Phosphorescent Ir Research Articles

Synthesis and characterization of phosphorescent cyclometalated Ir and Pt heteroleptic complexes using cyclophane-based chelates

The synthesis and characterization of a series of neutral cyclometalated Pt(II) and Ir(III) heteroleptic complexes containing cyclophane substituted chelates is reported. The complexes have the general formula of (C^N)Pt(O^O) or (C^N)2Ir(O^O) where C^N is a monoanionic cyclophane ligand 2-([2.2]-paracyclophane-4-yl)pyridyl (pCpy) or 1-([2.2]-paracyclophane-4-yl)pyrazolyl (pCpz) and (O^O) is an ancillary ligand; acetylacetonato (acac) or dipivolylmethanato (dpm). The pCpyH and pCpzH ligand precursors were obtained as racemates due to the planar chirality of the cyclophane ring system. Two diastereomers of (pCpz)2Ir(acac), one with C1 symmetry (ΛRS, ΔSR) and the other with C2 symmetry (ΛRR, ΔSS) were isolated, whereas only one C1 diastereomer was obtained for (pCpy)2Ir(acac). Inherent strain in the cyclophane core and transannular interaction between the bridged phenyl rings leads to a 250–400mV cathodic shift in the oxidation potentials relative to analogous complexes with ppy or ppz ligands. The destabilization of the HOMO leads to a corresponding red shift in the absorption and emission spectra from the complexes with pCpy and pCpz ligands. The Pt and Ir complexes with pCpy ligands are strongly emissive at room temperature, whereas complexes with pCpz ligands are only weakly emissive as they still suffer from temperature-dependent nonradiative deactivation to metal-centered ligand field states.

Heteroleptic orange light-emitting iridium complexes containing phenylbenzothiazolate ligands

The new heteroleptic orange light-emitting iridium complexes were synthesized and their luminescent properties were investigated. The iridium (III) complexes, Ir(4-Me-2,3-dpq)2(pbt-R) and Ir(ppy)2(pbt-R), were prepared, where 4-Me-2,3-dpq, ppy and pbt-R (R=H, F, OMe) represent the ligands of 4-methyl-2,3-diphenylquinoline, 2-phenylpyridine and phenylbenzothiazole derivatives, respectively. The photoabsorption and photoluminescence properties of the iridium complexes were studied, and their band gaps were measured with cyclic voltammetry. The emission maxima of these complexes occurred between 550 and 630 nm with full widths at half maximum (FWHM) of 75–105 nm. The luminescence mechanisms of Ir(4-Me-2,3-dpq)2(pbt-R) and Ir(ppy)2(pbt-R) were found to be different, considering their contrasting PL patterns. Owing to their rather large FWHMs, the combination luminescence ranges of these orange phosphorescent complexes with blue phosphorescent Ir(F2-ppy)2(pic) were also investigated for the application to the white organic light-emitting diodes.

Cyclometallated iridium phosphors with amino acid ancillary ligand for intracellular imaging

Two new iridium complexes, (dfppy)2Ir(l-alanine) (dfppy=2-(2,4-difluorophenyl)pyridine) and (piq)2Ir(l-alanine) (piq=1-phenylisoquinoline) were prepared with l-alanine as ancillary ligand. The two complexes show bright greenish-blue and red emission respectively. Theoretic study demonstrated that the emission nature of these complexes is mainly determined by the main ligand. And their improved aqueous solubility and the retained quantum yield favor their application in cell imaging. Intracellular imaging suggested that these two complexes have fine cell membrane permeability and is mainly distributed in cytoplasm. This study displayed a new strategy to design aqueous soluble phosphorescent cyclometallated Ir(III) complex via introducing amino acid as ancillary ligand.

Real-time tracking mitochondrial dynamic remodeling with two-photon phosphorescent iridium (III) complexes

Mitochondrial fission and fusion control the shape, size, number, and function of mitochondria in the cells of organisms from yeast to mammals. The disruption of mitochondrial fission and fusion is involved in severe human diseases such as Parkinson's disease, Alzheimer's disease, metabolic diseases, and cancers. Agents that can real-time track the mitochondrial dynamics are of great importance. However, the short excitation wavelengths and rapidly photo-bleaching properties of commercial mitochondrial dyes render them unsuitable for tracking mitochondrial dynamics. Thus, mitochondrial targeting agents that exhibit superior photo-stability under continual light irradiation, deep tissue penetration and at intrinsically high three-dimensional resolutions are urgently needed. Two-photon-excited compounds employ low-energy near-infrared light and have emerged as a non-invasive tool for real-time cell imaging. Here, cyclometalated Ir(III) complexes (Ir1–Ir5) are demonstrated as one- and two-photon phosphorescent probes for the real-time imaging and tracking of mitochondrial fission and fusion. The results indicate that Ir2 is well suited for two-photon phosphorescent tracking of mitochondrial fission and fusion in living cells and in Caenorhabditis elegans (C. elegans). This study provides a practical use for mitochondrial targeting two-photon phosphorescent Ir(III) complexes.

