Azobenzene derivatives carrying a nitroxide radical

Article abstract of DOI:10.1021/jo062266f

Several trans-azobenzene derivatives carrying a nitroxide (aminoxyl) radical (2a, 6a-12a) were prepared, and their photoisomerization reactions to the corresponding cis-isomers were investigated. Although no fruitful results could be obtained for the phot

Full text of DOI:10.1021/jo062266f

Azobenzene Derivatives Carrying a Nitroxide Radical
Shin’ichi Nakatsuji,*,† Masahiro Fujino,† Satoko Hasegawa,† Hiroki Akutsu,†
Jun-ichi Yamada,† Vladimir S. Gurman,‡ and Andrey Kh.Vorobiev*,‡
Department of Material Science, Graduate School of Material Science, UniVersity of Hyogo,
3-2-1 Kouto, Kamigori, Hyogo 678-1297, Japan, and Department of Chemistry,
Moscow State UniVersity, Leninskie Gory, Moscow 119992, Russia
nakatuji@sci.u-hyogo.ac.jp; VorobieV@excite.chem.msu.ru
ReceiVed NoVember 2, 2006
Several trans-azobenzene derivatives carrying a nitroxide (aminoxyl) radical (2a, 6a-12a) were prepared,
and their photoisomerization reactions to the corresponding cis-isomers were investigated. Although no
fruitful results could be obtained for the photoisomerizations of the derivatives with para-subsituents
(9a-12a), the unsubstututed derivatives at the para-position (2a, 6a, 7a, 8a) were found to show
photoisomerizations by irradiation to give the corresponding cis-isomers (2b, 6b, 7b, 8b), being isolated
as relatively stable solid materials, and the change of the intermolecular magnetic interactions was
apparently observed by the structural change for each photochromic couple.
Introduction
organic and organometallic photofunctional spin systems have
been reported until now, and they include several impressive
examples of diarylethene derivatives,4 spin systems with a
ferrocene moiety,5 or metal complexes with a spiropyran
photochromic unit.6 In the course of our studies to develop novel
organomagnetic materials, we have been interested in preparing
multifunctional spin systems with conductivity, photofunction-
ality, or a liquid crystalline property by using stable radicals,
especially nitroxide radicals, as spin sources. As for the spin
systems with photofunctionality, we have so far proposed several
photoresponsive spin systems by using such photochromic
systems as norbornadiene/quadricyclane, spiropyran/merocya-
nine, anthracene/dimer, or naphthopyran/merocyanine. Azoben-
There is a continuing trend in the field of molecular-based
magnetic materials to develop multifunctional spin systems,1
and the exploitation of organic photofunctional materials has
attracted in this context much attention in recent years.2 As a
precedent example, Iwamura and his collaborators prepared a
trans-azobenzene derivative carrying two nitronyl nitroxide
groups and reported it to show UV as well as EPR spectral
change upon irradiation in solution.3 Since then, a variety of
† University of Hyogo.
‡ Moscow State University.
(1) For recent reviews on molecular-based magnetic materials, see: (a)
Magnetic Properties of Organic Materials; Lahti, P. M., Ed.; Marcel Dekker,
Inc.: New York, Basel, 1999. (b) Molecular Magnetism; Itoh, K., Kinoshita,
M., Eds.; Kodansha/Gordon and Breach Science Publishers: Tokyo, 2000.
(c) Structure and Bonding, Vol. 100, π-Electron Magnetism: From Molecule
to Magnetic Materials; Veciana, J., Ed.; Springer-Verlag: Berlin, 2001.
(d) Magnetism: Molecules to Materials; Miller, J. S., Drillon, M., Eds.;
Wiley-VCH: Weinheim, Germany, 2001-2005; Vols. I-V.
(2) Cf. Natatsuji, S. Chem. Soc. ReV. 2004, 33, 348.
(4) Cf. Matsuda, K.; Irie, M. J. Photochem. Photobiol., C 2004, 5, 169
and references therein.
(5) (a) Ratera, I.; Ruiz-Molina, D.; Vidal-Gancedo, J.; Wurst, K.; Daro,
N.; Le´tard, J.-F.; Rivira, C.; Veciana, J. Angew. Chem., Int. Ed. 2001, 40,
919. (b) Ratera, I.; Ruiz-Molina, D.; Vidal-Gancedo, J.; Novoa, J. J.; Wurst,
K.; Le´tard, J.-F.; Rivira, C.; Veciana, J. Chem.sEur. J. 2004, 10, 603.
(6) Beˆnard, S.; Riviere, E.; Yu, P.; Nakatani, K.; Delouis, J. F. Chem.
Mater. 2001, 13, 159.
(3) Hamachi, K; Matsuda, K.; Itoh, T.; Iwamura, H. Bull. Chem. Soc.
