Red to near-infrared organometallic phosphorescent dyes for OLED applications

doi:10.1016/j.jorganchem.2013.09.035

Journal of Organometallic Chemistry

Volume 751, 1 February 2014, Pages 261-285

Dedicated to the 50th anniversary of Journal of Organometallic Chemistry

https://doi.org/10.1016/j.jorganchem.2013.09.035 Get rights and content

Highlights

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A survey of red to near-infrared phosphorescent dyes was provided.
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The structure–property–activity relationships of these dyes were discussed.
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The phosphorescent OLED applications of these red dopants were described.

Abstract

In modern research on organic light-emitting diodes (OLEDs), cyclometalated iridium(III) complexes represent one of the most studied class of compounds. The high emission efficiency caused by the strong spin-orbit coupling in the presence of heavy metals leads to the mixing of singlet and triplet manifolds so that both the singlet and triplet excitons can be harvested. For OLEDs to be useful in displays application, true red, green, and blue emissions of sufficient luminous efficiencies and proper chromaticity are required. In recent years, the development of materials for phosphorescent red OLEDs has indeed gone through several important evolutional stages. However, the luminescent quantum yields of red-emitting iridium(III) phosphors tend to be intrinsically low which are governed by the energy gap law for triplet states in which the luminescence quantum yields tend to decrease with an increase in the emission wavelength. Many red organic dyes currently in use do not show a good compromise between device efficiency and color purity. In general, a dilemma facing red OLEDs was realized in which efficient and bright dopants are not red enough, and red-enough dopants are not efficient and bright. In this review article, we highlight the recent progress and current challenges of efficient OLEDs based on cyclometalated iridium(III) dyes which exhibit saturated red and near-infrared electroluminescence. Optimization of the phosphorescent red OLED efficiency/color purity trade-off and extension of the work to other organometallic phosphors are also presented and discussed.

Graphical abstract

An overview of the recent progress in the molecular design, synthesis and OLED applications of red phosphorescent dyes is presented in this review article.

Introduction

Since the first commercialization of organic light-emitting diodes (OLEDs) in 1997 by the pioneering company in Japan, OLEDs are considered to be the next generation of flat panel displays [1]. Some advantages of OLEDs which make them perfect candidates to replace the widely used liquid crystal displays and plasma display panels include high-efficiency, low-voltage, full color and ease of fabrication as large area flat panel displays in electronic devices [2]. OLEDs based on phosphorescent transition-metal complexes are attracting significant attention since they can greatly improve electroluminescence (EL) performance as compared with the conventional fluorescent OLEDs [1]. According to spin statistics, EL from small molecular fluorophors cannot exceed a maximum quantum yield of 25%, but in phosphorescent complexes, their EL can theoretically achieve quantum yields up to 100% since both triplet and singlet excitons can be harvested for the emission [3]. Among all the phosphors, cyclometalated iridium(III) complexes are acquiring the mainstream position in the field of organic displays because of their highly efficient emission properties, relatively short excited state lifetime and excellent color tunability over the entire visible spectrum [4]. Besides, other metal ions, such as Pt(II), Os(III) and Zn(II) can also be introduced into the emitter molecules to give efficient luminescence. Their developments are supported by the broad diversity of the possible structures surrounding around the metal ions. In particular, extensive research efforts have been made recently for gaining more achievements in phosphorescent white OLEDs as they can be used for the next generation solid-state lighting. White emission can be achieved by mixing three primary colors (red, green and blue) or two colors from an orange emitter complemented with a blue emitter [5]. To achieve highly efficient white OLEDs, there is a great demand for efficient and bright true red color phosphors.

