Takafumi Sassa

Thermally-induced phase transition of pseudorotaxane crystals: changes in conformation and interaction of the molecules and optical properties of the crystals.

This paper presents a pseudorotaxane that acts as a thermally driven molecular switch in the single-crystal state. Crystals of the cationic pseudorotaxane consisting of dibenzo[24]crown-8 (DB24C8) and N-(xylylammonium)-methylferrocene as the cyclic and axle component molecules, respectively, undergo crystalline-phase transition at 128 °C with heating and 116 °C with cooling, according to differential-scanning-calorimetry measurements. X-ray crystallographic analyses revealed that the phase transition was accompanied by rotation of the 4-methylphenyl group of the axle component molecule and a simultaneous shift in the position of the PF(6)(-) counteranion. Crystalline phase transition changes the conformation and position of the DB24C8 molecule relative to the ammonium cation partially; the interaction between the cyclic component and the PF(6)(-) anion in the crystal changes to a greater extent. Moreover, there are changes in the vibration angle (θ) and birefringence (Δn) on the (001) face of the crystal transitionally; θ is rotated by +12°, and Δn is decreased from 0.070 to 0.059 upon heating across the phase transition temperature. The phase transition and accompanying change in the optical properties of the crystal occur reversibly and repeatedly upon heating and cooling processes. The switching rotation of the aromatic plane of the molecule induces a change in the optical anisotropy of the crystal, which is regarded as a demonstration of a new type of optical crystal. Partial replacement of the PF(6)(-) anion with the bulkier AsF(6)(-) anion forms crystals with similar crystallographic parameters. An increase in the AsF(6)(-) content decreases the reversible-phase-transition temperature gradually down to 99 °C (T(end)) and 68 °C (T(exo)) ([AsF(6)(-)]:[PF(6)(-)] = 0.4:0.6).

Influence of the liquid carbazole concentration on charge trapping in C60 sensitized photorefractive polymers

We study by two-beam coupling experiments the photorefractive properties of a poly(Nvinylcarbazole) (PVK) matrix plasticized with different concentrations of 9-(2-ethylhexyl)carbazole (EHCz), so-called liquid carbazole, and doped with the photosensitizer C60 and the electro-optic push-pull chromophore 4-piperidinobenzylidene malonitrile. The steady state and dynamic photorefractive performances of these materials are optimized by tuning the concentration ratio between PVK and EHCz. In parallel, the trap density values are determined by a spectroscopic method based on the strong absorption of the C60− anion in the near infrared region and are compared with those obtained from the photorefractive measurements. The results obtained from both techniques are in good agreement and show the important role played by the liquid carbazole in the charge trapping processes and the photorefractive properties of C60 sensitized photorefractive polymers.

Enhanced photorefractive two-beam coupling in low-T~g polymeric materials with a new device structure

Large two-beam coupling gain with a low driving voltage and a small grating spacing was observed by use of a novel device structure in a photorefractive polymer with a low glass-transition temperature (Tg). A net gain (Γ−α)L close to 1 was observed for an external electric field of ∼40 V/μm and a grating spacing of 0.42 μm. The enhanced coupling strength was the result of optimized parameters and improved, effective electro-optical coefficient, phase shift, space-charge field, and effective interaction length compared with those of a conventional device structure. Moreover, mechanism for this enhancement of coupling strength was found to be applicable to typical efficient low-Tg polymeric materials.

In Situ KPFM Imaging of Local Photovoltaic Characteristics of Structured Organic Photovoltaic Devices

Here, we discuss the local photovoltaic characteristics of a structured bulk heterojunction, organic photovoltaic devices fabricated with a liquid carbazole, and a fullerene derivative based on analysis by scanning kelvin probe force microscopy (KPFM). Periodic photopolymerization induced by an interference pattern from two laser beams formed surface relief gratings (SRG) in the structured films. The surface potential distribution in the SRGs indicates the formation of donor and acceptor spatial distribution. Under illumination, the surface potential reversibly changed because of the generation of fullerene anions and hole transport from the films to substrates, which indicates that we successfully imaged the local photovoltaic characteristics of the structured photovoltaic devices. Using atomic force microscopy, we confirmed the formation of the SRG because of the material migration to the photopolymerized region of the films, which was induced by light exposure through photomasks. The structuring technique allows for the direct fabrication and the control of donor and acceptor spatial distribution in organic photonic and electronic devices with minimized material consumption. This in situ KPFM technique is indispensable to the fabrication of nanoscale electron donor and electron acceptor spatial distribution in the devices.

