Control of the mesoscopic organization of conjugated thiophene oligomers, induced by self-assembly properties

In order to analyse the dependence of charge transport in conjugated materials on their structural organization, three sexithiophene derivatives were studied, unsubstituted 6T and its di-hexyl substituted compounds, both on the terminal α positions (αωDH6T) and as pendant groups in β position (β, β′DH6T). X-ray diffraction characterization of vacuum evaporated thin films of these oligomers shows that 6T and α,ωDH6T consist of layered structures in a monoclinic arrangement, with all-trans planar molecules standing on the substrate. A large increase of molecular organization at the mesoscopic level is observed when passing from 6T to α,ωDH6T, as shown by a much longer range ordering. Electrical characterizations show that the conductivity of α,ωDH6T is anisotropic, with a ratio of 120 in favor of the conductivity parallel to the substrate plane, ie along the stacking axis. The charge carrier mobility, determined on field-effect transistors realized from these conjugated oligomers, also shows a large increase when passing from 6T to α,ωDH6T, reaching a value of 5 × 10^-2 cm² V^-1 s^-1, in contrast with β,β′DH6T which presents very low conductivity and mobility. These results are attributed to the self-assembly properties brought by alkyl groups in the α,ω position.