Multistable molecular chain magnet in electrospun polymer fibers

Abstract

New composite materials with multi-switchable thermochromic and vapochromic properties have been produced by the incorporation of a coordination chain {NH₄[Ni(cyclam)][Fe(CN)₆]·5H₂O}_n (cyclam = 1,4,7,11-tetraazacyclotetradecane) into the electrospun fibers of poly(ε-caprolactone) (PCL) or poly(2-vinylpyridine-co-styrene) (P2VP-PS). The embedded coordination compound is characterized by reversible metal-to-metal charge transfer, which can be triggered by changes in temperature, humidity, or pressure, and under ambient conditions, it can exist in three phases differing in color and magnetic susceptibility. However, its crystalline form is brittle and easily damaged by contact with liquid water. The composite mats combine the switchability of the coordination chain with the favorable mechanical properties of the organic polymer matrix. Most importantly, the use of the hydrophobic PCL polymer provides full protection from water damage. The morphology of the mats and the influence of the polymer matrix on the properties of the embedded compound have been studied by electron microscopy (SEM and TEM), X-ray absorption (XAS) and Raman spectroscopy, PXRD, dynamic vapor sorption (DVS), and magnetic measurements.

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1. Introduction

Bistable molecular materials with switchable properties are good candidates for applications in sensing or information storage.^1–6 Switching in coordination systems can be achieved through various methods, but the most significant effects arise when the process involves conversion between different electronic states of the metal centers. This leads to easily observable changes in color and magnetic moment. Spin crossover (SCO) involving the interconversion between the high spin (HS) and low spin (LS) states is the most common switching mechanism.^7–9 SCO compounds exhibiting abrupt transitions with defined hysteresis at room temperature, multi-responsivity, and durability during repeated phase transitions are promising for potential applications.^10,11 Metal-to-metal charge transfer (MMCT) processes, less common than SCO, induce phase transitions and have been observed in CN-bridged systems, particularly involving Co^II/III–Fe^III/II redox couples.^12–14 Thermal charge transfer can also occur between metal centers and redox-active ligands, affecting electronic states, structure, color, and magnetic properties, similar to SCO. CT and SCO processes exhibit high solid-state cooperativity and are easily influenced by changes in intermolecular interactions, which allow tuning of phase transition characteristics through the exchange of lattice solvent,¹⁵ guest ions,¹⁶ or peripheral ligand substituents.¹⁷

Incorporating switchable molecular magnets into real electronic devices requires a fabrication methodology of robust materials that can be handled and integrated without compromising functionality, which currently remains a challenge. One of the approaches allowing the development of reproducible switching structures is the integration of a bistable molecular system into an organic polymer matrix.^18–21 According to the recent literature, there are many examples of SCO-based composites,^22–30 while there is a lack of examples of composites based on CT compounds. The simplest method to fabricate functional SCO particle–polymer composites is drop-casting.^31–33 Using this technique, composites that can be used as mechanical actuators, optical sensors, or artificial muscles were obtained.^34,35

Polymer/molecule-based magnetic nanoparticle composites can be efficiently produced through electrospinning methods.^36,37 This technique enables the fabrication of fibers with diameters ranging from nanometers to micrometers. The key advantages of electrospinning include its cost-effectiveness, the ability to produce materials with high surface area to volume ratios, and its versatility across various materials. Among composite preparation methods, electrospinning is particularly promising because it can integrate magnetic nanoparticles into mats of electrospun fibers, retaining both magnetic functionality and mechanical properties. The significant elongation forces during fiber stretching can help in overcoming issues commonly encountered in traditional methods, such as drop casting. The setup comprises a syringe needle connected to a high-voltage power supply, a syringe pump, and a grounded collector. Although this technique has been used to produce SCO–polymer composite fibers from solutions or particle–polymer suspensions, only a limited number of successful examples have been documented to date.^38–41

Our work aimed to obtain thermochromic and vapochromic composite materials based on the unique three-way switchable {(NH₄)[Ni(cyclam)][Fe(CN)₆]·5H₂O}_n (cyclam = 1,4,7,11-tetraazacyclotetradecane) coordination chain (1).⁴² The most remarkable feature of this compound is its ability to undergo a thermal MMCT phase transition, resulting in room temperature bistability. The switching between the blue low-temperature (LT) Ni^III–Fe^II phase and the red high-temperature (HT) Ni^II–Fe^III phase is abrupt and characterized by broad hysteresis (384–312 K). In addition, the reversible removal of crystallization water molecules gives a yellow dehydrated (deh) phase, in which the thermal MMCT process is inhibited, and the Ni^II–Fe^III oxidation states are stabilized across the entire temperature range. The serious problem hindering the practical application of this compound is its sensitivity to liquid water, which causes its immediate degradation. By embedding 1 in electrospun organic polymer fibers, we aimed at enhancing its mechanical properties and stability, thereby expanding its potential as a molecule-based sensor.

2. Materials and methods

2.1 Materials

All chemicals and solvents of reagent grade (Sigma-Aldrich) were used as supplied. [Ni(cyclam)(NO₃)₂] was synthesized according to the literature procedure.³⁰ For the electrospinning process, the following polymers were used: poly(ε-caprolactone) (PCL) with average M_w = 80 000 and poly(2-vinylpyridine-co-styrene) (P2VP-PS) with average M_w = 220 000.

2.2 Synthesis

{(NH₄)[Ni(cyclam)][Fe(CN)₆]·5H₂O}_n (1) in HT form was synthesized according to the literature protocol without modification,⁴² resulting in millimeter-size crystals (1c). Elemental analysis: % calculated for C₁₆H₃₈FeN₁₁NiO₅: C, 33.19%; H, 6.61%; N, 26.61%; found: C, 33.08%; H, 6.70%; N, 26.42%.

Subsequently, the crystalline sample (1c) was manually ground in an agate mortar until it fully transitioned to the LT form under applied pressure, providing a fine powder of 1g. Elemental analysis: % calculated for C₁₆H₃₈FeN₁₁NiO₅: C, 33.19%; H, 6.61%; N, 26.61%; found: C, 33.51%; H, 6.45%; N, 26.38%.

The powder (1g) was transferred into a 30 ml vial filled with chloroform (∼20 ml) to create a dark green stable suspension, which was sonicated using an ultrasonication homogenizer probe (Sonoplus GM 4200, Bandelin electronic GmbH & Co. KG) operating at 30% power amplitude without pulsation for a minimum duration of 45 minutes, divided into 15-minute intervals. Then the suspension was left in a Petri dish to evaporate the solvent, resulting in a light green powder of 1p particles (or it was used directly to prepare the polymer suspension for electrospinning). Elemental analysis: % calculated for C₁₆H₂₈FeN₁₁Ni: C, 39.30%; H, 5.77%; N 31.51%; found: C, 38.98%; H, 5.81%; N, 21.25%.

2.3 Preparation of solutions for electrospinning

Chloroform–methanol (3 : 1) solution. Details of the preparation of electrospinning solutions are given in Table S1. The manually ground crystals of 1g were placed in a 30 ml vial, and chloroform was added to achieve a stable suspension, followed by further sonication with a sonication probe to ensure uniform dispersion of the particles. Next, an appropriate amount of methanol was added. PCL or P2VP-PS was added to the suspension, and the vials were placed on a gyromixer for 24 hours.

DMF solution. In the case of DMF solution, manually ground crystals 1g were transferred to a 6 ml vial and covered with 4 ml of DMF. Due to the small volume of the solvent, sonication was conducted in five brief intervals of 2 minutes. Next, P2VP-PS was added to the suspension, and the vial was placed on a gyromixer for 24 hours.

