Shiyoshi Yokoyama

Selective assembly on a surface of supramolecular aggregates with controlled size and shape

The realization of molecule-based miniature devices with advanced functions requires the development of new and efficient approaches for combining molecular building blocks into desired functional structures, ideally with these structures supported on suitable substrates. Supramolecular aggregation occurs spontaneously and can lead to controlled structures if selective and directional non-covalent interactions are exploited. But such selective supramolecular assembly has yielded almost exclusively crystals or dissolved structures; the self-assembly of absorbed molecules into larger structures, in contrast, has not yet been directed by controlling selective intermolecular interactions. Here we report the formation of surface-supported supramolecular structures whose size and aggregation pattern are rationally controlled by tuning the non-covalent interactions between individual absorbed molecules. Using low-temperature scanning tunnelling microscopy, we show that substituted porphyrin molecules adsorbed on a gold surface form monomers, trimers, tetramers or extended wire-like structures. We find that each structure corresponds in a predictable fashion to the geometric and chemical nature of the porphyrin substituents that mediate the interactions between individual adsorbed molecules. Our findings suggest that careful placement of functional groups that are able to participate in directed non-covalent interactions will allow the rational design and construction of a wide range of supramolecular architectures absorbed to surfaces.

Electrical control of the structure and lasing in chiral photonic band-gap liquid crystals

This letter describes the electrical control of structure and lasing in the photonic bandgaps of cholesteric liquid crystals (CLCs). Photoexcitation of dye-doped CLC cells with a linearly polarized laser gives rise to laser emission at the edge(s) of the chiral photonic band gap. Applying voltages to the optically pumped CLC cells enables reversible switching of the laser action as a result of the structural changes in the chiral photonic band gap.

Intermolecular Coupling Enhancement of the Molecular Hyperpolarizability in Multichromophoric Dipolar Dendrons

Nonlinear optical dendritic macromolecules, called azobenzene dendrons, were synthesized, and their conformational properties and molecular nonlinear optical properties were clarified by second-order nonlinear optical measurement. The synthesized molecules were modified by introducing 1−15 numbers of azobenzene branching units as the nonlinear optical chromophore and by placing aliphatic chains at the end the dendritic chains. In these topologically complex molecules, each chromophore contributed coherently to the macroscopic nonlinear optical activity. The first-order molecular hyperpolarizability of the azobenzene dendron having 15 chromophoric units was measured to be 3010 × 10-30 esu using the hyper-Rayleigh scattering method. This level of molecular hyperpolarizability was much higher than that for an azobenzene monomer (150 × 10-30 esu). The polarized nonlinear optical measurement provided structural information on the dendrons and indicated that each chromophore was oriented noncentrosymmetrically ...

Phototunable photonic bandgap in a chiral liquid crystal laser device

This letter describes the phototunable photonic bandgap of cholesteric liquid crystal (CLC) doped with a fluorescent dye to generate the distributed feedback effect. Photoirradiation of the dye-doped CLC cell under UV light at 254 nm resulted in continuous changes in the chiral photonic bandgap from 550 to 720 nm due to photolysis reaction of the cholesteryl iodide embedded in the CLC host. We showed that the laser oscillation wavelength could be controlled in a wide wavelength range from 610 to 700 nm by photoassisted adjustment of CLC photonic bandgaps.

Nonplanar adsorption and orientational ordering of porphyrin molecules on Au(111)

Saddle-shaped deformation of planar porphyrin molecules is accomplished by rotations of four phenyl-based substituents, which results from optimum adsorption onto Au(111) surface. The nonplanar macrocyclic conformation is clearly visualized by using low-temperature scanning tunneling microscopy and confirmed by molecular orbital calculations. Inside of the supramolecular molecular islands, we find that two different orientations of the nonplanar porphyrins are randomly distributed. An orientational ordering is obtained after short thermal excitations, which should be associated with steric intermolecular interactions between substituents.

Conformation selective assembly of carboxyphenyl substituted porphyrins on Au (111).

The selective assembly of carboxyphenyl substituted porphyrins on the Au (111) surface has been studied using scanning tunneling microscopy. We find that conformational isomers of the porphyrins are induced upon adsorption and are selectively assembled into hydrogen-bonded supramolecular clusters or wires on the surface. The conformation selective assembly is attributed to the coplanar intermolecular interactions between hydrogen-bonded carboxyphenyl groups.

Laser emission from high-gain media of dye-doped dendrimer

We prepared high-gain media for laser emission by using a conventional laser-dye (DCM) and dendrimer. A dendrimer can encapsulate the laser dye, and therefore increase its concentration up to 9.0 mM with limited self-aggregation and intermolecular quenching. The optical confinement is attributed to gain guiding under optical excitation. The laser spectrum clearly indicated periodic resonant modes though the laser feedback was caused in an unusual way. Above the lasing threshold, the spectral linewidth became less than 0.1 nm. The laser-dye-doped dendrimer showed a particular ability to decrease the lasing threshold intensity by increasing the dye concentration.

Supernarrowing mirrorless laser emission in dendrimer-doped polymer waveguides

A supernarrowing laser spectrum is observed for mirrorless polymer waveguide lasers containing rhodamine B cored dendrimers. The moderate scattering from the aggregated dendrimers in the host polymer matrix is responsible for the mirrorless lasing. However, the long amplification path through the waveguide produces a directional beam and supernarrowing a spectrum in scattering lasers.

Complementary metal–oxide–semiconductor compatible athermal silicon nitride/titanium dioxide hybrid micro-ring resonators

Micro-ring resonators have been widely utilized in silicon photonics. However they often exhibit a high sensitivity to ambient temperature fluctuations. In this letter, we have demonstrated a complementary metal–oxide–semiconductor compatible athermal micro-ring resonator made from titanium dioxide (TiO2) and silicon nitride (SiNx). We have exploited the negative thermo–optic coefficient of TiO2 to counterbalance the positive coefficient of SiNx. By a precise control over the TiO2 layer thickness, an athermal condition remarkably consistent with the simulation can be achieved. Therefore, a SiNx–TiO2 hybrid micro-ring resonator with a temperature dependent wavelength shift of 0.073 pm/ °C has been realized.

Theoretical study of aryl-ring rotation in arylporphyrins

We carried out quantum chemical calculations for the aryl-ring rotations in phenylporphyrin and ortho-methoxyphenylporphyrin to determine the reaction path and the potential-energy barrier so that one can provide an insight into how such rotations can occur. It was found that along the reaction path, the structure changes from a planar porphyrin-ring at the stable state to a considerably distorted one at the transition state, and that the small potential-energy barrier, which is consistent with experiments, largely stems from the deformation of the porphyrin ring. An increase in the potential-energy barrier resulting from the substitution at the ortho position of the aryl group was also found to originate mainly from an additional deformation of the porphyrin ring at the transition state. These findings are in contrast with the calculated result for the aryl-ring rotation in the biphenyl, in which the direct steric hindrance between the aryl rings rather than the deformation makes large contributions to the overall potential energy barrier. We concluded that the deformation of the porphyrin ring is essential for the occurrence of the aryl-ring rotation in the arylporphyrins.