Exploring Benzothieno[3,2‐<i>b</i>]benzothiophene <i>S</i>‐Oxides for Organic Electronics, Fluorescence‐Based Applications, and Asymmetric Synthesis

A series of benzothieno[3,2-b]benzothiophene S-oxides has been efficiently synthesized via one- or two-step selective oxidation of benzothieno[3,2-b]benzothiophene (BTBT). These BTBT S-oxides form highly ordered structures that display optical anisotropy, are thermally stable up to temperatures above 210 °C, and do not show phase transitions. They exhibit long excited state lifetimes (0.181-0.869 ms) and are more resistant to oxidative degradation than their parent unoxidized BTBT. Some of these compounds demonstrate high quantum yields, exceeding 98 % in solution and 71 % in the solid state. As evidenced by quantum-chemical calculations, BTBT-based sulfoxides have sufficiently high pyramidal inversion energy barriers that prevent their stereomutation at room temperature. Additionally, fully S-oxidized BTBT meets the energy-splitting criterion required for the singlet fission process, with the T₁ energy level that makes it suitable for device application.