Self-organized honeycomb structures of Mn12 single-molecule magnets

Article abstract of DOI:10.1021/jp906520j

In this paper, Mn₁₂-based ordered honeycomb structures were successfully constructed from a simple solution casting process at high relative humidity through the modification of fatty acids to Mn₁₂ clusters. Mn₁₂-fatty aci

Full text of DOI:10.1021/jp906520j

14674
J. Phys. Chem. B 2009, 113, 14674–14680
Self-Organized Honeycomb Structures of Mn12 Single-Molecule Magnets
Hang Sun,† Wen Li,† Lance Wollenberg,§ Bao Li,† Lixin Wu,*,† Fengyan Li,‡ and Lin Xu‡
State Key Laboratory of Supramolecular Structure and Materials, Department of Chemistry, Jilin UniVersity,
Changchun 130012, People’s Republic of China, Key Laboratory of Polyoxometalate Science of Ministry of
Education, Department of Chemistry, Northeast Normal UniVersity, Changchun 130024, People’s Republic of
China, and Pharmaceutical and Pharmacological Sciences, West Virginia UniVersity, P.O. Box 9530,
Morgantown, West Virginia 26506
ReceiVed: July 10, 2009; ReVised Manuscript ReceiVed: September 11, 2009
In this paper, Mn12-based ordered honeycomb structures were successfully constructed from a simple solution
casting process at high relative humidity through the modification of fatty acids to Mn12 clusters. Mn12-fatty
acid complexes maintain typical features of a single-molecule magnet as confirmed by IR spectra and
magnetization hysteresis studies. Investigation of the effects of concentration, velocity of humid airflow, solvent,
substrate, and alkyl chain length of the Mn12 complex on the morphology of the honeycomb structures
demonstrated wide generality and high reproducibility of the formation of Mn12-based self-organized
honeycomb-patterned films. Both two-dimensional and three-dimensional honeycomb structures were obtained
by adjusting the concentration of the complex solution. Mn12-based, honeycomb-patterned films maintain a
paramagnetic response at room temperature, and thus give rise to a spatially distributed magnetic pattern on
the substrate, which can be imaged by magnetic force microscopy. Importantly, the single-molecule magnetic
property of the Mn12 complex at low temperature is well maintained in the honeycomb-patterned film, which
represents a promising outlook for high-density information storage and quantum computing applications.
Introduction
of SMMs on a given surface are essential.8 The Mn12 family
[Mn12O12(O2CR)16(H2O)x] (R ) substituted group) consists of
a structure containing a basic cluster of four Mn(IV) and eight
Mn(III) centers coordinated by an outer shell of 16 peripheral
carboxylate groups, representing a typical SMM system which
has been widely studied for the deposition of Mn12 complexes
to substrates.9 For example, Mn12 clusters were reported to
anchor on different surfaces such as Au(111) and Si(100)
through ionic or covalent interactions, and a Si/SiO2 surface
through photolithography or lithographically controlled wetting
methods.10 However, Gatteschi and co-workers found that
functionalized Mn12 complexes deposited on gold surfaces by
covalent interaction lose their SMM behavior by using X-ray
absorption spectroscopy and X-ray magnetic circular dichroism
methods, even though the molecules seem to be structurally
undamaged.11 By using a substituted Mn12 complex comprised
of aromatic biphenyl moieties, self-organized magnetic rings
were readily obtained through directed formation by condensed
water droplets.12 However, it is difficult to control the size and
arrangement of the ring structures unless premodification to the
substrate is carried out. By varying the peripheral carboxylate
ligands, the Mn12 complex becomes wettable and stable at the
interface between the water droplets and the organic solution,
at which point the ordered honeycomb structure based on the
Mn12 complex could be generated through arrayed water droplets
as the template.
Controlled condensation of water vapor on the cooling surface
of volatile polymer solutions leads to patterned structures of
arrayed water droplets.1 A process generally referred to as
“breath figures” has attracted much interest in the past few years,
providing a simple approach to generating patterned structures
with periodicity, in a size range of 50 nm to 20 µm.2 By using
this method, a variety of polymer materials have been success-
fully employed for the fabrication of honeycomb structures
exhibiting various functionalities such as superhydrophobic
surfaces, microlens arrays, cell culture substrates, and patterned
templates.3 Normally, stabilization of the condensed water
droplets is essential for the patterned structure to achieve high
regularity, which seriously limits the materials applied for this
method.2b Therefore, to integrate diverse functions into patterned
structures, development of new materials is essential, with recent
attention concentrated on the functional patterns of nanomate-
rials with unique optical, magnetic, and biological properties.4
For example, both honeycomb and ringlike structures were
reported by surface modified Au nanoparticles and quantum
dots.5
Single-molecule magnets (SMMs), the nanoscale magnetic
clusters of a sharply defined size, exhibit large spin ground states
with high axial magnetic anisotropy, resulting in an energy
barrier for spin reversal.6 Interesting magnetic properties of
SMMs, such as out-of-phase ac magnetic susceptibility signals
and stepwise magnetization hysteresis loops, offer a potential
access to ultimate high-density information storage and quantum-
computing applications.7 To achieve a true molecular device
based on SMMs, spatially and geometrically controlled patterns
Following previous studies on surface patterning of functional
hybrid complexes,13 we herein designed and synthesized a type
of Mn12-fatty acid complex, and demonstrated the self-
organization of the Mn12 complexes into highly ordered
honeycomb structures over different surfaces by simple solution
casting of Mn12-fatty acid complexes at high relative humidity.
