Giant analogues of tetrathiafulvalene in Langmuir and Langmuir–Blodgett films

Abstract

The behaviour of new giant tetrathiafulvalene (TTF) analogues has been studied at the gas/water interface and in Langmuir–Blodgett films. These compounds, denoted G-TTFC_n, have a dihydro-TTF core which bears two conjugated side-arms with various alkyl chains (from methyl to octyl). Langmuir films of these compounds are quite stable versus time and are characterized by a collapse pressure that increases as the length of the alkyl chains decreases. The ability of these compounds to form Langmuir films should then be due to their extended TTF core. Except for G-TTFC₁, transfer of the monolayer onto a solid substrate is easily done at low surface pressure. Doping of the LB films by iodine vapours changes their spectroscopic (EPR, electronic and vibrational) behaviour and leads to rather poor conducting films. Conductivity of G-TTFC₄ films (σ_RT≈ 4 × 10^–7 S cm^–1) is two orders of magnitude lower than for G-TTFC₂(σ_RT≈ 4 × 10^–4 S cm^–1), showing the influence of the molecular packing on the electrical properties of such films. Different results are obtained upon mixing these G-TTFC_n compounds with pentadecyltetracyanoquinodimethane (C₁₅TCNQ) in various proportions, which gives rise to charge transfer complexes of adjustable stoichiometry. It is shown that these extended π-donor molecules give new insights on the redox processes that can lead to conducting films.