Cationic Iridium(III) Complex Containing both Triarylboron and Carbazole Moieties as a Ratiometric Fluoride Probe That Utilizes a Switchable Triplet–Singlet Emission

A novel cationic Ir^III complex [Ir(Bpq)₂(CzbpyCz)]PF₆ (Bpq=2‐[4‐(dimesitylboryl)phenyl]quinoline, CzbpyCz = 5,5′‐bis(9‐hexyl‐9H‐carbazol‐3‐yl)‐2,2′‐bipyridine) containing both triarylboron and carbazole moieties was synthesized. The excited‐state properties of [Ir(Bpq)₂(CzbpyCz)]PF₆ were investigated through UV/Vis absorption and photoluminescence spectroscopy and molecular‐orbital calculations. This complex displayed highly efficient orange‐red phosphorescent emission with an emission peak of 583 nm and quantum efficiency of Φ=0.30 in dichloromethane at room temperature. The binding of fluoride ions to [Ir(Bpq)₂(CzbpyCz)]PF₆ can quench the phosphorescent emission from the Ir^III complex and enhance the fluorescent emission from the N^N ligand, which corresponds to a visual change in the emission from orange‐red to blue. Thus, both colorimetric and ratiometric fluoride sensing can be realized. Interestingly, an unusual intense absorption band in the visible region was observed. And the detection of F^? ions can also be carried out with visible light as the excitation wavelength. More importantly, the linear response of the probe absorbance change at λ=351 nm versus the concentration of F^? ions allows efficient and accurate quantification of F^? ions in the range 0–50 μM .     

Modification of Luminescent Iridium(III) Polypyridine Complexes with Discrete Poly(ethylene glycol) (PEG) Pendants: Synthesis,Emissive Behavior,Intracellular Uptake,and PEGylation Properties

We report the synthesis, characterization, and photophysical properties of a new class of luminescent cyclometalated iridium(III) polypyridine poly(ethylene glycol) (PEG) complexes [Ir(N^C)₂(N^N)](PF₆) (HN^C=Hppy (2‐phenylpyridine), N^N=bpy? CONH? PEG1 (bpy=2,2′‐bipyridine; 1 a ), bpy? CONH? PEG3 ( 1 b ); HN^C=Hpq (2‐phenylquinoline), N^N=bpy? CONH? PEG1 ( 2 a ), bpy? CONH? PEG3 ( 2 b ); HN^C=Hpba (4‐(2‐pyridyl)benzaldehyde), N^N=bpy? CONH? PEG1 ( 3 )) and their PEG‐free counterparts (N^N=bpy? CONH? Et, HN^C=Hppy ( 1 c ); HN^C=Hpq ( 2 c )). The cytotoxicity and cellular uptake of these complexes have been investigated by the MTT assay, ICPMS, laser‐scanning confocal microscopy, and flow cytometry. The results showed that the complexes supported by the water‐soluble PEG can act as biological probes and labels with considerably reduced cytotoxicity. Because the aldehyde groups of complex 3 are reactive toward primary amines, the complex has been utilized as the first luminescent PEGylation reagent. Bovine serum albumin (BSA) and poly(ethyleneimine) (PEI) have been PEGylated with this complex, and the resulting conjugates have been isolated, purified, and their photophysical properties studied. The DNA‐binding and gene‐delivery properties of the luminescent PEI conjugate 3 ‐PEI have also been investigated.     

