Luminescent tetradentate platinum(II) complexes bearing asymmetric ligands with mixed oxygen, nitrogen and carbon donor atoms : design, syntheses, photophysical studies and applications

Abstract

Along the development of organic light-emitting diode (OLED) emitters, luminescent Pt(II) complexes have been proved to be a promising candidate in fabricating phosphorescent OLEDs (P-OLEDs). Good emitters for P-OLEDs are expected to be highly emissive and disfavour the triplet-triplet annihilation in the solid or thin film state. In this thesis, three types of Pt(II) complexes supported by tetradentate ligands, namely, O^N^C^N, N^C^N^N-, O^N^N^O and their derivatives are reported. The comprehensive syntheses, characterization and photophysical properties in different physical states are studied and discussed. Several complexes have been preliminarily applied as emitting materials in OLED. A tertiary amine linker was introduced to O^N^C^N ligand; the resulting N-(O^N^C^N) ligand enriches the 3D configuration of the ligand geometry which suppresses the intermolecular interaction. The structurally robust Pt(II) complexes supported by the N-(O^N^C^N) ligand exhibit high thermal stabilities with Td up to 412 oC, and yellow to red-orange emission (λmax = 511–605 nm) with desirable quantum yield up to 97% in degassed solution. X-ray crystallography reveals the distorted square planar geometry of the complexes without short Pt…Pt contact. Theoretical calculations and time-resolved spectroscopy indicated insignificant excited state distortion and efficient inter-system crossing. Yellow light emitting OLED based on the complex achieved maximum external quantum efficiency (EQE), current efficiency and power efficiency up to 26%, 100.0 cd A-1 and 118 lm W-1, respectively, which are comparable to the corresponding values of the best reported yellow platinum(II) OLEDs and iridium(III) OLEDs without out-coupling enhancement. A series of Pt(II) complexes supported by quinolinolato-related tetradentate (O^N^N^O) ligands were prepared. Two distinctive synthetic routes with high versatility on ligand design were developed. Focuses have been placed on tunability of emission energies upon sagacious design on ligands. Bathochromic shift and improved quantum efficiency were observed by the incorporation of an electron-donating substituent (λmax: from 582 to 655 nm) and enhancing the rigidity of the ligand framework (φem: from 0.01 to 0.63). Time-resolved photoluminescent measurements unveiled no notable excited state molecular interaction of the complexes which renders the complexes eligible to be emissive dopant materials for high colour purity in devices. Pyrazolate-based tetradentate N^C^N^N- organoplatinum(II) complexes featured by tertiary amine linker have been synthesized. The complexes bearing the rigid and strong chelation ligand show resounding thermal stability with Td up to 392 oC and high emission quantum yields up to 74%. The crystal structure of the isoquinoline-containing complex illustrated distorted square planar geometry and alternating staggered packing along one axis with short Pt···Pt distance was observed. Red, yellow and green emitting OLEDs doped with the complexes were prepared with maximum EQE up to 18.53% and power efficiency up to 59.42 lm W-1. Preliminary studies on cancer cells suggested the potential of the complexes to be an imaging agent with cellular luminescence and low cytotoxicity.