Cyclometalated iridium and platinum complexes as singlet oxygen photosensitizers: quantum yields, quenching rates and correlation with electronic structures

Host–guest luminescent materials based on highly emissive species loaded into versatile sol–gel hosts

By mostly focusing on the findings of our group, this concise review provides insights into the development of promising new host–guest optical materials based on sol–gel assemblies of versatile hosts and highly luminescent guests.

Photophysical dynamics of the efficient emission and photosensitization of [Ir(pqi)2(NN)]+complexes

Rational photophysical investigation through experimental and theoretical analyses reveals the photophysical dynamics of the highly-emissive [Ir(pqi)₂(NN)]⁺complex series, with remarkable emission quantum yields and efficient generation of singlet oxygen.

Nanoscale Metal-Organic Layers for Deeply Penetrating X-ray-Induced Photodynamic Therapy

We report the rational design of metal-organic layers (MOLs) that are built from [Hf6 O4 (OH)4 (HCO2 )6 ] secondary building units (SBUs) and Ir[bpy(ppy)2 ]+ - or [Ru(bpy)3 ]2+ -derived tricarboxylate ligands (Hf-BPY-Ir or Hf-BPY-Ru; bpy=2,2'-bipyridine, ppy=2-phenylpyridine) and their applications in X-ray-induced photodynamic therapy (X-PDT) of colon cancer. Heavy Hf atoms in the SBUs efficiently absorb X-rays and transfer energy to Ir[bpy(ppy)2 ]+ or [Ru(bpy)3 ]2+ moieties to induce PDT by generating reactive oxygen species (ROS). The ability of X-rays to penetrate deeply into tissue and efficient ROS diffusion through ultrathin 2D MOLs (ca. 1.2 nm) enable highly effective X-PDT to afford superb anticancer efficacy.

AcetylacetonateBODIPY-Biscyclometalated Iridium(III) Complexes: Effective Strategy towards Smarter Fluorescent Photosensitizer Agents

Biscyclometalated Ir^III complexes involving boron-dipyrromethene (BODIPY)-based ancillary ligands, where the BODIPY unit is grafted to different chelating cores (acetylacetonate for Ir-1 and Ir-2, and bipyridine for Ir-3) by the BODIPY meso position, have been synthesized and characterized. Complexes with the BODIPY moiety directly grafted to acetylacetonate (Ir-1 and Ir-2) exhibit higher absorption coefficients (ϵ≈4.46×10⁴  m^-1  cm^-1 and 3.38×10⁴  m^-1  cm^-1 at 517 nm and 594 nm, respectively), higher moderate fluorescence emission (φ_fl ≈0.08 and 0.22 at 528 nm and 652 nm, respectively) and, in particular, more efficient singlet oxygen generation upon visible-light irradiation (φ_Δ ≈0.86 and 0.59, respectively) than that exhibited by Ir-3 (φ_Δ ≈0.51, but only under UV light). Phosphorescence emission, nanosecond time-resolved transient absorption, and DFT calculations suggest that BODIPY-localized long-lived ³ IL states are populated for Ir-1 and Ir-2. In vitro photodynamic therapy (PDT) activity studied for Ir-1 and Ir-2 in HeLa cells shows that such complexes are efficiently internalized into the cells, exhibiting low dark- and high photocytoxicity, even at significantly low complex concentration, making them potentially suitable as theranostic agents.

Synthesis and characterization of phosphorescent two-coordinate copper(i) complexes bearing diamidocarbene ligands

[(DAC)₂Cu][BF₄] (1) shows a high photoluminescence quantum efficiency (Φ_PL) in the solid state and in solution under both N₂ and O₂.

Preparation of metalated azine complexes of iridium(iii)

The preparation of half-sandwich κ¹-aldazine and κ¹-ketazine complexes of iridium and their metalation reaction affording chelate κ²-azine derivatives are described. The photoluminescence properties of the chelate complexes are also described.

