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 1–5 fused with thienopyrrole or indole moieties. They were synthesized by the Stille coupling reactions followed by the PPh₃-mediated reductive cyclizations as key steps. X-Ray crystallographic studies showed that isomeric 1b and 2b form significantly different packing from each other, and 1a and 4a afford supramolecular networks via multiple hydrogen bonding with water molecules. Thienopyrrole-fused compounds 1b and 2b displayed bathochromically shifted intramolecular charge-transfer (CT) bands and low oxidation potentials as compared to indole-fused analog 3b and showed moderate to good fluorescence quantum yields (Φ_f) up to 0.73. In 3b–5b, 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_f values of 3–5 (0.04–0.50) were found to be significantly dependent on the substituents in the indole moieties. The OFET properties with 1b and 2b as an active layer were also disclosed.