In recent years, Polydopamine (PDA) materials have garnered substantial attention inthe fields of chemistry and materials science due to their adaptable properties, which encompass metal-ion chelation, facile functionalization, strong adhesion, and the capacity for scavenging free radicals. Created through the autoxidation of dopamine precursor, polydopamine emerges as a nanomaterial suspendable in colloidal form. Initially conceived as a versatile biomimetic coating akin to the adhesive foot protein found in mussels, PDA later evolved into nanoparticles suitable for scavenging free radicals and serving as contrast agents in biomedical applications, with its uses continuously expanding. At present, PDA nanoparticles offer flexibility in their size, ranging from ten to several hundred nanometers. Size tunability is used as a means to add to the already considerable control over PDA structure-function and enhancing its immense potential across diverse fields. In chapter 2, the controlled synthesis of amorphous, organic, bio-inspired polydopamine (PDA) nanoparticles in varying atmospheric conditions was investigated. Specifically, we examine the role of oxygen (O2) partial pressure on PDA particles' distribution during the synthesis process. Our systematic exploration reveals that precise control of O2 content significantly impacts the quality, speed, and reliability of PDA nanoparticle production. Results indicate that 40% or higher O2 compositions lead to superior outcomes, including reduced polydispersity index (PDI) values and enhanced reproducibility. Notably, introducing an 80% O2 composition significantly accelerates the reaction, reducing the synthesis time from the typical 24 h under ambient conditions to just over 30 min. This breakthrough suggests the potential for large-scale or continuous reaction schemes, offering substantial time and resource savings for applications involving PDA nanoparticles and similar materials. In chapter 3, a multifaceted exploration of PDA is presented, starting with the synthesis of a diverse spectrum of PDA nanoparticle sizes ranging from dPDA = 29 to 731 nm, followed by the determination of average particle mass. Establishing a correlation between particle mass and volume led to a linear fit represented by the equation ? = 1.5 × 10^(-21)x with R^(2) value of 0.981. Subsequently, the chapter detailed the fabrication process of PDA-chitosan thin films, assessing their purity, optical quality, and the potential influence of bubbles and aggregate formations on the overall spectra. With a consistent film homogeneity between 99.8% and 98.3% with increased PDA concentration, fundamental trends observed in the spectra were consistent throughout. The study further explored the impact of composite materials in fundamental studies and potential applications of PDA-chitosan films for radiation protection across the UV to NIR spectrum. It was revealed that PDA was most effective at shorter wavelengths with higher energies, while chitosan and water exhibited greater influence at longer wavelengths with lower energies, rendering the composite material effective across a broad range of wavelengths. Moreover, the chapter highlighted the advantageous attributes of chitosan, including its antimicrobial properties, flexibility, and rehydration capabilities, positioning PDA-chitosan films as strong contenders for applications in wound healing. Lastly, the investigation showcased the versatility of PDA and its adaptability to various polymer matrices with different polarities. The successful introduction of PDA into resin, followed by 3D printing and polishing, resulted in the creation of transparent materials with designer structures. The chapter proposes expanding this project to create a more extensive collection of composite materials, aiming to deepen the understanding of PDA and increase their accessibility in our daily lives.

Polydopamine (PDA) materials have provoked great attention in chemistry and materials science since 2007 due to their versatile properties including metal-ion chelation, easy functionalization, adhesion, and free radical scavenging ability. Produced via mild oxidation of dopamine monomer to form a colloidally-suspendable nanomaterial, polydopamine was initially developed as a versatile, biomimetic coating material, similar to the foot protein of mussels. Subsequently, PDA nanoparticles were developed as free radical scavengers and biomedical contrast agents amongst an ever-expanding list of applications. Currently, PDA nanoparticles are tunable in size within the range from tens to several hundred nanometers, and they can be easily loaded with metal ions and functionalized with different types of molecules. Therefore, PDA materials have great potentials in applications such as biomedicine, catalysis, and environmental remediation.

In chapter 2, Fe3+-loaded PDA materials were studied as magnetic resonance imaging (MRI) contrast agents. An amorphous metal-chelated polymer nanoparticle presents a significant challenge for characterizing the source of MRI contrast as structural data is complex and characterization methods limited. Tunable concentrations of Fe3+ were achieved and the structure and magnetic interaction were analyzed comprehensively by magnetometry and electron paramagnetic resonance. These characterizations indicate the antiferromagnetic coupling in Fe3+ centers and optimal Fe3+ concentrations can be predicted to improve the contrast performance of PDA materials.

