Article

Synthesis and Photoinduced Magnetic Properties of a Mn12 Single Molecule Magnet by the cis-trans Isomerism of Azobenzene

Bull. Korean Chem. Soc 01/2009; 30(5). DOI: 10.5012/bkcs.2009.30.5.1143

ABSTRACT

Mn12O12(azo-L)16(H2O)4] (1), a new Mn12 single molecule magnet containing a photochromic azobenzene ligand, has been successfully synthesized by substitution of acetate ligand of Mn12 with 6-[4-{4-hexyloxy-phenyl(azo)}-phenoxy]hexanoic-1-acid. The reversible photoisomerization of the azobenzene group was confirmed by UV-visible absorption spectroscopy. The temperature and field dependence of dc susceptibility and the temperature and the frequency dependence of ac susceptibility were measured for the cis and the trans isomer of 1. The magnetization value of the cis isomer in dc measurement is higher than that of the trans isomer. The cis isomer of 1 has a slower relaxation because cis-trans photoisomerization of the azobenzene group in peripheral ligands induces changes in its structure and dipole moment.

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Available from: Jinkwon Kim, Jun 01, 2015
Synthesis and Photoinduced Magnetic Properties of a Mn12 Bull. Korean Chem. Soc. 2009, Vol. 30, No. 5 1143
Synthesis and Photoinduced Magnetic Properties of
a Mn12 Single Molecule Magnet by the cis-trans Isomerism of Azobenzene
Sheby M. George and Jinkwon Kim
*
Department of Chemistry, Kongju National University, Kongju, Chungnam 314-701, Korea.
*
E-mail: jkim@kongju.ac.kr
Received March 5, 2009, Accepted March 26, 2009
[Mn
12
O
12
(azo-L)
16
(H
2
O)
4
] (1), a new Mn12 single molecule magnet containing a photochromic azobenzene
ligand, has been successfully synthesized by substitution of acetate ligand of Mn12 with 6-[4-{4-hexyloxy-
phenyl(azo)}-phenoxy]hexanoic-1-acid. The reversible photoisomerization of the azobenzene group was
confirmed by UV-visible absorption spectroscopy. The temperature and field dependence of dc susceptibility
and the temperature and the frequency dependence of ac susceptibility were measured for the cis and the
trans isomer of 1. The magnetization value of the cis isomer in dc measurement is higher than that of the
trans isomer. The cis isomer of 1 has a slower relaxation because cis-trans photoisomerization of the azobenzene
group in peripheral ligands induces changes in its structure and dipole moment.
Key Words: Single-molecule magnets, Magnetic properties, Azobenzene, Photochroism, Mn12
Introduction
Optically switchable magnetic materials have become of
quite substantial importance in the field of high-density
information storage media.
1-3
One of the promising strategies
to achieve photochemically active magnetic materials is to
hybridize magnetic clusters with photochromic organic
materials to control their properties by light illumination.
4
The
photoswitching of the hybrid materials can be induced as the
electronic, and the magnetic properties of the photochromic
organic materials are changed by structural distortion such as
isomerization.
5
There have been several attempts to control the magnetic
properties of nanoparticles using photoresponsive ligands,
6
but few attempts have been done with molecular magnetic
clusters such as single molecule magnets (SMMs). The
SMMs have been considered promising materials for future
application in data storage and quantum computing
7
because
they have a very large coercive field, quantum tunneling of
magnetization (QTM), and quantum phase interference.
8,9
The best known examples of SMMs are the Mn12 clusters
having the general formula of [Mn
12
O
12
(O
2
CR)
16
(H
2
O)
4
].
10
In
this paper, we report synthesis and magnetic characterizations
of [Mn
12
O
12
(azo-L)
16
(H
2
O)
4
] (1) containing an azobenzene
group in its peripheral ligands.
Experimental
Materials. All chemicals were obtained commercially and
used as received. All solvents used are of HPLC grade.
[Mn
12
O
12
(O
2
CMe)
16
(H
2
O)
4
]2CH
3
COOH4H
2
O (Mn12ac) was
prepared as reported.
11
Synthesis of Ligand (azo-L). The azo compound 6-[4{4-
hexyloxyphenyl(azo)}-phenoxy]hexanoic-1-acid was synthesized
with a slight modification of the literature method.
