Atsushi Tojima

Orientational order study of monolayers at the air–water interface by Maxwell-displacement current and optical second harmonic generation

The dielectric polarization of organic monolayers at the air–water interface has been analyzed, assuming that the monolayers have C∞ symmetry. The formula of polarization of organic monolayers is derived and it is expressed using orientational order parameters. It is shown that Maxwell-displacement current measurement coupled with optical second harmonic generation measurement is helpful for the determination of these orientational order parameters. Monolayers of 4′-n-pentyl-4-cyanobiphenyl on the air–water interface were examined during monolayer compression and the orientational order parameters were determined.

Instrument equipped with Maxwell displacement current and optical second-harmonic generation measurement system

An experimental instrument equipped with the Maxwell displacement current (MDC) and optical second-harmonic generation measurement system has been developed for the study of the dielectric property of organic monolayers on the surface of water. In this instrument, the generation of MDC across monolayers is recorded by monolayer compression. At the same time, second-harmonic signals generated from monolayers by the laser irradiation are monitored. Using this experimental instrument, the orientational order of 4-octyl-4′-n-cyanobiphenyls monolayers was examined, and one of the most important parameters to characterize the physical properties of monolayers, the orientational order parameters S1 and S3 defined by Legendre polynomials, was determined.

Detection of phase transition of monolayers at the air-water interface by compression using Maxwell displacement current and optical second harmonic generation

The spontaneous and nonlinear dielectric polarization of Langmuir monolayers is expressed using orientational order parameters. With the theoretical calculations, the Maxwell displacement current (MDC) measurement coupled with optical second harmonic generation (SHG) measurement is shown to be helpful for the study of orientational orders and phase transitions in monolayers. Spontaneous and nonlinear polarization induced in monolayers are detected by MDC and SHG, respectively. Using the MDC-SHG measurement, monolayers of 4′-n-octyl-4-cyanobiphenyl on the water surface were examined by monolayer compression. The phase transitions such as from planar isotropic phase to polar and tilting orientational alignment phases were clearly revealed.

Second-harmonic generation and Maxwell displacement current spectroscopy of chiral organic monolayers at the air-water interface

The molecular orientation and chiral properties of the S-citronelloxy-cyanobiphenyl and 4′-hexyloxy-4-cyanobiphenyl monolayer at the air–water interface were investigated with optical second-harmonic generation (SHG) and Maxwell displacement current (MDC) measurements during monolayer compression. MDC was used to detect the phase transition from planar alignment phase to polar orientational phase in monolayers, and SHG was used to detect the orientation and the chirality of monolayers. The vectorial formulas for the SH process of the monolayer composed of chiral molecules were expressed using tensor components of the nonlinear optical (NLO) susceptibility, where the relationship between the NLO susceptibility and the molecular hyperpolarizabilities was also derived. According to the experiment, a polarized angle dependence measurement could reveal the chirality of this monolayer. The tensor component of the second-order NLO susceptibility, which is related to the chirality of the monolayer, was confirmed ...

Orientational order study of 4-alkyl-4′-cyanobiphenyl Langmuir films by Maxwell displacement current and optical second harmonic generation measurements

Abstract Maxwell displacement current (MDC) measurement coupled with second harmonic generation (SHG) measurement was employed for the orientational order study of Langmuir films. 4-alkyl-4′-cyanobiphenyl ( n CB, n =5, 6, 7 and 8) Langmuir films were examined, and the orientational order parameters were determined. The difference in the generation between MDC and SHG was revealed, and was discussed based on the difference in the contribution of n CBs. It is shown that the measurement employed here is very helpful for the orientational study of Langmuir films.

Detection of the reversible flow behavior of 4-heptyloxy-4′-cyanobiphenyl monolayer at air–water interface by compression and expansion with Maxwell displacement current and optical second harmonic generation

Abstract The flow behavior of 4-heptyloxy-4′-cyanobiphenyl (7OCB) monolayer is studied during compression and expansion by Maxwell displacement current (MDC) and second harmonic generation (SHG) measurement. It is found that in-plane molecular flow induced reorientation of 7OCB monolayer in L2 and L2′ phases of the monolayer is reversible. It is concluded that the MDC-SHG method is available for exploring the flow behavior of monolayers.

Generation of Maxwell-displacement current due to tilting phase transition of amphiphile monolayers on water surface

Generation of Maxwell-displacement current (MDC) across monolayers on the water surface due to the untilting to tilting (U --> t) phase transition of floating amphiphile monolayer is studied. This generation of MDC is analyzed using one parameter, the tilt angle theta(t) from the water surface normal. Here, this parameter is used to express the distortion of orthogonally hexagonal orientation of the molecules that causes the distortion of the projection of the hexagon on the water surface. We discuss the generation of a change in MDC across fatty acid monolayers observed in the condensed region in association with the U --> t phase transition

Determination of orientatinal order parameters of 4-alkyl-4 '-cyanobiphenyl monolayers at the air-water interface by Maxwell-displacement-current and second harmonic generation measurements

Abstract Using Maxwell displacement current (MDC) and second harmonic generation (SHG) measurements, the linear and nonlinear dielectric properties of organic monolayer films on a material surface can be examined. In this study, we have developed a novel experimental system which can detect MDC and SHG signals, and examined the molecular motion of 4-cyano-4′-n-alkyl-biphenyls (5CB) on the water surface by monolayer compression. The orientational order parameters expressed by using Legendre polynomials were successfully determined from both MDC and SHG signals, and the difference in the determined order parameters was discussed in association with the molecular motion of 5CB monolayers.

Detection of phase transition of 4-alkoxy-4′-n-cyanobiphenyl Langmuir films by Maxwell displacement current and optical second-harmonic generation measurements

Abstract Monolayers of 4-alkoxy-4′-n-cyanobiphenyl (nOCB, n=5–8) at the air–water interface have been investigated using Maxwell displacement current (MDC) and optical second-harmonic generation (SHG) measurements by monolayer compression. The generation of MDC and SHG from 5, 6 and 7OCB monolayers was similar, but different from that of 8OCB monolayers, possibly due to the strong interaction between 8OCB molecules leading to aggregation. That is, p-p and s-p SH signals were generated by monolayer compression and p-s and s-s SH signals were generated by further compression of 5, 6 and 7OCB monolayers, but these four signals were simultaneously generated for 8OCB monolayers. We could also detect p-s and s-s SH signals due to shear induced reorientation by monolayer compression.

Detection of phase transition of 4-heptyloxy-4

We investigated 4-heptyloxy-4′-n-cyanobiphenyl (7OCB) Langmuir films by monolayer compression using Maxwell displacement current (MDC) and optical second harmonic generation (SHG) measurements. In the region with a slope in the surface pressure – area (π-A) isotherm, it was found that the s-s and p-s SH signals were suddenly generated by monolayer compression, although the change of MDC was very gentle. This result reveals the appearance of phase transition accompanying the monolayer structural change from C∞v to Cs-symmetry, due to the shear induced azimuthal in-plane reorientation in monolayers.