Optical properties of MOVPE-grown a-plane GaN and AlGaN

a-Plane GaN and AlGaN were grown on r-plane sapphire by low-pressure metal-organic vapor epitaxy (LP-MOVPE), and the effects of reactor pressure (from 40 to 500 Torr) and growth temperature (from 1020 to 1100 °C) on the crystalline quality and surface morphology of a-plane GaN were studied. The a-plane GaN grown under 40 Torr had a smooth-surface morphology but a poor crystalline quality; however, the a-plane GaN grown under 500 Torr had higher crystalline quality and optical properties, whose full-width at half-maximum of the X-ray rocking curve (XRC-FWHM) and intensity of yellow luminescence (YL) were smaller. Furthermore, the optical properties of a-plane GaN were investigated by photoluminescence (PL) in detail. We also studied the emission properties of a-plane Al_0.35Ga_0.65N grown at room temperature.     

Influence of C60 Aggregation on Pseudorotation in the Excited Triplet State Probed by Multifrequency Time-Resolved EPR

The influence of C₆₀ aggregation on time-resolved (TR) electron paramagnetic resonance (EPR) of C₆₀ in the excited triplet state was investigated by multifrequency EPR techniques. Temperature-independent X-band (9.7 GHz) TR-EPR spectra were observed in a fresh toluene solution, while temperature-dependent ones were reported in literatures. The experimental spectra in this study indicated that the pseudorotation of pristine C₆₀ in frozen toluene solution is not frozen out even at lower temperatures. Careful investigations of TR-EPR and its decay kinetics demonstrated that the pseudorotation can be affected by C₆₀ aggregation. A comparison between X- and W-band (94.9 GHz) results indicated that the aggregation can be accelerated by a capillary effect. Three decay constants were extracted from the analysis of the decay kinetics. The fastest component was ascribed to the pseudorotation, which was independent of temperature in the range of 10–40 K. The temperature dependences of the decay kinetics showed that the pseudorotation is not affected by C₆₀ aggregation at higher temperatures.     

Half-cycle pulses in the mid-infrared from a two-color laser-induced filament

Four-wave mixing (FWM) of femtosecond near-infrared laser pulses and its second harmonic in the filamentation regime is shown to give rise to ultrashort field waveforms in the mid-infrared with pulse widths as short as a half of the field cycle and produce ultrabroadband supercontinuum spectra stretching from the mid-IR to the terahertz region. Generation of 7-fs pulses centered at 4.35 μm is demonstrated by a two-color filamentation experiment, where the 25-fs, 800-nm fundamental-wavelength output of a Ti: Sapphire laser is mixed with its second harmonic. The spectral and temporal properties of the mid-IR waveforms, as well as their emission pattern, are consistent with the FWM scenario of frequency conversion generalized to include the Kerr effect and ionization-induced refractive-index modulation.     

Recognition of aromatic compounds by π pocket within a cage-shaped borate catalyst

Taking shape: the ability of a Lewis acid catalyst to distinguish between aromatic and aliphatic hydrocarbon moieties was accomplished by using cage-shaped borate catalysts B(OC(6)H(3)Aryl)(3)CH having a π pocket derived from aryl substituents surrounding the boron center. The catalyst predominantly activated aromatic aldehydes over aliphatic ones for reaction.     

Phase-transfer behavior of cross-linked poly(acrylic acid) particles prepared by dispersion polymerization from ionic liquid to water

The phase-transfer behavior of poly(acrylic acid) (PAA) particles from the hydrophobic ionic liquid N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide phase to the water phase in the particle state, which we reported previously, was examined in more detail. PAA particles were prepared in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Bmim][TFSA]) and the organic solvent chloroform and were extracted. The transfer of PAA particles to water in the particle state was also observed in [Bmim][TFSA] systems. In contrast, the transfer phenomenon was not observed in the chloroform system. It was clarified that water/oil interfacial tension γ(wo) is an important parameter in the extraction of PAA in the particle state from the viewpoint of free energy. When the cationic surfactant tetradecyltrimethylammonium bromide, aqueous solution was used as the extraction medium, the PAA particles were extracted in the particle state from chloroform to water, in which γ(wo) became as low as that of the ionic liquid. This suggests that the phase-transfer phenomenon of PAA particles in the particle state was induced by the ionic liquid's unique property of low interfacial tension with water despite its high hydrophobic character.     

Lewis acid-mediated radical cyclization: stereocontrol in cascade radical addition-cyclization-trapping reactions

An efficient approach for achieving radical cyclizations by using hydroxamate ester as a coordination tether with Lewis acid was studied. The chiral Lewis acid-mediated cascade radical addition-cyclization-trapping reaction proceeded smoothly with good enantio- and diastereoselectivities, providing various chiral γ-lactams.     

