Size and Surface Effects in the Ultrafast Dynamics of Strongly Cooperative Spin-Crossover Nanoparticles.

Cooperative photoinduced switching of molecular materials at the nanoscale is still in its infancy. Strongly cooperative spin-crossover nanomaterials are arguably the best prototypes of photomagnetic and volume-changing materials that can be manipulated by short pulses of light. Open questions remain regarding their non-equilibrium dynamics upon light excitation and the role of cooperative elastic interactions in nanoscale systems that are characterized by large surface/volume ratios. Femtosecond-resolved broadband spectroscopy is performed on nanorods of the strongly cooperative Fe-triazole, which undergoes a reversible low-spin to high-spin (HS) phase transition ≈360 K. Supported by density functional theory and mechano-elastic Monte Carlo simulations, a marked difference is observed in the photoswitching dynamics at the surface of the nanoparticles compared with the core. Surprisingly, under low excitation (<2%) conditions, there occurs a transient increase in the HS population at the surface on the picosecond time scale, while the HS population in the core decays concomitantly. These results shed light onto the importance of surface properties and dynamical size limits of nanoscale photoresponsive nanomaterials that can be used in a broad range of applications.