Cation modules as building blocks forming supramolecular assemblies with planar receptor-anion complexes

Article abstract of DOI:10.1021/ja312214a

Ion-based materials were fabricated through ion pairing of planar receptor-anion complexes and cation modules as negatively and positively charged building blocks, respectively. Anion receptors that could not form soft materials by themselves provided mesophases upon anion binding and subsequent ion pairing with aliphatic cation modules. The mesogenic behaviors were affected by structural modification of both the cation module and the anion receptor. Synchrotron X-ray diffraction measurements suggested the formation of columnar mesophases with contributions from charge-by-charge and charge-segregated arrangements. Flash-photolysis time-resolved microwave conductivity measurements further revealed a higher charge-carrier mobility in the assembly with a large contribution from the charge-segregated arrangement than in the charge-by-charge-based assembly.

Full text of DOI:10.1021/ja312214a

Communication
pubs.acs.org/JACS
Cation Modules as Building Blocks Forming Supramolecular
Assemblies with Planar Receptor−Anion Complexes
Bin Dong,† Tsuneaki Sakurai,‡ Yoshihito Honsho,‡ Shu Seki,‡ and Hiromitsu Maeda*,†
†College of Pharmaceutical Sciences, Ritsumeikan University, Kusatsu 525-8577, Japan
‡Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita 565-0871, Japan
S
* Supporting Information
ABSTRACT: Ion-based materials were fabricated through
ion pairing of planar receptor−anion complexes and cation
modules as negatively and positively charged building
blocks, respectively. Anion receptors that could not form
soft materials by themselves provided mesophases upon
anion binding and subsequent ion pairing with aliphatic
cation modules. The mesogenic behaviors were affected by
structural modification of both the cation module and the
anion receptor. Synchrotron X-ray diffraction measure-
ments suggested the formation of columnar mesophases
with contributions from charge-by-charge and charge-
segregated arrangements. Flash-photolysis time-resolved
microwave conductivity measurements further revealed a
higher charge-carrier mobility in the assembly with a large
contribution from the charge-segregated arrangement than
in the charge-by-charge-based assembly.
Figure 1. (a) Anion receptors 1a, 1b, 2a, and 2b and their anion-binding
mode. (b) Structures of the cation modules.
3,4,5-trialkoxy-substituted aryl moieties in the receptors. In view
of these studies along with the soft materials comprising modified
anions,7 anion receptors that cannot form soft materials by
themselves would afford soft materials when combined with
cationic species possessing such aliphatic units. Ion-based
materials formed by van der Waals interactions of modified
cations as counterions of receptor−anion complexes have not
been previously reported. Herein, the use of cation modules such
as benzyltrialkylammonium chlorides (Bn3NCl and 16Bn3NCl, n
= 2 and 4) and benzylpyridinium chlorides (BPyCl and
16BPyCl) (Figure 1b) to construct ion-based supramolecular
assemblies with 1a, 1b, 2a, and 2b was examined. Columnar
mesophases based on charge-by-charge and charge-segregated
assemblies were formed with the appropriate combinations of
planar receptor−anion complexes and cation modules.
he arrangement of charged species in an appropriate
Tfashion can potentially provide assemblies exhibiting
fascinating electronic properties that cannot be observed in
those comprising electronically neutral molecules.1,2 Charged
species, electron-rich anionic and electron-deficient cationic
states, can be used to form assembled structures on the basis of
electrostatic attractive and repulsive interactions between
opposite and identical charges, respectively. Other parameters,
such as van der Waals interactions, enable the arrangement of the
charged species in various packing structures in the assemblies.
