title: Order versus disorder in chiral tetrathiafulvalene-oxazoline radical-cation salts: structural and theoretical investigations and physical properties.
abstract: Electrocrystallization experiments with the chiral ethylenedithio-tetrathiafulvalene-methyl-oxazoline (EDT-TTF-OX) donors (R)-, (S)-, and (rac)-1 have provided two series of mixed-valence salts with the PF(6) (-) and [Au(CN)(2)](-) anions. Within each series the cell parameters are the same for the three R, S, and rac compounds, except for the space group, which is centrosymmetric triclinic P$\bar 1$ for the racemic forms and noncentrosymmetric P1 for the enantiopure salts. In the racemic salt [(rac)-1](2)PF(6) the two enantiomers crystallize disordered on the same crystallographic site with a site occupational factor of 0.6:0.4, whereas this type of disorder is not possible in the enantiopure salts. Both s-cis and s-trans conformations, when taking into account the mutual orientation of the TTF and oxazoline moieties, are present in this first series. In sharp contrast, in the series of salts [1](2)[Au(CN)(2)], only the s-trans conformation is observed with no structural disorder. Theoretical calculations at the DFT level of theory revealed a very small energy difference between the two stable planar s-cis and s-trans conformations, which are both energy minima in either neutral or oxidized states. Single-crystal conductivity measurements showed metallic-like behavior for all the salts down to 220-250 K with a smooth increase in resistivity at lower temperatures. The conductivity at room temperature is 5 S cm(-1) for [(rac)-1](2)PF(6), in which disorder was observed, whereas for [(R)-1](2)PF(6) and [(S)-1](2)PF(6) the average value is around 100 S cm(-1). In the second series of salts the conductivity at room temperature is 125-130 S cm(-1) for [(rac)-1](2)[Au(CN)(2)] and [(R)-1](2)[Au(CN)(2)]. Extended Hückel band structure calculations revealed identical features for the three salts of the [1](2)[Au(CN)(2)] series and are consistent with the electronic structures of quasi-one-dimensional conductors.
authors.affiliation: Université d'Angers, CNRS, Laboratoire de Chimie et Ingénierie Moléculaire, CIMA UMR 6200, UFR Sciences, Bât. K, 2 Bd. Lavoisier, 49045 Angers, France.
authors.forename: Augustin M
authors.initials: AM
authors.lastname: Madalan
authors.forename: Céline
authors.initials: C
authors.lastname: Réthoré
authors.forename: Marc
authors.initials: M
authors.lastname: Fourmigué
authors.forename: Enric
authors.initials: E
authors.lastname: Canadell
authors.forename: Elsa B
authors.initials: EB
authors.lastname: Lopes
authors.forename: Manuel
authors.initials: M
authors.lastname: Almeida
authors.forename: Pascale
authors.initials: P
authors.lastname: Auban-Senzier
authors.forename: Narcis
authors.initials: N
authors.lastname: Avarvari