Yasukazu Hirao

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.

Alternating covalent bonding interactions in a one-dimensional chain of a phenalenyl-based singlet biradical molecule having Kekulé structures.

A novel naphthoquinoid singlet biradical (2a) stabilized by phenalenyl rings is prepared by a multistep procedure and is investigated in terms of covalent bonding interactions. The molecule 2a gives single crystals, in which a 1D chain is formed with a very short π-π contact at the overlapping phenalenyl rings. The unpaired electrons in 2a are involved in covalent bonding interactions not only within the molecule but also between the molecules in the 1D chain, and a linear conjugation is made of the alternating intra- and intermolecular covalent bonding interactions through conventional π-conjugation and multicenter bonding, respectively. The linear conjugation causes a lower-energy shift of the optical transition band in the crystal, but the transition energy is higher than that of the benzoquinoid singlet biradical (1a). This optical behavior and the magnetic susceptibility measurements reveal that the intermolecular covalent bonding interaction in the 1D chain of 2a is greater in strength than the intramolecular one, despite the fact that a fully conjugated Kekulé structure can be drawn for 2a.

Resonance balance shift in stacks of delocalized singlet biradicals.

Recently we succeeded in the isolation of delocalized singlet biradicals utilizing the spin-delocalizing character of the phenalenyl radical. We demonstrated that the singlet biradical 1 has strong spin–spin interactions between molecules through the overlap of phenalenyl rings in the onedimensional (1D) chain even though the closed-shell Kekul structure 1 can be drawn as a resonance contributor (Scheme 1). Huang and Kertesz gave further insight into the spin–spin interactions from a theroretical point of view and showed that the spin–spin interaction between the molecules was predicted to be stronger than that within the molecule. These experimental and theoretical findings are associated with very fundamental issues: Do delocalized singlet biradicals actually have open-shell character? Are the electrons coupled within a molecule involved in covalent bonding between molecules? In this study we will demonstrate that intraand intermolecular spin–spin interactions strongly correlate and can be altered in magnitude by an applied external field. Our proposal is based on the experimentally determined molecular structure of 2, a temperature-dependent reflection spectrum of 2, and a pressuredependent reflection spectrum of 1. Methyl groups at the 2and 10-positions in 2, where the frontier molecular orbital has very small coefficients, are expected to alter the distance between the overlapping phenalenyl rings with respect to the analogous separation in 1, and as a result, the magnitude of the intermolecular spin–spin interaction should be affected. The synthesis of 2 is outlined in Scheme 2. The 3,10and 3,11-bis(bromomethyl) compounds 3 were synthesized according to the previously reported procedures. The individual isomers were not isolated because both were expected to lead to the single compound 2. Bis(2-methylpropionic acid) derivatives 5 were obtained in three steps by standard methods. Friedel–Crafts cyclization of the acyl chloride derivatives of 5 with AlCl3 afforded diketones 6. These were reduced with NaBH4 and subsequently dehydrated with a catalytic amount of p-toluenesulfonic acid to afford the dihydro compounds 8. Dehydrogenation of 8 with p-chloranil afforded the hydrocarbon 2 as green prisms. Compound 2 was found to be stable in the solid state at room temperature. The small HOMO–LUMO gap of 2, which is an essential factor for a singlet biradical electronic structure, was confirmed by electrochemical and optical methods. The cyclic voltammogram of 2 shows four reversible redox waves: E 2 = + 0.51, E 1 =+ 0.11, E red 1 = 1.09, and E 2 = 1.62 (V vs. ferrocene/ferrocenium couple (Fc/Fc), see Figure S1 in the Supporting Information), which led to an electrochemical HOMO–LUMO gap of 1.20 eV. The electronic absorption spectrum of 2 in CH2Cl2 shows an intense low-energy band at 756 nm (13200 cm = 1.64 eV, e= 115000, f= 0.605, see Scheme 1. Resonance structures of 1 and 2. The arrows in the biradical structure represent antiparallel spins.

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, crystal structure, and physical properties of sterically unprotected hydrocarbon radicals.

We have prepared and isolated neutral polycyclic hydrocarbon radicals. A butyl-substituted radical gave single crystals, in which a π-dimeric structure, not a σ-bonded dimer, was observed, even though steric protection was absent. Thermodynamic stabilization due to the highly spin-delocalized structure contributes effectively to the suppression of σ-bond formation.

Theoretical consideration of singlet open-shell character of polyperiacenes using Clar's aromatic sextet valence bond model and quantum chemical calculations

Singlet open-shell character of polyperiacenes has been elucidated by theoretical analysis based on the Clars aromatic sextet valence bond model and quantum chemical calculations. Anti-ferromagnetic ground state of large polyperiacenes was relevant to the sufficient stabilization energy derived from aromatic sextet formation to overcome the energetic penalty associated with π-bond cleavages.

Design and synthesis of new stable fluorenyl-based radicals.

Organic neutral radicals have long fascinated chemists with a fundamental understanding of structure-reactivity relationships in organic reactions and with applications as new functional materials. However, the elusive nature of these radicals makes the synthesis, isolation, and characterization very challenging. In this work, the synthesis of three long-lived, fluorenyl-based radicals are reported. The geometry and electronic structures of these radicals were systematically investigated with a combination of various experimental methods, besides density functional theory (DFT) calculations, which include X-ray crystallographic analysis, electron spin resonance (ESR), electron nuclear double resonance (ENDOR), cyclic voltammetry, and UV-vis-NIR measurements. Their half-life periods (τ(1/2)) in air-saturated solution under ambient conditions were also determined. Surprisingly, all three radicals showed remarkable stabilities: τ(1/2) = 7, 3.5, and 43 days.

Trimacrocyclic arylamine and its polycationic states

A novel trimacrocyclic arylamine was found to be accessible to the different spin-states by consecutive electrochemical or chemical oxidation.

Dual Association Modes of the 2,5,8-Tris(pentafluorophenyl)phenalenyl Radical

The 2,5,8-tris(pentafluorophenyl)phenalenyl radical was obtained by a straightforward synthesis in 11 steps from 2,7-dibromonaphthalene. This radical crystallized as a σ dimer from a solution in MeCN and as a π-stack from a melted liquid. The π stack was not confined to dimerization, but extended into a uniform 1D stack with an interplanar distance of 3.503 Å. This unique duality in association mode arose from the thermodynamic stability of the phenalenyl moiety.