Toward Organization of Cyano-Bridged Coordination Polymer

Dec 16, 2008 - Size controlled cyano-bridged coordination polymer nanoparticles Mn1.5[Cr(CN)6] have been synthesized and organized at the nanolevel by...
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Langmuir 2009, 25, 1138-1147

Toward Organization of Cyano-Bridged Coordination Polymer Nanoparticles within an Ionic Liquid Crystal Joulia Larionova,*,† Yannick Guari,*,† Christophe Blanc,‡ Philippe Dieudonne´,‡ Alexei Tokarev,† and Christian Gue´rin† Institut Charles Gerhardt Montpellier and Laboratoire des Colloı¨des, Verres et Nanomate´riaux, UniVersite´ Montpellier II, Place E. Bataillon, 34095 Montpellier, France ReceiVed September 12, 2008. ReVised Manuscript ReceiVed NoVember 14, 2008 Size controlled cyano-bridged coordination polymer nanoparticles Mn1.5[Cr(CN)6] have been synthesized and organized at the nanolevel by using the room temperature ionic liquid crystal (ILC) C12-MIMBF4. The as-obtained material was studied by transmission electron microscopy (TEM), differential scanning calorimetry (DSC), optical microscopy, and X-ray diffraction. These analyses reveal the presence of a long-range organization of cyano-bridged nanoparticles at the nanoscale level within the ILC phase. The magnetic study of these nanoparticles reveals an appearance of a nanocluster-glass-like regime caused by magnetostatic interactions between neighboring nanoparticles. The properties of these organized nanoparticles have been compared with the properties of nanoparticles of the same composition and stoichiometry obtained and randomly dispersed into the isotropic IL C10-MIMBF4.

1. Introduction The large scale synthesis of long-range arrays of magnetic nanoparticles is an important issue in the development of nanostructured materials attractive both from a fundamental point of view and from their technological applications in magnetic storage and biomedicine.1 In these materials, magnetic properties of individual nanoparticles can be modulated by coupling magnetostatic interactions with neighboring nanoparticles in order to provide some collective behavior with unique cooperative2 or spin-related transport phenomena.3 In addition, in contrast to random mixtures of nanoparticles, ordered arrays can provide uniformity of packing, stoichiometry, and rigorous control of the interparticle distance. The commonly chosen approach to promote nanoparticle self-assembly consists of the deposition on a solid surface4 or on a liquid-liquid interface5 of nanoparticles surrounded with appropriate ligands to favor strong interligand interactions. Another approach, called templated self-assembly, makes use of substrates lithographically patterned with grooves into which nanoparticles can be self-assembled.6 In recent years, the self-organization of magnetic nanoparticles in two- or three-dimensional assemblies has also been performed * To whom correspondence should be addressed. Fax: 00(33)467143852. E-mail: [email protected] (J.L.); [email protected]. (Y.G.). † Institut Charles Gerhardt Montpellier. ‡ Laboratoire des Colloı¨des. (1) (a) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes, P. Science 2004, 303, 821–823. (b) Corr, S. A.; Byrne, S. J.; Tekoriute, R.; Meledandri, C. J.; Brougham, D. F.; Lynch, M.; Kerskens, Ch.; O’Dwyer, L.; Gun’ko, Y. K. J. Am. Chem. Soc. 2008, 130, 4214–4215. (c) Shevchenko, E.; Talapin, D. V.; Murray, C. B.; O’Brien, S. J. Am. Chem. Soc. 2006, 128, 3620–3637. (2) Puntes, V. F.; Krishnan, K. M.; Alivisatos, P. Appl. Phys. Lett. 2001, 78, 2187–2189. (3) Back, C. T.; Murray, C. B.; Sandstrom, R. L.; Sun, S. Science 2000, 290, 1131–1134. (4) (a) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55–59. (b) Shevchenko, E. V.; Talapin, D. V.; Murray, C. B.; O’Brien, S. J. Am. Chem. Soc. 2006, 128, 3620–3637. (c) Yamamuro, S.; Farrell, D. F.; Majetich, S. A. Phys. ReV. B. 2002, 65, 224431/1–224431/9. (d) Petit, C.; Taleb, A.; Pileni, M.-P. AdV. Mater. 1998, 10, 259–261. (e) Ethirajan, A.; Wiedwald, U.; Boyen, H.-G.; Kern, B.; Han, L.; Klimmer, A.; Weigl, F.; Ka¨stle, G.; Ziemann, P.; Fauth, K.; Cai, J.; Behm, R. J.; Romanyuk, A.; Oelhafen, P.; Walther, P.; Biskupek, J.; Kaiser, U. AdV. Mater. 2007, 19, 406–410. (5) (a) Lin, Y.; Skaff, H.; Emrick, T.; Dinsmore, A. D.; Russell, T. P. Science 2003, 299, 226–229. (b) Duan, H.; Wang, D.; Sobal, N. S.; Giersig, M.; Kurth, D. G.; Mo¨hwald, H. Nano Lett. 2005, 5, 949–952.

by using liquid crystals (LCs).7 It should be noted that the rules governing the appearance and stability of the various phases of LCs are now well-understood and the fluid nature of their mesophases makes these materials intrinsically defect tolerant, that is favorable to the nanoparticle integration and organization. In this connection, several works on the incorporation, the selfassembling, as well as the in situ synthesis and organization of magnetic nano-objects at the nanoscale in a thermotropic LC phase have been reported.8 However, to the best of our knowledge, magnetic coordination polymer nanoparticles have never been synthesized and organized into a LC. Coordination polymer nanoparticles are a relatively new type of nano-objects which have attracted an increasing interest in the recent 8 years due to their fundamental interest and their potential applications. As metallic or metal oxide nano-objects,9 due to the very important surface/core atoms ratio and size reduction, these ultrasmall nanoparticles presenting a size lower than 10 nm often exhibit an appearance of new interesting size-dependent physical and chemical properties, which are different in comparison to the properties of their bulk analogues.10 The first work on the synthesis of cubic shaped nanocrystals (∼12-50 nm) of cyano-bridged homometallic “Prussian Blue” nanopar(6) (a) Cheng, J. Y.; Ross, C. A.; Smith, H. I.; Thomas, E. L. AdV. Mater. 2006, 18, 2505–2521. (b) Angelakeris, M.; Crisan, O.; Papaioannou, E.; Vouroutzis, N.; Tsiaoussis, I.; Pavlidou, E.; Crisan, A. D.; Kostic, I.; Sobal, N.; Giersig, M.; Flevaris, K. K. Mater. Sci. Eng., C 2005, 23, 873–878. (c) Darling, S. B.; Yufa, N. A.; Cisse, A. L.; Bader, S. D.; Sibener, S. J. AdV. Mater. 2005, 17, 2446–2450. (7) (a) Collings, P. J. Liquid Crystals: Nature’s Delicate Phase of Matter, 2nd ed.; Princeton University Press: Princeton, NJ, 2002. (b) Singh, S. Liquid Crystals Fundamentals; World Scientific: Singapore, 2000. (c) Hamley, I. W. Introduction to Soft Matter; John Wiley & Sons, Ltd.: Hoboken, NJ, 2000. (8) (a) Song, H. M.; Kim, J. C.; Hong, J. H.; Lee, Y. B.; Choi, J.; Lee, J. I.; Kim, W. S.; Kim, J.-H.; Hur, N. H. AdV. Funct. Mater. 2007, 17, 2070–2076. (b) Brochard, F.; De Gennes, P. G. J. Phys. (Paris) 1970, 31, 691–695. (c) Rault, J.; Cladis, P. E.; Burger, J. P. Phys. Lett. 1970, 32A, 199–203. (d) Loudet, J. C.; Barois, P.; Poulin, P. Nature 2000, 407, 611–613. (e) Martinez-Miranda, L. J.; McCarthy, K.; Kurihara, K. K.; Harry, J. J.; Loel, A. Appl. Phys. Lett. 2006, 89, 161917-1. (f) Da Cruz, C.; Sandre, O.; Cabuil, V. J. Phys. Chem. B 2005, 109, 14292–14299. (g) Song, H. M.; Kim, H. C.; Hong, J. H.; Lee, Y. B.; Choi, J.; Lee, J. I.; Kim, W. S.; Kim, J.-H.; Hur, N. H. AdV. Funct. Mater. 2007, 17, 2070–2076. (9) (a) Feldheim, C.; Foss, A. Metal Nanoparticles: Synthesis, Characterization, and Applications; Marcel Dekker, Inc.: New York, Basel, 2002. (b) Schmid, G. Nanoparticles: From Theory to Application; VCH: Weinheim, 2004. (10) Dujardin, E.; Mann, S. AdV. Mater. 2004, 16, 1125–1129, and refs 11-18 .

