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.

Two-photon absorption properties of simple neutral Ir(III) complexes

A series of neutral Ir(2-phenylpyridine)3 derivatives substituted on the para-position of the pyridyl ligands with a π-conjugated substituent possessing different donor abilities has been prepared. Their two-photon absorption properties have been determined using the Z-scan technique. Such simple iridium(III) neutral complexes, which are easy to synthesize, show good two-photon absorption activity, with relevant TPA cross sections (the best is 750 GM), giving rise to multifunctional chromophores, since they present also high second-order NLO properties.

A Golgi-Targeted Platinum Complex Plays a Dual Role in Autophagy Regulation for Highly Efficient Cancer Therapy

Regulating autophagy to control the homeostatic recycling process of cancer cells is a promising anticancer strategy. Golgi apparatus is a substrate of autophagy but the Golgi-autophagy (Golgiphagy) mediated antitumor pathway is rarely reported. Herein, we have developed a novel Golgi-targeted platinum (II) complex Pt3, which is ca. 20 times more cytotoxic to lung carcinoma than cisplatin and can completely eliminate tumors after intratumoral administration in vivo. Its nano-encapsulated system for tail vein administration also features a good anti-tumor effect. Mechanism studies indicate that Pt3 induces substantial Golgi stress, indicated by the fragmentation of Golgi structure, down-regulation of Golgi proteins (GM130, GRASP65/55), loss of Golgi-dependent transport and glycosylation. This triggers Golgiphagy but blocks the subsequent fusion of autophagosomes with lysosomes, that is a dual role in autophagy regulation, resulting in loss of proteostasis and apoptotic cell death. As far as we know, Pt3 is the first Golgi-targeted Pt complex that can trigger Golgi stress-mediated dual-regulation of autophagic flux and autophagy-apoptosis crosstalk for highly efficient cancer therapy.

A novel ruthenium polypyridyl complex for the selective imaging and photodynamic targeting of the Golgi apparatus

A well-designed heteroleptic ruthenium(ii) polypyridyl complex demonstrated stable target-specific in vitro Golgi apparatus imaging abilities in HeLa cell lines. After utilizing photodynamic therapy via UV excitation, the Ru-SL complex could be triggered to generate singlet oxygen (1O2) and red fluorescence signals. 1O2 was highly cytotoxic and could induce DNA damage and the disappearance of the Golgi apparatus. The red fluorescence signals disappeared gradually, suggesting that the live or dead state of the cells can be estimated from the fluorescence signal intensity.

Dual Emissive Ir(III) Complexes for Photodynamic Therapy and Bioimaging

Photodynamic therapy (PDT) is a cancer treatment still bearing enormous prospects of improvement. Within the toolbox of PDT, developing photosensitizers (PSs) that can specifically reach tumor cells and promote the generation of high concentration of reactive oxygen species (ROS) is a constant research goal. Mitochondria is known as a highly appealing target for PSs, thus being able to assess the biodistribution of the PSs prior to its light activation would be crucial for therapeutic maximization. Bifunctional Ir(III) complexes of the type [Ir(C^N)₂(N^N-R)]⁺, where N^C is either phenylpyridine (ppy) or benzoquinoline (bzq), N^N is 2,2'-dipyridylamine (dpa) and R either anthracene (1 and 3) or acridine (2 and 4), have been developed as novel trackable PSs agents. Activation of the tracking or therapeutic function could be achieved specifically by irradiating the complex with a different light wavelength (405 nm vs. 470 nm respectively). Only complex 4 ([Ir(bzq)₂(dpa-acr)]⁺) clearly showed dual emissive pattern, acridine based emission between 407-450 nm vs. Ir(III) based emission between 521 and 547 nm. The sensitivity of A549 lung cancer cells to 4 evidenced the importance of involving the metal center within the activation process of the PS, reaching values of photosensitivity over 110 times higher than in dark conditions. Moreover, complex 4 promoted apoptotic cell death and possibly the paraptotic pathway, as well as higher ROS generation under irradiation than in dark conditions. Complexes 2-4 accumulated in the mitochondria but species 2 and 4 also localizes in other subcellular organelles.