12
4-Hydroxy-4'-hexyloxyazobenzene. 4-(hexyloxy)-aniline
(2.44 g, 12.6 mmol) was added to 3.2 mL of 12 M HCl in 10
mL distilled water and stirred at room temperature for 30 min.
The mixture was then cooled to 0
o
C and a solution of NaNO
2
(0.869 g, 12.6 mmol) in 15 mL of distilled water was added
drop by drop. To the resulting solution, was added phenol
(1.98 g, 21.1 mmol) dissolved in 12.6 mL of 2 N NaOH drop
wise. The yellow mixture was stirred for 2 h and the precipi-
tate was filtered and dried in air. The product was further
purified by column chromatography with hexane/ethylacetate
(8:2) R
f
0.27. Yield 2.2 g (59%).
1
H NMR (CDCl
3
) δ 7.84 (m,
4H), 6.9 (m, 4H), 5.36 (s, 1H), 4.03 (t, 2H), 1.8 (m, 2H), 1.47
(m, 2H), 1.36 (m, 4H), 0.93 (m, 3H).
6-[4-{4-Hexyloxyphenyl(azo)}-phenoxy]hexanoic-1-acid
Methyl Ester. To a mixture of 4-hydroxy-4'-hexyloxyazo-
benzene (1 g, 3.36 mmol) and anhydrous K
2
CO
3
(0.5788 g) in
100 mL of acetone was added 18C6 (0.16 g, 0.67 mmol) and
6-bromohexanoic acid methyl ester (0.7015 g, 3.36 mmol) in
50 mL acetone. The mixture was heated to reflux for 48 h and
then solvent was evaporated. The residue was partitioned
between dichloromethane-water solvents. Dichloromethane
layer was collected and dried over Na
2
SO
4
. The yellow solid
was purified by repeated precipitation from methanol until
there was no impurity on TLC. Hexane/EtOAc (8:2), R
f
0.43.
Yield 1.2623 g (88.2 %).
1
H NMR (CDCl
3
) δ 7.84 (m, 4H), 6.9
(m, 4H), 4.01 (m, 4H), 3.66 (s, 3H), 2.34 (t, 2H), 1.8 (m, 4H),
1.65 (m, 2H), 1.45 (m, 4H), 1.3 (t, 4H), 0.9 (m, 3H).
6-[4-{4-Hexyloxyphenyl(azo)}-phenoxy]hexanoic-1-acid
(azo-L). The ester was hydrolyzed using KOH at refluxing
condition in THF/H
2
O/EtOH mixed solvents until there was
no ester on TLC. The solution was acidified with dilute HCl
with pH of 4 or 5. The precipitate was collected by filtration.
R
f
0.43 (Hexane/EtOAc (1:1)).
1
H NMR (DMSO-d
6
) δ 7.8 (d,
4H), 7.09 (d, 4H), 4.07 (m, 4H), 2.5 (t, 4H), 2.25 (t, 2H), 1.75
(m, 4H), 1.6 (m, 2H), 1.45 (m,4H), 1.34 (m, 2H), 0.88 (m, 3H).
Synthesis of [Mn
12
O
12
(azo-L)
16
(H
2
O)
4
] (1). A slurry of
Mn12ac (206 mg, 0.1 mmol) in dichloromethane (30 mL) was
stirred with azo-L (680 mg, 1.65 mmol) for 4 h at room
temperature. As the substitution progressed, all the reactants
started to dissolve in solvent and finally to give a dark brown
solution. Azeotropic distillation with toluene (20 mL) was
Page 1
1144 Bull. Korean Chem. Soc. 2009, Vol. 30, No. 5 Sheby M. George and Jinkwon Kim
λ
(nm)
Abs.
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
300 350 400 450 500 550 600
Figure 1. Changes in the UVvis spectra of 1 up on illuminatio
n
with UV for 5 min at room temperature. (a) is for the trans
isomer and (b) is for the cis isomer.
Cycle index
Abs.
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Figure 2. Plot of absorption at 360 nm showing the reversible cis
trans isomerism of 1.