A phosphorescent Ir(iii) complex with formamide for the luminescence determination of low-level water content in organic solvents

A novel formamide-containing Ir(iii) complex was designed and synthesized for the determination of water content with low LOD and LOQ by a time-resolved luminescence technique.

Solution-processable iridium phosphors for efficient red and white organic light-emitting diodes with low roll-off

A new red phosphorescent material Ir(dmppm)2(dmd) is synthesized and successfully used to fabricate solution-processed red and white organic light-emitting diodes.

Semiconducting polymer dots with phosphorescent Ir(III)-complex for photodynamic cancer therapy

Semiconducting polymer dots with phosphorescent Ir(III)-complex for photodynamic cancer therapy

Protein Labelling with Versatile Phosphorescent Metal Complexes for Live Cell Luminescence Imaging.

To take advantage of the luminescent properties of d(6) transition metal complexes to label proteins, versatile bifunctional ligands were prepared. Ligands that contain a 1,2,3-triazole heterocycle were synthesised using Cu(I) catalysed azide-alkyne cycloaddition "click" chemistry and were used to form phosphorescent Ir(III) and Ru(II) complexes. Their emission properties were readily tuned, by changing either the metal ion or the co-ligands. The complexes were tethered to the metalloprotein transferrin using several conjugation strategies. The Ir(III)/Ru(II)-protein conjugates could be visualised in cancer cells using live cell imaging for extended periods without significant photobleaching. These versatile phosphorescent protein-labelling agents could be widely applied to other proteins and biomolecules and are useful alternatives to conventional organic fluorophores for several applications.

Highly efficient solution-processed pure red phosphorescent organic light-emitting diodes using iridium complexes based on 2,3-diphenylquinoxaline ligand

We synthesized two highly efficient red phosphorescent Ir(III) complexes, bis[2,3-diphenylquinoxalinato-N,C2′]iridium(III) pyrazinate (dpq)2Ir(prz) (1) and bis[2,3-diphenylquinoxalinato- N,C2′]iridium(III) 5-methylpyrazinate (dpq)2Ir(mprz) (2), which are both based on the use of 2,3-diphenylquinoxaline as the ligand, and investigated their photophysical, electrochemical, and electroluminescence (EL) properties. Both Ir(III) complexes are soluble in common organic solvents and uniform thin films can be readily spin-coated onto substrates. Phosphorescent organic light-emitting diodes (PhOLEDs) produced using the ITO/PEDOT:PSS/TCTA:TPBi:Ir(III) complex/cathode configuration, had a maximum external quantum efficiency of 7.32% and a luminance efficiency of 7.15 cd/A with a CIE coordinate of (0.70, 0.30) for compound 2. CIE coordinates remained stable on increasing current density. To the best of our knowledge, the efficiencies of these PhOLEDs are higher than those previously reported for solution-processed pure red PhOLEDs.

Analyzing degradation effects of organic light-emitting diodes via transient optical and electrical measurements

Although the long-term stability of organic light-emitting diodes (OLEDs) under electrical operation made significant progress in recent years, the fundamental underlying mechanisms of the efficiency decrease during operation are not well understood. Hence, we present a comprehensive degradation study of an OLED structure comprising the well-known green phosphorescent emitter Ir(ppy)3. We use transient methods to analyze both electrical and optical changes during an accelerated aging protocol. Combining the results of displacement current measurements with time-resolved investigation of the excited states lifetimes of the emitter allows for a correlation of electrical (e.g., increase of the driving voltage due to trap formation) and optical (e.g., decrease of light-output) changes induced by degradation. Therewith, it is possible to identify two mechanisms resulting in the drop of the luminance: a decrease of the radiative quantum efficiency of the emitting system due to triplet-polaron-quenching at trapped charge carriers and a modified charge carrier injection and transport, as well as trap-assisted non-radiative recombination resulting in a deterioration of the charge carrier balance of the device.

Design and Synthesis of Pyrimidine-Based Iridium(III) Complexes with Horizontal Orientation for Orange and White Phosphorescent OLEDs.

