Luminescent rhenium(I), ruthenium(II), and iridium(III) polypyridine complexes containing a poly(ethylene glycol) pendant or bioorthogonal reaction group as biological probes and photocytotoxic agents

Self-assembly of a ruthenium-based cGAS-STING photoactivator for carrier-free cancer immunotherapy

The cGAS (cyclic GMP-AMP synthase)-STING (stimulator of interferon genes) pathway promotes antitumor immune responses by sensing cytosolic DNA fragments leaked from nucleus and mitochondria. Herein, we designed a highly charged ruthenium photosensitizer (Ru1) with a β-carboline alkaloid derivative as the ligand for photo-activating of the cGAS-STING pathway. Due to the formation of multiple non-covalent intermolecular interactions, Ru1 can self-assemble into carrier-free nanoparticles (NPs). By incorporating the triphenylphosphine substituents, Ru1 can target and photo-damage mitochondrial DNA (mtDNA) to cause the cytoplasmic DNA leakage to activate the cGAS-STING pathway. Finally, Ru1 NPs show potent antitumor effects and elicit intense immune responses in vivo. In conclusion, we report the first self-assembling mtDNA-targeted photosensitizer, which can effectively activate the cGAS-STING pathway, thus providing innovations for the design of new photo-immunotherapeutic agents.

Systematic review on antibacterial photodynamic therapeutic effects of transition metals ruthenium and iridium complexes

The prevalence of antibiotic-resistant pathogenic bacteria poses a significant threat to public health and ranks among the principal causes of morbidity and mortality worldwide. Antimicrobial photodynamic therapy is an emerging therapeutic technique that has excellent potential to embark upon antibiotic resistance problems. The efficacy of this therapy hinges on the careful selection of suitable photosensitizers (PSs). Transition metal complexes, such as Ruthenium (Ru) and Iridium (Ir), are highly suitable for use as PSs because of their surface plasmonic resonance, crystal structure, optical characteristics, and photonics. These metals belong to the platinum family and exhibit similar chemical behavior due to their partially filled d-shells. Ruthenium and Iridium-based complexes generate reactive oxygen species (ROS), which interact with proteins and DNA to induce cell death. As photodynamic therapeutic agents, these complexes have been widely studied for their efficacy against cancer cells, but their potential for antibacterial activity remains largely unexplored. Our study focuses on exploring the antibacterial photodynamic effect of Ruthenium and Iridium-based complexes against both Gram-positive and Gram-negative bacteria. We aim to provide a comprehensive overview of various types of research in this area, including the structures, synthesis methods, and antibacterial photodynamic applications of these complexes. Our findings will provide valuable insights into the design, development, and modification of PSs to enhance their photodynamic therapeutic effect on bacteria, along with a clear understanding of their mechanism of action.

Recent Development of Rhenium-Based Materials in the Application of Diagnosis and Tumor Therapy

Rhenium (Re) is widely used in the diagnosis and treatment of cancer due to its unique physical and chemical properties. Re has more valence electrons in its outer shell, allowing it to exist in a variety of oxidation states and to form different geometric configurations with many different ligands. The luminescence properties, lipophilicity, and cytotoxicity of complexes can be adjusted by changing the ligand of Re. This article mainly reviews the development of radionuclide 188Re in radiotherapy and some innovative applications of Re as well as the different therapeutic approaches and imaging techniques used in cancer therapy. In addition, the current application and future challenges and opportunities of Re are also discussed.

Replacement of the phosphodiester backbone between canonical nucleosides with a dirhenium carbonyl "click" linker-a new class of luminescent organometallic dinucleoside phosphate mimics

The first-in-class luminescent dinucleoside phosphate analogs with a [Re₂(μ-Cl)₂(CO)₆(μ-pyridazine)] "click" linker as a replacement for the natural phosphate group are reported together with the synthesis of luminescent adenosine and thymidine derivatives having the [Re₂(μ-Cl)₂(CO)₆(μ-pyridazine)] entity attached to positions 5' and 3', respectively. These compounds were synthesized by applying inverse-electron-demand Diels-Alder and copper(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition reactions in three or four steps. The obtained compounds exhibited orange emission (λ_PL ≈ 600 nm, Φ_PL ≈ 0.10, and τ = 0.33-0.61 μs) and no toxicity (except for one nucleoside) to human HeLa cervical epithelioid and Ishikawa endometrial adenocarcinoma cancer cells in vitro. Furthermore, the compounds' ability to inhibit the growth of Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacterial strains was moderate and only observed at a high concentration of 100 μM. Confocal microscopy imaging revealed that the "dirhenium carbonyl" dinucleosides and nucleosides localized mainly in the membranous structures of HeLa cells and uniformly inside S. aureus and E. coli bacterial cells. An interesting finding was that some of the tested compounds were also found in the nuclei of HeLa cells.

