Ir(III)-Based Phosphors with Bipyrazolate Ancillaries; Rational Design, Photophysics, and Applications in Organic Light-Emitting Diodes

NHC-Based Deep-Red Phosphorescent Iridium Complexes Featuring Three-Charge (0, -1, -2) Ligands

N-heterocyclic carbene (NHC)-based phosphorescent iridium complexes have attracted extensive attention due to their good optical properties and high stability in recent years. However, currently reported NHC-based iridium complexes can easily achieve emission of blue, green, or even ultraviolet light, while emission of red or deep-red light is relatively rare. Here, we report a new family of NHC-based deep-red iridium complexes (Ir1, Ir2, Ir3, and Ir4) featuring three-charge (0, -1, -2) ligands. The single-crystal structures confirm that all complexes exhibit a trans-C-N configuration between the NHC carbon atom of the monoanionic (-1) ligand and the nitrogen atom of the neutral (0) ligand and that there are abundant intermolecular and intramolecular interactions in the crystalline state. Notably, all complexes exhibited an effective deep-red emission (650-664 nm). Moreover, the iridium complexes (Ir2 and Ir4) based on benzimidazol-2-ylidene (pmb) exhibited a higher emission efficiency and longer emission lifetime than the corresponding iridium complexes (Ir1 and Ir2) based on imidazol-2-ylidene (pmi), respectively. Density functional theory calculations demonstrate that the pmb ligand of Ir3 and Ir4 is more involved in the excited state than the pmi ligand of Ir1 and Ir2, which is caused by the stronger electron-donating ability of the pmb ligand. Considering better optical properties, Ir2 and Ir4 were eventually used as dopant emitters of the optical light-emitting diode (OLED) devices to obtain good maximum external quantum efficiency (8.5 and 10.1%) in the deep-red region (628 and 624 nm) with Commission Internationale deL'Eclairage (CIE) coordinates of (0.65, 0.34) and (0.63, 0.36), respectively, with a low turn-on voltage (2.4 V). This research provides an important idea for the design and optoelectronic applications of NHC-based deep-red phosphorescent iridium complexes.

Three-Charge (0, -1, -2) Ligand-Based Binuclear and Mononuclear Deep-Red Phosphorescent Iridium Complexes Bearing Benzo[d]oxazole-2-Thiol Ligand

A new class of three-charge (0, -1, -2) ligand-based binuclear and mononuclear iridium complexes bearing benzo[d]oxazole-2-thiol ligand have been synthesized. Notably, the binuclear complexes (IrIr1 and IrIr2) can be generated at low temperatures by reacting the iridium complex precursors (2a and 2b) with equal amounts of the benzo[d]oxazole-2-thiol ligands, while the corresponding mononuclear complexes (Ir1 and Ir2) are formed at high temperatures. X-ray diffraction analysis shows that the benzo[d]oxazole-2-thiol ligand plays an unusual and interesting bridging role in binuclear complexes and induces rich intermolecular and intramolecular interactions, while in mononuclear complexes, it forms an interesting four-membered ring coordination. More importantly, all complexes experienced efficient deep-red emission in the 628-674 nm range, and the mononuclear complexes have higher luminescent efficiency and longer excited state lifetime than the binuclear complexes. As a result, organic light-emitting diode devices incorporating two mononuclear complexes (Ir1 and Ir2) as guest material of the light-emitting layer can obtain good maximum external quantum efficiency (3.5% and 5.5%) in the deep-red region (629 and 632 nm) with CIE coordinates (0.61, 0.33) and (0.62, 0.34), along with a low turn-on voltage (2.8 V).

Deep-Red/Near-Infrared to Blue-Green Phosphorescent Iridium(III) Complexes Featuring Three Differently Charged (0, -1, and -2) Ligands: Structures, Photophysics, and Organic Light-Emitting Diode Application

