Dual dopable poly(phenylacetylene) with nitronyl nitroxide pendants for reversible ambipolar charging and discharging

Article abstract of DOI:10.1246/cl.2011.184

An electrode-attached layer of poly(phenylacetylene) bearing a pendant nitronyl nitroxide group per repeating unit, obtained by the Rh-catalyzed polymerization of 2-(4-ethynylphenyl)-4,4,5,5- tetramethylimidazoline-l-oxyl 3-oxide, underwent oxidation and

Full text of DOI:10.1246/cl.2011.184

doi:10.1246/cl.2011.184
184
Published on the web January 15, 2011
Dual Dopable Poly(phenylacetylene) with Nitronyl Nitroxide Pendants
for Reversible Ambipolar Charging and Discharging
Kenichi Oyaizu, Takashi Sukegawa, and Hiroyuki Nishide*
Department of Applied Chemistry, Waseda University, Tokyo 169-8555
(Received November 22, 2010; CL-100977; E-mail: nishide@waseda.jp)
An electrode-attached layer of poly(phenylacetylene) bearing
R
R
R
- e-
+ e-
- e-
+ e-
-O
+ O-
O
+ O-
N
O
+ O-
N
a pendant nitronyl nitroxide group per repeating unit, obtained by
the Rh-catalyzed polymerization of 2-(4-ethynylphenyl)-4,4,5,5-
tetramethylimidazoline-l-oxyl 3-oxide, underwent oxidation and
reduction at 0.80 and ¹0.84 V vs. Ag/AgCl, respectively. The
magnetically determined unpaired electron density of 92% was
coulometrically reproduced, which supported the presumption
that the radical survived during the course of the polymerization
to allow both positive and negative charging of the pristine neutral
polymer substantially per repeating unit. Galvanostatic Coulomb
titration revealed the charge storage capability of the polymer,
which demonstrated usefulness as organic electrode-active
material with unprecedented ambipolar chargeability.
N+
N
N
N
n-type
p-type
¹
Scheme 1. Reversible 1e redox reaction of nitronyl nitroxides.
SiMe3
HOH2N
SO4
NH2OH
2-
SiMe3
[Pd(PPh3)2Cl2]
CuI, TEA
Br
K2CO3
NaOAc
CHO
CHO
CHO
1
2
n
NaIO4
[Rh(cod)Cl]2, TEA
Recent progress in the chemistry of charge transport by
nonconjugated polymers with ultimate density of redox sites1 has
demonstrated that these polymers are promising as electroactive
materials for various electronic devices such as rechargeable
batteries,2 hybrid capacitors,3 solar cells,4 electrochromic cells,5
sensors,6 and memory devices.7 We have focused on various
aliphatic polymers bearing organic robust radicals as pendant
groups per repeating unit. The so-called “radical polymers” are
characterized by reversible charging/discharging behaviors at
+ O-
N
O
+ O-
N
O
HO
OH
N
N
N
N
3
4
5
Scheme 2. Synthesis of 5.
Iwamura et al. reported the synthesis of conjugated poly-
(phenylacetylene) carrying nitronyl nitroxide pendants by the Rh-
catalyzed coordination polymerization of 2-(4-ethynylphenyl)-
4,4,5,5-tetramethylimidazoline-l-oxyl 3-oxide, with a view to
obtain a magnetic polymer.14 However, the expected ferromag-
netic coupling among the radical centers through the conjugated
chain was not operative, due to the limited spin delocalization into
the conformationally twisted skeleton. We anticipated that such
structural features, regarded as negative for the spin coupling,
should turn out to be positive for the blocking of the redox cou-
pling.15 Here we revisit the polymer, focusing on the unprece-
dented ambipolar chargeability, which is accomplished by the re-
dox isolation of the radical pendants from the conjugated system.
Polymer 5 was synthesized according to a previous method,14
with slight modifications as follows (Scheme 2). Sonogashira
coupling of ethynyltrimethylsilane with 4-bromobenzaldehyde
followed by desilylation and annulation with 2,3-bis(hydroxy-
amino)-2,3-dimethylbutane produced 2-(4-ethynylphenyl)-4,4,5,5-
tetramethylimidazolidine-1,3-diol (3), which was carefully oxi-
dized with sodium periodate at 0 °C to give a deep blue crystalline
4 with an unpaired electron density of 100% based on a SQUID
magnetization experiment. The monomer 4 in THF (0.3 M) was
polymerized using 1% chloro(1,5-cyclooctadiene)rhodium(I) di-
mer as the catalyst and 25% triethylamine. After vigorous stirring
at room temperature for 25 min, 5 was obtained as a deep bluish
and powdery solid in a reasonable yield (78%), which was almost
insoluble but swellable in electrolyte solutions.
¹1
a typical mass specific capacity of 111 mA h g for 2,2,6,6-
tetramethylpiperidin-N-oxyl (TEMPO)-substituted polymethacry-
late (PTMA), with an excellent rate capability allowing full
charging and discharging within a few seconds.8 Such perform-
ance has led to the development of the “radical battery” as a new
class of thin, flexible, light, and yet high-power storage device.9
The radical polymers have typically been examined as
cathode-active materials, because many of them exhibit redox
potentials near 0.8 V vs. Ag/AgCl10 where the electrically neutral
radicals undergo positive (i.e., p-type) charging into cations. Radi-
cal batteries with a configuration of (¹)Li«electrolyte«PTMA(+)
produce an emf of 3.6 V based on the gap between the redox
potentials.8 One could expect that the battery configuration would
be much more simplified by employing a nitronyl nitroxide
pendant which undergoes both p- and n-type (i.e., negative)
charging reversibly (Scheme 1), because of its potential capability
of playing a dual role as cathode- and anode-active material.11
However, such challenges have been impeded by the difficulties
in preparing vinyl polymers with pendant nitronyl nitroxide
groups, which undergo side reactions with the propagating end.
Similar difficulties have also been encountered in the polymer-
ization of galvinoxyl-substituted styrene and appear to be inherent
in radical monomers with n-type charging capability. Indeed,
galvinoxyl-substituted polystyrene has only been prepared by
the polymerization of hydrogalvinoxyl precursor, followed by a
polymer-analogous reaction to generate the radical.12 Develop-
ment of a straightforward method to polymerize n-type radical
monomers has been an important subject of research.13
While 4 gave a five-line ESR signal centered at g = 2.0068
(Figure 1a), a featureless spectrum was obtained for the soluble
Chem. Lett. 2011, 40, 184-185
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

