Phosphorescent Iridium Complexes Research Articles

A New Facial Homoleptic Tris‐cyclometalated Iridium(III) Complex with Oxygen‐bridged Triarylamine Units

AbstractA new facial homoleptic tris‐cyclometalated phosphorescent iridium(III) complex Ir(ppyNO)3 featuring three oxygen‐bridged triarylamine units has been synthesized via an efficient Suzuki–Miyaura cross‐coupling reaction of oxygen‐bridged triarylamine unit based boron reagent 1‐b with corresponding tribrominated iridium(III) complex Ir‐Br. It can not only show an effective green light emission, but also display a narrower full width at half maximum (FWHM=57 nm) and better solubility than that of the classical complex fac‐Ir(ppy)3 (FWHM=82 nm, ppy=2‐phenylpyridine). Furthermore, a longer emission lifetime (τ=1201 ns) of Ir(ppyNO)3 in polystyrene (PS) film can be found than that (τ=45 ns) in CH2Cl2 solution. In addition, the lowest triplet state (T1) of Ir(ppyNO)3 can exhibit both metal‐to‐ligand charge transfer (3MLCT) and intraligand charge transfer (3ILCT) excited state character according to DFT calculations.

A phosphorescent iridium complex as a probe for diatom cells’ viability

Diatoms are unicellular photosynthetic algae that autonomously fabricate a porous organized biosilica shell refined in billion years of evolution. They represent an inexhaustible source of low cost, biocompatible mesoporous silica. Despite the major advances in the genomic field, studies on diatom cell biology are still hampered by a lack of cellular tools. In particular, cell staining assays of diatoms viability are still limited or not well performant. Here we provide a phosphorescent organometallic iridium complex (Ir-Fcx) suitable to act as staining agent to detect diatoms viability.

Saturated red phosphorescent Iridium(III) complexes containing phenylquinoline ligands for efficient organic light-emitting diodes

Saturated red iridium phosphors are strongly desired for application in organic light-emitting diodes (OLEDs). Most of the reported iridium complexes with 2-arylquinoline or its derivatives as cyclometalating ligands are orange-red phosphorescent with peak wavelength below 620 nm. Herein trifluoromethyl (CF3) or Fluorine (F) is introduced into the 6-site of quinoline ring to design and synthesize the cyclometalated ligands 2-phenyl-6-CF3/F-quinoline (CF3-phq or F-phq) and heteroleptic iridium complexes Ir(CF3-phq)2(acac) (Ir-1), Ir(CF3-phq)2(tmd) (Ir-2), Ir(F-phq)2(acac) (Ir-3) and Ir(F-phq)2(tmd) (Ir-4). Owing to the electron-deficient feature of CF3 and F, the electron affinity of these complexes are increased, the corresponding LUMOs are pulled downwards, and the phosphorescence of CF3-containing iridium complexes are saturated red with peaks at 627–632 nm in photoluminescence. At the same time, by varying the ancillary ligand from acetylacetonate (acac) to 2,2,6,6-tetrametylheptane-3,5-dionate (tmd), the emitting performance are dramatically improved. As a result, the Ir(CF3-phq)2(tmd) based red OLED realized pure red electroluminescence with Commision Internationale de L'Eclairage (CIE) coordinates of (0.67, 0.33) with a high brightness of 12695 cd m−2 and a maximum external quantum efficiency of 14.96%, respectively, which maintained as high as 9.62% at a practical luminance of 1000 cd m−2. This is the first example of saturated red iridium phosphors with structure modification at the 6-site of quinoline ring of 2-phenylquinoline ligand.

Mitochondria-targeted phosphorescent cyclometalated iridium(III) complexes: synthesis, characterization, and anticancer properties.

