Article pubs.acs.org/Langmuir
Hybrid Fibrillar Xerogels with Unusual Magnetic Properties Athmane Boulaoued,† Jean-Louis Bantignies,‡ Rozenn Le Parc,‡ Christophe Goze-Bac,‡ Philippe Mésini,† Thi-Thanh-Tam Nguyen,†,§ Abdelaziz Al Ouahabi,† Pierre Lutz,† and Jean-Michel Guenet*,† †
Institut Charles Sadron, CNRS UPR22-Université de Strasbourg, 23 rue du Loess, F-67034 Strasbourg Cedex 02, France Laboratoire Charles Coulomb (L2C), UMR 5221 CNRS-Université de Montpellier, 34095 Montpellier, France
‡
S Supporting Information *
ABSTRACT: We report on the preparation of a hybrid nanomaterial made up of 1D filaments of an antiferromagnetic self-assembling bicopper complex encapsulated in polymer nanofibrils. The encapsulation process is achieved through the heterogeneous nucleation of the growth of polymer fibrils obtained by thermoreversible gelation as shown by calorimetry experiments. Neutron scattering experiments confirm that the filaments of a bicopper complex retain their 1D character after encapsulation in the fibrils. Superconducting quantum interference device experiments show that the bicopper complex, originally in the gapped spin state in the 3D bulk mesophase, displays a gapless behavior once encapsulated. Extended absorption fine structure and infrared results further highlight the difference in the molecular arrangement of the bicopper complex between the bulk mesophase and the encapsulated state, which may account for the magnetic behavior. This material, which is largely disordered, differs totally from the usual magnetic systems where this effect is observed only on highly crystalline systems with long-range order. Also, this hybrid material is very easy to prepare from its basic constituents and can be further processed in many ways.
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nucleation process using an “impurity”, which allows the system to overcome the surface energy “barrier” and so to trigger the growth of a phase. As a result, the “impurity” is usually located in the center of the phase it has helped to create. In isotactic polystyrene (iPS) thermoreversible gels,12,13 the hysteresis observed between the gel formation and the gel melting temperature suggests the occurrence of a homogenous nucleation process, so that an appropriate impurity should allow one to increase the gelation temperature. These gels exhibit a fibrillar morphology with a rather large micrometer mesh size where fibrils possess cross sections of a few nanometers. Filaments of the bicopper complex (designated as CuS8E in what follows) were contemplated for playing the role of an impurity and eventually for being jacketed by polymer chains so as to produce nanocables (Figure 1). As the filaments start growing at a higher temperature than the gelation temperature of iPS in a common solvent,12 namely, trans-decahydronaphthalene, heterogeneous nucleation may occur. This was effectively observed with a racemic mixture of the bicopper complex (CuS8ER), and magnetic properties were observed to be at variance with those measured in the bulk mesophase.11 In this paper, we report a series of experiments carried out by
INTRODUCTION Spins in low dimensions are of great interest as they are responsible for exotic phenomena in the realm of magnetism, such as quantum Hall effect and high Tc superconductivity.1−4 The occurrence of high Tc superconductivity in cuprate systems possibly due to the low dimensionality of the copper atom arrangements5−7 prompted us to prepare hybrid nanomaterials made up of filaments of a bicopper complex (Figure 1) embedded in a fibrillar polymer matrix, where fibrils are randomly dispersed objects forming a three-dimensional network. These one-dimensional filaments form in organic solvents using a self-assembling process8,9 (see Figure 1). These filaments are not stable as the interaction between molecules is near kT, so that after a few hours, they eventually transform into 3D mesophases.8,9 We previously showed that these filaments could be stabilized by encapsulation in polymer fibrils from a thermoreversible gel through a heterogeneous nucleation process.10,11 The occurrence of an organized phase in a homogenous system, such as crystallization, takes place through a nucleation and growth process. In a very pure component, the nucleation process, designated as homogenous nucleation, arises from the production of nuclei large enough to overcome the surface energy “barrier”. As a result, the crystallization/ formation temperature is always lower than the melting temperature. Conversely, one can dramatically increase the crystallization/formation temperature through a heterogeneous © XXXX American Chemical Society
