Cyclometallated Organoiridium Complexes as Emitters in Electrophosphorescent Devices

Abstract

A brief overview is presented on cyclometallated iridium complexes that are used as phosphorescent dopants in organic light -emitting devices (OLEDs). Comparisons between devices made using either Pt(octaethylporphyrin) or Ir(phenylpyridyl) 3 as dopants are used to illustrate some parameters needed for optimal performance of electrophosphorescent OLEDs. The molecular properties responsible for the high luminescent effi ciency of the Ir complexes are then described. Interactions between the triplet ligand state and the singlet and triplet metal -to -land charge transfer states are responsible for photophysical characteristics of the complexes. Examples are given of various ligand combinations used to control the phosphorescent energy and effi ciency. This knowledge enables one to systematically tune the emission colors of these complexes from the near -UV to the near -infrared spectral regions. Organic Light -Emitting DevicesThe future holds tremendous opportunity for the low cost and high performance offered by organic LEDs. The term organic light -emitting device (OLED) refers to any light -emitting diode that is composed of either molecular or polymeric materials. Full -color displays that use OLEDs may eventually replace liquid -crystal displays (LCDs) in a range of mobile applications, ranging from cellular phones to laptop computers. The low power dissipation and high brightness of OLEDs make them ideal for such portable applications. OLED displays can be deposited on fl exible plastic or metal foils, eliminating the fragile and heavy glass substrates used in LCDs and other fl at panel displays. Moreover, OLED displays emit light without the pronounced directionality inherent in LCD viewing, all with effi ciencies signifi cantly higher than typical LCD panels. It is possible that portable and lightweight roll -up OLED displays will someday cover our walls. Another important application for OLEDs is in illumination. The high effi ciencies and excellent color qualities for white OLEDs make them promising candidates for replacing conventional fl uorescent and incandescent light sources.

Recent Developments in the Application of Phosphorescent Iridium(III) Complex Systems

“…Only a few selected examples will be described in the following, and the reader is referred to recent literature published elsewhere. [1,[8][9][10][11][12][13]16,60] The two configurational isomers of tris-cyclometallated Ir III complexes notably show differences in their photophysical properties, i.e., the fac-isomers feature over an order of magnitude longer lifetimes and higher quantum efficiencies than their meridional counterparts. [23] On the other hand, Sun and co-workers described mer-Ir(mppy) 3 (mppy ¼ 2-phenyl-4-methyl-pyridine) as an exception to this general concept of only low quantum efficiencies for mer-isomers.…”

“…[23,29] The lower thermodynamic stability of the kinetically favored meridional formation is primarily due to the strongly trans-influencing aryl groups opposite to each other (in the fac-isomer all three aryl groups are opposite to pyridyl or other neutral donor groups). [16] The direct route towards fac-Ir(C^N) 3 starting from the Ir(acac) 3 precursor, where acac is acetoacetonate (path d), or IrCl 3 Á xH 2 O is a common approach for phenylpyridine (Hppy) and its derivatives. [30] …”

“…In general, the emission color can be tuned via the combination of cyclometallating and ancillary ligands coordinated to the Ir III core. [1,16] A variety of emissive complexes covering the whole visible spectra-from blue, over For the synthesis of neutral bis-cyclometallated acetoacetonate and picolinate complexes, the most commonly used conditions are often rather harsh. Mixtures of precursor complex, the designated ligand, base (e.g., Na 2 CO 3 ), and high boiling alcohols (e.g., 2-ethoxyethanol) are stirred under reflux for several hours usually generating the desired complex in good yields and purity.…”

“…[9] A general overview of the most common synthetic strategies towards the various types of phosphorescent Ir III complexes is depicted in Scheme 1. [16] The m-dichloro bridged dimer [Ir(C^N) 2 -m-Cl] 2 , conveniently prepared from a reaction of the respective ligand and IrCl 3 Á xH 2 O, [21] plays a central role in the coordination chemistry of these complexes. The chloro-bridge can be split by chelating ligands leading to neutral (L^X ¼ b-diketonates, picolinates, etc.)…”

“…Tris-cyclometallated complexes or neutral bis-cyclometallated derivatives with ancillary ligands, such as acetylacetonates or picolinates, exhibit high potentials for modern OLED applications. [1,16] Furthermore, charged bis-cylometallated complexes, mainly with oligopyridines as ancillary ligand, are particularly interesting as emitters in LECs, [1,17] biolabeling [18][19][20] or photocatalytic applications. [9] A general overview of the most common synthetic strategies towards the various types of phosphorescent Ir III complexes is depicted in Scheme 1.…”