From Colorless to Near-Infrared S-Heteroarene Isomers: Unexpected Cycloaromatization of Cyclopenta[ b]thiopyran Catalyzed by PtCl2.

S-heteroarenes containing cyclopenta[ b]thiopyran moieties were synthesized via intramolecular ring-expanding cycloisomerization from 1-acetyl-2-thienyl-substituted precursors catalyzed by PtCl2. In comparison to the other two S-heteroarene isomers experiencing common 6-endo and 5-exo cyclization processes, the aromatic cyclopenta[ b]thiopyran derivative demonstrates intriguing photophysical and electrochemical properties, such as near-infrared absorption, low oxidation potential, and excellent electrochemical stability. Therefore, this work provides an effective pathway to incorporate cyclopenta[ b]thiopyran as building blocks for organic semiconductors with unique properties.