Charge-based and charge-free molecular assemblies comprising π-extended derivatives of anion-responsive acyclic oligopyrroles

Article abstract of DOI:10.1039/c2cc17282h

Assemblies comprising anion complexes of π-extended oligopyrrole derivatives and counter cations, as well as those of anion-free receptor molecules, exhibited the formation of mesophases based on columnar structures using electrostatic and π-π interactions. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2cc17282h

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COMMUNICATION
Charge-based and charge-free molecular assemblies comprising
p-extended derivatives of anion-responsive acyclic oligopyrrolesw
Yuya Bando, Shohei Sakamoto, Ippei Yamada, Yohei Haketa and Hiromitsu Maeda*
Received 22nd November 2011, Accepted 5th January 2012
DOI: 10.1039/c2cc17282h
Assemblies comprising anion complexes of p-extended oligopyrrole
derivatives and counter cations, as well as those of anion-free
receptor molecules, exhibited the formation of mesophases based
on columnar structures using electrostatic and p–p interactions.
Among various functional materials,1 ion-based materials
such as ionic liquid crystals can be fabricated by incorporation
of charged species into the building components.2,3 Effective
use of the interactions among the charged species would provide
more ordered assembled structures suitable for tunable electronic
and optical materials, but only a few examples of the soft materials
that consist of cations and anions in ordered arrangements have
been reported.4 As charged building subunits, p-conjugated
planar species are appropriate for the formation of charge-based
materials owing to their effective stacking interactions. One of the
protocols for efficiently constructing planar anionic structures is
the complexation of p-conjugated receptor molecules with halide
anions.5 The receptor–anion complexes as planar anions would
afford various assemblies in combination with appropriate
cations. In fact, based on the p-conjugated anion receptors, i.e.,
boron complexes of 1,3-dipyrrolyl-1,3-propanediones (1a–d,
Fig. 1a),6,7 charge-by-charge assemblies, which consist of
alternately stacking positively and negatively charged species,
have been constructed with the receptor–anion complexes and
appropriate cations.4,8 Tuning of assembled structures and
properties of such charge-by-charge assemblies, along with
anion-free assemblies, can be achieved by the modification of
anion receptors. Thus far, formation of assembled structures
has been on the basis of stacking of the core p-planes and van
der Waals interactions among the aliphatic chains at aryl
moieties connecting to pyrrole a-positions.4,7b,c However,
modification at pyrrole b-positions also seems to be an efficient
method for providing promising soft materials, which would
exhibit different assembled structures and properties from
a-substituted derivatives, mainly because of the orientations
of aliphatic chains. Benzo-fusion and b-aryl-substitution as
Fig. 1 (a) Structures and anion-binding scheme of dipyrrolyldiketone
BF2 complexes 1a–d and (b) pyrrole-b-p-extended derivatives 2a–d,
3a–d and 4b–d.
seen in 2a7d and 3a7f (Fig. 1b) are appropriate candidate strategies
for introducing alkyl chains into b-positions for fabricating supra-
molecular assemblies as soft materials. In this communication, we
report the synthesis and assemblies of benzo-fused and b-aryl-
substituted derivatives of anion receptors along with directly
b-alkoxy-substituted derivatives, which form assembled structures
as charge-based materials, and those in the anion-free forms.
Similar to 2a,7d benzo-fused receptors 2b–d were obtained
from the precursor ethano-bridged derivatives 20b–d, which were
prepared from the corresponding alkyloxycarbonyl-substituted
bicyclo[2.2.2]octadiene-fused pyrroles.9 Similar to 3a,7f b-aryl-
substituted 3b–d were also prepared by reaction of the corres-
ponding 3,4-diarylpyrroles10 with malonyl chloride and by
subsequent treatment with BF3ꢀOEt2. Mono-alkoxy-substituted
aryl rings in 3b–d were introduced in order to set the numbers of
alkyl chains to those of 2b–d. Furthermore, directly b-alkoxy-
substituted derivatives 4b–d were also synthesized from
3,4-dialkoxypyrroles.11 The exact structures of 3a, 3b and 4b were
elucidated by single-crystal X-ray analysis (Fig. 2).z In contrast to
1a, 1b,7b 2a7d and 4b, b-aryl-substituted 3a and 3b showed no
significant stacking structures of p-planes (Fig. S8, ESIw). Small
differences in the absorption maxima (lmax) owing to differences
in the alkyl chain lengths in CH2Cl2 were observed among the
analogues 1b–d (516, 521 and 521 nm),7b 2b–d (529, 530 and
530 nm), 3b–d (485, 486 and 486 nm) and 4b–d (452, 457
and 457 nm); these maxima are comparable to those of 1a
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
w Electronic supplementary information (ESI) available: Synthetic
procedures and analytical data, anion-binding behaviour and CIF files
for the single-crystal X-ray structural analysis of 20b, 3a, 3b and 4b. CCDC
852919–852922. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cc17282h
c
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Fig. 2 Single-crystal X-ray structures (top and side views) of (a) 3a,
(b) 3b and (c) 4b. Atom colour code: brown, pink, yellow, green, blue
and red refer to carbon, hydrogen, boron, fluorine, nitrogen and
oxygen, respectively.
