Shin ichi Kuroda

Substituent‐dependent self‐assembly: Two‐dimensional aggregate formation in cyanine dye‐adsorbed Langmuir‐Blodgett films

Interface‐adsorbed complex Langmuir‐Blodgett (LB) films of arachidic acid and water‐soluble, mesosubstituted cyanine dyes were fabricated using the diffusion‐adsorption method to examine the substituent‐dependent self‐assembly at the water‐monolayer interface. Either a J aggregate or an H aggregate was found to be formed in the LB films depending on the substituent introduced, the ethyl or methyl group. The H aggregate is accompanied by Davydov splitting, which is characterized as the split of the band into two bands, whose transition moments are orthogonal to each other. Measurements of photoluminescence and microscopic spectroscopy were performed to identify the type of the aggregates. The molecular arrangements in the aggregates were estimated based on Davydov’s theory and the extended dipole model. Dependence of photoelectric properties of the LB films on the type of aggregates has been studied employing Schottky‐type cells.

Flow Orientation in Langmuir-Blodgett Films of a Charge-Transfer Complex (TMTTF)3(C14TCNQ)2

The occurrence of flow orientation during the deposition of Langmuir-Blodgett films of (TMTTF)3(C14TCNQ)2 is shown through studies of in-plane molecular orientation in films deposited by two types of the vertical dipping method, with the substrates parallel and perpendicular to the barrier, using ESR and polarized UV-visible spectroscopies. Larger in-plane anisotropy is observed toward the edge of the substrates, which is consistent with the prediction of the recent theory of flow orientation during the deposition process. Furthermore, in-plane anisotropy in the films deposited by the horizontal lifting method is detected, suggesting the existence of a compression orientation at the air-water interface.

Formation of Herringbone Structure with Davydov Splitting in Cyanine Dye-Adsorbed Langmuir-Blodgett Films

Interface-adsorbed complex Langmuir-Blodgett (LB) films of arachidic acid and a water-soluble cyanine dye were fabricated using the diffusion-adsorption method. Formation of an H-aggregate with absorption band split was observed in the LB film. This split is assignable as Davydov splitting, which is characterized as the split of the band into two bands, whose transition moments are orthogonal to each other, due to formation of herringbonelike aggregates. The estimation of the molecular arrangement in the aggregate was performed based on Davydovs theory and the extended dipole model.

Modification of aggregate formation in arachidic‐acid–cyanine‐dye complex Langmuir–Blodgett films by substituent groups

Water‐soluble cyanine dye derivatives can be adsorbed from aqueous solution to an arachidic acid monolayer at the air‐water interface. Interface‐adsorbed complex Langmuir–Blodgett films of arachidic acid and four different derivatives of thiacarbocyanine dyes were fabricated using this method, and their optical properties were examined employing a linearly polarized incident. Clear differences in both spectral shape and in‐plane anisotropy were found among the films of different derivatives, each associated with an aggregate form characteristic of the substituent group. They can be classified as a dimer with Davydov splitting, a monomer, and a J aggregate. Estimation of the configuration of the molecules in the dimers and measurement of the photoluminescence spectrum for the J aggregate were performed to clarify the nature of aggregates. The results suggest the possibility of systematic control of aggregate formation in the system by introducing substituent groups for dyes.

Flow‐orientation effect in batch‐produced Langmuir–Blodgett films: Observation of the unsteady flow around the stagnation point

The in‐plane anisotropy of the Langmuir–Blodgett films of a merocyanine‐fatty acid mixed system has been further studied for two different cases of batch production with the substrates aligned face to face and side by side, respectively, to clarify the limitations of the flow‐orientation model based on the ideal‐fluid approximation. For the side‐by‐side case, satisfactory coincidence is found between the model and experiment. The actual dichroic behavior deviates from that predicted for the face‐to‐face case with smaller intersubstrate distances. It was speculated that the stagnation point is associated with anomaly in velocity which is responsible for this discrepancy. In order to examine this, the flow of the monolayer on the water surface during the deposition has been observed using sulphur powder as marker in addition to the optical measurements. The actual flow deviates from that expected from the model and is associated with unsteady motion around the stagnation point as previously speculated. It is indicated that the ideal‐fluid approximation is inappropriate for the face‐to‐face case with smaller distances, leading to the prediction inconsistent with the actual dichroic behavior.

