A series of alkyl chain end-capped oligofluorene−thiophenes have been prepared with high yields using Suzuki or Stille coupling reactions. The electronic and optical properties of the thin films deposited at different substrate temperatures have been investigated. Morphological studies using transmission electron microscopy (TEM) revealed well-interconnected microcrystalline domains in these thin films. X-ray diffraction measurements of the vacuum-evaporated films showed enhanced crystalline order with increasing substrate deposition temperature. Thermal analysis as well as electrochemical measurements of the materials indicated that the new oligomers have high thermal and oxidative stability. Highly ordered polycrystalline vacuum-evaporated films with charge carrier mobility as high as 0.12 cm2 V-1 s-1 have been achieved with 5,5‘-bis(7-hexyl-9H-fluoren-2-yl)-2,2‘-bithiophene (DHFTTF). Thin film field-effect transistor (TFT) devices made from these materials showed remarkable stability even after UV (366 nm) irradiation for more than 48 h in air. The semiconductors exhibit high on/off ratios (up to 105) and no significant decrease in mobility and on/off ratio over several months in air with exposure to ambient light. Finally, bright emission colors from greenish yellow to orange-red were observed in this new series of oligomer solid films excited with UV light (366 nm). In addition, a comparative study of the newly synthesized oligomers with α,α‘-dihexylsexithiophene (DH6T), one of the most widely investigated oligothiophenes, is presented. The current approach to the molecular design can be applied toward the rational design of new TFT materials.

The characterization of self-assembled monolayer molecular arrays, which serve as active layers in electronic devices, is an important step toward understanding molecular-scale electronics. To correlate the properties of self-assembled monolayers with their molecular structures, a series of π-conjugated monoand dithiols and aromatic dithiols with an oxygen or sulfur atom between two aromatic units have been designed and synthesized. Their optical properties were determined by UV-vis spectroscopy. Their selfassembled monolayer films on gold surfaces were characterized by cyclic voltammetry, grazing incidence Fourier transform infrared spectroscopy (GI-FTIR), and contact angle and ellipsometry measurements. Increasing the chain length from two to four phenyl rings showed a more than linear increase of the intensity of the aromatic CdC ring stretch modes in GI-FTIR, indicating that the longer p-phenylene system is oriented toward the surface normal. Similar to oligophenylenes, when the number of repeat units for oligothiophene is increased, a more than linear increase of the intensity of the CdC stretch and C-H bend modes implies that the longest oligothiophenedithiol molecule is oriented close to the surface normal. Ellipsometry showed a smaller deviation from the calculated monolayer thickness with increasing number of thiophene or phenylene rings, corroborating the GI-FTIR data. We conclude that the conjugated phenyleneand thiophene-based dithiols demonstrate a less tilted molecular orientation with respect to the surface normal with increasing chain length.