Recent advances of iridium(III) metallophosphors for health-related applications

Mitochondria-targeted and near-infrared phosphorescent Ir(III) complexes for specific detection of Hg2+ and photodynamic therapy

S: Mercury ions (Hg2+) are highly toxic and prone to bioaccumulation, showing a strong attraction to proteins and enzymes that contain sulfur. Even minute quantities of Hg2+ can lead to severe health issues. Given that mitochondria are a primary target organelle of Hg2+, it is essential to create a probe that can accurately detect Hg2+ within intracellular mitochondria. In this study, we developed two innovative Ir(III) complex probes that emit near-infrared light. The crystal structure of Ir2 was determined using X-ray techniques, which reveals that Ir2 contains a pyridine group capable of recognizing Hg2+ and targeting mitochondria, allowing for the precise identification of Hg2+ both in vitro and within the mitochondria of living cells. Additionally, these two novel near-infrared phosphorescent Ir(III) complexes demonstrate significant capabilities in producing ROS including singlet oxygen, ·O2- and ·OH, which renders them effective photosensitizers under visible light exposure for photodynamic therapy (PDT). This research offers a promising approach for detecting Hg2+ in vitro and in the mitochondrial microenvironment of living cells, which have some implications for the future development of pertinent transition metal complexes for mitochondria-targeted photodynamic therapy in cancer cells.

Cyclometalated Iridium(III) Schiff Base Complexes for Chemiluminogenic Bioprobes

Chemiluminogenic bioimaging has emerged as a promising paradigm due to its independence from light excitation, thereby circumventing challenges related to light penetration depth and background autofluorescence. However, the availability of effective chemiluminophores remains limited, which substantially impedes their bio-applications. Herein, we discovered for the first time that cyclometalated iridium(III) Schiff base complexes can unexpectedly generate chemiluminescence. Notably, the chemiluminescence reaction was rapid, with a half-life of only 0.86 s, significantly faster than previously reported examples. Unlike conventional chemiluminescent scaffolds, the distinguishing feature of the chemiluminogenic iridium(III) complex is its unique intramolecular imine-to-amide conversion upon reaction with reactive oxygen species (ROS). Intriguingly, the chemiluminogenicity of these complexes is not influenced by the cyclometalating ligands but is closely associated with the Schiff base ligand, allowing for tuning of the emission colors via altering the cyclometalating ligands. Additionally, we formulated one of the Schiff base complexes (1) as water-soluble chemiluminogenic nanoparticles (CLNPs) and successfully employed them as activatable chemiluminescence bioprobes for precise and rapid imaging of hypochlorite-related biological events both in vitro and in vivo. We believe that this significant finding of the development of chemiluminogenic Schiff base complexes will greatly facilitate the designing of innovative chemiluminophores for theranostic applications.

Exploiting a New Strategy to Prepare Water-Soluble Heteroleptic Iridium(III) Complexes to Control Electrochemiluminescence Reaction Pathways in Aqueous Solution

The direct derivatization of heteroleptic iridium(III) complexes with sulfonate groups removes the limitations of prior strategies for the preparation of water-soluble analogues, in which complexes were prepared from a narrow range of ligands with suitably polar or charged functional groups. Phenylpyridine, phenylpyrazole, and phenylbenzothiazole ligands were selectively sulfonated opposite to their cyclometalated carbon within iridium(III) complexes containing bipyridine or N-heterocyclic carbene ancillary ligands. Altering reaction conditions enabled additional sulfonation of the benzothiazole fragments. Informed by the electrochemical and photophysical properties of the parent complexes in organic solvents, we adopted this strategy to design six novel luminophores that proceeded through three different sets of coreactant electrochemiluminescence (ECL) reaction pathways under aqueous conditions. The intensity of the indirect coreactant ECL of one of the iridium(III) luminophores was enhanced by over an order of magnitude by introducing a redox mediator, extending this promising analytical approach beyond the conventional [Ru(bpy)3]2+ electrochemiluminophore.

Rationally Designed Ir(III) Complex with an Exceptionally Strong Binding to Human Serum Albumin for Targeted Photodynamic Therapy

The application of iridium(III) complexes in photodynamic therapy (PDT) is often limited by their poor selectivity for cancerous cells, necessitating high drug doses that increase the risk of side effects. The development of efficient drug delivery systems such as albumin conjugation is therefore crucial to enhance the tumor-targeted delivery of photosensitizers. To date, the vast majority of metal complexes exhibit weak to moderate binding with human serum albumin, limiting the feasibility of this approach. To overcome this limitation, the rational design through molecular docking and density functional theory calculations of a novel Ir(III) complex as a strong albumin-binding photosensitizer is described. The herein reported compound has the highest albumin binding constant ever reported for an iridium complex, and it showed to photocatalytically produce reactive oxygen species upon blue light irradiation. The presented compound as well as structural derivatives could have high potential in tumor-targeted photodynamic therapy.

A comprehensive survey of cytotoxic active half-sandwich Ir(III) complexes: structural perspective, and mechanism of action

Iridium(III) complexes, particularly those with piano-stool structures, have drawn a lot of interest recently as possible anticancer drugs. These complexes, which have displayed enhanced cytotoxicity and cytoselectivity compared with clinically approved drugs like cisplatin, oxaliplatin, and carboplatin, hold promising prospects for further anticancer research. Our review aims to explore the complex interplay between cytotoxic properties, cellular uptake efficiency, and intracellular distribution properties of this class of Ir(III) complexes, considering the variation of the coordination site atoms. We provide an overview of the majority of research on mono- and polynunclear half-sandwich Ir(III) complexes with mono- and bidentate ligands, focusing on the impact of altering the leaving group, tethers, substituents on the cyclopentadienyl ring and ligand, spacers, and counter ions on the cytotoxicity and mode of action.

Leveraging Metal Complexes for Microsecond Lifetime-Based Chloride Sensing

Chloride is the most abundant anion in cells and plays many critical roles in maintaining cellular homeostasis. However, current chloride indicators are rare with inherent sensitivity in their emission properties, such as vulnerability to pH changes or short emission lifetimes. These limitations restrict their application in aqueous media and imaging. In this work, we employed a transition-metal complex bearing pyridinium as a recognition unit for chloride and studied the phosphorescence emission properties. Iridium(III) complex 1 was synthesized as an alternative chloride-sensitive luminophore. The conjugable design also allows customization for the desired applications. Complex 1 exhibited high sensitivity and selectivity in chloride sensing across different physiological environments, regardless of pH fluctuation and ionic strength. Additionally, complex 1 featured a microsecond emission lifetime. The chloride sensing ability of complex 1 can be measured through both the luminescence intensity and long-lived phosphorescent lifetime, providing an alternative potential route for chloride imaging. The analogue 1b was successfully applied in the imaging of Cl- in cellular environments and showed dose-dependent responses in both live and fixed cells.

