Horst Orendi

Photochromic command surface induced switching of liquid crystal optical waveguide structures

We report on optical waveguide structures containing a thin liquid crystal (LC) film held between two photochromic command surfaces. The command surfaces consist of three monomolecular layers of a polymer with azobenzene side chains deposited according to the Langmuir–Blodgett–Kuhn technique. When exposed to light of appropriate wavelength, the command surfaces undergo a trans↔cis photoisomerization process that induces a reversible change in the liquid crystalline orientation. Such an orientation change of the LC alters the optical properties of the optical waveguide. We present experiments on the dynamics of the LC orientation process. The transition is shown to be continuous, with the degree of orientation dependent on the ratio of the cis‐trans chromophore concentration ratio in the command surface.

Correlation between the Molecular Structure and Photoresponse in Aliphatic Self-Assembled Monolayers with Azobenzene Tailgroups

We have compared the structural and photoisomerization properties of self-assembled monolayers (SAMs) comprising either the trans or cis isomers of azobenzene terminated dithiolane with in-chain amide unit, viz., 4-(phenyldiazenyl)phenyl-4-(1,2-dithiolane-3-yl)-butylcarboxamide ( 1). These films were prepared on Au(111) from solutions of both isomers. Structure and composition of the SAMs were studied by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The photoresponse of the films was monitored in real time by ellipsometry. SAMs fabricated from the trans isomer were found to be densely packed and highly ordered. These films did not show any discernible photoresponse upon irradiation with UV light, which, under favorable conditions, triggers the trans- cis isomerization. In contrast, films prepared from solutions containing predominantly the cis isomer were loosely packed and mostly disordered but exhibited reversible photoreactivity. The results confirm that steric effects, i.e., available free volume, play a dominant role for the photoresponse of aliphatic SAMs bearing the photoactive azobenzene group. The crystal structure of 1 ( trans isomer) exhibits a row-like aggregation of neighboring molecules by weak hydrogen bonds and can be taken as a model for the arrangement of 1 in the monolayer films. Further, in addition to the surface coordination behavior, we have also mimicked the chemisorption of the 1,2-dithiolane moiety onto the gold substrate in molecular coordination chemistry in oxidative addition reactions with the zero-valent platinum complex [Pt(PPh 3) 4].

PHOTOINDUCED ALIGNMENT OF AZOBENZENE MOIETIES IN THE SIDE-CHAINS OF POLYGLUTAMATE FILMS

Abstract The ‘trans↔cis’ reversible photoisomerization process tends to align azobenzene derivatives perpendicular to the polarization direction of the pumping beam. It is shown that in the trans→cis optical transition the cis state is aligned perpendicular to the pumping light polarization. This is shown for spin-coated films of ‘hairy-rod’ polyglutamate with azobenzene moieties in the side chains. Photoselection in both trans and cis states is demonstrated using the attenuated total reflection method.

ANISOTROPIC ALIGNMENT OF A NEMATIC LIQUID-CRYSTAL CONTROLLED BY A POLARIZATION-SENSITIVE LANGMUIR-BLODGETT COMMAND LAYER

Abstract We investigate the homeotropic⇔planar switching in the alignment of a nematic liquid crystal (LC) controlled by Langmuir-Blodgett command layers of polymer containing photochromic azobenzene molecules in the side chain. Waveguide spectroscopy is used to probe the changes in the optical properties of the LC induced by the cis ⇹ trans photoisomerization of the azobenzene units. This optical method provides a high sensitivity for studying the orientation of the LC both within and perpendicular to the plane of waveguide cell. The study of the dynamics of the switching induced by polarized uv light shows that initially the LC molecules follow the movement of the azobenzene units but that eventually they align with the dipping direction. An explanation for this effect based on the movement of the azobenzene units themselves is proposed.

Command surface controlled liquid crystalline waveguide structures as optical information storage

We report on optical waveguide structures in which light propagates within a liquid crystal (LC) thin film. In this configuration, the orientation, and therefore the optical properties of the LC waveguide structure are controlled by two photochromic command surfaces, consisting of an ultra thin film of a polymer with azobenzene side chains [K. Ichimura, Y. Suzuki, T. Seki, A. Hosoki, and K. Aoki, Langmuir 4, 1214 (1988)]. When exposed to light of appropriate wavelength, the azobenzene side chains undergo a trans↔cis photoisomerization process inducing a commensurate change in the LC alignment and therefore, in the set of refractive indices of the LC film. Using this effect we could reversibly write information into the LC cell. The size of our test structure was in the range of 50 μm. The stored information was read out by optical waveguide microscopy [W. Hickel and W. Knoll, Appl. Phys. Lett. 57, 1286 (1990)]; the storing times were in the range of several hours.

Command surface induced switching of the optical properties of liquid crystalline thin film structures

Abstract We report on optical waveguide structures with light propagation in a nematic liquid crystalline thin film. The liquid crystal (LC) is sandwiched between two so-called photochromic command layers, consisting of a polymer with azobenzene side chains. When exposed to light of appropriate wavelength, the azobenzene side chains undergo a transcis isomerization process, inducing a homeotropicparallel change in the LC alignment. These orientational changes affect the set of refractive indices of the LC thin film and, hence, can be detected by optical waveguide methods. As a possible application of the command surface effect in optical waveguide structures, we present an optical switching and information storage device.

Optical switching of azobenzene sidechain molecules observed by surface plasmon spectroscopy

The use of surface plasmons is reported for the optical characterization of thin organic films, in particular thin polymer films at a metal dielectric interface. The basic properties of PSP are presented, including excitation and the technique of surface plasmon microscopy, which allows to obtain an image. As an example studies are presented on the trans-cis photoisomerization of azobenzene sidechain polymers in ultra thin films, prepared by the Langmuir-Blodgett-Kuhn technique. The transition from the trans to the cis state induces a change in the refractive index of the film. These changes were detected and their kinetics measured by a surface plasmon spectroscopy technique. A refractive index pattern could be written into the films and the areas of different refractive indices have been detected by surface plasmon microscopy.

Ellipsometric studies of photochromic ultrathin polymer films

We present ellipsometric measurements on Langmuir-Blodgett monolayer assemblies of photochromic polymers; the polymer we used was a statistical copolymer with polymethacrylate mainchains and azobenzene sidegroups. Ellipsometry gives a value of n equals 1.557 for the refractive index and a film thickness of 1.58 nm per monolayer. When exposed to light of appropriate wavelength, the azobenzene groups undergo a trans (reversible reaction) cis photoisomerization process which was monitored by ellipsometry. We found an average change in film thickness of 0.02 nm per monolayer during the trans(reversible reaction)cis transition process, whereas the data obtained do not show any significant change in refractive index.