Multifunctional Spin-carrying Anthraquinone Derivatives
Yuuki Shibata,
1
Hiroki Akutsu,
1
Jun-ichi Yamada,
1
Masaharu Satoh,
2
Uma S. Hiremath,
3
Channabasaveshwar V. Yelamaggad,
3
and Shin’ichi Nakatsuji*
1
1
Graduate School of Material Science, University of Hyogo, Kamigori, Hyogo 678-1297
2
Murata Manufacturing Co., Nagaokakyo, Kyoto 617-8555
3
Centre for Liquid Crystal Research, Jalahalli, Bangalore-560013, India
(Received April 2, 2010; CL-100320; E-mail: nakatuji@sci.u-hyogo.ac.jp)
Several nitroxide radicals derived from 2,3,6,7-tetraalkoxy-
9,10-anthraquinone core have been prepared and their magnetic,
redox as well as battery properties are studied. Notably, one
of the derivatives with a mono-PROXYL-radical substituent
exhibits a fairly stable multistep charge-discharge process and a
heat-responsive magnetic behavior.
Considerable attention has been paid in recent years to the
development of organic functional radicals such as organic
photoresponsive radicals,
1
organic liquid crystal (LC) radicals,
2
or organic radical batteries.
3
During the course of our studies
toward the development of organic multifunctional spin systems
based on nitroxide radicals,
4
we have designed and prepared
various organic radicals with liquid crystal and/or heat-
responsive properties.
4,5
For example, we have recently reported
the first discotic radicals exhibiting magnetoresponsive colum-
nar (Col) mesomorphism.
5
In continuation of this work, we have
now turned our attention toward the development of anthraqui-
none-based nitroxide radicals having alkoxy tails. Such a
molecular design stemmed from the fact that anthraquinones
with long alkoxy chians display Col LC behavior.
6
Moreover,
the redox nature of both nitroxide group and anthraquinone ring
is expected to confer relevant battery properties on the resulting
systems. Here we report the synthesis and characterization of
a number of organic radicals derived from 2,3,6,7-tetraalkoxy-
9,10-anthraquinone where the number and nature of the nitro-
xide radicals have been varied.
The target radical compounds 2a, 2b, 3a, and 3b (Chart 1)
were obtained in good yields by condensing 1,5-dihydroxy-
2,3,6,7-tetraoctyloxy-9,10-anthraquinone (1)
6
with the required
equivalence of 4-carboxy-PROXYL (2,2,5,5-tetramethyl-1-pyr-
rolidinyloxyl) or 4-carboxy-TEMPO (2,2,6,6-tetramethyl-1-pi-
peridinyloxyl) in the presence of DCC and DMAP. The
unsymmetrically substituted radical 4 was prepared by condens-
ing mono-PROXYL-substituted compound 2a with 4-carboxy-
TEMPO in a moderate yield.
7
The redox data of each derivative were estimated by cyclic
voltammetry and are summarized in Table 1.
It is apparent from the data that they are amphoteric
compounds displaying both reduction and oxidation potentials.
Their reduction potentials indicate that they are weaker acceptors
than the previous benzoquinone derivatives with TEMPO-
substituent(s),
8
even though the monosubstituted derivatives 2a
and 3a are somewhat stronger acceptors than disubstituted
derivatives 2b, 3b, and 4.Similar oxidation potentials due to
the TEMPO group are observed within the five compounds but
theirelectron-donating abilities are weakened because of the
attachment of a carbonyl group. Taking such redox data into
consideration, we examined the compounds as possible sub-
strates for organic radical batteries.
The charge-discharge profiles of the anthraquinone deriv-
atives were measured with a coin cell which was fabricated by
stacking cathode and Li-metal anode with porous polyolefin
separator film. A cathode was formed by pressing the compo-
sites of an anthraquinone derivatives (10 wt %), carbon fiber
(80 wt %), and fluorinated polyolefin binder (10 wt %). A
composite solution of ethylene carbonate (30 vol%)/diethyl
carbonate (70 vol%) containing 1M of LiPF
6
was used as an
electrolyte. The charge-discharge profileof 2a is shown in
Figure 1. After the initial charging, discharge of one-electron
oxidation occurred at 3.6 V followed by one-electron reduction
at 2.9V, which is supposed to have originated from the redox
properties of the PROXYL group. Further discharging below ca.
2 V may be attributed to the redox of the anthraquinone moiety
with a capacity of over 120 A h kg
¹1
, indicating the applicability
of this compound as a cathode-active materialfor a rechargeable
battery. Thus, multistep discharging is found in 2a and the
charge-discharge process is fairly stable, although the response
is graduallydiminished through repetition. On the contrary, such
a process in the corresponding disubstituted compound 2b was
unstable (SI-1), since the discharging capacity was found to be
largely lost from the second cycle.
O
O
COO
C
8
H
17
O
C
8
H
17
O
OC
8
H
17
OC
8
H
17
OCO
N
O
N
O
O
O
COO
C
8
H
17
O
C
8
H
17
O
OC
8
H
17
OC
8
H
17
OCO
N
O
N
O
O
O
COO
C
8
H
17
O
C
8
H
17
O
OC
8
H
17
OC
8
H
17
OH
N
O
O
O
COO
C
8
H
17
O
C
8
H
17
O
OC
8
H
17
OC
8
H
17
OH
N
O
O
O
OH
C
8
H
17
O
C
8
H
17
O
OC
8
H
17
OC
8
H
17
OH
1
3a 3b
2a 2b
O
O
OC
8
H
17
OC
8
H
17
C
8
H
17
O
C
8
H
17
O
4
COO
N
O
OCO
N
O
Chart 1.
Table 1. Electrochemical properties of radicals 2-4
a
Compound E
1
RED
E
2
RED
E
1
OX
2a ¹0.93 ¹1.31 0.84
2b ¹1.04 ¹1.47 0.88
3a ¹0.98 ¹1.43 0.81
3b ¹1.10 ¹1.58 0.78
4 ¹1.06 ¹1.42 0.85
TEMPO 0.70
a
V vs. SCE, 0.1 M n-Bu
4
NClO
4
in PhCN.
Published on the web May 22, 2010 671
doi:10.1246/cl.2010.671
© 2010 The Chemical Society of Japan Chem. Lett. 2010, 39, 671-673 www.csj.jp/journals/chem-lett/