) at room temperature and the band calculation suggests that they are band insulator.

In this study, we synthesized 2-(4-chloro-5-methyl-1,3-dithiol-2-ylidene)-5-(4,5-dimethyl-1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene (ClMe3-TTP). Results show that the cation radical salts of ClMe3-TTP with PF6− and AsF6− anions show high conductivities (101 S cm−1) at room temperature and semiconducting behavior with low activation energy (24–27 meV). A crystal structure analysis of the PF6− and AsF6− salts reveals a 3:1 donor–anion ratio of these salts. The ClMe3-TTP molecules form one-dimensional columns along the a axis, and tape-structures, connected by the side-by-side S···S interactions, also form along the a axis. The calculated intracolumn and intratape overlap integrals are relatively large; however, the overlap integrals between the one-dimensional columns and the tapes are approximately 1/2–1/5 the value of the intracolumn and intratape overlap integrals. The band calculations suggest that they have one-dimensional electronic structures, indicating that they would comprise a Mott insulator.

The electron donor 2‐isopropylidene‐1,3‐dithiolo[4,5‐d]‐4,5‐ethylenediselenotetrathiafulvalene (3) provides two new molecular conductors, (3)2[M(CN)2] (M = Ag and Au). These salts crystallize in the triclinic P$\bar {1}$ space group. The donor sheet structure of these salts is the so‐called β‐type molecular array, in which the donor molecules form a head‐to‐tail dimer. The formation of considerable dimerization and the 2:1 donor–anion ratio result in an effectively half‐filled electronic nature. These salts show metallic conducting behavior from room temperature. The metallic conducting behavior of (3)2[Ag(CN)2] was retained down to 8 K; however, the resistivity of (3)2[Au(CN)2] increased at around 50 K. The magnetic susceptibility of (3)2[Au(CN)2] suggests that the strength of the electron correlation for this salt is enhanced below 50 K.

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.