Azobenzene derivatives with a long alkyl chain and aminoxyls

Article abstract of DOI:10.1039/b408432b

A trans-azobenzene derivative with a long alkyl chain and a TEMPO radical showed photo-induced isomerization to become the corresponding cis-isomer with a significant change of its intermolecular magnetic interaction from a weak ferromagnetic one based on

Full text of DOI:10.1039/b408432b

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Azobenzene derivatives with a long alkyl chain and aminoxyls{
Masahiro Fujino, Takamitsu Amano, Hiroki Akutsu, Jun-ichi Yamada and Shin’ichi Nakatsuji*
Department of Material Science, Graduate School of Material Science, University of Hyogo, 3-2-1 Kouto,
Kamigori, Hyogo 678-1297, Japan. E-mail: nakatuji@sci.u-hyogo.ac.jp; Fax: 81 791 58 0164;
Tel: 81 791 58 0163
Received (in Cambridge, UK) 4th June 2004, Accepted 22nd July 2004
First published as an Advance Article on the web 3rd September 2004
A trans-azobenzene derivative with a long alkyl chain and a
TEMPO radical showed photo-induced isomerization to
become the corresponding cis-isomer with significant
a
change of its intermolecular magnetic interaction from a
weak ferromagnetic one based on the CW model to a
relatively strong antiferromagnetic one based on the ST
model with the J-value of 36.7 K.
Considerable interest is currently focused on the development of
photo-responsive molecular-based magnetic materials1 and
searches for new organic photo-responsive spin systems have
been actively carried out by several research groups along these
lines in recent years.2 In the course of our studies for the
development of novel organic spin systems, we have been interested
in preparing multifunctional spin systems with conductivity, photo-
functionality, liquid crystalline properties or pressure-functionality
by using stable radicals, especially aminoxyl radicals, as spin
sources.3 As for the spin systems with photo-functionality, we have
so far proposed several photo-responsive spin systems by using
such photochromic systems as norbornadiene/quadricyclane,
spiropyran/merocyanine, anthracene/dimer, or naphthopyran/
merocyanine systems.4 In this communication, we wish to report
our latest results on the development of new photochromic spin
systems, azobenzene derivatives with aminoxyl radicals and a long
alkoxy as well as an ester group in between. The introduction of a
long alkoxy group was initially designed to add a possible heat-
responsive property5 to the photo-functionality expected from the
existence of an azobenzene core.
When 4-hydroxyazobenzene 1a was reacted with bromounde-
canoxy-carbonyl-PROXYL (2,2,5,5-tetramethyl-1-pyrrolidinyloxy)
2a6 or the corresponding TEMPO (2,2,6,6-tetramethyl-1-piperidi-
nyloxy) derivative 2b6 prepared from bromoundecanol and either
4-carboxy-PROXYL or 4-carboxy-TEMPO in the presence of
potassium carbonate (1 equiv.) in THF–DMF (1 : 3) at 130 uC, the
corresponding azobenzene derivative with a PROXYL-substituent
(3a) or with a TEMPO-subsitituent (3b) was obtained in 72 and
71% yield, respectively. In a similar manner, bis-aminoxyl-
substituted azobenzene derivatives 4a and 4b were obtained
when 4,4’-dihydroxyazobenzene 1b,7 prepared by the treatment
of p-hydroxybenzenediazonium chloride with copper sulfate
pentahydrate, was used as a substrate and a bimolecular amount
of the aminoxyl-substituted-bromide 2a or 2b was added as the
reagent for the derivatization, though their yields were lower than
for the mono-substituted derivatives (29 and 31%).
(CW) model was found in the spins of TEMPO-based monoradical
3b (Table 1). On the other hand, an antiferromagnetic interaction
was observed for both of the biradical compounds (4a, 4b) and
while the weak intermolecular magnetic interaction based on the
CW model was observed in the PROXYL derivative 4a, the
magnetic behavior based on the ST model and with a notably large
J-value was observed in the spin–spin interaction of the TEMPO
derivative 4b.
Although no single crystal was obtained for each of the
biradicals 4a and 4b and their structure–property relation could not
be recognized as yet, the structure of the TEMPO derivative 4b is
anticipated to have a similar structural motif to the biradicals with
naphthalene as well as a biphenyl core because of their similar
magnetic behaviors together with the J-values of ca. 240 K.8 On
the other hand,
a single crystal of the PROXYL-based
monoradical 3a was obtained by the recrystallization from
n-hexane–methanol and its structure with a trans-configuration
in the azobenzene moiety is apparent from the preliminary X-ray
analysis{ (see also ESI). The oxygen–oxygen distance of the
˚
neighboring spin centers amounts to 4.10 A, which is relevant to the
behavior of the ST model with a weak antiferromagnetic
interaction of J ~ 24.2 K. A single crystal suitable for X-ray
analysis{ could also be obtained by the recrystallization of the
corresponding TEMPO derivative 3b from n-hexane–methanol
and the crystal structure viewed along the a-axis is shown in Fig. 1.
Thus, the molecules are forming a sheet-like structure on the
bc-plane and the nearest oxygen–oxygen distance of the neighbor-
˚
ing spin centers is estimated to be 5.97 A, which is fairly far apart
for giving strong intermolecular magnetic interaction and is
DSC measurements for each sample thus prepared did not show
the appreciable endothermic peak expected for the existence of a
