Triangular arylene ethynylene macrocycles: Syntheses, optical, and thermotropic liquid crystalline properties

Article abstract of DOI:10.1039/c2sm06144a

A series of triangular, shape-persistent arylene-ethynylene macrocycles (AEMs) of related structures were synthesized and studied, with a focus on their mesomorphic behavior in correlation with their chemical structure. Generally, these discotic molecules decorated with flexible side chains demonstrated a propensity to form thermotropic liquid-crystalline (LC) phases. Characterized by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD), four of the eight investigated macrocycles manifested thermodynamically stable mesophases, featuring discotic nematic or columnar structures. Longer alkyl side chains were found more conducive to mesophases, and the alkoxycarbonyl functionality was a more effective side-chain linkage at inducing and stabilizing the LC states than the alkoxy side group. The size and structure of the cyclic aromatic backbone influenced both the occurrence and type of mesophase exhibited.

Full text of DOI:10.1039/c2sm06144a

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PAPER
Triangular arylene ethynylene macrocycles: syntheses, optical, and
thermotropic liquid crystalline properties†
*
Tian Li, Kan Yue, Qifan Yan, Helin Huang, Hao Wu, Ningbo Zhu and Dahui Zhao
Received 17th June 2011, Accepted 29th November 2011
DOI: 10.1039/c2sm06144a
A series of triangular, shape-persistent arylene–ethynylene macrocycles (AEMs) of related structures
were synthesized and studied, with a focus on their mesomorphic behavior in correlation with their
chemical structure. Generally, these discotic molecules decorated with flexible side chains demonstrated
a propensity to form thermotropic liquid-crystalline (LC) phases. Characterized by differential
scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD), four
of the eight investigated macrocycles manifested thermodynamically stable mesophases, featuring
discotic nematic or columnar structures. Longer alkyl side chains were found more conducive to
mesophases, and the alkoxycarbonyl functionality was a more effective side-chain linkage at inducing
and stabilizing the LC states than the alkoxy side group. The size and structure of the cyclic aromatic
backbone influenced both the occurrence and type of mesophase exhibited.
favorable for columnar LC phase formation. The discotic
mesogens, typically p-conjugated aromatic skeletons, possess
a distinct tendency to stack face-to-face into columns, which are
phase-segregated from the side chains driven by intermolecular
forces such as aromatic stacking, van der Waals, hydrophobic
interactions, and so on. Hierarchically ordered supramolecular
architectures of various organization patterns have been
observed with such columns through self-assembly. So far, the
most intensively studied discotic core structures include
substituted benzene,6 triphenylene,7 perylene8 and coronene5d,9
derivatives, porphyrin and phthalocyanine derivatives,10 mac-
rocycles,11 etc. Systems featuring metal–ligand coordination,12
dipole–dipole,13 hydrogen bonding,14 or donor–acceptor15
interactions have also been reported.
Introduction
Conjugated organic molecules emerged as a type of versatile
functional material due to their appealing electronic and optical
properties.1,2 Recently, both theoretical and experimental inves-
tigations have revealed that, in addition to geometry and
chemical structure of the molecules, the molecular packing motif
strongly influences the charge-carrier mobility of the system.3–5
To achieve optimal device performance, the ability to form
supramolecular structures with long-range order, facilitating
energy or carrier transport, is one of the critical issues that need
to be addressed.3 The presence of liquid-crystalline (LC) phases is
considered a desirable property for organic semiconductive
molecules, especially for solution-processed systems. This is
because owning to the fluid and dynamic nature, LC materials
are capable of minimizing and self-healing structural defects
(e.g., grain boundaries) while spontaneously forming ordered
structures of micro- to macroscopic dimensions.4 Columnar
phases are considered particularly favorable for semi-
conductivity applications, as intermolecular electronic coupling
effected through molecular orbital overlapping is most efficient
in columnar architectures and one-dimensional charge-carrier
transport is thereby vastly facilitated.5
For discotic molecules, it is known that the interplay and
balance of the rigidity of the core and flexibility of the side chains
play a pivotal role in governing the LC properties. Specifically,
studies with a wide range of molecules manifesting various dis-
cotic LC phases showed that a number of structural attributes
are critical for determining the LC state characteristics,4
including the chemical structure, size, geometry, and symmetry
of the aromatic core, the length, number, and structure of side
chains, as well as the functional linkage connecting the core with
the side chains. With regard to the complexity of the influential
factors, systematic investigations are warranted for a better
understanding of the correlation between the molecular structure
and LC phases characteristics, and relevant knowledge is indis-
pensable for designing new molecules that exhibit both LC states
and optimal semiconductive properties.
