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

An Ultimate Stereocontrol in Asymmetric Synthesis of Optically Pure Fully Aromatic Helicenes

The role of the helicity of small molecules in enantioselective catalysis, molecular recognition, self-assembly, material science, biology, and nanoscience is much less understood than that of point-, axial-, or planar-chiral molecules. To uncover the envisaged potential of helically chiral polyaromatics represented by iconic helicenes, their availability in an optically pure form through asymmetric synthesis is urgently needed. We provide a solution to this problem present since the birth of helicene chemistry in 1956 by developing a general synthetic methodology for the preparation of uniformly enantiopure fully aromatic [5]-, [6]-, and [7]­helicenes and their functionalized derivatives. [2 + 2 + 2] Cycloisomerization of chiral triynes combined with asymmetric transformation of the first kind (ultimately controlled by the 1,3-allylic-type strain) is central to this endeavor. The point-to-helical chirality transfer utilizing a traceless chiral auxiliary features a remarkable resistance to diverse structural perturbations

Large Converse Piezoelectric Effect Measured on a Single Molecule on a Metallic Surface

The converse piezoelectric effect is a phenomenon in which mechanical strain is generated in a material due to an applied electrical field. In this work, we demonstrate the converse piezoelectric effect in single heptahelicene-derived molecules on the Ag(111) surface using atomic force microscopy (AFM) and total energy density functional theory (DFT) calculations. The force–distance spectroscopy acquired over a wide range of bias voltages reveals a linear shift of the tip–sample distance at which the contact between the molecule and tip apex is established. We demonstrate that this effect is caused by the bias-induced deformation of the spring-like scaffold of the helical polyaromatic molecules. We attribute this effect to coupling of a soft vibrational mode of the molecular helix with a vertical electric dipole induced by molecule–substrate charge transfer. In addition, we also performed the same spectroscopic measurements on a more rigid <i>o</i>-carborane dithiol molecule on the Ag(111) surface. In this case, we identify a weaker linear electromechanical response, which underpins the importance of the helical scaffold on the observed piezoelectric response

Large Converse Piezoelectric Effect Measured on a Single Molecule on a Metallic Surface

The converse piezoelectric effect is a phenomenon in which mechanical strain is generated in a material due to an applied electrical field. In this work, we demonstrate the converse piezoelectric effect in single heptahelicene-derived molecules on the Ag(111) surface using atomic force microscopy (AFM) and total energy density functional theory (DFT) calculations. The force–distance spectroscopy acquired over a wide range of bias voltages reveals a linear shift of the tip–sample distance at which the contact between the molecule and tip apex is established. We demonstrate that this effect is caused by the bias-induced deformation of the spring-like scaffold of the helical polyaromatic molecules. We attribute this effect to coupling of a soft vibrational mode of the molecular helix with a vertical electric dipole induced by molecule–substrate charge transfer. In addition, we also performed the same spectroscopic measurements on a more rigid <i>o</i>-carborane dithiol molecule on the Ag(111) surface. In this case, we identify a weaker linear electromechanical response, which underpins the importance of the helical scaffold on the observed piezoelectric response

Tailored Formation of N‑Doped Nanoarchitectures by Diffusion-Controlled on-Surface (Cyclo)Dehydrogenation of Heteroaromatics

Surface-assisted cyclodehydrogenation and dehydrogenative polymerization of polycyclic (hetero)aromatic hydrocarbons (PAH) are among the most important strategies for bottom-up assembly of new nanostructures from their molecular building blocks. Although diverse compounds have been formed in recent years using this methodology, a limited knowledge on the molecular machinery operating at the nanoscale has prevented a rational control of the reaction outcome. We show that the strength of the PAH–substrate interaction rules the competitive reaction pathways (cyclodehydrogenation <i>versus</i> dehydrogenative polymerization). By controlling the diffusion of N-heteroaromatic precursors, the on-surface dehydrogenation can lead to monomolecular triazafullerenes and diazahexabenzocoronenes (N-doped nanographene), to N-doped oligomeric or polymeric networks, or to carbonaceous monolayers. Governing the on-surface dehydrogenation process is a step forward toward the tailored fabrication of molecular 2D nanoarchitectures distinct from graphene and exhibiting new properties of fundamental and technological interest