Heteroleptic Ir Complexes Research Articles

Cyclometalated Ir(III) complexes as potential electron acceptors for organic solar cells.

Cyclometalated iridium(iii) complexes have been investigated as promising electron donor (D) materials in organic solar cells (OSCs) due to their unique octahedral configuration for optimized morphology and their significantly long lifetimes potentially for enhanced exciton dissociation. However, the application as electron acceptor (A) materials has never been reported. In order to fill this blank, herein, two cyclometalated heteroleptic Ir complexes, TRIr and 2TRIr, based on electron donating-accepting type organic ligands with different π-conjugation lengths are reported as electron acceptor materials in comparison with their corresponding main organic ligands. The two Ir complexes exhibit suitable HOMO/LUMO energy levels of -5.55/-3.47 eV and -5.44/-3.48 eV, which are ∼0.1 eV higher in the HOMO and ∼0.15 eV deeper in the LUMO than the TR and 2TR ligands, respectively. 2TRIr with extended ligand π-conjugation displays a poor triplet feature, while TRIr demonstrates obvious metal-to-ligand charge transfer (MLCT) transition absorption, with a triplet component photoluminescence (PL) lifetime of 85 ns in neat films. When blended with PBDB-T in bulk heterojunction (BHJ) OSCs, the power conversion efficiencies (PCEs) are 2-3 times higher than their relevant ligands, with values of 1.20% and 1.62% for TRIr and 2TRIr, and 0.58% and 0.47% for the TR and 2TR ligand-based devices, respectively. TRIr and 2TRIr based active layer blends exhibit poorer hole and electron mobilities, whereas compared with their relatively linear planar ligands, both of the two octahedral Ir complexes exhibit an optimized surface morphology for less bimolecular recombination and more efficient exciton dissociation, thus contributing to improved photovoltaic performance.

Optical and Electrical Measurements Reveal the Orientation Mechanism of Homoleptic Iridium-Carbene Complexes.

Understanding and controlling the driving forces for molecular alignment in optoelectronic thin-film devices is of crucial importance for improving their performance. In this context, the preferential orientation of organometallic iridium complexes is in the focus of research to benefit from their improved light-outcoupling efficiencies in organic light-emitting diodes (OLEDs). Although there has been great progress concerning the orientation behavior for heteroleptic Ir complexes, the mechanism behind the alignment of homoleptic complexes is still unclear yet. In this work, we present a sky-blue phosphorescent dye that shows variable alignment depending on systematic modifications of the ligands bound to the central iridium atom. From an optical study of the transition dipole moment orientation and the electrically accessible alignment of the permanent dipole moment, we conclude that the film morphology is related to both the aspect ratio of the dye and the local electrostatic interaction of the ligands with the film surface during growth. These results indicate a potential strategy to actively control the orientation of iridium-based emitters for the application in OLEDs.

Unveiling the Role of Dopant Polarity in the Recombination and Performance of Organic Light‐Emitting Diodes

AbstractThe recombination of charges is an important process in organic photonic devices, because the process influences the device characteristics such as the driving voltage, efficiency, and lifetime. Here, by using various homoleptic and heteroleptic Ir complexes as dopants, it is reported that the stationary dipole moment (μ0) of the dopant rather than the trap depth (ΔEt) is a major factor determining the recombination mechanism in dye‐doped organic light‐emitting diodes (OLEDs). Dopants with large μ0 (e.g., homoleptic Ir(III) dyes) induce large charge trapping on them, resulting in high driving voltage and trap‐assisted recombination‐dominated emission. On the other hand, dyes with small μ0 (e.g., heteroleptic Ir(III) dyes) show Langevin recombination‐dominated emission characteristics with much less charge trapping on them no matter what ΔEt is, leading to lower driving voltage and higher efficiencies. This finding will be useful in any organic photonic devices such as phosphorescent or thermally assisted delayed fluorescent dye sensitized fluorescent OLEDs where trapping and recombination mechanisms play key roles.

