<p> This dissertation includes two topics, which are related to each other: (1) Rupture force measurement of single triazole molecule by atomic force microscope (AFM) and (2) Solid phase stepwise synthesis of polyethylene glycols (PEGs). </p><p> The reaction of an alkyne and an azide to form a 1,2,3- triazole has found wide applications. However, the stability of triazole under mechanical stress is unclear. We investigated the cycloreversion at the single molecular level using atomic force microscopy. A mica surface functionalized with a molecule containing a 1,2,3-triazole moiety in the middle and an activated ester at the end was prepared. An AFM tip with amino group was ramped over the surface, which temporarily linked the tip to the surface through amide bond formation. During retraction, the triazole broke and the forces were recorded. Most of the forces were below 860 pN. The resulting alkyne were labeled with gold nanoparticles (AuNPs). AFM imaging revealed AuNPs at the predefined locations, proving that 1,2,3-triazole could rupture with force below 860 pN. </p><p> We used commercially available functionalized polyethylene glycol (PEGs) for functionalizing of mica surface and AFM tip. These PEGs were polydispersed admixtures with different size of tags and not ideal for many applications. There is a need to develop a new method to synthesize a monodispersed PEGs. We believe that if monodisperse PEGs were used, our data would be much better. In addition, monodispersed PEGs can form single molecular conjugates during chemical reactions which allow precise characterization of all PEGs conjugates. This lead us to conceive an idea to prepare monodisperse PEGs. </p><p> Significant efforts have been made in the area, to achieve monodisperse PEGs using solution phase but the drawbacks include the need of multiple column chromatography, low overall yield, and contamination by depolymerized products. To overcome all shortcomings, we have developed a new technology to synthesize monodispersed PEGs on solid support. The Wang resin, which contains the 4-benzyloxy benzyl alcohol function, was used as the support. On this solid support, tetraethylene glycol monomer was added stepwise. Polyethylene glycol (PEG) with eight and twelve glycol units and its derivatives were synthesized. The synthetic cycle consists of deprotonation, Williamson ether formation (coupling), and detritylation. Cleavage of the PEG from solid support was achieved with trifluoroacetic acid. From this method, we prepared highly pure products having different functionalities at the two ends without any chromatographic purification in the entire synthesis.</p><p>

The shape of a crystalline organic solid has a major impact on its downstream processing and on its end-product quality, issues that are becoming increasingly important in the specialty chemical industry. In particular, shape affects the key washing and filtering steps in solution crystallization, and it determines solids agglomeration and dissolution characteristics. Traditional models for predicting crystal shape are based exclusively on the internal crystal structure. They are able to predict sublimation-grown crystals, but are not able to account for the effects of the environment (i.e., solvent and impurities) that dominate solution-grown shapes. Detailed kinetic theories of crystal growth—the spiral, two-dimensional nucleation, and rough growth mechanisms—have also been developed, but have not been widely employed. They require as input certain face-specific and solvent-specific properties that are generally unknown. This work explores all of the traditional methods and detailed kinetic theories in an attempt to develop a new method for predicting crystal shape. The detailed kinetic theories are simplified, yielding relative face growth rate expressions that depend primarily on kink and edge free energies—two microscopic properties of crystal faces. A method for estimating these properties is also proposed. It depends on the solvent's surface free energy, which is often known, and the crystal's internal energy, which can be determined from simple molecular mechanics calculations. The method is used to predict the shape of adipic acid grown from water, ibuprofen grown from polar and non-polar solvents, and biphenyl grown from toluene. An extension to this technique is developed in order to estimate hydrogen bonding between solvents and crystals: it is applied to succinic acid grown from water and isopropanol, and to paracetamol grown from water and acetone. This approach appears to be the first practical technique that can successfully predict solution-grown organic crystal shapes.

