Femtosecond photoswitching dynamics and microsecond thermal conversion driven by laser heating in FeIII spin-crossover solids

•An overview spin-state photo-switching in FeIII spin-crossover crystals is provided.•Ultrafast optical spectroscopy reveal a sequence of conversion.•Lattice expansion and heating induce additional spin-state conversion.•The photo-response is linear with excitation density.•The thermal conversion is strong in cooperative materials close to phase transition. In this paper we review time-resolved studies of ultrafast light-induced spin-state switching, triggered by a femtosecond laser flash, and the following out-of-equilibrium dynamics in FeIII spin-crossover crystals. The out-of-equilibrium dynamics involves several steps, resulting from the ultrafast molecular photoswitching of low-spin (LS) to high-spin (HS) states in solids. First, the transient HS state is reached within 200fs, and may rapidly decay into the stable LS state of the system. A second process at longer delay, associated with volume expansion, drives additional conversion to the HS state during the so-called elastic step occurring at nanosecond time scale. Finally, the laser heating process induces a temperature jump in the crystal that may result in a significant thermal population of the HS state on microsecond time scale. The photoswitching mechanism is of local nature and has linear dependence on the excitation fluence, whereas the heating effect can macroscopically perturb the LS/HS equilibrium. We discuss similarities and differences between photoswitching dynamics in solution and in different crystals for which the thermal spin conversion is of more or less pronounced cooperative nature.