Dual-Emissive Cyclometalated Iridium(III) Polypyridine Complexes as Ratiometric Biological Probes and Organelle-Selective Bioimaging Reagents

Effect of the Appended Morpholinyl Group on Photophysical Behavior of Mono- and Bis-cyclometalated Terpyridine Iridium(III) Chromophores

This paper provides extensive studies of [IrCl(Ph-py)(morph-C6H4-terpy-κ3N)]PF6 (1A), [Ir(Ph-py)2(morph-C6H4-terpy-κ2N)]PF6 (2A), [IrCl(Ph-py)(Ph-terpy-κ3N)]PF6 (1B), and [Ir(Ph-py)2(Ph-terpy-κ2N)]PF6 (2B) designed to demonstrate the possibility of controlling the photophysical properties of mono- and bis-cyclometalated complexes [IrCl(Ph-py)(R-C6H4-terpy-κ3N)]PF6 and [Ir(Ph-py)2(R-C6H4-terpy-κ2N)]PF6 through a remote electron-donating substituent introduced into the 4'-position of 2,2':6',2″-terpyridine (terpy) via the phenyl linker. The attachment of the morpholinyl (morph) group was evidenced to induce dramatic changes in the emission characteristics of the monocyclometalated Ir(III) systems with meridionally coordinated R-C6H4-terpy ligand (κ3N). In solution, the obtained complex [IrCl(Ph-py)(morph-C6H4-terpy-κ3N)]PF6 was found to be a rare example of dual-emissive Ir(III) systems. Within the series [Ir(Ph-py)2(R-C6H4-terpy-κ2N)]PF6 bearing the R-C6H4-terpy ligand bound to the central ion in a bidentate coordination mode, the appended electron-donating morpholinyl group induced a minor effect on the emission maximum, but it was found to be an effective tool for extending the excited-state lifetime, further prolonging with the increase of solvent polarity. The results of this work are of high significance for better understanding the push-pull effect and dual-emission phenomena in Ir-based luminophores, as well as developing chromophores with prolonged emission lifetimes.

Luminescent Iridium-Terpyridine Complexes with Various Bis-Cyclometalated Ligands

A series of luminescent bis-cyclometalated iridium complexes with 2,2':6',2″-terpyridine (tpy), [Ir(C^N)2(tpy)]PF6 (C^N = 2-phenylpyridinate (ppy) for 1; benzo[h]quinolinate (bzq) for 2; 1-phenylisoquinolinate (piq) for 3; and 2-phenylbenzothiazolate (pbt) for 4), have been synthesized and structurally characterized. Single-crystal X-ray diffraction analyses reveal that the tpy ligands of 1-4 are coordinated to the iridium center in a bidentate fashion, and the uncoordinated pendant pyridine rings in the tpy ligands of 1-4 form intramolecular π-π stacking interactions with a phenyl moiety of C^N ligands. In addition, the pendant pyridine ring in the tpy ligand of 1 forms an intramolecular hydrogen bonding interaction, unlike in 2-4. Of interest, the photophysical properties of 1-4 are strongly influenced by the C^N ligands; 1 shows a luminescence band at 572 nm, with a short lifetime (τ) value of 80 nsec and a lower absolute luminescence quantum yield (Φ) of 3.72%, whereas 3 exhibits an intense luminescence band at 588 nm with a long τ value of 1965 nsec and a moderate Φ value of 9.57%. The density functional theory calculations revealed that the luminescence originates from the triplet metal-ligand to ligand charge transfer (3MLL'CT) excited state.

