Femtosecond Spin-State\nSwitching Dynamics of Fe(II)\nComplexes Condensed in Thin Films

The tailoring of spin-crossover films has made significant\nprogress\nover the past decade, mostly motivated by the prospect in technological\napplications. In contrast to spin-crossover complexes in solution,\nthe investigation of the ultrafast switching in spin-crossover films\nhas remained scarce. Combining the progress in molecule synthesis\nand film growth with the opportunities at X-ray free-electron lasers,\nwe study the photoinduced spin-state switching dynamics of a molecular\nfilm at room temperature. The subpicosecond switching from the <i>S</i> = 0 low-spin ground state to the <i>S</i> =\n2 high-spin state is monitored by analyzing the transient evolution\nof the Fe L<sub>3</sub> X-ray absorption edge fine structure, i.e.\nelement-specifically at the switching center of the Fe(II) complex.\nOur measurements show the involvement of an intermediate state in\nthe switching. At large excitation fluences, the fraction of high-spin\nmolecules saturates at ≈50%, which is likely due to molecule–molecule\ninteraction within the film.