Skip to main content

Multiphosphorescent emitters based on Tris-Heteroleptic Iridium Complexes

Final Report Summary - MULTHIC (Multiphosphorescent emitters based on Tris-Heteroleptic Iridium Complexes)

This project main aim was to develop and explore a new family of phosphorescent iridium(III) complexes as single centre dual phosphorescent emitter. In particular, the goal of the project was to obtain a complex with a first phosphorescent band below 500 nm and a second around 600 nm in order to have white emission from a single complex.
Cyclometallated iridium complexes have attracted significant attention in the industrial and academic research groups due to their synthetic versatility, thermal stability, relatively short excited state lifetimes, generally high photoluminescence efficiency, and excellent emission wavelength tunability. Therefore, these complexes have been widely exploited as emitters in solid-state display and lighting applications such as phosphorescent organic light-emitting diodes (PhOLEDs) and light-emitting electrochemical cells (LECs). While these technologies are well developed for monochromatic devices, several challenges have still to be overcome for white light production. Two of these challenges are at the heart of the project: the complexity of the device architecture, which impacts on the costs, and the colour drift upon device aging, which impacts on the stability. Both challenges arise largely because of the use of multiple emitters (blue, green, and red) combined to obtain white light. This project was intended to resolve these issues by developing a new family of dual phosphorescent iridium(III) complexes that can directly emit white light, without the need to combine multiple emitters. The strategy revolved around tris-hetroleptic iridium complexes in which the ligands can undergo controlled geometric deformation in the excited state leading to two emitting triplet states.
In this project we have developed improved synthetic methodology with significantly good yields for the preparation of cyclometallated iridium complexes bearing N-heterocyclic carbenes, phenylimidazoles and pyrazoles. It was mainly based on the right choice of iridium precursor and preparation of key intermediates with improved purity. Application of the methodologies to the synthesis of tris-heteroleptic complexes generally allowed for improved yields compared to previously reported methodologies. However it was found that the results were very much dependent on the types of cyclometallating ligands used. As a result, at this stage, no satisfying general synthetic methods could be devised.
Nevertheless, using the specific synthetic methodologies developed in the initial stages of the project, we have prepared series of novel cyclometallated iridium complexes of type (C^C:)3Ir, [(C^N)2Ir(N^N)]PF6 and [(C1^N1)(C2^N2)Ir(C^C:)]. Initial photophysical measurements significantly improved our understanding of the impact of the geometric deformation of the ligand in the excited state. As a result, in the latest stage of the project, we have been successfully designing and preparing tris-heteroleptic iridium complexes with a first phosphorescent band below 500 nm and a second around 600 nm.
From an individual perspective, the Research Fellow has received training in new areas of research, which will be very useful for his future career. In particular he has acquired basics and beyond knowledge of organometallic synthesis and photochemistry. He has improved his practical skills in performing organic and inorganic synthesis using Schlenk techniques and purification methods. This project has also allowed him to improve his experience in modern analytical methods, in particular mass spectrometry, X-Ray single crystal analysis and NMR for structural confirmation and elucidation. He also gained experience in UV-visible and emission spectrometer to study the photophysical properties of synthesized complexes. His participation in the supervision of undergraduate and graduate students has provided him with leadership qualities and has enhanced his capabilities to work and communicate in English. The participation in international and national conferences and in the internal group meetings has also improved his knowledge transfer skills.
As a conclusion, we have been successful in achieving the main overarching goal of the project, which was to develop and explore a new family of dual phosphorescent tris-heteroleptic iridium complexes. Our initial hypothesis is demonstrated by the preparation of such a complex with a first emission band below 500 nm and a second around 600 nm. This exciting result opens up new avenues for the design of single centre white emitters Most importantly, as a result of this work the Research Fellow has broaden his intellectual knowledge and practical skills, which have allowed him to consolidate his future career.