This strategy exhibits several advantages for the self-organiza-
tion of Mn12 clusters including the following: (1) The modifica-
* To whom correspondence should be addressed. E-mail: wulx@jlu.edu.cn.
† Jilin University.
§ West Virginia University.
‡ Northeast Normal University.
10.1021/jp906520j CCC: $40.75  2009 American Chemical Society
Published on Web 10/13/2009

Mn12 Single-Molecule Magnets
J. Phys. Chem. B, Vol. 113, No. 44, 2009 14675
SCHEME 1: Chemical Structure of Fatty Acids Applied
To Modify the Mn12 Clusters: Stearic Acid (St),
Hexadecanoic Acid (He), and Tetradecanoic Acid (Te)
calculated number of crystallized water is ca. 1. Combining the
TGA and elemental analysis, Mn12St should correspond to the
formula Mn12O12(C17H35COO)11(CH3COO)5H2O. TGA mea-
surement was used to verify the mass ratio of [Mn12O12] core
and peripheral stearic ligands correctly. This technique serves
to decompose the organic component at 600 °C, and all the
inorganic residuals consist of a mixture of Mn2O3 and MnO2
with equal molar ratio (assuming the valence of Mn is
invariable). TGA analysis demonstrates 22.84 wt % remaining
after heating the sample to 600 °C. This experimental value
obtained from TGA is in perfect agreement with the calculated
value of 22.87 wt % from the given structural formula above.
IR (KBr, cm-1) for Mn12He: ν ) 3524, 2919, 2850, 1713,
1583, 1531, 1503, 1468, 1455, 1443, 1414, 1380, 1321, 1254,
1178, 1113, 721, 674, 640, 613, 568, 552, 523. Anal. Calcd for
Mn12He (C228H444O46Mn12, 4581.19): C, 59.78; H, 9.77. Found:
C, 59.63; H, 9.80. The TGA (temperature range 30-150 °C)
corresponds to the loss of crystallized water (0.88%), and the
calculated number of crystallized water is about 2. Combining
the TGA and elemental analysis, Mn12He corresponds to the
formula Mn12O12(C15H31COO)14(CH3COO)2(H2O)2. TGA mea-
surement was used to verify the mass ratio of the [Mn12O12]
core and peripheral hexadecanoic ligands. Analysis of these
results demonstrates 20.24 wt % remaining after heating the
sample to 600 °C. This experimental result obtained from TGA
is in reasonable agreement with the calculated value of 21.38
wt % from the given structural formula noted above.
tion of fatty acids provides the Mn12 cluster an amphiphilic
property in organic solvents. (2) Long alkyl chains of fatty acids
enhance the stability of the Mn12 cluster when in contact with
water, in contrast to the rapid decomposition of simple clusters.9f
(3) Modification of Mn12 clusters facilitates good organization
and typical SMM features in surface patterning. We investigated
the effects of complex concentration, velocity of humid airflow,
solvent type, substrate composition, and the alkyl chain length
of the Mn12 complex on the morphology of the honeycomb-
patterned films. Furthermore, we tested the magnetic properties
of the Mn12-based self-organized honeycomb-patterned film.
Subsequently, the results presented here provide a novel
approach for the development of Mn12 clusters into patterned
structures.
IR (KBr, cm-1) for Mn12Te: ν ) 2922, 2852, 1710, 1574,
1541, 1443, 1377, 1316, 1263, 1116, 1088, 1028, 959, 721, 610.
Anal. Calcd for Mn12Te (C212H420O50Mn12, 4428.83): C, 57.49;
H, 9.56. Found: C, 57.43; H, 9.60. The TGA (temperature range
30-150 °C) corresponds to the loss of crystallized water
(2.68%), and the calculated number of crystallized water is about
6. Combining the TGA and elemental analysis, Mn12Te should
correspond to the formula Mn12O12(C13H27COO)15(CH3COO)
(H2O)6. TGA measurement was used to verify the mass ratio
of the [Mn12O12] core and peripheral tetradecanoic ligands.