Two phosphorescent Ir(III) complexes Ir(ppm)2(acac) and Ir(dmppm)2(acac) were synthesized and characterized with emission ranged at 584/600 nm and high photoluminescence quantum yields (PLQYs) of 0.90/0.92, respectively. The angle-dependent PL spectra analysis reveals that the two orange iridium(III) complexes embodied horizontal orientation property. The high photoluminescence quantum yield and high horizontal dipoles ratio determine their excellent device performance. The devices based on Ir(ppm)2(acac) and Ir(dmppm)2(acac) achieved efficiencies of 26.8% and 28.2%, respectively, which can be comparable to the best orange phosphorescent devices reported in the literature. Furthermore, with the introduction of FIrpic as sky-blue emitter, phosphorescent two-element white organic light-emitting devices (OLEDs) have been realized with external quantum efficiencies (EQEs) as high as 25%, which are the highest values among the reported two-element white OLEDs.

Novel Thieno[3,2-<i>c</i>]Pyridine Based Ir Complexes for Solution/Evaporation Hybrid OLED

A series of novel thieno [3,2-c] pyridine based phosphorescent Ir complexes have been successfully synthesized and characterized. The HTL-free devices were fabricated on ITO with a multilayered structure of ITO/PEDOT:PSS(45 nm)/host (TCTA or NPB):dopant (10~15 nm)/TmPyPb/LiF/Al by solution/evaporation hybrid process and showed relatively high efficacies. The high efficacy enables these emitters to serve as promising candidates in fabricating high quality RGB displays. The monochromatic large-area of 40 x 32 mm2 OLEDs had also been demonstrated.

Intracellular and in vivo oxygen sensing using phosphorescent Ir(III) complexes with a modified acetylacetonato ligand.

Small luminescent molecular probes based on the iridium(III) complex BTP, (btp)2Ir(acac) (btp = benzothienylpyridine, acac = acetylacetone) have been developed for sensing intracellular and in vivo O2. These compounds are BTPSA (containing an anionic carboxyl group), BTPNH2 (containing a cationic amino group), and BTPDM1 (containing a cationic dimethylamino group); all substituents are incorporated into the ancillary acetylacetonato ligand of BTP. Introduction of the cationic dimethylamino group resulted in an almost 20-fold increase in cellular uptake efficiency of BTPDM1 by HeLa cells compared with BTP. The phosphorescence intensity of BTPDM1 internalized in living cells provided a visual representation of the O2 gradient produced by placing a coverslip over cultured monolayer cells. The intracellular O2 levels (pO2) inside and outside the edge of the coverslip could be evaluated by measuring the phosphorescence lifetime of BTPDM1. Furthermore, intravenous administration of 25 nmol BTPDM1 to tumor-bearing mice allowed the tumor region to be visualized by BTPDM1 phosphorescence. The lifetime of BTPDM1 phosphorescence from tumor regions was much longer than that from extratumor regions, thereby demonstrating tumor hypoxia (pO2 = 6.1 mmHg for tumor and 50 mmHg for extratumor epidermal tissue). Tissue distribution studies showed that 2 h after injection of BTPDM1 into a mouse, the highest distribution was in liver and kidney, while after 24 h, BTPDM1 was excreted in the feces. These results demonstrate that BTPDM1 can be used as a small molecular probe for measuring intracellular O2 levels in both cultured cells and specific tissues and organs.

Computational prediction for emission energy of iridium (III) complexes based on TDDFT calculations using exchange-correlation functionals containing various HF exchange percentages.

The accurate prediction for the emission energies of the phosphorescent Ir (III) complexes is very useful for the realizing of full-color displays and large-area solid-state lighting in OLED fields. Quantum chemistry calculations based on TDDFT methods are most widely used to directly compute the triplet vertical excitation energies, yet sometimes the universality of these calculations can be limited because of the lack of experimental data for the relative family of structural analogues. In this letter, 16 literature emission energies at low temperature are linearly correlated with their theoretical values computed by TDDFT using exchange-correlation functionals containing various HF exchange percentage with the relation of E exp (em) = 1.2Ē calc (em). The relation is proven to be robust across a wide range of structures for Ir (III) complexes. These theoretical studies should be expected to provide some guides for the design and synthesis of efficient emitting materials.

Long-lived phosphorescent iridium(III) complexes conjugated with cationic polyfluorenes for heparin sensing and cellular imaging.

