Rational Design of Cyclometalated Iridium(III) Complexes for Three‐Photon Phosphorescence Bioimaging

Compared to 2PE (two‐photon excitation) microscopy, 3PE microscopy has superior spatial resolution, deeper tissue penetration, and less defocused interference. The design of suitable agents with a large Stokes shift, good three‐photon absorption (3PA), subcellular targeting, and fluorescence lifetime imaging (FLIM) properties, is challenging. Now, two Ir^III complexes (3PAIr1 and 3PAIr2) were developed as efficient three‐photon phosphorescence (3PP) agents. Calculations reveal that the introduction of a new group to the molecular scaffold confers a quadruple promotion in three‐photon transition probability. Confocal and lifetime imaging of mitochondria using Ir^III complexes as 3PP agents is shown. The complexes exhibit low working concentration (50 nm ), fast uptake (5 min), and low threshold for three‐photon excitation power (0.5 mW at 980 nm). The impressive tissue penetration depth (ca. 450 μm) allowed the 3D imaging and reconstruction of brain vasculature from a living specimen.     

Tuning the Cellular Uptake Properties of Luminescent Heterobimetallic Iridium(III)–Ruthenium(II) DNA Imaging Probes

The synthesis of two new luminescent dinuclear Ir^III–Ru^II complexes containing tetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐h:2′′′,3′′′‐j]phenazine (tpphz) as the bridging ligand is reported. Unlike many other complexes incorporating cyclometalated Ir^III moieties, these complexes display good water solubility, allowing the first cell‐based study on Ir^III–Ru^II bioprobes to be carried out. Photophysical studies indicate that emission from each complex is from a Ru^II excited state and both complexes display significant in vitro DNA‐binding affinities. Cellular studies show that each complex is rapidly internalised by HeLa cells, in which they function as luminescent nuclear DNA‐imaging agents for confocal microscopy. Furthermore, the uptake and nuclear targeting properties of the complex incorporating cyclometalating 2‐(4‐fluorophenyl)pyridine ligands around its Ir^III centre is enhanced in comparison to the non‐fluorinated analogue, indicating that fluorination may provide a route to promote cell uptake of transition‐metal bioprobes.     

Combined Two‐Photon Excitation and d→f Energy Transfer in a Water‐Soluble IrIII/EuIII Dyad: Two Luminescence Components from One Molecule for Cellular Imaging

The first example of cell imaging using two independent emission components from a dinuclear d/f complex is reported. A water‐stable, cell‐permeable Ir^III/Eu^III dyad undergoes partial Ir→Eu energy transfer following two‐photon excitation of the Ir unit at 780 nm. Excitation in the near‐IR region generated simultaneously green Ir‐based emission and red Eu‐based emission from the same probe. The orders‐of‐magnitude difference in their timescales (Ir ca. μs; Eu ca. 0.5 ms) allowed them to be identified by time‐gated detection. Phosphorescence lifetime imaging microscopy (PLIM) allowed the lifetime of the Ir‐based emission to be measured in different parts of the cell. At the same time, the cells are simultaneously imaged by using the Eu‐based emission component at longer timescales. This new approach to cellular imaging by using dual d/f emitters should therefore enable autofluorescence‐free sensing of two different analytes, independently, simultaneously and in the same regions of a cell.     

Synthesis and photophysical properties of Ir<Superscript>III</Superscript>-Ln<Superscript>III</Superscript> (Ln = Nd,Yb, Er) bimetallic complexes containing bipyrimidines as bridging ligands

Bipyrimidines have been chosen as (N∧N)(N∧N) bridging ligands for connecting metal centers. Ir^III-Ln^III (Ln = Nd, Yb, Er) bimetallic complexes [Ir(dfppy)₂(μ-bpm)Ln(TTA)₃]Cl were synthesized by using Ir(dfppy)₂(bpm)Cl as the ligand coordinating to lanthanide complexes Ln(TTA)₃·2H₂O. The stability constants between Ir(dfppy)₂(bpm)Cl and lanthanide ions were measured by fluorescence titration. The obvious quenching of visible emission from Ir^III complex in the Ir^III-Ln^III (Ln = Nd, Yb, Er) bimetallic complexes indicates that energy transfer occurred from Ir^III center to lanthanides. NIR emissions from Nd^III, Yb^III, and Er^III were obtained under the excitation of visible light by selective excitation of the Ir^III-based chromophore. It was proven that Ir(dfppy)₂(bpm)Cl as the ligand could effectively sensitize NIR emission from Nd^III, Yb^III, and Er^III.     