Analysis of these results demonstrates 20.93 wt % remaining
after heating the sample to 600 °C. The experimental result
obtained from TGA is in reasonable agreement with the
calculated value of 22.11 wt % from the given structural
formula.
Preparation of Honeycomb-Patterned Thin Films. Typi-
cally, honeycomb-patterned thin films were prepared by direct
casting 20 µL of Mn12-fatty acid complex in chloroform
solution (1.5 mg/mL) onto the glass substrate under a moist
airflow. The humid conditions were achieved by bubbling
nitrogen gas through a water-filled conical flask. The nitrogen
gas saturated with water vapor (temperature 25 °C) was exported
through a glass nozzle onto the surface of the sample solution
vertically. The velocity of humid airflow was controlled at
200-400 mL/min, which was monitored by a flow meter to
ensure reproducibility. The internal diameter of the glass nozzle
used to apply moist airflow is 0.6 cm, and the distance between
the solution surface and the nozzle is ca. 1.2 cm. White thin
films covering an area of ca. 1 cm2 were left remaining after
the complete evaporation of the solvent and water within 30-60
s. The control experiments without humid airflow have been
conducted under ambient atmosphere (relative humidity 20-30%)
and no porous structures were obtained, resulting in unpatterned
flat films deposited on the surface.
Experimental Section
Materials. [Mn12O12(O2CCH3)16(H2O)4]·8H2O (Mn12Ac) was
freshly prepared according to literature procedures.9a Stearic
acid, hexadecanoic acid, and tetradecanoic acid were purchased
from Tianjin Guangfu Chemical Works and used as received.
Preparation of Mn12-Fatty Acid Complexes. Mn12-fatty
acid complexes were prepared through a modified procedure
as reported in the literature,9 by a substitution reaction in the
presence of the desired fatty acid as well as the precursor
Mn12Ac, to produce the Mn12-stearic acid complex. A mixture
of Mn12Ac (0.10 g, 0.05 mmol) and stearic acid (0.49 g, 1.5
mmol) was added to 50 mL of a 1:1 (v/v) solution of
dichloromethane and toluene and was stirred for 24 h at 45 °C.
Then, the mixture was concentrated to remove resulting acetic
acid. The resulting solid and additional stearic acid (0.33 g, 1.0
mmol) were dissolved in 50 mL of a 1:1 (v/v) solution of
dichloromethane and toluene, stirred for 24 h at 45 °C, and then
concentrated to remove resulting acetic acid. To fully substitute
the acetate ligands, this procedure was repeated three times.
The resulting brown solid was dissolved in hot methanol,
filtered, and washed with hot methanol (total 100 mL) to remove
uncomplexed stearic acid. The brown powder was further dried
under vacuum until its weight remained constant, giving the
product of Mn12-stearic acid complex (Mn12St). Following
similar procedures, Mn12-hexadecanoic acid complex (Mn12He)
and Mn12-tetradecanoic acid complex (Mn12Te) were prepared
(Scheme 1). All three complexes were characterized by IR
spectra (Figure S1 in the Supporting Information), elemental
analysis, and thermogravimetric analysis (TGA) (Figure S2 in
the Supporting Information) as follows.
IR (KBr, cm-1) for Mn12St: ν ) 2920, 2850, 1583, 1569,
1531, 1468, 1455, 1443, 1429, 1380, 1317, 1261, 1098, 1025,
868, 804, 721, 705, 673, 641, 610, 568, 558. Anal. Calcd for
Mn12St (C208H402O45Mn12, 4282.65): C, 58.33; H, 9.45. Found:
C, 57.99; H, 8.99. The TGA (temperature range 30-150 °C)
corresponds to the loss of crystallized water (0.52%), and the
Measurements. Fourier transform infrared (FT-IR) spectral
measurements were completed on a Bruker IFS66 V FT-IR
spectrometer equipped with a DGTS detector (32 scans), using

14676 J. Phys. Chem. B, Vol. 113, No. 44, 2009
Sun et al.
KBr pellets, and the spectra were recorded with a resolution of
4 cm-1. Element analysis (C, H, N) was carried out on a Flash
EA1112 analyzer from ThermoQuest Italia SPA Thermogravi-
metric analysis (TGA) was conducted with a Perkin-Elmer TG/
DTA-7 instrument, and the heating rate was set at 10 °C min-1.
