Kenneth Yin Zhang

Smart responsive phosphorescent materials for data recording and security protection

Smart luminescent materials that are responsive to external stimuli have received considerable interest. Here we report ionic iridium (III) complexes simultaneously exhibiting mechanochromic, vapochromic and electrochromic phosphorescence. These complexes share the same phosphorescent iridium (III) cation with a N-H moiety in the N^N ligand and contain different anions, including hexafluorophosphate, tetrafluoroborate, iodide, bromide and chloride. The anionic counterions cause a variation in the emission colours of the complexes from yellow to green by forming hydrogen bonds with the N-H proton. The electronic effect of the N-H moiety is sensitive towards mechanical grinding, solvent vapour and electric field, resulting in mechanochromic, vapochromic and electrochromic phosphorescence. On the basis of these findings, we construct a data-recording device and demonstrate data encryption and decryption via fluorescence lifetime imaging and time-gated luminescence imaging techniques. Our results suggest that rationally designed phosphorescent complexes may be promising candidates for advanced data recording and security protection.

Development of luminescent iridium(III) polypyridine complexes as chemical and biological probes

A number of luminescent iridium(III) polypyridine complexes have been designed as molecular sensors owing to their rich photophysical properties such as intense, long-lived and environment-sensitive emission. In particular, many complexes exhibit emissive behavior that can be readily controlled using various Werner-type and cyclometalating ligands. In this Perspective, we review some recent examples of luminescent iridium(III) polypyridine complexes as probes for chemical and biological molecules using different strategies. The targets include proton, cations and anions, small molecules, nucleic acids and protein molecules. There is also a recent interest in luminescent iridium(III) polypyridine complexes as cellular probes and imaging reagents; selected examples in these areas are described.

Exploitation of the Dual-emissive Properties of Cyclometalated Iridium(III)–Polypyridine Complexes in the Development of Luminescent Biological Probes†

There has been fast-growing interest in utilizing iridium(III)– polypyridine complexes as new luminescent sensors for analytes, including protons, halide ions, metal cations, oxygen, and biomolecules. These complexes display changes in their emission intensities and lifetimes upon analyte binding. Although the emission maxima exhibit small shifts in some cases, the emission profiles and spectral characteristics of the luminescent probes basically remain the same. Compared to common metal-to-ligand charge-transfer (MLCT) emitters, such as the ruthenium(II)– and osmium(II)–polypyridine systems, iridium(III)–polypyridine complexes exhibit emissive states that are very sensitive to their ligands and local environment, resulting in distinct emission features. However, it appears that this behavior has not been utilized in the current array of sensors available. Whilst dual emission is not uncommon for iridium(III)–polypyridine complexes in glass at low temperature, it is very rare in fluid solutions under ambient conditions. We believe that an attractive approach to the development of new iridium(III)-based luminescent probes would be the utilization of novel complexes that display environment-responsive dualemissive properties. Herein we report a series of novel dual-emissive cyclometalated iridium(III)–polypyridine complexes that serve as luminescent sensors for various biological receptors. The complex [Ir(ppy-CH2NH-C4H9)2(bpy-CONH-C2H5)](PF6) (1; Hppy-CH2NH-C4H9 = 2-(4-(N-(n-butyl)aminomethyl)phenyl)pyridine; bpy-CONH-C2H5 = 4-(N-(ethyl)aminocarbonyl)-4’-methyl-2,2’-bipyridine; Scheme 1) was synthesized from the reaction of the aldehyde complex [Ir(ppyCHO)2(bpy-CONH-C2H5)](PF6) (Hppy-CHO = 4-(2-pyridyl)benzaldehyde) with n-butylamine in refluxing methanol, followed by reduction with NaBH3CN. Upon irradiation, 1 exhibited intense and long-lived luminescence (Table 1). Interestingly, it showed dual emission in fluid solutions at room temperature, with a high-energy (HE) structured band at about 500 nm (to = 1.1–2.5 ms) and a low-energy (LE) broad band/shoulder at approximately 593–619 nm (to = 0.1– 0.3 ms; Table 1). The possibility of emissive impurities in the samples was excluded on the basis of the characterization data. In degassed nonpolar solvents such as CH2Cl2, the emission intensity of the LE band was higher than or comparable to that of the HE band, whilst in more polar solvents such as CH3CN and CH3OH, it became much weaker; in aqueous buffer the spectrum was dominated by the HE band (Figure 1). The intensities of both the HE and LE emission features were reduced in aerated solutions, with the former being more sensitive to quenching by oxygen. As a result, the LE band became dominant in aerated solutions, except in the case of aqueous buffer. Addition of trifluoroacetic acid (TFA) to an aerated solution of the complex in CH2Cl2 shifted the LE emission band to a shorter wavelength (ca. 574 nm) and the HE feature was eventually embedded into the broad LE band. Interestingly, the amine-free analogue complex [Ir(ppy)2(bpy-CONH-C2H5)](PF6) (1a ; Hppy = 2-phenylpyridine) did not display dual emission in fluid solutions (Table 1). The only broad band of this complex at around 609–632 nm was insensitive to the presence of TFA and has been assigned to a charge-transfer (CT) state of mixed MLCT (dp(Ir)!p*(N^N)) and ligand-to-ligand Scheme 1. Structures of complexes 1–4.

