Cooperative elastic switching vs. laser heating in [Fe(phen)2(NCS)2] spin-crossover crystals excited by a laser pulse.

Spin-crossover crystals show multi-step responses to femtosecond light excitation. The local molecular photo-switching from low to high spin states occurs on the sub-picosecond timescale. It is followed by additional conversion due to elastic (ns) and thermal (mu s) effects. In [Fe(phen)(2)(NCS)(2)] crystals discussed herein, the thermal switching can be made unobtrusive for the investigation of cooperative elastic switching. We evidence a cooperative transformation induced by lattice expansion through elastic coupling between molecules in the crystal, where up to 3 molecules are transformed per photon.