In a highly interdisciplinary field, such as organic photovoltaics (OPVs), developing a predictive understanding of the relationship between molecular structures, morphology of the photoactive layer and the ultimate device performance is the key to unlocking the vast potential of this field. Although isolated examples of high-performance organic molecules are prevalent in the literature, the reasons for their superior performance are not well understood. The function of an OPV device is dependent of four key processes: (i) light absorption, (ii) charge separation, (iii) charge transport, and (iv) charge collection. While the first three are material-dependent factors, charge collection depends on the nature of the interfaces involved. We have thus investigated a new class of semiconductor molecules based on BODIPY dyes with the aim of understanding how variations in the molecular structure affect the optoelectronic and transport properties of the molecules. First-generation pi-conjugated polymers based on the BODIPY core possess broad and intense absorption spectra. Additionally, the frontier molecular orbital (FMO) energy levels of the polymers can be tuned by a judicious choice of the comonomers. Electron-deficient comonomers with electron affinities higher than that of the BODIPY core, predominantly afford n-type polymers. A unique feature of these semiconductors is their panchromatic absorption spectrum that spans throughout the visible region. Thus these polymers can be considered to be potential electron acceptors in all-polymer solar cells. Copolymerization of BODIPY with electron-rich comonomers, on the other hand, only results in p-type semiconductors. Furthermore, the highest occupied molecular orbital (HOMO) of these polymers is found to correlate with the ionization potential of the electron-rich monomer. Having said that, the lowest unoccupied molecular orbital (LUMO) energy level does not change. Thus for the first time, a correlation between theoretical calculations and experimental observations has been demonstrated for predicting the FMO energy levels of BODIPY-based semiconducting polymers. Second-generation copolymers based on an unsubstituted BODIPY core retain the broad absorption characteristics of the first-generation polymers. In addition, due to reduced electron density on the BODIPY core, the HOMO energy level of the resulting polymers is reduced thereby imparting enhanced oxidative stability to these polymers. Charge transport measurements through thick films (∼1 micron) reveal only p-channel activity with hole mobilities comparable to some of the high-performance polymers reported in literature. Preliminary bulk-heterojunction OPV devices fabricated with these polymers show modest power conversion efficiencies. We believe that understanding the morphology of the active layer in relation to the polymer structure will help improve future molecular designs and eventually, device performance.

All organic photovoltaics (OPVs) undergo four major processes to convert sunlight in electrical energy. The first process is the absorbance of sunlight. Due to the limit of available acceptor molecules, the burden of light absorbance weighs heavily on the donor material. This thesis focuses heavily on the development of dyes consisting of donor-acceptor dyads and triads for improved light harvesting in OPVs. Squaraine dyes show impressive light harvesting properties with absorbances in the UV to near IR region with extinction coefficients on the order of 105 M-1 cm-1. Unfortunately, improved light harvesting is not enough to insure optimized OPVs. Energy level tuning to increase VOC and insure efficient exciton dissociation is also required. Functionalizing squaraine dyes with electron donating or electron withdrawing groups allow for the systematic tuning of the HOMO energy levels. This tunability allows for the concurrent optimization of bandgap and VOC. Cyanine dyes have been explored for small molecule OPVs due to their impressive absorbance properties. The absorbance of ketocyanine dyes can be tuned by manipulating the strength of the acceptor moiety. Stronger acceptors are better able to stabilize the negative charge in the charge separated state of the dye. This stabilization allows for a greater contribution from the cyanine structure of the dye, thus red shifting the absorbance. Stronger acceptors also increase the communication between the two amine functionalities as demonstrated by cyclic voltammetry. Block copolymers show impressive morphological control through the tuning of the molecular weight of the blocks as well as the compatibility of the functional groups. This allows for the access of morphologies with small, well ordered, and continuous domains thought to be beneficial in the active layer of OPVs. Unfortunately, block copolymers often show inferior light harvesting compared to their conjugated polymer counterparts. Donor-acceptor systems are explored as sensitizers for block copolymer OPVs. Small molecules without twists or bends or acetylene linkers were found to be most effective for lowering the bandgap and aligning the energy levels.

Self-assembly of nanoparticles presents an excellent tool in the development of novel nanoscale structures and materials for creating high sensitive sensors, electronic and diagnostic devices, ultrahigh-density magnetic storage devices and many more. In these systems, the nanoparticle core imparts exceptional physical properties while their organic coatings regulate the assembly process. Moreover, organic coatings improve particle stability and solubility, as well as regulate charge and hydrophobicity. This thesis has focused on the engineering of nanoparticles’ surfaces using organic molecules and assembly of these particles through supramolecular interactions for various applications. Morphology of the nanoparticle assembly was tuned simply by varying the degree of fluorinated coating on particles’ surfaces and thus controlling their hydrophobicity. Surface engineered particles were also assembled at oil-water interfaces alone and with enzymes creating colloidal microcapsules for controlled release and catalysis respectively. The combination of the unique attributes of the nanoparticle cores and the function of the organic coating provides ample opportunities in the creation of multi-functional nano-materials that are useful in biological and materials applications.

This dissertation explores the self-assembly of diblock copolymers as a way to control the morphology of photoactive layer in organic solar cells. Heterojunction formation between electron donor and electron acceptor materials needs to be controlled on the nanometer scale to have high power conversion from organic photovoltaic cells. Two approaches were developed to direct the assembly of electron-donors and electron-acceptors into heterojunction structures. The first one involves the synthesis of acid cleavable diblock copolymer to create porous polymer films, which can be used as templates to form well ordered donor-acceptor heterojunctions on the nanometer scale. Here, we demonstrate that nanoporous templates could be prepared under moderate conditions, which do not interfere with photovoltaic device fabrication. The second approach involves the use of conjugated diblock copolymers as the structure directing agents. Incompatible packing of the side chains was investigated to provide microphase segregation in conjugated polymer/fullerene blends. Packing of the materials within the domain has been shown to be a very important parameter for photovoltaic performances.

