Takeji Takui

Synthetic organic spin chemistry for structurally well-defined open-shell graphene fragments

Graphene, a two-dimensional layer of sp(2)-hybridized carbon atoms, can be viewed as a sheet of benzene rings fused together. Three benzene rings can be combined in three different ways, to yield linear anthracene and angular phenanthrene, where the rings share two C-C bonds, and the phenalenyl structure where three C-C bonds are shared between the rings. This third structure contains an uneven number of carbon atoms and, hence, in its neutral state, an uneven number of electrons--that is, it is a radical. All three structures may be viewed as being sections of graphene. Extension of this concept leads to an entire family of phenalenyl derivatives--open-shell graphene fragments--that are of substantial interest from the standpoint of fundamental science as well as in view of their potential applications in materials chemistry, in particular quantum electronic devices. Here we discuss current trends and challenges in this field.

Organic tailored batteries materials using stable open-shell molecules with degenerate frontier orbitals

Secondary batteries using organic electrode-active materials promise to surpass present Li-ion batteries in terms of safety and resource price. The use of organic polymers for cathode-active materials has already achieved a high voltage and cycle performance comparable to those of Li-ion batteries. It is therefore timely to develop approaches for high-capacity organic materials-based battery applications. Here we demonstrate organic tailored batteries with high capacity by using organic molecules with degenerate molecular orbitals (MOs) as electrode-active materials. Trioxotriangulene (TOT), an organic open-shell molecule, with a singly occupied MO (SOMO) and two degenerate lowest-unoccupied MOs (LUMOs) was investigated. A tri-tert-butylated derivative ((t-Bu)(3)TOT)exhibited a high discharge capacity of more than 300 A h kg(-1), exceeding those delivered by Li-ion batteries. A tribrominated derivative (Br(3)TOT) was also shown to increase the output voltage and cycle performance up to 85% after 100 cycles of the charge-discharge processes.

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.

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.

A Synthetic Two-Spin Quantum Bit: g-Engineered Exchange-Coupled Biradical Designed for Controlled-NOT Gate Operations†

A quantum gate: A system of two coupled electron spins that is useful for simple quantum computing operations has been prepared by synthesis of a biradical 1 and co-crystallization with an isomorphous host molecule. The two weakly exchange-coupled quantum bits (target qubit blue and control qubit red) span four electron spin states. The electron spin transition is denoted by two black arrows.

Triple-stranded metallo-helicates addressable as Lloyd's electron spin qubits.

We have first achieved the synthesis of triple-stranded metallo-helicates composed of 4,4:2,2:4,4-quaterimidazole (Qim) and Mn(II) or Zn(II) ions, which serve as synthetic electron spin qubits (quantum bits). In the crystal structure, a hydrogen-bonding network through counteranions and/or crystal solvents was constructed by the outward N-H hydrogen-bonding functional groups intrinsic to the imidazole skeleton. Importantly, these helicates showed high stability even in a solution state at room temperature. These salient features of triple helicates of Qim are different from those of reported metallo-helicates. These chemical properties of the Qim-based triple helicates allow us to synthesize magnetically diluted single crystals composed of Mn(II) (S = 5/2) and diamagnetic Zn(II) complexes of Qim in an appropriate Mn(II)/Zn(II) ratio. The magnetically diluted crystals can afford to build up the prototype of electron-spin qubits of Lloyds one-dimensional periodic system, which gives a practical approach to scalable quantum computers/quantum information processing systems (QCs/QIPSs). The experiments have proven the practical capability of oligo(imidazole)s as a component of Lloyds system which has nonequivalent g-tensors within the helicate (g-engineering). The helical symmetry plays an important role in giving a prototype of the synthetic spin qubits of the formidable Lloyd model. This result links supramolecular chemistry to the field of QCs/QIPSs.

Nitroxide-substituted nitronyl nitroxide and iminonitroxide.

We report on the highly compact nitroxide-substituted nitronyl nitroxide 1 and iminonitroxide 2; they have isoelectronic structures with trimethylenemethane. These diradicals are stable under aerated conditions at room temperature and have large positive exchange interactions: J/k(B) = +390 K (H = -2JS(1)(/)(2)·S(1/2)) for 1 and J/k(B) ≈ +550 K for 2.