Investigation on Optical and Biological Properties of 2-(4-Dimethylaminophenyl)benzothiazole Based Cycloplatinated Complexes

The optical and biological properties of 2-(4-dimethylaminophenyl)benzothiazole cycloplatinated complexes featuring bioactive ligands ([{Pt(Me 2 N-pbt)(C 6 F 5 )}L] [L = Me 2 N-pbtH 1 , p -dpbH (4-(diphenylphosphino)benzoic acid) 2 , o -dpbH (2-(diphenylphosphino)benzoic acid) 3) , [Pt(Me 2 N-pbt)( o -dpb)] 4 ,  [{Pt(Me 2 N-pbt)(C 6 F 5 )} 2 (µ-PR n P)] [PR 4 P = O(CH 2 CH 2 OC(O)C 6 H 4 PPh 2 ) 2 5 , PR 12 P = O{(CH 2 CH 2 O) 3 C(O)C 6 H 4 PPh 2 } 2 6 ] are presented. Complexes 1-6 display 1 ILCT and metal perturbed 3 ILCT dual emissions. The ratio between both bands is excitation dependent, accomplishing warm-white emissions for 2 , 5 and 6 .  The phosphorescent emission is lost in aerated solutions owing to photoinduced electron transfer to 3 O 2 and formation of 1 O 2 , as confirmed in complexes 2 and 4 . They also exhibit photoinduced phosphorescence enhancement in non-degassed DMSO, due to local oxidation of DMSO by sensitized 1 O 2 , which causes a local degassing. Me 2 N-pbtH and the complexes exhibit specific accumulation in the Golgi apparatus although only 2 , 3 and 6 were active against A549 and HeLa cancer cell lines, being 6 highly selective respect to nontumoral cells. The potential photodynamic property of these complexes was demonstrated with complex 4 .

Luminescent Neutral Cyclometalated Iridium(III) Complexes Featuring a Cubic Polyhedral Oligomeric Silsesquioxane for Lipid Droplet Imaging and Photocytotoxic Applications

New neutral iridium(III) complexes featuring a cubic polyhedral oligomeric silsesquioxane (POSS) unit, [Ir(N^∧C)₂(L1-POSS)] [HN^∧C = 2-phenylpyridine (Hppy; 1), 2-phenylbenzothioazole (Hbt; 2), and 2-(1-naphthyl)benzothiazole (Hbsn; 3); L1-POSS = (E)-4-[(2-hydroxybenzylidene)amino]benzyl 3-heptakis(isobutyl)POSS-propyl carbamate], were designed and synthesized. Their POSS-free counterparts, [Ir(N^∧C)₂(L1)] [L1 = (E)-N-(4-hydroxymethylphenyl)-1-(2-hydroxyphenyl)methanimine; HN^∧C = Hppy (1a), Hbt (2a), and Hbsn (3a)], and the poly(ethylene glycol) (PEG) derivatives [Ir(N^∧C)₂(L1-PEG)] [L1-PEG = (E)-4-[(2-hydroxybenzylidene)amino]benzyl 3-[2-[ω-methoxypoly(1-oxapropyl)]ethyl]carbamate; HN^∧C = Hppy (1b), Hbt (2b), and Hbsn (3b)] were also prepared. The photophysical, photochemical, and biological properties of the POSS complexes were compared with those of their POSS-free and PEG-modified counterparts. Upon irradiation, all of these complexes displayed orange-to-red emission and long emission lifetimes under ambient conditions. The bsn complexes 3, 3a, and 3b exhibited the highest singlet oxygen (¹O₂) generation quantum yields (Φ_Δ = 0.85-0.86) in aerated CH₃CN. Laser-scanning confocal microscopy images revealed that complexes 1-3 and 1a-3a showed exclusive lipid-droplet staining upon cellular uptake, while the PEG derivatives 1b-3b displayed lysosomal localization. Complex 3 was utilized to study various lipid-droplet-related biological events including lipid-droplet accumulation under oleic acid stimulation, the movement of lipid droplets, and preadipocyte differentiation. Notably, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays indicated that the ppy complexes 1 and 1b and the bt complexes 2 and 2b were noncytotoxic both in the dark and upon irradiation at 450 nm for 5 min (IC₅₀ > 200 μM), while the bsn complexes 3, 3a, and 3b showed low dark cytotoxicity (IC₅₀ = 52.9 to >200 μM) and high photocytotoxicity (IC₅₀ = 1.1-5.3 μM). The cellular uptake, internalization mechanisms, and cell death pathways of these complexes were also investigated. This work not only offers promising luminescent probes for lipid droplets through the structural modification of iridium(III) complexes but also paves the way to the construction of new reagents for theranostics.