The optical photographs were taken with an Olympus BX-51
optical microscope (OM). Scanning electron microscopic (SEM)
images were collected on a JEOL JSM-6700F field emission
scanning electron microscope. Magnetization hysteresis data
were collected at 2 K, between +5 and -5 T, cooling the
samples at zero field with a magnetometer (Quantum Design
MPMSXL-5) equipped with a SQUID sensor. The powder
sample of Mn12St complex and the crystal sample of Mn12Ac
complex were measured directly. The honeycomb-patterned film
of Mn12St complex was removed from the substrate and
collected for measuring. Each sample is first cooled to 1.8 K in
zero field, then a 100 G field is applied and magnetization is
measured as the temperature is increased to 10 K to give the
zero-field-cooled (ZFC) data. This is then followed by cooling
the sample from 2.0 to 10.0 K with the 100 G field maintained
to give the field-cooled (FC) data. Atomic force microscopy
(AFM) and magnetic force microscopy (MFM) measurements
were carried out on a commercial instrument (Digital Instrument,
Nanoscope III, Dimension 3000TM) at room temperature in air.
The AFM images were obtained in tapping mode using
phosphorus or antimony (n) doped Si tips. For MFM measure-
ment, cobalt-coated tips were used operating in the lift mode.
Each image was confirmed by measurements with three films
in at least five different well-separated 20 × 20 µm2 sites.
Figure 1. Magnetization hysteresis loops of the Mn12Ac complex
(black circles), the Mn12St complex (red squares), and its honeycomb-
patterned film (blue triangles) at 2 K.
and 1455 cm-1, demonstrating the basic cluster structure of
[Mn12O12] is invariable during the process of ligand exchange.
The bands at 2920 and 2850 cm-1, which are assigned to the
antisymmetric and symmetric stretching vibrations of the
methylene group, respectively, indicate that the alkyl chain
conformation of Mn12St is well-ordered.15 The other two
complexes, Mn12He and Mn12Te, also show characteristic
vibration bands similar to those of the Mn12St complex. Similar
to Mn12St, Mn12He complexes display antisymmetric and
symmetric stretching vibrations of the methylene group at 2919
and 2850 cm-1, indicative of ordered alkyl chains, whereas the
vibrations appear at 2922 and 2852 cm-1 for Mn12Te, implying
relatively disordered alkyl chains.
To further verify that the magnetic properties of Mn12 clusters
can be maintained in Mn12-fatty acid complexes, herein we
utilize the Mn12St complexes as an example to perform the
magnetization hysteresis study by comparison with the Mn12Ac
complex. The magnetization hysteresis loops for the Mn12St
complex in the powder state and the Mn12Ac complex in its
crystal state have been measured at 2 K as shown in Figure 1.
The samples were first magnetically saturated in a +5 T field,
and then the field was swept down to -5 T and cycled back to
+5 T. The field-dependent magnetization of Mn12Ac complex
shows a rapid increase at low field to reach a value of 11.38 µB
at 0.9 T. Upon increasing the field intensity, the magnetization
increases smoothly and reaches 14.94 µB at the highest field of
the experiment (5 T). However, saturation is not reached in a 5
T field. This is in contrast to the situation found in a single
crystal of Mn12Ac complexes, and reflects the fact that the
Mn12Ac complex is not well oriented with respect to the applied
field in our case.9c,16 Mn12St complex also shows a magnetization
unsaturation at a high magnetic field, suggesting that the Mn12St
complex is randomly oriented with respect to the applied field,
which is consistent with its powder state. Furthermore, the
magnetization of the Mn12St complex is lower than that of the
Mn12Ac complex at the highest field of 5 T, which could be
ascribed to a different spin ground state of the molecules after
the substitution of St ligands in the Mn12Ac complex. Previous
work has shown that carboxylic ligands have an important
influence on the spin ground state of the Mn12 cluster, and this
may result in what we have observed in our experiment.17
Mn12Ac complex clearly shows a hysteresis with a coercive field
of 0.84 T. As for the Mn12St complex, the coercive field
decreases to 0.4 T due to the partial substitution of acetate
ligands by diamagnetic fatty acids, and the hysteresis clearly
confirms the magnetic anisotropy in the Mn12St complex. The
hysteresis loop for the Mn12St complex does not show steplike
features, indicative of quantum tunneling of magnetization
(QTM) between the energy states of the molecule, in contrast
to what is observed for the crystal of the Mn12Ac complex.16
Results and Discussion
Characterization of Mn12-Fatty Acid Complexes. Mn12-
fatty acid complexes were prepared by a substitution reaction
in the presence of the desired fatty acid along with the precursor
Mn12Ac through a modified procedure as reported in the
literature.9 However, not all the acetate ligands of the Mn12
cluster are substituted by the desired fatty acids after four
successive attempts of the substitution reaction. There are only
11 stearic ligands in Mn12St, 14 hexadecanoic ligands in
Mn12He, and 15 tetradecanoic ligands in Mn12Te, as confirmed
by elemental analysis and TGA. The number of coordinated
fatty acids does not increase in seven sequential substitution
reactions repeated for all three complexes. Considering the
decreased solubility of the Mn12 cluster in the reaction solution
after the modification with fatty acids, as well as the similar
coordination constants of acetic acid and fatty acid to Mn, the
partial substitution of acetate ligands by fatty acids in Mn12
clusters is reasonable. The number of coordinated fatty acids
in Mn12 clusters decreases with increasing alkyl chain length
of fatty acids since the solubility of Mn12-fatty acid complexes
decreases with increasing alkyl chain length. As there are eight
axial and eight equatorial ligand positions in Mn12 clusters,9a,14
under the present case, we could not identify the positions of
the acetate ligands in the Mn12 cluster that were not substituted
by fatty acids, because all three Mn12-fatty acid complexes are
too difficult to crystallize.