Highly Efficient Triplet Photosensitizers: A Systematic Approach to the Application of IrIII Complexes containing Extended Phenanthrolines

A series of Ir^III complexes, based on 1,10‐phenanthroline featuring aryl acetylene chromophores, were prepared and investigated as triplet photosensitizers. The complexes were synthesized by Sonogashira cross‐coupling reactions using a “chemistry‐on‐the‐complex” method. The absorption properties and luminescence lifetimes were successfully tuned by controlling the number and type of light‐harvesting group. Intense UV/Vis absorption was observed for the Ir^III complexes with two light‐harvesting groups at the 3‐ and 8‐positions of the phenanthroline. The asymmetric Ir^III complex (with a triphenylamine (TPA) and a pyrene moiety attached) exhibited the longest lifetime. Red emission was observed for all the complexes in deaerated solutions at room temperature. Their emission at low temperature (77 K) and nanosecond time‐resolved transient difference absorption spectra revealed the origin of their triplet excited states. The singlet‐oxygen (¹O₂) sensitization and triplet‐triplet annihilation (TTA)‐based upconversion were explored. Highly efficient TTA upconversion (Φ_UC=28.1 %) and ¹O₂ sensitization (Φ_Δ=97.0 %) were achieved for the asymmetric Ir^III complex, which showed intense absorption in the visible region (λ_abs=482 nm, ?=50900 m ^?1 cm^?1) and had a long‐lived triplet excited state (53.3 μs at RT).     

Mesomorphism and Photophysics of Some Metallomesogens Based on Hexasubstituted 2,2′:6′, 2′′‐Terpyridines

The luminescent and mesomorphic properties of a series of metal complexes based on hexacatenar 2,2′:6′,2′′‐terpyridines are investigated using experimental methods and density functional theory (DFT). Two types of ligand are examined, namely 5,5′′‐di(3,4,5‐trialkoxyphenyl)terpyridine with or without a fused cyclopentene ring on each pyridine and their complexes were prepared with the following transition metals: Zn^II, Co^III, Rh^III, Ir^III, Eu^III and Dy^III. The exact geometry of some of these complexes was determined by single X‐ray diffraction. All complexes with long alkyl chains were found to be liquid crystalline, which property was induced on complexation. The liquid‐crystalline behaviour of the complexes was studied by polarising optical microscopy and small‐angle X‐ray diffraction. Some of the transition metal complexes (for example, those with Zn^II and Ir^III) are luminescent in solution, the solid state and the mesophase; their photophysical properties were studied both experimentally and using DFT methods (M06‐2X and B3LYP).     

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

To take advantage of the luminescent properties of d⁶ 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.     

Iridium(III) Sensitisers and Energy Upconversion: The Influence of Ligand Structure upon TTA-UC Performance

Six substituted ligands based upon 2-(naphthalen-1-yl)quinoline-4-carboxylate and 2-(naphthalen-2-yl)quinoline-4-carboxylate have been synthesised in two steps from a range of commercially available isatin derivatives. These species are shown to be effective cyclometallating ligands for Ir^III, yielding complexes of the form [Ir(C^N)₂(bipy)]PF₆ (where C^N=cyclometallating ligand; bipy=2,2′-bipyridine). X-ray crystallographic studies on three examples demonstrate that the complexes adopt a distorted octahedral geometry wherein a cis-C,C and trans-N,N coordination mode is observed. Intraligand torsional distortions are evident in all cases. The Ir^III complexes display photoluminescence in the red part of the visible region (668–693 nm), which is modestly tuneable through the ligand structure. The triplet lifetimes of the complexes are clearly influenced by the precise structure of the ligand in each case. Supporting computational (DFT) studies suggest that the differences in observed triplet lifetime are likely due to differing admixtures of ligand-centred versus MLCT character instilled by the facets of the ligand structure. Triplet–triplet annihilation upconversion (TTA-UC) measurements demonstrate that the complexes based upon the 1-naphthyl derived ligands are viable photosensitisers with upconversion quantum efficiencies of 1.6–6.7 %.     

Towards Novel Photodynamic Anticancer Agents Generating Superoxide Anion Radicals: A Cyclometalated IrIII Complex Conjugated to a Far‐Red Emitting Coumarin

Although cyclometalated Ir^III complexes have emerged as promising photosensitizers for photodynamic therapy, some key drawbacks still hamper clinical translation, such as operability in the phototherapeutic window and reactive oxygen species (ROS) production efficiency and selectivity. In this work, a cyclometalated Ir^III complex conjugated to a far‐red‐emitting coumarin, Ir^III–COUPY, is reported with highly favourable properties for cancer phototherapy. Ir^III–COUPY was efficiently taken up by HeLa cells and showed no dark cytotoxicity and impressive photocytotoxicity indexes after irradiation with green and blue light, even under hypoxia. Importantly, a clear correlation between cell death and intracellular generation of superoxide anion radicals after visible light irradiation was demonstrated. This strategy opens the door to novel fluorescent photodynamic therapy agents with promising applications in theragnosis.     

