Structure of two polymorphs of the TTF-TCNE charge-transfer complex and the degree of ionicity

Abstract

Charge-transfer (CT)π complexes between tetrathiafulvalene (TTF, donor) and tetracyanoethylene (TCNE, acceptor) have been studied. Two polymorphs (α and β) were obtained depending on the solvent used. X-Ray analysis has shown that both polymorphs are monoclinic with space group P2₁/n, are present in dimeric form [(TTF)₂(TCNE)₂] and possess very similar molecular dimensions. The crystal structure of both polymorphs is composed of mixed stacks of [TTF]₂ and [TCNE]₂ dimers in a (TTF)₂—(TCNE)₂—(TTF)₂—(TCNE)₂ one-dimensional arrangement. The interaction between TTF and TCNE is very strong in both polymorphs, as indicated by the low interplanar spacing (α phase = 3.13 Å, β phase = 3.15 Å) and low dihedral angle (α phase = 1.26°, β phase = 2.20°) but they differ for the hydrogen-bonding network and lamellar sheets. A method based on the C—S bond lengths provides for these two CT complexes a degree of ionicity or charge transfer (δ) of 0.54 e, in good agreement with that obtained using IR frequencies (δ= 0.5 ± 0.1 e) and very similar to that of TTF–TCNQ (δ= 0.59 e). Nevertheless, in contrast with TTF–TCNQ, these polymorphs exhibit a very high resistivity (≈ 10⁹Ω cm) that may be explained by the dimerization process and by the mixed-stack formation.