Jpn. 1998, 71, 2937.
10.1021/jo062266f CCC: $37.00 © 2007 American Chemical Society
Published on Web 02/20/2007
J. Org. Chem. 2007, 72, 2021-2029
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Nakatsuji et al.
CHART 1
SCHEME 1
zene derivatives with stable radicals have also been attractive
candidates for photofunctional spin systems, and even if there
was initial apprehension of difficulty for the isolation of cis-
isomers of azobenzene derivatives as inferred in another case,3
we took the plunge to prepare some targeted compounds of the
derivatives I (Chart 1). Among them, azobenzene derivatives
carrying nitroxide radicals as well as long alkyl groups have
particularly been focused upon, since the introduction of long
alkyl groups is supposed to add a possible heat-responsive
property as previously shown for compounds II7 to inherent
photofunctionality expected from the existence of an azobenzene
core. It was anticipated through earlier studies that the specific
assemblies of such molecules as II or III8 with supramolecular
structures might be relevant to the formation of the mesogenic
phase and/or unusual thermomagnetic properties. Although no
fruitful photoresponsive property could be realized in the
biradical compound IV with an azobenzene moiety as well as
long alkyl groups, probably because of the unstable nature of
the corresponding cis-isomer, fairly large intermolecular mag-
netic interaction has been observed in the compound due mainly
to its specific assembly structure in the solid state.9 On the other
hand, some cis-azobenzenes with a nitroxide group have been
found to be isolable in the solid state, exhibiting differences in
their magnetic properties compared with the corresponding
trans-isomers. We herein describe the details of the preparation
of such azobenzene derivatives carrying a nitroxide group that
show photoresponsive properties together with the differences
in their magnetic properties derived from the structural differ-
ences.10
Results and Discussion
Preparation of trans-Azobenzene Derivatives with a Ni-
troxide Group. The preparation of trans-azobenzene derivatives
(7) Nakatsuji, S.: Ikemoto, H.; Akutsu, H.; Yamada, J.; Mori, A. J. Org.
Chem. 2003, 68, 1708.
(8) Nakatsuji, S.:Mizumoto, M.; Ikemoto, H.; Akutsu, H.; Yamada J.
Eur. J. Org. Chem. 2002, 1869.
(9) (a) Amano, T.; Akutsu, H.; Yamada, J.; Nakatsuji, S. Chem. Lett.
2004, 33, 382. (b)Nakatsuji, S.; Amano, T.; Akutsu, H.; Yamada, J. J. Phys.
Org. Chem. 2006, 19, 333.
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Azobenzene DeriVatiVes Carrying a Nitroxide Radical
FIGURE 1. (Left) Time dependence of the absorption spectra for trans-isomer 2a to cis-isomer 2b every 5 min from the original absorption in
dichloromethane. (Right) Change in the absorbance of the forward and backward reactions of the photoisomer couple 2a/2b at 327 nm.
SCHEME 2
with a nitroxide group (2a, 6a-12a) was carried out as shown
in Scheme 1. A TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy)-
substituted ester derivative (2a) was prepared in 66% yield by
the condensation of 4-carboxyazobenzene (1) with 4-hydroxy-
TEMPO using DCC and DMAP as the condensing reagents.
Similarly, an isomeric ester (6) was obtained by using 4-hy-
droxyazobenzene (3) and 4-carboxyl-TEMPO in 74% yield. The
treatment of 3 with (bromoundecanoxy)carbonyl-TEMPO9 or
the PROXYL (2,2,5,5-tetramethyl-1-pyrrolidinyloxy) derivative9
gave the corresponding azobenzene derivatives with a TEMPO
(7a) or PROXYL (8a) substituent in 72% and 71% yield,
respectively. A couple of azobenzenes with p-dimethylamino
or p-nitro substituents and with a TEMPO or PROXYL group
(9a-12a) could similarly be prepared from the corresponding
hydroxyazobenzenes 4 and 5, although the yields were relatively
low (9a, 16%; 10a, 41%; 11a, 30%; 12a, 68%). The thermo-
tropic properties of a series of azobenzenes thus prepared were
examined by DSC measurements, but the endothermic peaks
of their melting points could only be discerned for all of the
compounds even those with a long alkoxy chain, indicating the
absence of any liquid crystalline phase in the radicals.
at around 327 nm were found to decrease gradually while a
new broad absorption at around 440 nm with an isosbestic point
at 390 nm was developed in turn, indicating occurrence of
photoisomerization in the solution (Figure 1, left). In spite of
the initial apprehension and the fruitless experience for the
photoisomerization of biradical compound IV,9 the correspond-
ing cis-isomer 2a could fortunately be isolated after purification
by short-column chromatography on SiO2 as a relatively stable
solid substance when kept in the dark and stored in a refrigerator.
