J. Phys. Chem. C 2010, 114, 11723–11729
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Superparamagnetic-to-Diamagnetic Transition in Hydroxo-Bridged Trinuclear Copper(II) Complex Nanorods S. Giri and S. K. Saha* Department of Materials Science, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700032, India ReceiVed: February 8, 2010; ReVised Manuscript ReceiVed: June 11, 2010
Although magnetic interaction in trinuclear Cu(II) complex is a widely studied subject, the field remains totally unexplored in their nanophase. Here, single crystalline nanorods of hydroxo-bridged trinuclear Cu complex with partial cubane (Cu3O4) core ([Cu3(µ3-OH)(SE)3](ClO4)2 · 0.5H2O, SE being the condensed form of N,N-dimethylethylenediamine and salicylaldehyde) is synthesized in the nanochannel of anodic alumina template. Considering the spin Hamiltonian for a trinuclear system, the magnetic exchange interaction shows a weak intramolecular antiferromagnetic interaction (J) -2 cm-1) and a weak intermolecular ferromagnetic interaction (zj′ ) 0.72 cm-1). At 2 K, both bulk as well as nanorod samples exhibit superparamagnetic behavior with domain size 1.5 nm; however, at higher temperature, remarkable changes in magnetic properties such as field induced diamagnetic transition are noticed for nanorods. Due to directional growth of nanorods along the c-axis (nanochannel axis), out of two types of hydrogen bonds only that acting along the c-axis with a donor-acceptor distance of 3.094 Å is more effective in the magnetic field. These effects of giant diamagnetism in crystalline nanorods are attributed to the charge delocalization across the H-bonds acting along the c-axis. Introduction 1-7
The study of molecule-based magnets has attracted considerable interest after the discovery of a molecular compound exhibiting a spontaneous magnetization below a critical temperature. To design a molecule-based magnet1-7 parallel alignment of neighboring spins is essential. Much attention has therefore been paid to synthesize multinuclear transition metal complexes with various ligands which bridge the metal ions exhibiting either ferromagnetic8-16 or antiferromagnetic17-21 interaction between the metal ions depending upon the ligands, bridging angles, etc. In recent years much effort has been spent on the synthesis of multinuclear metal complexes as functional and structural models of multinuclear enzymes. Besides biological application, polynuclear metal complexes are of great interest in the area of magnetochemistry. In most cases the dominant magnetic interaction between metal centers of known multinuclear copper(II) complexes is antiferromagnetic,17-21 whereas there are some examples in which the interaction is ferromagnetic.8-16 During the past few years the magnetic exchange coupling in several oxo- and hydroxo-bridged cyclic trinuclear complexes with a partial cubane (Cu3O4) core has been tested.8,13,15,17,19,21 The three unpaired electrons on this core interact magnetically through superexchange coupling in Cu-O-Cu pathways and the controlling factors to become either ferromagnetic or antiferromagnetic depend on the Cu-O-Cu angle and the degree of noncoplanarity of “O” from the plane containing the Cu centers.8,19 So far most of the studies concerning magnetic properties in multinuclear transition metal complexes deal with the exchange coupling whether ferromagnetic or antiferromagnetic between Cu centers; however, the overall magnetism exhibited by bulk crystals as well as their nanophase has not yet been studied in detail. There has been growing interest in * To whom correspondence should be addressed. E-mail: cnssks@ iacs.res.in.