Molecules of a planar, shape-persistent (i.e., discotic) central
core decorated with flexible side chains at the periphery are most
Beijing National Laboratory for Molecular Sciences, Department of
Applied Chemistry and the Key Laboratory of Polymer Chemistry and
Physics of the Ministry of Education, College of Chemistry, Peking
University, Beijing, 100871, China. E-mail: dhzhao@pku.edu.cn
† Electronic supplementary information (ESI) available: Synthetic
details, optical spectra, and more DSC and XRD characterization
results. See DOI: 10.1039/c2sm06144a
Among various discotic LC mesogens, shape-persistent mac-
rocycles represent a unique group.16–19 Syntheses and character-
izations of shape-persistent macrocycles have been conducted for
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decades.20,21 Having a unique interior void, these macrocycles
feature a nano-sized tubular cavity when assembling into
columnar arrangements. Intra-annularly functionalized nano-
tubes have been designed and exploited for a variety of purposes,
including host–guest interactions, covalent organic frameworks,
light-harvesting systems, sensors, and so on.22 Nonetheless, up
till now only limited examples of shape-persistent macrocycles
manifesting discotic LC phases have been reported. Moore and
coworkers first studied a set of phenylene–ethynylene macro-
cycles with different peripheral substitutions exhibiting columnar
Results
Chemical structures and syntheses of the AEMs
Chemical structures of all investigated AEMs in this study are
depicted in Chart 1. The previously reported AEMs 123a and 223b
had the same cyclic framework of oligo(o-phenyleneethynylene-
alt-p-phenyleneethynylene). AEMs 3 and 4 had an identical
backbone, which comprised three 4,40-biphenylene units con-
nected by three o-phenylene bis(ethynylene) groups. Whereas in
AEMs 5 and 6, the triangular scaffold was constituted by 3,6-
phenanthrylene bis(ethynylene) units in combination with
p-phenylene groups. All AEMs 1–6 had six alkoxy side chains
attached to the cyclic backbone, two on each vertex of the
triangle. Each pair of macrocycles with the same backbone
differed by the length of the side chains. Hexyloxy side chains
were attached to AEMs 1, 3, and 5, while longer dodecyloxy side
chains were tethered to AEMs 2, 4 and 6.
phases.16 Hoger’s research group demonstrated that AEMs with
€
the interior occupied with alkyl side chains gave rise to discotic
nematic or columnar phases.17 Tew introduced two types of side
chains of different polarity to ortho-phenylene ethynylene mac-
rocycles, both displayed LC phases.18 Kato et al. reported a dis-
cotic LC AEM having glutamic acid and alkyl side chains on the
exterior and oligoethylene glycol side chains in the interior.19
However, precedents of AEMs that exhibit both electronic
functions and discotic LC phases have been rarely reported.
Recently, we studied a triangle-shaped AEM that displayed an
extraordinary photoconducting capacity.23a To further investi-
gate pertinent functions and explore the applications of such
macrocycles, we subsequently designed and synthesized a series of
AEMs of related structures (Chart 1, 1–8). Here we report a study
on the correlation between the molecular structure and LC
properties by systematically varying the chemical features of the
macrocycles. Specifically, the influences of side chains, size and
structure of the aromatic skeleton, as well as the linkage joining
the side chains with the shape-persistent core were examined.
To unravel the effect of side chain linkage on the LC proper-
ties, we also designed two other triangular AEMs with carboxylic
ester-linked side chains, i.e., macrocycles 7 and 8. Both of these
macrocycles had a cyclic oligo(o-phenyleneethynylene-alt-p-
phenyleneethynylene) backbone. In AEM 7, each of the three
o-phenylene units was tethered with a hexyloxy side chain via
a carbonyl linker.24 AEM 8 adopted the same ester side-chain
linkages, but this molecule was decorated with six hexyloxy
carbonyl groups, one on each of the o- and p-phenylene units.
An overview of the synthetic routes to AEMs 1–8 is illustrated
in Scheme 1 (see the ESI for synthetic details†). Two different
cyclization strategies were employed to obtain the eight target
Chart 1 Chemical structures of studied AEMs.
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macrocycles.20 AEMs 1 to 6 were realized via route 1, in which the
final step was designed to couple two different short, open-chain
precursors into a cyclic structure under relatively dilute condi-
tions. One of the two fragments had aryliodide functionality on
both ends, and the other was terminated with two acetylene
groups. The two molecules were coupled into a macrocycle via two
consecutive Sonogashira–Hagihara reactions in one pot, i.e., an
intermolecular cross-coupling followed by an intramolecular
ring-closure reaction. The evident advantage of this cyclization
protocol was the shorter synthetic route (compared to route 2),
which compensated for the relatively low yield of the final step.
But such a double cross-coupling method was only suitable for
symmetrical macrocycles, like AEMs 1–6. Due to the lower
symmetry of AEMs 7 and 8 (C3h), a different synthetic scheme was
designed. In route 2, the reactant for the final step was a longer
oligomer containing all subunits that needed to be cyclized. The
oligomer chain ends were asymmetrically functionalized with
aryliodide and terminal ethynylene respectively, which were then
intramolecularly cross-coupled to accomplish the cyclization.
Synthetic route 2 was apparently more sophisticated and tedious
than route 1, for preparing the longer cyclization precursor.
Employment of protective/masking groups was necessary to
ensure the compatible accommodation of two complementarily
reactive functional groups. Nonetheless, the yield of the final
cyclization step was typically superior to that of route 1.
Furthermore, the sequence of repeating units in the backbone
could be precisely controlled via route 2. Upon the completion of
syntheses, the structures of AEMs 1–8 were characterized and
confirmed by 1H and 13C NMR spectra, mass spectroscopy, and
elemental analyses (see the Experimental Section).
Optical characterizations in solution
The absorption and emission spectra of AEMs 1–8 were recorded
in dichloromethane solutions (Fig. S1†) and relevant data are
summarized in Table 1. Generally, macrocycles of the same
Scheme 1 Synthetic schemes for AEMs 1–8.
