Elsevier

Journal of Luminescence

Volume 145, January 2014, Pages 259-262
Journal of Luminescence

Dual emitter IrQ(ppy)2 for OLED applications: Synthesis and spectroscopic analysis

https://doi.org/10.1016/j.jlumin.2013.07.067Get rights and content

Highlights

  • Mixed-ligand of IrQ(ppy)2 synthesis which gives green and red phosphorescence due to the MCLT processes coming from two types of ligands.

  • Absorption, photoluminescence, infrared spectroscopy and cathodoluminescence measurements for characterization of IrQ(ppy)2 organometallic compound.

  • Experimental results have been compared with the output files obtained from Density Functional Theory by using the Gaussian 03W software.

Abstract

The synthesis of organometallic compound with iridium and two types of ligands, quinoline and phenylpyridine, was done successfully. The absorption spectra of this compound have shown broad peaks in a visible region assigned to metal-to-ligands charge transfer and in UV region assigned to intraligand absorptions. The photoluminescence spectra exhibit dual character in which the red emission is more intense than the green one. In cathodoluminescence measurements, under electron beam, the powder obtained after recrystallization from dichloromethane, shows similar behaviors with photoluminescence spectra. The cathodoluminescence images have shown a luminescent crystalline powder with triclinic structure. This compound exhibits combined vibrational modes, which proves the presence in the same molecule of both ligands. Density Functional Theory calculation was involved in order to identify the main vibrations of this compound.

Introduction

The new OLED technologies involve a thin organometallic layer in a sandwich structure, between electrodes [1], [2], [3]. This active monolayer is usually obtained by dispersion of organometallic compound into conducting and transparent thin film-polymer. The phosphorescence of organometallic compound is generally given by a single color, but the using of a double-emission layer can give two colors [4]. To obtain different colors from the same thin layer, mixtures of organometallic compounds with specific phosphorescence are necessary, either coming from metal-to-ligand charge transfer processes or from the host material [5]. Another way, is to create mixed-ligands of Ir(III) complexes which give two phosphorescent colors, like IrQ(ppy)2.

Ir(III) complex (8-hydroxyquinolinat bis(2-phenylpyridyl) iridium IrQ(ppy)2 combines the red emission which comes from quinoline (Q) ligand and the green one, originated from phenylpyridine. To produce mixed-ligands based on Ir3+ metal, each type of ligand requires one step reaction.

The aim of this paper is focused on the synthesis of this compound, followed by the spectroscopic analysis (absorption in solution, photoluminescence and vibrational spectroscopy) and some structural aspects of IrQ(ppy)2. Several attempts were previously made in order to obtain dual phosphorescence in which either the compound gives only the red color [6] or both colors with different ratio between the green and red color [7], [8], [9]. In fact, the best way to produce a dual emission is the improving of synthesis by adding some catalysts in the second step of reaction. In our case, Na2CO3 increases the rate of the chemical reaction by reducing the activation energy, remaining unchanged at the end of reaction.

Density Functional Theory (DFT) was involved in optimizing structures of the intermediate and the final compounds obtained in the two step reaction, by using Gaussian 03W software, and to identify the main vibrations of the two compounds [10]. The calculated vibrational properties are in a good agreement with the chosen models for the intermediate and the final compounds.

The spectroscopic analyses were combined with some modern structural methods like cathodoluminescence images and spectrum, in order to compare the photoluminescence with the luminescence excited with electrons.

Section snippets

IrQ(ppy)2 synthesis

To synthesize IrQ(ppy)2 compound, a classical two step reaction procedure was used [6], [8]. In the first step, iridium chloride hexahydrate reacts with phenylpiridine, forming a [(C^N)2Ir–µ–Cl]2 bridged dimer, further called intermediate compound. The yellow precipitate was cooled and washed with ethanol and then dried in vacuum. The chemical yield varies between 75–80%. The first step of synthesis follows the chemical reaction:

In the second step, the reaction of the intermediate compound with

Results and discussions

The intermediate compound consists from a bimolecular structure in which two Ir(ppy)2 molecules are weakly bounded (bridging) with two chlorine atoms. In order to clarify the correspondence between the theoretical model of the intermediate compound and the experimental obtained powder, the FTIR spectrum of the [Ir(ppy)2–Cl]2 was compared with the calculated frequencies for this bimolecular structure, obtained with Gaussian 03W (Fig. 1). The main vibrations are 3058 (C=C), 1605, 1581, 1476,

Conclusions

The two step synthesis produces an iridium organometallic compound with different ligands: phenylpyridine and quinoline. In the first step, a yellow powder with iridium bimolecular structure was obtained and used in the second step of reaction. The chemical yield was between 75% and 80% for the first step and 55–60% in the second step of reaction. An important role is played by the Na2CO3 catalyzer which increases the rate of the chemical reaction by reducing the activation energy and allows

Acknowledgments

This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS-UEFISCDI, Project number PN-II-ID-PCE-2011-3-0620. The work has been funded by the Sectorial Operational Program Human Resources Development 2007–2013 of the Romanian Ministry of Labor, Family and Social Protection through the Financial Agreement POSDRU/107/1.5/S/76903.

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