Molecularly Ordered Low Molecular Weight Azobenzene Dyes and Polycation Alternate Multilayer Films: Aggregation, Layer Order, and Photoalignment

Abstract

Although the incorporation of small-molecule azobenzene dyes in polymer thin films has been investigated in the past, the application of the layer-by-layer adsorption technique has enabled incorporation of molecularly ordered dye layers without applying the Langmuir−Blodgett methodology. A problem with small-molecule dyes is their tendency to aggregate and even phase separate in polymer matrixes. With the alternate layer-by-layer technique, we have found that the aggregation properties of a charged chromophore are self-limiting and are promoted by adsorption to oppositely charged surfaces. The molecular assembly process of the dye−polycation pairs was investigated by a number of surface-sensitive spectroscopic and microscopic techniques. Comparison of the layer thickness with the molecular dimensions of the dyes suggests the formation of well-packed monomolecular layers depending on the size, spatial orientation, and aggregation limit of each pair. The polycation charge density and salt concentration play an important role in influencing aggregation both before and after adsorption to surfaces. Initial studies with the dye Direct Red 80 showed interesting photoalignment properties with linearly polarized UV−vis light. This method allows the preparation of functional ultrathin films of small-molecule azobenzene dyes for various optical applications.