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backbone structure and side-chain linkers generated very similar
spectra. The difference in side-chain length had negligible effect
on the electronic spectra. However, alteration of the backbone
structure evidently affected both the band shape and transition
energy. Specifically, from the spectra of AEMs 1–6, it was clearly
seen that variation of the side-chain length did not change the
absorption maxima but slightly changed the fluorescence prop-
erties. All macrocycles displayed two major peaks in their emis-
sion spectra. The one of shorter wavelength was hardly affected
by the side-chain length, while the one of longer wavelength
exhibited a bathochromic shift of 2–3 nm upon extension of the
side chains. When the p-phenylene units were replaced with 4,40-
biphenylene groups, the absorption band and the lower-energy
emission peak manifested noticeable hypsochromic shifts, sug-
gesting that the dihedral angle between the two phenyl rings in
the biphenylene units slightly disrupted the conjugation. On the
other hand, switching the triangle vertex from o-phenylene to
3,6-phenanthrylene group brought about a perceptible bath-
ochromic shift in the absorption maxima but a minor blue shift
of the emission band. This result indicated that the 3,6-phenan-
thrylene unit was slightly more effective at narrowing the band
gap than the o-phenylene group, while the smaller Stokes shift
was attributed to the rigidity of the phenanthrylene moiety. The
effect of side-chain linker was inferred by comparing the spectra
of AEMs 1 and 7. It was noted that changing the dialkoxy to
single alkyl carboxylate side chain resulted in a hypsochromic
shift in both absorption and emission spectra, by approximately
10 and 20 nm, respectively. However, when additional carbox-
ylate groups were attached to the p-phenylene groups on each
side of the triangle, bathochromic shifts occurred with both the
absorption and emission spectra, as illustrated by AEM 8. These
results revealed that attaching carboxylate groups to different
positions of the conjugated cyclic backbone had varied conse-
quences in electronic transitions.
enthalpy changes were determined based on the DSC measure-
ments. For certain transitions with a small enthalpy change
undetectable by DSC, the transition temperatures were deter-
mined based on the POM observations. The mesophase struc-
tures were revealed by the variable-temperature X-ray diffraction
(XRD) measurements, in combination with information
provided by POM. Samples for DSC and XRD characterizations
were obtained as precipitates from mixed solvents of dichloro-
methane and ethanol, while thin films drop-cast from dichloro-
methane solutions were used for POM characterizations. All
thermotropic phase transition results are summarized in Table 2,
and the mesophase structure data are listed in Table 3.
The properties of macrocycle 1 were first examined. As the self-
assembled structures of this molecule obtained from solution was
previously reported to exhibit optimal photoconductivity, which
was lost upon thermal annealing,23a its behavior prior to thermal
treatment was also investigated. The crystalline sample precipi-
tated from solution displayed multiple transitions during the first
heating cycle, as shown by DSC (Fig. S2†). Interestingly,
a unique monotropic mesophase was observed in this process
(see detailed characterizations in the ESI†). Nonetheless, all
thermal transitions observed in the first heating cycle were irre-
versible. In the cooling scan, only one exothermic peak was
detected at 196 ꢀC by DSC, which was confirmed by XRD to be
a transition from the isotropic liquid to a crystalline phase. This
crystal structure directly melted into the isotropic state at 210 ꢀC
(Fig. S4†).
AEM 2 had an identical cyclic backbone with that of AEM 1,
but longer dodecyloxy side chains were attached to it. During the
first heating scan, the solution-processed sample of SEM 2 also
exhibited a metastable LC phase, in a relatively wide temperature
range between 65 and 170 ꢀC (Fig. S8†). A typical spherulitic
texture was observed under POM, which identified the hexagonal
columnar structure (Fig. S9†). Nonetheless, the LC structure was
not reproduced during the cooling or second heating cycle.
Instead, an amorphous state with a glass transition was observed
(Fig. S10–S13†).
Thermotropic properties
AEM 3 displayed no LC mesophase. In DSC traces, only
a crystal-to-crystal transition was observed at 215 ꢀC during the
first heating scan, and no thermal transition was detected in
subsequent cooling and heating cycles (Fig. S14†) before the
The thermal stabilities of all studied AEMs were analyzed by
thermogravimetric analysis (TGA) under a nitrogen atmosphere,
and subsequent characterizations were carried out below their
respective decomposition temperatures. The thermal transitions
exhibited by the macrocycles were characterized by differential
scanning calorimetry (DSC) and polarized optical microscopy
(POM). Phase transition temperatures and corresponding
ꢀ
compound decomposed at 372 C.
Keeping the same oligo(o-phenyleneethynylene-alt-bipheny-
leneethynylene) backbone structure, when the side chains were
changed to longer dodecyloxy groups, AEM 4 exhibited a rect-
angular columnar LC phase (Fig. 1). The crystalline sample
precipitated from solution exhibited a melting point and entered
a LC phase at 164 ꢀC, which went isotropic at 198 ꢀC (Fig. S15†).
