PHYSICAL-PROPERTIES OF MIXED LANGMUIR-BLODGETT CONDUCTING FILMS BASED ON A TETRATHIAFULVALENE DERIVATIVE

Electroactive molecules such as (ethylenedithio)bis(octadecylthio)tetrathiafulvalene [EDTTTF(SC18)2] are useful molecular blocks for building up Langmuir-Blodgett conducting films when they are mixed with a fatty acid such as behenic acid and then doped with iodine vapors. To understand the molecular organization in these layers, we have investigated the IR electronic and vibrational absorption spectra, the resonance Raman scattering of the iodine species and the dc electrical conductivity as a function of the fatty acid content, and the number of deposited layers. These investigations have led us to propose a model of heterogeneous mixed film where clusters of EDTTTF(SC18)2 are surrounded by fatty acid bilayers, as demonstrated by using a 3D percolation theory. Inside a conducting cluster we have shown that the iodine intercalation process occurs in two stages: The insertion process lends to a complete oxidation of the TTF-type molecules, which form charged dimers associated with I3-ions, without any noticeable structural modification. A thermally activated process leads to a structural modification, where regular chains of triiodide are associated with an array of mixed-valence EDTTTF(SC18)2 stacks. Besides using a vibronic model which considers both electron-molecular vibrations and electronic correlations in these systems, we have been able to interpret quantitatively the optical absorption spectra. For that purpose we have used a specific calculation which takes account of the multireflections always present in thin films. We have shown that we have a mixed-valence system with its own molecular organization, which can be used as a component in a chemical sensor or perhaps for nonlinear optical devices.