Tetraarylethylene having two nitroxide groups: Redox-switching of through-bond magnetic interaction by conformation change

Article abstract of DOI:10.1039/b410858b

Reversible redox-switching of through-bond magnetic interaction has been achieved by conformation change of the tetraarylethylene moiety upon redox input: intramolecular magnetic interaction between two nitroxide groups is dead after oxidation, whereas it

Full text of DOI:10.1039/b410858b

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Tetraarylethylene having two nitroxide groups: redox-switching of
through-bond magnetic interaction by conformation change{
Akihiro Ito,a Yoshiaki Nakano,a Tatsuhisa Katob and Kazuyoshi Tanaka*a
Received (in Cambridge, UK) 16th July 2004, Accepted 3rd September 2004
First published as an Advance Article on the web 29th November 2004
DOI: 10.1039/b410858b
strong exchange interaction (J) through the p-conjugated
network7a is expected between the two radical centers. On the
other hand, the through-bond magnetic interaction in the dication
2 disappears the interception of p-conjugation between the two
nitroxide groups originating from rotational motion about the
olefinic bond (Scheme 1). In other words, the TPE moiety of 22+
is considered to have a twisted tetramethyleneethane (TME)
structure, in which singlet and triplet states are virtually
degenerate (J # 0).7
Effective conversion of through-bond magnetic interaction after
and before oxidation of 2 postulates a lower oxidation potential of
the TPE moiety than the two nitroxide groups. The cyclic
voltammetry of 2 in 1 mM benzonitrile solution was carried out
(Table 1). In comparison with the first oxidation potential of
the non-nitroxide-substituted TPE (3: cis–trans mixture)8 and the
N-tert-butyl-N-phenylnitroxide (4),9
Reversible redox-switching of through-bond magnetic interac-
tion has been achieved by conformation change of the
tetraarylethylene moiety upon redox input: intramolecular
magnetic interaction between two nitroxide groups is dead
after oxidation, whereas it was alive before.
Sterically congested hydrocarbon molecules often exhibit remark-
able structural changes upon redox input.1 Upon oxidation of
tetraphenylethylene (TPE), the CLC double bond is elongated to
a C–C single bond and concomitantly rotational motion about
the olefinic bond takes place. X-Ray structural analyses of
the isolated mono- and di-cations of TPE derivatives revealed
that the rotational angle increases on going from cation to
dication.2 In conjunction with this intriguing structural change,
TPE has attracted much attention from the viewpoint of new
electrochromic systems.3 In particular, tetrakis(4-dimethyl-
aminophenyl)ethylene (1) can be reversibly converted to the
diamagnetic dication (12+) in which the two cyanine moieties4
are generated by rotational motion about the olefinic bond.3
Hence, 1 is recognized as a novel organic electrochromic
system with long wavelength absorptions and high extinction
coefficients.
it was confirmed that the first two consecutive reversible one-
electron transfer steps correspond to the oxidation of the TPE
moiety, while the third step two-electron transfer process to the
oxidation of the two nitroxide groups. The observed peak
separations of 74, 70, and 69 mV for the first to third redox
processes were larger than the theoretical value of 56.5 mV
(25 uC).10 All these peak separations are probably caused by the
cis–trans mixture of the present sample.
On the other hand, the drastic structural change in TPE
including the change of p-conjugation connection pathway can be
utilized as a redox-switchable magnetic coupling unit. To date,
although a large number of studies have been made on the redox-
generation of non-Kekule´ high-spin organic molecules,5 little
is known about the control of through-bond magnetic
interactions using interconversion of the p-conjugation pathway.6
To create a new type of organic electrochromic system having
a function of a redox-switch of through-bond magnetic interac-
tions, we have designed the novel TPE derivative carrying two
nitroxide radicals (2), which was synthesised as a mixture of
cis- and trans-isomers (ESI{). In the neutral form of 2, a
The EPR spectrum of 2 in a frozen toluene matrix (123 K)
showed a fine-structured spectrum characteristic for the spin triplet
species [Fig. 1(a)]. The forbidden DMS 5 ¡ 2 resonance was also
detected in a half-field region of the allowed DMS 5 ¡ 1
resonance, indicating the existence of spin triplet species.11
Moreover, it was clarified that the two nitroxide groups of 2
are antiferromagnetically coupled from the temperature depen-
dence of the molar magnetic susceptibility (xM) of the
powder sample measured by the SQUID magnetometer (Fig. 2).
The xMT value decreases with decreasing temperature from
0.75 emu K mol21 corresponding to two independent spins of
1/2 to 0 emu K mol21, corresponding to two antiferromagnetically
coupled spins of 1/2. The measured data were analyzed on
{ Electronic supplementary information (ESI) available: synthetic details
for 2. See http://www.rsc.org/suppdata/cc/b4/b410858b/
*a51053@sakura.kudpc.kyoto-u.ac.jp
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 403–405 | 403

