Complex Materials for Molecular Spintronics Applications: Cobalt Bis(dioxolene) Valence Tautomers, from Molecules to Polymers

Abstract

Using first principles calculations, we predict a complex multifunctional behavior in cobalt bis(dioxolene) valence tautomeric compounds. Molecular spin-state switching is shown to dramatically alter electronic properties and corresponding transport properties. This spin state dependence has been demonstrated for technologically relevant coordination polymers of valence tautomers as well as for novel conjugated polymers with valence tautomeric functionalization. As a result, these materials are proposed as promising candidates for spintronic devices that can couple magnetic bistability with novel electrical and spin conduction properties. Our findings pave the way to the fundamental understanding and future design of active multifunctional organic materials for spintronics applications. ■ INTRODUCTIONMolecular spintronics seeks to exploit the electronic structural diversity of molecules to harness the spin of the electron in novel and improved computing and information storage applications. 1−4 Significant material focus in this area has been placed on the class of conjugated organic semiconductors (e.g., rubrene and tris(hydroxyquinolato)-aluminum) that have found applications in organic optoelectronics devices. 5 These materials, while often diamagnetic, have been reported to show dramatic spin-dependent charge transport effects including giant magnetoresistance (GMR) 6,7 and tunneling magnetoresistance (TMR). 8−10 However, recent years have seen net advances in the use of novel molecular materials synthesized for specific spin properties. This includes design and applications of single molecular magnets, which can have large spin quantum numbers and can be integrated into thin films 11−13 suitable for device applications. 14,15 In addition, the conjugated ferrimagnetic coordination polymer V(TCNE)x 16 has very recently been employed as a thin film electrode in an all-organic TMR-based spin valve device. 17 A molecular design motif with significant promise for spintronics is the spin crossover phenomenon 18 in which a transition between high-spin and low-spin states of a coordination compound can be externally tuned by temperature, light, or applied pressure. This is very common in a number of Fe(II) compounds with nitrogen-containing ligands and has been suggested for many years as a route to molecular memory devices due to the prevalence of hysteretic effects in the spin transition. 19 Recently, both experiment 20 and theory 21,22 have suggested spin state dependence of electrical transport in Fe(II) spin crossover compounds making them prime candidates for molecular spintronics.A chemical elaboration of the spin crossover phenomenon can be established through the use of radical dioxolene ligands complexed to a cobalt coordination center in a class of compounds called valence tautomers (VTs). 23,24 In valence tautomers, a high-spin to low-spin transition on the Co is accompanied by an intramolecular electron transfer from the Co to a redox active ligand.This suggests a very dramatic coupling betwee...

Valence tautomerism and spin crossover in pyridinophane–cobalt–dioxolene complexes: an experimental and computational study

“…These compounds can be reversibly interconverted between forms with different chemical and physical properties upon application of a stimulus, such as heat, which makes them of interest for future applications in displays, sensors, photoresponsive devices or molecular electronics/spintronics. [1][2][3] The switching in SCO complexes is due to a spin state transition at a single metal center from low spin (LS) to high spin (HS), which is well-established for complexes of Fe(II), Fe(III), Co(II) and Mn(III). 1,4,5 Very important for SCO is the ligand donor set around the metal center, as the ligand field splitting governs the thermodynamics of the transition.…”

“…1 closest intermolecular distance 2 SHAPE index for octahedral geometry OC-6, calculated in SHAPE 2.1 51. 153 …”

Theoretical calculation and prediction for experimental design to obtain spin crossover complexes

“…Therefore, d 6 complexes show spin OFF and ON behavior which is more interesting for applications such as switching devices, sensors, spintronic, information storage, and molecular scale switches devices. [8][9][10][11] Among spin crossover reported Fe(II) species, [Fe(phen) 2 (NCS) 2 ] has been one of the most widely studied. [12][13][14][15][16] It has been the model compound for a broad series of comparably established compounds such as a [FeL 2 X 2 ] (L is N-N dinitrogen donor chelates and X is ionic ligands such as SCN -, SeCN -, N -3 , N(CN) 2 -, TNCQ -, [NCBH 3 ] 2 , and etc.…”

“…Nearly octahedral d 6 SCO complexes show diamagnetic behavior in LS configuration and paramagnetic, S = 2, behavior in HS configuration. Therefore, d 6 complexes show spin OFF and ON behavior which is more interesting for applications such as switching devices, sensors, spintronic, information storage, and molecular scale switches devices . Among spin crossover reported Fe(II) species, [Fe(phen) 2 (NCS) 2 ] has been one of the most widely studied .…”

Magnetism and variable temperature and pressure crystal structures of a linear oligonuclear cobalt bis-semiquinonate

“…Valence tautomerism (VT) is a phenomenon with various potential applications, most notably as miniature switches. [1][2][3][4][5] Cobalt coordination complexes that contain one or two molecules belonging to the non-innocent ligand group cis-dioxolenes (diox) are particularly well-studied in this context since they possess a combination of structural and electronic fea-tures that favour the VT mechanism to occur at moderately high temperatures. [6][7][8][9] The VT transition involves a metal-to-ligand charge transfer and an accompanying change in the spin-value of the metal.…”