The photoinduced switching of Fe(II)-based spin-crossover complexes from singlet to quintet takes place at ultrafast time scales. This a priori spin-forbidden transition triggered numerous time-resolved experiments of solvated samples to elucidate the mechanism at play. The involved intermediate states remain uncertain. We apply ultrafast x-ray spectroscopy in molecular films as a method sensitive to spin, electronic, and nuclear degrees of freedom. Combining the progress in molecule synthesis and film growth with the opportunities at x-ray free-electron lasers, we analyze the transient evolution of the Fe L3 fine structure at room temperature. Our measurements and calculations indicate the involvement of an Fe triplet intermediate state. The high-spin state saturates at half of the available molecules, limited by molecule-molecule interaction within the film.
@article{arxiv.2312.01483,
title = {Femtosecond spin-state switching dynamics of spin-crossover molecules condensed in thin films},
author = {Lea Kämmerer and Gérald Kämmerer and Manuel Gruber and Jan Grunwald and Tobias Lojewski and Laurent Mercadier and Loïc Le Guyader and Robert Carley and Cammille Carinan and Natalia Gerasimova and David Hickin and Benjamin E. Van Kuiken and Giuseppe Mercurio and Martin Teichmann and Senthil Kumar Kuppusamy and Andreas Scherz and Mario Ruben and Klaus Sokolowski-Tinten and Andrea Eschenlohr and Katharina Ollefs and Carolin Schmitz-Antoniak and Felix Tuczek and Peter Kratzer and Uwe Bovensiepen and Heiko Wende},
journal= {arXiv preprint arXiv:2312.01483},
year = {2026}
}