A cyclometalated trinuclear Ir(iii)/Pt(ii) complex as a luminescent probe for histidine-rich proteins

A trinuclear luminescent organometallic Pt–Ir–Pt complex acts as an efficient protein staining agent due to reversible binding to histidine-rich proteins.

Cyclometalated rhodium and iridium complexes with imidazole containing Schiff bases: Synthesis, structure and cellular imaging

Cyclometalated rhodium(III) and iridium(III) complexes (1-4) of two Schiff base ligands L1 and L2 with the general formula [M(ppy)2(Ln)]Cl {M = Rh, Ir; ppy = 2-phenylpyridine; n = 1, 2; L = Schiff base ligand} have been synthesized. The new ligands and the complexes have been characterized with spectroscopic techniques. Electrochemistry of the complexes revealed anodic behavior, corresponding to an M(III) to M(IV) oxidation. The X-ray crystal structures of complexes 2 and 4 have also been determined to interpret the coordination behavior of the complexes. Photophysical study shows that all the complexes display fluorescence at room temperature with quantum yield of about 3 × 10-2 to 5 × 10-2. The electronic absorption spectra of all the complexes fit well with the computational studies. Cellular imaging studies were done with the newly synthesized complexes. To the best of our knowledge, this is the first report of organometallic complexes of rhodium(III) and iridium(III) with Schiff base ligands explored for cellular imaging. Emphasis of this work lies on the structural features, photophysical behavior, cellular uptake and imaging of the fluorescent transition metal complexes.

Two-Photon Image Tracking of Neural Stem Cells via Iridium Complexes Encapsulated in Polymeric Nanospheres

Iridium(III) complexes have been shown to be promising probes in two-photon imaging to real-time track the transplanted cells in stem-cell-based therapy. Here, we report on polymeric nanocapsules loaded with red phosphorescence dye of bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate) iridium(III) (Ir(MDQ)₂acac) with excellent stability created by the double emulsion method. The Ir(MDQ)₂acac nanocapsules present high biocompatibility and an efficient fluorescent labeling rate when incubated with cultured mouse neural stem cells (NSCs). More importantly, the Ir(MDQ)₂acac nanocapsules had both one- and two-photon imaging properties with stable phosphorescence lasting for 72 h. Furthermore, data from in vivo tracking in nude mice demonstrated that the photoluminescence from Ir(MDQ)₂acac nanocapsules in NSCs could be stably monitored for up to 21 days. Our data shed light on the potential clinical application of iridium complexes encapsulated in polymeric nanospheres for two-photon imaging in real-time tracking of the transplanted stem cells.

Effects of Varying the Benzannulation Site and π Conjugation of the Cyclometalating Ligand on the Photophysics and Reverse Saturable Absorption of Monocationic Iridium(III) Complexes

