Takehiko Chiba

Liquid crystals of 4-octyloxy-N-(benzylidene)aniline derivatives bearing trifluoromethyl or trifluoromethoxy end groups

Abstract Seven new derivatives of 4-octyloxy-N-(4-substituted benzylidene) aniline have been synthesized. 4-Trifluoromethyl and 4-trifluoromethoxy derivatives exhibit stable smectic B and A phases, respectively, while both the 4-methyl and 4-methoxy derivatives have monotropic nematic phases. Fundamental liquid crystalline properties such as entropies of the phase transitions, microscopic textures, smectic layer spacings, orientational order parameters, and molecular dipole moments were determined. It has been revealed that moderately polar nature of these mesogens act to stabilizing monolayer smectic states. The smectic A phase of 4-trifluoromethoxy derivative exhibit very high orientational order. None of the disubstituted compounds, 3,5-bis(trifluoromethyl), 3,5-dimethyl, and 3,5-dimethoxy derivatives were mesogenic. The effect of terminal trifluoromethylation on the liquid crystalline properties is discussed.

On Thermal Anomalies Due to Phase Transitions in Doubly and Singly Reentrant Liquid Crystals, CBOBP and OBBC

Differential thermal analysis experiments are carried out on two reentrant liquid crystals, CBOBP and OBBC, whose phase sequences are Crystal–S A1 –RN–S Ad –N–I and Crystal–RN–S Ad –N–I, respectively. The reentrant phase transition (RN/S Ad ) is accompanied by a significantly smaller thermal anomaly than the ordinary ones (S A1 /RN and S Ad /N). S A1 /RN had a larger thermal anomaly than S Ad /N. Comparatively strong temperature-dependence of the heat capacity is suggested in RN. All these findings support the picture of competing incommensurate smectic periodicities and the phenomenological optimum density theory.

Modification of electronic structure by charge transfer and molecular motion in titanium disulfide-ethylenediamine intercalation compound

An attempt at modifying the electronic conduction mechanism by guest-host charge transfer and thermally excited molecular motion is reported. Ethylenediamine was intercalated into a semiconducting layered material, titanium disulfide, and a significant increase in charge carriers was evidenced by metallic conductivity, Pauli paramagnetism, and conduction electron spin resonance. Rotational motion of the guest molecule was evidenced by the proton spin-lattice relaxation measurement. However, no anomalous behaviors were found on the electronic properties which are attributable to motion-induced modification of the electronic state.

Proton NMR study of CBOOA, NBOOA, and their chain-deuteriated homologues in smectic Ad and nematic mesophases

Abstract The nature of the smectic Ad to nematic phase transition and the modes of molecular motion were investigated for the homologous mesogens, CBOOA, NBOOA, and their chain-deuteriated compounds. In NBOOA, the SA d[sbnd]N phase transition was found to be first order with a pronounced pretransitional decrease of the orientational order parameter, , both above and below the transition point, while in CBOOA the anomaly in was very small. Despite the different behaviours in terms of the orientational order, the dynamical aspects of molecular motion, probed by the-NMR relaxation, are quite similar. The motional mode characteristic of the N phase is order director fluctuation, but translational self-diffusion becomes the dominant relaxation mechanism in the SA d phase. It is also recognized that the relative importance of these mechanisms changes in a continuous manner as a function of temperature.

Two-dimensional metallic hydrogen lattice in potassium-hydrogen-graphite ternary systems

Abstract Alkali metal-graphite intercalation compounds absorb hydrogen to form alkali metal hydride-graphite intercalation compounds. We investigate the electronic properties and the structures of potassium hydride-graphite intercalation compounds by means of proton nuclear magnetic resonance, Hall effect and UV photoelectron spectroscopy. Among the alkali metal hydride intercalation compounds, potassium hydride intercalation compounds, the structure of which consists of ionic K + H  K + sandwich triple atomic layers between graphitic sheets, were found to have novel two-dimensional metallic hydrogen sheets in the intercalate space.

Effect of terminal polar substituents on the nature of smectic A liquid crystals of a series of 4-(4″-octyloxybenzoyloxy)benzylidene-4′-substituted anilines

Abstract The nature of smectic A liquid crystals and the phase transitions from smectic A to nematic phases are studied in the homologous series of 4-(4″-octyloxy-benzoyloxy)benzylidene-4′-substituted anilines, in which the substituents are H, F, Cl, Br, I, NO2 and CN. Measurements have been performed for the dipole moment of the molecule, the smectic A lattice period, the temperatures and the entropies of phase transitions, and the temperature dependence of the orientational order parameter, all of which provide information on the intermolecular pair potentials. It is shown that the introduction of a terminal polar substituent brings about counteracting contributions to the stability of the smectic A state. It is suggested that the reentrant nematic phase transition can appear in the strongly polar mesogens if the extent of antiparallel association of the molecules increases on lowering the temperature.