10.1021/la803001x CCC: $40.75  2009 American Chemical Society Published on Web 12/16/2008

Cyano-Bridged Coordination Polymer Nanoparticles

ticles stabilized within reverse micelles was reported by Mann and co-workers.11 After that, the synthesis of coordination polymer nanoparticles of different size has been performed by using a reverse micelle technique,12 by stabilization of nanoparticles within various matrixes, such as polymers,13 biopolymers,14 structured alumina,15 amorphous and mesostructured silica,16 or by using stabilizing ligands17 in solutions. These methods allow the synthesis of nanocomposites or stable colloids in which the nanoparticles of adjustable size are homogeneously dispersed either into a matrix or into a solution. Along this line of thought, we recently reported on the synthesis and study of a large range of cyano-bridged homo- and heterometallic coordination polymer nanoparticles within the isotropic 1-butyl-3-methyl imidazolium ionic liquid (IL), which act both as stabilizing agent and as solvent.18 In this system, the ultrasmall magnetic nanoparticles of controlled size of 3-6 nm are randomly dispersed into the IL and the magnetostatic interactions between these nanoparticles may be modulated by their concentration. By the appropriate choice of N-alkyl substituent and counteranions, it is possible to generate ILs that possess properties of ionic liquid crystals (ILCs) and attempt to organize coordination polymer nanoparticles at the nanoscale level. In the present work, we report on the in situ organization into a two-dimensional array of Mn1.5[Cr(CN)6] nanoparticles within the ILC 1-dodecyl-3-methylimidazolium tetrafluoroborate, [C12-MIM]BF4, their textural and structural characteristics, and their magnetic properties. A special emphasis is given on the comparison of these organized nanoparticles with nanoparticles of the same chemical composition which are randomly dispersed in an isotropic IL, [C10-MIM]BF4.

2. Experimental Section Synthesis. K3[Cr(CN)6], [Mn(H2O)6](NO3)2, and [C12-MIM][Cl] were purchased from Acros and Aldrich, respectively. AgBF4 was purchased from Alfa Aesar. [C12-MIM][BF4] and [C10-MIM][BF4] (11) (a) Vaucher, S.; Li, M.; Mann, S. Angew. Chem., Int. Ed. 2000, 39, 1793– 1796. (b) Vaucher, S.; Fielden, J.; Li, M.; Dujardin, E.; Mann, S. Nano Lett. 2002, 2, 225–229. (12) (a) Catala, L.; Gacoin, T.; Boilot, J.-P.; Riviere, E.; Paulsen, C.; Lhotel, E.; Mallah, T. AdV. Mater. 2003, 15, 826–829. (b) Yamada, M.; Arai, M.; Kurihara, M.; Skamoto, M.; Miyake, M. J. Am. Chem. Soc. 2004, 126, 9482–9483. (c) Moulik, S. P.; De, G. C.; Panda, A. K.; Bhownik, B. B.; Das, A. R. Langmuir 1999, 15, 8361–8367. (d) Giordano, C.; Longo, A.; Ruggirello, A.; Turco Liveri, V.; Venezia, A. M. Colloid Polym. Sci. 2004, 283, 265–267. (e) Cao, M.; Wu, X.; He, X.; Hu, C. Chem. Commun. 2005, 2241–2243. (13) (a) Uemura, T.; Kitagawa, S. J. Am. Chem. Soc. 2003, 125, 7814–7815. (b) DeLongchamp, D. M.; Hammond, P. T. AdV. Funct. Mater. 2004, 14, 224– 232. (c) Catala, L.; Gloter, A.; Stephan, O.; Rogez, G.; Mallah, T. Chem. Commun. 2006, 1018–1020. (d) Brinzei, D.; Catala, L.; Louvain, N.; Rogez, G.; Ste´phan, O.; Gloter, A.; Mallah, T. J. Mater. Chem. 2006, 16, 2593–2599. (e) Catala, L.; Mathonie`re, C.; Gloter, A.; Sstephan, O.; Gacoin, T.; Boilot, J.-P.; Mallah, T. Chem. Commun. 2005, 746–748. (f) Uemura, T.; Ohba, M.; Kitagawa, S. Inorg. Chem. 2004, 43, 7339–7345. (14) (a) Guari, Y.; Larionova, J.; Molvinger, K.; Folch, B.; Gue´rin, Ch. Chem. Commun. 2006, 2613–2615. (b) Domingez-Vera, J. M.; Colacio, E. Inorg. Chem. 2003, 42, 6983–6985. (c) Guari, Y.; Larionova, J.; Corti, M.; Lascialfari, A.; Marinone, M.; Poletti, G.; Molvinger, K.; Gue´rin, Ch. Dalton Trans. 2008, 3658– 3660. (d) Gabez, N.; Sanchez, P.; Dominguez-Vera, J. M. Dalton Trans. 2005, 2492–2494. (15) (a) Zhou, P.; Xue, D.; Luo, H.; Chen, X. Nano Lett. 2002, 2, 845–847. (b) Zhou, P. H.; Xue, D. S. J. Appl. Phys. 2004, 96, 610–614. (16) (a) Moore, J. G.; Lochner, E. J.; Ramsey, C.; Dalal, N. S.; Stiegman, A. E. Angew. Chem., Int. Ed. 2003, 42, 2741–2743. (b) Vondorova, M.; Klimczuk, T.; Miller, V. L.; Kirby, B. W.; Yao, N.; Cara, R. J.; Bocarsly, A. Chem. Mater. 2005, 17, 6216–6218. (c) Clavel, G.; Guari, Y.; Larionova, J.; Gue´rin, Ch. New J. Chem. 2005, 29, 275–279. (d) Folch, B.; Guari, Y.; Larionova, J.; Luna, C.; Sangregorio, C.; Innocenti, C.; Caneschi, A.; Gue´rin, Ch. New J. Chem. 2008, 32, 273–282. (e) Folch, B.; Larionova, J.; Guari, Y.; Datas, L.; Gue´rin., Ch. J. Mater. Chem. 2006, 16, 4435–4442. (17) (a) Chelevaeva, E.; Guari, Y.; Larionova, J.; Trifonov, A.; Gue´rin, Ch. Chem. Mater. 2008, 20, 1367–1375. (b) Arai, M.; Miyake, M.; Yamada, M. J. Phys. Chem. C 2008, 112, 1953–1962.

