Akihito Konishi

Synthesis and Characterization of Teranthene: A Singlet Biradical Polycyclic Aromatic Hydrocarbon Having Kekulé Structures

A teranthene derivative has been successfully isolated in a crystalline form for the first time. Geometrical considerations and physical property investigations indicate that the molecule possesses prominent biradical character in the ground state.

Synthesis and characterization of quarteranthene: elucidating the characteristics of the edge state of graphene nanoribbons at the molecular level.

The characteristics of the edge state, which is a peculiar magnetic state in zigzag-edged graphene nanoribbons (ZGNRs) that originates from electron-electron correlation in an edge-localized π-state, are investigated by preparing and characterizing quarteranthene molecules. The molecular geometry that was determined from the X-ray analysis is consistent with a zigzag-edge-localized structure of unpaired electrons. The localized electrons are responsible for the peculiar magnetic (room-temperature ferromagnetic correlation), optical (the lowest-lying doubly excited state), and chemical (peroxide bond formation) behaviors. On the basis of these distinguishing properties and a careful consideration of the valence bonding, insight into the edge state of ZGNRs can be gained.

An extremely simple dibenzopentalene synthesis from 2-bromo-1-ethynylbenzenes using nickel(0) complexes: construction of its derivatives with various functionalities.

Nice and easy: A very simple synthesis for dibenzopentalenes, which starts from 1-bromo-2-ethynylbenzenes, has been developed. It uses Ni(0) complexes (see scheme), from which a relatively stable Ni(II) complex as an important intermediate has been isolated. Dibenzopentalenes with various functional groups can be prepared by the procedure, and their electronic properties are consistent with theoretical calculations.An extremely simple dibenzopentalene synthesis from readily available 2-bromo-1-ethynylbenzenes using a nickel(0) complex is described. Although the yields are moderate, the formation of three C-C bonds in a single process and the high availability of the starting materials are important advantages of this reaction. The corresponding aryl-nickel(II) complex as an important intermediate was isolated as relatively stable crystals, and the structure was confirmed by X-ray crystallographic analysis. The high stability of this complex should play a key role in this reaction. The reaction is applicable to the preparation of dibenzopentalenes bearing various functional groups. Their electronic properties are consistent with theoretical calculations. The cyclic voltammograms of these compounds reveal highly amphoteric redox properties. In particular, the electron-donating property of a tetramethoxy derivative is greater than that of oligothiophenes and dibenzodithiophenes and almost comparable to that of pentacene.

Synthesis and electronic structure of bisanthene: A small molecular-sized graphene with zigzag edges

The electronic structure of bisanthene has been considered as a bottom-up approach to provide experimental evidence for the edge-state in graphene ribbons. After several steps of chemical reactions and following recrystallization, bisanthene and its tert-butyl group substituted derivative were obtained as crystals. Physical measurements demonstrated no appreciable open-shell electronic structure, while chemical reactivity toward oxygen suggested the possibility of the presence of the spin-polarized electronic state at meso-positions.

Synthesis and Characterization of Dibenzo[a,f]pentalene: Harmonization of the Antiaromatic and Singlet Biradical Character

Mesityl derivatives of the unknown dibenzopentalene isomer dibenzo[a,f]pentalene were synthesized. The molecular geometry and physical properties of dibenzo[a,f]pentalene were investigated. Dibenzo[a,f]pentalene combines a large antiaromatic and appreciable singlet open-shell character, properties not shared by well-known isomer dibenzo[a,e]pentalene.

Anthenes: Model systems for understanding the edge state of graphene nanoribbons

Abstract The edge state, which is a peculiar magnetic state in zigzag-edged graphene nanoribbons (ZGNRs) originating from an electron–electron correlation in an edge-localized π-state, has promising applications for magnetic and spintronics devices and has attracted much attention of physicists, chemists, and engineers. For deeper understanding of the edge state, precise fabrication of edge structures in ZGNRs has been highly demanded. We focus on anthenes, which are peri-condensed anthracenes that have zigzag and armchair edges on the molecular periphery, as model systems for understanding, and indeed prepare and characterize them. This paper summarizes our recent studies on the origin of the edge state by investigating anthenes in terms of the relationship between the molecular structure and spin-localizing character.

