Core-cyanated distyryl-bithiophene: Synthesis and impact on charge transport in field-effect transistors

We report on the synthesis of two new distyryl-oligothiophenes with cyano groups at different positions of the ethylene linkage: E,E-5,5′-bis((2-cyano-2-phenyl)-ethenyl)-2,2′-bithiophene 1 and E,E-5,5′-bis((1-cyano-2-phenyl)-ethenyl)-2,2′-bithiophene 2. The impacts of cyanated substitutions on molecular orbitals, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), of these new oligomers investigated by the experimental data are confronted in a theoretical study. Despite lower LUMO level, introduction of electron-withdrawing groups in conjugated cores creates a local hindrance in the π-electron delocalization. Detrimental effects are observed on thin-film morphology studied by scanning electron microscopy and the crystalline structure characterization by X-ray diffraction. Organic thin-film transistors made from 1 showed only a low p-channel transistor activity in air whereas devices made from 2 showed neither a n- or a p-channel activity. Analysis of these materials reveals a direct relationship between the molecular structure and solid state properties.