Carbohelicenes and thiahelicene from phthalaldehydes through Perkin approach

Synthesis and structural features of helical nanographene molecules comprising of seven benzene rings are examined. Thus dibutyl-dicarboxylate functional [7]helicene and its two regioisomers, dinaphtho[1,2–a:1′,2′–h]anthracene and naphtho[2,1–c]pentahelicene, have been synthesized in two steps through Perkin approach using napthalene-2-acetic acid and ortho- or meta-phthalaldehydes. The feasibility of this approach to construct sulfur doped twisted dithiaarenes is also investigated by using thiophene-3-acetic acid. While dithiaarenes from meta-phthalaldehyde remains challenging, synthesis and characterization of planar anthra[1,2–b:5,6–b']dithiophene and twisted 1,12-dithiapentahelicene is successful from ortho-phthalaldehyde. Conformational analysis with DFT calculation shows unique helicity preference in such doubly helical carbon nanostructures. Absorption and emission behavior of these π-extended molecules shows enhanced conjugation.