2.4 Electrospinning process

The composite solution was introduced into a plastic syringe from which it was electrospun with the applied voltage of either 11 kV (PCL and P2VP-PS solutions based on a chloroform–methanol mixture) or 13 kV (P2VP-PS solutions based on DMF) at room temperature and relative humidity ranging from 36 to 47% at a constant flow rate of 1.5 ml h⁻¹. The fiber mats were randomly collected on a metallic plate collector placed at a distance of 10.5 cm from the tip of the syringe needle.

2.5 Characterization

SEM and TEM studies. TEM images were obtained using a Tecnai G2 (200 kV) transmission electron microscope fitted with an energy-dispersive X-ray (EDX) microanalyzer coupled with a high-angle-annular dark-field detector (HAADF). Microstructure and composition analysis of the fibers was performed using a Tescan Vega 3 scanning electron microscope equipped with an X-ray energy dispersive spectrometer (EDAX Bruker). The size of the particles and the diameters of the fibers were measured using ImageJ software.

X-ray absorption spectroscopy (XAS). The Fe and Ni K-edge X-ray absorption spectra were recorded at the ASTRA beamline of the SOLARIS National Synchrotron Radiation Centre (Kraków, Poland). The incident photon beam was generated by a double-bend achromatic 1.3 Tesla bending magnet and monochromatized using a Ge(220)-modified Lemonnier-type double crystal monochromator. The resultant monochromatic beam, with dimensions of 5 × 1 mm (horizontal × vertical direction) at the sample position, was shaped by slits. Spectra were recorded in transmission mode with ionization chambers, and the sample chamber was filled with N₂ gas at atmospheric pressure. For calibration and alignment purposes, Fe and Ni reference foils, procured from the EXAFS Company, were positioned between the second and third ionization chambers during measurements at the Fe and Ni K-edges. The samples of interest were sealed in a polyethylene (PE) foil bag under a protective layer of NH₄Cl solution. A custom-made sample holder allowed for temperature control of the sample in the range of 250–450 K. Liquid nitrogen was used for cooling the system, while a resistive heater was used as a heat source. A Pt-100 thermocouple was used as a temperature sensor, and the temperature was controlled using a Lakeshore temperature controller. Before measurements, the samples were scanned with an X-ray camera to ensure sample homogeneity at the measurement spot. Post-acquisition, the data were processed and analyzed using the Demeter software package.⁴³

Raman spectroscopy. Raman measurements were performed using a Confocal micro-Raman spectrometer (Nicolet Almega XR) equipped with a 532 nm laser. The measurement was performed with 1% laser power to minimize the burning effect on the samples. Composites were directly electrospun onto the Si (100) wafers in small amounts, similar to preparation for SEM imaging. The measurements of composites were performed with 10% of laser power.

Powder X-ray diffraction (PXRD). The PXRD data of ground crystals, particles after sonification, and composites were collected at 295 K using a Panalytical X'PERT PRO diffractometer with a Cu-Kα radiation source. The measurements were performed in the Bragg–Brentano geometry. Composites were directly electrospun onto a Si(100) wafer of 1.5 cm × 1.5 cm dimensions, ensuring sample uniformity and a flat surface that could be directly mounted on the diffractometer for data collection. To aid background correction during analysis of the composite patterns, additional data were collected on electrospun mats of the individual polymers (P2VP and PCL). The PXRD data were analyzed using the Profex software suite,⁴⁴ employing both Rietveld and Le Bail refinement methods.

Sorption study. The process of controlled sorption and desorption of water molecules was performed using a Surface Measurement System Dynamic Vapor Sorption DVS-Resolution apparatus. The water sorption isotherms were measured at 25 °C in the range of 0–96% RH of water vapor with a 2% step. At each step, the sample weight was equilibrated (dm/dt = 0.001% min⁻¹).

Magnetic measurements. Variable temperature (2–315 K, applied field: 1000 Oe) DC magnetization was measured using a Quantum Design MPMS XL7 SQUID magnetometer. 1c (m = 5.11 mg), 1g (m = 6.52 mg), 1p (m = 4.33 mg) and the composite samples 2 (m = 21.56 mg), 3 (m = 11.74 mg), and 3d (m = 16.32 mg) were sealed in a PE foil bag for the magnetic measurements. To ensure the stability and reproducibility of the thermal hysteretic characteristics, multiple temperature cycles were conducted. For further analysis, the data from the third cycle were used. Diamagnetic contributions were corrected using Pascal's constants. For the composite samples, the magnetic susceptibility correction was performed using the Curie–Weiss law according to ref. 45.

3. Results and discussion

3.1. Crystal size reduction

The particles of the bistable {NH₄[Ni(cyclam)][Fe(CN)₆]·5H₂O}_n chain (1) were prepared using a top-down strategy, necessitated by the fact that the compound is initially obtained as millimeter-sized crystals. Compound 1 is formed through a reaction between the [Ni(cyclam)]²⁺ complex (cyclam = 1,4,8,11-tetraazacyclotetradecane) and [Fe(CN)₆]³⁻ carried out in an aqueous solution of NH₄Cl. The final product is obtained after the initial precipitation of a 2D honeycomb-like network of the formula {[Ni(cyclam)]₃[M(CN)₆]₂·zH₂O}_n (4), which is the thermodynamically favored product formed in water solution without the addition of NH₄Cl.⁴²

In the top-down approach, the crystals of 1 were manually ground in an agate mortar until all the sample was transformed into the blue LT form, due to the applied pressure. The resulting powder (1g) was transferred into a glass vial and covered with chloroform, forming a yellowish-green suspension indicative of dehydration of the compound. The suspension was sonicated for 45 minutes in three intervals to further reduce the particle size and improve their dispersion (see Fig. S1). Fig. 1 shows the SEM images taken for the 1c, 1g, and 1p samples. A comparison of particle sizes indicates that sample 1p, obtained after sonication, has smaller average (1.023 μm) and median (0.629 μm) particle sizes compared to sample 1g (1.797 μm and 1.045 μm, respectively). These results confirm that sonication effectively reduced agglomeration and improved particle dispersion (Table 1).

Fig. 1 SEM images of 1c (A), 1g (E–G), and 1p (H–J). Photographs of 1c in the high temperature phase (B), the low temperature phase (C), and the dehydrated phase (D).

Table 1 Summary of sample symbols

	Sample	Form	Average size
NiFe chain	1c	Crystal	24 mm
	1g	Manually ground powder	1.797 μm
	1p	Sonicated particles	1.023 μm

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	Sample	Polymer	Solvent
Composites	2	PCL	CHCl3 + MeOH
	3	P2VP-PS	CHCl3 + MeOH
	3d	P2VP-PS	DMF

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	Sample	Formula	Ref.
	4	{[Ni(cyclam)]3[M(CN)6]2·zH2O}n	46

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Rietveld refinements of PXRD data for 1g and 1p confirmed phase purity, with no detectable impurities. However, a residual fraction (∼18 wt%) of the low-temperature (LT) phase consistently persists at 295 K. Cooling the sample for one hour converts it fully to the LT phase, whereas subsequent mild heating restores a predominant high-temperature (HT) phase accompanied by an ∼18 wt% LT phase.

The atomic parameters of 1g and 1p are nearly identical and closely match those reported for single crystals.⁴² The lattice parameters of the LT phases of both 1g and 1p are also presented in SI Table S2, which are also very similar to those reported from the single-crystal X-ray diffraction data.⁴² Clinographic views of the crystal structure along three crystallographic directions are presented in Fig. 2(A–C), and Fig. 2(D) shows the extended fragment of a single Fe–Ni chain. Although the low-temperature (LT) phase is smaller in size, it is isostructural to the HT phase.

