Discotic columnar mesophases derived from 'rod-like' π-conjugated anion-responsive acyclic oligopyrroles

Article abstract of DOI:10.1039/c0cc00551g

Rod-like π-conjugated anion-responsive acyclic oligopyrroles have been reported to form stacked disk-like components that result in the formation of discotic columnar mesophases as thermotropic liquid crystals.

Full text of DOI:10.1039/c0cc00551g

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Discotic columnar mesophases derived from ‘rod-like’ p-conjugated
anion-responsive acyclic oligopyrrolesw
Hiromitsu Maeda,*ab Yoshitaka Terashima,a Yohei Haketa,a Atsushi Asano,c
Yoshihito Honsho,c Shu Seki,bc Masahiro Shimizu,d Hidetomo Mukaid and Kazuchika Ohta*d
Received 23rd March 2010, Accepted 23rd April 2010
First published as an Advance Article on the web 19th May 2010
DOI: 10.1039/c0cc00551g
Rod-like p-conjugated anion-responsive acyclic oligopyrroles
have been reported to form stacked disk-like components that
result in the formation of discotic columnar mesophases as
thermotropic liquid crystals.
interactions such as hydrogen bonding to afford disk-like
aggregates, are found to afford discotic columnar mesophases.
According to literature and by employing modified
procedures,5b,c we prepared icosyloxy-substituted 1f along
with singly and doubly N-blocked 2b,e and 3b,e. The peri-
pheral modified 2b,e and 3b,e are key components for the
examination of the effects of noncovalent interactions by
pyrrole NH that result in the formation of p-conjugated unit
aggregates. The absence of one pyrrole NH affords smaller
anion-binding constants (Ka). The Ka values of 2b in
CH2Cl2—estimated by examining UV/vis absorption spectral
changes due to the addition of anions in the form of tetra-
butylammonium (TBA) salts—are 890 (Clꢀ), 140 (Brꢀ), 18 000
(CH3CO2ꢀ) and 4200 (H2PO4ꢀ) Mꢀ1. Under similar conditions,
small changes by anion binding in fluorescence were also
observed. Further, the solid-state assembled modes of
N-blocked molecules, which may provide insights into the
organized structures of soft materials from analogue derivatives,
are elucidated using single-crystal X-ray analysis of methoxy-
substituted 2b and 3b (Fig. 2).z The singly blocked 2b forms a
hydrogen-bonding dimer using pyrrole NH sites, whereas, in
the case of 3b, such a hydrogen-bonding interaction is not
observed. In both cases, 1D chain structures based on stacking
interactions, as observed in 1b,5b are constructed. The offset
arrangements of p-units in the solid-state structures are
presumed to be due to the less efficient van der Waals inter-
actions between the side alkyl moieties; these moieties are
observed to be due to short methyl units in all the molecules of
1b, 2b and 3b.
One of the essential parameters for determining the states and
properties of organized structures fabricated by noncovalent
interactions is the geometry of molecules. In contrast to
crystals and solution states, soft materials such as liquid
crystals and supramolecular gels comprising low-molecular-
weight molecules are remarkable because of their mobile but
anisotropic properties.1,2 p-Conjugated molecules are the
building subunits of such soft materials. These p-conjugated
molecules exhibit planar geometry because of which they are
useful for forming stacking assemblies. Among the various
p-conjugated molecules, dipyrrolyldiketone BF2 complexes
(e.g., 1a,b, Fig. 1a), which exhibit anion-responsive behaviours3
by inversion (flipping) of pyrrole rings (Fig. 1b),4,5 afford soft
materials such as supramolecular gels from aliphatic compounds
(1c–e)5b and water-supported organized structures from
amphiphilic receptors;5c these materials are afforded on the
basis of their core p-conjugated units. In general, p-conjugated
units are crucial parts for forming organized structures, whose
phases significantly depend on the geometries of these units. In
our study, the core p-conjugated unit of the anion receptors
exhibits a ‘rod-like’ shape and comprises two pyrrole rings and
a boron-containing six-membered ring connected by C–C
single bonds along with attached aryl rings. Such rod-like
p-conjugated units not only behave as calamitic mesogens
but also form disk-like components consisting of several
p-conjugated units; these disk-like components form stacking
structures under appropriate conditions. In fact, anion-
responsive p-conjugated units, which undergo intermolecular
The introduction of aliphatic chains to the pyrrole-based
p-conjugated molecules affords soft materials on the basis of
two interactions worked for p-units and alkyl moieties. First,
we examined the liquid crystal properties of hexaalkoxy-
substituted 1c–f using differential scanning calorimetry (DSC),
a College of Pharmaceutical Sciences, Institute of Science and
Engineering, Ritsumeikan University, Kusatsu 525-8577, Japan.
