π‑Extended Thiadiazoles Fused with Thienopyrrole or Indole Moieties: Synthesis, Structures, and Properties

We report the syntheses, structures, photophysical properties, and redox characteristics of donor–acceptor-fused π-systems, namely π-extended thiadiazoles <b>1</b>–<b>5</b> fused with thienopyrrole or indole moieties. They were synthesized by the Stille coupling reactions followed by the PPh<sub>3</sub>-mediated reductive cyclizations as key steps. X-Ray crystallographic studies showed that isomeric <b>1b</b> and <b>2b</b> form significantly different packing from each other, and <b>1a</b> and <b>4a</b> afford supramolecular networks via multiple hydrogen bonding with water molecules. Thienopyrrole-fused compounds <b>1b</b> and <b>2b</b> displayed bathochromically shifted intramolecular charge-transfer (CT) bands and low oxidation potentials as compared to indole-fused analog <b>3b</b> and showed moderate to good fluorescence quantum yields (Φ<sub>f</sub>) up to 0.73. In <b>3b</b>–<b>5b</b>, the introduction of electron-donating substituents in the indole moieties substantially shifts the intramolecular CT absorption maxima bathochromically and leads to the elevation of the HOMO levels. The Φ<sub>f</sub> values of <b>3</b>–<b>5</b> (0.04–0.50) were found to be significantly dependent on the substituents in the indole moieties. The OFET properties with <b>1b</b> and <b>2b</b> as an active layer were also disclosed.