In turn, the backward reaction took place simply by the exposure
of the powdery solid or a solution of the cis-isomer 2a to
diffused light or by illumination with a fluorescent lamp at
ambient temperature for several hours, and hence, a reversible
system could be constructed in principle in this case (Scheme
2). The forward and backward reactions could be repeated
several times without appreciable decay (Figure 1, right) for
the isomer couple 2a and 2b. The photoisomerization of the
ester isomer 6a proceeded quite similarly (cf. SI-1, Figure A;
SI ) Supporting Information), and the corresponding cis-isomer
6b could also be isolated as a relatively stable solid. Even the
azobenzenes with a long alkyl group and a TEMPO or a
PROXYL group (7a or 8a) were also found to occur by a sim-
ilar irradiation to give the corresponding cis-isomers 7b and
8b as relatively stable solids,10 while irradiation on the para-
substituted derivatives 9a-12a under various conditions re-
sulted in photobleaching and/or recovery of the starting ma-
terials in solution, and hence, the isolation of the corre-
sponding cis-isomers was unsuccessful so far as examined in
these cases.
Investigation of Photochemical Isomerizations of Azoben-
zene Derivatives with a Nitroxide. When trans-azobenzene
with a TEMPO substituent (2a) was irradiated in dichloro-
methane solution with light of 365 nm, the absorption maxima
(10) Portions of this work have appeared as preliminary communica-
tions: (a) Fujino, M.; Amano, T.; Akutsu, H.; Yamada, J.; Nakatsuji, S.
Chem. Commun. 2004, 2310. (b) Amano, T.; Fujino, M.; Akutsu, H.;
Yamada, J.; Nakatsuji, S. Polyhedron 2005, 24, 2614.
J. Org. Chem, Vol. 72, No. 6, 2007 2023

Nakatsuji et al.
TABLE 1. UV-Vis Data for Azobenzene Derivatives 6a, 7a, and 2aa
trans isomer
cis isomer (estimated value)
compd
λmax
ꢀ
λmax
ꢀ
λmax
ꢀ
azobenzene
447
444
435
457
480 ( 10
560 ( 15
720 ( 20
485 ( 15
318.5
323.5
348
2.0 104 ( 1000
2.3 104 ( 1000
2.0 104 ( 1000
2.7 104 ( 1000
433
436
441
436
1100 ( 15
∼1270
6a
7a
2a
∼1420
325.5
∼1000
a In methyltetrahydrofuran at 293 K.
To compare the efficiencies for photoisomerizations of three
azobenzenes (2a, 6a, 7a), we next examined their photorespon-
sive properties by kinetic measurements. The quantitative
spectral data necessary for calculation of the quantum yields
are summarized in Table 1 (cf. SI-2, Figure B). These data show
that nitroxide substituents induce some bathochromic shift of
the absorption bands. The largest shift of the ππ* absorption is
observed in the case of compound 7a, containing a long alkyl
spacer between the azobenzene and nitroxide moieties. This
interesting phenomenon obviously is a result of intramolecular
complexation of the azobenzene and nitroxide fragments of the
molecule in solution, as it is known that the nitroxide group is
inclined to complexation with molecules in almost all classes
of organic substances.11
The kinetics of reversible photochemical trans-cis and cis-
trans isomerization is described by the following equation:
FIGURE 2. Change in the absorbance of the solution of 6a in
methyltetrahydrofuran in the course of irradiation at 365 nm.
At
tAc + At
absorption coefficients at wavelength λ, and
dT
dt
[1 - 10-(A +A )] +
t
c
) -I0φ
1
τ )
(4)
Ac
cAc + At
I0φ
[1 - 10-(A +A )] (1)
t
c
I0l(φtꢀt + φcꢀc)
The value of τ in eq 3 can be determined in the course of kinetic
experiments. Figure 2 shows the photoinduced change of the
UV-vis spectrum of 6a in methyltetrahydrofuran in more detail.
Typical kinetic curves are presented in Figure 2.
where T is the concentration of the trans-isomer in solution, t
is the time of light irradiation, I0 is the intensity of irradiation,
φt and φc are the quantum yields of the trans-cis and cis-
trans photoreactions, correspondingly, and At and Ac are the
absorbances of light by the trans- and cis-isomers, correspond-
ingly. The thermal cis-trans isomerization is neglected in
eq 1.
One can see from Figure 2 that the absorbance of the solution
at 365 nm becomes very small after long irradiation. This
observation leads to the following two consequences. The
concentration of the trans-isomer after long irradiation by light
of 365 nm can be neglected, and the absorption coefficient of
the cis-isomer can be estimated using such a solution. Estimated
values for the compounds studied are presented in the SI. On
the other hand, the condition φtꢀt . φcꢀc allows us to determine
of the quantum yield of trans-cis isomerization using eq 4.
The results thus obtained are summarized in Table 2.
