Small organic molecules : building blocks of functional materials

The potential of phosphorescent organic light emitting diodes (phOLEDs) in full-color flat-panel display solid state lighting devices is fueling the interest in high triplet energy materials. 4,4′-Bis(9-carbazolyl)–biphenyl is one of the most widely used host material in phOLEDs. Because of its triplet energy of 2.56 eV, it is a suitable hole transporting material for green phosphorescent emitters, such as tris(2-phenylpyridine)iridium(III). However, the commonly used dye bis[(4,6-difluorophenyl)-pyridinato-N,C2]-picolinateiridium(III) in blue phOLEDs exhibits a triplet energy of 2.65 eV. Therefore, new high-triplet energy host materials are required to allow for efficient OLED-devices. The design of blue pixel host materials is based on the idea of reducing the level of conjugation between the carbazole subunit and the biphenyl backbone. Therefore, a series of CBP derivatives was synthesized and studied with spatially restricted degrees of freedom in their biphenyl-N-carbazole junctions by introducing spatial demanding methyl moieties either in positions 1 and 8 of the carbazole unit or in positions 3,5,3’,5’ of the biphenyl backbone. Moreover, the electronic features of the carbazole synthons were investigated by attaching electron-withdrawing or electron-donating groups in the positions 3 and 6 or positions 2 and 7 of the carbazole subunits

New Bipolar Organic Materials for Optoelectronic Applications

The literature surrounding organic small-molecule donor-acceptor systems is summarised for a range of optoelectronic applications (OLEDs, OPVs, OFETs etc.). There is a focus on the key building blocks: 1,3,4-oxadiazole (OXD), diphenylamine (DPA), carbazole (Cbz) and fluorene (F). The incorporation of such moieties into various donor-acceptor systems is discussed with further reference to selected alternative organic donor and acceptor systems. The syntheses of novel bipolar molecules based on a donor-spacer-acceptor (DPA/Cbz-F-OXD) structure and the incorporation of these molecules into single-layer OLEDs is presented. It is demonstrated how the emission colour can be tuned from green to deep blue by systematic manipulation of the structure. A significant result is that high efficiency accompanied with pure, deep blue emission in single-layer OLEDs can be achieved with this structural motif. The incorporation of these materials as part of a simple two-component blend to produce white OLEDs is presented and the modification of the materials to improve electron-transport properties is discussed. The synthesis of DPA-bridge-OXD wire systems is presented with the use of oligo-p-phenyleneethynylene units as a bridge of varying length to investigate the effect on charge transfer between the donor and acceptor. Photophysical studies demonstrate the change in absorption, emission and fluorescence lifetimes as the length scale of the molecules is altered. The synthesis of a series of planarised and twisted DPA-bridge-OXD systems based upon phenylene linkers is discussed. Finally, a series of DPA-F-OXD-anchor molecules is presented for incorporation into DSSC devices. The synthesis of these materials is described and the suitability of various anchoring groups for DSSCs is analysed through photophysical and device studies

Synthesis of Organic Semiconductors Based on Fused Heteroacenes for Optoelectronic Applications

