Ambipolar organic field-effect transistors based on N-Unsubstituted thienoisoindigo derivatives

To investigate a hybrid of isoindigo and thienoisoindigo (TIIG), a new series of unsymmetrical TIIG analogs, in which one of the outer thiophene rings of TIIG is replaced by benzene (CS) or pyridine (NS) are prepared. In addition, the π-skeleton extension effects are studied by examining α-substituted TIIG derivatives with thienyl (Dth-TIIG), furyl (Dfu-TIIG), and 1-phenyl-5-pyrazolyl groups (Bis(1-ph-5-py)-TIIG). These materials exhibit ambipolar transistor properties as expected from the energy levels. Dth-TIIG and Dfu-TIIG show hole mobilities exceeding 0.05 cm² V⁻¹ s⁻¹, and electron mobilities about 0.04 cm² V⁻¹ s⁻¹, which are close to those of the diphenyl derivative. These molecules are arranged nearly perpendicularly to the substrates in the thin films, and Dth-TIIG has a brickwork-like structure, whereas Dfu-TIIG has uniform columns. CS and Bis(1-ph-5-py)-TIIG have uniform stacking structures, but NS has a dimerized stacking structure due to the slightly bent molecular plane that results from largely unbalanced electron density.