Wavelength-Dependent Singlet Oxygen Generation in Luminescent Lanthanide Complexes with a Pyridine-Bis(Carboxamide)-Terthiophene Sensitizer

Lanthanide ion (Ln^III) complexes, [Ln(3Tcbx)₂]³⁺ (Ln^III=Yb^III, Nd^III, Er^III) are isolated with a new pyridine-bis(carboxamide)-based ligand with a 2,2′:5′,2′′-terthiophene pendant (3TCbx), and their resulting photophysical properties are explored. Upon excitation of the complexes at 490 nm, only Ln^III emission is observed with efficiencies of 0.29 % at 976 nm for Ln^III=Yb^III and 0.16 % at 1053 nm for Ln^III=Nd^III. Er^III emission is observed but weak. Upon excitation at 400 nm, concurrent ¹O₂ formation is seen, with efficiencies of 11 % for the Yb^III and Nd^III complexes and 13 % for the Er^III complex. Owing to the concurrent generation of ¹O₂, as expected, the efficiency of metal-centered emission decreases to 0.02 % for Yb^III and 0.05 % for Nd^III. The ability to control ¹O₂ generation through the excitation wavelength indicates that the incorporation of 2,2′:5′,2′′-terthiophene results in access to multiple sensitization pathways. These energy pathways are unraveled through transient absorption spectroscopy.     

Triphenylamine and carbazole‐modified iridiumIII 2‐phenylpyridine complexes: Synthesis,anticaner application and targeted research

Four half‐sandwich iridium^III (Ir^III) triphenylamine or carbazole‐modified 2‐phenylpyridine (TPA/Cz‐PhPy) complexes ([(η⁵‐Cp*)Ir(C^N)Cl]) were synthesized and characterized. Compared with cisplatin, these complexes show higher activity to A549, HepG2 and HeLa cells, with the IC₅₀ values changed from 2.5 ± 0.1 μM to 14.8 ± 2.6 μM. Additionally, complexes could effectively prevent the migration of cancer cells. Ir^III TPA/Cz‐PhPy complexes could bind to protein and transport through serum protein, catalyze the oxidation of nicotinamide‐adenine dinucleotid (NADH) and induce the accumulation of reactive oxygen species, and eventually lead to apoptosis, which was also confirmed by flow cytometry. Moreover, prominent targeted fluorescence property confirmed that Ir^III TPA/Cz‐PhPy complexes were involved in non‐energy dependent intracellular uptake mechanism, effectively accumulated in lysosomes and damage the integrity of acidic lysosomes, and eventually induce cell death. Above all, TPA/Cz‐appended half‐sandwich Ir^III phenylpyridine complexes are promising anticancer agents with dual functions, including migration inhibition and lysosomal damage.     

meso‐(2‐Pyridyl)‐boron(III)‐subporphyrin: Perimeter Iridium(III) Coordination

Peripherally metalated porphyrinoids are promising functional π‐systems displaying characteristic optical, electronic, and catalytic properties. In this work, 5‐(2‐pyridyl)‐ and 5,10,15‐tri(2‐pyridyl)‐B^III‐subporphyrins were prepared and used to produce cyclometalated subporphyrins by reactions with [Cp*IrCl₂]₂, which proceeded through an efficient C?H activation to give the corresponding mono‐ and tri‐Ir^III complexes, respectively. While the mono‐Ir^III complex was obtained as a diastereomeric mixture, a C₃‐symmetric tri‐Ir^III complex with the three Cp*‐units all at the concave side was predominantly obtained in a high yield of 90 %, which displays weak NIR phosphorescence even at room temperature in degassed CH₂Cl₂, differently from the mono‐Ir^III complexes.     