Synthesis, Structure, and Properties of Nontrivial Iridium(III) Complexes Based on Anthracene-Decorated Benzimidazole Ligand

Reactions of iridium trichloride hydrate with bulky 2-(9-anthracenyl)-1-phenyl-benzimidazole (anbi) in the presence of N-donor ligands afforded a number of unique noncyclometalated complexes, while attempts to prepare a common μ-chloro-bridged bis-cyclometalated dimer systematically gave a monocyclometalated complex cis-[Ir(C,N-anbi)(N-anbi)Cl2] instead. The obtained complexes were characterized by 1H NMR, high-resolution mass spectrometry, single-crystal and powder X-ray diffraction, UV-vis spectroscopy, and cyclic voltammetry. The noncyclometalated complexes fac-[Ir(N-anbi)(N^N)Cl3)], where N^N are 4,4'-disubstituted 2,2'-bipyridines, are octahedral and contain the anthracene and 2,2'-bipyridine units in a close cofacial arrangement. These complexes were found to be exceptionally inert to the chloride ligand exchange even in the presence of silver triflate, forming a rare trinuclear Ir-μ-Cl3-Ag-μ-Cl3-Ir structure instead. In the monocyclometalated complex, the Ir(III) ion is pentacoordinated in a rare square-pyramidal geometry, where the bulky anthracene fragment is involved in the steric shielding of the metal center. This is in line with the results of gas-phase density functional theory calculations, demonstrating that the experimentally observed structure is energetically most preferable. The monocyclometalated complex is deeply colored due to intense charge-transfer absorption bands in the range 450-650 nm with ε = 2000-5000 M-1 cm-1, superior to the noncyclometalated complexes. The synthesis, structures, and properties of the new complexes are discussed in the context of the related mono-, bis-, and noncyclometalated iridium(III) compounds.

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.

Dual-Emissive Iridium(III) Complexes and Their Applications in Biological Sensing and Imaging

Phosphorescent iridium(III) complexes are widely recognized for their unique properties in the excited triplet state, making them crucial for various applications including biological sensing and imaging. Most of these complexes display single phosphorescence emission from the lowest-lying triplet state after undergoing highly efficient intersystem crossing (ISC) and ultrafast internal conversion (IC) processes. However, in cases where these excited-state processes are restricted, the less common phenomenon of dual emission has been observed. This dual emission phenomenon presents an opportunity for developing biological probes and imaging agents with multiple emission bands of different wavelengths. Compared to intensity-based biosensing, where the existence and concentration of an analyte are indicated by the brightness of the probe, the emission profile response involves modifications in emission color. This enables quantification by utilizing the intensity ratio of different wavelengths, which is self-calibrating and unaffected by the probe concentration and excitation laser power. Moreover, dual-emissive probes have the potential to demonstrate distinct responses to multiple analytes at separate wavelengths, providing orthogonal detection capabilities. In this concept, we focus on iridium(III) complexes displaying fluorescence-phosphorescence or phosphorescence-phosphorescence dual emission, along with their applications as biological probes for sensing and imaging.

Cell-penetrating peptides noncovalently modified red phosphorescent nanoparticles for high-efficiency imaging

The application of long-lived phosphorescence probes in time-resolved luminescence imaging is limited by their low quantum yield in aqueous solutions. However, sensitization of thermally activated delayed fluorescence (TADF) materials can compensate for this limitation while addressing the issue of insufficient proportion of their own long lifetime. In this study, we utilized the characteristics of phosphorescence and TADF materials simultaneously by doping the receptor iridium complex PMD-Ir into the donor TADF polymer PCzDP-20 through donor-receptor doping method, and successfully prepared highly efficient red phosphorescent nanoparticles. The quantum yield of the nanoparticles obtained by this method reaches up to 30%, and the luminescence lifetime can reach several thousand nanoseconds. Additionally, due to the low concentration doping of PMD-Ir, the risk of transition metal toxicity is greatly reduced. Furthermore, we used non-covalent modification with amphiphilic cell-penetrating peptides (CPPs) to increase the cell membrane permeability of the nanoparticles. The CPPs modified nanoparticles achieve in vivo confocal imaging of zebrafish and intracellular time-resolved imaging by its significantly improved bioimaging capabilities. The functional nanoparticles designing method fully utilizes the characteristics of PMD-Ir, PCzDP-20, and CPPs, solving the problems of low quantum yield and poor membrane permeability of Ir-complex nanoparticles. This will greatly promote the development of time-resolved luminescence imaging.