Fig. 2 Crystal structure of the hydrated sample 1 viewed through the crystallographic a (A), b (B), and c (C) directions; the green lines indicate a unit cell boundary. (D) An extended fragment of a single chain.

The χ_MT(T) profiles of 1c, 1g, and 1p reveal a systematic evolution of the thermal hysteresis with decreasing particle size (Fig. 3). The thermal transition temperatures determined from the χ_MT vs. T measurements are summarized in Table 3. While crystalline 1c exhibits a sharp, well-defined transition, miniaturization leads to a gradual broadening of the hysteresis and a loss of abruptness at the transition edges. At 300 K, χ_MT for 1c reaches 1.95 cm³ K mol⁻¹, which is in good agreement with the expected Curie constant of 1.84 cm³ K mol⁻¹ for Ni^II (S = 1 and g = 2.2) and low-spin Fe^III ( and g = 2.6). Following the charge-transfer process, χ_MT decreases to 0.38 cm³ K mol⁻¹, consistent with low-spin Ni^III ( and g = 2.13) and diamagnetic low-spin Fe^II (S = 0), and remains nearly constant down to ∼25 K, below which it increases slightly to 0.65 cm³ K mol⁻¹ at 2 K, likely due to weak ferromagnetic interactions between Ni^III ions mediated by diamagnetic linkers or hydrogen-bonding networks.

Fig. 3 Temperature dependence of χT of 1c, 1g, and 1p in the temperature range of 260–315 K measured under an applied magnetic field of 1 kOe.

As the particle size decreases, the χ_MT value at room temperature slightly decreases (1.89 cm³ K mol⁻¹ for 1g and 1.83 cm³ K mol⁻¹ for 1p), while the low-temperature χ_MT increases (0.47 cm³ K mol⁻¹ and 0.61 cm³ K mol⁻¹ for 1g and 1p, respectively). This trend reflects a weakening of cooperative interactions caused by reduced structural coherence and increased disorder at smaller particle sizes, which leads to a more gradual and spatially heterogeneous progression of the charge-transfer transition. Nevertheless, the overall MMCT switching remains robust, with well-preserved bistability. The observed smoothing and broadening of the hysteresis thus provide evidence of size-dependent modulation of cooperativity, while highlighting the resilience of the charge-transfer process under confinement and microstructural disorder.

To directly probe the electronic structures of the metal ions in 1c and 1p, XAS measurements at the Fe and Ni K-edges were performed at temperatures outside the bistability region. Fig. 4 shows the Fe and Ni K-edge XAS spectral plots of samples 1c and 1p in the HT and LT phases, as well as the differences in spectral weights between the two phases. The antagonistic features are observed with the complementary roles played by the two metal ions during the charge transfer process. While the Fe difference spectrum shows first negative peaks (at 7116 eV and 7127 eV) and then positive peaks at around 7132 eV, the reverse signature is observed for Ni with the first two maxima (at 8338 eV and 8345 eV) and then a minimum above 8357 eV.

Fig. 4 X-ray absorption spectra recorded for 1c (A and B) and 1p (C and D) in the high-temperature (red) and low-temperature (blue) states, and their difference (black) at the Fe and Ni K-edges.

3.2. Preparation of composites and their morphology

The particles obtained by sonication of 1p were used as a magnetic filler in two polymer matrices with distinct properties: polycaprolactone (PCL) and the block copolymer poly(2-vinylpyridine-co-styrene) (P2VP-PS). PCL is a semi-crystalline aliphatic polyester valued for its biodegradability, low glass transition temperature (around −60 °C), good mechanical strength, and relatively low melting point (∼60 °C). Its hydrophobic nature, due to the aliphatic chains in the monomers, results in low affinity to water, making it insoluble in water and alcohols but soluble in various organic solvents such as chloroform and dichloromethane.^47–49 On the other hand, the block copolymer P2VP-PS, composed of polystyrene (PS) and poly(2-vinylpyridine) (P2VP) segments, combines the properties of hydrophobic polystyrene, which imparts rigidity and mechanical reinforcement, with hydrophilic poly(2-vinylpyridine), whose behavior is pH-dependent due to the protonation of nitrogen in the aromatic pyridine ring.^50–55 This amphiphilic character allows interesting interactions with nanoparticles, including coordination and confinement effects, as demonstrated for various nanoparticle systems. Although P2VP-PS is less explored in the form of electrospun fibers compared to PCL, its unique chemical functionalities make it a promising material for biomedical applications, drug release systems, and filtration membranes.

The fibers were obtained by electrospinning from organic polymer solutions in a mixture of chloroform and methanol. Due to the fragility of 1p in methanol, which can cause its recrystallization to the 2D compound 4, fine powder of 1p was dispersed in chloroform. Then the appropriate amount of methanol was added. Next, the organic polymers were introduced to the resulting suspension, and the vials were placed on a gyromixer to ensure slow dissolution of the polymers and prevent aggregation of the 1p particles. The electrospinning process resulted in composite mats with 1p embedded in the PCL (2) or P2VP-PS matrix (3). We tested three different concentrations of 1p in the organic polymer (approximately 5%, 10%, and 15% by weight). Fig. 5 presents the pictures of the 1p powder and the electrospun composites 2 and 3 in their HT, LT, and dehydrated states.

Fig. 5 Illustration of thermo- and vapochromism: photographs of the 1p powder and electrospun composites 2 and 3 in three different phases stable under ambient conditions.

For the composites in the P2VP-PS polymer matrix, DMF was used as an alternative solvent. The size reduction of 1 was performed in the amount of DMF necessary for electrospinning solution preparation without the solvent evaporation step. The manually ground crystals were sonicated in DMF to give the green suspension of 1p. The appropriate amount of P2VP-PS was added, and the mixture was agitated until the polymer dissolved. The resulting electrospun composite mat was denoted as 3d.

The morphology of the composite mats 2, 3, and 3d with the highest load of 1p is shown in Fig. 6. All mats are composed of continuous, intertwined, and crisscrossed fibers, forming a porous 3D network-like structure. The predominant diameter distribution of 2 fibers falls within the range of 0.27–0.67 µm in the fiber area outside the magnetic grains and 0.4–1.7 µm at the site of thickening due to the presence of 1p particles. Sample 3 exhibits an increased proportion of fibers with diameters ranging from 0.8 to 1.3 µm (0.95–2.4 µm at the site of 1p grains) compared to 2. The thinnest fibers are obtained for sample 3d, where the average fiber size is equal to 0.595 (±0.183) µm outside and 0.890 (±0.295) µm at the site of 1p particles. In electrospinning, the choice of solvent significantly influences the fiber diameter and morphology.^56–61 Specifically, chloroform and DMF, when used alone or in combination, influence the resulting fiber characteristics through their volatility, boiling point, and dielectric properties. Chloroform, used in samples 2 and 3, is a relatively volatile solvent with a low dielectric constant. These properties limit the extent of jet stretching and favor faster solvent evaporation, typically resulting in thicker fibers, as observed in both PCL-based and P2VP-PS-based mats. In contrast, sample 3d, based on P2VP-PS electrospun from DMF, exhibits significantly thinner fibers. DMF has a higher boiling point and dielectric constant, which promotes greater jet elongation and slower evaporation during fiber formation, leading to finer fiber diameters. These observations confirm the strong influence of solvent properties on fiber morphology, even when the same polymer is used.

Fig. 6 Scanning electron microscopy (SEM) images of the 2 (A), 3 (C), and 3d (E) composites and scanning electron microscopy-backscattered electron (SEM-BSE) images of composites 2 (B), 3 (D), and 3d (F).