(2,9-Dimethyl-1,10-phenanthroline)bis-[2-(pyridin-2-yl)phen-yl]iridium(III) hexa-fluoro-phosphate and (2,9-dimethyl-1,10-phenanthroline)bis-[5-methyl-2-(pyridin-2-yl)phen-yl]iridium(III) hexa-fluoro-phosphate-diethyl ether-aceto-nitrile (1/0.61/0.78)

The title compounds, [Ir(C14H12N2)(C11H8N)2]PF6 (1) and [Ir(C14H12N2)(C12H10N)2]PF6·0.61C2H10O·0.78CH3CN (2), crystallize in the space groups Pbca and P1, respectively, each structure containing one monocationic Ir complex and one PF6 - anion in the asymmetric unit. The anion and solvent in compound 2 are disordered. The Ir-N(phenanthroline) bond lengths of ca. 2.21 Å indicate a greater steric effect of the 2,9-dimethyl-1,10-phenanthroline ligand compared to 1,10-phenanthroline. Both structures show offset parallel inter-molecular π-π inter-actions between the pyridine rings of the phenanthroline ligands, and that of 1 also exhibits similar inter-actions between the phenyl and pyridine rings of the phenyl-pyridine ligands.

Photodissociation of Cr(CO) 4 bpy $$ \mathrm{Cr}{\left(\mathrm{CO}\right)}_4\mathrm{bpy} $$ : A Non-Adiabatic Dynamics Investigation

Carbonyl complexes ofd 6 $$ {d}^6 $$ metals with an α-diimine ligand exhibit both emission and ligand-selective photodissociation from MLCT states. Studying this photodissociative mechanism is challenging for experimental approaches due to an ultrafast femtosecond timescale and spectral overlap of multiple photoproducts. The photochemistry of a prototypical systemCr(CO) 4 bpy $$ \mathrm{Cr}{\left(\mathrm{CO}\right)}_4\mathrm{bpy} $$ is investigated with non-adiabatic dynamic simulations. Obtained 86 fs lifetime of the brightS 3 $$ {S}_3 $$ state and 13% quantum yield are in good agreement with experimental data. The present simulations suggest a ballistic mechanism of photodissociation, which is irrespective of the occupied electronic state. This is in contrast to the previously established mechanism of competitive intersystem crossing and dissociation. Selectivity of axial photodissociation is shown to be caused by the absence of an avoided crossing in the equatorial direction.

Anthracene-Based Endoperoxides as Self-Sensitized Singlet Oxygen Carriers for Hypoxic-Tumor Photodynamic Therapy

Singlet oxygen is a crucial reactive oxygen species (ROS) in photodynamic therapy (PDT). However, the hypoxic tumor microenvironment limits the production of cytotoxic singlet oxygen through the light irradiation of PDT photosensitizers (PSs). This restriction poses a major challenge in improving the effectiveness of PDT. To overcome this challenge, researchers have explored the development of singlet oxygen carriers that can capture and release singlet oxygen in physiological conditions. Among these developments, anthracene-based endoperoxides, initially discovered almost 100 years ago, have shown the ability to generate singlet oxygen controllably under thermal or photo stimuli. Recent advancements have led to the development of a new class of self-sensitized anthracene-endoperoxides, with potential applications in enhancing PDT effects for hypoxic tumors. This review discusses the current research progress in utilizing self-sensitized anthracene-endoperoxides as singlet oxygen carriers for improved PDT. It covers anthracene-conjugated small organic molecules, metal-organic complexes, polymeric structures, and other self-sensitized nano-structures. The molecular structural designs, mechanisms, and characteristics of these systems will be discussed. This review aims to provide valuable insights for developing high-performance singlet oxygen carriers for hypoxic-tumor PDT.

Lanthanide-Sensitized Upconversion Iridium Complex via Triplet Energy Transfer

Cyclometalated iridium (Ir) complexes demonstrate impressive capabilities across a range of fields, including biology and photocatalysis, due to their tunable optical characteristics and structure flexibility. However, generating upconversion luminescence of Ir complexes under near-infrared light excitation is challenging. Herein, by employing lanthanide-doped upconversion nanoparticles (UCNPs) as the sensitizer, a new strategy is demonstrated to gain upconversion luminescence of Ir complexes via triplet energy transfer. This design relies on a rationally designed hybrid of core-shell structured NaYbF4:Tb@NaTbF4 UCNPs and new Ir phosphonate complexes, in which UCNPs can migrate upconverted energy to the surface of nanoparticles through Tb3+-mediated energy migration and then sensitize the upconversion luminescence of Ir complexes upon 980 nm excitation. Both experimental and theoretical investigations highlight the significance of triplet energy transfer from excited Tb3+ ions to the triplet state of Ir complexes in the sensitization of upconversion luminescence of Ir complexes. These findings may open exciting avenues for fabricating hybrid Ir materials with new functions and driving the development of UCNP-based nanomaterials.

Modeling and NMR Data Elucidate the Structure of a G-Quadruplex-Ligand Interaction for a Pu22T-Cyclometalated Iridium(III) System

Cyclometalated iridium(III) complexes are increasingly being developed for application in G-quadruplex (GQ) nucleic acid biosensors. We monitored the interactions of a GQ structure with an iridium(III) complex by nuclear magnetic resonance (NMR) titrations and subsequently compared the binding site inferred from NMR with binding positions modeled by molecular docking and molecular dynamics simulations. When titrated into a solution of G-quadruplex Pu22T, compound 1(PF6), [Ir(ppy)2(pizp)](PF6), where ppy is 2-phenylpyridine and pizp is 2-phenylimidazole[4,5f][1,10]phenanthroline, had the greatest impact on the hydrogen chemical shifts of G5, G8, G9, G13, and G17 residues of Pu22T, indicating end-stacking at the 5' tetrad. In blind cross-docking studies with Autodock 4, end-stacking at the 5' tetrad was found as the lowest energy binding position. AMBER molecular dynamics simulations resulted in a refined binding position at the 5' tetrad with improved pi stacking. For this model system, Pu22T-1, molecular docking and molecular dynamics simulations are tools that are able to predict the experimentally determined binding position.

