Search

Materials based on spin crossover (SCO) molecules have centred the attention in Molecular Magnetism for more than forty years as they provide unique examples of multifunctional and stimuli-responsive materials, which can be then integrated into electronic devices to exploit their molecular bistability. This process often requires the preparation of thermally stable SCO molecules that can sublime and remain intact in contact with surfaces. However, the number of robust sublimable SCO molecules is still very scarce. Here we report a novel example of this kind. It is based on a neutral iron (II) coordination complex formulated as [FeII(neoim)2], where neoimH is the ionogenic ligand 2-(1H-imidazol-2-yl)-9-methyl-1,10-phenanthroline. In the first part a comprehensive study, which covers the synthesis and magneto-structural characterization of the [FeII(neoim)2] complex as a bulk microcrystalline material, is reported. Then, in the second part we investigate the suitability of this material to form thin films through high vacuum (HV) sublimation. Finally, the retainment of all present SCO capabilities in the bulk when the material is processed is thoroughly studied by means of X-ray absorption spectroscopy. In particular, a very efficient and fast light-induced spin transition (LIESST effect) has been observed, even for ultrathin films of 15 nm., Comment: 27 pages, 2 schemes, 5 figures, 1 table

Among the different types of switchable molecular compounds, sublimable Fe(II) SCO molecules provide a suitable platform to develop smart devices that respond to external stimuli. Here we report the synthesis, crystallographic structure and magnetic properties of three new neutral Fe(II) SCO molecules belonging to the {Fe[H2B(pz)2]2(L)} family with bidentate-alpha-diimine ligands L = 3-(pyridin-2-yl)-[1,2,3]triazolo[1,5-a]pyridine (tzpy), 5,5,6,6-tetrahydro-4H,4H-2,2-bi(1,3-thiazine) (btz) and 4,4,5,5-tetrahydro-2,2-bithiazole (bt) (1, 2 and 3, respectively), as well as two solvated forms of 1 and 3. All three desolvated compounds present thermal- and light-induced SCO transitions with different degrees of cooperativity and effectiveness. Furthermore, 1 and 2 are demonstrated to be sublimable under HV conditions affording homogeneous thin films 200 nm thick (TF1 and TF2) that retain the chemical integrity of the original molecules regardless the deposition surface. The SCO behaviour of the films is characterized by XAS technique revealing the partial retainment of both thermal- and light-induced spin transitions, yet losing the cooperativity. Finally, SCO/2D horizontal hybrid devices based on CVD-graphene are produced using these films. Being the first ones of this type utilizing molecules of {Fe[H2B(pz)2]2(L)} family, with L = tzpy and btz, the devices have allowed the successful detection of the thermal SCO transition through the electric properties of the CVD-graphene., Comment: 20 pages, 8 figures

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 measurements 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. Whereas, 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., Comment: 10 pages, 6 figures

Hybrid devices based on spin-crossover (SCO)/2D heterostructures grant a highly sensitive platform to detect the spin transition in the molecular SCO component and tune the properties of the 2D material. However, the fragility of the SCO materials upon thermal treatment, light irradiation or contact with surfaces and the methodologies used for their processing have limited their applicability. Here, we report an easily processable and robust SCO/2D hybrid device with outstanding performance based on the sublimable SCO [Fe(Pyrz)2] molecule deposited over CVD-graphene, which is fully compatible with electronics industry protocols. Thus, a novel methodology based on growing an elusive polymorph of [Fe(Pyrz)2] (tetragonal phase) over graphene is developed that allows us to electrically detect a fast and effective light-induced spin transition in the devices (~50% yield in 5 minutes). Such performance can be enhanced even more when a flexible polymeric layer of PMMA is inserted in between the two active components in a contactless configuration, reaching a ~100 % yield in 5 minutes., Comment: 25 pages, 5 figures

The adsorption mechanism of SO2 in the Hofmann-like coordination polymer Fe(pz)[Pt(CN)4] is studied using inelastic neutron scattering and density functional theory calculations. We find that the most important spectral change upon gas adsorption is the blueshift of the low energy peak found at 100 cm-1, a feature that is fully confirmed by the computed neutron-weighted phonon density of states. Our calculations suggest that the origin of this change is twofold: i) an increase in the force constant of the cyanide out-of-plane movement due to the binding of the gas onto the Pt(CN)4 plane, and ii) the hampered rotation of the pyrazine due to steric hindrance. The high energy region of the neutron scattering data whose spectral weight is dominated by the internal vibrations of the pyrazine is negligibly affected by the presence of the gas as expected from a physisorption type of binding.

The devil is in the detail: Periodic and aperiodic spin-state concentration waves form during "Devil's staircase"-type spincrossover in a new bimetallic 2D coordination polymer {Fe[(Hg(SCN)3)2](4,4'-bipy)2}n.The unprecedented bimetallic 2D coordination polymer {Fe[(Hg(SCN)3)2](4,4'-bipy)2}n exhibits a thermal high-spin (HS)$low-spin (LS) staircase-like conversion characterized by a multi-step dependence of the HS molar fraction gHS. Between the fully HS (gHS=1) and LS (gHS=0) phases, two steps associated with different ordering appear in terms of spin-state concentration waves (SSCW). On the gHS=0.5 step, a periodic SSCW forms with a HS-LS-HS-LS sequence. On the gHS=0.34 step, the 4D superspace crystallography structural refinement reveals an aperiodic SSCW, with a HS-LS sequence incommensurate with the molecular lattice. The formation of these different long-range spatially ordered structures of LS and HS states during the multi-step spin-crossover is discussed within the framework of "Devil's staircase"-type transitions. Spatially modulated phases are known in various types of materials but are uniquely related to molecular HS/LS bistability in this case., Comment: in Angewandte Chemie International Edition, Wiley-VCH Verlag, 2016