Resonating Valence-Bond Ground State in a Phenalenyl-Based Neutral Radical Conductor

Abstract

An organic material composed of neutral free radicals based on the spirobiphenalenyl system exhibits a room temperature conductivity of 0.3 siemens per centimeter and a high-symmetry crystal structure. It displays the temperature-independent Pauli paramagnetism characteristic of a metal with a magnetic susceptibility that implies a density of states at the Fermi level of 15.5 states per electron volt per mole. Extended Hückel calculations indicate that the solid is a three-dimensional organic metal with a band width of approximately 0.5 electron volts. However, the compound shows activated conductivity (activation energy, 0.054 electron volts) and an optical energy gap of 0.34 electron volts. We argue that these apparently contradictory properties are best resolved in terms of the resonating valence-bond ground state originally suggested by Pauling, but with the modifications introduced by Anderson.

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 .…”

Near-infrared absorption of π-stacking columns composed of trioxotriangulene neutral radicals

“…13) and 3.65 Å, 19 due to bulky protection groups, the latter of which shows a band around 800 nm. 19 π-Stacked structures of π-conjugated PNL dimers (1/2 unpaired electron per PNL unit) [20][21][22] and those of singlet-diradical bisphenalenyls 23 (one unpaired electron per PNL unit) exhibit NIR absorption bands. Bis(dithiazolyl) radicals 24,25 form 1D π-stacks with intercolumnar S•••S and S•••N contacts, showing lowenergy absorption bands around~2500 nm (~0.5 eV).…”

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

“…Since then synthetic efforts have led to the crystallization and characterization of several types of PLY derivatives by chemical modification [16][17][18][19][20][21][22][23]. Heteroatom functionalization of neutral radicals such as thio-substituted PLY [24][25][26], azaphenalenyls [12,27,28], and bisphenalenyl boron complexes [29,30] have played a crucial role in the recent progress in phenalenyl chemistry and the important developments in heterocyclic radicals [31][32][33][34][35].…”

“…The results are best interpreted in terms of a resonating valence bond (RVB) ground state [29,30,47,49,50] for this compound and this description is also applicable to the π-chain structures found for compounds 8, 9, 19, and 20 [29,51] which essentially crystallize as infinitely repeating forms of the π-dimers 1 and 3. These compounds were also stable in their highly symmetric 1D structures and gave rise to some of the highest conductivities attained for this class of compounds: σ RT = 0.…”