No significant correlation between the fiber diameter and the nanoparticle concentration was evident. The 1p particles were not observed outside the fibers, indicating that they were thoroughly incorporated into the fibers during the electrospinning process. The homogeneity of the 1p distribution in both polymers was also confirmed by Scanning Electron Microscopy (SEM) analysis coupled with energy dispersive X-ray (EDX) analysis (Fig. S6–S8). For a further in-depth understanding of the internal composition of the composite fibers, we conducted energy dispersive X-ray spectroscopy (EDS) coupled with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Fig. 7) on the 3d composite nanofibers. The elemental distributions of nickel and iron confirmed the presence of 1p within the P2VP-PS nanofibers, validating the successful fabrication of the composites.

Fig. 7 HAADF-STEM images (A) and the corresponding elemental distribution maps of Fe (B) and Ni (C) in the 3d composite fibers.

3.3 Raman spectroscopy

Raman spectroscopy is often employed to probe the morphology of organic polymer chains. The comparison between the Raman spectra of 1c, 1p, electrospun fibers composed of pure PCL and P2VP-PS, and composite fibers 2 and 3 is presented in Fig. 8. In the crystalline sample 1c, the stretching of CN ligands (ν(C≡N)) appears as a double band with maxima at 2125 cm⁻¹ and 2140 cm⁻¹. The spectra of 1p also exhibit a double band, with maxima at 2126 cm⁻¹ and 2145 cm⁻¹, showing a slight shift compared to the crystalline form. These bands are also retained in the composite fibers 2 and 3. The stretching vibration of the carbonyl group in PCL chains, observed in the range of 1700–1750 cm⁻¹, serves as a qualitative indicator of PCL crystallinity. This band was deconvoluted into two Gaussian lines centered at 1718 cm⁻¹ and 1729 cm⁻¹ for pure PCL fibers. The first peak corresponds to the crystalline phase, whereas the latter is associated with the amorphous fraction. For the composite fibers 2, the peak ascribed to the amorphous phase is found at 1741 cm⁻¹, indicating that non-crystalline conformers are more disordered in the composite than in the pure PCL fibers.⁶²

Fig. 8 Raman spectra of 1c, 1p, electrospun PCL and P2VP-PS fibers, and composite fibers 2 and 3 (A); C≡N stretching regions of 1c, 1p, 2 and 3 (B); and analysis of the C=O stretching bands in PCL and composite fibers 2 (C).

3.4 Sorption studies

To check if the switchability between the hydrated HT phase and the anhydrous phase is retained in the composites, the water vapor sorption was studied for 1p, pure PCL, and P2VP-PS, as well as composites 2, 3, and 3d, using the dynamic vapor sorption (DVS) technique (Fig. 9 and S9). The sorption–desorption isotherms measured for the pure PCL mat show very small water intake (below 0.5%) and no hysteresis, which is consistent with the hydrophobic character of PCL. On the other hand, the pure P2VP-PS mat shows considerable water intake, with a mass increase of about 7% and a fairly broad hysteresis. The sorption–desorption isotherms measured for 1p exhibit wide hysteresis spanning over the range of 20–80% relative humidity (RH), characterized by distinct sharp steps of water intake and release. This behavior arises from structural changes associated with the phase transition between the hydrated and dehydrated phases and was also observed for the crystalline samples of 1.⁴² The sorption behavior observed for the composites is the combination of the characteristics of pure polymers and 1p. Sharp changes in mass related to water sorption/desorption occur at similar RH, indicating that they originate from the embedded 1p particles. In the first sorption isotherms of the composites, the phase transition between the dehydrated and hydrated phases is shifted towards higher RH values compared to 1p. This phenomenon is more pronounced in the PCL-based composites 2, where sharp water intake in the first cycle occurs at 92% RH. In subsequent cycles, the sorption-induced phase transitions in the composite materials appeared at the same RH as in 1p. The reversibility of the dehydration process was tested by the same procedure as for the embedded compound,⁴² with several cycles of desorption and sorption at 0 or 97% RH at 298 K. The recorded profile shows excellent reversibility, with no signs of material wear.

Fig. 9 Water sorption isotherms of pure PCL fibers (A), pure P2VP-PS fibers (C), and the 2 (B) and 3 (D) composites.

The characteristic sharp decrease in mass between 22 and 20% RH in the desorption isotherm, connected with the phase transition, was used to estimate the 1p content in the 2, 3, and 3d nanocomposites. The mass drop during desorption in 1p is equal to 16.88%. Based on this, the proportion method was used to calculate the 1p content in the composites (Table S1). The results obtained for composites 3 and 3d based on P2VP-PS are consistent with those used in the synthesis. For the PCL-based composites 2, the calculated 1p content is approximately half of the value used for the synthesis. This discrepancy may be due to the polymer blocking water access to the 1p particles, resulting in incomplete participation in water sorption. This hypothesis is supported by the fact that the pure PCL mat practically does not absorb water. Larger 1p particles may contribute to water sorption, while those deeply embedded within the fibers do not.

3.5 PXRD

To study the influence of the polymer matrix on the structural properties of embedded particles, PXRD measurements were performed for the composite mats prepared by incorporating 1p into the P2VP polymer prepared with either DMF solvent (3d) or a mixture of chloroform and methanol (3), and 1p embedded in the PCL polymer in a mixture of chloroform and methanol (2). The diffraction patterns and the Le Bail fit results of fully hydrated composite mats are presented in Fig. 10(C–E). These data confirm the presence of only the HT phase in the composites, unlike the ground crystals (1g) and particles after sonification (1p), where a residual LT phase persists. The contributions from the P2VP and PCL polymers to the PXRD of respective nanocomposite mats were addressed by using the diffraction patterns of pure P2VP and PCL polymer mats as the background, in addition to a polynomial.

Fig. 10 Powder X-ray diffraction (PXRD) patterns obtained at 295 K illustrating the structural characteristics of 1g, 1p, and the composites. (A and B) Rietveld refinements for 1g and nanocrystals 1p. (C–E) Experimental and Le Bail fits for the hydrated nanocomposite mats prepared in different polymers/solvents. (F) Reference patterns: simulated PXRD patterns of hydrated crystals 1p, a recrystallized 2D honeycomb phase 4, and experimentally measured dehydrated 1p. (G–I) Evolution of the PXRD patterns of nanocomposites during the hydration/dehydration cycles (freshly deposited, humid air exposure, immersion in water, and subsequent drying). Obs. = observed data, calc. = calculated profile, diff. = difference curve, and bg = polymer background contribution.

The reference diffraction patterns for the HT phase of 1 ⁴² and the 2D network 4 ⁴⁶ were calculated from the published single crystal data and are presented in Fig. 10(F), along with the measured PXRD of dehydrated 1p. Direct contact of 1p with water transforms it into the 2D network {[Ni(cyclam)]₃[Fe(CN)₆]₂·zH₂O}_n(4). Diffractograms for P2VP-based (3d and 3) and PCL-based (2) mats during hydration cycles are shown in Fig. 10(G–I). Freshly deposited mats initially contain dehydrated 1p. Upon exposure to humid air, the PCL mats (2) partially transform to the hydrated phase, whereas the P2VP mats (3 and 3d) achieve complete transformation. Water immersion fully hydrates the PCL-based fibers within 1 hour, while the P2VP mats undergo partial decomposition to the 2D network (4), which is more pronounced in the DMF-based fibers due to their smaller diameter and higher porosity. Subsequent drying at 40 °C reverts embedded 1p to its dehydrated form, but the decomposition products prevent complete switchability in the P2VP mats. These results highlight that PCL encapsulation both preserves phase-switching ability and protects against water-induced degradation (Table 2).