Tunability of triplet excited states and photophysical behaviour of bis-cyclometalated iridium(III) complexes with imidazo[4,5-f][1,10]phenanthroline

This is a comprehensive study of the photophysical behaviour of heteroleptic iridium(III) complexes with imidazo[4,5-f][1,10]phenanthroline (imphen) as an ancillary ligand, represented by the general formula [Ir(N∩C)2(imphen)]PF6. As cyclometalating ligands, 2-phenylpyridine (Hppy), 2-phenylquinoline (Hpquin), 2-phenylbenzothiazole (Hpbztz), and 2-(2-pyridyl)benzothiophene (pybzthH) were used. The impact of structural modifications of cyclometalating ligands was widely explored by a combination of steady-state and time-resolved optical techniques accompanied by theoretical calculations. We evidenced that the cyclometalating ligands induce essential changes in the nature of the emissive excited state and the emission characteristics of [Ir(N∩C)2(imphen)]PF6. While the complex [Ir(ppy)2(imphen)]PF6 (1) is a typical 3MLLCT emitter, the lowest triplet states of [Ir(pquin)2(imphen)]PF6 (2), [Ir(pbztz)2(imphen)]PF6 (3) and [Ir(pybzth)2(imphen)]PF6 (4) have a predominant 3LCN∩C character. The phosphorescence colour of the investigated Ir(III) complexes changes from greenish-yellow to red, their quantum yields vary from 56 to 2%, and their triplet excited-state lifetimes fall in the 743-3840 ns range. The highest photoluminescence quantum yield was revealed for 2 in CH2Cl2, while complex 3 in MeCN shows the most pronounced increase in the lifetime. Both complexes 2 and 3 show an increased efficiency of singlet oxygen generation. The herein discussed structure-property relationships are of high significance for controlling photoinduced processes in heteroleptic iridium(III) complexes with the imphen-based ancillary ligand, and making further progress in effectively tuning the emission energies, quantum yields and excited-state lifetimes of these systems by structural modifications of cyclometalating ligands, especially the π-conjugation, the position of the N-donor and the presence of sulfur heteroatoms.

Dual phosphorescent emissions from conformers of iridium complex rotors

The chiral iridium rotors Ir(ppy)2(pyX)Cl (X = CC-SiR3, R = alkyl) remarkably contain two distinct rotational conformers in the ground (S0) and excited (T1) states that can be detected by NMR and emission measurements respectively at variable temperatures. The observed phosphorescent emissions, vibronic (involving L = ppy) and broad (L = pyX), arise from different triplet ligand to metal charge transfers from the two rotational conformers at distinct 3MLCT excited states. Both conformers exist in these Ir(ppy)2(pyX)Cl rotors due to the electron-withdrawing, conjugated substituent X.

Tuning The Intracellular Distribution of [3+2+1] Iridium(III) Complexes In Bacterial And Mammalian Cells By iClick Reaction With Biomolecular Carriers Functionalized With Alkynone Groups

Three iridium(III) triazolato complexes of the general formula [Ir(triazolatoR,R')(ppy)(terpy)]PF6 with ppy=2-phenylpyridine and terpy=2,2':6',2''-terpyridine were efficiently prepared by iClick reaction of [Ir(N3)(ppy)(terpy)]PF6, with alkynes and alkynones, which allowed facile introduction of biological carriers such as biotin and cholic acid. In contrast to the precursor azido complex, which decomposed upon photoexcitation on a very short time scale, the triazolato complexes were stable in solution for up to 48 h. They emit in the spectral region around 540 nm with a quantum yield of 15-35 % in aerated acetonitrile solution and exhibit low cytotoxicity with IC50 values >50 μM for most complexes in L929 and HeLa cells, demonstrating their high suitability as luminescent probes. Cell uptake studies with confocal luminescence microscopy in prokaryotic Gram-positive S. aureus and Gram-negative E. coli bacteria as well as eukaryotic mammalian L929 and HeLa cells showed significant uptake in particular of the cholic acid conjugates iridium(III) moiety and distinct intracellular distribution modulated by the nature of the peripheral functional groups that can easily be modified by the iClick reaction.

Biocompatible Re-Containing Block Copolymers for Intracellular pH Mapping in the PLIM Mode

Monitoring of intracellular pH is of great importance since deviation of this parameter from the "normal" magnitudes can be considered as an indicator of various pathologies. Thus, the development of new efficient and biocompatible sensors suitable for application in biological systems and capable of quantitative pH estimation remains an urgent chemical task. Herein, we report the synthesis of a series of phosphorescent rhenium [Re(NN)(CO)2(PR3)2]+ complexes based on the NN diimine ligands containing pH-responsive carboxylic groups and styrene-containing phosphine ligands. The complexes, which display the highest pH sensitivity, were copolymerized with polyvinylpyrrolidone using the RAFT protocol to impart water solubility and to protect the chromophores from interaction with molecular oxygen. The resulting copolymers show an emission lifetime response onto pH variations in the physiological range. Cellular experiments with Chinese hamster ovary cells (CHO-K1) reveal easy internalization of the probes in cell culture and an approximately uniform distribution in cells, with some preference for location in acidic compartments (late endosomes and lysosomes). Using nigericin to homogenize intra- and extracellular pH, we built a calibration of lifetime versus pH in live CHO-K1 cells. Analysis of the phosphorescence lifetime imaging microscopy (PLIM) data confirms the applicability of the obtained sensors for monitoring the intracellular pH in cell cultures.

2-Phenylbenzothiazolyl iridium complexes as inhibitors and probes of amyloid β aggregation

The aggregation of amyloid β (Aβ) peptides is a significant hallmark of Alzheimer's disease (AD), and the detection of Aβ aggregates and the inhibition of their formation are important for the diagnosis and treatment of AD, respectively. Herein, we report a series of benzothiazole-based Ir(III) complexes HN-1 to HN-8 that exhibit appreciable inhibition of Aβ aggregation in vitro and in living cells. These Ir(III) complexes can induce a significant fluorescence increase when binding to Aβ fibrils and Aβ oligomers, while their measured log D values suggest these compounds could have enhanced blood-brain barrier (BBB) permeability. In vivo studies show that HN-1, HN-2, HN-3, and HN-8 successfully penetrate the BBB and stain the amyloid plaques in AD mouse brains after a 10-day treatment, suggesting that these Ir(III) complexes could act as lead compounds for AD therapeutic and diagnostic agent development.