E-mail: maedahir@ph.ritsumei.ac.jp; Fax: +81 77 561 2659
b PRESTO, Japan Science and Technology Agency (JST), Kawaguchi
332-0012, Japan
c Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, Suita 565-0871, Japan
d Interdisciplinary Graduate School of Science and Technology,
Shinshu University, Ueda 386-8567, Japan.
E-mail: ko52517@shinshu-u.ac.jp
w Electronic supplementary information (ESI) available: Synthetic
procedures and analytical data, anion-binding behaviour, CIF files
for the X-ray structural analysis of 2b and 3b. CCDC 768243–768244.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c0cc00551g
Fig. 1 (a) Structures of anion-responsive dipyrrolyldiketone BF2
complexes 1a–f, 2b,e and 3b,e and (b) anion-binding mode of 1a.
ꢁc
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Fig. 2 Single-crystal X-ray structures of (a) 2b (one of the two
independent structures in (i) and (ii)) and (b) 3b ((i) top and (ii) side
view and (iii) assembled structure). Atom colour code: brown, pink,
yellow, blue, and red refer to carbon, hydrogen, boron, nitrogen and
oxygen, respectively.
polarized optical microscopy (POM) and X-ray diffraction
(XRD) analysis. DSC measurements of 1c–f reveal the meso-
phases at r.t.–191.0 1C, r.t.–183.3 1C, 36.7–172.5 1C and
65.6–156.0 1C, respectively. POM images of the mesophases
of 1c–e show ribbon-like textures and that of 1f shows a
mosaic-like texture (Fig. 3a). Further, XRD analyses of 1c–f
in the mesophases (Fig. 3b) suggest that the thermotropic
liquid crystalline states are formed due to the discotic hexa-
gonal columnar (Colh) phases that are, in turn, composed of
dimeric assemblies (Z = ca. 2 for r = 1) as building units. The
unit parameter a of 1c–f is 31.2, 36.7, 39.8 and 46.0 A,
respectively, and the unit parameter h of the same is 4.45,
4.52, 4.45 and 4.50 A, respectively. The increase in the values
of a in 1c to 1f is consistent with the increase in the corres-
ponding alkyl chains. The h values are derived from stacking
structures. The formation of stacking units comprising two
rod-like molecules is rare and remarkable6 and can be
achieved by weak interactions such as hydrogen bonding7
and dipole–dipole interactions8 at pyrrole NH and BF2 moieties.
In these p-conjugated systems, pyrrole NH sites that are useful
for anion binding behave as the interaction sites for disk-like
stacking components (Fig. 3c). We considered the mesophases
formed because of the stacking structures of p-conjugated
molecules and conducted flash photolysis time-resolved micro-
wave conductivity (FP-TRMC) measurements.9 At 25 1C, the
conductivities of the liquid crystal materials 1c and 1d were
Fig. 3 (a) POM images of (i) 1c (183.7 1C), (ii) 1d (178.8 1C), (iii) 1e
(169.5 1C) and (iv) 1f (155 1C), (b) XRD pattern of 1e (170 1C) and
(c) possible formation model of columnar structures on the basis of
p-conjugated acyclic oligopyrroles.
reveal the occurrence of mesophases at 44.4–102.9 1C (2e)
and 50.0–79.0 1C (3e). On the basis of the dendritic texture in
the POM image of 2e in the absence of XRD analysis due to a
weak diffraction pattern, we speculated that singly N-blocked
2e forms Colh phases such as those observed in the images of
1c–f. On the other hand, the POM image of 3e shows a flake-
like texture (Fig. 4a) and XRD analysis of the doubly
N-blocked 3e reveals a rectangular columnar (Colr (P21/a))
phase of a = 64.7 A, b = 30.2 A and h = 4.5 A without the
formation of a dimer structure (Z = ca. 2 for r = 0.8)
(Fig. 4b). Although the exact correlation between the meso-
phases in 2e and 3e and the solid-state structures of 2b and 3b
is currently unclear, the results in 2e and 3e are observed to be
correlated with the number of pyrrole NH sites.11
estimated at 0.093 and 0.22 cm2 Vꢀ1
values are fairly high and are comparable to that of the crystal
state such as 1e (0.25 cm2 Vꢀ1 ꢀ1). The lifetime of free charge
s
ꢀ1, respectively; these
s
carriers in discotic liquid crystal materials has been system-
atically discussed in terms of orthogonal displacement of
discotic molecules in a previous study.10 The value of charge
carrier mobility along the stacking axes increased with the
suppression of thermal fluctuation of the displation; this value
reached its maximum when the material was in a frozen state.