Fig. 3 POM images of (a) 2d and (b) 3d at 41 and 90 1C (1st cooling),
respectively.
with a = 4.20 nm and c = 0.45 nm based on a dimeric
assembly (Z = 2 for r of 0.74). In contrast, 3d formed a Colh
structure based on dimeric assemblies at 60–98 1C (1st heating) and
a rectangular columnar (Colr) structure at 93–23 1C (1st cooling),
and 4d formed a lamellar structure at 70 1C (2nd heating). Because
of the introduction of long alkyl chains at pyrrole b-positions,
various types of assembled structures were observed depending on
the positions of the aliphatic chains. The structures of mesophases
are in sharp contrast to those of the a-aryl derivatives 1c,d,
which form liquid crystals of Colh based on the self-assembled
dimers as circular building subunits.7c
(500 nm),7b 2a (528 nm),7d 3a (469 nm)7f and a parent receptor
(432 nm),7a respectively. The differences in the positions of
substituents are correlated with the HOMO–LUMO gaps of
3.084 eV (1a),7b 2.937 eV (2a)7d and 3.342 eV (3a)7f calculated at the
B3LYP/6-31+G(d,p)//B3LYP/6-31G(d,p) level.12 Fluorescence
emission maxima (lem) of a parent receptor (451 nm),7a 1b
(558 nm),7b 2b (552 nm),7d 3b (541 nm)7f and 4b (475 nm) in
CH2Cl2 (each excited at lmax) afforded Stokes shifts at 0.1809,
0.0977, 0.2646 and 0.1328 eV, respectively. UV/vis absorption
spectral changes of 3b upon the addition of anions as tetra-
butylammonium (TBA) salts revealed binding constants (Ka)
of 4200, 250, 120 000 and 51 000 Mꢁ1 for Clꢁ, Brꢁ, AcOꢁ and
H2PO4ꢁ, respectively (Fig. S14, ESIw). These values are smaller
than those of 1b,7b presumably because of the distorted
conformation of 3b for anion binding. In contrast, the corres-
Next, preparation of charge-based materials comprising
b-substituted receptors 2c,d, 3c,d and 4c,d was examined by
means of anion binding using appropriate salts. In the previous
study, a Clꢁ complex of 1d as a planar 4,8,12-tripropyl-4,8,
12-triazatriangulenium cation ((TATAC3)+) salt13 was found
to be appropriate for fabrication of charge-by-charge-based
thermotropic liquid crystals.4 In the case of the b-substituted
receptors, Clꢁ complexes of 2c,d, 3c,d and 4c,d were prepared by
treating the receptors with equivalent amounts of (TATAC3)+ꢀ
Clꢁ and subsequent removal of solvents. DSC measurements
ponding Ka values of 4b are 30, 14, 1600 and 6100 Mꢁ1
,
respectively (Fig. S15, ESIw), which are the smallest values among
those of all the receptors considered in this study, because of
the electron-donating substituents and the lower preference of
pyrrole-inverted structures as speculated by density functional
theory (DFT) calculations at the B3LYP/6-31G(d,p) level.y
Long alkyl chains in 2c,d, 3c,d and 4c,d enable the receptors
to form supramolecular assemblies as soft materials on the
basis of the interactions among p-planes and those among
alkyl chains. For example, in solids cast from solutions, 2d was
found to form H-aggregates, whereas 3d and 4d formed
J-aggregates, as revealed by UV/vis absorption spectra (Fig. S6,
ESIw). Solid-state assembled modes of 2c,d, 3c,d and 4c,d were
revealed in detail by synchrotron X-ray diffraction (XRD)
measurements of the precipitates from CH2Cl2/MeOH
(2c,d and 3c,d) or CHCl3/MeOH (4c,d). Benzo-fused 2d formed
a lamellar structure at 25 1C with a d value of 5.25 nm (100),
corresponding to a proposed model consisting of N(–H)ꢀꢀꢀF
hydrogen-bonding dimers. Differential scanning calorimetry
(DSC) measurements of 2d, 3d and 4d elucidated the formation
of mesophases at 33–166, 23–94 and 63–77 1C, respectively, during
cooling processes (Fig. S16, ESIw). In contrast, 2c and 3c formed
amorphous-like states below 209 and 93 1C as clear points,
whereas 4c formed a crystal state below 92 1C. These results
suggest that introduction of hexadecyloxy chains is required to
provide stable mesophases in the wide ranges of temperatures.
