Fine Tuning of π-Stack Separation Distances of Semiquinone Radicals Affects Their Magnetic and Electric Properties

Abstract

Two salts of tetrachloro-(chloranil, Cl 4 Q) and tetrabromosemiquinone (bromanil, Br 4 Q) radical anions with an organic cation Nmethylpyridinium (N-MePy), displaying a variety of tunable electrical and magnetic properties, are characterized. These systems comprise π-stacks of equidistant semiquinone radical anions, without Peierls deformation and diamagnetic spin coupling. Both are stable up to 150 and 120 °C in air, respectively, and are semiconductors with conductivities of 10 −6 −10 −7 (Ω cm) −1 in the single crystals. The room-temperature concomitant crystallization of two polymorphs with different magnetic properties (antiferromagnetic and Peierls-distorted diamagnetic) has been observed for Cl 4 Q•N-MePy. The semiquinone radical organic salts represent a novel class of potential (multi)functional materials with tunable properties.

Partially Covalent Two-Electron/Multicentric Bonding between Semiquinone Radicals

“…Two types of semiquinone radical π-stacks: [27] a) diamagnetic stacks of closely interacting dimers and b) antiferromagnetic stacks of equidistant radicals. Short (< 3.…”

“…Perhalogenated and cyano-substituted semiquinone crystals are easily obtained and are often stable in air and at room temperature, [27][28][29] and they are extensively used in the design and synthesis of organic molecular magnets and (semi)conductors. [12,27] It is possible to tune their magnetic properties and conductivity by crystal engineering. [27] In these systems, the crucial component is the conjugated π-electron system; interactions between π-systems of the radicals play the key role in determining magnetic and electrical properties.…”

“…[12,27] It is possible to tune their magnetic properties and conductivity by crystal engineering. [27] In these systems, the crucial component is the conjugated π-electron system; interactions between π-systems of the radicals play the key role in determining magnetic and electrical properties. [28,[30][31][32][33][34][35][36][37] Two types of stacks of semiquinone radicals have been described: [27] (1) stacks with alternating short (< 3.3 Å) and long (> 3.5 Å) inter-planar separations comprising dimers with diamagnetically coupled spins and (2) stacks of equidistant radicals with antiferromagnetic coupling (Figure 1).…”

Iodide···π Interactions of Perhalogenated Quinoid Rings in Co-crystals with Organic Bases

“…While preparing solid-state semiquinone systems, 17,[19][20][21][22] we serendipitously obtained a series of co-crystals of tetrachloro-and tetrabromoquinone (Cl4Q and Br4Q, respectively, Scheme 1) with iodide salts of organic cations (mostly derivatives of N-methylpyridinium, Scheme 2), in which close contacts between the quinoid ring and the iodide anion occur almost without exception.…”

Alkali Salts of Nitranilic and Cyanochloranilic Acids

“…Quinones are known to form relatively stable semiquinone radicals; [24][25][26] in some complexes of chloranilic acid (3,6-dichloro-2,5-dihydroxyquinone) charge transfer from the metal ion to the quinoid ring was observed. [5,27] Electron-withdrawing substituents also enhance acidity of hydroxyl groups.…”