Langmuir, Vol. 25, No. 2, 2009 1139 were synthesized according to previously published procedures.19 The quantity of water was controlled by the Karl Fischer method and maintained at 0.02 wt %. Synthesis of [C12-MIM]3[Cr(CN)6]. [C12-MIM]3[Cr(CN)6] was synthesized by a metathesis reaction from [C12-MIM][Cl] and K3[Cr(CN)6] in water using the following procedure: An aqueous solution (50 mL) of K3[Cr(CN)6] (3.10 g, 3 mmol) and an aqueous solution (20 mL) of [C12-MIM][Cl] (2.65 g, 9 mmol) were mixed, and the yellow precipitate was filtered, washed with a small quantity of cold water, and dried in Vacuo. [C12-MIM]3[Cr(CN)6] was obtained as a yellow powder in 84% yield. Elemental analysis: Calc. for C54CrH93N12: C, 67.39; Cr, 5.40; N, 17.46; H, 9.74. Found: C, 66.58; Cr, 5.61; N, 17.22; H, 9.88. IR (KBr disk, cm-1): 3151, ν(NCH) of NC(H); 3070, νas ring HCCH; 2115, ν(CN); 1575, ν(CH3N), ν(CH2N) of ring in plane; 1470, δ(HCH) of CH3(N); 1172, ν(CH3N), ν(CH2N) of ring in plane; 622, ν(CH3N), ν(CH2N) of ring out of plane. Q-TOF MS; ESI- (m/z): 91 ([Cr(CN)5]2-), 156 ([Cr(CN)4]-); ESI+ (m/z): 251 ([C12-MIM]+). Synthesis of Colloids Mn1.5[Cr(CN)6]/[Cn-MIM][BF4] (n ) 10, 12). In a typical synthesis, a [C12-MIM][BF4] solution (1 mL) of [Mn(H2O)6](NO3)2 (0.22 mmol) was added to a [C12-MIM][BF4] solution (4 mL) of [C12-MIM]3[Cr(CN)6] (0.15 mmol) at 60 °C. The white solution changes the color without any visible precipitate to light yellow. The solutions were stirred during 2 h at 60 °C and cooled to room temperature. The Mn/Cr ratio was found from EDS as equal to 1.5 for both samples. Elemental analysis found after precipitation by EtOH for Mn1.5[Cr(CN)6]/[C10-MIM][BF4]: Mn, 4.35; Cr, 2.72; C, 49.58; N, 11.58; H, 7.29. For Mn1.5[Cr(CN)6]/ [C12-MIM][BF4]: Mn, 4.87; Cr, 3.17; C, 51.52; N, 11.67; H, 7.55. Physical Measurements. IR spectra were recorded on a PerkinElmer 1600 spectrometer with a 4 cm-1 resolution. UV-vis spectra were recorded on a Cary 5E spectrometer in solution or in solid state in a KBr disk. Transmission electron microscopy (TEM) observations were carried out at 100 kV (JEOL 1200 EXII). Samples for TEM measurements were prepared using ultramicrotomy techniques from a frozen drop of solutions and then deposited on copper grids. An evaluation of the M/M′ ratio was performed by using an environmental secondary electron microscope FEI Quanta 200 FEG coupled with an electron dispersive spectroscope Oxford INCA detector. Magnetic susceptibility data were collected with a Quantum Design MPMSXL SQUID magnetometer. The data were corrected for the sample holder. The melting points, clearing points, and glass transition temperatures were determined by differential scanning calorimetry (NETZSCH PSC 204 F1 Phoenix equipped with dinitrogen cryostatic cooling, 5-15 mg samples, 2 °C min-1 heating and cooling rates), calibrated using an indium primary standard. The samples were first heated to 70 °C and then cooled to -50 °C; the DSC measurements were then performed from -50 to 70 °C for the second and third cycles. The point at which the deviation from baseline starts is determined as the temperature transition. Optical characterization of the compounds and the detection of mesophases were performed with a polarizing microscope (Leitz 12 POL S) equipped with a 1024 pixel × 768 pixel Sony CCD camera and an Instec hot stage regulated at 0.1 °C. Powder samples were deposited between slides and coverslips and inserted into the hot stage at room temperature. The temperature was then slowly increased (typically 1 °C min-1), and the phase transitions were detected from the texture changes observed between crossed polarizers. Once in the isotropic phase, the temperature was decreased, and the phase transitions under cooling were similarly detected. X-ray diffraction measurements on dried powders were carried out in 1.5 mm diameter glass capillaries in a transmission configuration. A copper rotating anode X-ray source (functioning at 4 kW) with a multiplayer focusing Osmic monochromator giving high flux (108 photons/s) and punctual collimation (18) (a) Clavel, G.; Larionova, J.; Guari, Y.; Gue´rin, Ch. Chem.sEur. J. 2006, 12, 3798–3804. (b) Larionova, J.; Guari, Y.; Sayegh, H.; Gue´rin, Ch. Inorg. Chim. Acta 2007, 360, 3829–3836. (c) Larionova, J.; Guari, Y.; Tokarev, A.; Chelebaeva, E.; Luna, C.; Sangregorio, C.; Caneschi, A.; Gue´rin, Ch. Inorg. Chim. Acta 2008, 361, 3988–3996. (19) (a) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; VCH: Weinheim, 2008. (b) Holbrey, J. D.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1999, 13, 2133–2139.

1140 Langmuir, Vol. 25, No. 2, 2009 Scheme 1. Molecular Structures of (a) [Cn-MIM][BF4] with n ) 10, 12 and (b) [C12-MIM]3[Cr(CN)6]

was employed. An image plate 2D detector was used. X-ray diffractograms were obtained, giving the scattering intensity as a function of the wave vector q. A Mettler oven equipped with mylar windows was used for measurements at temperatures between 22 and 100 °C. Scattered intensity was corrected by exposition time, transmission, and intensity background coming from scattering by an empty capillary. Elemental analyses were performed by the Service Central d’Analyses (CNRS, Vernaison, France). The samples were heated at 3000 °C under He. Oxygen was transformed in CO and detected by using an IR detector. Manganese and chromium were determined with a high resolution ICP-MS using a ThermoFischer element. Mass spectroscopy analyses were performed on a Micromass Q-Tof instrument in electrospray ionization mode.