Photoinduced Processes in Rhenium(I) Terpyridine Complexes Bearing Remote Amine Groups: New Insights from Transient Absorption Spectroscopy

Photophysical properties of two Re(I) complexes [ReCl(CO)3(R-C6H4-terpy-κ2N)] with remote amine groups, N-methyl-piperazinyl (1) and (2-cyanoethyl)methylamine (2), were investigated. The complexes show strong absorption in the visible region corresponding to metal-to-ligand charge transfer (1MLCT) and intraligand-charge-transfer (1ILCT) transitions. The energy levels of 3MLCT and 3ILCT excited-states, and thus photoluminescence properties of 1 and 2, were found to be strongly affected by the solvent polarity. Compared to the parent chromophore [ReCl(CO)3(C6H5-terpy-κ2N)] (3), both designed complexes show significantly prolonged (by 1–2 orders of magnitude) phosphorescence lifetimes in acetonitrile and dimethylformamide, contrary to their lifetimes in less polar chloroform and tetrahydrofuran, which are comparable to those for 3. The femtosecond transient absorption (fsTA) measurements confirmed the interconversion between the 3MLCT and 3ILCT excited-states in polar solvents. In contrast, the emissive state of 1 and 2 in less polar environments is of predominant 3MLCT nature.

Cathodoluminescence imaging of cellular structures labeled with luminescent iridium or rhenium complexes at cryogenic temperatures

We report for the first time the use of two live-cell imaging agents from the group of luminescent transition metal complexes (IRAZOLVE-MITO and REZOLVE-ER) as cathodoluminescent probes. This first experimental demonstration shows the application of both probes for the identification of cellular structures at the nanoscale and near the native state directly in the cryo-scanning electron microscope. This approach can potentially be applied to correlative and multimodal approaches and used to target specific regions within vitrified samples at low electron beam energies.

The cyclometalated iridium (III) complex based on 9-Anthracenecarboxylic acid as a lysosomal-targeted anticancer agent

9-Anthracenecarboxylic acid (9-Ac) was reported early as a chloride channel inhibitor and was found to exhibit significant anti-proliferative activity on leukemic cells, but has not been researched in solid tumor cells. Herein, a 9-anthraceneic acid derivative was introduced into the cyclometalated Iridium (III) species to construct a novel Iridium (Ir) complex Ir-9-Ac, [Ir(ppy)₂(9-Ac-L)]PF₆ (ppy = 2-phenylpyridine, 9-Ac-L = N-((4'-methyl-[2,2'-bipyridin]-4-yl)methyl)anthracene-9-carboxamide), which could accumulated in lysosomes. Ir-9-Ac showed good cytotoxic activity against several tumor cell lines, notably on A549 cells. Besides Ir-9-Ac could inhibit the cell colony formation and growth of the 3D cell spheroids, demonstrating the potential to suppress tumors in vivo. This design provided a platform for the design of cyclometalated Iridium (III) anticancer complexes.

Determination and Imaging of Small Biomolecules and Ions Using Ruthenium(II) Complex-Based Chemosensors

Luminescence chemosensors are one of the most useful tools for the determination and imaging of small biomolecules and ions in situ in real time. Based on the unique photo-physical/-chemical properties of ruthenium(II) (Ru(II)) complexes, the development of Ru(II) complex-based chemosensors has attracted increasing attention in recent years, and thus many Ru(II) complexes have been designed and synthesized for the detection of ions and small biomolecules in biological and environmental samples. In this work, we summarize the research advances in the development of Ru(II) complex-based chemosensors for the determination of ions and small biomolecules, including anions, metal ions, reactive biomolecules and amino acids, with a particular focus on binding/reaction-based chemosensors for the investigation of intracellular analytes' evolution through luminescence analysis and imaging. The advances, challenges and future research directions in the development of Ru(II) complex-based chemosensors are also discussed.

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.

Novel NIR-Phosphorescent Ir(III) Complexes: Synthesis, Characterization and Their Exploration as Lifetime-Based O2 Sensors in Living Cells

