A novel route towards water-soluble luminescent iridium(iii) complexes via a hydroxy-bridged dinuclear precursor

Unexpected reactivity of cyclometalated iridium(III) dimers. Direct synthesis of a mononuclear luminescent complex

A new synthetic method has been developed for the preparation of unexpected emissive iridium(III) complexes (A and B), directly obtained from the established [Ir(ppy)2(μ-Cl)]2 dimer, under reaction conditions in which such compounds are usually considered stable. Complex A ([Ir(ppy)2(Oppy)], where Hppy = 2-phenylpyridine and HOppy = 2-(o-hydroxyphenyl)pyridine) was obtained from the dimer without the addition of further ancillary ligands in the reaction environment, but in the presence of a basic water environment in 2-ethoxyethanol as solvent at 165 °C. The complex evidences the unexpected insertion of an oxygen atom between the iridium(III) center and the carbon atom of one ppy moiety. Under specific reaction conditions, the mer-[Ir(ppy)3] complex (B) was obtained. The presence of the right amount of water is important to maximize the formation of A relative to B. Both compounds were fully characterized by NMR spectroscopy and mass spectrometry (MS), and the X-ray structure of A was also determined. DFT calculations were used to shed light on the reaction mechanism leading to the unexpected formation of A, suggesting that the Oppy ligand is generated intramolecularly once the [Ir(ppy)2(μ-OH)]2 dimer is formed. The process is probably assisted by a redox reaction involving the second iridium(III) center in the dimer. The electrochemical and photophysical properties of complexes A and B were investigated in comparison with the well-known fac-[Ir(ppy)3] analogue (C). Complex A displays a green emission (λmax = 545 nm) with a photoluminescence quantum yield (PLQY) of nearly 40%, whereas the oxygen-free counterpart B is poorly emissive, exhibiting an orange emission (λmax = 605 nm) with a PLQY below 10%. These findings may pave the way for the direct synthesis of neutral luminescent complexes with the general formula [Ir(C^N)2(OC^N)], even using dimers with non-commercial or highly substituted C^N ligands, without the need for synthesizing the corresponding hydroxyl-substituted ancillary ligand, which may be hardly obtainable.

Nanostructured Iridium Oxide: State of the Art

Iridium Oxide (IrO2) is a metal oxide with a rutile crystalline structure, analogous to the TiO2 rutile polymorph. Unlike other oxides of transition metals, IrO2 shows a metallic type conductivity and displays a low surface work function. IrO2 is also characterized by a high chemical stability. These highly desirable properties make IrO2 a rightful candidate for specific applications. Furthermore, IrO2 can be synthesized in the form of a wide variety of nanostructures ranging from nanopowder, nanosheets, nanotubes, nanorods, nanowires, and nanoporous thin films. IrO2 nanostructuration, which allows its attractive intrinsic properties to be enhanced, can therefore be exploited according to the pursued application. Indeed, IrO2 nanostructures have shown utility in fields that span from electrocatalysis, electrochromic devices, sensors, fuel cell and supercapacitors. After a brief description of the IrO2 structure and properties, the present review will describe the main employed synthetic methodologies that are followed to prepare selectively the various types of nanostructures, highlighting in each case the advantages brought by the nanostructuration illustrating their performances and applications.

Luminescent Self-Assembled Monolayer on Gold Nanoparticles: Tuning of Emission According to the Surface Curvature

Until now, the ability to form a self-assembled monolayer (SAM) on a surface has been investigated according to deposition techniques, which in turn depend on surface-coater interactions. In this paper, we pursued two goals: to form a SAM on a gold nanosurface and to correlate its formation to the nanosurface curvature. To achieve these objectives, gold nanoparticles of different shapes (spheres, rods, and triangles) were functionalized with a luminescent thiolated bipyridine (Bpy-SH), and the SAM formation was studied by investigating the photo-physics of Bpy-SH. We have shown that emission wavelength and excited-state lifetime of Bpy-SH are strongly correlated to the formation of specific aggregates within SAMs, the nature of these aggregates being in close correlation to the shape of the nanoparticles. Micro-Raman spectroscopy investigation was used to test the SERS effect of gold nanoparticles on thiolated bipyridine forming SAMs.

