Synthesis and characterization of the neutral [pi]-radical molecular conductors

Abstract

1,3,2- and 1,2,3-dithiazolyl (DTA) radicals have been investigated as building blocks for neutral radical conductors (NRCs). Several new derivatives have been prepared and structurally characterized; their ESP, CV and transport properties have been interpreted in terms of theoretical calculation results.

The electronic properties of 1,3,2-DTA radicals are strongly influenced by the electron-withdrawing power of the fused aromatic residues on 4,5-positions. The shift in reduction potentials from NDTA to TDTA in over 1 V and more importantly, the redox potential which corresponds to the disproportionation energy of the radicals also decreases. This trend follows the theoretical prediction and can be related to a lower Coulombic barrier to charge transport between radical centers in the solid state. However, charge correlation effects still outweigh the electronic stabilization provided by interannular orbital overlap. The large amount of free spins are trapped in the radical molecules and the materials remain Mott insulators with pressed pellet conductivities <10-6 S cm-1.

The solid state packing patterns of 1,3,2-DTA radicals are clearly related to their molecular structures. While NDTA favors a herringbone packing, replacement of the structure-maker CH by N, as in QDTA and TDTA, leads to slipped B stack structures. The dimerization phase transition of 1,3,2-TDTA at low temperature (around 150 K) reveals the weakness of the interaction within the radical stacks. A multiple A tectonic plate rt-slippage mechanism was proposed to account for the interconversion of the two phases.

In comparison to their 1,3,2-counterparts, 1,2,3-DTA radicals are inherently better candidates for the design of neutral radical conductors. Guided by the theoretical calculations, many new 1,2,3-DTA derivatives have been explored. In the solid state, 1,2,3-TDTA adopts a slipped B-dimer stack in which the dimers are coupled in a head-to-tail arrangement. The Coulombic barrier is reduced by the improved charge correlation effect and the radical exhibits the highest room temperature conductivity (1 x 10-4S cm-1) yet obtained for a single-component sulfur based molecular material. Moreover, the decoupled spins at elevated temperatures contribute to an increased conductivity.

A range of 1,2,3-DTA derivatives based on the quinoxaline framework have been synthesized and investigated. Three QDTA radicals were purified and spectroscopically characterized.