Table 2 Comparison of structural parameters obtained from powder X-ray diffraction data of the samples prepared by mortar and pestle grinding (1g) and sonication (1p). The structural parameters of (1p) powders embedded in the P2VP-PS or PCL polymer and prepared with either DMF or a mixture of chloroform and methanol solvents are also shown. For reference, single-crystal data are included from ref. 42. The powder data in this work and the single-crystal data in ref. 42 were collected at 295 K

Parameter	1g	1p	3d	3	2	Single crystal
a (Å)	14.271(1)	14.273(1)	14.251(4)	14.261(9)	14.259(5)	14.2585
b (Å)	12.965(2)	12.957(1)	12.941(1)	12.949(7)	12.918(4)	12.9512
c (Å)	10.220(2)	10.220(1)	10.215(1)	10.222(6)	10.210(3)	10.2150
α, γ (°)	90	90	90	90	90	90
β (°)	133.38(2)	133.42(4)	133.44(3)	133.37(3)	133.44(2)	133.39

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3.6 Magnetic properties

The transition between the LT and HT states in the 1p-loaded PCL and P2VP-PS nanocomposites is evident due to their pronounced thermochromism, as shown in Fig. 5. To evaluate the magnetic states in the HT and LT phases, we performed magnetic susceptibility measurements following the temperature sequence 260 K → 315 K → 260 K (Fig. 11).

Fig. 11 Comparison of the χ_MT(T) plots measured for the powder 1p sample and electrospun composites 2, 3, and 3d.

Analysis of variable-temperature magnetic susceptibility indicates that MMCT for the 1p powder is observed with transition temperatures at approximately T_½↓ = 282 K and T_½↑ = 310 K during cooling and heating cycles, respectively. Upon incorporation of 1p into the P2VP-PS fibers, both transition temperatures shift to lower values, with a decrease in T_½↓ observed regardless of the solvent used during electrospinning. Moreover, the hysteresis widths for composites 3 and 3d (32 K, 29 K, respectively) are notably larger than that of the PCL-based composite 2, suggesting a clear influence of the polymeric environment. Such behavior is consistent with numerous studies on spin-crossover (SCO) composites, where the host matrix does not act as an inert support but significantly modifies the switching characteristics. Through stress and strain coupling associated with the spin-state volume change, the matrices can shift transition temperatures, alter cooperativity, and broaden or narrow hysteresis loops. These matrix–particle interactions, well described by mechano-elastic models, have been shown to impact even micrometer-sized SCO particles and can also feed back into the mechanical properties of the polymer host itself.^63–66 The comparison between the MMCT composites embedded in P2VP-PS and PCL clearly demonstrates how the stiffness of the polymeric matrix governs the switching behavior. The P2VP-PS composites exhibit a substantially broader thermal hysteresis compared to the PCL-based material. This can be rationalized by considering the mechanical nature of the two polymers. P2VP-PS is a rigid, glassy copolymer with a high glass transition temperature and stiff aromatic backbones, which efficiently transmit the lattice strain generated during the charge-transfer phase transition. The resulting strong particle–matrix coupling amplifies cooperative interactions between the MMCT particles and manifests in an enlarged hysteresis width. In contrast, PCL is a soft, semicrystalline polyester with a very low glass transition temperature. Its compliant, flexible chains can absorb and dissipate the strain associated with the spin-state volume change, thereby reducing interparticle elastic communication. Consequently, the hysteresis observed in the PCL composites is narrower, reflecting weaker cooperativity.

Comparison of the hysteresis loops obtained for samples 3 and 3d demonstrates that the choice of solvent also influences the magnetic properties of the composites. In the case of the polar solvent DMF, the low-temperature χ_MT value is significantly higher than that observed for the fibers prepared from chloroform/methanol solutions. This effect is most likely related to partial recrystallization of the NiFe chain to the 2D network 4 induced by DMF, leading to an increased contribution of the Ni^II ions in the low temperature phase. The magnetic parameters of the studied samples are summarized in Table 3.

Table 3 Data of the magnetic measurements for composite samples (10%). The sweep rate is 2 K min⁻¹

Sample	T ½↓ [K]	T ½↑ [K]	Hysteresis width [K]
1c	283	308	25
1g	283	308	25
1p	282	310	28
2	281	306	25
3	274	306	32
3d	277	306	29

---

4. Conclusions

New composite materials with thermochromic and vapochromic properties were successfully obtained by incorporation of the bimetallic coordination chain {(NH₄)[Ni(cyclam)][Fe(CN)₆]·5H₂O}_n (1) into electrospun organic polymer fibers. The materials retain their unique three-state stability under ambient conditions with temperature- and humidity-driven switching, which characterizes the embedded compound. At the same time, they show improved stability and favorable mechanical properties rendered by the organic polymer matrix.

Although no protocol for producing uniform nanoparticles of 1 could be established, due to specific synthetic conditions arising from the presence of the guest NH₄⁺ cation in the structure, we developed an efficient particle size-reduction approach based on appropriate solvent selection and sonication, which can be applied to the large crystals of 1. Thus, the obtained fine particles (1p) show a relatively wide size distribution around 1 μm. Nonetheless, they could be effectively incorporated into the electrospun fibers. The simple and versatile electrospinning-based strategy allowed the fabrication of two types of composites based on PCL and P2VP-PS polymer matrices, which differ in morphology and hydrophilicity. In both composites, the reversible thermal MMCT process, which allows switching between the red HT and blue LT phases, can be observed with only a slight shift of the hysteresis to lower temperatures in comparison with the embedded coordination compound 1. Likewise, both composites show humidity-driven switching between either of the hydrated HT or LT phases and the dehydrated yellow phase.

The key advancement of this study is the demonstration that the PCL matrix provides complete protection against water-induced degradation, overcoming the most serious limitation of compound 1 and significantly enhancing its practical applicability. Embedding in a polymer matrix also solves the problem of brittleness of the crystalline form of 1, which excludes any possibility of mechanical processing. Our work shows that combining organic polymers with MMCT-active coordination compounds enables the development of robust, flexible, and environmentally responsive switchable materials, opening new opportunities for their integration into future sensing, memory, and smart material technologies.

Author contributions

Aleksandra Pacanowska: data curation, investigation, validation, visualization, and writing – original draft; Gaja Wota: data curation and writing – review & editing; Małgorzata Jasiurkowska-Delaporte: data curation and investigation; Naveen Kumar Chogondahalli Muniraju: formal analysis, investigation, validation, and writing – review & editing; Wojciech Sas: data curation, investigation, validation, and writing – review & editing; Kamila Komędera: data curation, validation, and writing – review & editing; Wojciech Tabiś: formal analysis, funding acquisition, and writing – review & editing; Alexey Maximenko: data curation, investigation, and writing – review & editing; Grzegorz Gazdowicz: data curation; Paweł Czaja: data curation, investigation, and writing – review & editing; Marcin Perzanowski: data curation and investigation; Marzena Mitura-Nowak: investigation; Beata Nowicka: conceptualization, funding acquisition, investigation, supervision, and writing – original draft; Magdalena Fitta: conceptualization, formal analysis, funding acquisition, supervision, writing – original draft, and writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Additional data related to this paper may be requested from the corresponding authors.

Supplementary information is available. See DOI: https://doi.org/10.1039/d5nr03887a.