Iridium(III) complexes as therapeutic and bioimaging reagents for cellular applications

There is an emerging interest in applying inorganic and organometallic transition metal complexes to biomolecular and cellular studies. The cytotoxic effects of these complexes on various cancer and normal cells have been examined. Many of these complexes display intense, long-lived, and environment-sensitive emission, rendering them useful live-cell imaging reagents. Of particular interest are iridium(III) complexes, which possess a diversity of molecular structures with intriguing biological activity and photophysical properties. In this review article, we summarize recent work using iridium(III) complexes as anticancer drugs and cellular imaging reagents. We focus on the cytotoxic activity, cellular uptake efficiency and mechanisms, and intracellular distribution properties of these complexes. Additionally, we describe the applications of luminescent iridium(III) complexes in intracellular sensing for ions and small molecules, gene-delivery, and cancer cell detection.

A Mitochondria‐Targeted Photosensitizer Showing Improved Photodynamic Therapy Effects Under Hypoxia

Organelle-targeted photosensitizers have been reported to be effective photodynamic therapy (PDT) agents. In this work, we designed and synthesized two iridium(III) complexes that specifically stain the mitochondria and lysosomes of living cells, respectively. Both complexes exhibited long-lived phosphorescence, which is sensitive to oxygen quenching. The photocytotoxicity of the complexes was evaluated under normoxic and hypoxic conditions. The results showed that HeLa cells treated with the mitochondria-targeted complex maintained a slower respiration rate, leading to a higher intracellular oxygen level under hypoxia. As a result, this complex exhibited an improved PDT effect compared to the lysosome-targeted complex, especially under hypoxia conditions, suggestive of a higher practicable potential of mitochondria-targeted PDT agents in cancer therapy.

Luminescent Dendritic Cyclometalated Iridium(III) Polypyridine Complexes: Synthesis, Emission Behavior, and Biological Properties

Luminescent dendritic cyclometalated iridium(III) polypyridine complexes [{Ir(N--C)(2)}(n)(bpy-n)](PF(6))(n) (HN--C = 2-phenylpyridine, Hppy, n = 8 (ppy-8), 4 (ppy-4), 3 (ppy-3); HN--C = 2-phenylquinoline, Hpq, n = 8 (pq-8), 4 (pq-4), 3 (pq-3)) have been designed and synthesized. The properties of these dendrimers have been compared to those of their monomeric counterparts [Ir(N--C)(2)(bpy-1)](PF(6)) (HN--C = Hppy (ppy-1), Hpq (pq-1)). Cyclic voltammetric studies revealed that the iridium(IV/III) oxidation and bpy-based reduction occurred at about +1.24 to +1.29 V and -1.21 to -1.27 V versus SCE, respectively, for all the complexes. The molar absorptivity of the dendritic iridium(III) complexes is approximately proportional to the number of [Ir(N--C)(2)(N--N)] moieties in one complex molecule. However, the emission lifetimes and quantum yields are relatively independent of the number of [Ir(N--C)(2)(N--N)] units, suggesting negligible electronic communications between these units. Upon photoexcitation, the complexes displayed triplet metal-to-ligand charge-transfer ((3)MLCT) (dpi(Ir) --> pi*(bpy-n)) emission. The interaction of these complexes with plasmid DNA has been investigated by agarose gel retardation assays. The results showed that the dendritic iridium(III) complexes, unlike their monomeric counterparts, bound to the plasmid, and the interaction was electrostatic in nature. The lipophilicity of all the complexes has been determined by reversed-phase high-performance liquid chromatography (HPLC). Additionally, the cellular uptake of the complexes by the human cervix epithelioid carcinoma (HeLa) cell line has been examined by inductively coupled plasma mass spectrometry (ICP-MS), laser-scanning confocal microscopy, and flow cytometry. Upon internalization, all the complexes were localized in the perinuclear region, forming very sharp luminescent rings surrounding the nuclei. Interestingly, in addition to these rings, HeLa cells treated with the dendritic iridium(III) complexes showed specific labeled compartments, which have been identified to be the Golgi apparatus. Furthermore, the cytotoxicity of these iridium(III) complexes has been evaluated by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assay.

