Hiroo Nakahara

J-aggregation of photochromic spiropyran in Langmuir-Blodgett films

Abstract The possibility of controlling the photochromic reactions of two different spiropyrans was investigated using the Langmuir-Blodgett technique. The spiropyran having a long chain formed an unstable monolayer without UV light and a stable monolayer with UV light. The spiropyran having two long chains, however, could form more stable monolayers with and without irradiation and the most stable monolayer was formed on mixing with n-octadecane in a molar ratio of 1:2. The assemblies of this monolayer, under UV irradiation at temperatures above 35 °C, exhibited a sharp and intense band at a longer wavelength, which could be assigned to the formation of J-aggregates. The half-decay period in the dark was 104 times longer than that of conventional spiropyrans. The possibilities of three-dimensional high density optical memory with this photochromic spiropyran are suggested.

Monolayers and multilayers of anthraquinone derivatives containing long alkyl chains

Abstract Monolayers of seven anthraquinone derivatives containing one or two stearylamino-groups at different positions of the ring system, spread at the air/water interface, have been studied by pressure-area isotherms and qualitative tests for rheological behaviors. Further, preparation of multilayers of these anthraquinone derivatives on solid surfaces has been examined, using two techniques of the vertical dipping and the horizontal lifting methods, and the thickness per single layer was measured by the optical method. From a consideration of molecular models, probable orientations of the anthraquinone ring in these films are discussed. It has been concluded that different types of orientation of the anthraquinone nuclei can be realized in the surface films of these long-chain dyes, depending on the chemical modifications, such as variation in the numbers and relative positions of the long substituent groups, and also on the physical conditions for deposition, such as surface pressures. It is suggested that the multilayers of these anthraquinone derivatives with long alkyl chains may provide a model of much interest for the spectroscopic researches on dye aggregates.

Studies on molecular orientation in multilayers of long-chain anthraquinone derivatives by polarized infrared spectra

Abstract With seven anthraquinone derivatives containing one or two stearylamino groups at different positions, orientation of the chromophore in the built-up multilayers has been investigated by angular dependence of the polarized infrared spectra. The multilayers were prepared by the horizontal lifting method and the polarized infrared spectra were measured as functions of the angles for the film samples inclined variously against the incident beam and also rotated with respect to the plane of polarization. In the results, it has been found that orientation of the anthraquinone nuclei depends markedly on the numbers and relative positions of the long substituents, in accordance with the previous conclusion from pressure—area characteristics of the monolayers and the film thickness. It should be noted that in some cases the chromophores with long alkyl chains have a tendency to orient depending not only on the vertical direction to the water surface but also on the direction of monolayer compression in the two-dimensional plane, in other words, these exhibit the double-axial orientation in the surface films. The multilayers with a well-defined orientation of chromophores seem to be useful for the study on electronic spectra of dye aggregates.

Ultraviolet photoemission study of oligothiophenes: π-band evolution and geometries

Ultraviolet photoelectron spectroscopy (UPS) has been applied to the investigation of the electronic structure of oligothiophenes with 4–8 thiophene rings. In a series of α‐linked oligomers (αn with n being the number of rings), a systematic evolution of the π band is observed. Several peaks which correspond to the π band are observed in the region of 0.7–3 eV below the Fermi level (EF), and the bandwidth becomes broader with increasing n. The nonbonding π band is observed at 3.5 eV below EF and its energy is almost independent of the number of thiophene units. UPS spectra of α7 and α8 are fairly similar to the spectra of polythiophene, showing that these oligomers are good model compounds of the polymer. The ionization threshold energy of α7 and polythiophene was observed to be 5.3 eV. The effect of irregularity on the π‐electron system was also studied by using oligomers which contain a β linkage or a vinylene group at the middle of the molecule. The UPS spectra showed that the β linkages significantly ...

Photoisomerization of the azobenzenes included in Langmuir-Blodgett films of cyclodextrins

A new type of photochemical Langmuir-Blodgett (LB) film containing azobenzenes (p-phenylazobenzoic acid (PAB), methyl red (MR) and methyl orange (MO)) without long alkyl chains was prepared by the host-guest interaction with an amphiphilic cyclodextrin derivative. The azobenzenes in the LB film of the host- guest complex exhibited an excellent cis-trans photoisomerization, while trans-to- cis photoisomerization was restricted for LB films of long chain azobenzene derivatives alone. The cavity of cyclodextrin provides a favourable environment for the photochemical reaction leading to an increase in the molecular area in the rigid LB film. The half-life times of cis isomers of PAB, MR and MO in the host-guest complex LB films were 35 h, 55 s and 13 min respectively at 20°C.