the synthesis of nanoscale metal organic framework (NMOF) and nanoscale supramolecular coordination polymers (NSCP), due to novel properties such as biosensing,22 magnetic resonance imaging,23 multimodal contrast enhancing agent,24 valencetautomerism,25 nanophase of molecular magnets,26-28 etc. exhibited by particles with nanometric dimensions. Although much effort has been paid to investigate magnetic interaction in trinuclear Cu(II) complex in their bulk phase, the field remains totally unexplored in their nanophase. Remarkable changes of physical and chemical properties of materials are associated with their nanophase. In the present study we have synthesized hydrogen-bonded trinuclear Cu complex with hydroxo-bridging, both in their bulk form as well as in nanorods shape to explore the effect of dimensionality of the materials on magnetic interaction. Here, single domain superparamagnetic interaction with domain size equal to 1.5 nm is observed at 2 K in both the bulk as well as nanorods. In nanorods, due to directional growth along the c-axis, field induced charge delocalization in hydrogen bonds causes a magnetic transition with giant diamagnetism; however, no such changes are observed in the case of bulk samples. Details are reported in this paper. Results and Discussion Figure 1a shows the structural view of the [(CuL)3(OH)]2+ cation of the complex, which in their crystal phase belongs to a nonpolar P21/c space group reported earlier.8 The complex contains one dipositive trinuclear cation with a partial cubane core and two noncoordinated perchlorate anions. In addition, there are two lattice water molecules. The trinuclear cation consists of a triply bridging hydroxo group, which is coordinated to each of the three copper centers in which the oxygen occupies the apex of the pyramid and the three copper atoms occupy the corners of an equilateral triangle. To investigate the crystallographic understanding of the desired trinuclear Cu(II) complex, XRD measurements are carried out on the as-synthesized bulk
10.1021/jp101211w 2010 American Chemical Society Published on Web 06/22/2010
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Figure 1. (a) Structural view of [(CuL)3(OH)]2+ cation where H-atoms are omitted for clarity. (b) Powder diffraction pattern of polycrystalline bulk sample.
polycrystalline sample. Figure 1b shows the XRD pattern, which is very similar to that reported in the literature.8 Magnetic interaction between the Cu centers in hydroxobridged trinuclear Cu complex depends mainly on the noncoplanarity of the hydroxo group. It is reported that ferromagnetic interaction is exhibited between Cu centers if the hydroxo group is more out of plane containing the Cu centers contrary to antiferromagnetic interaction if the hydroxo group is nearer to the plane. To investigate the magnetic exchange interaction between three Cu centers we have considered the spin Hamiltonian for a trinuclear system given by eq 1, where J is the exchange coupling constant. The expression for the magnetic susceptibility deduced from the Hamiltonian considering intercluster interaction zJ′ is given by
ˆ ) -2J(Sˆ1Sˆ2 + Sˆ1Sˆ3 + Sˆ2Sˆ3) H
(1)
( kT3J ) + 1 3J exp( ) + 1 kT
(2)
χtrimer
χM )
Ng2β2 ) 4kT
(
5 exp
)
χtrimer + χd 2zj' χ 1trimer Ng2β2
{ ( ) }
Figure 2. Variation of molar susceptibilities with temperature for (a) bulk at 1 T and (b) nanorods with 500 Oe. The red line represents the theoretical curve obtained from eq 3.
(3)
where N, g, β, and k have their usual meanings. Panels a and b of Figure 2 show the variation of molar susceptibility with temperature for both bulk as well as nano samples and the values of J and J′ are shown in Table 1. The theoretical curves are obtained from eq 3 and the points are the experimental data.13 In the fitting procedure R2 represents the degree of fitness. The molar susceptibilities fitted by eq 3 in the case of nano sample at different fields are given in the Supporting Information. It is seen that the present trinuclear Cu complex exhibits a weak intramolecular antiferromagnetic interaction (J) -2 cm-1) and a weak intermolecular ferromagnetic interaction (zj′ ) 0.72 cm-1). As the intramolecular exchange interaction (J) is negative so the ground state should be a doubly degenerate doublet state (2E) with Stotal ) 1/2. Panels a and b of Figure 3 show the magnetization vs field at different temperatures for both bulk and nanorods. It is seen that at 2 K both samples show magnetic saturation with no
hysterysis loop, which means that the behavior is superparamagnetic in nature. Figure 3c gives the magnetic saturation with magnetic field for both bulk and nanorods at 2 K. The experimental data are fitted by the expression given by eq 4 to extract saturation magnetization (Ms) and magnetic moment (µ) per domain as 9.7 emu/g and 2.8 × 10-20 erg/Oe, respectively, for bulk sample.29
kT [ ( µH kT ) ( µH )] dM 18kT ( dH ) ×
M(T) ) Ms coth
dmag(max) )
[
]
(4)
1/3
H)0
π
FMs2
(5)
From eq 5, the domain size has been calculated to ∼1.5 nm, which ctheonsists of 4 molecules. Magnetic moment per domain extracted from the fitting procedure is about 2.8 × 10-20 erg/ Oe, which is higher than the value of Bohr Magneton (9.27 × 10-21 erg · Oe1-). As there are 4 molecules in each domain and