Determination of the quantum yields of cis-trans photo-
isomerization in the course of irradiation by light of 436 nm is
more complicated as there is no wavelength with pure absor-
bance by one component. As a result the photostationary
condition φcꢀcCs ) φtꢀtTs was used, and the ratio φcꢀc/φtꢀt was
determined as
Using variable
-(At+Ac)
t
1 - 10
u )
dt
∫
0
At + Ac
eq 1 can be simplified:
dT/du ) I0(φcAc - φtAt)
(2)
Taking into account that At ) ꢀtlT and Ac ) ꢀclC, where C is
the concentration of the cis-isomer, l is the optical length, and
ꢀt and ꢀc are the absorption coefficients of the trans- and cis-
isomers, correspondingly, the solution of eq 2 can be presented
as
Ts
Cs
φcꢀc
)
(5)
φtꢀt
φtꢀt
0φtꢀt + φcꢀc
∆Aλ(u) ) (ꢀλc - ꢀtλ)lT
(1 - e-u/τ
)
(3)
Then the quantum yields were calculated using eqs 4 and 5.
The results are also presented in the same table. The quantum
yields for azobenzene itself were determined as well for
comparison. One can see that literature values in different
solvents are in good agreement with the values obtained in the
present work. The results show that compounds 2a, 6a, and 7a
are more photoreactive as compared with azobenzene itself. It
where ∆Aλ is the change of absorbance of the solution at the
wavelength of registration λ, ꢀλc - ꢀtλ is the difference in the
(11) Cf. Buchachenko A. L. Kompleksy radikaloV i molekulyarnogo
kisloroda s organicheskimi molekulami (Complexes of Radicals and
Molecular Oxygen with Organic Molecules); Nauka: Moscow, 1984.
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Azobenzene DeriVatiVes Carrying a Nitroxide Radical
TABLE 2. Quantum Yields of Photoisomerization, Ot and Oc
irradiation 365 nm
irradiation 436 nm
compd
trans f cis
0.16
cis f trans
trans f cis
0.33
cis f trans
0.77
0.83
∼1.0
0.56 (0.46-0.58)a
6a
7a
2a
0.30
0.69
0.055
∼0.29
azobenzene
0.1 (0.11-0.16)a
0.26 (0.24-0.31)a
0.22 (0.21-0.27)a
a Lit.: Zimmerman G., Chow L.-Y., Paik U.-J. J. Am. Chem. Soc., 1956, 80, 3528. Bortulus P., Monti S. J. Phys. Chem. 1979, 83, 648.
TABLE 3. Solid-State Magnetic Data of Azobenzene Derivatives
with Nitroxide
magnetic
interaction
compd
Ca/emu K mol-1
Θb/K
(J/kB)/K
2a
6a
7a
8a
antiferromagnetic
antiferromagnetic
ferromagnetic
antiferromagnetic
antiferromagnetic
antiferromagnetic
ferromagnetic
antiferromagnetic
ferromagnetic
ferromagnetic
-47.6c (100)
-1.89d (91)
0.38 (100)
0.36 (95)
0.09
-4.2c (83)
9a
-12
10a
11a
12a
2b
6b
7b
8b
-3.5c (92)
0.16d (84)
0.35 (93)
0.38 (100)
0.38 (100)
-0.22
0.11
0.32
-36.7c (64)
-7.7c (76)
FIGURE 3. EPR spectrum of 6a in methyltetrahydrofuran at 77 K
and the simulated one.
antiferromagnetic
antiferromagnetic
a Curie constant. Numbers in parentheses denote the estimated spin
concentrations, by using a theoretical Curie constant of 0.375 emu K mol-1
.
is noteworthy that the largest quantum yields were found for
compound 7a with a long alkyl chain. We presume that this is
a result of the influence of the nitroxide group with a long alkyl
chain on photophysical processes in the exited state of the
molecule.
b Weiss temperature. c Exchange interactions obtained by fitting with the
singlet-triplet model. d Exchange interactions obtained by fitting with the
1-D Heisenberg model. Numbers in parentheses denote estimated spin
concentrations.
Magnetic Properties of Azobenzene Derivatives with a
Nitroxide. The magnetic data of the azobenzenes in solution
are obtained by EPR measurements, and 1:1:1 triplet spectra
were always observed in all of the radicals examined irrespective
of the difference in substituent, geometry (trans and cis), and
spin source (TEMPO or PROXYL) with similar g factors of
ca. 2.006 and similar coupling constants aN of around 1.5 mT
due to a nitroxide moiety (cf. SI-3, Figure C). A typical EPR
spectrum at 77 K in degassed methyltetrahydrofuran and the
simulated one for 6a are shown in Figure 3. To see some
difference in the magnetic behavior in solution between the
isomer couples, anisotropic parameters were obtained in the
course of numerical simulation of the spectrum of 6a at 77 K,
and the data for the three isomer couples including compound
6a (2a/2b, 6a/6b, and 7a/7b) show that the spectra of all
compounds in both the trans- and cis-forms coincide within
experimental errors (cf. SI-4, Table A).
