Reversible light-induced magnetization change in an azobenzene-attached pyridylbenzimidazole complex of iron(II) at room temperature

Article abstract of DOI:10.1039/b817196n

Reversible photomagnetic effects were demonstrated at room temperature by the use of a new iron(ii) complex t₁₀₀c₀-[Fe(1) ₃](BF₄)₂·3H₂O (1 = phenyl(2-pyridin-2-yl-3H-benzoimidazol-5-yl)diaz

Full text of DOI:10.1039/b817196n

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Reversible light-induced magnetization change in an azobenzene-attached
pyridylbenzimidazole complex of iron(II) at room temperature†
Yuta Hasegawa, Shoko Kume and Hiroshi Nishihara*
Received 30th September 2008, Accepted 7th October 2008
First published as an Advance Article on the web 25th November 2008
DOI: 10.1039/b817196n
Reversible photomagnetic effects were demonstrated at room temperature by the use of a new iron(II)
complex t100c0-[Fe(1)3](BF4)2·3H2O (1 = phenyl(2-pyridin-2-yl-3H-benzoimidazol-5-yl)diazene), based
on the strategy of the so-called “ligand-driven light-induced spin change” (LD-LISC) effect. The
complex showed thermal spin crossover in both solid and solution states, showing a transition
temperature T1/2 = 279(16) K in acetone. trans-cis Photoisomerization of azobenzene moieties using
different energy lights afforded reversible photomagnetic effects, which were directly determined using
the Evans 1H NMR method.
all LD–LISC complexes reported to date that utilize stilbene as the
Introduction
photochromic part, difficulty was encountered in changing the iso-
Spin crossover (SCO) can be observed under thermo-, piezo-,
merization ratio in the photostationary state (PSS) using different
and photo-induction, and has been extensively studied owing
energy lights. In contrast, azobenzene has notable aspects as a pho-
to unique optical, magnetic, and dielectric properties associated
with changes between high-spin (HS) and low-spin (LS) states.1
tochromic molecule because the trans–cis isomerization ratio can
be largely changed in the PSS of its p–p* and n–p* band excitation,
and no obvious side reaction occurs, unlike in the case of stilbene.7
Herein, we report electronic and magnetic properties of the first
azo-type LD–LISC complex, t100c0-[Fe(1)3](BF4)2·3H2O (t100c0-2)
(1 = phenyl(2-pyridin-2-yl-3H-benzoimidazol-5-yl)diazene, txcy
refers to a mixture of trans and cis isomers in the ratio of x : y),
which shows magnetization changes upon UV and visible light
irradiation in acetone. This is the first example of azo-containing
iron(II) LD–LISC complexes, and also the first example that
exhibits magnetization changes reversibly using different energy
lights, providing a new strategy for the development of opt- and
magneto-molecular devices (Scheme 1).
This bistability enables the use of SCO materials in molecular
devices responsive to external stimuli. The synergic combination
of SCO and other functional materials, such as liquid crystalline,
nanoporous, and electrical conductive materials, are current
research targets, with the aim of synchronizing these properties
with well-developed SCO chemistry.2
In this context, a new type of SCO material can be created via
combination with a photochromic ligand, which induces changes
between HS and LS states through ligand photoisomerization
(ligand-driven light-induced spin change, LD–LISC).3,4 LD–LISC
was reported for the first time by Zarembowitch et al. in the 1990’s,
and has the following different characteristics compared with
other SCO materials: (1) it observable even at room temperature
in contrast to light-induced excited spin state trapping (LIESST)
that deactivates above ca. 120 K;5 (2) is based on a single-molecule,
which means that no cooperativity is necessary, and it is thus
observable in various matrices such as in solution,3 in dispersed
media,4 and potentially in the solid state.