Red‐ and Green‐Emitting Iridium(III) Complexes for a Dual Barometric and Temperature‐Sensitive Paint

A new dual luminescent sensitive paint for barometric pressure and temperature (T) is presented. The green‐emitting iridium(III) complex [Ir(ppy)₂(carbac)] (ppy=2‐phenylpyridine; carbac=1‐(9H‐carbazol‐9‐yl)‐5,5‐dimethylhexane‐2,4‐dione) was applied as a novel probe for T along with the red‐emitting complex [Ir(btpy)₃], (btpy=2‐(benzo[b]thiophene‐2‐yl)pyridine) which functions as a barometric (in fact oxygen‐sensitive) probe. Both iridium complexes were dissolved in different polymer materials to achieve optimal responses. The probe [Ir(ppy)₂(carbac)] was dispersed in gas‐blocking poly(acrylonitrile) microparticles in order to suppress any quenching of its luminescence by oxygen. The barometric probe [Ir(btpy)₃], in turn, was incorporated in a cellulose acetate butyrate film which exhibits good permeability for oxygen. The effects of temperature on the response of the oxygen probe can be corrected by simultaneous optical determination of T, as the poly(acrylonitrile) microparticles containing the temperature indicator are incorporated into the film. The phosphorescent signals of the probes for T and barometric pressure, respectively, can be separated by optical filters due to the ≈75 nm difference in their emission maxima. The dual sensor is applicable to luminescence lifetime imaging of T and barometric pressure. It is the first luminescent dual sensor material for barometric pressure/T based exclusively on the use of Ir^III complexes in combination with luminescence lifetime imaging.     

A Quadrupolar Bis-Triarylborane Chromophore as a Fluorimetric and Chirooptic Probe for Simultaneous and Selective Sensing of DNA,RNA and Proteins

A water-soluble tetracationic quadrupolar bis-triarylborane chromophore showed strong binding to ds-DNA, ds-RNA, ss-RNA, as well as to the naturally most abundant protein, BSA. The novel dye can distinguish between DNA/RNA and BSA by fluorescence emission separated by Δ =3600 cm⁻¹, allowing for the simultaneous quantification of DNA/RNA and protein (BSA) in a mixture. The applicability of such fluorimetric differentiation in vitro was demonstrated, strongly supporting a protein-like target as a dominant binding site of 1 in cells. Moreover, our dye also bound strongly to ss-RNA, with the unusual rod-like structure of the dye, decorated by four positive charges at its termini and having a hydrophobic core, acting as a spindle for wrapping A, C and U ss-RNAs, but not poly G, the latter preserving its secondary structure. To the best of our knowledge, such unmatched, multifaceted binding activity of a small molecule toward DNA, RNA, and proteins and the selectivity of its fluorimetric and chirooptic response makes the quadrupolar bis-triarylborane a novel chromophore/fluorophore moiety for biochemical applications.     

A Two‐Dimensional Coordination Compound as a Zinc Ion Selective Luminescent Probe for Biological Applications

A 2D coordination compound {[Cu₂(HL)(N₃)]?ClO₄}_∞ ( 1 ; H₃L=2,6‐bis(hydroxyethyliminoethyl)‐4‐methyl phenol) was synthesized and characterized by single‐crystal X‐ray diffraction to be a polymer in the crystalline state. Each [Cu₂(HL)(N₃)]⁺ species is connected to its adjacent unit by a bridging alkoxide oxygen atom of the ligand to form a helical propagation along the crystallographic a axis. The adjacent helical frameworks are connected by a ligand alcoholic oxygen atom along the crystallographic b axis to produce pleated 2D sheets. In solution, 1 dissociates into [Cu₂(HL)₂(H₃L)]?2H₂O ( 2 ); the monomer displays high selectivity for Zn²⁺ and can be used in HEPES buffer (pH 7.4) as a zinc ion selective luminescent probe for biological application. The system shows a nearly 19‐fold Zn²⁺‐selective chelation‐enhanced fluorescence response in the working buffer. Application of 2 to cultured living cells (B16F10 mouse melanoma and A375 human melanoma) and rat hippocampal slices was also studied by fluorescence microscopy.     

