Three thienoquinonoid-extended analogues of bis(alkylenedithio)tetrathiafuIvalenes, which are symmetric and unsymmetric conjugation-elongated donors, are synthesized and TCNQ complexes of these donors are proved to exhibit a metallic temperature dependence or a fairly high room temperature conductivity even when measured on compressed powder samples.Bis(ethy1enedithio)tetrathiafulvalene (BEDT-TTF) has been recognized as the most promising synthetic component of organic superconductors owing to its ability to produce twodimensional conducting layers with high superconducting transition temperatures (Tc).2 Recently, it has been pointed out that the T,s of BEDT-TTF superconductors are proportional to the volume of the most effective space (V,,,) for the carrier distribution in the donor layer, which is composed of the c 6 s S skeleton and intra-donor-layer overlaps between the atomic orbitals of the c 6 s S ~c-systems.~ Donor molecules with a more elongated n-system than that of BEDT-TTF have been required3 in order to increase the Vmes and then to obtain high T, organic superconductors. 2,5-bis(4,5-Ethylenedithio-1,3-dithiol-2-ylidene)-2,5-dihydrothiophene (BEDT-BDTT, l), that is the 2,5-thienoquinonoid-extended analogue of BEDT-TTF, would be a promising donor to satisfy such a requirement, since the large CloS9 moiety of BEDT-BDTT may contribute effectively to increase the V,,. in the conducting donor-layer of its metallic salts. With this motivation, herein we report the BEDT-lTF 1 BEDT-BDlT; m = n = 2 2 EP-BDTT; m = 2, n = 3 3 BPDT-BDlT; m = n = 3 4
2,5-Bis(1,3-dithiol-2-ylidene)-2,5-dihydroselenophene (BDTS) and its ethylenedithio (EDT-BDTS) and bis(ethylenedithio) (BEDT-BDTS) derivatives, novel extended donors with a central selenium atom, have been synthesized by convenient methods. Of these, EDT-BDTS and BEDT-BDTS having outer chalcogen atoms afforded highly conducting molecular complexes with TCNQ (σ = 13 Scm−1 on a compressed pellet).
Conjugation-extended tetrathiafulvalene analogues, 2,5-bis(1,4-dithiafulven-6-yl)thiophene (2a), 2,5-bis[2,3-bis(methoxycarbonyl)-1,4-dithiafulven-6-yl]thiophene, and 2,5-bis(1,4-dithiafulven-6-yl)furan, involving an aromatic heterocyclic linking group have been synthesized by the Wittig condensation reaction of 2,5-thiophene- and 2,5-furandicarbaldehydes with 1,3-dithiol-2-ylidenetributylphosphorane and its 4,5-bis(methoxycarbonyl) derivative. The electrochemical redox properties of these compounds in solution have been investigated by cyclic voltammetry in order to evaluate the effect of the linking group on the donating ability and the thermodynamic stability of the radical cations. Whereas the radical cations are not so stable, complexes of 2a with 2,3-dichloro-5,6-dicyano-p-benzoquinone and iodine were found to show relatively high conductivities ((1.0—1.1) × 10−3 S cm−1) on a compressed pellet at room temperature.
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