Carbon-bridged oligo(phenylenevinylene)s: Stable φ-systems with high responsiveness to doping and excitation

Article abstract of DOI:10.1021/ja309318s

The high responsiveness of φ-conjugated materials to external stimuli, such as electrons and photons, accounts for both their utility in optoelectronic applications and their chemical instability. Extensive studies on heteroatom-stabilized φ-conjugated systems notwithstanding, it is still difficult to combine high performance and stability. We report here that carbon-bridged oligo(p-phenylenevinylene)s (COPV-n) are not only more responsive to doping and photoexcitation but also more stable than the conventional p-phenylenevinylenes and poly(3-hexylthiophene), surviving photolysis very well in air, suggesting that they could serve as building blocks for optoelectronic applications. Activation of the ground state by installation of bond angle strain toward the doped or photoexcited state and the flat, rigid, and hindered structure endows COPVs with stimuli-responsiveness and stability without recourse to heteroatoms. For example, COPV-6 can be doped with an extremely small reorganization energy and form a bipolaron delocalized over the entire φ-conjugated system. Applications to bulk and molecular optoelectronic devices are foreseen.

Full text of DOI:10.1021/ja309318s

Article
pubs.acs.org/JACS
Carbon-Bridged Oligo(phenylenevinylene)s: Stable π‑Systems with
High Responsiveness to Doping and Excitation
Xiaozhang Zhu,†,∥ Hayato Tsuji,*,†,‡ Juan T. Lopez Navarrete,§ Juan Casado,§ and Eiichi Nakamura*,†
́
†Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
‡JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
§Department of Physical Chemistry, University of Mal
́ ́
aga, Campus de Teatinos s/n, Malaga 29071, Spain
S
* Supporting Information
ABSTRACT: The high responsiveness of π-conjugated materials to
external stimuli, such as electrons and photons, accounts for both
their utility in optoelectronic applications and their chemical
instability. Extensive studies on heteroatom-stabilized π-conjugated
systems notwithstanding, it is still difficult to combine high
performance and stability. We report here that carbon-bridged
oligo(p-phenylenevinylene)s (COPV-n) are not only more responsive
to doping and photoexcitation but also more stable than the
conventional p-phenylenevinylenes and poly(3-hexylthiophene),
surviving photolysis very well in air, suggesting that they could
serve as building blocks for optoelectronic applications. Activation of the ground state by installation of bond angle strain toward
the doped or photoexcited state and the flat, rigid, and hindered structure endows COPVs with stimuli-responsiveness and
stability without recourse to heteroatoms. For example, COPV-6 can be doped with an extremely small reorganization energy
and form a bipolaron delocalized over the entire π-conjugated system. Applications to bulk and molecular optoelectronic devices
are foreseen.
addition, COPV-1 was found to serve as an effective molecular
wire in a metal-free sensitizer in dye-sensitized solar cells
(DSSCs), which recorded among the highest open-circuit
voltage and power conversion efficiency for metal-free dyes.5
Being aware that doping and photoexcitation weaken the
vinylene bonds in PPVs, we surmised that it is the ring strain of
the bicyclo[3.3.0]octatriene σ-framework that increased the
materials’ performance. To validate this idea and to gain ready
access to a broader range of materials, we developed a general
synthesis of COPVs and examined their behavior upon positive
doping and photoexcitation. We report here an efficient
synthesis of higher oligomers of COPVs, their remarkably
high stability as well as their polarons, bipolarons, and
photoexcited states, and on the physicochemical evidence of
strain-driven weakening of the vinylene bonds. The COPV
oligomers are far more stable under photolysis in air than that
of the parent PVs and P3HT [poly(3-hexylthiophene)], which
quickly decomposed under the same conditions. Unlike those
of heteroatom-stabilized oligomers and polymers, the dication
of COPV-6 is delocalized over the entire π-system and behaves
as a bipolaron instead of a polaron pair.6
INTRODUCTION
■
One major concern in the practical application of organic
optoelectronic devices is the stability of the π-conjugated
organic materials against heat, light, and oxygen. Because the
responsiveness of π-systems to external stimuli inevitably
generates chemically reactive species, such as radical cations,
radical anions, and photoexcited states, high stimuli-responsive-
