Tunable spin-state bistability in a spin crossover molecular complex.

The spin crossover (SCO) transitions at both the surface and over the entire volume of the [Fe{HB(pz)}(bipy)] polycrystalline films on AlO substrates have been studied, where pz  =  pyrazol-1-yl and bipy  =  2,2'-bipyridine. For [Fe{HB(pz)}(bipy)] films of hundreds of nm thick, magnetometry and x-ray absorption spectroscopy measurements show thermal hysteresis in the SCO transition with temperature, although the transition in bulk [Fe{HB(pz)}(bipy)] occurs in a non-hysteretic fashion at 157 K. While the size of the crystallites in those films are similar, the hysteresis becomes more prominent in thinner films, indicating a significant effect of the [Fe{HB(pz)}(bipy)]/AlO interface. Bistability of spin states, which can be inferred from the thermal hysteresis, was directly observed using temperature-dependent x-ray diffraction; the crystallites behave as spin-state domains that coexist during the transition. The difference between the spin state of molecules at the surface of the [Fe{HB(pz)}(bipy)] films and that of the molecules within the films, during the thermal cycle, indicates that both cooperative (intermolecular) effects and coordination are implicated in perturbations to the SCO transition.