Michael Wolf

Professor

Research Interests

Materials Chemistry
Inorganic Chemistry
Polymer chemistry
Photochemistry

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photochemistry
transient absorption
Polymer synthesis

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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.

Investigation of novel photo-cross-linking reactions for the preparation of functional polymeric materials (2022)

Investigations of a number of photochemical cross-linking reactions are reported for the preparation of functional polymeric materials. Reactions that require constant inputs of light and do not proceed through radical or ionic mechanisms are studied, and the final properties of these materials are characterized. This work aims to expand the scope of available photo-cross-linking reactions for the preparation of functional materials. The incorporation of photodimerizable anthracene into a thermally healable vinylogous urethane copolymer enables light-induced post-polymerization UV cross-linking. This allows for tuning of the glass transition temperature over a 20 °C range, control over the rheological properties, and tuning the scratch healing temperature. Pendant attachment of anthracene moieties onto a flexible siloxane polymer (PDMS) results in macroscopic polymer crystallization upon photo-cross-linking, giving a heterogeneous material with up to 1 mm diameter crystallites embedded inside a flexible matrix. A thorough investigation of the growth of the polymer crystallites through microscopic and diffusion techniques revealed unique crystallite growth initiating from the bottom of the film and progressing upwards towards the irradiation source. The application of photogenerated singlet oxygen is applied to the cross-linking of amine siloxanes. Oxidative coupling of amine groups to imine groups results in facile cross-linking at room temperature in oxygen rich environments using visible light with no measurable side reactions. Solvent free precursors were prepared by covalent attachment of the sensitizer to the PDMS backbone. As coatings, these materials demonstrate contact antimicrobial activity against bacterial cells, being more effective against E. Coli than methicillin-resistant Staphylococcus aureus (MRSA), as well as inducing cell lysis in mammalian cells. This system was further developed as a dual-functional textile coating, utilizing both contact antimicrobial activity as well as photodynamic disinfection using singlet oxygen, which results in >99% bacterial disinfection in 90 minutes and a 10× decrease in SARS-CoV-2 viral infectivity in two hours. Singlet oxygen was also used to photo-cross-link thiol containing siloxanes through the oxidative preparation of a diaryl-telluroxide catalyst, effectively linking low thiol-content PDMS through disulfide formation. These polymers exhibit up to 400% elongation, and can be reductively cleaved back into liquids using NaBH4 with >90% mass recovery.

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Photofunctional molecular crystals (2022)

Novel solid state photofunctional materials have been designed, and their structure-properties relationships are investigated. This thesis focuses on two distinct photofunctional properties: generating photomechanical motion in photoswitch-based crystals and ‘afterglow’ emission from organic alloy crystals and novel organic emitters.A novel hemithioindigo (HTI) photoswitch is functionalized with an anthracene group, and its photophysical properties are investigated in the solution and solid state. When irradiated with light, the photoswitch exclusively undergoes photoisomerization. In the crystalline state, E- isomer crystals isomerize and undergo a photosalient effect upon exposure to green light, while the Z- isomer undergoes a thermosalient event when heated.[2+2] Photodimerization is also employed to generate photomechanical motion in hemithioindigo photoswitch analogs. Hemithioindigo can undergo solid state reactions generating photoproducts not observable in solution. Three hemithioindigo photoswitch analogs are designed and used to elicit photomechanical motion in the crystalline state. Additionally, further mechanical motion is observed when previously irradiated crystals are heated. Crystallography and 1H NMR spectroscopy were used to provide insights into the mechanism of photomechanical motion. These proof-of-concept photomechanical responses in crystalline hemithioindigo photoswitches demonstrate their potential use as light-driven microactuators. The facile synthesis of HTIs enables tuning the photomechanical system based on the desired application.Solid state host-guest chemistry is employed to produce a series of seven organic alloys of two organic emitters with contrasting photophysical properties. One emitter has a long lifetime with ‘afterglow’ emission, while the other has a high photoluminescence efficiency. By carefully tuning the composition of the components between 30% - 100%, interesting photophysical properties emerge in addition to the fine-tunability of the system, which are probed using steady-state and time-resolved photoluminescence spectroscopy at variable temperatures. This provides a new perspective of the rules established for developing organic long-lived emissive materials for host-guest doped systems.Finally, novel organic carbazole-thiophene-based organic emitters are designed, synthesized, and characterized. The repositioning of the carbazole around the thiophene as well as introducing a secondary carbazole moiety results in solid state thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP).

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Sulfur-bridged chromophore dimers: photophysics, photochemistry and applications (2021)

The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.

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Photofunctional materials: photochromism, thermally activated delayed fluorescence and room temperature phosphorescence (2020)

Novel photofunctional molecules have been designed, synthesized and characterized. Their interactions with light are categorized into three types: photochromism, thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP). It is shown in this thesis that these properties can be tuned and manipulated by rational molecular design.A series of Cu(I) complexes bearing dithienylethene backbones show tunable photoreaction quantum yield by altering the ancillary ligand while a phosphine-oxide containing organic compound synthesized from one of the Cu(I) precursors is found to exhibit turn-on aggregation-induced emission. Their photochemical and photophysical properties are analyzed by UV-Vis and photoluminescence spectroscopy in addition to computational simulations. Using simple synthetic modifications such as oxidation and methylation, three structurally similar quinolinium-containing diarylethene compounds give rise to various photochemical and photophysical properties including turn-on fluorescence, red-green-blue emission switching and RTP, respectively. UV-Vis and photoluminescence spectroscopy as well as lifetime measurements are employed to study these properties. Potential applications in cellular imaging and super-resolution imaging are demonstrated. Pyridinium and quinolinium salts are designed to show TADF and RTP in the solid state with different counter anions. An iodide compound only shows low-temperature phosphorescence and bromide salts are found to exhibit TADF with RTP contributions and. However, compounds with other anions only display RTP. By using steady-state and time-resolved photoluminescence spectroscopy at variable temperatures, these properties are probed in depth. Three sulfur-bridged carbazole compounds were synthesized, to gain an understanding of electronic effects on RTP. The oxidation state at the sulfur center can be changed to modulate the phosphorescence efficiency in the solid state, in addition to fluorescence properties in the solution state.

