Spin-crossover Molecules Research Articles

Spin transition in arrays of gold nanoparticles and spin crossover molecules.

We investigate if the functionality of spin crossover molecules is preserved when they are assembled into an interfacial device structure. Specifically, we prepare and investigate gold nanoparticle arrays, into which room-temperature spin crossover molecules are introduced, more precisely, [Fe(AcS-BPP)2](ClO4)2, where AcS-BPP = (S)-(4-{[2,6-(dipyrazol-1-yl)pyrid-4-yl]ethynyl}phenyl)ethanethioate (in short, Fe(S-BPP)2). We combine three complementary experiments to characterize the molecule-nanoparticle structure in detail. Temperature-dependent Raman measurements provide direct evidence for a (partial) spin transition in the Fe(S-BPP)2-based arrays. This transition is qualitatively confirmed by magnetization measurements. Finally, charge transport measurements on the Fe(S-BPP)2-gold nanoparticle devices reveal a minimum in device resistance versus temperature, R(T), curves around 260-290 K. This is in contrast to similar networks containing passive molecules only that show monotonically decreasing R(T) characteristics. Backed by density functional theory calculations on single molecular conductance values for both spin states, we propose to relate the resistance minimum in R(T) to a spin transition under the hypothesis that (1) the molecular resistance of the high spin state is larger than that of the low spin state and (2) transport in the array is governed by a percolation model.

Analysis of first order reversal curves in the thermal hysteresis of spin-crossover nanoparticles within the mechanoelastic model

The recently obtained spin-crossover nanoparticles are possible candidates for applications in the recording media industry as materials for data storage, or as pressure and temperature sensors. For these applications, the intermolecular interactions and interactions between spin-crossover nanoparticles are extremely important, as they may be essential factors in triggering the transition between the two stable phases: the high-spin and low-spin ones. In order to find correlations between the distributions in size and interactions and the transition temperatures distribution, we apply the FORC (First Order Reversal Curves) method, using simulations based on a mechanoelastic model applied to 2D triangular lattices composed of molecules linked by springs and embedded in a surfactant. We consider two Gaussian distributions: one is the size of the nanoparticles and another is the elastic interactions between edge spin-crossover molecules and the surfactant molecules. In order to disentangle the kinetic and non-kinetic parts of the FORC distributions, we compare the results obtained for different temperature sweeping rates. We also show that the presence of few larger particles in a distribution centered around much smaller particles dramatically increases the hysteresis width.

Ultrafast light-induced spin-state trapping photophysics investigated in Fe(phen)2(NCS)2 spin-crossover crystal.

Few photoactive molecules undergo a complete transformation of physical properties (magnetism, optical absorption, etc.) when irradiated with light. Such phenomena can happen on the time scale of fundamental atomic motions leading to an entirely new state within less than 1 ps following light absorption. Spin crossover (SCO) molecules are prototype systems having the ability to switch between low spin (LS) and high spin (HS) molecular states both at thermal equilibrium and after light irradiation. In the case of Fe(II) (3d(6)) complexes in a nearly octahedral ligand field, the two possible electronic distributions among the 3d split orbitals are S = 0 for the LS diamagnetic state and S = 2 for the HS paramagnetic state. In crystals, such photoexcited states can be long-lived at low temperature, as is the case for the photoinduced HS state of the [Fe(phen)2(NCS)2] SCO compound investigated here. We first show how such bistability between the diamagnetic and paramagnetic states can be characterized at thermal equilibrium or after light irradiation at low temperature. Complementary techniques provide invaluable insights into relationships between changes of electronic states and structural reorganization. But the development of such light-active materials requires the understanding of the basic mechanism following light excitation of molecules, responsible for trapping them into new electronic and structural states. We therefore discuss how we can observe a photomagnetic molecule during switching and catch on the fly electronic and structural molecular changes with ultrafast X-ray and optical absorption spectroscopies. In addition, there is a long debate regarding the mechanism behind the efficiency of such a light-induced process. Recent theoretical works suggest that such speed and efficiency are possible thanks to the instantaneous coupling with the phonons of the final state. We discuss here the first experimental proof of that statement as we observe the instantaneous activation of one key phonon mode precluding any recurrence towards the initial state. Our studies show that the structural molecular reorganization trapping the photoinduced electronic state occurs in two sequential steps: the molecule elongates first (within 170 femtosecond) and bends afterwards. This dynamics is caught via the coherent vibrational energy transfer of the two main structural modes. We discuss the transformation pathway connecting the initial photoexcited state to the final state, which involves several key reaction coordinates. These results show the need to replace the classical single coordinate picture employed so far with a more complex multidimensional energy surface.

