A series of quinoidal oligothiophenes terminated with carbonyl groups (nTDs, n = 2-4) are studied as p-type organic semiconductors for the active materials in organic field-effect transistors (OFETs) both by the theoretical and experimental approaches. The theoretical calculations clearly show their high-lying highest occupied molecular orbital (HOMO) energy levels (EHOMOs), small reorganization energies for hole transport (λholes), and large contribution of sulfur atoms to HOMOs, all of which are desirable for p-type organic semiconductors. Thus, we synthesized nTDs from the corresponding aromatic oligothiophene precursors and then evaluated their physicochemical properties and structural properties. These experimental evaluations of nTDs nicely proved the theoretical predictions, and the largest 4TDs in the series (4,4'''-dihexyl- and 3',4,4″,4'''-tetrahexyl-5H,5'''H-[2,2':5',2″:5″,2'''-quaterthiophene]-5,5'''-dione) can afford solution-processed OFETs showing unipolar p-type behaviors and hole mobility as high as 0.026 cm2 V-1 s-1.
Keywords: molecular design; organic field-effect transistor; organic synthesis; p-type organic semiconductor; quinoidal oligothiophene; theoretical calculation.