Molecular engineering of organic semiconductors: Design of self-assembly properties in conjugated thiophene oligomers

Article abstract of DOI:10.1021/ja00072a026

In order to analyze the correlation between charge transport and structural properties in conjugated oligomers, sexithiophene, 6T, was substituted by hexyl groups, both on the terminal α positions (α,ωDH6T) and as pendant groups in the β position (β,β′DH6T). Structural characterizations by X-ray diffraction show that vacuum-evaporated thin films of 6T and α,ωDH6T consist of layered structures in a monoclinic arrangement, with all-trans planar molecules standing on the substrate. When compared to 6T, α,ωDH6T is mainly characterized by a very large increase of molecular organization at the mesoscopic level, evidenced by a much longer range ordering. Electrical characterizations indicate that the conductivity of α,ωDH6T is largely anisotropic, with a ratio of 120 in favor of the conductivity parallel to the substrate plane, i.e. along the stacking axis. The charge carrier mobility, determined on field-effect transistors fabricated from these conjugated oligomers, also shows an increase by a factor of 25 when passing from 6T to α,ωDH6T, reaching a value of 5 × 10^-2 cm² V^-1 s-¹. In contrast, β,β′DH6T presents very low conductivity and mobility, the latter being below detection limit. These results are attributed to the self-assembly properties brought by alkyl groups in the α,ω position.