Therefore, the choice of charged species is essential for the
fabrication of materials with unique assembled structures and
resulting properties.3 As candidates for the components of ion-
based materials, anion complexes of pyrrole-based π-conjugated
receptors (e.g., 1a, 1b, 2a, and 2b in Figure 1a)4 have been shown
to be planar anionic species that are suitable for the formation of
assembled structures with countercations.5−7 In this case, it was
found that the geometries of the cationic species predominantly
modulate the assembled structures. In particular, the introduc-
tion of planar cations to the receptor−anion complexes afforded
charge-by-charge assemblies comprising alternately stacking
positively and negatively charged species5a and charge-
segregated assemblies with nanoscale-segregated structures of
charged species6 as soft materials, such as thermotropic liquid
crystals. Bulky alkylammonium cations were also found to act as
counterparts of the receptor−anion complexes, forming
mesophases.5b In these examples, the soft materials were
supported by van der Waals interactions between the aliphatic
Solid-state assemblies of the receptor−Cl− complexes with
cation modules possessing unsubstituted benzyl units were
revealed by single-crystal X-ray analysis (Figure 2).8 In the solid
state of 1a·Cl−−B43N+, a bridging CH and an inverted pyrrole
NH associate with a Cl− anion with C/N(−H)···Cl− distances of
3.919 and 3.169 Å, respectively. The other pyrrole NH at the
opposite side is bound to another Cl− anion (N(−H)···Cl− =
3.368 Å) in the absence of pyrrole inversion, resulting in a Cl−-
bridged one-dimensional (1D) chain in a perfectly linear array
with a Cl−···Cl− distance of 9.814 Å. The dihedral angles between
the core plane and the two pyrrole rings were estimated as 7.80
Received: December 14, 2012
Published: January 9, 2013
© 2013 American Chemical Society
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Journal of the American Chemical Society
Communication
of 5.316 and 3.427 Å, respectively. In addition, there seem to be
less specific interactions between the neighboring 1D chains in
both complexes, except for weak C−H···Cl− hydrogen bonding.
The edge-to-edge stacking of the 1D chains in 2a·Cl−−B23N+
can be considered as the contribution of charge-segregated
assembly. It is noteworthy that the anion-binding modes with
partial or no pyrrole inversion in the solid state are derived from
the crystals based on cationic species with unsubstituted benzyl
units and would be different from those in solution and in soft
materials.
N-Based aliphatic cation-module salts 16Bn3NCl (n = 2 and 4)
and 16BPyCl were obtained by reacting the corresponding
tertiary amines (triethylamine, tributylamine, and pyridine) with
tri-C16H33O-substituted benzyl chloride.10 Subsequently, salts of
receptor−Cl− complexes with these cation modules were
prepared by evaporating solutions of the anion receptors and
cation-module chlorides in 1:1 molar ratios in CH2Cl2. The
products were identified by 1H NMR spectroscopy and
elemental analysis. The cation-module chloride salts are white
solids, whereas their solid-state [1 + 1]-type complexes with 1a,
1b, 2a, and 2b are yellow, red, orange, and red solids with
absorption maxima at 451/446/450, 523/514/520, 441/440/
446, and 501/498/500 nm, respectively. Their solid-state
emission peaks are blue-shifted relative to the anion-free
receptors because stacking of the receptors is interrupted by
aliphatic cation modules upon anion binding.
Figure 2. Single-crystal X-ray structures of (a) 1a·Cl−−B43N+ and (b)
2a·Cl−−B23N+: (i) Cl−-bridged 1D chain and (ii) charge-by-charge-
based packing diagram. Atom color code: brown, C; pink, H; yellow, B;
yellow-green (spherical), Cl; green, F; blue, N; red, O. Solvent
molecules in (b) have been omitted for clarity.
and 7.58°, indicating a fairly planar structure for the receptor.
Furthermore, the charge-by-charge-based arrangement compris-
ing alternately stacking 1a·Cl− and B43N+ layers at an angle of
3.25° with respect to the perpendicular line was observed. In the
charge-by-charge assembly, the distances between 1a·Cl− units
and between Cl− ions are 8.011 and 8.520 Å, respectively, and the
phenyl moiety in B43N+ is located between the uninverted
pyrrole units of the receptors with distances of 4.056 and 3.695 Å.
The distances between the proximal Cl− and the cation N exhibit
inequivalent values of 4.402 and 4.364 Å. The charge-by-charge
assembly of 1a·Cl−−B43N+ is similar to but more ordered than
that of 1a·Cl−−(C4H9)4N+,4a suggesting that the introduction of
a π-conjugated moiety in the cationic species is an effective way
to form alternately stacking structures as observed in 1a2·Cl−−
The Cl− complexes of 1a, 1b, 2a, and 2b as 16Bn3N+ (n = 2 and
4) and 16BPy+ salts showed different thermal behaviors from the
individual components.11 The mesogenic temperature ranges
varied with the structural changes of the cation modules as well as
those of the anion receptors, as observed by differential scanning
calorimetry (DSC) and polarized optical microscopy (POM)
(Table 1). The delicate balance between the positively and
negatively charged components, such as the size of the ionic part
in the cation module and the substituents on the anion receptor,
enabled self-organization into mesophases.2c,5b During POM
measurements, for example, pseudo-focal-conic fanlike, flakelike,
fiberlike, and needlelike textures were observed in 1a·Cl−−
16B23N+, 1b·Cl−−16B43N+, 2a·Cl−−16B23N+, and 2b·Cl−−
16B43N+, respectively, upon the second heating after cooling
from the isotropic liquid phase (Iso) (Figure 3).