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Symmetric and non-symmetric stimuli-responsive flexible Lewis pair systems (2020)

Herein an investigation of the photophysical properties of stimuli-responsive molecules incorporating a triarylborane Lewis acid and phosphine oxide Lewis base pair on a bithiophene backbone, called flexible Lewis pairs (FlexLPs), is presented. The FlexLPs change conformation between the Lewis acid-base bound adduct form and unbound open form with surrounding solvent, where strongly hydrogen bond donating solvent stabilizes the open form through formation of a hydrogen bond with the Lewis base. The ability to synthesize diverse stimuli-responsive molecular motifs incorporating FlexLPs is achieved by installing a reactive terminal alkyne functional group on a FlexLP molecule. The application of this FlexLP molecule as a versatile stimuli-responsive ligand that can be incorporated into more complex molecular architectures, such as dimers, transition-metal complexes, and functionalized nanoparticle materials, is thus demonstrated by utilizing different alkyne coupling chemistry. Synthesis of a dimeric molecule with two FlexLP pairs connected through a conjugated bithiophene-alkyne-alkyne-bithiophene backbone is described and compared to a monomeric FlexLP molecule. The monomer and dimer FlexLP demonstrate changing emission colours and lifetimes with varying solvent, proposed to be from a change in FlexLP conformation. In the closed form a π–π* energy transition occurs resulting in blue-green emission colour, while in the unbound form a charge transfer event occurs resulting in yellow-orange emission colour. Monometallic and bimetallic Pt(terpyridine)-containing complexes functionalized with the FlexLP ligand are explored as solvent-responsive optical switches. A low energy CT band between the acid-base pair ligand and terpyridine ligand appears for both complexes, Absorption spectroscopy reveals this CT band is solvent- and temperature-responsive, and is dependent on the FlexLP ligand conformation.Investigation of the photophysical properties of two emissive Pt-functionalized FlexLP complexes reveal both complexes fluoresce despite being able to populate the excited triplet states of the complexes. Transient absorption spectroscopy reveals that the FlexLP ligand conformation has a significant effect on the triplet-state lifetime.Functionalization of FlexLPs onto nanoparticles is explored for applications as water-sensing emissive fluorophore tags for biomedical imaging applications. The emission colour and lifetime change in response to water concentration in surrounding solvent, evidenced by fluorescence and emission lifetime spectroscopy. Preliminary fluorescence microscopy studies are also performed.

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Synthesis of nanostructured catalysts for low temperature methane combustion (2020)

The number of natural gas vehicles (NGVs) is increasing rapidly due to the growing concern for the environmental impact of gasoline and diesel-powered automobiles. However, the main component of natural gas, methane, is a potent greenhouse gas. Thus, removal of unburned methane from the exhaust of NGVs is important and this requires catalysts for methane oxidation at low temperature (less than 500-550 °C). In this thesis, different types of nanostructured catalysts were developed. Noble metal catalysts, like Pd-based catalysts, and non-precious metal catalysts, such as spinel catalysts, were synthesized, characterized and examined for catalytic methane combustion. To enhance the catalytic activity, NiCo₂O₄ catalysts with a unique bowtie-structure were synthesized. The morphology, growth mechanism, catalytic and kinetic performance were explored. These NiCo₂O₄ catalysts exhibited excellent activity for methane oxidation, but were unable to maintain their performance in the presence of water vapor.In addition, active site distribution plays an important role on catalytic activity. Taking this into account, MnO₂ aerogels supporting precious or non-precious metal oxide catalysts were explored. Both types of catalysts showed enhanced catalytic activity. The hydrothermal stability and sulfur tolerance were studied for PdO/MnO₂, and the deactivation mechanism was explored. Besides outstanding activity, commercial catalysts must be stable in practical usage. In order to improve the stability of Pd-based catalysts, nanostructured materials were prepared by embedding Pd into CeO₂ inside the channels of a mesoporous host, SBA-15. It was found that the materials showed excellent catalytic activity and improved activity after hydrothermal treatment. However, their performance in the presence of water vapor still needs to be enhanced.Furthermore, to solve the problem of poor water stability and sulfur tolerance, CoCr₂O₄ composites were selected as catalysts. Nanospheres of CoCr₂O₄ were prepared by a straightforward solvothermal method. These materials achieved 100% methane conversion below 500 °C and they also displayed excellent stability even in the presence of 10% water vapor and 5 ppm SO₂. Furthermore, the CoCr₂O₄ catalysts were scaled up and coated onto cordierite monoliths via a modified wash-coating method. The coated monolith showed excellent activity and stability, and these materials have potential to be applied for NGVs.

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Tuning the electronic properties of coordination complexes with sulfur-bridged ligands (2019)

