Structure-distortion-induced photomagnetic effect in azobenzene/ polyoxometalate Langmuir-Blodgett films

Article abstract of DOI:10.1039/c3dt51402a

We have prepared photomagnetic Langmuir-Blodgett films composed of an amphiphilic azobenzene and a magnetic polyoxometalate. The obtained film possesses a well-organized layered structure where polyoxometalate anions are sandwiched by azobenzene cations in a single-layered manner. Reversible photoisomerization of azobenzene was achieved even at low temperature, accompanying intensity changes in the d-d transition of polyoxometalate anions. The photomagnetic effect was observed reversibly upon alternate UV and visible light irradiation. Based on polarized spectroscopy, the observed photomagnetic effect is ascribed to the structure-distortion of polyoxometalate layers.

Full text of DOI:10.1039/c3dt51402a

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Structure-distortion-induced photomagnetic effect in
azobenzene/polyoxometalate Langmuir–Blodgett
films†
Cite this: Dalton Trans., 2013, 42, 16014
Takashi Yamamoto,*a Yasushi Umemurab and Yasuaki Einaga*a
We have prepared photomagnetic Langmuir–Blodgett films composed of an amphiphilic azobenzene
and a magnetic polyoxometalate. The obtained film possesses a well-organized layered structure where
polyoxometalate anions are sandwiched by azobenzene cations in a single-layered manner. Reversible
photoisomerization of azobenzene was achieved even at low temperature, accompanying intensity
changes in the d–d transition of polyoxometalate anions. The photomagnetic effect was observed rever-
sibly upon alternate UV and visible light irradiation. Based on polarized spectroscopy, the observed
photomagnetic effect is ascribed to the structure-distortion of polyoxometalate layers.
Received 29th May 2013,
Accepted 12th July 2013
DOI: 10.1039/c3dt51402a
www.rsc.org/dalton
molecules or ions from the subphase to produce multicompo-
Introduction
nent assemblies.17 Langmuir–Blodgett (LB) films can be
Photomagnetism has been one of the most attractive topics for obtained by transferring the Langmuir monolayers onto the
the molecule-based magnetism community.1 In principle, the solid substrate. This method is called the LB method, in which
phenomenon is attributed to photoinduced electron transfer, the number of layers and their sequence can be controlled at
leading to changes in the spin state. For example, in Co–Fe the molecular level.18 On the basis of these features, two-
Prussian Blue, excitation of an intervalence charge transfer dimensional assemblies of magnetic materials have been fab-
band results in trapping the metastable high-spin state.2 The ricated as a part of LB films.19–21 Confinement of reactants to
design of such photomagnetic materials is one of the main the air–water interface directs lateral propagation of a two-
challenges of material science because of potential appli- dimensional network.
cations in optical memory and switching devices.3–7 However,
Polyoxometalates (POMs) are discrete anionic metal oxides
the number of successful examples is still small, and it is that can be classified as useful building blocks for functional
desirable to propose a possible concept in order to obtain an materials.22–24 POMs have attracted much interest because of a
optically switchable magnetic material. On the other hand, wide variety of physical and chemical properties, ranging from
organic photochromic molecules have been used as building conductivity, magnetism, catalysis, electrochromism, and so
blocks for fabricating a photoswitchable molecular system.8 on.25–28 POMs are advantageous for integrating with a broad
Along these lines, we have focused on integration of magnetic range of molecules, which, in turn, are considered to be dis-
materials into organic photochromic molecules to facilitate crete building blocks for development of functional materials.
phototuning of the magnetization.9–15 This strategy has Thus, POMs could be extended to organic–inorganic hybrids
offered prospects for phototunable magnetic materials.
by using non-covalent and/or covalent interactions. For example,
The air–liquid interface is often used to direct assembly Coronado et al. have developed fascinating Langmuir–
processes.16 Traditional Langmuir monolayers can form Blodgett films of POM.29–31 They have demonstrated that
two-dimensional molecular crystals or can selectively bind the clever use of an air–liquid interface could be quite effective
in fabricating anisotropic magnetic thin films. Moreover, they
also show the possibility of designing a multi-functional mole-
aDepartment of Chemistry, Keio University, 3-14-1 Hiyoshi, Yokohama 223-8522,
Japan. E-mail: takyama@chem.keio.ac.jp, einaga@chem.keio.ac.jp;
cular assembly using POMs.