The detailed structures were examined by synchrotron X-ray
diffraction (XRD) analysis. The XRD profile of 1a·Cl−−
16B23N+ at 90 °C upon second heating, which exhibited d
spacings of 3.83, 2.23, 1.92, and 1.46 nm corresponding to (100),
(110), (200), and (210), respectively, suggested the formation of
a hexagonal columnar (Colh) structure (Figure 4a(ii)). Its
distinct difference from that at low temperature (25 °C; Figure
4a(i)) was the peak at around 0.4 nm derived from the alkyl
chains, which was broad or sharp at high or low temperatures,
4f
TATA+ and 1b·Cl−−TATA+,5a wherein TATA+ is a planar
triazatriangulenium cation.9 The solid state of the β-F derivative
2a·Cl−−B23N+, contains a more planar unit, with dihedral angles
of 2.46 and 3.79° between the core plane and the two pyrrole
rings, neither of which is inverted. In this case, hydrogen bonds
are formed only between the pyrrole NH and Cl−, with
N(−H)···Cl− distances of 3.161 and 3.149 Å, possibly because of
the greater polarization of the NH than in β-H derivative.4b Cl−-
bridged 1D chains with a Cl−···Cl− distance of 9.895 Å and a
Cl−···Cl−···Cl− angle of 115.97° are present. Moreover, a less
ordered charge-by-charge assembly is formed, with a Cl−···Cl−
distance of 9.250 Å and a Cl−···Cl−···Cl− angle of 125.47° in one
column consisting of alternately stacking 2a·Cl− and B23N+
layers. The distances between the proximal Cl− and the cation N
also have inequivalent values of 4.949 and 4.521 Å. The phenyl
moiety in B23N+ is located between the core plane of one
receptor and the pyrrole ring of another receptor with distances
a
Table 1. Phase Transitions of Cation-Module Chloride Salts and Their Ion Pairs in the Presence of 1a, 1b, 2a, and 2b
receptor
16B23NCl
16B43NCl
16BPyCl
b
b
f
b
c
c
b
b
b
f
none
1a
Cr 38.7 Cr′ 63.7 Colh 129.4 Iso
Cr 42.4 Cr′ 54.0 Cr′ 76.4 Iso [Cr 38.5 Colh 81.0 Iso] Cr 55.2 Cr′ 81.1 Colh 125.9 Iso
b
f
d
b
f
Cr 50.1 Colh 132.5 Iso
Cr 43.8 Iso
Cr 54.3 Colh 133.7 Iso
c
c
c
b
f
c
c
f
1b
Cr 37.1 Cr′ 45.6 Iso [Cr 40.4 Iso]
Cr 38.6 Colh 143.1 Iso
Cr 48.3 Cr′ 127.3 Iso
b f
c
b
b
b
b
2a
Cr 6.5 Cr′ 35.7 Colh 102.4 Iso
Cr 39.1 Cr′ 59.6 Cr′ 92.1 Iso
Cr 39.3 Colh 53.1 Iso
e
e
e
e
e
d
e
d
c
b
b
2b
Cr 29.1 Cr′ 51.3 Iso [Cr 30.7 Cr′ 49.4 Cr′ 54.1 Iso] Cr 30.7 M 69.8 Iso [Cr 36.6 M′ 74.2 Iso]
Cr 30.2 Cr′ 45.7 Iso
a
Phases and transition temperatures (in °C, the onset of the peak) from DSC upon second heating (5 °C/min) are listed. The entries in brackets
b
show the transitions upon first cooling, which exhibit phases different from those upon second heating. Basically as a hexagonal columnar structure.
c
d
e
f
Unidentified structure. Basically as a lamellar structure. Basically as a rectangular columnar structure. Value obtained from POM measurements.