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TABLE 1: Parameters Obtained from Least Square Fitting of Experimental χM9T Vs 9T Curve field (Oe)
g
J
zj′
χd
R2
10000 (bulk) 500 (nano) 1000 (nano) 5000 (nano) 20000 (nano)
2.11456 1.99298 2.02485 2.0 2.02485
-2.0686 -0.50725 -1.59734 -0.38616 -1.12082
0.72168 0.14366 1.2716 0.06698 0.5748
0 0.00042 -0.01018 -0.02902 -0.03628
0.99915 0.99988 0.99947 0.99895 0.9989
a weak ferromagnetic interaction is operative between the molecules the net magnetic moment per domain increases to 2.8 × 10-20 erg/Oe. In the case of nanorods, saturation magnetization and magnetic moment have been extracted from eq 4 as 10.9 emu/g and 3.3 × 10-20 erg/Oe, respectively. In this case also the domain size is found to be equal to the bulk value (1.5 nm). Therefore, in both cases, bulk as well as nano phase, the domain size is equal to 1.5 nm and each domain consists of 4 molecules in which a weak intermolecular ferromagnetic interaction is operative. This means the system behaves like a superparamagnetic with domain size equal to 1.5 nm. Usually, superparamagnetism originates due to magnetic anisotropy in materials.
Figure 3d shows the FC-ZFC curves for nanorods from which it is seen that the blocking temperature is outside the measured temperature range (below 2 K). This is due to the fact that in our complex, the ferromagnetic interaction is very weak giving rise to a much smaller value of anisotropy energy barrier. This is also evident from panels a and b of Figure 3 in which a superparamagnetic-to-paramagnetic transition is noticed only with a slight increase in temperature from 2 to 10 K. At 10 K, the behavior is paramagnetic as the magnetization is linear with field. Due to the very small magnetic anisotropy energy barrier a slight increase in thermal energy completely destroys the magnetic ordering to turn the system paramagnetic. Therefore, at temperatures 10 K and above, the bulk sample has paramag-
Figure 3. Variation of magnetization with field at different temperatures for (a) bulk crystal and (b) nanorods. (c) Variation of magnetization with field at 2 K for both bulk and nanorods. The solid line represents theoretical curve obtained from eq 4. (d) ZFC and FC curves for nanorods at 500 Oe.
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Figure 4. Variation of molar susceptibilities with temperature for (a) nanorod with 0.05, 0.1, and 2 T and (b) bulk at 1 T.
netic behavior; however, in the case of nanorods, a diamagnetic transition with a giant negative moment value is observed at higher temperatures, which depend on the applied field. To understand this effect, we have included a diamagnetic term χd to eq 3 and temperature-dependent magnetic susceptibility has been investigated at different fields as shown in Figure 4a. It is seen that at low magnetic field (500 Oe) the magnetization curve is similar to that of bulk as shown in Figure 4b and exhibits no diamagnetic transition. At higher magnetic fields, viz. 0.1 and 2 T, susceptibility values become negative after temperatures at around 125 and 25 K respectively. However, in the case of bulk sample the susceptibility value remains positive throughout the whole temperature range even at 2 T. This field-induced diamagnetism has been reported in conjugated polymers and other systems like TaS2 and is attributed to the charge delocalization.30,31 In the case of conjugated polymers polarons are usually the charge carrier and they behave like a paramagnetic system; however, at higher doping level bipolarons are formed and under the application of magnetic field the system behaves like a diamagnetic system due to charge delocalization. Panels a and b of Figure 5 show the TEM micrographs of a typical nanorod and its high resolution lattice image from which it is seen that the distance between two lattice planes has been calculated to 4.67 Å, which corresponds to (-1-2-2) planes and making an angle around ∼46° with the nanotube axis. This means that within the nanochannel there is a preferential growth along the (-1-2-2) direction that makes an angle of ∼46° with the nanotube axis. Figure 5c gives the 3D crystal structure where the (-1-2-2) plane is shown by a white solid line, which makes an angle of ∼45° with the c-axis. This means that the c-axis is almost along the nanotube axis. From this 3D crystal structure it is seen that there are strong hydrogen bonding interactions: one between the hydrogen atom of the µ3-OH group and an oxygen atom (O) of a perchlorate ion where donor-acceptor distance is 3.094 Å and extended almost along the c-axis and another one between a hydrogen atom of a water molecule and an oxygen atom of the other perchlorate ion with donor-acceptor distance 3.065 Å and extended perpendicular to the c-axis. Due to preferential growth in these nanorods along the c-axis (axis of nanochannel), out of two types of hydrogen bonds only that along the c-axis is predominant. To understand the charge delocalization in hydrogen bonds, FTIR spectra have been
Figure 5. (a) TEM micrograph of a single nanorod. (b) High resolution lattice image showing the crystalline phase. (c) Molecular packing diagram of complex showing (-1-2-2) plane and H-bonds.