Cyclometalated iridium(III) complexes represent a promising approach to developing new anticancer metallodrugs. In this work, three phosphorescent cyclometalated iridium(III) complexes Ir1-Ir3 have been explored as mitochondria-targeted anticancer agents. All three complexes display higher antiproliferative activity than cisplatin against the cancer cells screened, and with the IC50 values ranging from 0.23 to 5.6μM. Colocalization studies showed that these complexes are mainly localized in the mitochondria. Mechanism studies show that these complexes exert their anticancer efficacy through initiating a series of events related to mitochondrial dysfunction, including depolarization of mitochondrial membrane potential (MMP), elevation of intracellular reactive oxygen species (ROS) levels, and induction of apoptosis. Mitochondria-targted cyclometalated iridium complexes induce apoptosis through depolarized mitochondria, elevation of intracellular ROS and activated caspase.

A theoretical study: Green phosphorescent iridium(III) complexes with low‐efficiency roll‐off

A series of heterocyclic Ir(III) complexes used in organic light‐emitting diode (OLED) materials with low‐efficiency roll‐off performance have been studied theoretically. Their electronic structures, spectral properties, and their application value in OLEDs are discussed. The geometries, electronic structures, lowest‐lying singlet absorptions, and triplet emissions of (dmdppr‐dmp)2Ir(dibm), and the theoretically designed models of (dmdppr‐dmp)2Ir(acac), (dmdppr‐dmp)2Ir(tpip), (dmdppr‐Fdmp)2Ir(dibm), (dmdppr‐Fdmp)2Ir(acac), and (dmdppr‐Fdmp)2Ir(tpip) were investigated with density‐functional‐theory‐based approaches, where dibm denotes 2,6‐dimethy‐3,5‐heptanedionato‐κ2‐O,O′, acac denotes acetylacetonate, and tpip denotes tetraphenylimido‐diphosphinate.

An Efficient Probe of Cyclometallated Phosphorescent Iridium Complex for Selective Detection of Cyanide.

A cyclometallated phosphorescent iridium-based probe to detect CN– was prepared through a cyanide alcoholize reaction based on the C^N type main ligand and N^N type ancillary ligand (2-phenyl pyridine and 1,10-phenanthroline-5-carboxaldehyde, respectively). The efficient probe exhibited good sensitivity in response to CN– in an CH3CN and H2O (95/5) mixture within a 1.23 μM detection limit. The response of PL is directly in line with the concentration of CN– from 0 to 2.0 equiv. The PL investigation of other reactive anions proved the great selectivity to CN–. Additionally, upon adding 1.0 equiv. of cyanide, the formation of cyanohydrin was correctly elucidated in 1H NMR, FT-IR, and mass spectra studies. The conspicuous results indicate that the iridium complex has the potential possibility of application in other biosystems related to CN–.

Phosphorescent Iridium(III) Complexes with Acyclic Diaminocarbene Ligands as Chemosensors for Mercury.

A new application for bis(cyclometalated) iridium(III) species containing ancillary acyclic diaminocarbene ligands, viz. for sensing of mercury(II) ions, is disclosed. A family of bis(cyclometalated) iridium(III) species supported by both parent isocyanide and acyclic diaminocarbene ligands was prepared, and their electrochemical and photophysical properties were evaluated, revealing efficient blue-green phosphorescence in solution with quantum yields of up to 55%. We uncovered that the photophysical properties of these complexes are dramatically altered by the presence of metal ions and that the complex [Ir(ppy)2(CN){C(NH2)(NHC6H4-4-X)}] with an ADC ligand reacts selectively with Hg2+ ions, enabling its use for sensing of mercury(II) ions in solution. The limit of detection was as low as 2.63 × 10-7 M, and additional mechanistic studies indicated the formation of an unusual dinuclear iridium(III) cyclometalated intermediate, bridged by a mercury dicyano fragment as a driving force of mercury sensing.