185
1.2
1.0
0.8
0.6
0.4
0.2
0
0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
(a)
(b)
(a)
(b)
0.4
0.3
0.2
0.1
0
15
10
5
0.5 mA
-0.4
330
335
H/mT
340
345
-1.2
-0.8
E/V vs. Ag/AgCl
1 mA
1.2
χ
0.4
0.8
χ
E/V vs. Ag/AgCl
0
2 µA
0
100
200
300
T/K
0
20
40
60
0
20
40
60
Q/mC
Q/mC
-2.0
-1.0
0
1.0
0
100
200
300
E/V vs. Ag/AgCl
T/K
Figure 2. (a) Cyclic voltammograms (5 mV s¹1) and charging/dis-
charging curves for the p-type reaction of the carbon nanocomposite
electrode of 5 (0.59 mg) with a composition of 5/VGCF/PVdF (1/8/1
in w/w/w), where VGCF was a vapor-grown carbon fiber and PVdF
was poly(vinylidene fluoride). The electrolyte conditions were as in
Figure 1. (b) Repeat of (a) for the n-type reaction of 5 (0.32 mg) in the
presence of tetrabutylammonium hydroxide (0.1 M).
¹1
Figure 1. (a) ESR spectrum and cyclic voltammogram (0.1 V s
)
obtained for 1 mM CH2Cl2 and CH3CN solutions of 4, respectively.
The electrolyte was tetrabutylammonium perchlorate (0.1 M). (b) Plots
of »molT vs. T and 1/»mol vs. T with Curie-Weiss fitting for 5 (19.6
mg) obtained by SQUID measurements, where »mol was based on mol
of the radical. Dotted line represents the theoretical value of »molT for
S = 1/2.
maintaining the redox cyclability, is the topics of our continuous
research.
part of 5 due to the spin-exchange interaction between the locally
populated unpaired electrons. The unpaired electron density in 5
was determined from the 1/»mol vs. T plots (Figure 1b), based on
the Curie-Weiss rule according to 1/»para = T/C ¹ ª/C where C
is a Curie constant defined as Neg2®B2S(S + 1)/(3kB). The slope
of the 1/»mol vs. T plots gave an unpaired electron density of
Ne = 2.14 © 1021 spin/g, which corresponded to 92% of the
existing nitronyl nitroxide units in 5. The paramagnetic (S = 1/2)
nature of 5 at room temperature also allowed determination of the
unpaired electron density by integrating the ESR signal using 4 as
a standard, which agreed with that from the SQUID measurement
and was comparable with that of the Iwamura polymer (95%).12
Cyclic voltammogram for 4 showed two reversible responses
at 0.80 and ¹0.86 V vs. Ag/AgCl (Figure 1a), which were
ascribed to the p- and n-type reactions, respectively (Scheme 1).
The redox response remained intact after continuous potential
cycling for more than 100 times, which demonstrated the
robustness of the radical, the cation, and the anion. Similar
ambipolar charging was observed for a carbon nanocomposite
electrode of 5 prepared on a current collector (Figure 2).
Galvanostatic coulometry revealed the plateau voltages for both
p- and n-type charging, which agreed with their redox potentials.
An intriguing aspect is the unprecedented ambipolar ultimate
doping of the polymer with a capacity of 96 mA h g¹1. The
charging capacity of 92% with respect to the redox site, obtained
by the integration of the cyclic voltammogram, coincided with the
unpaired electron density, which demonstrated that the radical
survived during the Rh-catalyzed polymerization. It may be noted
that the charging behavior of 5 is reminiscent of the p- and n-
doping of polyacetylene.16 However, the charging at the redox-
isolated pendant nitronyl nitroxide was characterized by the higher