During the cooling cycle, the isotropic liquid was slightly super-
cooled and the LC phase appeared at 190 ꢀC. A much lower
crystallizing temperature of 42 ꢀC was observed, giving rise to
a structure different to that obtained from solution but similar to
that in the LC state (Fig. S18†). The DSC traces became
reversible after the second heating cycle. For this molecule, the
crystallization and melting processes took place with very small
enthalpy changes, although the transition was discernible from
the change of birefringent texture under POM (Fig. S19†). On
the other hand, the LC-isotropic transition was accompanied
with a significant enthalpy change, suggesting a relatively
Table 1 Optical properties of AEMs 1–8a
Absorbance lmax
(nm)
Extinction coefficient
(L molꢁ1 cmꢁ1
Emission lmax
(nm)
)
1
2
3
4
5
6
7
8
358
358
353
353
366
366
346
351
1.8
1.4
1.3
1.7
1.9
1.9
1.6
1.5
427, 453
428, 455
427, 449
426, 452
422, 446
423, 449
406, 433
413, 439
a
Spectra were recorded at room temperature in CH2Cl2 at ca. 5 ꢂ 10ꢁ6
mol Lꢁ1
.
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Table 2 Phase transition temperatures and enthalpy data
a
)
Transition T/ꢀC (enthalpy/kJ molꢁ1
AEM
1st cooling
2nd heating
1
2
4
6
7
8
Iso 196 (ꢁ18.3) / Cr
Cr 210 (20.7) / Iso
Iso 156 (ꢁ1.8) / O ꢁ16 (ꢁ2.7) / G
Iso 190 (ꢁ27.3) / Colrd 42 (ꢁ0.5) / Cr
Iso 195b / ND 162 (ꢁ34.1) / O
Iso 177 (ꢁ12.4) / Colrd 156 (ꢁ10.3) / Cr
Colho 81 (ꢁ13.8) / Cr
G ꢁ11 (2.7) / O 164 (2.7) / Iso
Cr 58 (0.4) / Colrd 197 (27.9) / Iso
O 169 (34.2) / ND 198b / Iso
Cr 177 (11.2) / Colrd 186 (7.0) /ND 191b / Iso
Cr 141 (19.7) / Colho / 335 (dec.)c
a
Data were obtained from DSC analyses at a scan rate of 5 ꢀC minꢁ1; Cr ¼ crystal phase; O ¼ amorphous state; G ¼ glass state; Colh ¼ columnar
hexagonal; Colr ¼ columnar rectangular (d ¼ disordered; o ¼ ordered); Iso ¼ isotropic; ND ¼ discotic nematic; dec. ¼ decomposition temperature.
b
c
The transition was only observed with POM and XRD. The temperature corresponds to 5% weight loss in TGA measurement at a heating rate
of 10 ꢀC minꢁ1
.
ordered mesophase but poorly organized crystalline structure.
These characteristics were consistent with the XRD patterns
collected from respective phases. Sharp peaks were detected in
the LC phase while poorly resolved diffractions emerged from
the crystalline state. The XRD profile of the LC state revealed
a series of diffractions corresponding to (200), (110), and (210) of
a rectangular columnar phase (Fig. 1d). Since no diffraction was
observed from plane (001), this mesophase was considered
disordered along the main axis of the columns. The presence of
a diffraction with the Miller index matching (h + k ¼ 2n + 1)
excluded the c2mm symmetry, so the Colr phase was identified to
be of p2gg symmetry.25 The broken fan-like texture observed
under POM in the LC temperature range also corroborated the
rectangular columnar feature (Fig. 1b and 1c).
On the other hand, the analogous AEM 6 having the same
backbone but longer side chains exhibited a discotic nematic
(ND) phase (Fig. 2). Upon heating the compound to 169 ꢀC,
a Schlieren texture characteristic of the nematic phase was
observed under POM (Fig. 2b). The recorded WAXD pattern
confirmed the nematic phase by showing only one diffuse peak in
the wide-angle regime (Fig. 2c). DSC clearly revealed the tran-
sition to the LC phase with a distinct endothermic peak, but the
transition to the isotropic state was not detectable by monitoring
the heat flow (Fig. S23 and S24†). A small enthalpy change is
typical for the transition between nematic and isotropic states,
consistent with the minimal degree of order in the nematic phaꢀse.
As the birefringent Schlieren texture disappeared at ca. 198 C
upon heating, the clearing point was identified based on the
POM observations. For this molecule, nearly identical DSC
traces were obtained from the first and second heating scans.
This observation indicated that AEM 6 formed similar molecular
structures from solution and upon thermal treatment.
For AEM 5 with a cyclic backbone composed of alternative p-
phenyleneethynylene and 3,6-phenanthryleneethnylene units
decorated with short hexyloxy side chains, no mesophase was
observed in repeated heating and cooling cycles between 0 and
250 ꢀC. Different crystal-to-crystal phase transitions were
observed by DSC during the first and second heating scans
(Fig. S20 and S21†), revealing the effect of solution-processing
on the sample.
