Sequential activation of molecular and macroscopic spin‐state switching within the hysteretic region following pulsed light excitation
Résumé
Molecular spin-crossover (SCO) compounds constitute a promising class of photoactive materials exhibiting efficient photo-induced phase transitions (PIPTs), driven by cooperative elastic interactions between the switchable molecules. Taking advantage of the unique, picture-perfect reproducibility of the spin-transition properties in the compound [Fe(HB(1,2,4-triazol-1-yl)3)2], we have dissected the spatiotemporal dynamics of the PIPT within the thermodynamic metastability (hysteretic) region of a single crystal, using pump-probe optical microscopy. Beyond a threshold laser excitation density, complete PIPTs were evidenced, with conversion rates up to 200 switched molecules per absorbed photon. We show that the PIPT takes place through the sequential activation of two (molecular and macroscopic) switching mechanisms, occurring on sub-μs and ms timescales, governed by the intramolecular and free energy barriers of the system, respectively. The main finding here is that the thermodynamic metastability has strictly no influence on the sub-ms switching dynamics. Indeed, before this ms timescale, the response of the crystal to the laser excitation involves a gradual, molecular conversion process, as if there was no hysteresis loop. Consequently, in this regime, even a 100% photo-induced conversion may not give rise to a PIPT. These results provide new insight on the intrinsic dynamical limits of the PIPT in SCO solids (and other types of bistable materials), which is an important issue, from a technological perspective, for achieving fast and efficient photo-control of the functionalities of condensed matter.
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