To verify the intermolecular magnetic interactions of the
azobenzene derivatives with a nitroxide group, magnetic
measurements of the solid samples were carried out by using a
SQUID susceptometer in the temperature range from 2 to 300
K, and the data are summarized in Table 3, in which the data
for the trans-isomers are given in the upper columns and those
for the cis-isomers in the lower ones. Thus, an antiferromagnetic
interaction being well expressed by the singlet-triplet (ST)
model12 is observed in the radical 2a with a fairly large exchange
interaction of J/kB ) -47.6 K. ST behaviors are also revealed
in the radicals 8a and 10a with a PROXYL substituent, but the
J/kB values (-4.2 K for 8a and -3.5 K for 10a) of the radicals
are much smaller than that found in 2a. On the other hand,
antiferromagnetic interactions being based on the Curie-Weiss
(CW) model are observed in the trans-azobenzenes 9a and 12a,
while weak ferromagnetic interaction of CW behavior is found
for the trans-azobenzene 7a. The magnetic behavior of the
radical 6a and 4-nitro-substituted azobenzene with TEMPO
(11a) can be well analyzed by the one-dimensional Heisenberg
model13 with an antiferromagnetic exchange interaction of
J/kB ) -1.89 K for 6a and a small ferromagnetic one of
J/kB ) 0.16 K for 11a (vide infra; cf. SI-5, Figure D, and SI-6,
Figure E).
While the existence of a fairly strong antiferromagnetic
interaction based on the ST model is clarified in the trans-isomer
of ester derivative 2a, a weak ferromagnetic interaction with
CW behavior is observed in the corresponding cis-isomer 2b
as shown in Figure 4, indicating the occurrence of an apparent
change in the intermolecular magnetic interactions between the
photoisomer couple originating possibly from the change in the
molecular/crystal structures.
Although weak intermolecular magnetic interactions are
preserved between the photoisomer couple 6a and 6b, apparent
antiferromagnetic interaction observed in the spins of 6a is
changed to a ferromagnetic one in those of 6b. As regards the
photoisomer couple 8a and 8b with a PROXYL group, the
magnitudes of the exchange interactions (J/kB values) are
apparently altered by the structural change as indicated in Table
3. Namely, the ST behavior with a weak antiferromagnetic
(13) Bonner J. C., Fisher, M. E. Phys. ReV. A 1964, 135, 640. In fact,
the radical molecules in the crystal are found to consist of two kinds of
chain structures: one gives rise to ferromagnetic interaction and the other
to antiferromagnetic interaction, and the best fit to the experimental data is
obtained in the present case by considering overwhelmingly the latter one
with the 1-D Heisenberg model.
(12) Bleaney, B., Bowers, K. D. Proc. R. Soc. London, Ser. A 1952,
214, 451.
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Nakatsuji et al.
FIGURE 4. Magnetic suscptibility data of trans-isomer 2a (left, ø-T
data) and the corresponding cis-isomer 2b (right, øT-T data). The solid
line in the left panel is the theoretical curve fitted by the ST model
(see the text).
FIGURE 6. Magnetic data for trans-isomer 7a (left) and cis-isomer
7b (right) between 2 and 400 K and during the heating (open square)
and cooling (closed triangle) processes. The reheating data (plus signs)
are also indicated with almost no difference from the cooling data for
both compounds.
in the heating and cooling processes is not clarified yet, and
although the occurrence of a change in the molecular config-
uration of the trans-isomer 7a cannot be considered from the
magnetic data, a change of the molecular pacing is presumed
to happen by thermal phase transition to give different J/kB and
ø values, respectively. Such a heat-responsive property is
considered to be due partially to the existence of a long alkoxy
group in the radical 7b as seen in the previous examples II and
III, although this is not the case for the trans-isomer 7a. Thus,
the photoisomer couple 7a and 7b is regarded as a kind of
photoresponsive as well as heat-responsive spin system.
