Tunable emission of polymer light emitting diodes bearing green-emitting Ir(III) complexes: The structural role of 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-carbazole ligands

doi:10.1016/j.dyepig.2009.10.017

Dyes and Pigments

Volume 85, Issue 3, May 2010, Pages 143-151

https://doi.org/10.1016/j.dyepig.2009.10.017 Get rights and content

Abstract

9-((6-Phenylpyridin-3-yl)methyl)-9H-carbazole and 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-carbazole were synthesized as ligands by attaching a carbazolyl group to the pyridine in 2-phenylpyridine and 2-(4-fluorophenyl)pyridine, respectively. Four different Ir(III) complexes were prepared using a simple procedure, the solubility of which was significantly greater than that of conventional green-emitting Ir(ppy)₃. Among the many devices fabricated, that which contained 10% of Ir(Cz-ppy)₁(Cz-Fppy)₂ in PVK/PBD (70:30 wt%), exhibited an external quantum efficiency of 6.80%, luminous efficiency of 20.23 cd/A, and maximum brightness of 17520 cd/m². In particular, the electronic property of the new 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-carbazole ligand can manipulate the triplet energy level of the Ir(III) complex to finely tune the emission spectra.

Introduction

Of the various types of light-emitting organic materials available, phosphorescent materials have been recognized as superior candidates because both singlet and triplet excitons can generate unique light emissions, with a theoretical internal quantum efficiency of 100% [1], [2], [3]. In particular, cyclometalated Ir(III) complexes show high phosphorescent efficiencies and are one of the most promising classes of phosphorescent dyes used in organic light-emitting diodes (OLEDs) [4], [5], [6], [7], [8], [9], [10], [11]. Along with advancements in processing solutions for the fabrication of OLEDs, novel, highly soluble materials displaying phosphorescence have been reported recently [12], [13], [14]. An extensively used technique for enhancing the solubility of Ir(III) complexes is via the attachment of lengthy alkyl moieties or their introduction into dendrimers [15], [16], [17], [18]. A dendritic architecture improves both solubility and the site-isolation effect by employing various peripheral groups around the emission center [19], [20].

In addition to the above methods, a very promising Ir(III) complex containing the tetraphenylsilyl group has been reported to exhibit excellent luminous efficiency using a conventional polymer-based [poly(N-vinyl carbazole), PVK] OLED. The design and synthesis of a highly phosphorescent tris-cyclometallated homoleptic Ir(III) complex [Ir(TPSppy)₃] (TPSppy = 2-(4′-(triphenylsilyl)biphenyl-3-yl)pyridine) with a silane-based dendritic substituent have been demonstrated. It has also been suggested that aryl silane was quite effective at emitting UV light for improving device efficiency compared to a long alkyl substituent [21].

Liu et al. reported PVK based light-emitting diodes fabricated with pinene-substituted Ir(III) phosphorescent dopants; three different Ir(III) complex dyes were demonstrated for which a photoluminescent spectral shift was observed with ligand modification. In addition, the pinene substitution induced steric hindrance to molecular structure of the dopant that aided suppression of triplet–triplet annihilation between Ir(III) dyes and consequently improved device performance [22]. These findings suggested that by substituting an electron withdrawing fluorine atom within the ligand, the HOMO energy levels could be tuned, thereby increasing the bandgap energy.

The objective of this work was to explore a method for tuning emission color by combining novel ligand design and good solubility and miscibility of the host polymer. This paper deals with three different highly soluble Ir(III) complexes bearing the 9-((6-phenylpyridin-3-yl)methyl)-9H-carbazole and 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-carbazole ligands. Green electrophosphorescence polymer light-emitting diodes (PLEDs) were fabricated by doping Ir(Cz-ppy)₂(Cz-Fppy)₁, Ir(Cz-ppy)₁(Cz-Fppy)₂, or Ir(Cz-Fppy)₃ into PVK blended with 5-4-tert-butylphenyl-1,3,4-oxadiazole (PBD), an electron transport molecule. For reference, the Ir(Cz-ppy)₃ was previously reported as being employed to compare the performances of photoluminescence (PL) spectrum and electrophosphorescent devices fabricated under identical conditions [23]. Two different ligands were systematically tethered to form a complex for investigating the effect of an electron withdrawing substituent in the ligand on the photophysical and electrophosphorescent properties.