In chapter 3 and 4, fluorocarbon-functionalized PDA and Fe3+-loaded PDA NPs were investigated as ultrasound contrast agents. Traditional ultrasound imaging uses microbubbles with perfluorocarbon core and lipid shell to enhance an ultrasound signal. Herein, a balance between polarity and fluorophilicity is achieved to generate water-dispersible nanomaterials that also stabilize perfluorocarbon liquids. Results show strong and long-term US imaging capability, with chelation of Fe3+ introducing enhanced photoacoustic imaging capability.

In chapter 5, a facile method to generate magnetic exchange-bias between ferromagnetic and antiferromagnetic nanomaterials using nanoparticle emulsion clusters is developed. Magnetic CoFe2O4/CoO nanoclusters are studied as proof-of-concept for exchange-bias behavior. This system shows a record exchange bias field (3200 Oe; 5K) for a nanoparticle-based system. This study gives a roadmap for extending synthetic methods beyond the superparamagnetic range and into composite single-domain materials for high-temperature magnetic materials with exchange-bias behavior.

A magnetostructural study on the erbium–cyclooctatetraenide (Er–COT) motif was conducted. This molecular fragment was shown to generate magnetic anisotropy, and consequently slow magnetic relaxation, in a series of example coordination complexes. The generality of this effect led to creation of the idea that certain metal-ligand pairs can act as magnetic synthons toward the development of single-molecule and molecular magnets.

Chapter 1 provides a brief introduction to the study of single-molecule magnetism. It includes a discussion on the historical development of the field, as well as some of the basic theoretical framework required to understand magnetic relaxation on a molecular level. The chapter concludes with the motivation for and introduction of the metal-ligand pair anisotropy design (MLPA) principle.

Chapter 2 discusses the development of a technique for measuring the magnetic relaxation time. Relaxation time measurements have historically been carried out using two separate approaches depending on the timescale being studied. A new impedance-based method toward measuring a system's relaxation dynamics at the long timescale is presented which obviates the necessity of two separate measurement approaches.

Chapter 3 presents the results of a magnetostructural study conducted on a series of mononuclear Er–COT complexes. A combined magnetic and computational evaluation of Er(COT)I(THF)₂, Er(COT)I(Py)₂, Er(COT)I(MeCN)₂, and Er(COT)(Tp*) (THF = tetrahydrofuran, Py = pyridine, MeCN = acetonitrile, Tp* = tris(3,5-dimethyl-1-pyrazolyl)borate) reveal that each behave as a single-molecule magnet and thus a degree of coordinative robustness within the Er–COT metal-ligand pair is evinced.

Chapter 4 introduces the idea of coupling Er–COT units. Therein a dinuclear erbium complex, [Er(COT)(μ-Cl)(THF)]₂, is shown to display a net-ferromagnetic interaction between metal centers while also maintaining magnetic anisotropy. A brief discussion is given on the relevance of this result toward making higher-order structures.

Chapter 5 discusses the results of a magnetic coupling optimization study performed on a series of structurally similar MLPA compounds. Mononuclear Er(COT)I(DMPE) (DMPE = 1,2-bis(dimethylphosphino)ethane), and dinuclear [Er(COT)(μ-I)]₂(μ-DPPE), [Er(COT)(μ-I)]₂(μ-DPPM), and [Er(COT)(μ-I)(MDPP)]₂ (DPPE = 1,2-bis(diphenylphosphino)ethane, DPPM = bis(diphenylphosphino)methane, MDPP = methyldiphenylphosphine) were prepared. The effects of nuclearity and anisotropy axis orientation on the low-temperature relaxation time resulted in a million-fold improvement across this series.

Synthetic melanins are incredibly versatile materials that have captured the attention of chemists and materials scientists due to the remarkable properties they share with natural melanins, such as adhesion, sequestration of metals and organic molecules, and biocompatibility. The facile and sustainable production of these materials involves oxidation of the catechol moiety of the dopamine monomer in mild and alkaline conditions, inducing the polymerization of the starting material into polydopamine. These materials have myriad applications, in areas such as energy (e.g. lithium ion batteries, supercapacitors), water treatment (e.g. metal and organic pollutant removal, desalination), and biomedical sciences (e.g. drug delivery, imaging contrast agents). The structure and function of these polydopamine materials is discussed in further detail in Chapter 1. Chapter 2 describes the materials and methods used to prepare synthetic melanins for characterization via magnetometry to probe the effects of small variations employed in the synthetic procedures on the electronic structure of these materials. In Chapter 3, the results of this work are discussed, showing evidence of the buffer solution’s influence on the magnetic behavior of the samples. Finally, Chapter 4 proposes future studies for the continuation of this project.