By altering the oxidation state at the sulfur atom of sulfur-bridged ligands we have shownthat photophysical properties of coordination complexes can be changed. A new class of sulfur-bridged dipyridyl ligand has been developed for effective “two-level” tuning of the emissioncolour of Ir(III) complexes. Tuning can occur via the classical method of altering substituents atthe pyridyl ring, and/or through oxidizing the sulfur. Changing the oxidation state of the sulfuratom serves as a switch to alter the emissive state from that of mainly ³LC character (blue-greenemission) to one of ³MLCT/³LLCT character (yellow emission), evidenced from photophysicalcharacterization and DFT calculations.The larger bite angle of the sulfur-bridged dipyridyl ligands, in conjunction with additionalcoordination sites afforded by the oxygen atoms at the sulfur, has led to interesting binding modeswhen bound to copper(I) in heteroleptic diimine-diphosphine species. On increasing stericconstraints about the copper(I) center, bimetallic adducts are isolated, with thermally activateddelayed fluorescence the mechanism of photoluminescence.Of the copper(I) species reported in this body of work, one was found to exhibit remarkablethermochromic properties not-before seen in a monometallic copper(I) complex. When isolated asan amorphous powder, Cu-Me-DPSO shows a yellow solid state emission, however when heatedto 180 ºC a crystalline powder is formed which shows an orange luminescence. This crystallinepowder displays a reversible thermochromic solid state emission, from orange at room temperatureto yellow at low temperatures. Using solid state variable temperature excitation and absorptiondata, this phenomenon is attributed to a change in coordination geometry about the copper atomin the excited-state.With a view to further understanding the impact of sulfur oxidation state on the excitedstateproperties of coordination complexes, two new Ru(II)-Re(I) dyads were synthesized, for useas CO₂ photocatalysts. Connected by a sulfur-bridged bisphenanthroline bridge, oxidation state atthe sulfur can be changed which could modulate the electron transfer from the photo-excited Ru(II)to the catalytic Re(I) center. The synthesis, characterization and preliminary photocatalytic resultsare discussed.

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Sulfur-bridged chromophores: polymers, copolymers and ligands for metal complexes (2018)

The impact of the incorporation of sulfur bridges and their oxidation on the electronic and photophysical properties of macromolecules was investigated through the synthesis of oligomers, short polymers and copolymers. In oligomers, the energies of the frontier orbitals and the band gap can be modulated with the oxidation state of the bridge along with the length of the molecule. The sulfone containing oligomers are emissive in solution and in a polymer matrix while the compounds with sulfide bridges are non-emissive. Polymers containing sulfide groups exhibit conjugated-like behavior where the length of the polymers influences the maximum absorption and emission wavelengths. The photophysical and electronic properties of the sulfone-bridged polymers do not depend on their molecular weight and behave like distinct chromophores. Incorporation of sulfur bridges in copolymers containing electron rich or electron poor comonomers influences the energies of the frontier orbitals and the photophysical behavior. Sulfide containing copolymers have smaller band gaps while the sulfone containing copolymers are more emissive. Incorporating monomers containing a N^N bidentate motif, the copolymers coordinate metal ions and the emission of the polymer decreases.Sulfur-bridged thiazoles containing a N^N bidentate motif are used as ligands for metal complexes.Homoleptic and heteroleptic complexes incorporating sulfide or sulfone N^N ligands were obtained with ruthenium(II) and copper(I) metal centers. Their structures were elucidated using NMR spectroscopy and single crystal X-ray diffraction. The ruthenium(II) heteroleptic complexes were found to undergo photoejection when irradiated in coordinating solvent with UV light. The complexes were found to be nonemissive in fluid solution at room temperature and are emissive in the solid state and at low temperature.

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The preparation and study of photocatalysts: from core-shell palladium-titanium dioxide nanoparticles to iridium(III) complexes (2018)

Titanium dioxide nanoparticles (NPs) and iridium(III) complexes have been prepared and studied as photocatalysts towards enhanced efficiency and mechanistic understanding of photocatalysis. Core-shell palladium-titanium dioxide NPs, Pd@TiO₂, were prepared using monodisperse Pd@SiO₂ core-shell NPs as a template. The Pd cores and porous, high surface area TiO₂ shells are expected to prevent Pd loss and increase surface-substrate interactions, respectively, thereby improving photocatalytic efficiency. Carbon dioxide was photocatalytically reduced in water by the Pd@TiO₂ NPs with methane being the major product.Iridium(III) complexes were tailored to increase the excited state lifetime through minor ligand modification. [Ir(ppy)₂phen]PF₆, [Ir(ppy)₂dtbbpy]PF₆, [Ir(ppy)₂dmbpy]PF₆, and [Ir(ppy)₂bpy]PF₆ were prepared where ppy = 2-phenylpyridine, bpy = 2,2ʹ-bipyridine, dmbpy = 4,4ʹ-dimethyl-2,2ʹ-bipyridine, dtbbpy = 4,4’-di-tert-butyl-2,2ʹ-bipyridine and phen = 1,10-phenanthroline. Their excited state lifetimes range from 0.3 µs to 0.7 µs and correlate to the rate of single-electron transfer (SET) from excited state to substrate, CF₃SO₂Cl (1.9 × 10⁸ M‾¹ s‾¹ to 9.3 × 10⁸ M‾¹ s‾¹), but the rate of final product formation is unchanged. The photocatalyzed trifluoromethylation of quinoline was used as a prototypical reaction. The unchanged rate of product formation indicates that SET involving the excited state is not rate-limiting in this system.Further increase in the SET rate was attempted by attaching pyrene onto a ligand in the complex for Reversible Electron Energy Transfer (REET) to increase the photocatalyst excited state lifetime more significantly. [Ir(npy)₂bpyethylpyr]PF₆, [Ir(npy)₂bpypyr]PF₆, and [Ir(npy)₂dmbpy]PF₆ were prepared where npy = 2-(naphthalen-1-yl)pyridine, dmbpy = 4,4ʹ-dimethyl-2,2ʹ-bipyridine, bpyethylpyr = 4-methyl-4ʹ-[2-(pyren-1-yl)ethyl]-2,2ʹ-bipyridine and bpypyr = 4-(1ʹʹ-pyrenyl)-2,2ʹ-bipyridine. The excited state lifetimes are 13.8 µs, 4.8 µs and 3.2 µs, respectively. Excited state lifetime and transient absorption studies indicate that only the complex with pyrene separated from bpy by an alkyl bridge displays REET. The rates of SET and product formation in the trifluoromethylation of quinoline are slower upon incorporation of pyrene. This is attributed to new, less reactive excited states and increased photocatalyst size, which slows down diffusion.The findings presented herein reveal the importance of photophysical and structural properties, such as photocatalyst size and excited state lifetime, in SET and photocatalytic efficiency, thereby contributing to guided optimization of photocatalytic systems.

