Spectroscopic properties of conducting TCNQ Langmuir-Blodgett films obtained via a homodoping process

We report the preparation and the spectroscopic properties of Langmuir-Blodgett films obtained by mixing in the same monolayer a polar amphiphilic octadecyl tetracyanoquinodimethane (Cj~TCNQ) and an ionic semi-amphiphilic (Cj~s~dithiolium-TCNQ compound. A cluster approach is used for the interpretation of the electronic charge transfer band and of the vibronic features in infrared as a result of electron-molecular vibration (EMV) linear coupling. Different assumptions used previously for an estimate of EMV coupling constants from infrared spectra are analyzed and a new formula relating positions of vibronic bands in absorption and EMV coupling constants is proposed. Experimental and theoretical absorption curves are compared and discussed. It allows us to conclude in favor of the presence of mixed-valence dimers which are at the origin of the conduction