Here, we focus on a new LD–LISC complex with an azo-
type ligand, in which 2-(2¢-pyridyl)benzimidazole (pybim) and
azobenzene are connected in a p-conjugated fashion. Iron(II)
complexes with pybim or its derivatives are known to show SCO
near room temperature, and these SCO properties are tunable by
changing environmental conditions due to electronic interaction
through hydrogen bonding.6 To our knowledge, there exist no
LD–LISC complexes that show a photomagnetization change
reversibly by the use of different energy lights. This is because for
Scheme 1 An iron(II) LD–LISC complex with azobenzene-attached
ligands showing reversible magnetization change at room temperature.
From magnetic susceptibilities determined by the Evans 1H NMR method,
HS fraction ratios, [HS]/([LS] + [HS]), are estimated to be 0.55 and 0.58
for t100c0-2 (left) and t67c33-2 (right), respectively, in acetone at 293 K.
Department of Chemistry, School of Science, The University of Tokyo, 7-3-
1, Hongo, Bunkyoku, Tokyo 113-0033, Japan. E-mail: nisihara@chem.s.u-
tokyo.ac.jp; Fax: +81(03)5841-4346; Tel: +81(03)5841-4346
Results and discussion
Synthesis
† Electronic supplementary information (ESI) available: IR spectra of
t
100c0-2 and [Fe(pybim)3](BPh4)2·3H2O (Fig. S1); quantitative titration
t100c0-1 was newly synthesized from an amino precursor8 and
analyses for t100c0-1 and t6c94-1 (Fig. S2); detailed description of Evans
1H NMR method (Fig. S3). See DOI: 10.1039/b817196n
nitrosobenzene by a general method (Scheme 2).9 The iron(II)
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Scheme 2 Synthetic procedures of t100c0-1 and t100c0-2.
complex, t100c0-2, was prepared from Fe(BF4)2·6H2O and 3 equiv.
of t100c0-1 mixed in acetone under a nitrogen atmosphere. A 1 : 3
complex containing three water molecules as crystal solvent was
successfully isolated, and characterized by elemental analysis and
IR spectra (Fig. S1).10
by the use of a reference solution that contains the same quantity of
1 (see ESI). Here, cMT becomes 3.75 cm3 K mol-1 in the pure HS
state, and 0 cm3 K mol-1 in the pure LS state. The temperature
dependence of cMT was well fitted to van’t Hoff equation,
affording the crossover enthalpy DH0 = 27(1) kJ mol-1, crossover
entropy DS0 = 98(4) kJ K-1 mol-1, and transition temperature
T1/2 = 279(16) K. T1/2 of t100c0-2 is near room temperature, which
is highly promising for the operation of LD–LISC at ambient
conditions.
SCO properties
t100c0-2 exhibited gradual SCO in the solid state, as shown in
Fig. 1(a) in the form of a cMT vs T curve (cMT is the molar
magnetic susceptibility). At 350 K, cMT was 3.62 cm3 K mol-1,
comparable to an expected value for an iron(II) complex that
remains in the pure HS form. Upon cooling, the cMT value
decreased gradually, reaching 0.73 cm3 K mol-1 at 30 K. Below
30 K, cMT dropped to reach 0.43 cm3 K mol-1 at 2 K, attributed
to zero-field splitting. No hysteretic phenomenon was observed
through this SCO process.
In the solution state, magnetization was estimated by a para-
magnetic shift of the reference tetramethylsilane (TMS) signal in
the 1H NMR spectra (Evans 1H NMR method) using acetone-d6
containing 0.03% TMS as the solvent (Fig. 1(b)).11 The effects of
solvent correction and diamagnetic contribution were minimized
LD–LISC properties
1
The absorption and H NMR spectral changes of t100c0-1 upon
photoirradiation were examined (Fig. 2(A) and (B)). In the initial
trans state, two absorption bands were observed at 359 and 445 nm,
which can be assigned to p–p*(azo) and n–p*(azo) transitions,
respectively. Upon UV (365 nm) irradiation, a decrease in the
p–p* band and an increase in the n–p* band were observed.
