Old Donors for New Molecular Conductors: Combining TMTSF and BEDT-TTF with Anionic (TaF6)1−x/(PF6)x Alloys

Abstract

Tetramethyl-tetraselenafulvalene (TMTSF) and bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) are flagship precursors in the field of molecular (super)conductors. The electrocrystallization of these donors in the presence of (n-Bu4N)TaF6 or mixtures of (n-Bu4N)TaF6 and (n-Bu4N)PF6 provided Bechgaard salts formulated as (TMTSF)2(TaF6)0.84(PF6)0.16, (TMTSF)2(TaF6)0.56(PF6)0.44, (TMTSF)2(TaF6)0.44(PF6)0.56 and (TMTSF)2(TaF6)0.12(PF6)0.88, together with the monoclinic and orthorhombic phases δm-(BEDT-TTF)2(TaF6)0.94(PF6)0.06 and δo-(BEDT-TTF)2(TaF6)0.43(PF6)0.57, respectively. The use of BEDT-TTF and a mixture of (n-Bu4N)TaF6/TaF5 afforded the 1:1 phase (BEDT-TTF)2(TaF6)2·CH2Cl2. The precise Ta/P ratio in the alloys has been determined by an accurate single crystal X-ray data analysis and was corroborated with solution 19F NMR measurements. In the previously unknown crystalline phase (BEDT-TTF)2(TaF6)2·CH2Cl2 the donors organize in dimers interacting laterally yet no organic-inorganic segregation is observed. Single crystal resistivity measurements on the TMTSF based materials show typical behavior of the Bechgaard phases with room temperature conductivity σ ≈ 100 S/cm and localization below 12 K indicative of a spin density wave transition. The orthorhombic phase δo-(BEDT-TTF)2(TaF6)0.43(PF6)0.57 is semiconducting with the room temperature conductivity estimated to be σ ≈ 0.16–0.5 S/cm while the compound (BEDT-TTF)2(TaF6)2·CH2Cl2 is also a semiconductor, yet with a much lower room temperature conductivity value of 0.001 to 0.0025 S/cm, in agreement with the +1 oxidation state and strong dimerization of the donors.

Organic Molecular Conductors Based on Tetramethyl-TTP: Structural and Electrical Properties Modulated by the Anion Size and Shape

“…) , and (TMTCF) 2 X (X = ClO 4 , PF 6 , AsF 6 , etc. ), − where TMTCF is TM-tetrachalcogenafulvalene (C = S or Se). In the stack, the donors are crystallographically dimerized (interactions a 1 and a 2).…”

A New Charge-Ordered Molecular Conductor: κ-(BEDT-TTF)2K+(18-crown-6)[CoII(NCS)4]∙(H2O)

“…The salt demonstrates semicon-ductive type of R(T) dependence from ambient temperature (297 K) down to measured temperatures. Room temperature resistivity is 3 Ω•cm that is a usual value for semiconductive ET salts with formal +0.5 charge per BEDT-TTF radical cation (e.g., recent δ 0 -(BEDT-TTF) 2 (TaF 6 ) 0.43 (PF 6 ) 0.57 has ρ = 2-6 Ω•cm [90], and α -(BEDT-TTF) 2 [Cr(C 2 O 4 ) 2 (2,2bipy)]•CHCl 2 CH 2 Cl has ρ = 2 Ω•cm [91]). However, the measured value for the charge gap retrieved from fitting the linear part of the Arrhenius plot was 0.57 eV (Figure 9, inset).…”

Solid-solution (alloying) strategies in crystalline molecular conductors

“…67 Solution NMR has been reported on one recent example for (TMTSF)2(TaF6)1−x(PF6)x solid solutions involving TaF6 − and AsF6 − anions thanks to 19 F NMR measurements of solutions prepared by dissolving small amounts of each material in DMSO-d6. 76 The integration of the signal of PF6 − , appearing as a doublet at −70 ppm with respect to the signals of fluorine atoms connected to Ta(V) provided the ratio TaF6 − /PF6 − in the bulk material. Satisfyingly, the ratios estimated by this method were in good agreement with those obtained by the refinement of the single crystal diffraction data.…”