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

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

Curcumin-based ionic Pt(ii) complexes: antioxidant and antimicrobial activity

Four new Pt(ii) ionic complexes assembled from N-donor ligands and curcumin display interesting antioxidant and antimicrobial properties.

Insertion of Unsaturated C-C Bonds into the O-H Bond of an Iridium(III)-Hydroxo Complex: Formation of Phosphorescent Emitters with an Asymmetrical β-Diketonate Ligand

A synthetic methodology to prepare iridium(III) emitters of the class [3b+3b+3b'] with two ortho-metalated 1-phenylisoquinolines and an asymmetrical β-diketonate has been discovered. The abstraction of the chloride ligands of the dimer [Ir(μ-Cl){κ²-C,N-(C₆H₄-isoqui)}₂]₂ (1, C₆H₅-isoqui = 1-phenylisoquinoline) with AgBF₄ in acetone and the subsequent addition of water to the resulting solution affords the water solvate mononuclear complex [Ir{κ²-C,N-(C₆H₄-isoqui)}₂(H₂O)₂]BF₄ (2), which reacts with KOH to give the dihydroxo-bridged dimer [Ir(μ-OH){κ²-C,N-(C₆H₄-isoqui)}₂]₂ (3). Treatment of the latter with dimethyl acetylenedicarboxylate leads to Ir{κ²-C,N-(C₆H₄-isoqui)}₂{κ²-O,O-[OC(CO₂CH₃)CHC(OCH₃)O]} (4), as a result of the anti-addition of the O-H bond of a mononuclear [Ir(OH){κ²-C,N-(C₆H₄-isoqui)}₂] fragment to the C-C triple bond of the alkyne and the coordination of one of the carboxylate substituents to the metal center. Complex 3 also reacts with α,β-unsaturated ketones. The reaction with 3-(4-methylphenyl)-1-phenylprop-2-en-1-one affords Ir{κ²-C,N-(C₆H₄-isoqui)}₂{κ²-O,O-[OC(C₆H₅)CHC(p-C₆H₄Me)O]} (5), whereas methyl vinyl ketone gives a mixture of Ir{κ²-C,N-(C₆H₄-isoqui)}₂{κ²-O,O-[OC(CH₃)CHCHO]} (6) and Ir{κ²-C,N-(C₆H₄-isoqui)}₂{κ²-O,O-[OC(CH₃)CHC(CH═CH₂)O]} (7). Complexes 5 and 6 are the result of the addition of the O-H bond of the mononuclear [Ir(OH){κ²-C,N-(C₆H₄-isoqui)}₂] fragment to the C-C double bond of the α,β-unsaturated ketones and the coordination of the carbonyl group to the iridium center, to generate O,O-chelates which lose molecular hydrogen to aromatize into the asymmetrical β-diketonate ligands. Complexes 4-7 are phosphorescent emitters in the red spectral region (599-672 nm) in doped poly(methyl methacrylate) (PMMA) film at 5 wt % at room temperature and 2-methyltetrahydrofuran at room temperature and 77 K. They display short lifetimes (0.8-2.5 μs) and quantum yields in both doped PMMA films and in 2-methyltetrahydrofuran at room temperature depending on the substituents of the β-diketonate: about 0.6-0.5 for 4 and 6 and ca. 0.35 for 5 and 7.

Electrochromic behaviour of Ir(iii) bis-cyclometalated 1,2-dioxolene tetra-halo complexes: fully reversible catecholate/semiquinone redox switches

