Understanding spin-dependent vibrational frequencies in Fe(II) metal organic coordination complexes

We investigated the compositional and temperature (T) dependences of vibrational frequencies in Hofmann-type Fe(L)₂[M(CN)₄] spin-crossover (SCO) coordination polymers in which {M = Ni, Pd or Pt with L = pyridine (py)}, or {L = 3-Cl-py or 3-methylpy with M = Ni}, using Raman spectroscopy. The SCO-driven peak shifts (in wavenumber) ranged up to ∼170 cm⁻¹, manifesting significant spin-dependent structural evolutions. Furthermore, there appear clear HS signatures even at T ≪ T_SCO for L = 3-Cl-py or 3-methylpy implying the steric effects of the organic ligands on the HS trapping. Meanwhile, for L = py, such HS trapping at the low temperature was not significant although some spectra taken under high laser fluence exhibit light-induced excited spin state trapping (LIESST) effect. The mechanism of the LIESST is discussed in detail in terms of the M d – C 2sp hybridization effects.