investigated in both nanorods and bulk samples as shown in Figure 6. In the bulk sample, the appearance of a broad band centering at 3523 cm-1 due to the stretching vibration of the -OH group substantiates the involvement of the -OH group in H-bonding. In the case of nanorods this broad band increases in intensity and shifts toward the lower wavenumber region
Hydroxo-Bridged Trinuclear Cu(II) Complex Nanorods
Figure 6. FTIR spectrum showing increasing peak intensity of H-bonds at the nano phase compare to bulk.
(3444 cm-1). This is due to the fact that the growth of the crystal is restricted to one dimension inside the nanopore. If onedimensional growth of the unit cell occurs along the c-axis, the concentration of H-bonds increases compared to that of the 3D crystal due to H-bonds connected between unit cells along the c-axis only while in the other two directions, i.e. along the aand b-axis, there are no such H-bonds connected between unit cells. Therefore, due to preferential growth, concentration, and stability of H-bonds increases, as a result, the band intensity increases and shifts toward lower energy.
J. Phys. Chem. C, Vol. 114, No. 27, 2010 11727 Under the application of magnetic field these hydrogen bonds are stable and charge delocalization occurs along the H-bonds. Such type of charge delocalization in H-bonds under the application of magnetic field has also been reported earlier by other workers.32 This charge delocalization along the hydrogen bonds causes diamagnetism in case of nanorods. Therefore, in the case of nanorods there is a competition between the magnetic interaction of three Cu centers and diamagnetism due to field induced charge delocalization across the H-bonds along the c-axis to control the overall magnetic response. At the transition temperature, diamagnetic contribution due to charge delocalization dominates over magnetic interaction between CuII centers and with increasing magnetic field, transition temperature decreases as charge delocalization across the H-bond increases, as a result diamagnetic contribution dominates over magnetic interaction of Cu centers at a comparatively lower temperature. To understand charge delocalization across H-bonds and their interaction we have therefore investigated temperature-dependent dielectric spectra over the frequency range 100 Hz to 1 MHz. Panels a and b of Figure 7 show the dielectric modulus spectra for bulk sample and nanorods, respectively, in which solid lines represent theoretical curves obtained by modulus spectra analysis33 and points are the experimental data. We have analyzed
Figure 7. Variation of dielectric modulus spectra with frequency of (a) bulk and (b) nano phase of MOF. The solid lines represent theoretical curves as obtained from eqs 10 and 11. (c) Variation of relaxation time with inverse of temperature to estimate the ac activation energy.
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Giri and Saha dielectric primittivity with frequency for both bulk and nanorods from which it is seen that for nanorods the permittivity value increases to a very high value due to H-bonds acting along the c-axis, the direction along which the dielectric measurements are carried out. Conclusion
Figure 8. Plot of permittivity as a function of frequency in bulk (triangular points) and nano (square points) phase.
TABLE 2: gi and λi Parameters Obtained from Analysis of Modulus Spectra in the Case of Bulk and Nanorod Sample s bulk
nanorod
gi
λi
gi
λi
0.009 0.065 0.002 0.0025 1 × 10-3 0.06 0.1069 0.1 0.139 0.405 0.079 0.024 1 × 10-3 0.01
46.5 44.5 25.5 10.5 0.4 0.37 15.6 0.51 0.45 1.7 0.32 5.5 4.6 0.21
0.0021 0.0073 0.0195 0.0485 0.0285 0.0955 0.155 0.305 0.184 0.082 0.065 0.006 0.0019 1 × 10-4
2 1.9 0.038 4.51 260 3.1 19.1 1.17 0.165 0.39 0.0115 5.5 0.007 0.01
Materials and Methods
dielectric modulus spectra using stretched nonexponential decay function33 given by eq 6.