Temperature (K)
18
16
14
12
10
8
6
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Figure 3. ZFCFC plots of the trans (empty circles) and the cis
(filled circles) isomer at 0.1 T.
performed 9 times to remove completely the released acetic
acid. The resulting dark brown residue was dissolved again in
dichloromethane and treated with hexane. The crystalline
solids were collected by filtration. FTIR data(KBr): 1599(s),
1580(s), 1500(s), 1467(m), 1420(m), 1393(m), 1316(w),
1296(w), 1247(vs), 1147(s), 1105(w), 1046(w), 1024(w),
936(w), 840(s), 727(w), 715(w), 669(w), 640(m), 612(w),
551(m). Calcd. for C
384
H
504
O
80
N
32
Mn
12
(Mw, 7507.5): C,
61.43; H, 6.77; N, 5.97. Found: C, 61.39; H, 6.71; N, 6.01.
Characterization. Variable-temperature magnetic suscepti-
bility measurements were carried out on powder samples of
the complexes using a Quantum Design MPMS-XL magne-
tometer equipped with a 50 kG magnet. The ac susceptibility
data were collected in the frequency range of 20-1500 Hz at
the ac field of 3 Oe. Corrections for the diamagnetism of the
complexes were estimated using Pascal’s constants, and
magnetic data were corrected for diamagnetic contributions
of the sample holder.
Elemental analysis was carried out by an Elementar vario
EL III analyzer. Infrared spectra were recorded from KBr
pellets with a Perkin-Elmer spectrum BX spectrophotometer.
NMR spectra were recorded at VARIAN MERCURY400
spectrometer. Absorption spectra of the samples were measured
on SCINCO S-4100 spectrophotometer using dichloromethane
solutions of 1. Photoisomerization of 1 was carried out in
dichloromethane solution using a high-pressure Hg-Xe lamp
(Oriel, 500 W) equipped with a UV35 band-pass filter and a
sharp cut Y46 glass filter (Shimadzu Co.) for UV and visible
light irradiation, respectively. The power densities of the
incident light were adjusted to 10 mW/cm
2
for UV and 50
mW/cm
2
for visible light using an optical power meter
(Melles Griot, 13PEM001).
Results and Discussion
Fig. 1 shows UV/Vis absorption spectra of 1 before and
after UV or visible illumination for 5 min at room
temperature. In general, azobenzene exhibits π-π* and n-π*
transition bands at around 360 nm and at around 460 nm,
respectively. After 5 min of UV illumination, the intensity of
the peak at 360 nm decreased and the peak at 460 nm
increased, indicating photoisomerization from trans to cis.
After the subsequent illumination of visible light, the reverse
process proceeded. The reversible cis-trans photoisomeri-
sations were repeated several times by the alternating
illumination of UV and visible light (Fig. 2).
The magnetic properties of the powder sample of 1 were
studied using a SQUID magnetometer. Magnetic measure-
ments were taken on the trans isomer first, and then the
sample was irradiated with UV light of 360 nm for 5 min in
solution phase and was then dried in vacuo under darkness
and magnetic measurements were taken for the cis isomer.
Fig. 3 shows the ZFC-FC plots of the trans isomer (empty
circle) and the cis isomer (filled circles) of 1. Both isomers
have a blocking temperature at around 3 K, but the magneti-
zation value for the cis isomer is considerably higher than the
trans isomer. The structural isomerism by UV illumination
brings about changes in the dipole moment of the azobenzene
moieties, which seem to have a direct effect on the magnetic
properties of these materials.
6b,6c,13
In general, the photo-
Page 2
Synthesis and Photoinduced Magnetic Properties of a Mn12 Bull. Korean Chem. Soc. 2009, Vol. 30, No. 5 1145
2
00
( / )exp( / )
=
4 exp( / )
PP PB
PPB
NdEdHEkT
M
EkT
ππ
π
Σ
Σ
∫∫
sin θdθd
φ
×
(2
)
Temperature (K)
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
2 4 6 8 10
Figure 4. Temperature dependence of out of phase AC susceptibility
signals of trans (above) and cis (below) form of 1.
H/T
M/N
µ
B
M/N
µ
B
17
16
15
14
13
12
11
10
9
8
16
15
14
13
12
11
10
9
8
0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
1T
2T
3T
4T
5T
1T
2T
3T
4T
5T
Figure 5. Plot of M/N
µB
vs H/T for the cis (below) and the trans
(above) form of 1 at indicated applied fields. The solid lines are fits
of the data.
isomerization of the azobenezene derivatives, particularly the
trans-to-cis isomerization reaction, is accompanied by an
increase in molecular volume.
