Materials Chemistry of Self-Assembling Dyes

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Organic dyes are ubiquitous in our environment and new dyes with improved properties for specific technologies are constantly in demand. The Eichhorn group develops dyes that assemble into specific molecular arrangements similar to conventional liquid crystals. This type of control over the structure of materials at molecular length scales (nanostructure) is as important for the adjustment of properties of dye films and particles as the molecular structure of the dye itself. In this proposal new design criteria are proposed for organic dyes that (1) self-assemble into anisotropic materials with strong interactions between aromatic groups and (2) form one molecule thick films of defined nanostructures. Materials with these structures are expected to generate unprecedented properties that benefit their application in a wide range of different technologies that are commercially important, such as organic semiconductors, high pressure lubricants, optical films, and sensors. Topic 1 challenges the paradigm flexible side-chains are required for the design of organic molecules and polymers consisting of larger conjugated/aromatic structures because these rigid aromatic structures are otherwise insoluble and difficult to process. Judicious choice of the types of side-chains and their attachment points has also been shown to enhance control over the arrangement and alignment of these molecules in a material. However, the utilization of flexible side-chains has important disadvantages as it often lowers thermal stability of the molecules, excludes certain purification and processing methods (e.g. sublimation), disfavors certain packing arrangements, and hinders charge transport in applications as organic semiconductors. The main objective of the work proposed here is to develop rational molecular design criteria for side-chain free polycyclic aromatic dyes that can be processed into similar molecular arrangements and identify materials properties that are superior to those of side-chain containing analogues. Research proposed for topic 2 builds on the group's recent success in controlling the alignment of dye molecules in thin films at interfaces. Polycyclic aromatic dyes commonly form aggregates of stacking molecules and self-assemble with an edge-on orientation at interfaces. This molecular arrangement alters their light absorption and often suppresses their fluorescence. It also generates a rather dense layer on a surface that hinders the dye molecules from interacting with molecules in the environment, such as analytes, and the substrate surface. Eichhorn's group has developed amphiphilic dyes that orient flat-on with regard to an interface and the main objective of the research proposed here is to optimize the molecular design criteria for these types of dye molecules, to cross-link their thin films for enhanced stability, and to demonstrate their advantages for applications in sensing, surface coatings, and functional passivation of nanoparticles.