Herein, we report on the photomagnetic Langmuir–
Blodgett films composed of an amphiphilic azobenzene (AZ, a
photochromic organic molecule) and a CoII-ion-containing
polyoxometalate anion (Co–POM, a ferromagnetic cluster)
(Scheme 1). The film was characterized by X-ray diffraction (XRD),
UV-Vis absorption spectroscopy (UV-Vis), and superconducting
Fax: +81-45-566-1697, +81-45-566-1697; Tel: +81-45-566-1790, +81-45-566-1704
bDepartment of Applied Chemistry, National Defense Academy, 1-10-20 Hashirimizu,
Yokosuka 239-8686, Japan
†Electronic supplementary information (ESI) available: Synthesis of materials;
linear FT-IR dichroism spectroscopy; preparation and characterization of the
model film. See DOI: 10.1039/c3dt51402a
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previously reported case, the lift-off area on isotherms shifted
towards the smaller area when POM solution was used as the
subphase, indicating hybridization of DODA and POM. As a
result, the electrostatic repulsion between DODA head groups
was compensated with a POM layer, which permits a higher
molecular density for a given surface pressure.
As a densely packed hybrid monolayer was obtained,
we proceeded to prepare Langmuir–Blodgett films. Transfer
(Y-type) of the monolayer was carried out at a surface pressure
of 30 mN m−1 by a vertical dipping method with a speed of
10 mm min−1. The transfer ratio was found to be 0.9–1.0, indi-
cating an ideal transfer.
Scheme 1 Structure of (top) AZ and (bottom) Co–POM.‡
Structure
In order to determine the structure of LB films, we performed
XRD and linear dichroism spectroscopy.
First, we measured XRD patterns in a θ–2θ mode to confirm
a layered structure of the film. Fig. 2 shows XRD patterns of
the film and three diffraction peaks were given at 2θ = 1.40,
2.95, and 4.35°. These peaks are ascribed to the (001), (002),
and (003) reflections from the Co–POM layer, respectively.
Thus, LB films obviously possess the layered structure and,
from the (001) diffraction peak, the basal spacing of the unit
layer (AZ/Co–POM/AZ) is calculated to be 62.7 Å.
Fig. 1 Surface pressure–area (π–A) isotherms of AZ at the air–water interface:
(red) on pure water and (blue) on Co–POM aqueous solution (1.0 μM).
Next, we estimate a tilt angle of AZ in LB films using a
linear UV-Vis dichroism measurement.33 Fig. 3 shows UV-Vis
quantum interference device (SQUID) magnetometry. As a
result, we could prepare a highly organized film, which exhi-
bits a photomagnetic effect derived from photoisomerization-
induced structure-distortion.
Results and discussion
Film preparation
Fig. 1 shows π–A isotherms of hybrid monolayers when a solu-
tion of AZ was spread onto a surface of Co–POM aqueous solu-
tion (1.0 μM) and also onto pure water. The π–A isotherm on
pure water (Fig. 1, red) shows that AZ by itself forms mono-
layers in an extended state at the air–water interface. When
Co–POM aqueous solution was used as the subphase, the π–A
isotherm was modified, shifting towards a smaller area per
molecule compared with that on pure water (Fig. 1, blue).
Moreover, the isotherm shows a steep increase in surface
pressure, which corresponds to a dense packing of AZ. This
tendency indicates that positively charged AZ monolayers
could be electrostatically adsorbed onto Co–POM at the air–
water interface.
Fig. 2 XRD patterns for AZ/Co–POM films. 150 Co–POM layers were trans-
ferred onto each side of a glass substrate.
Such dependence on isotherms has also been reported pre-
viously in the case of a dioctadecyldimethylammonium cation
(DODA) and a polyoxometalate anion (POM).32 Briefly, a shift
in the lift-off area on isotherms corresponds to changes in the
density of amphiphiles at the air–water interface. In the
Fig. 3 (Left) Linear UV-Vis dichroism spectra for AZ/Co–POM films: (red)
p-polarized and (blue) s-polarized spectra. 150 Co–POM layers were transferred
onto each side of a quartz substrate. (Right) A schematic illustration of the film
structure.
