Dynamical limits by downsizing for the photo-induced switching in a molecular material revealed by time-resolved X-ray diffraction

Cooperative molecular switching at the solid state is exemplified by spin crossover phenomenon in crystals of transition metal complexes. Time-resolved studies with temporal resolutions that separate molecular level dynamics from macroscopic changes, afford clear distinction between the time scales of the different degrees of freedom involved. In this work we use 100 ps X-ray diffraction to follow simultaneously the molecular spin state and the structure of the lattice during the photoinduced low spin to high spin transition in microcrystals of [FeIII(3-MeO-SalEen)2]PF6. We show the existence of a delay between the crystalline volume increase driven by the propagation of collective volumic strain waves, and the cooperative macroscopic switching of molecular state. Such behaviour is different from the expectation that phase transformation only requires atomic displacements in the unit cell, that can occur simultaneously with propagation of a volumic strain. Model simulations and discussions of the physical picture explain the phenomenon with thermally activated kinetics governed by local energy barriers separating the molecular states.