Neutral cyclometalated Ir(iii) complexes of general formula [Ir(ppy)2(O^O)squi], where ppy = 2-phenylpyridine and (O^O)squi = TBC (tetrabromocatechol) or TCC (tetrachlorocatechol) in their semiquinone (squi) monoanionic redox state, were synthesized by chemically oxidizing the anionic parent complexes NBu4[Ir(ppy)2(O^O)cat], in which (O^O)cat represents the corresponding ancillary dioxolene ligand in its dianionic catecholate (cat) redox state. This chemical oxidation leads to the modification of both the photophysical and the magnetic properties of the complexes. While the NBu4[Ir(ppy)2(O^O)cat] complexes are diamagnetic (D) and yellow-orange solids, the corresponding oxidized complexes [Ir(ppy)2(O^O)squi] display paramagnetic (P) properties and are characterized by a dark-green color. The conversion between the two forms (squi vs. cat) is electrochemically and chemically fully reversible. Indeed, the anionic NBu4[Ir(ppy)2(O^O)cat] complexes are quantitatively restored by chemical reduction of the neutral [Ir(ppy)2(O^O)squi] parents. These complexes therefore represent interesting redox based switches between multi-parameter states since they allow switching from a neutral paramagnetic to an anionic diamagnetic form together with a significant change in chromicity. Taking advantage of the significant color difference between the oxidized and the reduced form, an electrochromic cell was prepared with [Ir(ppy)2(TBC)squi] and its spectroelectrochemical properties are reported.

Electropolymerizable IrIII Complexes with β-Ketoiminate Ancillary Ligands

A series of electropolymerizable cyclometallated Ir^III complexes were synthesized and their electrochemical and photophysical properties studied. The triphenylamine electropolymerizable fragment was introduced by using triphenylamine-2-phenylpyridine and, respectively, triphenylamine-benzothiazole as cyclometalated ligands. The coordination sphere was completed by two differently substituted β-ketoiminate ligands deriving from the condensation of acetylacetone or hexafluoroacetylacetone with para-bromoaniline. The influence of the -CH₃ /-CF₃ substitution to the electrochemical and photophysical properties was investigated. Both complexes with CH₃ substituted β-ketoiminate were emissive in solution and in solid state. Highly stable films were electrodeposited onto ITO coated glass substrates. Their emission was quenched by electron trapping within the polymeric network as proven by electrochemical studies. The -CF₃ substitution of the β-ketoiminate leads instead to the quenching of the emission and inhibits electropolymerization.

Photofunctions of iridium(iii) complexes in vesicles: long-lived excited states and visible-light sensitization for hydrogen evolution in aqueous solution

Utilization of DPPC vesicles allows water-insoluble photoactive Ir(iii) complexes to be dispersed in bulk aqueous solution.

Phosphorescent cationic iridium(iii) complexes dynamically bound to cyclodextrin vesicles: applications in live cell imaging

We report cationic Ir(iii) complexes functionalized with adamantyl groups designed to bind to β-cyclodextrin vesicles (CDV) with high affinity (K a = 1 × 106 M-1). The emission of the complexes is tuned by changing the nature of the cyclometalating ligands. The host-guest adduct of CDV and Ir(iii) complexes shows increased and significantly blue-shifted emission due to the lower mobility of the Ir(iii)-complexes residing in the less polar environment of the vesicle surface. Ir(iii)-decorated CDV are efficiently taken up by cells and can be used in live cell imaging. The CDV act as carriers to transport the phosphorescent complexes into cells where they selectively stain mitochondria.

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.

High Order in a Self-Assembled Iridium(III) Complex Gelator Towards Nanostructured IrO2 Thin Films

The preparation and characterization of a new metallogelator based on the Ir^III discrete cyclometalated complex [(ppy)₂ Ir(bpy)](CH₃ CH₂ OCH₂ CO₂ ) are reported, where H(ppy) is 2-phenylpiridine and bpy is 2,2'-bipyridine, which is used as an ancillary ligand. The compound is able to self-assemble in water in a range of concentrations between 3 % and 6 % w/w, creating a luminescent ordered supramolecular gel. The gel and xerogel architectures were investigated through polarized optical microscopy (POM), SEM and TEM microscopies coupled with powder X-ray diffraction. The gel supramolecular organization is characterized by columnar tetragonal strands, already present at high dilution conditions, of cations surrounded by counteranions. These strands, in turn, are self-assembled in an oblique columnar cell upon gelification. The xerogel thin films obtained upon complete dehydration maintained the gel supramolecular order and can be used as a precursor for the preparation of nanostructured IrO₂ thin films.