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High-Repetition Rate Optical Pump–Nuclear Resonance Probe Experiments Identify Transient Molecular Vibrations after Photoexcitation of a Spin Crossover Material

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posted on 25.03.2021, 14:35 by Sakshath Sadashivaiah, Juliusz A. Wolny, Lena Scherthan, Kevin Jenni, Andreas Omlor, Christina S. Müller, Ilya Sergueev, Marcus Herlitschke, Olaf Leupold, Hans-Christian Wille, Ralf Röhlsberger, Volker Schünemann
Phonon modes play a vital role in the cooperative phenomenon of light-induced spin transitions in spin crossover (SCO) molecular complexes. Although the cooperative vibrations, which occur over several hundreds of picoseconds to nanoseconds after photoexcitation, are understood to play a crucial role in this phase transition, they have not been precisely identified. Therefore, we have performed a novel optical laser pump–nuclear resonance probe experiment to identify the Fe-projected vibrational density of states (pDOS) during the first few nanoseconds after laser excitation of the mononuclear Fe­(II) SCO complex [Fe­(PM-BiA)2(NCS)2]. Evaluation of the so obtained nanosecond-resolved pDOS yields an excitation of ∼8% of the total volume of the complex from the low-spin to high-spin state. Density functional theory calculations allow simulation of the observed changes in the pDOS and thus identification of the transient inter- and intramolecular vibrational modes at nanosecond time scales.