Chirality Driven Metallic versus Semiconducting Behavior in a Complete Series of Radical Cation Salts Based on Dimethyl-Ethylenedithio-Tetrathiafulvalene (DM-EDT-TTF)

Enantiopure (S,S) and (R,R) dimethyl-ethylenedithio-tetrathiafulvalene
(DM-EDT-TTF) 1 donors are synthesized by cross coupling
followed by decarboxylation reactions. In the solid state the methyl
groups are arranged in axial positions within sofa-type conformation
for the six-membered rings. Crystalline radical cation salts formulated
as [(S,S)-1]2PF6, [(R,R)-1]2PF6, and [(rac)-1]2PF6 are obtained by electrocrystallization.
When the experiment is conducted with enantioenriched mixtures both
enantiopure and racemic phases are obtained. The monoclinic enantiopure
salts, containing four independent donors in the unit cell, show semiconducting
behavior supported by band structure calculations of extended Hückel
type. The racemic salt contains only one independent donor in the
mixed valence oxidation state +0.5. Under ambient pressure the racemic
material is metallic down to 120 K, while an applied pressure of 11.5
kbar completely suppresses the metal–insulator transition.
Band structure calculations yield an open Fermi surface, typical for
a pseudo-one-dimensional metal, with unperfected nesting, thus ruling
out the possibility of charge or spin density modulations to be at
the origin of the transition. Raman spectroscopy measurements, in
agreement with structural analysis at 100 K, show no indication of
low-temperature charge ordering in the racemic material at ambient
pressure, thus suggesting Mott-type charge localization for the observed
metal–insulator transition.