3. Results and Discussion 3.1. Synthesis and Structural Characterizations. 1-Dodecyl-3-methylimidazolium tetrafluoroborate, [C12-MIM]BF4 (Scheme 1a), was used as a reaction medium due to its liquid crystalline behavior in its molten state and low melting and clearing points permitting the synthesis of coordination polymer nanoparticles at relatively low temperature (60 °C).19 The isotropic 1-decyl-3-methylimidazoliumtetrafluoroborateIL,[C10-MIM]BF4, was also used at this reaction temperature for the synthesis of the same coordination polymer nanoparticles in order to study the influence of the IL’s nature on the nanoparticles synthesis. The precursor [C12-MIM]3[Cr(CN)6] (Scheme 1b) was synthesized by a metathesis reaction from [C12-MIM]BF4 and K3[Cr(CN)6]. Its infrared spectrum presents, beside the bands characteristic of [C12-MIM]+, the stretching vibrations of the nonbridging cyano groups ν(CN) at 2125 and 2117 cm-1.20 This precursor is soluble in [C10-MIM]BF4 and in [C12-MIM]BF4 at 60 °C that gives the possibility to perform in both cases the in situ self-assembling reaction in an isotropic phase. Mixing solutions of [C12-MIM]3[Cr(CN)6] and Mn(NO3)2 · 4H2O in [C12-MIM]BF4 containing 0.02% wt water at 60 °C lead to the formation of a yellow transparent solution Mn1.5[Cr(CN)6]/ [C12-MIM]BF4. After cooling at room temperature, this solution became an opaque yellow solid. However, upon heating at 60 °C, it reversibly became a transparent solution (Scheme 2 and Supporting Information Figure S1). The as-obtained material is stable for months. The Mn/Cr ratio determined by elemental analysis and EDS is equal to 1.5. A similar procedure was performed for the synthesis of the nanoparticles Mn1.5[Cr(CN)6]/ [C10-MIM]BF4 by using [C10-MIM]BF4 that retains its yellow viscous liquid aspect when cooling to room temperature. The infrared spectra of both samples beside the bands characteristic of [Cn-MIM]BF4 (n ) 10, 12) present the stretching vibrations of the bridging cyano groups at 2152 and 2090 cm-1, proving the formation of the cyano-bridged network (Supporting (20) Nakamoto, N. Infrared and Raman Spectra, Wiley, New York, 1986.

LarionoVa et al. Scheme 2. Schematic Representation of the Synthesis of Cyano-Bridged Coordination Polymer Nanoparticles Mn1.5[Cr(CN)6]/[Cn-MIM][BF4] (n ) 10, 12)

Information Figure S2). The high frequency band can be attributed to the stretching of the CN ligand bridged between Mn2+ and Cr3+ in the Mn2+-CN-Cr3+ mode and the low frequency band can be assigned to the presence of the linkage isomer with the Mn2+-NC-Cr3+ coordination mode, as it was reported for the bulk cyano-bridged coordination polymer.18b,20 3.2. Textural Characterizations. In order to determine the presence of the nanoparticles in these samples, TEM measurementswereperformed.AdropoftheheatedsolutionMn1.5[Cr(CN)6]/ [C12-MIM]BF4 was cooled down to 25 °C and deposited onto a copper grid for TEM observations. Figure 1a shows a representative TEM image of the sample in which uniformly sized nonaggregated spherical nanoparticles with a mean diameter of ∼4.0 (0.6) nm may be clearly observed (Figure 1b). These nanoparticles are arranged into anisotropic arrays formed by parallel chains of nanoparticles with a regular interchain period of ∼5-6 nm and a regular intrachain period of approximately twice the nanoparticle’s radius, that is, 4.0(0.6) nm, as can be seen from the inset in Figure 1a. In order to verify the influence of the temperature on the nanoparticles, this sample was heated at 60 °C for 1 h. No changes in the shape, size, and organization of the nanoparticles even after several heating-cooling cycles were observed by TEM measurements, suggesting significant stability of the obtained system. Efficient methods for studying IL transition temperatures are differential scanning calorimetry (DSC) and polarized optical microscopy (POM). It is known that pure [C12-MIM]BF4 displays liquid-crystalline behavior in its molten state and melting and clearing points temperatures are very sensitive to the presence of water or different impurities.19 A DSC trace for the mesomorphic pure [C12-MIM]BF4 containing 0.02 wt % water reveals the presence of a peak with the characteristically large enthalpy for the crystal (C)-liquid crystal (LC) transition at 12.22 °C (-77.04 J g-1) and a peak with a small enthalpy for the liquid crystal (LC)-isotropic liquid (L) transition at 50.02 °C (-0.91 J g-1) for the heating cycle. Two respective transitions at 14.18 °C (53.57 J g-1) and at 50.8 °C (0.59 J g-1) were observed for the cooling cycle (Table 1, Figure 2a). These measurements

Figure 1. (a) TEM image of the Mn1.5[Cr(CN)6]/[C12-MIM]BF4 sample (inset: magnification ×10) and (b) its corresponding size distribution. Scale bar ) 100 nm.

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are coherent to the DSC analysis previously reported for [C12MIM]BF4.19 The splitting of the peaks attributed to the C-LC transition for both, the heating and cooling cycles, may be explained by the presence of different effects, such as supercooling of the IL, relatively high heating and cooling speed rate in our experiments (2 °C min-1), and the presence of several conformation forms of the [C12-MIM]BF4 dodecyl chain.21 The presence of C-LC and LC-L transitions was also confirmed by POM images. Heating from the crystalline solid (up to 12.22 °C), the appearance of an enantiotropic mesophase was observed showing a texture characteristic to a lamellar LC phase. In this phase, the regions of focal conic and oily streak texture were observed, which is indicative of a layered smectic (SmA) mesophase (Supporting Information Figure S3a).19 Upon heating up to 50.02 °C, it changes to an isotropic liquid (Supporting Information Figure S3b). The DSC trace of the Mn1.5[Cr(CN)6]/[C12-MIM]BF4 sample shown in Figure 2b also presents a large enthalpy peak at 13.89 °C (-50.17 J g-1) characteristic of the C-LC transition and a small enthalpy peak at 50.82 °C (-0.61 J g-1) characteristic of the LC-L transition for the heating cycle. The broad transition between 0 and 13 °C may be attributed to the reorganization prior to melting and the melting of water molecules present in the sample. The cooling cycle presents one peak at 14.32 °C

(48.32 J g-1) characteristic of the C-LC transition and a peak at 51.79 °C (-0.63 J g-1) characteristic of the LC- L transition (Table 1 and Figure 2). An additional peak at 0 °C appearing in the cooling cycle may be attributed to crystallization of water molecules added with the molecular precursors used.22 A comparison with DSC measurements performed with [C12MIM]BF4 shows that no dramatic changes in the transition temperatures were observed after formation of the nanoparticles in [C12-MIM]BF4 except in the presence of water. Indeed, after synthesis of the nanoparticles, the water content in the ionic liquid/nanoparticle system increases from 0.02 to ∼0.38 wt %. POM measurements have been performed in crystalline, liquid crystalline, and isotropic phases for the Mn1.5[Cr(CN)6]/[C12MIM]BF4 sample under polarized light in order to confirm our conclusions (Figure 3). Figure 3a shows the POM image of the sample at 2 °C in its crystalline phase showing strong birefringence of the sample. It should be noted that, at 0 °C with a waiting time of ∼10 min, the POM images show the growth of starlike crystals, which may be attributed to initial crystallization of water molecules (Supporting Information Figure S4). At 40 °C, in the liquid crystalline phase, the POM image (Figure 3b) shows that birefringent patterns are present in the overall sample, indicating a high degree of organization characteristic of a SmA phase. The presence of the lamellar phase is evidenced by typical smectic textures such as oily streaks. At 60 °C, above the clearing point, the presence of an isotropic phase characteristic of vanishing of birefringence is clearly observed (Figure 3c). However, a small birefringence is still observed, suggesting that short-range nanoparticle organization may be preserved even in the isotropic ILC phase. The sample was also characterized by X-ray diffraction measurements. In the ionic liquid crystalline phase, the X-ray diffraction performed for [C12-MIM]BF4 shows a single intense peak observed at 3.02° in 2θ (2.92 nm) characteristic of the SmA lamellar periodicity (Figure 4a, dotted line). The Mn1.5[Cr(CN)6]/ [C12-MIM]BF4 sample displays on behalf of this peak a small peak at 2.42° in 2θ (3.65 nm) that could be related to the nanoparticle’s intrachain period (Figure 4a, solid line) that corresponds to approximately twice the nanoparticle’s radius, that is, 4.0 (0.6) nm, in accordance with the dense nanoparticle packing observed by TEM. No peak is however visible at 1.69° in 2θ that shall correspond to the nanoparticle’s interchain period (ca. 5-6 nm). Such an absence of a peak could be explained by an interchain period which is not well-defined due to layers of IL of variable width resulting in a low position correlation. Below the C-LC transition, the Mn1.5[Cr(CN)6]/[C12-MIM]BF4 sample shows diffraction peaks at 1.83° in 2θ (4.83 nm), 2.91° in 2θ (3.03 nm), and 3.66° in 2θ (2.41 nm) characteristic of crystalline [C12-MIM]BF4 and an additional peak at 2.42° in 2θ (3.65 nm), indicating that the nanoparticle ordering is still preserved (Figure 4b). The nanoparticle behavior obtained in the isotropic IL is different. The TEM image of the Mn1.5[Cr(CN)6]/[C10-MIM]BF4 sample shows the presence of uniformly sized spherical shaped nanoparticles, which are nonaggregated and homogeneously dispersed in the IL (Figure 5a). The single size distribution of these nanoparticles is centered at 2.70 (0.7) nm, that is slightly smaller than that in the case of Mn1.5[Cr(CN)6]/[C10-MIM]BF4 (Figure 5c). However, after heating of these nanoparticles at 60 °C for 1 h, the TEM image shows the presence of spherical shaped nanoparticles with a size distribution centered at 3.9 (0.7) nm (Figure 5b, d), indicating that the postsynthetic heating induces