AEM
7 was a macrocycle with an oligio(o-phenyl-
eneethynylene-alt-p-phenyleneethynylene) backbone. Instead of
dialkoxy side chains, each of the three o-phenylene units was
attached with a hexyl carboxylate group, making the molecule of
Table 3 X-Ray diffraction data
a
dexp (dcalcd) (A)
b
ꢀ
ꢀ
r/R (A)
AEM
Phase (lattice parameters)
Miller indices (hkl)
ꢀ
ꢀ
4
Colrd (a ¼ 81.6 A, b ¼ 36.1 A)
(200)
(110)
(210)
40.8
33.0
27.1 (27.0)
21/53
6
7
ND
19/51
16/32
ꢀ
ꢀ
Colrd (a ¼ 54.1 A, b ¼ 27.9 A)
(200)
(110)
(310)
(020)
(620)
(040)
27.1
24.8
15.0 (15.1)
13.9 (14.0)
7.6 (7.6)
7.0 (7.0)
31.0
ND
ꢀ
ꢀ
8
Colho (a ¼ 27.4 A, c ¼ 3.6 A)
(100)
(110)
(200)
(210)
(001)
23.7
16/32
13.7 (13.7)
11.9 (11.9)
8.9 (8.9)
3.6
a
b
Data were determined based on diffractions in XRD profiles. r represents the calculated radius of the shape-persistent cyclic framework and R
represents the calculated molecular radius of the macrocycle including side chains in the extended conformation (data were estimated based on
B3LYP/6-31G calculations, cf. Fig. S40, ESI).
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Fig. 2 (a) DSC traces of AEM 6 at a rate of 5 ꢀC minꢁ1; (b) optical
microscopic image of AEM 6 showing a Schlieren texture characteristic
of nematic phase (at 184 ꢀC under crossed polarizers, ꢂ400); (c) WAXD
profile of AEM 6 at 180 ꢀC in the cooling cycle.
and 187 ꢀC, respectively. Different LC textures were observed
below and above the latter temperature under POM. Between
176 and 187 ꢀC, a pseudo-focal conic fan-shape texture appeared,
which gradually transformed into the Schlieren texture at 187 ꢀC
(Fig. 3b and 3c). XRD confirmed the first mesophase to be
a rectangular columnar phase (Colrd) by showing multiple
diffractions of (200), (110), (310), (020), etc. (Fig. 3d). Again, no
diffraction was detected along the main axis of the columns,
suggesting disorder in that direction. The other mesophase dis-
playing the Schlieren texture was assigned to the discotic nematic
phase (ND). Only one diffuse peak in the small-angle region was
detected by XRD in the nematic phase, with a d-spacing corre-
lated to the size of the macrocycle. Again, the clearing temper-
Fig. 1 (a) DSC traces of AEM 4 at a rate of 5 ꢀC minꢁ1; (b) optical
microscopic images of AEM 4 at 188 ꢀC during the 1st heating scan and
(c) at 90 ꢀC in the cooling cycle (under crossed polarizers, ꢂ200); (d)
XRD pattern of 4 at 100 ꢀC during the cooling scan (the asterisks indicate
diffractions from the aluminum foil used for supporting the sample); (e)
a schematic representation of the rectangular columnar packing with
lattice constants indicated.
ꢀ
ature around 191 C was only detected by POM but not DSC.
Thus, the ND phase had a very narrow temperature range. In the
cooling process, the ND phase was not observed and only the
Colr phase was detected. Evidently, the narrow nematic phase
was easily masked by even slightly super-cooled isotropic state.
For AEM 7, the DSC trace of the first heating scan was mostly
reproduced in subsequent cooling and heating scans, except for
the large enthalpy change of the crystal-to-crystal transition,
which disappeared in subsequent scans (Fig. S27 and S28†).
These results indicated that, although a minor solution-pro-
cessing effect persisted with this macrocycle, the two mesophases
manifested were thermodynamically stable states.
C3h symmetry. Based on the combined characterization results of
POM, DSC, and XRD, two different mesophases were identified
with this macrocycle (Fig. 3). Upon heating a sample precipitated
from solution, a crystal-to-crystal transition was first displayed
at ca. 148 ꢀC (Fig. S26†). This transition manifested itself as
a large endothermic peak in the DSC trace. Subsequently, two
more endothermic peaks occurred at higher temperatures of 176
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XRD profile showed multiple peaks in the small-angle region in
the LC state, in accordance with diffractions of (100), (110),
(200), and (210) in a hexagonal columnar phase (Fig. 4c and
4d).26,27 Additionally, a diffraction corresponding to a d-spacing
ꢀ
of 3.6 A was identified with this macrocycle in the wide-angle
region, and it was clearly attributable to the p–p stacking
distance of the aromatic cores. Hence, of all the AEMs investi-
gated in this study, AEM 8 was the only one that exhibited an
ordered columnar phase displaying the (001) diffraction peak.
This peak was relatively sharp at lower temperatures and became
somewhat diffused as the temperature was elevated. In the
meanwhile, the diffraction peaks in the small-angle regime were
slightly shifted to longer d-spacing values (Fig. S33†). These
observations were indicative of certain extent of loosening of the
hexagonal lattice, along with reduced order in the p–p stacking
dimension at increased temperatures. A very similar hexagonal
columnar structure was observed with the crystalline state. No
signature of solution processing history was demonstrated by
this molecule, as shown by the completely reproduced DSC
traces recorded in the first and subsequent heating cycles
(Fig. S30†).
Fig. 3 (a) DSC traces of AEM 7 at a scan rate of 5 ꢀC minꢁ1; optical
microscopic images of AEM 7 showing textures characteristic of Colr and
N phases at (b) 180 ꢀC and (c) 190 ꢀC, respectively (under crossed
polarizers, ꢂ200); (d) XRD pattern of AEM 7 at 170 ꢀC in the 1st cooling
cycle.
