Dendron Design, Synthesis, And Application In Nanocrystal Assembly

Katherine C. Elbert, University of Pennsylvania

Abstract

Inorganic nanocrystals (NC) exhibit a host of properties based on their size, shape, composition, and assembly within thin films. While surface ligands are a key component in NC synthesis, they are often overlooked during system design. Herein, we aim to highlight the powerful effects of intentional surface ligand design and synthesis, specifically drawing inspiration from the rich history of supramolecular assembly.The first section of this dissertation explores how dendron ligands known to assemble in supramolecular chemistry affect the assembly of NCs. We take advantage of the cone shape of dendrons and initially explore how dendron ligands can enhance NC stability by creating a dense organic shell around the NCs. This organic shell allows us to effectively size-match NCs of differing diameters to co-assemble them into one crystalline film, achieving substitutional doping. While dendrons provide a uniform organic shell around spherical NCs, the shape of the organic shell changes on anisotropic NC surfaces. For rhombic plate NCs, a unique architecture is observed within multilayer NC films, due to a non-uniform organic shell that guides the NCs to assemble following a lock-and-key mechanism. This non-uniform ligand shell can be extrapolated and applied to additional anisotropic NCs, including cubes and triangular plates. The addition of a dendron ligand, creating this shell, allows for co-assembly of these two anisotropic NCs, the first example of non-stoichiometric co-assembly of anisotropic NCs into ordered thin films. Given the large impact of these relatively simple dendron structures, we synthesized novel ligand structures that incorporate key components of the ligands used in the first section of this thesis. These new ligands are shown to be used as bridging ligands between two differing NCs, offering an alternative route for co-assembly into micron scale structures. Furthermore, dendrons are shown to be used on the periphery of stimuli-responsive ligands, which finely control the interparticle spacing between NCs within a lattice. Separately, due to the synthetic flexibility of our ligands, we can explore how incorporation of electronically or optically active moieties can be used to probe electron transfer or plasmon-enhanced fluorescence. In the final section of this thesis, we investigate the assembly of the surface ligands themselves, putting forth models of chirality effects on resulting NC assembly structures and inspirations for further experiments to elucidate patchy or striped NC surfaces.