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The prominent polynuclear spin-crossover complex [Fe(NH2trz)3](2ns)2 (polymer) (NH2trz = 4-amino-1,2,4-triazole, 2ns− = 2-naphthalenesulfonate) as obtained from synthesis exists in a metastable state characterized by a columnar hexagonal packing (Colh) 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 (Colr) 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(NH2trz)3](2ns)2 in dimethylformamide (DMF) can be converted into gels by increasing the concentration of [Fe(NH2trz)3](2ns)2 or by addition of toluene, while preserving the spin-crossover behavior. Gels prepared by the latter method were converted into xerogels by CO2 critical-point drying, or into transparent thermochromic films by slow evaporation of the solvent. For the xerogels an initial Colh packing is transformed into the more stable Colr 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 Colr packing by annealing at lower temperatures (150 °C).

Gels, xerogels and films of polynuclear iron(ii)–aminotriazole spin-crossover polymeric complexes