Energy EngineeringOrganic semiconductors have attracted much interest for future electronic devices such as organic light emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic photovoltaics (OPVs), dye-sensitized solar cells (DSSCs) and so on due to many potential merits like light weight, low cost, flexibilty and solution processablity. Despite considerable studies for material science and device physics related to organic semiconductors, a better understanding of the efficient molecular design and systemical optimization is highly required for further development of organic electronics. So far, organic semiconductors based on rigid and π-conjugated molecular backbones that synthesized by powerful cross-coupling reaction methodologies have exhibited promising device performance. Among those, especially fused acenes or hetercyclic architecture has many advantages such as strong intermolecular interaction, increased coplanarity, reduced bandgap and good charge carrier transport. However, restricted synthetic methods hinder various modification of electronic properties of organic semiconductors with fused ring systems. In this work, synthesis of organic semiconductors based on such fused or cyclized heteroacenes and their optoelectronic applications are presented. Firstly, a thiophene-fused ladderized heteroacene dye is designed and synthesized via efficienct synthetic pathways. The fused material is used as a metal-free organic photosensitizer for DSSCs and the light harvesting property is characterized. Secondly, a nitrogen-fused cycle, carbazole-based semicondcutor PCDTBT, which is one of promising photoactive p-type polymer in organic bulk-heterojuction solar cells, is modified via incorporation of different electron-deficient units such as bisbenzothiadiazole, naphthothiadiazole and fluorinated benzothiadiazoles for systemical studies of structure-property relationship. The synthetic procedures and characterization of optical and electronic properties in OPVs are presented for each resulting PCDTBT derivative. Finally, a highly π-extended heteroacene, dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene (DTBDT), with α-positions availability is prepared via the synthesis of a key intermediate. The homopolymer and copolymers are synthesized for the first time and their properties are fully characterized. All polymers based on DTBDT are applied as p-type materials for OFETs.ope

Design and synthesis of next-generation organic semiconductors based on benzo[1,2-d:4,5-d′]bisoxazole

Benzobisazoles are a class of molecules that initially found their use in high-performance materials as high tensile strength fibers. Recent modifications to the syntheses of benzobisazoles have allowed for the materials to be studied as an n-type material to be used in organic semiconductors, more specifically organic light-emitting diodes (OLEDs). The high molecular stability required to produce blue light gives an opportunity for benzobisazoles to fulfill the requirement. Prior work on benzobisazoles, more specifically, the oxygen analog benzobisoxazole, has been used to try to achieve blue (<450 nm) but fell short in terms of efficiency due to molecular design choices. The following describes new design strategies such as utilizing single-bond linkage between the electron rich and deficient molecules, as well as transitioning from polymer to small molecules to fine-tune the properties of the materials for semiconductor applications. Utilizing a new design strategy, we demonstrate the ability to blue-shift the emission on two benzobisoxazole-based polymers by adopting single bond linkage between the benzobisoxazole and electron rich moieties fluorene and carbazole and achieve a usable brightness (> 1000 Cd/m2) when incorporated into OLEDs. With further modification of the benzobisoxazole core piece by adding dual conjugation along both axes to produce small molecules, we were able to achieve a deeper blue emission at higher efficiencies due to the reduced conjugation and aggregation than our previous systems experienced. Development of the small molecules led us to adopt a modular synthetic strategy for the high-efficiency material design of benzobisoxazole-based materials. In combination with Density Functional Theory calculations, we show the viability of performing computer-backed molecular design to develop materials to be used in all types of semiconductor applications. From calculations, we synthesize benzobisoxazole cruciforms that have both electron rich and electron deficient moieties. These products we then compared to experimental data to confirm the validity of computer-based rational design of molecules for not only blue OLEDs but for all semiconductor applications. The extremely high number of possible combinations of electron rich and electron deficient moieties allows for extensive future studies for the most optimal substituents for proper energy leveling tuning

Luminescent Cyclometalated Platinum and Palladium Complexes with Novel Photophysical Properties

abstract: Organic light emitting diodes (OLEDs) is a rapidly emerging technology based on organic thin film semiconductors. Recently, there has been substantial investment in their use in displays. In less than a decade, OLEDs have grown from a promising academic curiosity into a multi-billion dollar global industry. At the heart of an OLED are emissive molecules that generate light in response to electrical stimulation. Ideal emitters are efficient, compatible with existing materials, long lived, and produce light predominantly at useful wavelengths. Developing an understanding of the photophysical processes that dictate the luminescent properties of emissive materials is vital to their continued development. Chapter 1 and Chapter 2 provide an introduction to the topics presented and the laboratory methods used to explore them. Chapter 3 discusses a series of tridentate platinum complexes. A synthetic method utilizing microwave irradiation was explored, as well as a study of the effects ligand structure had on the excited state properties. Results and techniques developed in this endeavor were used as a foundation for the work undertaken in later chapters. Chapter 4 introduces a series of tetradentate platinum complexes that share a phenoxy-pyridyl (popy) motif. The new molecular design improved efficiency through increased rigidity and modification of the excited state properties. This class of platinum complexes were markedly more efficient than those presented in Chapter 3, and devices employing a green emitting complex of the series achieved nearly 100% electron-to-photon conversion efficiency in an OLED device. Chapter 5 adapts the ligand structure developed in Chapter 4 to palladium. The resulting complexes exceed reported efficiencies of palladium complexes by an order of magnitude. This chapter also provides the first report of a palladium complex as an emitter in an OLED device. Chapter 6 discusses the continuation of development efforts to include carbazolyl components in the ligand. These complexes possess interesting luminescent properties including ultra-narrow emission and metal assisted delayed fluorescence (MADF) emission.Dissertation/ThesisPh.D. Materials Science and Engineering 201

