We synthesized iron(II)-triazole spin crossover compounds of the type [Fe(atrz)₃]X₂ and incorporated and deposited them on electrospun polymer nanofibers. For this, we used two separate electrospinning methods with the goal of obtaining polymer complex composites with intact switching properties. In view of possible applications, we chose iron(II)-triazole-complexes that are known to exhibit spin crossover close to ambient temperature. Therefore, we used the complexes [Fe(atrz)₃]Cl₂ and [Fe(atrz)₃](2ns)₂ (2ns = 2-Naphthalenesulfonate) and deposited those on fibers of polymethylmethacrylate (PMMA) and incorporated them into core–shell-like PMMA fiber structures. These core–shell structures showed to be inert to outer environmental influences, such as droplets of water, which we purposely cast on the fiber structure, and it did not rinse away the used complex. We analyzed both the complexes and the composites with IR-, UV/Vis, Mössbauer spectroscopy, SQUID magnetometry, as well as SEM and EDX imaging. The analysis via UV/Vis spectroscopy, Mössbauer spectroscopy, and temperature-dependent magnetic measurements with the SQUID magnetometer showed that the spin crossover properties were maintained and were not changed after the electrospinning processes.

The development of molecular switches with tunable properties has garnered considerable interest over several decades. A novel spin‐crossover (SCO) material based on iron(II) complexes incorporating 4‐acetylpyridine (4‐acpy) and [Hg(SCN)₄]²⁻ anions was synthesized and formulated as [Fe(4‐acpy)₂][Hg(μ‐SCN)₄] (1). Compound 1 is crystallized in a three‐dimensional network in the non‐centrosymmetric orthorhombic space group Pna2₁ with two octahedral [Fe(4‐acpy)₂(NCS)₄] entities featuring two distinct Fe centers (Fe1 and Fe2). Crystallographic, magnetic, and Mössbauer measurements reveal an incomplete SCO exclusively at Fe2, with transition temperature T_1/2≈102 K. Photomagnetic studies conducted at 10 K with lasers ranging from 405 to 1310 nm evidence light‐induced excited spin‐state trapping (LIESST) and reverse‐LIESST effects, with a unique near‐infrared‐responsive LIESST phenomenon at 1064 and 1310 nm. Advanced photocrystallographic studies at 40 K provide precise structural evidence for these metastable states. The optical and vibrational properties consistently corroborate with magnetic and photomagnetic studies. Additionally, temperature‐ and light‐dependent terahertz (THz) absorptions are associated with phonon vibrations around Fe2 centers, through SCO behavior, as supported by ab initio calculation. The Fe(II)−Hg(II) systems can be promising benchmarks for exploring synergistic switching effects in structural, magnetic, and spectroscopic properties.

Complexes like iron (II)-triazoles exhibit spin crossover behavior at ambient temperature and are often considered for possible application. In previous studies, we implemented complexes of this type into polymer nanofibers and first polymer-based optical waveguide sensor systems. In our current study, we synthesized complexes of this type, implemented them into polymers and obtained composites through drop casting and doctor blading. We present that a certain combination of polymer and complex can lead to composites with high potential for optical devices. For this purpose, we used two different complexes $$[\text {Fe}(\text {atrz})_{3}](2\, \text {ns})2$$[ Fe ( atrz ) 3 ] ( 2 ns ) 2 and $$[\text {Fe}(\text {atrz})_{3}]\text {Cl}_{1.5}(\text {BF}_{4})_{0.5}$$[ Fe ( atrz ) 3 ] Cl 1.5 ( BF 4 ) 0.5 with different polymers for each composite. We show through transmission measurements and UV/VIS spectroscopy that the optical properties of these composite materials can reversibly change due to the spin crossover effect.

The development of molecular switches with tunable properties has garnered considerable interest over several decades. A novel spin‐crossover (SCO) material based on iron(II) complexes incorporating 4‐acetylpyridine (4‐acpy) and [Hg(SCN)₄]²⁻ anions was synthesized and formulated as [Fe(4‐acpy)₂][Hg(μ‐SCN)₄] (1). Compound 1 is crystallized in a three‐dimensional network in the non‐centrosymmetric orthorhombic space group Pna2₁ with two octahedral [Fe(4‐acpy)₂(NCS)₄] entities featuring two distinct Fe centers (Fe1 and Fe2). Crystallographic, magnetic, and Mössbauer measurements reveal an incomplete SCO exclusively at Fe2, with transition temperature T_1/2≈102 K. Photomagnetic studies conducted at 10 K with lasers ranging from 405 to 1310 nm evidence light‐induced excited spin‐state trapping (LIESST) and reverse‐LIESST effects, with a unique near‐infrared‐responsive LIESST phenomenon at 1064 and 1310 nm. Advanced photocrystallographic studies at 40 K provide precise structural evidence for these metastable states. The optical and vibrational properties consistently corroborate with magnetic and photomagnetic studies. Additionally, temperature‐ and light‐dependent terahertz (THz) absorptions are associated with phonon vibrations around Fe2 centers, through SCO behavior, as supported by ab initio calculation. The Fe(II)−Hg(II) systems can be promising benchmarks for exploring synergistic switching effects in structural, magnetic, and spectroscopic properties.

Two iron(III) spin crossover complexes [Fe(⁵Cl-L)(NCS)] (1) and [Fe(⁵Cl-L)(NCSe)] (2) were synthesized with the pentadentate Schiff base ligand ⁵Cl-L and thiocyanato and selenocyanato as coligands. ⁵Cl-L, as an asymmetric {N₃O₂} donor Schiff base, was synthesized by a condensation reaction of 5-chlorosalicyladehyde using the asymmetric N-(2-aminoethyl)-1,3-propanediamine. The complexes exhibited a spin crossover at 280 (1) and 293 K (2), respectively, and were subjected to electron spin resonance (ESR) and Mössbauer spectroscopy at 77, 295 and 325 K. Ab initio CASSCF calculations followed by the NEVPT2 method were applied for predicting the g-tensor components as well as Mössbauer parameters.

In this paper we study a series of solid iron(II)‐triazole spin crossover complexes of the formula [Fe(NH₂trz)₃]Br_2(x−1)Cl_2x, where the counterions bromide and chloride are present in the same structure in different ratios. Thus, the spin transition temperature also changes in the temperature range between the pure complexes [Fe(NH₂trz)₃]Br₂ and [Fe(NH₂trz)₃]Cl₂. Since these switching temperatures of the pure complexes are rather far apart, this allows us to examine how the SCO behavior of iron(II) triazole complexes with mixed counterions changes with different ratios. To achieve this, the complexes were synthesized with both counterions simultaneously using the salts FeBr₂ and FeCl₂ ⋅ 4 H₂O. The complexes were analyzed by IR and XRD measurements to confirm a successful synthesis. Energy dispersive X‐ray spectroscopy (EDX) was further used to validate the results and to verify a homogeneous distribution of the mixed anions in all samples. This study reveals a linear switching behavior of the complexes depending on the substitution ratio with a temperature range between the homo ionic samples.