<div>Organic mixed ion-electron conducting (OMIEC) polymers are capable of transporting both electrons and ions. This unique functionality underpins many emerging applications, including biosensors, electrochemical transistors, and batteries. The fundamental operating principles and structure-function relationships of OMIECs are still being investigated. Computational tools such as coarse-grained molecular dynamics (CGMD), which use simpler representations than in atomistic modeling, are ideal to study OMIECs, as they can explore the slow dynamics and large length scale features of polymers. Nevertheless, methods development is still required for CGMD simulations to accurately describe OMIECs.</div><div><br></div><div>In this thesis, two CGMD simulation approaches have been adopted. One is a so-called "top-down" approach to develop a generic model of OMIECs. Top-down models are phenomenological but capable of exploring a broad space of materials variables, including backbone anisotropy, persistence length, side-chain density, and hydrophilicity. This newly developed model was used to interrogate the effect of side-chain polarity and patterning on OMIEC physics. These studies reproduce experimentally observed polymer swelling while for the first time clarifying several molecular factors affecting charge transport, including the role of trap sites, polaron delocalization, electrolyte percolation, and suggesting side-chain patterning as a potential tool to improve OMIEC performance.</div><div><br></div><div>The second strategy pursued in this thesis is bottom-up CGMD modeling of specific atomistic systems. The bottom-up approach enables CGMD simulations to be quantitatively related to specific materials; yet, the sources of error and methods for addressing them have yet to be systematically established. To address this gap, we have studied the effect of the CG mapping operator, an important CG variable, on the fidelity of atomistic and CGMD simulations. A major observation from this study is that prevailing CGMD methods are underdetermined with respect to atomistic training data. In a separate study, we have proposed a hybrid machine-learning and physics-based CGMD framework that utilizes information from multiple sources and improves on the accuracy of ML-only bottom-up CGMD approaches. </div>

<p>Radical polymers bearing open-shell moieties at their pendant sites offer potential advantages in processing, stability, and optoelectronic properties compared to conventional doped conjugated polymers. The rapid development of radical-containing polymers has occurred across various applications in energy storage devices and electronic systems. However, significant gaps still exist in understanding the key structure-property-function relationships governing charge transport phenomena in these materials. Most reported radical conductors primarily rely on (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) radicals, which raises fundamental questions about the ultimate limits of charge transport capabilities and the impact of radical chemistry choice on material deficiencies. Moreover, an understanding gap persists when it comes to connecting the computable electronic features of individual units and the charge transport behavior of these materials in condensed phases. This dissertation seeks to address these gaps by developing a molecular understanding of charge transport in radical-bearing materials through a combined computational and experimental approach.</p> <p><br></p> <p>The initial stage of this dissertation investigated the impact of dimeric orientations and interactions on charge transport by conducting a density functional theory (DFT) study on a diverse set of open-shell chemistries relevant to radical conductors. The results revealed the anomalously high reorganization energies of the TEMPO radical due to strong spin-localization, which may result in inefficient charge transfer. Additionally, a significant mismatch was identified between dimeric conformations favored by intermolecular interactions and those maximizing charge transfer. This study provided new insights into the impact of steric hindrance and spin delocalization on elementary charge transfer steps and suggests opportunities for exploiting directing interactions to enhance charge transport in these materials.</p> <p><br></p> <p>Building upon these findings, we established a direct relationship between the molecular architecture and intrinsic charge transport properties. To accomplish this, single-molecule characterization methods (i.e., break junction techniques) were implemented to study the nanoscale charge transport properties of radical-containing oligomeric nonconjugated molecules. Temperature-dependent measurements and molecular modeling revealed that the presence of radicals improves tunneling at the nanoscale. Integrating open-shell moieties into nonconjugated molecular structures significantly enhances charge transport, thereby characterizing charge transport through radicals at the individual level and opening new avenues for implementing molecular engineering in the field of nanoelectronics.</p> <p><br></p> <p>To further connect the electronic properties of repeat units with the condensed-phase charge transport behavior of radical polymers, a quantum chemical study was carried out to explicitly evaluate the interplay between polymer design, open-shell chemistries, and intramolecular charge transport. After comprehensive conformational sampling of the configurational space of radical polymers, we determined their anticipated intrachain charge transport values by utilizing graph-based transport metrics. We show that charge transport in radical polymers primarily hinges on the choice of radical chemistry, which in turn affects the optimal selection of backbone chemistry and spacer group to ensure proper radical alignment and prevent undesired trap states. These findings highlight the potential for a substantial synthetic exploration in radical polymers for radical conductors.</p> <p><br></p> <p>In summary, this dissertation provides compelling evidence of radical-mediated charge transport and suggests potential design guidelines to enhance the charge transfer behavior of radical-containing polymer materials. Furthermore, these findings inform future research directions in fine-tuning molecular engineering and modular design to enable the development of radical-based materials and their end-use applications in organic electronics.</p>

