Hysteretic Spin and Charge Delocalization in a Phenalenyl-Based Molecular Conductor

We have investigated the solid-state electronic structure and properties of a phenalenyl-based butyl-substituted neutral radical, 3, that shows a hysteretic phase transition just above room temperature. We quantitatively analyzed the electron density distribution of this radical throughout both branches of the hysteretic phase transition using solid-state X-ray structures and found two distinct electronic states in the hysteresis loop that accompanies the phase transition. The bistability of the two electronic states was observed through a number of measurements, including IR transmittance spectra of single crystals in the vicinity of the phase transition. By comparing the changes in the crystal structures of 3 and the related ethyl-substituted radical 1 (which exhibits no hysteresis) at various temperatures, we show that the change in the interplanar π-π distance within dimers is the most important structural parameter in determining the physical properties of the radicals. The large change in the C-H···π interaction in 3 occurs in concert with the spin redistribution during the phase transition, but these factors are not responsible for the hysteresis effect. We suggest that the presence of a high-temperature state inside the hysteretic loop during the cooling cycle is due to thermodynamic stability, while the existence of the low-temperature state during the heating cycle is due to the presence of a large energy barrier between the two states (estimated to be greater than 100 kJ/mol) that results from the large-amplitude motion of the phenalenyl rings and the associated lattice reorganization energy that is required at the phase transition.