β”-(CNB-EDT-TTF)4BF4; Anion Disorder Effects in Bilayer Molecular Metals.

The preparation and characterization of new salts based on the dissymmetrical TTF derivative CNB-EDT-TTF (cyanobenzene-ethylenedithio-tetrathiafulvalene) and BF₄⁻ anions, are reported. Depending on the electrocrystallization conditions salts with different stoichiometries, (CNB-EDT-TTF)BF₄ and <italic>β</italic>”-(CNB-EDT-TTF)₄BF₄, can be obtained. The 1:1 salt is an electrical insulator isostructural to the ClO₄ analogue previously described. The 4:1 salt is a new member of the family of 2D metals of this donor with different small anions X, (CNB-EDT-TTF)₄X, characterized by a bilayer arrangement of the donors and it was obtained in a monoclinic polymorph with a <italic>β</italic>”-type donor packing pattern. The small anions in this compound are severely disordered between the donor bilayers, which present slightly larger lattice parameters than the isostructural ClO₄ analogue. Both electrical conductivity and thermoelectric power measurements in single crystals denote metallic properties as predicted by electronic band structure calculations. As a consequence of the anion disorder the metallic regime of the electrical conductivity denotes electronic localization effects with a progressive increase of resistivity below ~25 K. Because of the larger lattice parameters the intermolecular interactions and electronic bandwidth are decreased compared to other (CNB-EDT-TTF)₄X salts. The large and positive thermoelectric power <italic>S</italic> of this compound (~110 μV/K in the range 100–330 K) and its electrical conductivity <italic>σ</italic> = 20 S/cm at room temperature lead to a power factor <italic>S</italic>²<italic>σ</italic> = 24 μW/K²m, quite large among molecular conductors, placing these compounds as potential candidates for thermoelectric materials. [ABSTRACT FROM AUTHOR]