<p>   Organic electronics has been a highly researched field owing to the low cost, biocompatibility, mechanical flexibility, and superior performance relative to their inorganic counterparts in some applications. Significant advancement has been achieved across various device platforms including organic light-emitting diodes (OLEDs), organic field effect transistors (OFETs), and organic solar cells, for instance. Recently, soft materials that can conduct both charge and ions simultaneously (i.e., organic mixed conductors) have been a major catalyst in the fields of biosensors and energy storage. Extensive research efforts in the organic electronics field are being invested to establish the relevant structure-property relationships to design and develop higher performing organic mixed conductors. Simultaneously, these materials are utilized in developing prototype biosensors with the aim of superior performance, lower cost, and better patient comfort and outcomes than currently available technologies. Following suit, this dissertation is dedicated to furthering organic electronics on both fundamental and applied fronts. Specifically, this work examines a novel class of redox-active macromolecules, radical polymers, as the organic electrochemical transistor (OECT) active layer. In addition, wearable ocular biosensors utilizing soft materials to realize design innovation are presented.</p> <p>   For the first part of the present dissertation, radical polymer-based blends are evaluated for mixed electron and ion conduction in OECTs. Traditional macromolecular design motifs for OECT active layer materials have been a closed-shell macromolecular backbone for electron conduction with charge-neutral hydrophilic side chains (e.g., triethylene glycol) for ion conduction. When poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (PTEO) is blended with poly(3-hexylthiophene) (P3HT), 2,2,6,6-tetramethylpiperidin-N-oxy (TEMPO) radicals in PTEO act as an independent voltage regulator that modulates the ionic and hence electronic transport of the OECT devices. Electrochemical analysis of the blend films reveals that the ionic transport and hence electrochemical doping of the P3HT phase occur when the applied bias matches the onset oxidation potential of TEMPO radicals in PTEO even though that of P3HT is lower than that of TEMPO oxidation. By optimizing the blend ratio, figure-of-merit (i.e., μC*) values over 150 F V–1 cm–1 s–1 at loadings as low as 5% PTEO (by weight) are achieved, placing the performance on the same order as top-performing conjugated polymers despite the mediocre performance of pristine P3HT (<10 F V–1 cm–1 s–1). These findings suggest that introduction of open-shell moieties in the OECT active layer as a secondary redox-active species may significantly improve OECT performance metrics and offer a new paradigm for future macromolecular designs.</p> <p>   In the second part of the dissertation, novel design strategies for wearable ocular electroretinography (ERG) sensors are presented. Typically, wearable sensors are custom-made contact lenses fabricated in a bottom-up fashion where the pre-fabricated sensor component is either embedded in the contact lens body or sandwiched between two. The present work instead utilizes commercially available contact lenses, and the corneal electrode is integrated via electropolymerization of poly(3,4-ethylenedioxythiophene):iron(III) p-toluenesulfonate (PEDOT:Tos) on the lens surface. Electrochemical analysis of the PEDOT:Tos reveals that the measured impedance is several orders of magnitude lower than that of noble metals (e.g., Au) used as the working electrode in commercial electrodes. The mechanical and chemical stability along with the soft form factor of the present design strategy enables high-fidelity recording of ERG signals in human subjects without the need for topical anesthesia.</p> <p>   Following the similar strategy, a new seamless wearable ocular sensor integration strategy utilizing polydopamine (PDA) conformal coating is demonstrated. In this work, we utilize its strong adhesive property originating from the van der Waals interactions between catechol moieties of PDA and various hydrophilic functional groups (e.g., hydroxy, ether, etc.) already present in commercial contact lens materials. The facile integration demonstrates high peeling strength (> 55 J m-2), chemical and mechanical stability. A series of <em>in vivo</em> assessments demonstrates high accuracy, reliability, and user comfort of the fabricated wearable sensor in both animal and human subjects. The findings suggest that the PDA-assisted integration strategy may be applied in designing various future-generation wearable ocular electrophysiological sensors.</p>