Blue‐Emitting Iridium(III) Complexes for Light‐Emitting Electrochemical Cells: Advances, Challenges, and Future Prospects

AbstractLight‐emitting electrochemical cells (LECs) are one of the most promising technologies for solid‐sate lighting. Among them, LECs based on phosphorescent iridium(III) complexes have attracted significant research interest in the past 15 years, because of their high efficiency and tunable emission color across the entire visible spectrum. To fabricate white LECs for lighting, high‐performance blue LECs are the first prerequisite. Huge efforts have been devoted to improving the performances of blue LECs based on iridium(III) complexes either by developing new blue‐emitting complexes or by engineering the devices. Nevertheless, blue LECs have still shown much lower performances (brightness, efficiency, stability, etc.) compared to the red, orange‐red, yellow, and green counterpart devices. In particular, a single blue LEC with satisfactory blue‐color purity, high efficiency, and high stability is still missing. Here, the advances in blue‐emitting iridium(III) complexes for LECs and the device engineering on LECs using the complexes are reported. The challenges ahead are discussed, and future prospects are outlined.

Unravelling the Mechanism of Excited-State Interligand Energy Transfer and the Engineering of Dual Emission in [Ir(C∧N)2(N∧N)]+ Complexes.

Fundamental insights into the mechanism of triplet-excited-state interligand energy transfer dynamics and the origin of dual emission for phosphorescent iridium(III) complexes are presented. The complexes [Ir(C∧N)2(N∧N)]+ (HC∧N = 2-phenylpyridine (1a-c), 2-(2,4-difluorophenyl)pyridine (2a-c), 1-benzyl-4-phenyl-1,2,3-triazole (3a-c); N∧N = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (pytz, a), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (pymtz, b), 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (pyztz, c)) are phosphorescent in room-temperature fluid solutions from triplet metal-to-ligand charge transfer (3MLCT) states admixed with either ligand-centered (3LC) (1a, 2a, and 2b) or ligand-to-ligand charge transfer (3LL'CT) character (1c, 2c, and 3a-c). Particularly striking is the observation that pyrimidine-based complex 1b exhibits dual emission from both 3MLCT/3LC and 3MLCT/3LL'CT states. At 77 K, the 3MLCT/3LL'CT component is lost from the photoluminescence spectra of 1b, with emission exclusively arising from its 3MLCT/3LC state, while for 2c switching from 3MLCT/3LL'CT- to 3MLCT/3LC-based emission is observed. Femtosecond transient absorption data reveal distinct spectral signatures characteristic of the population of 3MLCT/3LC states for 1a, 2a, and 2b which persist throughout the 3 ns time frame of the experiment. These 3MLCT/3LC state signatures are apparent in the transient absorption spectra for 1c and 2c immediately following photoexcitation but rapidly evolve to yield spectral profiles characteristic of their 3MLCT/3LL'CT states. Transient data for 1b reveals intermediate behavior: the spectral features of the initially populated 3MLCT/3LC state also undergo rapid evolution, although to a lesser extent than that observed for 1c and 2c, behavior assigned to the equilibration of the 3MLCT/3LC and 3MLCT/3LL'CT states. Density functional theory (DFT) calculations enabled minima to be optimized for both 3MLCT/3LC and 3MLCT/3LL'CT states of 1a-c and 2a-c. Indeed, two distinct 3MLCT/3LC minima were optimized for 1a, 1b, 2a, and 2b distinguished by upon which of the two C∧N ligands the excited electron resides. The 3MLCT/3LC and 3MLCT/3LL'CT states for 1b are very close in energy, in excellent agreement with experimental data demonstrating dual emission. Calculated vibrationally resolved emission spectra (VRES) for the complexes are in excellent agreement with experimental data, with the overlay of spectral maxima arising from emission from the 3MLCT/3LC and 3MLCT/3LL'CT states of 1b convincingly reproducing the observed experimental spectral features. Analysis of the optimized excited-state geometries enable the key structural differences between the 3MLCT/3LC and 3MLCT/3LL'CT states of the complexes to be identified and quantified. The calculation of interconversion pathways between triplet excited states provides for the first time a through-space mechanism for a photoinduced interligand energy transfer process. Furthermore, examination of structural changes between the possible emitting triplet excited states reveals the key bond vibrations that mediate energy transfer between these states. This work therefore provides for the first time detailed mechanistic insights into the fundamental photophysical processes of this important class of complexes.