(21) Nishikawa, K.; Wang, S.; Katayanagi, H.; Hayashi, S.; Hamaguchi, H.O.; Koga, Y.; Tozaki, K. J. Phys. Chem. B 2007, 111, 4894–4900.

(22) Inoue, T.; Dong, B.; Zheng, L.-Q. J. Colloid Interface Sci. 2007, 307, 578–581.

Table 1. Thermal Data from DSC and POM Observations for [C12-MIM][BF4] and Mn1.5[Cr(CN)6]/[C12MIM][BF4] Samplesa

samples

cycle

Tm of water, °C

[C12-MIM][BF4] heating cooling Mn1.5[Cr(CN)6]/ heating broad 0-13 [C12MIM][BF4] cooling 0 a

TC-LC, °C (enthalpy, J g-1)

TLC-L, °C (enthalpy, J g-1)

12.22 (-77.04) 50.02 (-0.91) 14.18 (53.57) 50.80 (0.59) 13.89 (-50.17) 50.82 (-0.61) 14.32 (48.32)

51.79 (0.63)

Enthalpy (J g-1) is given in parentheses.

Figure 2. (a) DSC traces for [C12-MIM][BF4] for the second heating (lower) and cooling (upper) cycles. (b) DSC traces for Mn1.5[Cr(CN)6]/ [C12-MIM]BF4 for the second heating (lower) and for the second cooling (upper) cycles.

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Figure 3. Optical microscopy images (crossed polarizers) for the Mn1.5[Cr(CN)6]/[C12-MIM]BF4 sample taken at (a) 2 °C, (b) 40 °C, and (c) 60 °C. Scale bar ) 100 µm.

Figure 4. X-ray diffractograms of (a) the [C12-MIM]BF4 (dotted line) and Mn1.5[Cr(CN)6]/[C12-MIM]BF4 (solid line) samples in their liquid crystalline phases (40 °C), and (b) the Mn1.5/[Cr(CN)6]/[C12-MIM]BF4 sample (solid line) in its crystalline phase (5 °C) below the C-LC transition. Arrows indicate the peak related to the nanoparticle ordering.

a slight increase of the nanoparticle size that can be explained by the Ostwald ripening mechanism.23 The DSC trace of the isotropic ionic liquid [C10-MIM]BF4 shows a single solid-liquid transition at 2.12 °C (-49.88 J g-1) and at -42.05 °C (18.19 J g-1) for the heating and cooling cycles, respectively (Supporting Information Table S1 and Figure S5a). The Mn1.5[Cr(CN)6]/[C10-MIM]BF4 sample shows a similar behavior with a melting point at 2.67 °C (-60.24 J g-1) and at -41.29 °C (22.40 J g-1) for the heating and cooling cycles, respectively (Supporting Information Table S1 and Figure S5b), indicating that, as in the previous case, nanoparticle formation does not induce a remarkable effect on the transition temperature of the IL. In summary, the TEM, DSC, and POM measurements performed on the Mn1.5[Cr(CN)6]/[C12-MIM]BF4 sample show the presence of a two-dimensional organization of spherical nanoparticles of 4.00 (0.60) nm in diameter at the nanolevel in the liquid crystalline and crystalline phases. These nanoparticles are organized into parallel chains with an interchain period of ∼5-6 nm and an intrachain period of ∼4.0 nm. On the other hand, in the case of the isotropic [C10-MIM]BF4 IL, these measurements show the formation of spherical Mn1.5[Cr(CN)6]/ [C10-MIM]BF4 nanoparticles of 2.70 (0.70) nm randomly dispersed into the IL. Clearly, the presence of the lamellar ILC phase induces the coordination polymer nanoparticle organization. In both cases, the nanoparticle’s formation mechanism may be envisaged considering the possible local structuring of the IL even in the liquid isotropic phase. Previously, it was proposed that [Cn-MIM]BF4 ionic liquids present hydrogen bonding between the imidazolium ring protons and F atoms of the

counteranion.24 The water molecules at low content replace the C(sp2)-H · · · F interactions with hydrogen bonds involving water as an acceptor toward the cation and as a donor toward the [BF4]ion25 without changing the local structure of the imidazolium entities of the ionic liquid.26 In other words, the water molecules are intercalated between BF4- and the imidazolium ring in order to form local hydrophilic pockets (Scheme 3). Consequently, it is reasonable to consider that, for both ILs, [C12-MIM]BF4 and [C10-MIM]BF4 in their isotropic phases (at 60 °C), the hydrophilic building blocks [C12-MIM]3[Cr(CN)6] and Mn(NO3)2,4H2O react and form the nanoparticles in these local hydrophilic pockets. However, upon cooling, [C12-MIM]BF4 presents a lamellar phase inducing a long-range organization of the nanoparticles at the nanolevel, which is also preserved in the crystalline phase. Such organization may be schematically described as an alternating of hydrophobic layers constituted of alkyl chains of the IL and hydrophilic layers constituted of nanoparticles, water, [BF4]anions, and the imidazolium ring entities (Scheme 3). The interspacing between the nanoparticle centers corresponds to the length of a double layer of [C12-MIM]BF4 plus two half-layers of nanoparticles that could be estimated to be ∼5.2 nm. This value fits well with the value obtained from TEM observations, that is, 5-6 nm (see inset in Figure 1a).27 On the other hand, the isotropic phase of Mn1.5[Cr(CN)6]/[C10-MIM]BF4 is transformed directly to the crystalline phase without organization of the particles. 3.3. Magnetic Properties. The magnetic properties of the Mn1.5[Cr(CN)6]/[C12-MIM]BF4 sample have been studied by direct current (dc) and alternating current (ac) modes by using a SQUID magnetometer working in the temperature range between 1.8 and 350 K and compared with the magnetic properties of the Mn1.5[Cr(CN)6]/[C10-MIM]BF4 sample obtained in the isotropic ionic liquid. The zero-field-cooled (ZFC)/field-cooled (FC) magnetization curves performed for the Mn1.5[Cr(CN)6]/ [C12-MIM]BF4 sample in the 2-50 K range are shown Figure 6a. The ZFC curve is obtained by recording the magnetization when the sample is heated under a field of 100 Oe after being cooled in zero magnetic field. The FC data were obtained by cooling the sample under the same magnetic field after the ZFC (23) Antonietti, M.; Kunag, D.; Smarsly, B.; Azhou, Y. Angew. Chem., Int. Ed. 2004, 43, 4988–4992. (24) Noda, A.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2001, 105, 4603–4610. (25) Mele, A.; Tran, C. D.; De Paoli Lacerda, S. H. Angew. Chem., Int. Ed. 2003, 42, 4364–4366. (26) Mele, A.; Romano, G.; Giannone, M.; Ragg, E.; Fronza, G.; Raos, G.; Marcon, V. Angew. Chem., Int. Ed. 2006, 45, 1123–1126. (27) The total volume of a bilayer of IL and two half layers of nanoparticles can be expressed as V ) lS + C(4/3)RS where l is the IL bilayer length, R is the nanoparticle radius, and C is the nanoparticle packing. The interspacing between two nanoparticle layers can then be expressed as d ) l + C(4/3)R. Given that l ) 2.8 nm, R ) 2.0 nm, and C ) 0.9, then d ) 5.2 nm.