Luminescence Lifetime-Based Sensing Platform Based on Cyclometalated Iridium(III) Complexes for the Detection of Perfluorooctanoic Acid in Aqueous Samples

Luminescence lifetimes are an attractive analytical method for detection due to its high sensitivity and stability. Iridium probes exhibit luminescence with long excited-state lifetimes, which are sensitive to the local environment. Perfluorooctanoic acid (PFOA) is listed as a chemical of high concern regarding its toxicity and is classified as a "forever chemical". In addition to strict limits on the presence of PFOA in drinking water, environmental contamination from industrial effluent or chemical spills requires rapid, simple, accurate, and cost-effective analysis in order to aid containment. Herein, we report the fabrication and function of a novel and facile luminescence sensor for PFOA based on iridium modified on gold surfaces. These surfaces were modified with lipophilic iridium complexes bearing alkyl chains, namely, IrC6 and IrC12, and Zonyl-FSA surfactant. Upon addition of PFOA, the modified surfaces IrC6-FSA@Au and IrC12-FSA @Au show the largest change in the red luminescence signal with changes in the luminescence lifetime that allow monitoring of PFOA concentrations in aqueous solutions. The platform was tested for the measurement of PFOA in aqueous samples spiked with known concentrations of PFOA and demonstrated the capacity to determine PFOA at concentrations >100 μg/L (240 nM).

Dual-Emissive Monoruthenium Complexes of N(CH3)-Bridged Ligand: Synthesis, Characterization, and Substituent Effect

Three monoruthenium complexes 1(PF6)2-3(PF6)2 bearing an N(CH3)-bridged ligand have been synthesized and characterized. These complexes have a general formula of [Ru(bpy)2(L)](PF6)2, where L is a 2,5-di(N-methyl-N'-(pyrid-2-yl)amino)pyrazine (dapz) derivative with various substituents, and bpy is 2,2'-bipyridine. The photophysical and electrochemical properties of these compounds have been examined. The solid-state structure of complex 3(PF6)2 is studied by single-crystal X-ray analysis. These complexes show two well-separated emission bands centered at 451 and 646 nm (Δλmax = 195 nm) for 1(PF6)2, 465 and 627 nm (Δλmax = 162 nm) for 2(PF6)2, and 455 and 608 nm (Δλmax = 153 nm) for 3(PF6)2 in dilute acetonitrile solution, respectively. The emission maxima of the higher-energy emission bands of these complexes are similar, while the lower-energy emission bands are dependent on the electronic nature of substituents. These complexes display two consecutive redox couples owing to the stepwise oxidation of the N(CH3)-bridged ligand and ruthenium component. Moreover, these experimental observations are analyzed by computational investigation.

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.

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.

Aggregation-Induced Near-Infrared Emission and Electrochemiluminescence of an Iridium(III) Complex for Ampicillin Sodium Sensing

A new iridium(III) complex was synthesized and characterized. Its photophysical properties and aggregation-induced emission and electrochemiluminescence in the near-infrared range were studied. The large conjugated cyclometallic ligand 1,2-phenylbenzoquinoline (pbq) was selected to form the Ir-C bond with the metal iridium(III) center and provide near-infrared emission of the complex. The auxiliary ligand 4,4'-diamino-2,2'-bipyridine (dabpy) can form hydrogen bonds, which was beneficial for the generation of aggregation-induced emission. The complex was aggregated into small spherical nanoparticles in 80% water and fascinating nanorings in 90% water. The sensing of ampicillin sodium (AMP) antibiotic by the iridium(III) complex were also investigated by photoluminescent and electrochemiluminescent methods. The complex showed a good selectivity toward AMP antibiotic compared to sodium phenylacetate and other eight antibiotics. The detection limits for AMP antibiotic was 0.76 μg/mL. This work provided a new strategy for the design of iridium(III) complex-based aggregation-induced emission and electrochemiluminescence probes for the sensing application.