Construction of Polycyclic π‐Conjugated Systems Incorporating an Azulene Unit Following the Oxidation of 1,8‐Diphenyl‐9,10‐bis(phenylethynyl)phenanthrene

Polycyclic aromatic hydrocarbons (PAHs) that incorporate either heptagons or pentagons consist of non-planar molecular structures with unusual optoelectronic properties, and the design of a relatively simple and efficient method to construct these highly fused π-conjugated systems with odd-membered rings is in high demand. This work describes the use of silver(I) cations to promote the efficient synthesis of azulene-embedded PAH 2, which is a structural isomer of tribenzo[fg,ij,rst]pentaphene 3, via tandem oxidative transannulation between the phenyl and arylethynyl moieties. This method involves a carbophilic interaction of the silver(I) cation with the acetylene units, which facilitates an electron transfer in the initial step. The synthesized PAH 2 and the protonated cation 2 H⋅BF4 were fully characterized by X-ray crystallographic analysis, electronic absorption, electrochemical measurement, and quantum chemical calculation. The azulene-embedded PAH 2 exhibited a low-energy absorption band and amphoteric redox events, which were characterized as non-alternant characteristics originating from the azulene unit.

C3-Symmetric Boron Lewis Acid with a Cage-Shape for Chiral Molecular Recognition and Asymmetric Catalysis

Chiral Lewis acids play an important role in the precise construction of various types of chiral molecules. Here, a cage-shaped borate 2 was designed and synthesized as a chiral Lewis acid that possesses a unique C3 -symmetric structure composed of three homochiral binaphthyl moieties. The highly symmetrical structure of 2 with homochirality was clearly elucidated by X-ray crystallographic analysis. The peculiar chiral environment of 2⋅THF exhibited chiral recognition of some simple amines and a sulfoxide. Moreover, the application of 2⋅THF to hetero-Diels-Alder reactions as a chiral Lewis-acid catalyst afforded the enantioselective products, which were obtained through an entropy-controlled pathway according to the analysis of the relationship between optical yield and reaction temperature. In particular, the robust chiral reaction field of 2⋅THF allowed the first example of an asymmetric hetero-Diels-Alder reaction with a simple diene despite the requirement of high temperature.

1,8‐Diphenyl‐9,10‐Bis(arylethynyl)phenanthrenes: Synthesis, Distorted Structure, and Optical Properties

The synthesis and optical properties of 1,8-diphenyl-9,10-bis(arylethynyl)phenanthrenes, which are distorted phenanthrenes, are reported. The presence of the two phenyl groups at the 1,8-positions of phenanthrene significantly distorts the molecular geometries, as was evidenced by X-ray crystallography. The congested substitution pattern in the K region results in a distorted aromatic framework, which leads to a redshift in the emission spectrum. These observations are in stark contrast to 9,10-bis(phenylethynyl)phenanthrene with no phenyl groups at the 1,8-positions. A large Stokes shift suggested extensive structural relaxation between the phenyl and arylethynyl units in the excited state, which was supported by theoretical calculations.

Organic Chemistry of Graphene Framework

Graphene, a sheet of carbon atoms arranged in a honeycomb lattice, has attracted much attention from chemists, physicists, and material scientists, because their peculiar electronic properties promise novel functions that radically change our lives in the coming years. Great contribution of physics has been disclosing electronic structures of graphene, whereas some of them can be understood by bottom-up approach, that is, by relating to chemistry of polycyclic aromatic hydrocarbons. This chapter mainly focuses on the origin of the magnetic properties of polycyclic aromatic hydrocarbons in terms of aromatic sextet formation and spin localization, in relation to “edge state” of graphene and graphene nanoribbons.