Triggering Two-Step Spin Bistability and Large Hysteresis in Spin

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Article Cite This: Chem. Mater. 2017, 29, 8875-8883

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Triggering Two-Step Spin Bistability and Large Hysteresis in Spin Crossover Nanoparticles via Molecular Nanoengineering Giorgio Zoppellaro,* Jiří Tuček, Juri Ugolotti, Claudia Aparicio, Ondrej Malina, Klára Č épe, and Radek Zbořil* Regional Centre for Advanced Technologies and Materials, Palacky University, Šlechtitelů 27, 78371 Olomouc, Czech Republic S Supporting Information *

ABSTRACT: The local entrapment of the spin crossover complex Fe(II)-tris[2-(2′-pyridyl)benzimidazole] into the pluronic polymeric matrix (P123, PEG20−PPG70−PEG20, MW ∼ 5800) yielded the formation of magnetic nanoparticles of ∼26 nm (SCO-Np). Formation of SCO-Np was driven by the emergence of noncovalent interactions between the aromatic −NH group of the benzimidazole moieties present in Fe(II)-tris[2-(2′-pyridyl)benzimidazole] with the aliphatic ether (−O−) groups of the pluronic polymeric matrix. The nanoparticles show spin crossover behavior, two-step spin bistability, and wide magnetic hysteresis, expressed in the temperature range of 170−280 K (ΔTmax = 38 K). The neat SCO molecules, Fe(II)-tris[2-(2′-pyridyl)benzimidazole], on the contrary show only first-order spin transition and negligible hysteresis. The developed matrix-confinement approach of SCO molecules shown in this work yielded an unprecedented and significant improvement of the magnetic cooperativity compared to the neat spin crossover system, despite the decreased dimension of the magnetic domain in the nanosized architecture.

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INTRODUCTION In spin crossover complexes (SCO), the energy gap between the high-spin (HS) and low-spin (LS) states of the metal centers lies within the range of thermal energies, kbT.1,2 The occurrence of this gap is governed by the interplay of ligand field strength (Δ) and the mean spin-pairing energy (P), which can induce rearrangements of the electron distribution in the t2g and eg metal/s orbitals. This rearrangement takes place when external stimuli, such as temperature, pressure, electric field, and light, are applied to the system.3−8 In addition to the magnetic HS↔LS transition, other physical effects, most notably structural modifications in the metal−donor bond lengths and angles, occur in the molecular system during the evolution of the spin crossover phenomenon.2 When the local changes experienced at the single-molecule level are effectively transmitted throughout the lattice, the bulk SCO material may exhibit macroscopic properties, such as collective first-order, multistep spin transitions, and hysteretic behaviors.1,6 These properties can be used to construct advanced materials in the form of memory devices, electrical (spin) switches, and sensors.9−13 Most of the reported SCO molecules contain d4−d7 metal centers, which have been organized into a large array of molecular architectures such as mononuclear complexes, gridlike structures, and polymeric Hofmann clathrate materials.1 Although many studies have described the SCO behavior of systems as a macroscopic property, fewer magnetic nanostructures capable of retaining spin bistability at the level of nanodomain have been reported.14−27 This dearth is attributed to several synthetic challenges that must be © 2017 American Chemical Society

overcome for the successful assembly of SCO nanostructures; for example, in nanoparticles, size-reduction effects lead to a decrease in the magnetic cooperativity with suppression of the SCO behavior.1,28 The emergence of this phenomenon has been demonstrated in several cases,1 for example, by Mallah et al. and Real et al. for SCO nanoparticles based on the pyrazine [Fe(pz){Pt(CN)4}] coordination polymers.15,16 Matrix effects link the single-molecule behavior of the nanoparticle to its surrounding and can either enhance or suppress the spin crossover phenomenon in a manner that remains difficult to envision a priori. Therefore, selection of the matrix for the assembly of the SCO nanocomposite represents, from the chemical-combination perspective, a synthetic conundrum.29−36 In this work, spin crossover nanoparticles (mean size of 26 nm) were synthesized using the spin crossover complex FeII-tris[2(2′-pyridyl)benzimidazole] (coded hereafter Fe(II)-SCO) as key building-block component (Figure 1a). Formation of the SCO nanoparticles (coded hereafter SCO-Np) was realized via the noncovalent bottom-up approach, and based on in situ formation and entrapment of the Fe(II)-SCO molecules into the triblock polymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Figure 1b, coded P123). We demonstrate that the size reduction of the magnetic domain, from micrometer needles-like structures of the neat Received: August 28, 2017 Revised: September 28, 2017 Published: September 28, 2017 8875