ness and high chemical stability of organic materials appear to
be fundamentally incompatible. One common strategy to solve
this problem is to stabilize the π-system with heteroatoms;1
however, this strategy is not necessarily ideal because the
polarized carbon−heteroatom bonds localize polarons and
excitons in the molecule and create new decomposition
pathways. For example, heteroatom stabilization of otherwise
unstable poly(p-phenylenevinylene)s (PPVs) and oligo(PV)s
(OPVs) has led to the dramatic development of new research
areas.2 However, these compounds are structurally flexible and
are not sufficiently responsive to doping and photoexcitation
nor are they stable under ambient conditions or in the doped
and photoexcited states.3 To reconcile performance and
stability, we decided to eliminate entirely heteroatoms from
the π-system and implement a structural feature that can
stabilize the doped and excited states. We tested our idea on a
PV monomer and a dimer (COPV-1 and -2 in Figure 1a), in
which we buttressed the σ-framework of a PV unit with one-
carbon bridges and found that they exhibit ambipolar carrier
mobilities of 5−6 × 10−3 cm2/V s for both hole and electron,4 a
value close to the upper limit for amorphous materials. In
RESULTS AND DISCUSSION
Synthesis of COPVs. We synthesized COPV-n (n = 3−6)
by an iterative synthetic strategy exploiting the one we
■
Received: September 20, 2012
Published: October 29, 2012
© 2012 American Chemical Society
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Figure 1. COPV-1−6 and COPV-4 cations. Ar = 4-octylphenyl. (a) Molecular structures. (b) Single-crystal X-ray analysis of COPV-4 (ORTEP
drawing; 30% probability for thermal ellipsoid; hydrogen atoms, disordered component, and solvent atoms are omitted for clarity). (c)
Representative structural data (in Å and °; substituents are omitted) of COPV-4 (X-ray), and [COPV-4]•+ (bracket; calculated at the UB3LYP/6-
31G** level). The OPV skeleton is highlighted in bold. Upon one-electron oxidation, the vinylene bond becomes shorter by 2.6%, and similarly the
C−C bonds between the phenylene and vinylene groups become slightly longer.
previously developed for COPV-1 and -2 (Supporting
Information).4 Each step took place smoothly and reproducibly
and produces versatile intermediates, such as COPV-3(I2), that
are useful for synthesizing longer oligomers, polymers, and
other derivatives of COPVs. One PV unit is derived from one
diarylacetylene and one arylmethanone unit. We incorporated
several 4-octylphenyl groups to minimize intermolecular
interactions (Figure 1b) and to enhance the solubility of the
compounds. All compounds have C2 symmetry, showed readily
discernible NMR signals, and were further characterized by
mass spectrometry and elemental analysis. COPV-6 has a
molecular weight of 4007.89 and is 4.2 nm in length.
the vinylene bonds, as revealed by the low Raman frequency of
the double-bond stretching (see below). Nonetheless, the
equilibrium bond length of 1.348 Å is rather close to the
normal CC double-bond length of 1.34 Å. The conjugated
core of the molecule is slightly distorted from planarity because
of either crystal packing or Jahn−Teller distortion (Figures 1b
and S21a). The crystal structure illustrates the steric protection
of the π-system by the 4-octylphenyl groups (Figure 1b), and in
a single crystal, they separate each molecule far enough to avoid
intermolecular interaction, as compared with nonbridged OPV-
47 (Figure S21b), and we consider that it is the case in solution
and on the surface of a substrate.5
X-ray crystallographic structure of COPV-4 shows excep-
tionally large bond angles around the vinylene carbon atoms,
136.6° and 137.6° (Figure 1c). Such large bond angles weaken
Stability. The most notable property of COPVs relevant to
materials applications is high thermal and photochemical
stability. COPVs can be kept on a shelf in air without
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decomposition at least for one year. COPV-6 melts at 220 °C,
and its TG-DTA analysis under nitrogen showed 5 wt % loss at
432 °C. Such stability of COPVs stands in contrast to the
instability of unsubstituted PVs with several repeating units
(>2).
correlation in turn indicates that the octyl group in the Ar
group does not affect the transition energies of COPV
framework, which is supported by the fact that COPV-1,
bearing the Ar groups and its congener bearing four phenyl
groups,4a shows exactly the same UV−vis spectra. The slope8 is
much larger than the value of 1.31 reported for OPVs
(dipropoxy-substituted oligo(phenylenevinylene)s at 25 °C).