Received: September 29, 2016 Revised: November 9, 2016 Published: November 14, 2016 A
DOI: 10.1021/acs.langmuir.6b03572 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. (a) Bicopper complex molecule, (b) the way the molecules pile up (the copper atom of one molecule interacts with the oxygen of the next molecules as highlighted by dashed lines), (c) fibrillar morphology of the network, (d) schematic representation of the encapsulation of a filament within a polymer fibril of the thermoreversible network (in the zoomed-in image: pink = bicopper complex filament and gray = polymer chains). of about 10%. Neutron detection and counting were achieved using a built-in two-dimensional sensitive detector that is composed of 64 × 64 cells (details are available on http://www.ill.eu). By varying the sample−detector distance (3, 10, and 34 m), the accessible q-range was 0.05 < q (nm−1) < 2.25, where q = (4π/λ) sin(θ/2), where θ is the scattering angle. The position sensitive detector was calibrated by light water. The absolute intensity, Iabs(q), was obtained after the usual signal processing whether one is dealing with an essentially deuterated sample (in TdecaD) or with an essentially hydrogenous sample (TdecaH) and using the water cross section dΣ/dΩ = 0.985 cm−1 determined experimentally for D11 at λm = 0.6 nm and the contrast factor for the labeled species (see ref 19 for further details). SQUID. The magnetic susceptibility was determined in a temperature range from 2 to 350 K using a SQUID MPMS-XL7T magnetometer device from Quantum design (superconducting quantum interference device). We used a dual-channel mode and a zero-field cooled procedure at a magnetic field of 1 Tesla. Data were corrected from diamagnetic contributions of the iPS matrix and the sample holder (measured using SQUID) and of CuS8E molecules (calculated using Pascal’s tables). Further details are given in the Supporting Information. EXAFS. EXAFS measurements were carried out using synchrotron radiation of the XAFS beamline at Elettra Synchrotron (Basovizza, Italy).20 The storage ring was operating at 2.0 GeV in the top-up mode with a typical current of 300 mA. Experiments at the Cu K-edge were performed using a fixed exit monochromator equipped with a pair of Si(111) crystals. Harmonics were rejected by using the cutoff of the reflectivity of the platinum mirror placed at 3 mrad with respect to the beam upstream of the monochromator and by detuning the second crystal of the monochromator by 30% of the maximum. The data were recorded in the transmission mode at low temperature (14 K) using N2- and Ar-filled ion chambers for measuring the incident (I0) and the transmitted (I1) beams, respectively. The sample was mounted on a liquid He cryostat. A thermocouple (chromel−gold/iron 0.07%) was fixed to the cryostat cold finger, at a distance of about 1 mm from the pellet, to monitor its actual temperature. The temperature was varied in the interval from 14 to 300 K, at 25 or 50 K steps. The spectra were recorded at the lowest temperature. The photon energy was calibrated from the spectrum of a copper foil at each scan, assigning 8979 eV to the pre-edge peak. This allows for a continuous monitoring of the energy during the consecutive scans. No energy drifts of the monochromator were observed during the experiments. The samples were dried gel (aerogel) of the CuS8E + iPS hybrid, and the solid pellet of CuS8E powder prepared by mixing the enantiomer with a cellulose filler (concentration), providing an edge jump μx ≈ 1.1 at the Cu K-edge. The spectra were recorded using sampling steps of 0.2 eV for the XANES domain and a variable sample step, giving Δkmax = 0.05 Å−1, for the EXAFS domain. A maximum integration time of 5.0 s per point was used in both cases. The energy resolution was approximately 0.5 eV.
means of different techniques on an enantiomer form of CuS8E (CuS8EE), that confirms not only previous results but also throws some light on the unusual variation in the magnetic susceptibility χM with respect to T, particularly, the way bicopper complex molecules pile up and their geometry is altered in the encapsulated state. Indeed, unlike the bulk mesophase where the complex intramolecular copper spins are coupled antiferromagnetically and χM falls to zero for T ≈ 0 K, after encapsulation χM is never equal to zero near 0 K, thereby, CuS8E molecules exhibit intermolecular interactions as well.
