The Synthesis, Crystal Structure, Electrical Conductivity and Band Electronic Structure of (BPDT-TTF)₂lCl₂

Abstract

(BPDT-TTF)₂ICl₂ is a synthetic metal salt composed of the electron donor molecule bis(propylenedithio)tetrathiafulvalene (BPDT-TTF, C₁₂H₁₂S_k) and the linear iododichloride anion, ICl₂ ⁻. The crystal structure of (BPDT-TTF)₂ICl₂, determined at 298 K and 120 K, closely resembles those of the organic metals and semiconductors based on bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF, C₁₀H₈H₈), in that all of these materials contain sheetlike donor molecule networks sandwiched between anion layers. In contrast to the (BEDT-TTF)₂X salts, however, there is a total absence of S···S contact distances less than ∼3.6 Å (the van der Waals radius sum) within the donor-molecule network of (BPDT-TTF)₂ICl₂. In addition, each stack of BPDT-TTF molecules is also strongly dimerized. The lack of short S···S contact distances at both 298 K and 120 K appears to be a consequence of the bulky propylene groups of BPDTTTF compared to the smaller ethylene groups of the BEDT-TTF molecule. Based upon four-probe a.c. resistivity measurements, (BPDT-TTF)₂ICl₂ is a semiconductor. From our tight-binding band calculations on (BPDT-TTF)₂ICl₂, and the fact that this salt is nonmetallic, it is concluded that each (BPDT-TTF)₂ ⁺ dimer cation carries an unpaired electron. The lack of short intermolecular S···S contacts, and the strong dimerization in each BPDT-TTF stack, is considered to be a consequence of the crystal packing of the relatively bulky propylene groups of the BPDT-TTF molecule around each ICI₂ ⁻ anion resulting in semiconductive electrical properties.

Keywords:

• organic conductors
• crystal structure
• electrical properties
• band electronic structure
• BPD T- TTF