Acknowledgements

This work was supported by the NCN within the OPUS project 2021/43/B/ST5/02216. We thank the National Synchrotron Radiation Centre SOLARIS for the provision of synchrotron radiation facilities (project ID: 221059). The development of the ASTRA beamline was supported by the Polish Ministry of Science and Higher Education within the project 1/SOL/2021/2: “Support for research and development with the use of research infrastructure of the National Synchrotron Radiation Centre SOLARIS” and a grant from the Priority Research Area SciMat under the Strategic Programme Excellence Initiative at Jagiellonian University. We thank the “Excellence Initiative-Research University” program of AGH University in Krakow. WT acknowledges the support from the Polish National Agency for Academic Exchange under “Polish Returns 2019” Programme No. PPN/PPO/2019/1/00014.

References

1. R. Torres-Cavanillas, M. Gavara-Edo and E. Coronado, Bistable Spin–Crossover Nanoparticles for Molecular Electronics, Adv. Mater., 2024, 36(1), 2307718,  DOI:10.1002/adma.202307718.
2. A. Tissot, X. Kesse, S. Giannopoulou, I. Stenger, L. Binet, E. Rivière and C. Serre, A Spin Crossover Porous Hybrid Architecture for Potential Sensing Applications, Chem. Commun., 2019, 55(2), 194–197,  10.1039/C8CC07573E.
3. Z.-S. Yao, Z. Tang and J. Tao, Bistable Molecular Materials with Dynamic Structures, Chem. Commun., 2020, 56(14), 2071–2086,  10.1039/C9CC09238B.
4. R. Turo-Cortés, F. J. Valverde-Muñoz, M. Meneses-Sánchez, M. C. Muñoz, C. Bartual-Murgui and J. A. Real, Bistable Hofmann-Type Fe II Spin-Crossover Two-Dimensional Polymers of 4-Alkyldisulfanylpyridine for Prospective Grafting of Monolayers on Metallic Surfaces, Inorg. Chem., 2021, 60(12), 9040–9049,  DOI:10.1021/acs.inorgchem.1c01010.
5. R. Torres-Cavanillas, M. Morant-Giner, G. Escorcia-Ariza, J. Dugay, J. Canet-Ferrer, S. Tatay, S. Cardona-Serra, M. Giménez-Marqués, M. Galbiati, A. Forment-Aliaga and E. Coronado, Spin-Crossover Nanoparticles Anchored on MoS2 Layers for Heterostructures with Tunable Strain Driven by Thermal or Light-Induced Spin Switching, Nat. Chem., 2021, 13(11), 1101–1109,  DOI:10.1038/s41557-021-00795-y.
6. F. Novio, E. Evangelio, N. Vazquez-Mera, P. González-Monje, E. Bellido, S. Mendes, N. Kehagias and D. Ruiz-Molina, Robust Spin Crossover Platforms with Synchronized Spin Switch and Polymer Phase Transition, Sci. Rep., 2013, 3(1), 1708,  DOI:10.1038/srep01708.
7. M. Feng, Z.-Y. Ruan, Y.-C. Chen and M.-L. Tong, Physical Stimulus and Chemical Modulations of Bistable Molecular Magnetic Materials, Chem. Commun., 2020, 56(89), 13702–13718,  10.1039/D0CC04202A.
8. M. A. Halcrow, Mix and Match – Controlling the Functionality of Spin-Crossover Materials through Solid Solutions and Molecular Alloys, Dalton Trans., 2024, 53(33), 13694,  10.1039/D4DT01855A.
9. D. Gentili, N. Demitri, B. Schäfer, F. Liscio, I. Bergenti, G. Ruani, M. Ruben and M. Cavallini, Multi-Modal Sensing in Spin Crossover Compounds, J. Mater. Chem. C, 2015, 3(30), 7836–7844,  10.1039/C5TC00845J.
10. M. D. Manrique-Juarez, S. Rat, F. Mathieu, D. Saya, I. Séguy, T. Leïchlé, L. Nicu, L. Salmon, G. Molnár and A. Bousseksou, Microelectromechanical Systems Integrating Molecular Spin Crossover Actuators, Appl. Phys. Lett., 2016, 109(6), 5131,  DOI:10.1063/1.4960766.
11. K. Senthil Kumar and M. Ruben, Emerging Trends in Spin Crossover (SCO) Based Functional Materials and Devices, Coord. Chem. Rev., 2017, 346, 176,  DOI:10.1016/j.ccr.2017.03.024.
12. C. Mathonière, Metal–to–Metal Electron Transfer: A Powerful Tool for the Design of Switchable Coordination Compounds, Eur. J. Inorg. Chem., 2018, 2018(3–4), 248–258,  DOI:10.1002/ejic.201701194.
13. D. Aguilà, Y. Prado, E. S. Koumousi, C. Mathonière and R. Clérac, Switchable Fe/Co Prussian Blue Networks and Molecular Analogues, Chem. Soc. Rev., 2016, 45(1), 203–224,  10.1039/C5CS00321K.
14. C. P. Berlinguette, A. Dragulescu-Andrasi, A. Sieber, J. R. Galán-Mascarós, H.-U. Güdel, C. Achim and K. R. Dunbar, A Charge-Transfer-Induced Spin Transition in the Discrete Cyanide-Bridged Complex {[Co(Tmphen) 2 ] 3 [Fe(CN) 6 ] 2 }, J. Am. Chem. Soc., 2004, 126(20), 6222–6223,  DOI:10.1021/ja039451k.
15. M. Fumanal, F. Jiménez-Grávalos, J. Ribas-Arino and S. Vela, Lattice-Solvent Effects in the Spin-Crossover of an Fe(II)-Based Material, The Key Role of Intermolecular Interactions between Solvent Molecules, Inorg. Chem., 2017, 56(8), 4474–4483,  DOI:10.1021/acs.inorgchem.7b00017.
16. N. Ozaki, H. Tokoro, Y. Miyamoto and S. Ohkoshi, Humidity Dependency of the Thermal Phase Transition of a Cyano Bridged Co–W Bimetal Assembly, New J. Chem., 2014, 38(5), 1950–1954,  10.1039/C3NJ01656K.
17. B. Dey, S. Mehta, A. Mondal, J. Cirera, E. Colacio and V. Chandrasekhar, Push and Pull Effect of Methoxy and Nitro Groups Modifies the Spin-State Switching Temperature in Fe(III) Complexes, ACS Omega, 2022, 7(43), 39268–39279,  DOI:10.1021/acsomega.2c05380.
18. A. Enriquez-Cabrera, A. Rapakousiou, M. Piedrahita Bello, G. Molnár, L. Salmon and A. Bousseksou, Spin Crossover Polymer Composites, Polymers and Related Soft Materials, Coord. Chem. Rev., 2020, 419, 213396,  DOI:10.1016/j.ccr.2020.213396.
19. D. Nieto-Castro, A. W. Graf, F. Gispert-Guirado and J. R. Galán-Mascarós, Magnetic and Electrical Bistability in Hybrid Composites of Conducting Organic Polymers with [Fe(NH 2 -Trz) 3 ] n [SO 4 ] n, J. Mater. Chem. C, 2023, 11(33), 11325–11332,  10.1039/D3TC00595J.
20. M. Piedrahita-Bello, J. E. Angulo-Cervera, A. Enriquez-Cabrera, G. Molnár, B. Tondu, L. Salmon and A. Bousseksou, Colossal Expansion and Fast Motion in Spin-Crossover@polymer Actuators, Mater. Horiz., 2021, 8(11), 3055–3062,  10.1039/D1MH00966D.
