A π-stacking terthiophene-based quinodimethane is an n-channel conductor in a thin film transistor

A terthiophene-based quinodimethane, 3′,4′-dibutyl-5,5″-bis(dicyanomethylene)-5,5″-dihydro-2,2′:5′,2″-terthiophene (1) was synthesized and crystallized. Compound 1 has a planar quinoid geometry that is stabilized by dicyanomethylene groups at each end of the molecule. In the crystal each molecule is part of a dimerized face-to-face π-stack, with intermolecular spacings of 3.47 and 3.63 Å, respectively. Cyclic voltammetry showed that 1 could be reversibly reduced and oxidized in methylene chloride solution. Thin film transistors (TFTs) were prepared by vacuum evaporation of 1 onto SiO₂(300 nm)/Si substrates, followed by evaporation of Ag source and drain contacts. The doped Si substrate served as the gate electrode. X-ray diffraction and atomic force microscopy indicate the films are polycrystalline, with the long axes of the molecules approximately perpendicular to the substrate. The TFT measurements revealed n-channel conduction in films of 1, with room-temperature electron field effect mobilities as high as 0.005 cm²/Vs. The butyl side chains give 1 appreciable solubility in a range of common solvents, and preliminary TFT results on films cast from chlorobenzene show electron mobility as high as 0.002 cm²/Vs. These results indicate that π-stacked quinoidal thiophene oligomers are a promising new class of soluble n-channel organic semiconductors.