Polarized optical microscopy (POM) images of 2d, 3d and 4d
showed broken fan textures during cooling processes (Fig. 3 and
Fig. S18, ESIw). Furthermore, synchrotron XRD analysis at
variable temperatures revealed the phase properties and structures
of the mesophases. At 41 1C after cooling from an isotropic
liquid (Iso), 2d showed a hexagonal columnar (Colh) phase
of (TATAC3 +
)
salts of 2dꢀClꢁ and 4dꢀClꢁ revealed the
formation of mesophases at 24–102 and 57–70 1C, respectively,
whereas those of 2cꢀClꢁ, 3cꢀClꢁ, 3dꢀClꢁ and 4cꢀClꢁ provided no
mesophases (Fig. S17, ESIw). POM images of these salts showed
broken fan textures (Fig. 4a). Furthermore, a synchrotron XRD
measurement of 2dꢀClꢁ-(TATAC3 +
)
suggested the formation
of a tetragonal columnar (Coltet) phase with a = 4.21 nm and
c = 0.73 nm based on a tetrameric assembly (Z = 4.02 for r of 1.0)
(Fig. 4b). In this case, it can be speculated that the receptors face the
centre of the circular unit owing to the anions being arranged on the
inside, presumably because of the alkyl chains at the fused-benzo
units at pyrrole b-positions (Fig. 4c). The packing diagram
of 2dꢀClꢁ-(TATAC3 +
)
is different from that of 1dꢀClꢁ-
(TATAC3)+, which forms a Colh phase on the basis of a
circular unit comprising four stacking ion pairs.4 The diffraction
of 0.73 nm corresponds to the stacking ion pair in 2dꢀClꢁ-
(TATAC3)+, suggesting the formation of a charge-by-charge
assembly. In the benzo-fused receptors, long alkyl chains are
necessary, as can be seen in the disordered structures of
2cꢀClꢁ-(TATAC3 +
)
possessing shorter alkyl chains than
2dꢀClꢁ-(TATAC3 +
)
(Fig. S29 and S30, ESIw). In contrast to the
benzo-fused receptor, 3cꢀClꢁ-(TATAC3 +
)
and 3dꢀClꢁ-(TATAC3 +
)
showed no clear XRD patterns, suggesting that b-aryl substituents
of the receptor–anion complexes were inappropriate for stacking
assemblies with planar cations (Fig. S31 and S32, ESIw).
Furthermore, 4cꢀClꢁ-(TATAC3 +
)
and 4dꢀClꢁ-(TATAC3 +
)
also formed complex structures, presumably because of the
c
2302 Chem. Commun., 2012, 48, 2301–2303
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(from MeOH/CH2Cl2/hexane): C15H17BF2N2O6, Mw
= 370.11,
%
triclinic, P1 (no. 2), a = 8.030(5), b = 9.349(5), c = 11.595(5) A,
a = 85.300(19), b = 102.28(2), g = 81.74(2)1, V = 823.0(7) A3, T =
123(2) K, Z = 2, Dc = 1.494 g cmꢁ3, m(Mo-Ka) = 0.128 mmꢁ1, R1 =
0.0380, wR2 = 0.0947, GOF = 1.072 (I 4 2s(I)). CCDC 852922.
y Relative energies between the stable and preorganized pyrrole-
inverted geometries of 1a, 2a, 3a and 4b are 8.71,7b 7.86,7d 9.867f and
11.75 kcal molꢁ1, respectively. Dihedral angles between two pyrrole
rings in the preorganized pyrrole-inverted geometries of 1–3a and 4b
are 0.701,7b
0.011,7d 36.6817f and 9.831, respectively.
