Conducting Langmuir-Blodgett films based on semiamphiphilic tetracyanoquinodimethane salts: mechanism of the homodoping process

Abstract

The homodoping process is a new approach to obtain a conducting Langmuir-Blodgett (LB) film in which a valence system is realized thanks to a controlled ratio between ionic and neutral compounds. In order to generalize this technique and to understand its formation mechanism we carried out a series of experiments with 3-methylthio, 4–5bis (octadecylthio) dithiolium salts with different anions (X = I −, TCNQ −, TCNQF 4 − where TCNQ denotes tetracyanoquinodimethane). The LB films were obtained from a Langmuir monolayer containing one of these salts and the neutral molecule C 18TCNQ. Two series of experiments were performed and analysed through IR spectroscopy techniques, as follows. (i) Starting from a mixture of 1–2 dithiolium-X:C 18TCNQ (1:2) we showed that the LB films obtained are independent of the anion used. As revealed by the IR absorption spectra which show both charge transfer band and TCNQ-type vibronic modes, the same final material is obtained. A chemical reaction at the gas-water interface giving rise to a mixed-valence cluster such as (C 18TCNQ) 2 - has to be assumed. (ii) Using a special in situ IR technique, polarization modulation Fourier transform IR spectroscopy on surfaces, we carried out a comparative series of experiments on pure H 2O and D 2O as subphases. These in situ spectra, obtained at different surface pressures, allow us to demonstrate that the mixed-valence clusters are spontaneously formed on the water surface through the presence of vibronic modes attributed to (C 18TCNQ) 2 dimers. We therefore conclude that a specific redox process is occurring at the gas-water interface to give rise to these conducting monolayers.