6c
Previous magnetic study on the
organic-inorganic hybrid material composed of Mn12ac and
azobenzene in PMMA shows no significant change in dc
susceptibility upon light illumination. It is mainly due to the
structural change in solid matrix is less plausible.
14
Furthermore, in this composite contribution of
electromagnetic field between Mn12ac and azobenzene may
be negligible because there are no direct chemical bonds
between them.
15
Out-of-phase ac magnetic susceptibility (χ
M
”) measure-
ments were carried out in the region of 2 to 10 K at the zero dc
field. As displayed in Fig. 4, both isomers predominantly
show a peak in the 4-7 K region with a smaller peak in the 2-4
K. However, the peak in the lower temperature region becomes
weaker as the trans isomer is converted into the cis isomer.
Illumination of UV light brought down the intensity ratio of
the lower temperature peak to that at the higher temperature
peak from 1:2 to 1:2.5. Peak maxima are accurately deter-
mined by fitting the peaks to a Lorentzian function. A
least-squares fit of ac susceptibility relaxation data to
Arrhenius equation gives U
eff
= 68.88 K for the trans isomer
and U
eff
= 72.1 K for the cis isomer. If two isomers have the
same core geometry and the same ground spin, the main
origin for the different relaxation rates may be different
transverse magnetic fields by dislocations. The local rotations
of the easy axis duo to dislocations result in a transverse
magnetic field for an external field applied along the c axis of
the crystal.
16,17
The trans isomer might have a higher transverse
anisotropy and field than those of the cis isomer.
Plots of reduced magnetization (M/Nµ
B
) as a function of
H/T, where N is Avogadro’s number and µ
B
is the Bohr
magneton, were also obtained for the both isomers of 1
(Figure 5). The data were fitted to the basic thermodynamic
expression given in Equation (2), which takes the full power
of the average of the magnetization into account.
18
In Equation (2), k
B
is the Boltzmann constant and Ep is the
eigenenergy obtained by diagonalization of the spin
Hamiltonian matrix, including axial zero-field splitting and
Zeeman interactions. The data were fitted using ANISOFIT
19
to give S = 10, g = 1.97, D = 0.31 cm
1
, and E = 0.0009 cm
1
for the trans isomer while S = 10, g = 1.93, D = 0.34 cm
1
,
and E = 0.0004 cm
1
for the cis isomer, where D is the axial
zero field splitting parameter and E is the rhombic zero field
splitting parameter. The cis isomer has a larger uniaxial
anisotropy than the trans isomer, which is consistent with the
observation in ac susceptibility measurements.
Figure 6 shows the M-H hysteresis loop of the cis and trans
Page 3
1146 Bull. Korean Chem. Soc. 2009, Vol. 30, No. 5 Sheby M. George and Jinkwon Kim
Figure 6. MH hysteresis loop of the trans (empty circles) and cis
(filled circles) isomers of 1 at 2 K.
isomers of 1. Small plateaus for both the isomers are observed
in the hysteresis loop because of quantum tunneling of
magnetization. The cis isomer shows a larger coercive field
(H
c
) compared to the trans isomer. The difference in the
coercive field is about 518 Oe, indicating a faster relaxation of
the trans isomer.
The physical phenomena exhibited by the Mn12, like
SMMs, are readily affected by the small changes in its
molecular structure. The trans-to-cis photoisomerizaion of
the azobenzene is usually accompanied by an increase in its
molecular volume. This change induces a large structural
change in the molecular arrangement, which leads to changes
in its magnetic properties. The slow relaxation shown by the
cis form compared to the trans is an overall result of change in
the change in packing and structural rearrangement caused by
the azobenzene group with UV illumination. The nature of the
azobenzene group to occupy large volume in its cis form
rather than the well-packed trans form may also result in an
increase in the intermolecular distance of Mn12.
In summary, we successfully synthesized new Mn12 SMM
which carries photo responsive ligands and responds to the
UV-vis illumination with a significant change in its pro-
perties.
Acknowledgments. This work was supported by the
Ministry of Education, Science and Technology (Grant no.
M1030000023405J000023410). S. M. George is thankful to
KRF for research fellowships.
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