Measuring the total photon economy of molecular species through fluorescent optical cycling

The total photon economy of a chromophore molecular species represents a study of how absorbed photons partition among various electronic states and ultimately dissipate their excited energy into the environment. A complete accounting of these rates and pathways would allow one to optimize chromophores and their environments for applications. We describe a technique, fluorescent optical cycling (FOC), which allows for simultaneous observation of prompt and delayed emission during and after multiple pulsed excitation, ultimately granting access to multi-state photophysical rates. We exercise control over the excitation pulse train, which allows us to "optically shelve" long-lived intermediate states without the use of diode or flashlamp excitation. By recording all photon arrival times in the visible and shortwave infrared, we can simultaneously resolve fluorescence, phosphorescence, and singlet oxygen sensitization in a single experiment. We use FOC to examine the photophysics of dual emitting bis(di-R-phosphino)alkanethiophene-pyridine-platinum ([Pt(thpy)(dppm)]+) under different solvation conditions, revealing changes in intersystem crossing and phosphorescent rates induced by the external heavy atom effect. Coupling FOC with decay associated Fourier spectroscopy (DAFS), we demonstrate simultaneous correlated spectral and lifetime data in this dual emitting complex. Finally, FOC combined with superconducting nanowire single photon detectors (SNSPDs) allows us to observe the shortwave infrared region (SWIR) phosphorescence of singlet oxygen sensitized by Rose Bengal. Overall, FOC provides a powerful tool to simultaneously study multiple photophysics across timescales, even in weakly populated electronic states.

Shining New Light on Biological Systems: Luminescent Transition Metal Complexes for Bioimaging and Biosensing Applications

Luminescence imaging is a powerful and versatile technique for investigating cell physiology and pathology in living systems, making significant contributions to life science research and clinical diagnosis. In recent years, luminescent transition metal complexes have gained significant attention for diagnostic and therapeutic applications due to their unique photophysical and photochemical properties. In this Review, we provide a comprehensive overview of the recent development of luminescent transition metal complexes for bioimaging and biosensing applications, with a focus on transition metal centers with a d6, d8, and d10 electronic configuration. We elucidate the structure-property relationships of luminescent transition metal complexes, exploring how their structural characteristics can be manipulated to control their biological behavior such as cellular uptake, localization, biocompatibility, pharmacokinetics, and biodistribution. Furthermore, we introduce the various design strategies that leverage the interesting photophysical properties of luminescent transition metal complexes for a wide variety of biological applications, including autofluorescence-free imaging, multimodal imaging, organelle imaging, biological sensing, microenvironment monitoring, bioorthogonal labeling, bacterial imaging, and cell viability assessment. Finally, we provide insights into the challenges and perspectives of luminescent transition metal complexes for bioimaging and biosensing applications, as well as their use in disease diagnosis and treatment evaluation.

IrCytoToxDB: a dataset of iridium(III) complexes cytotoxicities against various cell lines

Iridium(III) complexes nowadays became rising stars in various health-related applications. Thus, there is a necessity to assess cytotoxicity of the synthesized molecules against cancer/normal cell lines. In this report, we present a dataset of 2694 experimental cytotoxicity values of 803 iridium complexes against 127 different cell lines. We specify the experimental conditions and provide representation of the complexes molecules in machine-readable format. The dataset provides a starting point for exploration of new iridium-based cellular probes and opens new possibilities for predictions of toxicities and data-driven generation of new organometallic anticancer agents.

Cyclometalated iridium(III) tetrazine complexes for mitochondria-targeted two-photon photodynamic therapy

The fast-moving field of photodynamic therapy (PDT) has provided fresh opportunities to expand the potential of metallodrugs to combat cancers in a light-controlled manner. As such, in the present study, a series of cyclometalated Ir(III) complexes modified with a tetrazine functional group (namely, Ir-ppy-Tz, Ir-pbt-Tz, and Ir-dfppy-Tz) are developed as potential two-photon photodynamic anticancer agents. These complexes target mitochondria but exhibit low toxicity towards HLF primary lung fibroblast normal cells in the dark. When receiving a low-dose one- or two-photon PDT, they become highly potent towards A549 lung cancer cells (with IC50 values ranging from 24.0 nM to 96.0 nM) through the generation of reactive oxygen species (ROS) to induce mitochondrial damage and subsequent apoptosis. Our results indicated that the incorporation of tetrazine with cyclometalated Ir(III) matrices would increase the singlet oxygen (1O2) quantum yield (ΦΔ) and, meanwhile, enable a type I PDT mechanism. Ir-pbt-Tz, with the largest two-photon absorption (TPA) cross-section (σ2 = 102 GM), shows great promise in serving as a two-photon PDT agent for phototherapy.

A Photodynamic and Photochemotherapeutic Platinum-Iridium Charge-Transfer Conjugate for Anticancer Therapy

The novel hetero-dinuclear complex trans,trans,trans-[PtIV(py)2(N3)2(OH)(μ-OOCCH2CH2CONHCH2-bpyMe)IrIII(ppy)2]Cl (Pt-Ir), exhibits charge transfer between the acceptor photochemotherapeutic Pt(IV) (Pt-OH) and donor photodynamic Ir(III) (Ir-NH2) fragments. It is stable in the dark, but undergoes photodecomposition more rapidly than the Pt(IV) parent complex (Pt-OH) to generate Pt(II) species, an azidyl radical and 1O2. The Ir(III)* excited state, formed after irradiation, can oxidise NADH to NAD⋅ radicals and NAD+. Pt-Ir is highly photocytotoxic towards cancer cells with a high photocytotoxicity index upon irradiation with blue light (465 nm, 4.8 mW/cm2), even with short light-exposure times (10-60 min). In contrast, the mononuclear Pt-OH and Ir-NH2 subunits and their simple mixture are much less potent. Cellular Pt accumulation was higher for Pt-Ir compared to Pt-OH. Irradiation of Pt-Ir in cancer cells damages nuclei and releases chromosomes. Synchrotron-XRF revealed ca. 4× higher levels of intracellular platinum compared to iridium in Pt-Ir treated cells under dark conditions. Luminescent Pt-Ir distributes over the whole cell and generates ROS and 1O2 within 1 h of irradiation. Iridium localises strongly in small compartments, suggestive of complex cleavage and excretion via recycling vesicles (e.g. lysosomes). The combination of PDT and PACT motifs in one molecule, provides Pt-Ir with a novel strategy for multimodal phototherapy.