As compared to other colors, the design and synthesis of efficient red emitters is intrinsically more difficult, which is in accordance with the energy gap law [6], [7]. Many red emitters suffer from poor compromise between device efficiency and color purity. The lower luminosity of a red device is due to its characteristic red emission in a spectral region where the eye has poor sensitivity. Moreover, the wide bandgap host used in OLED device and narrow bandgap red-emitting guest have a significant difference in the HOMO and/or LUMO levels between the guest and host materials. Thus, the guest molecules are thought to act as deep traps for electrons and holes in the emitting layer, causing an increase in the driving voltage of the device. Furthermore, self-quenching or triplet–triplet annihilation for red dopant molecules is an inevitable problem in such host-guest systems especially at high doping concentrations. Therefore, from a practical standpoint, a solution to the above issues based on materials design or/and device optimization is highly desirable.

This review will comprehensively survey new red organometallic materials that have been used in phosphorescent OLEDs (PHOLEDs) in the past few years. Besides, since development of compounds with emission in the near-infrared (NIR) wavelength window (>700 nm) is rapidly emerging as an important area in a variety of biological and biomedical [8], [9], [10], telecommunications [11] and defense applications [12], a general overview of some molecular design strategies towards the various types of NIR-emitting metal complexes is also given. Their structure–activity relationship, photophysical and electroluminescence properties will also be discussed.

Section snippets

Cyclometalating ligands derived from 1-phenylisoquinoline

Although a number of red phosphors have been synthesized, isoquinoline-type Ir(III) complexes, particularly those with 1-phenylisoquinoline derivatives, are still the most studied one. Quinoline/isoquinoline-based compounds have received much attention due to their high electron affinities [13], [14]. The molecular design of red phosphorescent complexes with 1-phenylisoquinoline (piq) ligand is based on the fact that the highest occupied molecular orbital (HOMO) is principally composed of a

Red phosphors containing other transition metals

The research on red phosphors based on Ir(III) complexes has drawn increasing attention due to their high device efficiencies, however, metal complexes derived from other transition elements, such as Re(I), Os(II), Pt(II) and Zn(II), can also be utilized as red emitters. Burn and Samuel synthesized a series of (1,10-phenanthroline)rhenium(I)tricarbonyl complexes Re-1 to Re-6 with different dendritic generation which comprised of biphenyl units and 2-ethylhexyloxy surface groups [74]. All the

Near-infrared (NIR) emitters

In addition to red emitters, there is a growing interest in the NIR emission from metal complexes through photoluminescence [90], [91], [92], [93], [94], [95] and electroluminescence [96], [97], [98], [99], [100]. NIR OLEDs are of interest due to their applications in a number of areas, including infrared signaling and displays, telecommunications and wound healing [8], [11]. The most common approach that has been taken to develop NIR emitting devices is to employ lanthanide complexes. However,

Conclusions

In this review, we summarize the recent advances in different classes of molecular phosphorescent dyes suitable for use as red or NIR triplet emitters in vacuum-deposited or spin-coated PHOLEDs. Functionalization of metal complexes in terms of the metal center and/or ligand structure can effectively tune the HOMO and LUMO levels of the dopant which would produce red or NIR dopant with desirable photophysical, thermal, redox and EL properties. All of these factors would have dramatic effects on

Acknowledgments

W.-Y. Wong acknowledges the financial support from the National Basic Research Program of China (973 Program) (2013CB834702), Hong Kong Baptist University (FRG2/11-12/156), Hong Kong Research Grants Council (HKBU203011 and HKUST2/CRF/10) and Areas of Excellence Scheme, University Grants Committee of HKSAR, China (Project No. [AoE/P-03/08]). We also thank the 985 Program and 111 Project from Minzu University of China (CUN985-07-08 and 111 Project B08044) for financial support.

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ReviewRed to near-infrared organometallic phosphorescent dyes for OLED applications

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Cyclometalating ligands derived from 1-phenylisoquinoline

Red phosphors containing other transition metals

Near-infrared (NIR) emitters

Conclusions

Acknowledgments

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Review
Red to near-infrared organometallic phosphorescent dyes for OLED applications