We studied the spin state dependence of the electrical conductivity of the spin crossover compound [Fe(Htrz)₂(trz)](BF₄) (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.

A series of framework coordination polymers reveals the use of 1,2,4,5-tetrazines as efficient bridging ligands towards silver(I) and copper(I) ions. All four nitrogen atoms were functional as lone pair donors leading to an unprecedented μ₄-coordination of the ligands (1,2,4,5-tetrazine, ttz; 3,6-dimethyl-1,2,4,5-tetrazine, Me₂ttz) in [Ag(ttz)(X)] (X = NO₃, 1; ClO₄, 2), [Ag₂(Me₂ttz)(NO₃)₂] (3), [Ag₂(Me₂ttz)(H₂O)₂(ClO₄)₂] (4), [Ag₃(Me₂ttz)(H₂O)₂(CF₃SO₃)₃] (5) and [Cu₄Cl₄(Me₂ttz)] (6). In 1 and 2, μ₄-tetrazines and silver ions (AgN₄, Ag–N 2.42–2.53 Å) compose a 3D framework of {4²;8⁴} topology. Structures 3 and 4 were based on disilver–tetrazine ribbons, while in 5μ₄-tetrazines interconnect silver–triflate chains. In 6, μ₄-ligands connect inorganic layers sustained by copper–chloride squares, hexa- and octagons (1.974(2) and 1.981(2) Å). Multiple N-coordination to metal ions enhances the ability of the electron deficient tetrazine system for anion⋯π binding. Compounds 1 and 2 exhibit very short interactions of this type with corresponding O⋯C(N) separations down to a record value of 2.78 Å and O⋯π 2.61 Å. For 6, π-acidity of tetrazine was reflected by contacts Cl⋯π of 3.30 Å. Results of high level ab initio calculations (RI-MP2/aug-cc-pVTZ) were in good agreement with experimental results, and were suggestive of the progressive enhancement of the π-acidity by increasing the number of Ag(I) ions N-coordinated to tetrazine.

Nitrogen donor tetradentate ligands 4,4′-bipyridazine (bpdz) and pyridazino[4,5-d]pyridazine (pp) were prepared by inverse electron demand Diels–Alder cycloaddition reactions of 1,2,4,5-tetrazine. Examination of their behaviour towards silver(I) ions revealed a special potential of the ligands for the design of 3D coordination frameworks involving characteristic polynuclear and polymeric silver(I)–pyridazine motifs and multiple coordination of the ligands. Ag₄(pp)₅(ClO₄)₄ and Ag₄(pp)₅(SiF₆)(BF₄)₂·4H₂O adopt a unique 3D trinodal 4,4,5-connected topology based upon five-fold coordination of the metal ions and tetradentate bridging function of the organic modules. Complexes Ag₃(L)₃(SO₃CF₃)₃·nH₂O and Ag₄(L)₃(X)₄·nH₂O (L = bpdz, pp; X = BF₄ ⁻, 0.5SiF₆ ²⁻) illustrate formation of highly-connected frameworks incorporating trinuclear clusters as an origin of the net connectivity. In the carboxylate complexes Ag₂(L)(R_FCOO)₂ (R_F = CF₃, C₂F₅, C₃F₇) the pyridazine and acido ligands act as complementary linkers for generation of 3D frameworks involving helicate motifs. Fused bicyclic pyridazine pp is a unique system combining very efficient σ_N-donor ability and pronounced π-acidity. The coordination frameworks commonly exhibit strong anion–π interactions, including unprecedented examples of double anion–π,π binding that occur between pyridazino[4,5-d]pyridazine as a double π,π-receptor for geometry complementary SiF₆ ²⁻ anions.

Composites of the spin crossover complex [Fe(trz)(H-trz)₂](BF₄) (H-trz = 1,2,4-4H-triazole and trz = 1,2,4-triazolato) dispersed in a poly(methylmethacrylate) (PMMA) matrix were synthesized and investigated for their spin crossover properties by optical reflectivity, Raman spectroscopy and calorimetry. These composite films were used to fabricate bilayer cantilevers that can perform efficient and tuneable mechanical actuation based on the spin transition. A prototype device that uses the spin transition phenomenon to convert electrical energy into mechanical motion through Joule heating is described. This device is used to perform oscillatory actuation driven by a modulated current. The ability to tune the performance of this electromechanical system is demonstrated by varying the working temperature, the applied ac current and its frequency.

The investigation of size-reduction effects on the spin crossover properties of a class of transition metal complexes has recently become an area of intensive research. However, to avoid inter-particle interactions, ensemble averaging and matrix effects it is necessary to develop methods for the systematic study of individual nano-objects. To this aim thin films and nano-patterns of the compound [Fe^II(hptrz)₃](OTs)₂ doped with acridine orange were elaborated by spin coating and soft-lithography, respectively. The luminescence intensity was found to change significantly upon the spin transition even in isolated nano-objects of ca. 150 nm size, allowing us to monitor in a massively parallel way the spin crossover phenomenon in large arrays viafluorescence microscopy.