4750 Chem. Mater. 2009, 21, 4750–4752 DOI:10.1021/cm902158k
Ordering Magnetic Molecules within Nanoporous Crystalline Polymers Alexandra R. Albunia,† Concetta D’Aniello,† Gaetano Guerra,*,† Dante Gatteschi,‡ Matteo Mannini,‡,§ and Lorenzo Sorace*,‡ †
Dipartimento di Chimica and INSTM Research Unit, Universit a di Salerno, Via Ponte don Mellilo, 84084 Fisciano (SA), Italy, ‡Dipartimento di Chimica and INSTM Research Unit, Universit a di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy, and § ISTM-CNR, URT di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy Received July 16, 2009 Revised Manuscript Received September 8, 2009
The development of molecule-based materials and devices requires that functional molecules can be organized on appropriate substrates and/or embedding media, in an ordered manner. In this perspective, the possibility of using nanoporous crystalline polymers1 as a framework for active molecules in alternative to inorganic2 or metalorganic3 or gel4 host structures is appealing, as it takes advantage of their intrinsic properties (transparency, manufacturing capability, flexibility). In recent years, two nanoporous crystalline phases of syndiotactic polystyrene, s-PS, have been obtained that are capable of hosting in their cavities several low-molecular-mass guest molecules.5,6 This leads to the formation of different kinds of cocrystals and induces on the guest molecules different kinds of long-range order. In particular, for s-PS cocrystals defined as intercalates,6 the host molecules stay inside layers intercalated with ac layers of alternated enantiomorphic helices (Figure 1). Films *Corresponding author.
(1) (a) De Rosa, C.; Guerra, G.; Petraccone, V.; Pirozzi, B. Macromolecules 1997, 30, 4147–4152. (b) Petraccone, V.; Ruiz de Ballesteros, O.; Tarallo, O.; Rizzo, P.; Guerra, G. Chem. Mater. 2008, 20, 3663– 3668. (2) (a) Kuznicki, S. M.; Bell, V. A.; Nair, S.; Hillhouse, H. W.; Jacubinas, R. M.; Braunbarth, C. M.; Toby, B. H.; Tsapatsis, M. Nature 2001, 412, 720–724. (b) Hayashi, H.; C^ote, A. P.; Furukawa, H.; O'Keeffe, M.; Yaghi, O. M. Nat. Mater. 2007, 6, 501–506. (3) (a) Pan, L.; Adams, K. M.; Hernandez, H. E.; Wang, X.; Zheng, C.; Hattori, Y.; Kaneko, K. J. Am. Chem. Soc. 2003, 125, 3062–3067. (b) Kitaura, R.; Seki, K.; Akiyama, G.; Kitagawa, S. Angew. Chem., Int. Ed. 2003, 42, 428–431. (c) Millward, A. R.; Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 17998–17999. (4) (a) Lopez, D.; Guenet, J. M. Eur Phys. J.B 1999, 12, 405–411. (b) Guenet, J. M.; Poux, S.; Lopez, D.; Thierry, A.; Mathis, A.; Green, M. M.; Liu, W. Macromol. Symp. 2003, 200, 9–20. (5) (a) Chatani, Y.; Inagaki, T.; Shimane, Y.; Iijtsu, T.; Yukimori, T.; Shikuma, H. Polymer 1993, 34, 1620–1624. (b) De Rosa, C.; Rizzo, P.; Ruiz de Ballesteros, O.; Petraccone, V.; Guerra, G. Polymer 1999, 40, 2103–2110. (c) Tarallo, O.; Petraccone, V.; Daniel, C.; Guerra, G. Cryst. Eng. Comm. 2009No. DOI:10.1039/b904675p. (6) (a) Petraccone, V.; Tarallo, O.; Venditto, V.; Guerra, G. Macromolecules 2005, 38, 6965–6971. (b) Tarallo, O.; Petraccone, V.; Venditto, V.; Guerra, G. Polymer 2006, 47, 2402–2410. (c) Malik, S.; Rochas, C.; Guenet, J. M. Macromolecules 2006, 39, 1000–1007.
