Quaterthiophenes with terminal indeno[1,2-b]thiophene units as p-type organic semiconductors

Quaterthiophenes 4T, Oct-4T, and Tol-4T based on a central 2,2'-bithiophene core alpha,omega-terminated with 4,4-unsubstituted and 4,4-disubstituted n-octyl or p-tolyl indeno[1,2-b]thiophene have been synthesized by Stille or Miyaura-Suzuki couplings. Compound 4T was also synthesized by an alternative route involving a soluble precursor bearing solubilizing trimethylsilyl groups which have been eliminated in the last step. The electronic properties of the compounds have been analyzed by cyclic voltammetry, UV-vis absorption and fluorescence emission spectroscopy. Thermal evaporation of 4T and Oct-4T leads to crystalline thin films and UV-vis absorption and X-ray diffraction data for these films suggest that the molecules adopt a quasi-vertical orientation onto the substrate. Strong pi-pi intermolecular interactions have been observed for 4T but not for molecules Oct-4T due to the presence of n-octyl chains. Sublimed thin films of Tol-4T show an amorphous character. The characterization of field-effect transistors fabricated from these three materials gave a hole-mobility of 2.2 x 10(-2) cm2 V(-1) s(-1) with an on/off ratio of 2.2 x 10(4) for 4T while no field-effect was observed for Oct-4T and Tol-4T.