Investigation of the N^C Ligand Effects on Emission Characteristics in a Series of Bis-Metalated [Ir(N^C)2(N^N)]+ Complexes

A series of bis-metalated phosphorescent [(N^C)2Ir(bipyridine)]+ complexes with systematic variations in the structure and electronic characteristics of the N^C ligands were synthesized and characterized by using elemental analysis, mass spectrometry, NMR spectroscopy and X-ray crystallography. Investigation of the complexes’ spectroscopic properties together with DFT and TD DFT calculations revealed that metal-to-ligand charge transfer (MLCT) and intraligand (LC) transition play key roles in the generation of emissive triplet states. According to the results of theoretical studies, the 3LC excited state is more accurate to consider as an intraligand charge transfer process (ILCT) between N- and C-coordinated moieties of the N^C chelate. This hypothesis is completely in line with the trends observed in the experimental absorption and emission spectra, which display systematic bathochromic shifts upon insertion of electron-withdrawing substituents into the N-coordinated fragment. An analogous shift is induced by expansion of the aromatic system of the C-coordinated fragment and insertion of polarizable sulfur atoms into the aromatic rings. These experimental and theoretical findings extend the knowledge of the nature of photophysical processes in complexes of this type and provide useful instruments for fine-tuning of their emissive characteristics.

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.

Rapid and reliable triple-emissive detection of 2,6-dichloro-4-nitroaniline as a pesticide based on a high-nuclear Cd(ii)–Sm(iii) nanocluster

A 56-metal Cd(ii)–Sm(iii) nanocluster (molecular sizes: 4.5 × 2.7 × 2.7 nm) was constructed for the rapid and reliable triple-emissive detection of 2,6-dichloro-4-nitroaniline (DCN) as a pesticide.

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.

Recent development and application of cyclometalated iridium(III) complexes as chemical and biological probes

Iridium complexes have been widely applied as molecular sensors because of their rich photophysical properties, including large Stokes shifts, long emission lifetimes, environment-sensitive emissions, and high luminescence quantum yields. In this paper, we review the recent development and application of iridium complexes as probes for ions, anions, gaseous species, organic molecules, small biomolecules, biomacromolecules, and subcellular organelles. Our outlook for iridium-based probes is also discussed.

Dual emission from an iridium(III) complex/counter anion ion pair

[Ir(tpy)2](PF6)3 (tpy = 2,2':6',2''-terpyridine) dissolved in CH3CN was found to exhibit dual color luminescent emission depending on the excitation wavelength. Specifically, blue and green emissions were obtained with excitation at 350 and 410 nm, respectively. Because the associated emission spectra were consistent with those of [Ir(tpy)2]Cl3 in water and [Ir(tpy)2](PF6)3 in the crystalline state, respectively, this dual emission is attributed to emissions from the [Ir(tpy)2]3+ cation and its ion pair [Ir(tpy)2]3+·PF6-. The emission is assigned to the 3π-π* transition of the ligands based on time-dependent density functional theory (TD-DFT) calculations. Conversely, [Ir(tpy)2]I3 in CH3CN shows emission due to [Ir(tpy)2]3+ but not [Ir(tpy)2]3+·I-, while crystalline [Ir(tpy)2]I3 emits red luminescence at 77 K that is inconsistent with that from [Ir(tpy)2]3+. Since the emission energies of crystalline [Ir(tpy)2]X3 (X- = Cl-, Br- or I-) show a good correlation with the electron affinity of X, the emissions are assigned to a counter anion to complex ion charge-transfer transition. This hypothesis is supported by TD-DFT calculations regarding [Ir(tpy)2]3+·X-.

