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, --, --, and --, with different isomeric locations of sulfur atoms in two terminal thiophene rings were efficiently synthesized using dithieno[2,3-:3',2'-]thiophene (-), dithieno[2,3-:2',3'-]thiophene (-), and dithieno[2,3-:3',4'-]thiophene (-) as building blocks via Suzuki cross-coupling and intramolecular cyclization reactions. Aside from these racemic [7]helicenes, two novel heterocyclic isomers, namely, trithienothiepines and , were simultaneously obtained during the intramolecular cyclization. Two novel deprotonations of bi-s and cyclization for synthesizing target compounds showed high selectivity and efficiently constructed both s and s. X-ray crystallographic analyses revealed that the s have typical helical molecular structures. The isomeric location of sulfur atoms in the two terminal thiophene rings in --, --, --, and 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, , and were examined and theoretically calculated. Results indicated that the isomeric location of sulfur atoms plays a key role in tuning intramolecular π-electronic conjugation.