AEM 8 possessed an identical aromatic backbone with that of
AEM 7 and was also of C3h symmetry, but it had an ester-linked
hexyl side chain tethered to each of the six phenylene rings. Such
a structural modification gave rise to rather distinct thermotropic
behavior from that of 7. AEM 8 manifested a very stable
columnar LC phase with a wide temperature range. Based on the
DSC results (Fig. 4a), the onset of the mesophase emerged at 141
ꢀC and no isotropic state was observed before the molecule
decomposed at 335 ꢀC. In the cooling process, the mesophase
Fig. 4 (a) DSC traces of AEM 8 at a rate of 5 ꢀC minꢁ1; (b) optical
microscopic image of AEM 8 (at 170 ꢀC under crossed polarizers, ꢂ200);
(c) WAXD of AEM 8 at 180 ꢀC in the 1st cooling cycle; (d) a schematic
representation of the molecular packing in the discotic hexagonal
columnar phase.
ꢀ
was significantly super-cooled and crystallized at 81 C. Under
the POM, fan-like LC textures were observed to coexist with
large areas of homeotropic domains in the mesophase, suggesting
a preferred uniaxial alignment of the columns (Fig. 4b). The
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stable columnar mesophase, featuring hexagonally arranged
columns of stacked macrocycles. This LC state exhibited a lower
melting point and a much higher clearing point than AEM 7. The
extraordinary stability of the LC phase was very likely related to
the presence of a greater number of ester functionalities, the
inclusion of which was rather effective at promoting the meso-
phase. Imaginably, with more ester-linked side chains, intermo-
lecular interactions were further strengthened, which is
consistent with a significantly elevated LC-to-isotropic transition
temperature of AEM 8 compared to that of AEM 7. However,
the fact that macrocycle 7 manifested a Colr phase (even though
within a very narrow temperature range) before entering the ND
state while AEM 8 exhibited a Colh phase led to the proposition
that core–core correlation among different columns was sup-
pressed in AEM 8 relative to that in AEM 7. This notion was
related the presence of side chains appended to the p-phenylene
rings. Imaginably, the additional side chains of AEM 8 may
allow a greater magnitude of rotational motion of the macro-
cycles around the main axis of the columns without significantly
disrupting the interactions among side chains of different mole-
cules. Such motions likely resulted in reduced directionality of
molecular orientation with respect to neighboring columns. It
was also considered that since AEM 8 was the only macrocycle
studied herein that displayed order along the columns, this
phenomenon might also be correlated to the side chains on the
p-phenylene rings. These side chains possibly prohibited the
rotation of the p-phenylene units around the ethynylene groups,
by imparting stronger aromatic stacking interactions, and more
importantly intermolecular interactions among side chains. Such
rotational motions of the p-phenylene groups, which might exist
in all other macrocycles, likely hampered ordered stacking of the
molecules within columns.
Discussion
Generally, the designed triangular AEMs exhibited a distinct
propensity to form LC phases (Fig. 5). The current study
confirmed that the side chains were pivotal for LC state occur-
rence. Longer alkyl chains were much more favorable for the
mesomorphic behavior. Among AEMs 3–6, neither 3 nor 5 with
shorter hexyloxy groups exhibited mesophases, whereas both
analogous AEMs with longer dodecyloxy side chains (i.e., 4 and
6) manifested LC phases. In addition to the side chain length, the
functional group joining the backbone with side chains also
played a critical role in governing the LC properties. Unlike
AEMs with hexyloxy side chains, both macrocycles having ester-
linked short hexyl groups (7 and 8) displayed mesophases.
Evidently, the ester linkage was more effective at inducing the LC
phases. It is known that the more polar and electron-with-
drawing carbonyl group is capable of bringing about stronger
intermolecular p–p stacking interactions.16 Also, compared with
the ether linker, the carbonyl group was in conjugation with the
aromatic skeleton, facilitating a more extended, planar frame-
work, which not only further strengthened the interactions
among mesogens but possibly allowed for a great extent of lateral
molecular mobility while maintaining significant attractive
intermolecular forces.
By comparing the properties of macrocycles 2, 4, and 6,
information regarding the effect of the aromatic backbone on the
LC state was unravelled. The fact that AEM 2 possessed an
amorphous state whereas AEM 4 formed a Colr arrangement
suggested that enlargement of the triangular mesogen by
replacing the p-phenylene unit with biphenylene group favored
mesophase formation, likely resulting from stronger intermo-
lecular and inter-columnar interactions in AEM 4. Expanding
the core structure in a different fashion led to disparate conse-
quences. AEM 6 on the other hand manifested a narrow ND
phase. The melting point of AEM 6 was much higher than that of
AEM 4, but the clearing point was similar, thus showing
a considerably compressed LC-phase temperature range. These
observations indicated that the aromatic surface and structure
had a pronounced effect on mesophase formation. The type of
mesophase was also strongly dependent on the chemical features
of the aromatic backbone.
Experimental section
Methods and materials
All chemicals were used as received unless otherwise indicated.
Oxygen or moisture sensitive reactions were performed under
a nitrogen atmosphere using the standard Schlenk technique.
