Intermolecular control of thermoswitching and photoswitching phenomena in two spin-crossover polymorphs

International audienceWe discuss here the important role of inter-molecular coupling for the thermal and lightinduced molecular state switching in the solid state. Investigations were performed on the two crystalline polymorphs of the spin crossover [Fe(PM BiA)2(NCS)2] material. In addition to structural studies at thermal equilibrium, light-induced phenomena were investigated through photo-crystallography, photo-magnetic and dynamical optical measurements. Strong similarities between the thermal-equilibrium and the out-ofequilibrium light-induced transformations are observed in each polymorph: strong cooperative phenomena in one polymorph versus weak cooperative ones in the second polymorph. These different responses of the two crystalline forms of the compound to external perturbations are discussed at the microscopic level in terms of Ising-like model and two-mode description of on-site molecular potentials