2a). In addition, the diffuse
˚
scattering halo at 4.4 A corresponds to the mean distance
˚
between the fluid alkyl chains, and the diffraction at 3.5 A
has a value that is typical of the distance relating to the
πÀπ interactions between aromatic cores, suggesting the
formation of a highly ordered lamellar columnar meso-
phase (LCol) as shown in the model proposed (Figure 2a,
˚
inset). The d-spacing at 14.7 A in the middle-angle range
˚
Compounds 2c and 2d, which share the exact same
backbone as 2a and 2b but bear six instead of four side
chains, were designed accordingly. Indeed, both of them
exhibit LC phases. For example, when 2c is subjected to
agrees well with the value of 14.0 A as observed in the
crystal structure of similar tetramer derivatives,9c which
can be related to the width of the aromatic cores of the
molecule 1a.
The first observation of mesophase for 1a is particularly
interesting since its corresponding lower oligomers such as
trimeric and dimeric analogues never indicated any sign of
mesomorphism. Besides, the wide temperature range
(>170 °C) over which the LCol phase is stable for 1a is
(12) Cormier, R. A.; Gregg, B. A. Chem. Mater. 1998, 10, 1309.
(13) (a) Tang, B. Y.; Ge, J. J.; Zhang, A.; Calhoun, B.; Chu, P.; Wang,
H.; Shen, Z.; Harris, F. W.; Cheng, S. Z. D. Chem. Mater. 2001, 13, 78.
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3586
Org. Lett., Vol. 14, No. 14, 2012

DSC analysis, it shows two enantiotropic phase transitions
at 28.9 and 192.9 °C in the second heating circle. POM
image shows a lancet texture15 (Figure 1b), which is often
observed in rod-like mesogen, and attributed to smectic
phase. However, the XRD pattern recorded at 187 °C
less ordering nematic phase is interesting since the opti-
mized structure of 3 as indicated in molecular modeling
shows a nonplanar conformation11 compared to 2d with a
flat core. We speculate that side chain packing should
function cooperatively with the less ordered aromatic
packing in 3 due to the helical conformation to determine
the final outcome of LC properties. The TEG chains in 3b
would interfere with the packing of the core and prevent
the crescent molecules from efficient columnar packing,
which is otherwise less likely to occur in 2d having the alkyl
chains and a flat core or in 3a where the core packing is
similar to 3b, but alkyl chains dominate to assist inter-
molecular packing. Therefore, the presence of more flex-
ible TEG chains favors the less ordered organization of the
˚
shows five reflections at 32.9, 25.2, 16.9, 14.6, and 11.2 A,
which are indexed in sequence as (110, 200), (210), (220),
(320), and (600) reflections, revealing a rectangular colum-
˚
nar (Colr) phase with the cell parameter of a = 65.8 A and
˚
b = 38.0 A (Figure 2b). By substituting 2c with TEG, the
clearing point on cooling experienced a drastic reduction
from 185.7 °C in 2c to a much lower temperature of 94.2 °C
in 2d, disclosing the pronounced polarity effect of periph-
eral side chains upon the LC property. The presence of the
mobile TEG chains possibly induces a large disturbance
for the ordered packing and thus results in decreased
clearing point.12 The observation of a stable LC phase in
2c and 2d, which failed to be observed in 2a and 2b, clearly
indicates that the number of side chains is crucial to the
success of the designed LC molecules.
N
D phase in 3b.13b To our knowledge, so far few discotic
systems could exhibit ND phase in the low temperature
range.1c
In summary, a series of shape-persistent aromatic oli-
goamides including tetramers, pentamers, and hexamers
shows liquid crystalline birefringence over a wide tempera-
ture range. Both the constitutional components of crescent
oligoamides and side chains are crucial to the success of
designed molecules to show LC properties. With their
tunable crescent backbones, these compounds of various
size exhibit rich mesomorphic properties ranging from
lamellar, rectangular columnar, to discotic nematic meso-
phases, demonstrating their ability to serve as a new class
of mesogens of liquid crystals. So far no examples of these
crescent aromatic oligoamides have been found to exhibit
liquid crystalline properties. It is expected that further
structural modification and functionalization may provide
versatile LC materials based on these crescent oligoamides
with controlled change of mesophase patterns, thus en-
riching the family of LC materials with tailored new and
special properties. Detailed studies on these aspects are
currently being investigated in our laboratory and will be
reported in due course.
Figure 2. X-ray diffractograms for (a) the LCol phase of 1a at
140 °C and (b) the Colr of 2c at 178 °C, cooling from isotropic
phase at 2 °C minÀ1
.
Further extension of the backbone based on 2c and 2d
leads to hexamers 3a and 3b. Both of them showed fluidity
and birefringence on POM (Figure 1c and d). Compound
3a exhibits the schlieren texture. The XRD pattern re-
corded at 90°C shows the reflections that all point toa Colr
mesophase. It is noteworthy that 3a shows a broad melting
transition at 30.3 °C, nearly room temperature and a wide
temperature mesophase range (30.3 to 145.0 °C in the
second heating), which is particularly intriguing in terms
of applications. In the case of 3b bearing polar side chains,
the DSC profile exhibits a phase transition at 117.4 °C on
heating with ΔH = 2.9 kJ molÀ1, and the mesophase
appears at 116 °C with ΔH = À3.9 kJ molÀ1 on cooling.
The enthalpy change falls within the range for the isotropic
enthalpy of nematic phase (1À5 kJ molÀ1).1c In addition,
the VT XRD patterns persist in a wide temperature range
(30À235 °C), embracing only one broad and diffuse peaks
in wide-angle regime,11 which supports the ND phase. The
observation that3aformsColr mesophaseand3bproduces
Acknowledgment. The authors acknowledge NSFC
(21172158, 20774059), NSAF (11076018), the Doctoral
Program of the Ministry of Education of China
(20090181110047), and Open Project of State Key Labora-
tory of Supramolecular Structure and Materials
(SKLSSM201112) for funding this work, and the Analy-
tical & Testing Center of Sichuan University for NMR
analysis (Dr. Pengchi Deng). The authors also thank Prof.
Bing Gong from The State University of New York at
Buffalo for the helpful suggestions.
Supporting Information Available. General experimen-
tal methods, synthesis of compounds, NMR spectra,
TGA, VT FTIR, DSC, POM, XRD, and ab inito mole-
cular modeling of 3. This material is available free of
charge via the Internet at http://pubs.acs.org.
(15) Yin, S.; Li, W.; Wang, J.; Wu, L. J. Phys. Chem. B 2008, 112, 3983.
The authors declare no competing financial interest.
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