Spin Crossover of Spiro-Biphenalenyl Neutral Radical Molecular Conductors

We present ab initio molecular and solid-state calculations at the level of density functional theory (DFT) for the ethyl-substituted spiro-biphenalenyl neutral radical organic conductor. We find that the phase transition of this material is accompanied by a spin crossover (low-spin, LS, to high-spin, HS), and consequently a different band becomes the conduction band. The energy gap (E_g) increases from 0.12 eV of the low-temperature polymorph to 0.23 eV of the high-temperature polymorph corresponding to a different occupancy causing a change in the number of the available charge carriers, explaining the change of conductivity by 2 orders of magnitude at the phase transition. These gap values are also consistent with structural, IR, electrical conductivity, and magnetic susceptibility data of Itkis et al. The proximity of the monomers in the stacking dimers is closely related to the spin crossover in this material.