The effect of interligand energy transfer on the emission spectra of heteroleptic Ir complexes.

In order to understand the causes of the emission shape and colour changes of heteroleptic Ir3+ complexes containing 2-(2,4-difluorophenyl)pyridine (dfppy) as the main ligands, we introduced two types of ancillary ligands: (1) non-luminescent ancillary ligands, namely tetrakis(pyrazolyl)borate (bor) and picolinate (pic), which were employed for the preparation of Ir(dfppy)2(bor) and Ir(dfppy)2(pic), respectively, and (2) luminescent ancillary ligands, namely 1,10-phenanthroline (phen), bipyridine (bpy), and 2,3-dipyridylpyrazine (dpp), which were employed for the preparation of Ir(dfppy)2(phen), Ir(dfppy)2(bpy), and Ir(dfppy)2(dpp), respectively. In a glassy matrix at 77 K, the Ir complexes showed well-structured emission spectra, except Ir(dfppy)2(dpp). The vibronic structures in the emission spectra of Ir(dfppy)2(bor) and Ir(dfppy)2(pic) were maintained even at 300 K. However, Ir(dfppy)2(phen), Ir(dfppy)2(bpy), and Ir(dfppy)2(dpp) showed markedly red-shifted and broad emission spectra. The anomalous rigidochromism was attributed to an interligand energy transfer (ILET), and showed a strong temperature dependence. The excited states of dfppy are higher than those of phen, bpy, and dpp; thus, ILET occurs from dfppy to the other ligands lying in lower energy states. The ILET dynamics were probed directly using femtosecond transient absorption (TA) spectroscopy after the excitation of dfppy. As the time delay increased, the intensity of the TA band of dfppy decreased, while those of the bands related to the phen, bpy, and dpp ancillary ligands increased. On the other hand, no changes in the TA spectra were observed for Ir(dfppy)2(bor) and Ir(dfppy)2(pic). The TA spectral behaviours can be explained in terms of the relative ordering of the emissive states for cyclometalating and ancillary ligands.

Efficient Synthesis of Tris-Heteroleptic Iridium(III) Complexes Based on the Zn2+-Promoted Degradation of Tris-Cyclometalated Iridium(III) Complexes and Their Photophysical Properties.

We report on the efficient synthesis of tris-heteroleptic iridium (Ir) complexes based on the degradation of tris-cyclometalated Ir complexes (IrL3, L: cyclometalating ligand) in the presence of Brønsted and Lewis acids such as HCl (in 1,4-dioxane), AlCl3, TMSCl, and ZnX2 (X = Br or Cl), which affords the corresponding halogen-bridged Ir dimers (μ-complexes). Tris-cyclometalated Ir complexes containing electron-withdrawing groups such as fluorine, nitro, or CF3 moieties on the ligands were less reactive. This different reactivity was applied to the selective degradation of heteroleptic Ir complexes such as fac-Ir(tpy)2(F2ppy) (fac-12) (tpy: 2-(4'-tolyl)pyridine and F2ppy: 2-(4',6'-difluorophenyl)pyridine), mer-Ir(tpy)2(F2ppy) (mer-12), and mer-Ir(mpiq)2(F2ppy) (mer-15) (mpiq: 1-(4'-methylphenyl)isoquinoline). For example, the reaction of mer-12 with ZnBr2 gave the heteroleptic μ-complex [{Ir(tpy)(F2ppy)(μ-Br)}2] 27b as a major product, resulting from the selective elimination of the tpy ligand of mer-12, and treatment of 27b with acetylacetone (acacH) afforded the corresponding tris-heteroleptic Ir complex Ir(tpy)(F2ppy)(acac)18. In addition, another tris-heteroleptic Ir complex 35a having 8-benzenesulfonylamidoquinoline (8BSQ) ligand was synthesized. Mechanistic studies of this degradation reaction and the photochemical properties, especially a dual emission, of these newly synthesized tris-heteroleptic Ir complexes are also reported.

