Homo and Hetero Molecular 3D Nanographenes Employing a Cyclooctatetraene Scaffold (original) (raw)
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Helically Arranged Chiral Molecular Nanographenes
Journal of the American Chemical Society, 2021
A benchtop solution-phase synthesis of molecular nanographenes composed of two orthogonal dibenzo[fg,ij]phenanthro[9,10,1,2,3-pqrst]pentaphene (DBPP) moieties covalently connected through a tetrafluorobenzene ring is described. The helical arrangement of these three covalently linked molecular fragments leads to the existence of a chiral axis which gives rise to a racemic mixture, even with the molecular moieties being symmetrically substituted. X-ray diffraction studies show that both enantiomers cocrystallize in a single crystal, and the racemic mixture can be resolved by chiral HPLC. Asymmetric substitution in DBPP moieties affords a pair of diastereoisomers whose rotational isomerization has been studied by 1H NMR. Additionally, the electrochemical and photophysical properties derived from these new molecular nanographenes reveal an electroactive character and a significant fluorescent behavior.
Synthesis of corannulene-based nanographenes
Communications Chemistry, 2019
Corannulene (C 20 H 10) is a polycyclic hydrocarbon in which five six-membered rings surround a central five-membered ring to construct a bowl-like aromatic structure. Here we examine the development of synthetic strategies that allow for the growth of the peripheral aromatic rings as a means to extend the aromatic area of the central corannulene nucleus and provide access to unique nanocarbon molecules. s p 2-hybridised structures of carbon have fascinated the research community for a very long time. In 1985, buckminsterfullerene, otherwise known as C 60 , was discovered (Fig. 1a) 1. In this ball-shaped molecule, the curvature in the structure stems from the presence of fivemembered rings. In 1991, carbon nanotubes arrived on the scene 2. Here, the structure is cylindrical and composed of only rolled-up six-membered rings. In 2004, a sheet-like single layer from graphite-graphene-was isolated 3. All of these materials were shown to have extraordinary electronic and mechanical properties due to their unique curved or planar sp 2-hybridised aromatic structures. Inspired by these discoveries, chemists have been developing strategies to access such aromatic hydrocarbons through rational ('bottom-up') synthetic approaches. Scott's 12-step chemical synthesis of fullerene C 60 from a rationally designed precursor is a testament to the ingenuity and resourcefulness of organic chemists 4. In planar structures, nanographenes (well-defined cutouts of graphene with nano-scale dimensions) can now be prepared on a regular basis with a very diverse portfolio 5. It is expected that combining the planar structure of graphene with the curvature of fullerenes may produce hybrid materials with interesting properties 6-8. To induce non-planarity into nanographenes, a practical approach would be to introduce a fivemembered ring such as in the case of fullerene, C 60. A perfect building block that allows for such a structural arrangement to happen is corannulene (1)-a molecule in which five six-membered rings surround a central five-membered ring to give a bowl-like structure (Fig. 1b) 9-20. Corannulene also offers many beneficial features as a molecular building block. It has high solubility in common organic solvents. It can be derivatized in a well-defined manner. Due to synthetic ease, the derivatives can be prepared on a multigram scale. These attributes are important as they allow for the scalable preparation of carefully designed corannulene-based building blocks and the subsequent synthesis, purification and structural analysis of the larger (fused) aromatic systems. Recently, therefore, there has been a surge in employment of corannulene as a core molecule in the synthesis of extended aromatic structures. Our aim in this review article is to discuss
Open-Shell Nonbenzenoid Nanographenes Containing Two Pairs of Pentagonal and Heptagonal Rings
Journal of the American Chemical Society, 2019
Non-benzenoid carbocyclic rings are postulated to serve as important structural elements toward tuning the chemical and electronic properties of extended polycyclic aromatic hydrocarbons (PAHs, or namely nanographenes), necessitating a rational and atomically-precise synthetic approach toward their fabrication. Here, using a combined bottom-up in-solution and on-surface synthetic approach, we report the synthesis of non-benzenoid open-shell nanographenes containing two pairs of embedded pentagonal and heptagonal rings. Extensive characterization of the resultant nanographene in solution shows a low optical gap, and an open-shell singlet ground state with a low singlet-triplet gap. Employing ultra-high-resolution scanning tunneling microscopy and spectroscopy, we conduct atomic-scale structural and electronic studies on a cyclopenta-fused derivative on a Au(111) surface. The resultant 5-7 rings embedded nanographene displays an extremely narrow energy gap of 0.27 eV and exhibits a pronounced open-shell biradical character close to one (y 0 =0.92). Our experimental results are supported by mean-field and multiconfigurational quantum chemical calculations. Access to large nanographenes with a combination of non-benzenoid topologies and open-shell character should have wide implications in harnessing new functionalities toward the realization of future organic electronic and spintronic devices.
Enantioselective Synthesis of Inherently Chiral Molecular Nanographenes
Research Square (Research Square), 2023
The introduction and precise control of stereogenic elements in chemical structures is always a challenging and highly demanding task. The vast majority of available asymmetric methods require the presence of a heteroatom in the starting substrates acting as an anchor point for the successful transfer of chiral information. This eventually emerges, typically, in the formation of a new stereogenic centre while the heteroatom remains in the product. For this reason, compounds constituted only by carbon atoms, such as optically active molecular nanographenes have, so far, been obtained as racemates followed by chiral chromatographic separations. Here, we report an enantioselective strategy which uses three pivotal stereocontrolled key synthetic steps to introduce and extend three different types of stereogenic elements, namely central, axial and helicoidal chirality, into a polycyclic aromatic C sp2-based structure. The resulting helicity of six ortho-fused rings embedded in the triindane-based nanographene ensures stereochemical stability and good chiroptic activity.
New advances in chiral nanographene chemistry
Organic Chemistry Frontiers
This review will highlight several recent and lesser-reviewed works related to the preparation of chiral nanographenes, specifically focusing on structures prepared by non-Scholl methods.
Angewandte Chemie, 2021
We report a new family of hexa‐peri‐hexabenzocoronene (HBC)‐based helical nanographenes incorporating π‐extended carbo[5]helicenes bearing an octagonal carbocycle. This family represents a new kind of highly distorted saddle‐helix hybrid nanographenes. For the first time, the eight‐membered ring becomes a constituent of both a carbo[5]helicene and a HBC and thus, the negative curvature is responsible for twisting both units. This novel chiral motif, namely, oct‐[5]helicene results in the largest torsion angle recorded so far for a carbo[5]helicene (θ=79.5°), as it has been suggested by DFT‐calculations and confirmed by X‐ray crystallography. Consequently, the barriers of isomerization become exceptionally high for a [5]helicene unsubstituted in the fjord region since neither racemization nor decomposition were observed at 200 °C for 1 or 3 during 5 h. Therefore, racemic resolutions allowed subsequent chiroptical studies showing the ECD and CPL responses of this novel family of chira...