ChemInform Abstract: Computational Design of Magnetic Metal‐organic Complexes and Coordination Polymers with Spin‐switchable Functionalities

AbstractReview: 38 refs.

Computational design of magnetic metal-organic complexes and coordination polymers with spin-switchable functionalities

Abstract

Solvate-dependent spin crossover and exchange in cobalt(II) oxazolidine nitroxide chelates.

Two oxazolidine nitroxide complexes of cobalt(II), [Co(II)(L(•))2](B(C6F5)4)2·CH2Cl2 (1) and [Co(II)(L(•))2](B(C6F5)4)2·2Et2O (2), where, L(•) is the tridentate chelator 4,4-dimethyl-2,2-bis(2-pyridyl)oxazolidine N-oxide, have been investigated by crystallographic, magnetic, reflectivity, and theoretical (DFT) methods. This work follows on from a related study on [Co(II)(L(•))2](NO3)2 (3), a multifunctional complex that simultaneously displays magnetic exchange, spin crossover, and single molecule magnetic features. Changing the anion and the nature of solvation in the present crystalline species leads to significant differences, not only between 1 and 2 but also in comparison to 3. Structural data at 123 and 273 K, in combination with magnetic data, show that at lower temperatures 1 displays low-spin Co(II)-to-radical exchange with differences in fitted J values in comparison to DFT (broken symmetry) calculated J values ascribed to the sensitive influence of a tilt angle (θ) formed between the Co(dz(2)) and the trans-oriented O atoms of the NO radical moieties in L(•). Spin crossover in 1 is evident at higher temperatures, probably influenced by the solvate molecules and crystal packing arrangement. Complex 2 remains in the high-spin Co(II) state between 2 and 350 K and undergoes antiferromagnetic exchange between Co-radical and radical-radical centers, but it is difficult to quantify. Calculations of the magnetic orbitals, eigenvalue plots, and the spin densities at the Co and radical sites in 1 and 2 have yielded satisfying details on the mechanism of metal-radical and radical-radical exchange, the radical spins being in π*NO orbitals.

Pressure and temperature induced high spin–low spin phase transition: Macroscopic and microscopic consideration

The behavior under pressure of the high spin–low spin phase transition in the coordination compounds containing 3d ions is analyzed using thermodynamic and microscopic approaches. For thermodynamic approach the mean field model with interactions between spin-crossover molecules is considered. Microscopic model takes into account the interaction of d electrons of the transition metal ions with full symmetric distortions of the ligands. The relationship of the thermodynamic interaction parameters with microscopic ones is installed and shown how the quantum–mechanical interactions form the cooperativity of the system. Within the microscopic model the temperature and pressure dependences of the high spin fraction in 2-D compounds {Fe(3-Fpy)2[M(CN)4]} (M=Pd, Pt) are simulated and microscopic parameters are evaluated. It is concluded that different experimental behaviors of the temperature and pressure induced spin transitions are determined by different variations of the inelastic and elastic energies under pressure, and vibrational component of the free energy drives the ST equally with electronic part.

磁性分子自旋态检测与调控的STM研究

Detection and manipulation of single molecular spin states are one of the hottest fields in physics, chemistry and information technology. This review mainly focuses on progress recently achieved in this field investigated by STM. Study of magnetic molecules on single-crystal metal surfaces and insulator thin films is summarized. Especially, reversible switching of spin-crossover molecules, magnetic phthalocyanine molecules, surface preparation of single molecule magnets and their transport properties are addressed.

First glimpse of the soft x-ray induced excited spin-state trapping effect dynamics on spin cross-over molecules

The dynamics of the soft x-ray induced excited spin state trapping (SOXIESST) effect of Fe(phen)2(NCS)2 (Fe-phen) powder have been investigated by x-ray absorption spectroscopy (XAS) using the total electron yield method, in a wide temperature range. The low-spin (LS) state is excited into the metastable high-spin (HS) state at a rate that depends on the intensity of the x-ray illumination it receives, and both the temperature and the intensity of the x-ray illumination will affect the maximum HS proportion that is reached. We find that the SOXIESST HS spin state transforms back to the LS state at a rate that is similar to that found for the light induced excited spin state trapping (LIESST) effect. We show that it is possible to use the SOXIESST effect in combination with the LIESST effect to investigate the influence of cooperative behavior on the dynamics of both effects. To investigate the impact of molecular cooperativity, we compare our results on Fe-phen with those obtained for Fe{[Me2Pyrz]3BH}2 (Fe-pyrz) powder, which exhibits a similar thermal transition temperature but with a hysteresis. We find that, while the time constant of the dynamic is identical for both molecules, the SOXIESST effect is less efficient at exciting the HS state in Fe-pyrz than in Fe-phen.