Synthesis,One‐ and Two‐Photon Photophysical and Excited‐State Properties,and Sensing Application of a New Phosphorescent Dinuclear Cationic Iridium(III) Complex

A new phosphorescent dinuclear cationic iridium(III) complex ( Ir1 ) with a donor–acceptor–π‐bridge–acceptor–donor (D? A? π? A? D)‐conjugated oligomer ( L1 ) as a N^N ligand and a triarylboron compound as a C^N ligand has been synthesized. The photophysical and excited‐state properties of Ir1 and L1 were investigated by UV/Vis absorption spectroscopy, photoluminescence spectroscopy, and molecular‐orbital calculations, and they were compared with those of the mononuclear iridium(III) complex [Ir(Bpq)₂(bpy)]⁺PF₆^? ( Ir0 ). Compared with Ir0 , complex Ir1 shows a more‐intense optical‐absorption capability, especially in the visible‐light region. For example, complex Ir1 shows an intense absorption band that is centered at λ=448 nm with a molar extinction coefficient (ε) of about 10⁴, which is rarely observed for iridium(III) complexes. Complex Ir1 displays highly efficient orange–red phosphorescent emission with an emission wavelength of 606 nm and a quantum efficiency of 0.13 at room temperature. We also investigated the two‐photon‐absorption properties of complexes Ir0 , Ir1 , and L1 . The free ligand ( L1 ) has a relatively small two‐photon absorption cross‐section (δ_max=195 GM), but, when complexed with iridium(III) to afford dinuclear complex Ir1 , it exhibits a higher two‐photon‐absorption cross‐section than ligand L1 in the near‐infrared region and an intense two‐photon‐excited phosphorescent emission. The maximum two‐photon‐absorption cross‐section of Ir1 is 481 GM, which is also significantly larger than that of Ir0 . In addition, because the strong B? F interaction between the dimesitylboryl groups and F^? ions interrupts the extended π‐conjugation, complex Ir1 can be used as an excellent one‐ and two‐photon‐excited “ON–OFF” phosphorescent probe for F^? ions.     

Iridium(III) Complex-Based Activatable Probe for Phosphorescent/Time-Gated Luminescent Sensing and Imaging of Cysteine in Mitochondria of Live Cells and Animals

This study reports an activatable iridium(III) complex probe for phosphorescence/time-gated luminescence detection of cysteine (Cys) in vitro and in vivo. The probe, [Ir(ppy)₂(NTY-bpy)](PF₆) [ppy: 2-phenylpyridine; NTY-bpy: 4-methyl-4′-(2-nitrovinyl)-2,2′-bipyridine], is developed by incorporating a strong electron-withdrawing group, nitroolefin, into a bipyridine ligand of the Ir^III complex. The luminescence of the probe is quenched owing to the intramolecular charge transfer (ICT) process, but switched on by a specific recognition reaction between the probe and Cys. [Ir(ppy)₂(NTY-bpy)](PF₆) shows high sensitivity and selectivity for Cys detection and good biocompatibility. The long-lived emission of [Ir(ppy)₂(NTY-bpy)](PF₆) allows time-gated luminescence analysis of Cys in cells and human sera. These properties make it convenient for the phosphorescence and time-gated luminescence imaging and flow cytometry analysis of Cys in live samples. The Cys images in cancer cells and inflamed macrophage cells reveal that [Ir(ppy)₂(NTY-bpy)](PF₆) is distributed in mitochondria after cellular internalization. Visualizations and flow cytometry analysis of mitochondrial Cys levels and Cys-mediated redox activities of live cells are achieved. By using [Ir(ppy)₂(NTY-bpy)](PF₆) as a probe, in vivo sensing and imaging of Cys in D. magna, zebrafish, and mice are then demonstrated.     

Ferrocene-derivative Electrochemical Probe for the Selective Detection of Carcinoma-associated STn Antigen

Ferrocene-derivatives have a well-defined electrochemical behaviour, which can be changed upon binding to other molecules (including nonelectroactive species), as the conjugate presents a slower diffusion to the electrode surface. Therefore, the binding event can be easily detected by monitoring the decrease in the current intensity of the ferrocene-derivative oxidation peak. An electrochemical probe composed of ferrocenecarboxylic acid conjugated with Sambucus nigra agglutinin was prepared and used to selectively detect the STn antigen, a pan-carcinoma glycobiomarker. The probe showed high selectivity towards the STn antigen present in glycoproteins and demonstrated effectiveness in the analysis of serum samples from carcinoma patients.     

