Dominik Maskowicz

The development of multistep spin crossover materials is of considerable interest for molecular information processing and sensing applications and remains synthetically and mechanistically challenging. Herein, we present the first iron(II) two-dimensional Hofmann structure containing 1,2,4-triazole derivatives and [Au(CN)₂]^- units, namely, Fe(MeOPhtrz)₂[Au(CN)₂]₂ (1, MeOPhtrz = (E)-1-(2-methoxyphenyl)-N-(4H-1,2,4-triazol-4-yl)methanimine). The complex exhibits a temperature-induced three-step spin crossover behavior, confirmed by magnetic susceptibility, differential scanning calorimetry, Raman spectroscopy, single-crystal X-ray diffraction (SXRD), and optical microscopy. SXRD reveals a pseudothree-dimensional structure assembled through multiple intermolecular interactions, including hydrogen bonding, π-π stacking, and π-Au interactions. These interactions contribute to an anisotropic supramolecular framework that induces a multistep spin crossover process. The sequential spin transition is likely driven by the differential rigidity along the crystallographic axes and the varied response of Fe-N bond lengths, leading to distinct transition steps. This study highlights the significance of supramolecular interactions in governing spin crossover properties and opens new avenues for the design of 2D Hofmann-like materials with tunable functionalities.

Hydroxyapatite (HAp) is one of the most widely studied materials for utilization in the development of artificial implants. Research is mainly aimed at the production and modification of HAp coatings for simplification of the deposition process, cost reduction, and increase in biocompatibility. In this paper, the authors deposited HAp synthetic microparticles by means of matrix-assisted pulsed laser evaporation (MAPLE) on Ti6Al4V alloy plate substrates and obtained uniform HAp coatings without further treatment or modifications. The authors utilized a tunable pulsed laser to adjust its wavelength to the selected solvents, in order to optimize the process for deposition speed and quality. The following solvents were used as matrices: deionized water, isopropyl alcohol, and a 3:2 mixture of isopropanol:acetonitrile. Obtained coatings were examined by means of scanning electron microscopy, Raman spectroscopy, X-ray diffraction, and profilometry in order to evaluate coating quality, uniformity, and structural integrity. MAPLE deposition allowed the acquisition of approx. 200 nm thick coatings for water and isopropanol matrices and approx. 320 nm for isopropanol:acetonitrile matrix, which indicates an increase in deposition rate by 37%. The obtained coatings meet requirements for further biocompatibility testing, material modification, and composite synthesis.

Paramount spin-crossover properties of the 3D-Hoffman metalorganic framework (MOF) [Fe(pz)₂Pt(CN)₄] are generally described on the basis of the ligand field theory, which provides adequate insight into theoretical and simulation analysis of spintronic complexes. However, the ligand field approximation does not take into account the 3D periodicity of the actual complex lattice and surface effects and therefore cannot predict a full-scale periodic structure without utilizing more advanced methods. Therefore, in this paper, the electronic properties of the exemplar MOF were analyzed from the band structure perspective in low-spin (LS) and high-spin (HS) states. The density-of-states spectra determined for both spin-up and spin-down electrons of Fe d⁶ orbitals indicate spin-orbital splitting and delocalization for HS due to spin polarization in the iron atom ligand field. Presence of the surface states in the real crystal causes a red shift of the metal-metal charge transfer (MMCT) and metal-ligand charge transfer (MLCT) peaks for both HS and LS states. The addition of residual water molecules and disorder among the pyrazine rings reveal additional influences on the positions of the pyrazine band and, therefore, on the absorption spectra of the crystal. The results show a magnification of the peak correlated with the MLCT in the HS state and a significant red shift of the LS characteristic absorption band. The presented approach involving band structure analysis delivers a more complete image of the electronic properties of the [Fe(pz)₂Pt(CN)₄] crystalline network and can be a landmark for insightful studies of other MOFs.

