We report on thermally evaporated, crystalline films of [Fe(HB(tz)3)2] (tz = triazolyl), which display spin transition around 338 K.

Using ultrafast optical absorption spectroscopy, we have studied the room-temperature spinstate switching dynamics induced by a femtosecond laser pulse in high-quality thin films of the molecular spin-crossover complex [Fe(HB(tz)3)2] (tz = 1,2,4-triazol-1-yl). These measurements reveal that the early, sub-picosecond, low-spin to high-spin photoswitching event, with linear response to the laser pulse energy, can be followed under certain conditions by a second switching process occurring on a tens of nanoseconds timescale, enabling nonlinear amplification. Such out-of-equilibrium dynamics is discussed in light of characteristic timescales associated with the different switching mechanisms, i.e. the electronic and structural rearrangements of photo-excited molecules, the propagation of strain waves at the material scale and the thermal activation above the molecular energy barrier. Importantly, size reduction effects are evidenced on the second switching step. Notably, this nonlinear process appears to be completely suppressed in the thinnest (50 nm) film due to the efficient heat transfer to the substrate, allowing the system to retrieve the thermal equilibrium state on the 100-ns timescale.These results provide a first milestone towards the assessment of the physical parameters that