Impact of Molecular Distortions on Bistability in Spin Crossover Complexes

Abstract

We present a simple model of molecular distortions in spin crossover complexes, based on crystal field theory and transition state theory. This allows us to model the effects of molecular distortions on T_1/2, the characteristic temperature of thermal crossover, and T_LIESST, the maximum temperature at which trapped excited high spin (HS) complexes are stable. T_1/2 is a purely thermodynamic quantity, determined by the relative free energies of the HS and low spin (LS) states (ΔG = G_HS – G_LS). The average distortion across HS and LS species $[\overline{\mathrm{\Sigma }}=({\mathrm{\Sigma }}_{\mathrm{H}\mathrm{S}}+{\mathrm{\Sigma }}_{\mathrm{L}\mathrm{S}})/2]$ and the change in distortion between spin states (ΔΣ = Σ_HS – Σ_LS) have a significant impact on ΔG. However, the inner coordination sphere stiffness (k) has little impact on ΔG. Therefore, $\overline{\mathrm{\Sigma }}$ and ΔΣ have large effects on T_1/2 whereas k does not. T_LIESST is largely determined by the height of the barrier (E_b) between the metastable HS state and the LS state. E_b is strongly affected by ΔΣ, $\overline{\mathrm{\Sigma }}$, and k; thus T_LIESST is strongly dependent on all of these quantities. Thus, increasing both the relative and absolute distortion of SCO complexes increases T_LIESST and decreases T_1/2, providing a route to high temperature spin-state switching via molecular distortions.