Design Strategies and Synthesis of Thiophene-based Organic Materials

Abstract

Organic semiconductors have attracted researchers’ attention due to their fascinating electronic and photophysical properties. These types of materials have been incorporated into a wide variety of electronic devices since they are light-weight, solution processable, and they have tunable properties. To design conjugated materials for different applications, it is important to understand how chemical modification alters electronic and photophysical properties and several strategies have emerged as highly effective means to accomplish this goal. Incorporating electron-withdrawing substituents onto the periphery of π-systems can improve the oxidative stability ofthese organic structures while increasing electron affinity. The replacement of elements (particularly bridging elements within the π-system) also serves as an effective means to alter the electronic structure and influence packing in the material. Finally, ring fusion to afford polycyclic aromatic systems can alter frontier orbital energies and tune the band gap. These strategies have all been used in conjunction with cross-coupling to create a wide range of conjugated polymers. In this thesis, Chapter 1 provides general introduction of organic semiconductors; the chemical strategies for tuning the properties will be discussed. A controlled method to synthesize conjugated polymers – Catalyst-Transfer Polycondensation (CTP) – will also be introduced. In Chapter 2, an electron-poor thiophene derivative, 2,5-bis(tributylstannyl)thiophene 1,1-dioxide,was prepared and utilized in a series of Stille cross-coupling reactions to afford thiophene 1,1- dioxides with either electron-donating or electron-withdrawing substituents. It was found that thiophene 1,1-dioxides bearing electron-withdrawing groups undergo facile reductions. Chapter 3discusses the synthesis and characterization of electron-poor thiophene 1,1-dioxides bearing different number of cyanated phenyl groups. The electron-accepting nature of these compounds was evaluated by cyclic voltammetry, and highly reversible reductions were observed for several derivatives. Those dioxides were examined as electron relays in a photocatalytic water reduction reaction, and they showed potential to boost the efficiency. Chapter 4 highlights our work on the synthesis of periodic π-conjugated polymers based on group 16 heterocycles (furan, thiophene, and selenophene) with controlled chain lengths and low dispersities using Kumada Catalyst-Transfer Polycondensation (KCTP). The optical gap and redoxpotentials of these copolymers were fine-tuned by altering the heterocycle sequence, and atomic force microscopy revealed nanofibrillar morphologies for all the materials. In Chapter 5, we attempted to expand the monomer scope of CTP. The limited investigations of CTP with electrondeficientmonomers led us to explore polymerization of a popular electron-poor building block, benzotriazole, using variety of Ni and Pd catalysts. The effects of extra ligand addition and halogen substitution to improve chain-growth behavior were examined. Chapter 6 provides a short outlook of CTP.