Synthesis, characterization and application studies of some phosphorescent charged iridium (III) and platinum (II) complexes

Abstract

Transition metal complexes are poised to realize high performance in diverse applications including light-emitting devices, chemical sensing and bioimaging. Within several decades, the development of transition metal complexes with heavy atoms such as Ir(III), Pt(II) and Os(II) complexes, has been promoted by revolutionary approaches, and systematic investigations have led to various optical properties. However, both blue and red luminophores are critical components for red-green-blue displays and white lighting devices, but to date both kinds of luminophores still suffer from low stability and low PLQY (photoluminescence quantum yield). This thesis aims at understanding the physical background of these issues, and providing potential solutions. To begin with, a brief introduction is organized in the following manner. A discussion of the triplet harvesting process leading to high efficiency of triplet emitters, followed by typical energy transitions which are important for organo-transition metal complexes. Thus, color-tuning strategies could be applied to modify the energy transfer process, leading to diverse ionic Ir(III) complexes with different optical properties. Some potential applications based on Ir(III) and Pt(II) complexes are introduced. Chapter 2 lists the chemicals and the instrumentation methods applied in this thesis. In Chapter 3, four red-emissive cationic Ir(III) complexes with one green-emissive anionic Ir(III) complex were designed and investigated, and their electrochemical and photophysical studies have been carried out. The molecular structure of one complex is confirmed by the single crystal structural analysis. Moreover, based on the optimized theoretical basis, The HOMO and LUMO energy level of the Ir(III) complexes were identified and these corresponding energy levels are found to be suitable for hydrogen generation. By applying these ionic complexes as the photosensitizer, the hydrogen generation ability has been investigated with/without the engagement of Pt-cocatalyst. In Chapter 4, cationic Ir(III) complexes bearing non-conjugated ancillary ligands and their photoluminescence properties were investigated. Three ancillary ligands with chalcogen atoms were chosen, as the outermost electrons of oxygen, sulfur or selenium would tune the energy level in varying degrees. Furthermore, six new Ir(III)-based cationic complexes possessing btq and dFppy (where btq is 2-(benzo[b]thiophen-2-yl)quinolone and dFppy is 2,4-difluorophenylpyridine) cyclometalated ligands combined with the non-conjugated ancillary ligands were synthesized, and their electrochemical and photophysical studies have been carried out, together with the investigation of the crystal structure and DFT calculations. Based on those theoretical basis, it is confirmed that the HOMOs could be influenced by chalcogen atoms on the non-conjugated ancillary ligands. Meanwhile, the singlet oxygen generation ability and the light stability of these complexes were investigated. In particular, high cytotoxicity to cancer cells, as well as their strong luminescence efficiencies in buffer solution, make them to be promising candidates for anti-cancer and bioimaging agents. In Chapter 5, initial attempts of studying Ir(III)-based micro-structures via self-assembly were made. In order to achieve fine architectures with regular size, molecular structures of two cationic Ir(III)-based complexes were determined by the single crystal X-ray diffraction, and their properties were fully characterized by UV-vis, emission, NMR and cyclic voltammetry studies. Finally, self-assembled 1D and 2D micro-structures were obtained on the basis of two metallophosphors. One-dimensional micro-ribbons, micro-rods and 2D micro-sheets were obtained by facile modification on the proportion of solvents. The diverse nanostructures were confirmed by microscopy images and XDR patterns. This method will provide a new strategy based on the manipulation of model solvent to guide the self-assembly. To obtain more insights of the nature of anionic complexes, a collection of anionic Pt(II) complexes were introduced in Chapter 6 and, single crystals for three complexes suitable for X-ray analysis were obtained to confirm the molecular structures. In the solution state, they exhibit the emission color ranging from green to blue with PLQY up to 1.25%. Notably, OLED devices of three complexes have been successfully fabricated, and this is the first time the solution-processed OLED devices based on anionic Pt(II) phosphors was obtained. The as-prepared OLEDs display green to blue phosphorescence with the maximum EQE of 3.7%. Although the device architecture can be future improved, these results reveal that the new series of anionic Pt(II) complexes are promising candidates for future development of high-performance electroluminescence devices.