Based on Meier’s equation,9 the maximum absorption of an
infinite chain (λ∞) was determined to be 544 nm (Figure 3d),
which is 63 nm longer than that of OPVs, and the maximum
effective conjugation length is 13 PV units, which is larger than
the value of 11 units reported for OPVs.
The photoluminescence properties of COPVs support the
similarity of the structures between the ground and the excited
states. The emission spectra are close to those of mirror images
of the absorption spectra (Figure 2a vs 2b), proving that most
of the intensity in the absorption bands is due to a single
electronic transition (HOMO → LUMO). The emission
maximum wavelengths of COPVs are progressively longer
(Figure 2b, and data in rows 3−6 of Table 1). Their Stokes
shifts are very small and decrease progressively from 1309 to
501 cm−1 as n increases (Table 1, row 6), which are markedly
smaller than those of OPVs (4036 to 2675 cm−1). These values
translate into reorganization energies (λ(M/M*)) from 0.096
to 0.062 eV. The fluorescence quantum yields were almost
unity irrespective of the chain length.10 The fluorescence
lifetimes (τ) are 3.15−1.37 ns and are significantly longer than
those of the corresponding OPVs, 0.62−0.25 ns.11 The
lifetimes of COPVs exhibit an excellent linear relationship
with the reciprocal number of repeating units (1/n) (Figure 2e,
R2 = 0.998). These data indicate the absence of nonradiative
decay pathways which are predominant for OPVs.
Photostability is particularly important. COPV-4 survives
photolysis at 30 °C with a half-life of 1578 min in air-saturated
methylcyclohexane solution under illumination with a 300 W
xenon lamp (intensity of ca. 1 × 106 W/m2). Comparison of
the zero-order decay of the strongest absorption in the visible
light region indicated that P3HT decomposes 8.3 times faster
than COPV-4. Similarly, 1,4-bis[(E)-styryl]benzene decom-
poses over 100 times faster with a half-life of only 12 min. As
discussed below, this stability originates from the lack of
nonradiative decay of the singlet excited state of COPVs.
Absorption, Emission, and Electrochemical Properties
of COPVs. The spectral properties support very effective
conjugation in COPVs because of the rigid planar structure.
UV−vis absorption spectra of the neutral COPVs exhibited
intense, well-defined vibronic bands (ε up to 1.8 × 105 L/
mol·cm, Figure 2a, and the top two rows of Table 1 together
COPVs could be oxidized up to stable tetracations under
cyclic voltammetric (CV) conditions (Table 1, row 8). COPV-
1 shows one reversible oxidation process at 0.86 V (vs Fc+/Fc)
at scan rates between 25 and 400 mV·s−1 with the cathodic/
anodic peak current ratios (ia/ic) equal to 1.0. The CV curves
did not change after 50 cycles and up to a potential of 2.1 V
without any electropolymerization. COPV-2 shows one
reversible oxidative process at 0.53 V as opposed to the more
difficult, irreversible oxidation of E-stilbene and 1,4-bis[(E)-
styryl]benzene at 1.01 and 0.74 V, respectively. We recorded
reversible two-electron oxidations for COPV-3 and COPV-4,
and reversible three-electron oxidation for COPV-5. COPV-6
exhibited an unprecedented reversible four-electron oxidation
waves at 0.18, 0.41, 0.76, and 0.98 V within the window of the
tetrabutylammonium perchlorate/dichloromethane system.
The reorganization energies λ(M/M+) (B3LYP and
UB3LYP) calculated for COPV-2−6 are 0.237, 0.211, 0.190,
0.168, and 0.147 eV, respectively. These values are much
smaller than nonbridged counterparts.12 The value of λ(M/M+)
for COPV-6, 0.147 eV, is similar to that for pentacene (ca.
0.100 eV).13
Absorption of Cationic COPVs. The high stability of the
cations of COPVs14 allowed us, for the first time among
heteroatom-free PV-type polymers and oligomers, to measure
the spectra of the radical cation and the dication of a series of
COPVs. We did not find any sign of decomposition of radical
cations and dications during the measurements at room
temperature.
A typical example is shown in Figure 3 for COPV-4, which
was oxidized in degassed anhydrous dichloromethane by
titration with a solution of one-electron oxidant, thianthrenium
Figure 2. Spectral properties of COPV-1−6 in dichloromethane (ca.
