End-Capping π-Conjugated Systems with Medium-Sized Sulfur-Containing Rings: A Route Towards Solution-Processable Air-Stable Semiconductors.

The sulfur-containing nine-membered heterocycle thiacyclononene (TN) was evaluated as a new type of end-capping group for π-conjugated systems. A systematic study on TN-capped α-oligothiophenes (TNnTs; n=4-7) revealed that the capping with TN, which adopts a bent conformation, imparts the resulting oligothiophenes with drastically increased solubility at approximately 140 °C and high electrochemical stability, whereas the electronic structure remains virtually unperturbed. The even-numbered oligothiophenes TN4T and TN6T form characteristic offset herringbone-type packing structures on account of the steric repulsion between the TN rings and the presence of intermolecular nonbonding S⋅⋅⋅S interactions. This packing mode in combination with the high solubility enabled the solution-process fabrication of field-effect transistors based on TN6T, which exhibited a high performance without degradation even upon exposure to air.