Structures and transport properties of new molecular conductors based on TMEO-ST-TTP

Several radical cation salts based on TMEO-ST-TTP (2-[4,5-bis(methylthio)-1,3-diselenol-2-ylidenel-5-(4,5-ethylendioxy-1,3- dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene) have been prepared. X-ray structure analyses of (TMEO-ST-TTP)(2)X (X = PF6, AsF6, TaF6) reveal that they have the so-called beta-type array of donors with dimerized stacks. A tight-binding band calculation suggests that the present salts have a quasi-one-dimensional Fermi surface. On the other hand (TMEOST-TTP)Au(CN)(2) has strongly dimerized one-dimensional electronic structure. (TMEO-ST-TTP)(2)X (X=PF6, AsF6 TaF6) exhibit a high conductivity of sigma(rt)=10(0)-10(1) S cm(-1) and retain metallic conductivity down to 4.2K, while the tetrahedral (ReO4-) and linear (I-3(-) and Au(CN)(2)(-)) anions give semiconductors. Thermoelectric power of (TMEO-STTTP)(2)PF6 shows T-linear temperature dependence characteristic of a metal. (C) 2002 Elsevier Science (USA).