mesogenic phase but only the endothermic peaks corresponding to
melting points. Next, the temperature dependence data of their
magnetic susceptibilities were measured by a SQUID suscept-
ometer between the temperatures of 2 and 300 K. Regarding the
magnetic properties of monoradicals, the behavior being well
expressed by the singlet–triplet (ST) model with a small J-value was
apparent in the interaction of the PROXYL-based monoradical 3a,
while weak ferromagnetic interaction based on the Curie–Weiss
Table 1 Magnetic data of azobenzenes with aminoxyls 3a/3b, 4a/4b
and 5
Magnetic
interaction
Cc/emu
Compound
K mol21
hd/K
Je/K
24.2
3a
3b
4a
4b
5
antiferromagnetica
ferromagneticb
—
—
0.38
0.76
—
10.09
22.72
—
—
—
238.5
236.7
antiferromagneticb
antiferromagnetica
antiferromagnetica
—
—
a Fitting for ST model. b Fitting for CW model. c Curie constant.
{ Electronic supplementary information (ESI) available: a table with a
summary of the crystal data and diagrams of the crystal structures for 3a
and 3b. See http://www.rsc.org/suppdata/cc/b4/b408432b/
d Weiss temperature. e Exchange interaction.
2 3 1 0
C h e m . C o m m u n . , 2 0 0 4 , 2 3 1 0 – 2 3 1 1
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Scheme 1
in the spins; the trans-isomer 3b could be changed to the relatively
strong antiferromagnetic interaction based on the ST model in the
cis-isomer 5 with a J-value as large as 236.7 K. The successful
isolation of the cis-isomer 5 as a stable solid material is thought to
be due to the formation of an efficient supramolecular structure in
the crystal lattice of this isomer enabling it, at the same time, to give
the relatively strong spin–spin interaction based on the ST model.
In this context, novel organic spin systems with intriguing
properties would be developed by using a supramolecular approach
as exemplified in this work and therefore related work is still in
progress along these lines.
This work was supported by a Grant-in-Aid for Scientific
Research (No. 13440213) from the Japan Society for the
Promotion of Science (JSPS), which is gratefully acknowledged.
Notes and references
{ Crystal data for 3a: C32H46N3O4, M ~ 536.74, triclinic, space group P-1,
˚
a ~ 6.6750(7), b ~ 11.850(2), c ~ 20.811(2) A, a ~ 75.756(7), b ~
3
˚
81.813(3), c ~ 75.557(2)u, V ~ 1539.3(3) A , T ~ 296 K, Z ~ 2, m(Mo-
Ka) ~ 0.076 mm21, 8022 reflections measured, 4337 unique, R ~ 0.177,
wR ~ 0.081. For 3b: C33H48N3O4, M ~ 550.76, monoclinic, space group
Fig. 1 Crystal structure of 3b viewed along the a-axis.
˚
P21/n, a ~ 5.973(6), b ~ 42.691(9), c ~ 12.856(7) A, b ~ 100.52(7)u, V ~
relevant to the observed magnetic interaction in this radical. The
weak ferromagnetic interaction observed in this radical can be
understood by considering the spin polarization effect between the
spin centers through the hydrogen-bonds between the oxygen atom
3223(3) A , T ~ 303 K, Z ~ 4, m(Mo-Ka) ~ 0.074 mm21, 6455 reflections
3
˚
measured, 6205 unique, R ~ 0.060, wR ~ 0.056. CCDC numbers 240929
and 240930. See http://www.rsc.org/suppdata/cc/b4/b408432b/ for crystal-
lographic data in .cif or other electronic format. Crystal data for the
compounds are available as ESI.
of a spin center and a hydrogen atom on a methyl group of the
8
˚
neighboring molecule, which amounts to 3.17 A (see ESI).
While any attempt to isomerize and isolate the PROXYL-based
radicals 3a, 4a or TEMPO-based biradical 4b as a stable solid form
has so far been unsuccessful, the photochemical isomerization of
the TEMPO-based monoradical 3b was found to proceed
efficiently, giving the corresponding cis-isomer as a relatively
stable solid material. Thus, irradiation of 3b with light of 365 nm in
dichloromethane showed an apparent absorption spectral change
and gave after a period of exposure of 4 h the desired cis-isomer 5,
which could be purified by a short column chromatography on
SiO2 affording an orange yellow solid in 40% yield which was
found to be persistent when kept in the dark and stored in a
refrigerator. The reverse reaction was found to occur when the cis-
isomer 5 (lmax in the solid-state: 435 and 307 nm) was exposed in
the solid-state to diffused sunlight for 1 d to recover the original
trans-isomer 3b (lmax in the solid-state: 350 nm) in 50% yield and
thus the reversible system could be constructed in principle
(Scheme 1).
1 For recent examples: S. Karasawa, H. Kumada, N. Koga and
H. Iwamura, J. Am. Chem. Soc., 2001, 123, 9685; I. Ratera,
D. Ruiz-Molina, J. Vidal-Gancedo, K. Wurst, N. Daro, J.-F. Le`tard,
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3 cf. S. Nakatsuji, Adv. Mater., 2001, 13, 1719.
4 T. Kaneko, H. Akutsu, J. Yamada and S. Nakatsuji, Org. Lett., 2003, 5,
2127 and references cited therein.
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Chem., 2003, 68, 1708.
6 T. Amano, H. Akutsu, J. Yamada and S. Nakatsuji, Chem. Lett., 2004,
33, 382.
7 R. Willstatter and M. Benz, Chem. Ber., 1906, 39, 3492.
8 cf. J. Veciana, J. Cirujeda, C. Rovira and J. Vidal-Gancedo, Adv. Mater.,
1995, 7, 221; K. Tagashi, R. Imachi, K. Tomioka, H. Tsuboi, T. Ishida,
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4369.
At the same time, the distinct change of the intermolecular
interaction was disclosed by the structural change, that is, the weak
ferromagnetic interaction based on the CW model being observed
C h e m . C o m m u n . , 2 0 0 4 , 2 3 1 0 – 2 3 1 1
2 3 1 1