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Figure 5. TEM images of Mn1.5[Cr(CN)6]/[C10-MIM]BF4 nanoparticles (a) before and (b) after warming at 60 °C and (c,d) their corresponding size distributions. Scale Bars ) 50 nm. Scheme 3. Schematic Representation of the Growth and Organization of Coordination Polymer Nanoparticles within [C12-MIM][BF4]

experiment and recording the change in sample magnetization with temperature. The ZFC/FC curves show irreversible behavior: the ZFC curve exhibits a maximum at Tmax ) 20.04 K (Table 2), while the FC curve increases as the temperature decreases and never reaches saturation. The FC and ZFC curves coincide at high temperatures and start to separate at Tsep ) 27.00 K. The FC/ZFC magnetization curves for the sample Mn1.5[Cr(CN)6]/

[C10-MIM]BF4 also present an irreversibility of the FC and ZFC curves with a peak on the ZFC curve at Tmax ) 15.88 K and Tsep ) 18.00 K (Figure 6b, Table 1). The field dependence of the magnetization performed for the Mn1.5[Cr(CN)6]/[C12-MIM]BF4 nanoparticles at 2 K shows a hysteresis effect with a coercive field of 3.4 kOe (inset of Figure 6a). The Mn1.5[Cr(CN)6]/[C10MIM]BF4 sample shows at 2 K a similar shape of the hysteresis with a closed value of the coercive field of 3.2 kOe (inset of Figure 6b).18c In order to determine the magnetic regime of the nanoparticles in these samples, alternating current (ac) susceptibility measurements have been performed. The temperature dependence of the in-phase, χ′ (absorptive), and out-of-phase, χ′′ (dispersive), components of the ac susceptibility measured in zero static field for frequencies varying from 1 to 1488 Hz for the Mn1.5[Cr(CN)6]/ [C12-MIM]BF4 sample is shown in Figure 7a and b. At 1 Hz, both χ′ and χ′′ responses present a peak at 23.57 and 18.61 K, respectively, which shifted toward higher temperatures as the frequency increased, and at 1488 Hz these peaks are observed at 26.95 and 22.73 K, respectively. The temperature dependence of the ac susceptibility performed for the Mn1.5[Cr(CN)6]/[C10MIM]BF4 nanoparticles shows similar frequency dependent behavior with χ′ and χ′′ peaks shifted toward higher temperatures as the frequency increases (Figure 7c, d). Such frequency dependent behavior in both samples is characteristic of the presence of the nanoparticles, and it was not observed for the bulk Mn1.5[Cr(CN)6] analogues.28 In these nanoparticle-containing colloidal solutions, the low temperature magnetic transitions may be attributed to three limit cases: (i) blocking process of isolated (28) (a) Verdaguer, M.; Mallah, T.; Cadet, V.; Castro, J.; He´lary, C.; The´baut, S.; Veillet, P. Int. Conf. Coord. Chem. 1993, 4, 19–24. (b) Mallah, T.; Ferlay, S.; Souiller, A.; Verdaguer, M. NATO ASI Ser. 1996, C484, 597–614. (c) Ohkoshi, S.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Phys. ReV. B 1997, 56, 11642–11652. (d) Ohkoshi, S.; Hashimoto, K. Phys. ReV. B 1999, 60, 12820–12825.

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Figure 6. (a) Field-cooled and zero-field-cooled magnetization (FC/ ZFC) versus temperature curves performed with an applied field of 100 Oe for Mn1.5[Cr(CN)6]/[C12-MIM]BF4 samples. Inset: Field dependence of the magnetization for Mn1.5[Cr(CN)6]/[C12-MIM]BF4 samples. (b) Field-cooled and zero-field-cooled magnetization (FC/ZFC) versus temperature curves performed with an applied field of 100 Oe for Mn1.5[Cr(CN)6]/[C10-MIM]BF4 samples. Inset: Field dependence of the magnetization for Mn1.5[Cr(CN)6]/[C10-MIM]BF4 samples.

or weakly interacting superparamagnetic nanoparticles;29 (ii) nanocluster-glass-like transition caused by relatively strong magnetostatic interparticle interactions and by randomness,30 or (iii) intraparticle spin-glass-like regime due to the particle surface spin disorder.30 The temperature dependence of the relaxation time extracted from the maximum of the χ′′ component of the ac susceptibility was fitted with an Arrhenius law, τ ) τ0 exp(Ea/kBT), where Ea is the average energy barrier for the reversal of the magnetization, τ0 is the attempt time, and kB is the Boltzmann constant. According to the Ne´el model, this law governs the temperature dependence of the relaxation of the magnetization of noninteracting superparamagnetic systems (case (i)).29a For the Mn1.5[Cr(CN)6]/[C12MIM]BF4 sample, the values of the energy barrier, Ea/kB, and of the pre-exponential factor, τ0, are equal to 710 (4) K and 3.24 × 10-18 s, respectively (Supporting Information Figure S6 and Table 2). For the Mn1.5[Cr(CN)6]/[C10-MIM]BF4 sample, the Ea/kB and τ0 parameters are equal to 443 (7) K and 1.03 × 10-15 s (Supporting Information Figure S6 and Table 2). The obtained values of τ0 are smaller than those observed for pure superparamagnetic systems which are in the range of 10-9-10-12 s and thus have no physical meaning. However, similar small values of τ0 were often found for systems containing interacting metal or metal oxide nanoparticles31 or coordination polymer nanoparticles,17a,18a and they are normally interpreted as the signature (29) (a) Ne´el, L. Ann. Geophys. 1949, 60, 661–678. (b) Dormann, J. L.; Bessais, L.; Fiorani, D. J. Phys. C 1988, 21, 2015–2024. (30) Mydosh, J. A. Spin Glasses; Taylor and Francis: Washington, DC, 1993.