We have designed and synthesized a new family of neutral phosphorescent iridium(III) complexes (Ir1-Ir6) featuring three differently charged (0, -1, and -2) ligands, in which biphenyl (bp) is used as a dianionic (-2) ligand, 4,6-difluorophenylpyridine (dfppy) or 1-phenylisoquinoline (piq) is used as a monoanionic (-1) ligand, and 2,2'-bipyridyl (bpy), 1,10-phenanthroline (phen), 1,2-bis(diphenylphosphanyl)benzene (dppb), or 1,2-bis(diphenylphosphanyl)ethane (dppe) is used as a neutral (0) ligand. The X-ray structures confirm that three coordination carbon atoms of all complexes assume a facial geometry, which can be beneficial to the stability of the structure. More importantly, the emitting color of the complexes can be tuned from deep red/near-infrared (NIR) (680-710 nm) to blue-green (466-496 nm) with different monoanionic (-1) ligands and neutral (0) ligands. Interestingly, the complex Ir5 shows a significant aggregation-induced phosphorescent emission effect, while Ir6 with a similar structure shows an opposite aggregation-caused quenching effect, mainly due to slight differences in the neutral (0) ligand structure. Notably, all deep red/NIR-emitting complexes (Ir1-Ir4) exhibit a distinct charge transfer (CT) excited state from the dianionic (-2) ligand to the neutral (0) ligand according to density functional theory calculations, whereas the excited state of blue-green-emitting complexes (Ir5-Ir6) displays the CT from the dianionic (-2) ligand to the monoanionic (-1) ligand. Considering better stability and optical performance, the deep red-emitting complexes (Ir2 and Ir4) with a simple structure are used as emitting layers of organic light-emitting diode devices and achieved good maximum external quantum efficiency (4.9 and 5.8%) peaking at 676 and 655 nm, respectively, with a very low turn-on voltage (2.5 V). This research provides a good strategy for the design of phosphorescent iridium complexes based on three differently charged (0, -1, and -2) ligands and their optoelectric applications.

Three Types of Charged Ligands Based Carboxyl-Containing Iridium(III) Complexes: Structures, Photophysics, and Solution Processed OLED Application

A novel family of three types of charged (0, -1, -2) ligands based phosphorescent iridium(III) complexes with different carboxyl-containing dianionic (-2) ligands have been synthesized. Their single-crystal structures show that all neutral complexes (Ir1, Ir2, and Ir3) show a trans-N^N configuration between dianionic (-2) and monoanionic (-1) ligands, which is in contrast with the trans-N^C configuration in cationic complex Ir4, which has an interesting hydrogen bond in the solid state. Notably, Ir4 shows higher luminescence efficiency and an obvious blue shift emission relative to those in Ir1, Ir2, and Ir3. DFT calculations demonstrate that all neutral complexes (Ir1, Ir2, and Ir3) exhibit ligand-to-ligand charge transfer (LLCT) excited state character from the dianionic (-2) ligand to the neutral (0) ligand, which are completely different from the cationic complex Ir4 that exhibits an LLCT excited state from the monoanionic (-1) ligand to the neutral (0) ligand. Considering better solubility, Ir1 was eventually used in solution-processed OLED and achieved moderate efficiency (6.6%, 14.3 cd A^-1, 2.8 lm W^-1) with an orange light displaying CIE_x,y coordinates of (0.53, 0.46). This work provides a new strategy to construct three types of charged (0, -1, -2) ligands based phosphorescent iridium(III) complexes and extends the range of iridium complex luminescent materials.

Three types of charged ligand-based neutral phosphorescent iridium(III) complexes featuring nido-carborane: synthesis, structures, and solution processed organic light-emitting diode applications

In view of the fact that coordination configurations and special functional groups are both important for the optical properties of phosphorescent iridium complex materials, we have prepared a novel family of three types of charged ligand (0, -1, and -2) based neutral phosphorescent iridium(III) complexes (Ir1-Ir4) featuring nido-carborane. Single crystal structures indicate that complexes (Ir2, Ir3 and Ir4) with nido-carborane as a functional group at different substitution sites all show a trans-C^C configuration between dianionic (-2) and monoanionic (-1) ligands, which are different from the trans-N^C configuration in complex Ir1 with nido-carborane as a coordination skeleton, which has an interesting Ir-B coordination bond. Notably, Ir2, Ir3 and Ir4 all show obvious yellow light emission, while Ir1 does not emit light either in solution or in the solid state. DFT calculations demonstrate that complexes Ir2, Ir3 and Ir4 exhibit an unusual ligand-to-metal charge transfer (LMCT) excited state character due to the strong electron-donating character of nido-carborane. Considering its better solubility and luminescence properties, Ir3 was successfully applied in solution-processed organic light-emitting diodes and an effective yellow emission was achieved. This work provides a new strategy for the investigation of three types of charged ligand (0, -1, and -2) based phosphorescent iridium complex materials by constructing new dianionic ligands with nido-carborane.