Davydov splitting in arachidic acid-cyanine dye complex Langmuir-Blodgett films

A water‐soluble cyanine dye is adsorbed from the solution to form a salt with an arachidic acid monolayer at the air‐water interface. Complex monolayers thus formed were deposited onto substrates by the vertical dipping method to form Langmuir–Blodgett (LB) films. The optical properties of these films were examined employing a linearly polarized incident light. The films showed a large in‐plane anisotropy of light absorption in both visible (electronic transition) and infrared (vibrational transition) regions. The in‐plane anisotropy of the LB film is explained as due to the alignment of dimers, each showing Davydov splitting, by flow orientation during dipping. The configuration of the dye molecules in the dimer was estimated using Davydov’s theory and the extended dipole model. The layered structure of the film was determined by the x‐ray diffraction technique.

In‐plane anisotropy in batch‐produced Langmuir–Blodgett films: Side‐by‐side and face‐to‐face arrays

In‐plane anisotropy of Langmuir–Blodgett (LB) films is affected by the arrangement of substrates when many samples are batch produced in one single trough. We examined the dichroic behavior for two different cases of the batch production. In the case of substrates aligned side by side, the dichroic behavior is similar to that in the single‐substrate production and agrees with the flow orientation model. In the face‐to‐face case, however, the tendency is quite different from both the single‐substrate production and the side‐by‐side case, clearly deviating from the theoretical predictions. This discrepancy is suggested to be due to a dead water area which should be formed around the stagnation point.

Electron spin resonance in Langmuir-Blodgett films of a merocyanine dye and its isotope-substituted analogs

Abstract Stable radical species existing in mixed Langmuir-Blodgett films of a merocyanine dye and arachidic acid are studied through their hyperfine couplings using the isotope substitution technique. One of the species shows a nitrogen hyperfine structure possibly arising from one of the two nitrogen sites in the molecule. With the employment of two dye analogs in which either one of the 14N nuclei is replaced by 15N, the observed nitrogen site has been identified to be the one in benzothiazole. That is, the wave function of the radical is directly associated with the dye chromophore. The photo-induced spectra have shown a similar change as the dark spectra upon 15N-substitution, indicating that the dark and photo-induced signals arise from a common species. These facts together with the recently observed correlation between the anisotropic behaviour of ESR and optical J-band strongly suggest an intermolecular charge transfer in J-aggregates as the origin of radical generation.

Theory of flow orientation effects in Langmuir-Blodgett films: examination of the local thermal equilibrium approximation

Abstract The angular distribution function for the case of one single substrate in a large trough is numerically calculated allowing for the deviation from the local thermal equilibrium. The function obtained for the line-sink potential on the water surface is found to disagree with that of the previous model derived using the local thermal equilibrium approximation. As for the transfer from the water surface to the substrate, two different models are introduced, the abrupt connection and a smooth connection. The smooth connection model leads to a satisfactory agreement with the predictions from the previous model, and, at the same time, to a semi-quantitative coincidence with the experimentally observed dichroic behavior for the previously obtained values of the viscoelastic parameters. These results indicate that the line-sink potential with local thermal equilibrium approximation, if not strictly relevant to the process on the water surface, is effective for describing the angular distribution in LB films.

Acid Vapor Treatment of Fatty Acid Salt LB Films—Evidence for Conservation of the Lamellar Structure—

The width of each (0, 0, l) X-ray diffraction peak of a fatty acid salt LB film has been found to be invariant with the treatment with hydrochloric acid vapor in the first stage when the peak height decreases drastically. The IR measurements indicated that each fatty acid molecule in the LB film remains unchanged both in orientation and in conformation. These findings strongly suggest that the LB lamellar structure is conserved with the metal ions liberated from the original sites of the material.