A series of monocationic iridium(III) complexes, [Ir(C^N)₂(pqu)]⁺PF₆^- [pqu = 2-(pyridin-2-yl)quinoline, C^N = 2-phenylquinoline (1), 3-phenylisoquinoline (2), 1-phenylisoquinoline (3), benzo[ h]quinoline (4), 2-(pyridin-2-yl)naphthalene (5), 1-(pyridin-2-yl)naphthalene (6), 2-(phenanthren-9-yl)pyridine (7), 2-phenylbenzo[ g]quinoline (8), 2-(naphthalen-2-yl)quinoline (9), and 2-(naphthalen-2-yl)benzo[ g]quinoline (10)], were synthesized in this work. These complexes bear C^N ligands with varied degrees of π conjugation and sites of benzannulation, allowing for elucidation of the effects of the benzannulation site at the C^N ligand on the photophysics of the complexes. Ultraviolet-visible (UV-vis) absorption and emission of the complexes were systematically investigated via spectroscopic techniques and time-dependent density functional theory calculations. Their triplet excited-state absorption and reverse saturable absorption (RSA) were studied by nanosecond transient absorption (TA) spectroscopy and nonlinear transmission techniques. The fusion of phenyl ring(s) to the phenyl ring or the 4 and 5 positions of the pyridyl ring of the C^N ligand resulted in red-shifted UV-vis absorption and emission spectra in complexes 2, 5-7, 9, and 10 compared to those of the parent complex 0, while their triplet lifetimes and emission quantum yields were significantly reduced. In contrast, the fusion of one phenyl ring to the other sites of the pyridyl group of the C^N ligand showed an insignificant impact on the energies of the lowest singlet (S₁) and triplet (T₁) excited states in complexes 1, 3, and 4 but noticeably affected their TA spectral features. The fusion of the naphthyl group to the 5 and 6 and positions at the pyridyl ring did not influence the S₁ energy of complex 8 but altered the nature of the T₁ states in 8 and 10 by switching them to the benzo[ g]quinoline-localized ³π,π* state, which resulted in completely different emission and TA spectra in these two complexes. The site-dependent variations of the ground- and excited-state absorption induced strong but varied RSA from these complexes for 4.1-ns laser pulses at 532 nm, with the RSA strength decreasing in the trend of 3 > 7 ≈ 4 ≈ 9 ≈ 6 > 8 ≈ 1 ≈ 2 ≈ 5 > 10.

Monitoring mitochondrial viscosity with anticancer phosphorescent Ir(iii) complexes via two-photon lifetime imaging

Precise quantitative measurement of viscosity at the subcellular level presents great challenges. Two-photon phosphorescence lifetime imaging microscopy (TPPLIM) can reflect micro-environmental changes of a chromophore in a quantitative manner. Phosphorescent iridium complexes are potential TPPLIM probes due to their rich photophysical properties including environment-sensitive long-lifetime emission and high two-photon absorption (TPA) properties. In this work, a series of iridium(iii) complexes containing rotatable groups are developed as mitochondria-targeting anticancer agents and quantitative viscosity probes. Among them, Ir6 ([Ir(ppy-CHO)₂(dppe)]PF₆; ppy-CHO: 4-(2-pyridyl)benzaldehyde; dppe: cis-1,2-bis(diphenylphosphino)ethene) shows satisfactory TPA properties and long lifetimes (up to 1 μs). The emission intensities and lifetimes of Ir6 are viscosity-dependent, which is mainly attributed to the configurational changes in the diphosphine ligand as proved by ¹H NMR spectra. Ir6 displays potent cytotoxicity, and mechanism investigations show that it can accumulate in mitochondria and induce apoptotic cell death. Moreover, Ir6 can induce mitochondrial dysfunction and monitor the changes in mitochondrial viscosity simultaneously in a real-time and quantitative manner via TPPLIM. Upon Ir6 treatment, a time-dependent increase in viscosity and heterogeneity is observed along with the loss of membrane potential in mitochondria. In summary, our work shows that multifunctional phosphorescent metal complexes can induce and precisely detect microenvironmental changes simultaneously at the subcellular level using TPPLIM, which may deepen the understanding of the cell death mechanisms induced by these metallocompounds.