A near-infrared phosphorescent iridium(iii) complex for fast and time-resolved detection of cysteine and homocysteine.

Thiol-containing amino acids, cysteine (Cys) and homocysteine (Hcy), play crucial roles in the biosystem; their abnormal contents in the cells are linked to many diseases. Herein, we designed and synthesized a novel near-infrared (NIR) phosphorescent iridium(iii) complex-based probe (FNO1) that can detect Cys and Hcy in real-time in the biosystem. Due to the advantages of the iridium complex, the FNO1 probe had excellent chemical stability and photostability, high luminescence efficiency, and long luminescence lifetime. In addition, the probe showed a fast response, high sensitivity, and low cytotoxicity. As verified by high resolution mass spectra (HR-MS) and density functional theory (DFT) calculations, the detection was achieved through the addition of the α,β-unsaturated ketone group in FNO1 by the nucleophilic thiol group in Cys and Hcy. Through time-resolved emission spectroscopy (TRES) and in the presence of a strongly fluorescent dye rhodamine B, the FNO1 probe could detect Cys and Hcy due to its long luminescence lifetime (260/197 ns). Finally, owing to its NIR-emitting properties, the FNO1 probe was successfully applied in the imaging of Cys and Hcy in living cells, zebrafish, and mice.

Phosphorescent iridium(III) complex for efficient sensing of hypochlorite and imaging in living cells

In consideration of the strong oxidizing power of hypochlorite (ClO−), which could cleave CN moiety, a cyclometalated iridium (III) complex (Ir-Ts) modified hydrazide group as the response unit was synthesized to sensitively and selectively detect ClO− under neutral condition. Upon addition of ClO−, a 21-fold emission enhancement at 574 nm was observed and phosphorescent product was formed due to the cleavage of CN moiety. The probe Ir-Ts displayed rapid response (<15 s) and high selectivity toward ClO− with a low detection limit of 86 nM. More importantly, bioimaging of ClO− was further studied in living cells.

Conjugated Regulation of Phosphorescent Iridium (III) Complex Constructed from Spiro Ligand and Its Electroluminescent Performances

Conjugated Regulation of Phosphorescent Iridium (III) Complex Constructed from Spiro Ligand and Its Electroluminescent Performances

Synthesis and Characterization of High-Efficiency Red Phosphorescent Iridium(III) Complexes with 1-(4-(Trifluoromethyl)phenyl)isoquinoline Ligand

Two new tfmpiq-based bis-cyclometalated iridium(III) complexes, [(tfmpiq)2Ir(imdzppo)] (2a) and [(tfmpiq)2Ir(idzpo)] (2b) (where tfmpiq = 1-(4-(trifluoromethyl)phenyl)isoquinoline, imdzppo = 2-(imidazo[1,2-a]pyridin-2-yl)phenol, idzpo = 2-(2H-indazol-2-yl)phenol), have been synthesized and fully characterized. The single crystal structure of 2b has been determined. The relationship between the structures and photophysical properties of both complexes are considered, and the DFT calculations have been used to further support the deduction. These Ir(III) complexes emit red light with quantum yields of 39.9-51.9% in degassed CH2Cl2 solution at room temperature. Also, their emission originates from a hybrid 3MLCT/3LLCT/3LC excited state. All these results show that iridium(III) complexes 2a-2b are suitable for red-phosphorescent materials in OLEDs.