A series of [Ir(N^C)2(N^N)]+ NIR-emitting orthometalated complexes (1-7) has been prepared and structurally characterized using elemental analysis, mass-spectrometry, and NMR spectroscopy. The complexes display intense phosphorescence with vibrationally structured emission bands exhibiting the maxima in the range 713-722 nm. The DFT and TD DFT calculations showed that the photophysical characteristics of these complexes are largely determined by the properties of the metalating N^C ligands, with their major contribution into formation of the lowest S1 and T1 excited states responsible for low energy absorption and emission, respectively. Emission lifetimes of 1-7 in degassed methanol solution vary from 1.76 to 5.39 µs and show strong quenching with molecular oxygen to provide an order of magnitude lifetime reduction in aerated solution. The photophysics of two complexes (1 and 7) were studied in model physiological media containing fetal bovine serum (FBS) and Dulbecco's Modified Eagle Medium (DMEM) to give linear Stern-Volmer calibrations with substantially lower oxygen-quenching constants compared to those obtained in methanol solution. These observations were interpreted in terms of the sensors' interaction with albumin, which is an abundant component of FBS and cell media. The studied complexes displayed acceptable cytotoxicity and preferential localization, either in mitochondria (1) or in lysosomes (7) of the CHO-K1 cell line. The results of the phosphorescence lifetime imaging (PLIM) experiments demonstrated considerable variations of the sensors' lifetimes under normoxia and hypoxia conditions and indicated their applicability for semi-quantitative measurements of oxygen concentration in living cells. The complexes' emission in the NIR domain and the excitation spectrum, extending down to ca. 600 nm, also showed that they are promising for use in in vivo studies.

pH-Responsive Emission of Novel Water-Soluble Polymeric Iridium(III) Complexes

The synthesis and characterization of water-soluble copolymers containing N,N-dimethylacrylamide (DMAM) and a vinylic monomer containing an Iridium(III), Ir(III), complex substituted with the quinoline-based unit 2-(pyridin-2-ylo)-6-styrene-4-phenylquinoline (VQPy) as ligand are reported. These copolymers were prepared through pre- or post-polymerization complexation of Ir(III) with the VQPy units. The first methodology led to copolymer P1 having fully complexed VQPy units, whereas the latter methodology allowed the preparation of terpolymers containing free and Ir(III)-complexed VQPy units (copolymer P2). The optical properties of the copolymers were studied in detail through UV-Vis and photoluminescence spectroscopy in aqueous solution. It is shown that the metal-to-ligand charge transfer (ΜLCT) emission is prevailing in the case of P1, regardless of pH. In contrast, in the case of terpolymer P2 the MLCT emission of the Ir(III) complex is combined with the pH-responsive emission of free VQPy units, leading to characteristic pH-responsive color changes under UV illumination in the acidic pH region.

A new family of one-dimensional bromo-bridged Ir(IV)-Cu(II) complexes based on the hexabromoiridate(IV) metalloligand

By using the iridium(IV) complex (NBu₄)₂[IrBr₆] (1) as a metalloligand towards a Cu(II) metal ion, three novel Ir(IV) one-dimensional (1D) compounds of formula {IrBr₅(μ-Br)Cu(Meim)₄}_n (2), {IrBr₅(μ-Br)Cu(Viim)₄}_n (3) and {IrBr₅(μ-Br)Cu(Buim)₄}_n (4), [Meim = 1-methylimidazole; Viim = 1-vinylimidazole; Buim = 1-butylimidazole] have been prepared and structurally and magnetically characterised. Compounds 2, 3 and 4 crystallise in the triclinic, monoclinic and orthorhombic crystal systems with space groups P1̄, C2/c and Pccn, respectively. Each Ir(IV) ion in 1-4 is six-coordinate and bonded to six bromide ions in a quasi regular octahedral geometry. In compounds 2-4, the Cu^II ion shows an axially elongated octahedron with four N atoms, from four monodentate imidazole derivative ligands, that form the equatorial plane and two bromide ions that occupy the axial positions. Cu(II) and Ir(IV) ions are linked through bridging bromide anions generating Ir(IV)-Cu(II) chains [with intrachain Cu(II)⋯Ir(IV) distances covering the range of ca. 5.10-5.42 Å]. In the crystal lattice of 2 and 3 are observed significant intermolecular Ir-Br⋯Br-Ir contacts and π⋯Br interactions, which organize arrangements that contribute to stabilizing the crystal structure of these Ir(IV)-based compounds. DC magnetic susceptibility measurements reveal that 1 displays magnetic behaviour typical of noninteracting mononuclear centres with S = 1/2. Besides, antiferromagnetic behaviour (2 and 3) and ferromagnetic (4) exchange coupling occur between the Cu(II) and Ir(IV) metal ions in the one-dimensional bromo-bridged compounds 2-4. Moreover, the study of the AC magnetic susceptibility shows a field-induced slow relaxation of the magnetisation for 1, indicating the presence of the single-ion magnet (SIM) phenomenon for the magnetically isolated hexabromoiridate(IV) complex.