Sulfate ion motion in ammonium sulfate crystal as studied by low field proton magnetic relaxation of 17O enriched species

Proton T1D and T1 ρ are measured on ammonium sulfate crystal with 0.8% and 7.3% 17O content, at 30 and 10 MHz. A marked reduction is observed in T1D and T1 ρ compared with normal specimens at temperatures above about 200 K. At 30 MHz, T1D showed a minimum at about 240 K while two minima were observed for T1 ρ in the similar temperature range. Both T1D and T1 ρ at 10 MHz showed different behaviors from those measured at 30 MHz. These experimental results are interpreted in terms of H–O dipole modulation by the SO2−4 reorientation and by the fluctuation of the z component of 17O spin due to strong quadrupole relaxation. From the temperature dependence of the relaxation times, an activation energy of 21 kJ mol−1 and a preexponential factor of 3×10−11 s are obtained for the correlation time of the SO2−4 ion reorientation in the paraelectric phase. The correlation time exhibited a discontinuous change at Tc. Anomalous temperature dependence of the correlation time is observed in the ferroelectric phase. This b...

Structural phase transitions in deuterated ammonium hexachloroplumbate(IV), , studied by nuclear quadrupole resonance

The NQR frequency and the spin - lattice and spin - spin relaxation times were measured as functions of temperature, in order to study the structural phase transitions in deuterated ammonium hexachloroplumbate(IV) . It was shown that the phase transition at 81 K is of a second-order type with an overdamped soft mode, while the deuteration-induced transition at 38 K is of first order. Three NQR frequencies (17.99, 17.86 and 16.35 MHz at about 15 K) were observed below 38 K suggesting an orthorhombic symmetry in the deuteration-induced low-temperature phase. The averaged value of these frequencies almost coincides with the frequency extrapolated from that (17.348 MHz at 39 K) above 38 K. The splitting (about 1.6 MHz) of the frequency below 38 K is quite large compared with that (0.45 MHz) of , which shows a similar phase transition induced by deuteration at 27.5 K. This indicates a large distortion of the crystal lattice in the present compound.

Single crystal EPR and polycrystalline 1H NMR study of a ferromagnetically coupled dinuclear copper(II) complex in [P(C6H5)4][CuCl3]

EPR has been observed in a single crystal of [P(C6H5)4][CuCl3] at room temperature. The spectrum was characteristic of a spin one system, indicating that the copper(II) ions are pairwise coupled in the crystal. The present results agree fairly well with a previous study by Bencini etal. (Inorg. Chem., 1985, 24, 704). From the temperature dependence of the 1H NMR spin–lattice relaxation time, the copper dimer Cu2Cl62- is suggested to have a triplet ground state in accordance with an analysis of the magnetic susceptibility measurements by Estes etal. (Inorg. Chem., 1978, 17, 3657). The exchange interaction between the nearest triplet-state dimers is estimated to be of the order of 10-1 K.

NMR · NQR and DTA · DSC Studies of Phase Transitions in Pyridinium Tetrachloropalladate(II) and Pyridinium Tetrachloroplatinate(II)

Abstract From the measurements of DTA • DSC and the temperature dependences of 35Cl NQR frequencies, phase transitions were detected at 150 K, 168 K, and 172 K for (pyH)2 [PtCl4], and at 241 K for (PyH)2 [PdCl4]. In order to elucidate the motional state of the constituent ions in the crystals in connection with the structural phase transitions, the 35Cl NQR and 1H NMR spin-lattice relaxation times and the second moment of the 1H NMR line were measured as functions of temperature. For both compounds, the potential wells for the cationic reorientation are suggested to be highly nonequivalent at low temperatures. Above 168 K, the pyridinium ions in (pyH)2[PtCl4] are expected to reorient between almost equivalent potential wells. As for (pyH)2[PdCl4], it is expected that the orientational order of the cation still remains even above the second order transition at 241 K. A change of the potential curve from two-unequal to three-unequal wells is proposed as a possible mechanism of the second order transition. The activation energies for the cationic motion in the respective model potential are derived for both compounds at high and low temperatures.