Kenji Higashiguchi

Direct Observation of Polymer-Stabilized Blue Phase I Structure with Confocal Laser Scanning Microscope

The three-dimensional structure of polymer-stabilized blue phase with the order of optical wavelength was nondestructively investigated by a confocal laser scanning microscope. The periodical patterns corresponding to the bcc lattice were observed not only on the surface but also in the internal region. The visualization mechanism was expected to be back scattering caused by liquid crystal order.

Optically and Electrically Driven Organic Thin Film Transistors with Diarylethene Photochromic Channel Layers

We achieved drain-current switching of diarylethene-channel field-effect transistors with light- and electric-field effects. The drain current was reversibly changed by alternating ultraviolet and visible light irradiation. Stress is placed on the fact that the on/off ratio realized by light irradiation was 1 × 10(2) (1 × 10(4)%) and this value is much larger than those in other photochromism-based transistors. These results indicate that the drain current was effectively controlled by light irradiation. Furthermore, the on and off states modulated by light were maintained without light irradiation even after 1 week, exhibiting that our transistor works as an optical memory. We clarified that the light-driven modulation can be attributed to the transformation in the π-conjugation system accompanied by photoisomerization. These findings have the potential to attain high-performance optoelectrical organic devices including optical sensors, optical memory, and photoswitching transistors.

Photoinduced Macroscopic Morphological Transformation of an Amphiphilic Diarylethene Assembly: Reversible Dynamic Motion

Self-assembled microstructures of an amphiphilic diarylethene featuring an alkyl chain and triethylene glycol groups showed a photoinduced reversible morphological change in water. Reversible photoisomerization of the core diarylethene gave rise to a reversible morphological transformation between colorless microspheres and colored fibers. When colorless microspheres were irradiated with UV light, colored fibers were formed, and when the colored fibers were irradiated with visible light, the spheres were restored to their original positions where the spheres originally existed. This system showed reversible morphological change through not only photoirradiation but also temperature change. These behaviors can be interpreted as a phase transition between the sphere and fiber states. The dynamic process of the phase transition was monitored by polarized optical microscopy (POM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). It was revealed that the formation of fibers upon UV irradiation occurred radially at the surface of the sphere and the formation of the spheres upon visible-light irradiation occurred at the middle of the fiber. The unique photoinduced mechanical motion provides useful information for the design of sophisticated photoactuators.

Self-assembly and aggregate-induced enhanced emission of amphiphilic fluorescence dyes in water and in the solid state.

1-Cyano-1,2-bis(biphenyl)ethene (CNBE) derivatives with a hexa(ethylene glycol) group as an amphiphilic side chain were synthesized and the self-assembling character and fluorescence behavior were investigated. The amphiphilic derivatives showed aggregate-induced enhanced emission (AIEE) in water and in the solid state. The fluorescence quantum yield increased as the rigidity of the aggregates increased (i.e., in ethyl acetate<in water<in the solid state). As determined from measurements of fluorescence spectra, fluorescence quantum yields, and fluorescence lifetimes, a key factor for the enhanced emission is suppression of the nonradiative decay process arising from restricted molecular motion. Additionally, the difference in the emission rate constant is not negligible and can be used to interpret the difference in fluorescence quantum yield in water and in the solid state.

Photochromic reactivity of a dithienylethene dimer

Photochromic reactivity of a dithienylethene dimer, 5-(2-(3,4-dimethyl-2-thienyl))-3,3,4,4,5,5-hexafluorocyclopent-1-enyl-2-(2-(2,4-dimethyl-3-thienyl))-3,3,4,4,5,5-hexafluorocyclopent-1-enyl-3,4-dimethylthiophene, was studied in solution as well as in the crystalline phase. In solution two kinds of closed-ring form isomers were reversibly produced upon irradiation with ultraviolet light, while in the crystalline phase only one kind of closed-ring form isomer was produced. The different reactivity was attributed to the difference in the distance between two reactive centers in the crystal.

Evaluation of the β Value of the Phenylene Ethynylene Unit by Probing the Exchange Interaction between Two Nitronyl Nitroxides

A series of nitronyl nitroxide radicals having different lengths of phenylene ethynylene molecular wire were synthesized to investigate the decay constant of p-phenylene ethynylene. By the measurement and simulation of the ESR spectra of the biradicals, it was found that the exchange interaction decreased with a decay constant (β) of 0.39 Å(-1) as the length of the molecule increased. This result indicates that the spin-spin exchange interaction between neutral radicals has a decay constant similar to that of the molecular conductance. This value of the decay constant indicates that a hopping mechanism does not take place in the measurement of the exchange interaction between neutral radicals even when the molecular wire has enough length to show hopping conduction of electrons.

Evaluation of the β Value of the Phenylene Unit by Probing Exchange Interaction between Two Nitroxides

Oligophenylene molecular rods with bicyclo[2.2.2]octane having two nitronyl nitroxide radicals were synthesized to investigate the decay constant of p-phenylene. By the measurement and simulation of the ESR spectra of the biradicals with different rod length, it was found that the exchange interaction was decreased with the decay constant β of 0.51 ± 0.01 Å(-1). This result indicates that the spin-spin exchange interaction between neutral radicals has a decay constant similar to the molecular conductance.

π-Conjugation of two nitronyl nitroxides-attached diarylethenes.

Unusual blue shift of the absorption maxima of two nitronyl nitroxide attached diarylethene through phenyl units (DAE-phe-NN) with increasing number of phenyl units is examined by time dependent density functional theory (TDDFT). The extended π-conjugation between nitronyl nitroxide and diarylethene is rather suppressed by the bridge phenyl units. In comparison, the red shift found in two nitronyl nitroxide attached diarylethenes through thiophene units (DAE-thio-NN) with increasing number of thiophene units is due to the longer π-conjugation induced by smaller dihedral angles between diarylethene and bridge and between bridges.

Crystal to crystal transformation in the solid state

Either co-milling or simple mixing of benzoquinone (BQ) and diol crystals of varied melting point temperature produces adduct crystals through very slow and gradual molecular diffusion processes in the solid state. Their formation process was compared and discussed based on the crystal structures of the starting diols and the diol–BQ adducts.

Photoisomerization-induced manipulation of single-electron tunneling for novel Si-based optical memory.

We demonstrated optical manipulation of single-electron tunneling (SET) by photoisomerization of diarylethene molecules in a metal-insulator-semiconductor (MIS) structure. Stress is placed on the fact that device operation is realized in the practical device configuration of MIS structure and that it is not achieved in structures based on nanogap electrodes and scanning probe techniques. Namely, this is a basic memory device configuration that has the potential for large-scale integration. In our device, the threshold voltage of SET was clearly modulated as a reversible change in the molecular orbital induced by photoisomerization, indicating that diarylethene molecules worked as optically controllable quantum dots. These findings will allow the integration of photonic functionality into current Si-based memory devices, which is a unique feature of organic molecules that is unobtainable with inorganic materials. Our proposed device therefore has enormous potential for providing a breakthrough in Si technology.