These changes were reversed to recover the initial spectrum
upon irradiation of blue light (436 nm). These spectral changes
coincide with spectral characteristics of trans–cis isomerization of
azobenzene. The trans/cis ratio was estimated from the 1H NMR
spectra, confirming almost complete transformation to the cis
isomer with 365 nm light (94% cis molar ratio, t6c94-1, in the PSS),
and a relatively good recovery of the trans isomer with 435 nm light
(28% cis molar ratio, t28c72-1, in the PSS). The photogenerated cis
isomer was thermally reversed to the trans isomer within 2 weeks
at room temperature.
In the UV-vis absorption spectrum of t100c0-2 (Fig. 3(a)), three
bands were observed at 355, 440, and 550 nm. The first two
bands are attributed to p–p*(azo) and n–p*(azo) transitions,
as they have wavelengths and intensities similar to those of
t100c0-1. The intensity of the band at 550 nm increased upon
cooling (Fig. 4(a)), inducing a significant color change from
yellow to brown (Fig. 4(b)). Therefore, the absorption band
can be assigned to an 1MLCT band in the LS state, and the
absorption enhancement is ascribed to an increase in the LS
state by thermal SCO. Upon irradiation at 365 nm to t100c0-2,
Fig. 1 (a) cMT vs T curve of t100c0-2 in the solid state with an applied field
of 0.5 T. (b) cMT vs T curve of t100c0-2 (1.0 ¥ 10-3 mol dm-3) in acetone-d6
determined by Evans 1H method.
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Fig. 3 (a) A UV-vis absorption spectrum of t100c0-2 in acetone (solid line,
1.0 ¥ 10-4 mol dm-3), and its spectral changes upon irradiation with 365 nm
(dotted line) and 436 nm (broken line) light in the PSS at 295 K. (b) cMT
changes of t100c0-2 in acetone-d6 (1.0 ¥ 10-3 mol dm-3), containing 1.5 equiv.
of t100c0-1, upon repeated irradiation with 365 nm and 436 nm light in the
PSS at 293 K.
Fig. 2 (A) A UV-vis absorption spectrum of t100c0-1 in acetone (solid line,
1.0 ¥ 10-4 mol dm-3), and its spectral changes upon irradiation with 365 nm
(dotted line) and 436 nm (broken line) light in the photostationary state
at 295 K. (B) An 1H NMR spectrum of t100c0-1 in acetone-d6 (a), and its
spectral changes upon irradiation with 365 nm (b), 436 nm (c), and after
2 weeks in the dark at 295 K (d).
a decrease in the p–p* band and an increase in the n–p*
band were observed, indicating trans-to-cis photoisomerization.
Concomitantly, 1MLCT absorption decreased, suggesting that
LS-to-HS spin state change was induced by photoisomerization.
These spectral changes were recovered to the initial spectrum
upon irradiation at 436 nm. During these processes, isosbestic
points (420 and 512 nm) were fixed at the same position. The
cis molar ratio estimated from the absorption change is 33% in
the PSS of 365 nm light irradiation (t67c33-2), and 13% in the
PSS of 436 nm light irradiation (t87c13-2). If the three azobenzene
moieties in a complex are photoisomerized independently as we
observed for azobenezenes-attached tris(bipyridine)cobalt,9 one
of three moieties on average is converted to a cis isomer in the PSS
of 365 nm light irradiation. Direct steric interaction among azo
moieties appears to be unlikely, because the moieties are far apart
from each other.12
Fig. 4 (a) UV-vis absorption spectra of t100c0-2 (1.0 ¥ 10-4 mol dm-3) in
acetone in the temperature range of 192–300 K. (b) Photographs of t100c0-2
in acetone at 290 and 223 K.
photoirradiation, we added excess free ligand t100c0-1 (1.5 equiv.
to 2). The initial cMT value, 2.09 cm3 K mol-1, increased to
2.20 cm3 K mol-1 after irradiation at 365 nm for 20 min. The
increase in magnetization suggests conversion to a HS-richer
state accompanied by trans-to-cis photoisomerization. Upon
irradiation at 436 nm for 20 min, the cMT value recovered to close
to the initial value, confirming that the reversible magnetization
change was induced by photoisomerization. Repeated irradiation
showed zig-zag responses, while cMT values were decreased as a
whole, indicating that these responses were not fully reversible.