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Explorations and development of bis(N-heterocyclic carbene) pincer complexes for the electrocatalytic reduction of carbon dioxide (2017)

A variegated series of bis(N-heterocyclic carbene) pincer complexes have been tested for the ability to act as homogeneous CO₂ reduction electrocatalysts. Structure-activity relationships were investigated in order to enable the rational design of improved electrocatalysts and provide a platform for further development of mono- and bimetallic electrocatalysts within the pincer motif.Pyridine- and lutidine-linked bis-NHC palladium pincer complexes were screened for CO₂ reduction capability with trifluoroethanol, acetic acid, and trifluoroacetic acid as proton sources. The lutidine-linked pincer complexes were found to electrocatalytically reduce CO₂ to CO at potentials as low as -1.65 V vs. ferrocene in the presence of trifluoroacetic acid. The one-electron reduction of these complexes is shown to be chemically reversible, resulting in a monometallic species in solution. Computational models indicate charge transfer from a redox-active ligand upon interaction of the reduced species with CO₂, thus potentially addressing a source of deactivation in earlier pincer electrocatalysts. The presence of Lewis acids in solution was also investigated, assisting reactivity with CO₂.The NHC moieties of these lutidine-linked Pd pincer complexes were modified by phenanthro- and pyreno-annulation to investigate the effect of an extended NHC π-system on their electrochemical reactivity with CO₂. The polyannulated NHC groups are shown to be additional sites for redox-activity in the pincer ligand, enabling increased electron donation and activation of CO₂. Following this, modification of the pyridyl para position is reported (R = OMe, H, Br, and COOR), allowing the first reduction potential to be tuned over a 1 V range in relation to the substituent's Hammett σp constant, and labilizing the trans ligand in the case of electron-donating substituents, thus improving activity for one-electron reduced species. Finally, analogous Ni and Pt bis-NHC pincer complexes are synthesized, characterized, and compared to Pd, with the Pd bis(benzimidazol-2-ylidene) pincer complexes exhibiting the best performance with faradaic yields for CO production approaching 50% in the presence of trifluoroacetic acid. The remaining current resulted in the production of H₂, thus producing a CO-rich synthesis gas mixture as the overall product.

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Thiophene-Containing Photofunctional Molecules: Pacman Complexes, Metallacycles and Flexible Lewis Pairs (2016)

Three types of photofunctional molecules based on thiophene-containing conjugated backbones have been designed, synthesized and characterized. It is shown that their light absorption and emission properties can be manipulated and applied for applications such as fluorescent imaging. Binuclear Pt(II) terpyridine Pacman complexes with flexible thiophene-containing bisacetylide ligands are shown to exhibit structural folding/unfolding controllable by temperature change and different solvent environments. The structural folding and unfolding give rise to structural changes of the thiophene-containing backbone and different interactions between two metal-containing moieties, which are analyzed by DFT calculations and evidenced by UV-Vis and NMR spectroscopy. Metallacycles with a cis-diphosphino Pt(II) metal center and different thiophene-containing bisacetylides are designed to show room-temperature fluorescence and phosphorescence dual emission with different intensity ratios. Their absorption and emission properties are explained by DFT and TD-DFT analysis of ground state and singlet and triplet state energies and geometries. An intramolecular Lewis pair system between a Lewis acidic -BMes2 group and a Lewis basic phosphine oxide group based on a flexible bithiophene backbone is reported. Evidenced by NMR and IR spectra, the system is found to exhibit fast equilibrium between an open structure with unbound Lewis acid and a closed structure with Lewis adduct. The equilibrium is manipulated by lowering the temperature to favor the closed structures, or adding strong hydrogen bond donors that favor the open forms. The difference in coordination state of the boron center in these two states gives rise to an interesting single-component-two-state system with drastically different emission colors between states. The scope of such system was explored and different emission colors of the open and closed forms of the Lewis pairs are achieved by changing the backbone conjugation or strength of electron-donating groups. Furthermore, this system has been used as a two-color fluorescent dye system for fluorescence imaging of hydrophobic/hydrophilic environments in biological or medical applications.

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Supported and Unsupported Palladium Nanoparticles for Heterogeneous Catalysis (2015)

The development of new methods to prepare Pd-containing nanomaterials for catalysis are reported. Monodisperse, catalytically active Pd0 nanoparticles were prepared using a one-pot procedure, and new insights into the mechanism of the formation of these catalytically active Pd0 nanoparticles were obtained. A number of key intermediates and byproducts were determined using NMR and IR spectroscopies. Furthermore, addition of Lewis bases such as TOPO and DMSO to the reaction mixture greatly reduced the temperature at which highly monodisperse nanoparticles were formed. This effect was shown to be applicable to other Pd precursors in preparing Pd0 nanoparticles.Spherical Pd0@m-SiO2 core-shell nanoparticles were prepared and characterized by a number of techniques, including TEM, PXRD, TGA, and XPS. These nanoparticles consist of a Pd0 core enclosed by a mesoporous silica shell and are prepared using a simple, scalable one-pot procedure. The reaction conditions were crucial in controlling the morphology and pore diameter of the nanoparticles synthesized. Acid treatment of the Pd0@m-SiO2 nanoparticles was found to be the most suitable method in removing CTAB. The morphology, surface area, and pore diameter of the core-shell nanoparticles remained intact after removal of CTAB.A new ceria-containing core-shell material, PdO@m-CeO2, was prepared via templating from Pd0@m-SiO2 and PdO@m-SiO2 nanoparticles. The absence of Pd0 and presence of Pd2+ was explained by the possible formation of a solid solution composed of Ce1-xPdxO2-δ when Pd0 was the core. The catalytic activity of the nanoparticles was examined by performing the catalytic oxidation of methane. As well, the silica channels of the Pd0@m-SiO2 nanoparticles were used as selector to separate molecules based on their size. Size-selective hydrogenation was investigated using the porous silica shell of the Pd0@m-SiO2 nanoparticles as a selector, where the porous shell controlled the selectivity by the size of the substrates. Hydrogenation of a small molecule, 1-hexene, in CDCl3 using acid-treated Pd0@m-SiO2 nanoparticles occurred quickly, while the hydrogenation of a larger substrate, O-allyl cholesterol proceeded more slowly. However, similar results were observed using commercially available Pd/C as the catalyst. Narrowing the pore diameter of Pd0@m-SiO2 nanoparticles showed drastic difference in reaction rates between 1-hexene and O-allyl cholesterol.