V, Kar B, Das U, Paira P. Target-specific mononuclear and binuclear rhenium(i) tricarbonyl complexes as upcoming anticancer drugs

Metal complexes have gradually been attracting interest from researchers worldwide as potential cancer therapeutics. Driven by the many side effects of the popular platinum-based anticancer drug cisplatin, the tireless endeavours of researchers have afforded strategies for the design of appropriate metal complexes with minimal side effects compared to cisplatin and its congeners to limit the unrestricted propagation of cancer. In this regard, transition metal complexes, especially rhenium-based complexes are being identified and highlighted as promising cancer theranostics, which are endowed with the ability to detect and annihilate cancer cells in the body. This is attributed the amazing photophysical properties of rhenium complexes together with their ability to selectively attack different organelles in cancer cells. Therefore, this review presents the properties of different rhenium-based complexes to highlight their recent advances as anticancer agents based on their cytotoxicity results.

In this review, rhenium-based complexes are highlighted as promising cancer theranostics, which are endowed with the ability to detect and annihilate cancer cells in the body.

Amino group-driven distinguishing homocysteine from cysteine and glutathione in photoluminesecent signal of the iridium(III) complexes

In this work, six iridium(III) complexes have been designed, synthesized and characterized. The molecular structures of complex 1 ([(pba)₂Ir(bpy-2N(CH₃)₂)]PF₆), 2 ([(pba)₂Ir(bpy-2NH₂)]PF₆) and 3 ([(pba)₂Ir(bpy-2CH₃)]PF₆) were determined by single crystal X-ray diffraction. Upon addition of Hcy (homocysteine) to the solution of complex 1, a luminescent variation from orange red to green was observed by the naked eye, corresponding to a large blue shift from 604 nm to 498 nm (~106 nm). While the emission intensity of complex 1 was almost no change after addition of other common amino acids including Cys (cysteine) and GSH (glutathione). The aldehyde group of complex 1 formed a new thiazinane/thiazolidine ring with Hcy/Cys confirmed by ¹H NMR and high-resolution mass spectrometry. And the new product 1-Hcy had a higher quantum yield than 1-Cys. Theoretical calculations showed that the HOMO (highest occupied molecular orbital) of 1-Hcy was located on the newly formed six-membered thiazinane ring, which was different from the HOMO of 1-Cys. Compared with the other iridium(III) complexes, we can speculate that the large blue shift and enhancement of the emission intensity of the complex 1 were related to the strong electron donating ability of the modified amino groups on bipyridine ligand. This will provide an idea for the design of ratio-based luminescence probes for Hcy in future.

Tuning the organelle specificity and cytotoxicity of iridium(III) photosensitisers for enhanced phototheranostic applications

Luminescent cyclometallated iridium(III) complexes with a polyhedral oligomeric silsesquioxane (POSS) unit were designed as efficient theranostic agents that displayed tuneable organelle-targeting properties, minimal dark cytotoxicity and substantial photocytotoxicity even under hypoxic conditions.

Luminescent monomeric and dimeric Ru(ii) acyclic carbene complexes as selective sensors for NH3/amine vapor and humidity

A new class of luminescent bis(bipyridyl) Ru(ii) pyridyl acyclic carbene complexes with environmentally-sensitive dimerization equilibrium have been developed. Owing to the involvement of the orbitals of the diaminocarbene ligand in the emissive excited state, the phosphorescence properties of these complexes are strongly affected by H-bonding interactions with various H-bonding donor/acceptor molecules. With the remarkable differences in the emission properties of the monomer, dimer, and H-bonded amine adducts together with the change of the dimerization equilibrium, these complexes can be used as luminescent gas sensors for humidity, ammonia, and amine vapors. With the responses to amines and humidity and the corresponding change in the luminescence properties, a proof-of-principle for binary optical data storage with a reversible concealment process has been described.

Red Light-Emitting Water-Soluble Luminescent Iridium-Containing Polynorbornenes: Synthesis, Characterization and Oxygen Sensing Properties in Biological Tissues In Vivo

New water-soluble polynorbornenes P1-P4 containing oligoether, amino acid groups and luminophoric complexes of iridium(III) were synthesized by ring-opening metathesis polymerization. The polymeric products in organic solvents and in water demonstrate intense photoluminescence in the red spectral region. The polymers P1 and P3 with 1-phenylisoquinoline cyclometalating ligands in iridium fragments reveal 4-6 fold higher emission quantum yields in solutions than those of P2 and P4 that contain iridium complexes with 1-(thien-2-yl)isoquinoline cyclometalating ligands. The emission parameters of P1-P4 in degassed solutions essentially differ from those in the aerated solutions showing oxygen-dependent quenching of phosphorescence. Biological testing of P1 and P3 demonstrates that the polymers do not penetrate into live cultured cancer cells and normal skin fibroblasts and do not possess cytotoxicity within the concentrations and time ranges reasonable for biological studies. In vivo, the polymers display longer phosphorescence lifetimes in mouse tumors than in muscle, as measured using phosphorescence lifetime imaging (PLIM), which correlates with tumor hypoxia. Therefore, preliminary evaluation of the synthesized polymers shows their suitability for noninvasive in vivo assessments of oxygen levels in biological tissues.

