Abstract

Organic semiconductors (OSC) have become an integral part of modern society and have found applications in various electronics such as organic photovoltaic cells (OPVC), organic light-emitting diodes (OLED) and flexible organic field-effect transistors (FOFET). Although poly-3-hexylthiophene (P3HT) is one of the most widely used OSCs, interest in oligothiophenes has surged recently as they can be used as effective models to study P3HT. The most widely used methods for the synthesis of oligothiophenes are Kumada, Stille and Suzuki cross-coupling (CC) reactions. These methods are well-explored and robust however, the Stille and Kumada processes come with a variety of disadvantages as well as environmental and health risks such as requiring harsh conditions and producing toxic metallic waste. In this work, we aimed to investigate and compare two different cross-coupling methodologies for the synthesis of oligothiophenes that do not produce harmful byproducts while also having simple and convenient reaction procedures. Using both decarboxylative cross-coupling (DCC) and Suzuki cross-coupling, symmetric oligothiophenes of lengths between 3 and 12 thiophene units have been sequentially synthesized at scales between 0.1 mmol and 1.4 mmol. Individual functionalized monomers were coupled to di-halogenated thiophene cores utilizing a double Pd-catalyzed cross-coupling reaction. The resulting oligomers were then di-brominated using multi-solvent systems with average reaction times of 30 minutes leading to complete conversions without the need for purification. Combining the DCC/Suzuki cross-coupling with the dibromination reaction, an odd sequence of 3, 5, 7, 9 and 11-unit oligothiophenes as well as an even sequence of 4, 6, 8, 10 and 12-unit oligothiophenes were sequentially synthesized in very good yields (70% to 92%). To our knowledge, 4 of the synthesized compounds have not been previously reported, namely 9T, 11T, DiBr-7T and DiBr-9T.