Superparamagnetic nanoparticles represent an exciting area of research within the broader field of nanoscale materials. Synthetic control over physical properties, such as nanoparticle size, shape, and crystalline phase strongly impact the magnetic properties, ultimately dictating the functionality in potential applications.

Chapter 1 provides a brief background of nanoscale magnetic materials. It discusses the high degree of control possible through colloidal nanoparticle synthesis techniques, as well as common challenges encountered. The chapter includes specific examples on the structure-property relationship, including implications for magnetism, concluding with a summary of superparamagnetism.

Chapter 2 discusses the efforts to enhance the reproducibility in magnetic nanoparticle synthesis. The chapter introduces two novel forms of iron oleate precursor that offer long-term stability, well-defined stoichiometry, and large-scale availability, addressing issues with reproducibility. These advancements enable the synthesis of magnetite nanoparticles with tunable sizes ranging from 4 to 16 nm and low size dispersity, providing consistent results in the superparamagnetic size regime.

Chapter 3 details the development of a new technique for the quantification and parametrization of superparamagnetic nanoparticle magnetization curves. The size-dependent magnetic properties were elucidated by fitting the data to a statistical model, leading to a better understanding of these materials' behavior and potential for applications. It emphasizes the challenges in interpreting magnetization data and the importance of simplified models in advancing the field.

Chapter 4, in collaboration with Angelica Orlova, presents the extension of the statistical analysis of magnetization data to a molecular system. Three ErCOT2 compounds with varying structures, including the angle, and spacing between magnetic units, were analyzed. The temperature dependence and effect of dipolar coupling between ErCOT2 units on the magnetization curve were analyzed. Each curve was deconvoluted into its components, which were tracked and quantified across the parameter space.

Colloidal nanoparticles are an exciting class of materials for their ability to act as tunable building blocks for larger scale materials. The application of nanotechnology to magnetic materials depends on the controlled synthesis of uniform nanoparticles with targeted magnetization and magnetic anisotropy. The work presented in this dissertation has two focuses: one, the development of synthetic techniques which allow for the consistent production of uniform nanoparticles and of novel heterostructured nanoparticles with emergent magnetism, and two, the application of these nanoparticles into bulk magnetic and magnetoresistive assemblies. A common issue which plagues the study of nanoparticles is the variability of synthesis from written protocol to laboratory practice or even from batch to batch. A set of techniques are detailed which have been developed to ensure consistent recreation of reaction conditions which allow for separate LaMer ‘burst’ nucleation and growth regimes during synthesis. High-quality, single-phase nanoparticles synthesized using the preceding techniques were further used as seeds in the synthesis of novel heterostructured nanoparticles exhibiting enhanced exchange bias. Collective magnetism was studied in assemblies of ferrimagnetic Fe3O4 and antiferromagnetic CoO nanoparticles. In typical samples consisting only of permanent magnetic nanoparticles, dipolar interactions between the nanoparticles frustrate the orientation of their magnetic moments, often producing a superspin glass state. Conversely, antiferromagnetic nanoparticles, in their dipolar interactions with ferro- or ferrimagnetic nanoparticles, induce a uniaxial magnetic anisotropy and create a superferromagnetic state in the collective magnetism of the nanoparticle assembly Synthetic control over nanoparticle morphology was exploited to engineer nanoparticles for the assembly of granular magnetoresistance devices. Magnetoresistance measurements on a series of pellets of differently sized CoFe2O4 nanoparticles revealed a size threshold beyond which magnetoresistance was greatly diminished. Additionally, the magnetoresistance of CoFe2O4 was found to be superior to that of Fe3O4 despite the latter’s popularity in magnetoresistance research. Magnetoresistance measurements were also performed on mixed nanoparticle films cast from nanoparticle inks. Tuning of the CoFe2O4 to Fe3O4 ratio in the films successfully produced pseudo spin valve magnetoresistance, improving both the magnitude and responsivity of the films’ magnetoresistance.

Gadolinium systems offer a unique platform to investigate magnetically coupled ions, the analysis of said systems is of interest to the field of molecular magnetism. In Chapter 1 of this thesis, we introduce gadolinium species as a focus of study as well as some selected appearances in molecular magnetism literature. We also give a brief primer on magnetic parameterization and isotropy that are critical to our analysis. In Chapter 2 we describe the motivation for the synthesis of the phosphine scaffolded Gd(COT)I series and give a detailed overview of synthetic and characterization methods. Chapter 3 gives a magnetic analysis of the phosphine series which displays evidence of antiferromagnetic coupling, as well as a comparison to a few selected ferromagnetically coupled dinuclear gadolinium complexes.