F(t) ≈ exp[-(t/τ0)β]
Single crystalline nanorods of nonpolar trinuclear Cu complex with partial cubane structure have been synthesized in nanoporous anodic alumina template. Magnetic interaction has been investigated over the temperature range from 2 to 300 K and compared with the polycrystalline bulk sample. It is seen that both bulk as well as nanorods show superparamagnetic interaction at 2 K; however, at higher temperature a marked change is observed in magnetic behavior. In the case of nanorods, a giant diamagnetic transition which depends on magnetic field is observed. This effect of giant diamagnetism is attributed to the charge delocalization across H-bonds acting along the c-axis (nanochannel axis), which makes an angle of ∼45° with the crystal growth direction.
(6)
where the exponent β gives the distribution of relaxation times and lies between 0 and 1. A smaller value of β represents larger distribution and vice versa. β values are obtained for bulk and nanorods as 0.73 and 0.42, respectively. From the dielectric modulus spectra it is seen that the relaxation phenomenon in this complex does not arise due to migration/localized motion of the heavy ions like ClO4-, rather it originates due to localized motion of the permanent dipoles created by H-bonds. The DC resistivity of the sample is ∼109 ohm · cm, which means there is no long-range migration of any anion/cation; rather the ac conductivity originates due to localized motion of H-bonds. The activation energy calculated from the temperature-dependent modulus peak shift shown in Figure 7c is 1.18 eV, which agrees well with H-bonded charge transport.34 Usually, the activation energy for protonic conduction varies between 0.5 and 1.2 eV depending upon the nature of the hydrogen bonding network as well as the environment of the hydrogen ion. In our case the activation energy is a little bit higher as it does not form network structure throughout the crystal. From the microscopic theory of relaxation,35,36 it was pointed out that the value of β measures the strength of the cooperative nature of the relaxing units, which means a β value equal to 1 indicates a completely isolated unit giving rise to Debye-type relaxation whereas a low β value arises due to strongly interacting relaxing units with a cooperative nature of relaxation (non-Debye type). In the case of our nanorods, a low β value (0.42) indicates the cooperative nature of H-bonds and low dimensionality of the system. Figure 8 shows the variation of
To synthesize hydroxo-bridged trinuclear Cu complex8 with partial cubane (Cu3O4) core ([Cu3(µ3-OH)(SE)3](ClO4)2 · 0.5H2O, with SE being the condensed form of N,N-dimethylethylenediamine and Salicylaldehyde), N,N-dimethylethylenediamine (0.5 mmol) in methanol (10 mL) is added to salicylaldehyde (0.5 mmol). The mixed solution is refluxed for 1 h and cooled to room temperature and then Cu(ClO4)2 · 6H2O (0.5 mmol) in 3 mL of methanol is added slowly. In the resulting solution 0.5 mmol of triethylamine is added, then the solution is filtered and kept overnight to produce deep green poly crystals of Cu complex. To synthesize nanorods the crystals are dissolved in a minimum volume of methanol. Nanoporous alumina template (100 nm pore diameter) heated at 120 °C is immediately dipped into