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Communication
alkyl chain length in the anion module.7 Herein, the use of
dendritic cation modules as building blocks with 1a, 1b, 2a, and
2b afforded 2D columnar mesophases, implying the importance
of the ionic parts.
The (001) diffraction peak, assignable to the stacking distance
between the adjacent assembled units (c), was not observed in all
of the cation-module-based mesophases. In the Colh mesophase
of 1b·Cl−−16B43N+ (Figure 4b), the c value of 0.74 nm implies
that one assembled unit consists of four 1b·Cl−/16B43N+ pairs
(Z ≈ 4 for ρ = 0.7; Z is the expected number of ion-pair
components in a unit cell and ρ is the corresponding density) and
that charge-by-charge arrangements are formed with alternately
stacking cations and receptor−anion complexes, as observed in
the single crystal of 1a·Cl−−B43N+.5,7 As for the β-F derivatives,
the c value was located around 0.38 nm in 2a·Cl−−16B23N+ (Z ≈
2 for ρ = 1, Colh; Figure 4c) and 2b·Cl−−16B43N+ (Z ≈ 4 for ρ =
0.7, Colr (C2/m)), which is comparable to the π−π stacking
distance, strongly suggesting the local stacking of identical
charged species, further producing a column.6 The β-substituents
on the anion receptors are responsible for these charge-
segregated assemblies, as the fluorine moieties withdraw
electrons by an inductive effect and make the intermolecular
interactions among the anionic complexes sufficiently robust for
stacking structures.13 On the other hand, the enhanced van der
Waals interactions among the long alkyl chains in the cation
modules also contribute to the different packing modes in the
mesophases, compared with the assembled structure in the single
crystal of 2a·Cl−−B23N+.
Figure 3. Representative POM textures of (a) 1a·Cl−−16B23N+ at 90
°C, (b) 1b·Cl−−16B43N+ at 125 °C, (c) 2a·Cl−−16B23N+ at 85 °C, and
(d) 2b·Cl−−16B43N+ at 55 °C upon second heating after cooling from
Iso.
To evaluate the potential uses of ion-based materials with
charge-by-charge or charge-segregated structures, flash-photol-
ysis time-resolved microwave conductivity (FP-TRMC) meas-
urements were performed for 1b·Cl−−16B43N+ and 2a·Cl−−
16B23N+. This method is a powerful technique for probing the
local motion of charge-carrier species.14 Kinetic traces of ϕ∑μ
were obtained by TRMC (Figure 5), where ϕ denotes the
Figure 4. Representative XRD patterns of (a) 1a·Cl−−16B23N+ (i) at 25
°C after once melting to Iso and (ii) at 90 °C upon second heating, (b)
1b·Cl−−16B43N+ at 90 °C, and (c) 2a·Cl−−16B23N+ at 85 °C upon
second heating. Schematic illustrations of the assembled arrangements
are shown at the right in (b) and (c). Anions are shown as green spheres,
anion receptors as pink partial disks, and cation modules as fanlike
shapes based on the optimized structures; the charge-by-charge and
charge-segregated assemblies are represented by thick and thin disk
components, respectively.
Figure 5. FP-TRMC transient signals for 1b·Cl−−16B43N+ (bottom)
and 2a·Cl−−16B23N+ (top) at room temperature after once melting
upon excitation at 355 nm with a power of 10 mW. Schematic
illustrations of the motion of the charge carriers are shown at the right.
