Doctor of Philosophy in Chemistry (PhD)
Deep blue thermally activated delayed fluorescence: Unlocking their potential as energy transfer visible light photocatalysts
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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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
Polymer organic electronics are pervasive in materials chemistry and life sciences. These materials demonstrate mechanical properties ideal for solution processing and flexible electronic applications. The molecular weight and dispersity of a polymer can also have a profound effect on the properties of the material. Consequently, the use of controlled polymerization techniques is critical in the preparation of organic electronics. Herein, controlled polymerization techniques are used to prepare semiconducting materials demonstrating unique charge-transport and photophysical properties.Thermally activated delayed fluorescent (TADF) materials have been used widely in bioimaging, photocatalysis and organic light emitting diodes (OLEDs). TADF materials can, in theory, achieve full conversion of electricity to light making them particularly useful in OLEDs. However, many of these materials demonstrate low rates of reverse intersystem crossing process. These rates can improve when utilizing TADF materials demonstrating through space rather than through bond charge transfer. Thus, in this work a library of through-space charge transfer materials is explored. However, the charge transfer character of the emission in TADF materials results in broad emission bands. The colour purity can be improved by including an appropriate fluorophore to which the TADF moiety can transfer energy, termed thermally assisted fluorescence (TAF). Surface-initiated polymerization (SIP) is an approach to prepare densely grafted polymers on a surface. These “polymer brushes” often demonstrate properties that differ from “free” polymers. Thus, the incorporation of luminophores into polymer brushes could result in unique photophysical properties. Additionally, the use of SIP approaches enable optoelectronically relevant materials to be grown directly from substrates. However, polymer brushes containing TADF and TAF moieties have yet to be explored. Thus, in this work, controlled surface-initiated polymerization techniques are used to prepare polymer brushes incorporating TADF and TAF that show promise for use in lighting and spatially resolved sensing applications. Finally, these SIP approaches are leveraged to investigate unique polymer brush architectures known as bottlebrush brushes. These materials enable high surface coverage of the polymer coating on a substrate while still allowing mass transport of molecules through the brushes affording a molecular wire-type structure. This architecture shows promise as a polyelectrolyte material.
Ensuring long-term photostability in the design of luminescent materials remains a challenge that is often addressed at the molecular level. This consideration must be balanced with other optical properties such as emission wavelength, photoluminescence quantum yield (ΦF or PLQY), and colour purity, as well as physical properties such as solubility and thermal stability. Molecular rigidification is a promising means to improve photostability, ensuring suppression of molecular vibrations and photodegradation pathways. This strategy often enhances PLQY and colour purity as well, while requiring demanding synthetic procedures to prepare rigid and robust molecular motifs. By leveraging these advanced optical properties in conjunction with the design of purely organic materials exhibiting thermally activated delayed fluorescence (TADF), these materials can find use in a wide range of applications such as emitters in organic light emitting diodes (OLEDs), and as dyes for time-resolved imaging (TRI) in biology. Work described in this thesis involves the preparation of a range of luminophores exhibiting prompt nanosecond fluorescence, phosphorescence, and thermally activated delayed fluorescence, with tailored design features to render them useful for optoelectronic and biological applications. Deep-blue to deep-red emissive fluorophores were prepared using 1,3,4-oxadizole (ODA), s-heptazine (HAP), s-triazine (TRZ), and dibenzodipyridophenazine (BPPZ) motifs, with molecular rigidification explored using hexamethylazatriangulene (HMAT) as a planarized triarylamine electron-donating substituent. As some of these materials exhibited long-lived emission lifetimes and non-linear optical properties such as two-photon excited fluorescence (2PEF), efforts were made to render them water-dispersible in nanoparticle formulations for in vitro biological imaging applications. A range of strategies was examined to improve the versatility of luminophores using synthetic polymeric systems. In particular, living and controlled polymerization methods were used for incorporation of luminophores into polymers, resulting in solution-processable phosphorescent platinum (II) metallopolymers for OLED applications, and fluorescent polymers suitable for the preparation of polymer dot (Pdot) nanoparticles as bioimaging probes. Additionally, the challenge of rendering oxygen-sensitive TADF molecules water-dispersible was addressed using commercial surfactants for glassy organic dot (g-Odot) nanoparticles, in an effort to further explore a universal approach toward using any high-performance TADF material as a bioimaging probe.
Fluorescence imaging is a critical tool for visualizing cellular structures and complex biological processes; however, background autofluorescence from molecules and structures within the cell can severely reduce the signal-to-noise ratio (SNR) and imaging quality. Time-resolved imaging (TRI) and two-photon excited fluorescence (2PEF) microscopy are two techniques that can be used to remove background autofluorescence and improve SNR. Imaging probes with long lifetime emission are required to utilize TRI techniques. Purely organic materials with properties such as thermally activated delayed fluorescence (TADF) exhibit long lifetime photoluminescence. Additionally, to image living samples, luminescent imaging probes should ideally absorb and emit within the biological transparency window (650–1350 nm), but efficient TADF emission within this range is rare.Work described in this thesis uses materials exhibiting TADF or 2PEF to develop biological imaging probes with emission within the biological transparency window. These materials were synthesized as small molecules or polymerized with a semiconductor host and formed into polymer nanoparticles called polymer dots (Pdots) that retain the photophysical properties required for TRI or 2PEF microscopy. New deep-red/near-infrared (NIR) emissive TADF monomers were synthesized and the mechanism behind TADF was explored using density functional theory (DFT). Pdots with a cell-penetrating peptide mimic shell were demonstrated to efficiently enter multiple cell lines in under 30 minutes while retaining high cell viability. Proof-of-concept experiments for the use of these Pdots in TRI demonstrated that the TADF emission can be separated from background fluorescence. This work represents some of the first demonstrations of Pdots containing red to NIR emissive TADF polymers for cellular imaging. For materials exhibiting 2PEF, it was shown that the electron donor moieties effect on planarity directly correlates to the relative strength of 2PEF within a series.Additionally, in the closely related field of semiconductor polymers, reactive semifluorinated polymers were investigated for the efficient synthesis of a series of polymers based on p-type and n-type semiconductor motifs commonly found in organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), and organic photovoltaics (OPVs). Experiments demonstrating that semifluorinated polymers can provide a useful building block in the synthesis of organic electronic materials were conducted.
