Separation of Spectroscopically Uniform Nanographenes

Integration of Nanographenes and Organic Chemistry - Toward Nanographene-based Two-Dimensional Materials

Graphene and its relatives have received considerable attention from the fields of physics and chemistry since the isolation of pristine graphene sheets. Nanographenes (NGs) are graphene fragments that are a few to tens of nanometers in diameter. Compared to graphene and its relatives, such as graphene oxides, NGs can be handled more easily, and their large π surface and oxygen functional groups on the edge allow postsynthetic modifications. The study of NGs is gradually shifting from the development of synthetic procedures to postsynthetic modification. From the structural point of view, NGs can be regarded as two-dimensional carbon polymers. Their unique structures and affinity for organic molecules make NGs excellent scaffolds for two-dimensional materials, which are now an important topic in organic and polymer chemistry. In this conceptual article, we introduce the position of NGs from the perspective of two-dimensional substances and briefly summarize both the structural features of NGs and the effects of functionalization on their physical properties. These are valuable when producing reasonable strategies for their postsynthetic modifications.

Nanographene - A Scaffold of Two-Dimensional Materials

Substances can be divided into 0D to 3D species based on the number of repeating units (atom, ion, and molecule) and their arrangements in space (point, linear, layer, and solid). Discrete substances belong to 0D species, polymers are examples of 1D species, and molecular crystals are 3D species. Most of the substances belong to one of these species. On the other hand, those categorized into 2D species wherein the repeating units organize a layer are less explored. 2D species have a surface and edges. The incorporation of these structural features into a molecular design can realize multifunctionalized systems that are difficult to achieve by conventional organic synthesis. The development of 2D species is, therefore, the frontier of organic, inorganic, and polymer chemistry. Nanographenes (NGs) are suitable scaffolds for realizing 2D species due to several factors, such as chemical stability and oxygen-containing functional groups on the surface and on the edge, allowing postsynthetic modifications. Our group has utilized NGs with tens of nanometers in diameters for developing 2D species. Carboxy groups on the edge enable us to install various substituents into NGs, offering NG-based functional materials. These studies demonstrate that the integration of NGs with organic chemistry can widen the scope of their applications other than optical materials that are a main application of NGs. We introduce our recent studies on the development of NG-based functional materials realized by postsynthetic modifications. We hope that this account will contribute to the development of the chemistry of 2D species.

Self-Assembly of Nanographenes

Suitably decorated small aromatic systems can organize stacked structures that display interesting properties arising from their unique morphologies. Although nanographenes produced by top-down methods have graphitic domains and can in principle be applied for such supramolecular systems, to our knowledge, no such example has been reported thus far. This is partly because of their limited solubility in organic solvents and partly because of their wide lateral size distribution. To realize nanographene-based supramolecular aggregates, nanographenes carrying alkyl chains with narrow lateral size distributions are employed. We find that the nanographenes undergo self-assembly and that self-assembly is regulated by concentration, solvent polarity, temperature, and sonication. Optical measurements and AFM images indicate that stacked structures are possible candidates for aggregates. A molecular mechanics calculation models the interactions in the aggregates. The nanographenes showed concentration-dependent morphologies on mica, stacked structures at low concentrations and polymer-like network structures on mica at higher concentrations.

Chemically Functionalized Two-Dimensional Carbon Materials

Nanographenes (NGs), also known as graphene quantum dots, have recently been developed as nanoscale graphene fragments. These nanocarbon species can be excited with UV light and emit light from the UV-to-visible region. This photoemission has received great attraction across multiple scientific fields. NGs can be produced by cutting off carbon sources or fusing small organic molecules to grow graphitic structures. Furthermore, the organic synthesis of NGs has been intensely studied. Recently, the number of research papers on postsynthetic modification of NGs has gradually increased. Installed organic groups can tune the properties of NGs and provide new functionalities, opening the door for the development of sophisticated carbon-based functional materials. This review sheds light on recent progress in the postsynthetic modification of NGs and provides a brief summary of their production methods.

A protocol for size separation of nanographenes

Top-down methods are convenient preparative methods for nanographenes, although the products consist of graphene fragments with a broad size distribution. We show that a combination of dialysis membranes (50, 25, 15, 8, and 2 kD) can conveniently separate nanographenes into five size distributions. The separated nanographenes can be employed as starting materials for carbon-based functional materials.

Top-down methods are convenient preparative methods for nanographenes, although the products consist of graphene fragments with a broad size distribution. We developed a simple protocol for size separation of nanographenes.