Pressure Effect Studies on the Spin Transition of Microporous 3D Polymer [Fe(pz)Pt(CN)4]

Observation of TLIESST above Liquid Nitrogen Temperature and Disclosure of Hidden Hysteresis in Multiresponsive Hofmann-type Coordination Polymers

Photoresponsive spin-crossover (SCO) molecules are an important class of bistable magnetic molecules with intriguing potential in device applications. The light-induced excited spin state trapping (LIESST) and the combined application of light and temperature can provide access to the metastable region of the SCO profile. The primary obstacle in utilizing light stimuli is the manifestation of light-induced trappings at extremely low temperatures. Herein, we report two novel multiresponsive 2D Hofmann-type coordination polymers exhibiting light-induced excited spin state trapping above liquid nitrogen temperature (TLIESST = 82 and 81 K). Stimulating the samples in conjugation with light and temperature successfully unveils hysteresis, which is otherwise concealed. Apart from light and temperature, we found that the SCO phenomenon is also responsive to external hydrostatic pressure and exhibits modulation of the hysteresis width and transition temperature shifts with changes in pressure.

Anomalous Pressure Response of Temperature-Induced Spin Transition and a Pressure-Induced Spin Transition in Two-Dimensional Hofmann Coordination Polymers

Spin transition (ST) compounds have been extensively studied because of the changes in rich physicochemical properties accompanying the ST process. The study of ST mainly focuses on the temperature-induced spin transition (TIST). To further understand the ST, we explore the pressure response behavior of TIST and pressure-induced spin transition (PIST) of the 2D Hofmann-type ST compounds [Fe(Isoq)2M(CN)4] (Isoq-M) (M = Pt, Pd, Isoq = isoquinoline). The TISTs of both Isoq-Pt and Isoq-Pd compounds exhibit anomalous pressure response, where the transition temperature (T1/2) exhibits a nonlinear pressure dependence and the hysteresis width (ΔT1/2) exhibits a nonmonotonic behavior with pressure, by the synergistic influence of the intermolecular interaction and the distortion of the octahedral coordination environment. And the distortion of the octahedra under critical pressures may be the common behavior of 2D Hofmann-type ST compounds. Moreover, ΔT1/2 is increased compared with that before compression because of the partial irreversibility of structural distortion after decompression. At room temperature, both compounds exhibit completely reversible PIST. Because of the greater change in mechanical properties before and after ST, Isoq-Pt exhibits a more abrupt ST than Isoq-Pd. In addition, it is found that the hydrostatic properties of the pressure transfer medium (PTM) significantly affect the PIST due to their influence on spin-domain formation.

Is the surface of Hofmann-like spin-crossover {Fe(pz)[Pt(CN)4]} the same as its bulk?

Temperature dependent X-ray photoemission spectroscopy (XPS) has been employed to examine the spin-crossover (SCO) transition in the nanocrystals of 3D Hoffman-like {Fe(pz)[Pt(CN)4]}. Consistent with the existing literature, the temperature-dependent variations in the Fe 2p core-level spectrum provide unambiguous evidence of the spin-state transition in this SCO complex. One of the many possible reasons behind a lack of discernible temperature-driven shifts in the binding energies of both the N 1s core-level components could be the immunity of its HS electronic configuration to thermal fluctuations. The high-spin fraction versus temperature plot, extrapolated from the XPS measurements, reveals that the surface of the nanocrystals of {Fe(pz)[Pt(CN)4]} is in the high-spin state at room temperature, rendering it promising for room-temperature spintronics and quantum information science applications.

Vapochromic effect in switchable molecular-based spin crossover compounds

Coordination complexes based on transition metal ions displaying [Ar]3d⁴-3d⁷ electronic configurations can undergo the likely most spectacular switchable phenomena found in molecular coordination chemistry, the well-known Spin Crossover (SCO). SCO phenomena is a detectable, reproducible and reversible switch that occurs between the high spin (HS) and low spin (LS) electronic states of the transition metal actuated by different stimuli (i.e. light, temperature, pressure, the presence of an analyte). Moreover, the occurrence of SCO phenomena causes different outputs, one of them being a colour change. Altogether, an analyte in gas form could be detected by naked eye once it has triggered the corresponding HS ↔ LS transition. This vapochromic effect could be used to detect volatile molecules using a low-cost technology, including harmful chemical substances, gases and/or volatile organic compounds (VOCs) that are present in our environment, in our home or at our workplace. The present review condenses all reported iron coordination compounds where the colour change induced by a given molecule in its gas form is coupled to a HS ↔ LS spin transition. Special emphasis has been made on describing the nature of the post-synthetic modification (PSM) taking place in the material upon the analyte uptake. In this case, three types of PSM can be distinguished: based on supramolecular contacts and/or leading to a coordinative or covalent bond. In the latter, a colour change not only indicates the switch of the spin state in the material but also the formation of a new compound with different properties. It is important to indicate that some of the SCO coordination compounds discussed in the current report have been part of other spin crossover reviews, that have gathered thermally induced SCO compounds and the influence of guest molecules on the SCO behaviour. However, in the majority of examples in these reviews, the change of colour upon the uptake of analytes is not associated with a spin transition at room temperature. In addition, the observed colour variations have been mainly discussed in terms of host-guest interactions, when they can also be induced by a PSM taking place in different sites of the molecule, like the Fe(II) coordination sphere or by chemically altering its inorganic and/or organic linkers. Therefore, we present here for the first time an exhaustive compilation of all systems in which the interaction between the coordination compounds and the vapour analytes leads to a colour change due to a spin transition in the metal centre at room temperature.

