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 PPh(3)-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.

π-Extended thiadiazoles 4-8 fused with various electron-donating heteroaromatic moieties have been designed and synthesized. Just like thiadiazoles 1-3 synthesized previously, 4-8 exhibit intramolecular charge-transfer (CT) interactions, moderate-to-good fluorescence quantum yields of up to 0.78, and electrochemical amphoterism. In comparison with 1-3, the benzannulation in thiadiazoles 4-7 moderately extends the π conjugation and significantly increases the stability of the cationic species formed upon electrochemical oxidation. The fluorescence quantum yields increase remarkably from 3 to 6 and 7 due to the efficient suppression of nonradiative intersystem crossing resulting from the benzannulation. The properties of 4-8 strongly reflect the different species annulated to the pyrrole rings, namely benzothiophene, naphthalene, and benzofuran. Eleven crystals, including poly- and pseudopolymorphic crystals of 1 (1-Crys.(Y) and 1-Crys.(G)), 2 (2-Crys.(O) and 2-Crys.(G)), 4 (4-Crys.(O) and 4-Crys.(G)), and 6 (6-Crys.(O) and 6-Crys.(G)), were obtained and characterized by X-ray crystallography. The fluorescence colors and efficiencies are distinct for each poly- and pseudopolymorph of 1, 2, 4, and 6. It has been suggested that both the extent of the electronic interactions in the π-stacked dimers and the presence of excitonic interactions originating in the 1D face-to-face slipped columns affect the fluorescence wavelengths of the poly- and pseudopolymorphs.