DOI: 10.1021/acs.chemmater.7b03633 Chem. Mater. 2017, 29, 8875−8883

Article

Chemistry of Materials

wrapped around the Fe(II)-SCO molecule, is given in Figure 2a, together with the electrostatic-potential map surfaces of

Figure 1. Schematic of the sequence used for the synthesis of the spin crossover complex FeII-tris[2-(2′-pyridyl)benzimidazole], termed Fe(II)-SCO (a) and (b) the spin crossover nanoparticles, SCO-Np, based on the Fe(II)-SCO complex and the P123 polymer.

Fe(II)-SCO material to nanodomains for SCO-Np, does not hamper magnetic cooperativity; on the contrary, in the nanoparticles, it is enhanced. SCO-Np not only retains the spin crossover behavior of the neat FeII-tris[2-(2′-pyridyl)benzimidazole] complex but also shows in addition two-step spin transition and exhibits clear hysteretic behavior, with T1↓ at 210 K (cooling) and T1↑ at 248 K (heating), with hysteresis width (ΔTmax) of 38 K, and T2↑↓ at 123 K. The neat Fe(II)SCO complex undergoes only first-order spin transitions and shows negligible hysteresis (T1↓ at 150 K and ΔTmax ∼ 2 K). Therefore, the multistep spin transitions and clear hysteretic properties associated with the HS↔LS crossover process in SCO-Np is attributed to the morphological organization of the entrapped spin crossover molecules and their interaction with the polymer matrix. The results reported represent a rare example of highly enhanced magnetic cooperativity of spin crossover molecules arranged into nanodomains obtained through matrix-assisted confinement.

Figure 2. (a) Electrostatic potential energy map of Fe(II)-SCO (DFT/UB3LYP/6-31G*) before (left) and after (right) interaction with the poly(propylene glycol) residue (truncated at C60O21H122) (UHF/PM6). The folded structure drawn on the right side results from a conformational search performed via the MMFF94/Monte Carlo method, followed by geometry optimization via the UHF/PM6 method. A positive (2+) charge was set for both structures, following the removal of the perchlorate (ClO4−) counteranions. (b) Energy plot of the stable conformers obtained via MMFF94/Monte Carlo using an energy window of 40 kJ/mol and panel (c) the structural overlay showing the folded structures corresponding to the energy plot.

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RESULTS AND DISCUSSION Synthesis, Morphological Organization, and Electronic Properties of the SCO-Np System. The neat Fe(II)-SCO spin crossover complex was synthesized using a modified version of the procedure described in the literature (Figure 1a).37,38 The synthesis of SCO-Np is schematically illustrated in Figure 1b. SCO-Np was obtained by in situ assembly of Fe(II)-SCO molecules in the EtOH/CH2Cl2 mixture in the presence of the triblock polymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (coded henceforth P123, PEG20−PPG70−PEG20, MW ∼ 5800), followed by slow hydration of the raw material and separation of the spin crossover nanoparticles by centrifugation. The detailed synthetic steps used to produce SCO-Np are provided in the Experimental Section. The successful entrapment of Fe(II)-SCO molecules by P123, which leads to the formation of SCO-Np, is driven by electrostatic interactions between the two building-block components. The Fe(II)-SCO entrapment is promoted by the formation of short contacts between the aromatic −NH group of the benzimidazole moieties of Fe(II)-SCO and the aliphatic ether (−O−) groups of the pluronic polymer backbone (d = 1.731−1.794 Å). The resulting folded structure, in which the P123 polymer is tightly