respectively. The intercolumnar distance (a) was estimated to be
4.42 nm, in accordance with the optimized structure.12 Similarly,
XRD revealed the Colh structures in the mesophases of 1b·Cl−−
16B43N+ and 2a·Cl−−16B23N+ (at 90 and 85 °C, respectively,
upon second heating; Figure 4b,c). In contrast, 2b·Cl−−
16B43N+ at 55 °C upon second heating showed a complicated
XRD pattern. The diffraction peaks in the low-angle regions
corresponding to d spacings of 4.91, 3.60, 2.46, 1.64, 1.23, and
1.17 nm could be assigned as (200), (110), (400), (600), (800),
and (810), respectively, indicating a rectangular columnar (Colr,
C2/m) arrangement. As previously reported, the complexes of 1a
and 1b with dendritic anion modules (modified gallates)
provided 1D lamellar mesophases with properties tuned by the
quantum yield of charge-carrier generation and ∑μ is the sum of
the charge-carrier mobilities. Conductivity transients observed
for both samples on a quartz plate showed a prompt rise in the
signals, indicating that the singlet excited states of those
compounds were responsible for free carrier generation. The
maximum values of ϕ∑μ were 1.2 × 10−5 and 5.6 × 10−5 cm2 V−1
s−1 for 1b·Cl−−16B43N+ and 2a·Cl−−16B23N+, respectively,
suggesting that charge-segregated assemblies with a smaller
stacking distance realize a higher degree of delocalization of the
charge carriers along the columnar axis.15 From the ϕ values of
0.23 and 0.25% estimated from a current integration method,14b
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Journal of the American Chemical Society
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17014. (g) Ichikawa, T.; Yoshio, M.; Hamasaki, A.; Taguchi, S.; Liu, F.;
Zeng, X. B.; Ungar, G.; Ohno, H.; Kato, T. J. Am. Chem. Soc. 2012, 134,
2634.
(3) A recent review of ion-based materials comprising planar charged
species: Dong, B.; Maeda, H. Chem. Commun. 2013, DOI: 10.1039/
c2cc34407f.
the corresponding mobilities ∑μ were calculated to be 0.05
0.01 and 0.22 0.03 cm2 V−1 s−1 for 1b·Cl−−16B43N+ and
2a·Cl−−16B23N+, respectively. The increase in mobility for
charge-segregated assemblies offers the possibility of enhancing
the efficiency of charge-carrier transport in organic electronic
devices.
(4) Some examples: (a) Maeda, H.; Kusunose, Y. Chem.Eur. J. 2005,
11, 5661. (b) Maeda, H.; Ito, Y. Inorg. Chem. 2006, 45, 8205. (c) Maeda,
H.; Haketa, Y.; Nakanishi, T. J. Am. Chem. Soc. 2007, 129, 13661.
(d) Maeda, H.; Bando, Y.; Shimomura, K.; Yamada, I.; Naito, M.;
Nobusawa, K.; Tsumatori, H.; Kawai, T. J. Am. Chem. Soc. 2011, 133,
9266. (e) Haketa, Y.; Sakamoto, S.; Chigusa, K.; Nakanishi, T.; Maeda,
H. J. Org. Chem. 2011, 76, 5177. (f) Haketa, Y.; Takayama, M.; Maeda,
H. Org. Biomol. Chem. 2012, 10, 2603. (g) Haketa, Y.; Bando, Y.;
Takaishi, K.; Uchiyama, M.; Muranaka, A.; Naito, M.; Shibaguchi, H.;
Kawai, T.; Maeda, H. Angew. Chem., Int. Ed. 2012, 51, 7967.
(5) Examples of charge-by-charge-based soft materials: (a) Haketa, Y.;
Sasaki, S.; Ohta, N.; Masunaga, H.; Ogawa, H.; Mizuno, N.; Araoka, F.;
Takezoe, H.; Maeda, H. Angew. Chem., Int. Ed. 2010, 49, 10079.
(b) Dong, B.; Terashima, Y.; Haketa, Y.; Maeda, H. Chem.Eur. J.
2012, 18, 3460. (c) Bando, Y.; Sakamoto, S.; Yamada, I.; Haketa, Y.;
Maeda, H. Chem. Commun. 2012, 48, 2301.
In summary, the non-covalent association of planar anion
receptors and cation-module salts afforded various supra-
molecular assemblies that existed as solid and soft materials.
Both the ionic parts of the cation modules and the substituents
on the anion receptors were powerful contributors to the
modulation of the assembled structures in mesophases. Their
appropriate complexation can result in charge-by-charge
assemblies or charge-segregated assemblies. Derived from
favorable overlap of charged π-planes, ion-based materials with
charge-segregated arrangements provided charge-carrier mobi-
lity 1 order of magnitude higher than charge-by-charge-based
materials, making superior electric-conducting materials. Highly
ordered arrangement of charged species is a key factor to exhibit
the enhanced performance. Compared with the restricted
anionic species that can efficiently bind anion receptors, a variety
of cationic species was more advantageous for fine-tuning ion-
based assemblies comprising receptor−anion complexes and
thus for preparing useful types of functional materials.
(6) Charge-segregated assemblies: Haketa, Y.; Honsho, Y.; Seki, S.;
Maeda, H. Chem.Eur. J. 2012, 18, 7016.