Sensitized near-infrared emission of Yb from an Ir–Yb bimetallic complex

A new cyclometalated ligand 1,3-dimethyl-5-phenyl-1H-[1,2,4]-triazole (pdt) was designed and synthesized. And the corresponding Ir^III complex Ir(pdt)₂(phen5f) (phen5f stands for 4,4,5,5,5-pentafluoro-1-(1′,10′-phenanthrolin-2′-yl)-pentane-1,3-dionate) was obtained. According to the measurement of the lowest triplet state energy level of Ir(pdt)₂(phen5f), it is suitable for sensitizing NIR (near-infrared) lanthanide ions instead of Eu^III. The bimetallic complex [(pdt)₂Ir(μ-phen5f)YbCl₂ · 2CH₃CH₂OH · H₂O]Cl was synthesized by the approach of “complexes as ligands”. Data showed that the emission quenching was observed in the solid state when the Ir^III–Yb^III complex was compared with the Ir^III complex, which implied that energy transfer might occur from Ir^III complex-ligand to Yb^III ion. Upon irradiation of the MLCT (metal-to-ligand charge transfer) absorption of Ir(pdt)₂(phen5f), the characteristic emission of Yb^III was obtained with the peak around 978 nm.     

Encapsulation and Enhanced Luminescence Properties of IrIII Complexes within a Hexameric Self‐Assembled Capsule

Encapsulation and luminescence studies of [Ir(ppy)₂(bpy)]Cl (ppy=2‐phenylpyridinate, bpy=2,2′‐bipyridine) within a hexameric resorcinarene capsule are reported. One Ir^III complex cation was encapsulated within the capsule, as demonstrated by NMR and dynamic light scattering (DLS) studies. The emission color of the Ir^III complex was drastically changed from orange to yellow by encapsulation, in contrast with the lack of significant changes in the absorption spectrum. The hexameric capsule effectively hampers the non‐radiative pathway to increase both the luminescence quantum yield and the exited state lifetime. The luminescent properties of the encapsulated Ir^III complex depend on the ratio of Ir^III complex to the resorcinarene monomer as well as the concentration of resorcinarene monomer owing to the reversible process of self‐assembly of the hexameric capsule. Quenching experiments revealed that the Ir^III complex in the capsule was effectively separated from quenchers.     

Tm3+‐Sensitized NIR‐II Fluorescent Nanocrystals for In Vivo Information Storage and Decoding

In vivo fluorescence imaging in the second near‐infrared window (NIR‐II) affords deep‐tissue penetration and high spatial resolution. Herein, we present a new type of Tm³⁺‐sensitized lanthanide nanocrystals with both excitation (1208 nm) and emission (1525 nm) located in the NIR‐II window for in vivo optical information storage and decoding. Taking advantage of the tunable fluorescence lifetimes, the optical multiplexed encoding capacity is enhanced accordingly. Micro‐devices with QR codes featuring the NIR‐II fluorescence‐lifetime multiplexed encoding were implanted into mice and were successfully decoded through time‐gated fluorescence imaging technology.     

Cyclometalated Mono‐ and Dinuclear IrIII Complexes with “Click”‐Derived Triazoles and Mesoionic Carbenes

Orthometalation at Ir^III centers is usually facile, and such orthometalated complexes often display intriguing electronic and catalytic properties. By using a central phenyl ring as C?H activation sites, we present here mono‐ and dinuclear Ir^III complexes with “click”‐derived 1,2,3‐triazole and 1,2,3‐triazol‐5‐ylidene ligands, in which the wingtip phenyl groups in the aforementioned ligands are additionally orthometalated and bind as carbanionic donors to the Ir^III centers. Structural characterization of the complexes reveal a piano stool‐type of coordination around the metal centers with the “click”‐derived ligands bound either with C^N or C^C donor sets to the Ir^III centers. Furthermore, whereas bond localization is observed within the 1,2,3‐triazole ligands, a more delocalized situation is found in their 1,2,3‐triazol‐5‐ylidene counterparts. All complexes were subjected to catalytic tests for the transfer hydrogenation of benzaldehyde and acetophenone. The dinuclear complexes turned out to be more active than their mononuclear counterparts. We present here the first examples of stable, isomer‐pure, dinuclear cyclometalated Ir^III complexes with poly‐mesoionic‐carbene ligands.     

Electrically Amplified Circularly Polarized Luminescence by a Chiral Anion Strategy

The development of circularly polarized electroluminescence (CPEL) is currently hampered by the high difficulty and cost in the syntheses of suitable chiral materials and the notorious chirality diminishment issue in electrical devices. Herein, diastereomeric Ir^III and Ru^II complexes with chiral (±)-camphorsulfonate counteranions are readily synthesized and used as the active materials in circularly polarized light-emitting electrochemical cells to generate promising CPELs. The addition of the chiral ionic liquid (±)-1-butyl-3-methylimidazole camphorsulfonate into the active layer significantly improves the device performance and the electroluminescence dissymmetry factors (≈10⁻³), in stark contrast to the very weak circularly polarized photoluminescence of the spin-coated films of these diastereomeric complexes. Control experiments with enantiopure Ir^III complexes suggest that the chiral anions play a dominant role in the electrically-induced amplification of CPELs.     

