Thiophene-diazine molecular semiconductors: synthesis, structural, electrochemical, optical, and electronic...

The synthesis, structural, electrochemical, optical, and electronic structure properties of a new azine-thiophene semiconductor family are reported and compared to those of analogous oligothiophenes. The new molecules are: 5,5'-bis(6-(thien-2-yl)pyrimid-4-yl)-2,2'-dithiophene (1), 5,5'-bis(6-(5-hexylthien-2-yl)pyrimid-4-yl)-2,2'-dithiophene (3), and 5,5'-bis(6-(thien-2-yl)pyridazin-3-yl))-2,2'-dithiophene (2). Electrochemical experiments demonstrate that introduction of electron-poor heteroaromatic rings into the oligothiophene core significantly enhances electron affinity. Thin-film transistors were fabricated with these materials and evaluated both in vacuum and in air. We find that although diazine substitution is important in tuning oligothiophene orbital energetics, these oligomers are p-channel semiconductors and the field-effect transistor (FET) charge transport properties are remarkably similar to these of unsubstituted oligothiophenes. The combined computational-experimental analysis of the molecular and thin film properties indicates that these diazine-containing oligothiophenes essentially behave as pi-extended bithiophenes. Interestingly, despite strong intermolecular interactions, high solid-state fluorescence efficiencies are observed for these new derivatives. Such emission characteristics suggest that these materials behave as more extended pi systems, which should be advantageous in light-emitting transistors.