Dimeric Phenalenyl-Based Neutral Radical Molecular Conductors

Abstract

We report the preparation, crystallization, and solid-state characterization of ethyl (3)- and butyl (4)-substituted spiro-biphenalenyl radicals. Both of these compounds are found to be conducting face-to-face pi-dimers in the solid state but with different room-temperature magnetic ground states. At room temperature, 4 exists as a diamagnetic pi-dimer (interplanar separation of approximately 3.1 A), whereas 3 is a paramagnetic pi-dimer (interplanar separation of approximately 3.3 A), and both compounds show phase transitions between the paramagnetic and diamagnetic forms. Electrical resistivity measurements of single crystals of 3 and 4 show that the transition from the high-temperature paramagnetic pi-dimer form to the low-temperature diamagnetic pi-dimer structure is accompanied by an increase in conductivity by about 2 orders of magnitude. This behavior is unprecedented and is very difficult to reconcile with the usual understanding of a Peierls dimerization, which inevitably leads to an insulating ground state. We tentatively assign the enhancement in the conductivity to a decrease in the on-site Coulombic correlation energy (U), as the dimers form a super-molecule with twice the amount of conjugation.

Hydrogen bond-promoted metallic state in a purely organic single-component conductor under pressure

“…First, note that the electrical conductivity at room-temperature, s rt ¼ 1/r rt , is significantly high, B3.5 and 19 S cm À 1 for j-S and j-Se, respectively. These values are one or two orders of magnitude higher than the highest reported value, s n B10 À 1 S cm À 1 , in the purely organic single-component systems, to our best knowledge 20,23,24,26,27 . As temperature is decreased, the electrical resistivity of these systems exponentially increases, which is a typical semiconducting behaviour, r(T) p exp(D/2k B T), and consistent with the results of the electronic band calculations.…”

“…In general, the neutral-radical system falls into the Mott insulating states, due to the intrinsic half-filled nature of the valence band consisting of the SOMO. Nevertheless, the relatively high electrical conductivity at room-temperature, s n B10 À 6 -10 À 1 S cm À 1 , has been observed in several types of neutral-radical-based single-component molecular conductors [19][20][21][22][23][24][25][26][27][28][29][30][31][32] . So far, among the purely organic single-component systems, the metallic behaviour has been exclusively achieved under high physical pressure for some neutral-radical conductors 32,33 .…”

Synthesis, Structure and Solid State Properties of Cyclohexanemethylamine Substituted Phenalenyl Based Molecular Conductor

“…The absorptions in the mid-IR, between 650 and 4000 cm −1 , are due to the molecular vibrations of the radical 11. The nature of the spectra are common to that of other neutral radicals and the optical energy gap E g is comparable to that of the spiro-biphenalenyl boron radicals reported previously [38,44]. The optical gap is approximately two times higher than the activation energy ∆ = 0.22 eV of the electrical conductivity which is similar to the case of conventional intrinsic semiconductors.…”

“…Similar to 5, the compounds 16, 17 and 18 have very weak intermolecular interactions in the solid state, and the magnetic susceptibility could be fit to the Curie−Weiss function with θ = −55 K (16), −70 K (17), and −14 K (18), or to the antiferromagnetic 1D Heisenberg model; the conductivities span a broad range: σ RT = 0.04 (16), 0.01 (17), and 7 × 10 −6 S/cm (18) [37]. Shorter chain analogs of 5, the ethyl (1) and butyl (3) compounds crystallized as face-to-face π-dimers and showed conductivities of σ RT = 0.01 (1), and 0.02 (3) S/cm [38]. These dimeric neutral radicals (1 and 3) simultaneously exhibit bistability in three physical channels: magnetic, electrical and optical, which has been rarely realized in a single system [39][40][41][42].…”

A 1,3,2-oxazaborolidine dimer derived from (S)-α,α-diphenylprolinol

“…Both B-O distances in (II) are identical to within the s.u. (Lang et al, 1997;Kim et al, 1999;Chi et al, 1999Chi et al, , 2001). The observed B2-N1 distance of 1.618 (3) Å falls into the range of typical B-N distances [1.591-1.649 Å].…”