Ayumi Ishii

Structural and spectroscopic properties of a copper(I)–bis(N-heterocyclic)carbene complex

The structural and spectroscopic properties of a Cu(I) complex bearing a methylene-linked bis(N-heterocyclic carbene) ligand, [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) were investigated. X-ray single crystal structure analysis revealed that the complex is binuclear similar to the corresponding silver(I) complex. In [Cu(2)(mu-Me-mbim)(2)](PF(6))(2), cation-pi interaction between copper and the adjacent carbene carbon is observed. On the other hand, the copper-copper interaction is very weak in the crystal and almost negligible in solution. The absorption spectrum of [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) in methanol shows a strong absorption band (epsilon = 23 000 dm(3) mol(-1) cm(-1)) and a weaker shoulder (epsilon = 6200 dm(3) mol(-1) cm(-1)) at 261 nm and 300 nm, respectively. From molecular orbital calculations using TD-DFT, these absorption bands are assigned to the metal-centered transitions with some contribution from the NHC orbitals. The powdered sample of [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) shows bright blue-green phosphorescence with a high quantum yield (43%). The phosphorescence is of dual-emission character at room temperature with peak maxima at 374 nm and 482 nm whereas it changes to a single emission band centered around 500 nm at 77 K. Molecular orbital calculations indicate that the luminescence derives from the triplet MC and MLCT mixed excited states. A methanolic solution of [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) shows yellow-green phosphorescence with a peak maximum at 542 nm. Unlike in the solid state, no dual-emission was observed. These results suggest that the dual emission is caused by differences in the contribution of metal-metal interactions at room temperature in the solid state. The differences in the absorption and emission properties between [Cu(2)(mu-Me-mbim)(2)](PF(6))(2) and the related Cu(I)-diphosphine complex, [Cu(2)(mu-dcpm)(2)](BF(4))(2) are discussed.

Gate-tunable atomically thin lateral MoS2 Schottky junction patterned by electron beam

Among atomically thin two-dimensional (2D) materials, molybdenum disulfide (MoS2) is attracting considerable attention because of its direct bandgap in the 2H-semiconducting phase. On the other hand, a 1T-metallic phase has been revealed, bringing complementary application. Recently, thanks to top-down fabrication using electron beam (EB) irradiation techniques, in-plane 1T-metal/2H-semiconductor lateral (Schottky) MoS2 junctions were demonstrated, opening a path toward the co-integration of active and passive two-dimensional devices. Here, we report the first transport measurements evidencing the formation of a MoS2 Schottky barrier (SB) junction with barrier height of 0.13-0.18 eV created at the interface between EB-irradiated (1T)/nonirradiated (2H) regions. Our experimental findings, supported by state-of-the-art simulation, reveal unique device fingerprint of SB-based field-effect transistors made from atom-thin 1T layers.

Luminescence behaviour in acetonitrile and in the solid state of a series of lanthanide complexes with a single helical ligand

Luminescence mechanisms of EuIII, TbIII, GdIII and NdIII complexes with a hexadentate ligand (abbreviated to EuL, TbL, GdL, and NdL, respectively), which have two bipyridine moieties bridged by an ethylenediamine unit, have been examined. Our molecular design is that each complex forms a single helical polar structure based on the chelate ring to retain solubility in solutions. EuL and NdL show comparably bright emission from ff transitions both in acetonitrile solution and in the solid state. To understand the mechanism of the emission in detail, the energy level of the triplet (T) state of the ligand L has been estimated based on the phosphorescence measurements of GdL, because GdIII shows no ff emission. The donor level of the T state of L and the acceptor level of EuIII or NdIII can overlap, indicating that the excited photon localized on L has been used for the efficient ff emission, while not for ππ* emission. For TbL, the luminescence quantum yield is significantly dependent on temperature and the state: in the solid state of TbL, the quantum yield of ff emission is over 90% at 77 K, while no luminescence is observed at room temperature, and in solution TbL shows no emission. This observation suggests that the emissive f-level of TbIII and the energy donor level of the excited T state of L are in thermal equilibrium. The described lanthanide complexes are stable and retain their molecular structure even in solutions and show characteristic luminescence behaviour based on the energy relaxation process of each lanthanide ion. Furthermore the HoIII complex with L (HoL) has been prepared and its structure has been analyzed. HoL has a twisted arrangement of the bipyridine moiety surrounding HoIII due to the small ionic radius of HoIII.

