Doctor of Philosophy in Chemistry (PhD)
Red-Emitting Thermally Activated Delayed Fluorescent Materials for Sensor Applications
1) Luminescent materials design for applications in organic electronics.
2) Fluorescent polymer nanoparticles for biological sensing with high sensitivity.
3) Ultrastable fluorophores for catalysis, electroluminescence, and two-photon fluorescence.
Academic: A first-class grade point average (80%+) majoring in chemistry or a closely related discipline. Senior-level courses in chemistry are an asset.
Research: Undergraduate research experience, including a senior thesis project and one or more terms as a summer research assistant. Experience in one or more areas of synthetic chemistry (organic, inorganic, polymer) are an asset.
Soft skills: i) Interest in broad, multidisciplinary areas of chemistry and materials science; ii) A desire to work collaboratively as part of a team to accomplish shared goals.
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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.
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.
Organic semiconductors, which commonly contain π-conjugated systems, have many advantages over inorganic semiconductors. These advantages have generated significant research interest and have allowed for their successful use in a variety of electronic and optoelectronic devices, such as organic light emitting diodes. Herein, we demonstrate a novel method for the synthesis of nitrogen-containing π-conjugated luminescent materials. The typical synthetic methods suffer from disadvantages such as harsh reaction conditions, expensive metal catalysts, and stoichiometric reagents. By using hydroamination, an atom-economic catalytic route to amine-containing compounds, we demonstrate an alternative synthetic method which circumvents these limitations. We also demonstrate a novel synthetic route to polymeric organic semiconducting materials with a fiber-like morphology. Many of the existing methods suffer from the limitation of requiring very specific conditions and monomers. Employing the grafting-from method of bottlebrush synthesis, RAFT and SET-LRP polymerization techniques are used to produce long fiber-like nanowires from arbitrary semiconducting monomers. These cylindrical nanofibers can be applied to a diverse range of organic semiconductors, with potential applications in macroscale optoelectronic devices or as functional nanoscale objects.
Bottlebrush copolymers have shown promise as building blocks for self-assembled nanomaterials due to their reduced chain entanglement relative to linear polymers and their ability to self-assemble with remarkably low critical micelle concentrations (CMCs). Concurrently, the preparation of bottlebrush polymers from organic electronic materials has recently been described, allowing multiple optoelectronic functions to be incorporated along the length of single bottlebrush strands. Here we successfully synthesized well-defined bottlebrush diblock copolymers containing soluble n-butyl acrylate blocks and carbazole-based organic semiconductors with control over the backbone length ratio. Then the successful incorporation of highly fluorescent dye molecules into the BBCP was achieved by using the CzBA polymer as an organic semiconductor host to facilitate energy transfer. We also describe the self-assembly of these molecular bottlebrushes, which self-assemble in selective solvent to give spherical micelles with CMCs below 54 nM. These narrowly dispersed structures were stable in solution at high dilution over periods of months, and could further be functionalized with fluorescent dyes to give micelles with quantum yields of unity. These results demonstrate that bottlebrush-based nanostructures can be formed from organic semiconductor building blocks, opening the door to the preparation of fluorescent or redox-active micelles from giant polymeric surfactants.