Yasujiro Kawabata

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.

Intrinsically conducting langmuir-blodgett film of TMTTF-octadecyltcnq

Abstract Conducting Langmuir-Blodgett films were fabricated by use of an amphiphilic charge transfer complex, TMTTF-octadecylTCNQ ( 1 ). The film exhibited a bulk conductivity of 0.1 S cm−1 in the lateral direction without any doping or further treatments. High anisotropy of the conductivity was observed in the film: the conductivity in the normal direction was 10−14 S cm−1. The film was stable for at least three months in air at room temperature.

Conductive Langmuir-Blodgett films based on alkylammonium-metal(4,5-dimercapto-1,3-dithiol-2-dithiolene)2

Abstract Conductive Langmuir-Blodgett (LB) films of alkylammonium-metal(dmit)2 (where H2dmit ≡ 4,5-dimercapto-1,3-dithiol-2-thione) system are described. Mono-alkyl-substituted ammonium to tetra-alkyl-substituted ammonium were used as the counter-cation and nickel, gold, palladium and platinum were used as the central metal cation. These complexes were transferred onto solid substrates with or without icosanoic acid as a matrix. The LB films were oxidized either by bromine oxidation or electrochemical oxidation. The majority of the LB films showed a conductivity of the order of 0.1–1 S cm−1. The LB film of tridecylmethylammonium-Au(dmit)2, however, showed a conductivity of 30 S cm−1 after electrochemical oxidation. Moreover, this film showed metallic behaviour down to around 200 K with a weak temperature dependence. The IR spectrum of the oxidized film was consistent with the metallic behaviour of the film.

Langmuir-Blodgett films of amphiphilic cyclodextrins

Abstract Surface pressure-area isotherms for 12 amphiphilic cyclodextrins (CDs) with alkyl chains were recorded. CDs with long alkyl chains of more than eight carbon atoms form stable monolayers at the air-water interface. The limiting areas per molecule depend on the number of glucose units and the substitution of the secondary hydroxy groups. These areas are in good agreement with the values calculated from the model where the base of the cylindrical part of the molecule is packed closely in a two-dimensional layer. Some of the stable condensed monolayers were deposited as Y- or Z-type Langmuir-Blodgett (LB) films. The molecular orientation in the LB films determined by polarized IR spectra supported the above model.

Photoelectric effects in heterojunction Langmuir-Blodgett film diodes☆

Abstract Photoelectric characteristics of heterogeneous Langmuir-Blodgett (LB) film diodes have been studied using mixed monolayers of surfactant derivatives of semiconducting dyes and arachidic acid (C 20 ). The current-voltage and capacitance-voltage curves measured were qualitatively the same as those of amorphous dye devices. Evaluation of the photoelectron decay range showed that the dye chromophore modifies the LB films to act as electron transport layers. A simple model is proposed, which assumes the formation of energy bands at the chromophore-containing interfaces and shows an analogy between the LB film diodes and amorphous organic devices. With this model, the loss of photoelectrons at an unmodified interface of C 20 is evaluated as 28% and 52% for the hydrophobic and the hydrophilic regions, respectively.

Photoresponsive conductivity in Langmuir-Blodgett films

Abstract An amphiphilic charge transfer complex with azobenzene introduced as a switching unit into the hydrophobic part was synthesized. The occurence of the reversible cis-trans photoisomerization of azobenzene in the Langmuir-Blodgett (LB) films depends on the state of aggregation of azobenzene. The conductivity in the LB film increases by ca. 20% on irradiation with UV light and reverts to the initial state on irradiation with visible light. The photoresponsive switching in the conductivity can be repeated more than 100 times without any noticeable fatigue.

Exciton states of polysilanes as investigated by electro-absorption spectra

Abstract Electro-absorption (EA) spectra have been investigated for thin films of poly(di-n-hexylsilane) which shows the thermochromic phase transition associated with the conformational change in Si-backbones. The EA spectra for the trans-planer polysilanes (low-temperature phase) have revealed the one-photon forbidden A g state at 4.2 eV in addition to the Stark-shifted lowest B u exciton at 3.4 eV. The drastic change in EA spectra is observed on the phase transition, which is interpreted as the influence of the backbone conformation on the extended exciton states.

Application of Excimer Laser Polymer Ablation to Alignment of Liquid Crystals: Periodic Microstructure on Polyethersulfone

In view of applications for excimer laser ablation, it was found that the highly periodic microstructure on polyethersulfone, which was produced by XeCl excimer laser ablation, was effective for alignment of nematic liquid crystals. The alignment of the liquid crystal molecule depends on the orientation of the microstructure, which is easily controlled.

Structure and electronic transport of highly conductive Langmuir-Blodgett films of tridecylmethylammonium-Au(dmit)2

Electronic transport measurements are reported for Langmuir-Blodgett (LB) films of tridecylmethylammonium-Au(dmit)2 for 300-4 K. The conductivity after electrochemical oxidation is 30-50 S/cm at room temperature, increases with decreasing temperature, and begins decrease at about 200 K. The conductivity at lower temperatures is explained by two-dimensional variable range hopping (2D VRH). The thermoelectric power (S) is positive and well fitted by a linear combination of the temperature dependence of 2D VRH (Sv\proptT1/3) and that of metal (Sm\proptT). These results can be explained by a model in which highly conductive metallic regions are separated by thin weakly conductive regions. The UV/visible absorption spectra and the X-ray diffraction patterns indicate that a large structural change occurs accompanied by electrochemical oxidation.

Highly conducting Langmuir-Blodgett films of the TMTTF-CnTCNQ system☆

Highly conducting Langmuir-Blodgett (LB) films of the tetramethyltetrathia-fulvalene- alkyltetracyanoquinodimethane (TMTTF-Cn TCNQ, where Cn represents CnH2n+1) system are reported. The electron affinities of the acceptors CnTCNQ (n=14, 18 or 22) in solution are almost the same and lie between those of TCNQ and dimethylTCNQ. These TCNQ derivatives form solid charge transfer complexes with TMTTF. The limiting areas of the complexes, normalized to the number of TCNQ derivatives at the air-water interface, are almost the same irrespective of the donor-to-acceptor ratio and of the length of the alkyl chain, indicating that the areas are governed by the TCNQ moiety. The monolayers of TMTTF-CnTCNQ (n=14 or 18) are transferred onto solid substrates as LB films. Polarized UV-visible absorption spectra indicate that the long axis of TCNQ is parallel to the film surface. These LB films exhibit lateral conductivities as high as 0.4 S cm-1 and 0.1 S cm-1 when n=14 and n=18 respectively.