Human-eyes-friendly white electroluminescence from solution-processable hybrid OLEDs exploiting new iridium (III) complex containing benzoimidazophenanthridine ligand

This paper primarily deals with the development of highly efficient phosphorescent iridium (III) complex based on benzoimidazophenanthridine as main ligand. The potential of this complex, as phosphorescent emitter for optoelectronic applications, was screened by different methods, including luminescence spectrometry, cyclic voltammetry, thermogravimetric analysis, differential scanning calorimetry as well as photoelectron emission spectrometry. The possible application of the developed complex in single-colour vacuum-deposited organic light emitting diodes (OLEDs) exhibiting warm orange electrophosphorescence was demonstrated. The device was characterized with CIE1931 colour coordinates of (0.539, 0.459), colour temperature of 2088 K and maximum external quantum efficiency of 17.5%. Good performance in OLEDs can be partly explained by efficient orange phosphorescence with quantum yield of 60% at room temperature. This paper also deals with the development of white hybrid solution-processable organic light-emitting devices, exploiting ultralow concentration of newly synthesized iridium (III) complex as phosphorescent dopant in combination with blue fluorescencent emitter(host), and green emitter exhibiting thermally activated delayed fluorescence emitter. It was designed and fabricated by spin coating white hybrid devices, and was characterized by high quality (human-eyes-friendly) electroluminescence achieving CIE1931 coordinates of (0.335, 0.392), colour temperature of 2910 K and colour rendering index of 72 as well as high maximum external quantum efficiency of 8.7%.

New yellow/orange-emitting heteroleptic iridium(III) complexes with 5,7-difluoro-2-phenylbenzothiazole ligand

Two new phosphorescent iridium(III) complexes, (dfbt)2Ir(pic) and (dfbt)2Ir(acac) (dfbt = 5,7-difluoro-2-phenylbenzothiazole, pic = picolinate, acac = acetylacetonate), have been synthesized and characterized. Upon photoexcitation, both complexes exhibit strong yellow–orange emission in CH2Cl2 solution at room temperature with absolute quantum yields up to 99.0% and emission lifetimes up to 1.23 μs. The spectroscopic property has been calculated by density functional theory and time-dependent density functional theory, thus suggesting that the lowest absorption and the emission originate from the metal-to-ligand charge transfer/ligand-centered transition.

Theoretical investigation on a series of phosphorescent heteroleptic cyclometalated iridium (III) complexes containing substituted 2-(3-sulfonylfluorophenyl)pyridine ligands

By using the density functional theory/time-dependent density functional theory methods, the electronic structure and photophysical properties of five heteroleptic cyclometalated Ir(III) complexes have been studied. The lowest absorption wavelengths are located at 350 nm for 1, 352 nm for 2, 354 nm for 3, 400 nm for 4 and 352 nm for 5, respectively. The phosphorescence wavelength of complexes 1–5 are localized at 445, 473, 474, 490 and 466 nm, respectively, simulated in CH2Cl2 medium at CAM-B3LYP level. It is expected that the theoretical results could help the experimentalists in synthesizing new excellent emitters for organic light-emitting diodes.

Phosphorescent iridium(III) complex based photoluminescence sensor for sensitive and selective detection of picric acid

The sensitive and selective detection of picric acid is of great importance to the society safety and human health. In this paper, a novel blue emissive iridium(III) complex FIrPicOMPBHCz containing carbazole moieties has been successfully synthesized and applied in photoluminescence (PL) sensing towards nitroaromatic explosives (e.g. PA, TNB, DNP, TNT, DNT, NP, NBA, NB) detection. The PL emission of FIrPicOMPBHCz could be quenched by the explosives with different extents. Among these, the quenching constant KSV towards PA is the highest, which is 1.9 × 104 M−1, with one to two orders of magnitude higher than that of other nitroaromatics, thus demonstrating a selective PL detection of PA. The sensing mechanism is based on the excited state electron transfer process, by analyzing the UV–vis absorption, excitation and PL spectra and cyclic voltammogram results. In addition, the paper strip tests indicate a great potential of FIrPicOMPBHCz as a solid PL sensor for PA.

