Photochemically Induced Changes of Optical Anisotropy and Surface of LB- Multilayers Built up by an Amphiphilic and Liquid Crystalline Copolymer Containing Azobenzene Moieties

Orientation Direction Control in Liquid Crystalline Photoalignable Polymeric Films by Adjusting the Free-Surface Condition

Adjusting the free-surface condition facilely controls the in-plane and out-of-plane orientations in liquid crystalline polymer (LCP) films. Top coating with aromatic molecules onto LC polymethacrylate films with N-benzylideneaniline (NBA) or 4-methoxybiphenyl (MB) side groups (PNBAM or PMBM) and subsequent annealing generate a random planar orientation while simultaneously removing the coated aromatic molecules, whereas annealing noncoated films induces a homeotropic orientation of the mesogenic side groups. Additionally, irradiating a top-coated PNBAM film with linearly polarized (LP) 365 nm light induces an in-plane molecular reorientation of the NBA side groups without changing the orientation in the homeotropically oriented region. Changes in the surface topology of the LCP films due to the reorientation processes are investigated in detail. Inkjet coating with aromatic molecules and LP 365 nm light exposure precisely controls the in-plane and out-of-plane alignment pattern in a PNBAM film.

Photo-induced structural changes of azobenzene Langmuir-Blodgett films

Structural changes of the Langmuir-Blodgett (LB) films of azobenzene accompanied by photoisomerization are described. First, photoisomerization is explained in terms of 'free volume'. In the polyion complex monolayers of amphiphiles having two azobenzene units at the air-water interface, the area per molecule depends on the polycation species. The fraction of cis-azobenzene in the LB films at the photostationary state under the illumination with UV light increased with increasing area per molecule, which is consistent with the concept of free volume. Second, a counter example of the concept of free volume is presented. Three-dimensional cone-shaped structures developed with trans-to-cis photoisomerization in the polyion complex LB film of a water-soluble amphiphilic azobenzene. These structures appeared and disappeared reversibly by alternate illumination with UV and visible light. The results indicate that the two-dimensional LB film structure exerts significant modification by photoisomerization. This is against the concept of free volume because this concept does not consider the possibility that the two-dimensional LB film structures may change into three-dimensional ones. Finally, photo-induced J-aggregate formation of non-photochromic and photochromic dyes is described. Two cyanine dyes were each mixed with an amphiphilic azobenzene in the LB films. These cyanine dyes are known to form J-aggregates in single-component LB films. In the mixed LB films, the J-aggregate formation was suppressed to some extent. The alternate illumination of the films with UV and visible light caused the photoisomerization of azobenzene in the mixed LB films, which triggered the J-aggregate formation of the cyanine dyes. The J-aggregate formation was accompanied by the development of three-dimensional cone-shaped structures from the film surface. When an amphiphilic merocyanine was mixed with the azobenzene in the LB films, J-aggregate formation was also induced by the alternate illumination with UV and visible light. This J-aggregate formation was also accompanied by a large morphological change: circular domains changed into fractal-like ones. The J-aggregate formation of the dyes and the concomitant morphological change were irreversible. In these cases, the photoisomerization of azobenzene served as a trigger to induce self-organization of the dye molecules.