Tomoyuki Akutagawa

Ferroelectricity and polarity control in solid-state flip-flop supramolecular rotators

Molecular rotation has attracted much attention with respect to the development of artificial molecular motors, in an attempt to mimic the intelligent and useful functions of biological molecular motors. Random motion of molecular rotators--for example the 180 degree flip-flop motion of a rotatory unit--causes a rotation of the local structure. Here, we show that such motion is controllable using an external electric field and demonstrate how such molecular rotators can be used as polarization rotation units in ferroelectric molecules. In particular, m-fluoroanilinium forms a hydrogen-bonding assembly with dibenzo[18]crown-6, which was introduced as the counter cation of [Ni(dmit)(2)](-) anions (dmit(2-) = 2-thioxo-1,3-dithiole-4,5-dithiolate). The supramolecular rotator of m-fluoroanilinium exhibited dipole rotation by the application of an electric field, and the crystal showed a ferroelectric transition at 348 K. These findings will open up new strategies for ferroelectric molecules where a chemically designed dipole unit enables control of the nature of the ferroelectric transition temperature.

A molecular metal with ion-conducting channels

Metallic behaviour is well known in charge-transfer complexes that contain stacks of planar, partially oxidized (or reduced) π-conjugated molecules. Electronic conduction occurs in the partially occupied, delocalized π bands formed by intermolecular orbital overlap, and some of these materials exhibit superconductivity,. Counter-ions, present to achieve charge neutrality, usually play a passive role, although in some cases they couple to the electronic structure, for example by imposing a new structural periodicity (a superlattice) by orientational ordering. The development of molecular solids that can simultaneously support the transport of both electrons and ions is important for several fields, including the development of solid-state batteries,, electroluminescent devices and biomimetic systems,. Crown ethers are promising components for such systems, as they provide cavities through which ion motion might occur. Here we report that the charge-transfer salt Li0.6(15-crown-5-ether)[Ni(dmit)2]2·H2O exhibits both electron and ion conductivity: the stacks of the nickel complex (dmit is an organic molecule) provide a pathway for electron conduction, and stacks of the crown ethers provide channels for lithium-ion motion. Evidence for the latter above 250 K is provided by NMR and conductivity studies. We also see evidence for coupling of the electron and ion motions. This compound might serve as a model for the development of other hybrid electronic/ionic conducting materials.

Temperature dependence of band gap energies of GaAsN alloys

The temperature dependence of band gap energies of GaAsN alloys was studied with absorption measurements. As the N concentration in GaAsN increased, the temperature dependence of the band gap energy was clearly reduced in comparison with that of GaAs. The redshift of the absorption edge in GaAsN for the temperature increase from 25 to 297 K was reduced to 60% of that of GaAs for the N concentration larger than ∼1%. The differential temperature coefficient of the energy gap at room temperature was also reduced to 70% of that of GaAs. The main factor for this reduced temperature dependence in GaAsN was attributed to the transition from band-like states to nitrogen-related localized states with detailed studies of the temperature-induced shift of the absorption edge.

Mesomorphic behaviour of 1,3-phenylene bis[4-(4-alkoxyphenyliminomethyl)benzoates] and related compounds

Abstract DSC and X-ray diffraction studies on a series of 1,3-phenylene bis[4-(4-alkoxyphenyliminomethyl)benzoates] are presented. The only mesophase exhibited by the methoxy to hexadecyloxy homologues is of the smectic C type. The reversal of the iminomethyl linkage reduces drastically the incidence of a mesophase; that is, only the first five homologous members exhibit a smectic C phase in the series of 1,3-phenylene bis[4-(4-alkoxybenzylideneamino)benzoates]. Almost all the members in the second series become nematogenic by the chloro substitution at the 4-position of the 1,3-phenylene moiety. In addition, a smectic C phase is observable for the ethoxy to pentyloxy and also the tetradecyloxy and hexadecyloxy members. The second chloro-substituent introduced to the 6-position of the same central ring eliminates completely the smectic C phase and enhances the nematic thermal stability.

[Ni(dmit)2] salts with supramolecular cation structure

Abstract The dithiolate complex, [Ni(dmit) 2 ] (dmit=2-thioxo-1,3-dithiol-4,5-dithiolate) is an organic π-molecule which gives highly conducting salts with closed shell cations or organic donors. We have found that crown ether macrocycles can be incorporated into the salts in which the crown ethers include alkali metal cations forming supramolecular cation (SC) structures. The SC structural diversity was observed to depend on the size of the crown ether and cation. Three kinds of crown ethers, 12-crown-4, 15-crown-5 and 18-crown-6 in addition to an acyclic polyether analog of pentaetyleneglycol were incorporated in the conducting [Ni(dmit) 2 ] crystals. The structure of these crystals will be described with emphasis on both the structures of SC and Ni(dmit) 2 conducting layers. In one example, the crown ether forms a channel in which the cation has a translational freedom. The cation, otherwise merely forms a periodic Coulomb potential in the crystal, can influence the electrical conduction through the motion in the channel. The monovalent [Ni(dmit) 2 ] salts, which are possible candidates for molecular magnets will be described on a salt having SC structure.

