Investigating the kinetics of spin-crossover transitions using Raman spectroscopy

The room-temperature spin-crossover iron(II) complex Fe(1-bpp-COOC₂H₅)₂(BF₄)₂CH₃CN exhibits reversible switching between the low-spin and high-spin states. The observation of coexisting spin domains within polycrystalline samples of Fe(1-bpp-COOC₂H₂5)₂(BF₄)₂CH₃CN allows for real-time video recording of domain boundary propagation under an optical microscope. Quantitative analysis of this motion provides valuable information regarding the phenomenological transition involved in the spinstate switching process. To elucidate the intricate dynamics of this spin transition, a synergistic approach combining temperature-dependent Raman spectroscopy with ab initio computational methods is employed. Within this investigation, a comprehensive classification of Raman vibrational modes has been achieved, categorizing them based on their spin-state dependence and vibrational characteristics, symmetries, including stretching, bending, and tilting motions. Notably, low-frequency Raman modes associated with the iron center and its nitrogen ligand environment provide crucial insights into the local coordination and its role in the spin-crossover mechanism. This work reveals distinct spin-state-induced structural changes, including bond stretching and softening, which manifest as unique spectroscopic fingerprints for each spin state.