Design, Synthesis and Electroluminescence Applications of Photostable Blue Bis-tridentate Iridium (III) Phosphors


Student thesis: Doctoral Thesis

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Award date24 Jun 2020


Blue emitter is of vital importance for organic light-emitting diodes (OLEDs). Ideal blue emitters should deliver desired color purity, high performance and good stability. Novel bis-tridentate Ir(III) complex is considered a promising choice to meet these requirements. In this thesis, four series of blue bis-tridentate Ir(III) phosphors were designed, synthesized, and applied in OLEDs.

First, in the chromatic ligand, O, N and C atom were deliberately inserted between the central pyridine and phenyl ring and thus broke the conjugation of the 6-pyrazolyl-2-phenylpyridine(pzpyph) ligand. Fused six/five-membered metallicycle structures adapted in the new complexes released distortions of the octahedral arrangement from bis-tridentate Ir(III) complex with conjugated dianionic chelate, making the bis-tridentate architecture more suitable for high energy emission. Emission colors of the complexes were tuned from green to sky blue and blue. Extending the conjugations with fused aromatic structure in HOMO areas increased the HOMO and LUMO separations of complexes and enhances photostabilities of the complexes. The small ∆ESTs induced by HOMO and LUMO separations enabled higher energy emission for similar energy gaps and faster phosphorescent rate simultaneously. OLEDs based on complex 4 and 5 showed decent performance with external quantum efficiency (EQE) of 19.44 and 18.79% respectively.

Second, by functionalizing the carbazolyl moiety with two trifluoromethyl groups at the 3- and 6-position, the emission of complex 4 in Chapter 2 was successfully tuned from greenish-blue to blue in Cz-1–Cz-3. The new bis-tridentate Ir(III) phosphors exhibit superior photophysical properties and high photostabilities versus both green and true-blue-emitting reference compounds, i.e., fac-[Ir(ppy)3] and mer-[Ir(pmp)3]. OLEDs based on Cz-2 exhibit EQE of 21.6%, EQE of 15.1% at 100 cd m-2, and with color index of (0.17, 0.25).

Third, complex 2 in Chapter 2 has deep blue emission, long radiative decay, and thus low photostability has modified in this part of work. Replacement of pyrazolate with triazolate in phenoxyl based dianioic ligands maintained high quantum yields (> 92%) in both the solution and solid states and, concurrently, reduced the radiative lifetime (τrad) from 19.8 to 2.5 µs. Theoretical calculation reveals that considerable contributions of triazolate moiety in their frontier orbitals and increased metal involvements in T1 → S0 transition of tazPx-based Ir(III) complexes are two main factors responsible for maintaining blue emission and shortened τrad. Moreover, enhanced photostabilities in degassed toluene and competitive OLEDs performance with maximum EQE of 19.2% and CIE coordinates of (0.17, 0.22) were observed for triazolate-based Ir(III) complex Px-33.

Last, by modifying a fluorene-based dianionic chelate with tert-butyl, methoxyl, pyrrolidinyl and triazolyl, five blue bis-tridentate Ir(III) phosphors were successfully designed and prepared. Emission of the new phosphors were gradually blue shifted from sky blue to deep blue with increasing of electron donating abilities of the substituent groups. Photophysical properties and theoretical study reveal widening the energy gap of bis-tridentate Ir(III) phosphors is accompanied with ∆EST enlarging and decreased MLCT contribution, which made the phosphorescent inefficient. The robust skeleton and efficient emission endowed complex Flu-1–4 with excellent photostabilities. A deep blue OLED was successfully fabricated based on Flu-4 and presented a decent performance of EQEmax = 22.3% with CIE coordinates of (0.17, 0.19), which is comparable to recently reported high performance deep blue phosphors.