Theoretical Study of the Fe(btr)₂(NCS)₂ Spin-Crossover Complex with Reparametrized Density Functionals

Under various constraints, such as temperature or pressure variations, spin crossover (SCO) solids can be switched from the low-spin (LS) state, diamagnetic S = 0 to the high-spin (HS) state, paramagnetic S = 2 conversely. In many cases, the elastic interactions between the SCO units are strong enough to induce a hysteresis at the thermal spin transition which occurs as a first-order phase transition. Such “switchable” molecular solids are promising in terms of optical data storage. In this study, we explore the functional for density functional theory (DFT) calculations that reasonably reproduce the first-order transition, LS/HS energy, electronic state and IR spectral assignment of Fe(II) SCO compounds [Fe(btr)₂(NCS)₂]. A suitable functional was selected by comparing the experimental value of the LS/HS enthalpy difference. The configuration coordinate diagram of LS/HS shows that the LS state has higher stability than the HS state. It was found that the average Fe - ligand distance in the HS state is 0.22 Å longer than the LS state, and the structure is greatly changed. Moreover, the vibrational intensity of C ≡ N group and Fe are greatly different depending on the spin state and on the intramolecular interactions for the structure configuration around Fe.