5 research outputs found
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)<sub>2</sub>/(<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)<sub>2</sub>(PPh<sub>3</sub>)<sub>2</sub> 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