Investigating the Kinetics of Spin Crossover Transitions using Raman Spectroscopy

Abstract

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-COOC₂H₅)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 spin-state 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.