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formed with s-PS/active-guest cocrystals have been proposed as advanced chromophore,7 fluorescent,8 nonlinear optical,9 photoreactive,10 and magnetic materials.11 The last are particularly appealing guests, because the current trend in molecular magnetism12 is directed toward the use of substrates for organizing molecules, for instance, single molecule magnets, SMM, in environments that are structured like crystals but allow possibilities of addressing individual molecules.13 A macroscopic polymer film hosting iso-oriented magnetic molecules would be the dream of many researchers, allowing to easily measure the anisotropy of isolated molecules.12b We started the exploration of s-PS polymer films use to obtain organized arrays of magnetic molecules by studying the simple and well-characterized nitroxide radical compound, 2,2,6,6tetramethyl-piperidinyl-N-oxyl (TEMPO). In this communication, we show that indeed TEMPO can be ordered in a s-PS film, opening perspectives for ordered organizations of magnetic molecules. A preliminary study of a TEMPO/s-PS cocrystals was reported by Kaneko et al.,11 with only preliminary characterization of the cocrystalline structure and no evidence of the anisotropy of the obtained magnetic sublattice. Films exhibiting a cocrystalline s-PS/TEMPO phase have been obtained by TEMPO sorption from concentrated acetone solution into uniaxially stretched δ-form films. The nanoporous δ-phase of s-PS1a exhibits cavities located between layers of alternated enantiomorphic helices (left scheme in Figure 1). The stretching procedure1 allows us to iso-orient the axes of the polymer helices (i.e., the unique monoclinic c axes of the crystallites) along the draw direction (right scheme in Figure 1). In the Supporting Information, a detailed preparation procedure is reported. The X-ray diffraction pattern of the cocrystalline film with maximum content of TEMPO and of the precursor δ-form film are shown in Figure S1 (corresponding reflections are collected in Table S1 of the Supporting Information). The low-diffraction angle reflection corresponding to a large spacing (d = 1.36 nm) (7) Uda, Y.; Kaneko, F.; Tanigaki, N.; Kawaguchi, T. Adv. Mater. 2005, 17, 1846–1850. (8) (a) De Girolamo Del Mauro, A.; Carotenuto, M.; Venditto, V.; Petraccone, V.; Scoponi, M.; Guerra, G. Chem. Mater. 2007, 19, 6041. (b) Itagaki, H.; Sago, T.; Uematsu, M.; Yoshioka, G.; Correa, A.; Venditto, V.; Guerra, G. Macromolecules 2008, 41, 9156–9164. (9) (a) Daniel, C.; Galdi, N.; Montefusco, T.; Guerra, G. Chem. Mater. 2007, 19, 3302–3308. (b) Rizzo, P.; Daniel, C.; De Girolamo Del Mauro, A; Guerra, G. Chem. Mater. 2007, 19, 3864–3866. (10) (a) Stegmaier, P.; De Girolamo Del Mauro, A.; Venditto, V.; Guerra, G. Adv. Mater. 2005, 17, 1166–1168. (b) D'Aniello, C.; Musto, P.; Venditto, V.; Guerra, G. J. Mater. Chem. 2007, 17, 531–535. (11) Kaneko, F.; Kajiwara, A.; Uda, Y.; Tanigaki, N. Macromol. Rapid Commun. 2006, 27, 1643–1647. (12) (a) Magnetism: Molecules to Materials; Miller, J. S., Drillon, M., Eds.; Wiley-VCH: New York, 2001-2004; Vol. 1-4. (b) Gatteschi, D.; Sessoli, R.; Villain, J. Molecular Nanomagnets; Oxford University Press: Oxford, U.K., 2006. (13) Gatteschi, D.; Cornia, A.; Mannini, M.; Sessoli, R. Inorg. Chem. 2009, 48, 3408–3419.
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Figure 1. Schematic presentation of the along the chain (c axis) projection, showing the a and b crystalline axes of the (left) nanoporous δ crystalline phase, the cavities are indicated as ellipses; (center) s-PS/ TEMPO intercalate cocrystalline phase. The axial orientation of the host polymer phase (c monoclinic axis preferentially parallel to the draw direction), present in the macroscopic films is shown in the right panel.
