Controlling Emitting Dipole Orientation with Methyl Substituents on Main Ligand of Iridium Complexes for Highly Efficient Phosphorescent Organic Light‐Emitting Diodes

Abstract

the heteroleptic molecular structure of the Ir (III) complexes and strong Coulombic interaction between the dopant and host molecules are the driving forces of the preferred EDO. [ 4,5 ] In this regard, the structure of the emitting molecules, as well as the organic host molecules, must infl uence the EDO. The effect of the ancillary ligands in the heteroleptic Ir complexes (HICs) on the EDO and the photoluminescence quantum yields (PLQY) in doped fi lms were reported recently, exhibiting that bis(phenyl-pyridine)Ir III (2,2,6,6-tetramethylheptane-3,5-diketonate) (Ir(ppy) 2 tmd) results in higher PLQY and Θ values than Ir(ppy) 2 acac, which in turn has higher values than Ir(ppy) 3 . Unfortunately, the reason why different ancillary ligands resulted in different EDOs has not been reported yet and their discussion was limited to rather qualitative description and there are few reports on the quantitative and systematic investigation of the effect of the structure of the emitting molecules on the EDO in fi lms. Furthermore, the effect of the main ligands in HICs has not been investigated up to now to our best knowledge. The understanding of the structure-property relationship will eventually lead us to identify phosphorescent dyes with high Θ in fi lms to fabricate OLEDs with high EQE.In this study, we investigated the effect of main ligands of HICs on the EDO in doped fi lms and related the EDO to the angle between the TDM of emission dipoles and the C 2 axis of the molecules. For this purpose, we designed and synthesized four iridium complexes where the hydrogen atom(s) at different positions of the cyclometalated ligands of Ir(ppy) 2 tmd were systematically replaced by methyl group(s) keeping the ancillary ligand the same as tmd. We used the same host for the dopants to minimize the host effect, i.e., to minimize the variation of the intermolecular interaction between the dopants and the host. We found that the methyl substitution resulted in different directions of the TDM in the iridium complexes. Moreover, the molecules with higher angle between the C 2 axis and the TDM in the molecules resulted in a higher horizontal portion of the emitting dipole orientation in the emitting layer. Based on the observation, we were able to develop a new green emitter having high Θ of 80% and PLQY of 97% and resultantly demonstrate a green OLED exhibiting an unprecedented high EQE of 34.1%, power effi ciency (PE) of 157.6 lm W −1 , and current effi ciency (CE) of 120.5 cd A −1 using the new emitter.Chemical structures of the green-emitting phosphorescent dyes investigated in this study are shown in Figure 1 a. All the structures are based on Ir(ppy) 2 tmd having high Θ and

Unraveling the orientation of phosphors doped in organic semiconducting layers

“…2a, b . Three heteroleptic iridium complexes of Ir(ppy) 2 tmd, Ir(3′,5′,4-mppy) 2 tmd 18 , and Ir(dmppy-ph) 2 tmd 19 possessing high Θ values were adopted to investigate the effect of the phosphor molecular structure. The molecular C 2 symmetry axis toward the center of the ancillary ligand from the origin located at the Ir atom was set as m z , the orthogonal vector to m z normal to the molecular Ir-O-O plane was set as m x , and m y was determined by a cross product of m z and m x in the dopants.…”

“…Interestingly enough, it is only in recent years has attention turned to the orientation of emitting dipoles of iridium-based phosphors, the most verified light-emitting dyes with high photoluminescence quantum yield and variety of chromatic spectrum as doped in the emissive layers; probably because their iridium-centered spherical shape and the amorphous surrounding nature in the emissive layers are far from having strong molecular alignments. Recently, some heteroleptic Ir complexes exhibiting efficient electroluminescence in organic light-emitting diodes are reported to possess preferred horizontal emitting dipole orientations (EDOs) 13 – 16 , 18 – 20 . However, it was difficult to assert the reason why the spherical-shaped phosphors have a propensity toward preferred molecular alignment in the emissive layers.…”

Understanding and predicting the orientation of heteroleptic phosphors in organic light-emitting materials

“…Previous reports have suggested that simultaneous host and dopant interactions in aligned matrices arise from the formation of donor/host aggregates based on the component electrostatics 22,29,30 . The complexes studied here have very similar electrostatic surfaces (Fig.…”

“…Past reports have speculated that large dipole moments present in the tris-cyclometalates lead to aggregation that suppresses dopant interactions with the host matrix 16,[26][27][28] . Other reports have proposed electrostatic interactions between electronegative regions in the (C ∧ N) 2 Ir(O ∧ O) complexes and electropositive host structures give rise to macroscopic order and thus dopant (and host) alignment 22,29,30 . We will show that neither the dopant dipole-based mechanism nor component electrostatics adequately describe the alignment process and propose a mechanism based on the inherent asymmetry of (C ∧ N) 2 Ir(O ∧ O) and some fac-Ir(C ∧ N) 3 complexes that explain the many instances of observed dopant alignment in amorphous host materials.…”

Blue-Emitting Arylalkynyl Naphthalene Derivatives via a Hexadehydro-Diels–Alder Cascade Reaction

“…It is worth noting that for simple fluorescent emitters, a maximum theoretical efficiency of ~5% is expected. 15 Peak η EQE values for all three compounds are realized for devices having an emissive layer composition of 20% of 10 . This does not correlate directly with the PL efficiency η PL , which increases monotonically with dilution, likely reflecting a role played by the emitter in charge transport.…”