Iridium(III) Complexes with [-2, -1, 0] Charged Ligand Realized Deep-Red/Near-Infrared Phosphorescent Emission

A series of [-2, -1, 0] charged-ligand based iridium(III) complexes of [Ir(bph)(bpy)(acac)] (1), [Ir(bph)(2MeO-bpy)(acac)] (2), [Ir(bph)(2CF₃ -bpy)(acac)] (3), [Ir(bph)(bpy)(2^t Bu-acac)] (4) and [Ir(bph)(bpy)(CF₃ -acac)] (5), which using biphenyl as dianionic ligand [-2], acetylacetone (or its derivatives) as monoanionic ligand [-1], and 2,2'-bipyridine (or its derivatives) as neutral ligand [0] were designed and synthesized. The chemical structures were well characterized. All of the ligands have simple chemical structures, thus further making the complexes have excellent thermal stability and are easy to sublimate and purify. Phosphorescent characteristics with short emission lifetime were demonstrated for these emitters. Notably, all of the complexes exhibit remarkable deep red/near infrared emission, which is quite different from the reported [-1, -1, -1] charged-ligand based iridium(III) complexes. The photophysical properties of these complexes are regularly improved by introducing electron-donating or -withdrawing groups into [-1] or [0] charged-ligand. The related organic light-emitting diodes exhibited deep red/near infrared emission with acceptable external quantum efficiency and low turn-on voltage (<2.6 V). This work provides a new idea for the construction of new type phosphorescent iridium(III) emitters with different valence states of [-2, -1, 0] charged ligands, thus offering new opportunities and challenges for their optoelectronic applications.

Probing Electron Excitation Characters of Carboline-Based Bis-Tridentate Ir(III) Complexes

In this work, we report a series of bis-tridentate Ir(III) metal complexes, comprising a dianionic pyrazole-pyridine-phenyl tridentate chelate and a monoanionic chelate bearing a peripheral carbene and carboline coordination fragment that is linked to the central phenyl group. All these Ir(III) complexes were synthesized with an efficient one-pot and two-step method, and their emission hue was fine-tuned by variation of the substituent at the central coordination entity (i.e., pyridinyl and phenyl group) of each of the tridentate chelates. Their photophysical and electrochemical properties, thermal stabilities and electroluminescence performances are examined and discussed comprehensively. The doped devices based on [Ir(cbF)(phyz1)] (Cb1) and [Ir(cbB)(phyz1)] (Cb4) give a maximum external quantum efficiency (current efficiency) of 16.6% (55.2 cd/A) and 13.9% (43.8 cd/A), respectively. The relatively high electroluminescence efficiencies indicate that bis-tridentate Ir(III) complexes are promising candidates for OLED applications.

Roles of Ancillary Chelates and Overall Charges of Bis-tridentate Ir(III) Phosphors for OLED Applications

A series of charge-neutral bis-tridentate Ir(III) complexes (1, 3, and 4) were prepared via employing three distinctive tridentate prochelates, that is, (pzp^tBph^FO)H₂, [(phpyim)H₂·(PF₆)], and [(pimb)H₃·(PF₆)₂], which possess one dianionic pzp^tBph^FO, together with a second monoanionic tridentate chelate, namely, (pzp^tBph^FO)H, phpyim, and pimb, respectively. Moreover, a homoleptic, charge-neutral complex 2 was obtained by methylation of chelating (pzp^tBph^FO)H of 1 in basic media, while closely related cationic complexes 5-7 were obtained by further methylation of the remaining pyrazolate unit of previously mentioned neutral complexes 2-4, followed by anion metatheses. All of these Ir(III) metal complexes showed a broadened emission profile with an onset at ∼450 nm, a result of an enlarged ligand-centered ππ* transition gap, but with distinct efficiencies ranging from 0.8% to nearly unity. Comprehensive spectroscopic and computational approaches were executed, providing a correlation for the emission efficiencies versus energy gaps and between the metal-to-ligand charge transfer/ππ* emitting excited state and upper-lying metal-centered dd quenching state. Furthermore, Ir(III) complexes 3 and 4 were selected as dopant emitters in the fabrication of sky-blue phosphorescent organic light-emitting diodes, affording maximum external quantum efficiencies of 16.7 and 14.6% with CIE_x,y coordinates of (0.214, 0.454) and (0.191, 0.404) at a current density of 10² cd/m², respectively. Hence, this research highlights an inherent character of bis-tridentate Ir(III) complexes in achieving high phosphorescence quantum yield at the molecular level.