Ratiometric oxygen probes with a cell-penetrating peptide for imaging oxygen levels in living cells

We investigated the photophysical and intracellular properties of two newly designed molecule-based ratiometric oxygen probes, 7DEAC-R8-BTQphen (R8) and 7DEAC-R12-BTQphen (R12), to evaluate their suitability for intracellular O2 measurements. R8 and R12 consist of a blue fluorescent coumarin derivative (7DEAC) and a red phosphorescent iridium(III) complex (BTQphen) connected respectively by an octa- or dodecaarginine linker to promote intracellular delivery of the probes. The probes showed oxygen-sensitive dual emissions in acetonitrile, lipid bilayer membrane, and in living cells. Energy state diagrams showing the relaxation processes of R8 and R12 in the excited state were constructed using emission spectra, quantum yield, and lifetime data. Efficient photon absorption at 405 nm due to the 7DEAC chromophore is followed by intramolecular energy transfer to the BTQphen moiety, resulting in 7DEAC fluorescence and BTQphen phosphorescence with an intensity ratio and spectral separation suitable for ratiometric measurements. The validities of O2 concentrations derived from the ratios between the phosphorescence and fluorescence intensities of R8 and R12 were verified in lipid membranes, as well as in solution by phosphorescence lifetime measurements of the BTQphen moiety. The cellular uptake efficiencies of R8 and R12 were significantly enhanced due to their oligoarginine linkers compared to previous probes with oligoproline linkers. These ratiometric oxygen probes allowed visualization of the O2 gradient produced by placing a thin coverslip onto a HeLa cell monolayer.

Complementary Approaches to Imaging Subcellular Lipid Architectures in Live Bacteria Using Phosphorescent Iridium Complexes and Raman Spectroscopy

A family of three neutral iridium(III) tetrazolato complexes are investigated as bacterial imaging agents. The complexes offer a facile tuning of the emission colour from green (520 nm) to red (600 nm) in aqueous media, while keeping the excitation wavelength unchanged. The three complexes do not inhibit the bacterial growth of Bacillus Cereus, used as a model in this study, and exhibit extremely fast cellular uptake. After a minute incubation time, the nontoxic complexes show subcellular localisation in spherical structures identified as lipid vacuoles. Confocal Raman imaging has been exploited for the first time on live bacteria, to provide direct and label-free mapping of the lipid-enriched organelles within B. cereus, complementing the use of luminescent probes. Examination of the Raman spectra not only confirmed the presence of lipophilic inclusions in B. cereus but offered additional information about their chemical composition, suggesting that the lipid vacuoles may contain polyhydroxybutyrate (PHB).

Bimodal Visualization of Endogenous Nitric Oxide in Lysosomes with a Two-Photon Iridium(III) Phosphorescent Probe.

Nitric oxide (NO) is a fundamental signaling molecule that shows complex effects on the catabolic autophagy process, which is closely linked with lysosomal function. In this study, a new lysosome-targeted, pH-independent, and two-photon phosphorescent iridium(III) complex, Ir-BPDA, has been investigated for endogenous NO detection and imaging. The rational design of the probe, as the addition of the morpholine moieties and the substitution of a benzyl group in the amino group in Ir-BPDA, facilitates its accumulation in lysosomes and makes the reaction product with NO, Ir-BPDA-NO, insusceptible in its phosphorescence intensity and lifetime against pH changes (pH 4-10), well suited for lysosomal NO detection (pH 4-6). Furthermore, Ir-BPDA exhibits a fast and 50-fold response to NO in phosphorescence intensity and a two-photon cross-section as high as 60 GM after the reaction, as well as a notably increased phosphorescence lifetime from 200.1 to 619.6 ns. Thus, accompanied by its photostability, Ir-BPDA enabled the detection of NO in the lipopolysaccharide-stimulated macrophages and zebrafish model, revealing the endogenous lysosomal NO distribution during inflammation in vivo by means of both TPM and PLIM imaging techniques.