Photoinduced Control over the Self-Organized Orientation of Amorphous Molecular Materials Using Polarized Light

Novel chiral amorphous molecular materials containing photoresponsive azobenzene moieties were designed and synthesized. These materials led to the formation of smooth and uniform thin films without grain boundaries. Photoirradiation of the thin film with linearly and elliptically polarized light caused trans-cis photoisomerization of the azobenzene moieties and resulted in an anisotropic orientation of the materials. Maximum change in a value of birefringence, Deltan, after irradiation of the linearly polarized light is about 0.08 with a response time of 10 s. However, when irradiation was ceased after photoinduced orientation, the value of Deltan decreased due to relaxation of the azobenzene moieties. Furthermore, these materials exhibited a high efficiency of a photoinduced polarization rotation over 30 deg mum(-1) after irradiation with the elliptically polarized light for 60 s. We also found that the efficiency depends on ellipticity of the incident light and on the thickness of the sample. The origin of the large change in the molecular orientation is the anisotropic arrangement of the azobenzenes in the terminal groups upon irradiation with the linearly and elliptically polarized light.

Electron donor and acceptor spatial distribution in structured bulk heterojunction photovoltaic devices induced by periodic photopolymerization.

Donor and acceptor spatial distributions were directly formed in a surface relief grating of structured bulk heterojunction (BHJ) photovoltaic devices by simple periodic photopolymerization. Enhanced photocurrents were observed in the structured BHJ photovoltaic devices and formation of the D/A spatial distribution was confirmed by Kelvin probe force microscopy. This technique enables the fabrication of structured BHJ photovoltaic devices with solution-processable organic semiconductors, and has tremendous potential for controlling D/A spatial distribution in organic optoelectronics devices.

Surface waves in photorefractive polymer films

We observed the temporal development of surface waves and investigated their power propagation loss in typical photorefractive polymer films sandwiched between ITO glass substrates. We found that amplified scattered waves generated in a pumped region started to develop into surface waves from a point where they reached the substrate through the self-bending effect. The surface waves propagated over a distance of 1.7 mm, thereby confining the power to a region at a distance of 30 microns from the substrate. Considerable propagation power loss of the surface waves was observed at a low pumping power of the beam; however, the power loss decreased considerably when the beam had high power.

Photorefractive effect from photo-induced orientation of a novel carbazole derivative

Two-beam coupling gain obtained from an orientational index grating was observed without applying an external field in a photorefractive polymeric material with ow glass transition temperature. A novel carbazole derivative with a photorefractive function was used as the material doped with a sensitizer for photocarrier generation. The mechanism to form the grating and to give the gain was investigated by two-beam coupling measurements.

Phase-matched second harmonic generations in poled polymer films based on poly (methyl methacrylate) containing a p-aminophenyl-cyclobutenedione moiety as the side chains

Abstract Poled polymer films containing a nonlinear optical active moiety (cyclobutenedione derivative) as the side chains were prepared and applied to frequency doublings in guided-wave structures. A (p-aminophenyl) cyclobutenedione containing monomer, 1-{4-[N-(2- methacryloyloxyethyl)-N-methyl-aminol-phenyl}-2-methylamino-cyclobutene -3,4-dione [CD1-MA was newly synthesized. For less than 20 wt% concentration, amorphous copolymers of CD1-MA with methyl methacrylate showed the glass transition at around 125°C. The copolymers were crystalline at higher dye concentrations. The d33 value of the poled polymer film with 10 wt% of CD1-MA concentration was estimated as 8.1 pm.V−1 by the Maker fringe method. Second harmonic waves of Nd:YAG laser (1064 nm) were observed in three-layer optical waveguides (air / poled polymer / pyrex glass) for the collinear type and the Cerenkov type phase matchings.

Laser irradiation durability of photorefractive ferroelectric liquid crystal blends containing terthiophene photoconductive chiral dopants

Ferroelectric liquid crystal blends composed of a smectic liquid crystalline mixture, a photoconductive chiral dopant, and an electron trap reagent exhibit significant photorefractivity together with rapid responses. As such, they allow the dynamic amplification of moving optical signals. In the present work, the photochemical durability of a photorefractive ferroelectric liquid crystal blend sandwiched between two transparent electrodes was investigated. A series of photoconductive chiral dopants was prepared and the durability of blends incorporating these dopants during laser irradiation was examined. The photorefractivity of the liquid crystal blend with application of a DC electric field was found to decay on increasing the laser irradiation time. However, no evidence for a photochemical reaction of the photoconductive chiral dopant was obtained. The effect of the conduction of photogenerated ionic species on the photorefractivity decay was clarified. It was concluded that the observed decay of the photorefractivity was caused by the adsorption of photogenerated ionic species on the electrode.