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Bis-cyclometallated Iridium (III) Complexes Bearing Pyridineimine and Salicylimine Ancillary Ligands: Synthesis, Characterization and Applications (2014)

Two stable diastereomeric atropisomers of a cyclometallated iridium complex containing a pyrene functionalized pyridineimine ligand have been isolated. These are the first fully characterized examples of metal containing atropisomers in which the rotational axis is not between two chelating atoms. The atropisomers can be converted thermally via a rocking motion of the pyrene moiety.A new mechanism for enhanced phosphorescence emission in the solid state (EPESS) in cyclometallated Ir complexes with the general formula [Ir(C^N)₂(N^O)] involving distortion of the six-membered chelate ring of the ancillary ligand is proposed. Photophysical and computational studies show that neither π-stacking nor restricted rotation cause the observed EPESS in these complexes and that ligand distortions in the triplet excited state are responsible for EPESS.Bis-cyclometallated Ir(III) complexes with the general formula Ir(ppz)₂(N^XPyrene) where X = N or O are shown. Modifications on the ancillary ligand containing pyrene drastically affect the emission lifetimes observed (2 μs to 104 μs). Extended emission lifetimes in these complexes compared to model complexes result from reversible electronic energy transfer or the observation of pyrene (³LC) phosphorescence. A combination of steady state and time-resolved spectroscopic techniques are used to observe reversible electronic energy transfer between the cyclometallated iridium core and the pyrene moiety in the complexes [Ir(ppz)₂(N^NPyrene)][PF₆]. Replacing the N^NPyrene ligand with an N^OPyrene ligand results in long-lived pyrene phosphorescence. Reversible electronic energy transfer as well as 3pyrene emission is observed and characterized in a PMMA film.

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Oligothiophene coordination polymers and cyclic trinuclear complexes (2014)

Spacial control within solid state materials is a method for controlling their properties. This thesis demonstrates the structural and functional control of oligothiophenes within coordination polymers and cyclic trinuclear complexes.Solvothermal and room temperature reaction conditions were used to synthesize oligothiophene metal-organic frameworks (71–85, 92–98). Appendage of phenyl and n-hexyl groups to the β-position of oligothiophene linkers induces structural changes in both the local and extended structures of the coordination polymers. Frameworks are sensitive to the linker functionality: phenyl groups promote the formation of 1D and 2D coordination polymers (74, 77, 80, 85, 92, 98) while aggregation of n-hexyl groups directs the local and extended structure of 2D and 3D materials (75, 78, 81, 94). Manganese(II) terthienyl coordination polymers (96 and 97) exist as isomers that form under solvothermal and post-solvothermal conditions, respectively.The photoluminescent properties of the coordination polymers generally matches those of the proligand. A bathochromic shift in oligothiophene-based emission occurs in 83 while compounds 75 and 81 undergo hypsochromic shifts. Quenching of oligothiophene emission in compound 81 occurs via incomplete energy transfer. The magnetic susceptibility of manganese(II) compounds reflects the local structure, and a spin-canting transition is present the acentric compound 96. Collapse of the framework of 94 prohibits a spin-canting transition.Gold(I) thienyl pyrazolate cyclic trinuclear complexes form dimeric or polymeric species in the solid state. Metal-perturbed ligand-based phosphorescence with lifetimes on the order of 5 ms are found in gold(I) monothienyl pyrazolates (126, 127, 130, 131). Bithienyl complexes (128, 129, 132, 133) do not exhibit phosphorescence at 77 K. Density functional theory confirms the contributions of gold(I) ions to the electronic structure of the S1 and T1 states. Oxidative polymerization of monothienyl (130) and bithienyl (132 and 133) complexes with n-hexyl derivatives generates conductive thin films.

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The electrochemical properties of conducting polymers for energy storage applications (2014)

The synthesis and characterization of oligothiophene-capped Au and Cu nanoparticles (NPs) are reported. Homo- and co-polymer films of these NPs were prepared electrochemically and studied using cyclic voltammetry, electron microscopy, and absorption and emission spectroscopies. The Au NPs were capped by either 3'-thiol-substituted terthiophene (1) or 3'-phosphine-substituted terthiophene (2). The electrochemical oxidation of 1-capped Au NPs (4) resulted in a polymeric film which displayed good electroactivity whereas attempted electropolymerization of 2-capped Au NPs (5) was unsuccessful. Exposing the poly(4) films to an iodide/triiodide solution lead to etching of the Au and a decrease in the electroactivity of the polymer. A hybrid copolymer was formed by electropolymerizing ethylenedioxythiophene in solution with 4 (PEDOT-4). It was found that it was possible to selectively etch Au from PEDOT-4 films to yield porous PEDOT films, which were analyzed using elemental analysis methods and electron microscopy. The films after etching showed slightly improved charging and discharging kinetics, suggestive of improved ionic diffusion in the polymer. Cu NPs capped by 1 (7) were electrochemically oxidized to form a polymer film. The lower oxidation potential of 7 relative to 4 allowed for the formation of crystalline regions in the polymer film, and exhibited the characteristic XRD peaks of Cu (0). PEDOT-7 films were prepared which showed enhanced electroactivity over pure PEDOT films. An azidostyrylthiophene compound (9) was synthesized and was shown to be both thermally and photochemically reactive. PEDOT-9 copolymers were prepared and studied using cyclic voltammetry. Irradiation of the PEDOT-9 films show improved charging / discharging kinetics due to crosslinking of the polymer film by the reactive azide substituent.

