This dissertation focuses on the solution-state polymer assembly of conjugated polymers with specific attention to nano- and molecular-scale morphology. Understanding how to control these structures holds potential for applications in polymer-based electronics. Optimization of conjugated polymer morphology was performed with three objectives: 1) segregation of donor and acceptor materials on the nanometer length-scale, 2) achieving molecular-scale ordering in terms of crystallinity within distinct domains, and 3) maximizing the number and quality of well-defined donor/acceptor interfaces. Chapter 1 introduces the development of a mixed solvent method to create crystalline poly(3-hexyl thiophene) (P3HT) fibrils in solution. Chapter 2 describes fibril purification and approaches to robust and functional fibrils, while chapters 3 and 4 demonstrate the formation of hybrid nanocomposite wires of P3HT and cadmium selenide (CdSe) nanoparticles by two methods: 1) co-crystallization of free and P3HT-grafted CdSe for composite nanowires and 2) direct attachment of CdSe nanoparticles at fibril edges to give superhighway structures. These composite structures show great potential in the application of optoelectronic devices, such as the active layer of solar cells. Finally, ultrafast photophysical characterization of these polymers, using time-resolved photoluminescence and transient absorption, was performed to determine the aggregation types present in suspended fibrils and monitor the formation and decay of charged species in fibrils and donor-acceptor systems

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

The field of organic electronics has received considerable attention over the past several years due to the promise of novel electronic materials that are cheap, flexible and light weight. While some devices based on organic materials have already emerged on the market (e.g. organic light emitting diodes), a deeper understanding of the excited states within the condensed phase is necessary both to improve current commercial products and to develop new materials for applications that are currently in the commercial pipeline (e.g. organic photovoltaics, wearable displays, and field effect transistors). To this end, a model for pi-conjugated molecular aggregates and crystals is developed and analyzed. The model considers two types of electronic excitations, namely Frenkel and charge-transfer excitons, both of which play a prominent role in determining the nature of the excited states within tightly-packed organic systems. The former consist of an electron-hole pair bound to the same molecule while in the later the electron and hole are located on different molecules. The model also considers the important nuclear reorganization that occurs when the system switches between electronic states. This is achieved using a Holstein-style Hamiltonian that includes linear vibronic coupling of the electronic states to the nuclear motion associated with the high frequency vinyl-stretching and ring-breathing modes. Analysis of the model reveals spectroscopic signatures of charge-transfer mediated J- and H-aggregation in systems where the photophysical properties are determined primarily by charge-transfer interactions. Importantly, such signatures are found to be sensitive to the relative phase of the intermolecular electron and hole transfer integrals, and the relative energy of the Frenkel and charge-transfer states. When the charge-transfer integrals are in phase and the energy of the charge-transfer state is higher than the Frenkel state, the system exhibits J-aggregate characteristics including a positive band curvature, a red shifted main absorption peak, and an increase in the ratio of the first two vibronic peaks relative to the monomer. On the other hand, when the charge-transfer integrals are out of phase and the energy of the charge-transfer state is higher than the Frenkel state, the system exhibits H-aggregate characteristics including a negative band curvature, a blue shifted main absorption peak, and a decrease in the ratio of the first two vibronic peaks relative to the monomer. Notably, these signatures are consistent with those exhibited by Coulombically coupled J- and H-aggregates. Additional signatures of charge-transfer J- and H-aggregation are also discovered, the most notable of which is the appearance of a second absorption band when the charge-transfer integrals are in phase and the charge-transfer and Frenkel excitons are near resonance. In such instances, the peak-to-peak spacing is found to be proportional to the sum of the electron and hole transfer integrals. Further analysis of the charge-transfer interactions within the context of an effective Frenkel exciton coupling reveals that the charge-transfer interactions interfere directly with the intermolecular Coulombic coupling. The interference can be either constructive or destructive resulting in either enhanced or suppressed J- or H- aggregate behavior relative to what is expected based on Coulombic coupling alone. Such interferences result in four new aggregate types, namely HH-, HJ-, JH-, and JJ-aggregates, where the first letter indicates the nature of the Coulombic coupling and the second indicates the nature of the charge-transfer coupling. Vibronic signatures of such aggregates are developed and provide a means by which to rapidly screen materials for certain electronic characteristics. Notably, a large total (Coulombic plus charge-transfer) exciton coupling is associated with an absorption spectrum in which the ratio of the first two vibronic peaks deviates significantly from that of the unaggregated monomer. Hence, strongly coupled, high exciton mobility aggregates can be readily distinguished from low mobility aggregates by the ratio of their first two vibronic peaks. Analysis of the spatial dependence of the intermolecular interactions reveals that all four aggregate types (HH-, HJ-, JH-, JJ-) can be achieved by enforcing the appropriate crystalline packing arrangement. Such tunability is possible due of the different length scales over which the natures of the two coupling sources interconvert from J-like to H-like; whereas the nature of the Coulombic coupling is known to be sensitive to displacements on the order of half the molecular length, the nature of the charge-transfer mediated exciton coupling is sensitive to geometric displacements of approximately a carbon-carbon bond length. It is proposed that such sensitivity should allow for fine tuning of the total excitonic coupling via modifications in the packing structure, as determined, for example, by the side chains. Several examples of the different aggregate types are provided throughout this dissertation as the model is used to probe the excited state character of several relevant conjugated organic systems. Such examples include pentacene and 7,8,15,16-tetraazaterrylene (TAT) along with several derivatives from the perylene family. / Chemistry

The ability of the nucleic acids to absorb ultraviolet light and remain relatively photostable is a property upon which life depends. The nucleobases, which are the primary chromophores, when irradiated display rapid radiationless decay back to the ground state, in general faster than is needed for photoreaction. Fluorescent analogs of these bases have structures similar to the nucleic acid bases, but display much longer excited state lifetimes. Theoretical investigations using quantum mechanical methods can provide insight into the precise mechanisms of these decay processes, and to the molecular specifics that contribute to them. The results of multi-reference configuration interaction (MRCI) ab initio investigations into these mechanisms are presented, with emphasis on cytosine and its fluorescent analog 5-methyl-2-pyrimidinone (5M2P). A comprehensive picture of the potential energy surfaces of these two bases is given, including stationary points and conical intersections, where radiationless transitions are promoted, between up to three state surfaces, as well as pathways connecting these points for each base. Cytosine is shown to have two different energetically accessible radiationless decay channels. The fluorescence of 5M2P is also demonstrated theoretically, with mechanism proposed. The potential energy surfaces of the two bases have many close similarities, with the different photophysical properties being attributed to subtle energetic differences between the two bases. Nonadiabatic coupling and the geometric phase effect are analyzed in detail near conical intersections in cytosine, including in a region close to a three-state conical intersection. A substituent effect study on the 2-pyrimidinone ring system shows that the presence, position and orientation of the amino group in cytosine is central to its photophysical properties, particularly its high absorption energy, and can be explained with a simple Frontier Molecular Orbital model. The effects of water solvent on the excitation energies of cytosine and uracil are theoretically investigated using two multi-reference ab initio methods, a quantum mechanical molecular mechanics method using MRCI (MRCI-QM/MM), and the fragment molecular orbital multiconfiguration self-consistent field method (FMO-MCSCF). The solvatochromic shifts calculated from both methods agree well with other more expensive methods and experimental data. The effects of water on the photophysical pathways of cytosine is also investigated using MRCI-QM/MM, including considerations of solvent reorganization. Results show that the overall effect of water on the decay mechanisms is small, with neither decay channel being significantly blocked or favored. / Chemistry