The use of organolithium reagents for the synthesis of 4-aryl-2-phenylpyridines and their corresponding iridium(iii) complexes

A versatile palladium-free route for the synthesis of 4-aryl-substituted phenylpyridines (ppy), starting from tert-butyl 4-oxopiperidine-1-carboxylate, to give 4 ligands (L1–4H) is reported. These ligands were coordinated to iridium to give the corresponding Ir(L)₂(A) complexes.

Development of a D–π–A pyrazinium photosensitizer possessing singlet oxygen generation

(D–π–)₂A pyrazinium dyes (OEJ-1 and OEJ-2) bearing a counter anion (X⁻ = Br⁻ or I⁻) have been newly developed as a photosensitizer possessing singlet oxygen (¹O₂) generation.

Single oxygen generation sensitized by spiro(dipyridinogermole)(dithienogermole)s

Three spiro(dipyridinogermole)(dithienogermole) derivatives (1–3), including newly prepared spiro(dipyridinogermole)[di(2-pyridyl)dithienogermole] (3), were examined as photosensitizers for singlet oxygen (¹O₂) generation in dichloromethane–methanol.

Photostable ester-substituted bis-cyclometalated cationic iridium(iii) complexes for continuous monitoring of oxygen

Bis-cyclometalated cationic iridium(iii) complexes bearing an ester group possess long luminescence lifetimes, good oxygen-sensitivity and excellent photostability.

A ligand substituted tungsten iodide cluster: luminescence vs. singlet oxygen production

The cluster (TBA)₂[W₆I₈(CF₃COO)₆] shows photoluminescence in the solid state and in solution, and singlet oxygen (a¹Δ_g) is generated in the presence of oxygen.

Developing strongly luminescent platinum(IV) complexes: facile synthesis of bis-cyclometalated neutral emitters

A straightforward, one-pot procedure has been developed for the synthesis of bis-cyclometalated chloro(methyl)platinum(IV) complexes with a wide variety of heteroaromatic ligands of the 2-arylpyridine type. The new compounds exhibit phosphorescent emissions in the blue to orange colour range and represent the most efficient Pt(IV) emitters reported to date, with quantum yields up to 0.81 in fluid solutions at room temperature.

Yellow/orange emissive heavy-metal complexes as phosphors in monochromatic and white organic light-emitting devices

Owing to the electron spin-orbit coupling (SOC) and fast intersystem crossing (ISC), heavy-metal complexes (such as iridium(III), platinum(II) and osmium(II) complexes, etc.) are phosphorescent emitters at room temperature. Since 1998, heavy-metal complexes as phosphors have received considerable academic and industrial attention in the field of organic light-emitting diodes (OLEDs), because they can harvest both the singlet (25%) and triplet (75%) excitons for emission during the electro-generated processes. Among all the visible colors (blue, green, yellow, orange and red), the yellow/orange heavy-metal complexes play an important role for realizing full-color OLEDs as well as high-efficiency white OLEDs, and thus the development of highly efficient yellow/orange heavy-metal complexes is a pressing concern. In this article, we will review the progress on yellow/orange heavy-metal complexes as phosphors in OLEDs. The general principles and useful tactics for designing the yellow/orange heavy-metal complexes will be systematically summarized. The structure-property relationship and electrophosphorescence performance of the yellow/orange heavy-metal complexes in monochromatic phosphorescent OLEDs (PhOLEDs) and white OLEDs (WOLEDs) will be comprehensively surveyed and discussed.