Ground state of a spin-crossover molecule calculated by diffusion Monte Carlo

Spin crossover molecules have recently emerged as a family of compounds potentially useful for implementing molecular spintronics devices. The calculations of the electronic properties of such molecules is a formidable theoretical challenge as one has to describe the spin ground state of a transition metal as the legand field changes. The problem is dominated by the interplay between strong electron correlation at the transition metal site and charge delocalization over the ligands, and thus it fits into a class of problems where density functional theory may be inadequate. Furthermore, the crossover activity is extremely sensitive to environmental conditions, which are difficult to fully characterize. Here we discuss the phase transition of a prototypical spin crossover molecule as obtained with diffusion Monte Carlo simulations. We demonstrate that the ground state changes depending on whether the molecule is in the gas or in the solid phase. As our calculation provides a solid benchmark for the theory we then assess the performances of density functional theory. We find that the low spin state is always over-stabilized, not only by the (semi-)local functionals, but even by the most commonly used hybrids (such as B3LYP and PBE0). We then propose that reliable results can be obtained by using hybrid functionals containing about 50% of exact-exchange.

Influence of Solvents on Spin Transition of \([\text{Fe(ABPT)}_2{(\text{C(CN)}_3)}_2]\) Spin-crossover Molecule

We present a density-functional study of the inuence of solvents on the geometricstructure and the electronic structure of the low-spin and high-spin states of [Fe(ABPT)2 (C(CN)3)2] molecule with abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, in order to explore more aboutthe way to control spin-crossover behavior of transition metal molecules. Our calculated resultsdemonstrated that the geometric structure of molecule under consideration is only slightly changedby solvents. However, typical quantities of the electronic structure and spin-transition of thismolecule such as the atomic charge, the magnetic moment of Fe ion, the HOMO-LUMO gap, andthe spin-state energy dierence are varied as a function of dielectric constant of solvents. Theyreach saturation values with increasing dielectric constant. These results should give some hintsto control spin transition temperature of spin-crossover molecules.

Geometric Structure, Electronic Structure, and Spin Transition of Fe<SUP>2+</SUP> Spin-crossover Molecules

We present a density functional study on the geometric structure, electronic structureand spin transition of a series of Fe2+ spin-crossover molecules, i.e., [Fe(abpt)2(NCS)2] (1),[Fe(abpt)2(NCSe)2] (2), and [Fe(abpt)2(C(CN)3)2] (3) with abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole in order to shed light on more about the dynamics of the spin-crossover phenomenon.All results presented in this study were obtained by using the exchange correlation PBE functional.For better accuracy, the hexadecapolar expansion scheme is adopted for resolving the charge densityand Coulombic potential. Our calculated results demonstrate that the transition between the low-spin (LS) and high-spin (HS) states of these Fe2+ molecules is accompanied with redistributionof atomic charge and reformation of molecular orbitals. These cause dierences in the kineticenergy, the electrostatic energy as well as the total energy between the LS and HS states. TheLS state is advantage in the kinetic energy in comparison to the HS state, while the HS state isadvantage in the electrostatic energy. Moreover, the coulomb attraction energy between the Fe2+ion and its surrounding anionic ions plays a crucial role for spin crossover occurring.

Giant Resistance Change across the Phase Transition in Spin-Crossover Molecules

The electronic origin of a large resistance change in nanoscale junctions incorporating spin-crossover molecules is demonstrated theoretically by using a combination of density functional theory and the nonequilibrium Green's function method for quantum transport. At the spin-crossover phase transition, there is a drastic change in the electronic gap between the frontier molecular orbitals. As a consequence, when the molecule is incorporated in a two-terminal device, the current increases by up to 4 orders of magnitude in response to the spin change. This is equivalent to a magnetoresistance effect in excess of 3000%. Since the typical phase transition critical temperature for spin-crossover compounds can be extended to well above room temperature, spin-crossover molecules appear as the ideal candidate for implementing spin devices at the molecular level.