An ALP-responsive, anionic iridium complex for specific recognition of osteosarcoma cells

Poor cellular permeability greatly hampers the utilization of anionic Ir(III) complexes, though efficiently emissive and remarkably stable, in cell-based diagnosis. To overcome this barrier, we present the development of an alkaline phosphatase (ALP)-responsive, anionic, and aggregation-induced emission (AIE)-active Ir(III) complex (Ir1) for specific recognition of osteosarcoma cells. Containing phosphate moieties, Ir1 exhibits a net -1 charge, enabling charge repulsion from the cell membrane and resulting in low cellular uptake and good biocompatibility in normal osteoblast cells. Upon ALP-mediated hydrolysis of phosphate groups, the resulting dephosphorylated product, Ir2, demonstrates a positive charge and increased lipophilicity, promoting cellular uptake and activating its AIE properties for specific recognition of osteosarcoma cells that express elevated levels of ALP. This study elucidates the role of ALP as an ideal trigger for enhancing the cellular permeability of phosphate ester-containing Ir(III) complexes, thus expanding the potential of anionic Ir(III) complexes for biomedical applications.

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Iridium(III)-based minor groove binding complexes as DNA photocleavage agents

Transition metal complexes containing the qtpy ligand (2':4,4'':4',4'''-quaterpyridyl) are known to be DNA intercalators or minor groove binders. In this study, new tricationic iridium(III) complexes of qtpy are reported. Both [Ir(bpy)2(qtpy)]3+1 and [Ir(phen)2(qtpy)]3+2 display good water solubility as chloride salts. The complexes possess high-energy excited states, which are quenched in the presence of duplex DNA and even by the mononucleotides guanosine monophosphate and adenosine monophosphate. Further studies reveal that although the complexes bind to quadruplex DNA, they display a preference for duplex structures, which are bound with an order of magnitude higher affinities than their isostructural dicationic RuII-analogues. Detailed molecular dynamics simulations confirm that the complexes are groove binders through the insertion of, predominantly, the qtpy ligand into the minor groove. Photoirradiation of 1 in the presence of plasmid DNA confirms that this class of complexes can function as synthetic photonucleases by cleaving DNA.

Identification of Cellular Protein Targets of a Half-Sandwich Iridium(III) Complex Reveals Its Dual Mechanism of Action via Both Electrophilic and Oxidative Stresses

Identification of intracellular targets of anticancer drug candidates provides key information on their mechanism of action. Exploiting the ability of the anticancer (C∧N)-chelated half-sandwich iridium(III) complexes to covalently bind proteins, click chemistry with a bioorthogonal azido probe was used to localize a phenyloxazoline-chelated iridium complex within cells and profile its interactome at the proteome-wide scale. Proteins involved in protein folding and actin cytoskeleton regulation were identified as high-affinity targets. Upon iridium complex treatment, the folding activity of Heat Shock Protein HSP90 was inhibited in vitro and major cytoskeleton disorganization was observed. A wide array of imaging and biochemical methods validated selected targets and provided a multiscale overview of the effects of this complex on live human cells. We demonstrate that it behaves as a dual agent, inducing both electrophilic and oxidative stresses in cells that account for its cytotoxicity. The proposed methodological workflow can open innovative avenues in metallodrug discovery.

Photoinduced One-Electron Chloride Oxidation in Water Using a Pentacationic Ir(III) Photosensitizer

A novel iridium(III) photosensitizer containing pyridinium-decorated terpyridines has been used for the photo-oxidation of chloride in water. Despite its abundance, the very positive one-electron reduction potential (E° Cl•/- = 2.1-2.4 V vs NHE) restricted its use in energy conversion schemes and artificial photosynthesis. The kinetics of the photoinduced electron transfer process were investigated through Stern-Volmer quenching experiments and nanosecond transient absorption spectroscopy, which provided unambiguous evidence that photoinduced chloride oxidation occurred with a quenching rate constant kq = 5.0 × 1010 M-1 s-1. Complementary spectroelectrochemistry and photolysis experiments confirmed the formation of the reduced photosensitizer and showcased the redox and photostability of the Ir(III) photosensitizer that holds great promise for the HX splitting approach.

Hybrid nanospheres of silica covalently containing yellow-emitting cationic iridium(III) complex: preparation and application in white light-emitting diodes

A yellow-emitting cationic iridium(III) complex [(dfppy)2Ir(TBD)]PF6 (TBD: N4,N4'-bis(3-(triethoxysilyl)propyl)-[2,2'-bipyridine]-4,4'-dicarboxamide; dfppy: 2-(2,4-difluorophenyl)pyridine) containing hydrolysable alkoxysilanes was synthesized. Then, a series of silica-based hybrid nanospheres with diameters of around 400 nm was prepared via the hydrolysis of this complex together with tetraethyl orthosilicate (TEOS, a silica source). When the amount of the complex used was 5.0 wt%, hybrid nanospheres showed the best photoluminescence (PL) properties, relative to the PL quantum yield of pure solid [(dfppy)2Ir(TBD)]PF6 (12.7%), that of hybrid nanospheres increased to 26.2%. Moreover, the thermal decomposition temperature (Td) of pure solid [(dfppy)2Ir(TBD)]PF6 was 331 °C, the Td of the complex in hybrid nanospheres increased to 447 °C. However, the yellow light emission was almost unchanged and was still located at 500-750 nm with a maximum wavelength (λem,max) of 577 nm. Under the excitation of blue-emitting chips (λem,max ≈ 455 nm), cold/neutral/warm white light-emitting diodes (WLEDs) with good luminous quality can all be fabricated using these hybrid nanospheres as phosphors in epoxy resin at different blending concentrations. Compared with two or three iridium(III) complexes being contained in silica-based particles as phosphors as described in literatures, in this study, silica-based hybrid nanospheres covalently containing only one yellow-emitting cationic iridium(III) complex as phosphors provide a more effective and simpler method for preparation high-performance WLEDs.

Site-Specific Anchoring a Luminescent Tag in a Protein with Non-Emissive Iridium(III) Complex

Site-specific modification of proteins with synthetic fluorescent tag effectively improves the resolution of imaging, and such a labeling method with negligible three-dimensional structural perturbations and minimal impact on the biological functions of proteins is of high interest to dissect the high-resolution activities of biomolecules in complex systems. To this end, several non-emissive iridium(III) complexes [Ir(C-N)2 (H2 O)2 ]+ OTF- (C-N denotes various cyclometalated ligands) were designed and synthesized. These complexes were tested for attaching a protein by coordinating to H/X (HisMet, HisHis, and HisCys) that are separated by i and i+4 in α-helix. Replacement of the two labile water ligands in the iridium(III) complex by a protein HisHis pair increases the luminescent intensity up to over 100 folds. This labeling approach has been demonstrated in a highly specific and efficient manner in a number of proteins, and it is also feasible for labeling target proteins in cell lysates.