Luminescent halido(aryl) Pt(IV) complexes obtained via oxidative addition of iodobenzene or diaryliodonium salts to bis-cyclometalated Pt(II) precursors

The synthesis of bis-cyclometalated halido(aryl) Pt(IV) complexes [PtX(Ar)(C^N)₂], with C^N = cyclometalated 4-(tert-butyl)-2-phenylpyridine (bppy), 2-(p-tolyl)pyridine (tpy), 2-(2-thienyl)pyridine (thpy), or 1-phenylisoquinoline (piq), X = I, Cl, or F, and Ar = Ph (for all C^N ligands) or t-BuPh (for C^N = tpy), and the photophysical properties of the chlorido and fluorido series is reported. The oxidative addition of iodobenzene to cis-[Pt(C^N)₂] precursors is demonstrated to occur in MeCN under irradiation with visible light to give complexes [PtI(Ph)(C^N)₂], presumably involving radical species that also produce the activation of the solvent to give cyanomethyl complexes [PtI(CH₂CN)(C^N)₂]. The introduction of an aryl ligand can also be achieved by reacting cis-[Pt(C^N)₂] with (Ar₂I)PF₆ (Ar = Ph, t-BuPh), which affords cationic intermediates of the type [Pt(Ar)(C^N)₂(NCMe)]⁺. The subsequent addition of an iodide or chloride salt gives the corresponding iodido- or chlorido(aryl) complexes. The fluorido(aryl) derivatives can be obtained from the iodido complexes by halide exchange using AgF. The chlorido- and fluorido(aryl) complexes display intense phosphorescence in deaerated CH₂Cl₂ solution and poly(methyl methacrylate) (PMMA) films at 298 K from triplet excited states primarily localized on the cyclometalated ligands (³LC) with a small MLCT admixture. Compared with the chlorido complexes, the fluorido derivatives consistently present significantly shorter emission lifetimes and higher radiative and nonradiative rate constants due to a greater MLCT contribution to the emissive state. In contrast, the introduction of the t-BuPh group did not induce significant changes in radiative rates with respect to the phenyl complexes.

Photochemistry of P,N-bidentate rhenium(i) tricarbonyl complexes: reactive species generation and potential application for antibacterial photodynamic therapy

In this work, we describe the photoisomerization of facial rhenium(i) tricarbonyl complexes bearing P,N-bidentate pyridyl/phosphine ligands with different chelating rings and anions: RePNBr, RePNTfO, and RePNNBr, which are triggered under irradiation at 365 nm in solutions. The apparent photodegradation rate constants (k app) depend on the coordinating ability of the solvent, being lowest in acetonitrile. The k app value increases as the temperature rises, suggesting a reactive IL excited state thermally populated from the MLCT excited state involved. Using the Eyring equation, positive activation enthalpies (ΔH ≠) accompanied by high negative values for the activation entropy (ΔS ≠) were obtained. These results suggest whatever the P,N-ligand or anion, the reaction proceeds through a strongly solvated or a compact transition state, which is compatible with an associative mechanism for the photoisomerization. A 100-fold decrease in the log10 CFU value is observed for E. coli and S. aureus in irradiated solutions of the compounds, which follows the same tendency as their singlet oxygen generation quantum yield: RePNBr > RePNTfO > RePNNBr, while no antibacterial activity is observed in the darkness. This result indicates that the generation of singlet oxygen plays a key role in the antibacterial capacity of these complexes.

Strategies to promote permeation and vectorization, and reduce cytotoxicity of metal complex luminophores for bioimaging and intracellular sensing

Transition metal luminophores are emerging as important tools for intracellular imaging and sensing. Their putative suitability for such applications has long been recognised but poor membrane permeability and cytotoxicity were significant barriers that impeded early progress. In recent years, numerous effective routes to overcoming these issues have been reported, inspired in part, by advances and insights from the pharmaceutical and drug delivery domains. In particular, the conjugation of biomolecules but also other less natural synthetic species, from a repertoire of functional motifs have granted membrane permeability and cellular targeting. Such motifs can also reduce cytotoxicity of transition metal complexes and offer a valuable avenue to circumvent such problems leading to promising metal complex candidates for application in bioimaging, sensing and diagnostics. The advances in metal complex probes permeability/targeting are timely, as, in parallel, over the past two decades significant technological advances in luminescence imaging have occurred. In particular, super-resolution imaging is enormously powerful but makes substantial demands of its imaging contrast agents and metal complex luminophores frequently possess the photophysical characteristics to meet these demands. Here, we review some of the key vectors that have been conjugated to transition metal complex luminophores to promote their use in intra-cellular imaging applications. We evaluate some of the most effective strategies in terms of membrane permeability, intracellular targeting and what impact these approaches have on toxicity and phototoxicity which are important considerations in a luminescent contrast or sensing agent.