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Ruthenium(II) complexes bearing polypyridyl ligands with amide bound thienyl groups for photochemical energy conversion (2013)

Mimicking natural photosynthesis requires long charge recombination lifetimes, spatial charge separation, and sufficient excited state energy to catalyze processes such as water splitting. Complexes bearing laminate acceptor ligands and ancillary diimine ligands with electron rich donor moieties are suitable candidates as reaction centers for photoinduced charge separation in artificial photosynthesis.In this work, new diimine ligands with thiophene oligomers appended via amide linkages are incorporated into metal polypyridyl complexes. Homoleptic Ru²⁺ complexes bearing 1,10-phenanthroline ligands with amide bound bithiophene units exhibit excited state behavior deviating significantly from that observed in [Ru(phen)₃][PF₆]₂ (30). The bithiophene unit fuels a long-lived excited state (τ ≈ 7 μs) in [Ru(phen-btL)₃][PF₆]₂ (32) through an energy reservoir effect, where ³LC and ³MLCT states are equilibrating in the excited state. A third ³ILCT state is found to equilibrate with the aforementioned states giving a rare three-state equilibrium where the third state is a charge separated state storing ΔG° ≥ 1.9 eV of energy.In order to introduce an aspect of vectorial charge separation into donor-chromophore-acceptor triads, 1,10-phenanthroline ligands bearing bithiophene moieties are introduced as donor ligands into Ru²⁺-based triads with laminate polypyridyl acceptor ligands; dipyrido[3,2-a:2',3'-c]phenazine (dppz), tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine (tpphz), and 9,11,20,22-tetraazatetrapyrido[3,2-a:2',3'-c:3'',2''-l:2''',3''']-pentacene (tatpp). Triads incorporating bithiophene amide diimine ligands and laminate acceptor ligands (50-52) have long-lived charge separated excited states (τes = 2.2 – 7.0 μs), where an electron is localized on the central portion of the acceptor ligand. Charge separated excited states in these triads transiently store an appreciable amount of energy (ΔG° ≈ 0.98 – 1.41 eV)

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Synthesis and Characterization of Late Transition Metal Oligothiophene Complexes for Light Harvesting Applications (2012)

The synthesis and characterization of late transition metal complexes combinedwith the β-substituted 3ʹ-(diphenylphosphino)-2,2ʹ:5ʹ,2ʺ-terthiophene (PT₃) to yieldmetal-oligothiophene hybrid complexes are reported. These new complexes (shown onthe following page) were studied with absorption, emission, and transient absorptionspectroscopies, electrochemistry and X-ray crystallography.Ru(II) and Os(II) bis(diimine) complexes (52-53 and 54-55, respectively)containing PT₃ in two different coordination modes (PS and PC bound) are reported. Thebinding mode is shown to affect the structural, photophysical and electronic properties ofthe complexes. Transient absorption spectra and lifetimes were obtained for all thecomplexes, and support a PT₃ ligand-based lowest excited state in the case of the PSbound complexes, and a charge separated lowest excited state in the PC boundcomplexes. The DFT calculations and experimental results agree well.Cyclometalated iridium (III) complexes (58-63) were synthesized usingmicrowave irradiation. In addition to the PS and PC coordination modes observed in thegroup 8 complexes, a third, monodentate, coordination mode was observed (P). Thelowest energy absorption band and emission band shift based on the PT₃ coordinationmode. The nature of the excited state lifetimes did not change with coordination mode;all were assigned as a PT3 ligand-based excited state, however, the excited state lifetimesare influenced by the coordination mode.Heteroleptic Cu(I) complexes (75-76) were synthesized and yielded three and fourcoordinate complexes. The electrochemical and photophysical properties of thesecomplexes vary with solvent. Minor changes are observed in the absorption spectra whenobtained in different solvents, but interesting differences are observed in theelectrochemical reversibility, excited state lifetimes, and profiles of the emission andtransient absorption spectra.

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Synthesis and Characterization of Photochromic Platinum-Coordinated Dithienylethenes (2011)

The syntheses and characterization of photochromic platinum-coordinated dithienylethenes (DTEs) are reported. Platinum acetylide complexes were investigated as potentially useful chromphores to integrate with photoresponsive DTEs. Acetylides were observed to be successful at promoting strong electronic interaction between the Pt atom and the DTE’s thienyl scaffold. As a result of platinum’s large spin-orbit coupling, these metal complexes exhibit a high propensity for generating excited states with triplet character upon photoexcitation. Consequently, platinum acetylide complexes were found to be effective sensitizers for population of DTE-localized triplet states via energy transfer between the two chromophores. Platinum terpyridine complexes were initially targeted because they exhibit long-lived excited states, whose lifetime could be potentially modulated by photoswitching of the appended DTE. In fact, selective irradiation of the Pt complex with visible light resulted in energy transfer from a metal-based excited state to the DTE chromophore, in either the ring-open or ring-closed state. The triplet excited state of the ring-open DTE was observed to be photoactive and underwent cyclization to the ring-closed form. The effect of the alkynyl linkage on energy transfer was studied by incorporating a longer, non-conjugated alkynyl linkage between the two chromophores. Lengthening the linker reduced excited-state interaction between the metal-based and DTE-localized states, but did not completely eliminate energy transfer. The high efficiency of metal-sensitized ring-closing in the Pt terpyridine system led to subsequent exploration of multifunctional systems, those containing more than one DTE. Symmetrical Pt(II)-bis(phosphine)bis(acetylide) complexes were used for the preparation of discrete model systems and extended oligomeric species. The Pt-acetylide linkage was successful in allowing photoswitching of multiple adjacent DTE chromophores while maintaining a conjugated pathway between DTE units. Optical and electrochemical characterization supported that adjacent DTEs are capable of electronic communication through the Pt center. Extended electronic delocalization observed when multiple adjacent DTEs are ring-closed makes this system suitable for applications utilizing π-conjugated materials.

