Zachary Hudson

Emerging methods for the fabrication of multifunctional nanostructures from soft matter have allowed for the synthesis of complex macromolecules with varied morphologies and physical properties. Using organic semiconductors as building blocks, nanostructures with novel emissive properties and interesting charge-transport behaviour are increasingly accessible. Self-assembly processes have been developed to promote nanofiber formation, giving access to previously inaccessible functional nanostructures. However, self-assembly routes to nanofibers are limited by the metastability of the resulting structures, leading to a demand for covalent methodologies for their assembly. This thesis presents an optimized method for the synthesis of polymeric organic semiconductors via controlled radical polymerization and methods by which these linear polymers can be used to generate complex, covalently bound nanofibers. We proposed that by controlling the composition and ordering of the polymeric components, multiblock bottlebrush copolymers could be prepared with photophysical and electronic properties that could not be achieved in a linear polymer morphology. Using this method, multicomponent fibers were prepared resembling nanoscale organic electronic devices such as two-component diodes. This bottlebrush framework can also be used to control the electronic interaction between multiple organic semiconductors within the brush. This property was exploited to control through-space charge transfer thermally activated delayed fluorescence (TSCT TADF) in bottlebrush fibers. The polymerization methodologies developed for the synthesis of organic semiconductors were also explored as a method to prepare luminescent copolymers composed of a host monomer and a series of emissive monomers. We propose that using this methodology, low-cost polymers can be prepared that exhibit the photophysical properties of the emissive dopant. This will allow us to prepare polymers that exhibit a range of interesting properties including deep blue electroluminescence, TADF, and ratiometric fluorescent oxygen sensing. Lastly, a pair of violet emitters were prepared using a novel planarized triphenylamine donor and a sulfone or sulfoxide acceptor. State-of-the-art deep blue emitters typically exhibit low photostability. We propose that stable emitters can be achieved using a planarized hexamethylazatriangulene donor in a donor-acceptor framework. The reduced torsional strain and locked planar rigidity increase the stability of this donor while reducing nonradiative decay resulting in highly emissive compounds with resistance to photobleaching.

View record

Synthesis of multicomponent nanoscale structures with precisely addressable function is critical to discover new phenomena and new applications in nanotechnology. Though self-assembly offers a route such materials, these methods often require building blocks with particular structural motifs, limiting the scope of nanomaterials that can be prepared. Work described in the thesis uses a bottom-up approach based on covalent chemistry to synthesize a series of bottlebrush copolymers (BBCPs) – polymeric side chains attached covalently to a linear backbone – from organic semiconductors. Methods are also presented for the efficient synthesis of planar thermally activated delayed fluorescence (TADF) materials and stable organic fluorophores, incorporating them into nanoscale systems for biological imaging. A series of acrylic monomers was synthesized based on p-type organic semiconductor motifs found commonly in organic electronic devices. These monomers were polymerized by Cu(0)-reversible deactivation radical polymerization (RDRP), the kinetics of which are described in detail. By combining Cu(0)-RDRP and ring opening metathesis polymerization, narrowly dispersed multiblock bottlebrush fibers were prepared from monotelechelic dye-functionalized acrylate polymers with polymerizable norbornene end-functions (macromonomers). This strategy was used to construct nanofibers with the structure of phosphorescent organic light emitting diodes (OLEDs) on single macromolecules, such that the photophysical properties of each component of an OLED could be independently observed. Red, green, and blue (RGB) luminescent macromonomers were prepared using Cu(0)-RDRP, which were used to prepare multiblock organic nanofibers structurally analogous to nanoscale RGB pixels and multilayer white OLEDs. Changes in energy transfer efficiency and interchromophore distance were quantified using a Förster resonance energy transfer model. Additionally, donor-acceptor dyes were prepared using a novel acceptor based on N-phenylbenzimidazole constrained in a coplanar fashion with a methylene tether (IMAC). Emitters were designed with a twisted conformation between donor and acceptor resulting in effective spatial separation of the highest occupied molecular orbital and lowest unoccupied molecular orbital and small singlet-triplet energy gaps to give TADF. In fluorescent IMAC derivatives, locking these chromophores into planar configurations was demonstrated to improve their cross-section for two-photon excited fluorescence and reduce the rate of photobleaching. Proof-of concept studies incorporating these dyes into water-soluble polymer dots suitable for biological imaging was also demonstrated.

View record