Retraction Note: Living annulative π-extension polymerization for graphene nanoribbon synthesis.

This article has been retracted. Please see the Retraction Notice for more detail: https://doi.org/10.1038/s41586-020-2950-0 .

Living annulative π-extension polymerization for graphene nanoribbon synthesis.

The properties of graphene nanoribbons (GNRs)1-5-such as conductivity or semiconductivity, charge mobility and on/off ratio-depend greatly on their width, length and edge structure. Existing bottom-up methods used to synthesize GNRs cannot achieve control over all three of these parameters simultaneously, and length control is particularly challenging because of the nature of step-growth polymerization6-18. Here we describe a living annulative π-extension (APEX)19 polymerization technique that enables rapid and modular synthesis of GNRs, as well as control over their width, edge structure and length. In the presence of palladium/silver salts, o-chloranil and an initiator (phenanthrene or diphenylacetylene), the benzonaphthosilole monomer polymerizes in an annulative manner to furnish fjord-type GNRs. The length of these GNRs can be controlled by simply changing the initiator-to-monomer ratio, achieving the synthesis of GNR block copolymers. This method represents a type of direct C-H arylation polymerization20 and ladder polymerization21, activating two C-H bonds of polycyclic aromatic hydrocarbons and constructing one fused aromatic ring per chain propagation step.

Double-Helix Supramolecular Nanofibers Assembled from Negatively Curved Nanographenes.

The layered structures of graphite and related nanographene molecules play key roles in their physical and electronic functions. However, the stacking modes of negatively curved nanographenes remain unclear, owing to the lack of suitable nanographene molecules. Herein, we report the synthesis and one-dimensional supramolecular self-assembly of negatively curved nanographenes without any assembly-assisting substituents. This curved nanographene self-assembles in various organic solvents and acts as an efficient gelator. The formation of nanofibers was confirmed by microscopic measurements, and an unprecedented double-helix assembly by continuous π-π stacking was uncovered by three-dimensional electron crystallography. This work not only reports the discovery of an all-sp2-carbon supramolecular π-organogelator with negative curvature but also demonstrates the power of three-dimensional electron crystallography for the structural determination of submicrometer-sized molecular alignment.

Step-Growth Annulative π-Extension Polymerization for Synthesis of Cove-Type Graphene Nanoribbons.

Graphene nanoribbons (GNRs), nanometer-wide strips of graphene, are attracting significant attention in materials science as candidates for next-generation carbon materials. As their physical properties mainly depend on their structures, the precise synthesis of structurally well-defined GNRs is highly desirable to control their properties. Herein, we report a step-growth annulative π-extension polymerization that allows for the rapid and modular synthesis of cove-type GNRs with pyrene and/or coronene diimide repeating units. The structures and photophysical properties of the separated GNRs were confirmed by various spectroscopic analyses. In addition, gas-blow-assisted uniform on-surface self-assembly of the GNRs was accomplished.

A Quest for Structurally Uniform Graphene Nanoribbons: Synthesis, Properties, and Applications.

Graphene nanoribbons are the class of next-generation carbon materials that are attracting many researchers in various research fields. Their unique properties, such as band gap, conductivity, carrier mobility, thermal conductivity, spin polarization, and on-off behavior, heavily depend on structural factors such as edge structure, width, and length. Therefore, the synthesis of graphene nanoribbons with control over these structural factors with atomic precision is crucially important. Among various synthetic approaches, bottom-up synthetic methods such as on-surface polymerizations and solution-phase polymerizations represent promising ways to control the width, length, and edge structure of graphene nanoribbons. In this Perspective, we introduce the recently reported bottom-up synthetic methods for graphene nanoribbons and the theoretical and experimental research on those physical properties and applications. Reviewing these researches along with highlighting advantages and limitations, we emphasize how structurally controlled synthesis is important and provides future outlooks in graphene nanoribbon science.

Carbon Nanosheets by Morphology-Retained Carbonization of Two-Dimensional Assembled Anisotropic Carbon Nanorings.

Two-dimensional (2D) carbon nanomaterials possessing promising physical and chemical properties find applications in high-performance energy storage devices and catalysts. However, large-scale fabrication of 2D carbon nanostructures is based on a few specific carbon templates or precursors and poses a formidable challenge. Now a new bottom-up method for carbon nanosheet fabrication using a newly designed anisotropic carbon nanoring molecule, CPPhen, is presented. CPPhen was self-assembled at a dynamic air-water interface with a vortex motion to afford molecular nanosheets, which were then carbonized under inert gas flow. Their nanosheet morphologies were retained after carbonization, which has never been seen for low-molecular weight compounds. Furthermore, adding pyridine as a nitrogen dopant in the self-assembly step successfully afforded nitrogen-doped carbon nanosheets containing mainly pyridinic nitrogen species.

Synthesis, properties, and crystal structures of π-extended double [6]helicenes: contorted multi-dimensional stacking lattice.

The synthesis and properties of a new π-extended double [6]helicene 2 and a dithia[6]helicene 3 are described. Compared to the previously reported parent double-helicene molecule 1, the introduction of n-butyl groups successfully improved the solubility, which allowed an experimental investigation into the electronic structure of 2 and 3 by photophysical measurements and cyclic voltammetry. The characteristic two-blade propeller structures of 2 and 3 were unambiguously determined by single-crystal X-ray diffraction analysis. The crystal packing structure of 2 exhibited a contorted two-dimensional stacking, whereby molecules of n-pentane were incorporated in the stacks. Despite the presence of n-butyl groups, 3 formed a unique three-dimensional stacking lattice in the crystal. Time-resolved microwave conductivity measurements revealed that the double helicenes (1-3) exhibited transient conductivities. An organic field-effect transistor fabricated using 3 was found to function as a p-type transistor.

Enhanced sensing response of oxidized graphene formed by UV irradiation in water.

A small amount of defects (less than 0.01%) were introduced into graphene by irradiating it with ultraviolet (UV) light in water. The chemisorbed oxygen species caused a limited amount of degradation in the charge carrier mobility, while the physisorbed water molecules caused both a reduction in the mobility and hole doping. The oxidation was nonuniform, owing to variations in the potential caused by the metal contacts. Raman spectroscopy measurements revealed that UV irradiation in water promoted mild oxidation of graphene's basal plane, which enhanced the electrical sensing response of the adsorption of water molecules. The enhanced electrical response was achieved by the high binding energy of the water molecules at the oxidized sites and the near-zero Dirac point voltage, easily obtained by desorbing the physisorbed water molecules.

Perfluorocycloparaphenylenes.

Perfluorinated aromatic compounds, the so-called perfluoroarenes, are widely used in materials science owing to their high electron affinity and characteristic intermolecular interactions. However, methods to synthesize highly strained perfluoroarenes are limited, which greatly limits their structural diversity. Herein, we report the synthesis and isolation of perfluorocycloparaphenylenes (PFCPPs) as a class of ring-shaped perfluoroarenes. Using macrocyclic nickel complexes, we succeeded in synthesizing PF[n]CPPs (n = 10, 12, 14, 16) in one-pot without noble metals. The molecular structures of PF[n]CPPs (n = 10, 12, 14) were determined by X-ray crystallography to confirm their tubular alignment. Photophysical and electrochemical measurements revealed that PF[n]CPPs (n = 10, 12, 14) exhibited wide HOMO-LUMO gaps, high reduction potentials, and strong phosphorescence at low temperature. PFCPPs are not only useful as electron-accepting organic materials but can also be used for accelerating the creation of topologically unique molecular nanocarbon materials.