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EXPERIMENTAL SECTION
Sample Preparation. The bicopper complex was prepared from the (−)-(R)-EHA enantiomer according to a method described in the literature.14,15 The (−)-(R)-EHA enantiomer was obtained by resolving racemic 2-ethylhexanoic acid using a method described in the literature.16,17 Further details are available in the Supporting Information. Hydrogenous (iPSH) and deuterated (iPSD) iPSs were synthesized through a method described in ref 18. Their weight-average molecular weight is MwiPSH = 4.8 × 105 with MwiPSH/MniPSH = 3.6 and MwiPSD = 6.6 × 105 with MwiPSD/MniPSD = 5.4, as determined using size exclusion chromatography. Samples were prepared by heating the mixtures of iPS, enantiomer bicopper complex, and trans-decahydronaphthalene at 130 °C until a homogenous solution was obtained. This solution was then quenched at 0 °C for a minimum of 30 min to obtain a gel (thermoreversible). For the neutron scattering experiments, two types of samples were used: iPSD + TdecaH + CuS8E, where TdecaH is a mixture of deuterated trans-decahydrophthalene and hydrogenous trans-decahydrophthalene in the proportion of 8:92 (v/v), to match the coherent signal of the bicopper complex and iPSD + TdecaD + CuS8E, where TdecaD is a mixture of deuterated trans-decahydrophthalene and hydrogenous trans-decahydrophthalene in the proportion of 91:9 (v/ v), to match the coherent signal of the deuterated polymer. The neutron scattering samples were prepared in quartz cells as above. Samples for extended X-ray absorption fine structure (EXAFS) experiments and the superconducting quantum interference device (SQUID) measurement were prepared again as above but were further dried by supercritical CO2 extraction. Differential Scanning Calorimetry. Pieces of gels were transferred into stainless steel differential scanning calorimetry (DSC) pans that were hermetically sealed. Thermal analysis was carried out using a Perkin-Elmer DSC 8500 instrument. Before any measurement, gels were melted in the DSC pan to erase all structures. The samples were then scanned from 120 to −20 °C at different cooling rates: 15, 10, 5, and 2.5 K/min. Small-Angle Neutron Scattering. The experiments were performed on a D11 camera located at the Institut Laue-Langevin (ILL, Grenoble, France). A wavelength of λm = 0.6 nm was used with a wavelength distribution characterized by a full-width at half-maximum B
DOI: 10.1021/acs.langmuir.6b03572 Langmuir XXXX, XXX, XXX−XXX
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Langmuir More details on the X-ray absorption spectroscopy analysis are available in the Supporting Information. Infrared Spectroscopy. Attenuated total reflectance (ATR) spectra were recorded with a Bruker Tensor 27 Fourier transform infrared spectrometer equipped with a room-temperature-deuterated, L-alanine-doped triglycine sulfate (DLaTGS) detector. An ATR accessory using a MVP-PRO Star diamond (monolithic diamond internal reflection element) (Harrick Scientific Products Inc., NY, USA) was used with a 2 mm aperture of the spectrometer and an angle of incidence θ of 45°. The spectra were obtained from the acquisition of 128 scans at 4 cm−1 resolution from 7000 to 400 cm−1 using a Blackman−Harris-3-term apodization. The spectra were not smoothed, and no baseline correction was applied.
