Thiophene and Selenophene Copolymers Incorporating Fluorinated Phenylene Units in the Main Chain:  Synthesis, Characterization, and Application in Organic Field-Effect Transistors

A series of thiophene oligomers bearing core phenylene and fluorinated phenylene units has been synthesized as potential semiconductor materials for organic field-effect transistors (OFETs). Polymerization of these compounds has been achieved using Stille and oxidative coupling methods. Functionalization of the phenylene unit with fluorine atoms has a marked effect on the self-assembly and electronic properties of the parent materials: the optical band gaps and highest occupied molecular orbital levels are affected with the introduction of fluorine atoms as a result of a combination of inductive effects and rigidification of the main chain. The design of these materials has focused on the self-assembly and solution processability of the materials. All the polymers are readily soluble in common organic solvents. Self-assembly and planarization of the fluorinated materials in the solid state are identified by a combination of X-ray diffraction studies, absorption spectroscopy, and cyclic voltammetry. The organizational behavior of the films is in contrast to the conformational freedom observed in solution (absorption spectroscopy) and in the gas phase (computational studies). Thin-film OFETs have been fabricated for the entire polymer series. Hole mobilities have been measured up to 10-3 cm2/(V·s), with high current modulation (on/off ratios up to 105) and low turn-on voltages (down to 2 V). For the Stille coupled polymers, replacement of the bridging thiophene unit with selenophene generally increases the hole mobility of the polymers.