Multinuclear 2-(Quinolin-2-yl)quinoxaline-Coordinated Iridium(III) Complexes Tethered by Carbazole Derivatives: Synthesis and Photophysics

Five mono/di/trinuclear iridium(III) complexes (1-5) bearing the carbazole-derivative-tethered 2-(quinolin-2-yl)quinoxaline (quqo) diimine (N^N) ligand were synthesized and characterized. The photophysical properties of these complexes and their corresponding diimine ligands were systematically studied via UV-vis absorption, emission, and transient absorption (TA) spectroscopy and simulated by time-dependent density functional theory. All complexes possessed strong well-resolved absorption bands at <400 nm that have predominant ligand-based ¹π,π* transitions and broad structureless charge-transfer (¹CT) absorption bands at 400-700 nm. The energies or intensities of these ¹CT bands varied pronouncedly when the number of tethered Ir(quqo)(piq)₂⁺ (piq refers to 1-phenylisoquinoline) units, π conjugation of the carbazole derivative linker, or attachment positions on the carbazole linker were altered. All complexes were emissive at room temperature, with 1-3 showing near-IR (NIR) ³MLCT (metal-to-ligand charge-transfer)/³LLCT (ligand-to-ligand charge-transfer) emission at ∼710 nm and 4 and 5 exhibiting red or NIR ³ILCT (intraligand charge-transfer)/³LMCT (ligand-to-metal charge-transfer) emission in CH₂Cl₂. In CH₃CN, 1-3 displayed an additional emission band at ca. 590 nm (³ILCT/³LMCT/³MLCT/³π,π* in nature) in addition to the 710 nm band. The different natures of the emitting states of 1-3 versus those of 4 and 5 also gave rise to different spectral features in their triplet TA spectra. It appears that the parentage and characteristics of the lowest triplet excited states in these complexes are mainly impacted by the π systems of the bridging carbazole derivatives and essentially no interactions among the Ir(quqo)(piq)₂⁺ units. In addition, all of the diimine ligands tethered by the carbazole derivatives displayed a dramatic solvatochromic effect in their emission due to the predominant intramolecular charge-transfer nature of their emitting states. Aggregation-enhanced emission was also observed from the mixed CH₂Cl₂/ethyl acetate or CH₂Cl₂/hexane solutions of these ligands.

Design and Synthesis of Cyclometalated Iridium(III) Complexes-Chromophore Hybrids that Exhibit Long-Emission Lifetimes Based on a Reversible Electronic Energy Transfer Mechanism

We report on the design and synthesis of triscyclometalated iridium (Ir) complexes that contain aryloxy groups at the end of diamino linkers, which exhibit an extraordinarily long-emission lifetime, and were prepared by regioselective substitution reactions of fac-tris-homoleptic cyclometalated Ir complexes, fac-Ir(tpy)₃ (tpy = 2-(4'-tolyl)pyridine). It was found that the Ir(tpy)₃ complex, equipped with approximately one to six 6-N,N-dimethylamino-2-naphthoic acid (DMANA) groups through the appropriate alkyl linkers, exhibited remarkably long-emission lifetimes of up to 216 μs in DMSO/H₂O at room temperature through a reversible electronic energy transfer effect between the Ir complex core and the organic chromophore moieties; however, under the same conditions, the lifetime of fac-Ir(tpy)₃ was 1.4 μs. Regarding the mechanistic aspects, the relationship between the emission lifetimes of the Ir complexes and the structures and numbers of the conjugated chromophores, linker lengths, solvents, positions of the chromophores on the Ir(tpy)₃ core, and related items are discussed.

Unravelling the Mechanism of Excited-State Interligand Energy Transfer and the Engineering of Dual Emission in [Ir(C∧N)2(N∧N)]+ Complexes