Control of assembly and magnetism of metal–dmit complexes by supramolecular cations

Abstract The assembly of monovalent species of [Ni(dmit) 2 ] (dmit 2− =2-thioxo-1,3-dithiole-4,5-dithiolate), which bear S =1/2 spin on each molecule, was controlled by using crown-ether based supramolecular cations. We could regulate the shape and valence of the supramolecular cations by changing the ring size of the crown ethers as well as the metal cations included. Since the structure of the ionic crystals is mainly determined by the Madelung energy and the Lennard-Jones potential (shape of molecules), a large diversity in crystal structure was observed. A relationship between the overlap of [Ni(dmit) 2 ] frontier orbitals and the magnetism is discussed.

Adsorption and Catalytic Properties of the Inner Nanospace of a Gigantic Ring-Shaped Polyoxometalate Cluster**

are of great interestbecause of their unique properties such as adsorption,separation, exchange, and catalysis, which are all associatedwith the presence of a functional nanospace. In this respect,polyoxometalate clusters, which are nanosized metal-oxidemacroanions built from molecular precursors with a range ofunique redox and acidic properties, are ideally suited for thedevelopment of functional nanospaces but have not yet beenthus explored.

Formation of oriented molecular nanowires on mica surface

Molecular “nanowire” structures composed of the charge transfer complex of a bis-tetrathiafulvalene substituted macrocycle and tetrafluorotetracyanoquinodimethane were constructed on mica substrates by employing the Langmuir–Blodgett technique. The nanowires transferred from a dilute aqueous potassium chloride subphase had typical dimensions of 2.5 nm × 50 nm × 1 μm. The nanowires are oriented to specific directions, corresponding to the directions of the potassium-ion array on the mica surface having sixfold symmetry. Such correlation between the nanowires and the substrate surface was also observed when a dilute aqueous rubidium chloride subphase was used. On the other hand, the correlation completely disappeared when the subphase contained divalent cations, indicating that the molecular nanowires orient by recognizing the monocation array on the mica surface. The nanowires formed by the vertical dipping method coexist with the monolayers. Only nanowire structures are, however, observed when we apply the horizontal lifting method. Based on the crystal structure of a related complex, a possible structure of the nanowires is presented. The conductivity of the nanowires was estimated to be of the order of 10−3 S⋅cm−1. The nanowires formed specific (regular) structures such as T-shape junctions, suggesting their use in construction of future molecular nanoscale devices.

Solid-State Molecular Rotators of Anilinium and Adamantylammonium in [Ni(dmit)2]-Salts with Diverse Magnetic Properties

Supramolecular rotators of hydrogen-bonding assemblies between anilinium (Ph-NH 3 (+)) or adamantylammonium (AD-NH 3 (+)) and dibenzo[18]crown-6 (DB[18]crown-6) or meso-dicyclohexano[18]crown-6 (DCH[18]crown-6) were introduced into [Ni(dmit) 2] salts (dmit (2-) is 2-thioxo-1,3-dithiole-4,5-dithiolate). The ammonium moieties of Ph-NH 3 (+) and AD-NH 3 (+) cations were interacted through N-H (+) approximately O hydrogen bonding with the six oxygen atoms of crown ethers, forming 1:1 supramolecular rotator-stator structures. X-ray crystal-structure analyses revealed a jackknife-shaped conformation of DB[18]crown-6, in which two benzene rings were twisted along the same direction, in (Ph-NH 3 (+))(DB[18]crown-6)[Ni(dmit) 2] (-) ( 1) and (AD-NH 3 (+))(DB[18]crown-6)[Ni(dmit) 2] (-) ( 3), whereas the conformational flexibility of two dicyclohexyl rings was observed in (Ph-NH 3 (+))(DCH[18]crown-6)[Ni(dmit) 2] (-) ( 2) and (AD-NH 3 (+))(DCH[18]crown-6)[Ni(dmit) 2] (-) ( 4). Sufficient space for the molecular rotation of the adamantyl group was achieved in the crystals of salts 3 and 4, whereas the rotation of the phenyl group in salts 1 and 2 was rather restricted by the nearest neighboring molecules. The rotation of the adamantyl group in salts 3 and 4 was evidenced from the temperature-dependent wide-line (1)H NMR spectra, dielectric properties, and X-ray crystal structure analysis. ab initio calculations showed that the potential energy barriers for the rotations of adamantyl groups in salts 3 (Delta E approximately 18 kJmol (-1)) and 4 (Delta E approximately 15 kJmol (-1)) were similar to those of ethane ( approximately 12 kJmol (-1)) and butane (17-25 kJmol (-1)) around the C-C single bond, which were 1 order of magnitude smaller than those of phenyl groups in salts 1 (Delta E approximately 180 kJmol (-1)) and 2 (Delta E approximately 340 kJmol (-1)). 1D or 2D [Ni(dmit) 2] (-) anion arrangements were observed in the crystals according to the shape of crown ether derivatives. The 2D weak intermolecular interactions between [Ni(dmit) 2] (-) anions in salts 1 and 3 led to Curie-Weiss behavior with weak antiferromagnetic interaction, whereas 1D interactions through lateral sulfur-sulfur atomic contacts between [Ni(dmit) 2] (-) anions were observed in salts 2 and 4, whose magnetic behaviors were dictated by ferromagnetic (salt 2) and singlet-triplet (salt 4) intermolecular magnetic interactions, respectively.

Supramolecular Silver Polyoxometalate Architectures Direct the Growth of Composite Semiconducting Nanostructures

Nanosilver on a string: Crystalline silver polyoxovanadate supramolecular architectures are employed as precursors for the synthesis of composite nanowires (see scheme). The nanostructures are composed of semiconducting vanadium oxide which forms wires with high aspect ratios, and are embedded with metallic silver nanoparticles.