10−6 M). (a) UV−vis absorption; (b) emission; (c) linear plot of
transition energies against 1/n; (d) exponential fits of absorption data
of COPVs; and (e) linear plot of the fluorescence lifetimes vs the
reciprocal of the number of COPV units (n). (f) Photographs of colors
under room light and under 360 nm irradiation.
with the calculated absorption wavelengths) assignable to the
HOMO → LUMO (S0 → S1) excitation (see Table S1). The
colors are shown in Figure 2f.
As shown in Figure 2c, the plot of the 0−0 transition energy
against the reciprocal number of repeating units (1/n, measured
at 25 °C) showed an excellent linear correlation (Figure 2c, R2
= 0.9996) with a slope of 1.76. The perfect linearity of the
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Table 1. Photophysical and Electrochemical Properties of COPVs
properties
unit
nm
COPV-1
COPV-2
COPV-3
COPV-4
COPV-5
COPV-6
a
b
b
λabs
323, 336,
352
376, 396, 419
413, 437, 466
435, 461, 492 (338)
450, 478, 512 (366)
459, 488,
522 (387)
b
c
ε × 10−4
cm−1 M−1
nm
1.39
385
4.84
433, 458
1.00
8.70
479, 511
1.00
11.9
507, 542
1.00
15.1
526, 566
1.00
18.6
536, 575
1.00
d
λem
e
ΦF
0.98
3.15
1309
f
τ
ns
cm−1
2.10
1.78
1.60
1.51
1.37
Stokes
shift
772
583
602
520
501
g
λ(M/M*)
eV
V
−
0.86
0.096
0.53
0.072
0.075
0.064
0.062
h
1/2
Eox
0.32, 0.76
0.27, 0.60
0.17, 0.46, 0.94
0.18, 0.41, 0.76,
0.98
a
a
RC, λabs
nm
−
−
575, 640 1020,
1082
589, 648, 730, 1232,
1498
632, 705, 794, 1460,
1831
673, 752, 833, 1664,
2157
771, 862, 1818,
2445
DC, λabs
nm
−
946, 1078
1119, 1324
1329, 1628
1526, 1938
a
b
Absorption maximum wavelengths of the neutral species, radical cations and dications measured in dichloromethane. Subgap absorption.
c
d
e
Extinction coefficients for the most intense absorption band. Fluorescence maximum wavelengths measured in dichloromethane. Fluorescence
f
quantum yield determined using absolute method. Fluorescence lifetime measured with the time-correlated single-photon counting operation mode.
g
h
Reorganization energy between ground and excited states. CV on a carbon electrode with Bu4NClO4 in dichloromethane (0.1 M, vs Fc+/Fc).
The absorption data for the cations and dications compiled
in Figure 4 and Table 1 (rows 9−10) showed excellent linear
correlation between the transition energies of RC1 and RC2
and 1/n (Figure 4a). The low-energy RC1 bands are more
sensitive to the size of the conjugation than the RC2 bands,
which agree with the calculated data (Table S1). The maximum
absorptions of the COPV dication (DC band) also showed a
very good 1/n dependence on the transition energies. Notably,
the planar framework of COPV here forces the two polarons
(radical cations) to interact strongly with each other to form a
bipolaron (dication). In conventional flexible conjugated
systems, the system twists itself to leave two positive polarons
out of conjugation, thus forming a polaron pair.15
Raman Analysis of Neutral and Positively Charged
COPVs. The strain-driven activation of the ground state of
COPV-3−6 is conclusively demonstrated by the Raman
analysis of the neutral and cationic molecules. The analysis
also highlights their unique electronic properties in which
positive doping promotes electron redistribution within the
rigid CC/C−C core without much structural change.