Prior studies of the thin film deposition of the metal-organic compound of Fe(pz)Pt[CN]₄ (pz = pyrazine) using the matrix-assisted pulsed laser evaporation (MAPLE) method, provided evidence for laser-induced decomposition of the molecular structure resulting in a significant downshift of the spin transition temperature. In this work we report new results obtained with a tunable pulsed laser, adjusted to water resonance absorption band with a maximum at 3080 nm, instead of 1064 nm laser, to overcome limitations related to laser–target interactions. Using this approach, we obtain uniform and functional thin films of Fe(pz)Pt[CN]₄ nanoparticles with an average thickness of 135 nm on Si and/or glass substrates. X-ray diffraction measurements show the crystalline structure of the film identical to that of the reference material. The temperature-dependent Raman spectroscopy indicates the spin transition in the temperature range of 275 to 290 K with 15 ± 3 K hysteresis. This result is confirmed by UV-Vis spectroscopy revealing an absorption band shift from 492 to 550 nm related to metal-to-ligand-charge-transfer (MLCT) for high and low spin states, respectively. Spin crossover is also observed with X-ray absorption spectroscopy, but due to soft X-ray-induced excited spin state trapping (SOXIESST) the transition is not complete and shifted towards lower temperatures.

The influence of guest molecules affecting the spin transition characteristics of iron(II) complexes represents a widely discussed topic because it opens the possibility of using this type of materials in many fields of science, especially if the material properties could be tuned in a controlled way. We report an experimental observation of the spin transition in thin films of a 3D Hoffman framework nanocrystalline material Fe(pyrazine)[Pt(CN)₄] deposited by nanosecond laser ablation at λ = 1064 nm of its cryo-cooled colloidal suspension in water and water-free organic solvents. For vacuum deposited films (thickness 120 nm), the substantial downshift in temperature, gradual spin transition, and shrinkage of the temperature hysteresis compared to the starting material are ascribed to incomplete removal of water molecules from the porous network and partial destruction of the crystalline site caused by laser heating. The destructive effect of laser irradiation occurring in vacuum conditions was not observed for deposition conducted in N₂ at atmospheric pressure. In this case, thin films reproducing properties of the reference material with transition temperature near 276 K and 12 K wide hysteresis were obtained. In addition, the changes in the spin switching characteristics associated with the exchange of guest molecules in the SCO crystal lattice were observed using a water-free solvent as a matrix for the laser based thin film deposition. The observed laser induced host–guest exchange indicates on the possibility of selective modification of thin layers of SCO materials to obtain their desired characteristics.

This work shows comparative analysis of surface-enhanced Raman scattering (SERS) activity of gold nanoparticles fabricated by chemical synthesis and laser ablation methods. The gold nanoparticles prepared by laser ablation (Au-LA) are more effective for SERS than those prepared chemically (Au-citr). The “analyte on Au film” configuration allows obtaining enhancement of Raman scattering up to 10⁴ in case of Au-LA nanoparticles and up to 10² in case of Au-citr. Also the “sandwich” configuration for Au-LA gives additional enhancement of SERS up to two times, and for Au-citr up to one order, that is consistent with theoretical calculations.

In this work, the fabrication of [Fe(pz)Pt(CN)₄] (pz = pyrazine) thin films by means of matrix‐assisted pulsed laser evaporation (MAPLE) was investigated. As starting material, a cryogenically cooled suspension of nanocrystalline [Fe(pz)Pt(CN)₄] (0.35wt.‐%) in a mixture of 1,1‐dichloroethane and polyethylene glycol (PEG) was used. Films of a thickness up to 150–200 nm were deposited on Si substrates by laser ablation at λ = 1064 nm of the cryogenically cooled material. PEG impregnated films exhibit a cooperative first‐order hysteretic spin crossover behavior around 155 K as detected by Raman spectroscopy. Most interestingly, deposition of a crystalline material was achieved through MAPLE deposition.