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of a magnetic moment correlation introduced by considerable dipole-dipole interparticle interactions. In order to investigate the probable presence of these interactions, the temperature dependence of the relaxation time was also fitted with the Vogel-Fulcher law, τ ) τ0 exp(Ea/kB (T - T0)), commonly used for magnetically interacting clusters.32 In comparison with the Arrhenius law adopted for isolated nanoparticles, the interparticle interactions here are introduced by the additional parameter, T0 (case (ii)). The best fit for the Mn1.5[Cr(CN)6]/[C12-MIM]BF4 sample gives satisfactory parameters of Ea/kB, τ0, and T0 equal to 215 (6) K, 2.107 × 10-11 s, and 9.0 (0.4) K, respectively, suggesting the presence of relatively strong interparticle interactions (Figure 8a and Table 2). In the case of the Mn1.5[Cr(CN)6]/[C10-MIM]BF4 sample, the Vogel-Fulcher fit gives parameters for Ea/kB, τ0, and T0 equal to 286 (5) K, 2.90 × 10-13 s, and 3.0 (0.5) K, respectively (Figure 8a and Table 2). In this last case, the T0 value is too small to be reasonable and the τ0 value is also smaller in comparison to the usual values observed for interacting clusters (∼10-9-10-12 s), indicating that this model is not appropriate. In order to verify if the dynamics in these systems exhibit critical slowing down as usually observed in canonical spinglass systems presenting strong interparticle interactions (case (ii)), we also use the conventional critical scaling law of spin dynamics, τ ) τ0[Tg/(Tmax - Tg)]zV and Tmax ) Tg[1 + (τ0f)1/zν], where Tg (*0) is the glass temperature, f is the frequency, and zV is a critical exponent.33 For the Mn1.5[Cr(CN)6]/[C12-MIM]BF4 sample, the best fit gives the following parameters: Tg ) 13.5 K, τ0 ) 1.76 × 10-6 s, and zν ) 11.4 (Figure 8b and Table 2). The obtained zν value is in the range 4-12 expected for classical spin-glass systems, while the value of τ0 is larger than that in conventional spin-glasses (∼10-13 s) but close to what is observed in the case of nanometric cluster-glass systems (∼10-6-10-11 s).32 Another situation is observed in the case of Mn1.5[Cr(CN)6]/ [C10-MIM]BF4 nanoparticles in which the parameters Tg ) 13.0 K, τ0 ) 1.08 × 10-7 s, and zν ) 19.1 were found (Figure 8b and Table 2). This high zν value indicates that this model is not appropriate in this case. The temperature dependences of ac susceptibility at 125 Hz were then performed with different applied direct current (dc) fields. For both samples, Mn1.5[Cr(CN)6]/[C12-MIM]BF4 and Mn1.5[Cr(CN)6]/[C10-MIM]BF4, the same tendency was observed: as the field increases, the peak intensity of both components, χ′ and χ′′, decreases. On the other hand, the peak temperature of χ′ is shifted toward higher temperatures while the peak temperature of χ′′ is shifted to lower temperatures as the field increases (Figure 9 and Supporting Information Figure S7). This behavior does not correspond to what is usually observed for classical spin-glass systems where the peak temperature of both components decreases as the applied field increases.34 However, such behavior was previously observed for cyano-bridged coordination polymer nanoparticles obtained in organic phases.17a The trace of the temperature maxima of the χ′ and χ′′ components as a function of H2/3 giving linear dependence in both cases perfectly corresponds to the so-called de Almeida-Thouless (AT) line35 given by the equation H ∝ (1 - Tmax/Tf)3/2 usually used for classical spin-glass systems (case (ii)) (Figure 10 and (31) (a) Djurberg, C.; Svedlindh, P.; Nordblad, P.; Hansen, M. F.; Bodker, F.; Morup, S. Phys. ReV. Lett. 1997, 79, 5154–5157. (b) Balaji, G.; Wilde, G.; Weissmuller, J.; Gabhiye, N. S.; Sankaranarayanan, V. K. Phys. Status Solidi B 2004, 241, 1589–1592. (32) Girtu, M. A. J. Opt. AdV. Mater. 2002, 4, 85–92. (33) Dekker, C.; Arts, A. F. M.; De Wijn, H. W.; Van Duyneveldt, A. J.; Mydosh, J. A. Phys. ReV. B 1989, 40, 11243–11251. (34) Tiwari, S. D.; Rajeev, K. P. Phys. ReV. B 2005, 72, 104433/1–104433/9. (35) Almeida, J. R. L.; Thouless, D. J. J. Phys. A 1978, 11, 983–995.

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Table 2. Magnetic Data of Mn1.5[Cr(CN)6]/[Cn-MIM][BF4] (n ) 10, 12) Samples Arrhenius law a

τ0, s

Vogel-Fulcher law τ0, s

Ea/kB, K Tg, K

τ0, s

samples

Tmax, K

Ea/kB, K

Mn1.5[Cr(CN)6]/ [C12-MIM][BF4] Mn1.5[Cr(CN)6]/ [C10-MIM][BF4]

20.04

710 (4)

3.24 × 10-18 9.0 (0.4) 2.11 × 10-11

215 (6)

13.5

1.76 × 10-6 11.4

21.6

3.4

15.88

443 (7)

1.03 × 10-15 3.0 (0.5) 2.90 × 10-13

286 (5)

13.0

1.08 × 10-7 19.1

16.7

3.2

a

Tg, K

power law zV

Tg, KAT line Hc, Oe

Obtained as a maximum value of the ZFC curve.

Figure 7. Temperature dependence of (a) in-phase, χ′, and (b) out-of-phase, χ′′, components of the ac susceptibility performed in zero field for the Mn1.5[Cr(CN)6]/[C12-MIM]BF4 sample. Temperature dependence of (c) in-phase, χ′, and (d) out-of-phase, χ′′, components of the ac susceptibility performed in zero field for Mn1.5[Cr(CN)6]/[C10-MIM]BF4 sample. Frequencies: 1 Hz (O), 125 Hz (b), 498 Hz (0), 998 Hz (9), and 1498 Hz (4).

Supporting Information Figure S8). The spin-glass transition temperature obtained by extrapolation of the AT line back to H ) 0 was found equal to 21.6 K for Mn1.5/[Cr(CN)6]/[C12-MIM]BF4 nanoparticles and to 16.7 K for Mn1.5[Cr(CN)6]/[C10-MIM]BF4 nanoparticles (Table 2). These values are closed to the Tmax values obtained by the maximum on the ZFC curves, which are equal to 20.04 and 15.88 K, respectively. Usually, the compliance of the data with the AT line is considered to be evidence for the existence of spin-glass behavior. In our case, both samples present a satisfactory agreement with the AT line, while the temperature dependence of the relaxation time fitted with Vogel-Folcher and critical slowing down models shows the presence of clusterglass behavior in the first case and not in the second one. It is thus reasonable to consider that, in the present cases, the AT line model is appropriate not only for the strong but also for the not strong interparticle interactions. Finally, we verify the presence of an aging phenomenon in Mn1.5[Cr(CN)6]/[C12-MIM]BF4 and Mn1.5[Cr(CN)6]/[C10-MIM]BF4 systems that allows us to unequivocally distinguish correlated dynamics from only individual particle relaxation. The aging phenomenon is lacking in superparamagnetic systems in which the relaxation is only governed by thermally activated dynamics of individual particles. On the other hand, an aging phenomenon occurs in the systems with collective transitions such as classical spin-glasses or cluster-glasses induced by strong interparticle interactions between nanoparticles.36 For these measurements, the samples were cooled in zero field from a temperature where

