A series of π-extended thiadiazoles fused with electron-donating heteroaromatic moieties: synthesis, properties, and polymorphic crystals.

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.