<p dir="ltr">Molecular materials exhibiting bistability between two states are intriguing candidates for next generation electronic devices. Two similar classes of materials, known as spin crossover (SCO) and valence tautomers (VT) respectively, are of particular interest due to their multifunctional properties, which are controllable via several external parameters, such as temperature, light irradiation, pressure, magnetic field, and electric field. In recent years, considerable research has been dedicated to better understanding the underlying principles that govern the behavior of these materials, so that their implementation into nano-based devices might be achieved.</p><p dir="ltr">In this report, a systematic study of the valence tautomer molecule [Co(sq)(cat)(3-tpp)₂] is presented. In the first chapter, the phenomenon of valence tautomerism (VT) occurring in coordination compounds is introduced and described from the perspective of Crystal Field Theory (CFT). Further, the molecular structure and physical properties of the [Co(sq)(cat)(3-tpp)₂] molecule are explored. The properties of the ferroelectric material Polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), and the 2-D Mxene Ti₃C₂ are also discussed.</p><p dir="ltr">The next section details equipment development and experimental methods. Thin films of VT molecules were prepared from solution via a drop-casting approach. For thin film analysis, we have developed a custom made, fully automated Vibrating Sample Magnetometer (VSM) with a sensitivity on the order of 1 × 10^-5 emu, as well as a fully automated, variable temperature, under vacuum electron transport stage, and a magneto-optic Kerr effect apparatus (MOKE). Additional experimental methods used to characterize the VT thin films include X-ray Absorption Spectroscopy (XAS), UV-visible Spectrometry (UV-Vis) and Differential Scanning Calorimetry. Experimental results obtained from these techniques are discussed and analyzed in the third section. PVDF-HFP polarization dependent isothermal spin state switching of [Co(sq)(cat)(3-tpp)₂] is also discussed as well as the effects of doping [Co(sq)(cat)(3-tpp)₂] with Ti₃C₂, followed by a conclusion and an outline of future work.</p>

<p>Neuromorphic computing endeavors to develop computational systems capable of emulating the brain’s capacity to execute intricate tasks concurrently and with remarkable energy efficiency. By utilizing new bioinspired computing architectures, these systems have the potential to revolutionize high-performance computing and enable local, low-energy computing for sensors and robots. Organic and soft materials are particularly attractive for neuromorphic computing as they offer biocompatibility, low-energy switching, and excellent tunability at a relatively low cost. Additionally, organic materials provide physical flexibility, large-area fabrication, and printability.</p><p>This doctoral dissertation showcases the research conducted in fabricating a comprehensive spiking organic neuron, which serves as the fundamental constituent of a circuit system for neuromorphic computing. The major contribution of this dissertation is the development of the organic, flexible neuron composed of spiking synapses and somas utilizing ultra-low voltage organic field-effect transistors (OFETs) for information processing. The synaptic and somatic circuits are implemented using physically flexible and biocompatible organic electronics necessary to realize the Polymer Neuromorphic Circuitry. An Axon-Hillock (AH) somatic circuit was fabricated and analyzed, followed by the adaptation of a log-domain integrator (LDI) synaptic circuit and the fabrication and analysis of a differential-pair integrator (DPI). Finally, a spiking organic neuron was formed by combining two LDI synaptic circuits and one AH synaptic circuit, and its characteristics were thoroughly examined. This is the first demonstration of the fabrication of an entire neuron using solid-state organic materials over a flexible substrate with integrated complementary OFETs and capacitors.</p>

The development of solution-processable semiconducting polymers has brought mankind’s long-sought dream of plastic electronics to fruition. Their potential in the manufacturing of lightweight, flexible yet robust, and biocompatible electronics has spurred their use in organic transistors, photovoltaics, electrochromic devices, batteries, and sensors for wearable electronics. Yet, despite the successful engineering of semiconducting polymers, we do not fully understand their molecular behavior and how it influences their doping (oxidation/reduction) properties. This is especially true for donor-acceptor (D-A) p-systems which have proven to be very efficient at tuning the electronic properties of organic semiconductors. Historically, chain-length dependent studies have been essential in uncovering the relationship between the molecular structure and polymer properties. Discussed here is the systematic investigation of a complete D-A molecular series composed of monodispersed and well-defined conjugated molecules ranging from oligomer (n=3-21) to polymer scale lengths. Structure-property relationships are established between the molecular structure, chain conformation, and redox-active opto-electronic properties for the molecular series in solution. This research reveals a rod-to-coil transition at the 15 unit chain length, or 4500 Da, in solution. The redox-active optical and electronic properties are investigated as a function of increasing chain-length, giving insight into the nature of charge carriers in a D-A conjugated system. This research aids in understanding the solution behavior of conjugated organic materials. <br>

<p> Radical species are essential in modern chemistry. In addition to fundamental chemistry, their unique chemical bonding and distinct physicochemical features serve critical functions in materials science in the form of organic electronics. Due to their high reactivity, radicals of the main group element are often transient. In recent years, remarkably stable radicals are often stabilized by π-delocalization, sterically demanding side groups, carbenes, and weakly coordinating anions. The impacts of modifications such as electron-donating, electron-withdrawing, and end-capping on the spin density distribution and thermodynamic and kinetic stability of archetypal radical-driven processes such as dimerization are not well understood. This dissertation aims to track the perturbation of spin density from EDG and EWG modifications, provide mechanistic insight into the radical-initiated reactions of conjugated radical cations, and establish correlations between molecular design and thermochemical properties and their resulting kinetic stability by computationally evaluating these characteristics against experimental data. The disclosed connections give useful new recommendations for the rational design of thermodynamically and kinetically stable novel materials.</p>