**Harukazu Yoshino 1,\*, Yoshiki Iwasaki 2, Rika Tanaka 3, Yuka Tsujimoto <sup>1</sup> and Chiaki Matsuoka <sup>1</sup>**


Received: 2 November 2020; Accepted: 5 December 2020; Published: 8 December 2020

**Abstract:** Three novel organic conductors (TMTSF)8(I3)5, (TMTSF)5(I3)2, and (TMTSF)4(I3)4·THF (THF = tetrahydrofuran) were synthesized and their crystal structures were characterized by X-ray diffraction analyses, where TMTSF denotes tetramethyltetraselenafulvalene. The crystals of both the (TMTSF)8(I3)5 and (TMTSF)5(I3)2 are composed of one-dimensional stacks of TMTSF trimers separated by TMTSF monomers. The crystal of the (TMTSF)4(I3)4·THF is composed of the TMTSF tetramers and I− <sup>3</sup> tetramers; and regarded as the elongated rock-salt structure. The electrical conductivity of the (TMTSF)8(I3)5 and (TMTSF)5(I3)2 is about 60 and 50 S·cm−<sup>1</sup> at room temperature, respectively. The electrical resistivity of (TMTSF)8(I3)5 is weakly metallic below room temperature and rapidly increases below 88 and 53 K on cooling suggesting two possible phase transitions. The electrical resistivity of (TMTSF)5(I3)2 is semiconducting below room temperature but shows an anomaly around 190 K, below which the activation energy becomes small. The application of hydrostatic pressure up to 1.7 GPa do not change these behaviors of (TMTSF)8(I3)5 and (TMTSF)5(I3)2 very much. A method to evaluate the non-integer valence of crystallographically independent TMTSF molecules is developed by using the DFT (density-functional-theory) and MP2 (Hartree–Fock calculations followed by Møller–Plesset correlation energy calculations truncated at second order) calculations. It is shown that the method gives the valence of the TMTSF molecules of the I3 salts consistent with their electrical properties.

**Keywords:** (TMTSF)8(I3)5; (TMTSF)5(I3)2; (TMTSF)4(I3)4·THF; organic conductor; crystal structure; electrical resistivity; high pressure; DFT; MP2