An important focus in molecular magnetism is the delicate interplay of single-ion anisotropy and coupling between magnetic centers when designing the magnetic energy landscape to yield intended magnetic properties. Throughout this dissertation, we explore how these two perturbations interplay to create this environment and how dynamic behavior in magnetic systems are affected. In Chapter 1, we explore the current design principles dictating the state-of-the-art in molecular magnetic design and outline the parameter space we seek to optimize. In Chapter 2, we explore the ability of highly charge-dense alkoxide bridging ligands to tightly bind highly anisotropic ErCOT units to couple them via the dipolar interaction and explore how this changes in asymmetric complexes. By introducing such a large perturbation, the beneficial aspects of coupling are tempered by increased mixing in states that are close in energy to each other. In Chapter 3, we describe the angular dependence of the dipolar interaction in a series of alkyl-bound ErCOT units by modulating the number of bridging ligands. By enforcing collinearity, we raise the energy separation within dipolar coupled states to increase the temperature dependence of relaxation mechanisms below the first excited state. In Chapter 4, we extend the model describing the dipolar interaction beyond the ground state to describe long-timescale relaxation in a series of halide-bridged ErCOT complexes. By using first-order perturbation theory, we can describe how magnetic states couple indpedently when well-separated and how magnetic relaxation is affected in turn. As a step in this direction we build from the established premise that the COT2− ligand is consistently able to stabilize uniaxial Ising-type magnetic anisotropy in the Er3+ ion along the metal-centroid vector. This premise allows a tangible method to translate spin-space models into molecule design using classical principals for axial moments. Extending from this, ErCOT-derived polynuclear complexes have a useful structural handle to predict and direct the individual centers’ anisotropies; therefore, they are ideal for tuning the throughspace dipolar coupling mechanism. We have carefully modified the magnetic environment as enforced by the bridging ligands across two series of complexes. In one, we show the lengthened relaxation when moving from an inversion-symmetric arrangement of anisotropy axes to a colinear one. In the other, we maintain an inversion-symmetric bridging motif and modify the crystal field strength of the bridging ligands, noting the effect of both coupling strength and single-ion properties of the individual magnetic centers.

The focus of this dissertation is the thorough analysis of the effects of dipolar coupling on magnetic relaxation behavior within erbium-based molecular magnets. Utilizing the Er-COT unit as the starting point and building block, we investigate intra- and inter-molecular dipolar coupling motifs generated with analogous highly anisotropic building block units. Each chapter holds a specific focus, organized as follows: Chapter 1 offers a brief introduction to molecular magnetism, followed by an overview of topics necessary towards understanding magnetic relaxation in the scope of this work, including energy perturbations, relaxation dynamics, and anisotropy; an introduction to the Er-COT unit and coupling schemes in lanthanide-based molecular magnetism, and the motivation for investigating dipolar coupled systems. The chapter concludes with extended chapter summaries for the remainder of this work. Chapter 2 focuses on the role and effects of intramolecular dipolar coupling in a series of compounds of increasing nuclearity. This work demonstrates the ability of intramolecular dipolar coupling to control quantum tunnelling of magnetization, and thus the rate and mechanism of magnetic relaxation. This chapter utilizes an expanded frequency space magnetometry technique to garner new insights into magnetic relaxation by visualizing, fitting, and analyzing multiple relaxation regimes. The chapter concludes with an intuitive model of thought based on a simple vector addition model, within which spin interactions can be estimated directly from a crystal structure. Chapter 3 discusses the effects of intermolecular dipolar coupling within a series of identical single-ion magnets within varied crystal packing environments. This work depicts the propensity of intermolecular dipolar coupling to drive dramatic differences in resulting magnetic behavior and seeks to shed light on the relationship between single-ion magnetism and solid-state magnetism. This chapter applies a novel fitting methodology to quantify additional parameters from isothermal magnetization data for downstream analysis. Chapter 4 presents the analysis of a highly symmetric, near-tetrahedral tetranuclear single-molecule magnet to discuss the effects of crystallographic symmetry on the resulting dipole-coupled spin-space symmetry. This chapter discusses the ensuing available rhombic dodecahedral quantum space of this molecule, composed of octahedral and cubic subspaces, and makes connections to theoretically proposed quantum Cayley networks upon hypercubes.