Field-effect transistors of the block co-oligomers based on thiophene and pyridine

We have investigated the structural and electrical characteristics of two kinds of block co-oligomers, 5,5″-Bis(2-pyridyl)-2,2′:5′, 2″-terthiophene (5A) and 2,5-Bis(2-(2′-thienyl)-pyridine-5-yl) thiophene (5B), which are composed of electron-donating thiophene and electron-withdrawing pyridine rings. At the view of building block units, the amount of the module units included in these molecules is completely equivalent to each other. X-ray diffraction patterns of 5A and 5B thin films grown on atomically flat α-Al ₂O ₃ (0001) substrates indicated a high degree of orientation along the c-axis. Field-effect transistors (FETs) of 5A and 5B thin films were fabricated and evaluated without exposure to air. The FETs based on 5A active layers exhibited p-type behavior with a mobility of ~ 10 ^-3 cm ²/V•s and an on-off ratio of 10 ⁴, while no switching behavior was observed in FETs based on 5B active layers. The lower highest occupied molecular orbital (HOMO) of 5B than that of 5A in the quantum-chemical calculation might explain these FET behaviors. Thus, HOMO, lowest unoccupied molecular orbital (LUMO) levels and FET actions of the co-oligomer molecules that consist of the same amount of building blocks can be controlled by the sequence of electron-donating and electron-withdrawing building blocks.