Molecular/Crystal Structures of trans-Azobenzene Deriva-
tives with a Nitroxide (2a, 6a, 7a, 8a, and 11a). A single
crystal suitable for X-ray analysis was obtained for the radical
2a by recrystallization from hexane/diethyl ether, and its
structure with a trans-configuration of the azobenzene moiety
is apparent from X-ray analysis (Figure 7). The oxygen-oxygen
distance of the neighboring spin centers amounts about to 3.5
Å, which is relevant to the behavior being based on the ST
model observed in the radical with a fairly strong antiferro-
magnetic interaction of J/kB ) -47.6 K.9,12 A single crystal
could also be obtained by recrystallization of the ester isomer
6a from hexane/diethyl ether, and the crystal structure is shown
in Figure 8. In this crystal, the molecules are found to be formed
of interconnecting chains, and its magnetic property is regarded
to be predominantly governed by antiferromagnetic chains, being
well analyzed by the 1-D Heisenberg model, with J/kB ) -
1.89 K (vide supra).13
The crystal structure of the TEMPO derivative 7a is shown
in Figure 9, and two molecules are depicted in the figure. Thus,
the molecules have long alkoxy chains attached to trans-
azobenzene cores and TEMPO groups at their ends. The
molecules form a sheetlike structure on the bc plane, and the
nearest oxygen-oxygen distance of the neighboring spin centers
is estimated to be ca. 6 Å, which is fairly far apart for giving
a strong intermolecular magnetic interaction and is relevant to
the observed very weak magnetic interaction in this radical.
In Figure 10, the crystal structure of the corresponding
PROXYL derivative 8a is indicated, and two molecules are
depicted in this case as well. Thus, the molecules also have
long alkoxy chains attached to trans-azobenzene cores and
PROXYL groups at their ends, and the O-O distance of the
FIGURE 5. Magnetic data of trans-isomer 7a (left, øT-T data) and
the corresponding cis-isomer 7b (right, ø-T data). The solid line in
the right panel is the theoretical curve fitted by the ST model (see the
text).
interaction (J/kB ) -4.2 K) in the trans-isomer 8a is changed
to ST behavior with an antiferromagnetic interaction of an
almost 2 times larger value (J/kB ) -7.7 K) in the cis-isomer
8b. A more impressive change in the intermolecular interactions
is found in the photoisomer couple 7a and 7b (Figure 5). That
is, the original CW behavior with a weak ferromagnetic inter-
action (Θ ) 0.09 K) is changed to ST behavior with a fairly
strong antiferromagnetic interaction with a J/kB value of ca. -37
K, suggesting closer contact of the spin centers evoked by the
structural change.14 Thus, a series of photoresponsive spin sys-
tems showing different changes in their intermolecular magnetic
interactions could successfully be constructed in these cases.
Interestingly, a heat-responsive magnetic property has also
been disclosed in the cis-isomer 7b. While no appreciable
change could be discerned in the trans-isomer 7a during the
heating process to 400 K (mp ca. 380 K) and successive cooling
process (Figure 6, left), an apparent change was observed in
the cis-isomer 7b during the processes, when heated over its
melting point (ca. 310 K) as shown in Figure 6 (right). Namely,
an appreciable increase of the ø values in the lower temperature
region together with a decrease of the exchange interaction to
J/kB ) -14 K from the original J/kB value of ca. -37 K is
observed during the cooling process in the isomer 7b. Although
the reason why the different magnetic behaviors are observed
(14) The discrepancy between the experimental and theoretical curves
in the low-temperature region is considered to be due to the presence of a
small amount of paramagnetic impurities and/or defects, and they are
neglected in this case.
2026 J. Org. Chem., Vol. 72, No. 6, 2007

Azobenzene DeriVatiVes Carrying a Nitroxide Radical
FIGURE 7. Crystal structure of 2a indicating oxygen-oxygen distances of the spin centers and the spins on them.
FIGURE 8. Crystal structure of 6a indicating 1-D magnetic chains with dotted lines.
FIGURE 9. Crystal structure of 7a (two molecules are depicted)
indicating oxygen-oxygen distances of the spin centers.
FIGURE 11. Crystal structure of 11a (four molecules are depicted)
indicating short contacts between the next neighbor molecules.
FIGURE 10. Crystal structure of 8a (two molecules are depicted)
indicating oxygen-oxygen distances of the spin centers.
spin centers between the molecules amounts to ca. 4.1 Å, which
is considered to be responsible for the ST behavior with a weak
antiferromagnetic interaction.
The crystal structure of p-nitroazobenzene derivative 11a
is shown in Figure 11, and there are a couple of short con-
tacts between the oxygen atom of a TEMPO group and the
neighboring molecule, forming on the whole a 1-D chain
structure and thus reflecting the magnetic behavior of the
1-D Heisenberg model observed in the derivative 11a (see
Table 3).
However, any attempt to prepare a single crystal of the
corresponding cis-isomers has so far been unsuccessful probably
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Nakatsuji et al.
C22H26N3O3: C, 69.45; H, 6.89; N, 11.04. Found: C, 69.23; H,
6.84; N, 11.08.
because of the unstable nature of the isomers, particularly in
solution.
