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 = 10¹ to 10² S cm^-1 on a single crystal. The anion shape and size of TMTTP conductors modulate the electrical properties. The PF₆^- and AsF₆^- salts exhibit metallic conductivity down to 10 K, while the ReO₄^- and AuI₂^- salts show metal-to-insulator (MI) transition at 126 and 11 K, respectively. Single-crystal X-ray structure analysis of (TMTTP)₂X (X = PF₆, AsF₆, BF₄, and ReO₄) and (TMTTP)₃AuI₂ reveals that the donor packing motif is categorized as the so-called β-type. The tight-binding band calculations of (TMTTP)₂X (X = ReO₄, BF₄, PF₆, and AsF₆) suggest these salts are characterized by quasi-one-dimensional Fermi surfaces. The band calculation based on X-ray structure analysis of (TMTTP)₂ReO₄ at 100 K demonstrates that the MI transition of this salt is associated with the charge ordering with the zigzag stripe pattern.