Fundamental insights into the mechanism of triplet-excited-state interligand energy transfer dynamics and the origin of dual emission for phosphorescent iridium(III) complexes are presented. The complexes [Ir(C^∧N)₂(N^∧N)]⁺ (HC^∧N = 2-phenylpyridine (1a-c), 2-(2,4-difluorophenyl)pyridine (2a-c), 1-benzyl-4-phenyl-1,2,3-triazole (3a-c); N^∧N = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (pytz, a), 1-benzyl-4-(pyrimidin-2-yl)-1,2,3-triazole (pymtz, b), 1-benzyl-4-(pyrazin-2-yl)-1,2,3-triazole (pyztz, c)) are phosphorescent in room-temperature fluid solutions from triplet metal-to-ligand charge transfer (³MLCT) states admixed with either ligand-centered (³LC) (1a, 2a, and 2b) or ligand-to-ligand charge transfer (³LL'CT) character (1c, 2c, and 3a-c). Particularly striking is the observation that pyrimidine-based complex 1b exhibits dual emission from both ³MLCT/³LC and ³MLCT/³LL'CT states. At 77 K, the ³MLCT/³LL'CT component is lost from the photoluminescence spectra of 1b, with emission exclusively arising from its ³MLCT/³LC state, while for 2c switching from ³MLCT/³LL'CT- to ³MLCT/³LC-based emission is observed. Femtosecond transient absorption data reveal distinct spectral signatures characteristic of the population of ³MLCT/³LC states for 1a, 2a, and 2b which persist throughout the 3 ns time frame of the experiment. These ³MLCT/³LC state signatures are apparent in the transient absorption spectra for 1c and 2c immediately following photoexcitation but rapidly evolve to yield spectral profiles characteristic of their ³MLCT/³LL'CT states. Transient data for 1b reveals intermediate behavior: the spectral features of the initially populated ³MLCT/³LC state also undergo rapid evolution, although to a lesser extent than that observed for 1c and 2c, behavior assigned to the equilibration of the ³MLCT/³LC and ³MLCT/³LL'CT states. Density functional theory (DFT) calculations enabled minima to be optimized for both ³MLCT/³LC and ³MLCT/³LL'CT states of 1a-c and 2a-c. Indeed, two distinct ³MLCT/³LC minima were optimized for 1a, 1b, 2a, and 2b distinguished by upon which of the two C^∧N ligands the excited electron resides. The ³MLCT/³LC and ³MLCT/³LL'CT states for 1b are very close in energy, in excellent agreement with experimental data demonstrating dual emission. Calculated vibrationally resolved emission spectra (VRES) for the complexes are in excellent agreement with experimental data, with the overlay of spectral maxima arising from emission from the ³MLCT/³LC and ³MLCT/³LL'CT states of 1b convincingly reproducing the observed experimental spectral features. Analysis of the optimized excited-state geometries enable the key structural differences between the ³MLCT/³LC and ³MLCT/³LL'CT states of the complexes to be identified and quantified. The calculation of interconversion pathways between triplet excited states provides for the first time a through-space mechanism for a photoinduced interligand energy transfer process. Furthermore, examination of structural changes between the possible emitting triplet excited states reveals the key bond vibrations that mediate energy transfer between these states. This work therefore provides for the first time detailed mechanistic insights into the fundamental photophysical processes of this important class of complexes.

Luminescent organic dyes containing a phenanthro[9,10-D]imidazole core and [Ir(N^C)(N^N)]+ complexes based on the cyclometalating and diimine ligands of this type

A family of diimine (N^N) and cyclometalating (N^C) ligands based on a phenanthro-imidazole aromatic system: 2-pyridyl-1H-phenanthro[9,10-d]imidazole (N^N); 2-R-1-phenyl-1H-phenanthro[9,10-d]imidazole, R = phenyl (N^C4), 3-iodophenyl (N^C5) and 4-nitrophenyl (N^C6) were prepared. It was found that N^C4 and N^C5 show π-π* fluorescence typical of aromatic systems of this sort, whereas the donor-acceptor architecture of N^C6 leads to strong emission solvatochromism and acidochromism, indicating the charge transfer character of the fluorescence observed. Six iridium(iii) complexes (1-6) [Ir(N^C#)2(N^N)]+, where # = 1-6 and N^C1 = 2-phenylpyridine, N^C2 = 2-(benzo[b]thiophen-2-yl)pyridine, and N^C3 = methyl 2-phenylquinoline-4-carboxylate, were also synthesized and characterized. The complexes obtained display moderate to bright phosphorescence with quantum yields up to 46% in degassed solution. The photophysical characteristics of 1-6 were studied in detail. DFT and TD DFT calculations were used for the assignment of electronic transitions responsible for the absorption and emission of these compounds. The variations in the cyclometalating ligand structure give rise to rich photophysics of the complexes obtained. It was found that the orbitals of both N^C and N^N ligands make a major contribution to the formation of emissive excited states and a delicate balance between the energy of the ligands' frontier orbitals determines the emission character.