View Article Online
Fig. 1 EPR spectra of (a) 2 in toluene at 123 K (inset: the forbidden
DMS 5 ¡ 2 resonance at 123 K), and (b) 2 after treatment with 2 equiv.
of TBA?SbCl6 in toluene/n-butyronitrile (1 : 1) at 123 K.
Scheme 1 Bisnitroxide 2 (trans-isomer) and the redox-switching of
through-bond magnetic interaction.
Table 1 Redox potentials (V) of 2 and its related compoundsa
Compound
E1u
E2u
E3u
2
3
4
0
0.25
0.49b
0.15b
0.41c
Fig. 2 Plot of xMT versus T for 2 at 500 G. The solid line represents the
a
0.1 M n-Bu4NBF4 in PhCN, potential vs. Fc/Fc+, Pt electrode,
25 uC, scan rate 100 mV s21
.
Two-electron oxidation process.
b
best theoretical fit [eqn. (1)] to the data.
c
Irreversible oxidation process represented by the anodic peak
potential.
and the third is the Curie term originating from a very small
quantity of spin doublet impurity such as mononitroxide
radicals. The best fitted values for f1, f2, J1, and J2 were
estimated to be f1 5 0.27, f2 5 0.71, J1/kB 5 266 K
(2 0.13 kcal mol21), and J2/kB 5 293 K (20.18 kcal mol21),
indicating intramolecular antiferromagnetic interaction for
both isomers. Moreover, the ratio of the two isomers in 2 can
be estimated at 28 : 72 from f1 and f2 values. From the DFT
calculations at the UB3LYP/6-31G* level,13 the ground states for
cis- and trans-isomers of 2 were predicted to be both singlet
state, and the singlet trans-isomer lies 0.24 kcal mol21 lower than
the singlet cis-isomer. This suggests that the trans-isomer is the
thermodynamically favorable isomer. In addition, the energy
differences between the singlet and triplet states DES–T (5 2J)
for cis- and trans-isomers were predicted to be 20.47 and
20.49 kcal mol21, respectively. This indicates that the trans-
isomer has a large J value as compared with the cis-isomer.
Therefore, f1 and f2 are likely to be assignable to the cis- and trans-
isomers, respectively.
the basis of eqn. (1),
2NAg2m2B
1
ꢀ
ꢁ
xMT~f1
2J1
BT
kB
3z exp { k
2NAg2m2B
1
ꢀ
ꢁ
z f2
(1)
2J2
BT
kB
3z exp { k
NAg2m2B
2kB
z ð1{f1{f2Þ
where f1 and f2 are the molar fractions of the two isomers, J1 and
J2 the magnetic exchange coupling constants (the positive and
negative values indicate ferromagnetic and antiferromagnetic
interactions, respectively) for the two isomers, NA the Avogadro
number, g the isotropic g-factor, kB the Boltzmann constant, and
mB the Bohr magneton. The first two terms represent the Bleany–
Bowers singlet–triplet model12 corresponding to the two isomers,
404 | Chem. Commun., 2005, 403–405
This journal is ß The Royal Society of Chemistry 2005

