Spatially Resolved Investigation and Control of the Bistability in Single Crystals of the [Fe(bbpya) (NCS)2] Spin Crossover Complex

The spin transition in single crystals of the [FeII(bbpya) (NCS)2] (bbpya = N,N-bis(2–2?-bipyrid-6-yl)amine) mononuclear complex was investigated by a combination of X-ray diffraction, Raman spectroscopy, as well as optical and atomic force microscopy (AFM) methods. These studies, performed around 440 K, revealed an extremely abrupt spin transition associated with a structural phase transition from a triclinic (low spin) to a monoclinic (mixed low spin/high spin) structure. Spatially resolved observations of this transition evidenced a clear phase separation associated with heterogeneous nucleation and the formation of a moving macroscopic interface whose velocity reached in some cases 300 ?m s–1. Using photothermal control it was possible to stabilize biphasic states of the crystal and then acquire AFM images of the phase boundary. A “sawtooth” like topography was repeatedly observed, which most likely emerges so as to minimize the elastic strain. Remarkably, a fine spatial control of the phase boundary could be also achieved using the AFM probe itself, through probe–sample convective heat exchange.