Synthesis, structures, and properties of fused thiophenes for organic field-effect transistors

Article abstract of DOI:10.1002/chem.200902755

A series of fused thiophenes composed of fused a-oligothiophene units as building blocks, end-capped with either styrene or l-pentyl-4-vinylbenzene groups, has been synthesized through Stille coupling reactions. The compounds have been fully characterized by means of ¹H NMR spectrometry, high-resolution mass spectrometry, and elemental analysis. The molecules present a trans-trans configuration between their double bonds, which has been verified and confirmed by Fourier-transform infrared spectroscopy and single-crystal X-ray diffraction analysis. The X-ray crystal structures showed π- π overlap and sulfur-sulfur interactions between the adjacent molecules. The decomposition temperatures were all found to be above 300 °C, indicating that compounds of this series possess excellent thermal stability. The fact that no phase transition occurs at low temperature indicates that they should be well-suited for application in devices. Moreover, they possess low HOMO energy levels, based on cyclic voltammetry measurements, and suitable energy gaps, as determined from their thin-film UV/Vis spectra. Thinfilm X-ray diffraction analysis and atomic force microscopy revealed high crystallinity on supporting substrates. In addition, as the substrate temperature has a significant influence on the morphology and the degree of crystallinity, the device performance could be optimized by varying the substrate temperature. These materials were found to exhibit optimal field-effect performance, with a mobility of 0.17 Cm₂V^-1S^-1 and an on/off ratio of 10⁵, at a substrate temperature of 70°C.