Fe(II)-SCO before (left structure) and after (right structure) interaction with a poly(propylene glycol) chain (truncated at C60O21H122). The structures were optimized by neglecting the presence of perchlorate counteranions (ClO4−), in order to ease the computational cost and because, in the merged SCONp system, a positive zeta potential is found to hold in solution (vide inf ra). Figure 2b shows the results (energy plot) obtained from theoretical calculations via MMFF94/Monte Carlo methods. The corresponding structural overlay of the conformers distribution is shown in Figure 2c. The morphological organization and chemical composition of the SCO-Np nanosystem obtained by TEM/STEM-EDS and SEM analysis are illustrated in Figure 3f−m, and have been compared with the neat Fe(II)-SCO complex, which forms large rod-shaped μm-sized crystals (Figure 3a−e). SCO-Np, differing from neat Fe(II)-SCO, is composed by small, nm-sized nanoparticles. The DLS size-distribution analysis, carried out on SCO-Np (H2O/EtOH) is given in the Supporting Information (Figure S1), showing a mean hydrodynamic size (d) of 26 nm. These nanostructures feature, 8876

DOI: 10.1021/acs.chemmater.7b03633 Chem. Mater. 2017, 29, 8875−8883

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Chemistry of Materials

Figure 3. (a) SEM micrographs of the Fe(II)-SCO complex in the form of needle crystals. Panels (b)−(e) show the SEM-EDS elemental mapping (oxygen, b; chlorine, c; carbon, d; iron, e). Panel (f) shows the SEM micrograph of SCO-Np. (g) TEM micrograph of SCO-Np. Panel (h) shows the HAADF image of a small nanoparticles assembly of SCO-Np. Panels (i)−(m) show the STEM-EDS elemental mapping with carbon in (i), chlorine in (j), oxygen in (k), iron in (l) and the elemental Fe/O/C overlay in (m). Panel (n) shows the STEM-EDS spectrum of SCO-Np in the low-energy window, with peaks associated with C, N, O, and Fe.

in addition, a positive zeta potential, (ξ), of +2.1 ± 0.6 mV and exhibit low conductivity in water (0.0281 mS/cm). Each SCONp nanoassembly contains approximately 1.19 × 105 molecules of Fe(II)-SCO molecules confined in the pluronic nanostructures, assuming an average volume of 7.36 × 107 Å3 (with r ∼ 260 Å) for the individual nanoparticle and 617 Å3 for the isolated Fe(II)-SCO molecule, as calculated from DFT theory (UB3LYP/6-31G*). In the so-formed structural organization, the pluronic wall defines a constraint environment for the entrapped Fe(II)-SCO molecules and forms a diamagnetic shield that may limit the magnetic cooperativity. Further details on the structural organization and chemical composition of

SCO-Np come from high-angle annular dark-field imaging (HAADF) and the energy-dispersive X-ray spectroscopic (STEM-EDS) analysis. Figure 3h shows the HAADF image of a small assembly of the nanoparticles deposited onto the copper grid and Figure 3n the low-energy EDS spectrum. The spatial distribution of elemental Fe (coded in red), O (coded in green), C (coded in blue), and Cl anion (coded in purple), as obtained from STEM-EDS analysis, is given in Figure 3i−n. Additional TEM micrographs are presented in the Supporting Information (Figure S1). From the STEM-EDS analysis, the Fe(II)-SCO molecules appear to be well encapsulated in the 8877

DOI: 10.1021/acs.chemmater.7b03633 Chem. Mater. 2017, 29, 8875−8883

Article

Chemistry of Materials

Comparison of the powder FT-IR spectrum of neat Fe(II)SCO with SCO-Np reveals, in the latter, the emergence of typical peaks belonging to the 2-(2′-pyridyl)benzimidazole backbone associated with the Fe(II)-SCO system (∼1600−900 cm−1) together with a clear peak linked to the presence of perchlorate counteranions (νCl‑O, 1055 cm−1); thus, the process of matrix entrapment occurred without chemical degradation of the Fe(II)-SCO molecules (Supporting Information, Figure S3). Additional evidence of the structural/chemical integrity of the Fe(II)-SCO organometallic scaffold after entrapment in the P123 matrix is given by X-ray powder diffraction analysis (PXRD). Figure 5 shows the diffraction pattern of SCO-Np

pluronic matrix, with homogeneous distribution of the key Fe element, associated univocally to the SCO complex. However, we also noticed that some loss of perchlorate anions occurred during the encapsulation process of Fe(II)SCO in P123, as seen from the Cl elemental distribution shown in Figure 3j. This effect has been further estimated semiquantitatively by analyzing the XEDS pattern in more details, within the energy window of 2.1−8.7 keV, and by assessing the relative abundance of the Cl and Fe elements (K-alpha peaks at 2.622 and 6.404 eV, respectively). As shown in Figure 4b, in

