Femtosecond spin-state photo-switching dynamics in an FeIII spin crossover solid accompanied by coherent structural vibrations

We investigate light-induced excited spin-state trapping (LIESST) dynamics of an Fe-III spin-crossover material from low (S = 1/2) to high (S = 5/2) spin states. Our results show that this process occurs only at the molecular level as evidenced by the linear dependence of the fraction of photo-switched molecules with the excitation density as well as with the initial fraction of low spin molecules. The inter-system crossing from photoexcited LS (S = 1/2) to HS (S = 5/2) occurs within approximate to 200 fs and is accompanied by coherent non-equilibrium vibrational relaxation in the photo-induced HS state. These results reveal similar dynamical features to those already reported for LIESST in Fe-II systems. The activation of coherent molecular vibrations is essential for rapidly reaching the HS potential on the timescale of molecular motions, whereas their fast damping allows an efficient trapping in the HS potential. The observed coherent oscillations are attributed to photoinduced molecules in the HS states, as supported by Raman spectroscopy at thermal equilibrium, and DFT analyses of molecular vibrations and TD-DFT calculations of optical absorption.