1 As selected book and review: (a) G. A. Ozin and A. C. Arsenault,
Nanochemistry: A Chemical Approach to Nanomaterials, RSC,
Cambridge, 2005; (b) B. M. Rosen, C. J. Wilson, D. A. Wilson,
M. Peterca, M. R. Imam and V. Percec, Chem. Rev., 2009,
109, 6275.
2 (a) T. Kato, N. Mizoshita and K. Kishimoto, Angew. Chem., Int.
Ed., 2006, 45, 38; (b) T. Kato, T. Yasuda, Y. Kamikawa and
M. Yoshio, Chem. Commun., 2009, 729.
Fig. 4 (a) POM image at 80 1C, (b) synchrotron XRD results at 80 1C
suggesting the formation of a Coltet structure and (c) a proposed Coltet
3 (a) M. Zhou, P. R. Nemade, X. Lu, X. Zeng, E. S. Hatakeyama,
R. D. Noble and D. L. Gin, J. Am. Chem. Soc., 2007, 129, 9574;
(b) L. Wu, J. Lal, K. A. Simon, E. A. Burton and Y.-Y. Luk,
J. Am. Chem. Soc., 2009, 131, 7430; (c) M. A. Alam,
J. Motoyanagi, Y. Yamamoto, T. Fukushima, J. Kim, K. Kato,
M. Takata, A. Saeki, S. Seki, S. Tagawa and T. Aida, J. Am. Chem.
Soc., 2009, 131, 17722; (d) T. Ichikawa, M. Yoshio, A. Hamasaki,
J. Kagimoto, H. Ohno and T. Kato, J. Am. Chem. Soc., 2011,
133, 2163.
4 For example: Y. Haketa, S. Sasaki, N. Ohta, H. Masunaga,
H. Ogawa, N. Mizuno, F. Araoka, H. Takezoe and H. Maeda,
Angew. Chem., Int. Ed., 2010, 49, 10079.
5 Selected books for anion binding: (a) Supramolecular Chemistry of
Anions, ed. A. Bianchi, K. Bowman-James and E. Garcıa-Espana,
´
Wiley-VCH, New York, 1997; (b) Fundamentals and Applications
of Anion Separations, ed. R. P. Singh and B. A. Moyer, Kluwer
Academic/Plenum Publishers, New York, 2004; (c) Anion Sensing,
Topics in Current Chemistry, ed. I. Stibor, Springer-Verlag, Berlin,
2005, vol. 255, p. 238; (d) J. L. Sessler, P. A. Gale and W.-S. Cho,
Anion Receptor Chemistry, RSC, Cambridge, 2006; (e) Recognition
of Anions, Structure and Bonding, ed. R. Vilar, Springer-Verlag,
Berlin, 2008, vol. 129, p. 242.
6 As a review: H. Maeda, in Anion Complexation in Supramolecular
Chemistry, Topics in Heterocyclic Chemistry, ed. P. A. Gale and
W. Dehaen, Springer-Verlag, Berlin, 2010, vol. 24, p. 103.
7 (a) H. Maeda and Y. Kusunose, Chem.–Eur. J., 2005, 11, 5661;
(b) H. Maeda, Y. Haketa and T. Nakanishi, J. Am. Chem. Soc.,
2007, 129, 13661; (c) H. Maeda, Y. Terashima, Y. Haketa,
A. Asano, Y. Honsho, S. Seki, M. Shimizu, H. Mukai and
K. Ohta, Chem. Commun., 2010, 46, 4559; (d) H. Maeda,
Y. Bando, Y. Haketa, Y. Honsho, S. Seki, H. Nakajima and
N. Tohnai, Chem.–Eur. J., 2010, 16, 10994; (e) Y. Haketa and
H. Maeda, Chem.–Eur. J., 2011, 17, 1485; (f) H. Maeda, Y. Bando,
K. Shimomura, I. Yamada, M. Naito, K. Nobusawa, H. Tsumatori
and T. Kawai, J. Am. Chem. Soc., 2011, 133, 9266; (g) Y. Haketa,
S. Sakamoto, K. Chigusa, T. Nakanishi and H. Maeda, J. Org.
Chem., 2011, 76, 5177.
structure of 2dꢀClꢁ-(TATAC3 +
) .
less p-extended geometry of the receptor molecules (Fig. S33
and S34, ESIw). These observations suggest that b-benzo-fused
2d is more effective in constructing charge-based columnar
assemblies than b-aryl- and b-alkoxy-substituted receptors.