‡Illustration of Co–POM drawn by using the software VESTA 3: K. Momma and
F. Izumi, J. Appl. Crystallogr., 2011, 44, 1272.
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absorption spectra recorded with polarized incident beams at
an incident angle of 60° from the surface normal. Both spectra
gave two intense absorption peaks at 252 nm and 350 nm. The
peak at 250 nm is ascribed to the π–π* transition of trans-AZ
(the short axis transition).34 In addition, the peak at 350 nm is
ascribed to the π–π* transition of trans-AZ (the long axis tran-
sition).34 We chose an absorption peak at 350 nm for a linear
dichroism investigation. Here, the dichroic ratio at 350 nm
(D350) is defined as the ratio of absorbance at 350 nm recorded
with the p-polarized beam (Ap) to that with the s-polarized one
(As): D350 = Ap/As. Theoretically, a relation between D350 and
φ is provided as follows:
2ð1 ꢀ sin2 αÞ þ ð3 sin2 α ꢀ 1Þ sin2 φ
Ap
D350
¼
¼
2 ꢀ 3 sin2 φ
As
where φ is a tilt angle from the surface normal for AZ, and α is
an angle of the incident beam from the surface normal. From
this equation, φ was estimated to be 48°. Additionally, a linear
FT-IR dichroism measurement revealed that AZ tilts 46° from
the surface normal (ESI†), which agrees well with the value
obtained from UV-Vis measurements.
From these results and taking the molecular length of AZ
into account, we could evaluate the thickness of the Co–POM
layer within the film to be 11.2 Å. This value roughly corres-
ponds to the diameter of the Keggin anions,35 which indi-
cates that Co–POM is arranged in a single layer as supposed
for a Y-type transfer. The transfer during the upper stroke
should involve large changes concerning Co–POM adsorbed
along the last hybrid monolayer deposited onto a solid sub-
strate. This reorganization could be explained by instability of
double layer formation of the negatively charged Co–POM.
Such a structure has been recently found in a single crystal of
dioctadecyldimethylammonium cations (DODA) and Lindqvist
hexamolybdate anions.36
Fig. 4 Changes in UV-Vis absorption spectra for AZ/Co–POM films by photo-
isomerization: (top, left) at room temperature and (bottom, left) at 10 K.
Changes in absorbance at 350 nm upon photoirradiation: (top, right) at room
temperature and (bottom, right) at 10 K; (full arrow) UV irradiation and (broken
arrow) visible light irradiation. In both spectra, the initial trans state (red) was
first irradiated with UV light to achieve a cis-rich photostationary (blue) state.
Then, it was subsequently irradiated with visible light to obtain a trans-rich
photostationary state (green). The overall trans–cis isomerization cycle was
repeated several times between two photostationary states. 300 Co–POM layers
were transferred onto each side of a quartz substrate.
Subsequently, the photochromic reaction of AZ in the
film was investigated at 10 K. Even at low temperature, the
cis–trans isomerization of AZ was observed similar to that
at room temperature. However, the fraction of cis-isomer
after UV irradiation was calculated to be ca. 35%, which is
much lower than that obtained at room temperature. This
is because the photoisomerization of the azobenzene mole-
cule, particularly trans-to-cis isomerization, accompanies the
Photochromic reaction
Photoisomerization of AZ in LB films was monitored by UV-Vis free volume change.40 In other words, the cis-isomer is much
absorption spectra at room temperature and 10 K (Fig. 4). As more bulky than the trans-isomer. At low temperature,
already mentioned, the spectrum exhibits an intense peak at such molecular motion was significantly restricted, and
350 nm, ascribed to the π–π* transition of trans-AZ (the long therefore the photochemical transformation was inhibited to
axis transition). At room temperature, before photo-irradiation, some extent. The overall trans–cis isomerization cycle was
AZ in the film mainly exists as a trans-isomer, because it is repeated several times between two photostationary states
thermodynamically more stable than a cis-isomer.37 Upon UV (Fig. 4, bottom).