21. Y. Koo and J. R. Galán-Mascarós, Spin Crossover Probes Confer Multistability to Organic Conducting Polymers, Adv. Mater., 2014, 26(39), 6785–6789,  DOI:10.1002/adma.201402579.
22. A. Enriquez-Cabrera, X. Yang, S. E. Alavi, L. Salmon and A. Bousseksou, Self-Healing Composite Polymers Based on Spin Crossover Materials for Multidirectional Preprogrammed Actuation, ACS Appl. Polym. Mater., 2024, 6(15), 9364–9374,  DOI:10.1021/acsapm.4c02098.
23. A. T. Ngo, D. Aguilà, J. P. Vale, S. Sevim, M. Mattera, J. Díaz-Marcos, R. Pons, G. Aromí, B. Jang, S. Pané, T. S. Mayor, M. Palacios-Corella and J. Puigmartí-Luis, On–the–Fly Synthesis of Freestanding Spin–Crossover Architectures With Tunable Magnetic Properties, Adv. Mater., 2025, 37, 2420492,  DOI:10.1002/adma.202420492.
24. M. Piedrahita-Bello, J. E. Angulo-Cervera, R. Courson, G. Molnár, L. Malaquin, C. Thibault, B. Tondu, L. Salmon and A. Bousseksou, 4D Printing with Spin-Crossover Polymer Composites, J. Mater. Chem. C, 2020, 8(18), 6001–6005,  10.1039/D0TC01532F.
25. F.-L. Zeng, X.-T. Jin, J. Zhao, S.-X. Zhang, C. Xue and Y.-H. Luo, Construction and Screening of Spin-Crossover-Sponge Materials Based on Iron(ii)-Triazole Coordination Polymers, Dalton Trans., 2024, 53(5), 2333–2340,  10.1039/D3DT03531J.
26. Y. Zan, M. Piedrahita-Bello, S. E. Alavi, G. Molnár, B. Tondu, L. Salmon and A. Bousseksou, Soft Actuators Based on Spin–Crossover Particles Embedded in Thermoplastic Polyurethane, Adv. Intell. Syst., 2023, 5(6), 2200432,  DOI:10.1002/aisy.202200432.
27. E. Mishra, W. Chin, K. A. McElveen, T. K. Ekanayaka, M. Z. Zaz, G. Viswan, R. Zielinski, A. T. N'Diaye, D. Shapiro, R. Y. Lai, R. Streubel and P. A. Dowben, Electronic Transport Properties of Spin-Crossover Polymer plus Polyaniline Composites with Fe 3 O 4 Nanoparticles, J. Phys.: Mater., 2024, 7(1), 015010,  DOI:10.1088/2515-7639/ad1b35.
28. S. E. Alavi, B. Martin, Y. Zan, X. Yang, M. Piedrahita-Bello, W. Nicolazzi, J.-F. Ganghoffer, L. Salmon, G. Molnár and A. Bousseksou, Dynamical Mechanical Analysis and Micromechanics Simulations of Spin-Crossover@Polymer Particulate Composites: Toward Soft Actuator Devices, Chem. Mater., 2023, 35(8), 3276–3289,  DOI:10.1021/acs.chemmater.3c00293.
29. Y. Lai, R. Carrasco, A. Enríquez-Cabrera, L. Routaboul and A. Bousseksou, Spin Crossover Composite Film as Recyclable Catalyst for Acetalization Reaction, Chem. – Eur. J., 2025, 31(19), e202404700,  DOI:10.1002/chem.202404700.
30. D. E. Berry, S. Girard and A. McAuley, The Synthesis and Reactions of Nickel(III) Stabilized by a Nitrogen-Donor Macrocycle, J. Chem. Educ., 1996, 73(6), 551,  DOI:10.1021/ed073p551.
31. I. A. Gural'skiy, C. M. Quintero, J. S. Costa, P. Demont, G. Molnár, L. Salmon, H. J. Shepherd and A. Bousseksou, Spin Crossover Composite Materials for Electrothermomechanical Actuators, J. Mater. Chem. C, 2014, 2(16), 2949–2955,  10.1039/C4TC00267A.
32. S. Rat, M. Piedrahita-Bello, L. Salmon, G. Molnár, P. Demont and A. Bousseksou, Coupling Mechanical and Electrical Properties in Spin Crossover Polymer Composites, Adv. Mater., 2018, 30(8), 1705275,  DOI:10.1002/adma.201705275.
33. J. E. Angulo-Cervera, M. Piedrahita-Bello, B. Martin, S. E. Alavi, W. Nicolazzi, L. Salmon, G. Molnár and A. Bousseksou, Thermal Hysteresis of Stress and Strain in Spin-Crossover@polymer Composites: Towards a Rational Design of Actuator Devices, Mater. Adv., 2022, 3(12), 5131–5137,  10.1039/D2MA00459C.
34. H. J. Shepherd, I. A. Gural'skiy, C. M. Quintero, S. Tricard, L. Salmon, G. Molnár and A. Bousseksou, Molecular Actuators Driven by Cooperative Spin-State Switching, Nat. Commun., 2013, 4(1), 2607,  DOI:10.1038/ncomms3607.
35. K. A. Vinogradova, D. P. Pishchur, I. V. Korolkov and M. B. Bushuev, Magnetic Properties and Vapochromism of a Composite on the Base of an Iron(II) Spin Crossover Complex, Inorg. Chem. Commun., 2019, 105, 82–85,  DOI:10.1016/j.inoche.2019.04.035.
36. J. Xue, T. Wu, Y. Dai and Y. Xia, Electrospinning and Electrospun Nanofibers: Methods, Materials, and Applications, Chem. Rev., 2019, 119(8), 5298–5415,  DOI:10.1021/acs.chemrev.8b00593.
37. D. Ji, Y. Lin, X. Guo, B. Ramasubramanian, R. Wang, N. Radacsi, R. Jose, X. Qin and S. Ramakrishna, Electrospinning of Nanofibres, Nat. Rev. Methods Primers, 2024, 4(1), 1,  DOI:10.1038/s43586-023-00278-z.
38. K. Tran, P. Sander, M. Seydi Kilic, J. Brehme, R. Sindelar and F. Renz, Facile Approach for the Fabrication of Vapor Sensitive Spin Transition Composite Nanofibers, Eur. J. Inorg. Chem., 2024, 27(33), e202400363,  DOI:10.1002/ejic.202400363.
39. C. Göbel, C. Hils, M. Drechsler, D. Baabe, A. Greiner, H. Schmalz and B. Weber, Confined Crystallization of Spin–Crossover Nanoparticles in Block–Copolymer Micelles, Angew. Chem., 2020, 132(14), 5814–5819,  DOI:10.1002/ange.201914343.
40. A. Pacanowska, A. Regueiro, M. Clemente-León, E. Coronado, A. Forment-Aliaga and M. Fitta, Spin-Crossover Nanoparticles in Electrospun Polymers: A Route to Bistable Materials for Smart Textiles, ACS Appl. Nano Mater., 2025, 8(32), 15999–16007,  DOI:10.1021/acsanm.5c02764.
41. J. Pawlak, J. Brehme, M. S. Kilic, K. Tran, J. Koch, M. Beyki, R. Sindelar, R. Patzke and F. Renz, Pressing of Functionalized Polymer Composite Materials to Improve Mössbauer Measurement Signals, Polymers, 2024, 16(10), 1311,  DOI:10.3390/polym16101311.
42. M. Reczyński, D. Pinkowicz, K. Nakabayashi, C. Näther, J. Stanek, M. Kozieł, J. Kalinowska-Tłuścik, B. Sieklucka, S. Ohkoshi and B. Nowicka, Room–Temperature Bistability in a Ni–Fe Chain: Electron Transfer Controlled by Temperature, Pressure, Light, and Humidity, Angew. Chem., Int. Ed., 2021, 60(5), 2330–2338,  DOI:10.1002/anie.202012876.