Importantly, all three Mn12-fatty acid complexes maintain
their cluster structures by comparing typical vibration absorp-
tions of [Mn12O12] clusters to those Mn12Ac complexes reported
in the literature, as exemplified by Mn12St (Figure S1 in the
Supporting Information). Mn12Ac shows intense vibration
absorptions at 1561 and 1450 cm-1, which are assigned to the
characteristic vibrations of Mn12 clusters.9 For the Mn12St
complex, similar intense vibration absorptions appear at 1569

Mn12 Single-Molecule Magnets
J. Phys. Chem. B, Vol. 113, No. 44, 2009 14677
reach the surface of the substrate. Through use of AFM
performed in tapping mode, ordered micrometer-sized porous
structures can be seen, typically at a depth of 0.5 µm, with a
uniform diameter of 1.6 µm and an interval of 1.5 µm between
adjacent holes. The Mn12St honeycomb-patterned film shows
similar characteristic vibration absorptions of the [Mn12O12]
cluster and the methylene group with its bulk compound (Figure
S1 in the Supporting Information).
The honeycomb-patterned films were successfully fabricated
over a wide concentration range from 1.5 to 5.0 mg/mL Mn12St
solution. However, the holes tend to be multilayered in the
honeycomb-patterned film with increasing concentration of the
Mn12St complex in chloroform. Specifically, the SEM and AFM
images of the honeycomb-patterned film casting from 5.0 mg/
mL Mn12St solution are shown in Figure 3. From the top view
of the SEM image, the pores in the second layer can be seen
clearly through the surface layer. SEM image of the cross section
indicates that the holes are multilayered, but do not reach the
bottom of the film. Tapping mode AFM images further confirm
the ordered micrometer-sized porous structures with a porous
sublayer, where the average diameter of the surface holes is ca.
1.2 µm separated by ca. 1.3 µm, which is smaller than those in
the case of 1.5 mg/mL complex solutions. By comparison of
honeycomb-patterned films prepared from 1.5 and 5.0 mg/mL
complex solutions, we can see that the concentration of the
complex solution has an impact on both the size and arrange-
ment of the pores. An increase in the concentration of the
complex solution usually results in the multilayered structure
accompanied by a decrease in the pore size;19 thus we could
adjust the structure of the honeycomb-patterned film to some
extent by changing the concentration of the complex solution.
No honeycomb morphology has been observed on the films
prepared under ambient atmosphere with relative humidity
below 30%, further confirming that the honeycomb structure
formation is driven by the water droplets condensing on the
surface of the evaporating solution.
Figure 2. Honeycomb structure images prepared from 1.5 mg/mL
Mn12St complex chloroform solution: (a) SEM image of top surface,
(b) local magnification of (a), (c) SEM image of cross section, and (d)
AFM image.
The absence of well-defined QTM steps can be primarily
explained by the different molecular environments and the
distribution of magnetization relaxation barriers existing in the
randomly oriented powder sample. Similar hysteresis loops
without QTM steps are a common feature of large SMMs, where
disordered solvents, counterions, and ligands result in the
inefficient packing of molecules.18 It is known that the shape
of the hysteresis loop is always sensitive to the morphology of
the sample and the remaining solvates. Here, the diamagnetic
stearic ligands, the unsymmetrical microenvironment of the Mn12
cluster derived from the partial substitution of stearic acids to
acetate ligands in the Mn12St complex, and the powder
morphology of the Mn12St complex all have an important
influence on the magnetization of the Mn12 cluster. All of these
possible factors are accounted for the observed differences of
magnetization hysteresis loops between the random oriented
powder sample of the Mn12St complex and the crystal of the
Mn12Ac complex. Furthermore, we performed the zero-field-
cooled (ZFC) and field-cooled (FC) susceptibility measurements
to estimate the blocking temperatures of the Mn12Ac complex
and the Mn12St complex, respectively (Figure S3 in the
Supporting Information). The divergence of the ZFC and the
FC susceptibilities below about 3.1 K for Mn12Ac complex
indicates the occurrence of irreversibility of magnetization. Thus,
the blocking temperature for the Mn12Ac complex is about 3.1
K, whereas for the Mn12St complex the blocking temperature
decreases to about 2.0 K. Thus, for the Mn12Ac complex and
the Mn12St complex, the blocking temperature decreases with
decreasing the saturation of the magnetization.