(7) Ion-based soft materials were prepared by complexing 1a or 1b
with anion modules (tetrabutylammonium salts of modified gallates
with aliphatic chains): Maeda, H.; Naritani, K.; Honsho, Y.; Seki, S. J.
Am. Chem. Soc. 2011, 133, 8896.
ASSOCIATED CONTENT
* Supporting Information
Synthesis procedures and additional data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(8) (a) Crystal data for 1a·Cl−−B43N+ (from CHCl3/iPr2O):
C30H43BClF2N3O2, MW = 561.93, triclinic, P1 (No. 1), a = 8.5203(2)
Å, b = 9.0253(3) Å, c = 9.8139(3) Å, α = 84.4633(17)°, β =
86.7504(17)°, γ = 85.1185(16)°, V = 747.53(4) Å3, T = 93(2) K, Z = 1,
Dcalcd = 1.248 g/cm3, μ(Cu Kα) = 1.489 mm−1, R1 = 0.0621, wR2 =
0.1379, GOF = 0.993 (I > 2σ(I)), CCDC 894188. (b) Crystal data for
2a·Cl−−B23N+ (from CHCl3/hexane): C24H27BClF6N3O2·1.5H2O,
MW = 576.77, monoclinic, C2/c (No. 15), a = 21.1495(4) Å, b =
16.7800(3) Å, c = 16.4444(3) Å, β = 114.7240(11)°, V = 5300.97(17)
Å3, T = 93(2) K, Z = 8, Dcalcd = 1.445 g/cm3, μ(Cu Kα) = 1.966 mm−1, R1
= 0.0486, wR2 = 0.1237, GOF = 1.033 (I > 2σ(I)), CCDC 894189.
(9) (a) Laursen, B. W.; Krebs, F. C. Angew. Chem., Int. Ed. 2000, 39,
3432. (b) Laursen, B. W.; Krebs, F. C. Chem.Eur. J. 2001, 7, 1773.
(10) (a) Percec, V.; Peterca, M.; Tsuda, Y.; Rosen, B. M.; Uchida, S.;
Imam, M. R.; Ungar, G.; Heiney, P. A. Chem.Eur. J. 2009, 15, 8994.
(b) Imam, M. R.; Peterca, M.; Edlund, U.; Balagurusamy, V. S. H.;
Percec, V. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 4165.
(11) The melting points of 1a, 1b, 2a, and 2b are 249.1, 298.8, 285.7,
and 299.0 °C, respectively.
■
S
AUTHOR INFORMATION
Corresponding Author
maedahir@ph.ritsumei.ac.jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grant-in-Aid for Young Scientists
(A) (23685032) from MEXT and the Ritsumeikan R-GIRO
Project (2008−2013). We thank Prof. Atsuhiro Osuka, Dr.
Naoki Aratani, and Mr. Hirotaka Mori (Kyoto University) for
single-crystal X-ray analysis; Dr. Noboru Ohta (JASRI/SPring-8)
for synchrotron radiation XRD measurements (BL40B2 at
SPring-8); Prof. Tomonori Hanasaki (Ritsumeikan University)
for DSC and POM measurements; and Prof. Hitoshi Tamiaki
(Ritsumeikan University) for various measurements.
(12) Optimized structures of the receptor−anion complexes (ref
4a−c,e) and cation modules were obtained using Gaussian 03: Frisch,
M. J.; et al. Gaussian 03, revision C.01; Gaussian, Inc.: Wallingford, CT,
2004.
(13) Reviews of fluorinated organic materials: (a) Babudri, F.; Farinola,
G. M.; Naso, F.; Ragni, R. Chem. Commun. 2007, 1003. (b) Berger, R.;
Resnati, G.; Metrangolo, P.; Weber, E.; Hulliger, J. Chem. Soc. Rev. 2011,
40, 3496.
(14) (a) Saeki, A.; Seki, S.; Sunagawa, T.; Ushida, K.; Tagawa, S. Philos.
Mag. 2006, 86, 1261. (b) Saeki, A.; Fukumatsu, T.; Seki, S.
Macromolecules 2011, 44, 3416. (c) Saeki, A.; Koizumi, Y.; Aida, T.;
Seki, S. Acc. Chem. Res. 2012, 45, 1193.
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