Molecular Engineering of Organic Semiconductors: Design of Self-Assembly Properties in Conjugated Thiophene Oligomers

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, α,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.