Structural and Electronic Response of Fe(II) Hofmann-Type Conjugated Coordination Polymers to Spin Crossover

Magnetic bistability (bi) in Fe(II) Hofmann-type conjugated coordination polymers (CCPs) offers a pathway to innovative applications in data storage and molecular switching. Upon spin crossover (SCO) induced by external stimuli, these CCPs undergo transitions between different spin states with disparate structural, thermodynamic, electronic, and magnetic properties. Here, we employ periodic density functional theory calculations to decipher the structure-property-function relationship in bi-CCPs with a focus on archetypal 2D [Fe(II)(py)₂Pt(CN)₄]. Employing a supercell consisting of four Fe SCO centers we cover a range of full to partial high-spin (HS) and low-spin (LS) states. Our results show that the fully HS state is a semiconductor global minimum, aligning with our experimental electrical conductivity measurements. Changing the spin states of the four Fe centers successively reveals that a full SCO from the HS to the LS state is hampered by a reduction in the unit cell volume, which increases the internal strain in the system. We attribute this shrinkage to the decreases of the equatorial Fe-N bond lengths (with (NC)₄Pt ligands), while the axial Fe-N bond lengths (with pyridine ligands) remain almost intact, showcasing the cooperativity effects. All five spin states are semiconductors with the smallest band gap of 1.727 eV being related to the state with 75% HS Fe centers. Investigating charge transport pathways in the system reveals dominant in-plane charge transport via a charge carrier hopping mechanism between π-stacked pyridine rings, while out-of-plane charge transport plays a minor role. The outcome of this study will provide valuable insights into the spin transition behavior in bi-CCPs and serve as a foundation for designing materials with tailored electronic and magnetic properties.