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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.

An iron(II) complex [Fe(L5)2][ClO4]2 of a new nonplanar tridentate ligand [L5 = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl](1-methylimidazole)] has been synthesized and its spin-transition properties have been investigated. X-ray structural analysis of the solvated compound [Fe(L5)2][ClO4]2.CH3CN at 100 K and at 298 K revealed a mononuclear Fe(II)N6 pseudooctahedron with the tridentate ligand in the meridional coordination mode. Magnetic susceptibility measurements revealed LS (1A1) <--> HS (5T2) transition starting above room temperature and remained incomplete even up to 350 K, however providing evidence for the hysteretic behaviour of the transition. Photo-magnetic measurements using a 532 nm laser revealed incomplete light-induced excited spin-state trapping (LIESST). Relaxation curves in the dark exhibited a stretched exponential shape, typical for broad distributions of relaxation times. The complete photo-magnetic data were modelled using a non-cooperative master equation including a tunnelling process with distributed parameters and accounting for the bulk absorption of light. We also measured the diffuse reflectance properties, with detailed investigation of the absorption spectra and discussed the obtained features with respect to structural properties, including the suggested presence of disorder.