Ir(2-phenylpyridine)2(benzene-1,2-dithiolate) anion as a diastereoselective metalloligand and nucleophile: stereoelectronic effect, spectroscopy, and computational study of the methylated and aurated complexes and their oxygenation products

The anionic complex [Ir(2-phenylpyridine)2(benzene-1,2-dithiolate)](-) ([IrSS](-)) is a nucleophile and metalloligand that reacts with methyl iodide and AuPR3(+) (R = Ph or Et) to form S-methylated complexes (thiother-thiolate and dithiother complexes) and S-aurated complexes, respectively. The reactions are completely diastereselective, producing only the enantiomers ΛS and ΔR or ΛSS and ΔRR. The diastereoselectivity is stereoelectronically controlled by the orientation of the highest occupied molecular orbital (HOMO) of [IrSS](-) arising from filled dπ-pπ antibonding interactions, and the chirality of the iridium ion. Methylation or auration removes the high-energy lone pair of the thiolate S atom, leading to low-lying HOMOs composed mainly of the Ir d-orbital and the 2-phenylpyridine π (ppyπ) orbital. The methylated and aurated complexes can be oxidized by H2O2 or peracid to give sulfinate-thiother, disulfoxide, and sulfinate-sulfoxide complexes, and the oxygenation further stabilizes the HOMO. All the complexes are luminescent, and their electronic spectra are interpreted with the aid of time-dependent density functional theory calculations. The thiother-thiolate complex exhibits ligand(S)-to-ligand(π* of ppy)-charge-transfer/metal-to-ligand-charge-transfer absorption (LLCT/MLCT) and a relatively low-energy (3)LLCT/MLCT emission, while the other complexes display (3)ππ*/MLCT emissions.

A luminescent poly(amidoamine)-iridium complex as a new singlet-oxygen sensitizer for photodynamic therapy

A polymer complex (1P) was synthesized by binding bis(cyclometalated) Ir(ppy)2(+) fragments (ppy = 2-phenylpyridyl) to phenanthroline (phen) pendants of a poly(amidoamine) copolymer (PhenISA, in which the phen pendants involved ∼6% of the repeating units). The corresponding molecular complex [Ir(ppy)2(bap)](+) (1M, bap = 4-(butyl-4-amino)-1,10-phenanthroline) was also prepared for comparison. In water solution 1P gives nanoaggregates with a hydrodynamic diameter of 30 nm in which the lipophilic metal centers are presumed to be segregated within polymer tasks to reduce their interaction with water. Such confinement, combined with the dilution of triplet emitters along the polymer chains, led to 1P having a photoluminescence quantum yield greater than that of 1M (0.061 vs 0.034, respectively, in an aerated water solution) with a longer lifetime of the (3)MLCT excited states and a blue-shifted emission (595 nm vs 604 nm, respectively). NMR data supported segregation of the metal centers. Photoreaction of O2 with 1,5-dihydroxynaphthalene showed that 1P is able to sensitize (1)O2 generation but with half the quantum yield of 1M. Cellular uptake experiments showed that both 1M and 1P are efficient cell staining agents endowed with two-photon excitation (TPE) imaging capability. TPE microscopy at 840 nm indicated that both complexes penetrate the cellular membrane of HeLa cells, localizing in the perinuclear region. Cellular photodynamic therapy tests showed that both 1M and 1P are able to induce cell apoptosis upon exposure to Xe lamp irradiation. The fraction of apoptotic cells for 1M was higher than that for 1P (74 and 38%, respectively) 6 h after being irradiated for 5 min, but cells incubated with 1P showed much lower levels of necrosis as well as lower toxicity in the absence of irradiation. More generally, the results indicate that cell damage induced by 1M was avoided by binding the iridium sensitizers to the poly(amidoamine).

Solvent dependent photosensitized singlet oxygen production from an Ir(iii) complex: pointing to problems in studies of singlet-oxygen-mediated cell death

The photophysics of an Ir(iii) complex with phenanthroline and phenylpyridine ligands depends appreciably on the local environment.

Phosphorescent biscyclometallated iridium(iii) ethylenediamine complexes functionalised with polar ester or carboxylate groups as bioimaging and visualisation reagents

Four new phosphorescent biscyclometallated iridium(iii) ethylenediamine complexes were designed as bioimaging and visualization reagents.

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.

Novel phosphorescent iridium(iii) complexes containing 2-thienyl quinazoline ligands: synthesis, photophysical properties and theoretical calculations

Highly efficient phosphorescent iridium complexes based on 2-thienyl quinazoline derivatives with various substituents are successfully prepared and fully characterized.