Ruthenium(II) complexes as bioorthogonal two-photon photosensitizers for tumour-specific photodynamic therapy against triple-negative breast cancer cells

Herein, we developed the first Ru(ii) complex-based bioorthogonal two-photon photosensitizers. Through bioorthogonal labelling, they realize effective tumour-specific photodynamic therapy against triple-negative breast cancer cells.

Nanoparticles Functionalised with Re(I) Tricarbonyl Complexes for Cancer Theranostics

Globally, cancer is the second (to cardiovascular diseases) leading cause of death. Regardless of various efforts (i.e., finance, research, and workforce) to advance novel cancer theranostics (diagnosis and therapy), there have been few successful attempts towards ongoing clinical treatment options as a result of the complications posed by cancerous tumors. In recent years, the application of magnetic nanomedicine as theranostic devices has garnered enormous attention in cancer treatment research. Magnetic nanoparticles (MNPs) are capable of tuning the magnetic field in their environment, which positively impacts theranostic applications in nanomedicine significantly. MNPs are utilized as contrasting agents for cancer diagnosis, molecular imaging, hyperfusion region visualization, and T cell-based radiotherapy because of their interesting features of small size, high reactive surface area, target ability to cells, and functionalization capability. Radiolabelling of NPs is a powerful diagnostic approach in nuclear medicine imaging and therapy. The use of luminescent radioactive rhenium(I), 188/186Re, tricarbonyl complexes functionalised with magnetite Fe3O4 NPs in nanomedicine has improved the diagnosis and therapy of cancer tumors. This is because the combination of Re(I) with MNPs can improve low distribution and cell penetration into deeper tissues.

Photoinduced DNA Cleavage and Photocytotoxic of Phenanthroline-Based Ligand Ruthenium Compounds

The photophysical and biological properties of two new phenanthroline-based ligand ruthenium complexes were investigated in detail. Their DNA interaction modes were determined to be the intercalation mode using spectra titration and viscosity measurements. Under irradiation, obvious photo-reduced DNA cleavages were observed in the two complexes via singlet oxygen generation. Furthermore, complex 2 showed higher DNA affinity, photocleavage activity, and singlet oxygen quantum yields than complex 1. The two complexes showed no toxicity towards tumor cells (HeLa, A549, and A375) in the dark. However, obvious photocytotoxicities were observed in the two complexes. Complex 2 exhibited large PIs (phototherapeutic indices) (ca. 400) towards HeLa cells. The study suggests that these complexes may act as DNA intercalators, DNA photocleavers, and photocytotoxic agents.

Biocompatible Ir(III) Complexes as Oxygen Sensors for Phosphorescence Lifetime Imaging

Synthesis of biocompatible near infrared phosphorescent complexes and their application in bioimaging as triplet oxygen sensors in live systems are still challenging areas of organometallic chemistry. We have designed and synthetized four novel iridium [Ir(N^C)2(N^N)]+ complexes (N^C-benzothienyl-phenanthridine based cyclometalated ligand; N^N-pyridin-phenanthroimidazol diimine chelate), decorated with oligo(ethylene glycol) groups to impart these emitters' solubility in aqueous media, biocompatibility, and to shield them from interaction with bio-environment. These substances were fully characterized using NMR spectroscopy and ESI mass-spectrometry. The complexes exhibited excitation close to the biological "window of transparency", NIR emission at 730 nm, and quantum yields up to 12% in water. The compounds with higher degree of the chromophore shielding possess low toxicity, bleaching stability, absence of sensitivity to variations of pH, serum, and complex concentrations. The properties of these probes as oxygen sensors for biological systems have been studied by using phosphorescence lifetime imaging experiments in different cell cultures. The results showed essential lifetime response onto variations in oxygen concentration (2.0-2.3 μs under normoxia and 2.8-3.0 μs under hypoxia conditions) in complete agreement with the calibration curves obtained "in cuvette". The data obtained indicate that these emitters can be used as semi-quantitative oxygen sensors in biological systems.