Construction of a 1-D Sm(iii) coordination polymer with a long-chain Schiff base ligand: dual-emissive response to metal ions

A 1-D coordination polymer [Sm(HL)₂(NO₃)]_n was constructed using a new flexible Schiff base ligand that has a long-chain (CH₂)₁₀ backbone, and it shows dual-emissive response to metal ions, especially to Co²⁺ at the ppm level.

Electroluminochromic Materials and Devices Based on Metal Complexes

Electroluminochromism (ELC) refers to an interesting phenomenon exhibited by a material whose luminescent properties can be reversibly modulated under an electrical stimulus. Such a luminescence-switching property has been widely used in various organic optoelectronic devices because it can simultaneously detect electrical and optical signals. Metal complexes are the promising candidates for ELC materials due to their sensitivity to an electrical stimulus. Herein, recent progress on electroluminochromic materials and devices based on various metal complexes has been summarized. Meanwhile, the applications of these complexes in data recording and security protection have also been discussed. Finally, a brief conclusion and outlook are presented, pointing out that the development of electroluminochromic metal complexes with excellent performance is important because they play a vital role in future intelligent optoelectronic devices.

Construction of a crystalline 14-metal Zn–Nd rectangular nanocluster with a dual-emissive response towards metal ions

A crystalline 14-metal Zn–Nd cluster [Zn₆Nd₈L₂(OAc)₂₀(O)₂(NO₃)₄(OC₂H₅)₄] (1) was constructed using a flexible Schiff base ligand with long-chain (CH₂)₂O(CH₂)₂O(CH₂)₂ backbone. 1 exhibits a nanoscale rectangular structure (10 × 14 × 18 Å). Besides the visible ligand-centered emission, 1 displays typical NIR luminescence of Nd³⁺. Interestingly, 1 shows both a ligand-centered and lanthanide fluorescence response towards metal ions, especially to K⁺ and Co²⁺ at the ppm level.

A crystalline 14-metal Zn–Nd nanocluster was constructed, and it shows ligand-centered and lanthanide fluorescence response towards metal ions.

Half-sandwich Iridium(III) Benzimidazole-Appended Imidazolium-Based N-heterocyclic Carbene Complexes and Antitumor Application

A series of half-sandwich iridium(III) benzimidazole-appended imidazolium-based N-heterocyclic carbene (NHC) antitumor complexes [(η⁵ -Cp^x )Ir(C^N)Cl]Cl, where Cp^x is pentamethylcyclopentadienyl (Cp*) or its biphenyl derivative (Cp^xbiph ) and C^N is a NHC chelating ligand, were successfully synthesized and characterized. The Ir^III complexes showed potential antitumor activity against A549 cells, at most three times more potent than cis-platin under the same conditions. Complexes could bind to BSA by a static quenching mode, catalyzing the change of NADH to NAD⁺ and inducing the production of reactive oxygen species (maximum turnover number, 9.8), which play an important role in regulating cell apoptosis. Confocal microscopy showed that the complexes could specifically target lysosomes in cells with a Pearson's co-localization coefficient 0.76 and 0.72 after 1 h and 6 h, respectively, followed an energy-dependent cellular uptake mechanism and damaged the integrity of lysosomes. At the same time, complexes caused a marked loss of mitochondrial membrane potential.

