Spin‐Crossover Nanocrystals with Magnetic, Optical, and Structural Bistability Near Room Temperature

Abstract

Nanocrystals of the three‐dimensional, spin‐crossover, porous coordination framework [Fe(pz)Pt(CN)4]⋅n H2O (pz=pyrazine; n≤2.5) have been synthesized from water‐in‐oil microemulsions. The surfactant‐free nanocrystals readily desorb water and the resulting anhydrous compounds exhibit thermally induced electronic bistability accompanied by a pronounced color change (see picture; HS=high spin, LS=low spin) close to room temperature.

Spin state switching in iron coordination compounds

“…In fact, determination of the critical particle size or film thickness that preserves a complete ST with hysteretic behavior is a vital factor in the nanominiaturization of a SCO material. Recent studies have involved the preparation of nanocrystals [359–360] and thin films [361–362] of the 3D coordination polymer [Fe(pz)Pt(CN) 4 ] (pz = pyrazine) [363] exhibiting size-dependent ST characteristics. Bulk microcrystalline samples undergo very cooperative ST with T 1/2 ↓ = 285 K and T 1/2 ↑ = 309 K. The corresponding nanocrystals of average dimensions 200 × 200 × 60 nm and 60 × 60 × 30 nm display practically complete spin transitions, demonstrating a decrease of the critical temperatures and hysteresis widths with decreasing size of the crystallites (Fig.…”

“…Bulk microcrystalline samples undergo very cooperative ST with T 1/2 ↓ = 285 K and T 1/2 ↑ = 309 K. The corresponding nanocrystals of average dimensions 200 × 200 × 60 nm and 60 × 60 × 30 nm display practically complete spin transitions, demonstrating a decrease of the critical temperatures and hysteresis widths with decreasing size of the crystallites (Fig. 31) [359]. This effect is even more pronounced for 10–20 nm sized nanoparticles, where only around 1/3 of the Fe(II) ions undergo ST characterized by a very narrow thermal hysteresis loop [360].…”

Solution and solid state properties of Fe(III) complexes bearing N-ethyl-N-(2-aminoethyl)salicylaldiminate ligands

“…This included the discovery of several new examples of SCO compounds [3,[9][10][11], the explanation of different types of SCO profiles [4,12] and the modification of SCO compounds to increase cooperativity and to direct their application into materials science [13]. Scientists have developed SCO networks [14][15][16], frameworks [17][18][19], gels [20][21][22], liquid crystals [23][24][25], nanoparticles and nanocrystals [26][27][28][29], nanowires [30], thin films [31][32][33] and have also applied patterning techniques to fabricate SCO devices [34]. Among complexes displaying SCO, [Fe(salEen) 2 ] + derivatives (salEen = N-ethyl-N-(2-aminoethyl)salicylaldiminate) are known to undergo thermal SCO sensitive to unit cell contents and supramolecular packing in the crystal [35].…”

Two New Iron(II) Spin‐Crossover Complexes with N4O2 Coordination Sphere and Spin Transition around Room Temperature

“…[1,2] Recent research activities in this field explore the possibility to combine this SCO bistability with additional properties (e.g., liquid crystalline properties, [3] magnetic exchange interactions [4] ), resulting in multifunctional SCO materials; [5] other research is based on the rational design of nanostructured SCO materials and their chemical and physical properties. [6] Of the possible types of spin transitions (gradual, abrupt, with hysteresis, step wise, incomplete), much of the interest is focused on the bistability in highly cooperative systems (hysteresis or memory effect), and as such, compounds can exist in two different electronic states depending on the history of the system. With regard to this, we recently characterised an iron(II) spin-crossover complex with a 70 K wide thermal hysteresis loop around room temperature on the basis of a 2D network of hydrogen bonds between the complex molecules.…”