Ordered Honeycomb Structures of Mn12-Fatty Acid
Complexes. The honeycomb structures of Mn12-fatty acid
complexes are constructed by casting their chloroform solutions
onto glass substrates under a moist airflow across the solution
surface. The films exhibit bright iridescent colors when viewed
with reflected light, indicating a periodic refractive index of
variation with the film thickness. Figure 2 shows representative
SEM and AFM images of the honeycomb-patterned film casting
from 1.5 mg/mL Mn12St chloroform solution under a humid
airflow of 200 mL/min. The top view of the SEM image reveals
the regular, spherical pores covering a large area. A close
examination through the magnified SEM image shows that the
holes are uniform and well-ordered. From the image of the cross
section, we can clearly see that the holes are monolayered and
To examine the generality of this supramolecular self-
organized method for the construction of Mn12St honeycomb
structures, we investigated the effects of the velocity of humid
airflow, as well as solvent type and substrate composition, on
the morphology of the honeycomb structures. As seen from the
comparison of Figure 2 and Figure S4 in the Supporting
Information, the average size of the holes in the honeycomb-
patterned film increases from 1.6 to 1.9 µm, and the wall
contracts from 1.5 to 1.2 µm, with increasing velocity of humid
airflow from 200 to 300 mL/min. By further increasing velocity
of humid airflow to 400 mL/min, the average size of the holes
increases to 2.2 µm with the walls contracting to 0.5 µm.
Furthermore, at this higher velocity of humid airflow, holes in
the honeycomb-patterned film show a relatively large size
distribution, and some visible defects appear. A high velocity
of humid airflow increases the evaporation rate of solvent, and
thus reduces the opportunities for the condensation and growth
of water droplets.20 At the same time, a high velocity of humid
airflow also brings additional moisture and can result in
contradictory effects. Here, the increase in velocity of humid
airflow seemed to aid in the condensation and swelling of the
water droplets on the casting solution even with an increased
evaporation rate of solvent. Additionally, the extremely high
velocity of humid airflow results in the unavoidable coalescence
of the condensed water droplets and the increased instability of
the evaporating solution, both of which result in the disorder
and defects observed in the honeycomb-patterned film.

14678 J. Phys. Chem. B, Vol. 113, No. 44, 2009
Sun et al.
Figure 3. Honeycomb structure images prepared from 5.0 mg/mL Mn12St complex chloroform solution: (a) SEM image of top surface, (b) SEM
image of cross section, and (c) AFM image.
TABLE 1: Vapor Pressure and Molecular Weight of the
Used Solvents (20 °C)
complexes show the same spectral features of their correspond-
ing bulk compounds, respectively (Figure S1 in the Supporting
carbon
Information). The results show that the Mn12-fatty acid
complexes with the shorter alkyl chains are unfavorable for the
construction of ordered honeycomb structures, which is con-
sistent with our previous studies on surfactant-encapsulated
polyoxometalate complexes and DNA-surfactant complexes.12
Furthermore, we have also tried to prepare Mn12-eicosanoic
acid complex for honeycomb pattern formation, but the obtained
complex shows low solubility in chloroform, dichloromethane,
benzene, etc., and thus cannot form films by solvent casting
techniques. Therefore, it is difficult to give an example of
honeycomb-patterned films fabricated from Mn12 complexes
with more hydrophobic fatty acid ligands to establish the trend
for the formation of ordered honeycomb-patterned films.