light irradiation, trans-to-cis isomerization occurred as
It should be noted that, accompanied by the above photo-
reflected by a decrease of absorbance at 350 nm. After reaching chromic reaction of AZ, we observed changes in UV-Vis absorp-
the photostationary state (cis-rich), the conversion efficiency tion spectra in the visible region (Fig. 5). As Co–POM contains
was estimated to be ca. 65%, assuming that the conversion four CoII ions, the d–d transition of CoII ions was observed as a
efficiency is 100% for a reaction in solution.38 In addition, the broad absorption peak between 500 and 900 nm. It should be
reverse process (i.e. cis-to-trans isomerization) proceeded to a considered that the d–d transition of WV ions also appears in
certain degree upon subsequent visible light irradiation, this region. However, it is unlikely that W ions possess the
leading to the other photostationary state (trans-rich). The electronic state of +V, because the ground electronic state of W
overall trans–cis isomerization cycle was repeated several times ions is +VI in Co–POM. In addition, reduction of Co–POM is
between two photostationary states (Fig. 4, top). Similar photo- also unlikely to occur under the conditions of film prepa-
chromic reactions have been reported in LB films containing ration. Therefore, the observed broad peak could be apparently
azobenzene derivatives.39
assigned to the d–d transition of CoII ions.
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Fig. 5 Changes in UV-Vis absorption spectra for AZ/Co–POM films in the
visible region at room temperature. For clarity, difference spectra are shown. A
broad peak is ascribed to the d–d transition of CoII ions (not to that of WV ions),
because the ground electronic state of W ions is +VI in Co–POM. As Co–POM
was revealed not to exhibit photochromism (ESI†), the observed changes were
apparently derived from photoisomerization of AZ. 300 Co–POM layers were
transferred onto each side of a quartz substrate.
Fig. 6 Plots of χT vs. T under an applied field of 5000 Oe: (red) AZ/Co–POM
films and (blue) Co–POM powders. χT values for AZ/Co–POM films were normal-
ized to that for Co–POM powders. 300 Co–POM layers were transferred onto
each side of a Mylar substrate.
It is also noteworthy that polyoxometalates are known to
exhibit photochromism in the presence of an electron donor.41
Briefly, polyoxometalates exhibit a ligand-to-metal charge
transfer (LMCT; oxygen to WVI ion) band in the UV region.
Upon excitation of this band, WVI ions are reduced to form
mixed-valence species, leading to an intervalence charge trans-
fer (IVCT) band between 600 and 900 nm. That is, if Co–POM
exhibits photochromism, absorbance should increase in the
visible region. However, in our LB films, absorbance decreased
upon UV light irradiation, concluding that the observed
Fig. 7 Changes in normalized χT for AZ/Co–POM films upon photoirradiation
changes in the visible region are derived from the d–d tran- at 2 K under an applied field of 5000 Oe: (full arrow) UV irradiation and (broken
sition of CoII ions. In any case, as it is important to check
whether Co–POM exhibits photochromism, we have prepared
arrow) visible light irradiation. Upon UV irradiation, normalized χT values
decreased and, on the other hand, normalized χT values recovered upon visible
light irradiation. 300 Co–POM layers were transferred onto each side of a Mylar
the model LB films by using a dioctadecyldimethylammonium
substrate.
cation (DODA) instead of AZ (structure characterization is
given in ESI†). As a result, we could not observe any photochro-
mic reaction in LB films of DODA and Co–POM (ESI†).
Subsequently, we examined the influence of photoirradia-
tion on the magnetic properties of the film at 2 K (Fig. 7).
Magnetic properties
Upon UV irradiation (λmax = 360 nm, 1.0 mW cm−2, 15 min)
Co–POM contains the magnetic Co4O16 cluster encapsulated
between two polyoxotungstate moieties [PW9O34]9−. CoII ions
are ferromagnetically coupled in the magnetic cluster, giving
rise to a highly magnetic ground state.42 The temperature
dependence of magnetization in LB films is plotted in Fig. 6.
