More Like this

1. Dynamics of Spin Crossover Molecular Complexes

   https://doi.org/10.3390/nano12101742  

   Ekanayaka, Thilini K.; Maity, Krishna Prasad; Doudin, Bernard; Dowben, Peter A. (May 2022, Nanomaterials)

   We review the current understanding of the time scale and mechanisms associated with the change in spin state in transition metal-based spin crossover (SCO) molecular complexes. Most time resolved experiments, performed by optical techniques, rely on the intrinsic light-induced switching properties of this class of materials. The optically driven spin state transition can be mediated by a rich interplay of complexities including intermediate states in the spin state transition process, as well as intermolecular interactions, temperature, and strain. We emphasize here that the size reduction down to the nanoscale is essential for designing SCO systems that switch quickly as well as possibly retaining the memory of the light-driven state. We argue that SCO nano-sized systems are the key to device applications where the “write” speed is an important criterion. 

   more » « less
2. High-temperature insulating ferromagnetic state in charge-disproportionated and spin-state-disproportionated strained SrCoO2.5 thin film

   https://doi.org/10.1063/5.0188767  

   Chowdhury, Sourav; Jana, Anupam; Rawat, Ritu; Yadav, Priyanka; Islam, Rajibul; Xue, Fei; Mandal, A K; Sarkar, Sumit; Mishra, Rajan; Venkatesh, R; et al (May 2024, APL Materials)

   Ferromagnetic insulators (FMIs) have widespread applications in microwave devices, magnetic tunneling junctions, and dissipationless electronic and quantum-spintronic devices. However, the sparsity of the available high-temperature FMIs has led to the quest for a robust and controllable insulating ferromagnetic state. Here, we present compelling evidence of modulation of the magnetic ground state in a SrCoO2.5 (SCO) thin film via strain engineering. The SCO system is an antiferromagnetic insulator with a Neel temperature, TN, of ∼550 K. Applying in-plane compressive strain, the SCO thin film reveals an insulating ferromagnetic state with an extraordinarily high Curie temperature, TC, of ∼750 K. The emerged ferromagnetic state is associated with charge-disproportionation (CD) and spin-state-disproportionation (SSD), involving high-spin Co2+ and low-spin Co4+ ions. The density functional theory calculation also produces an insulating ferromagnetic state in the strained SCO system, consistent with the CD and SSD, which is associated with the structural ordering in the system. Transpiring the insulating ferromagnetic state through modulating the electronic correlation parameters via strain engineering in the SCO thin film will have a significant impact in large areas of modern electronic and spintronic applications. 

   more » « less
3. Modular Design of Multifunctionality in TCNQ-Based Fe(II) Spin Crossover Complexes

   Üngör, Ö.; Choi, E. S.; Shatruk, M. (April 2021, National Meeting of the Anerican Chemical Society)

   null (Ed.)

   Fe(II) coordination complexes with ligands of an intermediate field strength often show witching between the high-spin (HS) and low-spin (LS) electronic configurations, known as spin crossover (SCO). This spin-state conversion is achieved by changes in temperature, pressure, or photoexcitation, which make SCO complexes promising materials for various applications that rely on bistable systems. Multifunctional materials that exhibit both spin-state switching and conductivity can be created by combining Fe(II) SCO complexes with organic TCNQ-type electron acceptors. In such complexes, TCNQ●d– radical anions are typically arranged in layers of one-dimensional stacks that provide conducting pathways (Fig. 1). The stacking distance can be affected by structural changes induced by the alteration in the electronic configuration and, thus, bond lengths at the Fe(II) center, resulting in synergy between SCO and conductivity. The synthesis of such materials can be approached in two ways: (1) by coordinating TCNQ●d– ligands directly to the Fe(II) center, which is partially protected by blocking ligands that limit the growth of extended structures or (2) by co-crystallizing completely blocked Fe(II) centers with free TCNQ●d– radicals. We will discuss several examples of the second approach, in which homoleptic Fe(II) cationic SCO complexes with tridentate 2,6-bispyrazolyl-pyridine (bpp) type ligands have been co-crystallized with fractionally-charged TCNQ●d– radical anions. The temperature- and solvent-dependent magnetic behavior and transport properties of these materials will be discussed. We will also present new pathways to improve the design of such molecule-based conductors with spin-state switching properties. To the best of out knowledge, we report the first examples of Fe(II) based conducting molecular materials with abrupt temperature-driven spin transitions. 

   more » « less
4. Spin‐Crossover Properties of an Iron(II) Coordination Nanohoop

   https://doi.org/10.1002/anie.202013374  

   Heras Ojea, María José; Van Raden, Jeff M.; Louie, Shayan; Collins, Richard; Pividori, Daniel; Cirera, Jordi; Meyer, Karsten; Jasti, Ramesh; Layfield, Richard A. (December 2020, Angewandte Chemie International Edition)

   Abstract Addition of the bipyridyl‐embedded cycloparaphenylene nanohoop bipy[9]CPP to [Fe{H₂B(pyz)₂}] (pyz=pyrazolyl) produces the distorted octahedral complex [Fe(bipy[9]CPP){H₂B(pyz)₂}₂] (1). The molecular structure of1shows that the nanohoop ligand contains a non‐planar bipy unit. Magnetic susceptibility measurements indicate spin‐crossover (SCO) behaviour with aT_1/2of 130 K, lower than that of 160 K observed with the related compound [Fe(bipy){H₂B(pyz)₂}₂] (2), which contains a conventional bipy ligand. A computational study of1and2reveals that the curvature of the nanohoop leads to the different SCO properties, suggesting that the SCO behaviour of iron(II) can be tuned by varying the size and diameter of the nanohoop. 

   more » « less
5. Pyridyl-Thioethers as Capping Ligands for the Design of Heteroleptic Fe(II) Complexes with Spin-Crossover Behavior

   https://doi.org/10.3390/magnetochemistry7100134  

   Üngör, Ökten; Igimbayeva, Dilyara; Dragulescu-Andrasi, Alina; Yergeshbayeva, Sandugash; Delgado, Teresa; Greer, Samuel M.; Donalson, Gabrielle; Jo, Minyoung; Erkasov, Rakhmetulla; Shatruk, Michael (October 2021, Magnetochemistry)

   Mononuclear heteroleptic complexes [Fe(tpma)(bimz)](ClO4)2 (1a), [Fe(tpma)(bimz)](BF4)2 (1b), [Fe(bpte)(bimz)](ClO4)2 (2a), and [Fe(bpte)(bimz)](BF4)2 (2b) (tpma = tris(2-pyridylmethyl)amine, bpte = S,S′-bis(2-pyridylmethyl)-1,2-thioethane, bimz = 2,2′-biimidazoline) were prepared by reacting the corresponding Fe(II) salts with stoichiometric amounts of the ligands. All complexes exhibit temperature-induced spin crossover (SCO), but the SCO temperature is substantially lower for complexes 1a and 1b as compared to 2a and 2b, indicating the stronger ligand field afforded by the N2S2-coordinating bpte ligand relative to the N4-coordinating tpma. Our findings suggest that ligands with mixed N/S coordination can be employed to discover new SCO complexes and to tune the transition temperature of known SCO compounds by substituting for purely N-coordinating ligands. 

   more » « less