Synthetically Tailored Cyanines for Imaging and Therapy

This dissertation focuses on two fundamental yet unanswered questions in human health and disease concerning both diagnostics and therapeutics. The first half addresses the lack of optimized near-infrared fluorophores for image-guided surgery. Fluorescence imaging cameras and intraoperative imaging systems lack appropriately engineered contrast agents that allow the detection of sensitive normal tissue (i.e. endocrine and exocrine glands) for surgical avoidance. After systematically probing various fluorophore classes, we discovered promise in cyanine-based contrast agents. By altering functional groups of cyanine fluorophores, we, for the first time, report the selective targeting of many endocrine glands, exocrine glands, cartilage and bone using NIR fluorescence to visualize the targeted tissue. Secondly, this dissertation elaborates a new pathway for developing functional and fluorescent chemotherapeutics based on the targeting, stabilization and imaging of G-quadruplex DNA – a folding pattern commonly associated with cancer cell proliferation through telomerase over-expression and oncogene promoter regions. Chromophores modified to assume quadruplex binding characteristics – planar cationic structures – have been developed that stabilize the quadruplex as evaluated through several complementary methods toward developing novel theranostic agents in the NIR visualization and treatment of human cancers

MOLECULAR DESIGN OF ORGANIC SEMICONDUCTORS FOR INTERFACIAL AND EMISSIVE MATERIAL APPLICATIONS

This dissertation describes the synthesis and characterization of functional optoelectronically active materials. Synthetic techniques were used to prepare polymers containing perylene diimide (PDI) or tetraphenylethylene (TPE) moieties in the polymer backbone. PDI-based structures were prepared with embedded cationic or zwitterionic moieties intended to tailor organic/inorganic interfaces in thin film photovoltaic devices. The aggregation-induced emission (AIE)-active TPE polymers were synthesized to study how AIE properties evolve in π-conjugated polymers. The syntheses discussed here focused on modulation of molecular architecture to give rise to materials with tailored optoelectronic properties. Chapter 1 provides a brief overview of the field of organic electronics and the key concepts underpinning this thesis research. Chapter 2 describes the synthesis of PDI-containing polyionenes and linear polymer zwitterions. Dual-functional PDI monomers containing tertiary amines at the imide position, and bulky bromide or phenyl groups at the aromatic core, afforded reactive and high solubility monomers. Polymers with ammonium bromide and sulfobetaine functionality in the backbone were prepared by reacting PDI monomers with the appropriate electrophiles. By controlling PDI content, macromolecular structures with tunable PDI-PDI interactions were achieved and studied spectroscopically. Chapter 3 focuses on integration of novel PDI-based materials into organic and perovskite solar cells as interfacial layers. The interfacial properties, morphology, and device enhancement were studied as a function of PDI incorporation in the polymer backbone. Trends in electronic properties and device performance were correlated to polymer structure and revealed a strong dependence on the selection of cationic vs. zwitterionic functionality. The PDI-containing polymers were found to enhance photovoltaic device performance, despite not being continuous conjugated structures, but rather having conjugated molecular segments in the polymer backbone. The effective work function modification of metal cathodes and energy level overlap with perovskite active layer permitted enhanced device performance when tertiary amine-functionalized PDI small molecules were incorporated into devices. Chapter 4 centers on the synthesis and characterization of conjugated polymers containing TPE. The optical properties of these materials were adjusted by controlling extent of vinylene groups in the polymer backbone. These vinylene-containing TPE polymers exhibited similar optical and electronic properties to poly(phenylene vinylene) (PPV) while maintaining the desirable AIE properties of TPE. Moreover, by controlling the mole percent of TPE in PPV copolymers aggregation-caused quenching (ACQ) was attenuated without perturbation of PPV’s optical properties. Finally, Chapter 5 projects an outlook of the discussed research. Emphasis is given to where research focus should be oriented to advance the technology beyond the academic space. The aim of this chapter is to highlight the impact of this work and its relationship to bringing the science closer to the general public. The experimental procedures of the materials synthesis, characterization, and device fabrication are then detailed in Chapter 6