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Synthesis and properties of gold(I) and Ruthenium(II) complexes of beta-substituted oligothiophenes (2011)

Oligothiophenes functionalized in the beta-position with ethynyl and aryl phosphine groups (57 – 59) are reported. Compounds 57 and 59 are used as bridging ligands with Au(I) (80 – 82, 93 – 95), while compounds 57 and 58 are used as tridentate ligands with Ru(II) (104 – 112). The optical and electronic properties of these compounds were studied with absorption and emission spectroscopy, transient absorption spectroscopy, X-ray crystallography, and electrochemistry. The lowest energy absorption bands and emission bands were found to be sensitive to the substituent and conjugation. The lowest energy pi-based absorption band in complex 80 was blue-shifted, while complexes 93 – 95 and 104 – 112 caused a bathochromic shift in the comparable band. Complexes 80, 93 and 94 showed ligand based emission at 295 K, complex 95 at temperatures
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Morphological control in conjugated polymers : synthesis and applications of polymer microspheres (2010)

The synthesis, characterization, and application of conjugated polymer colloidal microspheresare reported. Monodisperse mesoporous silica spheres were utilized as hard templates for thepreparation of poly(3,4-ethylenedioxythiophene) (PEDOT)–silica composites, which could inturn be etched with hydrofluoric acid to yield monodisperse conjugated polymer microparticles.The synthetic procedure was generalized to include other polymers such as polythiophene andpoly(N-methylpyrrole). The colloidal and electronic properties of these composites weresubsequently studied, and it was found that a balance between colloidal stability and electricalconductivity could be achieved when the mesopores of the silica template were partially filledwith polymer. These insights enabled the successful self-assembly of an opaline film of thePEDOT–silica composite. Bragg diffraction of visible light by the lattice planes of the opal wasthen demonstrated.The silica host was subsequently explored for its ability to control the phase separation ofpolymer blends. It was found that depending on the order of polymer addition, phase separationcould be suppressed. This resulted in an intimate mixture of the two constituent polymers, andhas potential impact in the field of photovoltaics. The colloidal microspheres were also examinedfor their utility as electrode materials in supercapacitors. The PEDOT microspheres were foundto have a mass specific capacitance higher than that of typical electropolymerized PEDOT films,indicating that the morphological control was translated into an improvement in deviceperformance.An ion exchange and in situ polymerization approach was developed in order to incorporatepoly(p-phenylenevinylene) into the mesoporous silica template. This yielded a luminescentcolloidal material that displayed enhanced optical properties relative to the unencapsulatediiipolymer. This is again due to the morphological constraints imposed on the polymer by themesoporous template; polymer chains are isolated from each other, thus shutting down access tonon-radiative decay pathways.Other template approaches were explored as a way of producing conjugated polymermicrospheres. Sodium dodecyl sulfate micelles were utilized as soft templates for the synthesisof carbohydrate-functionalized poly(p-phenyleneethynylene) particles. These microparticleswere examined for their protein-binding ability, and were found to extract the common lectinconcanavalin A from solution.

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Optical and Electronic Properties of Functional Polythiophenes (2010)

The synthesis and characterization of a series of functionalized head-to-tailregioregular poly-3-alkylthiophenes are reported. The influences of the structure andfunctional groups on the optical and electronic properties are investigated with NMR,absorption, emission and infrared spectroscopy, gel permeation chromatography, andcyclic voltammetry.A regioregular poly-3-alkylthiophene (Poly-1) was synthesized via GrignardMetathesis polymerization conditions. Poly-1 contains bromide groups as sites of latentreactivity along the polymer backbone through which additional reactions were carriedout post-polymerization. The bromide was converted to an azide group (Poly-2) whichwas further functionalized via Click chemistry with a variety of functional groups.Click chemistry was carried out using the Huisgen 1,3-dipolar cycloadditionreaction. Post-polymerization functionalization of Poly-2 provided a facile method forpreparing a variety of related functional polymers, each with identical average chainlength, average polydispersity and average distribution of monomers. Preparation ofPoly-3a, -3b, and -3c, demonstrated the utility of the Click reaction for modificationswith a variety of functional groups.A series of poly-3-alkylthiophene analogs (Poly-1 - Poly-11) were characterizedby absorption and emission spectroscopies and the spectra were found to be dependant onregioregularity along the polymer backbone. The UV-vis absorption maxima, varied withthe percentage of head-to-tail couplings in relation to the extent of conjugation.The series of dithienylethene functionalized oligo- (71) and polymer analogs(Poly-4, Poly-7 and Poly-11) displayed fluorescence quenching capabilities uponphotoinduced ring closing of the dithienylethene moiety via energy transfer. The extentof quenching was determined to be dependent on both the length and structure of thebackbone. Extended conjugation contributed to amplified fluorescence quenching asobserved by complete fluorescence quenching of Poly-4.The functionalization of the Poly-2 with the stable free nitroxide radical 2,2,6,6-tetramethylpiperdine-1-oxyl is described. The resulting polymer, Poly-12, wascharacterized with cyclic voltammetry and IR spectroscopy. Electrochemical depositionof thin films of Poly-12 onto various working electrodes is described. The thin films wereinvestigated for potential charge storage via galvanostatic charge/discharge cycles. IRspectroscopy revealed that the nitroxide radical had sensitized the polythiophenebackbone to oxidation, resulting in irreversible damage to the polymer and reducedcharge storage capacity.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Iron(II) coordination compounds with improved octahedral character and photophysical properties for solar energy harvesting (2022)

