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

Materials can be controlled via physical parameters like pressure or temperature. With the advent of ultrashort lasers pulses (typically femtosecond), light excitation has been added to the panel of available techniques for materials control. Two of the major aspects in so-called photo-induced phase transitions are reversibility, meaning the ability to switch back and forth between two states, and efficiency, corresponding to the ratio of the amount of transformed material on the quantity of provided light. One possible approach to amplify the photoresponse is based on the material elastic properties. In light-sensitive and volume-changing Spin Crossover (SCO) materials, the sudden generation (via a laser pulse) of a high enough fraction of photo-excited molecules (switched from Low spin to High spin state) creates local negative pressure. This drives lattice expansion that can induce additional switching of neighboring molecules through positive feedback effect. This cooperative effect is associated as expected with a well-defined threshold mechanism. To unambiguously discriminate between spin state conversion and volume change, we performed time-resolved x-ray diffraction study at ESRF synchrotron. The diffraction patterns measured from ps to µs time delays on nanocrystals powder films give a direct signature of the ultrafast volume expansion. Quantitative analysis of the powder spectra at ps time scale allows going deeper into the understanding of the cooperative aspects of the photo-induced spin conversion in these molecular materials. On the other hand, these observations were supplemented by Monte-Carlo simulations from a mechano-elastic model. These simulations corroborate the positive feedback effect of volume dilation on the spin conversion efficiency at nanosecond timescale.