dichloromethane
46.50
chloroform benzene
disulfide
39.24
vapor pressure
(kPa)
molecular weight
21.28
10.67
78.11
84.93
76.14
119.38
Films prepared from dichloromethane, carbon disulfide, and
benzene solutions also show honeycomb structures, but the holes
are less regular than those formed from chloroform (Figure S5
in the Supporting Information). Micropores in the film castings
from dichloromethane and carbon disulfide are smaller than
those formed from chloroform, while the film casting from
benzene shows a larger-sized honeycomb structure. The major
differences between the solvents applied are their differential
evaporation rates. The evaporation rate of the solvent depends
mostly on the vapor pressure above the solvent as well as the
molecular weight of the solvent (Table 1). Typically, higher
vapor pressure and lower molecular weight lead to faster
evaporation rates.21 With vapor pressure decreasing, solvent
volatility is lessened, allowing water droplets more time to
coalesce and grow larger on the surface of the solution,
consequently forming larger pores. Rapid evaporation of solvent
leads to the instability of the solution surface, and the
honeycomb-patterned film may be solidified well before the
arrangement of the condensed water droplets. On the other hand,
slow evaporation of solvent may lead to unavoidable coales-
cence, which decreases the order of the honeycomb-patterned
films. As a result, the proper evaporation rate of the solvent is
essential for the formation of ordered honeycomb-patterned
films. In addition to glass, mica and silicon were used as the
substrates for the preparation of Mn12St honeycomb-patterned
films (Figure S6 in the Supporting Information).22 An ordered
honeycomb-patterned surface was observed on mica, similar to
that on glass. However, on the less hydrophilic silicon substrate,
disordered holes with a relatively broad size distribution were
observed. The ordered honeycomb-patterned films are easily
prepared on hydrophilic substrates because of the favorable
spreading of the complex solution, as well as the steady
absorbance of the condensed water droplets onto the hydrophilic
substrates to assist in molding patterned structures.
Honeycomb-patterned films based on polymers and nano-
particle materials by using ordered condensed water droplets
as template have been extensively investigated.2a,23 Considering
the consistency of our results with those of polymer systems
and modified nanoparticles, a similar mechanism for the
formation of ordered honeycomb-patterned films of Mn12-fatty
acid complexes is proposed: (1) When the moist airflow passes
across a volatile solution of Mn12-fatty acid complex, the rapid
evaporation of the solvent induces a cooling of the solution
surface, allowing water vapor to condense into droplets on the
solution surface. (2) The water droplets grow with time by
molecular condensation until they reach a self-limiting narrow
size distribution, while convective currents on the surface and
short-range lateral capillary forces among the water droplets
drive the droplets into hexagonally packed arrays. Coalescence
of the water droplets is inhibited by Mn12-fatty acid complexes
adsorbed and precipitated at the interface between the chloroform-
containing solution and the water droplets. Additionally, this
mechanism provides a reasonable explanation for the formation
of multilayered films. It is proposed that water droplets will
continually sink into the solution at which point they will
condense and arrange themselves upon a previously fashioned
monolayer of water droplets, creating a structure similar to that
observed in Figure 3b. (3) After complete evaporation of the
solvent and the encapsulated water droplets, hexagonally
arranged pores are imprinted in the film of the Mn12-fatty acid
complex. Based on experimental results presented here, it is
obvious that the size and regularity of the pores can be controlled
by changing any experimental parameters that affect the above
three steps, such as humidity, solvent, temperature, and casting
volume.
Moreover, we have also tried Mn12 complexes with shorter
alkyl chains, Mn12He and Mn12Te, to fabricate honeycomb
structures (Figure S7 in the Supporting Information). Films
formed from solutions of 1.5 and 5.0 mg/mL Mn12He in
chloroform exhibit a type of honeycomb-patterned morphology;
however, pores in these honeycomb-patterned films do not show
an ordered hexagonal packing. In the case of Mn12Te, failure
to form uniformed honeycomb structures is demonstrated, and
only large disordered holes with a relatively broad size distribu-
tion are observed in the casting field at the 1.5 and 5.0 mg/mL
concentrations. The porous films of both Mn12He and Mn12Te
Magnetic Property of Ordered Honeycomb Structure.
MFM, a microscopy technique utilizing a magnetic tip, is a
powerful tool to study magnetic properties of patterned struc-
tures, allowing the combination of topographic imaging of an
object with the mapping of local distribution of magnetic
moments. Mn12 clusters are paramagnetic without a preferential

Mn12 Single-Molecule Magnets
J. Phys. Chem. B, Vol. 113, No. 44, 2009 14679
Figure 4. MFM images of Mn12St honeycomb-patterned film: (a) topography, (b) corresponding phase-contrast image at 300 nm lift height, and
(c) cross section along the line indicated in (a).
ordering of their magnetic moments at room temperature.
