Synthesis of All Thiophene-Based [7]Helicenes and Trithienothiepines with Isomeric Location of Sulfur Atoms Based on Intramolecular Selectivity of Deprotonation

Three unsymmetric thiophene-based [7]­helicenes, namely, endo-exo-UH-1, endo-top-UH-2, and exo-top-UH-3, with different isomeric locations of sulfur atoms in two terminal thiophene rings were efficiently synthesized using dithieno­[2,3-b:3′,2′-d]­thiophene (bb-DTT), dithieno­[2,3-b:2′,3′-d]­thiophene (bt-DTT), and dithieno­[2,3-b:3′,4′-d]­thiophene (bs-DTT) as building blocks via Suzuki cross-coupling and intramolecular cyclization reactions. Aside from these racemic [7]­helicenes, two novel heterocyclic isomers, namely, trithienothiepines TTTP-1 and TTTP-2, were simultaneously obtained during the intramolecular cyclization. Two novel deprotonations of bi-DTTs and cyclization for synthesizing target compounds showed high selectivity and efficiently constructed both UHs and TTTPs. X-ray crystallographic analyses revealed that the UHs have typical helical molecular structures. The isomeric location of sulfur atoms in the two terminal thiophene rings in endo-exo-UH-1, endo-top-UH-2, exo-top-UH-3, and TTTP-1 allowed multiple intermolecular interactions, such as S···S, S···C, and S···H interactions, resulting in different crystal-packing patterns. Moreover, the absorption behaviors of these [7]­helicenes, TTTP-1, and TTTP-2 were examined and theoretically calculated. Results indicated that the isomeric location of sulfur atoms plays a key role in tuning intramolecular π-electronic conjugation.