Molecular spin crossover phenomenon: recent achievements and prospects

Thermometry at the nanoscale

Non-invasive precise thermometers working at the nanoscale with high spatial resolution, where the conventional methods are ineffective, have emerged over the last couple of years as a very active field of research. This has been strongly stimulated by the numerous challenging requests arising from nanotechnology and biomedicine. This critical review offers a general overview of recent examples of luminescent and non-luminescent thermometers working at nanometric scale. Luminescent thermometers encompass organic dyes, QDs and Ln(3+)ions as thermal probes, as well as more complex thermometric systems formed by polymer and organic-inorganic hybrid matrices encapsulating these emitting centres. Non-luminescent thermometers comprise of scanning thermal microscopy, nanolithography thermometry, carbon nanotube thermometry and biomaterials thermometry. Emphasis has been put on ratiometric examples reporting spatial resolution lower than 1 micron, as, for instance, intracellular thermometers based on organic dyes, thermoresponsive polymers, mesoporous silica NPs, QDs, and Ln(3+)-based up-converting NPs and β-diketonate complexes. Finally, we discuss the challenges and opportunities in the development for highly sensitive ratiometric thermometers operating at the physiological temperature range with submicron spatial resolution.

Spin state dependence of electrical conductivity of spin crossover materials

We studied the spin state dependence of the electrical conductivity of the spin crossover compound [Fe(Htrz)(2)(trz)](BF(4)) (Htrz = 1H-1,2,4-triazole) by means of dc electrical measurements. The low spin state is characterized by higher conductance and lower thermal activation energy of the conductivity, when compared to the high spin state.

Status and perspectives in thin films and patterning of spin crossover compounds

Spin crossover compounds are a class of functional materials able to switch their spin state upon external stimuli. They were proposed as potential candidates for several technological applications that require highly controlled thin films and patterns. Here we present a critical overview of the most important approaches for thin film growth and patterning of spin-crossover compounds, giving special attention to Fe(II) based molecules, which are the most studied materials. We present both conventional approaches to thin film growth (Langmuir-Blodgett, constructive chemical approach, spin coating, drop casting and vacuum sublimation) and patterning (combined top-down/bottom-up method, soft and unconventional lithography). We critically discuss the application of thin film growth and fabrication techniques highlighting the most critical aspects and the perspectives opened by the recent progress.

Tuning the magneto-structural properties of non-porous coordination polymers by HCl chemisorption

Responsive materials for which physical or chemical properties can be tuned by applying an external stimulus are attracting considerable interest in view of their potential applications as chemical switches or molecular sensors. A potential source of such materials is metal-organic frameworks. These porous coordination polymers permit the physisorption of guest molecules that can provoke subtle changes in their porous structure, thus affecting their physical properties. Here we show that the chemisorption of gaseous HCl molecules by a non-porous one-dimensional coordination polymer instigates drastic modifications in the magnetic properties of the material. These changes result from profound structural changes, involving cleavage and formation of covalent bonds, but with no disruption of crystallinity.

Laser-induced artificial defects (LIADs): towards the control of the spatiotemporal dynamics in spin transition materials

Micrometer-sized defects, induced by laser ablation, radically change the spatiotemporal dynamics of a first-order structural phase transition, in this case of a spin crossover material. This type of "domain engineering" is thus based on artificial defects, such as that in the image, which can serve either as nucleation sites or as pinning sites. The subsequent growth of the nucleated domains can also be guided to some extent.

High-spin versus spin-crossover versus low-spin: geometry intervention in cooperativity in a 3D polymorphic iron(II)-tetrazole MOFs system

Reported here are three 3D metal-organic framework (MOF) polymorphs with the chemical formula [Fe(2)(H(0.67)bdt)(3)]·xH(2)O (H(2)bdt = 5,5'-(1,4-phenylene)bis(1H-tetrazole)), all of which are constructed from similar Fe(II)-tetrazole rod secondary building units (SBUs) via covalent links, but exhibit diverse spin states regulated by inter-chain cooperativity.

Lattice effects on the spin-crossover profile of a mononuclear manganese(III) cation

Six solvated salts of a mononuclear manganese(III) complex with a chelating hexadentate Schiff base ligand are reported. One member of the series, [MnL]PF(6)⋅0.5 CH(3)OH (1), shows a rare low-spin (LS) electronic configuration between 10-300 K. The remaining five salts, [MnL]NO(3)⋅C(2)H(5)OH(2), [MnL]BF(4)⋅C(2)H(5)OH(3), [MnL]CF(3)SO(3)⋅C(2)H(5)OH (4), [MnL]ClO(4)⋅C(2)H(5)OH (5) and [MnL]ClO(4)⋅0.5 CH(3)CN (6), all show gradual incomplete spin-crossover (SCO) behaviour. The structures of all were determined at 100 K, and also at 293 K in the case of 3-6. The LS salt [MnL]PF(6)⋅0.5 CH(3)OH is the only member of the series that does not exhibit strong hydrogen bonding. At 100 K two of the four SCO complexes (2 and 4) assemble into 1D hydrogen-bonded chains, which weaken or rupture on warming. The remaining SCO complexes 3, 5 and 6 do not form 1D hydrogen-bonded chains, but instead exhibit discrete hydrogen bonding between cation/counterion, cation/solvent or counterion/solvent and show no significant change on warming.

Iron(II) Mononuclear Materials Containing Functionalised Dipyridylamino-Substituted Triazine Ligands: Structure, Magnetism and Spin Crossover

The spin crossover effect in iron(II) materials containing the di-2-pyridylamine functional group has been investigated for the new nitrile-functionalised ligand DTAC (2,2′,2″,2″′-((6-(di(pyridin-2-yl)amino)-1,3,5-triazine-2,4 diyl)bis(azanetriyl))tetra acetonitrile). This ligand has successfully been incorporated into a family of materials of the general formula trans-[Fe(DTAC)2(anion)2], wherein we have systematically varied the trans-nitrogen donor anion from NCS, NCSe, N(CN)2 (dca; dicyanamide) to NCBH3 – thus forming the four mononuclear materials trans-[Fe(DTAC)2(NCS)2]·6MeCN (1), trans-[Fe(DTAC)2(NCSe)2]·6MeCN (2), trans-[Fe(DTAC)2(N(CN)2)2] (3) and trans-[Fe(DTAC)2 (NCBH3)2]·3MeCN (4)). We find that the materials with a weaker crystal field strength anion remain high spin over all temperatures (1 and 2) whereas the materials containing stronger crystal field strength anions undergo a thermally induced spin crossover (3 and 4). Structural analysis revealed that the packing interactions in the solid state and the degree of solvation also play a large role in the observed magnetic behaviour. Indeed, aged or rapidly precipitated samples of 2 show a spin transition above room temperature.

Coherent transport through spin-crossover single molecules

Coherent quantum transport calculations were performed for high- and low-spin states of a mononuclear Fe(II) complex showing spin-crossover behavior using density functional theory methods combined with the non-equilibrium Green functions procedure. The high-spin state has a larger conductivity than the low-spin state; furthermore, it behaves as a spin filter, giving a β-polarized current.