Multifunctional spin-carrying anthraquinone derivatives

Article abstract of DOI:10.1246/cl.2010.671

Several nitroxide radicals derived from 2,3,6,7-tetraalkoxy9,10- 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.

Full text of DOI:10.1246/cl.2010.671

doi:10.1246/cl.2010.671
Published on the web May 22, 2010
671
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
1Graduate School of Material Science, University of Hyogo, Kamigori, Hyogo 678-1297
2Murata Manufacturing Co., Nagaokakyo, Kyoto 617-8555
3Centre for Liquid Crystal Research, Jalahalli, Bangalore-560013, India
(Received April 2, 2010; CL-100320; E-mail: nakatuji@sci.u-hyogo.ac.jp)
Table 1. Electrochemical properties of radicals 2-4a
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.
RED
RED
OX
Compound
E1
E2
E1
2a
2b
3a
3b
4
¹0.93
¹1.04
¹0.98
¹1.10
¹1.06
¹1.31
¹1.47
¹1.43
¹1.58
¹1.42
0.84
0.88
0.81
0.78
0.85
0.70
TEMPO
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.
aV vs. SCE, 0.1 M n-Bu4NClO4 in PhCN.
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
their electron-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 1 M of LiPF6 was used as an
electrolyte. The charge-discharge profile of 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 2.9 V, 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 kg¹1, indicating the applicability
of this compound as a cathode-active material for a rechargeable
battery. Thus, multistep discharging is found in 2a and the
charge-discharge process is fairly stable, although the response
is gradually diminished 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.
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-
O
O
N
N
OH
O
COO
O
COO
O
C8H17
O
O
OC8H17
OC8H17
C8H17
O
O
OC8H17
OC8H17
C8H17
O
O
OC8H17
OC8H17
C8H17
C8H17
C8H17
O
OH
O
O
OCO
OH
1
2b
2a
N
O
O
O
N
O
N
N
COO
COO
COO
O
O
O
O
C8H17
O
OC8H17
OC8H17
C8H17
O
O
OC8H17
C8H17
O
O
OC8H17
OC8H17
C8H17
OC8H17
C8H17
C8H17O
O
OCO
OH
O
OCO
3b
3a
4
N
O
N
O
Chart 1.
Chem. Lett. 2010, 39, 671-673
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

672
Table 2. Magnetic data of radicals 2a, 2b, 3a, 3b, and 4
Compd.
Magnetic interactiona
Cb
ªc
2a
2b
3a
3b
4
F
AF
F
F
F
0.37
0.75
0.37
0.75
0.74
0.17
¹3.04
0.50
0.63
0.20
aCurie-Weiss model. F: Ferromagnetic interaction. AF: Anti-
ferromagnetic interaction. bCurie constant (emu K mol¹1).
cWeiss temperature (K).
Figure 1. Charge-discharge profile of 2a at a constant current
density of ca. 0.05 A g¹1 in a cell voltage range of 1.5-4.2 V in 7
cycles.
c
71.5°C
a
81.36°C
b
o
4.262(6) Å
Figure 2. The DSC traces obtained for the radical 2a during
the first heating (red-trace) and cooling (blue-trace) cycles.
Figure 3. Crystal structure of the radical compound 2b.
Hydrogen atoms are omitted for clarity.
The thermal behavior of the synthesized radicals was
examined with the aid of polarizing optical microscopy
(POM) and differential scanning calorimety (DSC). In the
DSC thermograms, two endothermic peaks were observed
during the heating cycle for 2a (at 72 and 81 °C as illustrated
in Figure 2), 2b (at 133 and 137 °C), and 3a (at 95 and 102 °C),
while only one exothermic peak or no peak was apparent during
cooling for the compounds (no peak for 2a, at 100 °C for 2b, and
at 81 °C for 3a) (SI-2-SI-5).9 Further examinations by using
POM revealed that the peaks observed in the DSC traces are not
due to the liquid crystal behavior of the compounds but can be
ascribed to originating from the solid-to-solid phase transitions
during the heating of each compound. The detrimental effect of
the bulky TEMPO or PROXYL may result in the absence of
liquid crystal phases in these radical compounds as suggested
from the crystal structures of 2b and 4.
The magnetic data for the radical compounds 2a, 2b, 3a, 3b,
and 4 obtained by SQUID measurements in the temperature
range of 2-300 K are summarized in Table 2. The Curie
constants of the radical compounds are close to the theoretical
values (0.375 for S = 1/2 spin or 0.75 for non-interacting S = 1
spin) indicating that the spins are alive and weak ferromagnetic
(F) intermolecular interactions predominantly occur for all the
compounds with the exception of 2b for which a negative Weiss
temperature (antiferromagnetic, AF, interaction) is evidenced.
Among the anthraquinone derivatives thus prepared, single
crystals suitable for X-ray analysis could be obtained for the
compounds 2b and 4 and their structures were solved by using
X-ray diffractomety.10
The bulky oxycarbonyl-PROXYL moiety together with
long octyl groups in a molecule of 2b extend above and below
the anthraquinone mean plane as shown in Figure 3. The
structural feature appears to be detrimental to the formation of
a liquid crystal phase in this compound. The molecules are
stacking in stepwise and slanted manner along the b axis.
A chiral structure with space group P1 is found in 4, based
possibly on the presence of different substituents (TEMPO and
PROXYL) in the crystal. Similar to the molecular structure of
2b, the bulky oxycarbonyl-TEMPO and -PROXYL together
with long octyl groups in 4 extend above and below the
anthraquinone mean plane to disturb the formation of LC phase
in the compound (Figure 4a). The molecules also display rather
similar stacking profile to that of 2b, that is, the anthraquinone
planes of the compound 4 are also stacking regularly and in a
slanted manner along the a axis.
Although it is not easy to see the origins of the magnetic
interactions observed in 2b and 4 from their crystal structures,
a relatively short oxygen-to-oxygen distance between the spin
centers (4.26 ¡) may contribute at least in part to the
antiferromagnetic interaction observed in 2b. In turn, a couple
of short contacts between the oxygen atom of a TEMPO group
and each methyl group of a neighboring molecule (3.66 and
3.88 ¡ in Figure 4b) may result in the ferromagnetic behavior
observed in 4.
In order to see the heat-responsive properties of this class of
radical compounds, the magnetic properties of compound 2a
were investigated as a representative example from 2 to 370 K
Chem. Lett. 2010, 39, 671-673
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