A direct access to heterobimetallic complexes by roll-over cyclometallation

Complexes of the type [Cp*Ir(N,N')Cl]⁺ (N,N' = 2-(2-dialkylaminopyrimidin-4-yl)pyrimidine) can undergo roll-over cyclometallation leading to a novel N,N'-donor site. Following this strategy heterobimetallic complexes including iridium(iii) and a Group X metal centre in the oxidation state +II were achieved.

Photophysical dynamics of the efficient emission and photosensitization of [Ir(pqi)2(NN)]+complexes

Rational photophysical investigation through experimental and theoretical analyses reveals the photophysical dynamics of the highly-emissive [Ir(pqi)₂(NN)]⁺complex series, with remarkable emission quantum yields and efficient generation of singlet oxygen.

Cyclometalated Ir(iii) complexes containing quinoline–benzimidazole-based N^N ancillary ligands: structural and luminescence modulation by varying the substituent groups or the protonation/deprotonation state of imidazole units

Four Ir(iii) complexes have been prepared, showing structural and luminescence modulation upon varying their substituent groups or protonation/deprotonation state.

Cyclometalated Ir(iii) complexes based on 2-(2,4-difluorophenyl)-pyridine and 2,2′-(2-phenyl-1H-imidazole-4,5-diyl)dipyridine: acid/base-induced structural transformation and luminescence switching, and photocatalytic activity for hydrogen evolution

1·PF₆and2show luminescence switching behaviors, and serve as photosensitizers for H₂evolution.

A Mesoionic Carbene as Neutral Ligand for Phosphorescent Cationic Ir(III) Complexes

Two phosphorescent Ir(III) complexes bearing a mesoionic carbene ligand based on 1,2,3-triazolylidene are obtained for the first time. A silver-iridium transmetalation of the in situ-generated mesoionic carbene affords the cationic dichloro complex [Ir(trizpy)2Cl2](+) (3, trizpy = 1-benzyl-3-methyl-4-(pyridin-2-yl)-1H-1,2,3-triazolylidene) that reacts with a bis-tetrazolate (b-trz) dianionic ligand to give [Ir(trizpy)2(b-trz)](+) (5). The new compounds are fully characterized by NMR spectroscopy and mass spectrometry, and the X-ray structure of 3 is determined. The electrochemical behavior is somewhat different compared to most standard cationic iridium complexes. The first oxidation process is shifted to substantially higher potential in both 3 and 5, due to peculiar and different ligand-induced effects in the two cases, which stabilize the highest occupied molecular orbital; reduction processes are centered on the mesoionic carbene ligands. Both compounds exhibit a mostly ligand-centered luminescence band in the blue-green spectral region, substantially stronger in the case of 5 versus 3, both in CH3CN solution and in poly(methyl methacrylate) matrix at room temperature. Optimized geometries, orbital energies, spin densities, and electronic transitions are determined via density functional theory calculations, which support a full rationalization of the electrochemical and photophysical behavior. This work paves the way for the development of Ir-based emitters with neutral mesoionic carbene ligands and anionic ancillary ligands, a new concept in the area of cationic Ir(III) complexes.

Heteroleptic Ir(iii) and Pt(ii) complexes based on 2-(2,4-difluorophenyl)-pyridine and bisthienylethene BrLH: the influence of the metal center on structures, luminescence and photochromism

Based on bisthienylethene BrLH, [Ir(dfppy)₂(BrL)]·3CH₃OH (1) and [Pt(dfppy)(BrL)]·CH₃OH (2) have been prepared. The two complexes are significantly different in structure, luminescence and photochromic behavior, due to their different metal centers.