Pressure-Induced Mixed States Caused by Spin-Elastic Interactions during First-Order Spin Phase Transition in Spin Crossover Compounds

Recently, the possibility of exploiting the phenomenon of spin transition (ST) has been intensively investigated; therefore, it is particularly important to study the behavior of ST under various stimuli. Here, the shape and content of the intermediate phase of ST in Hoffmann-like compounds [Fe(Fpz)₂M(CN)₄] (M = Pt, Pd) under external stimuli are studied. For this purpose, magnetic and Raman spectroscopy studies were carried out. In pressure-induced spin transition (PIST), a mixture of high-spin and low-spin states appears, while in temperature-induced spin transition (TIST), a homogeneous state occurs. The first-order ST induced by pressure has a hysteresis but is not abrupt. However, the temperature-induced spin transition at ambient pressure is hysteretic and abrupt. To investigate this difference, we discuss using a thermodynamic model that considers elastic interactions, showing that the slope of the hysteresis loop is related to the appearance of internal pressure, which is related to the difference in sample compressibility under high-spin and low-spin states.

Thin Films of Nanocrystalline Fe(pz)[Pt(CN)4] Deposited by Resonant Matrix-Assisted Pulsed Laser Evaporation

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.

Pressure Tunable Electronic Bistability in Fe(II) Hofmann-like Two-Dimensional Coordination Polymer [Fe(Fpz)2Pt(CN)4]: A Comprehensive Experimental and Theoretical Study

A comprehensive experimental and theoretical study of both thermal-induced spin transition (TIST) as a function of pressure and pressure-induced spin transition (PIST) at room temperature for the two-dimensional Hofmann-like SCO polymer [Fe(Fpz)₂Pt(CN)₄] is reported. The TIST studies at different fixed pressures have been carried out by magnetic susceptibility measurements, while PIST studies have been performed by means of powder X-ray diffraction, Raman, and visible spectroscopies. A combination of the theory of elastic interactions and numerical Monte Carlo simulations has been used for the analysis of the cooperative interactions in TIST and PIST studies. A complete (T, P) phase diagram for the compound [Fe(Fpz)₂Pt(CN)₄] has been constructed. The critical temperature of the spin transition follows a lineal dependence with pressure, meanwhile the hysteresis width shows a nonmonotonic behavior contrary to theoretical predictions. The analysis shows the exceptional role of the total entropy and phonon contribution in setting the temperature of the spin transition and the width of the hysteresis. The anomalous behavior of the thermal hysteresis width under pressure in [Fe(Fpz)₂Pt(CN)₄] is a direct consequence of a local distortion of the octahedral geometry of the Fe(II) centers for pressures higher than 0.4 GPa. Interestingly, there is not a coexistence of the high- and low-spin (HS and LS, respectively) phases in TIST experiments, while in PIST experiments, the coexistence of the HS and LS phases in the metastable region of the phase transition induced by pressure is observed for a first time in a first-order gradual spin transition with hysteresis.

A comprehensive experimental and theoretical study of both thermal-induced spin transition as a function of pressure and pressure-induced spin transition at room temperature for the two-dimensional Hofmann-like SCO polymer [Fe(Fpz)₂Pt(CN)₄] is reported. A complete (T, P) phase diagram for compound [Fe(Fpz)₂[Pt(CN)₄] has been constructed. The critical temperature of the spin transition follows a lineal dependence with pressure, meanwhile the hysteresis width shows a nonmonotonic behavior contrary to theoretical predictions.

Downsizing of Nanocrystals While Retaining Bistable Spin Crossover Properties in Three-Dimensional Hofmann-Type {Fe(pz)[Pt(CN)4]}-Iodine Adducts

Mastering nanostructuration of functional materials into electronic devices is presently an essential task in materials science. This is particularly relevant for spin crossover (SCO) compounds, whose properties are extremely sensitive to size reduction. Indeed, the search for materials displaying strong cooperative hysteretic SCO properties operative at the nanoscale close near room temperature is extremely challenging. In this context, we describe here the synthesis and characterization of 20-30 nm surfactant-free nanocrystals of the Fe^II Hofmann-type polymer {Fe^II(pz)[Pt^II,IVI_x(CN)₄]} (pz = pyrazine), which affords the first example of a robust three-dimensional coordination polymer, substantially keeping operational thermally induced SCO bistability at such a scale.