Delivery of Phosphorescent Anticancer Iridium(III) Complexes by Polydopamine Nanoparticles for Targeted Combined Photothermal-Chemotherapy and Thermal/Photoacoustic/Lifetime Imaging

Recently, phosphorescent iridium complexes have demonstrated great potential as anticancer and imaging agents. Dopamine is a melanin-like mimic of mussel adhesive protein that can self-polymerize to form polydopamine (PDA) nanoparticles that demonstrate favorable biocompatibility, near-infrared absorption, and photothermal effects. Herein, PDA nanoparticles are functionalized with β-cyclodextrin (CD) substitutions, which are further assembled with adamantane-modified arginine-glycine-aspartic acid (Ad-RGD) tripeptides to target integrin-rich tumor cells. The thus formed PDA-CD-RGD nanoparticles can deliver a phosphorescent iridium(III) complexes LysoIr ([Ir(ppy)2(l)]PF6, ppy = 2-phenylpyridine, L = (1-(2-quinolinyl)-β-carboline) to form a theranostic platform LysoIr@PDA-CD-RGD. It is demonstrated that LysoIr@PDA-CD-RGD can be applied for targeted combined cancer photothermal-chemotherapy and thermal/photoacoustic/two-photon phosphorescence lifetime imaging under both in vitro and in vivo conditions. This work provides a useful strategy to construct multifunctional nanocomposites for the optimization of metal-based anticancer agents for further biomedical applications.

Crystal structure of fac-{5-[(hexyl-aza-nium-yl)meth-yl]-2-(pyridin-2-yl)phenyl-κ2 N,C 1}bis-[2-(pyridin-2-yl)phenyl-κ2 N,C 1]iridium(III) chloride

The asymmetric unit of the title compound, fac-[Ir(C11H8N)2(C18H24N2)]Cl or fac-[Ir(ppy)2(Hppy-NC6)]Cl, contains two [Ir(ppy)2(ppy-NC6)](H+) cations, two Cl- anions and disordered solvent. In each complex mol-ecule, the IrIII ion is coordinated by two C,N-bidentate 2-(pyridin-2-yl)phenyl ligands and one C,N-bidentate N-[4-(pyridin-2-yl)benz-yl]hexan-1-aminium ligand, leading to a distorted fac-octa-hedral coordination environment. In the crystal, the mol-ecules are linked by N-H⋯Cl, C-H⋯π and π-π inter-actions, forming a three-dimensional supra-molecular structure. The hexyl group of one mol-ecule is disordered over two orientations with a refined occupancy ratio of 0.412 (13):0.588 (13). The acetone and hexane solvent mol-ecules were found to be highly disordered and their contribution to the scattering was masked using the solvent-masking routine smtbx.mask in OLEX2 [Rees et al. (2005 ▸). Acta Cryst. D61, 1299-1301]. These solvent mol-ecules are not considered in the given chemical formula and other crystal data.

Phosphorescent iridium(iii) complexes capable of imaging and distinguishing between exogenous and endogenous analytes in living cells

Many luminescent probes have been developed for intracellular imaging and sensing. During cellular luminescence sensing, it is difficult to distinguish species generated inside cells from those internalized from extracellular environments since they are chemically the same and lead to the same luminescence response of the probes. Considering that endogenous species usually give more information about the physiological and pathological parameters of the cells while internalized species often reflect the extracellular environmental conditions, we herein reported a series of cyclometalated iridium(iii) complexes as phosphorescent probes that are partially retained in the cell membrane during their cellular uptake. The utilization of the probes for sensing and distinguishing between exogenous and endogenous analytes has been demonstrated using hypoxia and hypochlorite as two examples of target analytes. The endogenous analytes lead to the luminescence response of the intracellular probes while the exogenous analytes are reported by the probes retained in the cell membrane during their internalization.

Pyridazine-bridged cationic diiridium complexes as potential dual-mode bioimaging probes

