We investigated by optical microscopy the propagation of the thermal spin transition in [Fe(btr) 2 (NCS) 2 ]·H 2 O·(btr = bis-triazole) single crystals. Using high-quality fresh crystals embedded in oil we could follow the transformation front of the on-cooling transition. We observed in detail a linear front propagating through the entire crystal at extremely slow velocity, ∼2.5 μm/s on average. The analysis of the temporal dependences of the optical densities (OD) at given positions within a Kolmogorov-Johnson-Mehl-Avrami (KJMA) law led to an exponent ∼0.6, typical for a process governed by planar diffusion. It is inferred that the spin transition is a multiscale process. The agreement with a previous structural investigation, analyzed in terms of an average KJMA law, is shown. We conclude that the stresses generated by the volume change associated with the spin transformation are the driving force for the propagation of the nucleation and growth mechanism. The role of defects is discussed and optical effects specific of the experiment are evidenced.

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The colorimetric analysis of images recorded with an optical microscope during the onset of the spin crossover transformation allows monitoring separately the involved electronic and structural aspects, through the separation of resonant absorption and scattering effects. Complementary information can also be obtained by using the polarized modes of the microscope. These potentialities are illustrated by the observation of [Fe(ptz)(6)](BF(4))(2) single crystals during the onset of the thermal transitions in the 110-140 K range. We characterized the interplay between the electronic (HS <--> LS) and structural (order <--> disorder) transformations. Elastic stresses and mechanical effects (hopping, self-cleavage) generated by the volume change upon electronic transition are also illustrated, with their impact on the photoswitching properties of the crystals.