Figure 5. PXRD diffractograms of (a) bulk Fe(II)-SCO, (b) the SCONp nanoparticles, and (c) the polymer matrix P123, recorded at T = 300 K. The radiation used was Cu Kα (45 kV, 40 mA, λ = 0.1541 nm). Figure 4. X-ray energy-dispersive spectroscopy (XEDS) patterns of (a) neat Fe(II)-SCO and (b) SCO-Np, after being deposited onto copper grids. The integrated areas under the Cl-Ka and Fe-Ka peaks have been highlighted. The signal-area ratio (Cl-Ka/Fe-Ka) is equal to ∼3.6 for Fe(II)-SCO, as shown in (a), and ∼0.6 for SCO-Np, as shown in (b).

(panel b) compared to neat Fe(II)-SCO (panel a) and the neat P123 polymer (panel c). The sharp diffraction peaks observed in SCO-Np are found at very similar scattering angles (deg) but with some variations in the relative intensities with respect to those expressed by neat Fe(II)-SCO. The effect indicates that, while the entrapped spin crossover material remains crystalline, some variations in the local arrangements of the pyridylbenzimidazole moieties occur in response to the matrix entrapment. UV/vis spectroscopy was additionally used as a sensitive probe for screening the extent of electronic perturbation of the Fe(II) d−d transition after matrix entrapment; a bathochromic shift in the d−d transition (λmax, nm), from 492 nm for the neat Fe(II)SCO to 502 nm for SCO-Np (Supporting Information, Figure S4) occurs upon entrapment into the polymer matrix. Finally, the thermogravimetric (TGA) analysis uncovered that the weight loss recorded as a function of temperature for Fe(II)-SCO and SCO-Np follows a similar trend. At 600 °C, these systems show a residual mass of 11.28% (Fe(II)-SCO), and 8.87% (SCO-Np), due to the formation of iron oxide (Figure S5). The result indicates that SCO-Np contains an approximate of 2.5% (weight) in P123 polymer and supports further the minor contribution witnessed on the scattering

SCO-Np, the intensity of the Fe-Ka peak was found consistently slightly stronger than the Cl-Ka peak, after probing several different areas of the deposited nanoparticles onto the copper grid, while in the neat Fe(II)-SCO material (Figure 4a) occurred the reversed property; therefore, neat Fe(II)-SCO contains statistically more perchlorate anions vs iron cation than SCO-Np. A similar phenomenon has been observed by Ruben et al. in bis-hydrazone based ligands, upon formation of [2 × 2] gridlike cobalt complexes carried out in polar solvents.39 In that scenario, loss of tetrafluoroborate anions was accompanied by the loss of protons from the N-H groups of the hydrazone-based ligands, rendering the reversible protonation/deprotonation process in solution of the cobalt complex strongly pH-responsive. Although an analogous deprotonation process cannot be excluded and, thus, may also occur in Fe(II)-SCO during synthesis of SCO-Np, the loss of N-H protons from the benzimidazole moieties as well as loss of perchlorate anions appears, in our case, not so prominent. 8878

