Structural and Electrical Properties of Novel Molecular Conductors Based on Extended-TTF Donors BDT-TTP and IAnions

With the aim of developing organic conductors with new types of molecular arrangements, some organic conductors composed of elongated (pi) donors BDT-TTP (2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene) and simple I- anions were prepared by electrocystallization. The crystal of (BDT-TTP)3I has a very unique three-dimensional donor arrangement, where six BDT-TTP molecules are crystallographically independent. Despite the three-dimensional intermolecular short S···S network, the examination of C=C bond lengths of six BDT-TTP molecules indicates an inhomogeneous charge distribution, which is unfavorable for the formation of metallic bands. The resistivity measurements showed the system to be a semiconductor with a very small activation energy. On the other hand, the needle-shaped crystal has (beta)-type donor arrangement and 2 : 1 stoichiometry. The crystal of (beta)-(BDT-TTP)2I retains its metallic state down to 2 K. Though an open Fermi surface was obtained by tight-binding band calculation, the fairly strong two-dimensionality of the electronic structure was suggested.