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

Abstract

Metrics & MoreArticle Recommendations * sı Supporting Information CONSPECTUS:The 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((L 1 ) 2 L 2 ), 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(L 1 L 2 L 3 ) 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 t...

Blue heteroleptic iridium(iii) complexes for OLEDs: simultaneous optimization of color purity and efficiency

“…2 Due to the spin-orbit coupling effect, heavy metal complexes can simultaneously harvest singlet and triplet excitons, leading to an internal quantum efficiency of up to 100%. 3,4 In particular, owing to their high photoluminescence quantum yields (PLQYs), short phosphorescence lifetimes, facile color adjustments and good stability, iridium(III) metal complexes are considered as one of the most promising lightemitting materials for practical applications. [5][6][7] However, there are still some thorny problems in the development of blue phosphorescent iridium(III) complexes with regard to color purity, stability and emission efficiency, as compared to green and red phosphorescent iridium(III) complexes.…”

“…To increase the ligand-centered triplet state (i.e. 3 LC) energy by appropriate ligand modification, e.g. by adding a heteroatom in the C-coordinating ring of the cyclometalating ligands, was proved favorable to cause a hypsochromic shift of phosphorescence.…”

“…by adding a heteroatom in the C-coordinating ring of the cyclometalating ligands, was proved favorable to cause a hypsochromic shift of phosphorescence. [14][15][16][17] In addition, adopting strong s-donating ligands such as carbenes as the ancillary ligands could raise the non-radiative d-d* excited-state so as to make it distant from the emitting triplet states such as 3 MLCT. Meanwhile, the carbene ligand with higher triplet energy could adjust the electron density distribution of the frontier molecular orbital to increase the contribution of 3 MLCT in the lowest lying triplet manifold.…”

Divergent Approach for Tris-Heteroleptic Cyclometalated Iridium Complexes Using Triisopropylsilylethynyl-Substituted Synthons

“… 10 It was not until we developed the approach of reacting two different 2-phenylpyridine-based ligands (L a and L b ) with IrCl 3 ·3H 2 O to form a mixture of iridium chloro-bridged dimers followed by reaction with acetylacetone to give a statistical mixture of Ir(L a ) 2 (acac), Ir(L a )(L b )(acac), and Ir(L b ) 2 (acac) species that Ir(L a )(L b )(acac) complexes could be separated by chromatography. 10 − 12 Following this, new approaches have been developed that initially use the degradation of a tris -cyclometalated to a mixed ligand iridium-chloro dimer, which was coordinated to an ancillary ligand to form a tris -heteroleptic complex. 13 − 15 Adamovich et al recently reported an approach that used an aryl- N -heterocyclic carbene ligand to form an intermediate iridium complex that favored the formation of mixed ligand chloro dimers, avoiding statistical distributions of products.…”

“…Despite bis -heteroleptic iridium complexes being well-established, there remain relatively few examples of tris -heteroleptic iridium complexes . It was not until we developed the approach of reacting two different 2-phenylpyridine-based ligands (L a and L b ) with IrCl 3 ·3H 2 O to form a mixture of iridium chloro-bridged dimers followed by reaction with acetylacetone to give a statistical mixture of Ir­(L a ) 2 (acac), Ir­(L a )­(L b )­(acac), and Ir­(L b ) 2 (acac) species that Ir­(L a )­(L b )­(acac) complexes could be separated by chromatography. − Following this, new approaches have been developed that initially use the degradation of a tris -cyclometalated to a mixed ligand iridium-chloro dimer, which was coordinated to an ancillary ligand to form a tris -heteroleptic complex. − Adamovich et al recently reported an approach that used an aryl- N -heterocyclic carbene ligand to form an intermediate iridium complex that favored the formation of mixed ligand chloro dimers, avoiding statistical distributions of products …”