Curved nanographene with five-, six- and seven-membered rings synthesized in two steps
Nanographenes can be considered molecular fragments of graphene, a 2D conductive material in which carbon atoms are connected in a honeycomb pattern. Because their electronic and photophysical properties vary depending on ...
For this, simple methods for synthesizing nanographenes that avoid complex multistep processes are desired. Now, a team of scientists from WPI-ITbM at Nagoya University and RIKEN has established a new two-step annulative π-extension (APEX) method that generates structurally diverse nanographenes. This research is published in the journal Angewandte Chemie International Edition.
Building unusual nanographene frameworks
At the molecular level, nanographenes are composed of polycyclic aromatic hydrocarbons (PAHs), networks of connected benzene rings—hexagon-shaped carbon molecules. APEX reactions provide chemists with a toolbox to connect small or large PAHs without having to prefunctionalize PAH substrates. With their new APEX method, the researchers could synthesize novel nanographenes featuring rare "defects" such as curvature, twisting and five- or seven-membered rings, rather than standard planar structures.
The two-step APEX method proceeds straightforwardly. Using a palladium catalyst and the oxidant ortho-chloranil, the researchers could selectively extend the structure of different PAHs at specific sites. They successfully generated several polyaryl intermediates.
From intermediates to curved molecules
Next, they subjected the polyaryl intermediates to an oxidative cyclization reaction. The reaction uses the oxidant dichloro-5,6-dicyano-p-benzoquinone (DDQ) and trifluoromethanesulfonic acid (TfOH) to synthesize new nanographenes with various frameworks. The frameworks consisted of 8–10 rings, including those with "cove" regions and highly fused hexagonal nanographenes. Unexpectedly, they also synthesized a nanographene consisting of five-, six- and seven-membered rings.
Nanographenes with diverse structures can be synthesized in two-steps using a new method. Credit: Issey Takahashi
(a) Structure of a nanographene (left) and conventional synthetic methods (right). (b) The new synthetic strategy targeted in this study to increase the structural diversity of nanographenes. Credit: WPI-ITbM
Top) A polyaryl intermediate with multiple structurally congested aromatic rings was synthesized by directly arylating polycyclic aromatic hydrocarbons (PAHs) with aryltrimethylsilanes as arylating agents, using a palladium complex and the oxidant ortho-chloranil. (Bottom) Various aryltrimethylsilanes bearing naphthalene frameworks were reacted with representative PAHs, phenanthrene and pyrene, as well as a strained nanocarbon molecule. Credit: WPI-ITbM
(a) Oxidative cyclization of the polyaryl intermediate enabled the synthesis of a variety of frameworks consisting of 8–10 rings, including complex fused nanographenes containing cove regions and highly fused hexagonal nanographenes. (b) During the cyclization reaction, in addition to conventional nanographenes with six-membered rings, a nanographene containing fused five-, six-, and seven-membered rings was generated. Credit: WPI-ITbM
X-ray crystallography of the nanographene containing fused five-, six-, and seven-membered rings. Top view of the molecular structure (left), side view (center), and illustration showing the "negative curvature," in which the two partial planes within the molecule are tilted by 34.8 degrees (right). Credit: WPI-ITbM