Tuning the Organelle-Specific Imaging and Photodynamic Therapeutic Efficacy of Theranostic Mono- and Trinuclear Organometallic Iridium(III) Complexes

The organelle-specific localization of mononuclear and trinuclear iridium(III) complexes and their photodynamic behavior within the cells are described herein, emphasizing their structure-activity relationship. Both the IrA2 and IrB2 complexes possess a pair of phenyl-benzothiazole derived from the -CHO moieties of mononuclear organometallic iridium(III) complexes IrA1 and IrB1, which chelates IrCp*Cl (Cp* = 1,2,3,4,5-pentamethylcyclopentadiene) to afford trinuclear complexes IrA3 and IrB3. Insights into the photophysical and electrochemical parameters of the complexes were obtained by a time-dependent density functional theory study. The synthesized complexes IrA2, IrA3, IrB2, and IrB3 were found to be nontoxic to human MCF7 breast carcinoma cells. However, the photoexcitation of complexes using LED light could effectively trigger intracellular reactive oxygen species (ROS) generation, leading to cell death. Furthermore, to check the organelle-specific localization of IrA2 and IrB2, we observed that both complexes could selectively localize in the endoplasmic reticulum. In contrast, trinuclear IrA3 and IrB3 accumulate in the nuclei. The photoexcitation of complexes using LED light could effectively trigger intracellular reactive oxygen species (ROS) generation, leading to cell death.

Rational design of efficient photosensitizers based on cyclometalated iridium(III) complexes with 2-arylbenzimidazole and aromatic 1,3-diketone ligands

A judicious selection of substituents in cyclometalating 2-arylbenzimidazoles and an ancillary aromatic 1,3-diketone enabled the creation of heteroleptic iridium(III) complexes demonstrating strong light absorption up to 500 nm (ε ≈ 10 000-12 000 M-1 cm-1). The complexes, which were studied by various spectroscopic techniques, single-crystal X-ray diffraction and cyclic voltammetry, displayed tunable absorption maxima depending on the nature of substituents and their positions. The experimental study was corroborated by quantum chemical calculations, which showed an increased contribution of intraligand charge transfer transitions to the visible light absorption in the case of complexes containing electron-withdrawing substituents in the ligands. Despite being of high intensity, some of these transitions are responsible for the formation of the excited states located at large distances from the 'anchoring' fragment incorporated in the ancillary ligand. In turn, incorporation of electron-donating substituents at the para-position to the Ir-C bonds increases the number of excited states located on the ancillary ligand. The destabilization of the HOMO, which is caused by the increase in the electron-donating ability of the substituents in the metalated rings, translated into negative shifts of the Ir4+/Ir3+ redox potential, affecting, in some cases, the degree of electrochemical reversibility of the complexes. Several complexes having strong light-harvesting characteristics and undergoing reversible oxidation in the appropriate potential range were used for coating the TiO2 photoanodes, which reached an efficiency of 2.15% upon irradiation with the standard AM 1.5 spectrum.

Luminescent iridium(III) porphyrin complexes as near-infrared-emissive biological probes

We report herein the design, synthesis and characterisation of a series of luminescent iridium(III) porphyrin complexes [Ir(ttp)(CH2CH2OH)] (H2ttp = 5,10,15,20-tetra-4-tolylporphyrin) (1), [Ir(tpp-Ph-NO2)(CO)Cl] (H2tpp-Ph-NO2 = 5-(4-((4-nitrophenoxy)carbonyloxymethyl)phenyl)-10,15,20-triphenylporphyrin) (2), [Ir(tpp-COOMe)(Py)2](Cl) (H2tpp-COOMe = 5-(4-methoxycarbonylphenyl)-10,15,20-triphenylporphyrin; Py = pyridine) (3) and [Ir(tpp-COOH)(Py)2](Cl) (H2tpp-COOH = 5-(4-carboxylphenyl)-10,15,20-triphenylporphyrin) (4). All the complexes displayed long-lived near-infrared (NIR) emission attributed to an excited state of mixed triplet intraligand (3IL) (π → π*) (porphyrin) and triplet metal-to-ligand charge transfer (3MLCT) (dπ(Ir) → π*(porphyrin)) character. The cytotoxicity of the complexes toward HeLa cells was examined by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assay. The cationic complexes 3 and 4 exhibited higher cytotoxic activity toward HeLa cells than their neutral counterparts 1 and 2. Cellular uptake studies by inductively coupled plasma-mass spectrometry (ICP-MS) and laser-scanning confocal microscopy (LSCM) indicated that complexes 3 and 4 showed higher cellular uptake efficiencies than complexes 1 and 2 due to their cationic charge, and they were enriched in the perinuclear region of the cells with negligible nuclear uptake. Additionally, the carboxyl complex 4 was used to label a model protein bovine serum albumin (BSA) via an amidation reaction. The resultant luminescent protein conjugate 4-BSA displayed similar photophysical properties and intracellular localisation behaviour to its parent complex. The results of this work will contribute to the development of luminescent iridium(III) porphyrin complexes and related bioconjugates as NIR-emissive probes for bioimaging applications.

A new family of luminescent [Pt(pbt)2(C6F5)L]n+ (n = 1, 0) complexes: synthesis, optical and cytotoxic studies

Given the widely recognized bioactivity of 2-arylbenzothiazoles against tumor cells, we have designed a new family of luminescent heteroleptic pentafluorophenyl-bis(2-phenylbenzothiazolyl) PtIV derivatives, fac-[Pt(pbt)2(C6F5)L]n+ (n = 1, 0) [L = 4-Mepy 1, 4-pyridylbenzothiazole (pybt) 2, 4,4'-bipyridine (4,4'-bpy) 3, 1,2-bis-(4-pyridyl)ethylene (bpe) 4 (E/Z ratio: 90/10), 1,4-bis-(pyridyl)butadiyne (bpyb) 5, trifluoroacetate (-OCOCF3) 6] and a dinuclear complex [{Pt(pbt)2(C6F5)}2(μ-bpyb)](PF6)27, in which the trans ligand to the metalated C-(pbt) was varied to modify the optical properties and lipophilicity. Their photophysical properties were systematically studied through experimental and theoretical investigations, which were strongly dependent on the identity of the N-bonded ligand. Thus, complexes 1, 3 and 6 display, in different media, emission from the triplet excited states of primarily intraligand 3ILCT nature localized on the pbt ligand, while the emissions of 2, 5 and 7 were ascribed to a mixture of close 3IL'(N donor)/3ILCT(pbt) excited states, as supported by lifetime measurements and theoretical calculations. Irradiation of the initial E/Z mixture of 4 (15 min) led to a steady state composed of roughly 1 : 1.15 (E : Z) and this complex was not emissive at room temperature due to an enhanced intramolecular E to Z isomerization process of the 1,2-bis-(4-pyridyl)ethylene ligand. Complexes 1-3 and 6 showed excellent quantum yields for the generation of singlet oxygen in aerated MeCN solution with the values of ϕ(1O2) ranging from 0.66 to 0.86 using phenalenone as a reference. Cationic complexes 1-3 exhibited remarkable efficacy in the nanomolar range against A549 (lung carcinoma) and HeLa (cervix carcinoma) cell lines with notable selectivity relative to the non-tumorigenic BEAS-2B (bronchial epithelium) cells. In the A549 cell line, the neutral complex 6 showed low cytotoxicity (IC50: 29.40 μM) and high photocytotoxicity (IC50: 5.75) when cells were irradiated with blue light for 15 min. These complexes do not show evidence of DNA interaction.

