Formation of two-dimensional metals by weak intermolecular interactions based on the asymmetric EDO-TTF derivatives

Abstract

Two mono-substituted EDO-TTF derivatives, 4,5-ethylenedioxy-4′-methyl-TTF (MeEDO-TTF, 2) and 4,5-ethylenedioxy-4′-methylthio-TTF (MeSEDO-TTF, 3), were synthesized in the moderate yields. These two donors showed similar electronic structures of the π-electron systems to that of the parent molecule EDO-TTF, according to the cyclic voltammetry and UV-vis spectroscopy. Despite their different compositions, the radical cation salts of these two donors exhibited similar optical spectra with a low-lying absorption band at around 3.3 × 10³ cm⁻¹, which was consistent with their good conductivities at room temperature. The transport properties of these salts ranged from stable metals, metal–insulator transition systems, to semiconductors. Most of the radical cation salts showed Pauli paramagnetic behavior at around room temperature. The crystal structures of the stable metallic salts, (MeEDO-TTF)₂X (X = BF₄, ClO₄), exhibited a novel donor packing pattern, and the calculated band structures showed two-dimensional feature, consistent with their transport properties.