Rapid synthesis of a curved nanographene

Researchers demonstrate a new method enables the quick synthesis of both planar and non-planar nanographenes

Nanographenes can be considered as 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 the size and shape of the molecule, nanographenes are expected to find applications in OLEDs, organic solar cells, organic field-effect transistors, and more. For this, simple methods to synthesizing nanographenes that avoid complex multi-step processes are desired. Now, a team of scientists from WPI-ITbM at Nagoya University and from RIKEN have established a new two-step annulative p-extension (APEX) method that generates structurally diverse nanographenes. This research was published online in the scientific journal Angewandte Chemie International Edition on August 25.

At the molecular level, nanographenes are composed of polycyclic aromatic hydrocarbons (PAHs), which are a network of connected benzene rings--hexagon-shaped carbon molecules. APEX reactions provide chemists a toolbox to connect PAHs together, small or large, without having to prefunctionalize PAH substrates. With their new APEX method, the researchers could synthesize novel nanographenes engendered with rare "defects" like curvature, twisting, and five- or seven-membered rings, rather than standard planar structures.

(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.

The two-step APEX method proceeds in a straightforward manner. Using a palladium catalyst and the oxidant ortho-chloranil, they could selectively extend the structure of different PAHs at specific sites. With this, they successfully generated several polyaryl intermediates.

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.

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.

(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.

The researchers further analyzed the rare nanographene containing fused five-, six-, and seven-membered rings with X-ray crystallography and revealed that it has a highly curved structure deviating significantly from a planar structure. It exhibits "negative curvature," in which the two partial planes within the molecule are tilted by 34.8 degrees.

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).

These results demonstrate that the two-step APEX method is an efficient way to synthesize known frameworks and is promising for discovering nonplanar nanographenes. The researchers are interested to expand their work in the future to create molecular nanocarbons with diverse shapes and electronic structures, including planar, curved, twisted, and porous structures.