Discrepancy between Proline and Homoproline in Chiral Recognition and Diastereomeric Photoreactivity with Iridium(III) Complexes

The chiral-recognition processes of homoproline (hpro) and [Ir(pq)₂(MeCN)₂](PF₆) (pq is 2-phenylquinoline; MeCN is acetonitrile) are investigated, in favor of formation of the thermodynamically stable diastereomers Λ-[Ir(pq)₂(d-hpro)] and Δ-[Ir(pq)₂(l-hpro)]. Moreover, the diastereoselective photoreactions of Δ-[Ir(pq)₂(d-hpro)] and Δ-[Ir(pq)₂(l-hpro)] are reported in the presence of O₂ at room temperature. Diastereomer Δ-[Ir(pq)₂(l-hpro)] is dehydrogenatively oxidized into imino acid complex Δ-[Ir(pq)₂(hpro-2H²)] (hpro-2H² is 3,4,5,6-tetrahydropicalinate), while diastereomer Δ-[Ir(pq)₂(d-hpro)] occurs by interligand C-N cross-coupling and dehydrogenative oxidation reactions, affording three products: Δ-[Ir(pq)(d-pqh)] [pqh is N-(2-phenylquinolin-8-yl)homoproline], Δ-[Ir(pq)₂(hpro-2H²)], and Δ-[Ir(pq)₂(d-hpro-2H⁶)] [hpro-2H⁶ is 2,3,4,5-tetrahydropicalinate]. The C-N cross-coupling and dehydrogenative oxidation reactions are competitive, and the dehydrogenative oxidation reactions are regioselective. By optimization of the photoreaction parameters such as the diastereomeric substrate, solvent, and temperature as well as base, each possible competitive product is selectively controlled. In addition, density functional theory calculations are performed to elucidate the distinctly chiral recognition between proline and hpro with an iridium(III) complex.

Metal complexes as optical probes for DNA sensing and imaging

Transition and lanthanide metal complexes have rich photophysical properties that can be used for cellular imaging, biosensing and phototherapy. One of the applications of such luminescent compounds is the detection and visualisation of nucleic acids. In this brief review, we survey the recent literature on the use of luminescent metal complexes (including Re^I, Ru^II, Os^II, Ir^III, Pt^II, Eu^III and Tb^III) as DNA optical probes, including examples of compounds that bind selectively to non-duplex DNA topologies such as quadruplex, i-motif and DNA mismatches. We discuss the applications of metal-based luminescent complexes in cellular imaging, including time-resolved microscopy and super-resolution techniques. Their applications in biosensing and phototherapy are briefly mentioned in the relevant sections.

Cyclometalated Iridium(III) Complexes as Mitochondria-targeting Photosensitizers against Cisplatin-resistant Cells†

Four iridium (III) complexes Ir1-Ir4 were synthesized and characterized. Possessing high singlet oxygen production ability and specific mitochondria-localization, Ir1 was developed as a mitochondria-targeting photosensitizer. Ir1 exhibited strong phototoxicity against cancer cell line A549 and its corresponding cisplatin-resistant one A549R. In contrast, Ir1 showed low cytotoxicity toward normal cell HLF. This selectivity resulted from the different uptake amount. With 405 nm irradiation, Ir1 induced mitochondria-mediated cell death in A549R cells, achieving the overcome of drug-resistant.

In-vitro and in-vivo monitoring of gold(III) ions from intermediate metabolite of sodium aurothiomalate through water-soluble ruthenium (II) complex-based luminescent probe

Real-time monitoring of drug metabolism in vivo is of great significance to drug development and toxicology research. The purpose of this study is to establish a rapid and visual in vivo detection method for the detection of an intermediate metabolite of the gold (I) drug. Gold (I) drugs such as sodium aurothiomalate (AuTM) have anti-inflammatory effects in the treatment of rheumatoid arthritis. Gold(III) ions (Au³⁺) are the intermediate metabolite of gold medicine, and they are also the leading factor of side effects in the treatment of patients. However, the rapid reduction of Au³⁺ to Au⁺ by thiol proteins in organisms limits the in-depth study of metabolism of gold drugs in vivo. Here we describe a luminescence Au³⁺ probe (RA) based on ruthenium (II) complex for detecting Au³⁺ in vitro and in vivo. RA with large Stokes shift, good water solubility and biocompatibility was successfully applied to detect Au³⁺ in living cells and vivo by luminescence imaging, and to trap the fluctuation of Au³⁺ level produced by gold (I) medicine. More importantly, the luminescent probe was used to the detection of the intermediate metabolites of gold (I) drugs for the first time. Overall, this work offers a new detection tool/method for a deeper study of gold (I) drugs metabolite.