In summary, anion-responsive p-conjugated molecules
possessing aliphatic substituents at pyrrole b-positions have
been synthesized in order to form assembled structures in the
presence of anions and counter cations and in the absence of them.
On the basis of the various receptor molecules, orientation of the
substituents attached to the core p-plane was found to be an
essential factor in determining assembled modes in the case of both
anion-free receptors and receptor–anion complexes. Modification
of receptor molecules would provide not only charge-by-charge
assemblies but also charge-separated assemblies. Preparation of
such functional charge-based materials is currently underway.
This work was supported by PRESTO/JST (2007–2011),
Grants-in-Aid for Young Scientists (B) (No. 21750155) and
(A) (No. 23685032) from the MEXT and Ritsumeikan
R-GIRO project (2008–2013). We thank Prof. Atsuhiro
Osuka, Dr Shohei Saito, Mr. Eiji Tsurumaki and Mr. Taro
Koide, Kyoto University, for single-crystal X-ray analysis,
Dr Noboru Ohta, JASRI/SPring-8, for synchrotron XRD
measurements (BL40B2 at SPring-8: 2010A1504, 2010A1621,
2011A1294 and 2011B1535), Prof. Tomonori Hanasaki,
Ritsumeikan University, for DSC and POM measurements
and Prof. Hitoshi Tamiaki, Ritsumeikan University, for
various measurements. Y.H. thanks JSPS for a Research
Fellowship for Young Scientists.
8 Charge-by-charge assemblies based on modified anions: H. Maeda,
K. Naritani, Y. Honsho and S. Seki, J. Am. Chem. Soc., 2011,
133, 8896.
9 S. Ito, H. Uno, T. Murashima and N. Ono, Tetrahedron Lett.,
2001, 142, 45.
Notes and references
10 (a) T. Fukuda, E. Sudo, K. Shimokawa and M. Iwao, Tetrahedron,
2008, 64, 328; (b) C. Zonta, F. Fabris and O. D. Lucchi, Org. Lett.,
2005, 7, 1003.
z Crystal data for 3a (from CH2Cl2/hexane): C35H25N2O2BF2, Mw
=
%
554.38, triclinic, P1 (no. 2), a = 10.810(4), b = 11.458(5), c =
12.236(6) A, a = 101.958(18), b = 107.036(17), g = 99.465(15)1,
V = 1375.9(10) A3, T = 123(2) K, Z = 2, Dc = 1.338 g cmꢁ3
,
¨
11 (a) A. Merz, R. Schropp and E. Dotterl, Synthesis, 1995, 795;
m(Mo-Ka) = 0.092 mmꢁ1, R1 = 0.0406, wR2 = 0.0952, GOF = 1.055
(I 4 2s(I)). CCDC 852920. Crystal data for 3b (from EtOAc/
heptane): C39H33BF2N2O6, Mw = 674.50, monoclinic, C2/c (no. 15),
a = 14.7974(4), b = 20.7489(5), c = 10.5484(2) A, b = 94.9674(15)1,
(b) T. Murashima, K. Hirai and Y. Uchihara, Tetrahedron Lett.,
1998, 39, 5397.
12 M. J. Frisch, et al., Gaussian 03, Revision C.01, Gaussian, Inc.,
Wallingford, CT, 2004.
13 (a) B. W. Laursen and F. C. Krebs, Angew. Chem., Int. Ed., 2000,
39, 3432; (b) B. W. Laursen and F. C. Krebs, Chem.–Eur. J., 2001,
7, 1773.
V = 3226.51(13) A3, T = 93(2) K, Z = 4, Dc = 1.7983 g cmꢁ3
,
m(Mo-Ka) = 0.835 mmꢁ1, R1 = 0.0441, wR2 = 0.1164, GOF =
1.092 (I 4 2s (I)). CCDC 852921. Crystallographic data for 4b:
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2301–2303 2303