Exploring excited-state tunability in luminescent tris-cyclometalated platinum(IV) complexes: synthesis of heteroleptic derivatives and computational calculations

The synthesis, structure, electrochemistry, and photophysical properties of a series of heteroleptic tris- cyclometalated Pt(IV) complexes are reported. The complexes mer-[Pt(C^N)2 (C'^N')]OTf, with C^N=C-deprotonated 2-(2,4-difluorophenyl)pyridine (dfppy) or 2-phenylpyridine (ppy), and C'^N'=C-deprotonated 2-(2-thienyl)pyridine (thpy) or 1-phenylisoquinoline (piq), were obtained by reacting bis- cyclometalated precursors [Pt(C^N)2 Cl2] with AgOTf (2 equiv) and an excess of the N'^C'H pro-ligand. The complex mer-[Pt(dfppy)2 (ppy)]OTf was obtained analogously and photoisomerized to its fac counterpart. The new complexes display long-lived luminescence at room temperature in the blue to orange color range. The emitting states involve electronic transitions almost exclusively localized on the ligand with the lowest π-π* energy gap and have very little metal character. DFT and time-dependent DFT (TD-DFT) calculations on mer-[Pt(ppy)2 (C'^N')](+) (C'^N'=thpy, piq) and mer/fac-[Pt(ppy)3](+) support this assignment and provide a basis for the understanding of the luminescence of tris-cyclometalated Pt(IV) complexes. Excited states of LMCT character may become thermally accessible from the emitting state in the mer isomers containing dfppy or ppy as chromophoric ligands, leading to strong nonradiative deactivation. This effect does not operate in the fac isomers or the mer complexes containing thpy or piq, for which nonradiative deactivation originates mainly from vibrational coupling to the ground state.

Singlet oxygen generation by cyclometalated complexes and applications

While cyclometalated complexes have been extensively studied for optoelectronic applications, these compounds also represent a relatively new class of photosensitizers for the production of singlet oxygen. Thus far, singlet oxygen generation from cyclometalated Ir and Pt complexes has been studied in detail. In this review, photophysical data for singlet oxygen generation from these complexes are presented, and the mechanism of (1) O2 generation is discussed, including evidence for singlet oxygen generation via an electron-transfer mechanism for some of cyclometalated Ir complexes. The period from the first report of singlet oxygen generation by a cyclometalated Ir complex in 2002 through August 2013 is covered in this review. This new class of singlet oxygen photosensitizers may prove to be rather versatile due to the ease of substitution of ancillary ligands without loss of activity. Several cyclometalated complexes have been tethered to zeolites, polystyrene, or quantum dots. Applications for photooxygenation of organic molecules, including "traditional" singlet oxygen reactions (ene reaction, [4 + 2] and [2 + 2] cycloadditions) as well as oxidative coupling of amines are presented. Potential biomedical applications are also reviewed.

Solution-processable electrochemiluminescent ion gels for flexible, low-voltage, emissive displays on plastic