Novel emission properties of melem caused by the heavy metal effect of lanthanides(III) in a LB film

A novel emissive molecular system is constructed by the intercalation of the fluorophore melem (triamino-tri-s-triazine) within a Langmuir-Blodgett (LB) film of stearic acid with the periodic arrangement of lanthanides (Ln(III)), mainly Pr(III) with supporting of Eu(III). From emission spectra, decay curves, quantum yields and XPS measurements, it is clarified that the external heavy metal effect of Pr(III) on melem is much stronger in the film than in the bulk solid state, resulting in producing an unusual triplet state of melem. The triplet state of melem in the LB film donates the excitation energy to Pr(III) in the LB film, which is completely different from the energy transfer pathway of Pr-melem complex in the solid state through the singlet state of melem.

Cation‐Tuned Stimuli‐Responsive and Optical Properties of Supramolecular Hydrogels

Hierarchical self-assembly of an amphiphilic tris-urea in aqueous media is shown. A mixture of the amphiphilic tris-urea and an alkaline solution gave a viscous solution composed of fibrous aggregates. This viscous solution transformed into supramolecular hydrogels, which are capable of hierarchically organizing into higher-order aggregates in response to several cationic triggers. The resulting supramolecular hydrogels were relatively stiff and their storage moduli attained over 10(3)  Pa. The stimuli-responsive and optical properties of the resulting hydrogels were influenced by the cationic trigger. Proton and calcium ion triggers gave pH- and chemical stimuli-responsive hydrogels, respectively. A terbium ion trigger also provided a highly luminescent hydrogel through energy transfer from the tris-urea to terbium.

The key role of accurate lattice parameters in revealing subtle structural differences—a case study in the system [Ln(phen/phen-d8)2(NO3)3]

In the model system [Ln(phen/phen-d8)2(NO3)3] (Ln = Eu3+, Tb3+) the assignment of a new modification, based on the unit cell volumes, demonstrates the superior angular accuracy of synchrotron powder diffraction and the value of the lattice parameters refined thereof.

Organic Dye Adsorption by Amphiphilic Tris‐Urea Supramolecular Hydrogel

The development of an effective adsorbent for cleansing polluted water is required for environmental purification. In this respect, a supramolecular hydrogel constructed by the self-assembly of small molecules could be a strong candidate. Adsorption experiments of organic dyes were performed using supramolecular hydrogels of amphiphilic tris-urea 1. Cationic organic dyes were adsorbed efficiently; indeed, the adsorption of methylene blue was as high as 4.19 mol equivalents relative to 1. Two luminescence peaks were observed in the rhodamine 6G-adsorbed supramolecular hydrogels, and their ratios varied with the amount of dye adsorbed. Fluorescence microscopy images of the supramolecular hydrogel at lower dye levels exhibited fibrous fluorescence consistent with the fibrous aggregates of 1. According to these results, adsorption may proceed gradually, that is, occurring initially on the fibers and later in the aqueous spaces of the supramolecular hydrogel.

Making graphene luminescent by adsorption of an amphiphilic europium complex

We fabricated luminescent chemical vapor deposition-grown monolayer graphene sheets with an adsorbed europium complex, EuLC18, and characterized their luminescence properties. The EuLC18/graphene sheets clearly showed several photoluminescence peaks in a wavelength region from 580 to 694 nm, which were attributed to the ff transitions of the Eu ion. Luminescence was obtained via a photo-antenna effect, in which the ligands of EuLC18 absorbed the photo-excitation energy and transported it to the Eu excitation. Although the absolute luminescence quantum yield of the EuLC18/graphene sheet was as low as 0.5% due to the interaction between graphene and EuLC18, we demonstrated that graphene sheets can be made luminescent simply through adsorption of the luminescent Eu complex on the graphene surface.