Presently, detailed conformations of the building subunits,
that can exhibit ‘pyrrole inversion’, are unclear; however,
N-containing heterocycles (pyrrole rings) can form stable
disk-like components arranged in columnar structures.
Assembled properties observed in mesophases of the
p-conjugated oligopyrroles can be applicable in the formation
of supramolecular gels. In contrast to 1c–e that form solution
states (1c,d) or gelated materials (1e) from octane (10 mg mLꢀ1
)
at r.t.,5b icosyloxy-substituted 1f forms precipitates due to the
fairly strong van der Waals interactions between its longer
alkyl chains. Singly N-blocked 2e affords a solution, whereas
doubly N-blocked 3e forms precipitates by ambient cooling
and gels by rapid cooling after heating at 40.0 1C to solution
state. The UV/vis absorption and fluorescence emission
Next, the effects of pyrrole N sites are examined using
hexadecyloxy-substituted 2e and 3e. DSC measurements
ꢁc
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Notes and references
z Crystal data for 2b (from CH2ClCH2Cl/hexane): C30H31N2O8BF2,
Mw = 596.38, monoclinic, P21/c (no. 14), a = 12.997(2), b =
13.014(3), c = 33.825(8) A, b = 96.658(8)1, V = 5683(2) A3, T =
123(1) K, Z = 4, Dc = 1.394 g cmꢀ3, m(Mo-Ka) = 0.109 mmꢀ1, R1 =
0.0603, wR2 = 0.1386, GOF = 1.007 (I > 2s(I)). CCDC 768243.
Crystal data for 3b (from CHCl3/heptane): C31H33BF2N2O8, Mw =
610.40, monoclinic, C2/c (no. 15), a = 29.464(14), b = 7.038(4), c =
13.928(10) A, b = 102.28(2)1, V = 2822(3) A3, T = 123(2) K, Z = 4,
Dc = 1.437 g cmꢀ3, m(Mo-Ka) = 0.111 mmꢀ1, R1 = 0.0475, wR2 =
0.1137, GOF = 1.064 (I > 2s(I)). CCDC 768244.
1 Books and reviews for further information on liquid crystals:
(a) I. Dierking, Textures of Liquid Crystals, Wiley-VCH Verlag,
Weinheim, 2003; (b) Liquid Crystalline Functional Assemblies and Their
Supramolecular Structures, ed. T. Kato, Springer-Verlag, Berlin, 2008;
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¨
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¨
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Fig. 4 (a) POM image of 3e (80.2 1C) and (b) XRD pattern of 3e (70 1C).
maxima (lmax and lem) of 2e in octane (10ꢀ5 M) are 483 and
521 nm, respectively, and those of 3e in octane (10ꢀ5 M) are
468 and 510 nm, respectively. In contrast, 2e and 3e in
concentrated states (10 mg mLꢀ1) exhibit broad absorption
bands with lmax at 470 and 413 nm, respectively, and, excited
at each lmax, 2e and 3e exhibit broad emission bands with lem
at 649 and 640 nm, respectively. These optical properties
suggest that the stacking structures of 2e are formed in a
solution state and those of 3e are formed in a gel state.
In contrast to 3e, which shows no anion-responsive
behaviours4b,12 owing to the absence of NH sites, 2e shows
changes in emission and colour of the solution when 1 equiv.
of TBACl is added to the solution. On the basis of these
results, it is considered that the organized structures in the
solution and gelated states with ca. 1% (w/w) concentrations
are affected by the geometries of p-conjugated molecules.
In conclusion, anion-responsive p-conjugated oligopyrroles
form disk-like assemblies supported by weak noncovalent
interaction to afford discotic columnar mesophases.
Anion-responsive behaviours in mesophases such as those
observed in supramolecular gels are also of considerable
interest; however, these behaviours are complicated as they
are exhibited by soft materials containing multiple ionic
components. In fact, we have conducted a preliminary study
of the anion-responsive behaviours of the mesophases and the
essential effects of counter cations.13 Further, it is believed that
a modification of boron units14 would provide a variety of
mesophases. These investigations are currently underway.
This work was supported by Grant-in-Aid for Scientific
Research in a Priority Area ‘‘Super-Hierarchical Structures’’
(No. 19022036) and for Young Scientists (B) (No. 21750155)
from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) and Ritsumeikan Global Innovation
Research Organization (R-GIRO) project (2008–2013). We
thank Prof. Atsuhiro Osuka, Dr Yasuhide Inokuma,
Dr Shohei Saito and Mr Taro Koide, Kyoto University,
for X-ray analysis and Prof. Tomonori Hanasaki and
Prof. Hitoshi Tamiaki, Ritsumeikan University, for various
measurements. Y.H. thanks JSPS for a Research Fellowship
for Young Scientists.
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ꢁc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4559–4561 | 4561