Stable Green Electroluminescence from an Iridium Tris-Heteroleptic Ionic Complex

An ionic tris-heteroleptic iridium complex gives green light-emitting electrochemical cells (LECs) with unprecedented performances for this part of the visible spectrum. The devices are very bright (>1000 cd m^–2), efficient (∼3%), and stable (>55 h). The novel complex is prepared using a new and efficient synthetic procedure. We show that there is a mixed orbital formation originating from the two different orthometalating ligands resulting in photophysical properties that lie between those of its two bis-heteroleptic analogs. Therefore, tris-heteroleptic complexes provide new avenues for fine-tunning the emission properties and to bridge gaps between a series of bis-heteroleptic complexes

Correlating the Lifetime and Fluorine Content of Iridium(III) Emitters in Green Light-Emitting Electrochemical Cells

In light-emitting electrochemical cells, the lifetime of the device is intrinsically linked to the stability of the phosphorescent emitter. In this study, we present a series of ionic iridium­(III) emitters based on cyclometalating phenylpyridine ligands whose fluorine substituents are varied in terms of position and number. Importantly, despite these structural modifications, the emitters exhibit virtually identical electrochemical and spectroscopic properties, which allows for proper comparison in functional devices. Quantum-chemical calculations support the properties measured in solution and suggest great similarities regarding the electronic structures of the emitters. In electroluminescent devices, the initial luminance, efficiency, and efficacy are also relatively unaffected throughout the series. However, a shorter device lifetime is obtained upon increasing the fluorine content of the emitter, which suggests drawbacks of such electron-withdrawing substituents for the design of ionic iridium­(III) emitters

Acid-Induced Degradation of Phosphorescent Dopants for OLEDs and Its Application to the Synthesis of Tris-heteroleptic Iridium(III) Bis-cyclometalated Complexes

Investigations of blue phosphorescent organic light emitting diodes (OLEDs) based on [Ir­(2-(2,4-difluorophenyl)­pyridine)₂(picolinate)] (FIrPic) have pointed to the cleavage of the picolinate as a possible reason for device instability. We reproduced the loss of picolinate and acetylacetonate ancillary ligands in solution by the addition of Brønsted or Lewis acids. When hydrochloric acid is added to a solution of a [Ir­(C^∧N)₂(X^∧O)] complex (C^∧N = 2-phenylpyridine (ppy) or 2-(2,4-difluorophenyl)­pyridine (diFppy) and X^∧O = picolinate (pic) or acetylacetonate (acac)), the cleavage of the ancillary ligand results in the direct formation of the chloro-bridged iridium­(III) dimer [{Ir­(C^∧N)₂(μ-Cl)}₂]. When triflic acid or boron trifluoride are used, a source of chloride (here tetrabutylammonium chloride) is added to obtain the same chloro-bridged iridium­(III) dimer. Then, we advantageously used this degradation reaction for the efficient synthesis of tris-heteroleptic cyclometalated iridium­(III) complexes [Ir­(C^∧N¹)­(C^∧N²)­(L)], a family of cyclometalated complexes otherwise challenging to prepare. We used an iridium­(I) complex, [{Ir­(COD)­(μ-Cl)}₂], and a stoichiometric amount of two different C^∧N ligands (C^∧N¹ = ppy; C^∧N² = diFppy) as starting materials for the swift preparation of the chloro-bridged iridium­(III) dimers. After reacting the mixture with acetylacetonate and subsequent purification, the tris-heteroleptic complex [Ir­(ppy)­(diFppy)­(acac)] could be isolated with good yield from the crude containing as well the bis-heteroleptic complexes [Ir­(ppy)₂(acac)] and [Ir­(diFppy)₂(acac)]. Reaction of the tris-heteroleptic acac complex with hydrochloric acid gives pure heteroleptic chloro-bridged iridium dimer [{Ir­(ppy)­(diFppy)­(μ-Cl)}₂], which can be used as starting material for the preparation of a new tris-heteroleptic iridium­(III) complex based on these two C^∧N ligands. Finally, we use DFT/LR-TDDFT to rationalize the impact of the two different C^∧N ligands on the observed photophysical and electrochemical properties

Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium­(III) complexes Ir­(2,4-di-X-phenyl-pyridine)₂(picolinate) (X = H, F, Cl, Br) <b>HIrPic</b>, <b>FIrPic</b>, <b>ClIrPic</b>, and <b>BrIrPic</b> has been synthesized and characterized by NMR, X-ray crystallography, UV–vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with <b>FIrPic</b> and <b>ClIrPic</b>, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of <b>BrIrPic</b> and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin–orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for <b>ClIrPic</b> and <b>BrIrPic</b>, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect

Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium­(III) complexes Ir­(2,4-di-X-phenyl-pyridine)₂(picolinate) (X = H, F, Cl, Br) <b>HIrPic</b>, <b>FIrPic</b>, <b>ClIrPic</b>, and <b>BrIrPic</b> has been synthesized and characterized by NMR, X-ray crystallography, UV–vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with <b>FIrPic</b> and <b>ClIrPic</b>, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of <b>BrIrPic</b> and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin–orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for <b>ClIrPic</b> and <b>BrIrPic</b>, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect

Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium­(III) complexes Ir­(2,4-di-X-phenyl-pyridine)₂(picolinate) (X = H, F, Cl, Br) <b>HIrPic</b>, <b>FIrPic</b>, <b>ClIrPic</b>, and <b>BrIrPic</b> has been synthesized and characterized by NMR, X-ray crystallography, UV–vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with <b>FIrPic</b> and <b>ClIrPic</b>, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of <b>BrIrPic</b> and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin–orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for <b>ClIrPic</b> and <b>BrIrPic</b>, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect

Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium­(III) complexes Ir­(2,4-di-X-phenyl-pyridine)₂(picolinate) (X = H, F, Cl, Br) <b>HIrPic</b>, <b>FIrPic</b>, <b>ClIrPic</b>, and <b>BrIrPic</b> has been synthesized and characterized by NMR, X-ray crystallography, UV–vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with <b>FIrPic</b> and <b>ClIrPic</b>, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of <b>BrIrPic</b> and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin–orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for <b>ClIrPic</b> and <b>BrIrPic</b>, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect

Charged Bis-Cyclometalated Iridium(III) Complexes with Carbene-Based Ancillary Ligands

Charged cyclometalated (C^∧N) iridium­(III) complexes with carbene-based ancillary ligands are a promising family of deep-blue phosphorescent compounds. Their emission properties are controlled primarily by the main C^∧N ligands, in contrast to the classical design of charged complexes where N^∧N ancillary ligands with low-energy π* orbitals, such as 2,2'-bipyridine, are generally used for this purpose. Herein we report two series of charged iridium complexes with various carbene-based ancillary ligands. In the first series the C^∧N ligand is 2-phenylpyridine, whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One bis-carbene (:C^∧C:) and four different pyridine–carbene (N^∧C:) chelators are used as bidentate ancillary ligands in each series. Synthesis, X-ray crystal structures, and photophysical and electrochemical properties of the two series of complexes are described. At room temperature, the :C^∧C: complexes show much larger photoluminescence quantum yields (Φ_PL) of ca. 30%, compared to the N^∧C: analogues (around 1%). On the contrary, all of the investigated complexes are bright emitters in the solid state both at room temperature (1% poly­(methyl methacrylate) matrix, Φ_PL 30–60%) and at 77 K. Density functional theory calculations are used to rationalize the differences in the photophysical behavior observed upon change of the ancillary ligands. The N^∧C:-type complexes possess a low-lying triplet metal-centered (³MC) state mainly deactivating the excited state through nonradiative processes; in contrast, no such state is present for the :C^∧C: analogues. This finding is supported by temperature-dependent excited-state lifetime measurements made on representative N^∧C: and :C^∧C: complexes

Charged Bis-Cyclometalated Iridium(III) Complexes with Carbene-Based Ancillary Ligands

Charged cyclometalated (C^∧N) iridium­(III) complexes with carbene-based ancillary ligands are a promising family of deep-blue phosphorescent compounds. Their emission properties are controlled primarily by the main C^∧N ligands, in contrast to the classical design of charged complexes where N^∧N ancillary ligands with low-energy π* orbitals, such as 2,2'-bipyridine, are generally used for this purpose. Herein we report two series of charged iridium complexes with various carbene-based ancillary ligands. In the first series the C^∧N ligand is 2-phenylpyridine, whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One bis-carbene (:C^∧C:) and four different pyridine–carbene (N^∧C:) chelators are used as bidentate ancillary ligands in each series. Synthesis, X-ray crystal structures, and photophysical and electrochemical properties of the two series of complexes are described. At room temperature, the :C^∧C: complexes show much larger photoluminescence quantum yields (Φ_PL) of ca. 30%, compared to the N^∧C: analogues (around 1%). On the contrary, all of the investigated complexes are bright emitters in the solid state both at room temperature (1% poly­(methyl methacrylate) matrix, Φ_PL 30–60%) and at 77 K. Density functional theory calculations are used to rationalize the differences in the photophysical behavior observed upon change of the ancillary ligands. The N^∧C:-type complexes possess a low-lying triplet metal-centered (³MC) state mainly deactivating the excited state through nonradiative processes; in contrast, no such state is present for the :C^∧C: analogues. This finding is supported by temperature-dependent excited-state lifetime measurements made on representative N^∧C: and :C^∧C: complexes

Charged Bis-Cyclometalated Iridium(III) Complexes with Carbene-Based Ancillary Ligands

Charged cyclometalated (C^∧N) iridium­(III) complexes with carbene-based ancillary ligands are a promising family of deep-blue phosphorescent compounds. Their emission properties are controlled primarily by the main C^∧N ligands, in contrast to the classical design of charged complexes where N^∧N ancillary ligands with low-energy π* orbitals, such as 2,2'-bipyridine, are generally used for this purpose. Herein we report two series of charged iridium complexes with various carbene-based ancillary ligands. In the first series the C^∧N ligand is 2-phenylpyridine, whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One bis-carbene (:C^∧C:) and four different pyridine–carbene (N^∧C:) chelators are used as bidentate ancillary ligands in each series. Synthesis, X-ray crystal structures, and photophysical and electrochemical properties of the two series of complexes are described. At room temperature, the :C^∧C: complexes show much larger photoluminescence quantum yields (Φ_PL) of ca. 30%, compared to the N^∧C: analogues (around 1%). On the contrary, all of the investigated complexes are bright emitters in the solid state both at room temperature (1% poly­(methyl methacrylate) matrix, Φ_PL 30–60%) and at 77 K. Density functional theory calculations are used to rationalize the differences in the photophysical behavior observed upon change of the ancillary ligands. The N^∧C:-type complexes possess a low-lying triplet metal-centered (³MC) state mainly deactivating the excited state through nonradiative processes; in contrast, no such state is present for the :C^∧C: analogues. This finding is supported by temperature-dependent excited-state lifetime measurements made on representative N^∧C: and :C^∧C: complexes