Development of bimetallic emitters for organic light emitting diodes

Martinscroft, Ross (2019) Development of bimetallic emitters for organic light emitting diodes. Doctoral thesis, Northumbria University.

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Development of bimetallic emitters for organic light emitting diodes:

The chemical and photophysical properties of iridium (III) and platinum (II) complexes make them ideal candidates for dopants in organic light emitting diodes (OLEDs). To date, the majority of documented complexes have contained a single heavy metal atom. Only recently have multinuclear complexes emerged as efficient emitters with the addition of a secondary metal centre having shown to improve the photophysical properties of the complexes.

This project describes the preparation of novel complexes which incorporate iridium (III) and platinum (II) as central metal ions. The work is mainly focused on the synthesis of complexes rigidly linked by heterocycles such as pyrimidine and thiazolo-[5,4-d]-thiazole (TzTz). The use of these linking heterocycles and different auxiliary ligands led to novel complexes that displayed intense luminescence at room temperature.

Chapter 2 describes the adaptation of known dinuclear Ir(III) complexes for application in solution processable OLEDs. In particular the solubility of dinuclear iridium complexes with cyclometalated C^N ligands rigidly linked by pyrimidine was improved by the introduction of the hexamethylated auxiliary ligand (III) and the removal of the fluorine groups from the auxiliary ligand in the previously published complex, (I). The complex (II) shows intense red emission (λmax= 631 nm) with a high quantum yield and a short triplet state lifetime (ϕlum= 0.87, τ = 0.53 μs). The complex (II) was successfully used in solution processable OLEDs with external quantum efficiency of 8% was recorded and emission centred at λmax= 646 nm.

Chapter 3 describes the preparation of dianionic terdentate cyclometalating phenolate ligands and the associated complexes. These unprecedented phenolate terdentate assemblies have not previously been described in literature. The complexes (IV, V, VI, VII) showed encouraging photoluminescent properties. The mono-iridium complexes prepared showed quantum efficiencies in the range lum= 0.16-0.19 and short triplet state lifetimes in the range  = 0.46-0.58s. The work showed that phenolate coordination is considerably more favourable in comparison to picolinate auxiliary ligands. Several changes to the structure of the O^N^C ligands have been performed do not vastly alter the photophysical properties. Despite the encouraging results, our attempts to prepare dinuclear iridium complexes with O^N^C ligands were not successful.

Chapter 4 describes the preparation of symmetrical complexes as a strategy to remove chirality from the dinuclear structures. Two novel C^N^C cyclometalated di-iridium complexes were prepared with both showing intense luminescence at room temperature. A fluorine containing complex (VIII) and a tert-butylated complex (IX) were prepared showing quantum yields of ϕ= 0.89 and ϕ= 0.74 respectively. The synthetic procedure was optimised to achieve 44% yield. This complex was used in the fabrication of a device showing a peak external quantum efficiency of ηext= 19.7%.

Chapter 5 describes that changing the linking heterocycle from pyrimidine to thiazole-[5,4-d]-thiazole resulted in one of the first examples of a luminescent cyclometalated complex using TzTz as a linking group. Preliminary testing of the complex (X) prepared showed a peak emission wavelength of λmax= 675 nm, highlighting the effect that changing the heterocycle and increasing the π system has on the luminescent properties. Addition of a DMSO group to the dichlorobridged dimer (XI) also displayed luminescence - an abnormal observation in DMSO complexes.

Chapter 6 describes how the synthesis of two novel SALEN type platinum (II) complexes (XII, XIII) was investigated using 3,3’ and 5,5’-bis-1,2,4-triazines. It was found that the tertiary butyl had little effect on solubility and the position of the cyclopentene ring had a greater effect. A device was fabricated using the tert-butylated complex and showed a device efficiency of ηext= 10.4% and only required one emitter to produce the desired “candlelight” colour. The change of tertiary butyl to mesitylene was also investigated as a means of improving overall solubility.

Overall this research shows that although difficult to prepare, the bimetallic emitters are some of the best performing phosphorescent red emitters known to date. As a result show great potential for further development as dopants in OLEDs.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: luminescent metal complexes, irdium, complex synthesis, phosphorescent, terdentate auxiliary ligands and structures
Subjects: F100 Chemistry
F200 Materials Science
Department: Faculties > Health and Life Sciences > Applied Sciences
University Services > Graduate School > Doctor of Philosophy
Depositing User: John Coen
Date Deposited: 31 Mar 2022 07:33
Last Modified: 31 Mar 2022 08:01

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