Asymmetric Tris-Heteroleptic Cyclometalated Phosphorescent Iridium(III) Complexes: An Emerging Class of Metallophosphors

Abstract

ConspectusThe rapid developments of cutting-edge research on photofunctional organic semiconductor materials are greatly promoting the progress of science and technology. As one of the most important organic semiconductor materials, phosphorescent iridium(III) complexes are a promising class of organometallic emitters because of their rich emissive excited states together with excellent chemical stability, which have been widely used in various fields, such as organic electroluminescence, solar cells, photocatalysis, biosensing, bioimaging, cancer therapy, etc. The exploration of highly efficient phosphorescent iridium(III) complexes showing various structural features is blooming quickly. In general, the traditional iridium(III) phosphors usually contain at least two identical cyclometalating bidentate ligands. These iridophosphors with two identical bidentate ligands are termed as the bis-heteroleptic complex Ir((L1)2L2), where L denotes the free ligand or the deprotonated form of the free ligand. Those with three identical ligands are called homoleptic complex Ir(L)3. Recently, the iridium(III) complexes Ir(L1L2L3) supported by three different (cyclometalating) ligands are emerging as a novel and interesting category of phosphors. This class of iridophosphors usually refers to tris-heteroleptic cyclometalated phosphorescent iridium(III) complexes, and they usually show the asymmetry in their molecular structures. Compared with the case for the traditional iridium(III) phosphors, the independent ligand control provides tris-heteroleptic iridium(III) phosphors with more flexible molecular design/synthesis and excited-state fine-tuning ability, thereby resulting in more rich and appealing excited states and potential multifunctional applications.In this Account, we will highlight our recent efforts on the asymmetric tris-heteroleptic cyclometalated iridium(III) phosphors including the molecular design strategies, chemical synthesis, excited-state tuning, and structure–property relationships as well as their applications, especially in organic electroluminescence. Specifically, the molecular design of tris-heteroleptic iridium(III) phosphors focuses on the independent ligand design including the following three aspects: (1) the substituent functionalization engineering (SFE); (2) the ligand skeleton engineering (LSE); (3) the double metalation engineering (DME). For SFE, we mainly introduce the substituent based on the main-group element into the ligand of iridophosphors and also investigate the influence of the substituent position on the emissive excited states of iridophosphors. For instance, the main-group elements show unique electronic effects, which could contribute to the manipulation of the photoelectric properties of complexes (e.g., balanced charge transport ability, improved utilization of excitons, etc.). For LSE, different types of aromatic skeleton or various lengths of π conjugation of the ligands are also examined for the effective tuning of their excited states. For DME, the type and spatial orientation of the bridging/franking ligand will be considered. At the same time, the related optoelectronic applications (e.g., electroluminescence, optical power limiting, etc.) of this novel class of versatile iridophosphors are also discussed. Finally, we give some perspectives on this fascinating topic and also try to provide some potential research opportunities based on the current stage of asymmetric tris-heteroleptic cyclometalated phosphorescent iridium(III) complexes. It is believed that the emerging asymmetric tris-heteroleptic cyclometalated iridophosphors will open an important avenue for designing novel metallophosphor-based materials with tunable and appealing photophysical properties, thus offering new probabilities for potential multifunctional applications.