A new family of Langmuir–Blodgett films of tetracyanoquinodimethane charge transfer salts: Pristine and iodine doped conducting films

A new family of semiamphiphilic tetracyanoquinodimethane (TCNQ) ionic salts of nonconjugated cations, namely octadecyl-dimethylsulfonium, octadecyl-methyl-ethyl-sulfonium, and octadecyl-trimethylphosphonium labeled a, b, and c, respectively, has been synthetized. Langmuir–Blodgett (LB) films are built up from these three compounds, which are mixed with octadecylurea in a molar ratio 1/1, in order to improve the stability of the film at the water surface. The structural properties of the transferred films are thoroughly investigated by linear dichroism in the IR and UV–visible ranges, together with ESR spectroscopy. In the films of the three compounds, the polar sheets contain (TCNQ− • )2 dimers, which are the largely prevalent species. The molecular planes of these dimers are found to be roughly parallel to the substrate. Moreover, a strong in-plane orientation of the dimers is found in films of compound c. This phenomenon is thought to be related to an overall flow orientation of rod-shaped crystallites in the floating film, during the high speed downstroke of the substrate across the film. Advantage is taken from the high and well-controlled thinness of LB films to make iodine vapor diffuse into the films and convert the precursor films into conducting ones. The iodination process leads for the three salts to a stable ternary conducting compound located in the polar planes of the LB assembly. Correlatively, it induces a strong molecular reorganization in the films: the TCNQ molecules stand on edge with their long molecular axis perpendicular to the substrate. In spite of this considerable rearrangement arising from iodination, we notice that the strong in-plane anisotropy observed in pristine films of c is retained in conducting films of the iodinated c compound. Beside these structural results, the IR spectra of conducting films are reminiscent of these of highly conducting, but not metallic-like, organic conductors. The conductivity of the films is estimated from their IR spectra and found to be 20–50 Ω−1 cm−1. However, dc conductivity measurements provide a value at least ten times lower than the one estimated from IR spectra. This behavior is likely related to the polycrystalline nature of the films whose macroscopic inhomogeneities are evidenced by scanning electron microscopy.