2
q Iabs(q) = 4πqCCuS8μfil ×
J12 (qrfil) q2rfil 2
(1)
where J1 is a Bessel function of the first kind and order 1, rfil is the cross section radius of the cylinder, and μfil is the mass per unit length. The values found through fitting for these two parameters are rfil = 0.7 ± 0.1 nm and μfil = 1300 ± 100 g·mol−1·nm−1 clearly indicate that the self-assembled structure of the bicopper complex consists of rigid and straight filaments with one molecule per cross section. This outcome together with the calorimetry results is a strong support to the existence of encapsulated filaments. For bicopper concentrations higher than CCuS8E = 0.055 g/ cm3, the absolute scattered intensity has significantly increased in the low-q domain. This behavior can be fitted by considering two different entities: the encapsulated f ilaments and a f ibrillar network of the nonencapsulated, associated filaments (see Figure 2). The scattered intensity plotted by a Kratky plot can then be expressed through
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RESULTS AND DISCUSSION Calorimetry Experiments. Calorimetry experiments involve the determination of the minimum of the gel formation exotherms for different bicopper complex contents and at different cooling rates (results are presented in the Supporting Information). Extrapolation to zero cooling rates gives a discrepancy of about 10 °C between the pure polymer gel and the gel containing 0.057 g/cm3 of the bicopper complex. This result is in agreement with previous findings obtained with the racemic mixture.10 In addition, the gel formation enthalpy and the gel melting temperature remain virtually constant. These outcomes are consistent with the occurrence of a heterogeneous nucleation effect: filaments of the bicopper complex nucleate the growth of gel fibrils. Neutron Scattering. The neutron scattering experiments have been performed on samples with two differing solvent isotopic labeling so as to determine the structure of the bicopper complex and of the polymer, within the ternary system. Bicopper Complex Structure in the Ternary System. In the case of iPSD + TdecaD + CuS8E, where TdecaD is a 91:9 (v/v) mixture of deuterated trans-decahydrophthalene and hydrogenous trans-decahydrophthalene, the neutron signal is related only to the bicopper complex. Figure 2 shows the results of differing bicopper complex concentrations while maintaining the polymer concentration
⎡ J 2 (qrfil) J 2 (qrfib) q2Iabs(q) = 4πqCCuS8⎢Xμfil × 1 2 2 + Yμfib × 1 2 2 ⎢⎣ q rfil q rfib + Zμjunc ×
J12 (qrjunc) ⎤ ⎥ q2rjunc 2 ⎥⎦
(2)
where X, Y, and Z are the weight fractions of the encapsulated filaments, the bicopper complex fibers and the fiber network junctions, with X + Y + Z = 1, and μ and r with the appropriate subscripts represent the mass per unit length and the cross section radius of the different types of cylinders. Note that this approach has already been successfully used for syndiotactic poly(methyl methacrylate) gels.22 As a first approximation, one has μfib ≈ μfil
rfib 2 rfil 2
and μjunc ≈ μfil
rjunc 2 rfil 2
(3)
where μfib and μjunc are the mass per unit length of the fibers and of the network junctions, respectively. The scattered intensity then reads ⎡ J 2 (qrfil) r 2 q2Iabs(q) ≈ 4πqμfil CCuS8⎢X × 1 2 2 + Y fib2 ⎢⎣ q rfil rfil ×
J12 (qrfib) q2rfib 2
+Z
rjunc 2 rfil 2
×
J12 (qrjunc) ⎤ ⎥ q2rjunc 2 ⎥⎦
(4)
The fit is first performed so as to match the form of the scattering curve (see Figure 2). This gives the following values rfil = 0.67 nm, rfib = 4 nm, and rjunc = 10 nm. The value of μfil = 1300 g/mol·nm is obtained from the behavior at large q chiefly related to the encapsulated filaments. This eventually gives μfib = 7500 g·mol−1·nm−1 and μjunc = 46 800 g·mol−1·nm−1, which in turn yields X = 0.99, Y = 0.037, and Z = 0.022 (the sum is slightly larger than one but this stands within experimental uncertainties and also may arise from the approximation in relation (3)). Although the intensity arising from the bicopper complex network is important, especially in the low-q range, it turns out that the fraction of the CuS8E network remains at a relatively low level, whereas the fraction of encapsulated
Figure 2. Neutron scattering curves related to the bicopper complex filaments plotted by a Kratky plot (q2Iabs(q) vs q). Upper curve CCuS8E = 0.055 g/cm3; lower curve CCuS8E = 0.038 g/cm3. Solid lines represent the theoretical fits, and dotted lines represent the scattering functions for rfib = 4 nm and rjunc = 10 nm (see text for details).
