Materials science communication
Fabrication and characterization of OLED with Mg complex of 5-chloro-8-hydroxyquinoline as emission layer

https://doi.org/10.1016/j.matchemphys.2007.10.011Get rights and content

Abstract

Magnesium(II) ions were complexed with 5-chloro-8-hydroxyquinoline (L) to give an MgL2 chelate that emitted green fluorescent light in tetrahydrofuran (THF) solvent as well as in solid state when excited by UV light. The complex was found to produce electroluminescent (EL) and thus OLEDs were fabricated and characterized based on this green light emitting material. In these devices the hole transport layer was obtained using a thin film of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD), LiF as hole blocking layer and tri(8-hydroxyquinoline aluminum)(Alq3) as the electron transporting layer. The EL peak was at 539 nm with 19 nm red shift from PL peak. The EL devices fabricated had a maximum brightness of 425 cd m−2.

Introduction

Since organic light emitting devices (OLEDs) using 8-hydroxyquinoline aluminum (Alq3) as the emitting layer was reported to emit green light in 1987 [1], organic LEDs have received considerable attention due to their potential applications in full color, flat panel displays and other emissive devices [2], [3], [4], [5]. Recent years have witnessed significant progress with regard to low-voltage driving, high brightness, and multi- or full-color emission by the selection of emitting materials, easy fabrication of large area, thin-film devices, durability and thermal stability of OLEDs [6], [7], [8], [9].

The construction of reliable organic light emitting devices (OLEDs) for displays is a major challenge in the field of optoelectronics in which chemist and physicists are working together in the search for appropriate molecular materials [10]. Many OLEDs have been actively developed in recent years and now these devices are in the stage of practical application. Organic compounds and metal complexes have been playing an important role in the ongoing challenge to develop efficient solid-state electroluminescent materials [11], [12], [13], [14] to be processed as amorphous and homogenous thin films.

Detailed theoretical and experimental studies have been made to investigate the photo physical and morphological properties of 8-hydroxyquinoline-based species [15], [16], [17], [18]. The emission color and charge transporting properties of complexes can be tuned by varying the central metal ion or the ligand structure by introducing electron releasing or electron withdrawing groups in the basic 8-hydroxyquinoline. We synthesized a light emitting material based on a metal chelate of Mg with 5-chloro-8-hydroxyquinoline and present the electroluminescent (EL) properties of the metal-chelate complex in the OLEDs that showed emission of bright green color.

Section snippets

Synthesis of metal complex

The various chemicals used to synthesize metal complex were purchased from Fluka. Solvents were used as supplied. The magnesium complex was synthesized by method as shown in Scheme 1.

A solution of magnesium sulphate (1 mmol) in water was added to a solution of the ligand (2 mmol) in acetone and pH was adjusted to neutral. After stirring the mixture for 2 h at 60 °C on a magnetic stirrer, a crude product, which precipitated from the solution, was collected by filtration. The product was purified by

Characteristic of the complex

The 1H NMR spectral studies of the synthesized Mg complex showed that the peaks for aromatic hydrogens were present at 7.1 (s 1H), 7.5 (s 1H), 8.5 (s 2H) and 8.8 (s 1H). The results showed that the peaks had shifted significantly downfield against the 1H NMR peaks values of the ligand itself, taken from literature indicating the formation of the reported complex. The C, H, N analysis of the complex indicated the formula of the complex to be bis(5-chloro-8-hydroxyquinolinato)magnesium(II) (C9H5

Conclusion

An organic electroluminescent material, bis(5-chloro-8-hydroxyquinolinato)magnesium(II) was synthesized and characterized using different spectroscopic techniques. The excitation and emission spectra of the complex were studied in solution as well as in its solid state. The complex was employed as an emitting layer in fabricating OLEDs that had emission peak at 539 nm with a bright green color. The PL and EL spectra exhibited similar patterns except EL was red shifted by 19 nm from its PL peak

Acknowledgements

Anita Sharma and Devender Singh are thankful to the CSIR and the UGC, New Delhi for providing them the Junior Research Fellowships (award no. 99/382 (96)/2k2/EMR-I) and (project no. 12-5/2003 (SR)), respectively.

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