A novel diiridium complex [(N^C^N)₂Ir(bis-N^C)Ir(N^C^N)₂Cl]PF₆ (N^C^N = 2-[3-tert-butyl-5-(pyridin-2-yl)phenyl]pyridine; bis-N^C = 3,6-bis(4-tert-butylphenyl)pyridazine) was designed, synthesised and characterised. The key feature of the complex is the bridging pyridazine ligand which brings two cyclometallated Ir(iii) metal centres close together so that Cl also acts as a bridging ligand leading to a cationic complex. The ionic nature of the complex offers a possibility of improving solubility in water. The complex displays broad emission in the red region (λ_em = 520–720 nm, τ = 1.89 μs, Φ_em = 62% in degassed acetonitrile). Cellular assays by multiphoton (λ_ex = 800 nm) and confocal (λ_ex = 405 nm) microscopy demonstrate that the complex enters cells and localises to the mitochondria, demonstrating cell permeability. Further, an appreciable yield of singlet oxygen generation (Φ_Δ = 0.45, direct method, by ¹O₂ NIR emission in air equilibrated acetonitrile) suggests a possible future use in photodynamic therapy. However, the complex has relatively high dark toxicity (LD₅₀ = 4.46 μM), which will likely hinder its clinical application. Despite this toxicity, the broad emission spectrum of the complex and high emission yield observed suggest a possible future use of this class of compound in emission bioimaging. The presence of two heavy atoms also increases the scattering of electrons, supporting potential future applications as a dual fluorescence and electron microscopy probe.

A novel cell permeable, mitochondria localising, diiridium complex has a high emission yield and two heavy atoms to increase scattering of electrons, supporting potential future applications as a dual fluorescence and electron microscopy probe.

Water-soluble cyclometalated platinum(ii) and iridium(iii) complexes: synthesis, tuning of the photophysical properties, and in vitro and in vivo phosphorescence lifetime imaging

This paper presents synthesis and photophysical investigation of cyclometalated water-soluble Pt(ii) and Ir(iii) complexes containing auxiliary sulfonated diphosphine (bis(diphenylphosphino)benzene (dppb), P^P*) ligand. The complexes demonstrate considerable variations in excitation (extending up to 450 nm) and emission bands (with maxima ranging from ca. 450 to ca. 650 nm), as well as in the sensitivity of excited state lifetimes to molecular oxygen (from almost negligible to more than 4-fold increase in degassed solution). Moreover, all the complexes possess high two-photon absorption cross sections (400–500 GM for Pt complexes, and 600–700 GM for Ir complexes). Despite their negative net charge, all the complexes demonstrate good uptake by HeLa cells and low cytotoxicity within the concentration and time ranges suitable for two-photon phosphorescence lifetime (PLIM) microscopy. The most promising complex, [(ppy)₂Ir(sulfo-dppb)] (Ir1*), upon incubation in HeLa cells demonstrates two-fold lifetime variations under normal and nitrogen atmosphere, correspondingly. Moreover, its in vivo evaluation in athymic nude mice bearing HeLa tumors did not reveal acute toxicity upon both intravenous and topical injections. Finally, Ir1* demonstrated statistically significant difference in lifetimes between normal tissue (muscle) and tumor in macroscopic in vivo PLIM imaging.

Novel water-soluble iridium complexes with sulfonated diphosphine allow in vitro and in vivo lifetime hypoxia imaging.

Using a redox-sensitive phosphorescent probe for optical evaluation of an intracellular redox environment

A reducing intracellular environment is necessary for living cells. Here a redox-sensitive phosphorescent probe Ir-NO has been developed for evaluating the redox environment in living cells. Upon addition of reducing molecules, such as glutathione and ascorbic acid, the phosphorescent intensity of the probe is turned on, and the emission lifetime is elongated evidently. Furthermore, this probe has been used for optical imaging of the intracellular reducing environment by utilizing confocal laser scanning microscopy and phosphorescence lifetime imaging microscopy.

Long-Term Lysosomes Tracking with a Water-Soluble Two-Photon Phosphorescent Iridium(III) Complex

Lysosomes are the stomachs of the cells that degrade endocytosis and intracellular biomacromolecules and participate in various other cellular processes, such as apoptosis and cell migration. The ability of long-term tracking of lysosomes is very important to understand the details of lysosomal functions and to evaluate drug and gene delivery systems. For studying lysosomes, we designed and synthesized a water-soluble triscyclometalated iridium(III) complex (Ir-lyso) attaching morpholine moieties. The phosphorescent intensity of Ir-lyso is responsive to pH and decreases with an increase in the pH but not quenching in high pH. With excellent two-photon properties, Ir-lyso was used to light up the lysosomes in living cells and 3D tumor spheroids. Moreover, Ir-lyso could label lysosomes more than 4 days, so we developed this complex to act as a long-term probe for tracking lysosomes during cell migration and apoptosis. To the best of our knowledge, this is the first paradigm of metal complexes as the two-photon phosphorescent probe for long-term lysosomes tracking.

