Design, Synthesis and Applications of Bis-tridentate and Tris-bidentate Iridium (III) Phosphors


Student thesis: Doctoral Thesis

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Awarding Institution
Award date8 Mar 2022


Extensive research works on organo-iridium compounds have been carried out in the past two decades due to their promising potential in electrophosphorescent organic light-emitting diodes (OLEDs). Almost all of them were emphasized on the variation of skeletal arrangement or modification of substituents of the bidentate ligands to fine-turn the emission hue. In recent years, the rise of bis-tridentate iridium metal complexes, having well-established coordination bonding strength over the tris-bidentate counterparts, served as better and more efficient OLED emitters. This thesis reported the design, synthesis, and characterization of a series of tris-bidentate Ir(III) complexes and two series of bis-tridentate Ir(III) complexes, and their potential application in OLED fabrications.

Firstly, a series of tris-bidentate Ir(III) complexes bearing N-heterocyclic carbene (NHC) chelates were successfully designed and synthesized, to which the absorption and emission spectra were also measured. The employed complexes, namely, Ir(III) complexes m-timpz and f-timpz and m-t2impz and f-t2impz and m-t2empz and f-t2empz, exhibited blue (facile isomer) and green (meridional isomer) emission in degassed toluene with moderate to good photoluminescent quantum yield (46-74 %). Their emission peak wavelengths are mainly controlled by their tert-butyl and electronegative nitrogen atoms at the outer aromatic peripheral of the functional carbene moiety. Due to the electron-donating nature of the tert-butyl substituent on the cyclometalating phenyl group, m-t2impz and f-t2impz and f-t2empz give slight red-shifted emission in comparison to that of m-timpz and f-timpz. All complexes exhibited good thermal stability with decomposition temperature (Td) higher than 310 °C and, displayed promising properties for OLEDs applications.

Secondly, charge-neutral Ir(III) metal complexes bearing two tridentate chelates composed of two orthogonally aligned pyrazole-pyridine-pyrazole coordination fragments were designed and synthesized. It is notable that the substituents play an essential role in tuning both the photophysical and electrochemical properties. Upon introduction of CF3 substituents at one central pyridyl fragment and peripheral pyrazolate fragments of one and alternative tridentate chelate, phosphorescence can be fine-tuned from green to orange, together with an improved emission quantum yield, as recorded in degassed THF solution. Further replacement of N-H fragment of monoanionic chelate with a methyl substituent gives Ir(III) complexes [Ir(Bpz2pyMe)(Fpz2py)] (2Me) and [Ir(Bpz2FpyMe)(Fpz2py)] (3Me), showing decomposition temperatures (Td) greater than 300 °C. Therefore, this work paves a new avenue of better emissive iridium(III) complexes without the traditional cyclometalating aromatics, albeit with lowered emission efficiency in the solution state. As a proof-of-concept, Ir(III) complexes 2Me and 3Me were employed as the dopant emitters for OLED fabrication, giving a maximum external quantum efficiency (EQE) of 4.8% and 17.6%, and current efficiency of 14.22 and 50.33 cd·A-1, respectively.

Lastly, we designed a series of bis-tridentate Ir(III) metal complexes, comprising a dianionic pyrazole-pyridine-phenyl tridentate chelate and a monoanionic chelate bearing a peripheral carbene and carboline coordination fragment that linked to the central phenyl group. All these Ir(III) complexes were synthesized with an efficient one-pot and two-step methodology, and their emission hue was fine-tuned by variation of substituent at the central coordination entity (i.e. pyridinyl and phenyl group) of each of the tridentate chelates. Their photophysical and electrochemical properties, thermal stabilities and electroluminescence performances are examined and discussed comprehensively. The doped devices based on [Ir(cbF)(phyz1)] (Cb1) and [Ir(cbB)(phyz1)] (Cb4) give a maximum external quantum efficiency (current efficiency) of 16.6% (55.2 cd/A) and 13.9% (43.8 cd/A), respectively. The relatively high electroluminescence efficiencies indicate that bis-tridentate Ir(III) complexes are promising candidates for OLED applications.