43. B. Ravel and M. Newville, ATHENA, ARTEMIS, HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy Using IFEFFIT, J. Synchrotron Radiat., 2005, 12(4), 537–541,  DOI:10.1107/S0909049505012719.
44. N. Doebelin and R. Kleeberg, Profex: A Graphical User Interface for the Rietveld Refinement Program BGMN, J. Appl. Crystallogr., 2015, 48(5), 1573–1580,  DOI:10.1107/S1600576715014685.
45. D. Nieto-Castro, A. W. Graf, F. Gispert-Guirado and J. R. Galán-Mascarós, Magnetic and Electrical Bistability in Hybrid Composites of Conducting Organic Polymers with [Fe(NH 2 -Trz) 3 ] n [SO 4 ] n, J. Mater. Chem. C, 2023, 11(33), 11325–11332,  10.1039/D3TC00595J.
46. B. Nowicka, M. Reczyński, M. Balanda, M. Fitta, B. Gawel and B. Sieklucka, The Rule Rather than the Exception: Structural Flexibility of [Ni(Cyclam)]2+-Based Cyano-Bridged Magnetic Networks, Cryst. Growth Des., 2016, 16(8), 4736–4743,  DOI:10.1021/acs.cgd.6b00800.
47. J.-H. Choi, J.-J. Woo and I. Kim, Sustainable Polycaprolactone Polyol-Based Thermoplastic Poly(Ester Ester) Elastomers Showing Superior Mechanical Properties and Biodegradability, Polymers, 2023, 15(15), 3209,  DOI:10.3390/polym15153209.
48. S. W. Ibrahim, T. I. Hamad and J. Haider, Biological Properties of Polycaprolactone and Barium Titanate Composite in Biomedical Applications, Sci. Prog., 2023, 106(4), 1–31,  DOI:10.1177/00368504231215942.
49. D. S. Boucher, Solubility Parameters and Solvent Affinities for Polycaprolactone: A Comparison of Methods, J. Appl. Polym. Sci., 2020, 137(30), 48908,  DOI:10.1002/app.48908.
50. M. F. Schulz, A. K. Khandpur, F. S. Bates, K. Almdal, K. Mortensen, D. A. Hajduk and S. M. Gruner, Phase Behavior of Polystyrene−Poly(2-Vinylpyridine) Diblock Copolymers, Macromolecules, 1996, 29(8), 2857–2867,  DOI:10.1021/ma951714a.
51. P. Guo, W. Guan, L. Liang and P. Yao, Self-Assembly of PH-Sensitive Random Copolymers: Poly(Styrene-Co-4-Vinylpyridine), J. Colloid Interface Sci., 2008, 323(2), 229–234,  DOI:10.1016/j.jcis.2008.04.009.
52. Y. C. Kim, R. J. Composto and K. I. Winey, PH-Mediated, Size-Selective Adsorption of Gold Nanoparticles on Diblock Copolymer Brushes, ACS Nano, 2023, 17(10), 9224–9234,  DOI:10.1021/acsnano.3c00212.
53. T. Curtis, A. K. Taylor, S. E. Alden, C. Swanson, J. Lo, L. Knight, A. Silva, B. D. Gates, S. R. Emory and D. A. Rider, Synthesis and Characterization of Tunable, PH-Responsive Nanoparticle–Microgel Composites for Surface-Enhanced Raman Scattering Detection, ACS Omega, 2018, 3(9), 10572–10588,  DOI:10.1021/acsomega.8b01561.
54. B. J. Kim, J. Bang, C. J. Hawker, J. J. Chiu, D. J. Pine, S. G. Jang, S.-M. Yang and E. J. Kramer, Creating Surfactant Nanoparticles for Block Copolymer Composites through Surface Chemistry, Langmuir, 2007, 23(25), 12693–12703,  DOI:10.1021/la701906n.
55. Y.-S. Sun, K.-W. Wu and O. Shih, Tuning Perovskite Nanocrystal Synthesis via Amphiphilic Block Copolymer Templates and Solvent Interactions, ACS Appl. Mater. Interfaces, 2024, 16(45), 62664–62679,  DOI:10.1021/acsami.4c13822.
56. S. Pal, R. K. Srivastava and B. Nandan, Effect of Spinning Solvent on Crystallization Behavior of Confined Polymers in Electrospun Nanofibers, Polym. Cryst., 2021, 4(6), e10209,  DOI:10.1002/pcr2.10209.
57. L. Van der Schueren, B. De Schoenmaker, Ö. I. Kalaoglu and K. De Clerck, An Alternative Solvent System for the Steady State Electrospinning of Polycaprolactone, Eur. Polym. J., 2011, 47(6), 1256–1263,  DOI:10.1016/j.eurpolymj.2011.02.025.
58. G. H. F. Melo and U. Sundararaj, Influence of Mixed Solvent in the Morphology and Hydrophobicity of Electrospun Polystyrene Porous Fibers, Macromol. Rapid Commun., 2024, 45(21), 2400403,  DOI:10.1002/marc.202400403.
59. J. Lasprilla-Botero, M. Álvarez-Láinez and J. M. Lagaron, The Influence of Electrospinning Parameters and Solvent Selection on the Morphology and Diameter of Polyimide Nanofibers, Mater. Today Commun., 2018, 14, 1–9,  DOI:10.1016/j.mtcomm.2017.12.003.
60. E. Ewaldz, J. Randrup and B. Brettmann, Solvent Effects on the Elasticity of Electrospinnable Polymer Solutions, ACS Polym. Au, 2022, 2(2), 108–117,  DOI:10.1021/acspolymersau.1c00041.
61. Y. Xu, L. Zou, H. Lu and T. Kang, Effect of Different Solvent Systems on PHBV/PEO Electrospun Fibers, RSC Adv., 2017, 7(7), 4000–4010,  10.1039/C6RA26783A.
62. A. P. Kotula, C. R. Snyder and K. B. Migler, Determining Conformational Order and Crystallinity in Polycaprolactone via Raman Spectroscopy, Polymer, 2017, 117, 1–10,  DOI:10.1016/j.polymer.2017.04.006.
63. L. Stoleriu, A. Stancu, P. Chakraborty, A. Hauser and C. Enachescu, Analysis of First Order Reversal Curves in the Thermal Hysteresis of Spin-Crossover Nanoparticles within the Mechanoelastic Model, J. Appl. Phys., 2015, 117(17), 17B307,  DOI:10.1063/1.4914953.
64. A. Tissot, C. Enachescu and M.-L. Boillot, Control of the Thermal Hysteresis of the Prototypal Spin-Transition FeII(Phen)2(NCS)2 Compound via the Microcrystallites Environment: Experiments and Mechanoelastic Model, J. Mater. Chem., 2012, 22(38), 20451,  10.1039/c2jm33865c.
65. C. Enachescu, R. Tanasa, A. Stancu, A. Tissot, J. Laisney and M.-L. Boillot, Matrix-Assisted Relaxation in Fe(Phen)2(NCS)2 Spin-Crossover Microparticles, Experimental and Theoretical Investigations, Appl. Phys. Lett., 2016, 109(3), 031908,  DOI:10.1063/1.4959262.
66. R. Tanasa, J. Laisney, A. Stancu, M.-L. Boillot and C. Enachescu, Hysteretic Behavior of Fe(Phen)2(NCS)2 Spin-Transition Microparticles vs. the Environment: A Huge Reversible Component Resolved by First Order Reversal Curves, Appl. Phys. Lett., 2014, 104(3), 031909,  DOI:10.1063/1.4862748.

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