Luminescent materials for organic light-emitting diodes (OLEDs) and bioimaging

Ph.DDOCTOR OF PHILOSOPH

Low-Molecular Weight Molecules as Selective Contacts for Perovskite Solar Cells

La tecnologia fotovoltaica és una de les fonts d'energia neta i renovable més prometedores per reduir l'impacte ambiental dels combustibles fòssils en les últimes dècades. en aquest context, les perovskites són un material que ha atret recentment una atenció important a causa de la seva capacitat per aconseguir eficiències de conversió molt elevades. Les capes de càrrega selectiva juguen un paper crucial en el ràpid augment del rendiment del dispositiu i en l'estabilitat de les cel·les solars de perovskita. Recentment, l'aplicació de mono-capes auto-assemblades formades per molècules orgàniques que funcionen com a capes selectives de càrrega en cel·les solars de perovskita ha atret una gran atenció a causa d'avantatges com la rendibilitat, l'estabilitat i l'absència d'additius. L'objectiu d'aquesta tesi és el disseny i la síntesi de noves molècules que formen mono-capes auto-assemblades que funcionin com a capes selectives de forats en cel·les solars de perovskita per aconseguir una eficiència de conversió d'alta d'energia i una vida d'envelliment d'alt rendiment feta a mida.La tecnología fotovoltaica es una de las fuentes de energía limpia y renovable más prometedoras para reducir el impacto ambiental de los combustibles fósiles en las últimas décadas. en este contexto, las *perovskites son un material que ha atraído recientemente una atención importante a causa de su capacidad para conseguir eficiencias de conversión muy elevadas. Las capas de carga selectiva juegan un papel crucial en el rápido aumento del rendimiento del dispositivo y en la estabilidad de las celdas solares de *perovskita. Recientemente, la aplicación de *mono-capes auto-asemejadas formadas por moléculas orgánicas que funcionan como capas selectivas de carga en celdas solares de *perovskita ha atraído una gran atención a causa de ventajas como la rentabilidad, la estabilidad y la ausencia de aditivos. El objetivo de esta tesis es el diseño y la síntesis de nuevas moléculas que forman *mono-capes auto-asemejadas que funcionen como capas selectivas de agujeros en celdas solares de *perovskita para conseguir una eficiencia de conversión de alta de energía y una vida de envejecimiento de alto rendimiento hecha a medida.Photovoltaic technology is one of the most promising clean and renewable energy sources to reduce the environmental impact of fossil fuels in recent decades. In this context, perovskites are a material that has recently attracted significant attention due to their ability to achieve very high conversion efficiencys. Selective charge layers play a crucial role in rapidly increasing device performance and in the stability of perovskite solar cells. Recently, the application of self-assembly mono-caps made up of organic molecules that function as selective layers of charge in solar perovskite cells has attracted great attention due to advantages such as profitability, stability and the absence of additives. The goal of this thesis is the design and synthesis of new molecules that form self-assembly mono-layers that function as selective layers of holes in solar perovskite cells to achieve high-energy conversion efficiency and a high-performance aging life tailored to size