Dong Keun Oh

Ferroelectric liquid crystalline polyoxetanes bearing chiral dimesogenic pendants

Two new polyoxetanes bearing dimesogenic pendants were prepared and their liquid crystalline properties were studied by differential scanning calorimetry (DSC), polarizing microscopy, and X-ray diffractometry. The dimesogens consist of cholesterol and biphenyl moieties connected through polymethylene spacers. Their biphenyl ends were attached to the oxetane monomer through a hexamethylene unit and the dimesogenic monomers were subjected to cationic ring-opening polymerization. The molecular weights of the polymers were very low and the degree of polymerization was only about 6. The two polymers were found to be thermotropic and formed only the chiral smectic C (Sc*) phase and, thus, their ferroelectricity was studied. Their spontaneous polarization values at the Sc* phase were about 20 nC cm−2 with responsive times of around 0.3 ms at 7 °C below the clearing temperature.

Time-of-flight measurements of charge-carrier mobilities in a poly (p-phenylenevinylene) derivative carrying an electron-transporting moiety

A poly (p-phenylenevinylene) derivative carrying an electron-transporting moiety, poly [2-(4-biphenyl)-5-(4-tert-butylphenyl))-1,3,4-oxadiazole]-p-phenylenevinylene] (PPPDPV), has recently been synthesized and shown to yield much enhanced external quantum efficiency when employed in an electroluminescence (EL) device. As the mobility balancing of positive and negative charge carriers plays a main role in the EL efficiency, we have carried out time-of-flight (TOF) measurements for separate determination of the two charge-carrier mobilities in PPV- and PPPDPV-based EL devices.

Mechanism of enhanced quantum efficiency in light-emitting diode based on a poly(p-phenylenevinylene) derivative

Transient electroluminescence (EL) measurements were made for a indium-tin–oxide (ITO)/poly[(2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole)-p-phenylenevinylene] (PPPDPV)/Al organic light-emitting diode (OLED), where PPPDPV is a derivative of poly(pphenylenevinylene) (PPV) carrying the electron-transporting 2-(4-biphenyl)-5-(4-t-butylphenyl-1, 3,4-oxadiazole) moieties directly attached to the PPV backbone. As a result, we were able to explain the superior external quantum efficiency of a PPPDPV OLED device by a faster mobility as well as a longer EL lifetime in comparison to those for a PPV device.

Charge-carrier mobilities and charge conduction in a poly(p-phenylenevinylene) derivative carrying an electron-transporting moiety

A PPV derivative carrying an electron-transporting PBD moiety, with much enhanced external quantum eAciencies, has been synthesized and studied by means of time-of-flight (TOF) and J ‐V measurements. As a result, the mechanism of the enhanced external quantum eAciencies, i.e., the mobility balancing of positive and negative charge carriers, was revealed, as well as the spacecharge limited conduction with an exponential trap distribution. ” 2001 Elsevier Science B.V. All rights reserved.

Photomagnetism of A-DNAs Intercalated with Photoresponsive Molecules

We have reported that the natural DNA and DNAs intercalated with organic radicals were able to show the noble magnetic properties in dry state. Extending our study on the Magnetic properties of DNAs, we intercalated the photoresponsive proflavine (PF) molecules into DNAs when the intercalated DNA complexes were illuminated by a laser beam of 488 nm. Electron magnetic resonance (EMR) new signals appeared. We succeeded in measurement of photomagnetic properties of modified DNAs through electron magnetic resonance (EMR) spectroscopy and SQUID measurement. A significant increase in magnetization was observed for the photo-illuminated DNA/proflavine samples.

35Cl nuclear quadrupole resonance study of lattice dynamics in SnCl4⋅5H2O comprising uncoordinated water molecules

Lattice dynamics and bond characteristics in SnCl4⋅5H2O have been investigated using 35Cl nuclear quadrupole resonance (NQR). Two chemically inequivalent chlorine sites in SnCl4⋅5H2O were distinguished from the temperature dependences of the NQR frequency and from the spin–lattice (T1Q) and spin–spin relaxation times (T2Q). The anomalous temperature dependence of the NQR frequency of one of the two sites was attributed to a thermal weakening of the crystal field from uncoordinated water molecules. In addition, high temperature activated molecular motion was observed.

Derivation of dc Voltages in a Magnetic Multilayer Undergoing Ferromagnetic Resonance

In this work, we present a comprehensive and systematic approach for the derivation of the dc voltage generated by a magnetic multilayer undergoing ferromagnetic resonance, originally derived by Berger. Our alternative derivation applies especially in the limit of the spin diffusion length much longer than the carrier mean free path.

Study of charge conduction in a poly(p-phenylenevinylene) derivative carrying an electron-transporting moiety

Recently, we have reported transient electroluminesence (EL) measurements in an organic light-emitting diode based on a poly(p-phenylenevinylene) (PPV) derivative carrying an electron-transporting moiety, poly[(2-(4-biphenyl)-5-(4-tert-butylphenyl))-1,3,4-oxdianzole)-p-phenylenevinylene] (PPPDPV), in order to investigate the mechanism of the enhanced external quantum efficiency. In this work, we have carried out J-E (I-V) characteristics measurements in EL devices based on PPV and PPPDPV, respectively, which revealed a space-charge limited conduction (SCLC) with exponential trap distribution mechanism in both. The trap densities of majority carrier were calculated from the J-E (I-V) curves and were compared with the EL external quantum efficiencies.