Cyclometalated Iridium(III) Complexes as AIE Phosphorescent Probes for Real-Time Monitoring of Mitophagy in Living Cells

Mitophagy, which is a special autophagy that removes damaging mitochondria to maintain sufficient healthy mitochondria, provides an alternative path for addressing dysfunctional mitochondria and avoiding cellular death. In the present study, by coupling the triphenylamine group with 2-phenylimidazo[4,5-f][1,10]phenanthroline derivatives, we synthesized five Ir(III) complexes with an AIE property that are expected to fulfill requirements for real-time monitoring of mitophagy. Ir1-Ir5 were exploited to image mitochondria with a short incubation time by confocal microscopy and inductive coupled plasma-mass spectrometry (ICP-MS). Due to aggregation-induced emission (AIE), Ir1-Ir5 exhibited excellent photostability compared to MitoTracker Green (MTG). Moreover, Ir1-Ir5 manifested satisfactory photostability in the mitochondrial physiological pH range. In addition, the uptake mechanism of Ir1 was investigated using confocal microscopy and flow cytometry analysis. Finally, using both Ir1 and LysoTracker Green, we were able to achieve real-time monitoring of mitophagy.

Novel aminoalkyl tris-cyclometalated iridium complexes as cellular stains

Novel tris-cyclometalated luminescent iridium complexes capable of staining cells and showing in cellulo lifetimes in the microsecond regime are reported.

An emission-tunable fluorescent organic molecule for specific cellular imaging

A color-tunable fluorescent molecule was synthesized and applied in specific lysosomal imaging.

Two- and three-photon absorption and excitation phosphorescence of oligofluorene-substituted Ir(ppy)3

A series of oligofluorene-substituted Ir(ppy)₃ complexes with remarkable 2PA and 3PA as well as two- and three-photon excited phosphorescence properties are reported.

Synthesis, crystal structures and two-photon absorption properties of triphenylamine cyanoacetic acid derivative and its organooxotin complexes

Three novel organooxotin complexes (Z1, Z2 and Z3) exhibit large 2PA cross-section per molecular weight and can be used as potential anti-tumor agents.

Cyclometalated iridium(iii) complexes with imidazo[4,5-f][1,10]phenanthroline derivatives for mitochondrial imaging in living cells

A new series of cyclometalated iridium(iii) complexes with imidazo[4,5-f][1,10]phenanthroline derivatives, MitoIr1–MitoIr7, were developed to image mitochondria in living cells.

Functionalization of cyclometalated iridium(iii) polypyridine complexes for the design of intracellular sensors, organelle-targeting imaging reagents, and metallodrugs

We summarize the biological applications of selected organometallic iridium(iii) complexes as intracellular sensors, organelle-targeting imaging reagents, and metallodrugs.

Development of a cyclometalated iridium complex with specific intramolecular hydrogen-bonding that acts as a fluorescent marker for the endoplasmic reticulum and causes photoinduced cell death

Cyclometalated iridium complexes have important applications as phosphorescent probes for cellular imaging due to their photophysical properties. Moreover, these properties also make them potential candidates as photosensitizers for photodynamic therapy (PDT) of tumors and skin diseases. Treatment of MCF7 breast carcinoma cells with a heteroleptic phosphorescent cyclometalated iridium(III) complex C2 followed by confocal imaging indicates that the complex selectively localizes and exhibits high fluorescence in the endoplasmic reticulum. In an unprecedented approach, systematic alteration of functional groups or the metal core in C2 to synthesize a series of iridium(III) complexes (C1–C10) and an organometallic rhenium complex C11 with an imidazolyl modified phenanthroline ligand has indicated the functional groups and their interactions that are responsible for this selective localization. Remarkably, the exposure of the cells treated with C2 to irradiation at 405 nm for one hour led to membrane blebbing and cell death, demonstrating a photosensitizing property of the compound.