Enhanced Cooperativity in Supported Spin-Crossover Metal–Organic Frameworks

Abstract

A c c e p t e d m a n u s c r i p t Molecular electronics originally proposed in the 1950s and reinvigorated by the founding proposition of Aviram and Ratner 1 has led to intense activity on the study of electronic transport through single molecules, supported by the advent of scanning tunneling microscopy (STM) and break junction setups. 2,3 More recently, molecules entered the field of spintronics, i.e. the control and manipulation of spins, 4 and gave birth to molecular spintronics, [5][6][7][8] where the working principle of the targeted devices relies on bistability. Molecular bistability has been best served by molecular magnetism with two distinct families: spin-crossover (SCO) compounds and single molecule magnets (SMM). 9,10 Whereas SMM display bistability at rather low temperatures, 9,10 higher temperature regimes are reached for SCO materials. 11 In order to bridge the gap between functional molecules and practical devices, one has to deposit the objects on a surface while retaining the bistability property. With respect to the intense activity in the field of adsorbed SMM, 8,[12][13][14][15] SCO supported compounds have received much less attention. It is only recently that supported examples have emerged, starting from 3-dimensional (3D) materials with thin-films grown on gold. 11,16,17 0D systems followed based on single SCO molecules, [18][19][20][21][22][23][24] or molecular junctions. 25 However, 2D monolayer organized networks exhibiting SCO behavior have not been reported so far. Not only should low-dimensional SCO patterns offer new insights into the spin transition phenomenon, but such organized networks may also offer the opportunity to control the intermolecular interactions dictating the collective behavior of transiting centers (a.k.a. cooperativity). 26 With this goal in mind, a promising route might come from the use of self-assembled metalorganic framework (MOF). 27 Indeed, magnetic couplings 28 were reported in several 2D MOF based on 3d ions such as Fe, 29,30 Ni, 31 Mn 32,33 or Cu. 34 Recently, Umbach et al. have investigated a Au(111)-supported MOF based on Fe(II) ions and 2,4,6-tris(4-pyridyl)-1,3,5-triazine (T4PT) ligand. 30 Interestingly, in addition to the observation of a weakly ferromagnetic behaviour, the speculated structure of the system consists in FeN 6 units, an archetype arrangement of SCO systems. 35 In this letter, we use a model based on Fe(II) transiting unit to investigate the effect of dimensionality reduction and surface deposition. Our thermodynamic model shows that a metallic A c c e p t e d m a n u s c r i p t substrate is likely to enhance cooperativity by an order of magnitude. In the light of these findings, we then perform calculations based on density functional theory (DFT) to establish the structure of the synthetic Fe-T4PT MOF supported on a Au(111) surface. 30 We show that the most stable structure consists of FeN 3 Au 3 units rather than on the expected FeN 6 building blocks. Beyond the interpretation of experimental results, a chemical mo...

Spin‐Crossover Molecules on Surfaces: From Isolated Molecules to Ultrathin Films

“…Recent theoretical work based on the experimental investigation of an Fe II ‐based organometallic compound on gold has shown that SCO can indeed be engineered on gold by using suitable ligands with the electric polarization of the surface driving the spin switching process. [ 73 ] It is based on the premise that electrostatic interactions play an important role in governing SCO phenomena and may give rise to cooperative effects—a role mainly attributed to elastic forces in bulk materials. The success of SCM 4 in retaining SCO and LIESST even on metallic surfaces like Ag and Cu underlines the importance of engineering new, chemically robust compounds.…”

Evolution of cooperativity in the spin transition of an iron(II) complex on a graphite surface

“…A model based on interacting HS and LS domains (as opposed to HS and LS states) still yields a Gibb’s free energy similar to the S–D model 59 . Wavefunction ab initio calculations also reproduced the S–D model 57 , which has been used recently to explore the possibility of an enhanced cooperativity in surface-supported 2D metal-organic frameworks 60 . At equilibrium, the S−D model leads to the implicit equation: where Δ H and Δ S are the differences in enthalpy and entropy, respectively, between the HS and LS states; R is the universal gas constant.…”

Large-scale comparison of 3d and 4d transition metal complexes illuminates the reduced effect of exchange on second-row spin-state energetics

“…Approximate density functional theory (DFT) is widely used in studying the catalytic [1][2][3][4][5][6] and materials [7][8][9][10][11][12][13] properties of openshell transition-metal complexes. 14 The well-localized d or f electrons of open-shell transition-metal centers impart unique properties but also can lead to a significant number of low-energy spin and oxidation states that are challenging to describe on equal footing using approximate electronic structure methods.…”