virtually constant (0.04 g/cm3 ± 0.002). For bicopper concentrations lower than CCuS8E = 0.055 g/cm3, the absolute scattered intensity can be fitted by means of a solid cylindrical structure of length Lc with Lc ≫ rfil and for qLc ≫ 121 C
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Langmuir filaments is still very high. Note that the fit could be improved by introducing a slight cross section dispersity. This would, however, not change the conclusions. The difference between the mass per unit length of CuS8E filaments in the binary solutions (μfil = 2000 g·mol−1·nm−1) and in the ternary system (μfil = 1250 g·mol−1·nm−1), that is well beyond experimental uncertainties, suggests a different molecular stacking. Because the molar mass of a CuS8E molecule is 700 g·mol−1, one ends up with ∼1.8 CuS8E molecules/nm in the case of the hybrid system against ∼2.8 CuS8E molecules/nm in the binary solution. As can be found below, these conclusions are backed up by the EXAFS study of the molecular structure. Polymer Structure in the Ternary System. In the case of iPSD + TdecaH + CuS8E, where TdecaH is a 8:92 (v/v) mixture of deuterated trans-decahydrophthalene and hydrogenous trans-decahydrophthalene, the neutron signal is related only to the polymer.10 The results are shown in Figure 3. As
cross-sectional dimension, is quite consistent with the occurrence of a heterogeneous nucleation effect (Table 1). Table 1. Variation in Rmax as a Function of Bicopper Complex Concentration CCuS8E (g/cm3)
rmax (nm)
0 0.021 0.0395 0.062
11.2 10.0 7.5 6.6
Magnetic Properties: SQUID. The magnetic susceptibility per Cu(II) ion of the bulk mesophase and the encapsulated filaments display the behavior as reported in Figure 4. As was
Figure 4. Variation in the magnetic susceptibility (emu/Cu(II) ion) as a function of temperature (Kelvin). (○) represents the encapsulated state of CuS8E; (□) represents for the bulk mesophase of CuS8E. Lines correspond to fits as described in the text. Figure 3. Scattering curve plotted by a Kratky plot (q2Iabs(q) vs q) related to the iPS structure with Cpol = 0.04 ± 0.002 g/cm3, ● = CCuS8E = 0 g/cm3, ○ = CCuS8E = 0.021 g/cm3, and × = CCuS8E = 0.0395 g/cm3. The solid line shows that the intensity is still related to the polystyrene chain helical structure for the largest values of q (fit with equations of the type in relation (1) with rhelix = 0.45 nm). Inset, the same data plotted as q4Iabs(q) vs q in the transitional range (rmax−1 < q < rmax−1). Straight line represents a fit with eq 6 (see text for details).
