Electronic Structure, Vibrational Spectra and Probing of Spin-states of Tetra-coordinated Iron(II) Porphyrins

The nearly degenerate spin multiplets of the iron(II) porphyrins have endowed these biomacromolecules with potential for use in various applications such as, molecular spintronics, single molecule devices, and sensor technology. Achieving a definitive understanding of the spin crossover in these complexes is a challenging area to explore. In this work, we aim to provide a theoretical perspective of the magneto-structural correlations and their resulting spectroscopic signatures for a series of tetra-coordinated, square-planar iron(II) porphyrin and phthalocyanine derivatives. We systematically investigate the spin-state energies, vibrational, and pre-edge X-ray spectra of these complexes in three possible spin states (viz. low spin (S = 0), intermediate spin (S = 1) and high spin (S = 2)). Our calculations show that the triplet or intermediate spin state is the ground spin state for all the complexes. We also observe that the small structural difference in the dFe−N between the intermediate- and high-spin sate in these complexes can be captured using nuclear resonance vibrational spectra (NRVS). Furthermore, our results identify the position of the X-ray absorption (XAS) peaks as a signature of the underlying spin state.