Structural properties of mixed Langmuir-Blodgett films of EDTTTF(SC18)2and behenic acid

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Abstract
Molecular assemblies were studied at the air-water interface, as were neutral and oxidized Langmuir-Blodgett (LB) films in the mixed (1: n) molecular system of EDTTTF (SC18)2 (EDTTTF, ethylenedithiodioctadecylthiotetrathiafulvalene) and behenic acid (CH3(CH2)20 COOH). At the air-water interface, a possible molecular organization was discussed from the study of the area occupied by EDTTTF (SC18)2 and n molar fraction of behenic acid at a surface pressure of 30 mN m-1. EDTTTF (SC18)2 molecules form dimers and with increasing behenic acid content, a monolayer reorganization occurs. The formation of monomolecular layers including monomer EDTTTF (SC18)2 molecules was achieved for n>or=2. X-ray diffraction studies were concentrated on samples of Y-type LB multilayered films with n=0, 1 and 3. Throughout the mixed (1:n) system, stacking periodicity was dominantly observed from (001) reflections as 55+or-1 AA for neutral films, 72+or-1 AA for a brown-coloured film after exposure to iodine, and 46+or-1 AA for the stable, conducting and purple-coloured film after a thermal activation process. Possible molecular assemblies based upon centrosymmetrical bilayers were qualitatively discussed based on the calculation of the linear electron density profiles of pure and doped EDTTTF (SC18)2 LB films. The large variation of the stacking periodicity during the oxidation process was explained by the molecular inclination of EDTTTF (SC18)2 molecule associated with the insertion of I3- ions which intercalate between EDTTTF (SC18)2 layers.