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.
Charge-assisted halogen bonding is unambiguously revealed from structural and electronic investigations of a series of isostructural charge-transfer complexes derived from iodinated tetrathiafulvalene and tetracyanoquinodimethane derivatives, (EDT-TTFI2)2(TCNQF(n)), n=0-2, which exhibit variable degrees of ionicity. The iodinated tetrathiafulvalene derivative, EDT-TTFI2, associates with tetracyanoquinodimethane (TCNQ) and its derivatives of increasing reduction potential (TCNQF, TCNQF2) through highly directional C-I⋅⋅⋅N≡C halogen-bond interactions. With the less oxidizing TCNQ acceptor, a neutral and insulating charge-transfer complex is isolated whereas with the more oxidizing TCNQF2 acceptor, an ionic, highly conducting charge-transfer salt is found, both of 2:1 stoichiometry and isostructural with the intermediate TCNQF complex, in which a neutral-ionic conversion takes place upon cooling. A correlation between the degree of charge transfer and the C-I⋅⋅⋅N≡C halogen-bond strength is established from the comparison of the structures of the three isostructural complexes at temperatures from 300 to 20 K, thus demonstrating the importance of electrostatics in the halogen-bonding interaction. The neutral-ionic conversion in (EDT-TTFI2)2(TCNQF) is further investigated through the temperature dependence of its magnetic susceptibility and the stretching modes of the C≡N groups.
The sensitivity of electronic properties of organic conductors to minute modifications of their solidstate structure is investigated here within BEDT-TTF (ET) salts with organic bis-sulfonate anions, where specific hydrogen bonds between water molecules and sulfonate moieties are shown to dynamically control the organic slabs' electronic structure. While the mixed-valence, 2,6-naphthalene-bis-(sulfonate) salt, (ET) 4 (NBS)•4H 2 O, exhibits a charge order state already at room temperature, the corresponding salt with the 2,6-anthracene-bis(sulfonate) dianion, formulated as (ET) 4 (ABS)•4H 2 O, is metallic at RT and exhibits a metal− insulator transition at T MI = 85 K. The origin of the MI transition is revealed from a combination of temperature-dependent spectroscopic (Raman) measurements, X-ray structure elucidations (from 300 to 15 K), and theoretical investigations, demonstrating that the charge disproportionation observed below T MI is associated here with the progressive switching of bifurcated OH•••O hydrogen bonds between the sulfonate moieties of the anion and the trapped water molecules. These movements within the anion layer are transmitted through weaker C−H•••O interactions to the two A and B donor molecules, modifying the details of the overlap interactions within AA and BB pairs and opening a gap in the band structure.
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