To investigate the effect of ligand dissociation caused in the
photochemical process, complexation behaviours of t100c0-1 and
t6c94-1 to iron(II) were analysed with a quantitative titration
method (Fig. 5 and Fig S2). Logarithms of the first, second,
As absorption spectroscopic results described above strongly
suggest reversible LD–LISC in 2 at room temperature in the
solution state, we carried out direct detection of the magnetization
1
change upon photoirradiation by the Evans H NMR method
(Fig. 3(b)). To eliminate the possibility of ligand dissociation with
282 | Dalton Trans., 2009, 280–284
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results of this study provide a new strategy for the molecular design
of photo- and magneto-functional molecular systems suitable for
the development of molecular devices.
Experimental
Synthesis
Solvents and reagents were used as received from the com-
mercial sources. 2-Pyridin-2-yl-3H-benzoimidazol-5-ylamine was
prepared by the literature procedure.8
Fig. 5 UV-vis spectral changes of Fe(ClO4)2·6H2O upon stepwise addi-
tion of t100c0-1 (a) and t6c94-1 (b) in acetone at 295 K.
Phenyl(2-pyridin-2-yl-3H-benzoimidazol-5-yl)diazene (t100c0-1).
2-Pyridin-2-yl-3H-benzoimidazol-5-ylamine (2.00 g, 9.61 mmol)
was dissolved in a mixture of 50 cm3 of chloroform and 35 cm3 of
acetic acid. Nitrosobenzene (1.15 g, 10.7 mmol) was added to the
former stirred solution at 20 ◦C. The reaction mixture turned from
a yellow solution to a red suspension within 2 h, and was stirred for
15 h at 20 ◦C. After removing the solvent under vacuum, 200 cm3 of
chloroform was added to the residue and filtered. The filtrate was
chromatographed on an alumina column with chloroform-hexane
(7 : 3). The third orange band was collected. The product was
recrystallized from dichloromethane/hexane and was obtained as
yellow powder. Yield: 1.21 g (4.04 mmol, 42%). 1H NMR (DMSO-
d6 with 5 v% TFA, 293 K): d 8.87 (d, J = 4.6 Hz, 1H), 8.42 (d, J =
7.8 Hz, 1H), 8.21 (s, 1H), 8.17 (t, J = 7.8 Hz, 1H), 8.04 (d, 8.7 Hz,
1H), 7.94-7.91 (m, 3H), 7.71 (dd, J = 4.6, 7.6 Hz, 1H), 7.64-7.59
and third complex formation constants, log K1, log K2, and log
K3 were estimated to be 8.5, 7.5, and 5.7, respectively, from a
titration of t100c0-1 to iron(II) in acetone. The tendency of stepwise
complexation is a regular characteristic of a complex without
an azo moiety.13 Using the same procedure, the complexation
behaviour of t6c94-1, which was preformed upon UV irradiation to
an acetone solution of t100c0-1, was analysed. Log K1, log K2, and
log K3 were estimated to be 8.9, 7.0, and 5.8, respectively. These
values are almost identical to those of t100c0-1, showing that the
effect of ligand dissociation is largely negligible. These results also
confirm that more than 99.5% of the iron(II) ion forms as a 1 : 3
complex in both cases under the experimental conditions.
Comparison of the energy diagrams derived from spectroscopic
and electrochemical data indicates that the ligand field effect is
weaker in the cis isomer than that in the trans isomer. The complex
of the ligand without an azo moiety, [Fe(pybim)3](BPh4)2·3H2O,
shows an irreversible redox process for FeIII/II at 0.50 V vs Fc+/Fc in
acetonitrile with 0.1 mol dm-3 tetrabutylammonium perchlorate,
1
(m, 3H). 13C{ H} NMR (DMSO-d6 with 5 v% TFA, 293 K): d
152.1, 150.9, 150.5, 149.7, 143.9, 138.7, 136.8, 135.1, 131.9, 129.5,
127.3, 123.4, 123.0, 120.1, 116.0, 110.2. ESI-TOF mass (positive,
acetonitrile) m/z: 300.1 [t100c0-1 + H]+. Analytical data. Found: C,
72.03; H, 4.60; N, 23.41%. Calcd for C18H13N5: C, 72.23; H, 4.38;
N, 23.40%.