The crystal structure of bis-{3,5-di-fluoro-2-[4-(2,4,6-tri-methyl-phen-yl)pyridin-2-yl]phen-yl}(picolinato)iridium(III) and its 4-tert-butyl-pyridin-2-yl analogue

The crystal structures of bis-{3,5-di-fluoro-2-[4-(2,4,6-tri-methyl-phen-yl)pyridin-2-yl]phenyl-κ2 N,C 1}(picolinato-κ2 N,O)iridium(III), [Ir(C20H16F2N)2(C6H4NO2)], 1, and bis-[2-(4-tert-butyl-pyridin-2-yl)-3,5-di-fluoro-phenyl-κ2 N,C 1](picolinato-κ2 N,O)iridium(III), [Ir(C15H14F2N)2(C6H4NO2)], 2, are presented herein. These phospho-rescent cyclo-metallated iridium(III) compounds have been structurally investigated in order to better understand the nature of their blue-shifted emssions while maintaining high quantum yields. Compound 1 exhibits substantial twisting of the mesitylene rings out of the plane of the attached pyridine ring, with dihedral angles of 67.0 (1) and 78.7 (1)° between the best mean planes. For both compounds, the contribution of disordered solvent mol-ecule(s) was removed using the SQUEEZE [Spek (2015 ▸). Acta Cryst. C71, 9-18] routine in PLATON [Spek (2015 ▸). Acta Cryst. C71, 9-18]. 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.

Dual-Phosphorescent Iridium(III) Complexes Extending Oxygen Sensing from Hypoxia to Hyperoxia

Hypoxia and hyperoxia, referring to states of biological tissues in which oxygen supply is in sufficient and excessive, respectively, are often pathological conditions. Many luminescent oxygen probes have been developed for imaging intracellular and in vivo hypoxia, but their sensitivity toward hyperoxia becomes very low. Here we report a series of iridium(III) complexes in which limited internal conversion between two excited states results in dual phosphorescence from two different excited states upon excitation at a single wavelength. Structural manipulation of the complexes allows rational tuning of the dual-phosphorescence properties and the spectral profile response of the complexes toward oxygen. By manipulating the efficiency of internal conversion between the two emissive states, we obtained a complex exhibiting naked-eye distinguishable green, orange, and red emission in aqueous buffer solution under an atmosphere of N₂, air, and O₂, respectively. This complex is used for intracellular and in vivo oxygen sensing not only in the hypoxic region but also in normoxic and hyperoxic intervals. To the best of our knowledge, this is the first example of using a molecular probe for simultaneous bioimaging of hypoxia and hyperoxia.

"Two-in-one" organic-inorganic hybrid MnII complexes exhibiting dual-emissive phosphorescence

Unprecedented organic-inorganic hybrid complexes, [Mn(L)3]MnHal4, containing both four- and hexacoordinated Mn2+ ions were synthesized by reacting MnCl2 or MnBr2 with bis(phosphine oxide) ligands (L) such as dppmO2, dppeO2, and 2,3-bis(diphenylphosphinyl)-1,3-butadiene (dppbO2). In the [Mn(L)3]2+ cation of the complexes, the Mn2+ ion features a [MnO6] octahedral coordination environment (Oh), and the [MnHal4]2- anion adopts a tetrahedral geometry (Td). These "two-in-one" complexes exhibit strong long-lived luminescence (τav = 12-15 ms at 300 K) having interesting thermochromic behavior attributed to the thermal equilibrium between two emission bands. So, in an emission spectrum of the typical complex [Mn(dppbO2)3]MnBr4, the intense "red" (ca. 620 nm) and weak "green" (ca. 520 nm) bands, originating from Mn2+ ions in Oh and Td environments, respectively, are observed. Cooling from 300 to 77 K simultaneously leads to (i) redshift of both bands by ca. 20 nm, (ii) increasing their intensities, and (iii) causing a substantial change of their integral intensity ratio from about 4 : 1 to 2 : 1. As a result, the colour of the emission changes from orange (CIE 0.56, 0.45) at 300 K to deep red (CIE 0.62, 0.39) at 77 K. This behavior was rationalized using steady-state and time-resolved fluorescent spectroscopy at various temperatures. The high photoluminescence quantum yields (up to 61% at 300 K) and fascinating dual-emissive phosphorescence coupled with high thermal stability and solubility suggest a high potential of this novel class of emissive Mn2+ complexes as promising emitters for OLED devices and potential stimuli-responsive materials.