already reported, the behavior in the bulk mesophase state can be reproduced by the Bleaney−Bowers equation24 (based on the Heisenberg Hamiltonian: H = −2J × S1 × S2) derived for dimeric copper ions coupled by superexchange via the carboxyl ligands. Taking into account the presence of a fraction Z of noncoupled copper(II) ions (noted thereafter monocopper) using the Curie law, the slight upturn at low temperature could be reproduced. Therefore, the total model describing the copper spins in the CuS8E powder is
can be seen, the scattered intensity decreases significantly in the low-q range. Because we are dealing with a fibrillar polymer network with fibers of cross sections in the range of 2−20 nm in diameter, the low-q range is therefore sensitive to the evolution of the cross section size. Consequently, the average size decreases with increasing bicopper fraction, an outcome in agreement with a nucleation process (the higher the number of nuclei, the smaller the resulting structures). A fit is achieved using a model derived for thermoreversible networks characterized by an array of cross-section-polydispersed fibers with a distribution function, w(r), of the type23
w(r ) ≈ r
−λ
χCuS8 = (1 − Z)
+ Z·
⎡ KBT ⎢3 + exp ⎣
( )⎤⎥⎦ − 2J KBT
NA ·g p2μB 2 S(S + 1) 3KBT
(7)
where NA is Avogadro’s number, g and gp are the electron spin g-factors’ of the bicopper and the monocopper molecules, respectively. J is the coupling constant, μB is the Bohr’s magnetron, and S is the electron spin (S = 1/2). The best fit presented in Figure 4 yields 2J = −198 ± 5 cm−1 (corresponds to a spin gap of 138 K) and g = 2.22 (electron paramagnetic resonance experiments not shown here25 give g = 2.17). At low temperature, the ground state of the dimer spins is a singlet (Ms = 0). By increasing the temperature, the spins get excited to the triplet state (Ms = 1) separated from the ground state by a spin gap energy of 2J = −198 ± 5 cm−1, and a broad maximum susceptibility is reached at 180 K. Hence, CuS8E molecules in the powder state belong to the class of “spin gap” systems.
(5)
where the exponent is 0 < λ < 3 and is bracketed by two values of cross section radii rmax and rmin. In the transitional q range (rmax−1 < q < rmin−1), the scattered intensity reads for λ = 1 (a case that holds here) ⎡ 2 ⎤ q 4Iabs(q) ≈ ⎢πq − ⎥ rmax ⎦ ⎣
NA ·g 2 ·μB 2
(6)
4
Extrapolation to q Iabs(q) = 0 provides one with rmax (inset Figure 3), whose value decreases significantly with the increase in the bicopper content. This behavior, that is, a decrease in the D
DOI: 10.1021/acs.langmuir.6b03572 Langmuir XXXX, XXX, XXX−XXX
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Figure 5. (left) EXAFS spectra measured at the Cu K-edge (k3)χ(k) vs k at T = 30 K for the bulk mesophase (dashed line), and the encapsulated state (solid line). (right) FT modulus of the X-ray absorption spectra. (○) represents the bulk mesophase; (●) represents the encapsulated state. The calculated radial distributions for each atom are given as indicated. Note that in EXAFS the FT peak positions are always shifted to lower distance (in this case ∼0.05 nm) compared with the actual pair distance. The different Cu neighbor theoretical distances are given in the figure.
EXAFS. The X-ray absorption investigation involves the determination of the atomic environment of the copper atoms in the CuS8E bulk mesophase and in the CuS8E encapsulated state. The comparison of the EXAFS spectra measured at the Cu K-edge indicates a deep modification in the local environment of the copper after encapsulation (Figure 5a). The Fourier transform (FT) modulus of the EXAFS oscillations providing the pseudo radial atomic distribution around a copper atom is reported in Figure 5b. As was mentioned in the introduction, piling between two neighboring molecules occurs through a Cu → O interaction in the powder state.26 In the model of a polymeric copper−hexanoate complex,33 the copper site has the nearest-neighbor oxygen atoms at a distance of 0.194 nm (2 atoms), 0.197, and 0.202 nm. The next nearest neighbors of copper are oxygen atoms (at ∼0.221 nm intra and ∼0.303 nm next molecule), copper atoms (at ∼0.258 nm intra and ∼0.324 nm next molecule), and carbon atoms (between 0.280 and 0.293 nm and at ∼0.323 nm). The contributions from more remote atoms are not detected. Using these distances, the positions of the peak in the pseudo radial distributions of the CuS8E bulk mesophase can be theoretically reproduced. They agree with previous findings by Abied et al.26 In the encapsulated state, the main first FT peak, which is related to the nearest Cu−O spacing, is not affected by the encapsulation. The local structure parameters of this shell are similar before and after confinement (see Supporting