Asymmetric Synthesis of Nonracemic 2‑Amino[6]helicenes and Their Self-Assembly into Langmuir Films

Alternative ways of preparing nonracemic 2-amino[6]­helicene derivatives were explored. The enantioselective [2 + 2 + 2] cycloisomerization of a nonchiral triyne under Ni­(cod)₂/(<i>R</i>)-QUINAP catalysis delivered the enantioenriched (+)-(<i>P</i>)-2-aminodibenzo­[6]­helicene derivative in 67% ee. An ultimate “point-to-helical” chirality transfer was observed in the cyclization of enantiopure triynes mediated by Ni­(CO)₂(PPh₃)₂ affording (−)-(<i>M</i>)- or (+)-(<i>P</i>)-7,8-bis­(<i>p</i>-tolyl)­hexahelicen-2-amine in >99% ee as well as its benzoderivative in >99% ee. The latter mode of stereocontrol was inefficient for a 2-aminobenzo[6]­helicene congener with an embedded five-membered ring. The <i>rac</i>-, (−)-(<i>M</i>)-, and (+)-(<i>P</i>)-7,8-bis­(<i>p</i>-tolyl)­hexahelicen-2-amines formed Langmuir monolayers at the air–water interface featuring practically identical surface pressure vs mean molecular area isotherms. The corresponding Langmuir–Blodgett films on quartz or silicon substrates were characterized by UV–vis/ECD spectroscopy and AFM microscopy, respectively

Tuning the Diradical Character of Pentacene Derivatives via Non-Benzenoid Coupling Motifs

The development of functional organic molecules requires structures of increasing size and complexity, which are typically obtained by the covalent coupling of smaller building blocks. Herein, with the aid of high-resolution scanning tunneling microscopy/spectroscopy and density functional theory, the coupling of a sterically demanded pentacene derivative on Au(111) into fused dimers connected by non-benzenoid rings was studied. The diradical character of the products was tuned according to the coupling section. In particular, the antiaromaticity of cyclobutadiene as the coupling motif and its position within the structure play a decisive role in shifting the natural orbital occupancies toward a stronger diradical electronic character. Understanding these structure–property relations is desirable not only for fundamental reasons but also for designing new complex and functional molecular structures