the solution of methanol. Due to capillary action the solution entered into nanochannels and recrystallize in the form of single crystalline nanorods. After complete formation of nanorods, the template surfaces are cleaned with methanol and kept under vacuum desiccators for drying. Magnetic measurements have been carried out over the temperature range from 2 to 300 K, using Quantum Design MPMS XL (ever cool model). Electrical impedance analysis was carried out using Agilent E 4980A precision LCR Meter. IR spectroscopic measurements were done with a FTIR-8400S (Simadzu, America) FTIR spectrometer and transmission electron microscopy has been investigated with a JEOL 2010 transmission electron microscope. For this study template is ground in a mortar and nanorods are separated with use of ultrasonic vibration. The suspension containing the nanorods is poured on a Cu grid and dried in a vacuum oven. Powder diffraction of bulk polycrystalline sample has been carried out at room temperature, using SEIFERT XRD 3000P. Analysis of Modulus Spectra For analyzing the modulus spectra of the present system, we consider the stretched exponential relaxation function F(t) can be written as
F(t) ≈ exp[-(t/τ0)β]
(6a)
In the frequency domain the electric modulus M*(ω) is given by
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J. Phys. Chem. C, Vol. 114, No. 27, 2010 11729
M*(ω) ) Ms[1 - N*(ω)]
(7)
Ms ) 1/εR
(8)
where
and
(
m
N*(ω) )
∑ gi 1 +1jωτi i)1
)
(9)
The τi values in eq 9 are multiples of τp. These are selected to cover a suitable range of (t/τR) in which F(t) has an appreciable time dependence. The gi and λi (≡τi/ τp) values are determined by a linear least-squares fitting of experimental M′(ω) and M′′(ω) values respectively with the theoretical expressions as given below:
[
m
M'(ω) ) Ms 1 M''(ω) ) Ms
[
g
∑ 1 + ω2i λ 2τ 2 i)1
m
i
g λ ωτ
∑ 1 +i ωi 2λ p2τ 2 i)1
i
p
p
]
]
(10)
(11)
In the present analysis 14 terms have been used and a typical set of gi and λi values obtained in the case of bulk and nanorod sample is shown in Table 2. Acknowledgment. S.G. acknowledges CSIR for awarding a Fellowship [Grant No. 09/080 (0639)/2009-EMR-I] and S.K.S. acknowledges DST Unit on Nanoscience and Centre for Nanotechnology for supporting infrastructural facilities. The authors also acknowledge A. Ghosh, Department of Chemistry, University of Calcutta, for helpful discussions. Supporting Information Available: The molar susceptibilities fitted by eq 3 in the case of nano sample at different fields. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Ferlay, S.; Mallah, T.; Ouahes, R.; Veillet, P.; Verdaguer, M. Inorg. Chem. 1999, 38, 229. (2) Hatlevik, O.; Buschmann, W. E.; Zhang, J.; Manson, J. L.; Miller, J. S. AdV. Mater. 1999, 11, 914. (3) Miller, J. S. Inorg. Chem. 2000, 39, 4392. (4) Rentschler, E.; Gatteschi, D.; Cornia, A.; Fabretti, A. C.; Barra, A. L.; Shchegolikhina, O. I.; Zhdanov, A. A. Inorg. Chem. 1996, 35, 4427. (5) Ruiz-Perez, C.; Rodriguez-Martin, Y.; Hernandez-Molina, M.; Delgado, F. S.; Pasan, J.; Sanchiz, J.; Lloret, F.; Julve, M. Polyhedron 2003, 22, 2111. (6) Vaz, M. G. F.; Pinheiro, L. M. M.; Stumpf, H. O.; Alcantara, A. F. C.; Golhen, S.; Ouahab, L.; Cador, O.; Mathoniere, C.; Kahn, O. Chem.sEur. J. 1999, 5, 1486.