Ion gels comprising ABA triblock copolymers and ionic liquids have received much attention as functional materials in numerous applications, especially as gate dielectrics in organic transistors. Here we have expanded the functionality of ion gels by demonstrating low-voltage, flexible electrochemiluminescent (ECL) devices using patterned ion gels containing redox-active luminophores. The ECL devices consisted only of a 30 μm thick emissive gel and two electrodes and were fabricated on indium tin oxide-coated substrates (e.g., polyester) simply by solution-casting the ECL gel and brush-painting a top Ag electrode. The triblock copolymer employed in the gel was polystyrene-block-poly(methyl methacrylate)-block-polystyrene, where the solvophobic polystyrene end blocks associate into micellar cross-links in the versatile ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]). An ECL gel containing ~6.25 wt % Ru(bpy)3Cl2 (relative to [EMI][TFSI]) as the luminophore turned on at an AC peak-to-peak voltage as low as 2.6 V (i.e., -1.3 to +1.3 V) and showed a relatively rapid response (sub-ms). The wavelength of maximum emission was 610 nm (red-orange). With the use of an iridium(III) complex, Ir(diFppy)2(bpy)PF6 [diFppy = 2-(2',4'-difluorophenyl)pyridine; bpy = 2,2'-bipyridyl], the emitting color was tuned to a maximum wavelength of 540 nm (green). Moreover, when a blended luminophore system containing a 60:40 mixture of Ru(bpy)3(2+) and Ir(diFppy)2(bpy)(+) was used in the emissive layer, the luminance of red-orange-colored light was enhanced by a factor of 2, which is explained by the generation of the additional excited state Ru(bpy)3(2+)* by a coreactant pathway with Ir(diFppy)2(bpy)(+)* in addition to the usual annihilation pathway. This is the first time that enhanced ECL has been achieved in ion gels (or ionic liquids) using a coreactant. Overall, the results indicate that ECL ion gels are attractive multifunctional materials for printed electronics.

Homoleptic tris-cyclometalated platinum(iv) complexes: a new class of long-lived, highly efficient 3LC emitters

Tris-cyclometalated Pt(IV) complexes are reported for the first time. The facial isomers exhibit long-lived ³LC emissions with quantum yields up to 0.49, the highest ever found for Pt(IV) complexes, combined with a strong oxidizing character in the excited state.

An unprecedented cyclometallated platinum(ii) complex incorporating a phosphinine co-ligand: synthesis and photoluminescence behaviour

The first luminescent platinum(ii) complex bearing phosphinine co-ligands with a long-lived emission from a phosphinine-localised triplet excited state is reported.

When self-assembly meets biology: luminescent platinum complexes for imaging applications

Self-assembled luminescent structures based on platinum complexes. A new tool for bioimaging?

Synthesis and photochemical properties of pH responsive tris-cyclometalated iridium(III) complexes that contain a pyridine ring on the 2-phenylpyridine ligand

In our previous publication, it was reported that fac-Ir(atpy)3 3 (atpy = 2-(5'-amino-4'-tolyl)pyridine), which contains three amino groups at the 5'-position of the atpy ligands, exhibits a pH-dependent change in the color of the emitted radiation. Aqueous solution of 3 shows a weak red emission (at around 613 nm) under neutral or basic conditions, but the emission color changes to green (at around 530 nm) under acidic conditions, where the NH2 group is protonated to become an electron-withdrawing (NH3)(+) group. In this manuscript, we report on the preparation of some new pH-responsive Ir(III) complexes; fac-Ir(4Pyppy)3 5 and fac-Ir(3Pyppy)3 6 that contain three pyridyl groups at the 5'-position of the 2-phenylpyridine (ppy) ligand, and Ir(4Pyppym)3 7 and Ir(3Pyppym)3 8 that contain a pyridyl group at the same position of the 2-phenylpyrimidine (ppym) ligand. The introduction of three pyridyl groups on iodinated Ir(ppy)3 and Ir(ppym)3 was achieved via Suzuki-Miyaura cross-coupling reaction assisted by microwave irradiation. Solutions of the acid-free Ir(III) complexes 5, 6, 7, and 8 showed a strong green emission (at around 500 nm) in dimethylsulfoxide (DMSO). Protonation of three pyridyl groups of 5 and 7 causes a significant red-shift in the emission wavelength (at around 600 nm) with a decrease in emission intensity. The pH-dependent emission change of these complexes is also discussed. The generation of singlet oxygen ((1)O2) by the photoirradiation of the Ir complexes 5 and 6 was evidenced by the decomposition of 1,3-diphenylisobenzofuran (DPBF), the oxidation of thioanisole, and the oxidation of 2,2,5,5-tetramethyl-3-pyrroline-3-carboxamide (TPC). The induction of necrosis-like cell death of HeLa-S3 cells upon photoirradiation of 5 at 465 nm is also reported.