Supramolecular Assembly of An Organoplatinum(II) Complex with Ratiometric Dual Emission for Two-Photon Bioimaging

The organoplatinum(II) complex [Pt(C^N^N)(Cl)] (C^N^N=5,6-diphenyl-2,2'-bipyridine, Pt1) can assemble into nanoaggregates via π-π stacking and complementary hydrogen bonds, rather than Pt-Pt interactions. Pt1 exhibits ratiometric dual emission, including rare blue emission (λ_em =445 nm) and assembly-induced yellow emission (λ_em =573 nm), under one- and two-photon excitation. Pt1 displays blue emission in cells with an intact membrane due to its low cellular uptake. In cells where the membrane is disrupted, uptake of the complex is increased and at higher concentrations yellow emission is observed. The ratio of yellow to blue emission shows a linear relationship to the loss of cell membrane integrity. Pt1 is, to our knowledge, the first example of an assembly-induced two-photon ratiometric dual emission organoplatinum complex. The excellent and unique characteristics of the complex enabled its use for the tracking of cell apoptosis, necrosis, and the inflammation process in zebrafish.

Ultrafast processes: coordination chemistry and quantum theory

Coordination compounds, characterized by fascinating and tunable electronic properties, are capable of binding easily to proteins, polymers, wires and DNA. Upon irradiation, these molecular systems develop functions finding applications in solar cells, photocatalysis, luminescent and conformational probes, electron transfer triggers and diagnostic or therapeutic tools. The control of these functions is activated by the light wavelength, the metal/ligand cooperation and the environment within the first picoseconds (ps). After a brief summary of the theoretical background, this perspective reviews case studies, from 1st row to 3rd row transition metal complexes, that illustrate how spin-orbit, vibronic coupling and quantum effects drive the photophysics of this class of molecules at the early stage of the photoinduced elementary processes within the fs-ps time scale range.

Photofunctional transition metal complexes as cellular probes, bioimaging reagents and phototherapeutics

This critical review summarises the recent biological applications of transition metal complexes as cellular probes, bioimaging reagents and phototherapeutics.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Dinuclear Rhenium Complexes with a Bridging Helicene-bis-bipyridine Ligand: Synthesis, Structure, and Photophysical and Chiroptical Properties

By attaching pyridine groups to a diaza[6]helicene, a helical, bis-ditopic, bis-N N-coordinating ligand can be accessed. Dinuclear rhenium complexes featuring this bridging ligand, of the form [{Re(CO)3 Cl}2 (N N-N N)], have been prepared and resolved to give enantiopure complexes. These complexes are phosphorescent in solution at room temperature under one- and two-photon excitation. Their experimental chiroptical properties (optical rotation, electronic circular dichroism and circularly polarized emission) have been measured. They show, for instance, emission dissymmetry factors of c.a. ±3x10-3 . Quantum-chemical calculations indicate the importance of stereochemistry on the optical activity, pointing towards further design improvements in such types of complexes.

Simultaneous Ultrasensitive Detection and Elimination of Drug-Resistant Bacteria by Cyclometalated Iridium(III) Complexes

Antimicrobial resistance has become a major threat to public health due to the rampant and empirical use of antibiotics. Rapid diagnosis of bacteria with the desired sensitivity and selectivity still, however, remains an open challenge. We report a special class of water-soluble metal-based aggregation-induced emission luminogens (AIEgens), namely, cyclometalated iridium(III) polypyridine complexes of the type [Ir(PQ)₂(N^N)]Cl (1-3), where PQ = 2-phenylquinoline and N^N = 2,2'-bipyridine derivatives, that demonstrate dual capability for detection and elimination of drug-resistant bacteria in aqueous solutions. These AIEgens exhibit selective and rapid sensing of endotoxins, such as lipopolysaccharides (LPS) and lipoteichoic acid (LTA) released by the bacteria, with a detection limit in the lower nanomolar range. Targeting these naturally amplified biomarkers (approximately 1 million copies per cell) by iridium(III) complexes induces strong AIE in the presence of different Gram-negative and Gram-positive bacteria including carbapenem-resistant A. baumannii (CRAB) and methicillin-resistant S. aureus (MRSA) at concentrations as low as 1.2 CFU/mL within 5 min in spiked water samples. Detection of bacteria by the complexes is also visible to the naked eye at higher (10⁸ CFU/mL) cell concentrations. More notably, complexes 1 and 2 show potent antibacterial activity against drug-resistant bacteria with low minimum inhibitory concentrations (MICs) ≤ 5 μg/mL (1-4 μM) via ROS generation and cell membrane disintegrity. To the best of our knowledge, this work is the "first-in-class" example of a metal-based theranostic system that integrates selective, sensitive, rapid, naked-eye, wash-free, and real-time detection of bacteria using broad-spectrum antibiotics into a single platform. This dual capability of AIEgens makes them ideal scaffolds for monitoring bacterial contamination in aqueous samples and pharmaceutical applications.