Preparation of Nitroxide-Substituted trans-Azobenzenes 7a-
12a. The preparation of a TEMPO-substituted derivative (7a) is
described as an example: A stirred solution of 4-hydroxyazoben-
zene (0.20 g, 1.0 mmol), (bromoundecanoxy)carbonyl-TEMPO9
(0.44 g, 1.0 mmol), and potassium carbonate (0.42 g, 3.0 mmol) in
a mixed solvent of DMF/THF (3:1, 40 mL) was heated to reflux,
and heating was continued for 20 h. After being cooled to room
temperature, the reaction mixture was filtered and washed with THF,
and the filtrate was concentrated in vacuo to give a brown-yellow
solid, which was purified by column chromatography on silica gel
by using the solvent system of benzene and diethyl ether and then
recrystallized from a mixed solvent of hexane and methanol. The
derivative 7a was obtained as yellow needles (0.40 mg, 71%).
Mp: 106-108 °C. EPR data: see the SI. Anal. Calcd for
C33H49N3O4: C, 71.83; H, 8.95; N, 7.62. Found: C, 71.64; H, 8.77;
N, 7.83. In a similar manner, the derivatives 8a ((bromoundecan-
oxy)carbonyl-PROXYL9 is used in place of (bromoundecanoxy)-
carbonyl-TEMPO), 9a (4-(dimethylamino)-4′-hydroxyazobenzene
is used in place of 4-hydroxyazobenzene), 10a, 11a (4-nitro-4′-
hydroxyazobenzene is used in place of 4-hydroxyazobenzene), and
12a were prepared in 72%, 16%, 41%, 30%, and 68% yields,
respectively, and their data are as follows. (8a) Yellow needles.
Conclusions
We prepared a series of trans-azobenzene derivatives with
nitroxide substituents and investigated their photoisomerization
reactions as well as magnetic properties. The crystal structures
of some of the derivatives could be clarified, and their structure/
magnetic property relations were investigated. Among the trans-
azobenzenes prepared, the unsubstututed derivatives at the para-
position (2a, 6a, 7a, 8a) were found to undergo photoisomeriza-
tions by irradiation to give the corresponding cis-isomers (2b,
6b, 7b, 8b), being isolated as relatively stable solid materials.
The respective changes of the intermolecular magnetic interac-
tions were apparently observed by the structural changes for
all of the photochromic couples, and a kind of multifunctionality
with photoresponsive as well as heat-responsive properties was
disclosed in the photoisomer couple 7a/7b.
Experimental Section
Materials. 4-Carboxy- and 4-hydroxy-TEMPO radicals as well
as 4-carboxy-, 4-hydroxy-, and 4-nitro-4′-hydroxyazobenzenes used
as building blocks in this study are commercially available.
4-(Dimethylamino)-4′-hydroxyazobenzene15 was prepare by the
coupling of diazonium salt from 4-aminophenol with N,N-di-
methyaniline.
Mp: 133-136 °C. EPR (benzene): three lines, g ) 2.006, aN
)
1.42 mT. Anal. Calcd for C32H47N3O4: C, 71.47; H, 8.81; N, 7.82.
Found: C, 72.20; H, 8.94; N, 7.70. (9a) Yellow powdery solid.
Mp: 122-126 °C. EPR (benzene): three lines, g ) 2.006, aN
)
1.54 mT. Anal. Calcd for C35H53N4O4: C, 70.79; H, 9.00; N, 9.44.
Found: C, 70.77; H, 9.26; N, 9.48. (10a) Yellow solid. Mp: 117-
122 °C. EPR (benzene): three lines, g ) 2.006, aN ) 1.44 mT.
FAB-HRMS (m/z): calcd for C34H52N4O4 (M + 1) 580.3989, found
580.4046. (11a) Orange-yellow needles. Mp: 105-108 °C. EPR
(benzene): three lines, g ) 2.006, aN ) 1.54 mT. FAB-HRMS
(m/z): calcd for C33H47N4O6 595.3496, found 595.3522. (12a)
Orange-yellow solid. Mp: 96-100 °C. EPR (benzene): three lines,
g ) 2.006, aN ) 1.44 mT. FAB-HRMS (m/z): calcd for C32H45N4O6
581.3339, found 581.3367.
Instrumentation. Melting points of the solid samples are
uncorrected. The UV-vis spectra were measured in dichlo-
romethane or methyltetrahydrofuran solution at ambient temerature.
The FAB-MS spectral data were obtained by using m-nitrobenzyl
alcohol as the matrix and the appropriate polyethylene glycol sample
as the internal standard. The g values of the EPR data were
determined using Mn2+/MgO maker as an internal standard.
Susceptibility measurements were carried out using ca. 10 mg for
each powdered sample in the usual way.16 Photolysis experiments
for kinetic studies were performed with a mercury high-pressure
lamp (500 W). Light of the required wavelength was isolated using
standard glass filters. The intensity of incident light was determined
by ferrioxalate actinometry17 (for light of 365 nm) and Reinekate
salt actinometry18 (for light of 436 nm). These experiments were
performed with samples degassed by a repeated procedure of
freezing-pumping-melting.