Electron Paramagnetic Resonance Observation of Critical Behaviors in a Disordered Conducting Polymer System

While most polymers are good insulators, some of them known as conducting polymers show remarkable electrical conductivities upon doping with electron donors or acceptors. The electrical conductivity is generally known to be governed by the one-dimensional diffusion of the chargecarrying solitons or polarons, and the main charge transfer mechanism in conjugated conducting polymers is known to be the intrachain diffusion and interchain hopping of polarons and/or bipolarons. Electron paramagnetic resonance (EPR) is a powerful tool for spin dynamics, and have been employed to study various conducting polymers as they can sensitively reflect the spin carrier motions and interactions. In conjugated conducting polymers like poly[2buthoxy-5-methoxy-1,4-phenylenevinylene] (PBMPV) where the charge carriers are polarons and bipolarons, the EPR intensity is proportional to the density of the spincarrying polarons (spin 1=2, charge e). A pair of polarons can form spinless singlet bipolarons (spin 0, charge 2e), which are energetically more stable. The spin-carrying polarons can give rise to EPR signals, whereas the spinless singlet bipolarons cannot make EPR transitions. Thus EPR can serve as a sensitive probe of spin carriers in the conducting polymers. PBMPV is a derivative of the conjugated PPV (polyphenylenevinylene) polymer. It can be easily doped to controllable degrees of doping and provides a unique opportunity to systematically study the doping as well as conductivity dependence of various properties including the dopant kinetics and spin/charge dynamics. Recently, we have reported a critical behavior associated with the delocalization of the charge-carrier wave function in a series of I2-doped PBMPV conducting polymers by means of nuclear magnetic resonance (NMR) measurements. In this work, we have investigated the charge/spin dynamics in the same series of samples by means of EPR measurements. While there have been some indirect indications of bipolaron formation in some electrochemically synthesized conducting polymers through the EPR and susceptibility measurements, this quantitative and systematic work is believed to represent a comprehensive observation of the collective critical behavior of bipolaron formation and spin–charge carrier delocalization in an iodine-doped conjugated polymer system. The PBMPV conducting polymer samples were prepared by thermal elimination of polyelectrolyte precursor polymer films and iodine-doping, and the iodine concentration was determined from the weight gain after doping. While the doped PBMPV samples are known to have inherent inhomogeneities, relatively thick good quality samples have been obtained according to visual inspection. The electrical conductivity was measured by the standard four-in-lineprobe method. The room temperature EPR measurements were made at 9.4 GHz using an X-band spectrometer, and the magnetic susceptibilities were obtained by doubleintegrating the differential lineshapes. The room temperature electrical conductivity was measured as a function of the degree of I2-doping, and the conductivity displayed a characteristic increase with the doping. A rapid increase of the electrical conductivity was noticed around the dopings of pc1 1⁄4 10 3 I3 /RU (repeating unit) and pc2 1⁄4 10 1 I3 /RU, which can be attributed to the dynamics governing the charge conduction mechanism. Figure 1 shows the doping degree dependence of the spinto-charge ratio obtained from the spin susceptibility, assuming that the charge density is the same as the dopant density per repeating unit of the polymer. Figure 1 displays an initial decrease of the spin-to-charge ratio until it reaches a minimum of less than 10 2 around pc2, which can readily be attributed to the polaron–bipolaron recombination. It is also interesting to note the spin-to-charge ratio minimum, followed by an increase of the ratio, which may be explained by the dissociation of some of the spinless bipolarons to polarons. Figure 2 shows the EPR lineshapes for samples with various degrees of doping. It is also worth noting the abrupt increase of the peak-to-peak EPR linewidth, in Fig. 3, at the same doping degrees where the spin-to-charge ratio shows a minimum, which indicates that they arise from the same spin dynamics. The abrupt increase in the EPR linewidth at pc2 appears to be a lifetime broadening, i.e., line broadening due to the lifetime shortening, arising from the delocalization of the spin/charge-carrier wavefunctions at the percolation threshold. Thus, EPR is shown to reflect sensitively the spin dynamics in our systems. Figure 4 shows the temperature dependencies of the EPR spin susceptibility and the linewidth for a heavily doped sample with p 1⁄4 3 10 . The spin susceptibility undergoes a decrease with decreasing temperature before showing a typical Curie behavior below a critical temperature around which the linewidth shows a characteristic peak. This

35Cl NQR Study of Phase Soliton Formation and Residual Commensurations in K2ZnCl4

In order to investigate the dynamics of the ZnCl 4 tetrahedra in K 2 ZnCl 4 , 3 5 Cl nuclear quadrupole resonance was employed for the spin-lattice relaxation measurements of each of the triplicated Cl sites in the lock-in phase. The temperature dependence of the spin-lattice relaxation rate (1/T 1 Q ) indicates that the domain peak observed in the incommensurate phase arises from the ZnCl 4 tetrahedral site with a negligible reorientational motion. On the other hand, an activated reorientational motion of the other two ZnCl 4 tetrahedral sites appears to lead to the phase solitons. Molecular motions in Rb 2 ZnCl 4 and in Cs 2 ZnCl 4 , undergoing an incommensurate an not incommensurate phase transition, respectively, were also compared.