analogous to previously reported s-PS cocrystals clearly indicates the occurrence of an intercalate cocrystalline phase, with a long-range contiguity between guest molecules along the c axis (center scheme in Figure 1).6 In analogy with s-PS intercalate phases with guests having similar molecular volumes,6a it is reasonable to assume a monomeric-unit/guest molar ratio of 2:1. The TEMPO content in the film, as evaluated by thermogravimetric and magnetometric analysis, is close to 27 wt %, whereas the degree of crystallinity, as evaluated by FTIR measurements,14 is close to 35%. It is also worth noting, to support the quality of this cocrystal, that if guest molecules were only in the intercalate phase, the calculated TEMPO content would not be far from 24 wt %. The FTIR spectra in the range 1400-920 cm-1 taken with the polarization plane parallel and perpendicular to the draw direction, for both the uniaxially oriented δ-form s-PS film and the derived intercalate s-PS/TEMPO cocrystalline film, are shown in Figure 2. The absorption peaks characteristic of the helical polymer chain conformation,15 located at 1320, 1277, and 944 cm-1, are highly dichroic (Figure 2), indicating that the crystalline phase presents a high degree of axial orientation ( fcIR ≈ 0.95). Well-resolved absorption bands of TEMPO are also all dichroic: 1340 cm-1(//), 1258 cm-1(^), 1242 cm-1(//), and 1130 cm-1(^). This feature confirms that TEMPO molecules are mainly absorbed as guest of the cocrystalline phase with s-PS. The orientation of the guest molecules in the intercalate s-PS/TEMPO cocrystalline structure can be estimated by the linear dichroism of the peak at 1340 cm-1, whose transition moment vector is along the N-O bond.16 This analysis17 gave evidence that the angle between the N-O bonds of the TEMPO molecules and the polymer chain axis is ca. 45 ( 2. This means that the NO groups of two neighboring molecules (located at 0 and 1/2c in Figure 1), whose distance depending on their relative orientation is in the range 0.5-0.9 nm, are approximately orthogonal to each other. (14) Albunia, A. R.; Musto, P.; Guerra, G. Polymer 2006, 47, 234–242. (15) Torres, F. J.; Civalleri, B.; Meyer, A.; Musto, P.; Albunia, A. R.; Rizzo, P.; Guerra, G. J. Phys. Chem. B 2009, 113, 5059–5071. (16) Rintoul, L.; Micallef, A. S.; Bottle, S. E. Spectrochim. Acta, Part A 2008, 70, 713–717. (17) (a) Albunia, A. R.; Di Masi, S.; Rizzo, P.; Milano, G.; Musto, P.; Guerra, G. Macromolecules 2003, 36, 8695–8703. (b) Albunia, A. R.; Milano, G.; Venditto, V.; Guerra, G. J. Am. Chem. Soc. 2005, 127, 13114–13115.
Figure 2. FTIR spectra in the wavenumber range 1400-920 cm-1 taken with polarization plane parallel and perpendicular to the draw direction for a uniaxially stretched s-PS film: (a) δ-form; (b) intercalate s-PS/ TEMPO. The absorption peaks characteristic of the polymer chains are indicated by H (host), those of TEMPO are indicated by G (guest).
Angular-dependent X-band EPR spectra on cocrystalline films were recorded at variable temperature rotating a semicrystalline film around three laboratory axes, X, Y, and Z, where Z is the draw axis, corresponding to the dominant orientation of the local monoclinic axes (Figure 1, right). They show one band for every film orientation, with g values that are essentially isotropic (g = 2.005(7)) and in accord with the reported average values for TEMPO.18 The W-band EPR spectra, recorded by rotating the sample around X (see Figure S2 in the Supporting Information) provided further confirmation of the observed g value, with a single, isotropic band at g = 2.006. The observed g values correspond to the average of the reported values for TEMPO (g1 = 2.003; g2 =2.006; g3 = 2.010; g3 is observed parallel to the NO direction, g1 is perpendicular to the mean plane of the molecule) and may result in principle by either motional averaging or by exchange-coupling between differently oriented molecules, resulting in an accidentally isotropic value. Because an isotropic g value is observed also at low temperature, where the molecular motion can not be effective in averaging, the latter hypothesis has to be chosen. The existence of a weak but non-negligible exchange interaction between adjacent paramagnetic molecules is also witnessed by the temperature dependence of χT of the sPS/TEMPO cocrystalline film (see Figure S3 in the Supporting Information), showing clear signs of antiferromagnetic coupling. Because the structural information suggests the formation of a monodimensional magnetic structure, with a distance between adjacent N-O groups in the range 0.5-0.6 nm, the data were modeled using the Bonner-Fisher approximation for chains of S = 1/2.19 The best fit was obtained for an exchange coupling constant J=0.4 cm-1, which is compatible with structural data. Further, the magnitude of the exchange interaction is enough to average out the small g anisotropies both at X- and at W-band frequency. However a clear mark of (18) Kobayashi, H.; Ueda, T.; Miyakubo, K.; Eguchi, T.; Tani, A. Phys. Chem. Chem. Phys. 2008, 10, 1263–1269. (19) Bonner, J. C.; Fisher, M. E. Phys Rev. 1964, 135, A640–A658.
