title: Hysteretic spin and charge delocalization in a phenalenyl-based molecular conductor.
abstract: 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.
authors.affiliation: Department of Chemistry, University of California, Riverside, California 92521-0403, USA.
authors.forename: Sushanta K
authors.initials: SK
authors.lastname: Pal
authors.forename: Pradip
authors.initials: P
authors.lastname: Bag
authors.forename: Arindam
authors.initials: A
authors.lastname: Sarkar
authors.forename: Xiaoliu
authors.initials: X
authors.lastname: Chi
authors.forename: Mikhail E
authors.initials: ME
authors.lastname: Itkis
authors.forename: Fook S
authors.initials: FS
authors.lastname: Tham
authors.forename: Bruno
authors.initials: B
authors.lastname: Donnadieu
authors.forename: Robert C
authors.initials: RC
authors.lastname: Haddon