Section snippets

Materials

All commercially available starting materials and solvents were purchased from Aldrich, TCI, and ACROS Co. and used without further purification unless otherwise stated. HPLC grade dimethylformamide (DMF) and methylene chloride (MC) were purchased from Samchun chemical and distilled from CaH₂ immediately before use. All reactions were performed under an argon atmosphere unless otherwise stated. 9-(6-Chloro-pyridin-3-ylmethyl)-9H-carbazole (1), 9-(6-phenyl-pyridin-3-ylmethyl)-9H-carbazole (3a),

Materials synthesis

We demonstrated the synthetic route to the Ir(III) complex Ir(Cz-ppy)₃, which contains a carbazolyl substituent, in a previous report [23]. In this study, the 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-carbazole ligand was newly synthesized for preparing the heteroleptic and homoleptic Ir(III) complexes (see Fig. 1, Fig. 2, Fig. 3). For a new ligand, 9-((6-chloropyridin-3-yl)methyl)-9H-carbazole and 4-fluorophenylboronic acid were reacted via the Suzuki coupling method to yield

Conclusions

The new soluble iridium complexes, Ir(Cz-ppy)₂(Cz-Fppy)₁, Ir(Cz-ppy)₁(Cz-Fppy)₂, and Ir(Cz-Fppy)₃ were successfully synthesized, and their electrophosphorescence device performances were evaluated using a PVK:PBD blend host. The Ir(III) complexes had good solubility over Ir(ppy)₃, thus possessing greater processability.

It was intriguing that introducing one 3b instead of 3a yielded a hypsochromic shift of 5 nm in the PL spectrum of Ir(Cz-ppy)₂ (Cz-Fppy)₁, relative to that of the green-emitting

Acknowledgments

This research work was supported by LG display (2009–2010). Particularly, Prof. D. H. Choi thanks the financial support by the Seoul R&BD Program (2008–2009), and second stage of the Brain Korea 21 Project in 2009 (Korea Research Foundation)

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Tunable emission of polymer light emitting diodes bearing green-emitting Ir(III) complexes: The structural role of 9-((6-(4-fluorophenyl)pyridin-3-yl)methyl)-9H-carbazole ligands

Abstract

Introduction

Section snippets

Materials

Materials synthesis

Conclusions

Acknowledgments

Synthetic Metals

Tetrahedron

Organic Electronics

Highly efficient OLEDs with phosphorescent materials

Nearly 100% internal phosphorescence efficiency in an organic light-emitting device

Journal of Applied Physics

Highly efficient phosphorescent emission from organic electroluminescent devices

Nature

High quantum efficiency in organic light-emitting devices with iridium-complex as a triplet emissive center

Japanese Journal of Applied Physics

Amorphous iridium complexes for electrophosphorescent light emitting devices

Chemical Communications

New iridium complexes as highly efficient orange-red emitters in organic light-emitting diodes

Advanced Materials

Tuning the emission of cyclometalated iridium complexes by simple ligand modification

Journal of Materials Chemistry

High-efficiency polymer-based electrophosphorescent devices

Advanced Materials

Highly efficient phosphorescence from organic light-emitting devices with an exciton-block layer

Applied Physics Letters

Tris-cyclometalated iridium(III) complexes of carbazole(fluorenyl)pyridine ligands: synthesis, redox and photophysical properties, and electrophosphorescent light-emitting diodes

Chemistry-A European Journal

High-efficiency endothermic energy transfer in polymeric light-emitting devices based on cyclometalated Ir complexes

Applied Physics Letters

Solution-processable red phosphorescent dendrimers for light-emitting device applications

Advanced Materials