A series of novel Fe(II) coordination complexes were synthesized and investigated for enhanced structural and photophysical properties. Spacer sulfur atoms were introduced in a terpyridine ligand motif, and a near-perfect octahedral coordination environment resulted in the stabilization of the spin-crossover transition between LS and HS in the crystallographic state. Two nonequal Fe(II) centers were contained within the unit cell of the homoleptic complex Fe(TPS)₂, with bond lengths unambiguously attributed to different spin-states on the nonequal Fe(II) centers within the same crystal at 100 K, and a gradual increase to LS character at higher temperatures. Two pincer-type ligands were synthesized in an effort to improve the excited-state lifetime decay pathway in Fe(II) complexes. Both ligand motifs incorporated a coordinating amido bridging moiety, which resulted in panchromatic light absorption, with charge-transfer bands in the far-vis and NIR regions. Furthermore, computational calculations confirmed that both complexes’ HOMOs are composed of contributions from both metal and ligand orbitals, as opposed to metal-centered as in most classical complexes. A pincer ligand with flanking NHCs was successful in raising the ⁵MC energy to a noncompetitive height with respect to the ³MLCT state, resulting in deactivation of the excited state via the ³MC state. When the NHCs were substituted for pyridines in the second ligand framework, the decay pathway proceeded through the ³MLCT to the ³MC and ⁵MC states. In both cases, the excited state ³MLCT decays were more than an order of magnitude larger than those in the prototypical comparison complex, Fe(tpy)₂.

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Photophysics and photochemistry of pyridine-bridged chromophores (2021)

Organic π-conjugated materials are growing in popularity as they can be employed in flexible devices such as OLED displays, used in bioimaging, and for sensing applications. The incorporation of heteroatoms, in particular nitrogen, provides a simple synthetic handle that can be used to modify organic emitters through reactions such as protonation, oxidation, alkylation, and chelation, leading to a new class of molecules with different photophysical properties. This thesis explores a series of pyridine-containing luminophores arranged in donor-acceptor architectures and the changes in the photophysical properties when functionalized at the nitrogen center. First, a spectroscopic study examines the effect of substituent size at the nitrogen on the conformational and rotational properties of the molecule. Modification of the nitrogen by protonation, oxidation and alkylation are investigated and differences in photophysical properties are observed. Substitutions with groups larger than a hydrogen induce restricted rotation in specific molecules resulting in atropisomerism. NMR, UV-Vis, and emission spectroscopies are used to explain the impact of different substitutions on fluorescence quantum yield. Next, a new photoreaction is demonstrated in which a C–N bond is formed between pyridine and different polyaromatic hydrocarbons (PAHs). Protonation and chelation of the pyridine enables compounds to react under UV irradiation. The mechanism of this photoreaction is explored using kinetic experiments, revealing a photochemically driven single electron transfer (SET) reaction. The scope of this reaction is tested with different chromophores, electron-withdrawing/donating groups and under various reaction conditions. The formation of the C–N bond leads to planarization and rigidification of the photoproduct, leading to a redshift in emission wavelength and an increase in fluorescence quantum yield.

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Synthesis and characterization of sulfur-bridged terpyridine-like Ru(II) coordination complexes and dinuclear Ru(II) sulfoxide complexes (2019)

A new set of sulfur-bridged oligopyridine proligands was synthesized with each having different functional groups in the 4ʹ position. Three new homoleptic Ru(II) complexes with these new tridentate ligands were synthesized and were found to have no geometric strain about the Ru(II) metal center due to the expanded ligand cage about the metal. In addition, a heteroleptic Ru(II) complex with one of these new ligands and a terpyridine ligand was made. The heteroleptic complex was found to be emissive at 77 K while the homoleptic compounds were non-emissive. These photophysical and electrochemical properties of these new complexes were also probed, and Lever Electrochemical parameters assigned. Photoactive mononuclear and dinuclear Ru(II) sulfoxide complexes were synthesized. The mononuclear complexes were found to have photoswitching activity that is consistent with reversible photoisomerization. A dinuclear complex was synthesized to probe if two photoisomerizations on the same complex was feasible. The dinuclear complex was found to undergo an irreversible photoreaction, and no evidence of two photoisomerizations was found

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Inner and outer sphere ligand effects on palladium pincer complexes as electrocatalysts for CO2 reduction (2018)

A series of novel palladium pincer complexes have been investigated for activity towards the electrochemical conversion of CO₂ to CO. Controlled variation of the inner and outer ligand sphere has provided mechanistic insight and valuable information towards rational ligand design for this variety of catalyst. Stabilizing interactions between charged electrocatalytic intermediates and a host of cationic residues were explored through the synthesis and characterization of six isoelectronic palladium bis(N-heterocyclic carbene) (NHC) complexes bearing unique onium functionalities. The presence of a positively charged, pendant substituent was found to mediate electrode kinetics and facilitate CO₂ coordination to the catalytic center in a systematic fashion. Ultimately, the introduction of cationic moieties onto this system was shown to enhance catalytic selectivity for the conversion of CO₂ to CO by as much as 5 times that of an alkyl-bearing analog. A combination of electrochemical experiments and computational analysis demonstrate that catalyst performance is most benefited by a bulky onium unit tethered to the catalyst through a flexible linker. This behavior was interpreted as the preference for a wide, hydrophobic reaction pocket that allows for the unhindered formation of catalytic intermediates and mediated interaction with the solution. With the aim of gaining better insight towards the influence of inner-sphere ligands on selectivity in palladium bis(N-heterocyclic carbene) CO₂ reduction catalysts, a novel, mixed phosphine-NHC complex was synthesized with variation in the fourth coordinate ligand and screened for activity towards the transformation of CO₂ to CO. The newly created structures were demonstrated to catalyze the reduction of CO₂ to CO in the presence of acetic acid with no deactivation detected after several hours of electrolysis. In addition to exhibiting versatility in the palladium bis-(NHC) pincer motif, this marks the first report of a palladium pincer catalyst capable of sustained turnover of CO₂ to CO with the assistance of a weak acid.

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