Plasmonic resonance properties of a series of lithographically patterned gold nanorod arrays, spin coated by thin films of an iron(II)–triazole type spin crossover complex, were investigated upon heating/cooling and also under 633 nm laser irradiation. In both cases a reversible shift of the localised surface plasmon resonance wavelength was observed and quantitatively linked to the refractive index change accompanying the spin transition. These results show that molecular spin state switching can be very efficiently triggered by the photo-thermal effect, which – in turn – allows for an active tuning of the plasmon resonance.

Topographic images of [Fe(Htrz)₂(trz)](BF₄) nanoparticles were acquired across the first-order spin transition using variable-temperature atomic force microscopy (AFM) in amplitude modulation mode. These studies revealed a complex morphology of the particles consisting of aggregates of small nanocrystals, which expand, separate and re-aggregate due to the mechanical stress during the spin-state switching events. Both reversible (prompt or slow recovery) and irreversible effects (fatigue) on the particle morphology were evidenced and correlated with the spin crossover properties.

We report on atomic force microscopy (AFM) and near-field scanning optical microscopy (NSOM) investigations of single crystals of the spin crossover complex {Fe(pyrazine)[Pt(CN)₄]} across the first-order thermal spin transition. We demonstrate for the first time that the change in spin state can be probed with sub-micrometer spatial resolution through various topographic features extracted from AFM data. This original approach based on surface topography analysis should be easy to implement to any phase change material exhibiting sizeable electron–lattice coupling. In addition, AFM images revealed specific topographic features in the crystals, which were correlated with the spatiotemporal evolution of the transition observed by far-field and near-field optical microscopies.