As can be seen, the magnetic behavior of the film is similar to
that observed in the powdered sample of Co–POM, which indi-
cates that the ferromagnetic clusters are maintained within
the film. In detail, the χT product shows a sharp increase
below 50 K upon cooling, and a maximum at ca. 6.5 K. Below
this temperature, the χT product shows a decrease due to the
magnetic anisotropy. However, the χT product of the film is
relatively small compared with the powdered Co–POM, which
might be because of a slight distortion of the Co–POM frame-
work sandwiched by AZ layers. Such a distortion affects the
the normalized χT value decreased from 12.43 to 12.39 emu K
mol−1. In contrast, visible light irradiation (λ = 400–700 nm,
1.0 mW cm−2, 10 min) led to recovery of the χT value from
12.39 to 12.45 emu K mol−1. After this process, reversible
changes were repeated several times upon alternate irradiation
with UV and visible light. It should be noted that the photo-
magnetic effect was maintained for at least 2 h, even after
photoirradiation was stopped. As mentioned in the previous
section, the observed photomagnetic effect would be related to
changes in the d–d transition of CoII ions, accompanied by the
photochromism of AZ. Investigation of the mechanism is pro-
vided in the next section.
Photoinduced structural changes in LB films
symmetry of the magnetic cluster, leading to a reduction of We have observed the photomagnetic effect in LB films,
the magnetic interaction.
in which the d–d transition of CoII ions changed upon
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photochromic reaction of AZ. As shown in Fig. 5, the d–d tran-
sition of CoII ions changes in intensity, not absorption
maxima wavelength. We have previously reported a photomag-
netic effect in various systems where magnetic materials are
hybridized with azobenzene molecules. Briefly, in the case of
Prussian Blue intercalated into LB films of the amphiphilic
azobenzene, we have observed reversible changes in an inter-
valence charge transfer (IVCT) band of Prussian Blue by photo-
isomerization of azobenzene molecules.10 In other cases,
reversible changes in the CN stretching vibration (bridge-
mode: –FeII–CN–FeIII–) by photoisomerization were observed
in Prussian Blue/azobenzene reverse micelles and LB films,
respectively.11,13 It should be mentioned that the above spec-
tral changes are the shift in wavelength and wavenumbers.
Therefore, it is concluded that the photochromic reaction
of azobenzene molecules affects the electronic states of Prus-
sian Blue, leading to changes in magnetization values.
However, in the present case, the d–d transition of CoII
ions changed reversibly in intensity. Therefore, it is un-
likely that the electronic states of Co–POM were affected by
photoisomerization.
Fig. 8 Changes in linear UV-Vis dichroism spectra for AZ/Co–POM films in the
visible region at room temperature: (top, left) p-polarized and (bottom, left)
s-polarized spectra. For clarity, difference spectra are shown. In both polarized
spectra, we observed reversible changes in absorption peaks ascribed to the d–d
transition of CoII ions. As Co–POM was revealed not to exhibit photochromism
(ESI†), the observed changes were apparently derived from photoisomerization
For the above reason, we assume that the photomagnetic
effect would be ascribed to orientational changes of Co–POM.
Therefore, we performed XRD measurements before and after
photoisomerization of AZ. As a result, the basal spacing (d) of
LB films changes reversibly: an increase upon UV irradiation of AZ. 300 Co–POM layers were transferred onto each side of a quartz substrate.
(Right) A schematic illustration of photochromism-induced orientational distor-
and a decrease upon visible light irradiation. Although reversi-
ble changes were achieved repeatedly, we observed only slight
tion in a Co–POM single layer.
changes in the d value. This is because the efficiency of photo-
isomerization (ca. 65%) was relatively low in the film. In other
words, the obtained d values in XRD measurements were aver-
Conclusions
aged between the trans- and cis-rich states.
We have prepared photomagnetic Langmuir–Blodgett films
As the XRD measurement of LB films provides macroscopic
composed of an amphiphilic azobenzene and a magnetic poly-
information about the film structure, we examined linear
oxometalate. From XRD and linear dichroism spectroscopy, it
UV-Vis dichroism spectra to obtain microscopic information.
was confirmed that the film possesses the well-organized
Fig. 8 shows changes in UV-Vis absorption spectra recorded
layered structure where polyoxometalate anions are sand-
with polarized incident beams at an incident angle of 60°
wiched by azobenzene cations in a monolayered manner.