The Raman spectra of the neutral compounds, COPV-1 to
-6, in the solid state are shown Figure 5a. One conspicuous
feature is a much smaller number of Raman bands than for
Figure 3. UV−vis NIR spectral titration of COPV-4 (2.68 × 10−6 M)
in dichloromethane with the incremental addition of a solution of
THI•+ClO4 to 2 equiv. (a) From neutral to radical cation; and (b)
radical cation to dication.
conventional OPVs.16 In addition, the frequency and intensity
pattern significantly change as n increases. This is ascribed to:
(1) high local molecular symmetry of the conjugated core; (2)
small influence of the lateral aryl substitution; and (3) effective
electron−phonon coupling that enhances the Raman activity of
those vibrations coupled to the electronic excitations, leaving
unexcited molecular segments silent.17
perchlorate (THI•+ClO4, E0 = 0.76 V vs Fc+/Fc) in a stepwise
manner. Upon gradual addition of 1 equiv of oxidant, two
subgap absorption bands resulting from a radical cation
[COPV-4]•+ emerged at 794 nm (RC2 = radical cation band
2) and 1831 nm (RC1 band 1) with the disappearance of the
492 nm band from the neutral compound. This change
occurred with an isosbestic point at 516 nm. The appearance of
two bands at longer wavelength agrees with what one expects
for a radical cation. Upon further addition of the oxidant up to
2 equiv, one new structured absorption band at 1324 nm (DC
= dication band; [COPV-4]2+) appeared with the disappear-
ance of the RC1 and RC2 bands. This change occurred with
two isosbestic points at 860 and 1435 nm. The appearance of a
single new DC band between the RC1 and RC2 bands agrees
with the formation of a closed-shell dication.
We first compare COPV-2 with its analogue, (1,4-bis[(E)-
3,5-di-tert-butylstyryl]benzene) (OPV-2). While OPV-2 shows
three bands around 1630, 1590, and 1550 cm−1, COPV-2
shows similar bands with a shift toward lower frequencies 1614
(black), 1596 (red), and 1543 cm−1 (blue) as a result of more
effective conjugation. The 1596 cm−1 band is due to a collective
CC stretching mode of the phenyl rings. This intense band for
COPV-1−6 is rather insensitive to n, appearing at 1602, 1596,
1598, 1598, 1599, and 1598 cm−1.
The Raman band (black) at 1617, 1614, 1586, 1587, 1582,
and 1583 cm−1 belongs to the collective stretching modes of
the vinylene bonds and hence is most relevant to the present
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Figure 4. UV−vis NIR absorption spectra of neutral and positively charged COPVs in CH2Cl2 (black: neutral; blue: radical cation; red: dication).
(a) Linear plot of transition energies against 1/n for the radical cation (RC1 and RC2 bands) and dication (DC) of COPV-4. (b−f) Spectra of
neutral, radical cation, and dication of COPV-2−6.
that there is little change of the vinylene stretching frequency
for conventional OPV.16
We assign the third band (blue) to a second collective CC
stretching of the CC vinylene stretching modes coupled with
the benzene vibrations. The Raman spectra show a large
frequency downshift by −25 cm−1 from 1543 cm−1 in COPV-2
to 1518 cm−1 in COPV-6, but the conjugation effect saturates
gradually toward COPV-3. From the behavior of the three
characteristic bands, we conclude that that it is not the benzene
rings, but only the vinylene moiety that feels the ring strain of
the bicyclo[3.3.0] ring, of which they are a part.
Figure 5b displays the resonance Raman spectra of the
radical cations of five compounds ([COPV-2−6]•+). Compar-
ison with the neutral spectra indicates that the vinylene bond
weakens very much upon oxidation. As seen in the two vinylene
stretching bands at ca. 1530 and 1480 cm−1, the vinylene bonds
Figure 5. Raman spectra of COPV-1−6. (a) 1064 nm FT-Raman
are much weaker than those in the neutral compounds, whereas
spectra of the COPV compounds in the solid state. (b) 532 nm
resonant Raman spectrum of [COPV-2]•+ and 785 nm resonant
Raman spectra of [COPV-3]•+, [COPV-4]•+, [COPV-5]•+, and
[COPV-6]•+ in CH2Cl2 solutions (the asterisk denotes the Raman
band of the solvent). (c) Absorption spectra of COPV-6 (black),
[COPV-6]•+ (blue), and [COPV-6]2+ (red). (d) 1064 nm FT-Raman
spectrum of COPV-6, 785 nm resonant Raman spectrum of [COPV-
6]•+, and 1064 nm FT-Raman spectrum of [COPV-6]2+.
the benzene rings are insensitive (ca. 1590 cm−1).
Figure 5c shows the Raman spectra of the stable dication
[COPV-6]2+ together with that of COPV-6 and [COPV-6]•+.