Saccharide Sensing Based on Saccharide‐Induced Conformational Changes in Fluorescent Boronic Acid Polymers

Summary: A novel fluorometric saccharide detection strategy has been established by utilizing saccharide‐induced conformational changes in copolymers containing boronic acid and fluorescent units. The polymer chain changes its conformation due to an alteration of the charge state induced by binding with saccharides. The conformational changes are conveniently detected by monitoring excimer to monomer intensity ratio in the fluorescence spectra. Thus, the present strategy would be a promising way for the creation of highly selective and sensitive saccharide‐sensing systems.

Saccharide‐induced conformational change in copolymers containing boronic acid units.     

Cyclometalated Iridium(III) Complexes with Deoxyribose Substituents

Fundamental study of enzymatic nucleoside transport suffers for lack of optical probes that can be tracked noninvasively. Nucleoside transporters are integral membrane glycoproteins that mediate the salvage of nucleosides and their passage across cell membranes. The substrate recognition site is the deoxyribose sugar, often with little distinction among nucleobases. Reported here are nucleoside analogues in which emissive, cyclometalated iridium(III) complexes are “clicked” to C‐1 of deoxyribose in place of canonical nucleobases. The resulting complexes show visible luminescence at room temperature and 77 K with microsecond‐length triplet lifetimes. A representative complex is crystallographically characterized. Transport and luminescence are demonstrated in cultured human carcinoma (KB3‐1) cells.     

Ancillary Ligand Effects on Red-Emitting Cyclometalated Iridium Complexes

In this work, a series of ten new red-emitting heteroleptic iridium(III) complexes of the type Ir(C^N)₂(L^X) (C^N=cyclometalating ligand, L^X=monoanionic chelating ancillary ligand) is introduced. The suite of new complexes includes two different cyclometalating ligands and five different ancillary ligands, with the primary goal of investigating the effect of the ancillary ligand structure on the excited-state dynamics. The structural variety of the ancillary ligands permitted investigations of the effects of donor atom identity, chelate ring size, and substituents on the electronic structure and excited state properties. Electrochemical analysis showed that the ancillary ligand has a substantial effect on the energy of the HOMO, whereas the LUMO is left unperturbed. Photoluminescence spectra showed that the ancillary ligand can sometimes strongly influence the emission wavelength, but in all cases is an important determinant of the excited-state dynamics.     

An Efficient Probe for Rapid Detection of Cyanide in Water at Parts per Billion Levels and Naked‐Eye Detection of Endogenous Cyanide

A new molecular probe based on an oxidized bis‐indolyl skeleton has been developed for rapid and sensitive visual detection of cyanide ions in water and also for the detection of endogenously bound cyanide. The probe allows the “naked‐eye” detection of cyanide ions in water with a visual color change from red to yellow (Δλ_max=80 nm) with the immediate addition of the probe. It shows high selectivity towards the cyanide ion without any interference from other anions. The detection of cyanide by the probe is ratiometric, thus making the detection quantitative. A Michael‐type addition reaction of the probe with the cyanide ion takes place during this chemodosimetric process. In water, the detection limit was found to be at the parts per million level, which improved drastically when a neutral micellar medium was employed, and it showed a parts‐per‐billion‐level detection, which is even 25‐fold lower than the permitted limits of cyanide in water. The probe could also efficiently detect the endogenously bound cyanide in cassava (a staple food) with a clear visual color change without requiring any sample pretreatment and/or any special reaction conditions such as pH or temperature. Thus the probe could serve as a practical naked‐eye probe for “in‐field” experiments without requiring any sophisticated instruments.     

