Nature and Origin of Stable Metallic State in Organic Charge-Transfer Complexes of Bis(ethylenedioxy)tetrathiafulvalene

Abstract

The complex formation of bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF) with 29 organic electron acceptors and six organic anions yielded 37 charge-transfer (CT) complexes, about three-quarters of which exhibited metallic behavior. The BEDO-TTF molecule proves to be an excellent source for stable metals irrespective of the structure, shape, size, and electron affinity of counter components. The crystal structure of metallic BEDO-TTF complexes indicates a strong aggregation of donor molecules into a two-dimensional (2D) layered structure by the aid of both intermolecular CH···O and side-by-side heteroatom contacts. Band calculations based on an extended Hückel method demonstrate that the sulfur atoms of the BEDO-TTF molecule are dominant for 2D intermolecular overlap. The resulting 2D electronic bands with large band width serve to stabilize the metallic state even at low temperatures. A comparison between the structural properties of complexes of BEDO-TTF and those of its sulfur analog BEDT-TTF (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) demonstrates that the oxygen atoms of the BEDO-TTF molecule not only enhance the intermolecular S··S atomic contacts due to its small size, but also determine donor packing through multiple weak hydrogen bondings. The nicely combined roles of these heteroatoms are the origins of the strongly stabilized metallic state in BEDO-TTF complexes without aid of heavier selenium and/or further addition of sulfur atoms on TTF moiety.

Development of Conductive Organic Molecular Assemblies: Organic Metals, Superconductors, and Exotic Functional Materials

“…This indicates that a stable and high-dimensional metallic state is realized by the self-assembling ability of the donor molecules (see also Fig. 14), [234][235][236][237][238][239][240][241][242][243] which was confirmed by structural analysis. A similar wide range of ÁE(DA) was observed for 2D CT complexes of BEDO-DBTTF.…”

“…3.1.4 Ionicity Diagram: Figure 18 presents the ionicity diagrams for a 1,6-dithiapyrene system by Nakasuji, 233 and BO, 234 EOET, 235 and HMTTeF systems [236][237][238] by Saito et al The solid V-shaped line represents Eqs. 19 and 20 for the TTF Á p-quinone system, where ÁE(DA) is used instead of (I D À E A ).…”

“…The conduction in these metallic complexes having alternating columns is explained by the soliton or domain-wall mechanism (see section 4.3.3). 243 4) The complexes residing far above the V-shaped line have a molecular packing that is disadvantageous for CT interactions (such as clathrate, Group E 234,235 ). In summary, a metallic band structure is realized when the system has the partial CT state and molecules form uniform segregated columns or layers.…”

“…234,[239][240][241][369][370][371][372] As a result, the BO complexes hardly exhibit any phase transition including the superconducting one (only two superconducting salts with T c 1:5 K were found, see Table 15). 373,374 The structural features of 2D BO complexes are described in sections 3.…”

New BEDT-TTF Radical Cation Salt with Mixed Anions: α'-[BEDT-TTF]2[CuBr2]0.4[CuCl2]0.6

“…As with other BEDT-TTF salts, calculated HOMOs of BEDT-TTF are characterized by the same signs on each sulfur atom [1,[17][18][19]. Calculated overlap integrals (S) between the HOMOs are listed in Table 3 with a schematic drawing of the donor arrangement.…”