Selective Ag(I) binding, H2S sensing, and white-light emission from an easy-to-make porous conjugated polymer.

Separating silver (Ag(+)) from lead (Pb(2+)) is one of the many merits of the porous polymer framework reported here. The selective metal binding stems from the well-defined chelating unit of N-heterocycles, which consists of a triazine (C3N3) ring bonded to three 3,5-dimethylpyrazole moieties. Such a rigid and open triad also serves as the distinct building unit in the fully conjugated 3D polymer scaffold. Because of its strong fluorescence and porosity (e.g., BET surface area: 355 m(2)/g), and because of the various types of metal species that can be readily taken up, this versatile framework is especially fit for functionalization. For example, with AgNO3 loaded, the framework solid exhibits a brown color in response to water solutions of H2S, even at the dilution of 5.0 μM (0.17 ppm); whereas cysteine and other biologically relevant thiols do not cause notable change in color. In another example, tunable white-light emission was produced when an Ir(III) complex was doped (e.g., about 0.02% of the polymer weight) onto the framework. Mechanistically, the bound Ir(III) centers become highly emissive in the orange-red region, complementing the broad, bluish emission from the polymer host to result in the overall white-light quality: the color attributes of the emission are therefore easily tunable by the Ir(III) dopant concentration. With this exemplary study, we intend to highlight metal uptake as an effective approach to modify and enrich the properties of porous polymer frameworks and to stimulate interest in further examining metal-polymer interactions in the context of sensing, separation, catalyzes, and other applications.

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

In this Article, we present a series of cyclometalated iridium(III) polypyridine complexes of the formula [Ir(N^C)2(N^N)](PF6) that showed dual emission under ambient conditions. The structures of the cyclometalating and diimine ligands were changed systematically to investigate the effects of the substituents on the dual-emission properties of the complexes. On the basis of the photophysical data, the high-energy (HE) and low-energy (LE) emission features of the complexes were assigned to triplet intraligand ((3)IL) and triplet charge-transfer ((3)CT) excited states, respectively. Time-dependent density functional theory (TD-DFT) calculations supported these assignments and indicated that the dual emission resulted from the interruption of the communication between the higher-lying (3)IL and the lower-lying (3)CT states by a triplet amine-to-ligand charge-transfer ((3)NLCT) state. Also, the avidin-binding properties of the biotin complexes were studied by emission titrations, and the results showed that the dual-emissive complexes can be utilized as ratiometric probes for avidin. Additionally, all the complexes exhibited efficient cellular uptake by live HeLa cells. The MTT and Annexin V assays confirmed that no cell death and early apoptosis occurred during the cell imaging experiments. Interestingly, laser-scanning confocal microscopy revealed that the complexes were selectively localized on the cell membrane, mitochondria, or both, depending on the nature of the substituents of the ligands. The results of this work will contribute to the future development of dual-emissive transition metal complexes as ratiometric probes and organelle-selective bioimaging reagents.

A Phosphorescent Iridium(III) Complex‐Modified Nanoprobe for Hypoxia Bioimaging Via Time‐Resolved Luminescence Microscopy

Oxygen plays a crucial role in many biological processes. Accurate monitoring of oxygen level is important for diagnosis and treatment of diseases. Autofluorescence is an unavoidable interference in luminescent bioimaging, so that an amount of research work has been devoted to reducing background autofluorescence. Herein, a phosphorescent iridium(III) complex‐modified nanoprobe is developed, which can monitor oxygen concentration and also reduce autofluorescence under both downconversion and upconversion channels. The nanoprobe is designed based on the mesoporous silica coated lanthanide‐doped upconversion nanoparticles, which contains oxygen‐sensitive iridium(III) complex in the outer silica shell. To image intracellular hypoxia without the interferences of autofluorescence, time‐resolved luminescent imaging technology and near‐infrared light excitation, both of which can reduce autofluorescence effectively, are adopted in this work. Moreover, gradient O2 concentration can be detected clearly through confocal microscopy luminescence intensity imaging, phosphorescence lifetime imaging microscopy, and time‐gated imaging, which is meaningful to oxygen sensing in tissues with nonuniform oxygen distribution.

A carborane-triggered metastable charge transfer state leading to spontaneous recovery of mechanochromic luminescence

An efficient strategy was designed to realize spontaneous recovery of mechanochromic luminescence by carborane-functionalized anthracene derivatives. A metastable charge-transfer emission from anthracene to o-carborane is responsible for this process.