Reagent-grade tetrahydrofuran (THF) was distilled over sodium
and benzophenone, toluene was distilled over sodium, diiso-
propylamine and triethylamine were distilled over CaH2 prior to
use. NMR spectra were recorded on a Varian Mercury 200 (200
MHz), Mercury plus 300 (300 MHz), or Bruker ARX 400 (400
MHz) instrument, using CDCl3 as the solvent. Chemical shifts
were reported in parts per million (ppm) relative to TMS (0 ppm)
or CDCl3 (77.0 ppm) for 1H and 13C NMR spectra, respectively.
FTMS were recorded on a Bruker APEX IV mass spectrometer.
MALDI-TOF was recorded on a Bruker Daltonics Inc. BIFLEX
III mass spectrometer. Elemental analyses were performed using
a German Vario EL III elemental analyzer.
Additional information was obtained by comparing the
properties of AEMs 7 and 8. AEM 8 possessed a much more
UV-Vis absorption spectra were recorded on a Hitachi U-4100
spectrophotometer using 1-cm quartz cuvettes. Photo-
luminescence spectra were recorded on a Horiba Jobin Yvon
FluoroMax-4P spectrofluorometer in the right-angle geometry
and using 1 cm quartz cuvettes. Thermal stability of the AEMs
was studied using a TA Q600 SDT instrument at a heating rate of
10 K minꢁ1 under a N2 atmosphere. The thermal properties were
Fig. 5 LC phase temperature ranges of studied AEMs (based on data
from the second heating cycle).
2412 | Soft Matter, 2012, 8, 2405–2415
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characterized with a TA Q100 DSC calorimeter under a N2
atmosphere. The temperature and heat flow were calibrated at
varied heating and cooling rates using standard materials. Phase
transitions were also examined by Leica DML polarized optical
microscope (POM) with a Mettler hot stage (FP-90) with samples
contained in N2 atmosphere. The XRD experiments were per-
formed with a high flux SAXS instrument (SAXSess, Anton
Paar) equipped with Kratky block-collimation system (the q
range covered by the IP was from 0.06 to 29 nmꢁ1, q ¼ 4p(sinq)/l,
where the wavelength l was 0.1542 nm and q was the scattering
angle). A temperature control unit (Anton Paar TCS300) in
conjunction with the SAXSess was utilized to study the structure
evolution as a function of temperature. The temperature-
dependent 1D WAXD measurements were performed with
a Rigaku MultiFlex 2 kW tube-anode X-ray (Cu-Ka radiation)
generator coupled to a diffractometer at a scanning rate of 1ꢀ
minꢁ1. The peak positions were calibrated using silicon powder in
the high-angle region (>15ꢀ) and silver behenate in the low-angle
region (<15ꢀ). Background scattering was subtracted.
Calc. for C144H180O6: 2006.4 (m/z). Found: 2006.8 (m/z). Found:
C, 85.96; H, 8.99. Calc. for C144H180O6: C, 86.18; H, 9.04.
1
AEM 7 H NMR (300 MHz, CDCl3). d 8.04 (3H, d, J ¼ 1.7
Hz), 7.78 (3H, dd, J ¼ 8.7, 1.7 Hz), 7.51 (15H, m), 4.30 (6H, t, J ¼
6.8 Hz), 1.79 (6H, m), 1.55–1.30 (18H, m), 0.93 (9H, t, J ¼ 6.9
Hz). 13C NMR (50 MHz, CDCl3): d 165.2, 132.8, 131.6, 131.5,
131.3, 129.7, 129.4, 128.6, 125.6, 123.5, 123.0, 95.9, 93.8, 90.0,
89.9, 65.5, 31.5, 28.7, 25.7, 22.6, 14.0. MALDI-TOF: Calc. for
C69H60O6: 984.4 (m/z). Found: 984.8 (m/z). Found: C, 84.11; H,
6.09. Calc. for C69H60O6: C, 84.12; H, 6.14.
1
AEM 8 H NMR (300 MHz, CDCl3). d 8.29 (3H, d, J ¼ 1.5
Hz), 8.24 (3H, d, J ¼ 1.5 Hz), 8.02 (3H, dd, J ¼ 8.1, 1.5 Hz), 7.76–
7.65 (9H, m), 4.36 (6H, t, J ¼ 6.6 Hz), 4.27 (6H, t, J ¼ 6.6 Hz),
1.78 (12H, m), 1.26 (36H, m), 0.93 (9H, t, J ¼ 6.9 Hz), 0.82 (9H, t,
J ¼ 6.9 Hz). 13C NMR (50 MHz, CDCl3): d 165.2, 134.2,
134.13,134.07, 133.2, 132.7, 131.9, 130.3, 129.2, 129.1, 125.9,
123.3, 123.0, 94.9, 94.2, 92.9, 90.6, 65.8, 65.6, 31.5, 31.4, 29.6,
28.6, 25.6, 22.5, 22.4,14.0, 13.9. FTMS: Calc. for C90H96O12:
1368.7 (m/z). Found: 1391.7 (M + Na+). Found: C, 78.73; H,
7.06. Calc. for C90H96O12: C, 78.92; H, 7.06.
AEM structure characterizations
AEMs 1 and 2 were synthesized as previously reported.23ab
Structure characterization results of all other AEMs are as
follows.