they show a reversible behavior (100 K) to a measurement temperature, 13 K. After a wait time, tw, at 13 K, a small field of 2 Oe was applied and the magnetization was recorded versus time. The ZFC relaxation measurements have been performed with a zero field wait of 102, 103, and 104 s. For the Mn1.5[Cr(CN)6]/ [C12-MIM]BF4 sample, the relaxation curves display a clear wait time dependence, indicating that the magnetic dipole-dipole interactions impose aging effects usually observed in spin-glasses or cluster-glasses (Figure 11).36 On the contrary, the wait time dependence for the Mn1.5[Cr(CN)6]/[C10-MIM]BF4 sample is very small, indicating that the interparticle interactions in this case are not strong enough to induce an important aging effect (inset in Figure 11). On the other hand, we cannot completely exclude an influence of the particle surface spin disorder on the magnetic regime in these systems (case (iii)). However, the spin frustration influence on the magnetic regime was found mostly important for coordination polymer nanoparticles inserted into silica matrixes but less significant for nanoparticles in colloidal solutions.16d

4. Conclusion To summarize, the ILC 1-dodecyl-3-methylimidazolium tetrafluoroborate, [C12-MIM]BF4, was used for the first time as a stabilizing and structuring agent and as a solvent in the in situ synthesis and organization into a two-dimensional array of cyano-bridged Mn1.5[Cr(CN)6]/[C12-MIM]BF4 coordination polymer nanoparticles. An ensemble of TEM, DSC, POM,

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Figure 8. (a) Thermal variation of the relaxation time according to the Vogel-Fulcher law for Mn1.5[Cr(CN)6]/[C12-MIM]BF4 (b) and Mn1.5[Cr(CN)6]/[C10-MIM]BF4 (O) samples. (b) Thermal variation of the relaxation time according to a power law, τ ) τ0[Tg/(Tmax - Tg)]zV, for Mn1.5[Cr(CN)6]/[C12-MIM]BF4 (b) and Mn1.5[Cr(CN)6]/[C10-MIM]BF4 (O) samples.

and X-ray diffraction measurements shows the presence of a two-dimensional organization of the nanoparticles at the nanolevel in the liquid crystalline and crystalline phases. Spherical nanoparticles of 4.00 (0.60) nm in diameter are organized into a two-dimensional array formed by parallel chains of nanoparticles with a regular interchain separation between the nanoparticle centers in the 5-6 nm range corresponding to the length of a double layer of [C12-MIM]BF4 plus two half-layers of nanoparticles and an intrachain separation of ∼4.0 nm between the nanoparticle centers corresponding to the sum of the radii of the two nanoparticles. The heating of these nanoparticles does not induce any change in their size, shape, and organization, which is in agreement with the fact that some nanoparticle organization is preserved even in the isotropic phase, providing important stability of the system. The first point to note concerns the difference in the nanoparticle spatial distribution obtained in the ILC, [C12MIM]BF4, and in the isotropic IL, [C10-MIM]BF4. Indeed, by using the isotropic IL, spherical Mn1.5[Cr(CN)6]/[C10-MIM]BF4 nanoparticles with a slightly smaller size of 2.70 (0.70) nm, which are homogeneously dispersed in [C10-MIM]BF4, were obtained. The difference in the spatial arrangement of the nanoparticles in these samples is clearly attributed to the presence of the liquid crystalline phase of the [C12-MIM]BF4 ionic liquid, which induces an organization of the nanoparticles into the hydrophilic space of the double layers of IL. The second point to note concerns the difference in the magnetic properties of the organized and randomly dispersed (36) Jonsson, T.; Mattsson, J.; Djurberg, C.; Khan, F. A.; Nordblad, P.; Svedlindh, P. Phys. ReV. Lett. 1995, 75, 4138.

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Figure 9. Temperature variation of the (a) real and (b) imaginary parts of the ac susceptibility performed with different applied fields for the Mn1.5[Cr(CN)6]/[C12-MIM]BF4 sample. Applied field: 100 Oe (O), 250 Oe (3), 500 Oe (b), and 750 Oe (0).

Figure 10. Tmax (χ′) and Tmax (χ′′) plotted against H2/3 for the Mn1.5[Cr(CN)6]/[C12-MIM]BF4 sample. Solid lines show a linear fit to the data.

nanoparticles. Two factors may be responsible for this difference: the difference in the size of the nanoparticles and the strength of the magnetostatic interparticle interactions. First, in the case of Mn1.5[Cr(CN)6]/[C12-MIM]BF4, the size of the nanoparticles is 4.0 (0.6) nm, and in the case of Mn1.5[Cr(CN)6]/[C12-MIM]BF4 the size is equal to 2.7 (0.7) nm. For the isolated nanoparticles, the blocking temperature is proportional to the volume of the nanoparticle and then decreasing the nanoparticle’s size induces decreasing of the blocking temperature and the energy barrier. For this reason, it is reasonable to suppose that the difference in the size of the nanoparticles can contribute to the difference in their magnetic behavior. Second, the dynamic analysis of the ac susceptibility data of these samples with different models, such as the Vogel-Fulcher law, critical slowing down model, as well as the ZFC relaxation measurements, clearly indicates the presence of relatively strong interparticle interactions

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These interparticle interactions appear stronger than the dipolar interactions observed for homogeneously dispersed Mn1.5[Cr(CN)6] nanoparticles synthesized in the isotropic IL, [C10MIM]BF4, in which the presence of relatively weak magnetostatic interactions may be evidenced by the ZFC relaxation measurements. The difference in the magnetic regime of these samples is coherent with the observed anisotropic closepacking arrangement of the nanoparticles induced by the ILC, [C12-MIM]BF4. In this case, the interparticle distances and thus the interparticle interactions are controlled by the ILC organization giving rise to a pronounced collective effect for these strongly interacting Mn1.5[Cr(CN)6] nanoparticles.

Figure 11. Time dependence of the ZFC magnetization curves performed at 13 K with different wait times, tw, for the Mn1.5[Cr(CN)6]/ [C12-MIM]BF4 sample with an applied field of 2 Oe. Inset: Time dependence of the ZFC magnetization curves performed at 13 K with different wait times, tw, for the Mn1.5[Cr(CN)6]/[C10-MIM]BF4 sample with an applied field of 2 Oe.

producing a nanocluster-glass-like behavior in the case of organized Mn1.5[Cr(CN)6] nanoparticles within [C12-MIM]BF4.

Supporting Information Available: Thermal data from DSC and POM observation, photograph of sample Mn1.5[Cr(CN)6]/[C12MIM]BF4, IR spectra of Dmim3(Cr(CN)6) and Mn1.5[Cr(CN)6]/ DmimBF4, optical microscopy images of the [C12-MIM]BF4 sample, POM image of the Mn1.5[Cr(CN)6]/[C12-MIM]BF4 sample in crystalline phase, DSC traces of the isotropic liquids, thermal variation of relaxation times for Mn1.5[Cr(CN)6]/[C12-MIM]BF4 and Mn1.5[Cr(CN)6]/ [C10-MIM]BF4 samples, and temperature variation of real and imaginary parts of the ac susceptibility for the Mn1.5[Cr(CN)6]/[C12MIM]BF4 sample. This material is available free of charge via the Internet at http://pubs.acs.org. LA803001X