Reversible photoisomerization of azobenzene molecules was
achieved even at low temperature, accompanying intensity
changes in the d–d transition of cobalt(II) ions in a polyoxo-
from the surface normal. As the d–d transition of CoII ions
exhibits a broad peak, we provided difference polarized spectra
in Fig. 8. Clearly, both p- and s-polarized spectra exhibited
reversible changes in the d–d transition of CoII ions: a decrease
metalate. Moreover, at 2 K, the photomagnetic effect was observed
in intensity upon UV irradiation and recovery upon visible
reversibly upon alternate UV and visible light irradiation.
light irradiation. Again, as Co–POM lies flat along the AZ layer,
Based on investigation using polarized spectroscopy, the
observed photomagnetic effect is ascribed to the structure-
the d–d transition of CoII ions would direct to the almost same
angle. In this situation, if we assume that an orientation of
distortion of polyoxometalate layers. The present work demon-
Co–POM changes by photoisomerization of AZ, the d–d tran-
strates that confinement of functional materials could create a
novel function (the photomagnetic effect in the present case).
sition of CoII ions should change in polarized UV-Vis absorp-
tion spectra. Moreover, the observed changes were anisotropic
Furthermore, combination with other polyoxometalates
with respect to incident polarized beams. Along these lines,
enables the design of a multifunctional heterostructured film.
we suggest that changes in the d–d transition of CoII ions
are reflected by a structure-distortion of LB films upon
photochromic reaction. It should be noted that when we
prepare the AZ/Co–POM hybrid in solution, so-called surfac-
tant-encapsulated clusters, we observed neither a photo-
Experimental
Synthesis of materials
magnetic effect nor a structure-distortion despite reversible An amphiphilic azobenzene compound (AZ) was synthesized
photoisomerization.
according to previous reports.43,44 A polyoxometalate with a
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Co4O16 ferromagnetic cluster (Co–POM) was synthesized
according to a previous report.45 Detailed procedures are given
in ESI.†
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Langmuir–Blodgett film preparation
AZ was dissolved in a 9 : 1 mixture (by volume) of dichloro-
methane and ethanol to prepare a solution at 2.0 mM. The
solution was spread on a subphase of Co–POM aqueous solu-
tion (1.0 μM) at room temperature. A floating monolayer of AZ
was hybridized with Co–POM at the air–water interface. Fifteen
minutes later, the floating hybrid monolayers were compressed
up to a surface pressure of 30 mN m−1. After 30 min, the float-
ing hybrid monolayers were transferred onto a hydrophilic
substrate by a vertical dipping method with a speed of 10 mm
min−1. Langmuir–Blodgett films composed of AZ and Co–POM
were prepared by repeating this cycle.
11 M. Taguchi, K. Yamada, K. Suzuki, O. Sato and Y. Einaga,
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Instruments
Preparation of the floating hybrid monolayer and measure-
ments of the surface pressure–molecular area (π–A) isotherms
were carried out using a computer-controlled film balance
system (FSD-50, USI System, Japan). X-ray diffraction (XRD)
patterns were recorded on a Philips X’Pert MRD high-resolu-
tion X-ray diffractometer using Ni-filtered Cu Kα radiation.
UV-Vis absorption spectra were recorded on a V-560 spectro-
photometer (JASCO). UV-Vis measurements at low temperature
were performed using
a closed-cycle helium refrigerator
(Iwatani Co., Ltd). FT-IR absorption spectra were recorded on
an FT-IR 660Plus spectrometer (JASCO, Japan). UV irradiation
(filtered light, λmax = 360 nm, 1.0 mW cm−2) was carried out
using an ultrahigh-pressure mercury lamp (Spot Cure SP-7,
USHIO, Japan), and visible light irradiation (λ = 400–700 nm,
1.0 mW cm−2) was carried out using a xenon lamp (Optical
Modulex X 500, USHIO, Japan). The magnetic properties were
investigated using a superconducting quantum interference
device (SQUID) magnetometer (model MPMS-XL, Quantum
Design).
20 G. Romualudo-Torres, B. Agricole, P. Delhaes and
C. Mingotaud, Chem. Mater., 2002, 14, 4012.
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D. R. Talham, J. Am. Chem. Soc., 2002, 124, 10083.
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Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas (“Coordination Programming”
Area 2107, grant no. 24108736) from MEXT, Japan.
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