DOI: 10.1021/acs.chemmater.7b03633 Chem. Mater. 2017, 29, 8875−8883

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Chemistry of Materials

molecules remaining in the HS (S = 2) state, when the Fe(II) zero-f ield-splitting contribution is taken into consideration.38 In contrast to the behavior observed for Fe(II)-SCO, the χmT product of SCO-Np at 330 K accounts for 2.60 cm3 mol−1 K and decreases at first smoothly until 180 K and then sharply by lowering further the temperature, reaching the value of 0.11 cm3 mol−1 K at 5 K, where approximately 4% of molecules remained in the HS (S = 2) configuration (Figure 6b). The maximum hysteresis width is observed in the high temperature range, with estimated T1↓ at 210 K (cooling) and T1↑ at 248 K (heating), and ΔTmax of 38 K, while the second transition T2↑↓ occurs at 123 K. Upon recording additional cooling and heating cycles (Figure 6c, second cycle; Figure 6d, third cycle), the observed magnetic behavior revealed that the SCO-Np system is capable of retaining well the two-steps spin crossover behavior, without alteration of both transition temperatures and hysteresis width. The generation of multi-bistable spin crossover systems, as that reported here in SCO-Np, has been previously achieved by (i) engineering molecular building units that contain two or more spin active centers, such as binuclear, polynuclear, and gridlike complexes, (ii) via crystal engineering where mononuclear spin crossover complexes are crystallized in different asymmetric units, or (iii) by cocrystallization of different SCO molecules.40−51 Owing to a reduction in the number of interacting spin active centers, size-reduction effects lead (in general) to a decrease in the magnetic cooperativity of nanosized SCO materials, an effect that does not occur in SCONp.1 In addition, many studies have attributed the occurrence of steplike features and hysteresis in SCO materials to the existence of subtle physical and structural parameters. These properties have been interpreted mainly through exchange phenomena, stemming from “ferromagnetic-like” long-range and “antiferromagnetic-like” short-range elastic interactions in the lattice, as well as through static and/or dynamic Ising-like and atom-phonon coupling processes.1,52−57 Thus, the interplay of short-range versus long-range interactions in the lattice can generate various SCO domains, which have different transition temperatures (e.g., T1,2). In the SCO-Np system, we can suggest the occurrence of the following phenomena: (i) The SCO molecules entrapped in the P123 micelle, and directly interacting with the polymeric wall of the nanoparticle, have a different transition temperature from those SCO molecules segregated in the core region. The hysteresis of the high-temperature step is therefore thought to arise from the fraction of Fe(II)-SCO molecules in direct contact with the P123 matrix, property that is in harmony with known effects associated with surface rigidity observed on SCO nanoparticles.28 (ii) The transition temperatures T1 and T2 are primarily tuned by the matrix organization (size constraint through matrix’s boundary), the chemical nature of the encaging polymer (elasticity), and its ability to establish noncovalent interactions with the Fe(II)-SCO molecules. The presence of noncovalent interactions between P123 and the fraction of directly interacting Fe(II)-SCO molecules grants to the entire SCO-Np system enough plasticity, thereby preventing the core molecules to be frozen into one fixed spin configuration. (iii) The core Fe(II)-SCO molecules, which are not directly affected by noncovalent interactions with the polymer matrix, undergo HS−LS transition at similar temperature to that expressed by the neat Fe(II)-SCO material (T2↑↓ at 123 K for SCO-Np and T1↓ at 150 K (cooling) for Fe(II)SCO).

signals of SCO-Np (PXRD analysis) that arises from the presence of the P123 matrix. Magnetic Properties of the SCO-Np System Compared to Neat Fe(II)-SCO. The morphological architecture, crystallinity, and retained chemical integrity of Fe(II)-SCO molecules entrapped in the P123 polymer allow probing the relevance of the matrix and size-reduction effects in either suppressing, enhancing, or leaving unaltered the spin crossover property of the neat material (vide inf ra). The recorded magnetic behavior of the micrometer-sized Fe(II)-SCO crystals is shown in Figure 6a. The observed evolution of the χmT

Figure 6. Magnetic properties of the SCO systems. Panel (a) shows the temperature dependence of the magnetic moments of neat Fe(II)SCO. Panels (b)−(d) show the magnetic moment of SCO-Np (1st, 2nd, and 3rd cycles, respectively). The magnetic moments were recorded under cooling and heating rates (v) of 1 K/min and under an applied field of 1000 Oe. The arrows in the panels indicate the direction of the magnetic acquisition (arrow down, cooling; arrow up, heating).

values (Figure 6a) vs temperature T is consistent with that reported for the monohydrate perchlorate complex.37,38 Neat Fe(II)-SCO has a magnetic moment (χmT) of 3.17 cm3 mol−1 K at 300 K and a residual 0.24 cm3 mol−1 K at 5 K. The firstorder spin transition (T1↓ = 150 K, cooling), T1↑ = 152 K, heating) is accompanied by negligible hysteresis, which is characterized by the maximum width (ΔTmax) of ∼2 K. The residual moment occurring at 5 K corresponds to