View Article Online
On the other hand, when 2 equiv. of tris(4-bromophenyl)ami-
nium hexachloroantimonate (TBA?SbCl6) are added to 2 in
toluene/n-butyronitrile (1 : 1) solution at 195 K, the EPR spectrum
of 22+ was completely changed from that observed for neutral 2 to
one with typical anisotropic hyperfine structure for the randomly
oriented mononitroxide radical [Fig. 1(b)]. This strongly indicates
that the two nitroxide groups in 22+ are no longer coupled
and by CREST (Core Research for Evolutional Science and
Technology) of Japan Science and Technology Agency (JST).
Thanks are due to the Research Center for Molecular-Scale
Nanoscience, the Institute for Molecular Science for assistance in
obtaining the pulsed EPR spectra. Numerical calculations were
partly carried out at the Supercomputer Laboratory of the
Institute for Chemical Research of Kyoto University.
magnetically due to rotational motion about the olefinic bond.
14
Akihiro Ito,a Yoshiaki Nakano,a Tatsuhisa Katob and
Kazuyoshi Tanaka*a
Note that the principal value (AZZ
)
for the perpendicular
aDepartment of Molecular Engineering, Graduate School of
Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan.
E-mail: a51053@sakura.kudpc.kyoto-u.ac.jp
direction to the nitroxide plane of the hyperfine coupling tensor of
22+ takes a small value (19.28 G) as compared with the
corresponding value (27.99 G) of 11. This suggests that spin
density on the nitrogen atoms of the nitroxide groups decreases on
going from 2 to 22+, reflecting the spin delocalization over the
whole molecule.
bDepartment of Chemistry, Josai University, 1-1 Keyakidai, Sakado,
Saitama 350-0295, Japan
Notes and references
To check the reversibility between 2 and 22+, 22+ was treated
with an excess of hydrazine monohydrate. Consequently, the same
EPR spectrum as the as-prepared 2 [Fig. 1(a)] was retrieved from
the hyperfine-structured spectrum due to the independent nitroxide
radical of 22+ [Fig. 1(b)]. From the viewpoint of redox-switching of
through-bond magnetic interaction, on- and off-states correspond
to the cis–trans mixture of 2 and the dication 22+, respectively. In
addition, the reversibility was also confirmed spectroelectoro-
chemically. Absorption spectra for 22+ at a forward bias of + 0.4 V
vs. Fc/Fc+ and 2 at a reverse bias of 2 0.1 V vs. Fc/Fc+ are shown
in Fig. 3. Several electrochemical cycling experiments reversibly
reproduced the spectra corresponding to 2 and 22+ (Fig. 3). These
results strongly suggest that the present diradical 2 can operate as a
redox-switch of through-bond magnetic interaction. It is antici-
pated that current investigations will lead to more versatile
switching systems when the radical centers are incorporated into
TPE so as to create the required through-bond magnetic
interactions.
1 H. Bock, K. Ruppert, C. Na¨ther, Z. Havlas, H.-F. Herrmann, C. Arad,
I. Go¨bel, A. John, J. Meuret, S. Nick, A. Rauschenbach, W. Seitz,
T. Vaupel and B. Solouki, Angew. Chem., Int. Ed. Engl., 1992, 31, 550.
2 R. Rathore, S. V. Lindeman, A. S. Kumar and J. K. Kochi, J. Am.
Chem. Soc., 1998, 120, 6931.
3 S. Hu¨nig, M. Kemmer, H. Wenner, I. F. Perepichka, P. Ba¨uerle,
A. Emge and G. Gescheid, Chem. Eur. J., 1999, 5, 1969; S. Hu¨nig,
M. Kemmer, H. Wenner, F. Barbosa, G. Gescheid, I. F. Perepichka,
P. Ba¨uerle, A. Emge and K. Peters, Chem. Eur. J., 2000, 6, 2618.
4 J. Fabian and H. Hartmann, Light Absorption of Organic Colorants,
Springer, New York, 1980, pp. 162–197.
5 Representative examples are: S. Rajca and A. Rajca, J. Am. Chem. Soc.,
1995, 117, 9172; J. Sedo´, D. Ruiz, J. Vidal-Gancedo, C. Rovira,
J. Bonvoisin, J.-P. Launay and J. Veciana, Adv. Mater., 1996, 8, 748.
6 Recently, the control of through-bond magnetic interaction has been
achieved by using 1, 2-bis(3-thienyl)ethene as a photo-switchable spin
coupling unit, see: K. Matsuda and M. Irie, Chem. Eur. J., 2001, 7,
3466–3473.
7 (a) J. A. Crayston, J. N. Devine and J. C. Walton, Tetrahedron, 2000,
56, 7829; (b) W. T. Borden and E. R. Davidson, J. Am. Chem. Soc.,
1977, 99, 4587; (c) P. Dowd, W. Chang and Y. H. Paik, J. Am. Chem.
Soc., 1986, 108, 7416; (d) P. Nachtigall and K. D. Jordan, J. Am. Chem.
Soc., 1993, 115, 270; (e) K. Matsuda and H. Iwamura, J. Am.
Chem. Soc., 1997, 119, 7412.
The present work was supported by Grant-in-Aids for Scientific
Research from Japan Society for the Promotion of Science (JSPS)
8 Compound 3 was prepared from 4-bis(4-methoxyphenyl)aminophenyl
phenyl ketone by using the Mukaiyama method, see: T. Mukaiyama,
T. Sato and J. Hanna, Chem. Lett., 1973, 1041.
9 H. Lemaire, Y. Marechel, R. Ramasseul and A. Rassat, Bull. Soc.
Chim. Fr., 1965, 372.
10 J. Heinze, Angew. Chem., Int. Ed. Engl., 1984, 23, 831.
11 Preliminarily, we have also determined the spin-multiplicity for 2 (triplet)
and 22+ (doublet) on the basis of the pulsed EPR spectroscopy. For
determination of spin-multiplicity for high-spin molecules by using the
pulsed EPR technique, see: J. Isoya, H. Kanda, J. R. Norris, J. Tang
and M. K. Brown, Phys. Rev. B, 1990, 41, 3905; A. V. Astashkin and
A. Schweiger, Chem. Phys. Lett., 1990, 174, 595; A. Ito, H. Ino,
K. Tanaka, K. Kanemoto and T. Kato, J. Org. Chem., 2002, 67, 491.
12 B. Bleaney and K. D. Bowers, Proc. R. Soc. London, Ser. A, 1952, 214,
451.
13 The GAUSSIAN 98 (Revision A.9) program was used for the present
DFT calculations.
14 The separation between the lowest the highest field resonance peaks
corresponds to the value of 2AZZ, see: O. H. Griffith, P. W. Cornell and
H. M. McConnell, J. Chem. Phys., 1965, 43, 2909; L. J. Libertini and
O. H. Griffith, J. Chem. Phys., 1970, 53, 1359; O. Takizawa,
J. Yamauchi, H. Ohya-Nishiguchi and Y. Deguchi, Bull. Chem. Soc.
Jpn., 1973, 46, 1991.
Fig. 3 Absorption spectral change of 2 in CH2Cl2 at room temperature
upon several electrochemical cycling episodes between +0.4 and 20.1 V vs.
Fc/Fc+.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 403–405 | 405