The two vinylene stretching bands, 1583 and 1518 cm−1 in the
neutral compound, shift down to 1529 and 1480 cm−1 in the
radical cation and further to 1489 and 1428 cm−1 in the
dication. In contrast, the benzene modes around 1600 cm−1
changed very little but broadened significantly, probably
because of the formation of an extended quinoidal segment
that encompasses seven benzene rings (Figure 1c). The Raman
characteristics of the polaron and bipolaron indicate that the
radical and cation are delocalized over the entire π-system of
COPV-6. This interpretation is consistent with the effective
conjugation number of 13 PV units.
study. This band is very sensitive to n (by −34 cm−1 from
COPV-1−6). This line weakens slightly from COPV-1 to -2
(by −3 cm−1), and very much in COPV-3 (by −28 cm−1
COPV-2 to -3), in which the strained vinylenes are now
flanked by two PV units on both sides. The conjugation effect
saturates between COPV-3 and -6 (−4 cm−1 change). Note
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(c) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science
1995, 270, 1789−1791.
CONCLUSION
■
We have developed an efficient synthesis of new heteroatom-
free OPV derivatives (COPV-n) that are stable under
photolysis in air and doped with very small reorganization
energy to generate stable and fully delocalized positive polarons
and bipolarons. Photoexcitation occurs with an extremely small
reorganization energy. Both the ease of photoexcitation and the
stability of the cations originate from the fact that the ground
state is activated (or destabilized) by the bond angle strain
associated with the bicyclo[3.3.0]octatriene rings. The Raman
data indicated that the strain effects become particularly
prominent for COPV-n larger than COPV-3, where the
internal PV group(s) enjoys full conjugation with the two
peripheral PV rings. The rigid and flat σ-framework provides
efficient conjugation of the π-system up to 13 PV units and
allows the formation of a bipolaron in COPV-6. The stable
tetracation of COPV-6 observed under electrochemical
oxidation is an interesting subject for more detailed studies in
the future. In this vein, the well-defined structure and physical
properties of COPVs make them ideal subjects for theoretical
and physical studies. The present synthesis has made available
useful synthetic building blocks, such as COPV-3(I2), and will
arouse interest in the changes of materials’ properties created
by replacement of the conventional PV and polythiophene
derivatives in molecular wires18 and bulk organic optoelec-
tronics materials.19 A positive indication has already been
obtained for the monomer COPV-1 in the DSSC studies.
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ASSOCIATED CONTENT
* Supporting Information
Complete experimental details, CIF file of COPV-4. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
(14) van Hal, P. A.; Beckers, E. H. A.; Peeters, E.; Apperloo, J. J.;
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AUTHOR INFORMATION
Corresponding Author
tsuji@chem.s.u-tokyo.ac.jp; nakamura@chem.s.u-tokyo.ac.jp
■
Mullen, K.; Tao, N. Nat. Nanotechnol. 2011, 6, 226−231.
̈
Present Address
(19) (a) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H.
∥Organic Solids Laboratory, Beijing National Laboratory for
Moleuclar Sciences, Institute of Chemistry, Chinese Academy
of Sciences, Beijing 100190, P. R. China
Chem. Rev. 2010, 110, 6595−6663. (b) Li, C.; Liu, M.; Pschirer, N. G.;
Baumgarten, M.; Mullen, K. Chem. Rev. 2010, 110, 6817−6855.
̈
(c) van Hutten, P. F.; Krasnikov, V. V.; Hadziioannou, G. Acc. Chem.
Res. 1999, 32, 257−265. (d) Song, S.; Jin, Y.; Kim, S. H.; Moon, J.;
Kim, K.; Kim, J. Y.; Park, S. H.; Lee, K.; Suh, H. Macromolecules 2008,
41, 7296−7305.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank MEXT for financial support (KAKENHI for E.N.,
no. 22000008, H.T., no. 20685005). This work was partly
supported by a Grant-in-Aid for Scientific Research on
Innovative Areas (for H.T., no. 23108704, “π-Space”) from
MEXT, Japan, and Masashi Maruyama for crystallographic
analysis. J.T.L.N. and J.C. thank MEC (CTQ2009-10098) and
́
Junta de Andalucia (PO9-4708). X.Z. thanks the JSPS
Postdoctoral Fellowship for Foreign Researchers (no. P
09046).
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