Variable Cooperative Interactions in the Pressure and Thermally Induced Multistep Spin Transition in a Two-Dimensional Iron(II) Coordination Polymer

Two types of experiments conducted to investigate the effect of pressure on the spin crossover (SCO) properties of the 2D Fe(II) coordination polymer formulated {Fe[bipy(ttr)₂]}_n are reported, namely, (1) magnetic measurements performed at variable temperature and at fixed pressure and (2) visible spectroscopy at variable pressure and fixed temperature. The magnetic experiments carried out under a hydrostatic pressure constraint of 0.04, 0.08, and 0.8 GPa reveal a two-step spin transition behavior. The characteristic critical temperatures of the spin transition are shifted upward in temperature as pressure increases. The slope of the straight-line of the T_c vs P plot, dT_c/dP, is 775 K/GPa and 300 K/GPa, for the high temperature and the low temperature steps, respectively. These values are remarkably large and denote the extreme sensitivity of the material to the application of pressure. Indeed, the visible spectroscopic measurements performed at 293 K show that a complete spin transition is induced at pressures as low as 0.4 GPa. Moreover, the pressure-induced spin transition is reversible and shows an asymmetric hysteresis. An analysis of the cooperative interactions of the thermal- and pressure-induced spin transition in the framework of the model of elastic interactions reveals that the elastic energy of the lattice as well as the interaction parameter between the SCO centers change during the course of the spin transition. Consequently, the character of the spin transition varies from abrupt for the high temperature step to continuous for the low temperature step.

Modulation of Jahn-Teller distortion and electromechanical response in a Mn3+spin crossover complex

Structural, magnetic and electromechanical changes resulting from spin crossover between the spin quintet and spin triplet forms of a mononuclear Mn3+complex embedded in six lattices with different charge balancing counterions are reported. Isostructural ClO4-and BF4-salts (1) and (2) each have two unique Mn3+sites which follow different thermal evolution pathways resulting in a crossover from the spin quintet form at room temperature to a 1:1 spin triplet:quintet ratio below 150 K. The PF6-(3) and NO3-(4) salts which each have one unique Mn3+site show a complete conversion from spin quintet to spin triplet over the same temperature range. A complete two step spin crossover is observed in the CF3SO3-lattice (5) with a 1:1 ratio of spin quintet and spin triplet forms at intermediate temperature, while the BPh4-lattice (6) stabilizes the spin triplet form over most of the temperature range with gradual and incomplete spin state switching above 250 K. An electromechanical piezoresponse was detected in NO3-complex4despite crystallization in a centrosymmetric space group. The role of deformations associated with stress-induced spin triplet-spin quintet switching in breaking the local symmetry are discussed and computational analysis is used to estimate the energy gap between the two spin states.

Tuning of crystallization method and ligand conformation to give a mononuclear compound or two-dimensional SCO coordination polymer based on a new semi-rigid V-shaped bis-pyridyl bis-amide ligand

With the new semi-rigid V-shaped bidentate pyridyl amide compound 5-methyl-N,N'-bis(pyridin-4-yl)benzene-1,3-dicarboxamide (L) as an auxiliary ligand and the Fe^II ion as the metal centre, one mononuclear complex, bis(methanol-κO)bis[5-methyl-N,N'-bis(pyridin-4-yl)benzene-1,3-dicarboxamide-κN]bis(thiocyanato-κN)iron(II), [Fe(SCN)₂(C₁₉H₁₆N₄O₂)₂(CH₃OH)₂] (1), and one two-dimensional coordination polymer, catena-poly[[[bis(thiocyanato-κN)iron(II)]-bis[μ-5-methyl-N,N'-bis(pyridin-4-yl)benzene-1,3-dicarboxamide-κ²N:N']] methanol disolvate dihydrate], {[Fe(SCN)₂(C₁₉H₁₆N₄O₂)₂]·2CH₃OH·2H₂O}_n (2), were prepared by slow evaporation and H-tube diffusion methods, respectively, indicating the effect of the method of crystallization on the structure type of the target product. Both complexes have been structurally characterized by elemental analysis, IR spectroscopy and single-crystal X-ray crystallography. The single-crystal X-ray diffraction analysis shows that L functions as a monodentate ligand in mononuclear 1, while it coordinates in a bidentate manner to two independent Fe(SCN)₂ units in complex 2, with a different conformation from that in 1 and the ligands point in two almost orthogonal directions, therefore leading to a two-dimensional grid-like network. Investigation of the magnetic properties reveals the always high-spin state of the Fe^II centre over the whole temperature range in 1 and a gradual thermally-induced incomplete spin crossover (SCO) behaviour below 150 K in 2, demonstrating the influence of the different coordination fields on the spin properties of the metal ions. The current results provide useful information for the rational design of functional complexes with different structure dimensionalities by employing different conformations of the ligand and different crystallization methods.