Dual-emissive Iridium(III) Complexes as Phosphorescent Probes with Orthogonal Responses to Analyte Binding and Oxygen Quenching

Phosphorescent probes often show sensitive response toward analytes at a specific wavelength. However, oxygen quenching usually occurs at the same wavelength and thus hinders the accurate detection of analytes. In this study, we have developed dual-emissive iridium(III) complexes that exhibit phosphorescence responses to copper(II) ions at a wavelength distinct from that where oxygen quenching occurs. The complexes displayed colorimetric phosphorescence response in aqueous solutions under different copper(II) and oxygen conditions. In cellular imaging, variation in oxygen concentration over a large range from 5 % to 80 % can modulate the intensity and lifetime of green phosphorescence without affecting the response of red phosphorescence toward intracellular copper(II) ions.

Optical Resolution of Carboxylic Acid Derivatives of Homoleptic Cyclometalated Iridium(III) Complexes via Diastereomers Formed with Chiral Auxiliaries

We report on a facile method for the optical resolution of cyclometalated iridium(III) (Ir(III)) complexes via diastereomers formed with chiral auxiliaries. The racemic carboxylic acids of Ir(III) complexes (fac-4 (fac-Ir(ppyCO2H)3 (ppy: 2-phenylpyridine)), fac-6 (fac-Ir(tpyCO2H)3 (tpy: 2-(4'-tolyl)pyridine)), and fac-13 (fac-Ir(mpiqCO2H)3 (mpiq: 1-(4'-methylphenyl)isoquinoline))) were converted into the diastereomers, Δ- and Λ-forms of fac-9 (from fac-6), fac-10 (from fac-4), fac-11 (from fac-6), and fac-14 (from fac-13), respectively, by the condensation with (1R,2R)-1,2-diaminocyclohexane or (1R,2R)-2-aminocyclohexanol. The resulting diastereomers were separated by HPLC (with a nonchiral column) or silica gel column chromatography, and their absolute stereochemistry was determined by X-ray single-crystal structure analysis and CD (circular dichroism) spectra. Spectra of all diastereomers of the Ir(III) complexes are reported. Hydrolysis of the ester moieties of Δ- and Λ-forms of fac-10, fac-11, and fac-14 gave both enantiomers of the corresponding carboxylic acid derivatives in the optically pure forms, Δ-fac and Λ-fac-4, -6, and -13, respectively.

Development of Wireless Power-Transmission-Based Photodynamic Therapy for the Induction of Cell Death in Cancer Cells by Cyclometalated Iridium(III) Complexes

Photodynamic therapy (PDT), a noninvasive method for cancer therapy, involves the generation of reactive oxygen species (ROS) by the photochemical excitation of photosensitizers (PSs) to induce cell death in cancer cells. A variety of PS including porphyrin derivatives and metal complexes such as iridium (Ir) complexes have been reported. In clinical trials, red-near infrared (NIR) light (650-900 nm) is preferred for the excitation of PSs due to its deeper penetration into tissues compared with visible light (400-500 nm). To overcome this limitation, we established a PDT system that uses cyclometalated iridium(III) (Ir(III)) complexes that are excited with blue light in the wireless power transmission (WPT) system. To achieve this, we developed a light-emitting diode (LED) light device equipped with a receiver coil that receives electricity from the transmitter coil through magnetic resonance coupling. The LEDs in the receiving device use blue light (470 nm) to irradiate a given Ir(III) complex and excite triplet oxygen (³O₂) to singlet oxygen (¹O₂) which induces cell death in HeLa S3 cells (human cervical carcinoma cells). The results obtained in this study suggest that WPT-based PDT represents a potentially new method for the treatment of tumors by a non-battery LED, which are otherwise difficult to treat by previous PDT systems.

N-Heterocyclic Carbene-Iridium Complexes as Photosensitizers for In Vitro Photodynamic Therapy to Trigger Non-Apoptotic Cell Death in Cancer Cells

A series of seven novel iridium complexes were synthetized and characterized as potential photosensitizers for photodynamic therapy (PDT) applications. Among them, four complexes were evaluated in vitro for their anti-proliferative activity with and without irradiation on a panel of five cancer cell lines, namely PC-3 (prostate cancer), T24 (bladder cancer), MCF7 (breast cancer), A549 (lung cancer) and HeLa (cervix cancer), and two non-cancerous cell models (NIH-3T3 fibroblasts and MC3T3 osteoblasts). After irradiation at 458 nm, all tested complexes showed a strong selectivity against cancer cells, with a selectivity index (SI) ranging from 8 to 34 compared with non-cancerous cells. The cytotoxic effect of all these complexes was found to be independent of the anti-apoptotic protein Bcl-xL. The compound exhibiting the best selectivity, complex 4a, was selected for further investigations. Complex 4a was mainly localized in the mitochondria. We found that the loss of cell viability and the decrease in ATP and GSH content induced by complex 4a were independent of both Bcl-xL and caspase activation, leading to a non-apoptotic cell death. By counteracting the intrinsic or acquired resistance to apoptosis associated with cancer, complex 4a could be an interesting therapeutic alternative to be studied in preclinical models.

Acid-base-induced fac → mer isomerization of luminescent iridium(iii) complexes

Luminescent Ir(C^N)3 complexes (C^N = cyclometalated arylpyridine ligand) exist in the form of two stable isomers with distinct photophysical and electrochemical properties: fac and mer. Herein, we show that fac-Ir(C^N)3 complexes can be converted into the thermodynamically less stable mer forms by a consecutive reaction with first acid and then base. The chemically induced isomerization is fast, quantitative, and stereoselective, and it can be inversed by light. The new isomerization process opens the possibility to use highly luminescent Ir(C^N)3 complexes as molecular switches.