Nevertheless, application of a magnetic tip will magnetize Mn12
clusters at room temperature, and this collection of magnetic
moments of Mn12 clusters is powerful enough to generate a
detectable magnetic contrast with the diamagnetic surroundings
in MFM images.24 Figure 4 shows the MFM image of the
honeycomb-patterned surface casting from 1.5 mg/mL Mn12St
chloroform solution at room temperature. The image was
recorded in two scans over the same area. On the first scan, the
topography was recorded using “tapping mode”, and a similar
pattern to that shown in Figure 2d, albeit with a lower resolution
due to the use of a thicker cobalt-coated tip, was observed
(Figure 4a). Furthermore, from the cross section of the MFM
topography image, it can be seen that the walls of the holes are
not vertical but sloped with respect to the substrate (Figure 4c),
which is common in breath figure patterned films and consistent
with the shape of the water droplets.2b Subsequently, the tip
was retracted to a selected lift scan height in the range of
50-300 nm, at which height the interatomic van der Waals force
is virtually nonexistent and only long-range magnetic force
persists. The MFM image shown in Figure 4b with a cobalt-
coated tip at 300 nm clearly indicates a magnetic gradient
response corresponding to the molecular position found by
topographic imaging. The regions with deposited Mn12St
complex show a lighter phase than the uncovered areas,
indicating that the deposited material is magnetically active and
interacts in an attractive manner with the magnetized tip. It is
noted that the inner edge of the holes also demonstrates a
magnetic signal from the Mn12St complex in MFM phase-
contrast image because the walls of the holes are sloped. Only
the center of the hole, which is exposed substrate, shows a
different magnetic contrast than the Mn12St complex. Combining
the topography and phase-contrast results from the MFM image,
ca. 62% of the total area of the honeycomb film is covered by
Mn12St complex with a typical thickness of 0.5 µm. This
observation confirms the formation of a spatially distributed
magnetic pattern and thus confirms the role of magnetic Mn12St
complexes in the formation of the honeycomb-patterned surface.
Significantly, the typical magnetic properties of Mn12 single-
molecule magnets are maintained in the Mn12St honeycomb-
patterned film as confirmed by the magnetization hysteresis
study at 2 K. The magnetization hysteresis loop for the Mn12St
ordered honeycomb-patterned film shows both similar loop
features and the same coercive field of 0.4 T with those
randomly oriented Mn12 powder samples as seen in Figure 1.
Furthermore, the Mn12St honeycomb-patterned film shows
similar magnetization at the highest field of 5 T and the blocking
temperature. Thus, the solvent casting process under humid
conditions shows little influence on the single-molecule magnetic
property of the complex. Mn12St honeycomb-patterned surfaces
demonstrating typical single-molecule magnetic properties show
a promising perspective as magnetic quantum bits for informa-
tion storage or quantum computing.
Conclusions
The modification of fatty acids not only brings the Mn12
clusters an amphiphilic property, allowing themselves to stabilize
the condensed water droplets, but also protects the Mn12 clusters
from decomposition when in contact with water and helps
maintain the features of SMMs, thus forming ordered magnetic
patterns on substrates. Both two-dimensional and three-
dimensional honeycomb structures were obtained by adjusting
the concentration of the complex solution. The ordered honey-
comb structures are easily formed on the hydrophilic substrates.
The pore size of the honeycomb-patterned film can be scaled
down by decreasing the velocity of the humid airflow or
increasing the concentration of the complex solution and the
vapor pressure of the solvents. The proper hydrophobicity of
the fatty acid in the Mn12 complex was confirmed to be a
prerequisite for the formation of ordered honeycomb-patterned
films. Specifically, highly ordered honeycomb-patterned sur-
faces, with monolayered, closely packed, hexagonally patterned
holes were formed on glass substrates from a solution containing
1.5 mg/mL Mn12St in chloroform. Mn12-based honeycomb
structures exhibit a paramagnetic response at room temperature,
and thus give rise to a spatially distributed magnetic pattern on
the substrate, which can be characterized by MFM. Importantly,
the single-molecule magnetic property of the Mn12 complex at
low temperature is well maintained in the honeycomb-patterned
film, which can be effectively used as a permanent information
storage medium with magnetic readout, representing a promising
perspective for high-density information storage and quantum
computing applications.
Acknowledgment. The authors acknowledge financial sup-
port from the National Basic Research Program (2007CB808003),
National Natural Science Foundation of China (20731160002,
20973082, 50973042, 20703019), National Science Foundation
International Research Experience for Students (OISE-0824860),
PCSIRT of Ministry of Education of China (IRT0422), 111
Project (B06009), for supporting the visit and helpful discussion
from Prof. Bin Hu at the University of Tennessee, Open Project
of State Key Laboratory of Polymer Physics and Chemistry of
CAS, and West Virginia Graduate Student Fellowships in
Science, Technology, Engineering and Mathematics (STEM).
Supporting Information Available: FT-IR spectra and TGA
thermograms of Mn12-fatty acid complexes. SEM images and
optical micrographs of the honeycomb-patterned films prepared
under different conditions: velocity of humid airflow; substrate;
solvent; length of alkyl chain in Mn12 cluster. This material is
available free of charge via the Internet at http://pubs.acs.org.
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