Could you explain the significance of your article to the non-specialist?

For practical applications, especially for AMOLED (active matrix organic light-emitting diode), highly luminescent emitting materials are strongly required to generate efficient electroluminescence. In this sense, phosphorescent transition metal complexes, particularly cyclometalated Ir(III) complexes are very promising because they provide nearly 100% of internal quantum efficiency for the light emission by the efficient spin-orbit coupling. To further enhance the phosphorescence quantum yield with improved stability and color tunability, there have been vast amount of synthetic efforts to find highly phosphorescent transition metal complexes. In this article, we have synthesized a new cyclometalated homoleptic Ir(III) complex with aryl silane based dendritic ligands. The Ir(III) complex showed much higher solid state phosphorescent quantum yield (74 +/- 3%) and the particularly remarkable polymer-based OLED efficiency (32.8 cd/A) to be promising results for practical applications.

What has motivated you to conduct this article?

Material strategy for design and synthesis of new phosphorescent Ir(III) complexes has been mostly focused in two target properties; color tuning and phosphorescence quantum yield. Introduction of substituents with suitable electronic character makes it possible to obtain aimed phosphorescent color. However, tuning factors in phosphorescence quantum yield are generally uncertain. Moreover, the quantum yield is variable according to physical states, nature of matrix, and doping condition. We believed that bulky substituents, in the coordination environment of Ir(III) complex, could provide affirmative aspect to control hazardous inter-chromophoric interactions enhancing phosphorescence quantum yield in the solid state. However, similar previous studies employing alkyl substituent showed inferior device characteristics when the degree of wrapping by substituents was increased. So, we determined to introduce aryl silyl substituent in the Ir(III) complex.

Where do you see this work developing in the future?

It is thought that the design strategy to use a dendrimer can be an attractive solution for the highly phosphorescent Ir(III) complex. However molecular size and electronic character of the dendritic part should be appropriately tailored to optimize properties. Our work provides a favorable result that dendritic substituents are necessary to guarantee higher phosphorescence quantum yield in the solid state. However we believed that high generation of the dendritic substituent is not always required for the maximized device efficiency. This material design strategy will be adapted for further synthesis of new Ir(III) complexes.

Are there any particular challenges facing future research in this area?

As we know, high OLED efficiency is not singly determined by the performance of the luminescent emitting material such as Ir(III) complex. Energy level alignment between the emitting material and the host material is important for the efficient exciton generation in the emission center. Beside this, some other requirements such as the physical compatibility between an emitter and a host, thermal and electrochemical stability of the emitter, and balanced charge carrier transport in the host should be comprehensively tuned before practical application. We think all of these conditions can be successfully achieved by chemical methods. Proper material design and synthesis surely will offer high probability for breakthrough in this area.