2‐(2′‐Pyridyl)‐4,6‐diphenylphosphinine versus 2‐(2′‐Pyridyl)‐4,6‐diphenylpyridine: Synthesis,Characterization, and Reactivity of Cationic RhIII and IrIII Complexes Based on Aromatic Phosphorus Heterocycles

The bidentate P,N hybrid ligand 1 allows access for the first time to novel cationic phosphinine‐based Rh^III and Ir^III complexes, broadening significantly the scope of low‐coordinate aromatic phosphorus heterocycles for potential applications. The coordination chemistry of 1 towards Rh^III and Ir^III was investigated and compared with the analogous 2,2′‐bipyridine derivative, 2‐(2′‐pyridyl)‐4,6‐diphenylpyridine ( 2 ), which showed significant differences. The molecular structures of [RhCl(Cp*)( 1 )]Cl and [IrCl(Cp*)( 1 )]Cl (Cp*=pentamethylcyclopentadienyl) were determined by means of X‐ray diffraction and confirm the mononuclear nature of the λ³‐phosphinine–Rh^III and Ir^III complexes. In contrast, a different reactivity and coordination behavior was found for the nitrogen analogue 2 , especially towards Rh^III as a bimetallic ion pair [RhCl(Cp*)( 2 )]⁺[RhCl₃(Cp*)]^? is formed rather than a mononuclear coordination compound. [RhCl(Cp*)( 1 )]Cl and [IrCl(Cp*)( 1 )]Cl react with water regio‐ and diastereoselectively at the external P?C double bond, leading exclusively to the anti‐addition products [MCl(Cp*)( 1 H ? OH)]Cl as confirmed by X‐ray crystal‐structure determination.     

Anticancer IrIII–Aspirin Conjugates for Enhanced Metabolic Immuno-Modulation and Mitochondrial Lifetime Imaging

The chemo-anti-inflammatory strategy is attracting ever more attention for the treatment of cancer. Here, two cyclometalated Ir^III complexes Ir2 and Ir3 formed by conjugation of Ir1 with two antiphlogistics (aspirin and salicylic acid) have been designed. Ir2 and Ir3 exhibit higher antitumor and anti-inflammatory potencies than a mixture of Ir1 and aspirin/salicylic acid. We show that they can be hydrolyzed, accumulate in mitochondria, and induce mitochondrial dysfunction. Due to their intense long-lived phosphorescence, Ir2 and Ir3 can track mitochondrial morphological changes. Phosphorescence lifetime imaging shows that Ir2 and Ir3 can aggregate during mitochondrial dysfunction. As expected, Ir2 and Ir3 exhibit immunomodulatory properties by regulating the activity of immune factors. Both Ir2 and Ir3 can induce caspase-dependent apoptosis and caspase-independent paraptosis and inhibit several events related to metastasis. Moreover, Ir2 and Ir3 show potent tumor growth inhibition in vivo. Our study demonstrates that the combination of mitochondrial-targeting and immunomodulatory activities is feasible to develop multifunctional metal-based anticancer agents.     

Iridium(III) Complexes Bearing Pyrene‐Functionalized 1,10‐Phenanthroline Ligands as Highly Efficient Sensitizers for Triplet–Triplet Annihilation Upconversion

“Chemistry‐on‐the‐complex” synthetic methods have allowed the selective addition of 1‐ethynylpyrene appendages to the 3‐, 5‐, 3,8‐ and 5,6‐positions of Ir^III‐coordinated 1,10‐phenanthroline via Sonogashira cross‐coupling. The resulting suite of complexes has given rise to the first rationalization of their absorption and emission properties as a function of the number and position of the pyrene moieties. Strong absorption in the visible region (e.g. 3,8‐substituted Ir‐3 : λ_abs=481 nm, ?=52 400 m ^?1 cm^?1) and long‐lived triplet excited states (e.g. 5‐substituted Ir‐2 : τ_T=367.7 μs) were observed for the complexes in deaerated CH₂Cl₂. On testing the series as triplet sensitizers for triplet–triplet annihilation upconversion, those Ir^III complexes bearing pyrenyl appendages at the 3‐ and 3,8‐positions ( Ir‐1 , Ir‐3 ) were found to give optimal upconversion quantum yields (30.2 % and 31.6 % respectively).