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

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.