Structures and Conducting Properties of Molecular Conductors Based on Dimethyl-Substituted DTDA-TTP and DTDH-TTP

Abstract

Abstract Dimethyl substituted donors, 2-(4,5-dimethyl-1,3-dithiol-2-ylidene)-5-(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene (DMDT-DA-TTP) and 2-(4,5-dimethyl-1,3-dithiol-2-ylidene)-5-(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene (DMDT-DH-TTP), were successfully synthesized, and (DMDT-DA-TTP)2X (X− = PF6−, AsF6−, and SbF6−) and (DMDT-DH-TTP)2X (X− = PF6− and AsF6−) were prepared. All the DMDT-DA-TTP and DMDT-DH-TTP salts adopt β-type molecular packing with a head-to-tail molecular stacking. Although the overlap modes of donor molecules in the DMDT-DA-TTP and DMDT-DH-TTP salts are similar, the degree of dimerization of donor molecules is quite different. That is, electronically strong dimerization of the DMDT-DA-TTP molecules was assessed in the salts, while the DMDT-DH-TTP molecules have electronically almost uniform stacking in the salts. A tight-binding band calculation suggested that all the DMDT-DA-TTP salts have effectively half-filled energy bands with narrow bandwidths of 0.39–0.40 eV, while the bandwidths of DMDT-DH-TTP salts (0.96 eV) are relatively wide due to the lack of an energy gap. The calculated Fermi surfaces of all the salts are quasi-one-dimensional. In correspondence with the different band structures, all the DMDT-DA-TTP salts exhibited semiconducting behavior derived from the Mott-insulating nature. In contrast, both the DMDT-DH-TTP salts showed metallic temperature dependence down to 5–7 K.