Synthesis, photophysical and electrochemical properties, and protein-binding studies of luminescent cyclometalated iridium(III) bipyridine estradiol conjugates

A new series of luminescent cyclometalated iridium(III) bipyridine estradiol conjugates [Ir(N-C)2(N-N)](PF6) (N-N = 5-(4-(17alpha-ethynylestradiolyl)phenyl)-2,2'-bipyridine, bpy-est, HN-C = 2-phenylpyridine, Hppy (1 a), 1-phenylpyrazole, Hppz (2 a), 7,8-benzoquinoline, Hbzq (3 a), 2-phenylquinoline, Hpq (4 a), 2-((1,1'-biphenyl)-4-yl)benzothiazole, Hbsb (5 a); N-N = 4-(N-(6-(4-(17alpha-ethynylestradiolyl)benzoylamino)hexyl)aminocarbonyl)-4'-methyl-2,2'-bipyridine, bpy-C6-est, HN-C = Hppy (1 b), Hppz (2 b), Hbzq (3 b), Hpq (4 b), Hbsb (5 b)) was synthesized, characterized, and their photophysical and electrochemical properties studied. Upon photoexcitation, all the complexes displayed intense and long-lived emission in fluid solutions at 298 K and in low-temperature glass. The emission of complexes 1 a-3 a and 1 b-3 b was assigned to a triplet metal-to-ligand charge-transfer ((3)MLCT) (dpi(Ir)-->pi*(bpy-est and N-C-)) state mixed with some triplet intraligand ((3)IL) (pi-->pi*) (N-C- and N-N) character. However, the emissive states of the pq- and bsb- complexes 4 a, 4 b, 5 a, and 5 b showed substantial (3)IL (pi-->pi*) (pq-/bsb-) character. The lipophilicity of all the complexes was determined by reversed-phase HPLC. Upon binding to estrogen receptor alpha, all of these iridium(III) estradiol conjugates exhibited emission enhancement and lifetime extension, rendering them a novel series of luminescent probes for this receptor.     

Smart Poly(N‐isopropylacrylamide) Containing Iridium(III) Complexes as Water‐Soluble Phosphorescent Probe for Sensing and Bioimaging of Homocysteine and Cysteine

Homocysteine (Hcy) and cysteine (Cys) are two important kinds of amino acids in human bodies. Herein, we synthesized an iridium(III) complex‐functionalized poly(N‐isopropylacrylamide) and its hydrogel, which could be used as the excellent phosphorescent bioprobe for sensing Hcy and Cys. Their detection can be realized in aqueous system through the variations in absorption and photoluminescence spectra. Furthermore, living cell imaging experiments demonstrate that the phosphorescent bioprobe is membrane permeable and can monitor the changes of Hcy and Cys within living cells. In addition, the probe is also thermoresponsive, and its photoluminescence intensified with increasing temperature. These results suggests that this bioprobe has promising application in biomedical fields.     

Emissive Iridium(III) Complexes with Phosphorous‐Containing Ancillary

Ir(III) metal complexes and related emitters bearing all kind of cyclometalated chromophoric chelates and non‐chromophoric ancillary are extensively studied during the past three decades. Many of them have been found to display bright room temperature phosphorescence from triplet excited states in both solution and solid states, offering a possible application in contemporary optoelectronic technologies, including organic light emitting diodes, electrochemiluminescence, biological imaging and chemical sensing. Among reported materials, there are Ir(III) complexes with at least one phosphorus (P)‐containing ligand and/or ancillary chelate, together with cyclometalates or equivalents that are in control of the actual emission energy. Particularly, possession of P‐based donor can lead to divergent structural and photophysical properties compared to the traditional designs. This review aims to provide a literature overview as well as the authors’ personal account to the development of relevant Ir(III) based phosphors bearing these P‐donors. To the readers’ convenience, the contents are subdivided into six sessions, according to whether or not they are charge natural, or with mono‐ or dianionic electronic character, and in accordance to their divergent bonding modes, i. e. monodentate, bidentate and tripodal coordination. In many cases, the P‐based ancillaries offer an easy accessible route to the formation of efficient sky‐blue and true‐blue emitters due to their π‐accepting property, together with enlarged emission energy gap and destabilized upper lying quenching state.