reversibility and charge population than those for the doping of
polyacetylene. Indeed, the conjugated polyene chain in 5 was not
operative for the redox reaction or the electric conduction, which
was indicated by the lack of these properties for poly(phenyl-
acetylene)s with similar conformational constraints.
This work was partially supported by Grants-in Aid for
Scientific Research (Nos. 19105003, 21655043, 21550120, and
21106519), and the Waseda University Global COE Program
from MEXT, Japan.
References
1
a) K. Oyaizu, H. Nishide, Adv. Mater. 2009, 21, 2339. b) K. Oyaizu, Y.
Ando, H. Konishi, H. Nishide, J. Am. Chem. Soc. 2008, 130, 14459.
H. Nishide, T. Suga, Electrochem. Soc. Interface 2005, 14, 32.
H. Li, Y. Zou, Y. Xia, Electrochim. Acta 2007, 52, 2153.
a) S. M. Lindner, S. Hüttner, A. Chiche, M. Thelakkat, G. Krausch,
Angew. Chem., Int. Ed. 2006, 45, 3364. b) F. Kato, N. Hayashi, T.
Murakami, C. Okumura, K. Oyaizu, H. Nishide, Chem. Lett. 2010, 39,
464.
2
3
4
5
a) P. M. Beaujuge, J. R. Reynolds, Chem. Rev. 2010, 110, 268. b) P. M.
Beaujuge, C. M. Amb, J. R. Reynolds, Acc. Chem. Res. 2010, 43,
1396. c) Y. Takahashi, N. Hayashi, K. Oyaizu, K. Honda, H. Nishide,
Polym. J. 2008, 40, 763.
6
7
H. Fu, M. Policarpio, J. D. Batteas, D. E. Bergbreiter, Polym. Chem.
2010, 1, 631.
a) Y. Yonekuta, K. Susuki, K. Oyaizu, K. Honda, H. Nishide, J. Am.
Chem. Soc. 2007, 129, 14128. b) T. Suga, S. Takeuchi, T. Ozaki, M.
Sakata, K. Oyaizu, H. Nishide, Chem. Lett. 2009, 38, 1160.
a) K. Nakahara, J. Iriyama, S. Iwasa, M. Suguro, M. Satoh, E. J.
Cairns, J. Power Sources 2007, 165, 870. b) K. Nakahara, J. Iriyama,
S. Iwasa, M. Suguro, M. Satoh, E. J. Cairns, J. Power Sources 2007,
165, 398. c) K. Nakahara, J. Iriyama, S. Iwasa, M. Suguro, M. Satoh,
E. J. Cairns, J. Power Sources 2007, 163, 1110.
8
9
H. Nishide, K. Oyaizu, Science 2008, 319, 737.
10 a) S. Yoshihara, H. Isozumi, M. Kasai, H. Yonehara, Y. Ando, K.
Oyaizu, H. Nishide, J. Phys. Chem. B 2010, 114, 8335. b) K. Koshika,
N. Chikushi, N. Sano, K. Oyaizu, H. Nishide, Green Chem. 2010, 12,
1573. c) K. Koshika, N. Sano, K. Oyaizu, H. Nishide, Chem. Commun.
2009, 836. d) K. Oyaizu, T. Suga, K. Yoshimura, H. Nishide,
Macromolecules 2008, 41, 6646.
11 T. Suga, S. Sugita, H. Ohshiro, K. Oyaizu, H. Nishide, Adv. Mater.
2010, 22, in press. doi:10.1002/adma.201003525.
12 T. Suga, H. Ohshiro, S. Sugita, K. Oyaizu, H. Nishide, Adv. Mater.
2009, 21, 1627.
13 K. Oyaizu, A. Hatemata, W. Choi, H. Nishide, J. Mater. Chem. 2010,
20, 5404.
14 A. Fujii, T. Ishida, N. Koga, H. Iwamura, Macromolecules 1991, 24,
1077.
15 T. Ibe, S. Kaiho, K. Oyaizu, H. Nishide, Chem. Lett. 2010, 39, 356.
16 D. MacInnes, M. A. Druy, P. J. Nigrey, D. P. Nairns, A. G.
MacDiarmid, A. J. Heeger, J. Chem. Soc., Chem. Commun. 1981, 317.
A capacity of ca. 30% with respect to the theoretical capacity
obtained for the galvanostatic experiments (Figure 2) suggested
some limitation of electroneutralization during charging. Tuning
the swelling properties of the composite electrode to facilitate the
diffusion of electrolyte counterion for charge compensation, while
Chem. Lett. 2011, 40, 184-185
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/