Iridium-Functionalized Cellulose Microcrystals as a Novel Luminescent Biomaterial for Biocomposites

Microcrystalline cellulose (MCC) is an emerging material with outstanding properties in many scientific and industrial fields, in particular as an additive in composite materials. Its surface modification allows for the fine-tuning of its properties and the exploitation of these materials in a plethora of applications. In this paper, we present the covalent linkage of a luminescent Ir-complex onto the surface of MCC, representing the first incorporation of an organometallic luminescent probe in this biomaterial. This goal has been achieved with an easy and sustainable procedure, which employs a Bronsted-acid ionic liquid as a catalyst for the esterification reaction of -OH cellulose surface groups. The obtained luminescent cellulose microcrystals display high and stable emissions with the incorporation of only a small amount of iridium (III). Incorporation of MCC-Ir in dry and wet matrices, such as films and gels, has been also demonstrated, showing the maintenance of the luminescent properties even in possible final manufacturers.

Post-complexation Functionalization of Cyclometalated Iridium(III) Complexes and Applications to Biomedical and Material Sciences

Cyclometalated iridium(III) (Ir(III)) complexes exhibit excellent photophysical properties that include large Stokes shift, high emission quantum yields, and microsecond-order emission lifetimes, due to low-lying metal-to-ligand charge transfer (spin-forbidden singlet-triplet (³MLCT) transition). As a result, analogs have been applied for research not only in the material sciences, such as the development of organic light-emitting diodes (OLEDs), but also for photocatalysts, bioimaging probes, and anticancer reagents. Although a variety of methods for the synthesis and the applications of functionalized cyclometalated iridium complexes have been reported, functional groups are generally introduced to the ligands prior to the complexation with Ir salts. Therefore, it is difficult to introduce thermally unstable functional groups such as peptides and sugars due to the harsh reaction conditions such as the high temperatures used in the complexation with Ir salts. In this review, the functionalization of Ir complexes after the formation of cyclometalated Ir complexes and their biological and material applications are described. These methods are referred to as "post-complexation functionalization (PCF)." In this review, applications of PCF to the design and synthesis of Ir(III) complexes that exhibit blue -red and white color emissions, luminescence pH probes, luminescent probes of cancer cells, compounds that induce cell death in cancer cells, and luminescent complexes that have long emission lifetimes are summarized.

Phosphorescent Ir(III) Complexes for Biolabeling and Biosensing

Cyclometalated Ir(III) complexes exhibit strong phosphorescence emission with lifetime of submicroseconds to several microseconds at room temperature. Their synthetic versatility enables broad control of physical properties, such as charge and lipophilicity, as well as emission colors. These favorable properties have motivated the use of Ir(III) complexes in luminescent bioimaging applications. This review examines the recent progress in the development of phosphorescent biolabels and sensors based on Ir(III) complexes. It begins with a brief introduction about the basic principles of the syntheses and photophysical processes of cyclometalated Ir(III) complexes. Focus is placed on illustrating the broad imaging utility of Ir(III) complexes. Phosphorescent labels illuminating intracellular organelles, including mitochondria, lysosomes, and cell membranes, are summarized. Ir(III) complexes capable of visualization of tumor spheroids and parasites are also introduced. Facile chemical modification of the cyclometalating ligands endows the Ir(III) complexes with strong sensing ability. Sensors of temperature, pH, CO₂, metal ions, anions, biosulfur species, reactive oxygen species, peptides, and viscosity have recently been added to the molecular imaging tools. This diverse utility demonstrates the potential of phosphorescent Ir(III) complexes toward bioimaging applications.

Photo- and Electroluminescent Neutral Iridium(III) Complexes Bearing Imidoylamidinate Ligands

Imidoylamidinate-based heteroleptic bis(2-phenylbenzothiazole)iridium(III) and -rhodium(III) complexes [(bt)₂M(N^∩N)] (bt = 2-phenylbenzothiazole, N^∩N = N'-(benzo[d]thiazol-2-yl)acetimidamidyl (Ir1 and Rh1), N'-(6-fluorobenzo[d]thiazol-2-yl)acetimidamidyl (Ir2), N'-(benzo[d]oxazol-2-yl)acetimidamidyl (Ir3), N'-(1-methyl-1H-benzo[d]imidazol-2-yl)acetimidamidyl (Ir4); yields 70-84%) were obtained by the reaction of the in situ-generated solvento-complex [(bt)₂M(NCMe)₂]NO₃ and benzo[d]thia/oxa/N-methylimidozol-2-amines in the presence of NaOMe. Complexes Ir1-4 exhibited intense orange photoluminescence, reaching 37% at room temperature quantum yields, being immobilized in a poly(methyl methacrylate) matrix. A photophysical study of these species in a CH₂Cl₂ solution, neat powder, and frozen (77 K) MeOC₂H₄OH-EtOH glass matrix─along with density-functional theory (DFT), ab initio methods, and spin-orbit coupling time-dependent DFT calculations─verified the effects of substitution in the imidoylamidinate ligands on the excited-state properties. Electrochemical (cyclic voltammetry and differential pulse voltammetry) and theoretical DFT studies demonstrated noninnocent behavior of the imidoylamidinate ligands in Ir1-4 and Rh1 complexes due to the significant contribution coming from these ligands in the HOMO of the complexes. The iridium(III) species exhibit a ligand (L, 2-phenylbenzothiazole)-centered (³LC), metal-to-ligand (L', imidoylamidinate) charge-transfer (³ML'CT,³MLCT) character of their emission. The imidoylamidinate-based iridium(III) species were proved to be effective as the emissive dopant in an organic light-emitting diode device, fabricated in the framework of this study.

Iridium(III)-Cp*-(imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenol analogues as hypoxia active, GSH-resistant cancer cytoselective and mitochondria-targeting cancer stem cell therapeutic agents

Herein, we have introduced a series of iridium(III)-Cp*-(imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenol complexes via a convenient synthetic methodology, which act as hypoxia active and glutathione-resistant anticancer metallotherapeutics. The [Ir^III(Cp*)(L5)(Cl)](PF₆) (IrL5) complex exhibited the best cytoselectivity, GSH resistance and hypoxia effectivity in HeLa and Caco-2 cells among the synthesized complexes. IrL5 also exhibited highly cytotoxic effects on the HCT-116 CSC cell line. This complex was localized in the mitochondria and subsequent mitochondrial dysfunction was observed via MMP alteration and ROS generation on colorectal cancer stem cells. Cell cycle analysis also established the potential of this complex in mediating G2/M phase cell cycle arrest.