(7) Kahn, O. Molecular Magnetism; VCH Publishers: New York, 1993. (8) Bian, H. D.; Xu, J. Y.; Gu, W.; Yan, S. P.; Cheng, P.; Liao, D. Z.; Jiang, Z. H. Polyhedron 2003, 22, 2927. (9) Glaser, T.; Heidemeier, M.; Strautmann, J. B. H.; Bogge, H.; Stammler, A.; Krickemeyer, E.; Huenerbein, R.; Grimme, S.; Bothe, E.; Bill, E. Chem.sEur. J. 2007, 13, 9191. (10) Ruiz, R.; Julve, M.; Faus, J.; Lloret, F.; Munoz, M. C.; Journaux, Y.; Bois, C. Inorg. Chem. 1997, 36, 3434. (11) Tudor, V.; Kravtsov, V. C.; Julve, M.; Lloret, F.; Simonov, Y. A.; Averkiev, B. B.; Andruh, M. Inorg. Chim. Acta 2005, 358, 2066. (12) Albada, G. A.V.; Mutikainen, I.; Roubeau, O. S.; Turpeinen, U.; Reedijk, J. Eur. J. Inorg. Chem. 2000, 10, 2179. (13) Sarkar, B.; Ray, M. S.; Li, Y. Z.; Song, Y.; Figuerola, A.; Ruiz, E.; Cirera, J.; Cano, J.; Ghosh, A. Chem.sEur. J. 2007, 13, 9297. (14) Maspoch, D.; Ruiz-Molina, D.; Wurst, K.; Domingo, N.; Cavallini, M.; Biscarini, F.; Tejada, J.; Rovira, C.; Veciana, J. Nat. Mater. 2003, 2, 190. (15) Suh, M. P.; Han, M. Y.; Lee, J. H.; Min, S. K.; Hyeon, C. J. Am. Chem. Soc. 1998, 120, 3819. (16) Dul, M. C.; Ottenwaelder, X.; Pardo, E.; Lescouezec, R.; Journaux, Y.; Chamoreau, L. M.; Ruiz-Garcia, R.; Cano, J.; Julve, M.; Lloret, F. Inorg. Chem. 2009, 48, 5244. (17) Ray, M. S.; Chattopadhyay, S.; Drew, M. G. B.; Figuerola, A.; Ribas, J.; Diaz, C.; Ghosh, A. Eur. J. Inorg. Chem. 2005, 22, 4562. (18) Das, O.; Zangrando, E.; Paine, T. K. Inorg. Chim. Acta 2009, 362, 3617. (19) Mukherjee, P.; Drew, M. G. B.; Estrader, M.; Diaz, C.; Ghosh, A. Inorg. Chim. Acta 2008, 361, 161. (20) Yoon, J.; Mirica, L. M.; Stack, T. D. P.; Solomon, E. I. J. Am. Chem. Soc. 2004, 126, 12586. (21) Sarkar, B.; Ray, M. S.; Drew, M. G. B.; Figuerola, A.; Diaz, C.; Ghosh, A. Polyhedron 2006, 25, 3084. (22) Qiu, L. G.; Li, Z. Q.; Wu, Y.; Wang, W.; Xu, T.; Jiang, X. Chem. Commun. 2008, 11, 3642. (23) Taylor, K. M. L.; Jin, A.; Lin, W. Angew. Chem., Int. Ed. 2008, 47, 7722. (24) Rieter, W. J.; Taylor, K. M. L.; An, H.; Lin, W; Lin, W. J. Am. Chem. Soc. 2006, 128, 9024. (25) Imaz, I.; Maspoch, D.; Blanco, C. R.; Falcon, J. M. P.; Campo, J.; Ruiz-Molina, D. Angew. Chem., Int. Ed. 2008, 47, 1857. (26) Guari, Y.; Larionova, J.; Molvinger, K.; Folch, B.; Gue´rin, C. Chem. Commun. 2008, 24, 2613. (27) Sun, H.; Shi, H.; Zhao, F.; Qi, L.; Gao, S. Chem. Commun. 2005, 34, 4339. (28) Clavel, G.; Larionova, J.; Guari, Y.; Gue´rin, C. Chem.sEur. J. 2006, 12, 3798. (29) Kommareddi, N. S.; Tata, M.; John, V. T.; McPherson, G. L.; Herman, M. F.; Lee, Y. S.; O’Connor, C. J.; Akkara, J. A.; Kaplan, D. L. Chem. Mater. 1996, 8, 801. (30) Long, Y.; Chen, Z.; Shen, J.; Zhang, Z.; Zhang, L.; Xiao, H.; Wan, M.; Duvail, J. L. J. Phys. Chem. B 2006, 110, 23228. (31) Dai, S.; Yu, C.; Li, D.; Shen, Z.; Fang, S. Phys. ReV. B 1995, 52, 1578. (32) Hosoda, H.; Mori, H.; Sogoshi, N.; Nagasawa, A.; Nakabayashi, S. J. Phys. Chem. A 2004, 108, 1461. (33) Bhattacharyya, S.; Saha, S. K.; Chakravotry, D. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 1193. (34) Behrens, H.; Kappes, R.; Heitjans, P. J. Non-Cryst. Solids 2002, 306, 271. (35) Rajagopal, A. K.; Ngai, K. L.; Rendell, R. W.; Teitler, S. J. Stat. Phys. 1983, 30, 285. (36) Ngai, K. L.; Strom, U. Phys ReV B. 1983, 27, 6031.
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