1
and possesses an MLCT band at 497 nm. On the other hand,
t100c0-2 shows a corresponding redox process at 0.55 V in the same
1
solvent, and possesses an MLCT band at 535 nm. The higher
t100c0-[Fe(1)3](BF4)2·3H2O
(t100c0-2). t100c0-1
(400
mg,
oxidation potential in t100c0-2 indicates that the HOMO energy
level is decreased. Also, the red shift of the 1MLCT band indicates
that the p* energy level is decreased by the introduction of an
azo moiety. These data suggest that the ligand field strength was
increased by the introduction of an azo moiety because of the
high p-back donation ability of t100c0-1 in its iron(II) complex.
In fact, T1/2 of t100c0-2 (279(16) K) was higher than that of
[Fe(pybim)3](BPh4)2·3H2O (252(8) K) in acetone.6 In comparison,
1.34 mmol) was suspended in 6 mL of acetone. Fe(BF4)2·6H2O
(150 mg, 0.445 mmol) dissolved in 4 cm3 acetone was added to the
former stirred solution at 20 ◦C. The reaction mixture immediately
turned from orange to red, and was stirred at 20 ◦C. After s◦tirring
for 20 min, the reaction mixture was left to stand at -30 C for
a day. The solid was filtered and washed by 5 cm3 of ice-cold
ace◦tone and 10 cm3 of hexane. After drying under vacuum at
20 C, the title complex was obtained as an orange-red powder.
Yield: 260 mg (0.195 mmol, 43%). ESI-TOF mass (positive,
acetonitrile) m/z: 476.6 [t100c0-2]2+. Analytical data. Found: C,
55.02; H, 4.07; N, 17.88%. Calcd for C54H45B2F8FeN15O3: C,
54.89; H, 3.84; N, 17.78%.
1
the 1 : 3 complex with t6c94-1 shows an MLCT band at 513 nm,
suggesting lower ligand field strength due to the lower p-back
donation ability of t0c100-1 compared with t100c0-1 in its iron
complex. The bent structure of cis-azobenzene may restrict p-
conjugation in the ligand; the decrease in p-acceptor character
was the main contributor to the ligand field strength.
Note. The obtained product was sometimes hygroscopic due to
the defects of water molecules as crystal solvent. If this is the case,
the analytically pure t100c0-2 can be obtained by exposure to room
air for several days.
Conclusions
We have synthesized a novel iron(II) complex with an azo-
containing ligand, t100c0-2, which shows spin crossover in both
the solid and solution states. The complex showed photomagnetic
effects reversibly by using different energy lights, and the degree of
magnetization change is sufficiently large to be directly determined
using the Evans1H NMR method. These properties were not
found in the previously reported iron(II) LD–LISC complexes. The
Apparatus
UV-vis spectra were recorded with a Jasco V-570, 1H NMR spectra
were recorded with a Bruker NMR spectrometer (500 MHz),
Magnetic susceptibility were measured using an MPMS SQUID
susceptometer (Quantum Design, Inc.), and electrochemical data
were recorded with an ALS 750A electrochemical analyzer.
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´
Supply of monochromatic light
4 J. S. Kolb, M. D. Thomson, M. Novosel, K. S. David, E. Rivie`re,
M.-L. Boillot and H. G. Roskos, C. R. Chim., 2007, 10, 125–
Monochromic light was supplied by a high-pressure mercury lamp
(Ushio-500D). The bright lines were split by a monochromator
(Jasco CT-10T). The bandwidth of monochromic light was 14 nm
for 365 nm light and 12 nm for 436 nm light.
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Acknowledgements
This work was supported in part by Global COE program ‘Chem-
istry Innovation through Cooperation of Science and Enginering’,
MEXT, Japan.
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not succeeded in identifying them so far. Their magnetic behaviours,
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