Probing the Spin States of Tetra-coordinated Iron(II) Porphyrins by their Vibrational and Pre K-edge X-Ray Absorption Spectra

Tetra-coordinated iron(II) porphyrins are observed in triplet (intermediate spin, IS) ground state with low-lying quintet (high spin, HS) and singlet (low spin, LS) excited states. It is well known in the literature that the $d_{Fe-N}$ bond distances are dependent on the spin state such that the $d_{Fe-N}$ distances are larger for HS compared to the LS/IS states. This implies the existence of strong magneto-structural correlations. We address here whether it is possible to obtain the spin-state information of a molecular systems by exploiting the magneto-structural correlations i.e., probing the ground-state vibrational and structural properties, can we determine the spin state of a molecular systems? To answer this, we have studied a series of tetra-coordinated ion (II) porphyrins with various substitutions at the peripheral macrocyclic rings applying first-principles calculations. The spin-state dependent nature of the chemical bonds and spin polarization on the N-atoms plays a crucial role in the vibrational dynamics of the central Fe-atom. Theoretical nuclear resonance vibrational spectroscopy (NRVS) is adopted to investigate the vibrational dynamics and the Fe-atom associated spectral peaks and their spin-dependent features. We find that NRVS studies along with iron pre-K-edge X-ray absorption analysis convincingly provide the information on the spin state, which could be utilized to establish a new methodology to identify the spin states without adopting any magnetic probes.