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

Abstract

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 σrt = 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.