Information). Conversely, a deep modification in the local arrangement is observed for atoms located in the following coordination shells. In particular, the peak corresponding to the distance that is related to the copper atom belonging to the same CuS8E molecule seems to have shifted to a larger value closer to that between copper atoms belonging to two neighboring CuS8E molecules. The EXAFS results therefore suggest the occurrence of a significant modification in the stacking of the CuS8E complexes in the encapsulated state. This outcome is consistent with the SANS results that point to a less dense piling of the encapsulated CuS8E molecules and the analysis of the magnetic properties described above. A possible explanation of this modification may be due to constraints imposed by the surrounding polymer chains to the CuS8E filaments up to the point of deforming the “cage” structure of the CuS8E molecule.34 The deformation of the cage may occur either through stretching along the Cu−Cu direction or a shearing parallel to the Cu−O4 planes. These two processes would certainly entail a mutual receding of the
Our EXAFS investigation reported below of CuS8E molecules in the powder state (columnar phase) and that carried out by Abied et al.26 on analogous copper acetate show that the intermolecular copper distance is about 0.33 nm, giving rise to a zigzag chain with molecules shifted from the axe of the chain. A similar behavior has been found for the Cu 2 (C5H12N2)2Cl4 compound by Chaboussant et al.27 The magnetic susceptibility in the encapsulated state differs markedly from spin-gapped systems as it never goes to zero and is much higher than that observed in the bulk mesophase. Theoretical and experimental studies28−31 have shown that in contrast to the case of “spin gapped” systems showing a zero susceptibility at low T, “gapless spin” systems have a finite magnetic susceptibility even at low T, as revealed for instance in the KCuF3 crystal.32 Furthermore, the outcomes of small-angle neutron scattering (SANS) experiments have shown that the hybrid system CuS8E/iPS consists of individual filaments encapsulated within iPS fibrils with a different arrangement of CuS8E units. These findings suggest strongly that the encapsulation in iPS fibrils modifies the arrangement of the CuS8E molecules in such a way that an intermolecular exchange is involved in addition to the exchange between copper ions within the same molecule. Accordingly, the experimental data can be fitted with a model including three species: (i) a bicopper molecules with only intramolecular coupling (such as CuS8E powder alone). (ii) Antiferromagnetic spin chains with 2Jintra and 2Jinter are inter- and intra-dimer exchange constants, respectively. The alternation parameter α = 2Jintra/2Jinter is close/equal to 1 in the case of uniform chains (gapless spin), and amounts to α = 0 for noncoupled dimers (spin-gapped). The latest corresponds to the magnetic susceptibility observed in the CuS8E powder state. Finally, (iii) a small amount of paramagnetic monocopper ions should be taken into account by Curie’s law to reproduce the upturn at low T. The total susceptibility fit model is (details are given in the Supporting Information) χhybrid = χbicoppers + χAF spin chains + χmonocoppers
Subsequently, the best fit obtained shown in Figure 4 suggests that the system formed after encapsulation stands as 44% for uniform chains (α ≈ 1) with an inter- and intramolecular coupling constants 2J1 = 2J2 = −205 ± 5 cm−1, 54% for noninteracting bicopper molecules with 2J = −196 ± 3 cm−1, and about ∼1.2% for monocopper molecules. The fit parameters of each CuS8E species in the powder state and in the encapsulated state are summarized in the Supporting Information. E
DOI: 10.1021/acs.langmuir.6b03572 Langmuir XXXX, XXX, XXX−XXX
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It is worth stressing that the material considered here is a randomly dispersed array of fibrils, which is at variance with the usual magnetic spin gap or gapless compounds that possess highly organized structures (crystals). Provided that the formation thermodynamics is controlled, we believe that our elaboration approach via physical processes (gelation, nucleation, and so forth) could be a very promising alternative to tune the spin gap or other physical properties of supramolecular assemblies instead of the approaches using chemical engineering38−40 or external stimuli.28,41,42 Finally, this hybrid material is very easy to prepare from its basic constituents and can be further processed in many ways. For instance, a highly porous material can be obtained once the solvent has been properly extracted,43,44 which opens up new possible paths for applications in material sciences.
