Unprecedented Size Effect on the Phase Stability of Molecular Thin Films Displaying a Spin Transition

An unexpected upshift of the spin transition temperature by ca. 3 K is observed in thermally evaporated films of the [FeII(HB­(tz)3)2] (tz = 1,2,4-triazol-1-yl) complex when reducing the film thickness from ca. 200 to 45 nm. Fitting the experimental data to continuum mechanics and thermodynamical models allows us to propose an explanation based on the anisotropy of the transformation strain leading to ∼5 mJ/m2 higher 00l surface energy in the high-spin phase.