Conclusions
Eight triangle-shaped AEMs of related but different structures
were synthesized, and their optical and thermotropic properties
were systematically investigated. Four of the studied macrocycles
manifested LC phases, which were thoroughly characterized by
DSC, POM, and XRD techniques. A number of variants in
structure, e.g., the side chain length, side chain linkage, and
backbone scaffold, were examined regarding their influence over
the thermotropic LC behavior. Size and structure of the mesogen
core were important. With three different backbone structures
investigated, disparate mesophases were manifested by varied
aromatic backbones, showing that core–core interactions among
mesogens were decisive for the molecular arrangement and
strength of intermolecular interactions. Extension or expansion
of the aromatic framework appeared to be favorable for LC
phase formation in the investigated systems. The side chain
length was found to be one of the pivotal factors as well. Longer
dodecyloxy side chains were more conducive to mesophases than
shorter hexyloxy chains. Among the macrocycles studied, two
molecules having dodecyloxy side chains manifested thermody-
namically stable LC phases, while macrocycles having the same
mesogen core but tethered with shorter hexyloxy groups did not.
Side chain linkage was also critical in governing the mesophase.
Two types of side chain linkers were compared. Both macro-
cycles having ester side chain linkers exhibited LC phases, even
when the side chains were the shorter hexyl groups. Of all the
macrocycles examined here, a cyclic oligo(phenylene–ethyny-
lene) molecule of C3-symmetry having six side chains attached to
each of the phenylene units via an ester functionality displayed
the most stable LC phase featuring a hexagonal columnar
structure.
AEM 3 1H NMR (300 MHz, CDCl3). d 7.67 (24H, s), 7.05 (6H,
s), 4.05 (12H, t, J ¼ 6.6 Hz), 1.86 (12H, m), 1.55–1.35 (36H, m),
0.93 (18H, t, J ¼ 7.2 Hz). 13C NMR (50 MHz, CDCl3): d 149.3,
139.5, 131.9, 126.8, 122.9, 118.7, 115.9, 91.9, 89.7, 69.2, 31.6,
29.1, 25.7, 22.6, 14.0. FTMS: Calcd for. C102H108O6: 1429.8
(m/z). Found: 1452.8 (M + Na+). Found: C, 85.31; H, 7.66. Calc.
for C102H108O6: C, 85.67; H, 7.61.
AEM 4 1H NMR (300 MHz, CDCl3). d 7.68 (24H, s), 7.06 (6H,
s), 4.06 (12H, t, J ¼ 6.6 Hz), 1.86 (12H, m), 1.55–1.26 (108H, m),
0.89 (18H, t, J ¼ 6.6 Hz). 13C NMR (100 MHz, CDCl3): d 149.4,
139.7, 132.0, 126.9, 123.1, 118.8, 116.1, 92.0, 89.8, 69.4, 32.0,
31.5, 30.3, 29.71, 29.66, 29.42, 29.38, 29.2, 26.1, 22.7, 14.1.
FTMS: Calc. for C138H180O6: 1934.4 (m/z). Found: 1957.4
(M + Na+). Found: C, 85.51; H, 9.19. Calc. for C138H180O6: C,
85.66; H, 9.38.
AEM 5 1H NMR (300 MHz, CDCl3). d 8.77 (6H, s), 8.11 (6H,
d, J ¼ 8.4 Hz) 7.65 (6H, d, J ¼ 8.4 Hz) 7.63 (12H, s), 4.21(12H, t,
J ¼ 6.6 Hz), 1.92 (12H, m), 1.59–1.40 (36H, m), 0.96 (18H, t, J ¼
6.9 Hz). 13C NMR (75 MHz, CDCl3): d 143.8, 131.6, 129.3, 127.7,
126.5, 123.1, 122.3, 120.4, 92.1, 89.9, 73.7, 31.8, 30.5, 26.0, 22.7,
14.1. FTMS: Calc. for C108H108O6: 1501.8 (m/z). Found: 1501.8
(m/z). Found: 86.45; 7.26. Calc. for C108H108O6: C, 86.36; H,
7.25.
AEM 6 1H NMR (300 MHz, CDCl3). d 8.81 (6H, s), 8.15 (6H,
d, J ¼ 8.4 Hz) 7.69 (6H, d, J ¼ 8.4 Hz) 7.65 (12H, s), 4.21(12H, t,
J ¼ 6.6 Hz), 1.92 (12H, m), 1.57–1.29 (108H, m), 0.90 (18H, t, J ¼
6.6 Hz). 13C NMR (100 MHz, CDCl3): d 144.0, 131.7, 129.6,
129.5, 127.9, 126.6, 123.2, 122.5, 120.5, 92.1, 90.0, 73.8, 32.0,
30.6, 29.8, 29.7, 29.6, 29.4, 26.3, 22.7, 14.1. MALDI-TOF MS:
With a conjugated aromatic backbone, these macrocycles are
potentially functional materials. As exemplified in a previously
report, they may possess appealing semi-conductive proper-
ties.23a Upon further functionalization, e.g., modifying the
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8 (a) G. Zucchi, B. Donnio and Y. H. Geerts, Chem. Mater., 2005, 17,
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framework, the molecules may offer more diversified and opti-
mized functions. Their propensity to form LC phases, as
demonstrated in the current study, will undoubtedly benefit the
material’s application and device fabrication. Relevant studies
are underway.
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This work was supported by the National Natural Science
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Fok Ying-Tung Educational Foundation (No. 114008).
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^ ꢂ
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This journal is ª The Royal Society of Chemistry 2012
Soft Matter, 2012, 8, 2405–2415 | 2415