Preparation of TEMPO-Substituted Azobenzenes 2a and 6a.
To a stirred mixture of 4-carboxyazobenzene (1) (0.26 g, 1.2 mmol)
and 4-hydroxy-TEMPO (0.20 g, 1.2 mmol) in dichloromethane (30
mL) were added DCC (0.29 g, 1.4 mmol) and DMAP (0.17 g, 1.4
mmol) at ambient temperature. After being stirred for 1 d and after
the precipitated urea was filtered off, the reaction mixture was
concentrated in vacuo to give an orange solid, which then was
purified by column chromatography on silica gel with the solvent
system of hexane and diethyl ether and recrystallized from the same
system. The trans-azobenzene 2a was obtained as orange plates
(0.29 g, 66%). Mp: 127-130 °C. EPR data: see the SI. Anal.
Calcd for C22H26N3O3: C, 69.45; H, 6.89; N, 11.04. Found: C,
69.81; H, 6.95; N, 11.22. In a similar manner, the isomeric
azobenzene 6a was prepared by using 4-hydroxyazobenzene (3)
and 4-carboxy-TEMPO as starting materials and was obtained as
yellow needles by recrystallization from n-hexane and diethyl ether.
Mp: 151-154 °C. EPR data: see the SI. Anal. Calcd for
Preparation of Nitroxide-Substituted cis-Azobenzenes 2b, 6b,
7b, and 8b by Photochemical Isomerization. The preparation of
a TEMPO-substituted derivative (6b) is described as an example:
A dichloromethane solution (20 mL) of trans-azobenzene derivative
6a (0.10 g, 0.26 mmol) was irradiated by a lamp with light of 365
nm for 6 h. The solvent was then evaporated carefully under reduced
pressure in the dark to avoid cis to trans isomerization by light,
and the resulting solid was purified in the dark by short-column
chromatography on silica gel with the solvent system of n-hexane
and diethyl ether. After the elution of a small amount of less polar
trans-isomer 6a, relatively polar cis-isomer 6b could be isolated
as an orange powdery solid (77 mg, 77%). Mp: 110-113 °C. EPR
data: see the SI; FAB-HRMS (m/z): calcd for C22H26N3O3
380.1974, found 380.1977. In a similar manner, the derivatives 2b,
7b, and 8b were prepared in 38%, 40%, and 50% yield, respectively,
and their data are as follows. (2b) Mp: 134-135 °C. EPR data:
see the SI. FAB-HRMS (m/z): calcd for C22H26N3O3 380.1974,
found 380.1962. (7b) Mp: 34-37 °C. EPR data: see the SI. FAB-
HRMS (m/z): calcd for C33H49N3O4 (M + H) 551.3723, found
551.3698. (8b) Mp: 123-125 °C. EPR (benzene) three lines, g )
2.006, aN ) 1.44 mT. FAB-HRMS (m/z): calcd for C32H46N3O4
536.3488, found 536.3438.
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research (No. 13440213) from the Japan
Society for the Promotion of Science (JSPS) and a special grant
from the University of Hyogo and the Russian Foundation for
Base Reserch (06-03-32231).
(15) Burawoy, A.; Salem, A. G.; Thompson, A. R. J. Chem. Soc. 1952,
4793.
(16) Nakatsuji, S.; Takai, A.; Nishikawa, K.; Morimoto, Y.; Yasuoka,
N.; Suzuki, K.; Enoki, T.; Anzai, H. J. Mater. Chem. 1999, 9, 1747.
(17) Kurien K. C. J. Chem. Soc. B 1971, 2081.
(18) Wegner, E. E.; Adamson, A. W. J. Am. Chem. Soc. 1966, 88, 394.
2028 J. Org. Chem., Vol. 72, No. 6, 2007

Azobenzene DeriVatiVes Carrying a Nitroxide Radical
Supporting Information Available: Figures showing the
absorption change of 6a by irradiation (SI-1, Figure A), the
absorption spectra of 2a, 6a, and 7a (SI-2, Figure B), the EPR
spectra of 6a (SI-3, Figure C), and the magnetic suscepti-
bility data for 6a and 11a (SI-5, Figure D, and SI-6, Figure E), a
table of EPR data for three isomer couples (2a/2b, 6a/6b, and
7a/7b) (SI-4, Table A), a summary of the X-ray structure deter-
mination (SI-7), and CIF files of the crystallographic data (tables
of crystal data, bond lengths and angles, atomic coordinates, and
anisotropic thermal parameters) for 2a, 6a, 7a, 8a, and 11a. This
material is available free of charge via the Internet at http://pubs.
acs.org.
JO062266F
J. Org. Chem, Vol. 72, No. 6, 2007 2029