Kouzou Matsumoto

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 of tetrakis(2-pyridyl)methane: the first tetrapyridylmethane

Tetrakis(2-pyridyl)methane has been synthesized as the first member of tetrapyridylmethane family by nucleophilic aromatic substitution of 2-chloropyridine with tris(2-pyridyl)methyl anion in refluxing aromatic hydrocarbons; the use of 2-bromopyridine resulted in electron transfer giving rearranged dimer of tris(2-pyridyl)methyl radical.

Synthesis of tetrakis(2-thienyl)methane

Tetrakis(2-thienyl)methane was synthesized for the first time by sodium sulfide induced cyclization of 5,5,5-tris(2-thienyl)pentadiyne which was obtained from tris(2-thienyl)methyl cation, and the molecular structure was elucidated by X-ray analysis for the tetrabromo derivative.

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 functionalization of 3,3'-bis(spirodienone)-bridged 2,2'-bithiophene: a new building block for redox-active molecular switching materials.

Bithiophene derivatives bridged with a bis(spirodienone) unit were synthesized and characterized. Lithiation of the thiophene rings of an unsubstituted derivative proceeded without decomposition of the bis(spirodienone) skeleton. Palladium-catalyzed cross-coupling reactions (Suzuki-Miyaura, Sonogashira) with bromides afforded a variety of pi-extended derivatives. Bond breaking and formation under redox conditions were observed by cyclic voltammetry.

Synthesis and Identification of a Trimethylenemethane Derivative π-Extended with Three Pyridinyl Radicals

A trimethylenemethane (TMM) derivative 1(2*), in which the parent TMM is pi-extended by the symmetric insertion of three pyridine rings into the C-C bonds of TMM, has been synthesized by the alkali metal reduction of the isolated corresponding dication. Although the frozen-glass X-band cw-ESR spectrum of 1(2*) gave unresolved fine structures due to the small ZFS parameters, pulsed ESR two-dimensional electron spin transient nutation (2D-ESTN) spectroscopy unambiguously can afford to identify diradical 1(2*) as a triplet species.