Chiral Stable Phenalenyl Radical: Synthesis, Electronic‐Spin Structure, and Optical Properties of [4]Helicene‐Structured Diazaphenalenyl

Helicenes are ortho-annelated polycyclic aromatic molecules with nonplanar helical conjugation of the p-electron system. These aromatic molecules have attracted special attention not only because of the nonplanarity of the p-conjugated skeleton but also because of their inherent helical chirality. Recent developments in helicene chemistry have focused on the synthesis and optical resolution of new helicene derivatives as well as on asymmetric catalysts, molecular recognition, and chiroptical functionalities. However, almost all of the helicene derivatives investigated so far are intrinsically closed-shell electronic systems except for a few examples. Thus, the chemistry of open-shell helicenes has been little explored in the past; to our knowledge there have been no experimental studies on helicene-based nonplanar chiral neutral p-radical systems. As stable open-shell molecular systems possessing chirality, a number of nitroxide-based chiral neutral radical derivatives have been synthesized and characterized. These systems were constructed by introducing a substituent having one or more asymmetric carbon atoms to an achiral nitroxide radical skeleton based on, for example, pyrrolidine-1-oxyl and a-nitronylnitroxide. By taking advantage of the localized unpaired electronic spin on the N O moiety, chiral molecular magnets and paramagnetic organic chiral liquid crystals have extensively been implemented. 10] In contrast to the nitroxide radicals, in phenalenyl (PLY in Figure 1), a planar neutral hydrocarbon p-radical, the electronic spin is extensively delocalized over the whole molecular skeleton. As a consequence, phenalenyl derivatives exhibit unique physical properties and functionalities different from the those of spin-localized neutral radicals such as nitroxide derivatives. 12] Furthermore, our recent study revealed that the highly delocalized electronic structure inherent in phenalenyl also appears in a curved phenalenyl system with a corannulene substructure (Cor-PLY, Figure 1). 14] Cor-PLY is the first nonplanar phenalenyl derivative in the half-century-old phenalenyl chemistry. These studies have inspired us to design nonplanar pconjugated chiral phenalenyl radicals based on helicene. The chiral neutral p-radical is intriguing not only because of the spin delocalization in the three-dimensional (3D) helical p-conjugated network but also because of optical magnetic properties attributable to the combination of helicene chirality and delocalized electronic spin. Here we report the synthesis, electronic-spin structure, and optical properties of chiral diazaphenalenyl 1 (Figure 1), in which a benzene ring is fused to a nitrogen-containing [4]helicene-type structure. Thanks to two terminal methoxy groups and three tert-butyl groups, radical 1 exhibits high configurational and chemical stability, enabling us to experimentally characterize both chiral and racemic species of 1 with the help of theoretical calculations. Solution-phase ESR and ENDOR/TRIPLE studies indicate that the unpaired electron spin is extensively delocalized over the nonplanar helicenic p-conjugated skeleton. Furthermore, the racemization behavior of 1 was also successfully studied by time-dependent solution-phase circular dichroism (CD) measurements at several temperatures. We first prepared the helicenic neutral p-radical 1 as a racemic compound, rac-1, to evaluate its chemical stability and electronic-spin structure. The five-step synthesis from 1tert-butyl-3,5-dimethoxybenzene (2) is depicted in Scheme 1. Triarylmethyl cation salt 3 was obtained by nucleophilic reaction of lithiated 2 with diethyl carbonate followed by treatment with aqueous tetrafluoroboric acid. A double condensation reaction of 3 with N,N-dimethylethylenediamine gave the [4]helicene-type compound 4 in racemic Figure 1. Chemical structures of PLY, Cor-PLY, and 1 (the P enantiomer is arbitrarily chosen).

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.

Organic Rechargeable Batteries with Tailored Voltage and Cycle Performance

Made to order: Rechargeable batteries are fabricated by using organic electron acceptors and donors as active cathode materials. Their output voltage and cycle performance can be tuned by organic chemistry techniques. The output voltages are linked to both the redox potentials and the energy levels of the frontier molecular orbitals of the cathode materials, enabling to predict the output voltage at an early stage of the design.