Thiophene-fused Reactions Based on Trisulfide Radical Anion S<sub>3</sub><sup>•−</sup>

Thiophene-fused reactions (thienannulations) to the π-electron system are essential for the development of high-performance (high-mobility) organic semiconductors in terms of enhancing intermolecular interactions and charge delocalization. In the conventional thienannulations of ethynylated π-electron system, elemental sulfur and sodium sulfide are frequently used as sulfur sources. Meanwhile, the trisulfur radical anion (S3•−) has recently attracted attention as a new sulfur source that enables the development of unexplored organic semiconductors that are difficult to access based on the conventional method. This review highlights thienannulation based on S3•− for the synthesis of highly π-extended and unsymmetrical thienoacenes.