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Engineering charge transfer characteristics in donor- acceptor cyclometalated gold(III) and platinum(II) complexes: synthesis, photophysical investigations and device properties
Organic light-emitting diodes (OLEDs) have been gaining a lot of attention from both academia and industry, are now employed in full color flat-panel displays.1 OLEDs, are thin electronic devices that upon applying bias produce photons via radiative recombination of electron-hole pairs through an excited state of a molecular emitter material. This material is typically embedded in a suitable host matrix to prevent aggregation and, as a result, quenching.2 In addition to displays, OLEDs can also have a number of applications in lighting, and other optoelectronic devices due to their high quantum efficiency in electroluminescence (EL). OLEDs are considered a top contender for use in future lighting technology.3 Efficiency, driving voltage, lifetime, and colour are the four primary metrics that characterise an OLED's performance.4 A colorimetric system developed by the commission internationale de l’eclairage (CIE) can be used to characterise the colour of the emitted electroluminescence. For OLEDs to become a reality, a significant forward step has been made in the design and synthesis of high-performance organometallic complexes using phosphorescence or thermally activated delayed fluorescence (TADF), that can harvest all excitons for light emission to realise an internal quantum efficiency of unity.5, 6 As a result of their superior photophysical properties, organometallic complexes based on platinum(II) and iridium(III) are much sought after for the development of efficient light emitters for device applications. 7, 8 Compared to iridium(III) and platinum(II) complexes, which have been extensively investigated, gold(III) complexes for TADF emission have been understudied and have yet to find commercial applications. In addition, the different charge transfer nature of gold(III) complexes and transition metal complexes bearing persistent radicals have been only rarely documented. The primary objective of this thesis is to build innovative TADF emitters based on gold(III) and platinum(II) complex bearing a persistent radical with intriguing light emission and charge transfer capabilities. To investigate their light emission properties and prospective uses in optoelectronic devices for lighting technologies, detailed experimental studies supported by computational calculations as well as the construction of devices were employed. Chapter 1 describes an overview of TADF properties of existing gold(III) emitters based on alkyne, aryl and tetradentates, including their synthesis, DFT calculations and device properties. Key strategies employed and molecular attributes to achieve TADF is highlighted. In chapter 2, we present the tuning of charge-transfer excited states of luminescent cyclometalated (C^C^N) gold(III) amide complexes their synthesis, photophysical properties and devices and also correlate experimental findings with computational calculations. In order to achieve blue/ deep blue emission, highly stable, unique and unknown monocyclometalated gold(III) amide complexes with charge transfer characteristics are presented in chapter 3. The monocyclometalated framework provides flexibility with the introduction of ancillary ligands. The alkyne ligand provides the molecule with the key emission characteristics and the additional stability. The photophysical, electrochemical and computational calculations are discussed for this unique class of gold(III) complexes. . Monocyclometalated compounds containing two electron-donating units effectively improve the charge transfer properties and resulted TADF type emission. In chapter 4, C^N^C cyclometalated gold(III) complexes having a new ligand scaffold along with alkyne and aryl ancillary ligands is presented. The synthesis, photophysical and electrochemical properties is discussed in detail. In chapter 5, we report the synthesis, characterization and the photophysical properties of a stable Pt(II) complex bearing a persisting radical as the ancillary ligand. The complex displays room temperature emission with strong doublet character.
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