Preparation of Micron‐Sized Monodisperse Poly(ionic liquid) Particles

Micron‐sized monodisperse poly(ionic liquid) (PIL) particles, poly([2‐(methacryloyloxy)ethyl]trimethylammonium bis(trifluoromethanesulfonyl)amide), were prepared by dispersion polymerization at 70 °C in methanol with poly(vinylpyrrolidone) as a stabilizer. The obtained particle size could be controlled by addition of ethanol to the methanol medium while maintaining narrow monodispersity. The PIL particles exhibit unique properties; they can be observed by scanning electron microscopy without platinum coating, which is generally used to avoid an electron charge. Moreover, the solubility of the PIL particles can be easily changed by changing the counter anion, similar to the process for ionic liquids.     

Relationship between diffusivity of water molecules inside hydrating tablets and their drug release behavior elucidated by magnetic resonance imaging

We reported previously that sustained release matrix tablets showed zero-order drug release without being affected by pH change. To understand drug release mechanisms more fully, we monitored the swelling and erosion of hydrating tablets using magnetic resonance imaging (MRI). Three different types of tablets comprised of polyion complex-forming materials and a hydroxypropyl methylcellulose (HPMC) were used. Proton density- and diffusion-weighted images of the hydrating tablets were acquired at intervals. Furthermore, apparent self-diffusion coefficient maps were generated from diffusion-weighted imaging to evaluate the state of hydrating tablets. Our findings indicated that water penetration into polyion complex tablets was faster than that into HPMC matrix tablets. In polyion complex tablets, water molecules were dispersed homogeneously and their diffusivity was relatively high, whereas in HPMC matrix tablets, water molecule movement was tightly restricted within the gel. An optimal tablet formulation determined in a previous study had water molecule penetration and diffusivity properties that appeared intermediate to those of polyion complex and HPMC matrix tablets; water molecules were capable of penetrating throughout the tablets and relatively high diffusivity was similar to that in the polyion complex tablet, whereas like the HPMC matrix tablet, it was well swollen. This study succeeded in characterizing the tablet hydration process. MRI provides profound insight into the state of water molecules in hydrating tablets; thus, it is a useful tool for understanding drug release mechanisms at a molecular level.     

New Strategy for Controlling Biodegradability of Biodegradable Polyesters by Enzyme‐Catalyzed Surface Grafting

A novel preparation method for the core‐shell type biodegradable polyesters or biodegradable materials grafted with biodegradable polyesters was developed by alkaline surface treatment of biodegradable polyester films and subsequent enzymatic polymerization of aliphatic lactones, one example of which is shown in this study, i.e., the preparation of poly(L ‐lactide) (PLLA) film grafted with poly(ε‐caprolactone). It is revealed that only alkaline surface treatment or the combination of alkaline surface treatment and enzyme‐catalyzed grafting, the former and the latter, respectively accelerating and delaying the enzymatic degradation of PLLA, will give PLLA materials having a wide variety of biodegradability. Also, the specificity of the enzyme used for hydrolysis could be used to confirm the grafted chain species.

     

Surface Hydrophilicities and Enzymatic Hydrolyzability of Biodegradable Polyesters, 2

Control of the surface hydrophilicities and enzymatic hydrolyzability of hydrophobic aliphatic polyesters such as poly(ε‐caprolactone) (PCL) and poly(L ‐lactide) [i.e. poly(L ‐lactic acid) (PLLA)] was attempted by coating with hydrophilic poly(vinyl alcohol) (PVA). The PVA coating was carried out by immersion of the PCL and PLLA films in PVA solutions. The effects of PVA coating on the hydrophilicities were monitored by dynamic contact angle measurements, while the enzymatic hydrolyzability of the PVA‐coated PCL and PLLA films was evaluated by the weight losses after Rhizopus arrhizus lipase‐ and proteinase K‐catalyzed hydrolysis, respectively. It was found that the PVA coating successfully enhanced the hydrophilicities of the aliphatic polyester films and significantly suppressed enzymatic hydrolyzability of the aliphatic polyester films, excluding the PCL film coated at a very low concentration such as 0.01 g · dL^?1 and the crystallized PLLA film coated at 1 g · dL^?1, for which slight enhancement and no significant enhancement, respectively, were observed in the enzymatic hydrolyzability. Moreover, the hydrophilicities and enzymatic hydrolyzability of the aliphatic polyester films were controllable to some extent by varying the PVA solution concentration and the film crystallinity.

Advancing contact angle (θ_a) of PCL, PLLA‐C, and PLLA‐A films before and after the PVA coating by immersion in 1 g · dL^?1 solution.