Design of Heteroleptic Ir Complexes with Horizontal Emitting Dipoles for Highly Efficient Organic Light-Emitting Diodes with an External Quantum Efficiency of 38%

Orientation of emitting dipoles is one of the most important material properties influencing the efficiency of organic light-emitting diodes (OLEDs). Recently, even globular-shaped Ir complexes, especially heteroleptic Ir complexes, have been reported to have horizontal emitting dipole orientation (EDO) to improve the external quantum efficiency (EQE) over 30%. Still the relationship between molecular structure of Ir complexes and their EDO has not been fully understood yet. Here, we report that substituents at the para-position of the pyridine in the main ligands of Ir complexes play a pivotal role inducing the orientation of heteroleptic Ir complexes. Substitution of aliphatic and aromatic functional moieties at the position leads to high horizontal emitting dipole orientation with the horizontal dipole ratio up to 86.5% to realize unprecedentedly high-efficiency yellow and green OLEDs, with EQEs of 38% and 36%, respectively. Elongated and planar substituents with high electrostatic potential enlarge th...

Ligand-to-ligand charge transfer in heteroleptic Ir-complexes: comprehensive investigations of its fast dynamics and mechanism

To gain new insights into ligand-to-ligand charge-transfer (LLCT) dynamics, we synthesised two heteroleptic Ir(3+) complexes: (Ir(dfppy)2(tpphz)) and (Ir(dfppy)2(dpq)), where dfppy, tpphz, and dpq are 2-(4,6-difluorophenyl)pyridine, tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine, and 2,3-bis-(2-pyridyl)-quinoxaline, respectively. The tpphz and dpq ligands have longer π-conjugation than dfppy. Therefore, the excited ligand-centred (LC) state and the metal-to-ligand charge transfer (MLCT) state of dfppy are higher than those of tpphz and dpq. The LLCT dynamics from dfppy to tpphz (or dpq) was probed using femtoscond transient absorption (TA) spectroscopy after the selective excitation of dfppy. The TA band for the LC/MLCT state of dfppy is observed at 480 nm. Because of the LLCT process, the TA bands related to the MLCT states of tpphz and dpq ligands increased, whereas those of dfppy decreased. The time constants for the LLCT process were 17 ps for Ir(dfppy)2(tpphz) and 5 ps for Ir(dfppy)2(dpq). The MLCT emission of Ir(dfppy)2(tpphz) showed strong temperature dependence, indicating that the LLCT process has a significant energy barrier. In comparison, the temperature weakly influenced the emission of Ir(dfppy)2(dpq), and thus, its LLCT process may have a smaller barrier. The anomalous rigidochromism and photodynamic behaviours can be explained in terms of the barrier between cyclometalating and ancillary ligands.

Optimization of White Organic Electroluminescent Devices with Sensitized Heteroleptic Ir Complex

In this study, the heteroleptic tris-cyclometalated iridium complex, bis(2-phenylquinoline)(2-phenylpyridine)iridium(III) [Ir(pq)2(ppy)], used as red emitting dopant for white organic light-emitting diodes (WOLEDs) had higher maximum luminescence efficiency (13.80 cd/A) than that of the homoleptic tris-cyclometalated iridium complex, tris(2-phenylquinoline)iridium(III) [Ir(pq)3], due to the presence of a sensitizing ppy ligand. Thus, WOLEDs combining the Ir(pq)2(ppy) as a red emissive phosphorescent material and 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl (BCzVBi) as a blue emissive fluorescent material have been studied. The WOLEDs were optimized by adjusting the device structure, the optimized WOLED (device 10) had a maximum luminescence efficiency of 16.90 cd/A and CIE coordinates of (0.407, 0.337) at the applied voltage of 12 V.