Emissive polymers offer unique features over small molecules for applications in biological imaging and sensing. The ability to define polymer architecture and molecular weight through highly controlled polymerization methods allows for the development of materials such as nanoscale drug carriers, temperature-responsive matrices, and emissive polymer nanoparticles. This work centers around the controlled synthesis of emissive polymers with highly defined molecular weight through two living polymerization methods: Cu(0)-reversible deactivation radical polymerization (RDRP) and ring-opening metathesis polymerization (ROMP). Phosphorescent iridium(III)-based polymers were synthesized by Cu(0)-RDRP, and the kinetics of this polymerization are discussed in detail. The resulting polymers exhibited narrow molecular weight distributions and emission in the blue, green, and red regions. As heavy metal-based emitters often have low biocompatibility, purely organic materials based on a 1,8-naphthalimide (NAI) acceptor exhibiting thermally activated delayed fluorescence (TADF) were prepared and incorporated into star polymers. The addition of a water-soluble, biocompatible monomer, Nisopropylacrylamide (NIPAM) and a blue fluorescent emitter resulted in water-soluble polymers with temperature-controlled emission colour. Next, additional red TADF monomers based on NAI acceptors were developed for ROMP, which allows for the facile synthesis of block copolymers with highly defined interfaces. These monomers were incorporated into amphiphilic diblock copolymers with a guanidine-rich hydrophilic block, and a hydrophobic block resembling the emissive layer of organic light emitting diodes (OLEDs). These diblocks self-assembled into polymer dots (Pdots) in water, where the guanidine-rich corona provided excellent shielding ability of the encapsulated emitters and allowed for efficient uptake into multiple cell lines. The covalently bonded TADF dyes eliminated the previously seen problem of dye leakage from Pdots, and these materials were successfully used for cellular imaging. Lastly, highly rigid TADF emitters based on BPPZ acceptors were synthesized and incorporated into Pdots to improve brightness and photostability. These TADF emitters were encapsulated into guanidine-rich Pdot matrices as small molecules, allowing this system to act as a model for hydrophobic drug encapsulation. Confocal imaging with cancer cell lines was performed to evaluate the internalization of the Pdots, and it was demonstrated that the length of the guanidine corona affects the internalization of the Pdots by HeLa and Jurkat-T cells.
Photosensitizer molecules play a crucial role in materials and life sciences. Efforts to improve theirperformance and reduce the associated costs are vital for advancing environmentally friendly light-driven technologies. This thesis examines photosensitizers that make use ofthermally activated delayed fluorescence (TADF) and explores efforts to develop emitters withapplication-tailored properties. The key finding is the diversity of accessible excited state pathwaysin TADF sensitizers, which can be tuned by both molecular and supramolecular approaches to suita problem.Organic light emitting diodes (OLEDs) are the future of solid-state lighting, but the fullrealization of this technology will require new materials that can be manufactured cost-effectivelywith high performance for the conversion of electricity to light. Polymeric TADF emitters areattractive in this respect because they can function as the emissive elements of OLEDs and bedeposited by low-cost solution methods. This thesis makes use of 2,4,6-triphenylpyrimidine-basedacrylic monomers which exhibit bright, colour-tunable TADF, potentially suitable for use insolution-processed OLEDs when copolymerized with a host monomer. These same monomers can also function as sensitizers of fluorescent emission in terpolymers containing a fluorescentmonomer as an energy acceptor for bright and colour-pure TADF-sensitized fluorescence (TAF).The efficiency with which energy is transferred in TAF is found to depend on polymerarchitecture, with block copolymers limiting emission quenching interactions and rigid bottlebrushcopolymers providing optimal control over the interface between energy donor and acceptor at thenanoscale.TADF sensitizers are also attractive in photocatalysis because of the diversity of theiraccessible excited states. A series of TADF donor-acceptor conjugates based on 2,4,6-triphenylpyrimidine acceptor and 9,10-dihydro-9,9-dimethylacridine (DMA) donors wereprepared as photosensitizers for organocatalyzed organic atom transfer radical polymerization (OATRP). The donor is either unmodified DMA or functionalized with methylphenyl ormethoxyphenyl groups. The donor-modification strategy is found to dramatically improve OATRP performance by imparting redox reversibility and high rates of dissociative electrontransfer, as well as preventing unwanted side-reactivity in the excited state. With themethoxyphenyl derivative, methacrylic monomers could be polymerized with low dispersity(≤1.3), high initiator efficiency and chain-end fidelity, and molecular weights up to 40 kDa.
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.
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
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.