673
(a)
when compared to its disubstituted analog suggesting that the
increase in the number of radical moieties has no added
advantage in discharge. Furthermore, the monosubstituted
PROXYL radical shows heat-responsive magnetic properties.
Thus, our study reveals that the spin-carrying anthraquinones are
intriguing molecular systems exhibiting multiproperties and
further investigations on these and related systems are still in
progress.11
c
We thank Ms. Yukari Tanaka of the Office of Energy Device
Development, Murata Manufacturing Co., Ltd. for the electro-
chemical measurements of the cells with the radical compounds.
Partial support of this work by a Grant from Tokyo Ohka
Foundation for the Promotion of Science and Technology is also
gratefully acknowledged.
a
b
o
3.664(4) Å
(b)
3.879(4) Å
References and Notes
b
1
2
3
a) K. Matsuda, M. Irie, J. Photochem. Photobiol., C 2004, 5,
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18, 2872. b) R. Tamura, in Nitroxide: Application in
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c
o
a
4
5
Figure 4. (a) Crystal structure of radical compound 4 viewed
along the a axis. Hydrogen atoms are omitted for clarity. (b)
Crystal structure of 4 depicting four molecules, in which
hydrogen atoms are omitted for clarity.
C. V. Yelamaggad, A. S. Achalkumar, D. S. S. Rao, M.
Nobusawa, H. Akutsu, J. Yamada, S. Nakatsuji, J. Mater.
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6
7
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All the radical compounds gave satisfactory elemental
analysis data.
S. Nakatsuji, M. Nobusawa, H. Suzuki, H. Akutsu, J.
Yamada, J. Org. Chem. 2009, 74, 9345.
Only one exothemic/endothermic peak couple was observed
during heating/cooling process for 3b and 4 (SI-4 and SI-5).
(over the melting point). Apparent increase of »T-values over
the melting point (345 K) was revealed from the measurement
during heating up to 370 K and at the same time the original
weak ferromagnetic interaction observed in heating with ª =
0.17 was found to turn antiferromagnetic (ª = ¹0.81) on
cooling (SI-6). Thus, a heat-responsive magnetic property was
elucidated in this radical compound through the phase transi-
tions.
In summary, novel tetraalkoxy-substituted anthraquinones
differing in the number and nature of nitroxide radicals have
been prepared and characterized. They exhibit electrochemical
(redox) activity and paramagnetic behavior. Of the two radicals
investigated for their stable multistep discharge, the monosub-
stituted PROXYL compound appears to be relatively promising
10 Crystal data of 2b and 4 reported in this paper have been
deposited with Cambridge Crystallographic Data Centre.
Copies of the data can be obtained via www.ccdc.cam.ac.uk/
conts/retrieving.html.
11 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
Chem. Lett. 2010, 39, 671-673
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/