Complexities in the Molecular Spin Crossover Transition

International audienceVariable-temperature studies of the electronic structures of four different Fe(II) spin crossover molecules, [Fe(H2B(pz)2)2(bipy)] (pz = pyrazol-1-yl, bipy = 2,2′-bipyridine), [Fe(H2B(pz)2)2(phen)], [Fe(phen)2(NCS)2] (phen = 9,10-phenantroline), and [Fe(PM-AzA)2(NCS)2] (PM-AzA = 4-phenyldiazenyl-N-(pyridin-2-ylmethylene)aniline) by X-ray absorption spectroscopy (XAS), combined with electrical properties studies of the [Fe(PM-AzA)2(NCS)2] single crystal are presented. We show that both the XAS signature of the spin state of powdered samples and the dielectric permittivity of the [Fe(PM-AzA)2(NCS)2] single crystal change at significantly lower temperatures than the magnetometry, structure, and resistivity indicators of a spin crossover transition. The changes in electronic structure are in agreement with the expectations from density functional theory (DFT) results for the different molecular electronic structures associated with the high-spin and low-spin states. These findings suggest that the electronic structure phase ordering process does not simply follow the spin transition