The applications of conjugated polyelectrolytes in biosensing and bioimaging have attracted more and more research interests due to their excellent photophysical properties. In this work, a new series of conjugated polyelectrolytes containing long-lived phosphorescent Ir(III) complexes is designed and synthesized, which can be used for ratiometric and lifetime-based sensing of heparin utilizing the electrostatic interaction between cationic polymers and anionic heparin. By changing the ligand structures of Ir(III) complexes, the sensing performances of phosphorescent-conjugated polyelectrolytes (PCPEs) are optimized. In addition, the application of PCPEs in cellular imaging is carried out. These polymers can be applied for specific staining of cell membrane. Importantly, utilizing the long emission lifetime of phosphorescent signal of Ir(III) complexes, time-gated luminescent imaging is carried out, which can eliminate the short-lived background fluorescence interferences from the environment or biological samples.

White light emission from a two-component hybrid gel via an energy transfer process.

A two-component light-harvesting organogel containing a naphthalimide-based gelator (1) as a donor and a phosphorescent Ir(III) complex [Ir(bt)2(acac)] (bt = 2-phenylbenzothiazole and acac = acetylacetone) (Ir) as an acceptor was used to produce white-light-emitting organogels. The addition of complex Ir to the gel 1 had a certain effect on the self-assembly behaviour of molecule 1, but did not affect the gelation ability, mechanical strength and structure surface wettability of the gel. The optical properties of the two-component gel 1-Ir could be tuned via high intermolecular energy transfer efficiency between 1 and complex Ir, which was confirmed by geometry optimizations and harmonic vibrational analyses. The white-light-emitting organogel was obtained with the molar ratio of complex Ir in the range of 0.3-1.0. In particular, the gel 1-Ir with the addition of 0.5 equivalent of Ir could emit white-light with the Commission Internationale de L' eclairage (CIE) coordinates of 0.33 and 0.31 under the excitation of 374 nm light.

Phosphorescent chemosensor for Hg2+ based on an iridium(iii) complex coordinated with 4-phenylquinazoline and carbazole dithiocarbamate

A colorimetric phosphorescent chemosensor Ir(pqz)2(cdc) showed satisfactory selective, sensitive, naked eye detection and fluorescence change response to Hg2+ in MeCN.

Tetraphenylsilane group containing carbazoles as high triplet energy host materials for solution-processable PhOLEDs

A series of solution processable, wide band-gap host materials composed of carbazole and tetraphenylsilane groups were designed and synthesized. Their thermal, electrochemical, and photophysical properties were fully investigated. The introduction of bulky tetraphenylsilane and \textit{tert}-butyl groups around the carbazole led to high glass transition temperatures (T$_{g})$ between 120 and 204 $^{\circ}$C. The triplet energies (E$_{T})$ of the synthesized materials were examined by low temperature (77 K) photoluminescence studies and determined as E$_{T}>$ 2.6 for all compounds. Phosphorescent organic light-emitting devices with the ITO/PEDOT:PSS/EML/TPBi/Cs$_{2}$CO$_{3}$/Al device structure were fabricated by using synthesized materials as the host and two kinds of phosphorescent emitters, FIrpic and Ir(ppy)$_{3}$, as the guests. The highest luminous and power efficiency values obtained by using FIrpic were 3.6 cd A$^{-1}$ and 1.48 lm W$^{-1}$, respectively, with commission International de I'Eclairage (CIE) coordinates of (0.17, 0.36), whereas these values were 7.8 cd A$^{-1}$ and 2.9 lm W$^{-1}$ for the device structure when Ir(ppy)$_{3}$ was used as the guest [CIE (0.28, 0.62)].

Ir(III) complexes designed for light-emitting devices: beyond the luminescence color array.

In pursuing novel efficient lighting technologies and materials, phosphorescent cyclometallated Ir(iii) complexes have been prominent due to their wide color arrays and highly efficient electroluminescence. Their photophysical properties are strongly influenced by spin-orbit coupling exerted by the iridium core, usually resulting in intense, short-lived emission, which can be systematically tuned as a triumph of molecular engineering. This Perspective aims to present recent breakthroughs and state of the art on emissive Ir(iii) compounds, in particular a personal account on heteroleptic [Ir(N^C)2(L^X)](+) complexes, addressing the mechanistic concepts behind their luminescence. Their fascinating photophysical properties strengthen application in more-efficient light-emitting technologies, such as organic light-emitting diodes and light-emitting electrochemical cells.

Design, synthesis and characterization of a new blue phosphorescent Ir complex

An efficient phosphorescent dye was developed by grafting a typically used phosphor to a functional group with an alkyl chain to suppress the phase aggregation.