copper atoms within the CuS8E molecule. This point is still under discussion and so out of the scope of the present paper. Fourier Transform Infrared. Fourier transform infrared (FTIR) experiments (Figure 6) provide one with further
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03572. Syntheses of the racemic and enantiomeric R-(−)-2ethylhexanoic acid copper(II) complex, DSC experiments, determination of magnetic susceptibility using SQUID, and EXAFS data analysis (PDF)
Figure 6. Infrared spectra in the region of asymmetric and symmetric stretching vibrations of the carboxylate groups of iPS (black spectrum), CuS8E (blue spectrum), and encapsulated CuS8E in iPS (red spectrum).
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evidence of the cage deformation. In metal carboxylates, the stretching vibrations of carboxylate groups (νCO2) are reported to be sensitive to the type of carboxylate-to-metal complexation structure.35,36 The difference in wavenumber positions between asymmetric (νasCO2) and symmetric (νsCO2) stretching (ΔνCOO) is used to distinguish the type of bonding in alkanoate ligands.35 ΔνCOO values around 150−170 cm−1 correspond to a bridging bidentate mode of coordination, and values around 100 cm−1 are characteristic of chelating bidentate structures.33 As expected, Δν COO values for CuS8E (178 cm −1 ) and encapsulated CuS8E (175 cm−1) correspond to a bridging bidentate mode of coordination. It has been reported36 that O−C−O angles decrease with decreasing ΔνCOO. The decrease in the ΔνCOO values after encapsulation appears therefore to be consistent with a modification in the local arrangement of carboxylate groups, which gives further support to the hypothesis of a deformation of the “cage” structure of the CuS8E molecule due to the proximity of polymer chains. A shearing process might better account for the decrease in the O−C−O angles. It is worth stressing that the slight modification in the intramolecular J coupling constant of CuS8E after encapsulation is most probably a direct effect induced by the O−C−O angle alteration.28,37,38
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jean-Michel Guenet: 0000-0002-3829-8303 Present Address §
ICMPE CNRS 2-8 rue Henri Dunant, 94320 Thiais, France.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was carried out as a part of an ANR grant (MATISSE) no. 11-BS08-001 (French National agency for research). A.B. is indebted to the same program for a PhD grant. The authors are also indebted for technical assistance to C. Saettel in the DSC measurements, Ralf Schweins in the neutron scattering experiments on D11, Antonella Ladecola and Giuliana Aquilanti for XAS measurements, Tristan Dewolf for EXAFS data analysis, Corine Reibel for SQUID experiments, and Alexander Vielhauer for assistance in the synthesis of the polymer samples. The authors are grateful to ELETTRA Sincrotrone Trieste SCpA for the XAS measurements through the proposal # 20125181. ILL is acknowledged for granting beamtime on D11 camera for performing the neutron scattering experiments.
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CONCLUSIONS It has been further demonstrated herein that an enantiomeric form of a bicopper complex can be encapsulated in the polymer fibrils. The encapsulation process definitely alters the magnetic properties because the bicopper complex filaments behave as a “spin gap” system in the bulk mesophase, whereas a large fraction of a “gapless” moiety occurs in the encapsulated state. The EXAFS and FTIR investigations indicate that the copper atom arrangement differs in both states, which correlates the magnetic properties with the structure of the copper atoms.
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REFERENCES
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DOI: 10.1021/acs.langmuir.6b03572 Langmuir XXXX, XXX, XXX−XXX
