Polynuclear spincrossover (SCO) complexes prepared by the combination of [Fe(DMF)6](2+) and NH2trz (NH2trz = 4-amino-1,2,4-triazole) were studied (2ns(-) = counterion 2-naphthalenesulfonate). It is demonstrated that these [Fe(NH2trz)3](2ns)2 complexes can be dissolved-contrary to common reported experience-in N,N-dimethylformamide (DMF) and, therefore, can be conveniently processed by simple means. The resulting solutions were examined with UV/vis and X-ray absorption spectroscopy (XANES and EXAFS) as well as with small-angle X-ray scattering (SAXS). At a molar NH2trz/Fe(2+) ratio of 3/1, corresponding to the stoichiometric ratio of the ideal coordination compound, [Fe(NH2trz)3](2+) in the low-spin state was found to be in equilibrium with polynuclear species in the high-spin state. The equilibrium can be shifted virtually completely to the side of low-spin Fe(2+) by an excess of the ligand. The polymer therewith formed contains 100 or more Fe(2+) ions and is of a pronounced rigid-rod structure, with Fe-Fe distances around 3.32 Å (in comparison to 3.94 Å of the polynuclear species in the high-spin state). Reversible spin crossover takes place in solution upon a temperature increase to around 60 °C; this process is associated with a shift in equilibrium toward species shorter than the initial polynuclear species.
The prominent polynuclear spin-crossover complex [Fe(NH 2 trz) 3 ](2ns) 2 (polymer) (NH 2 trz ¼ 4-amino-1,2,4-triazole, 2ns À ¼ 2-naphthalenesulfonate) as obtained from synthesis exists in a metastable state characterized by a columnar hexagonal packing (Col h) of polynuclear rigid-rod complexes, and shows considerable fluctuations in the spin-crossover temperature (between 22 and 41 C upon heating and between 14 and 25 C upon cooling). This structure is converted into a more stable columnar rectangular packing (Col r) by annealing this material at 250 C, which is accompanied by a shift of the spin-crossover towards higher temperatures ($52 C upon heating and $33 C upon cooling). Furthermore, solutions of [Fe(NH 2 trz) 3 ](2ns) 2 in dimethylformamide (DMF) can be converted into gels by increasing the concentration of [Fe(NH 2 trz) 3 ](2ns) 2 or by addition of toluene, while preserving the spincrossover behavior. Gels prepared by the latter method were converted into xerogels by CO 2 criticalpoint drying, or into transparent thermochromic films by slow evaporation of the solvent. For the xerogels an initial Col h packing is transformed into the more stable Col r packing by the same annealing procedure applied to the powders. On the other hand, the slow evaporation of solvent used for the manufacture of films allowed establishment of a Col r packing by annealing at lower temperatures (150 C). Polynuclear complexes of the type [Fe(Rtrz) 3 ] n X 2n (Rtrz ¼ 4-R-1,2,4-triazole) have attracted particular attention, not only due to their exceptional structure but also because their spincrossover temperature can be varied by applying differently substituted ligands and counter anions X À. 13-15 Thus, also materials with spin-crossover near room temperature can be fabricated. 16-18 Unfortunately, processing of those complexes is oen hampered due to lacking solubility or thermal stability (usually, decomposition takes place prior to melting). 19 One possibility to overcome the poor solubility is to attach long side chains at the ligands or counter anions. 20-22 However, introducing large Scheme 1 Schematic representation of the structure of the spincrossover complex [Fe(NH 2 trz) 3 ](2ns) 2 (NH 2 trz: 4-amino-1,2,4-triazole, 2ns À : 2-naphthalenesulfonate).
Optical and electrical studies reveal spin switching bistability from polymer thin films.
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