Femtosecond Spin-State Switching Dynamics of Fe(II) Complexes Condensed in Thin Films.

Kämmerer, Lea; Kämmerer, Gérald; Gruber, Manuel; Grunwald, Jan; Lojewski, Tobias; Mercadier, Laurent; Le Guyader, Loïc; Carley, Robert et al. · ACS Nano · 2024

basic_science · Level V

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Abstract

The tailoring of spin-crossover films has made significant progress over the past decade, mostly motivated by the prospect in technological applications. In contrast to spin-crossover complexes in solution, the investigation of the ultrafast switching in spin-crossover films has remained scarce. Combining the progress in molecule synthesis and film growth with the opportunities at X-ray free-electron lasers, we study the photoinduced spin-state switching dynamics of a molecular film at room temperature. The subpicosecond switching from the <i>S</i> = 0 low-spin ground state to the <i>S</i> = 2 high-spin state is monitored by analyzing the transient evolution of the Fe L<sub>3</sub> X-ray absorption edge fine structure, i.e. element-specifically at the switching center of the Fe(II) complex. Our measurements show the involvement of an intermediate state in the switching. At large excitation fluences, the fraction of high-spin molecules saturates at ≈50%, which is likely due to molecule-molecule interaction within the film.