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

The tetramethyl derivative of bis-fused tetrathiafulvalene, 2,5-bis(4,5-dimethyl-1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene (TMTTP), has been successfully synthesized. Most of the radical cation salts consisting of TMTTP feature a high electrical conductivity of σ rt = 10 1 to 10 2 S cm -1 on a single crystal. The anion shape and size of TMTTP conductors modulate the electrical properties. The PF 6 - and AsF 6 - salts exhibit metallic conductivity down to 10 K, while the ReO 4 - and AuI 2 - salts show metal-to-insulator (MI) transition at 126 and 11 K, respectively. Single-crystal X-ray structure analysis of (TMTTP) 2 X (X = PF 6 , AsF 6 , BF 4 , and ReO 4 ) and (TMTTP) 3 AuI 2 reveals that the donor packing motif is categorized as the so-called β-type. The tight-binding band calculations of (TMTTP) 2 X (X = ReO 4 , BF 4 , PF 6 , and AsF 6 ) suggest these salts are characterized by quasi-one-dimensional Fermi surfaces. The band calculation based on X-ray structure analysis of (TMTTP) 2 ReO 4 at 100 K demonstrates that the MI transition of this salt is associated with the charge ordering with the zigzag stripe pattern.