Shuhei Ogata

Mechanoresponsive liquid crystals exhibiting reversible luminescent color changes at ambient temperature

Mechanochromic luminescent liquid crystals containing oligothiophene derivatives have been obtained. These liquid-crystalline (LC) molecules having bi-, ter-, and quarterthiophene moieties exhibit shear-induced phase transitions accompanied by changes in their luminescent colors at ambient temperature. The shear-induced LC phases and their luminescent colors spontaneously revert back to the initial LC phases and the initial luminescent colors during the thermal aging processes at ambient temperature because the shear-induced LC phases are metastable phases. These three compounds show various luminescent colors and cover a wide visible region. The terthiophene and quaterthiophene derivatives recover their initial luminescent colors faster than the bithiophene derivative, indicating that not only the luminescent colors but also the stimuli-responsive properties are tunable through simple change of the π-conjugated length of the luminescent mesogen.

Multiple Magnetic Relaxation Pathways and Dual-Emission Modulated by a Heterometallic Tb-Pt Bonding Environment

A heterometallic Tb-Pt complex, [Tb2 Pt3 (SAc)12 (H2 O)2 ] (SAc=thioacetate), was synthesized. Dual emission was modulated by the presence of a heterometallic Tb-Pt bonding environment. The heterometallic Tb-Pt bond lowers the symmetry of the Tb ion and enhanced the emission efficiency. In addition, the Tb-Pt complex shows field-induced multiple magnetic relaxation pathways. Furthermore, it served as an antenna for the observed dual emission. In other words, the heterometallic Tb-Pt bond has a significant effect on the luminescence and magnetic properties of the complex.

Slow Magnetic Relaxation in a Palladium-Gadolinium Complex Induced by Electron Density Donation from the Palladium Ion

Incorporating palladium in the first coordination sphere of acetato-bridged lanthanoid complexes, [Pd2 Ln2 (H2 O)2 (AcO)10 ]⋅2 AcOH (Ln=Gd (1), Y (2), Gd0.4 Y1.6 (3), Eu (4)), led to significant bonding interactions between the palladium and the lanthanoid ions, which were demonstrated by experimental and theoretical methods. We found that electron density was donated from the d8 Pd2+ ion to Gd3+ ion in 1 and 3, leading to the observed slow magnetic relaxation by using local orbital locator (LOL) and X-ray absorption near-edge structure (XANES) analysis. Field-induced dual slow magnetic relaxation was observed for 1 up to 20 K. Complex 3 and frozen aqueous and acetonitrile solutions of 1 showed only one relaxation peak, which confirms the role of intermolecular dipolar interactions in slowing the magnetic relaxation of 1. The slow magnetic relaxation occurred through a combination of Orbach and Direct processes with the highest pre-exponential factor (τo =0.06 s) reported so far for a gadolinium complex exhibiting slow magnetic relaxation. The results revealed that transition metal-lanthanoid (TM-Ln) axial interactions indeed could lead to new physical properties by affecting both the electronic and magnetic states of the compounds.

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.

Enhanced Quantum Yield of Fluorophores in Confined Spaces of Supermicroporous Silicas

The optical properties of polycyclic aromatic hydrocarbon fluorophores, such as pyrene and perylene, are controlled by confined spaces of meso- and supermicropores in a silica matrix. The quantum yield of fluorescence from monodispersed pyrene and perylene increases with limitation of the excimer formation in 1 nm supermicropores. When the pore diameter is close to the molecular size, solid-state fluorescence having a high quantum yield is achieved via the fluorophores in the confined spaces by suppressing the aggregation, the interaction with charge-transfer sites and the stabilization of the excited state.