Light-induced orientation phenomena in Langmuir-Blodgett multilayers

The double-layer structure of Langmuir-Blodgett (LB) multilayer assemblies formed by a thermotropic polyacrylate with a-hydroxyethyl and [[[[(2-methylbutyl)oxy]phenyl]azo]phenoxy]hexyl side groups is modified by annealing and on ultraviolet (UV) irradiation. Both procedures cause order-disorder transitions. The interaction with linearly polarized visible light results in a reversible reorientation of the azobenzene side groups maintaining the layered structure. in this way the optical anisotropy of the LB films is modified reversibly and continuously. In the original multilayer the photoreorientation process is restricted by the dense packing of the side groups. But the process takes place after weakening the intermolecular interactions. This has been achieved by a suitable preparation of the films, by annealing of the thermotropic polymer, or by a photochemically induced decrease of the supramolecular order. The photogeneration of anisotropy is governed by the molecular aggregation, the orientational order, the thermotropic ordering tendency of the polymer to establish a homeotropic alignment, and the irradiation conditions (wavelength, power density, polarization direction). Continued irradiation or annealing above the glass transition temperature (T-g) results in a decrease of absorbance indicating smaller tilt angles. The photoinduced dichroism is long-term stable but can be erased by irradiation and heating.