Langmuir-Blodgett films of oligo- and polythiophenes with well-defined structure

Abstract Using terthiophene and its derivatives, quater-, quinque-, sexi-, and α- or β-septithiophenes, Langmuir-Blodgett (LB) films including oligo- or polythiophenes with well-defined alignments have been obtained. Esters of terthiophene with a short alkyl chain form stable condensed monolayers on water and can be deposited by the LB method. α-polythiophenes without any hydrophilic groups can be incorporated in LB films of mixed monolayers with arachidic acid. In these monolayer assemblies, the long axes of oligo- or polythiophenes stand nearly vertically. LB films of the terthiophene ester derivatives showed a large dielectric constant and a high conductivity which were enhanced by iodine doping.

Monomolecular and multimolecular films of cellulose Esters with various alkyl chains

Abstract The preparation and characterization of monomolecular and multimolecular films of cellulose triesters with various chain lengths were studied in an attempt to obtain well-ordered Langmuir-Blodgett films of polymer molecules. Monolayers of cellulose triesters with medium chain length such as trioctanoate, tridecanoate and tridodecanoate exhibited a characteristic phase transition from expanded to condensed states. The mechanism of the phase transition is discussed on the basis of the temperature dependence of the surface pressure-area isotherms. X-type multilayers of cellulose tridodecanoate could be prepared by the horizontal lifting method. Polarized IR spectra suggested an ordered orientation of the molecules. X-ray photoelectron spectroscopy and contact-angle measurement support the suggestion that the dominant population of hydrophilic glucose rings with ester groups is at the outermost surface of the multilayers.

Conformational analysis of phosphatidylethanolamine in multilayers by infrared dichroism

Built-up films of L-alpha-dipalmitoyl phosphatidylethanolamine were prepared. Infrared dichroism was measured for the sample and analysed by a new method. This method has been developed for the determination of the directions of transition moments in a film sample, in which there is an axis of symmetry of perpendicular to the film plane. The directions of transition moments were determined for the six vibrations assigned to the CH2 antisymmetric stretching, CH2 symmetric stretching, CH2 scissoring, C=O stretching, PO2- antisymmetric stretching, and C-C-N+ antisymmetric stretching modes. The results indicate that hydrocarbon chains are inclined at about 75 degrees to the film plane and the polar groups orient parallel to the plane in the builtup film. A structural model of the phosphatidylethanolamine in the built-up film is proposed.

Orientation of chromophores in monolayers and multilayers of azobenzene derivatives with long alkyl chains

Abstract With five azobenzene derivatives containing one or two stearylamino groups at different positions, the monolayers on the water surface were studied by pressure-area isotherms. From the limiting areas together with the molecular model, probable orientations of the azobenzene nuclei in the monolayers were considered. These monolayers were transferred onto the solid (CaF 2 ) surface by the horizontal lifting method and the molecular orientations were examined by the infrared absorbances of characteristic bands for the multilayers as compared with those for the random samples. Further, the polarized infrared spectra were measured as functions of the angles for the film samples inclined variously against the incident beam and also rotated with respect to the plane of polarization. In the results, it was confirmed that different types of orientation of the azobenzene chromophore can be obtained in the surface films, depending on variation in the numbers and relative positions of the long-chain substituents. These monolayer assemblies with a well-controlled orientation of chromophores seem to be useful for the spectroscopic study on dye aggregates.

In-plane X-ray diffraction and polarized NEXAFS spectroscopic studies on the organized molecular films of fluorinated amphiphiles with vinyl esters and their comb-polymers

Layer structures and the molecular packing in the two-dimensional lattice were studied by using the in-plane X-ray diffractions in addition to the conventional small-angle XRD, and the polarized near-edge X-ray absorption fine structure (NEXAFS) spectra for the multilayers of fluorinated amphiphiles with vinyl groups and their comb-polymers containing fluorocarbons as the side-chains. These molecular lattices were significantly different from the bulk crystals due to the film compression, and NEXAFS results show that the molecular arrangements of the fluorocarbon side-chains were strongly influenced by atoms at the ω-position of the fluorocarbon-chains.