Investigation of nucleation and growth phenomena during the thermal and light induced spin transition in the [Fe(1-bpp)2][BF4]2 complex

Optical microscopy measurements have been realized on single crystals of the [Fe(1-bpp)2][BF4]2 complex (1-bpp=2,6-di(pyrazol-1-yl)pyridine). The thermal spin transition around 253 K occurs by a heterogeneous nucleation and growth mechanism and involves a clear phase separation and a small hysteresis. This very abrupt and complete thermal transition is preceded by a premonitory spin conversion, which implies only a small fraction (ca. 2–3 %) of the molecules. This peculiar behavior may be the sign of heterophase fluctuations. The light-induced spin transition from the stable low spin (LS) to the metastable high spin (HS) phase was achieved at 80 K by focusing laser light into a small volume fraction of the crystal. Under continuous irradiation this photo-converted HS “nucleus” then grows and the whole crystal converts to the HS phase providing evidence for a light-induced instability in the system due to long-range elastic interactions. The relaxation of the light-induced metastable HS phase at 83 K follows a sigmoidal decay – typical of cooperative spin crossover systems. Nevertheless the spatial development of this relaxation process appears very homogeneous and no phase separation could be detected within the resolution of the optical microscope.