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

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 CH···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.