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Figure 3. Left: Angular dependence of EPR line width in the ZX plane (blue circles, room temperature; red circles, 80 K; black circles, 5 K) and best fit curves; in the XY plane at room temperature (purple triangles). Right: Relative orientation of TEMPO molecules inside s-PS deduced by EPR data.
magnetic anisotropy is conserved on the linewidths, whose angular dependence is shown in Figure 3. The rotation around the Z axis is completely isotropic, whereas for the rotation X (and Y), the line width passes through a maximum parallel to Z, to a minimum around 51, and to a relative maximum parallel to Y (and X). This behavior is typical of one-dimensional magnets where the broadening induced by dipolar interactions is mainly due to interaction along the chains, thus passing through a minimum at the magic angle (θ = 54.74), where the dipolar interaction goes to zero.20 The observed angular dependence of the line width in the XZ plane could be fitted by ΔH(θ)=a|3cos2 θ - 1|4/3 þ b þ ccos2 θ, with best fit parameters a = 1.5 Oe, b = 14.2 Oe, c = -0.8 Oe. The first term is expected for a purely 1D magnetic system, whereas the presence of the second and third term may be attributed to the effect of the hyperfine interactions,21 and/or to the small fraction (about 10%) of TEMPO in the amorphous phase. As the shortest distance between neighboring chains is 0.87 nm, interchain interactions should indeed be negligible. The line shape analysis at different angles performed through the Dietz plot20(see Figure S4 in the Supporting Information) points out a predominant Lorentzian character of the resonance lines at all angles, thus confirming a partial deviation from the ideal 1D character. Finally, we note that an increase in the line width is observed below room temperature, varying from 15.7 Oe at room temperature to 25 Oe at 110 K and 27.2 Oe at 5 K (with field applied along X, see Figure S5 in the Supporting Information). The persistence of line width anisotropy at low temperature indicates that the crystal order is maintained. Important structural information can be obtained on the basis of the one-dimensional (20) Dietz, R. E.; Merritt, R. F.; Dingle, R.; Hone, D.; Silbernagel, B. G.; Richards, P. M. Phys. Rev. Lett. 1971, 26, 1186–1188. (21) Benelli, C.; Caneschi, A.; Gatteschi, D.; Pardi, L.; Rey, P. Inorg. Chem. 1989, 28, 3230–3234.
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hypothesis. In fact, the molecules whose g values are averaged, g = (g1 þ g2)/2, are the two magnetically nonequivalent molecules reported by the 21 axis of the s-PS crystal. The observed pattern of g values requires the local NO direction (corresponding to gx =2.010)18 on one molecule to be orthogonal to the normal to the TEMPO molecular plane (corresponding to gz = 2.003)18 on the neighboring symmetry-equivalent molecule. The angle of the local x axis with the draw axis Z must then be ca. 45 (Figure 3, right), as observed in FTIR analysis. The present data show how a simple and low-cost polymer as s-PS can act as host for magnetic molecules, keeping the features of a crystal for high concentrations of guests. Even if 1D character has been previously suggested for TEMPO molecules in phosphazene nanochannels,22 this is the first time the anisotropic behavior typical of low-dimensional magnetic materials can be directly probed for paramagnetic molecules in a porous host. Another exciting feature of s-PS, which we started to exploit, is that of providing the possibility, unique among polymers, of three different kinds of uniplanar orientations of the crystalline phase.23 In particular, it is possible to impose, by polymer processing, three different orientations with respect to the film plane, of the high density ac layers, formed by close-packing of alternated enantiomorphous s-PS helices. Two of these uniplanar orientations can be also combined with the axial orientation (common to all semicrystalline polymers) producing uniplanar-axial orientations,8b which should give in principle the opportunity to achieve for the magnetic guest molecules not only the axial order with respect to the polymer stretching direction (shown in the present paper) but also a 3D orientational order in the whole macroscopic film. Further work is in progress to characterize the sorption of TEMPO and other radicals in the various phases of s-PS. Acknowledgment. Prof. A. Cucolo, Dr. F. Bobba, and Dr. C. Daniel (University of Salerno) are gratefully acknowledged for discussions. We acknowledge the financial support of EU (Project MolSpinQiP-STREP 211284), Ente CaRiFi, CNR (Commessa PM.P05.011), and MIUR (PRIN2007). Supporting Information Available: Detailed preparation procedure, table of reflections, XRD patterns, magnetic measurements, W-band EPR spectra, Dietz plot, and temperature dependence of the line width (PDF).This material is available free of charge via the Internet at http://pubs.acs.org. (22) Kobayashi, H.; Ueda, T.; Miyakubo, K.; Eguchi, T.; Tani, A. Bull. Chem. Soc. Jpn. 2007, 80, 711–720. (23) Albunia, A. R.; Rizzo, P.; Tarallo, O.; Petraccone, V.; Guerra, G. Macromolecules 2008, 41, 8632–8642.