Magnetostructural investigations were carried out on pyrene-1-yl (Pyr) bearing nitronylnitroxide (NN) and iminoylnitroxide (IN) radicals. PyrNN gives two allotropes: one has spin-paired dyads with ΔE = J/k ≈ -178 K, and the other is only half spin-paired with ΔE = J/k ≈ -102 K and the other half paramagnetic. PyrIN also gives two allotropes, an anti conformation that is spin paired in the crystal lattice with ΔE = J/k = -410 K, and a syn conformation that is disordered and paramagnetic. PyrNN also was discovered to co-crystallize with C6F6 in 2:1 ratio to give chains of radical networks linked into networks exhibiting low dimensional 1-D or 2-D antiferromagnetic exchange behavior. Furthermore, PyrNN was discovered to form a 2:1:2 co-crystal with octafluoronaphthalene (OFN) and entrapped solvent dichloromethane (DCM), in which the radical is ``shepherded'' into forming chains of radical-radical contacts on the peripheries of (PyrNN-OFN-PyrNN)n pi-stacks, giving weak, low dimensional inter-radical antiferromagnetic (AFM) exchange interactions. Anthraquinone-substituted nitronylnitroxide radical (AntQNN) was synthesized and found to form two crystal polymorphs. Magnetostructural investigations carried on these indicated that both have antiferromagnetic (AFM) exchange behavior attributed to chain-type inter-radical contacts: one with J1D/k ≈ -3 K, and one with J1D/k ≈-17 K. Five different anthracene nitroxide-type biradicals were synthesized: 27AntdNN, 27AntdIN, 26AntdNN, 26AntdIN, and 9Br27AntdNN. Room temperature solution state, and frozen solution state electron spin resonance (ESR) studies were carried on all of these biradicals. Crystallographic packing information was successfully obtained for 27AntdIN, 26AntdNN, 26AntdIN, and 9Br27AntdNN. Magnetic susceptibility measurements were carried on 27AntdNN, 27AntdIN, 26AntdIN and 9Br27AntdNN. The NN derivatives showed both ferromagnetic (presumed intramolecular) and antiferromagnetic exchange interactions in the solid state. Four different anthraquinone nitroxide-type biradicals were synthesized: 27AntQdNN, 27AntQdIN, 26AntQdNN, 26AntdIN. ESR studies were carried on these biradicals, and showed that 27AntQdNN is not a stable organic radical. While ESR spectra confirmed that 27AntQdNN, 27AntQdIN are biradicals, ESR spectra with isolated monoradical behavior were obtained for 26AntQdNN, 26AntQdIN. Iodine substituted meta-phenylene nitroxide biradicals, IPhdNN, IPhNNIN, and IPhdIN were synthesized. Room temperature and frozen solution ESR studies showed triplet states with strong intramolecular spin interaction. Magnetic behavior and crystallography for IPhdIN (which incorporates DCM), showed halogen bonding between molecules that assists formation of chains between radical sites.

The orientation of nematic liquid crystals in electrical fields is commonplace in flat-panel watch and calculator displays. In 99% of these devices, the optical path and the electrical field are parallel to each other. When an alternating electrical field is applied perpendicular to the optical path unusual optical patterns result. The physics and properties of the wave-like optical pattern were previously unreported. The geometry and wave-like optical pattern are described in chapter two for a room-temperature nematic liquid crystal: 4-4$\sp\prime$-n-pentylcyanobiphenyl. Physical conditions of wave stability, i.e. temperature, field and frequency were investigated and discussed in terms of nematic liquid crystalline theory. It was determined that the wave pattern was stabilized at low temperatures and high frequencies. It was also concluded that the mechanism of deformation was dielectric rather than ionic. The macroscopic observations were quantified into two molecular orientation profiles in the succeeding chapter. The orientation profiles given are based on two possible cases of surface orientation for the nematic layer. The analysis of molecular orientation relies heavily on the theory of electrical deformation developed by Deuling. To understand the importance of the material parameters of the nematic in relationship to the optical effect, binary blends of 4-4$\sp\prime$-n-pentylcyanobiphenyl and 4-(4-alkylcyclo-hexyl) -cyanophenyl homologs were prepared. The complete phase behavior of these mixtures is the subject of chapter 4. A thorough physical characterization, regarding the dielectric and elastic properties, response times and threshold voltages, of four selected nematic fluids is given in chapter five. Finally, in chapter six, the effect of the blending on the electrical deformation of the nematic mesophase in a transverse-electrode cell is studied.

Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2006. / Source: Dissertation Abstracts International, Volume: 68-02, Section: B, page: 0975. Adviser: Jeffrey S. Moore. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 2974. Adviser: Paul J. Hergenrother. Includes bibliographical references (leaves 150-175). Available on microfilm from Pro Quest Information and Learning.