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

Magnetic bi-stability (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 transition between different spin states with disparate structural, thermodynamic, electronic, and magnetic properties. Here, we employ periodic density functional theory (DFT) 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 consist of four Fe SCO centers covering a range of full to partial high-spin (HS) and low-spin (LS) states. Our results show that the fully HS state is a wide band gap 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 HS to LS state is not possible in the extended material due to the high energy barriers of partially high-spin states (i.e. 75% HS or 25% HS states) which are accompanied by an overall reduction in the unit cell volume. We attribute this shrinkage to the decreases of the Fe-N bond lengths which in turn creates lattice strain. These two transient spin-states show a metallic behavior as opposed to the semiconductor behavior of the HS state promising potential applications for creating a system with switchable magnetic and electronic responses. Investigating charge transport pathways in the system reveals dominant intra-layer charge transport via both through-bond and through-space mechanisms. The outcome of this study will provide valuable insights into the spin transition behavior in bi-CCPs and will help as a foundation for designing materials with tailored electronic and magnetic properties.