Geometric structure, electronic structure, and spin transition of several FeII spin-crossover molecules

We present a density functional study on the geometric structure, electronic structure, and spin transition of a series of FeII spin-crossover (SCO) molecules, i.e., [Fe(abpt)2(NCS)2] (1), [Fe(abpt)2(NCSe)2] (2), and [Fe(dpbo)(HIm)2] (3) with dpbo = {diethyl(E,E)-2,2′-[1,2-phenylbis(iminomethylidyne)]bis[3-oxobutanoate](2–)-N,N’,O3,O3’}, and abpt = {4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole} in order to explore more about the way to control SCO behavior of transition metal complexes. Our calculated results show that the spin transition of these FeII molecules is accompanied with charge transfer between the Fe atom and ligands. This causes change in the electrostatic energy (ΔU) as well as the total electronic energy of SCO molecules. Moreover, our calculated results demonstrate an important contribution of the interionic interactions to ΔU, and there is the relation between ΔU and the thermal hysteresis behavior of SCO molecules. These results should be helpful for developing new SCO molecules.

Robust spin crossover and memristance across a single molecule

A nanoscale molecular switch can be used to store information in a single molecule. Although the switching process can be detected electrically in the form of a change in the molecule's conductance, adding spin functionality to molecular switches is a key concept for realizing molecular spintronic devices. Here we show that iron-based spin-crossover molecules can be individually and reproducibly switched between a combined high-spin, high-conduction state and a low-spin, low-conduction state, provided the individual molecule is decoupled from a metallic substrate by a thin insulating layer. These results represent a step to achieving combined spin and conduction switching functionality on the level of individual molecules.

Room‐Temperature Electrical Addressing of a Bistable Spin‐Crossover Molecular System

A switchable molecular device is constructed by contacting an individual nanoparticle based on spin-crossover molecules between nanometer-spaced electrodes. The switching and memory effects near room temperature are a consequence of the intrinsic bistability of the nanoparticle. Interestingly, for molecular spintronics, the spin crossover can also be induced by applying a voltage, showing that its magnetic state is electrically controllable.

Molecular Dynamics and Transfer Integral Investigations of an Elastic Anharmonic Model for Phonon-Induced Spin Crossover

Based on a vibronic description of the spin-crossover (SC) molecule, the electronic and elastic properties of a chain of anharmonically coupled SC molecules are studied by means of transfer integral (TI) and molecular dynamics (MD) simulations. The thermodynamical properties (high spin fraction and lattice deformation) are derived and show the existence of a first-order transition driven by the phonon field. The MD investigations of the velocity autocorrelation function evidenced that the density of phonon states shows a redshift and a broadening at high temperature, attributed to the enhancement of the phonon entropy. This result is discussed in relation to the thermal dependence of the dispersion relation omega(q) and that of the effective intermolecular elastic constant.

Dinuclear Iron(II) Spin Crossover Compounds: Singular Molecular Materials for Electronics

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Dinuclear iron(ii) spin crossover compounds: singular molecular materials for electronics

Dinuclear spin crossover molecules can adopt three different spin-pair states: a fully diamagnetic low spin state, [LS–LS], with both iron(II) atoms in the LS state; a paramagnetic mixed spin-pair state [LS–HS]; and an antiferromagnetically coupled [HS–HS] state. Stabilisation of the [LS–HS] state depends on a subtle balance between intra- and inter-molecular interactions in the solid state, consequently, the thermal dependence of the physical and structural properties can present one-step or two-step spin transitions. The former case involves the [LS–LS] ↔ [HS–HS] transformation while in the latter case the intermediate stage responsible for the plateau, at 50% conversion between the two steps, is observed. It may be due to the formation of a 50% mixture of [HS–HS] and [LS–LS] or to the existence of 100% [LS–HS] species. In some cases switching between the three spin-pair states has been observed upon the action of temperature, pressure or light, which implies competition between magnetic coupling and spin crossover phenomena. The results here reviewed represent a first step in the search for polynuclear spin crossover molecules of nanometric dimensions, which could be used as multi-stepped molecular switches.

Ground-state phases in spin-crossover chains

Spin-crossover molecules having a low-spin ground state and a low-lying excited high-spin state are promising components for molecular electronics. We theoretically examine one-dimensional spin-crossover chain molecules of the type of Fe-II triazole complexes. The existence of the additional low-spin/high-spin degree of freedom leads to rich behavior already in the ground state. We obtain the complete ground-state phase diagram, taking into account an elastic nearest-neighbor interaction, a ferromagnetic or antiferromagnetic exchange interaction between the magnetic ions, and an external magnetic field. Ground-state energies are calculated with high numerical precision using the density-matrix renormalization group. Besides pure low-spin, highspin, and alternating low-spin/high-spin phases we obtain a number of periodic ground states with longer periods, which we discuss in detail. For example, for antiferromagnetic coupling there exists a dimer phase with a magnetic unit cell containing two high-spin ions forming a spin singlet and a single low-spin ion, which is stabilized by the energy gain for singlet formation.