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

Charge-transfer (CT) pi complexes between tetrathiafulvalene (TTF, donor) and tetracyanoethylene (TCNE, acceptor) have been studied. Two polymorphs (alpha and beta) were obtained depending on the solvent used. X-Ray analysis has shown that both polymorphs are monoclinic with space group P2(1)/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 (alpha phase = 3.13 Angstrom, beta phase = 3.15 Angstrom) and low dihedral angle (alpha phase = 1.26 degrees, beta phase = 2.20 degrees) 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 (delta) of 0.54 e, in good agreement with that obtained using IR frequencies (delta = 0.5 +/- 0.1 e) and very similar to that of TTF-TCNQ (delta = 0.59 e). Nevertheless, in contrast with TTF-TCNQ, these polymorphs exhibit a very high resistivity (approximate to 10(9) Omega cm) that may be explained by the dimerization process and by the mixed-stack formation.